# Design and Construction of Large, Welded, Low-pressure Storage Tanks

API STANDARD 620 TWELFTH EDITION, OCTOBER 2013

### Special Notes API publications necessarily addr ess problems of a general nature. With respect to particular circumstances, local, state, and federal laws and regulations should be reviewed. Neither API nor any of API' s employees, subcontract ors, consultants, committees, or other assignees make any warranty or representation, either expres s or implied, with respect to the accu racy, completeness, or usefulness of the information contained herein, or assume any liability or responsibility for any use, or the results of such use, of any information or process disclosed in this publication. Neither API nor any of API' s employees, subcontractors, consultants, or other assignees represent that use of this publication would not infringe upon privately owned rights. API publications may be used by anyone desiring to do so. Ev ery effort has been made by t he Institute to assure the accuracy and reliability of the data cont ained in them; however, the Institute ma kes no representation, warranty, or guarantee in connection with this publicati on and hereby expressly disclaims any liability or re sponsibility for loss or damage resulting from its use or for the violation of any aut horities having jurisdiction with which this publication may conflict. API publications are published to fac ilitate the broad availability of proven, sound engineering and operating practices. These publications are not intended to obviate the need for applying sound engineering judgment regarding when and where these publicati ons should be utilized. The formulation and publication of API publications is not intended in any way to inhibit anyone from using any other practices. Any manufacturer marking equipment or materials in confor mance with the marking requirements of an API standard is solely responsible for complying with all the applicable requirements of that standard. API does not represent, warrant, or guarantee that such products do in fact conform to the applicable API standard.

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### Foreword Nothing contained in any API publication is to be construed as granting any right, by implication or otherwise, for the manufacture, sale, or use of any method, apparatus, or product covered by letters pa tent. Neither should anything contained in the publication be construed as insuring anyone agains t liability for infringement of letters patent. Shall: As used in a standard, “shall” denotes a minimum requirement in order to conform to the specification. Should: As used in a standard, “should” denotes a recommendation or that which is advised but not required in or der to conform to the specification. This document was produced under API standardization pr oced ures that ensu re appropriate notification and participation in the developmental process and is designated as an API standard. Questions concerning the interpretation of the content of this publication or comments and questi ons concerning the procedures under which this publication was developed should be directed in writ ing to the Director of Standards, American Petroleum Institute, 1220 L Street, NW, Washington, DC 20005. Requests fo r permission to reproduce or translate all or any part of the material published herein should also be addressed to the director. Generally, API standards are reviewed and revised, reaffirmed, or withdrawn at least ev ery five years. A one-time extension of up to two years may be added to this review cy cle. Status of the publicati on can be ascertained from the API Standards Department, telephone (202) 682-8000. A ca talog of API publications and materials is published annually by API, 1220 L Street, NW, Washington, DC 20005. Suggested revisions are invited and shoul d be submitted to the S tandards Department, API, 1220 L Street, NW, Washington, DC 20005, standards@api.org. Also see Annex A for further information regarding the submitting of suggestions for change.

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### Contents
Page 1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-1 1.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . 1-1 1.2 Coverage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . 1-1 1.3 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . 1-3 2 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .2-1 3 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-1 3.1 Stress and Pressure Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . 3-1 3.2 Capacity Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . 3-1 3.3 Tank Wall. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . 3-1 3.4 Welding Terms

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1 3.5 Other Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . 3-3 4 Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .4-1 4.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . 4-1 4.2 Plates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . 4-1 4.3 Pipe, Flanges, Forging, and Castings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . 4-8 4.4 Bolting Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . 4-10 4.5 Structural Shapes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . 4-10 5 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .5-1 5.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . 5-1 5.2 Operating Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . 5-1 5.3 Pressures Used in Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . 5-1 5.4 Loads. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . 5-2 5.5 Maximum Allowable Stress for Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . 5-3 5.6 Maximum Allowable Stress Values for Structural Members and Bolts. . . . . . . . . . . . . . . . . . . . . . . . . .5-11 5.7 Corrosion Allowance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . 5-15 5.8 Linings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . 5-15 5.9 Procedure for Designing Tank Walls. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . 5-15 5.10 Design of Sidewalls, Roofs, and Bottoms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . 5-17 5.11 Special Considerations Applicable to Bottoms That Rest Directly on Foun dat ions. . . . . . . . . . . . . . 5-27 5.12 Design of Roof and Bottom Knuckle Regions and Compr ession-ring Girders

. . . . . . . . . . . . . . . . . . 5-29 5.13 Design of Internal and External Structural Members . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-35 5. 14 Shapes, Locations, and Maximum Sizes of Wall Openings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5- 37 5.15 Inspection Openings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . 5-39 5.16 Reinforcement of Single Openings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . 5-39 5.17 Reinforcement of Multiple Openings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . 5-49 5.18 Design of Large, Centrally Located, Circular Openings in R oof s and Bottoms . . . . . . . . . . . . . . . . . 5-50 5.19 Nozzle Necks and Their Attachments to the Tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-52 5. 20 Bolted Flanged Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . 5-53 5.21 Cover Plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . 5-54 5.22 Permitted Types of Joints. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . 5-60 5.23 Welded Joint Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . 5-61 5.24 Plug Welds and Slot Welds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . 5-61 5.25 Stress Relieving. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . 5-62 5.26 Radiographic/Ultrasonic Examination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . 5-62 5.27 Flush-type Shell Connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . 5-64

vPage 6 Fabrication. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1 6.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-1 6.2 Workmanship. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-1 6.3 Cutting Plates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-1 6.4 Forming Sidewall Sections and Roof and Bottom Plates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-1 6.5 Dimensional Tolerances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-1 6.6 Details of Welding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-4 6.7 Qualification of Welding Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-4 6.8 Qualification of Welders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-5 6.9 Matching Plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-5 6.10 Cleaning Surfaces to be Welded . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-6 6.11 Weather Conditions for Welding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-6 6.12 Reinforcement on Welds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-7 6.13 Merging Weld With Plate Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-7 6.14 Aligning of Main Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-7 6.15 Repairing Defects in Welds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-7 6.16 Matching Plates of Unequal Thickness. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-7 6.17 Fitting Up of Closure Plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-8 6.18 Thermal Stress Relief . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-8 6.19 Peening Field Welds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-8 7 Inspection, Examination and Testing . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1 7.1 Responsibility of Inspector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-1 7.2 Qualifications of Inspectors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-1 7.3 Access for Inspector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-1 7.4 Facilities for Inspector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-1 7.5 Approval of Repairs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-1 7.6 Inspection of Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-1 7.7 Stamping of Plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-2 7.8 Measuring Thickness of Material. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-2 7.9 Inspection of Surfaces Exposed during Fabrication. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-2 7.10 Surface Inspection of Component Parts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-2 7.11 Check of Dimensions of Component Parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-2 7.12 Check of Chemical and Physical Property Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-2 7.13 Data Required from Manufacturer on Completed Tanks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-2 7.14 Check of Stress-relieving Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-2 7.15 Examination Method and Acceptance Criteria. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-3 7.16 Inspection of Welds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-5 7.17 Radiographic/Ultrasonic Examination Requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-6 7.18 Standard Hydrostatic and Pneumatic Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-8 7.19 Proof Tests for Establishing Allowable Working Pressures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-11 7.20 Test Gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-12 8 Marking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1 8.1 Nameplates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-1 8.2 Division of Responsibility. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-2 8.3 Manufacturer’s Report and Certificate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-2 8.4 Multiple Assemblies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-2 9 Pressure- and Vacuum-relieving Devices. .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1 9.1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9-1 9.2 Pressure-relieving Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9-1 9.3 Construction of Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9-1

viPage 9.4 Means of Venting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9-1 9.5 Liquid Relief Valves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9-1 9.6 Marking. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9-1 9.7 Pressure Setting of Safety Devices. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9-2

Annex A (informative) Inquiries and Suggestions for Change. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-1 Annex B (normative) Use of Materials That are Not Identified with Listed Specifications . . . . . . . . . . . . . . . B-1 Annex C (informative) Suggested Practice Regarding Foundations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-1 Annex D (informative) Suggested Practice Regarding Supporting Structures . . . . . . . . . . . . . . . . . . . . . . . . D-1 Annex E (informative) Suggested Practice Regarding Attached Structures (Internal and External). . . . . . . E-1 Annex F (informative) Examples Illustrating Application of Rules to Various Design Problems. . . . . . . . . . F-1 Annex G (informative) Considerations Regarding Corrosion Allowance and Hydrogen-induced Cracking G-1 Annex H (informative) Recommended Practice for Use of Preheat, Post-heat, and Stress Relief. . . . . . . . . H-1 Annex I (informative) Suggested Practice for Peening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-1 Annex J (Reserved for Future Use) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .J-1 Annex K (informative) Suggested Practice for Determining the Relieving Capacity Required . . . . . . . . . . . K-1 Annex L (normative) Seismic Design of Storage Tanks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-1 Annex M (informative) Recommended Scope of the Manufacturer’s Report . . . . . . . . . . . . . . . . . . . . . . . . . . M-1 Annex N (normative) Installation of Pressure-relieving Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . N-1 Annex O (informative) Suggested Practice Regarding Installation of Low-pressure Storage Tanks . . . . . . O-1 Annex P (informative) NDE and Testing Requirements Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . P-1 Annex Q (normative) Low-pressure Storage Tanks for Liquefied Gases at –325 ° F or Warmer . . . . . . . . . . Q-1 Annex R (normative) Low-pressure Storage Tanks Operating Between +40 ° F and –60 ° F . . . . . . . . . . . . . . R-1 Annex S (normative) Austenitic Stainless Steel Storage Tanks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S-1 Annex SC (normative) Stainless and Carbon Steel Mixed Materials Storage Tanks . . . . . . . . . . . . . . . . . . . SC-1 Annex U (normative) Ultrasonic Examination in Lieu of Radiography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . U-1 Annex X (normative) Duplex Stainless Steel Storage Tanks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . X-1 Figures 4-1 Isothermal Lines Showing 1-day Mean Ambient Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-3 4-2 Minimum Permissible Design Metal Temp erature for Pipe, Flanges, and Forgings without Impact Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-10 4-3 Governing Thickness for Impact Test Determination of Pipe, Flanges, and Forgings . . . . . . . . . . . . .4-12 5-1 Biaxial Stress Chart for Combined Tension and Compression, 30,000 lbf/in. 2 – 38,000 lbf/in. 2 Yield Strength Steels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-5 5-2 Method for Preparing Lap-welded Bottom Plates under the Tank Sidewall. . . . . . . . . . . . . . . . . . . . . .5-16 5-3 Detail of Double Fillet-groove Weld for Bottom Plates with a Nominal Thickness Greater than 1 / 2 in.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-17 5-4 Typical Free-body Diagrams for Certain Shapes of Tanks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-18 5-5 Compression-ring Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-31 5-6 Permissible and Non-permissible Details of Constructi on for a Compression-ring Juncture . . . . . .5-33
viiPage 5-7 Reinforcement of Single Openings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-40 5-8 Part 1—Acceptable Types of Welded Nozzles and Other Connections . . . . . . . . . . . . . . . . . . . . . . . . . 5-42 5-8 Part 2—Acceptable Types of Welded Nozzles and Other Connections . . . . . . . . . . . . . . . . . . . . . . . . . 5-41 5-8 Part 3—Acceptable Types of Welded Nozzles and Other Connections . . . . . . . . . . . . . . . . . . . . . . . . . 5-42 5-8 Part 4—Acceptable Types of Welded Nozzles and Other Connections . . . . . . . . . . . . . . . . . . . . . . . . . 5-43 5-9 Large Head Openings and Conical Shell-reducer Sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5- 51 5-10 Acceptable Types of Flat Heads and Covers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . 5-55 5-11 Spherically Dished Steel Plate Covers with Bolting Flanges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-5 7 5-1 2 Part 1—Flush-type Sidewall Connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . 5-65 5-12 Part 2—Flush-type Sidewall Connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . 5-66 5-13 Design Factors for Flush-type Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . 5-68 5-14 Rotation of Sidewall Connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . 5-70 6-1 Shaping of Plates for Steel Tanks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . 6-2 6-2 Shaping of Plates for Aluminum Tanks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . 6-2 6-3 Butt Welding of Plates of Unequal Thickness. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . 6-9 8-1 Nameplate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . 8-3 F-1 Reduction of Design Stresses Required to Allow for Biaxial Stress of the Opposite Sign. . . . . . . . . . F-2 F-2 Examples Illustrating the Use of a Biaxial St re ss Chart for Combined Tension and Compression, 30,000 – 38,000 Pounds per Square Inch Yield Strength Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F-7 F-3 Form for Use in Graphical Solutions of Prob lems Involving Biaxial T ension an d Compression, 30,000 – 38,000 Pounds per Square Inch Yield Strength Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F-8 F-4 Free-body Sketch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .F-10 F-5 Example of a Reinforced Opening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . .F-17 F-6 Example of a Reinforced Opening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . .F-21 F-7 Example of a Reinforced Opening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . .F-27 F-8 Example of a Reinforced Opening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . .F-28 Q-1 Typical Stiffening-ring Weld Details

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Q-13 Q-2 Radiographic/Ultrasonic Examination Requirements for Butt-welded Shell Joints in Primary and Secondary Liquid Containers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . Q-22 R-1 Typical Stiffening-ring Weld Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . .R-10 R-2 Radiographic/Ultrasonic Examination Requirements for Butt-welded Shell Joints in Primary and Secondary Liquid Containers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . .R-17 Tables 4-1 Minimum Requirements for Plate Specifications to be Used for Design Metal Temperatures . . . . . . . 4-4 4-2 Maximum Permissible Alloy Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . 4-6 4-3 Minimum Charpy V-notch Requirements for Plate Specimens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-7 5-1 Maximum Allowable Stress Values for Simple Tension. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5- 6 5-2 Maximum Allowable Efficiencies for Arc-welded Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1 0 5-3 Maximum Allowable Stress Values for Structural Members . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5- 12 5-4 Sidewall-to-Bottom Fillet Weld for Flat-bottom Cylindrical Tanks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5- 16 5-5 Factors for Determining Values of R 1 and R 2 for Ellipsoidal Roofs and Bottoms . . . . . . . . . . . . . . . . 5-21 5-6 Tank Radius Versus Nominal Plate Thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 -25 5-7 Allowable Tension Stresses for Uplift Pressure Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-2 9 5-8 Minimum Size of Fillet Weld . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . 5-34 5-9 Factors for Determining Values of k for Compression -ring Bracing . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-35 5-10 Dimensions of Flush-type Shell Connections (Inches) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 -64 6-1 Diameter Range Versus Radius Tolerance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . 6-3 6-2 Minimum Preheat Temperatures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . 6-6 6-3 Maximum Thickness of Reinforcement on Welds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 -7 6-4 Stress-relieving Temperatures and Holding Times . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . 6 -9 7-1 Maximum Thickness of Reinforcement on Welds for Radiography Examined Joints. . . . . . . . . . . . . . 7-3

viiiPage F-1 Computed Values of ( t – c ) R , s c , s t , and N for the Assumed Thicknesses: Example 1. . . . . . . . . . . . . . F-5 F-2 Computed Values of ( t – c ) R , s c , s t , and N for the Assumed Thicknesses: Example 2. . . . . . . . . . . . . . F-9 F-3 Cross-Sectional Area of Standard Angles: Example 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F-14 L-1Q Force Reduction Factors for ASD Methods, Annex Q Tanks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L -2 L-1R Force Reduction Factors for ASD Methods, Annex R Tanks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L -2 L-2 Impulsive Damping Ratio Adjustment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . L-5 Q-1 ASTM Standards for Product Temperature Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . Q- 2 Q-2 Charpy V-notch Impact Values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . Q-5 Q-3 Maximum Allowable Stress Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . Q-7 Q-4AMinimum Thickness for the Annular Bottom Plate: Steel Tanks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Q-12 Q-4BMinimum Thickness for the Annular Bottom Plate: Aluminum T anks

. . . . . . . . . . . . . . . . . . . . . . . . . Q-12 Q-5 Nominal Thickness of Primary and Secondary Liquid Container Cylindrical Sidewall Plates . . . . . Q-15 Q-6 Radius Tolerances for Primary and Secondary Liquid Container Shells . . . . . . . . . . . . . . . . . . . . . . . Q-16 R-1 Standards for Product Temperature Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . R-3 R-2 Minimum Charpy V-notch Impact Requirements for Product Temperature Material Plate Specimens (Transverse) and Weld Specimens Including the Heat-affected Zone . . . . . . . . . . . . . . . . R-4 R-3 Atmospheric Temperature Material Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . R-5 R-4 Minimum Permissible Design Metal Temperat ure for Atmospheric Te mperatur e Material Plates Used without Impact Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . R-6 R-5 Minimum Charpy V-notch Impact Requirement s for Atmospheric Tempera ture Material Plate Specimens (Transverse). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . R-7 R-6 Thickness Requirements for the Annular Bottom Plate (in.) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . R-9 S-1a ASTM Materials for Stainless Steel Components (SI Units) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S-2 S-1b ASTM Materials f or Stainless Steel Components (US Customary Units) . . . . . . . . . . . . . . . . . . . . . . . . S-3 S-2 Maximum Allowable Stress Values for Simple Tension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S-4 S-3 A llowable Stresses for Plate Ring Flanges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . S-4 S-4 Yield Strength Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . S-4 S-5 Modulus of Elasticity at the Design Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . S-4 U-1 Flaw Acceptance Criteria for UT Indications (May be Used for All Materials and 201LN) . . . . . . . . . . U-5 U-2 Alternate Flaw Acceptance Criteria for UT Indications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . U-6 U-3 Charpy V-notch Impact Values Required to Use Table U-2 for 9% Nickel Steel. . . . . . . . . . . . . . . . . . . U-6 X-1 ASTM Materials for Duplex Stainless Steel Components. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . X-2 X-2a Allowable S tresses for Tank Shells (SI) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . X-3 X-2b Allowable Stresses for Tank Shells (USC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . X-4 X-3a Yield Strength Values (SI) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . X-5 X-3b Yield Strength Values (USC). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . X-5 X-4a Modulus of Elasticity at the Maximum Operating Temperature (SI) . . . . . . . . . . . . . . . . . . . . . . . . . . . . X-6 X-4b Modulus of Elasticity at the Maximum Operating Temperature (USC) . . . . . . . . . . . . . . . . . . . . . . . . . . X-6 X-5a Minimum and Maximum Hot Forming Temperatures and Minimum Soaking Temperature (SI) . . . . . X-8 X-5b Minimum and Maximum Hot Forming Temperatures an d Minimum Soaking Temperature (USC) . . . X-8

ix1-1 

## Design and Construction of Large, Welded, Low -pressure Storage Tanks

SECTION 1—SCOPE
1.1 General The API Downstream Segment has prepared this standard to cover large, field-assemb led storage tanks of the type described in 1.2 that contain petroleum intermediates (gases or vapors) and finished products, as well as other liquid products commonly handled and stored by the various branches of the industry. The rules presented in this standard cannot cover all details of design and construction becaus e of the var i ety of tank sizes and shapes that may be constructed. Where complete ru les for a specific design are not given, the intent is for the Manufacturer—subject to the approval of the Purchaser’s author ized representative—to provide design and construction details that are as safe as those which would otherwise be provided by this standard. The Manufacturer of a low-pressure storage tank that will bear the API 620 nameplate sha ll ensure that the tank is constr ucted in accordance with the requirements of this standard. The rules presented in this standard are further intended to ensure that the applicati on of the nameplate shall be subject to the appr oval of a qualified inspector who has made the checks and in spections that are prescribed for the design, materials, fabrication, and testing of the completed tank. 

### 1.2 Coverage
1.2.1 This standard covers the design and construction of la rge, welded, low-pressure carbon steel above ground storage tanks (including flat-bottom tanks) that have a single vertical axis of revolution. This standard does not cover design procedures for tanks that have walls shaped in such a way that the walls cannot be generated in their entirety by the rotation of a suitable contour around a single vertical axis of revolution. 1.2.2 The tanks described in this standard are designed for metal temperatures not greater than 250 °F and with pressure s in their gas or vapor sp aces n ot more than 15 lbf/in. 2 gauge. 1.2.3 The basic rules in this standard provide for installa tion in areas where the lowest recorded 1-day mean atmospheric temperature is –50 °F. Annex S covers stainless steel lo w-pressure storage ta nks in ambien t temperature service in all areas, without limit on low tem peratures. Annex R covers low-pressure storage tanks for refrigerated products at temperatures from +40 °F to –60 °F. Annex Q covers low-pressure storage tanks for liquefied gases at temperatures not lower than –325 °F. 1.2.4 The rules in this standard are applicable to tanks that are intended to (a) ho ld or store liq uids with gases or vapors above their surface or (b) hold or store gases or vapors alone. These rules do not apply to lift-type gas holders. 1.2.5 Although the rules in this standard do not cover horizont al t anks, they ar e not in tended to preclude the application of appropriate portions to the design and cons truction of horizontal tanks designed in accordance with good engineering practice. The details for hor izontal tanks not covered by these ru les shall be equally as safe as the design and construction details provided for the tank shapes that are expressly covered in this standard. 1.2.6 Annex A describes how to submit inquiries and s uggestions for changes to this standard. 1.2.7 Annex B covers the use of plate and pipe materials t hat are not completely i dentified with any of the specifications listed in this standard.1-2 API S TANDARD 620
1.2.8 Annex C provides information on subgrade and foundat ion loading conditions and foundation construction prac tices. 1.2.9 Annex D provides information about imposed loads and stre sses fro m external support s attached to a tank wall. 1.2.10 Annex E provides considerations for the design of internal and external structural supports. 1.2.11 Annex F illustrates through examples how the rules in this standard ar e applied to various design problems. 1.2.12 Annex G provides considerations for service conditions that af fect the selection of a corrosion allowance; concerns for hydrogen-induced cracki ng effects are specifically noted. 1.2.13 Annex H covers preheat and post-heat stress-re lief practices for improved no tch toughness. 1.2.14 Annex I covers a suggested practice for peeni ng weldments to r educe internal stresses. 1.2.15 Annex J is reserved for future use. 1.2.16 Annex K provides considerations for determining the cap acity of t ank venting devices. 1.2.17 Annex L covers requirements for the design of storage t anks subject to seismic load. 1.2.18 Annex M covers the extent of information to be provided in the Manufacturer ’ s report and presents a suggested format for a tank certification form. 1.2.19 Annex N covers installation practices for pressure- and vacuum-relieving devices. 1.2.20 Annex O provid es consider ations for the safe operati on and maintenance of an installed tank, with attention given to m ar king, access, site drainage, fireproofing, water draw-off piping, and cathodic protection of tank bottoms. 1.2.21 Annex P summarizes the requirements for inspecti on by method of examinat ion and the reference paragraphs within the standard. The acceptance standards, inspector qualifications, and procedure requirements are also provided. This annex is not intended to be used al one to determine the inspection requirements within this standard. The specific requirements listed within each applicable section shall be followed in all cases. 1.2.22 Annex Q covers specific requirements for the material s, design, and fabr ication of tanks to be used for the storage of liquefied gases such as ethane, ethylene, and methane. 1.2.23 Annex R covers specific requirements for the materials, design, and fabr ication of tanks to be used for the storage of refrigerated products. 1.2.24 Annex S covers requirements for stainless steel t anks in non-refr iger ated service. 1.2.25 Annex SC provides requirements for mixed material tanks using stainless steel (including austenitic and duplex) and carbon steel in the same tank for walls, bottom plates, roof structur e, and other parts of a tank requiring added corrosion resistance. 1.2.26 Annex U covers detailed rules for the use of the ultr asonic examination (U T) met hod fo r the examination of tank seams. 1.2.27 Annex X covers materials, design, fabrication, er ection, and testi ng requirement s for aboveground, welded, duplex stainless steel storage tanks.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 1-3


### 1.3 Limitations 1.3.1 General The rules presented in this standard apply to vertical, cyli ndrical oil stor age t anks built according to API 650 as specifically allowed in 5.7.1. 8, F.1, and F.7 of that standard. These rules do not apply to tanks built according to rules established for unfired pressure vessels designated for an internal pressure greater than 15 lbf/in. 2 gauge. 1.3.2 Piping Limitations The rules of this standard are not applicable beyond the followi ng locations in piping connected internally or externally to the walls 1 of tanks constructed according to this standard. a) The face of the first flange in bolted flanged connections. b) The first threaded joint on the pipe outside the tank wall in threaded pipe connections. c) The first circumferential joint in welding-end pipe c onnections th at do not have a flange located near the tank.

1 The term wall refers to the roof, shell and bottom of a tank as defined in 3.3.2-1 

### SECTION 2—REFERENCES

The most recent editions or revisions of the following standards, codes, and specifications are cited in this standard.
Note: The reference below to ASCE 7-05 is not the latest version. API Specification 5L , Specificati on for Line Pipe API Recommended Practice 520, Sizing, Selection, and Installation of Pr essure-relieving De vices in Refiner ies— Part 2, Installation API Recommended Practice 582, Recom mended Practice W elding Guidelin e s for the Chemical, Oil, and Gas Industries API Standard 605 , Large-Diameter Carbon Steel Flanges (Nominal Pipe Sizes 26 Through 60; Classes 75, 150, 300, 400, 600, and 900) (out-of-print) API Standard 650, W elded T anks for Oil Storage API Standard 2000, Venting Atmosp heric and Low-Pressure Storage Tanks AAI 2 , Aluminum Design Manual, “Specification for Aluminum Structures” ACI 318 3 , Building Code Requirements for Structural Concrete and Commentary (ANSI/ACI 318) AISC 4 , Steel Construction Manual ANSI H35.2 5 , Dimensional Tolerances for Aluminum Mill Products ASCE Std 7-05 6 , Minimum Design Loads for Buildings and Other Structures ASME B1.20.1 7 , Pipe Threads, General Purpose (Inch) (ANSI/ASME B1.20.1) ASME B16.5, Pipe Flanges and F langed Fittings (ANSI/ASME B16.5) ASME B31.1, Power Pi pin g ASME B31.3, Process Pipi ng (ANSI/ASME B31.3) ASME B36.10M, W elded and Seamless Wrought S t eel Pipe (ANSI/ASME B36.10) ASME B96.1, Welded Aluminu m -Alloy Storage Tanks (ANSI/ASME B96.1) ASME Boiler and Pressure V essel Code, Section V, “Nondestructive Examination;” Section VIII, “Pressure Vessels, Division 1;” and Section IX, “Weldi ng and Brazing Qualifications” ASNT CP-189 8 , Standard for Qualification and Certification of Nondestr uctive Testing Personnel

2 Aluminum Association, 1525 Wilson Blvd, Suite 600, Arlington, Virginia 22209, www.aluminum.org. 3 American Concrete Institute, P.O. Box 9094, Farmington Hills, Michigan 48333, www.aci-int.org. 4 American Institute of Steel Construction, One East Wa cker Drive, Suite 700, Chicago, Illinois 60601, www.aisc.org. 5 American National Standards Institute, 25 West 43rd St reet, 4th floor, New York, New York 10036, www.ansi.org. 6 American Society of Civil Engineers, 1801 Alexander Bell Dr., Reston, Virginia 20191, www.asce.org. 7 ASME International, 3 Park Avenue, New York, New York 10016, www.asme.org.2-2 API S TANDARD 620
ASNT SNT-TC-IA , Personnel Qualification and Certif ication in Nondestructive Testing ASTM A6 9 , Standard Specification for General Requirements for Rolled Structural Steel Bars, Plates, Shapes, and Sheet Piling ASTM A20 , S tand ard Specification for General Requirem ents for Steel Plates for Pressure Vessels ASTM A27, Stand ard Specification for Steel Casti ngs, Carbon, for General Application ASTM A36, Stand ard Specification for Carbon Structural Steel ASTM A53, Stand ard Specification for Pipe, Steel, Black and Hot-Dipped, Zinc-Coated, Welded and Seamless ASTM A105, Stan dard Specification for Carbon Steel Forgings for Piping Applications ASTM A106, Stan dard Specification for Seamless Carbon Steel Pipe for High-Temperature Service ASTM A131, Stan dard Specification for Structural Steel for Ships ASTM A134, Stan dard Specification for Pipe, Steel, Electr ic-Fusion (Arc)-Welded (Sizes NPS 16 and Over) ASTM A139, Stan dard Specification for Electric-Fusion (Arc)-Welded Steel Pipe (NPS 4 and Over) ASTM A181 , Stan dard Specification for Carbon Steel Forgings, for General-Purpose Piping ASTM A182, Standard Specification for Forged or Rolled Alloy and Stainless Steel Pipe Flanges, Forged Fittings, and Valves and Parts for High-Temperature Service ASTM A193, S tandard Specification for Alloy- Steel and Stainless Steel Bolting for High Temperature or High Pressure Service and Other Special Purpose Applications ASTM A194, S tandard Specification for Carbon a nd Alloy Steel Nuts for Bolts for High-Pressure or High-Temperature Service, or Both ASTM A213, S tandard Specification for Seamless Ferritic and Aust en itic Allo y-Steel Boiler, Superheater, and HeatExchanger Tubes ASTM A240, S tandard Specification for Chromium and Chromium-nic kel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels and for General Applications ASTM A283, S tandard Sp ecification for Low and Intermediate Tensile Strength Carbon Steel Plates ASTM A285 , St anda rd Specification for Pressure Vessel Plates, Carbon Steel, Low- and Intermediate-Tensile Strength ASTM A307 , St a ndard Specification for Carbon Steel Bolts, Studs, and Threaded Rod 60,000 psi Tensile Strength ASTM A312 , S t andard Specification for Seamless, Welded, and Heavily Cold Worked Austenitic Stainless Steel Pipes ASTM A320, Sta ndar d Specification for Alloy-Steel and Stainless Steel Bolting for Low-Temperature Service ASTM A333, Sta ndar d Specification for Seamless and Welded Steel Pipe for Low-Temperature Service

8 American Society for Nondestructive Testing, 1711 Arlingate Lane, P.O. Box 28518, Columbus, Ohio 43228, www.asnt.org. 9 ASTM International, 100 Bar Harbor Drive, West Conshohocken, Pennsylv ania 19428, www.astm.org.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 2-3 ASTM A334, Standard Specification for Seamless and Welded Carbon and Alloy-Steel Tubes for Low-Temperature Service ASTM A350, S tandar d Specification for Carbon and Low-alloy Steel Forgings, Requiring Notch Toughness Testing for Piping Components ASTM A351 , S tandard Specification for Castings, Austeniti c, Austenitic, for Pressure-Containing Parts ASTM A353 , Standard Specificatio n for Pressure V essel Plat es, Alloy Steel, Double-Normalized and Tempered 9% Nickel ASTM A358 , St andard Specification for Electric-Fusion-Welded Austenitic Chromium-Nickel Stainless Steel Pipe for High-Temperature Service and General Applications ASTM A370 , S tan dard T est Methods and Definitions for Mechanical Testing of Steel Products ASTM A403 , Stan dard Specification for Wrought Au stenitic Stainless St eel Piping Fittings ASTM A479 , Standard Specification for S t ainl ess Steel Bars and Shapes for Use in Boilers and Other Pressure Vessels ASTM A480, S ta ndard Specification for General Requirements for Flat-Rolled St ainless and Heat-R esisting Steel Plate, Sheet, and Strip ASTM A516 , S tandard Specification for Pr essur e Vessel Plat es, Carbon Steel, for Moderate- and Lower-Temperature Service ASTM A522, S tandard Specification for Forged or Rolled 8 and 9% Nickel Alloy S t eel Flanges, Fittings, Valves, and Parts for Low-Temperature Service ASTM A524, S tan dard Specification for Seamless Carbon Steel Pipe for Atmospheric and Lower Temperatures ASTM A537, Stan dard Specification for Pressure Vessel Pl ates, Heat-Treated, Carbon-Manganese-Silicon Steel ASTM A553 , Stand ard Specification for Pressure V essel Pl ates, Alloy Steel, Quenched and Tempered 8 and 9% Nickel ASTM A573 , S tan dard Specification for Structural Carbon Steel Plates of Improved Toughness ASTM A633 , Stan dard Specification for Normalized High-St rength Low-Alloy Structural Steel Plates ASTM A645 , Standard Specification for Pres sure V essel Plates, 5% and 5 1 / 2 % Nickel Alloy Steel, Specially Heat Treated ASTM A662 , S tandard Specification for Pressure Vessel Pl ates, Carbon-Manganese-Silicon Steel, for Moderate and Lower Temperature Service ASTM A671, S tan dard Specification for Electric-Fusion-Welded Steel Pipe for Atmospheric and Lower Temperatures ASTM A673, Stan dard Specification for Sampling Procedure for Impact Testing of Structural Steel ASTM A678 , Stan dard Specification for Quenched-and-Tempered Carbon and High-Strength Low-Alloy Structural Steel Plates2-4 API S TANDARD 620 ASTM A737, Standard Specification for Pressure Vess el Plates, High-Strength, Low-Alloy Steel ASTM A841, Standard Specification for Steel Plates for Pressu re Vessels, Produced by Thermo-Mechanical Control Process (TMCP) ASTM A992, Standard Specification for Structural Steel Shapes ASTM B209, Standard Specification for Aluminum and Aluminum-Alloy Sheet and Plate ASTM B210 , Standard Specification for Aluminum and Aluminum-Alloy Drawn Seamless Tubes ASTM B211, Standard Specification for Aluminum and Aluminum-Alloy Rolled or Cold Finished Bar, Rod, and Wire ASTM B221, Standard Specification for Aluminum and Aluminu m-Alloy Extruded Bars, Rods, Wire, Profiles, and Tubes ASTM B241, Standard Specification for Aluminum and Alumi num-Alloy Seamless Pipe and Seamless Extruded Tube ASTM B247, Standard Specification for Aluminum and Aluminum-Alloy Die Forgings, Hand Forgings, and Rolled Ring Forgings ASTM B308, Standard Specification for Aluminum-A lloy 6061-T6 Standard Structural Profiles ASTM B444, Standard Specification for Nickel-Chromium-Molybdenum-Columium Alloys (UNS N06625 and UNS N06852) and Nickel-Chromium-Molybdenum-Silicon Alloy (UNS N06219) Pipe and Tube ASTM B619, Standard Specification for Welded Nickel and Nickel-Cobalt Alloy Pipe ASTM B622 , Standard Specification for Seamless Nickel and Nickel-Cobalt Alloy Pipe and Tube ASTM E23, Standard Test Methods for Notched Bar Impact Testing of Metallic Materials AWS A5.11 10 , Specification for Nickel and Nickel Alloy Weld ing Electrodes for Shielded Metal Arc Welding (ANSI/AWS A5.11) AWS A5.14, Specification for Nickel and Nickel-Alloy Bare Weld ing Electrodes and Rods (ANSI/AWS A5.14) CSA G40.21 11 , Structural Quality Steel EN 10025 12 , Hot Rolled Products of Structural Steels EN 10028, Flat Products Made of Steels for Pressure Purposes ISO 630 (1995 Edition) 13 , Structural steels

10 American Welding Society, 550 N.W. LeJeune Road, Miami, Florida 33126, www.aws.org. 11 Canadian Standards Association, 5060 Spectrum Way, Suite 100, Mississauga, Ontario L4W 5N6, Canada, www.csa.ca. 12 European Committee for Standardization, Avenue Marnix 17, B-1000, Brussels, Belgium, www.cen.eu. 13 International Organization for Standardization, 1, ch. de la Voie-Creuse, Case postale 56 , CH-1211 Geneva 20, Switzerland, www.iso.org.3-1 

### SECTION 3—DEFINITIONS
3.1 Stress and Pressure Terms 3.1.1 design pressure The maximum positive gauge pressure permissible at the top of a tank when the t ank is in oper ation. It is the basis for the pressure setting of the safety-relieving devices on the tank. The design pressure is synonymous with the nominal pressure rating for the tank as referred to in this standard (see 5.3.1). 3.1.2 maximum allowable stress value The maximum unit stress permitted to be used in the desi gn formulas given or provided for in this standard for the specific kind of material, characte r of loading, and purpose for a tank member or element (see 5.5 and 5.6). 

### 3.2 Capacity Terms 3.2.1 nominal liquid capacity The total volumetric liquid cap acity of a t ank (excl uding deadwood) between the plane of the high liquid design level and elevation of the tank grade immediately adjacent to the wall of the tank or such other low liquid design level as the Manufacturer shall stipulate. 3.2.2 total liquid capacity The tot al volumetric liquid cap acity of a tank (e xcluding deadwood) below the high liquid design level. 

### 3.3 Tank Wall The tank wall is any or all parts of the plates located in the surface of revolution that bounds the tank and serves to separate the interior of the tank from the surrounding atmos phere. Flat bottoms of cylindrical tanks are covered by the rules of 5.9.4. As such, the tank wall s include the sidewalls (or s hell), roof, and bottom of the tank but not any of the following elements located on or projecting from the walls: a) nozzles and manways or their reinforcement pads or cover plates; b) internal or external diaphragms, webs, trus ses, structural columns, o r other fr aming; c) those portions of a compression-ring angle, bar, or girder that project from the walls of the tank; d) miscellaneous appurtenances. 

### 3.4 Welding Terms The terms defined in 3.4.1 through 3.4.21 are commonly used welding terms mentioned in this standard. See 5.22 for descriptions of fusion-welded joints. 3.4.1 automatic welding W elding with equipment which per forms the welding operati on without adjustment of the controls by a welding operator. The equipment may or may not perform the loading and unloading of the work. 3.4.2 backing The material—met al, weld met al, carbon, granular flux, and so forth—that backs up the joint during welding to facilitate obtaining a sound weld at the root.3-2 API S TANDARD 620
3.4.3 base metal The met al or alloy to be welded or cut. 3.4.4 depth of fusion The distance that fusion extends into the base met al from the surface melted during welding. 3.4.5 filler metal Met al or alloy added in making a weld. 3.4.6 fusion The melting together of filler metal and base metal, or the melting of base meta l only, which results in coalescence. 3.4.7 heat-affected zone The portion of the base metal that has not been melted but whose mechanical properties or microstructures have been altered by the heat of welding or cutting. 3.4.8 joint penetration The mi nimum depth a gr oove weld extends from its fa ce into a joint, exclusive of reinforcement. 3.4.9 lap joint A joint between two overlapping members. An overlap is th e protrusion of weld metal beyond the bond at the toe of the weld. 3.4.10 machine welding W elding with equipment that per forms the welding operation under the constant observation and control of a welding operator. The equipment may or may not perform the loading and unloading of the work. 3.4.11 manual welding W elding wherein the entire welding oper ation is performed and controlled by hand. 3.4.12 oxygen cutting A gr oup of cutting processes wherein the severing of metals is effected by mean s of the chemical reaction of oxygen with the base metal at elevated temperatures. In the case of oxidation-resistant metals, the reaction is facilitated by use of a flux. 3.4.13 porosity The existence of gas pocket s or voids in metal. 3.4.14 reinforcement of weld W eld met al o n the face of a groove weld in excess of the metal necessary for the specified weld size.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 3-3
3.4.15 semiautomatic arc welding Arc welding with equipment that controls only the filler met a l feed. The advance of the welding is manually controlled. 3.4.16 slag inclusion Nonmet allic solid material entrappe d in weld metal or between weld metal and base metal. 3.4.17 undercut A groove melted into the base met a l adjacent to the toe of a weld and left unfilled by weld metal. 3.4.18 weld metal The portion of a weld that has been m e lted during welding. 3.4.19 welded joint A union of two or more members pr oduced b y the application of a welding process. 3.4.20 welder One who performs manual or semiautomatic welding. 3.4.21 welding operator One who operates machine welding equipment. 

### 3.5 Other Terms 3.5.1 Manufacturer The party having the prim a ry respons ibility for constructing the tank. 3.5.2 Purchaser The owner or the own er ’s designated agent , such as an engineering contractor. 3.5.3 Examiner A person who performs nondestructive exam inations specified by this standard and is qualified and certified as meeting the requirements of ASNT SNT-TC-1A and/or ot her requirements as determined by the Manufacturer or Purchaser for the method employed. 3.5.4 Inspector The Inspector is the represe nt ative of the Purchaser w ho is qualified according to 7.2 and who ensures compliance with this standard as described in 7.1.4-1 

### SECTION 4—MATERIALS 4.1 General
4.1.1 Material Specifications Materials used in the construction of AP I 620 tanks shall comply with the specif ications in this section (see Annexes Q, R, and S for specific material re quirements). Material produced to specifications other than those listed in this section may be used if the material is ce rtified to meet all the requirements of a material specificat ion listed in this section and that its use is approved by the Purchaser. 4.1.2 Materials That Cannot Be Completely Identified Any plate materials or tubular products on hand that cannot be completely identified with a specification listed in this standard, by records satisfactory to the inspector, may be used to construct tanks accord ing to the rules of this standard if the material passes the test prescribed in Annex B. 4.1.3 Accessory Pressure Parts All accessory pressure parts, such as pipe fittings, valves, flanges, nozzles , welding necks, welding cap s, manhole frames, a nd cover s, shall be made from materials provided fo r in this standard or in any accepted ANSI standard that covers the particular part. These parts shall be marked with the name or trademark of the manufacturer and any other markings that are required by the applicable standards. Such markings shall be considered the manufacturer’s guarantee that the product complies with the material specifications and standards i ndicated and is suitable for service at the rating indicated. The int ent of this paragraph will have been met if , in lieu of the detailed marking on the part itself, the accessory pressure parts have been marked in any permanent or temporary manner that serves to identify the part with the manufacturer’ s written listing of the particular it ems and if this listing is available for examination by the inspector. 4.1.4 Small Parts Cast, forged, or rolled parts of small size (which are ordinarily carried in stock and for which mill test report s or certificates are not customarily furnished) may be used if, in the opinion of the inspecto r, they are suitable for the purpose intended and that, if such parts are to be welded, they are of welding grade. 

### 4.2 Plates 4.2.1 General 4.2.1.1 All plates that are subject to pressure-imposed membr ane stress or ar e other wise important to the structural integrity of a tank, including bottom plat es welded to the cylindrical sidewall of flat-bottom tanks, shall conform to specifications selected to provide a high order of resistance to brittle fracture at the lowest temperature to which the metal in the walls of the tank is expected to fall on the col dest days of record for the locality where the tank is to be installed. 4.2.1.2 In all cases, the Purchaser shall s pecify th e design met al temperatur e, an d the plates used for the tank shall conform to one or more of the specifications listed in Table 4-1 as being acceptable for use at that temperature (refer to 4.2.5 for impact testing requirements). Except as other wise provided in the last sentence of this paragraph and in 4.2.2, the design metal temperature for materials in c ontact with nonrefrigerated fluids shall be assumed to be 15 °F above the lowest one-day mean ambient temperature for the locality involved, as determined from Figure 4-1. For locations not covered by Figure 4-1, authentic meteorologi cal data shall be used. Where no such data are available, the Purchaser shall estimate the temperature from the mo st reliable information at hand. Where special means, such as covering the outside of the tank with insulation or he ating the tank contents, are provided to ensure that the temperature of the tank walls never falls to within 15 °F of the lowest one day mean am bient temperature, the design4-2 API S TANDARD 620 metal temperature may be set at a higher level that can be ju stified by computations or by actual temperature data on comparable existing tanks. 4.2.1.3 Unless exempted per 4.2.2, notch toughness of specia lly designed plate flanges and cover plates shall be evaluated using governing thickness in Table 4-1. (S ee 4.3.6.3 for definition of governing thickness.) 4.2.2 Low-stress Design The following design criteria, relative to the use of T able 4-1, apply when t he actual stress under design conditions does not exceed one-third of t he allowable tensile stress. a) Consideration of the design met al temperat ure is not required in sele cting material from Table 4-1 for tank components that are not in contact with the liquid or vapor being stored and are not designed to contain the contents of an inner tank (see Q.2.4 and R.2.3). b) The design metal temperature may be increased by 30 °F in selecting material from Table 4-1 for tank component s that are exposed to the vapor fro m the liqui d or vapor being stored and are not designed to contain the contents of an inner tank. c) Excluding bottom plates welded to the cy lindr ical sidewall of flat- bottom tanks, the plates of a non-refrigerated flatbottom tank, counterbalanced in accordanc e with 5.11.2, may be constructed of any material selected from Table 4-1. 4.2.3 Plate Specifications 4.2.3.1 General The specifications listed in 4.2.3.2, 4.2.3.3, and 4.2.3.4 are approved for p l ates, subject to the modifications and limitations of this paragraph, 4.2.4, and Table 4-1. 4.2.3.2 ASTM Specifications The following ASTM specificat ions are approved for plates. a) A20. b) A36. c) A131 (structural quality only). d) A283 (Grades C and D only, with a maximum nominal thickness of 3 / 4 in.). e) A285 (Grade C only, with a ma ximum nominal thickness of 3 / 4 in.). f) A516, with the following API modifi cations as required: Mod 1 requires t he carbon content to be restricted to a maximum of 0.20 % by ladle analysis; a maxi mum manganese content of 1.50 % shall be permitted. Mod 2 requ ires the minimum manganese c ontent to be lowered to 0.70 % and the maximum increased to 1.40 % by ladle analysis. The carbon content shall be limited to a maximum of 0.20 % by ladle analysis. The steel shall be normalized. The silicon content may be increased to a maximum of 0.50 % by ladle analysis. g) A537, with the following modification: The minimum mangane se content shall be 0.80 % by ladle analysis. The maximum manganese content may be incr eased to 1.60 % b y ladle analysis if maximum carbo n content is 0.20 % by ladle analysis.El PasoSanta FeAmarilloSan AntonioHoustonDallasOklahomaCityLittle RockFort SmithShreveportJacksonNew OrleansMontgomeryBirminghamMobileMemphisNashvilleKnoxvilleChattanoogaSpringfieldJoplinWichitaTopekaKeokukKansasCitySt. LouisSpringfieldFortWayneColumbusLouisvilleClevelandDes MoinesNorth PlatteMinneapolisSioux FallsSioux CityAberdeenPierreMolineChurchillThe PasWinnepegKapuskasingInternationalFallsPort Aux BasquesSt.JohnsNEWFOUNDLANDBuchansFargoDuluthEdmontonMedicine HatHavreBillingsSheridanPrinceAlbertSaskatoonReginaWillistonHelenaClayoquotKamloopsSeattleBoiseSpokaneBakerPortlandPenticionNelsonCranbrookVictoriaVancouverChathamBangorHalifaxSt. JohnHuntsvilleOttawaPittsburghBuffaloAlbanyRaleighNorfolkColumbiaCharlestonSavannahJacksonvilleTampaMiamiPuebloDenverPocatelloSan DiegoLos AngelesSan FranciscoEurekaCompiled from U.S. Weather Bureau andMeteorological Div. Dept. of Transport ofDominion of Canada Records up to 1952.ChicagoDetroitArvida35353025202520151050101510454035252050–5–10–15–20–25–10–5Ð40–35–30Haileybury–20–30–30–25–20–15–10–50515–20–15Calgary30151050–5–10–15–25–30–20–35–40–45–5–20–25–30–35–40–45–55–50–45–20RichmondCharlestonCincinnatiWashingtonBaltimoreNew YorkPhiladelphiaHartfordBostonConcordCharlottestownSidneyQuebecSaranac LakeMontpelierLennoxvilleSault St. MarieSouthhamptonTorontoLondonLudingtonGreen BayMarquettePort ArthurSioux LookoutMilwaukeeCheyenneLanderRed BluffSacramentoFresnoRenoPrinceGeorgeSalt LakeCityTusconPhoenixHarrisburgSt. CatherinePortlandGanderWilmingtonAshvilleAtlantaIndianapolisWythvillePrinceRuppertMontrealAmherstProvidence–15–10Las VegasBismarkÐ45–25–25Grand CanyonFigure 4-1—Isothermal Lines Showing 1-day Mean Ambient Temperature D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 4-34-4 API S TANDARD 620 Table 4-1—Minimum Requirements for Plate Specifications to be Used for Design Metal Temperatures
Design Metal Temperature

(See 4.2.1) Plate Thickness Including Corrosion Allowance (in.) Permissible Specifications

Specification Grade Special Requirements (in Addition to 4.2.3) 65°F and over ≤ 3 / 4 Any listed in 4.2.3 — None ≤ 1 ASTM A36 — Note 6 > 1 G40.21 38W, 44W, 50W Note 1 25°F and over ≤ 1 / 2 Any listed in 4.2.3 — None ≤ 1 ASTM A36 — Note 5 ASTM A131 B None CSA G40.21 38W, 44W, 50W None –5°F and over ≤ 1 / 2 ASTM A131 B None CSA G40.21 38W, 44W, 50W None > 1 / 2 ASTM A516 55, 60, 65, 70 Note 1 ASTM A573 58, 65, 70 Note 1 ASTM A662 B and C Note 1 ASTM A737 B None ASTM A841 Class 1 None CSA G40.21 38W, 44W, 50W Note 2 ISO 630 (1995) E 275, E355 Quality D Note 1 EN 10025 S 275, S355 Quality J2 Notes 1 and 2 –35°F and over ≤ 1 / 2 ASTM A516 55, 60, 65, 70 None ASTM A537 Classes 1 and 2 None ASTM A573 58, 65, 70 None ASTM A633 C and D None ASTM A662 B and C None ASTM A678 A and B None ASTM A737 B None ASTM A841 Class 1 None CSA G40.21 38W, 44W, 50W Note 2 ISO 630 (1995) E 275, E355 Quality D None EN 10025 S 275, S355 Quality J2, K2 Note 2 ≤ 1 ASTM A516 55, 60, 65, 70 Note 3 ASTM A537 Classes 1 and 2 None ASTM A573 58 Note 3 ASTM A633 C and D None ASTM A662 B and C Note 3 ASTM A678 A and B None ASTM A737 B None ASTM A841 Class 1 None CSA G40.21 38W, 44W, 50W Notes 2 and 3 ISO 630 (1995) E275, E355 Quality D Note 3 EN 10025 S 275, S355 Quality J2, K2 Notes 2 and 3D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 4-5

h) A573. i) A633 (Grades C and D only). j) A662 (Grades B and C only). k) A678 (Grades A and B only). l) A737 (Grade B only). m) A841 (Class 1 only). 4.2.3.3 CSA Specification The following CSA specification is appr oved for plates: G40.21 (Grades 38W, 44W, and 50W only; if impact tests are required, these grades are designated 38WT, 44WT, and 50WT). Elements added for grain strengthening shall be restricted in ac cordance with Table 4-2. Plates shall have a tensile strength not more than 20 ksi above the minimum specified for the grade. Full y killed steel made to a fine grain practice must be spec ified when required. 4.2.3.4 ISO Specification The following ISO specificat ion is approved for plates: 630-1 (Grades E275 and E355 in Qualities C and D only). Elements added for grain refining or strengthening shal l be restricted in accordance with Table 4-2. > 1 ASTM A516 55, 60, 65, 70 Notes 3 and 4 ASTM A537 Classes 1 and 2 Note 4 ASTM A573 58 Notes 3 and 4 ASTM A633 C and D Note 4 ASTM A662 B and C Notes 3 and 4 ASTM A678 A and B Note 4 ASTM A737 B Note 4 ASTM A841 Class 1 None CSA G40.21 38W, 44W, 50W Notes 2, 3, and 4 ISO 630 (1995) E275, E355 Quality D Notes 3, and 4 EN 10025 S 275, S355 Quality J2, K2 Notes 2, 3, and 4 NOTE 1 All plates over 1 1 / 2 in. thick shall be normalized. NOTE 2 The steel shall be killed and made with fine-grain practice. NOTE 3 The plates shall be normalized or quench tembered (see 4.2.4.2). NOTE 4 Each plate shall be impact tested in accordance with 4.2.5. NOTE 5 The manganese content shall be within the range of 0.80 % to 1.20 %. The steel shall not be rimmed or capped. NOTE 6 The material supplied shall not be rimmed or capped. Table 4-1—Minimum Requirements for Plate Specifications to be Used for Design Metal Temperatures (Continued)
Design Metal Temperature

(See 4.2.1) Plate Thickness Including Corrosion Allowance (in.) Permissible Specifications

Specification Grade Special Requirements (in Addition to 4.2.3)4-6 API S TANDARD 620

4.2.3.5 EN Specification The following EN specification is approved for plates: EN 10025 (Grades S275 and S355 in Qualities J0, J2, and K2 only). Elements added for grain refining or strengthening shall be restricted in accordance with Table 4-2. 4.2.4 Plate Manufacture 4.2.4.1 All material for plates shall be made using the open-hearth, electric -furnace, or basic-oxygen process. Universal mill plates shall not be used. All plates for pressure parts, with t he exception of thos e whose thicknesses are established by the requirements of Table 5-6, shall be ordered on the basis of edge thickness to ensure that the plates furnished from the mill will not underrun the specified thickness by more than 0.01 inch. This stipulation shall not be construed to prohibit the use of plates purchased based on weight if it is est ablished by actual measurements (taken at a multiplicity of points along the edges of the plates) that the minimum thicknesses of the plates do not underrun the required design thickness by more than 0.01 in. 4.2.4.2 Subject to the Purchaser’s approval , thermo-mechanical-control-proces s (TMCP) plates (plates produced by a mechanical-thermal rolling process designed to enhance notch toughness) may alternatively be used where heat treated plates are normally required by Table 4-1 (note 1) because of thickness over 11⁄2 inch. In this case, each TMCP plate-as-rolled shall receive Charpy V-notch impact energy testing in accordance with 4.2.5. 4.2.5 Impact Test Requirements for Plates 4.2.5.1 Table 4-1 provides exemption and impact testing r equirements of plates for various grades for given thickness range and design metal temperature. Any materi al listed in Table 4-1 may be used for any thickness and temperature provided the material meets impact test requirements as specified in 4.2.5 and Table 4-3 and welding procedure requirements specified in 6.7. When the plate is not exempted from impact testing per Table 4-1 or 4.2.2, each plate shall be impact tested; plate refers to the unit plate rolled from a slab or directly from an ingot. The ASTM A370, Type A, Charpy V-notch test shall be used. The l ong dimension of the specimen shall be parallel to the direction of the expected maximum st ress. When the coincident stresses are approximately equal, the specimens shall be taken transverse to the final direction of the plate rolling. The mi nimum energy absorption values of Table 4-3 shall be satisfied. Table 4-2—Maximum Permissible Alloy Content Alloy % Notes Columbium 0.05 1, 2, and 3 Vanadium 0.10 1, 2, and 4 Columbium (0.05 % maximum) Plus Vanadium 0.10 1, 2, and 3 Nitrogen 0.015 1, 2, and 4 Copper 0.35 1 and 2 Nickel 0.50 1 and 2 Chromium 0.25 1 and 2 Molybdenum 0.08 1 and 2 NOTE 1 When not included in the material specification, the use of these alloys, or combinations thereof, shall be at the option of the plate producer, s ubject to the approval of th e Purchaser. These elements shall be reported when requested by the Purchaser. NOTE 2 The material shall conform to these requirements on product analysis subj ect to the product analyses tolerances of the specification. NOTE 3 Columbium, when added either singly or in combination with vanadium, shall be restricted to plates of 0.50-in. maximum thickness unless it is combi ned with a minimum of 0.15 % silicon. NOTE 4 When added as a supplement to vanadium, nitrogen (a maximum of 0.015 %) shall be reported and the minimum ratio of vanadium to nitrogen shall be 4:1.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 4-7 Table 4-3—Minimum Charpy V-notch Requirements for Plate Specimens
Group Specification Number Grade Range in Thickness (in.) Impact Value a (foot-pounds) Average Individual I (semikilled) A 36 3 / 16 to 1 13 9 A 36 e 3 / 16 to 1 13 9 A 131 A and B 3 / 16 to 1 13 9 A283 C and D 3 / 16 to 3 / 4 13 9 A285 C 3 / 16 to 3 / 4 13 9 ISO 630 (1995) E275 Quality C 3 / 16 to 1 1 / 2 13 9 EN 10025 S275 J0 3 / 16 to 1 1 / 2 13 9 II (fully killed) A 573 58 c 3 / 16 to 1 1 / 2 15 10 A 131 CS 3 / 16 to 1 1 / 2 15 10 A 516 55 and 60 3 / 16 to 2 15 10 A 516 55 and 60 d 3 / 16 to 1 / 2 15 10 ISO 630 (1995) E275 Quality D 3 / 16 to 1 1 / 2 15 10 EN 10025 S275 J2 3 / 16 to 1 1 / 2 15 10 CSA G40.21 38WT 3 / 16 to 2 15 10 III (fully killed and high strength) A 573 65 and 70 3 / 16 to 2 15 10 A 516 65 and 70 3 / 16 to 2 15 10 A 516 65 and 70 Mod 1 b 3 / 16 to 2 15 10 A 516 65 and 70 Mod 2 b 3 / 16 to 2 15 10 A 537 1 3 / 16 to 2 15 10 A 537 2 3 / 16 to 2 20 15 A 633 C and D 3 / 16 to 2 15 10 A 662 B and C 3 / 16 to 2 15 10 A 678 A 3 / 16 to 1 1 / 2 20 15 A 678 B 3 / 16 to 2 20 15 ISO 630-1 E355 Quality C and D 3 / 16 to 2 15 10 EN 10025 S355 J2 and K2 3 / 16 to 2 15 10 CSA G40.21 44WT 3 / 16 to 2 15 10 CSA G40.21 50WT 3 / 16 to 2 15 10 A 737 B 3 / 16 to 2 15 10 A 841 1 3 / 16 to 2 15 10

a The stated values apply to full-sized spec imens. For sub-size specimen acceptance criteria, see ASTM A20. An impact test temper ature lower than the design metal temperature may be used by the Manufacturer, but the impact value at the test temperature must comp ly with Table 4-3. When plate is selected, consideration must be given to the possible degradation of the impact properties of the plat e in the weld heat-affected zone. b See 4.2.3 for a complete description of this material.

c The steel shall be fully killed and made with fine-grain practice, without normaliz ing, for thicknesses of 3 / 16 in. to 1 1 / 2 in. d The manganese content shall be in the range from 0.85 % to 1.20 % by ladle analysis.

e The manganese content shall be in the range of 0.80 % to 1.20 %. The steel shall not be rimmed or capped.4-8 API S TANDARD 620
4.2.5.2 All other impact requirements of ASTM A20, Supplem entary Requirement S5, shall apply for all materials listed in Table 4-3, including specificat ions that do not refer to ASTM A20. 4.2.5.3 For thickness exceeding the range in Table 4-3, impact test requirement s shall be mutually agreed by the Manufactur er and the Purchaser. ASME Section VIII, Division 1, Part UG-84 may be used as a guide. 

### 4.3 Pipe, Flanges, Forging, and Castings 4.3.1 General All pipe, flanges, forgings, and castings used in the parts of the tanks that are subject to internal pressure shal l conform to applicable requirements of 4.3.2 to 4.3.6 inclusive. 4.3.2 Pipe 14
4.3.2.1 Carbon steel pipe shall conform to one of the following specifications : a) ASTM A53; b) ASTM A106; c) ASTM A134, excluding helic al ( spiral) welded pipe; d) ASTM A139, excluding helical (spiral) welded pipe; e) ASTM A333; f) ASTM A524; g) ASTM A671 (Grades CA, CC, CD, and CE only); h) API 5L (Grades A and B only). 4.3.2.2 When ASTM A134, A139, or A671 pipe is used, it shall comply with the following. a) The pipe shall be certified to have been pressure tested. b) The plate specification for the pipe s hall satisfy the requir ement s of 4.2.3, 4.2.4, and 4.2.5 that are applicable to that plate s pecification. c) Impact tests for qualifying the welding procedure for the pipe longitu dinal welds shall be perfor m ed in accordance with 4.7.1. 4.3.3 Built-up Fittings Built-up fittings, such as ells, t ees, and return bends, may be fabricated by fusion welding when they are designed according to the applicable par agraphs in this standard.

14 For design metal temperatures below –20 ° F, the materials shall conform to Tables R-1 and/or R-3.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 4-9
4.3.4 Flanges 4.3.4.1 Hub, slip-on welding neck and long we lding neck flanges shall conform to the m aterial requirements of ASME B16.5 for forges carbon steel flanges. Plate materi al used for nozzle flanges s hall have physical properties better than or equal to those required by ASME B16. 5. Plate flange material shall conform to 4.2.3. 4.3.4.2 For nominal pipe sizes greater than 24 in., flanges that conform to ASME B16.47, Ser ies B, m a y be used, subject to the Purchaser’s approval . Particular attention should be give n to ensuring that mating flanges of appurtenances are compatible. 4.3.5 Castings and Forgings Large castings and forgings (see Footnote 12 for both materi als) not co vered in 4.1.3 sha ll be of welding grade if welding is to be done on them, and they shall confo rm to one of the followi ng ASTM specifications: a) A27 (Grade 60-30, for structural parts only), b) A105, c) A181, d) A350. 4.3.6 Toughness Requirements Except as covered in 4.3.2.2, t he toughness requ irement s of pipe, flanges, and forgings shall be established as described in 4.3.6.1 through 4.3.6.4. 4.3.6.1 No impact testing is required for ASME B16.5 ferritic stee l flanges used a t minimum design metal temperature, no colder than –20 °F. Piping materials made according to ASTM A333 and ASTM A350 may be used at a minimum design metal temperatur es, no lower than the impact test temperature required by the ASTM specification for the applicable material grade, unless additional impact tests (see 4.3.6.4) are conducted. 4.3.6.2 Other pipe and forging materials shall be classified under the material g r oups shown in Figure 4-2 as follows: a) Group I—API 5L, Grades A, B, ASTMA 106, Grades A and B; ASTM A53, Grades A and B; ASTM A181; and ASTM A105; b) Gro up II—ASTM A524, Grades I and II. 4.3.6.3 The materials in the groups listed in 4.3.6.2 may be used at nominal thicknes ses, includin g corr osion allowance, at minimum design metal temperatures no lowe r than those shown in Figure 4-2 without impact testing (see 4.3.6.4). The governing thickness (see Figure 4-3) to be used in Figure 4-2 shall be as follows. a) For butt-welded joints, it is the nominal thickness of the thickest welded joint. b) For corner weld (groove or fillet) or lap we lds, it is the thinn er of the two parts joined. c) For nonwelded parts (such as bolted flanges), it is 1 / 4 of flat cover nominal thickness. 4.3.6.4 When impact tests are required by 4.3.6.2 or 4.3.6. 3, they shall be perfor med in accordance with the requirements, including minimum energy requirements of ASTM A333, Grade 1 for pipe, or ASTM A350 Grade LF1, for forgings at a test temperature no higher than the mi nimum design metal temperature. Except for the plateFigure 4-2—Minimum Permissible Design Metal Temperature for Pipe, Flanges, and Forgings without Impact T esting –60 –50 –40 –30 –20 –10 0 10 20 30 40 50 60

–60 –50 –40 –30 –20 –10 0 10 20 30 40 50 60

0.25 0.50 0.75 1.00Design metal temperature (oF)

Group II Group I

Thickness, including corrosion allowance (inches) 1.25 1.50 4-10 API S TANDARD 620

specified in 4.2.3, the material specif ied in 4.3 shall have a minimum Charpy V- notch impact strength of 13 ft-lb (full size specimen) at a temperature no higher than the minimum design metal temperature. 

### 4.4 Bolting Material Carbon steel bolts 15 may be used if they conform to the following, or to better 16 , specifications: a) ASTM A193, b) ASTM A307, c) ASTM A320.

15 For design metal temperatures below –20 ° F, the materials shall conform to Tables R-1 and/or R-3. 16 If better grades of bolts are used, higher bolt stress values are not recommended with full-faced gaskets.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 4-11 

### 4.5 Structural Shapes All structural shapes (see Footnote 12) that are subject to pressure-imposed loads or are otherwise important to the structural integrity of a tank shal l be made only by the open-hearth, electric-furnace, or basic-oxygen process and shall conform to one of t he following specifications: a) ASTM A36; b) ASTM A131; c) ASTM A633 (Grade A only); d) ASTM A992; e) CSA G40.21 (Grades 38W, 44W, and 50W only; if impact tests are required, these grades are designated 38WT, 44WT, and 50WT); f) ISO 630 (1995), Grade E275, Qualities B, C, and D; g) EN 10025, Grade S275, Qualities JR, J0, J2, and K2.Figure 4-3—Governing Thickness for Impact Test De termination of Pipe, Flan ges, and ForgingsTc

t nTfTcTf

t ntstsTc

t ntsTc

t nTfts

Long Welding–neck Flange Welding–neck Flange Ring–type Flange Slip–on Flange C L C L

C L C LTf

NOTE 1 Shell reinforcing plate is not included inthe illustration above. NOTE 2 t s = shell thickness; t n = nozzle neck thickness; T f = flange thickness; T c = bolted cover thickness. NOTE 3 The governing thickness for each component shall be as follows: Components Governing Thickness (whichever is less) Nozzle neck t n or t s Slip-on flange t n or T f Ring-type flange t n or T f Welding-neck flange t n or T f Long welding-neck flange t n or t s Bolted cover 1 / 4 T c 4-12 API S TANDARD 6205-1 

### SECTION 5—DESIGN
5.1 General 5.1.1 Scope of Rules The rules presented in this standard are intended to es tablish approved engineer ing practices for low-pressure storage tanks constructed of any shape within the scope of 1.2 and to provide the fundamental rules for design and testing, which can serve as a suffici ent basis for an inspector to judge the safety of any vessel and improve the application of the API 620 nameplate. Where these rules do not cover all details of design and construction, the Manufacturer, subject to the approval of the inspector, shall provide details of design and construction that will be as safe as those provided by this standard. 5.1.2 Pressure Chambers For tanks that consist of two or mo r e independent pressure chambers and have a roof, bottom, or other elements in common, each pressure part shall be des igned for the most severe combination of pressure or vacuum that can be experienced under the specif ied operating conditions. 5.1.3 Avoidance of Pockets Tank walls shall be shaped to avoid any pockets on t he inside where gases may become tr apped when the liquid level is being raised or on the outside where rainwater may collect. 5.1.4 Volume of Vapor Space The volume of the vapor space above the high liquid desi gn level upon which the nominal cap acity is based shall be not less than 2 % of the tot al liquid capacity (see 3.2.2). 5.1.5 Tests of New Design When a tank is of a new design and has (a) an unusual shape or (b) large branches or openings that may make the stress system aro und these locations in the tank wall unsymmetrical to a degree that, in the judgment of the designer, does not permit computation with a satisfactory assurance of safety, the tank shall be subjected to a proof test, and strain-gauge surveys shall be made as provided in 7.24. 

### 5.2 Operating Temperature The temperature of the liquids, vapor, or gases stor ed in, or entering, these tanks shall not exceed 250 °F (see 1.2.2). 

### 5.3 Pressures Used in Design 5.3.1 Above Maximum Liquid Level 5.3.1.1 Tank components, including those above the maximum liquid level, subject ed principally to gas pressure shall be designed for the following. a) A pressure not less than the relief valves’ set pressure. The maximum positive gauge pressure shall be under stood to be the nominal pressure rating for the tank (sometimes called the design pressure) and shall not exceed 15 lbf/in. 2 gauge. b) The maximum partial vacuum (also called the design va cuum) when the inflow of air (or another gas or vapor) through the vacuum re lief valves is at the tank design maximum in-breathing flow rate.5-2 API S TANDARD 620
5.3.1.2 If the maximum liquid level is below the top of the roof but the tank will be filled to the top of the roof during the hydrostatic test as pr ovided in 7.18.4, the tank shall be designed for both maximum liquid-level conditions. 5.3.1.3 A suitable margin shall be provided between the operating pressure in the gas or vapor sp ace and the relie f valve’s set pressure. This margin allo ws for pressure increases caused by variations in temperature, atmospheric pressure, or other factors that affect the pressure in the gas or vapor space. 5.3.1.4 The set pressure of the vacuum relief valve shall limit the vacuum pre ssure accumulation in the t ank to the design vacuum pressure. 5.3.2 Below Maximum Liquid Level All pressure containing elements of the tank below the maximum liquid level shall be designed for the most severe combination of gas pressure (or partial vacuum) and static liquid head affecting the element. 5.3.3 Weight for Liquid Storage The weight for liquid storage shall be assumed to b e the weight per ft 3 of the specified liquid contents at 60 °F. When the minimum weight is less than 48 lb/ft 3 , the provisions of 5.5.7 shall be fo llowed. This minimum weight does not apply to tanks used for gas storage only, or used for refrigerated liquid stor age as discussed in Annexes Q and R. 

### 5.4 Loads 5.4.1 Individual Loads a) dead load ( D

L ): the weight of the tank or tank component, incl uding any insulation, lining, or corrosion allowance unless otherwise noted. b) hydrost atic and pneumatic test s ( H

t ): the load due to conducting the tests specified in 7.18. c) loads from connected piping ( L p )

. d) loads from platforms and stairways ( L s ) (see Annex E). e) minimum roo f live load ( L r ): 20 lb/ft 2 on the horizontal projected area of the roof. f) pressure ( P

g ): the maximum positive gauge pressure given in 5.3.1. g) pressure ( P

v ): the maximum partial vacuum given in 5.3.1. The maximum partial vacuum shall be at least 1 in. w c . h) seismic ( E ): seismic loads given in Annex L. i) snow ( S ): The ground snow load (lb/ft 2 ) shall be determined from ASCE 7 Figure 7-1 or Table 7-1 unless the ground snow load is specified by the Purchaser. The design uniform roof snow load shall be 0.84 times the ground snow load. Alternately, the desi gn uniform roof snow load shall be det ermined from the ground snow load in accordance with ASCE 7 or a national standard. j) stored liqu id ( P

l ): gauge pressure (lb/in. 2 ) resulting from the liquid head of th e liquid with the density given in 5.3.3. All liquid levels from empty to the maximum liquid level shall be considered. k) wi nd ( W ): 

### T he design wind speed ( V 

### ) shall be either; 1) the 3-sec gust design wind speed (mph) determined from ASCE 7-05, Figure 6-1, multiplied by √ I 

### ; or 2) the 3-sec gust design wind speed determined from ASCE 7-10 for risk category specif ied by the Purchaser (typically risk category 2 applies—Figure 26.5-1b) multiplied by 0.78; or 3) the 3-sec gust design wind speed specified by theD ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-3 

### Purchaser. When wind is specified as measured by fastest mile, the speed shall be multiplied by 1.2. For tank components exposed to wind up to 80 ft above ground, the design wind pressures normal to the tank’s outside surface shall be the pressures below, multiplied by ( V /120) 2 . For tank components located more than 80 ft above ground, use ASCE 7 to determine wind pressures.

Surface Direction Average Pressure (lbf/ft 2 ) Maximum Pressure (lbf/ft 2 ) cylinder inward 16 31 sphere inward or outward 16 31 dome or cone roof or bottom outward 30 50
Alternatively, pressures may be determined in accordance with ASCE 7 or a national standard for the specific conditions for the tank being designed. Average wind pressure on the roof shall be used to design the roof to shell compr ession region and for overturning. Maximum wind pressure shall be used to design the roof and shell.
NOTE ASCE 7-10 wind velocities now have LRFD load factors and ri sk category (importance factor s) built in, whereas API 620 uses the working stress. The 0.78 factor ap plied to the ASCE 7-10 wind speed provides a conversion to working stress levels. 5.4.2 Load Combinations The tank shall be designed for the following load combinations. If the absence of any load other than dead load result s in a more severe condition, that load shall not be included in the combination. a) D L + P g

+ P l b) D L + W L + 0.7 P g c) D L + W L + 0.4 P v d) D L + P v + 0.4( L r

or S ) e) D L + 0.4 P v + ( L r

or S ) f) D L + 0.7 P g

+ P l + E + 0.1 S g) D L + H t h) D L + L s i) D L + L p + P g

+ P l 

### 5.5 Maximum Allowable Stress for Walls 17
5.5.1 General Higher localized shear and secondary bending stresses may exist in the walls of tanks designed and fabricated according to this standard, and th e prescribed test loadi ngs may result in some localized reshaping. This is permissible, since localized reshaping is ex pected as part of a legitimate fabric ation operation, if the reshaping is not

17 See Biaxial Stress Criteria for Large Low-Pressure Tanks , written by J. J. Dvorak a nd R. V. McGrath and published as Bulletin No. 69 (June 1961) by the Welding Research Council, 3 Park Avenue, 27th Floor, New York, New York 10016.5-4 API S TANDARD 620 so severe that upon release of the test pressure, plastic straining occurs in the opposite direction. This would tend to develop continuing plastic strain ing in subsequent normal operation. 5.5.2 Nomenclature 5.5.2.1 Variables relating to stresses common to the require ment s of 5.5.3 through 5.5

. 5 and Figure 5-1 are defined as follows:

t is the thickness of the wall, in inches;
R is the radius of the wall, in inches;

c is the corrosion allowance, in inches; S

ts is the maximum allowable stress for simple tension, in lbf/in. 2 , as given in Table 5-1; S

cs is the maximum allowable longitudi nal compressive stress, in lbf/in. 2 , for a cylindrical wall acted upon by an axial load with neither a tensile nor a compressiv e force acting concurrently in a circumferential direction (determined in accordance with 5.5.4.2 for the thick ness-to-radius ratio involved);

s ta is the allowable tensile stress, in lbf/in. 2 ; s ta lower than S ts because of the presence of a coexistent compressive stress perpendicular to it;

s ca is the allowable compressive stress, in lbf/in. 2 ; s ca is lower than S cs because of the presence of a coexistent tensile or compress ive stress perpendicular to it;

s tc is the computed tensile stress, in lbf/in. 2 , at the point under consideration;

s cc is the computed compressive stress, in lbf/in. 2 , at the point under consideration;

s t is the general variable for indicati ng a tensile stress, in lbf/in. 2 , which may be either an allowable or computed value depending on the context in which the variable is used;

s c is the general variable for indicating a compressive stress, in lbf/in. 2 , which may be either an allowable or computed value depending on the context in which the variable is used; N is the ratio of the tensile stress, s t , to the maximum allowable stress for simple tension, S ts ; M is the ratio of the compressive stress s c , to the maximum allowable compressive stress, S cs (see Figure F-1). 5.5.2.2 The term tank wall is defined in 3.3. Unless otherwise stipulated in this st andard, the stresses in nozzle and manway necks, reinforcing pads, flanges, and cover plates shall not exceed the va lues that apply for the walls of the tank. 5.5.3 Maximum Tensile Stresses 5.5.3.1 The maximum tensile stresses in the outside walls of a t ank, as determined for a ny of the loadings listed in 5.4 or any concurrent combination of such loadings that is expected to be encountered in the specified operation, shall not exceed the applicable stress values determined in accordance with provisions described in 5.5.3.2 and 5.5.3.3. 5.5.3.2 If both the meridional and la titudin al unit forces, T 1 and T 2 , are tensile or if one force is tensile and the other is zero, the computed tensile stress, S tc , shall not exceed the applicabl e value given in Table 5-1.Figure 5-1—Biaxial Stress Chart for Combined Tension and Compression, 30,000 lbf/in. 2 to 38,000 lbf/in. 2 Yield Strength Steels 16,000
14,000
12,000
10,000

8,000
6,000
4,000
2,000

0 0 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018 0.020 0.022 S cs

= 1,800,000 ( t - c )/ R S cs = 10,150 - 277,400 ( t - c )/

R S cs = 15,000 psi

A B C

N=0 Do not extrapolate beyond this lineCoexistent compressive stress,Sc, for biaxial tension-compression(pounds per square inch)

( t - c )/ R ratio (see Note 3) NOTE 1 At no time can a compressive stress for a particular value of ( t - c )/ R exceed S cs represented by curve 0ABC, no values of compressive stress or N are permitted to fall to the left or above this curve. NOTE 2 See Figure F-1 for relationship between factors M and N. NOTE 3 If compressive stress is latitudinal, use R = R 1 ; if compressive stress is meridional, use R = R 2 .Values of tensile stress factorN = Stc/Sts 0.10 0.20 0.30 0.40 0.50 0.60
0.70

0.80

0.90 N

= 0 S cs = MS cs -15,000 M (See Notes 1 and 2 ) D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-5

5.5.3.3 If the meridional force, T 1 , is tensile and the coexist ent latitudinal unit force, T 2 , is compressive or if T 2 is tensile and T 1 is compressive, the computed tensile stress, s tc , shall not exceed a value of the allowable tensile stress, s ta , obtained by multiplying the applicable stress value given in Table 5-1 by the appropriate value of N obtained from Figure 5-1 for the va lue of compressive stress ( s c = s cc ) and the co-related ratio of ( t – c )/ R involved. However, in cases where the unit force acting in compression does not exceed 5 % of the coexistent tensile unit force acting p erpendicular to it, the designer has the option of permitti ng a tensile stress of the magnitude specified in 5.5.3.2 instead of complying strictly wi th the provisions of this paragraph, (see F.1 for examples illustrating the determination of allowable tensile stress values, s ta , in accordance with this paragraph) . In no event shall the value of s ta exceed the product of the applicable joint efficiency for t ension as given in Table 5- 2 and the allowable stress for simple tension shown in Table 5-1. 5.5.4 Maximum Compressive Stresses 5.5.4.1 Except as provided in 5.12.4.3 for the compression-ring region, the maximum comp ressive stresses in the outside walls of a t ank, as determined for any of the loadings listed in 5.4 or any concurrent combination of loadings expected to be encountered in the specified operation, shall not exceed the applicable stress values determined in accordance with the provisions described in 5.5.4.2 through 5.5.4.8. These rules do not purport to apply when the circumferential stress on a cylindrical wall is compress ive (as in a cylinder acted upon by external pressure). However, values of S cs computed as in 5.5.4.2, with R equal R 1 when the compressive unit force is latitudinal or to R 2 when the compressive unit force is meridional, in some degree fo rm the basis for the rules given in 5.5.4.3, 5.5.4.4, and 5.5.4.5, which apply to walls of double curvature.5-6 API S TANDARD 620 Table 5-1—Maximum Allowable Stress Values for Simple Tension Specified Minimum Maximum Allowable Tensile Stress for Tension, S

ts (lbf/in. 2 , See Notes 2 and 3) Specification (See Note 1) Grade Notes Tensile Strength (lbf/in. 2 ) Yield Point (lbf/in. 2 ) Plates ASTM A36 — 4 58,000 36,000 16,000 ASTM A131 A 4, 5, and 6 58,000 34,000 15,200 ASTM A131 B 4 58,000 34,000 16,000 ASTM A131 CS 4 58,000 34,000 16,000 ASTM A283 C 4 and 5 55,000 30,000 15,200 ASTM A283 D 4, 5, and 6 60,000 33,000 15,200 ASTM A285 C 5 55,000 30,000 16,500
ASTM A516 55 — 55,000 30,000 16,500 ASTM A516 60 — 60,000 32,000 18,000 ASTM A516 65 — 65,000 35,000 19,500 ASTM A516 70 — 70,000 38,000 21,000
ASTM A537 Class 1 7 70,000 50,000 21,000 ASTM A537 Class 2 7 80,000 60,000 24,000 ASTM A573 58 4 58,000 32,000 16,000 ASTM A573 65 4 65,000 35,000 18,000 ASTM A573 70 4 70,000 42,000 19,300 ASTM A633 C and D 4 and 7 70,000 50,000 19,300
ASTM A662 B — 65,000 40,000 19,500 ASTM A662 C 7 70,000 43,000 21,000 ASTM A678 A 4 and 8 70,000 50,000 19,300 ASTM A678 B 4 and 7 80,000 60,000 22,100 ASTM A737 B 7 70,000 50,000 21,000 ASTM A841 Class 1 7 70,000 50,000 21,000
CSA G40.21 38W and 38WT 4 60,000 38,000 16,500 CSA G40.21 44W and 44WT 4 64,000 44,000 17,700 CSA G40.21 50W and 50WT 4 65,000 50,000 18,000 ISO 630 (1995) E275 Quality C, D 4 59,500 37,000 16,400 ISO 630 (1995) E355 Quality C, D 4 71,000 48,500 19,600 EN 10025 S275 Quality J0, J2 4 59,500 37,000 16,400 EN 10025 S355 Quality J0, J2, K2 4 68,100 48,500 18,800D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-7

Pipe Seamless API Spec 5L B — 60,000 35,000 18,000 ASTM A33 B — 60,000 35,000 18,000 ASTM A106 B — 60,000 35,000 18,000 ASTM A106 C — 70,000 40,000 21,000 ASTM A333 1 — 55,000 30,000 16,500 ASTM A333 3 — 65,000 35,000 19,500 ASTM A333 6 — 60,000 35,000 18,000 ASTM A524 I — 60,000 35,000 18,000 ASTM A524 I1 — 55,000 30,000 16,500 Electric-fusion Welded ASTM A134 A283 Grade C 4, 5, and 9 55,000 30,000 12,100 ASTM A134 A285 Grade C 5 and 9 55,000 30,000 13,200 ASTM A139 B 9 60,000 35,000 14,400 ASTM A671 CA55 9 55,000 30,000 13,200 ASTM A671 CC60 9 60,000 32,000 14,400 ASTM A671 CC65 9 65,000 35,000 15,600 ASTM A671 CC70 9 70,000 38,000 16,800 ASTM A671 CD70 7 and 9 70,000 50,000 16,800 ASTM A671 CD80 7 and 9 80,000 60,000 19,200 ASTM A671 CE55 9 55,000 30,000 13,200 ASTM A671 CE60 9 60,000 32,000 14,400 Forgings ASTM A105 — — 60,000 30,000 18,000 ASTM A181 I — 60,000 30,000 18,000 ASTM A181 II — 70,000 36,000 21,000 ASTM A350 LF1 — 60,000 30,000 18,000 ASTM A350 LF2 — 70,000 36,000 21,000 ASTM A350 LF3 — 70,000 40,000 21,000 Castings and Bolting ASTM A27 60-30 10 60,000 30,000 14,400 ASTM A36 For anchor bolting 11 58,000 36,000 15,300 Table 5-1—Maximum Allowable Stress Values for Simple Tension (Continued) Specified Minimum Maximum Allowable Tensile Stress for Tension, S

ts (lbf/in. 2 , See Notes 2 and 3) Specification (See Note 1) Grade Notes Tensile Strength (lbf/in. 2 ) Yield Point (lbf/in. 2 )5-8 API S TANDARD 620

ASTM A193 B7 11 125,000 105,000 24,000 ASTM A307 B for flanges and pressure parts 11 and 12 55,000

— 8,400
ASTM A307 B for structural parts and anchor 11 55,000

— 15,000

ASTM A320 bolting L7 11 125,000 105,000 24,000
Structural Shapes Resisting Internal Pressure ASTM A36

— 4 and 6 58,000 36,000 15,200 ASTM A131 A 4 and 6 58,000 34,000 15,200 ASTM A633 A 4 63,000 42,000 17,400 ASTM A992

— 4 and 6 65,000 50,000 15,200 CSA G40.21 38W and 38WT 4 and 6 60,000 38,000 15,200 CSA G40.21 44W and 44WT 4 and 6 64,000 44,000 15,200 CSA G40.21 50W and 50WT 4 and 6 65,000 50,000 15,200 ISO 630 (1995) E275 Quality B, C, D 4 and 6 59,500 37,000 15,200 ISO 630 (1995) E355 Quality B, C, D 4 and 6 71,000 48,500 15,200 EN 10025 S275 Quality JR, J0, J2 4 and 6 59,500 37,000 15,200 EN 10025 S355 Quality JR, J0, J2, K2 4 and 6 68,100 48,500 15,200 NOTE 1 All pertinent modifications and limit ations of specifications required by 4.2. through 4.6 shall be complied with. NOTE 2 Except for those cases where additional factors or limitatio ns are applied as indicated by references to Notes 4, 6, 10, and 12, the allowable tensile stress values given in th is table for materials other than bolting steel are the lesser of (a) 30% of the spe cified minimum ultimate tensile strength for the material or (b) 60 % of the specified minimum yield point. NOTE 3 Except when a joint efficiency factor is already reflected in the specified al lowable stress value, as indicated by the references to Note 10, or where the value of N determined in accordance with 5.5.3.3. is less than the app licable joint effici ency given in Table 5-2 (and, therefore, effects a greater reduction in allowable stress than would the pertinent join efficiency factor, if applied), the specified str ess values for welds in tension shall be multiplied by th e applicable joint efficiency factor, E , given in Table 5-2. NOTE 4 Stress values for structural quality steels include a quality factor of 0.92. NOTE 5 Plates and pipe shall not be used in thickness greater than 3 / 4 in. NOTE 6 Stress values are limited to those for steel that has an ultimate tensile strength of only 55,000 lbf/in. 2

. NOTE 7 Less than or equal to 2 1 / 2 in. thickness. NOTE 8 Less than or equal to 1 1 / 2 in. thickness. NOTE 9 Stress values for fusion-welded pipe include a welded-joint efficiency factor of 0.80 (see 5.23.3). Only straight-seam p ipe shall be used; the use of spiral -seam pipe is prohibited. NOTE 10 Stress values for castings include a quality factor of 0.80. NOTE 11 See 5.6. NOTE 12 Allowable stress based on Section VIII of the ASME Boiler and Pressure Vessel Code multiplied by the ratio of the design stress factors in this standard and Section V111 of the ASME Code , namely 0.30/0.25. Table 5-1—Maximum Allowable Stress Values for Simple Tension (Continued) Specified Minimum Maximum Allowable Tensile Stress for Tension, S

ts (lbf/in. 2 , See Notes 2 and 3) Specification (See Note 1) Grade Notes Tensile Strength (lbf/in. 2 ) Yield Point (lbf/in. 2 )D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-9
5.5.4.2 If a cylindrical wall, or a portion thereof, is acted upon by a longitudinal compressive force with neither a tensile nor a compressive force acting concurrently in a circumferential direction, the computed compressive stress, s cc , shall not exceed a value, S cs , established for the applicable thic kness-to-radius ratio as follows: for values of ( t – c )/ R less than 0.00667,
S

cs = 1,800,000 [( t – c )/ R ] for values of ( t – c )/ R between 0.006 67 and 0 .0175,
S

cs = 10,150 + 277,400 [( t – c )/ R ] for values of ( t – c )/ R greater than 0.0175,
S

cs = 15,000 5.5.4.3 If both the meridional and la titudinal unit forces, T 1 and T 2 , are compressive and of equal magnitude, the computed compressive stress, s cc , shall not exceed a value, s ca , established for the applicable thickness-to-radius ratio as follows: for values of ( t – c )/ R less than 0.00667,

s ca = 1,000,000 [( t – c )/ R ] for values of ( t – c )/ R between 0.006 67 and 0 .0175,

s ca = 5650 + 154,200 [( t – c )/ R ] for values of ( t – c )/ R greater than 0.0175,

s ca = 8340 5.5.4.4 If both the meridional and la titudinal unit forces, T 1 and T 2 , are compressive but of unequal magnitude, both the larger and smaller computed compressive stresses shall be limited to values that satisfy the following requirements: ( S 1 + 0.8 S s )/

S

cs ≤ 1.0
1.8 S s /

S

cs ≤ 1.0 where
S 1 is the larger stress, in lbf/in. 2 ; S

s is the small stress, in lbf/in. 2 ; S

cs is the maximum allowable longitudinal compressive stress, in lbf/in. 2 , determined as in 5.5.4.2 using R for the larger unit force in the first equation and fo r the smaller unit force in the second equation.
NOTE In the previous expres sions, if the unit force in volved is latitudinal, R shall be equal to R 1 ; if the force is meridional, R shall be equal to R 2 .Table 5-2—Maximum Allowable Efficiencies for Arc-welded Joints

Type of Joint Limitations Basic Joint Efficiency (%) Radiographed (See Note 1) Maximum Joint Efficienc y

(%; see Note 2) Butt joints, attained by double-welding or other means approved by the Pu rchaser, that will obtain the quality of deposited weld metal on the inside and outside weld surfaces that agrees with the requirements of Paragraph UW-35 in Section VIII of the ASME Code ; welds using metal backing strips that remain in place are excluded. None, for all double-welded joints, except for roofs abov e liquid level.

Roofs ab ove liquid level. 85

70 Spot Full (see Note 3)

— Spot Full (see Note 3) 85 10 0

70 85 100 Sin g le-welded butt joint with backing strip or equivalent o ther than those included above. Longitudinal or meridi onal circumference or latitudinal joints b etween plates not more than 1 1 / 4 in. thick; nozzle attachment welding without thickness limitation. Roofs above liquid level. 75

70 Spot Full (see Note 3)

— Spot Full (see Note 3) 75 85

70 75 85 Single-welded butt joint without backing strip. Nozzle attachment welding. 70

— 70 Double full-fillet lap joint (see Note 4). Longitudinal or meridional joints and equivalent (see Note 5) circum fere ntial or latitudinal joints between plates not more and 3 / 8 in. thick; joints of this type shall not be used for longitudinal or meridional joints that the provisions of 5.12.2 require to be butt-welded. Other circumferential or latitudinal joints be tween plates not more than 5 / 8 in. thick. 70

65 —

— 70

65 Single full-fillet lap joint (see Note 4). Longitud i nal or meridional joints and circumferential or la titudinal joints between plates not more than 3 / 8 in. thick; joints of this type shall not be used for longitudinal or meridional joints that the provisions of 5.12.2 require when the thinner plate joined exceeds 1 / 4 in. 35

— 35

Single full-fillet lap jo int s for head-to-nozzle joint s For att achment of heads convex to pressure not more than 5 / 8 in. required thickness, only with use of the fillet weld on the inside of the nozzle. 35

— 35

Nozzle-attachment fillet welds Attachment welding for nozzles and their reinforcemen ts. (Inc luded in the strength factors in 5.16.8.3) Pl ug welds (see 5.24.5) Attachment welding for nozzle reinforcements (see Note 6). 80

— 80 5-10 API S TANDARD 620

NOTE 1 See 5.26 and 7.15 for examination requirements. NOTE 2 Regardless of any values given in this column, the efficiency for lap-we lded joints between plates with surfaces of dou ble curvature that have a compressive stress across the joint from a negative value of P g or other external loading may be taken as unity; such compressive stress shall not exceed 700 lbf/in. 2 . For all other lap-welded join ts, the joint efficiency factor must be a pplied to the allowable compressive stress, S ca . The efficiency for full-penetration butt-welded joints, which are in co mpression across the entire th ickness of the connected plate s, may be taken as unity. NOTE 3 All main butt-welded joints (see 5.26.4.2) shall be comp letely radiographed or ultrasonically examined as specified in 5.26 and nozzle and reinforcement attachment welding shall be examined by the magnetic-particle me thod as specified in 7.15.2. NOTE 4 Thickness limita tions do not apply to flat bottoms supported uniformly on a foundation. NOTE 5 For the purposes of this table, a circumferential or latitudinal joins shall be consider ed subject to the same requirem ents and limitations as are longitudinal or meri dional joints when such a circumferential or latitu dinal joint is located (a ) in a spherical, tori s pherical or ellipsoidal shape or in any other surface of double curvature, (b) at the juncti on between a conical or dished roof (or bottom) and cylindrical s idewalls, as considered in 5.12.3 or (c) at a similar juncture at either end of a transition section or reduc er as shown in Figure 5-9. NOTE 6 The efficiency factors shown for f illet welds and plug welds are not to be appli ed to the allowable shearing stress val ues shown in Table 5-3 for structural welds.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-11
5.5.4.5 If the meridional unit force, T 1 , is compressive and the coexistent unit force T 2 , is tensile, except as otherwise provided in 5.5.4.6, or if T 2 is compressive and T 1 is tensile the computed compressive stress, s cc , shall not exceed a value of the allo wable compressive stress, s ca , determined from Figure 5-1 by entering the computed value of N and the value of t/R associated with the compressive unit stress and reading the value of s c that corresponds to that point. The value of s c will be the limiting value of s ca for the given conditions. (See F- 1 for examples illustrating the determination of allowable compressive stress values in accordance with this paragraph.) 5.5.4.6 When a local axial compressive buckling stress in a cylindrical shell is primarily due to a moment in the cylinder, then the allowable l ongitudinal compressive stress S cs

or s ca , as specified in 5.5.4.2 or 5.5.4.3, may be increased by 20 %. If the shell bending is due to wind (tank full or empty) or due to earthquake (tank empty), then in addition to the above allowed 20 % increase, the allowabl e buckling stress due to a moment can be increased an additional 1 / 3 . For tanks full or partially full of liquid and for an earthquake induced longitudinal compressive stress, the allowable compression stress need not be limited for biaxial stress as other wise may be required by Figure 5-1. For seismic design, the tank full is usually the worst case . For wind loading, the tank empty and with internal pressure is usually the worst case for local, bending induced compressive stress. 5.5.4.7 The allowable compressive stresses previously specified in 5.5.4 are predicated on butt-welded construction. If one or more of the main joints across which the compressive forc e acts are of the lap-welded type, the allowable compressive stress will be determined according to 5.5.4, but subject to the limitations of 5.12.2 and Table 5-2 (including Note 2). 5.5.4.8 Cylindrical shells can be checked for wind buckling to determine if t here is the need for intermediate wind girders using the rules of 5.10.6. If th e transition between the roof or bottom is a curved knuckle section (5.12.3) then 1 / 3 of the knuckle height shall be included as part of the unstiffened shell height. 5.5.5 Maximum Shearing Stresses The maximum shearing stresses in welds used for attaching manways and nozzles and their reinforcements or other attachments to the walls of a tank and in sections of manway or nozzle necks that serve as reinforcement attachment shall not exceed 80 % of the value of the applicable maximum allowable tensile stress, S ts , given in Table 5-1 for the kind of material involved. Such maxi mum shearing stresses are permissible only where the loading is applied in a direction perpendicular to the length of the weld and must be reduced where the loading is applied differently (see 5.16.8.3). 5.5.6 Maximum Allowable Stresses for Wind or Earthquake Loadings The maximum allowable stresses for design loadings co mbined with wind or earthquak e loadings shall not exceed 133 % of the stress permitted for the desig n loading condition; except as allowed in Annex L, this stress shall not exceed 80 % of the specified mi nimum yield strength for carbon steel. Fo r stainless steel and aluminum, see Q.3.3.6. 5.5.7 Allowable Stress for Tests Stresses for hydrostatic and hydrostatic-pneumatic tests s hall not exceed 130 % of the a llowable design tensile stress or 125 % of the allowable design compressive stress except as modified in 5.12.4.3. Ca lculations are required for product density less than 48 lb/ft 3 to verify test stresses will not exceed these limits. 

### 5.6 Maximum Allowable Stress Values for Structural Members and Bolts 5.6.1 Subject to the provisions of 5.6.5, the maximum stresses in internal or external diaphragms, webs, trusses, columns, and other framing, as determined for any of the loadi ngs listed in 5.4 or any concurrent combination of such loadings expected to be encountered in the specified operation, shall not ex ceed the applicable allowable stresses given in Table 5-3.5-12 API S TANDARD 620
Table 5-3—Maximum Allowable Stress Values for Structural Members

Structural Member Value for Members Not Subject to Pressure-im p osed Loads (lbf/in. 2 ) Value for Members Subject to Pressure-imposed Loads (lbf/in. 2 ) Tension Rolled steel, on net section
Butt welds on smaller cross- se ctional area, in or at edge o f weld (see 5.16.8.3, Item a) Bolts and other threaded parts on net area at roof of thread 18,000 18,000

18,000 Per Table 5-1
Per Table 5-1

Per Table 5-1

Compression (see Note 1) Axially loaded structural colu mns, structura l braci ng, and structural seco ndary members, on gross section Axially Loaded tubular columns, tubular bracing and tubular secondary members, on gross section (minimum permissible thickness of 1 /4 in.)
Butt welds on smaller cross- sectional area, in or at edge o f weld (crushing) Plate-girder stiffeners, on gross section 18,000 / [1 + ( l 2 /18,000 r 2 )] but not to exceed 15,000 18,000 Y / [1 + ( l 2 18,000 r 2 )] but not to exceed 15,000 Y

18,000

18,000 18,000 / [1 + ( l 2 /18,000 r 2 )] but not to exceed 15,000 18,000 Y / [1 + ( l 2 18,000 r 2 )] but not to exceed 15,000 Y

15,000

15,000 Bending (see Note 2) Tension on extreme fibres of rolled sections, plate girders, an d built-up members Compression o n extreme fibers of rolled sections, plate girders, and built-up members With ld/bt not in excess of 600 With ld/bt in excess of 600
St ress on extreme fibers of pins Members subjected to both axial and bending loads shall be proportioned so that maximum combined axial and bending stress will not exceed the permissible value for axial loading alone Stresses on extreme fibers of butt welds resulting from bending shall n ot exceed the values prescribed for tension and compression, respectively; such values for welds in tension must be multiplied by the applicable joint efficiency Stresses on extreme fibers of butt welds resulting from bending shall n ot exceed the values prescribed for tension and compression, respectively; such values for welds in tension must be multiplied by the applicable joint efficiency 18,000

18,000 10,800,000 /( ld/bt )
2 7,000 Per Table 5-1

Same as tens. val. from Table 5-1 [(600 ) (tension value from Table 5- 1)/[( ld/bt )]

20,000D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-13

Shearing (see Note 2) Pins and turned bolts in reamed or drilled holes Unfinished bolts Webs of beans and plate girders where h/t is not more th an 60, or wher e w eb is adequately stiffened, on gross section of web Webs of beams and plate finders where web is not adequately stiffened and h/t is more than 60, on gross section of web Fillet welds where load is perpendicular to the length of weld , on the se ction through the throat (see 5.16.8.3. Item b) Fillet welds where load is parallel to the length of weld, on the section through the throat (se e 5.16.8.3, Item b) Plug welds or slot welds, on effective faying-surface are a of weld (see 5.24 .5 and Table 5-2) Butt welds on least cross-sectional area, in or at edge of weld (see 5.16.8.3, Item a) 13,500 10,000 12,000

18,000/[1 + ( h 2 /7200 t 2 )]

12,600

9,000
11,700
14,400 12,000 8,000
2 / 3 tension value from Table 5-1

(Tension value from Table 5-1) /[l + ( h 2 /7200 t 2 )]

70% tension value from Table 5-1

50% tension value from Table 5-1

65% tension value from Table 5-1

80% tension value from Table 5-1

Bearing Pins and turned bolts in reamed or drilled holes Load applied to bolt at only one side of the member con nected Load distri buted uniformly, approximately, across thickness of the me mber connected Unfinished bolts

Load applied to bolt at only one side of the member con nected Load distributed un iformly, approximately, across th icknes s of th e member connected 24,400
30,000

16,000
20,000 1.33 × tension value from Ta ble 5-1

1. 67 × tension value from Table 5-1

0. 09 × tension value from Table 5-1

1. 1 ×tension value from T a ble 5-1
NOTE 1 The variables in the compressiv e stress equations are defined as follows: l = unbraced length of the column, in in.; r = corresponding least radius of gyration of the column, in inch es ; t = thickness of the tubular column, in inches ; Y = unity (1.0 ) for values of t/R equal to or greater than 0.015; Y = (2/3)[100( t/r )] {2 – (2/3)[100) t/r )]} for values of t/R less than 0.15. NOTE 2 The variables in the bending stress equations ar e de fined as follows: l = unsupported length of the member; for a c antilever beam not fully stayed at its ou ter end against translation or rotation, l shall be taken as twice the length of the compression flange, in inches; d = depth of the member, in inches; b = width of i ts compression flange, in inches; t = thickness of its com pression flange, in inches. NOTE 3 The variables in the shearing stress equations are def ined as follows:

h = clear distance be twee n web flanges, in inches; t = thickness of the web, in inches. Table 5-3—Maximum Allowable Stress Values for Structural Members (Continued)

Structural Member Value for Members Not Subject to Pressure-imposed Loads (lbf/in. 2 ) Value for Members Subject to Pressure-imposed Loads (lbf/in. 2 )5-14 API S TANDARD 620
5.6.2 The slenderness ratio (that is, the ratio of the unbraced length, l , to the least radius of gyration, r ) for structural members in compression and for tension members other than r ods shall not exceed the fo llowing values, except as provided in 5.6.3. a) For main compression members—120. b) For bracing and other secondary members in compression—200. c) For main tension members—240. d) For bracing and other secondary members in tension—300. 5.6.3 The slenderness ratio of main compression members inside a tank may exceed 120 but not 200, provide d that the member is not or dinarily sub ject to shock or vibration loads and that the unit stress under full design loadings does not exceed the following fraction of the stress value stipulated in Table 5-3 for the member’s actual l/r ratio: f = 1.6 – ( l /200 r ). 5.6.4 The gross and net sections of structural members s hall be determined as descr ibed in 5.6.4.1 through 5.6.4.5. 5.6.4.1 Th e gross section of a member at any point shall be determined by summing the pr oducts of the thickness and the gross wid t h of each element as measured normal to the axis of the member. The net section shall be determined by substituting for the gross width the net width which, in the case of a member that has a chain of holes extending across it in any diagonal or zigzag line, shall be computed by deducting from the gross width the sum of the diameters of all holes in the chain and adding the following quantity for each gauge space in the chain:

s 2 4 g -----
where

s is the longitudinal spacing (pitch), in inches, of any tw o successive holes;
g is the transverse spacing (gauge), in inches, of the same two holes. 5.6.4.2 In the case of angles, the gauge for holes in opposite legs shall be the sum of the gauges from the back of the angle minus the thickness. 5.6.4.3 In determining the net section across plug or slot welds, the weld metal shall not be considered as adding the net area. 5.6.4.4 For splice members, the thickness considered shall be onl y that p art of the thickness of the member that has been developed b y the welds or other attachments beyond the section considered. 5.6.4.5 In pin-connected tension members other than forged eyebar s, the net section across the pinhole, transverse to the axis of the member , shall be not less than 135 %; the net section beyond the pinhole, parallel to the axis of the member, shall be not less than 90 % of the net section of the body of the member. The net width of a pin- connected member acr oss the pinhole, transverse to the axis of the member, shall not exceed eight times the thickness of the member at the pin unless lateral buckling is prevented. 5.6.5 External structural, or tubular, columns and framing subject to stresses produced by combination of wind and other applicable loads specified in 5.4 may be proportion ed for unit stresses 25 % greater than those specified in Table 5-3 if the required section is not le ss tha n that required for all other applicable loads combined on the basis ofD ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-15 the unit stresses specified in Table 5-3. A corresponding increase may be applied to the allowable unit stresses in the connection bolts or welds for such members. 5.6.6 Allowable design stresses for bolts ar e est ablished that recognize possible stressing during initial tightening. For flange bolt s , these design allowable stresses also re cognize additional stressing during overload and testing. Where bolts are used as anchorage to resist the she ll uplift, see 5.11.2.3 for allowable stresses. 

### 5.7 Corrosion Allowance When corrosion is expected on any part of the tank wall or on any external or internal supporting or bracing members upon which the safety of the completed tank depends, additional metal thickness in excess of that required by the design computations shall be provided, or some satisfactory method of protecti ng these surfaces from corrosion shall be employed. The added thickness need not be the same for all zones of exposure inside and outside the tank (see Annex G). 

### 5.8 Linings When corrosion-resistant linings are attac hed to any element of the tank wall, in cluding nozzles, their thickness shall not be included in the computation for the required wall thickness. 

### 5.9 Procedure for Designing Tank Walls 5.9.1 Free-body Analysis Free-body analysis denotes a design procedure that determines the magnitude and dir ection of the forces that must be exerted by the walls of a tank, at the level selected for analysis, to hold in static equili brium the portion of the tank and its contents above or below the selected level as a free-body, as if it were isolated from the remaining portions of the tank by a horizontal plane cu tting the walls of the tank at the level under consideration. 5.9.2 Levels of Analysis Free-body analyses shall be made at successive levels from the top to the bottom of the t ank for the purpose of determining the magnitude and character of th e meridional and longitudinal unit forces that will exist in the walls of the tank at critical levels under all the various combinations of gas pressure (or partial vacuum) and liquid head to be encountered in service, which may have a controlling effect on the design. Several analyses may be necessary at a given level of the tank to establish the governing conditi ons of gas pressure and li quid head for that level. The thicknesses required in the main walls of the tank s hall then be computed by the applicable procedures given in 5.10.3. 5.9.3 Tank Shape and Capacity The analyses in 5.9.2 provide the exact shape and overall dimensions needed fo r the desired capacity o f the tank. Except for the more common shapes such as spheres and cylinders, the determination of optimum shapes and sizes is frequently a trial-and-error procedure requiring considerable experience and judgment. As a further preliminary to making the free-body analyses (see 5.9.2) of tanks that will be provided with inte rnal ties, diaphragms, trusses, or other members subject to pressure-im posed loads, studies must be made to es tablish the preferred arrangement of the members and the magnitude and nature of the loads they must carry under the various conditions of gas pressure and liquid level that will be encountered in operation (see 5.13). 5.9.4 Flat Bottoms of Cylindrical Tanks 5.9.4.1 Flat bottoms of cylindrical tanks that are unifo rmly supported on a ringwall, grade, or concrete-slab foundation are pressure-resisting membranes but are considered nonstressed because of support from the foundation.5-16 API S TANDARD 620
5.9.4.2 All bottom plates shall have a minimum nominal thickness of 1 / 4 in. exclusive of any corrosion allowance specified by the Purchaser for the bottom plate. (See Q.3.4.7 for an excepti on to this requirement.) 5.9.4.3 Bottom plates shall be ordered to a sufficient size so that when they are trimmed, at least a 1 in. width will project beyond the outside edge of the weld that attaches the bottom to the sidewall plate. 5.9.4.4 Unless otherwise specified by the Purchaser, lap-we lded bottom plates shall be furnished and installed to lap over the adjacen t plate a minim um of 1 in. Three-plate joints in tank botto ms shall not be closer than 12 in. from each other and 12 in. from the sidewall. 5.9.4.5 Lap-welded bottom plates under the sidewall shall have t he outer ends of the joints fitted and la p-welded to form a smooth bearing for the sidewall plates (see Figure 5-2).

Figure 5-2—Method for Preparing Lap-welded Bottom Plates under the Tank Sidewall Shell plate

Bottom plate

5.9.4.6 Bottom plates under the sidewall that are thicker than 3 / 8 in. shall be butt-welded. The butt-welds shall be made using a backing strip 1 / 8 in. thick or more, or they shall be butt-we lded from both sides. Welds shall be full fusion through the thickness of the bottom plate. The butt-we ld shall extend at least 24 in. inside the sidewall. 5.9.5 Sidewall-to-Bottom Fillet Weld 5.9.5.1 For bottom and annular plat e nominal thicknesses 1 / 2 in. and less, the attachment between the bottom edge of the lowest course sidewall plate and the bottom plate shall be a continuous fillet weld laid on each side of the sidewall plate. The size of eac h weld shall not be greater than 1 / 2 in., not less than the nom inal thickness of the thinner of the two plates joined (that is, the sidewall plate or the bottom plat e immediately under the sidewall), and not less than the values shown in Table 5-4. Table 5-4—Sidewall-to-Bottom Fillet Weld for Flat-bottom Cylindrical Tanks Maximum Thickness

of Shell Plate (in.) Minimum Size of Fillet Weld (in.) 0.1875 > 0 .1875 to 0.75

> 0.75 to 1.25 > 1.25 to 1.50 3 / 16 1 /

4 5 / 16 3 / 8D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-17
5.9.5.2 The plates of the first sidewall cour se shall be attached to the bottom pl ates under the sidewall by a fillet weld inside a nd outside as required by 5.9.5.1, but when th e sidewall material has a spec ified minimum yield strength greater than 36,000 lbf/in. 2 , each weld shall be made with a minimum of two passes. 5.9.5.3 For bottom plates under t he sidewall with a nominal thickness greater than 1 / 2 in., the attachment welds shall be sized so that either th e legs of the fillet welds or the groove depth plus t he leg of the fillet for a combined weld are of a size equivalent to the thickness of t he bottom plate under the sidewall (see Figure 5-3).

Figure 5-3—Detail of Double Fillet-groove Weld fo r Bottom Plates w ith a Nominal Thickness Grea ter tha n 1 / 2 in. (See 5.9.5.3) 1 / 2 in. maximum 1 / 4 in. minimum

1 / 2 in. maximum 1 / 4 in. minimum t minimum 45°

t > 1 / 2 in. Shell plate

5.9.6 Discontinuity of Junctures For tanks that have points of marked discontinuity in the di rection of the meridional tangent, such as the point s that occur at the juncture between a conical or dished roof (or bottom) and a cylindrical sidewa ll or at the juncture between a conical reducer and a cylindrical sidewall, the portions of the tank near these points shall be designed in accordance with the provisions of 5.12. 

### 5.10 Design of Sidewalls, Roofs, and Bottoms 5.10.1 Nomenclature The variables used in the formula s of 5.10 are defined as follows.
P is the tot al pressure, in lbf/in. 2 gauge, acting at a given level of the tank under a particular condition of loading,

= P l + P g ; P l is the gauge pressure resulting from the liquid head at the level under consideration in the tank (see 5.4.1b);
P

g

see 5.4.1g, P g

is positive except in computations used to in vestigate the ability of a tank to withstand a partial vacuum; in such computations, its value is negative;5-18 API S TANDARD 620

T

1 is the meridional unit force, in lbf/in . of latitudinal arc, in t he wall of the tank at the level under consideration. T

1 is positive when in tension;
T

2 is the latitudinal unit force, in lbf/in . of meridional arc, in the wall of the tank at the level under consideration. T

2 is positive when in tension (in cy lindrical sidewalls, the latitudinal unit forces are circumferential unit forces);
R 1 is the radius of curvature of the tank wall, in inches, in a meridional pl ane, at the level under consideration. R 1 is to be considered negative when it is on the side of the tank wall opposite from R 2 except as provided in 5.10.2.6;
R 2 is the length, in inches, of the nor mal to the tank wall at the level under consideration, measured from the wall of the tank to its axis of revolution. R 2 is always positive except as provided in 5.10.2.6;
W

is the total weight, in lb, of that portion of the t ank and its contents (either above the level under consideration, as in Figure 5-4, Panel b, or below it, as in Figure 5-4, Panel a) that is treated as a free-body in the computations for that level. Strictly speaking, the total weight w ould include the weight of all metal, gas, and liquid in the portion of the tank treated as des cribed; however, the gas we ight is negligible, and the metal weight may be negligible compared with the liquid weight. W shall be given the same sign as P when it acts in the same direction as the pressure on t he horizontal face of the free-body; it shall be given the opposite sign when it acts in the opposite direction.

Figure 5-4—Typical Free-body Diagrams for Certain Shapes of Tanks P

g T

1

W 1 W 1 T 1

T 1

T 1 R s

P

W

PWW R s Axis of revolution

Supports Free body

Panel a Axis of revolution

Free body

Level under consideration

W 1

R 2 R 1 T

1 W 1F/n F/n Panel b Supports

Axis of revolution

n = number of ties cut by this plane

Ties Free body

Level under consideration

Panel cD ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-19

F

is the summation, in lb, of the vertical components of the forces in any and all internal or external ties, braces, diaphragms, trusses, columns, skirts, or other structural devices orsupports acting on the freebody. F shall be given the same sign as P when it acts in the same direction as the pressure on the horizontal face of the free-body; it shall be given t he opposite sign when it acts in the opposit e direction. A t is the cross-sectional area, in inches 2 , of the interior of the tank at the level under consideration;
t is the thickness, in inches, of t he sidewalls, roof, or bottom of t he tank, including corrosion allowance;

c is the corrosion allowance, in inches; E is the efficiency, expressed as a dec imal, of the weakes t joint across which the stress unde r consideration acts [applicable values given in Table 5-2 shall be used except that, for (a) butt-welded joints in compression across their entire thickness and (b) t he lap-welded joints in compression specified in Note 3 of Table 5-2, E may be taken as unity]; S

ts is the maximum allowable stress for simple tension, in lbf/in. 2 , as given in Table 5-1;

s ta is the allowable tensile stress, in lbf/in. 2 , established as pres cribed in 5.5.3.3;

s ca is the allowable compressive stress, in lbf/in. 2 , established as prescribed in 5.5.4;

s tc is the computed tensil e stress, in lbf/in. 2 , at the point under consideration;

s cc is the computed compressive stress, in lbf/in. 2 , at the point under consideration. 5.10.2 Computation of Unit Forces 5.10.2.1 At each level of the tank selected for free-body anal ysis as specified in 5. 9 ( s ee typical diagrams in Figure 5-4) and for each condition of gas and li quid loading that must be investigated at that level, the magnitude of the meridional and latitudinal un it forces in the wall of the tank shall be computed from the following equations, except as provided in 5.10.2.6, 5.11, or 5.12. 18

T 1 R 2 2 ----- P W F + A t -------------- +    

= (1)

T

2 R 2 P T 1 R 1 ----- –    

= (2)

T

2 R 2 P 1 R 2 2 R 1 -------- –     R 2 2 R 1 -------- W F + A t --------------    

– = (3)
NOTE Footnote 16 is also app licable to Equation (1), Equation (2), and Equation (3).

18 Equation (2), Equation (5), and Equation (9) have been derive d from a summation of the normal-to-surface components of the T 1 and T 2 forces acting on a unit area of the tank wall subjected only to pressure P . To be technically correc t, the normal-to-surface components of other loads, such as metal, snow or insulation, should be added to or subtracted from P . For the usual internal design pressure, these added loads are small compared with P and can be mooted without signif icant error. Where the pressure P

is relatively small, as in the case of a partial vacuum loading, the other load components can have a substantial effect on th e calculated T 2 force and resultant thickness. Equation (3) and Equation (6) are correct only when P is the free-body pressure witho ut the normal-to-surface components of other loads. The example in F.3 calculates the required roof thicknesses under a small vacu um by consi dering the metal, insulation and snow loads in Equation (1) through Equation (5). The designer shoul d note that if these loads ha d been omitted, the calculated thicknesses would have been much less than the correct values. In Equation (1), Equation (4), Equation (8), and Equa tion (10), W is intended to include loads of insignificant value, such as metal weight. At points away from the vertic al centerline of the roof, the value of T 2 is required for the thicknes s calculations of Equation (18), Equation (20), and Equatio n (22) and the value of P in Equation (2), Equation (5), and Equation (9) must be modified by the normal components of the added loads for the correction determination of T 2 .5-20 API S TANDARD 620
5.10.2.2 Positive values of T 1 and T 2 indicate tensile forces; negative va lues indicate compressive forces. 5.10.2.3 Free-body analyses shall be made at t he level of each horizontal joint in the sidewalls, roof, and bottom of the tank and at any intermediate levels at which the cent er of curvature changes significantly. The maximum total pressure (liquid head plus gas pressure) that can exist at a given level will not necessa rily be the governing condition for that level. Sufficient analyses shall be made at eac h level to determine the combination of liquid head and gas pressure (or partial vacuum) that, in conjunction with the allowable tensile and compressiv e stresses, will control the design at that level. A tank may normally be operated at a fixed height of liquid contents, but the tank must be made safe for any conditions that might develop in filling or emptying the tank. This will necessitate a particularly careful investigation of sidewal ls of double curvature. 5.10.2.4 Mathematically exact instead of approximate values of R 1 and R 2 should be used in computations for ellipsoidal roofs and bottoms. The values for a point at a horizontal distance, x , from the vertical axis of a roof or bottom in which the length of the horizontal semiaxis, a , is two times the length of the vertical semiaxis, b , may be determined by multiplying length a by the appropriate factor selected from Table 5-5. Values for ellipsoidal shapes of other proportions may be computed using the following formulas:

R 1 b 2

a 4 ---- a 4 b 2 ---- 1 a 2 b 2 ---- –     + x 2 1.5 b 2 R 2 ( ) 3

a 4 ----------------- = =

R 2 a 4 b 2 ---- 1 a 2 b 2 ---- –     x 2 + 0.5

=

5.10.2.5 Equation (1) and Equation (2) are general formulas applicable to any tank that has a single vertical axis o f revolution and to any free-body in the tank that is isolated by a hor izontal plane which intersects the walls of the tank in only one circle (see 5.10.2.6). For tanks or segments of tanks of the shapes most commonly used, Equation (1) and Equation (2) reduce to the following simplified equations fo r the respective shapes indi cated in Items a through c. a) For a spherical tank or a spherical segment of a tank, R 1 = R 2 = R s (the spherical radius of the tank or segment), and Equation (1) and Equation (2) become the following:

T 1 R s 2 ---- P W F + A t -------------- +    

= (4)

T

2 R s P T 1

–= (5)

T

2 R 1 2 ----- P W F + A t -------------- –    

= (6)

NOTE See Footnote 16 for information applicable to Equation (4) through Equation (6). Furthermore, if the sphere is for gas pressure only and if ( W + F )/

A t is negligible compared with P g , Equation (4) and Equation (5) reduce to the following:
T

1 = T 2 = 1 / 2 P g R s (7) b) For a conical roof or bottom,
R 1 = infinity
R 2 = R 3 /cos αTable 5-5—Factors for Determining Values of R 1 and R 2 for Ellipsoidal Roofs and Bottoms (See 5.10.2.4) x/a u = R t /a v = R 2 /a 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00 2.000 1.994 1.978 1.950 1.91 1 1.861 1.801 1.731 1.651 1.562 1.465 1.360 1.247 1.129 1.006 0.879 0.750 0.620 0.492 0.367 0.250 2.000 1.99 8 1.993 1.983 1.970 1.95 3 1.931 1.906 1.876 1.842 1.803 1.759 1.709 1.653 1.591 1.521 1.442 1.354 1.253 1.137 1.000 D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-21

where R 3 is the horizontal radius of the base of the cone at the level under consideration;

α is the one-half the included apex angle of the conical roof or bottom. For this condition, Equation (1) and E quation (2) reduce to the following:

T 1 R 3 2 cos α ----------------     P W F + A t -------------- +    

= (8)

T

2 PR 3 cos α ----------- = (9)

NOTE See Footnote 16 for information applicable to Equation (8) and Equation (9). c) For cylindrical sidewalls of a vertical tank, R 1 = infinity; R 2 = R c , the radius of the cylinder; and Equation (1) and Equation (2) become the following:

T 1 R c 2 ----

-     P W F + A t -------------- +    

= (10)

T

2 PR c

= (11)
NOTE See Footnote 16 for information applicable to Equation (10). NOTE The variables in this table are defined as follows:

x = horizontal distance from point in roof or bottom to the axis of the revolution; a = horizontal semiaxis of the elliptical cross section; R 1 = ua ; R 2 = va .5-22 API S TANDARD 620 Furthermore, if the cylinder is for gas pressure only and ( W + F )/

A t

is negligible compared with P g , Equation (10) and Equation (11) reduce to the following:
T

1 = 1 / 2 P g R c (12)
T

2 = P g R c (13) 5.10.2.6 Where a horizontal plane that passes through a tank inte rsects the roof or bottom in more than one circle, thus isolating more than one free-body at that level, the formulas given in 5.10.2.1 and 5.10.2.5 apply only to the central free-body whose walls continue acro ss and are pierced by the axis of re volution. (An example of the kind of plane described would be one passed through the bottom of the tank shown in Figure 5-4, Panel c, just a short distance below the lower ends of the internal ties.) The me ridional and latitudinal unit forces acting along the edges of the annular free-body or bodies lying outside of the central free-body must be computed from formulas developed especially for the particular shape of fr ee-body cross section involved. This standard cannot provide formulas for all shapes of cross sections and conditions of loading that mi ght be used at these locations; however, for a toroidal segment that rests directly on its foundation (s ee 5.11.1) and has a constant meridional radius, R 1 , such as is used in the outer portion of the bottom of the tanks shown in Figure 5-4, Panel c, applicable equations fo r the meridional and latitudinal unit forces in the wall s of the segment are as follows:

T 1 P g R 1 1 R 1 2 R 2 -------- –    

= (14)

T

2 1 2 -- P g R 1 = (15)

The variables are defined in 5. 10.1; however, in this case, R 1 is always positive and R 2 is negative when it is on the tank wall on the side opposite from R 1 . 5.10.3 Required Thickness 5.10.3.1 The thickness of the tank wall at any given level shall be not less than the largest value of t as d etermined for the level by the methods prescribed in 5.10.3.2 through 5.10.3.5. In addition, provision shall be made by means of additional metal, where needed, for the loadings other than internal pressure or possible partial vacuum enumerated in 5.4. If the tank walls have points of marked discontinuity in the direction of the meridional tangent, such as occur at the juncture between a conical or dished roof (or bottom) and a cylindrical si dewall, the portions of the tank near these points shall be designed in accordanc e with the provisions of 5.12. 5.10.3.2 If the units forces T 1 and T 2 are both positive, indicating tension, for the governing combination of gas pressure (or partial vacuum) and liquid head at a given leve l of the tank, the larger of the two shall be used for computing the thickness required at that le vel, as shown in t he following equations:

t T 1 S ts E --------- c + or t T 2 S ts E --------- c += = (16)
In these equations, S ts

and E have the applicable values prescribed in Tables 5-1 and 5-2, respectively. 5.10.3.3 If the unit force T 1 is positive, indi cating tension, and T 2 is negative, indicating compression, for the governing combination of gas pressure (or partial vacuu m) and liquid head at a given le vel of the tank or if T 2 is positive and T 1 is negative, the thickness of t ank wall required for this condi tion shall be determined by assuming different thicknesses until one is found for which the simultaneous values of the computed tension stress, s tc , and the computed compressive stress, s cc , satisfy the requirements of 5.5.3.3 and 5. 5.4.5, respectively. The determination of this thickness will be facilitated by using a graphical solution such as the one illustrated in F-2. 19 Notwithstanding theD ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-23 foregoing provisions, if the unit force acting in co mpression in the case described does not exceed 5 % of the coexistent tensile unit force acting perpen dicular to it, the designer has the option of determining the thickness required for this condition by using the method specified in 5.10.3.2 instead of comp lying strictly with the provisions of this paragraph. The value of the joint efficiency factor, E , will not enter into this dete rmination unless the magnitude of the allowable t ensile stress, s ta , is governed by the product ES ts as provided in 5.5.3.3. 5.10.3.4 If the unit forces T 1 and T 2 are both negative and of equal magnitude for the governing condition of loading at a given level of the tank, the thi ckness of tank wall required for this condition shall be computed using Equation (17):

t T 1 S ca ------ c T 2 S ca ------ c +=+= (17) In this equation, s ca has the appropriate value for the thickness-to-r adius ratio involved, as prescribed in 5.5.4.3 and 5.5.4.6. Lap-welded joints shall be subject to the limit ations of 5.5.4.6 and Tabl e 5-2 (including Note 3). 5.10.3.5 If the unit forces T 1 and T 2 are both negative but of unequal magni tude for the governing condition of loading at a given level, the thickness of tank wall required for this condition shall be the largest of those thickness values, computed by the stepwise procedur e outlined in Items a through f, that show a proper correlation with the respective thickness-to-radius ratios invo lved in their computation (see Steps 2 and 4). a) Step 1. The values of Equation (18) and Equa tion (19) shall be computed as follows:

t T ′ 0.8 T ′′ + ( ) R ′ 1342 ------------------------------------ c + = (18)

t T ′′ R ′′ 1000 -------------- c += NOTE See Footnote 16 for information applicable to Equation (18).
(19) In both equations, the value of T  ́ shall be equal to the larger of the tw o coexistent unit forces; the value of T  ̋ shall be equal to the smaller of the two unit forces. R  ́ and R  ̋ shall be equal to R 1 and R 2 , respectively, if the larger unit force is latitudinal; conversely, R  ́ and R  ̋ shall be equal to R 2 and R 1 , respectively, if the larger unit force is meridional. b) Step 2. The corrosion allowance shall be deducted from each of the two thicknesses computed in S t ep 1, and the thickness-to-radius ratio, ( t – c )/ R , shall be checked for each thic kness based on the value of R used in computing it by either Equation (18) or Equation (19). If both such thickness-to-radius ratios are less than 0.00667, the larger of the two thicknesses computed in Step 1 will be the required thickness for the condition under consideration; otherwise, Step 3 shall be followed. c) Step 3. If one or both thickness-to-radius ratios determined in S tep 2 exceed 0.00667, the values of the following equation s shall be computed:

t T ′ 0.8 T ′′ + 15 000

, ----------------------     c + = (20)

t T ′′ 8340 ----------- c += (21)

NOTE See Footnote 16 for information applicable to Equation (20).

19 See Figure F-3, a copy of a chart used to make graphical solutions.5-24 API S TANDARD 620 d) Step 4. The corrosion allowance shall be deducted from each of the two thicknesses computed in Step 3, and the thickness-to-radius ratio, ( t – c )/ R , shall be checked for each th ickness using a value of R equal to R  ́ as defined in Step 1 in connection with the thickness determined from Equation (20) and a value of R equal to R  ̋ connection with the thickness determined from Equat ion (21). If both such thickness-to-r adius ratios are greater than 0.0175, the larger of the two thicknesses computed in Step 3 will be the required thic kness for the condition under consideration; otherwise, Step 5 shall be followed. e) Step 5. If one or more of the thickness-to-radius ratios determined in S tep 2 or Step 4 fall between 0.00667 and 0.0175 and the thickness involved was co mputed using Equation (18) or Equation (20), a thickness shall be found that satisfies the following equation:
10,150 t c

– ( ) 277 400 t c

– ( ) 2 , + R ′ ---------------------------------------------------------------------------- T ′ 0.8 T ′′ += (22)

NOTE See Footnote 16 for information applicable to Equation (22). If the thickness involved was computed using Equation (19) or Equation (21), a thickness s hall be found that satisfies the following equation:

5650 t c

– ( ) 154 200 , + t c

– ( ) 2 R ′′ ----------------------------------------------------------------------- T ′′ = (23)
f) Step 6. A tentative final selection of thickness shall be made from among t he thickness values computed in the previous steps (if the value has not b een finally est ablished earlier in the procedure). The values of s cc shall be computed for both T 1 and T 2 and checked to see that they satisfy the requirements of 5.5.4.4 and 5.5.4.6. If the tentative thickness does not satisfy t hese requirements, the necessary adjust ments shall be made in the thickness to make the values of s cc satisfy these requirements. 5.10.3.6 The procedure described in 5.10.3.5 is for the condition in which biaxia l compression with unit forces o f unequal magnitude is governing. In many ca ses, however, a tentative thicknes s will have been previously established by other design conditions and will need to be checked only for the external pressure or partial vacuum condition. In such cases, the designer has only to compute the values of s cc for both T 1 and T 2 and then check to see that these satisfy the requirements of 5.5.4.4, as specified in St ep 6. (See F.3 for examples i llustrating the application of 5.10.3.5.) 5.10.4 Least Permissible Thicknesses 5.10.4.1 Tank Wall The minimum thickness of the t ank wall at any level shall be the greatest of the following. a) A measu re of 3 / 16 in. plus the corrosion allowance. b) The calculated thickness in accordance with 5.10.3 plus the co rrosion allowance. c) The nominal thickness as shown in Table 5-6. The nominal thickness refers to the t ank shell as constructed. The thicknesses specified are based on erection requirements. 5.10.4.2 Nozzle Neck See 5.19.2 for the minimum th ickness of the nozzle neck.Table 5-6—Tank Radius Versus Nominal Plate Thickness Tank Radius (ft) Nominal Plate Thickness (in.) ≤ 25 > 25 to 60 > 60 to100

> 100 3 / 16 1 / 4 5 / 16 3 / 8 D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-25

5.10.5 External Pressure Limitations 5.10.5.1 The thicknesses computed using the formulas and procedures specified in 5.10, where P g

is a negative value equal to the partial vacuum for wh ich the tank is to be designed, will ens ure stability against collapse for tank surfaces of double curvature in which the meridional radius, R 1 , is equal to or less than R 2 or does not exceed R 2 by more than a very small amount. Data on the stability of sidewall surfaces of prolate spher oids are lacking; the formulas and procedures are not intended to be used for evaluati ng the stability of such surfaces or of cylindrical surfaces against external pressure. 5.10.5.2 This standard does not contain provisions for the design of cylindrical sidew alls that are subject to p artial inter nal vacuum in tanks constructed for the storage of gases or vapors alone. However, cylindrical sidewalls of vertical tanks designed in accordance with these rules for st oring liquids (with the thickness of upper courses not less than specified in 5.10.4 for the tank size involved and with increasing thickness from top to bottom as required for the combined gas and liquid loadings) may be safely subjected to a partial vacuum in the gas or vapor space not exceeding 1 ounce per square in. with the operating liquid leve l in the tank at any stage from full to empty. The vacuum relief valve or valves shall be set to open at a smaller partial vacuum so that the 1-ounce partial vacuum will not be exceeded when the inflow of air (or gas) through the valves is at the maximum specified rate. 5.10.6 Intermediate Wind Girders for Cylindrical Sidewalls 5.10.6.1 The maximum height of unstiffened si dewall, in f t, shall not exceed:

H 1 6 100 t ( ) 100 t D ----------     3

=

where
H

1 is the vertical distance between the intermediate wind gi rder and the top of the sidewall or in the case of formed heads the vertical distance between the inte rmediate wind girder and the head-bend line plus onethird the depth of the formed head, in ft;

t is the thickness of the top sidewall course, as or dered condition unless otherwise specified, in inches;
D is the nominal t ank diameter, in ft.
NOTE This formula is based on the following factors. a) A 3-sec gust design wind velocity, V , of 120 mph which impo ses a dy namic pressure of 25.6 lbf/ft 2 . The velocity is increased by 10 % for either a height above the ground or a gust fact or. The pressure is thus i ncrea sed to 31 lbf/ft 2 . An additional 5 lbf/ft 2 is added for internal vacuum. This pressure is intended by these rules to be the result of a 120 miles per hour 3-sec wind gust at approximately 33 ft above the ground. H 1 may be modified for other wind velocities , as specified by the Purchaser, by multiplying the formula by (120/ V ) 2 . When a design wind pressure, rather than a wind velocity, is stated by the Purchaser, the preceding increase factors should be added, unless t hey are contained within the design wind pressure.5-26 API S TANDARD 620
b) The formula is based on the wind pressure being uniform over the theoretical buckling mode in the tank sidewall which eliminates the necessity of a shape factor for the wind loading. c) The formula is based on the modified U.S. Model Basin formula fo r the critical uniform external p r essure on thin-wall tubes f ree from end loading, subject to the total pressure in Item a. d) When other factors are specified by the Purchaser which are greater than tho s e in (a) through (c), the total load on the side wall shall be modified accordingly and H 1 shall be decreased by the ratio of 36 lbf/ft 2 to the modified total pressure. e) The background for the criteria given in the note is covered in R

. V. McGrath, “Stability of API Standard 650 Tank Shells,” Proceedings of the American Petroleum Instit ute, Section III—Refining, American Petroleum Institute, New York, 1963, Vol. 43, pp. 458 to 469. 5.10.6.2 To determine the maximum height H 1 of the unstiffened sidewall, a ca lculation shall be made using the thickness of the top sidewall course. Next the height of the transformed si dewall shall be calc ulated as follows. a) Change the width ( W ) of each sidewa ll course into a transposed wid th ( W tr ) of each sidewall course, having the top sidewall thickness, by the following relationship:

W

tr w t uniform t actual --------------     5
=

where

t uniform is the thickness of the top sidewall course, as or dered condition in inches, unless otherwise specified;

t actual is the thickness of the sidewall course for wh ich transposed width is being calculated, as ordered condition in inches, unles s otherwise specified;
W is the actual course width, in ft;
W

tr is the transposed course width, in ft. b) The sum of the transposed width of each course will give the height of the transformed sidewall. 5.10.6.3 If the height of the transposed sidewall is greater than the maximum height, H 1 , an intermediate girder is required. a) For equal stability above and below the intermediate wind girder , the latter should be located at the mid-height o f the transposed sidewall. The location of the girder on the actual sidewall should be at the same course and relative position as on the transposed sidewa ll using the foregoing th ickness relationship. b) Other locations for the girder may be used provi ded the height of the unstiffened sidewall on the transposed sidewall does not exceed H 1 (see 5.10.6.5). 5.10.6.4 If half the height of the transposed sidewall exceeds the maximum height, H 1 , a second intermediate girder shall be used in order to reduce the height of uns tiffened sidewall to a height less than the maximum. 5.10.6.5 Intermediate wind girders shall not be att ached to the sidewall within 6 in. of a horizontal joint of the sidewall. When the preliminary location of a girder is within this distance from a horizontal joint, the girder shall preferably be located 6 in. below the joint, except that the maximum unstiffened sidewall height shall not be exceeded.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-27
5.10.6.6 The required minimum section modulus, in inches cubed, of the intermediate wind girder shall be determin ed by the equation:
Z

= 0.0001 D 2 H

1 NOTE This equation is based on a 3 sec gust wind velocity of 12 0 miles per hour. If speci fied by the Purchaser, other wind velocities may be used by multiplying the equation by ( V /120) 2 . Refer to Item a of notes to 5.10.6.1 for a description of the loads on the tank sidewall which are used for the 120 mile per hour 3 sec gust design wind velocity. 5.10.6.7 Where the use of a transposed sidewall permits the intermediate wind girder to be located at a height less than H 1 calculated by the formula in 5.10.6.1, the spacing to the mid-height of the transposed sidewall, transposed to the height of the ac tual sidewall, may be substituted for H 1 in the calculation for minimum section modulus if the girder is attached at the transposed location. 5.10.6.8 The section modulus of the intermediate wind girder shall be based up on the properties of the att ached members and may include a portion of the sidewall for a distance of 1.47( Dt )0.5 above and below the attachment to the sidewall, where t is the sidewall thickness at the attachment. 5.10.6.9 Intermediate stiffeners extending a maximum of 6 in. from the out side of the si dewall are permitted withou t need for an opening in the stif fener when the nominal stairway wi dth is at least 24 in. For greater outward extensions of a stiffener, the stairway shall be in creased in width to provide a minimum clearance of 18 in. between the outside of the stiffener and the handrail of the stairway, subject to the approval of the Purchaser. If an opening is necessary, the built up section shall have a section modulus gr eater than or equal to that required for the stiffener. 

### 5.11 Special Considerations Applicable to Bottoms That Rest Directly on Foundations 5.11.1 Shaped Bottom Where the bottom of a tank is a spherical segment or a spherical segment combin ed with one or more toroidal segments, or is conical in shape, and the entire bottom area rests directly on the tank foundation in such a way that the foundation will absorb the weight of the tank contents without significant movement, the liquid head may be neglected in computing the internal pressure, P , acting on the bottom and in computing the unit forces, T 1 and T 2 , in the bottom. Under these condi tions, the unit forces in the bottom of the tank may be computed considering that P in each case is equal to P g . 5.11.2 Flat-bottom Tanks with Counterbalance 5.11.2.1 General In tanks that have cylindrical sidewal ls and flat bottoms, the upl ift that r esult s from the pressure acting on the underside of the roof combined with the ef fect of design wind pressure, or seismic loads if specified, must not exceed the weight of the sidewalls plus the weight of that portion of the roof that is carried by the sidewalls when no uplifts exists unless the excess is counteracted by a counterbal ancing structure such as a concrete ringwall, a slab foundation, or another structur al system. The means for accomplishing this shall be a matter of agreement between the Manufacturer and the Purchaser. Simila r precautions must be taken with flat -bottomed tanks of other shapes. All weights used in such computations shall be based on net thicknesses of the material s, exclusive of corrosion allowance. 5.11.2.2 Counterbalancing Structure The counterbalancing structure, which may be a foundation or support system, shall be designed to resist uplift calculated as described in 5.11.2 based on 1.25 times the inte rnal design pressure plus the wind load on the shell and5-28 API S TANDARD 620 roof based on its projection on a vertical plane. If seismic loads are specified, uplift shall be calculated using internal design pressure plus the seismic loads . Wind and seismic loads need not be combined. 5.11.2.3 Anchorage The design of the anchorage and the attachments to the tank shall be a matter of agreement between the Manufacturer and the Purchaser and shal l satisfy the foll owing conditions. a) The design stresses shall satisfy al l of the conditions in Table 5-7. b) When corrosion is specified for the anchors, thicknes s shall be added to the anchors and the attachments. If bolts are used for anchors, the nominal diameter shall be not less than 1 in. plus a corrosi on allowance of at least 1 / 4 in. on the diameter. c) Attachments of anchors to the shell shall be designed using good engineering practice. d) Anchor materials and allo wable stresses shall be t hose permitted by Table 5-1.
NOTE The allowable stresses for stainless steel and alumin um anchors for the applicable loading conditions are found in Q.3.3.6, Q.8.1, and Table Q-3. 5.11.3 Flat-bottom Tanks without Counterbalancing Weight The detailed design of flat-bottom tank s without counterbalancing weight shall be a matter of agreement between the Manufacturer and the Purchaser and shal l satisfy the foll owing conditions. a) The bottom of a flat-bottom tank shall be designed to re main flat during all desi gn and test conditions. When the flat-bottom tank is designed without anchoring the shell to the counterbalancing wei ght, the bottom will be designed to carry all the weight and pressure forces dist ributed on the bottom and to transfer the uplift forces from the sidewall through the bottom plates. The uplift forces will be obtained from a free-body analysis as specified in 5.9 and 5.10. These forces shall be determined for the tank (deducting any specified corrosion allowance) for both a full and an empty condition and shall incl ude uplift from design wind velocity. T he largest values will be used for design. b) The bottom plates in the flat-bottom tank shall be desi gned as a strength member to span between main structural members (for example, grillage beams or other structur al members) and transfer the distributed pressure and liquid-weight forces to these main structural members. c) When the bottom plate is a bending strength member, single- fillet lap joints are not permitted in the bottom plate. d) Adequate provision shall be made at the sidewall to transfe r the uplift forces from the shell to the shear-carrying elements in the bottom structure. e) Consideration shall be given to protecting all bottom stru ctural elements from environmental corrosion. f) Anchorage shall be provided for resistance to wind and seismic forces and shall be designed in accordance with 5.11.2.2. 5.11.4 Additional Considerations Unless otherwise required, tanks that may be subject to sliding due to wind shall use a maximum allowable sliding friction of 0.40 times the force against the tank bottom.Table 5-7—Allowable Tension Stresses for Up lift Pressure Conditions (see 5.11.2.2) Source of Uplift Pressure Allowable Tension Stress a (lbf/in. 2 ) Tank design pressure
Tank design pressure plus w ind or earthqu ake Tank test pressure Allowable design stress, S ts (see Table 5-1) Smaller of 1.33 S ts or 80 % of the specified minimum yi eld strength Smaller of 1.33 S ts or 80 % of the specified minimum yi eld strength D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-29



### 5.12 Design of Roof and Bottom Knuckle Regions and Compression-ring Girders 5.12.1 Design Limitations The design rules in this section do not cover the junction between a conical reducer and cy lindrical sidewalls except as indicated o n Figure 5-9, Panel b. However, the provisions of this section shall be observed at such a juncture if the angle formed is nonreentrant. (See 5.18.3 fo r design of reentrant junctures.) 5.12.2 General When the roof or bottom of a pressure tank is a cone or p artial sphere (or nearly so) and is attached to cylindrical sidewalls, the membrane stresses in the roo f or bottom pull inward on the periph ery of the sidewalls. This pull results in circumferential compressive forces at the juncture, whic h may be resisted either by a knuckle curvature in the roof or bottom or by a limited zone at the juncture of the intersecting roof or bottom plates and sidewall plates, supplemented in some cases by an angle, a rectangular bar, or a horizontally disposed ring girder. All longitudinal and meridional joints in a knuckle region, or between those portions of plates that are considered to participate 20 in resisting compressive forces in a compression-ring region, and all radial joints in a compression- ring angle, bar, or girder shall be butt-welded. 5.12.3 Knuckle Regions 5.12.3.1 If a curved knuckle is provided, a ring girder or other for m of compression ring shall not be u s ed in connection with it, and there shal l be no sudden changes in the direction of a meridional line at any point. In addition, the radius of curvature of the knuckle in a meridional plane shal l be not less than 6 %, and preferably not less than 12 %, of the diameter of the sidewalls. Subject to the pr ovisions of 5.12.3.2, the thickness of the knuckle at all point s shall satisfy the requirement s of 5.10. Use of a knuckl e radius as small as 6 % of the sidewall diameter will frequently require an excessively he avy thickness for the knuckle regi on. The thickness requirement for such a region will be found more reasonable if a lar ger knuckle radius is used. 5.12.3.2 The designer should recognize that applying the eq uations in 5.10.2 to levels immediately above and below a point where two surfaces of diff ering meridio nal curvature have a common meridional tangent (for example, at the juncture between the knuckle regi on and the spherically dished portion of a tori spherical roof) will result in the a The allowable tension stress determined at the minimum net section or tensile stress area of the anchor.

20 If, for manufacturing reasons, it is uneconom ical or impractical to use butt-welded longitudinal or meridional joints for a dis tance on either side of the juncture as com puted using Equation (24) and Equation (25) and the thickness of the plates involved does not exceed the applicable limits for lap joints as set forth in Ta ble 5-2, the joints may be lap-welded provided that the plate s jointed in this way are not given credit for contributing to t he net cross-sectional area provi ded for resisting compressive fo rces in the compression-ring region. In such a case, however, computation of (a) force Q from Equation (26), (b) the width of the horizontal projection (see 5.12.5) and (c) the centroid of the composit e corner compression region (see 5.12.5) shall be made as though these plates did actually partic ipate in resisting the compressive force.5-30 API S TANDARD 620
calculation of two latitudinal unit forces, differing in magnitude and perhaps in sign, at the same point. The exact latitudinal unit force at this point will be intermedia te between the two calculated values, depending on the geometry of the tank wall in that area; the designer may adj ust the immediately adjacent thicknesses accordingly. 5.12.4 Compression Rings 5.12.4.1 The variables used in Equation (24) thr oug h Equation (27) are defined as follows:

w h is the width, in inches, of the roof or bottom plate considered to particip ate in resisting the circumferential force acting on the compression-ring region;

w c is the corresponding width, in inches , of the participat ing sidewall plate;

t h is the thickness, in inches, of the roof or bottom plate at and near the juncture of the roof or bottom and sidewalls, including co rrosion allowance;

t c is the corresponding thickness, in inc hes, of the cylindrical sidewalls at and near the juncture of the roof bottom and sidewalls;
R 2 is the length, in inches, of the normal to the roof or bottom at the juncture between the roof or bottom and the sidewalls, measured from the roof or bottom to the tank’s vertical axis of revolution;
R c is the horizontal radius, in inches, of the cylindrical sidewall at its juncture with the roof or bottom of the tank;
T

1 is the meridional unit force (see 5.10) in the roof or bottom of the tank at its juncture with the sidewall, in lbf/in. of circumferential arc;
T

2 is the corresponding latitudinal unit force (see 5.10) in the roof or bottom, in lbf/in. of meridian arc;
T

2 s is the circumferential unit force (see 5.10) in the cylindri cal sidewall of the tank at its juncture with the roof or bottom, in lbf/in. measured along an element of the cylinder;

α is the angle between the directio n of T 1 and a vertical line (in a conical su rface it is also one-half the total vertex angle of the cone); Q is the tota l circum ferential force, in lb, acting on a vertical cross section through the compression-ring region;
A c is the net area, in inches 2 , of the vertical cross se ction of metal required in the compression-ring region, exclusive of all corrosion allowances;
S

ts is the maximum allowable stress value for simple tension, in lbf/in. 2 , as given in Table 5-1;
E is the efficiency , expressed as a decimal, of meridional joints in t he compression-ring region in the event that Q should have a positive value, indicating tension (see Table 5-2).
5.12.4.2 If a curved knuckle is not provided, the circumferent ial compressive forces mentioned in 5.12.2 must be resisted by other means in the compre ssion-ring region of the tank walls. This region shall be understood to be the zone of the tank walls at the junctu re between the roof or bottom and the si dewalls, including the width of plate on each side of the juncture that is consi dered to participate in resisting these fo rces (see Figure 5-5). In no event shall the thickness of the wall plate on eit her side of the juncture be less t han the thickness needed to satisfy theFigure 5-5—Compression-ring Region w

c = 0.6 R c ( t c - c)

t c R c R 2 t h

w

h = 0.6 R

2 (

t

h - c)

Cylindrical sidewall of tank Roof of tankCompression-ring region D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-31

requirements of 5.10. The widths of plate making up the compression-ring region shall be computed using the following equations:

w h 0.6 R 2 t h c

– ( ) = (24)

w c 0.6 R c t c c

– ( ) = (25)
5.12.4.3 The magnitude of the total circumferential force acting on any ver tical cross section through the compression-ring region shall be computed as follows: Q T 2 w h T 2 s + w c T 1 – R c sin α = (26) The net cross-sectional area provided in the compression- ring region shall be not less than that found to be required by one o f the following equations:
A c = Q /15,000 or Q / S

ts

E (27) The selection of Equation (27) depends on whether the value of Q as determined by Equation (26) is negative or positive. 21 For the test overpressure condition, 20,000 may be substituted for 15,000 in Equation (27).

21 Because of the discontinuities and other conditions found in a compression-ring-region, biaxial-stress design criteria are not considered applicable for a compressive force determined as in Equation 26. Experience has show n that a compressive stress of the order of 15,000 lbf/in. 2 , as indicated in Equation 27, is permissible in this case, provided the requirements of 5.12.5 are satisfied.5-32 API S TANDARD 620
5.12.5 Details of Compression-ring Regions 5.12.5.1 If the force Q is negative, indicating compression, then the horizont al projecti on of the effective compression-ring region shall have a width in a radial dire ction not less than 0.015 times the horizontal radius of the tank wall at the level of the juncture between the roof or bottom and the sidewalls; if the projected width does not meet this requirement, appropriate corrective measures shall be applied as specified in this section. 5.12.5.2 Whenever the magnitude of t he circumferential force Q deter mined in accord ance with 5.12.4 is su ch that the area required by Equation (27) is not provided in a compression-ring region with plates of the minimum thicknesses established by the requirements of 5.10 or when Q is compressive and the horizontal projection of the width, w h , is less than specified in 5.12.5.1, the compression-ring region shall be reinforced by (a) thickening the roof or bottom and sidewall plates as required to provide a co mpression-ring region having the necessary cross-sectional area and width as determined on t he basis of the thicker plates, 22 (b) adding an angle, a rectangular bar, or a horizontally disposed ring girder at the j uncture of the roof or bottom an d sidewa l ls plates, or (c) using a combination of these alternatives. This additional area shall be arranged so that the centroid of the cr oss-sectional area of the composite corner compression region lies ideally in the hor izontal plane of the corner formed by the two members. In no case shall the centroid be off the plane by more t han 1.5 times the average thickness of the two members intersecting at the corner. 5.12.5.3 Such an angle, bar, or ring girder, if used, may be loca te d either insid e or outside the tank (see Figure 5-6) and shall have a cross section with dimensions that sati sfy the following conditions. a) The cross-sectional area within the limit of w c or w h makes up the deficiency between the area A c required by Equation (27) and the cross-sectional area provided by t he compression-ring region in the walls of the tank. b) The horizontal width of the angle, bar, or ring girder is not less than 0.01 5 times the horizontal radius, R c , of the tank wall at the level of the juncture of the roof or bottom and the sidewalls except that when the cross-sectional area to be added in an angle or bar is not more than one-half the total area required by Equation (27), the foregoing width requirement for this member may be disregarded if the horizontal pr ojection of the width, w h , of the participating roof or bottom plates alone is equal to or greater than 0.015 R c or, with an angle or bar located on the outside of a tank, the sum of the projection of the width, w h , and the horizontal width of the added angle or bar is equal to or greater than 0.015 R c . c) When bracing must be provided as spec ified in 5.12.5.8, the moment of inertia of the cross section around a horizont al axis shall be not less than that required by Equation (28). 5.12.5.4 When the vertical leg of an angle ring or a vertical fl ange of a ri ng girder is located on the sidewall of the tank, it may be built into the sidewall if its thickness is not less than that of the adj ourning wall plates. If this construction is not used, the leg, edge, or flange of the comp ression ring next to the tank shall make good contact with the wall of the tank around the ent ire circumference and shall be attac hed thereto along both the top and bottom edges by continuous fillet welds except as provided in 5.12.5.5. These welds sha ll be sufficiently sized to transmit to the compression-ring angle, bar, or girder that portion of the total circumferential force, Q , which must be carried thereby, assuming in the case of welds separated by the widt h of a leg or flange of a structural member as shown in Figure 5-6, details a and h, that only the weld nearest the r oof or bottom is effective. In no event, however, shall the size of any weld along either edge of a compression ring be less than the thickness of the thinner of the two parts joined or 1 / 4 in. (whichever is smaller), nor shall the size of the corner welds between the shell and a girder bar, such as shown in Figure 5-6, details d and e, be less than the applicable weld sizes in Table 5-8. The part thicknesses and weld sizes in Table 5-8 relate to dimensions in the as-welded condition before the deduction of corrosion allowances; with this exception, all other part thicknesses and weld sizes referred to in this paragraph relate to dimensions after the deduction of corrosion allowance.

22 Note that unless the effect of the unit forces T 2 and T 2 s on the resulting increments in width of participating plate may be safely neglected, the use of thicker plates involves recomputing not only T h and w c , but also Q and A for the increased plate thickness; hence, the design of the compression-ring-region in this case becomes a trial-and-er ror procedure.Figure 5-6—Permissible and Non-permissible Details of Construction for a Compression-ring Juncture t W h 16 t (max)

16 t (max) 16 t (max) 16 t (max) 16 t (max) W

c

t

t t tt t t t W

h L W h L W h W h L

W

c W c W c W c W

c W

c W c W c W

h W h W h

W

c W c W c 16 t (max) 16 t (max)

t t

C L

W

h W h W h W h A A A A B B B

B

 A  A

Neutral axis of angle Neutral axis of angle Neutral axis of angle Neutral axis of angle Neutral axis

of angle Neutral axis of angle Alternate roof connection Permissible

Permissible

Permissible where roof (or bottom) plate thickness is  1 / 4 in.

Not permissible Compression areas are shaded.

Bracket Detail a Detail b Detail c Detail d

Detail j Detail k Detail l Detail h-1 Detail h Detail i Detail i-1 Detail f Detail e Detail f-1 Detail g See 6.16

See 6.16 See 6.16 See 6.16 See Note 1

For details e through g, W h  L  16 t and W h 32 t

NOTE 1 When using the alternate roof position (the roof plate under the compression bar as shown in detail f-1), the Purchaser should consider the use of caulking on top of the fillet weld to ensure the drainage of rainfall. NOTE 2 See Table 5-2 for limitations concerning locations where various types of welded joints may be used and 5.12.2 for limits on material contributing to compression cross-sectional area. D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-335-34 API S TANDARD 620
5.12.5.5 If a continuous weld is not needed for strength or as a seal against corro sive element s, attachment welds along the lower edge of a compression ring on the outside of a tank may be intermittent if (a) the summation of their lengths is not less than one-half the circumference of the tank, (b) the unattached width of tank wall between the ends of welds does not exceed eight times the tank wall thicknes s exclusive of corrosion allowance, and (c) the welds are sized as needed for strength (if this is a factor), but in no case are they smaller than specified in Table 5-8. Table 5-8—Minimum Size of Fillet Weld Dimensions in inches. Thickness of the Thicker of the Two Parts Joined Minimum Size of Fillet Weld
≤ 1 / 4 > 1 / 4 to 3 / 4 > 3 / 4 to 1 1 / 4 > 1 1 / 4 3 / 16 1 /

4 5 / 16 3 / 8 5.12.5.6 The projecting part of a compression ring shall be placed as close as pos sible to the juncture between the roof or bottom plates and the sidewall plates. 5.12.5.7 If a compression ring on either the inside or outside of a t ank is shaped in such a way that liquid may be trapped, it shall be provided with adequate drain holes uni formly distributed along its l ength. Similarly, if a compression ring on the inside of a tank is shaped in such a way that gas would be trapped on the underside when the tank is being filled with liquid, adequate vent holes shall be provided along it s length. Where feasible, such drain or vent holes shall be not less than 3 / 4 inches in diameter. 5.12.5.8 The projecting part of a compression ring without an outer ver t ical fl ange need not be braced if the width of the projecting part in a radical vertical plane does not exceed 16 times its thickness. With this exception, the horizontal or near-horizontal part of the compression ring shall be braced at intervals around the circumference of the tank with brackets or other suitable members securely attached to both the ring and the tank wall to prevent that part of the ring from buckling laterally (vertically) out of its own plane. W hen bracing is required, the mom ent of inertia of the cross section of the angle, bar, or ring girder about a horizontal axis shall be not less than t hat computed by the following equation:

I

l 1.44 Q p R c 2 29 000 000 k , , -------------------------------- 0.00000005 ( ) Q p R c 2 k ------------ = = (28)

where
I 1 is the required moment of inertia, in inches to the fourth power, for the cross section of a steel 23 compression ring with respect to a horizontal axis through the centroid of the section (not t a king credit for any portion of the tank wall) except that in the case of an angle ring whose vertical leg is attached to or forms a part of the tank wall, the moment of inertia of the horizontal leg only shall be considered and shall be figured with respect to a horizontal axis through the centroid of the leg; Q

p is the that portion of the total circumferential force Q [see Equation (26)] that is carried by the compressionring angle, bar, or girder as computed from the ratio of the cross-sectional area of the compression ring to the total area of the compression zone;
R c is the horizontal radius, in inches, of the cylindrical sidewall of the tank at its juncture with the roof or bottom;

23 The value of I 1 a computed using Equation (28) is not applicable for materials other than steel.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-35

k is the constant whose value depends on the magnitude of the angle q subtended at the central axis of the tank by the space between adjacent brackets or other supports, the value of which shall be determined from Table 5-9 in which n is the number of brackets or other suppor ts evenly spaced around the circumference of the tank. In no case shall q be larger than 90 degrees. Table 5-9—Factors for Determining Values of k for Compression-ring Bracin g (See 5.12.5.8)

n θ (degrees) k 30 24 20 18 15 12 10 9 8 6 5 4 12 15 18 20 24 30 36 40 45 60 72 90 186.6 11 9.1 82.4 66.6 46.0 29.1 20.0 16.0 12.5 6.7 4.4 2.6 

### 5.13 Design of Internal and External Structural Members 5.13.1 General The provisions of 5.13.2 through 5.13.5 ar e limited to a discussion of the basic requirem en ts and principles involved. For reasons that will appear obvious, spec ific design formulas cannot be included. 5.13.2 Basic Requirements 5.13.2.1 Wherever the shape selected for a tank is such that the t ank, or some portio n thereof, would tend to assume an appreciably different shape under certain conditions of loading or whenever the shape is such that it is not feasible or economical to design the wa lls themselves to carry t he entire loads imposed by all possible combinations of gas and liquid loadings that may be enc ountered in service, suitable internal ties, columns, trusses, or other structural members shall be provided in the tank to preserve its shape and to carry the forces that are not carried directly by the walls of the tank. Ot her structural members may be needed on the out side of a tank to support or partly support the weight of the tank and its c ontents, and these shall be provided as requi red. All such internal and external members shall be designed in accordance with good structural engineering practices, using st resses as specified in 5.6. They shall be arranged and distributed in or on the tank and connected to the walls of the tank (in cases where such connections are needed) in such a way that reactions will not cause excessive localized or secondary stresses in the walls of the tank. When these members are rigidly attach ed to the wall of a tank by welding, the stresses in the member at the point of attachment shall be limited to the st ress value permitted in the wall of the tank (see Annex D). 5.13.2.2 In no event shall the nominal thickness, including the corr osion allowance, if any , of any part of any internal framing be less th an 0.17 in. 5.13.2.3 If any structural members (s uch as girders at node circles), t ank acce ssories, or other internals are placed to form gas pockets inside a tank, adequate and suitably located vent holes shall be provi ded so that these spaces will vent freely when the liquid level is raised beyond them. Similarly, if any such members, accessories, or other internals are shaped to hold liquid above them when the tank is being emptied, they shall be provided with adequate5-36 API S TANDARD 620 and suitably located drain holes. These vent and drain holes shall be not smaller than 3 / 4 inches in diameter and shall be distributed along the member. 5.13.3 Simple Systems In some cases the forces acting on structural members are st atically determina te; in other cases, the y are statically indeterminate. The external columns t hat are often used for supporting a s pherical tank are an example of the statically determinate class of members. If the columns are vertical, the forc e acting on each column is simply the combined weight of the tank and its c ontents divided by the number of columns. If the columns are inclined, this quotient must be divided by the cosine of the angle each column makes with the ve rtical to obtain the force acting in each column. To cite another case, where internal framing is needed inside a tank only to support the weight of the roof and such lo ads (i ncludin g external pressure load, if any) as may be superimposed upon it, the procedure for designing such framing is more or less stra ightforward, involving only a few assumptions. In other cases, however, whenever the internal framing serves to supplement the load-carrying capacity of the walls of the tank, the design procedure is more complex. 5.13.4 Complex Systems 5.13.4.1 The design rules in this standard do not cover specific requirement s for designing the inte rnal framing in all the various shapes of tanks that might be constructed, but an outline of t he procedure used in the design of internal framing for one special shape of tank, as shown in Figure 5-4, Panel c, should serve to il lustrate the general method of attack. In such a system of internal framing, the magnitude of the forces in the tension members, which tie the ring girders under the roof node circles to the respective gir ders above the bottom node circles, are determined by static, assuming for the purpose of a preliminary analysis that these tensi on members are replaced by a cylindrical shell if the members are vertical or by a coni cal frustum if the members are inclined. 5.13.4.2 Under these assumed conditions, the vertical components of the T 1 (meridional) unit forces in the roof plates at their juncture with the cylinder or frustum are transmitted directly to t he cylinder or frustum so that an upper ring girder is unnecessary in this hypothetical case if (a) the horizontal components of the T 1 unit forces in the roof or wall plates on opposite sides of the junctu re balance each other in the case of the cylindrical tie or (b) the difference between them is balanced by the horizontal components of the uni t forces in the top of the frustum in the case of the conical tie. 5.13.4.3 Similarly, at the lower end of the cylinder or frustu m, the sum mation of the vertical component s of the forces must be in balance with the vertical components of the forces in the cylinder or frustum, and the summation of the horizontal components of the forces acting at the juncture must be zero. Furthermore, the total vertical force acting along the edges of t he top of the cylinder or frustum must equal t he total vertical force acting along the edges of the bottom of the cylinder or fr ustum. In other words, the general layout of the tank must be such that the upward gas pressure over a predetermined portion of the roof is bal anced by the downward gas pressure over a predetermined portion of the bottom without undue el astic stressing or straining. 5.13.4.4 If the horizontal forces at the node circles are not ot herwise in equilibrium, ring gird ers must be pro v ided at these circles. The girders must be designed to carry the unbalanced components—either in tension or compression, as the case may be. 5.13.4.5 Having satisfied the conditions of stat ic equ ilibrium using a hypothetical cylinder or frustum for a tie, the designer must consider and provide for the real conditions where the cylinder or frustum is approximated by a number of uniformly spaced structural members, each of which, in addi tion to its primary function as a tie, serves also as a column to support its assigned portion of the roof and external loads. The torsional and vertical moments in the ring girders at the node circles must be provided for, keeping in mind that relatively small va riations from the nominal T 1 (meridional) roof forces will greatly reduce, if not completely offse t, the torsional moments in the girders.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-37
5.13.5 Internal Meridional Stiffeners 5.13.5.1 When curved meridional trusses or ribs are fasten ed to the sidewalls of a tank to prevent the T 1 (meridional) compressive forces from buckling the sidewalls, t he distribution of meridi onal forces between the sidewalls and trusses or ribs is to a degree indeterminate if the foundation supp ort for the overhanging portions of the sidewalls is so uniformly distributed ar ound the tank that there is no greater foundation-bearing intensity against the tank wall beneath the trusses or ribs. In this case, the total meridional forces that the sidewalls and trusses or ribs must carry, acting together, at any given level in the t ank may be computed from Equatio n (1) in 5.10.2.1, assuming for the purposes of these computations only that the cross-se ctional area of the trusses or ribs is distributed uniformly along the circumference of the sidewalls as an added si dewall thickness. In other words, the value of F in Equation (1) may be taken as not including the forces in these trusses or ribs, and the hypothetical value of the meridional unit force computed using Equation (1) may be regarded as the summation of all meridi onal forces acting on the composite section of sidewalls and trusse s or ribs at the level under considerat ion divided by the ci rcumference of the tank at that level. 5.13.5.2 The net cross-sectional area of me t al (exclusive of corr osion allowance) required per inch of tank circumference to resist these forces may then be determined by dividing the hypothetical value of the meridional unit forces acting on the composite section by allowabl e compressive stress. This area must then be apportioned between the sidewalls and the trusses of ribs, by trial-and-error computations, in such a way that (a) sufficient material is placed in the trusses or ri bs to enable them to serve their inten ded function of prevent ing buckling of the sidewalls in a vertical directi on (the trusses or ribs must also be proportioned and distributed around the circumference of the tank so t hat they will serve this function) and (b) suff icient thickness is pr ovided in the sidewalls to enable them to withstand not only their share of the meridional unit forces but also the entire latitudinal unit force T 2 as computed by the following equation:
T

2 = R 2 ( P

– T 1 /

R 1 ) In this equation, T 1 is the meridional unit force assumed to be act ually carried by the sidewalls and is obtained by multiplying the hypothetical value of t he meridional unit forces acting on the composite section by the ratio of the sidewall cross-sectional area to the com posite cross-sectional area at the level in question. Other variables used in the foregoing equation are defined in 5.10.1, and the thickness provided to resist this force T 2 must satisfy all of the requirements of 5.10.3 that involve this force. 5.13.5.3 No such uniform distribution of forces on the composit e section of side walls and tr usses or ribs actually occurs. However, the assumption of uniform distribution of 5.13.5.1 and 5.13.5.2 will give safe designs if the principles outlined are observed and the eccentricity of loading on the trusses or ribs is taken into account. (New designs shall be proved by strain-gauge surveys.) 5.13.5.4 In the case of a tank whose foundations and supports are designed and arranged so that the weight of the overhanging portions of the tank and its c ontents is transferred entirely to the trusses or ribs and from there to the foundations, the total vertical load on each truss or rib is determinate. The stress system in the tank wall is analogous to that in a large horizontal pipeline supported entirely on ring girders. In the latter case, design stresses comparable to those permitted in 5.13.5.2 may be used insofar as si dewall thicknesses are governed by forces acting in a meridional direction. 

### 5.14 Shapes, Locations, and Maximum Sizes of Wall Openings 5.14.1 The term opening as used in this sectio n, 5.16, 5.17, and 5.18 refers to the hole cut in a tank wall to accommodate a nozzle, manway, or other connection (rather than just the bore of the connection) except when the wall of a connection extends through the t ank wall and is attached to it with su fficient weld within the tank wall thickness to develop the strength in tension of that section of the wall of the c onnection which lies with in the tank wall thickness (that is, the strengt h of an area equal to twice the product of t he nozzle wall thickness and the tank wall thickness) in addition to whatever weldi ng is required at this location for reinforcement attachment. In the latter case,5-38 API S TANDARD 620
when the wall of a connection is attached to the tank wall in this way, opening refers to the figure formed by the imaginary line of intersection between th e inside surface of the connection and t he surface of the tank wall extended. 5.14.2 In all cases, requirements concerning openings shall be understood to refer to dim ensions tha t apply to the corroded condition. Unless otherwise spec ified, dimensions of openings genera lly refer to measurements taken along the chord of the tank wall curvature if the wall is curved in the direction involved; how ever, when there is more than approximately a 2 % difference between the length of chord and the length of the ar c that is subt ends in the t ank wall, the measur ement shall be taken along t he arc of the tank wall curvature. 5.14.3 The rules in this section shall also apply to openings in cy lindr ical shells that are adj acent to a relatively fla t bottom; as an alternative, the insert plate or reinforcing plate may extend to and intersect the bottom-to-shell joint at approximately 90°. Stress-relieving requirements do not apply to the weld to the bottom or annular plate. 5.14.4 All manholes, nozzle connections, or other connections in the sidewalls , r oofs, or bottoms of tanks constructed under these rules s hall be circular, elliptical, 24 or obround 25 in shape. Where elliptical or obround connections are employed, the long dimensions shall not exceed twice the shor t dimens ion, as measured alon g the outer surface of the tank; if the connection is in an area of unequal meridional and latitudinal stresses in the tank wall, the long dimension shall prefer ably coincide with the direct ion of the greater stress. 5.14.5 Each opening in the walls of a tank shall be locate d so that the distance bet ween the outer edge of its reinforcement 26 and any line of significant disconti nuity in the curvature of the t ank walls (such as the juncture between two nodes in a noded surface, the juncture between a dished or coni cal roof or bottom and cylindrical sidewalls, or th e juncture between a roof or bottom and cylindrical sidewalls, or the juncture between a roof or bottom knuckle and other portions of the tank) is not less than 6 in. or (if this be larger) eight times the nominal thickness (including corrosion allowance; if any) of the wall plate containing the opening, exc ept as permitted by 5.14.3. No part of the attachment for any openings shall be located closer than the larger of these dist ances to any part of the attachment for any lugs, columns, skirts , or other members attached to the tank for supporting the tank itself or for supporting important loads that are carried by th e tank. When any two adjacent openings are reinforced independently of each other, they shall be spaced so that the distance between the edges of their respective reinforcements will not at any point be less than the la rger of the foregoing specified distances (see 5.17). 5.14.6 Each opening shall be located so that any attachments and reinforcements will be, or may readily be made, fully accessible for inspection and repair on both the outside and inside of the tank except in the case of connections that for compelling reasons must be located on the underside of a tank bottom resting directly on the tank foundation. 5.14.7 Properly reinforced openings may be of any size 27 that can be located on the tank to comply with the requirements of 5.14.5 and 5.14.6 except that in no event shall the inside diameter (af t er allowing for corrosion) of any opening 28 other than those considered in 5.18 exceed 1.5 times t he least radius of curvature in that portion of the tank wall in which the opening is located. 5.14.8 Large openings shall be given special considerati on (see 5.16.7 and 5.18) . In the case of large opening s which have attachments that require s hop stress relief (see 5.25.1), shipping clearances, affecting the maximum size of assembly that can be shi pped, may control the size of the opening that can be used.

24 An opening made for a pipe or nozzle of circular cross section whose axis is not perpendicular to the tank wall shall be treate d as an elliptical opening for design purposes. 25 An obround figure is one that is formed by two parallel sides and semi-circular ends. 26 The term edge or reinforcement means the edge, or toe, or the out ermost weld that attaches the reinforcing pad to the wall of the tank. In the case of an opening that is not provided with a reinforcing pad, it means the neck of the nozzle or other connection extending from the opening to the tank wall. 27 Although no minimum size is prescribed, it is recommended that no nozzle smaller than 3 / 4 in. pipe be used on a tank constructed according to these rules. 28 In the case of elliptical or obround openings, the dimension of the opening in any given direction shall meet this requirement with respect to the radius of curvatur e of the tank wall in that direction.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-39


### 5.15 Inspection Openings Each tank shall be provided with at least two manhole openings to afford acce ss to its interior for inspection and repair. Manholes shall in no event be smaller than 20 in. along any inside dimension. All manholes shall be made readily accessible by platforms and ladders, stairways, or other suitable facilities. 

### 5.16 Reinforcement of Single Openings
5.16.1 General The requirements of this paragraph are illustrated in Figur e 5-7 and Figure 5-8. See 5.21.1.2, 5.2 1.1.3, 5.21.2.7, and 5.21.2.8 for provisions concerning reinforcem ent of openings in cover plates for nozzles.
5.16.2 Basic Requirements All openings in the walls of tanks constructed accord ing to these rules and all openings for branch connections 29 from nozzle necks welded to the tank wall shall be fully re inforced with the exception of the exclusions covered in 5.16.2.1 and 5.16.2.2.
5.16.2.1 Single openings in tanks do not require reinforcement other than that which is inher ent in their construction for the following conditions: a) 3 in., or less, pipe size we lded connections in tank walls 3 / 8 in. or less; b) 2 in., or less, pipe size wel ded connections in tank walls over 3 / 8 in.; c) threaded connections in which the hole in the t ank wall is not greater than 2 in. pipe size.
5.16.2.2 T he reinforcement required for openings in tank wa lls for external pressure co nditions need be only 50 % of that required in 5.16.5 where t has been determined for exter nal pressure conditions.
5.16.2.3 The requirements for full reinforcement shall not be c onstrued as requiring that a special reinforcing pad be provided where the necessary reinforcing metal is ava ilable in the nozzle neck or elsewhere around the opening as permitted by these rules. The amount of reinforcement required, the limiting dimensions within which metal may be considered to be effective as reinforcement, and the strength of the welding required for attaching the reinforcement are defined in 5.16.3. Reinforcement shall be provided in the specified amount and shall be distributed and attached to the wall of the tank in such a way that the requirement s are satisfied for all paths of potential failures through the opening extended in either a meridi onal or latitudinal direction.
5.16.2.4 The maximum amount of reinforcement will be needed in a plane that is perpendicular to the direction of princip al wa ll stress passed through the opening at the point where the centerline of the connection intersects the wall of the tank; for obround openings, that same amount must be pr ovided along the entire length of the parallel sides of the opening between the planes passing through the respective ce nters of the semicircular ends. However, these planes may not be the controlling sections with respect to possible fa ilure through the opening, inasmuch as failure might occur along another path (in the case of a cylindrical wall, paralle l to, but somewhat removed from, the aforesaid planes) by a combination of a tensile failure of the tank wall and shearing or tensile failure of the attachment welds.

29 The design rule in this section make no mention of openings for branch connections from nozzle ne cks, but the provisions shall be understood to apply to openings of th is type. For this purpose, the term tank wall shall refer to the neck of the main nozzle to which the branch connection is attached, and the term nozzle wall shall refer to the wall of the branch connection.Figure 5–7—Reinforcement of Single Openings t n

t r t2.5(tw- c)2.5(tw- c) ( d + 2 c )
( d + 2 c ) t n - c t rn e 3 H d + 2 c d c

Use the smaller dimension

Use the greater dimensionor2.5(tn - c) G

2 + ( t n - c ) - ( t w - c )

J K c(t - c)tre1e2 A 2 A 3

A 3 A 1 A 4 A 4

is the required thickness of a seamless wall (or solid plate); if the opening passes through a welded joint whose direction is parallel to the cross section under consideration, t r shall be given a value that allows for the difference, if any, between the specified efficiency of the joint and an efficiency of 100 %; is the required thickness of the nozzle neck, allowing for the efficiency of longitudinal joints, if any, on the nozzle neck; is the excess thickness if the opening is in solid plate; is the excess thickness if the opening passes through a welded joint that has an efficiency of less than 100 % and whose direction is parallel to the cross section under consideration; is the thickness of the nozzle neck available for reinforcement of the opening; is the area in excess thickness of the tank wall that is available for reinforcement; is the area in excess thickness of the nozzle neck that is available for reinforcement; is the cross-sectional area of welds available for reinforcement; is the cross-sectional area of material added as reinforcement. t w

t rn
e 1 e 2
e 3
A 2 A 3 A 1

A 4 NOTE See 5.16.5.1 and Figure 5-8 for definitions of other variables.

where 5-40 API S TANDARD 620D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-41
5.16.3 Size and Shape of Area of Reinforcement 5.16.3.1 The area of reinforcement for a given cross secti on of an opening shall be understood to be that ar ea in a plane normal to the surface of the tank and passing through the section under considerati on within which available metal may be deemed effective for reinforcing the opening. For surfaces that have straight elements, such as cylinders and cones, the areas of reinforcement will be rectangular in shape as i ndicated by lines GH, HK, GJ, and JK in Figure 5-7; however, on surfaces that are curved in two di rections, the lines GH and JK shall follow the contour of the tank surface. 5.16.3.2 The maximum length of the area of reinforcement shal l be the greater o f the foll owing limiting distances on each side of the axis of the opening, meas ured along the outside su rface of the tank. a) A distance equal to the diameter of the opening after co rr osion; in the case of non circ ular openings, a dist ance equal to the corresponding clear dimension is substituted fo r the diameter of the opening. b) A distance equal to the radius of the opening after corro sion plus the thickness of the nozzle wall plus the thickness of the t ank wall, all taken in the corroded condi tion; in the case of non circular openings, a distance equal to the corresponding half chord is s ubstituted for the radius of the opening. 5.16.3.3 The maximum width of the area of reinforcement, measur ed radially as applicable from either the inner or outer surface of the tank wall, or both, shall be not more than the smaller of the two following distances. a) A distance equal to 2.5 times the nominal thickness of the t ank wa ll less the corrosion allowance. b) A distance equal to 2.5 times the nominal thickness of the nozzle wall less its co rrosion allo wance plus the thickness of any additional re inforcement inside or outside the tank wa ll less its corrosion allowance if the reinforcement considered is inside the tank. 5.16.3.4 If the areas of reinforcement computed for two or more adjacent openings overlap, the openings shall be reinforced as pr ovided in 5.17. 5.16.4 Metal Considered to Have Reinforcing Value 5.16.4.1 Subject to the provisions of 5.16.7, the met al within the limit s of the area of reinforcement as described in 5.16.4.2 and 5.16.4.3 may be consi dered to act as reinforcement. 5.16.4.2 Metal thickness in the tank wall in e xcess of that r equired by 5.10 for 100 % joint ef ficiency may be considered to act as reinforcement when the entire opening is in solid plate or in excess of that required for the applicable design joint efficiency when any part of the opening passes through a joint that lies in approximately the same meridional or latitudinal direction 30 as the cross section of the opening for which the reinforcement requirements are being computed. In no case does this incl ude any metal provided for corro sion allo wance. If desired , the wall thickness may be arbitrarily increased to ma ke additional amounts of excess thickness available for reinforcement in the tank wall instead of adding rein forcement locally in the form of reinforcing pads. 5.16.4.3 All other metal attached to the tank wall in conformance with 5.16.8 may be consider ed to act as reinforcement, including those portions of fusion we lds and the nozzle wall that remain available for reinforcement of the opening after deducting applicable corr osion allowances and allowing for t he thickness of nozzle wall needed to satisfy minimum thickness and strength requirem ents for the nozzle wall itself (see 5.19).

30 If part of the opening passes through a joint whose directi on is approximately perpendicular to the cross section under consideration, the presence of the joint ma y be ignored in the computations for this cr oss section but must be taken into accou nt in computations for reinforcement requirements along cross sections parallel to this joint (see 5.16.5).Figure 5-8–Part 1—Acceptable Types of Welded Nozzles and Other Connections Backing strip, if used, may be removed after welding

t n

t 3 t w t n

t 3 t w t n t 3

t w

t n t 1

t w t 2 + c t n t 1

t w t 2 + c

t n t 1

t w t 2 + c t w t 1 t n

t 2 + c t 3 + c t 1 or t 2 not less than 0.7 t min or 1 / 4 in. t 1 + t 2 = 1.25 t min Panel a Panel b Panel c

Panel d Panel e-1

Panel e-2 Panel f 5-42 API S TANDARD 620Figure 5-8–Part 2—Acceptable Types of Welded Nozzles and Other Connections Panel h t n

t w t

3 t n t n

t w t 3 t 4

t w

t 5 + c d /4

t 5 + c t 3 + c t n t 3

t w t w t 4 t 4 t p t p t 5 t n

t w t 4 t p t 5 t n t 3

t 5 + c

t w t 4 t p t n t 3 t p t 3 t 4

t w t p t 5 + c t n

t 3 + c t 4 + c t 5 t n

t w 1 3 Panel g Panel i

Panel k Panel j Panel l

Panel n Panel m Panel o-1 Weld to shell Weld to shell D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-43Figure 5-8–Part 3—Acceptable Types of Welded Nozzles and Other Connections t n

t w 30o max 45o max

At least 1.5 ( t w - c ) t n 30o min 1 / 2 in. min 1 / 4 in. R min t 6

t 7 t w t 6 + t 7 = 0.2 t but is not greater than 1 / 4 in. t n

t w 3 / 4 in. R min

t n

t 4

t w t 5 + c

1 / 4 in. t 5 + c t 4 t n

t w

t 5 + c t 4 t w t p t 5 + c

t w t 3 t n

t 1

t 2 + c t 3

t 1
t 2 + c t 3 + c t 2 + c t 1 t 3 Panel o-2 Panel o-3 Panel o-4

Panel p Panel q Panel r

Panel s-1 Panel s-2 a Panel t-1 Panel t-2 a Panel u-1 Panel u-2 a Either method of attachment is satisfactory 5-44 API S TANDARD 620Figure 5-8–Part 4—Acceptable Types of Welded Nozzles and Other Connections 3 in. pipe size max

1 / 4 in. min t 5 (see 3.16.9.3)

t 5 + c t 4

t n

t h 2 t n 1.3 t n t n Tangent line Minimum 3 t h but need not exceed 1 1 / 2 in.

Single-fillet lap weld for heads convex to pressure

Panel u-3 Panel v a

Panel w
Figure 5-8 Notes (Parts 1 through 4):

t w is the nominal thickness of the tank wall, in inches, including corrosion allowance;
t n is the nominal minimum thickness of the nozzle neck, in inches, including corrosion allowance;
t p is the nominal thickness of the reinforcing pad, in inc hes, including corrosion allow ance if the pad is exposed to corrosion;

c is the corrosion allowance, in inches;
t min is the smaller of 3 / 4 in. or the thickness less the corrosion allowance of ei ther of the parts joined by a fillet weld or groove weld;
t 1 or t 2 is the a value not less than the smaller of 1 / 4 in. or 0.7 t min ; the sum t 1 + t 2 shall not be less than 1.25 t min ; t 3 is the smaller of 1 / 4 in. or 0.7( t n – c ), (inside corner welds may be further limited by a lesser length of projection of the nozzle wall beyond the inside face of the tank wall);
t 4 is a value not less than 0.5 t min ; t 5 is a value not less than 0.7 t min ; t h is the nominal head thickness, in inches. NOTE 1 The weld dimensions indicated in this figure are predi cated on the assumption that no corrosion is anticipated on the o utside of the tank. If outside corrosion is expected, the outside weld dimensions shall be increased accordingly. NOTE 2 Exposed edges shown as rounded may be finished by light grinding to at least a 1 / 8 in. radius or chamfered at 45 degrees to at least a 5 / 32 in. width.

a For 3-in. pipe size and smaller, see exemptions in 5.16.9.2. D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-455-46 API S TANDARD 620
5.16.5 Reinforcement Required 5.16.5.1 The total cross-sectional area of the reinforcement provided at any section thr ough an opening and within the limits of the area of reinforcement as defined in 5.16.3 shall be not less than the value computed by the following equation:
A r = ( d + 2 c )(t – c )( E  ́) where
A r is the area, in square inches, to be provided in th e reinforcement of the se ction under consideration; d is the inside clear dimension, in inches, across the opening at the section under consider ation before deducting the applicable corrosi on allowance (see 5.14.1);

c is the corrosion allowance, in inches, for the part under consideration;

t is the thickness, in inches, required by 5.10 for the pa rticular area of the t ank wall in which the opening is located for resisting the unit forc es that act in a direction perpendicular to the cross section under consideration;
E ́ is the factor whose value s hall be equal to the efficiency, E , of the main joint s along the edges of the tank wall plate containing the opening that are approximately parallel to t he cross section under consideration where the opening is in solid plate or passes only through a joint that is substant ially perpendicular to this cross section; and whose value shall be 1.00 where any part of the opening passes through a joint that is approximately parallel to the cross sect ion under consideration (for values of E , see Table 5-2). The value of E ́ , when not taken as unity, s hall be expressed as a decimal.
5.16.5.2 Consideration must be given to the reinforcement r equirement s for cross sections in both meridional and latitudinal directions, part icularly on noncircular openings that have appr eciable differences between their maximum dimensions in these two directions (see 5.16.2). 5.16.5.3 The equation in 5.16.5.1 assumes that all of the materials consider ed as reinforcement will have ultimate tensile strengths not less than the ultimate minimum tensile strength specified for the material in the tank wall. If some portion (such as the nozzle neck, if it is constructed of pipe) or all of the re inforcement material does not conform to this assumption, additional reinforcement shall be provided to fully compensate for the lower ultimate tensile strength; in no case shall any credit be taken for the additional strength of any ma terial of a tensile strength higher than that of the tank wall to be reinforced. 5.16.6 Distribution of Reinforcement Reinforcement shall be distributed so that the strength o f the rein forcement in each and every plane that constitutes a path of potential failure, as mentioned in 5.16.2, will be at least equal to the total load perpendicular to the same plane that would have been carried by the metal removed from the net wall thickness needed for that region of the tank if the metal had remained in the tank wall. The strength of the reinforcement is computed by multiplying the crosssectional area of the reinforcing materi al provided within the area of reinfo rcement in that plane by the maximum allowable unit stress value for the reinforcement material (this value shall not exceed t he allowable unit stress for the tank wall). In addition, the reinforcem ent shall preferably be shaped in section and welded to the tank wall in such a way that stress intensifications in the tank wall at the edges of the reinforcement will be kept as low as feasible.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-47
5.16.7 Distribution of Reinforcement for Large Openings 5.16.7.1 The rules previously given for the reinforcement of openings are primarily intended for openings no t larger than the following sizes. a) For surfaces that have a radius of curvature of 30 in. or less, the inside diame t er (width or length) of the openings shall not exceed the radius of curvature of the surface in wh ich the opening is located, nor s hall it exceed 20 inches in any case. b) For surfaces whose least radius of cu rvature is over 30 in., the inside d iam eter (width or length) of the openings shall not exceed 2 / 3 the least radius of curvature of the surface in which the opening is located, nor shall it exceed 40 in. in any case. 5.16.7.2 Openings larger than those just described, but still wit hin the limit s specified in 5.14.7, shall be given special consideration; except as otherwise provided in 5.18, the rein forcement shall meet all of th e requirements previously given. In addition, special attention shall be given to placing the major portion of the reinforcement as close as practicable to the edge of the opening while still providing a reasonably gradual transition in contour from the thickness of the tank wall to t he maximum thickness of the reinforc ement. Wherever practicable, about 2 / 3 of the required reinforcement shall be placed within a distance extending 1 / 4 of the dimension d (as defined in 5.16.5) on each side of the opening. 5.16.7.3 Fillet welds may be ground to concave contour, and the inside corners of the tank wall or nozzle neck along the edges of th e opening shall be rounded to a generous radius to reduce stress concentrations. Reinforcement may sometimes be more advantageously obtained by inserting a thicke r plate or plates in that portion of the wall of the tank where the nozzle is located. Ho wever, when this is done, considerati on shall be given to whether it would introduce an objectionable degree of restraint that might affect adjoining plat es. The degree to which these and other measures should be used will depend on t he particular application and the seve rity of the intended service. In extreme cases, appropriate proof testing may be advisable. 5.16.8 Strength Required In Welds 5.16.8.1 The reinforcement shall be attached using a method that develops the full strength required of the reinforcement at the centerline of t he opening and provides adequate protection agai nst failure that might occur in a plane (referred to herein as the critical plane) that is some what removed from the center of the opening as a result of tensile failure of the tank wall in co mbination with shearing or tensile failur e of the reinforcement attachment (see 5.16.2.4). To this end, the welds and other parts of the as sembly that serve as reinforcement attachment shall be properly located to transmit the stresses to the reinforc ement, taking credit for only those portions of the attachment that lie beyond the critical plane—i. e., on the side of the plane opposite the c enter of the opening. Similarly, the strength of the attachment between any two parts of attached reinforcement bey ond the critical plane shall be at least equal to the strength required in tension of the attached part or parts (see F.5 for examples illustrating the computation of reinforcement).
NOTE Although the location of the critical plane may be determined ana lytically in most cases, it is not essential that it be determined analytically for the purposes of this section; the in tent of the requirements in this section will be satisfied if ( a) the critical plane is assumed to be located as specified in Item 1 or 2 of this note and (b) suff icient welding and other attachments are pr ovided beyond the plane (that is, on the side of the plane opposite the center of the opening) to develop the strength of the attached reinforcement required at the centerline of the opening. Attachment welds shall be m ade continuous around t he entire periphery of the opening and reinforcement without any material reduction in size along the portions not credited as effective attachment in the computation. The critical plane locations to be assumed accordi ng to Item a of this note shall be established from a considerat ion of the relative magnitude of the coincident biaxial stresses in the tank wall and the shape of the opening as follows:

1) For an opening in a spherical surface or in a surface of some ot h er shape wh ere neither of the pr incipal biaxial stresses is le ss than 75 % of the other, the critical plane shall be one that is perpendicu lar to the direction of the tank wall stress for which the reinforcement is being investigated; for a round or elliptical opening, the critical plane shall pass through the center of the opening; for an obround opening, it shall pass through the center of one of the semicircular ends if a transverse section of th e opening is being analyzed or coincides with the lengthwise center line of the opening if a section in this direction is being analyzed.5-48 API S TANDARD 620
2) For an opening in a cylindrical or conical surface or in a surf ace of some othe r shape where one of the principal biaxial stresses is less than 75 % of the other, the critical plane shall be one that is parallel to the plane described in Item 1 for the shape of o pening involved but is located halfway between that plane and the edge of the opening (see Annex F). 5.16.8.2 The strength of welds that attach the reinforcement shall be the strength in shear or tension depending on the possible mode of failure of the weld. When either shear or tension stress may be considered, the computations resulting in the lesser strength shal l govern. Plug welds, wherever applicabl e, may be included in the strength of the attachment welding in conformance with 5.24. The thickness of the nozzle wall after corrosion may be included in the shear strength of the reinforcement attachment when the nozzle extends through the tank wall and is attached to it with a weld within the tank wall thickn ess that is sufficient to develop its str ength in shear, which may not require full penetration through the tank wall. Some of the attachment welding may be placed outside the limits of the area of reinforcement as defined in 5.16.3; although it is not credited as reinforcement, this welding may nevertheless be counted as attachment welding if it qualifies in other respects. Lengths of curved fillet welds shall be determined on the basis of their inner dimensions. 5.16.8.3 In addition to complying with the rules for attach ment welding given in this st andard, the following requirement s shall be met. a) The joint efficiencies of butt-welds shall be in accor dance with 5 .23 except that no credit shall be taken for radiographing unless the attachment welding itself can be and is properly radiographed. The strength of buttwelds shall be computed on the area in shear, wherever applicable, or the area in tension using the following stress values multiplied by the joint efficiency: 1) when the load is perpendicular to the weld, the applicable tens ion or she ar stress va lues for plate or forged steel given in Table 5-1 or specified in 5.5.5; 2) when the load is parallel to the weld, 75 % of the applicable tension or shear stress values for plates or forged steel given in Table 5- 1 or specified in 5.5.5; 3) for combined perpendicular and parall el loadings around the openings, 87.5 % of the applicable tension or shear stress values for plate or forged steel given in Table 5-1 or specified in 5.5.5. b) The strength of fillet welds shall be computed by multiplying the area of the minimum section through the throat of the weld by the applicable allowabl e stress value determined by comb ining the following factors: 80 % for shear strength of weld metal; an ef ficiency factor of approximately 85 %; and a load factor of 100 % for perpendicular loading, 75 % for parallel loading, or 87.5 % for combined perpendicular and parallel loading: 1) when the load is perpendicular to the weld, 70 % of the applicable tension stress value for plat e or forged steel given in T able 5-1; 2) when the load is parallel to the weld, 50 % of the applicable tension stress va lue for plate or forged steel given in Table 5-1; 3) for combined perpendicular and parallel loadings around openings, 60 % of the applicable tension stress value for plate or forged steel given in T able 5-1. 5.16.9 Minimum Dimensions of Attachment Welds 5.16.9.1 Supplementing the requirements of 5.16. 8, the dimensions of reinforcement atta chment welds shall comply with the following: a) where the thickness of the thi nner of two part s being joined is 3 / 4 in. or less, exclusive of corrosion allowance, the dimensions of the welds shal l be not less than the requirements indicated in Figure 5-8;D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-49 b) where the thickness of both parts is greater than 3 / 4 in., exclusive of corrosion allo wance, the dimensions of the welds shall be not less than the requirements indicated in Figure 5-8 using a value of 3 / 4 in. for t min . 5.16.9.2 Fittings shown in Figure 5-8, Panels s-2, t-2, u-2, and v , that do not exceed 3 in. pipe size may be attached by welds that are exempt from size requi rements other than those required by 5.16.8. 5.16.9.3 For fittings attached as show n in Figure 5-8, Panel u-3, the de pth of the groove weld, t 5 , shall be not less than the thickness of Schedule 160 pipe (see ASME B36.10M). 5.16.10 Telltale Holes in Reinforcing Plates Except for nozzles located on t he underside of a t ank bottom that re st s directly on the tank grade 31 and nozzles with reinforcements that are too narrow to permit compliance with the following provisions, si ngle- thickness reinforcing plates and saddle flanges or integral reinforcing pads on m anholes or nozzles attached to the outside of a tank shall be provided with at least one telltale hol e with a maximum actual diameter of 3 / 8 in. that shall be tapped for a preliminary compressed-air and solution-film test for soundness of attachment weld s around the manhole or nozzle and its reinforcement both inside and outside the tank. These telltale holes shall be left open when the tank is in service. The surface of the reinforcing plate, saddle flange, or pad adjacent to the tank wall shall be relieved slightly by grinding to be reasonably certain th at the test pressure will extend ent irely around the nozzle even though the reinforcement may be drawn tightly against the tank wall by the welding. 

### 5.17 Reinforcement of Multiple Openings 5.17.1 When either of the following conditions occurs for two or more adjacen t openings, the opening shall be provided with a combined reinforcement whose strength shall equal the combined strength of the reinforcement that would be required by 5.16 for the separate openings. a) The distance between the centers of any two adjacent openi ngs is less than two times their average diameter so that their required areas of reinforcement overlap. b) Any two adjacent openings are spaced so that if they ar e r einforced separately, the distance between the outer edges, or toes, of their reinforcing plate fillet welds (see Footnote 24) or insert welds is (1) less than 6 in. at any point, or if this be larger, (2) eight times the nominal thic kness of the fillet weld around the thicker reinforcing plate or eight times the nominal thic kness of the insert butt-weld 32 for an insert-type connecti on. In no case shall any portion of a cross section be considered to apply to more than on e opening, that is, to be evaluated more than once in a combined area. Curved sect ions that form the outer boundary of a combined reinforcement shall be connected by straight lines, large-radius reverse curves tangent to the curved sections, or a combination of these two elements; in no case shall ther e be any re-entrant angles therein. 5.17.2 When two or more adjacent openings will be provided with a com bined r einforcement, the minimum dist ance between the centers of any tw o of these openings shall pref erably be at least 1.5 time s their average diameter, and the area of reinforcement between t hem shall be at least equal to 50 % of the total required for these two openings on the cross section being considered. 5.17.3 When two adjacent ope nings, as cons idere d under 5.17.2, have a dist ance b etween centers less than 1 1 / 3 times their average diameter, no credit for reinforcement shall be given for any of the metal between these two openings.

31 Even in this case, telltale holes should be provided and attachment welds should be tested before the bottom plates are placed in position on the tank grade. 32 Where the periphery weld has been stress-rel ieved before the welding of the adjacent shell joint, the spacing may be reduced to 6 in. from the longitudinal or meridional joints or 3 in. from ci rcumferential or latitudinal joint provided that in either cas e the spacing is not less than 2 1 / 2 times the shell thickness.5-50 API S TANDARD 620
5.17.4 Any number of closely spaced adjacent openings, in any arrangement, may be reinforced for an assumed opening of a diameter enc losing all such openings. 

### 5.18 Design of Large, Centrally Located, Circular Openings in Roofs and Bottoms 5.18.1 General Large openings and reducers of the types illustr ated in Figure 5-9, which are centrall y located in the roof or bottom o f a tank with the axis of the connected cy lindrical neck coincident with the axis of revolution of the tank, are not limited as to size and need not be reinforced in accordance with 5.16 if the design of the neck extending from the opening or reducer, the regions of the roof or bottom around the openi ng, and the transition section between the roof or bottom and the neck meet all applicable requirement of 5.10 and the additional requirements specified in this section. In the case of reducers, the design of the region where the large end meets cylindrical sidewalls shall conform to the requirements of 5.12. A design procedure similar to that s pecified in 5.12 shall also be used for the region around the large end of a conical transition section t hat connects to the horizontally dispos ed surfaces of a roof or bottom instead of to the sidewalls. 5.18.2 Nomenclature Variables used in Equation (29) through Equation (32) are defin ed as follows: Q is the total circ umferen tial force, in lb, acting on a vertical cross section thr ough the juncture between the roof, bottom, or transition section and the neck extending from the opening at one side of the opening;
A c is the net area, in inches 2 , of the vertical cross section of metal required to resist Q , exclusive of all corrosion allowances;
R 2 is the length, in inches, of the norma l to the roof, bottom, or transition se ction at its juncture with the neck extending from the opening, measured from the surface of the roof, bottom, or transition section to the tank's vertical axis of revolution;
R n is the horizontal radius, in inches , of the cylindrical neck extending fr om the opening at the juncture with the roof, bottom, or transition section;
T

1 is the meridional unit force (see 5.10) in the roof, bottom, or transition section at its juncture with the cylindrical neck, in lbf/in. of circumferential arc;
T

2 is the corresponding latitudinal unit forc e (see 5.10) in the roof, bottom, or transition section, in lbf/in. of meridional arc (if the roof or bottom is of double curv ature) or per linear in. along an element of the cone (if the surface is that of a conical frustum);
T

2 n is the circumferential unit force (see 5.10) in the cylindrical neck at its juncture with the roof, bottom, or transition section, in lbf/in. m easured along an element of the neck;

α is the angle between the direction of T 1 and a vertical line (in a conical surface it is also one-half the vertex angle of the cone);
S

ts is the maximum allowable stress value for simple tension, in lbf/in. 2 , as given in Table 5-1;
E is the efficiency, e xpressed as a dec imal, of the least efficient joint cu tting across the section considered (see Table 5-2);

w h is the width, in inches, of the roof , bottom, or transition section plate c onsidered to participate in resisting the circumferential force Q ;Figure 5-9—Large Head Openings and Conical Shell-reducer Sections t n

t n

t n t n

R n R n R n

R 2 R 2 R 2 W

h

W

h W n

W

n R 1 Roof

See 5.18.3 for design of this region
See 5.12.4 for design of this region See 5.12.3 for design of this region See 5.18.3 for design of this region

See 6.15Centerline of tank

Panel a

Panel b Panel c D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-51

w n is the corresponding width, in inches , of the participating neck plate;

t h is the thickness, in inches, of the roof, bottom, or transition section plate at and near its juncture with the neck extending from the opening, including corrosion allowance;

t n is the corresponding thickness, in in ches, of the cylindrical neck at and near the juncture described for t h . 5.18.3 Knuckle Radius 5.18.3.1 A knuckle radius used for the j uncture between the roof, bottom, or tra nsition section and the neck extending from the opening shall be not less than 6 % of the diameter of the opening, and the thicknesses required at this location shall be computed in accordance with 5.10. The use of a knuckle radius as small as 6 % of the sidewall diameter will frequently require an excess ively heavy thickness for the knuckle region. The th ickness requirements for this region will be found more reasonable if a larger knuckle radius is used. 5.18.3.2 When a knuckle radius is not used at th is location, the stress si tuation at the juncture is the reverse of that found at the juncture (without a knuckl e) between a conical or dished roof and the sidewalls of a cylindrical tank because in this case the horizontal components of the T 1 meridional unit forces in t he roof, bottom, or transition section pull outward on the neck extending from the opening and increase the circum ferential tensile stresses acting5-52 API S TANDARD 620 at the juncture. In this case, the walls of the tank and neck of the opening at and near t heir juncture must be designed to withstand a total circumferential load, Q , on each side of the opening, as computed using the following formula: Q

= T 2 w h + T 2 nw n + T 1 R n sin α (29) 5.18.4 Cross-sectional Area The total cross-sectional area of metal required to resist t he circumferential force is s hown by the fo llowing e quation:
A c = Q / S

ts

E (30) The widths of plate available for prov iding this area and resisting the force Q on each side of the opening shall be computed using the following formulas:

w h 0.6 R 2 t h c

– ( ) = (31)

w n 0.6 R n t n c

– ( ) = (32)



### 5.19 Nozzle Necks and Their Attachments to the Tank 5.19.1 General 5.19.1.1 Nozzles to be used for pipe connections, handholes, or manholes may be constructed of pipe, pipe couplings, forged steel, cast steel , fabricated plate, or other suitable material conforming to the provisions of 4.1, 4.2.2, 4.3, or 4.5. 5.19.1.2 Nozzles may be integral with the tank wall or the wa ll of another nozzle or with a nozzle cover plate; or , subject to the limitations stated in t hese rules, nozzles may be attached direct ly to the wall of the tank or another nozzle or nozzle cover plate by threading, fusion welding, bearing against the insi de of the wall, studding, or bolting. 5.19.1.3 Openings for all nozzles in the wall of the tank or another nozzle shall be reinforced a s required by 5.16 or 5.17. Openings in nozzle cover plates need only be reinforced to the extent required by 5. 21.1.2, 5.21.1.3, 5.21.2.7, and 5.21.2.8. 5.19.1.4 Nozzles may be attached to a tank by any of the methods s hown in Figure 5-8 or by o ther methods that conform to sound design principles if the nozzle and its at tachment in each case meet the requirements of 5.16. 5.19.2 Minimum Thickness of Nozzle Neck The thickness of a nozzle neck shall be computed for the applicable loadings in 5.4, using al lowable stresses as specified in 5.5, and to this thickness shall be added the corrosion allowance. The mini mum thickness of nozzle neck to be used shall be at least equal to the required thicknes s so obtained; in no case shal l the net thickness of the nozzle neck, excluding corrosion allo wance, be less than the smaller of the following thicknesses: a) the net thickness, excluding corrosion allowa nce, of the tank wall adjac ent to the nozzle, disrega rding any added thickness that serves as reinforcement for the opening; b) the thickness of standard-we ight pipe (see ASME B36.10M).D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-53
5.19.3 Outer Ends of Nozzles 5.19.3.1 The outer ends of nozzles may be flanged, beveled fo r welding, or threaded except that threa ded ends shall not be used unless they are per mitted by and meet the requirements of 5.20.4. When bolted flanges are provided on nozzle sizes NPS 2 and greater, the minimum distanc e from face of flange to the tank wall shall be 8 in. 5.19.3.2 When a bolting flange is welded to the nozzle neck for its entire thickness, the corner fo rmed by the back of the flange and the nozzle wall shall be provided with a fillet weld. The fillet weld si ze shall be at l east 0.25 times the thickness of the nozzle wall, not including corrosion allowance, except that for re latively thick nozzle walls, the fillet weld shall be not less than 0.25 times the thickness of st andard-weight or extra-strong pipe, whichever is nearest to and less than the nozzle wall thickness. This fillet may be machined to a radius of the same size, but in no case shall it be less than 3 / 16 in. 5.19.3.3 When a bolting flange is welded to the nozzle neck, but not for its entire thickness, it shall be designed and atta ched in accordance with 5.20 in this standard and the provisions of Figure 4-4, Appendix 2, in Sect ion VIII of the ASME Code

. 

### 5.20 Bolted Flanged Connections 5.20.1 Bolted flanged connections conforming to ASME B16.5, Class 150, shall be used for connections to external piping and may be used for other flanged connections. Such flanges may be built up by fusion welding if the Manufacturer satisfies the inspector, by direct or comparative calculation, that the welded flanges are equivalent in strength to the one-piece flanges t hat they are intended to replace. 5.20.2 Bolted flanges for external piping c onnections other than those meeting t he requirement s of 5.20.1 shall be designed for a pressure of at least 50 lbf/in. 2 gauge in accordance with the applicab le provisions of Section VIII, Appendix 2, of the ASME Code , using for values of S f and S n the applicable allowable design stress values given in Table 5-1 of this standard (instead of t he allowable design stress values spec ified in Section VIII of the ASME Code ) and limiting the values for S h , S r , and S t as follows: S

h is the longitudinal hub stress, not greater than 1.5 S f

, except that for flanges of t he types illustrated in Figures 4-4 (7), (8), (8a), (8b), and (9) in Section VIII, Appendix 2, of the ASME Code , S h shall not exceed the smaller of 1.5 S f or 1.5 S n ; S

r is the radial flange stress, not greater than S f

; S

t is the tangential flange stress, not greater S f

. Also, ( S h + S r )/2 shall not be greater than S f , and ( S h + s t )/2 shall not be greater than S f . Design stress values for bolts shall not exceed the applicable va lues given in Table 5-1 in these rules, based on the area at the root of the thread. 5.20.3 Bolted flange connections, other than external piping connections, shall c onform to ASME B16.5, Class 150 or shall be designed in accordance with t he requirements of 5.20.2 except that they shall be designed for a pressure of at least 15 lbf/in. 2 gauge or the total pressure, P , on the wall of the tank and the level of the connection, whichever is greater. 5.20.4 Hubbed flanges may be welded to the ends of nozzle necks by any of the m ethods p ermitted for circumferential joints in the walls of t he tank; the attachment shall conform to t he requirement for circumferential joints of the type employed. 5.20.5 Flanges that do not exceed 12-in. pipe size for working pressures up to 50 lbf/in. 2 gauge or 4 in. pipe size for working pressures above 50 lbf/in. 2 gauge may be screwed to the end of a nozzl e neck if the number of full threads engaged conforms to or exceeds t he requirements of ASME B1.20.1. 5.20.6 Bolts and studs shall be at least 1 / 2 inch in diameter. If smaller bolts are used, they shall be of alloy steel.5-54 API S TANDARD 620


### 5.21 Cover Plates 5.21.1 Flat Cover Plates and Blind Flanges 5.21.1.1 The thickness of flat, unstayed co ver plates and blind flanges shall be determined by one of the following methods, but shall not be less than 1 / 2 in. plus corrosion allowance. a) Blind flanges that conform to ASME B16.5 and are of the appropriate pres sure- t emperature ratings and diameters given in the standard, or their equiva lent, shall be used when attached by bolting as shown in Figure 5-10, Panels b and c

. b) For sizes and designs not covered by ASME B16.5, the requ ired thickness of flat st eel cover plates or blind flanges shall be computed by the following formula using the appropriate value for C :

t d CP s ------- c or P st c

– ( ) 2

Cd 2 ------------------- =+= (33)

where

t is the minimum required thickness, in inches; d

is the diameter, in inches, measured as indicated in Figure 5-10;
C

is the 0.25 for plates rigidl y riveted or bolted to flanges as shown in Figure 5-10, Panel a (this applies in these rules to any kind of gasket material); = 0.30 for plates inserted into the en ds of nozzles and held in place by some suit able positive mechanical locking arrangement such as those shown in Figure 5-10, Panel d , e , or f if the design of all holding parts against failure by shear, tension, or compression resulting from the end force produced by the pressure is based on a factor of safety of at least four and that threaded joints, if any, in the nozzle wall are at least as strong as they are for standard pipe of the same diameter; = 0.30 + (1.49 Wh G / HG ) for plates bolted to flanges in such a manner that the setting of the bolts tends to dish the plate, where the pressure is on the same side of the plate as the bolting flange, as shown in Figure 5-10, Panels b and c ; W is the flange design bolt load, in lb [see AS ME Section VIII, Appendix 2, Para graph 2-5 (e)]; h G is the radial distance from the bol t-circle diameter to diameter G , in inches;
H is the total hydrostatic end force, in lb, as defined in ASME Section VIII, Appendix 2, Paragraph 2-3;
G is the diameter , in inches, at the location of the gasket load reaction, as defi ned in ASME Sectio n VIII, Appendix 2, Paragraph 2-3;
P is the design pressure, in lbf/in. 2 gauge (this shall be at least equal to the total pressure P on the wall of the tank at the level where the cover plate is located or shall be 15 lbf/in. 2 gauge, whichever is greater);

s is the maximum allowable stress value, S ts , in lbf/in. 2 , as given in Table 5-1;

c is the corrosion allowance, in inches.Figure 5-10—Acceptable Types of Flat Heads and Covers d

t

C

= 0.25 C = 0.30 1.40 Wh G HGd= ( =G) h G

C

= 0.30 1.40 Wh G HG + +

C = 0.30 d td= ( =G) h G

C

= 0.30 C = 0.30 d d Pressure

Threaded ring

Pressure

Sectional ring Panel a Panel b

Panel c Panel d

Panel e Panel f
NOTE The illustrations above are diagrammatic only. Other designs, which meet the requirements of 5.21, will be acceptable. t t

t

t D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-555-56 API S TANDARD 620
5.21.1.2 Unreinforced openings up to and including 2-in. pipe size are permissible in flat cover plates without incr easing the plate thickness if the edges of these openings are not closer to t he center of the plate than one-fourth the diameter d in Figure 5-10. When this condition is not me t, the plate thickness shall be increased by 40 % of the thickness required in a solid plate afte r the loss of corrosion metal. The soli d-plate thickness s hall be determined by deducting the corrosion allowance from the thickness computed using Equation (33). 5.21.1.3 Openings that do not exceed 50 % of dimension d shown in Figur e 5-10 may be made in flat cover plates if these openings are reinforced in accordance with 5.16 as though the cover plates were dished to the form of a spherical segment having a radius equal to diameter d . However, the reinforcement added to the cover plate shall compensate for not less than 50 % of the cross section of the metal remo ved for the opening in the cover plate. When the maximum diameter of the opening in the flat cover plate exceeds 50 % of dimension d shown in Figure 5-10, the cover plate shall be designed as a flange in accordance with t he rules for bolted flanges given in 5.20 of this standard and in ASME Section VIII, Appendix 2. 5.21.2 Spherically Dished Cover Plates 5.21.2.1 The variables used in the formulas in this section and Figure 5-11 are defined as follows:

t is the minimum required thickness of the cover plate af ter forming, incl uding corrosion allowance, in inches;

t f is the flange thickness, including corrosion allowance, in inches;
A is the outside diameter of flange, in inches;
B is the inside diameter of flange, in inches;
C

is the bolt-circle diameter, in inches;
D is the inside diameter of cover-plate skirt, in inches;
L is the inside spherical or crown radius, in inches;

r is the inside knuckle (tor us) radius, in inches;
P

is the design pressure, in lbf/in. 2 gauge (shall be at least equal to the total pressure P on the wall of the tank at the level where the cover plate is located or 15 lbf/in. 2 gauge, whichever is greater);

s is the maximum allowable stress value, S ts , in lbf/in. 2 , as given in Table 5-1;
M

o is the total moment determined as for loose-type fl anges (see ASME Section VI II, Appendix 2, Paragraph 2- 6) except that, for cover plates of the ty pe shown in Figure 5-10, Panel d, a moment H r h r shall be included (that may add or subtract);
H

r is the radial component of the memb rane load in the spherical segment = H D cot β 1 ; h r

is the level arm of force H r around the centroid of the flange ring, in lb. 5.21.2.2 The radius of dish, L , in tori spherical heads shall not exceed the outside diameter of the head skirt, and the knuckle radius, r , shall be not less than 6 % of the outside diameter (see Figure 5-11, Panel a). 5.21.2.3 In ellipsoidal heads, the inside depth of the head minus the wid th of the skir t shall be not less than 1 / 4 the inside diameter of the head skirt (see Figure 5-11, Panel a).Figure 5-11—Spherically Dished Steel Plate Covers with Bolting Flanges Edge of weld shall not overlap knuckle Not less than 2 t and in no case less than 1 / 2 in.

Knuckle radius Gasket Knuckle radius Gasket Ring gasket shown Preferably 2 t

minimum

1 / 2 C 1 / 2 B L L L

1 / 2 A
1 / 4 ( A + B ) Point of H

D action

Shown as welded; smooth weld both sides L 1 H D H

r h r

1 / 2 C 1 / 2 B Centroid Full-penetration weld

Use any suitable type of gasket 1 / 2 A

1 / 2 B 1 / 2 C

Preferably 2 t

minimum

Ring gasket shown 0.7 t minimum Loose-flange type Integral-flange type Panel a (See note 1) Panel b

Panel c Panel d NOTE 1 Ellipsoidal or hemispherical heads may also be used in the above type of cover plate. NOTE 2 In no case shall the radius of dish, L , be greater than the inside of the cover-plate bolting flange (Dimension R ).tt t1 t 1 D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-57

5.21.2.4 Cover-plate heads of hemispherical shape need not have an int egral skirt, but where a skirt is provided, the juncture between the skirt and the spherically dished portion of the head shall not project more than 1 / 2 in. beyond the weld between the head and the back face of the cover-plate flange unless the thickness of the skirt is at least equal to the thickness required for a seamless cy lindrical shell of the same diameter. 5.21.2.5 The thickness of circular dished co ver plate s with boltin g flanges, conc ave to the pr essure and conforming to the several types illustrated in Figure 5-11, shall be designed in accordance with the following requirements, but shall not be less than 1 / 2 in. plus corrosion allowance. a) For cover plates of the types shown in Figur e 5-11, Panel a, the thickness of the plate, t , shall be deter mined by the following application equation: for torispherical heads,

t = (0.885 PL / s ) + c ( 34) for 2:1 ellip soidal heads,

t = ( PD /2 s ) + c (35)5-58 API S TANDARD 620 for hemispherical heads,

t = ( PD /4 s ) + c (36) The cover pla te flange thickness and bolting for these types of cover plates shall comply at least with the applicable requirement s of Section VIII, Appendi x 2, Figure 4-4, of the ASME Code and shall be designed in accordance with the provisions of 5.20.2. b) For cover plates of the type shown in Figure 5-11, Panel c, the thickness of th e pl ate and flanges sha ll be determined by the foll owing applicable equation: for the thickness of the cover plate,

t = ( 5 PL /6 s ) + c (37) for the flange thickness using a ring gasket,

t f M o s ------ A B + BA B

– ( ) --------------------- = (38)
for the flange thickness using a full-face gasket,

t f 0.6 P s --- BA B + ( ) C B

– ( ) A B

– ---------------------------------------- = (39)
(The radial components of the membrane load in the sphe rical segment are assum ed to be resisted by the flange.) c) For cover plates of the type shown in Figure 5-11, Panel b, the plate thickness for a ring gasket is determined using the followin g equation:

t Q 1 1 7.5 M o PQBL --------------- ++ c + = (40) The plate thickness for a full-face gaske t is determined using the following equation:
t Q 1 1 3 C B

– ( ) B

QL ------------------------ - ++ c + = (41)

where

Q P s ---     L 4 --     1 1 6 C B

– C B

+ -------------     + ------------------------------- =

In no case shall the plate thickness be less than the value determined us ing the following equation:

t = (5 PL /6 s ) + cD ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-59 d) For cover plates of the type shown in Figure 5-11, Panel d, the thickness of the cove r plate is determined using the following equation:

t = (5 PL /6 s ) + c (42) (The factor 5 / 6 in this case includes an allowance of E = 0.8 at the circumferential weld.) The flange thickness is determined using the follow ing equation:

t f F 1 1 J F

2 ----- ++ c + = (43)

where

F PB 4 L 2 B 2

– 8 s A B

– ( ) ------------------------------- =

J M o sB ------     A B + ( ) A B

– ( ) ----------------- =

NOTE Inasmuch as H r h r may add to or subtract from the total moment dete rmined as for loose-type flanges, the moment in the flange ring when the internal pressure is zero may be the critical loading for the flange design (see 5.20.2). 5.21.2.6 The thicknesses of circular dished co ver plates with bolting flanges, conv ex to the pressure and conforming to the several types illustrated in Figure 5-11, shall be designed in accordance with t he requirements of 5.21.2.5 except that the pressure, P , used for computing the thickness of the cover plate, t , shall be not less than 1.67 P v , where P

v is defined as follows:
P

v is the maximum unbalanced pressure, in lbf/in. 2 , on the convex side of the pl ate during operation; however, if the pressure is 15 lbf/in. 2 or less, P v shall be 15 lbf/in. 2 or 25 % more than the maximum possible unbalanced pressure, whichev er is the smaller. The minimum thickness shall be as calculated plus corr osion allowance or 1 / 2 in. plus corrosion allowance, whichever is greater. Moreover, if the net plate thickness, t – c , determined previously in this p aragr aph, is found to be less than or equal to 0.01 times the inside diameter of the cover-plate fl ange, a check computation s hall be made to determine the thickness required by Equation (44). The plate shall be made not less than the thickness computed using the following equation:

t 4 L c P v 5350 ----------------- c += (44)

where
L c is the inside crown radius for dished (torispher ical) and hemispherical heads, in inches; or 0.9 D for 2:1 ellipsoidal heads, in which D is the inside diameter of the head, in inches. 5.21.2.7 Openings up to and including 2-in. pipe size may be made in the sp herical segment of a dished cover plate without increasing the thickness of the segment if the opening attachment is entirely clear of the fillets joining the spherical segment to the flange of the cover plate.5-60 API S TANDARD 620
5.21.2.8 Openings greater than 2-in. pipe size may be made in the spherical segment of a dish ed cover plate if these openings are reinforced in accordance with 5.16 or 5.17. 

### 5.22 Permitted Types of Joints 5.22.1 Definitions 5.22.1.1 The information in 5.22.1.2 through 5.22.1.6 covers fusion-welded joints permitted by this st andard. (See Table 5-2 for limitations of joints.) 5.22.1.2 Terms relating to weld joints shall be as defined in Section IX of th e ASME Code and the following. a) An angle joint is one located between two members located in intersecting planes between zero (a butt joint) and 90° (a corner joint). b) A slot weld is the same as a plug weld except that it is made through an elongated hole that has semicir cular ends. Fillet-welded holes should not be c onstrued as a plug or slot weld. 5.22.1.3 The reverse side of a double-welded butt joint shall be pr ep ared by chipping, grind i ng, or melting-out to ensure sound metal at the base of the weld metal first deposit ed before weld metal is applied from the reverse side. This operation shall be done to ensure complete pene tration and proper fusion in the final weld.
NOTE The proceeding requirements of this section are not intended to apply to any welding pr ocedure by which proper fusion and complete penetration are otherwise obt ained and by which unacceptable defects at the base of the weld are avoided. 5.22.1.4 If the backing strips for single-welded joints are not removed, all ends of stri ps abutting (including a Tjunction) shall be joined with a full-penetration weld. A backi ng strip need not be removed after the weld is completed unless the joint is to be radiographed and the backing-strip im age would interfere with interpretation of the resultant radiographs. 5.22.1.5 Double and single lap-joints shall have fu ll-fillet welds, the size of whic h is e qual to the thinner member joined. The surface overlap shall be not less than four times the thickness of the thinner plate, with a 1 in. minimum. 5.22.1.6 When full-penetration welds are specified for the circum fere ntial joints of dia m etrical trans itions, angle joints 30 degrees or less meet this requirement. All other requirements for a butt-welded joint apply. 5.22.2 Size of Weld 5.22.2.1 Groove Weld The size of a groove weld is determined by the joint penet ration, which is the depth of chamferin g plus the root penetration when r oot penetration is specified. 5.22.2.2 Fillet Weld 5.22.3 For equal-leg fillet welds, the leg length of the largest isosceles right tria ngle that can be inscribed within the fillet-weld cross section determines the size of the weld. For unequal-leg fillet welds, the l eg length of the largest right triangle that can be inscribed within the fillet-we ld cross section determines the size of the weld. 5.22.4 Throat of a Fillet Weld The throat of a fillet weld is the shortest distance from the r oot of the fillet wel d to i ts face. For a convex fillet weld, th e hypotenuse of the triangle which has the greatest area that can be inscribed within the fillet-weld cross section is considered the face.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-61
5.22.5 Heads Convex to Pressure Heads convex to pressure for the purpose of sealing manways may be attached to the manway neck using single fullfillet lap joints without plugs in accordance with Fi gure 5-8, Panel w, and the limitations of Table 5-2. 

### 5.23 Welded Joint Efficiency 5.23.1 General The efficiency of a welded joint is a joint efficiency factor us ed in design comput ation or in computations that relate the strengths of welded structures. The jo int efficiency factor is based on the assumption that the welds may contain defects within the limits permitted by these rules or may otherwise be of a quality somewhat below that of the parent material. Permissible joint efficiency fact ors are given in Table 5-2, where the factor is expressed as a percent; in computations it is expressed as a decimal. In the case of butt-welded joints and full-fillet lap-welded jo int s, the joint efficiency factor is assumed to exist between the working strength of the joint and t he working strength of the solid plate. In the case of fillet welds evaluated as spe cified in 5.16.8.3, Item b, plug we lds, and other att achment welding, the joint efficiency factor is assumed to exist between the working strength of the area of weld involved in the computations and the working strength of the same area of solid parent metal. 5.23.2 Maximum Joint Efficiencies The maximum joint efficiencies permitted in the design of t anks or tank p arts fabricated by an arc-welding process and the limitations on the use of the various ty pes of these joints are given in Table 5-2. 5.23.3 Welded Pipe Joint Efficiencies The allowable unit tensile stress values given in Table 5-1 fo r weld ed steel pip e reflect a welded joint efficiency factor of 0.80 for the longitudinal joints in that material. No further reduction for joint efficiency needs to be made in those joints. The low-pressure operating conditions for which these t anks are used will of ten ma ke the thickness of pipe materials, as determined by the cylindrical shell formula , of little signific ance; the girth joints that are subject to piping strains, including even moderate temperature effect s, may be the controlling factor. The joint efficiencies for such girth joints shall be taken from Table 5-2, but in appl ying these efficiencies to t he allowable stress values in Table 5-1 for welded pipe, allowance may be made for the fact that the allowable values already reflect a joint efficiency factor of 0.80, as stated in this paragraph. 

### 5.24 Plug Welds and Slot Welds 5.24.1 Plug welds and slot welds may be used in conjunction with other forms of welds for joints in structural attachment s and in reinforcements around openings. They shall be sized and spaced properly to carry their portion of the load but shall not in any case be considered to take more than 30 % of the total load to be transmitted by the joint of which they form a p art. 5.24.2 The diameter of plug-weld holes and the wid th of slot-weld holes in mem bers whose thickness is 1 / 2 in. or less shall be not less than 3 / 4 in.; for members more than 1 / 2 inches in thickness, the diameter, or width, of such holes shall be not less than the thickness of the member through which the hole is cut plus 1 / 4 in. 5.24.3 Except as otherwise provided in 5.24.2, the diameter , or width, of the holes shall not exceed twice the thickness of the member th rough which the hole is cut plus 1 / 4 in. In no case, however, does the dimension need to be greater 2 1 / 4 in.5-62 API S TANDARD 620
5.24.4 Plug-weld and slot-weld holes shall be completely filled with weld me tal when the thickne ss of the member through which the hole is cut is 5 / 16 in. or less. For thicker members, the holes shall be filled to a depth of at least onehalf the thickness of the member or one- third the hole diameter, or width, whichever is greater, but in no case shall they be filled less than 5 / 16 in. Fillet-welded holes are not consi dered to be plug welds or slot welds. 5.24.5 The effective shearing area of plug welds shall be considered to be the area of a circle whose diameter is 1 / 4 in. less than the diameter of the hole at the fraying surface. Th e effective shearing area of the semicircular ends of slot welds shall be computed on a comparable basis, and the effective area between the centers of the semicircular ends shall be taken as the product of the distance between such centers and a width that is 1 / 4 in. less than the width of the slot at the fraying surface. 

### 5.25 Stress Relieving 33
5.25.1 Definition Stress-relief heat treatment is the unifo rm heating of a structure or portion of a structure to a suf f icient temperature below the critical range to relieve the major portion of the residual stresses, followed by uniform cooling. 5.25.2 Field Stress Relief A tank built according to the rules of this standard is not usually thermally fiel d stress relieved af ter erection because its size and weight do not permit adequate support at the temp erature required for stress relieving. When a tank is not to be field stress relieved, the field-welding procedure shall be one that (a) has been proven satisfactory by experience or adequate experiments and (b) wi ll minimize locked-up re sidual stresses, which are thought to be one of the main causes of cracking in or adjacent to welds (see 6.19 and H.4). 5.25.3 Wall Thickness Tank sections that have a nominal th ickness of wall plate greater than 1 1 / 4 in. 34 at any nozzle or other welded attachment and nozzle necks w hose thickness at any wel ded joint therein exceeds ( D + 50)/120 shall be thermally stress relieved af ter welding. Thickness of compression ri ngs as defined in 5.12 (examples shown in Figure 5-6) are not considered in the determination of thermal stress relief requirements. In this formula diameters less than 20 in. shall be assumed to be 20 in. When thermal stress relief cannot be applied to welded assemblies of these parts after erection, all such assemblies, particularly around openi ngs and support attachments, shall be made in the shop and shall be thermally stress relieved before shipment. 5.25.4 Fillet-weld Attachments The requirement of 5.25.3 does not apply to fillet welds used for small nozzle or lug att achments when the welds have a size that is a) no greater than 1 / 2 in. for welds on a flat surface or circum ferential welds on a cylindrical or conical surface or b) no greater than 3 / 8 in. for longitudinal welds on surfaces of the latter two shapes or for any welds on surfaces that have double curvature. 

### 5.26 Radiographic/Ultrasonic Examination 5.26.1 General Examination for the quality of butt welds where required by 5.26 shall be made using either the radiogra phic method specified in 7.15.1 or alternativel y, by agreement between the Purchaser and the Manufacturer, using ultrasonic

33 Any proposed application of stress-relieving requirements and th e procedures to be followed in each case should be agreed upon by the Purchaser and the Manufacturer. Peening may be done if it is part of the welding procedure and is approved by the Purchaser (see 6.7 and 6.10). 34 For P-1 materials, the 1 1 / 4 in. thickness may be increased to 1 1 / 2 in., provided that a minimum preheat temperature of 200 ° F is maintained during welding.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-63 examination in lieu of radiography as spec ified in 7.15.3.1. When the term “exami nation” is used in 5.26, it shall be understood to refer to radiographi c or ultrasonic examination.
NOTE Ultrasonic testing (UT) and radiography (RT) are complement ary methods. UT can de tect some types of flaws better than RT and vice versa. Therefore, the acce ptance criteria provided in API 620 for the two methods differ. In the case of RT, a ll weld flaws require characterization as their type (e.g., crack, incomplete fusion, rounded indication, etc.) and acceptance cri teria vary according to the characterization. RT acceptance criteria in API 620 are based on definitions of good workmanship. In the case of UT, flaw characterization by type or shape is not requir ed (except for results of supplemental MT/PT). Rather the sizing of both the length and height of flaws is emphasized. The UT acceptance criteria provided in Annex U are based on fracture mechanics and assume that any flaw may have crack-like characteristics. 5.26.2 Definitions Radiography is the process of passing electromagnetic radiat ion, such as X-rays or Gamma rays through an object and obt a ining a record of its soundness upon a sensitized film. Ultrasonic examination is a method of detecting imperfections in material by p a ssing ultrasonic waves through the material and interpreting the refl ected and/or diffracted waves. 5.26.3 Examination Required by Wall Thickness Complete examination is required for all butt joints hav ing complete penetr ation and complete fusion wherever the thinner of the plates or the tank-w all thicknesses at the joint exceed 1 1 / 4 in. and the joint is subjected to tension stress greater than 0.1 times the specified minimum tensile strength of the material. 5.26.4 Examination Required for Joint Efficiency 5.26.4.1 The increased joint efficiency allowed in Table 5-2 for completely radiographed joints in a tank or tank sections may be used in the design calculations if the conditions descr ibed in 5.26.4.2 and 5.26.4.3 are met. 5.26.4.2 Main joints (all longitudinal and circumferential joints in the ta nk wall or meridional and latitudinal joints in walls of double curvature) are of the butt-welded type except for nozzle, m anhole, and support attachment welds to the tank wall, which need not be of the butt-welded type. 5.26.4.3 All butt-welded joints described in 5.26.4.2 are examined throughout their length, as pre scribed in 7.15, except under the following conditions. a) When parts of tanks do not require complete examination (see 5.26.3). In this case, circumferential joint s in cylindr ical or conical surfaces need to be prepared and ex amined for a distance of only 3 in. on each side of any intersection with a longitudinal joint. A ll joints in a spherical, torispherical, or ellipsoidal shape or in any other surface of double curvature shall be c onsidered longitudinal joints. For simi lar reasons, the juncture without a knuckle between a conical or dished roof or bottom, and cylindrical sidewalls a nd the circumferential joints without a knuckle at either or both ends of a transition secti on shown in Figure 5-9 shall be examined if the adjacent longitudinal joints are to receive credit for being radiographed. b) When welded butt joints in nozzle necks do not require co mplete examination (see 5. 26.3). This provisio n applies to their fabrication and is not necessarily the form of attachment to the tank. 5.26.5 Insert Style Shell Openings When shell openings (nozzles, manways, flush -typ e nozzles and flush-type cleanout doors) are installed using an insert style detail, the butt-weld joint around the periph ery of the insert plate s hall be examined as follows. a) For an insert plate that does not extend over the full height of a shell course, the butt-welded joint around the periphery of an insert plate shall be fully examined.5-64 API S TANDARD 620 b) For an insert plate that extends over the full height of a shell course, the butt- welded joints of the insert plate shall be examined to the extent required of main joints (longit udinal and circumferential joints in the tank wall or meridional and latitudinal joints in walls of double cu rvature) for other plates in the same course. 5.26.6 Exemptions Spot or full examination is not mandatory on tank bo ttoms that are uniformly suppor ted throughout (for example , concrete slab or compacted sand) or on components that are not subject to tension stre sses greater than 0.1 times the minimum tensile strength of the material. 

### 5.27 Flush-type Shell Connection 5.27.1 Cylindrical-sidewall, Flat-bottom Tanks 5.27.1.1 A low-pressure tank of this configuration may have flush-type connections at the lower edge of the shell. These connections can be made flush with the flat bot tom under the conditions descr ibed 5.27.1.2 through 5.27.1.4. 5.27.1.2 The design pressure for the gas vapor spac e of the t ank shall not exceed 2 lbf/in. 2 gauge. 5.27.1.3 The sidewall uplift from the internal design and test pressures, wind, and earthquake loads shall be counteracted, as noted in 5.1 1.2 , in such a manner that no upli ft will occur at the cylindrical sidewall, flat-bottom junction. 5.27.1.4 The longitudinal or meridional membr ane stress in the cylindrical sidewal l at the top of the opening for the flush-type connection shall not exceed 1 / 10 of the circumferential design stress in the lowest sidewall course that contains the opening. 5.27.2 Dimensions and Details 5.27.2.1 The dimensions and details of the connection shall conform to Table 5- 10, Figure 5-12, and the rules specified in this section. 5.27.2.2 The maximum width, b , of the flush-typ e connection opening in t he cylindrical sidewa ll shall not exceed 36 in. 5.27.2.3 The maximum height, h , of the opening in the cylindrical sidewall shall not exceed 12 in.
T

ab le 5-10—Dimensions of Flush-type Shell Connections (Inches)
Class 150 Nominal Flange

Size Height of Opening, h Width of Opening, b Arc Width of Sidewall Reinforcing

Plate, W Upper Corner Radius of Opening, r 1 Lower Corner Radius of Sidewall Reinforcing

Plate, r 2 8
12
16 18
20
24 8
12
12
12
12
12 8
12
20
22
25 36 38 52
64 66
69 89 4 6
6 6
6 6 14 18
18 18
18 18Figure 5-12—Part 1—Flush-type Sidewall Connection t n = 5 / 8 in. min One 1 / 4 in. telltale hole in reinforcing plate at about mid-height

SECTION B–B

SECTION A–A B h

2 in. min 6 in. min W/2 arc dimensions 6 in. min

L 15 in. min C

C Shell plate in flush connection = t d

Reinforcing plate = t d

B

3 in. Bottom reinforcing plate Shell plate of lowest shell course = t See Section C-C (Figure 5-12 Part 2 continued) Centerline of connection

t b t d t d 1 1

/ 4 in. min

Bottom reinforcing plate A A

Butt weld Nozzle transition to circular flange Butt weld 3 in. radius 5 in. min Full-fillet weld 60 ±30 Bottom plate

Bottom transition plate for minimum arc dimension of W + 60 in.

2 t

d + 10 in.

(See note)

Centerline of connection 24 in. min 1 1 / 4 in. min Notch as required to provide flush joint Full-penetration weld

1 / 4 3 / 16 r 2

r 1

b/2

12 in. min

NOTE The thickness of the thinner plate joined with a maximum of 1 / 2 inch. D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-65Figure 5-12—Part 2—Flush-type Sidewall Connection (See Note 1) t d (1 1 / 2 in. max) Round corner when t d > 1 1 / 2 in.

5 in. t d

t d b /2

t n = 5 / 8 in. min Nozzle transition

(See Note 2) Typical Detail for Connections With b = h t b t a = 1 / 2 in. min 2 t

d + 10 in. h Centerline of nozzle flange and shell opening Bottom plate Bottom reinforcing plate Bottom transition plate Full-penetration weld

1 4

Alternative butt-weld detail

Bottom transition plate

1 / 4 in. min 1 1 / 4 in. min Bottom reinforcing plate Full-penetration weld

Flanges per Table 5-10 t n t b 2 t

d + 10 in. h Centerline

of nozzle Bottom plate Round corner t d (1 1 / 2 in. max) t /2 5 in.
t d

t a h

t b t d Round corner when t d > 1 1 / 2 in.

<30 Nozzle transition

Nozzle neck (see 5.2.7.4.6) Back-chip and weld

(See Note 1)

1 4 Round corner

Full-penetration weld t n

SECTION C–C Typical Detail for Connections With b > h
NOTE 1 The thickness of the thinner plate joined with a maximum of 1 / 2 inch. NOTE 2 Flanges in sizes 1 1 / 2 to 24 in. shall conform to ASME B16.5. Flanges in sizes larger than 24 in. shall conform to ASME B16.47. 5-66 API S TANDARD 620D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-67
5.27.2.4 The thickness of the sidew all plate in the clean-out opening assembly sh all be at least as thick as the adjacent sidewall plate in t he lowest sidewall course. 5.27.2.5 The thickness of the sidewall reinforcing plate shall be of the same thickness as th e sidewall plate in the flush connection a ssembly

. 5.27.2.6 The thickness, t a , of the bottom transition plate in the assembly shall be 1 / 2 in. minimum, or when specified, the thickness of the bottom annular plate. 5.27.3 Stress Relieving The reinforced connection shall be completely pre assembled into a sidewall plate. The completed assembly , including the sidewall plate that contai ns the connections, shall be thermally st ress relieved at a temperature of 1100 °F to 1200 °F for a period of 1 hour per in. thickness of sidewall-plate thickness, t d . 5.27.4 Reinforcement 5.27.4.1 The reinforcement for a flush-type sidewall connection shall conform to the rules described in 5.27.4.2 through 5.27.4.6. 5.27.4.2 The required cross-sectional area of the reinforcemen t over the to p of the connection shall be not less than the value determined using the following equation:
K 1 ht/ 2 where
K 1 is the area coefficient, as given in Figure 5-13; h is the greatest vertical height of the clear opening, in inches;

t is the calculated thickness, in inches of the sid ewall course in which t he connection is located, exclusive of corrosion allowance. 5.27.4.3 The reinforcement in the plane of the si dewall shall be pro vided within a height, L , above the bottom of the opening. L shall not exceed 1.5 h except that L minus h shall be not less than 6 in. for small openings. Where this exception results in a height, L , greater than 1.5 h , only that portion of the reinfo rcement within a height of 1.5 h shall be considered effective.
NOTE L is the height of the shell reinforcing plate, in inches. 5.27.4.4 The required reinforcement may be provided by any one or by any combination of the following: a) the shell reinforcing plate; b) any thickness of the shell plate in the assembly great er than the calculated thickne ss of th e lowest side wa ll course, exclusive of corrosion allowance; c) that portion of the neck plate t hat has a length equ al to the thickness of the reinforcing plate.Figure 5-13—Design Factors for Flush-type Connections Minimum K 1

Minimum K 1

0 1.0
Key 1.1 1.2 1.25

1.00

0.75

0.50

0.25

1.3 1.4 K 1 Coefficient

K 1 = H

= D = Area coefficient for determining minimum reinforcement of a flush-type connection. Height of tank, in feet. Inside diameter of tank, in feet.[(H+ 29)D+ 770]385h 5-68 API S TANDARD 620

5.27.4.5 The width of the tank-bottom reinforcing plate at t he centerline of the opening shall be 10 in. plus the combined thickness of the sidewall plat e in the flush connection assembly and the sidewall reinforcing plate. The thickness of the bottom reinforcing plate, t b , in inches, shall be calculat ed using the following equation.

t b h 2 14 000

, ----------------- b 280 -------- H +=

where h is the vertical height of clear opening, in inches; b is the horizontal width of clear opening, in inches;
H is the height of tank, in ft. Th e minimum th ickness of the bottom reinforcing plate, t b , shall be 5 / 8 in. for H = 48; 11 / 16 for H = 56; and 3 / 4 in. for H = 64. 5.27.4.6 The thickness of the nozzle transition piece and the nozzle neck, t n, shall be a minimum of 5 / 8 in. External loads applied to the connection may require that t n be greater than 5 / 8 in. 5.27.5 Material Requirements All materials in the flush-type shell c onnection a ssembly shall conform to the requirements of Section 4. The plate materials of the sidewall cont aining the assembly, the sidewall reinforcing plate, the nozzle nec k attached to theD ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 5-69 sidewall, the transition piece, and the bottom reinforcing plat e shall meet the impact test requirements of 4.2.5 at design metal temperatures for the re spective thickness involved. Notch toughness evaluation fo r the bolting flange and the nozzle neck attached to the bolting flange shall be ba sed on the governing thicknes s as defined in 4.3.6.3 and used in Figure 4-2. Additionally, the yield strength and tens ile strength of the sidewall pl ate in the flush-type shell connection and the sidewall reinforcing pl ate shall be equal to or greater than the yield strength and tensile strength of the adjacent sidewall of the lowest shell course plate material. 5.27.6 Connection Transition The nozzle transition between the flush connection in t he shell and the circular pi pe flange shall be designed in a manner consistent with the rules given in this standard. Where the rules do not cover all details of design and construction, the Manufacturer shall provide details of design and construction t hat will be as safe as those provided by the rules of this standard (see 5.1.1). 5.27.7 Anchorage Where anchoring devices are used to resist the sidewall upli f t, they shall be spaced so that th ey will be located immedia tely adjacent to each side of the reinforcing plat es around the opening, while st ill providing the required anchorage for the tank sidewall. 5.27.8 Allowance for Sidewall or Piping Movement Adequate provision shall be made for free movement of connected piping to minimize thrusts an d moment s applied to the sidewall connection. Allowanc e shall be made for the rotation of the sidew all connection caused by the restraint of the tank bottom to the sidewall expansion from stress and te mperature as well as for thermal and elastic movement of the piping. In double-wall tanks, any insulation or other mate rial shall not restrain or tend to increase the movement of the sidewall connection. The rotation of the si dewall connection is illustrated in Figure 5-14. 5.27.9 Foundation The foundation in the area of a flush-ty pe connection shall be prepare d to support the bottom reinforcing plate of the connection. The foundation for a tank that rests on a concrete ringwall shall provide a uniform support for the bottom reinforcing plate as well as the remaining bottom plate under the tank sidewall. Differ ent methods of supporting the bottom reinforcing plate under a flush-ty pe connection are shown in Figure 5-14. 5.27.10 Nozzle Spacing Flush-type connections may be installed using a common reinfo r cing p ad. However , when th is type of c onstruction is employed, the minimum distanc e between nozzle centerlines shall be not less than 1.5( b 1 + b 2 + 2.5) in. or 2 ft, whichever is greater. The dimensions b 1 and b 2 shall be obtained from Table 5-10, Column 3, for the respective nominal flange sizes. Adjacent sidewall flush-type connections that do not share a common reinforcing plate shall have at least a 36 in. clearance between adj acent edges of their reinforcing pads. 5.27.11 Weld Examination All longitudinal butt-welds in the nozzle neck and transition piece if any , and the fi rst circumferential butt-weld in the neck closest to sidewall, excluding neck to flange weld sha ll receive 100% radiographic examination. The nozzle-totank sidewall and reinforcing plate welds and the sidewall-to -bottom reinforcing plate welds shall be examined their entire length using magnetic-particle exam ination. This magnetic-particle exam ination shall be performed on the root pass, on every 1 / 2 in. of deposited weld metal while the weld is being made, and on the completed weld.Figure 5-14—Rotation of Sidewall Connection Position of shell after elastic movement Initial shell radius = R

Shell radius = R + R

Height of bending in shell varies with tank radius and thickness

Transition plate
Bottom Reinforcing plate

Inside diameter of shell 3 in. min (See Details a and b) Initial centerline of connection
Angle of rotation Centerline of connection after elastic movement of shell

Inside of shell at centerline of opening

3 in. min 3" min Inside of shell at centerline of opening Notch to suit bottom reinforcing plate

W + 12 in. min, except as limited by curvature of foundation (see Detail b) Detail a Detail b

Details of Notch in Ringwall  5-70 API S TANDARD 6206-1 

### SECTION 6—FABRICATION
6.1 General This section covers details in fabricat ion practices that are cons idered essential in constr ucting large, welded tanks designed according to the rules in this standard. 

### 6.2 Workmanship 6.2.1 All work of fabricating API 620 tanks shall be done in accordance with this standard and with the permissible alter natives specified in the Purchaser ’s inquiry or order. The workmanship and finish shall be first class in every respect and subject to the closest inspec tion by the Manufacturer’s inspector, whether or not the Purchaser waives any part of the inspection. 6.2.2 When material requires straightening, the wo rk shall be done by pressing or another non injurious method prior to any layout or shaping. Heating or hammering is not permissible unless the mate rial is heated to a forging temperature during straightening. 

### 6.3 Cutting Plates 6.3.1 Plates, edges of heads, and other parts may be cut to shape and size by mechanical means such as machining, shearing, and grinding or by gas or arc cutt ing. After gas or arc cutt ing, all slag and detrimental discoloration of material that has been molten shall be re moved by mechanical means bef ore further fabrication or use. 6.3.2 All holes made in the tank wall, the edges of which are not to be fused by welds, should prefer ably be tool-cut. If open ings are manually flame-cut, the edges to remain unwelded shall be tool-cut or ground smooth (see Figure 5-8 for finish of unwelded exposed edges). 

### 6.4 Forming Sidewall Sections and Roof and Bottom Plates 6.4.1 Cylindrical Side Wall Sections Figure 6-1 for steel tanks and Figure 6-2 for aluminum tanks pr ovide criteria for shaping of cylindrical sidewalls to the curvature of the t ank prior to inst allation in the tank. Shaping of plates concurrently with inst allation in the tank shell is permitted if the diameter exceeds the limits in Figure 6-1 (for steel tanks) and Figure 6-2 (for aluminum tanks) or if the Manufacturer’s alternate procedure for any diameter has been accepted by the Purchaser. 6.4.2 Roof and Bottom Plates If curved, roof and bottom plates shall be formed to the r equired shape by any process t hat will not unduly imp a ir the mechanical properties of the material. 

### 6.5 Dimensional Tolerances 6.5.1 General Tank walls subject to membrane stresses that are more than 1 / 3 of the allowable design stress under service conditions shall conform to the tolerances descri bed in 6.5.2 through 6.5.6. The number and frequency of measurements are left to the judgement of the Manufacture r in order to produce an acceptable tank. Outer walls of double-wall tanks (see Annex Q and Annex R) that contain insulation and are not in contact with the design liquid are excluded. These tolerances may be waived or modified by agreement between the Purchaser and the Manufacturer.Figure 6-1—Shaping of Plates for Steel Tanks (See 6.4) 40 60 120

10 13 16 mm ft.

ALL ALLALLALL

m 5 12 18 36 in.

5 / 8

1 / 2

3 / 8

3 / 16 Shaping required prior to installation

Shaping not required

Tank diameterSide wall plate thickness

NOTE Any combination of diameter and thickness falling on or above the solid line require shaping prior to installation.

Figure 6-2—Shaping of Plates for Aluminum Tanks (See 6.4) ft.

Shaping required prior to installation

Shaping not required

ALL ALLALLALL

9 15 27 10 13 16 mm

Tank diameterSide wall plate thickness

NOTE Any combination of diameter and thickness falling on or above the solid line require shaping prior to installation. m 5 30 50 90

5 / 8

1 / 2

3 / 8

3 / 16 in. 6-2 API S TANDARD 620D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 6-3
6.5.2 Plumbness 6.5.2.1 For cylindrical sidewalls, the maximu m out-of-plumbness of the top of the shell relative to the bottom of the shell shall not exceed 1 / 200 of the total tank height. 6.5.2.2 The out-of-plumbness in one shell pl ate shall not exceed the permissib le variations for flatness and waviness specified in ASTM A6 or ASTM A20 , whichever is applicable for carbon and alloy steels. For stainless steels, ASTM A480 is applicable. For aluminum plates, Table 5.13 of ANSI H35.2 provides the dimensional flatness tolerance. 6.5.3 Roundness 6.5.3.1 For cylindrical sidewalls, the horizontal circular cross section of a large, low pressure stor age tank shall be sufficiently true to round so that the difference between the maximum and mi nimum diameters (measured inside or outside) at any section in a cylindrical wall shall not exceed 1 % of the average diameter or 12 in., whichever is less, except as modified for flat-bottom tank s for which the ra dii measured at 1 ft 0 in. above the bottom corner weld shall not exceed the tolerances listed in Table 6-1. 6.5.3.2 The skirts or cylindrical ends of formed tops or botto ms shall be sufficiently true to round so that the dif

ference between the maximum and mini mum diameters shall not exceed 1 % of the nominal diameter. 6.5.4 Local Deviations Local deviations from the theoretical shape, such as weld d iscontinuities and flat spot s, shall be limited as follows. a) Using a horizontal sweep board 36-in. long, peaking at vertical joints shall not exceed 1 / 2 in. This may be increased to 1 in. for aluminum shells (see Annex Q). Table 6-1—Diameter Range Versus Radius Tolerance Diameter Range (ft) Radius Tolerance (in.) < 40 ± 1 / 2 40 to < 150 ± 3 / 4 150 to < 250 ± 1 ≥ 250 ± 1 1 / 4 b) Using a vertical sweep board 36-in. long, bandi ng at horizontal joints shall not exceed 1 / 2 in. This may be increased to 1 in. for aluminum shells (see Annex Q). c) Flat spots in the vertical plane shall not exceed t he appr opriate plate flatness and waviness requirement s of 6.5.2.2. 6.5.5 Fitting Attachments All lugs, brackets, nozzles, manhol e frames, reinforcement ar ound openings, and other appurtenances shal l fit and conform to the curvature of the su rface to which they are attached. 6.5.6 Foundation 6.5.6.1 To achieve the tolerances outlined in 6.5, a level f oundation must be provided for the t ank erection. The foundation should have adequate bearing power to ma intain the levelness of the foundation.6-4 API S TANDARD 620
6.5.6.2 The top of the foundation with a concrete ringwall shall be level within ± 1 / 8 in. in any 30 ft of circumference and within ± 1 / 4 in. in the total circumference. Without a concrete ringwall, the foundation shall be within ± 1 / 2 in. of the design shape. 6.5.6.3 For concrete slab foundations, from the out side of the tank radial ly towar d the center, the first foot of the foundation (or width of the annular ring) shall comply with the concrete ringw all requirement. The remainder of the foundation shall be within ± 1 / 2 in. of the design shape. 6.5.7 Measurements When measurements are required by agreement between the Purch aser and the Manufacturer, they shall be taken before the hydrostatic test. Measurements of local deviations shall be taken dur ing construction. T hey shall be taken with a steel tape—making corrections for temperature, sag, and wind—when t he length being measured makes such corrections necessary. Deviation m easurements shall be taken on the surf ace of the plate and not on welds. 6.5.8 Double-curvature Roofs, Bottoms, and Sidewalls For double-curvature roofs, bottoms and sidewalls, the toler ances shall be as follows: T he surface shall not deviate outside the design shape by more than 1.25 % of D and inside the specified shape by more than 5 / 8 % of D where D is the nominal inside diameter of the roof (or botto m) under consideration. Such deviations shall be measured perpendicular to the design shape and shall not be abrupt but s hall merge smoothly into the adjoining surfaces in all directions. For a knuckle, D shall be considered to be twic e the radius of the knuckle. 

### 6.6 Details of Welding 6.6.1 General 6.6.1.1 Tanks and tank parts fabricated under these rules s hall be welded by the processes defined in 6.6.2

. Welding may be performed by manual, semi- automatic arc, or machine welding according to procedures described in ASME Section IX, and by welders and we lder operators qualified under 6.7 and 6.8. 6.6.1.2 Welding shall be fusion welding without the application of mechanical pressure or blows. 6.6.1.3 Peening is permitted in accordance with 6.19. 6.6.1.4 Pipe materials that have long itudinal joint s of the types permitte d by the specifications listed in 4.3 are allowed. 6.6.2 Welding Processes Tanks and their structural attachments shall be welded by the shielded met al-arc, gas met al-arc, gas tungsten-arc, oxyfuel, flux-cored-arc, submerged-arc, electroslag, or electrogas process using suitable equipment. Use of the oxyfuel, electroslag, or electrogas process shall be by agreement between t he Manufacturer and the Purchaser. Use of the oxyfuel process is not permitted when impact testing of the material is required. Welding may be performed by manual, semi-automatic arc, machine, or automatic weldi ng according to procedures described in ASME Section IX. Welding shall be performed in such a manner as to ensure complete fusion with the base metal. The Purchaser may designate applicable sections of API 582 for s upplementary welding guidelines and practices. 

### 6.7 Qualification of Welding Procedure 6.7.1 Each Welding Procedure Specification (W PS) shall be qualified in accordance wi th the latest practice as given in Section IX of the ASME Code . When impact tests are required by 4.2.5 or when required by appropriate annexes, the weld metal and heat affected zone shall be tested and the S upplementary Essential Variables in Section IX of the ASME Code shall be applied. In addition, the heat treated condition and the application or omission of fine grainD ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 6-5 practice for the base metal shall be an additional Suppl ementary Essential Variable. Plates produced by the controlled-rolled process are not to be considered as hav ing received any heat treatm ent. Welding procedures for ladder and platform assemblies, handrails, stairways, and ot her miscellaneous assemblies but not their attachments to the tank, shall comply with either AW S D1.1, AWS D1.6, or Section IX of the ASME Code , including the use of SWPSs. 6.7.2 Carbon steel materials not listed in Table QW-422 of Section IX of the ASME Code shall be considered as PNumber 1 material with group numbers assigned as follows, according to the minimum t ensile strength specified: a) <70 kips/in. 2 (Group 1); b) ≥ 70 kips/in. 2 but < 80 kips/in. 2 (Group 2); c) ≥ 80 kips/in. 2 (Group 3). 6.7.3 The required tests to qualify the Welding Procedure Spec ification (WPS) shall be c onducted by the fabricator. 6.7.4 The str ess- relieving requirements in the procedur es to be followed in each case should be agreed upon between the Manufacturer and the Purchaser. Peening may be done if it is part of the welding procedure and is approved by the Purchaser. 

### 6.8 Qualification of Welders 6.8.1 All welders assigned to manual or semi-automatic ar c welding, and welding operators assigned to machine welding, shall have successfully pa ssed the tests conducted by the fabricator, or Manufacturer, as prescribed for welder qualification in Section IX of the ASME Code . Tests conducted by one Manufactu rer shall not qualify a welder or welding operator to do work for any other Manufacturer. 6.8.2 The Manufacturer shall assign each welder or weldi ng oper ator an identifying number, letter , or symbol. Except fo r all lap-welded roof seams and flange-to-neck join ts, this identifying mark s hall be stamped, either by hand or machine, on all tanks adjacent to and at intervals of not more than 3 ft along the welds made by a welder or welding operator; alternatively, the Manufacturer may keep a reco rd of welders employed on each joint and shell-opening joint and omit the stamping. If such a record is kept, it shall be maintained until tests are completed and shall be available to the inspector. 6.8.3 The Manufacturer shall maintain a record of the wel ders employed, showing the date and result of tests and the identification mark assigned to each. These records shall be certified by the Manufacturer and shall be accessible to the inspector. 

### 6.9 Matching Plates 6.9.1 The plates that are being welded shall be accurately matched and retained in position during the welding operation. Tack welds may be used to hold the plate edges in line if the requirements of 6.9.1.1 through 6.9.1.4 are followed. 6.9.1.1 The tack welds in butt joints to be welded manually are removed before welding. 6.9.1.2 The tack welds in butt joints to be machine welded by a process that will re- melt the t ack welds shall be thoroughly cleaned of all welding slag and examined for soundness. 6.9.1.3 Tack welds in lap and fillet welded joints need not be removed if they a re sound and the subsequently applied weld beads are thoroughly fused into the tack welds.6-6 API S TANDARD 620
6.9.1.4 Tack welds, whether removed or left in place, s hall be made using a fillet-w eld or butt-weld procedure qualified in acco rdance with Section IX of the ASME Code . Tack welds to be left in place shall be made by welders qualified in accordance wi th Section IX of the ASME Code and shall be examined visually for defects; if welds are found to be defective, t hey shall be removed. 6.9.2 During assembly of the plates and subject to agr eement between the Manufacturer and the Purchaser , the welded joints in adjoining segments, which abut at a common transverse joint fr om opposite sides, need not be staggered unless specified by the Purchaser. When specifi ed, the stagger should be at l east five times the plate thickness of the thicker course. 

### 6.10 Cleaning Surfaces to be Welded 6.10.1 Immediately before any welding operation, the surface to be welded or to which weld metal is to be applied shall be cleaned thoroughly of all scale, sl ag, grease, and any oxide that would lo wer the quality of the deposited weld metal. A light oxide film resulting from flame cutting is not considered detrimental. 6.10.2 On all multilayer welding, each layer of weld metal shall be cleaned of slag and other deposit s before the next layer is applied. 6.10.3 The reverse side of double welded butt joints shall be prep ared by chipping, grindin g, or melting out to ensure sound metal at the base of the weld metal first deposit ed before weld metal is applied from the reverse side. This operation shall be done to ensure complete penetration and proper fusion in the final weld. When melting out is done, particular care shall be exercised to prevent contamination of the melted area by foreign materials, especially carbon.
NOTE The proceeding requirements of this section are not intended to apply to any welding procedu re by which proper fusion and complete penetration are otherwise obt ained and by which unacceptable defects at the base of the weld are avoided. 6.10.4 Cast steel surfaces to be welded must first be mach ined or chipped to remove foundry scale and to expose sound metal. 

### 6.11 Weather Conditions for Welding No welding of any kind shall be done (a) when the surfaces to be welded are wet from rain, snow, or ice, (b) when rain or snow is falling on such surfaces, or (c) during period s of high winds, unless the wel der and work are properly shielded. Preheat shall be applied when the metal temperature is below the temperature required by Table 6-2. In that case the base metal shall be heated to at least the temperatur e indicated in Table 6-2 with in 3 in. of the place where welding is to be started and maintained 3 in. ahead of the arc. Material P-Numbers and Groups shall be as designated in ASME IX, Table QW-422 or in API 620, 6.7.2 for materials not listed in Table QW-422. Table 6-2—Minimum Preheat Temperatures
Material P-No. and Group Thickness ( t ) of Thicker Plate (inches) Minimum Preheat Te mper ature
P-No. 1 Group 1 t ≤ 1.25 32 ° F 1.25 < t ≤ 1.50 50 ° F t > 1.50 200 ° F
P-No. 1 Groups 2 & 3 t ≤ 1.25 50 ° F 1.25 < t ≤ 1.50 100 ° F t > 1.50 200 ° FD ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 6-7


### 6.12 Reinforcement on Welds 6.12.1 Butt joints shall have complete joint penetration and comp lete fusion for the full length of the weld and shall be free from undercuts, overlaps, or abrupt ridges or valleys. To ensure that the weld grooves are completely filled so that the surface of the weld metal at any point does not fall below the surface of the adjoining plate, weld metal may be built up as reinforcement on each side of the plate. The thickness of the reinforcement on each side of the plate shall not exceed the thickness listed in Table 6-3, but the reinforcement need not be removed except when it exceeds the permissible thickness or when required in 7.15.1. 6.12.2 When a single-welded butt joint is made by using a backing strip that is lef t in place (see Table 5-2), the requ irement for reinforcement applies to only the side opposite the backing strip. 

### 6.13 Merging Weld With Plate Surface The edges of the weld shall merge smoot hly with the surface of the plate wi thout a sharp angle. Maximum permissible weld undercut is 1 / 64 in. for longitudinal or meridional butt join ts, similarly oriented permanent attachments, attachment welds for nozzles, manhol ds, flush-type openings, and the inside shell-to-flat bottom welds. For circumferential and latitudinal butt join ts, similarly oriented permanent attach ments, and annular ring butt joints, the maximum permissible undercutting is 1 / 32 in. 

### 6.14 Aligning of Main Joints Particular care shall be taken in matching up the edges of all plates within the toler ances of offset as follows: a) for plates 1 / 4 inch in thickness and less, 1 / 16 in.; b) for plates over 1 / 4 inch in thickness, 25 % of the plate thickness or 1 / 8 in., whichever is smaller. 

### 6.15 Repairing Defects in Welds Defects in welds shall be chipped, melted out, or machined out until sound metal is r eached on all sides. Subject to the approval of the inspector, the resulting cavi ty shall be filled with the weld metal and retested 

### 6.16 Matching Plates of Unequal Thickness For plates over 1 / 2 -in. thick in the sidewalls, roof, or bottom of a tank , if the thickness of two adjacent plates that are to be butt-welded together differ by more than 1 / 8 in., the thicker plate shall be trimmed to a smooth taper that extends for a distance at least four times the offset between the abutting su rfaces so that the adjoining edges will be of approximately the same thickness. The length of the required taper may include the width of the weld (see Figure 6-3).

Table 6-3—Maximum Thickness of Reinforcement on Welds Dimensions in inches. Maximum Reinforcement Plate Thickness Vertical Joints Horizontal Joints ≤ 1 / 2 3 / 32 1 / 8 > 1 / 2 thru 1 1 / 8 3 / 16 > 1 3 / 16 1 / 46-8 API S TANDARD 620


### 6.17 Fitting Up of Closure Plates For the closure of the final joints, plates of extr a width and length—not narrow strips or filler bars 35 —shall be used. The fitting up of the closure plates used and the proposed me thod of installation shall be subje ct to the inspector’s approval before the work is started, and the inspector sh all ensure that the closure plates meet all applicable requirements. 

### 6.18 Thermal Stress Relief 6.18.1 General thermal stress relief of an ent ire tank is not visualized for tanks of this type, but sections of t anks shall be stress relieved before erection w here required by the pr ovisions of 5.25. 6.18.2 Parts of a tank that require stress relief according to the rule s in 5.25 shall be st ress relieve d in an enclosed furnace before shipment from the f abricators’ shop s. The procedure used shall be as outlined in 6.18.2.1 through 6.18.2.5. 6.18.2.1 The temperature of the furnace shall not exceed 600 °F at the time the part or section of the tank is placed in it. 6.18.2.2 The rate of heating in excess of 600 °F shall be not more than 400 °F per hour divided by the maximum met al thic knes s, in inches, of the wall plate being heated, but in no case shall it be more than 400 °F per hour. 6.18.2.3 During the heating period, the temperature throughout the portion of the tank being heated shall not vary more than 250 °F within any 15-ft interval of length and w hen at the hold temperature not mor e than 1 50 °F throughout the portion of the t ank being heated. A minimum temperature of 1100 °F (except as permitted in 6.18.2.5) shall be maintained for a period of one hour per in. of meta l thickness (maximum metal thickness of the tank wall plates affected). During the heati ng and holding periods, the furnace atmo sphere shall be controlled to avoid excessive oxidation of the surface of the material being treated. The furnace shall be designed to prevent direct impingement of the flame on the material. 6.18.2.4 At temperatures over 600 °F, cooling shall be done in a closed furnac e or cooling chamber at a rate not gre ater than 500 °F p er hour divided by the maximum metal thickness, in in. of the plates affected; in no case shall the rate be more than 500 °F per h our. At temperatures of 600 °F and below, the material may be cooled in still air. 6.18.2.5 When stress relieving at a temperature of 1100 °F is impracticable, it is permissible to carry out the stressrelieving ope ration at lower temperatures for longer time periods in accordance with Table 6-4. 

### 6.19 Peening Field Welds 6.19.1 A tank fabricated according to these rules that is too large to be completely assembled and welded in a shop may be transported in sections and assembled in the field. Welds made after assembly in the field may require a special welding procedure in accordance with 5.25, and mechanical peening as described in Annex I may then be used on the field welds. 6.19.2 Peening of welds is not considered as ef fective as ther mal stress relief and is not to be substituted for thermal stress relief where the thermal stress relief is mandatory under the provision of 5.25.

35 Gaps of this kind may require removal of part of the adjoining plate to give proper widths. Fu ll consideration should be given to radiographic and magnetic-particle methods of examination as well as to the therma l stress relief or peening of these welds.Figure 6-3—Butt Welding of Plates of Unequal Thickness Weld (see note 1)

Panel a Preferred (Center lines

coincide) Panel b Permissible Panel c Preferred Taper may be inside or outside Hemispherical shape

	



! 	#$

&&&

'
&&&&''
&!   

Table 6-4—Stress-relieving Temperatures and Holding Times
Metal Temperature (° Fahrenheit) Holding Time (hours per i n. of thickness )
1 100 1 1050 2 1000 3
950 5 900 (minimum) 10 D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 6-9

NOTE For intermediate temperatures, the heating time shall be determined by straight line interpolation.7-1 

### SECTION 7—INSPECTION, EXA MINATION, AND TESTING 7.1 Responsibility of Inspector 7.1.1 The inspector shall ensure that all materials used in ta nks constructed according to the rules in this standard comply in all respects with the requirements of these ru les. This shall be done either by witnessing mill tests or verifying that the materials to be used are properly ident ified in the certified mill test reports supplied by the Manufacturer. 7.1.2 Tanks constructed according to the rules in this st andard shall be inspected and tested in accordance with the sections that follow. The inspector s hall carefully follow the fabrication and testing of each tank and shall make sure that they comply in all details with the design, fabrication, and tests specified in these rules. 7.1.3 All nondestructive examination shall be reviewed and approved by the Inspector. 

### 7.2 Qualifications of Inspectors 7.2.1 Inspectors for tanks constructed according to the rule s in this standard shall have had not less than 5 years experience in design, construction, main tenance and/or repair, or in the responsib le supervision of the construction, maintenance and/or repair of various ty pes of unfired pressure ve ssels and/or tanks, including at least 1 year of experience in the construction or supervi sion of the construction of vessels or tanks by fusion welding. Satisfactory completion of a suitable course of training approved by the Purchaser or the Purchaser’s agent may be substituted for 3 of the 5 years experience. However, training cannot replace more than 6 months of the required experience on fusion-welding construction. 7.2.2 Inspectors shall be employed by the Purchaser or by an organization regularly engaged in making inspections. The Inspector is the representative of the Purchaser. 7.2.3 The Manufacturer shall also provide inspection to hel p ensure that all requirements of these rules have been met before signing the certificate and Manufacturer’s report (see 8.3). 

### 7.3 Access for Inspector The inspector shall be permitted free access to all parts of the plant concerned with the manufacture of the tank during fabrication and to all parts of the plants of mate rial suppliers who are conc erned with the manufacture of materials to be used in the tank. 

### 7.4 Facilities for Inspector The Manufacturer shall afford the Inspector all reasonable facilities for testing and inspection and shall provide mutually agreeable advance notification to permit the Inspector to witness al l tests of the equipment and materials during fabrication, including all laborat ory tests of the material to be used and all hydrostatic and pneumatic tests at the site of erection. 

### 7.5 Approval of Repairs Approval by the inspector shall be required before and after any defects are repaired. Defe ctive material that cannot be satisfactorily repaired shall be rejected (see 6.16 for repair of defects in welds). 

### 7.6 Inspection of Materials The plates and other material for parts that will be subj ected to pressure-imposed l oads shall be inspected before being incorporated in the tank. Particular attention shall be given to all cut edges to ensur e that the material is free from serious laminations and other defects.7-2 API S TANDARD 620


### 7.7 Stamping of Plates Before plates required to be stamped by the steel mill are us ed, the inspector shall see t hat they bear the stamp. In laying out and cutting the plates, at least one set of the origin al material identific ation markings should, if possible, be left where it will be plainly visible when the tank is co mpleted. Should the identifyi ng marks be obliterated, one set shall be accurately transferred by the tank Manufacturer to a location that will be visible on the completed tank, or a coded marking shall be used to ensure identification of each piece of material during fabrication and subsequent identification of the markings on the completed tank. These latter markings shall be readi ly distinguishable from the mill markings. The inspector need not witness the transfer of the markings but shall be satisfied that the transfer of the markings has been made correctly. Care should be taken not to damage the plate by stamping the figures too deeply. To guard against incipient crac ks in plates less than 1 / 4 in. thick, the mill markings shall be transferred in some manner other than by die stamping. 

### 7.8 Measuring Thickness of Material All material shall be gauged or measured to determi ne whether the thickness meets the requirements. 

### 7.9 Inspection of Surfaces Exposed during Fabrication 7.9.1 The edges of plates, openings, and fitt ings exposed during fabrication shal l be inspected carefully to make sure that any defects have been uncovered, as well as to determine that the work has been performed properly. 7.9.2 Minor defects found may be repaired only after the ins pector approves the method an d extent of rep airs. Materials that have more than minor defects that cannot be satisfactorily repaired shall be rejected. 

### 7.10 Surface Inspection of Component Parts Before assembly, all sidewall plates or sections and roof and bottom plates shall be inspected for thickness, freedom from injurious defects, and soundness of any welded joints. 

### 7.11 Check of Dimensions of Component Parts All formed plates and curved sections shall be checked for conformance with the planned dimensions and cross section. For unusual repairs the inspector should keep a re cord of measurements taken at sufficient intervals to constitute a satisfactory record. 

### 7.12 Check of Chemical and Physical Property Data The Inspector shall check the material being assembled by the lists of the plates from the mill, their heat numbers, chemical analyses, and mechanical propertie s as given on mill reports and shall s ee that copies are available to be attached to the Manufacturer’s report (see 8.3). 

### 7.13 Data Required from Manufacturer on Completed Tanks If specified in the purchase order, the Manufacturer shal l supply marked copies of pl ans (or a separate sketch) showing the location of all pl ates, with a means of identif ying each plate with the heat num bers. These markings shall be checked by the inspector. A copy shall be attached to the Manufacturer’s report. 

### 7.14 Check of Stress-relieving Operation The inspector shall check any thermal stress-relieving oper ation and shall be satisfied t hat the temperature readings are accurate and that the procedure conforms to the applicable requirements of these rules.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 7-3


### 7.15 Examination Method and Acceptance Criteria 7.15.1 Radiographic Method 7.15.1.1 Except as modified in this section, the radiographic examination metho d employ ed shall be in accordance with Section V, Article 2 of the ASME Code . The requirements of T-285 in Section V, Article 2, are to be used only as a guide. Final acceptance of radiographs shall be based on the ability to see the prescribed image quality indicator (IQI) (penetrameter) and the specified hole or wire. The finished surface of the reinforcement at the location of the radiograph shall either be flush with the plate or have a reasonably uniform crown that does not exceed the values listed in Table 7-1. Table 7-1—Maximum Thickness of Reinforcement on Welds for Radiography Examined Joints Dimensions in inches.
Plate Thickness Maximum Thickness of Reinforcement
≤ 1 / 2 1 / 16
> 1 / 2 – 1 3 / 32

> 1 1 / 8

7.15.1.2 Before any welds are repaired, the radiogr aphs shall be submitted to the inspector. 7.15.1.3 The acceptability of welds examined by radiograph y shall be judged by the standards of Paragraph UW

51(b) in Section VIII of the ASME Code

. 7.15.1.4 Sections of welds that radiography has revealed to be unacceptable shall be repaired in accordance with 6.15 and re-radiographed repeating the original radiograph fr equency and with examination procedure in accordance with 7.15.1.1. Their acceptability shall be determined by the standards of 7.15.1.3. 7.15.1.5 After the structure is completed, the films shall be the property of the Purchaser un less otherwise agreed upon by the Purchaser and the Manufacturer. 7.15.1.6 Personnel who perform and evaluate radiographic examinati on according to this se ction shall be qualified and certified by the Manufacturer as meeting requirements of certification as generally outlined in ASNT SNT-TC-1A (including applicable supplements), Level II or Level III. Level I personnel may be used if they are given written acceptance/rejection procedures prepared by Level II or III personnel. This written procedure shall contain the applicable requirements of Section V, Article 2, of the ASME Code . In addition, all Level I personnel shall be under the direct supervision of Lev el II or III personnel. 7.15.2 Magnetic-particle Method Examination 7.15.2.1 When magnetic-particle examination is specified, the method of exami nation shall be in accordance with Section V , Article 7 of the ASME Code

. 7.15.2.2 Magnetic-particle examination shall be performed in accordance with a written procedure, which shall be in accordance with the requirement s of T-1 50, Section V, Article I of the ASME Code

. 7.15.2.3 The Manufacturer shall determine that each magnetic-par ticle examiner meets the following requirement s. a) The examiner has vision with correction, if necessary, to be able to read a Jaeger-Type No. 2 St and ard Chart at a distance of not less than 12 in. and is capable of disti nguishing and differentiating contrast between colors used. These requirements shall be checked annually.7-4 API S TANDARD 620 b) The examiner is competent in the techniques of the magnetic-particle ex amination method for which the examiner is certified, including making the ex aminations and interpreting and evaluati ng the results; however, where the examination method consists of more than one operation, the examiner may be certified as being qualified only for one or more of these operations. 7.15.2.4 The acceptance standards, defect removal, and repair shall be in accordance with Section VIII, Appendix 6, Paragraphs 6-3, 6-4, and 6-5 of the ASME Code

. 7.15.3 Ultrasonic Examination Method 7.15.3.1 Ultrasonic Examination in Lieu of Radiography When ultrasonic examination is app lied in order to fulfill the requirements of 5.26, Q.5.6, or R.5. 6, the provisions of Annex U shall apply. 7.15.3.2 Ultrasonic Examination NOT in Lieu of Radiography 7.15.3.2.1 When the radiographic method is applied in order to fulfill the requirements of 5.26, any ultrasonic examination specified shall be in accordance with this section. 7.15.3.2.2 The method of examination shall be in accordanc e with Section V, Arti cle 4, of the ASME Code

. Acceptance standards shall be by agreement between Purchaser and Manufacturer. 7.15.3.2.3 Ultrasonic examination shall be perform ed in accordance with a written pr ocedure that is certified by the Manufacturer to be in compliance with the applic able requirements of Section V of the ASME Code

. 7.15.3.2.4 Examiners who perform ultrasonic exam ination under this section shall be qualified and certified by the Manufacturer as meeting the requirements of certificat ion as generally outlined in ASNT SNT-TC-1A (including applicable supplements), Level II or Level III. Level I personnel may be used if they are given written acceptance/ rejection criteria prepared by Level II or III personnel . In addition, all Level I personnel shall be under the direct supervision of Level II or III personnel. 7.15.4 Liquid-penetrant Examination Method 7.15.4.1 When liquid-penetrant examination is specified, the method of examination shall be in accordance with Section V, Article 6, of the ASME Code

. 7.15.4.2 Liquid-penetrant examination shall be performed in accordance with a written procedure certified by the Manufacturer to be in compliance with applic able requirements of Section V of the ASME Code

. 7.15.4.3 The Manufacturer shall determine and certify that each liquid-penetrant examiner meets the following requirements. a) The examiner has vision with correction, if necessary, to be able to read a Jaeger-Type No. 2 Standard Chart at a distance of not less than 12 in. and is capable of disti nguishing and differentiating contrast between colors used. These requirements shall be checked annually. b) The examiner is competent in the te chniques of the liquid penetrant exami nation method for which the examiner is certified, including making the examination and interp reting and evaluating the results; however, where the examination method consists of more than one operation, the examiner may be certified as being qualified only for one or more of these operations. 7.15.4.4 The acceptance standards, defect removal, and repair shall be in accordance with Section VIII, Appendix 8, Paragraphs 8-3, 8-4, and 8-5 of the ASME Code

.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 7-5
7.15.5 Visual Examination Method 7.15.5.1 The Manufacturer shall determine and certify that each visual examiner meets the following requirement s. a) Has vision (with correction, if neces sary) to be able to read a Jaeger Type 2 standard chart at a distance of not less than 300 mm (12 in.) and is capable of passing a co lor contrast test. Examiner s shall be checked annually to ensure that they meet this requirement. b) Is competent in the technique of the visual examination, including p erformi ng the examination and interpreting and evaluating the results; however, w here the examination method consists of more than one operation, the examiner performing only a portion of the test need only be qualified for the portion that the examiner performs. 7.15.5.2 All welds shall be visually examined in acco rdance with 7

. 15.5.3 and 7.15.5.4. 7.15.5.3 A weld shall be acceptable by visual exam ination if examination shows the following. a) The weld has no crater cracks or other surface cracks. b) Undercut does not exceed the applicable limit in 6.13. c) The frequency of surface porosity in welds does not exceed one cluster (one or more pores) in each 4 in. o f length, and the maximum diameter of each cluster does not exceed 3 / 32 in. d) Complete fusion and required penetration exists at the joint between the weld metal and the base metal. 7.15.5.4 Welds that fail to meet the visual examination criteria of 7.15.5.2 shall be rewor k ed before hydrostatic testing in accordance with the following. a) Defects shall be repaired in accordance with 6.16. b) Rewelding shall be required if the re sulting thickness is below the mini mum required for design and hydrostatic test conditions. All defects in areas above the minimum thickness shall be feathered to at least 4:1 taper. c) The repair weld shall be examined visually for defects. 7.15.6 Examination Method for Spot Radiographic/Ultrasonic Examination 7.15.6.1 The procedure prescribed in 7.15.1.1 shall be follow ed as close ly as is practicable when the spot examination is made by radiography. A spot radiograph s hall not be considered equivalent to a recheck where complete radiography is mandatory and applied. 7.15.6.2 Spot radiographic or ultrasonic exam ination shall be not less than 6 in. extending along the weld. Spot radiography shall comply with the standards given in 7.15. 1.3. Where spot radiographic or ultrasonic examination is applied at joint intersections, the surface shall be prepar ed and examined for a distance of 3 in. on each side of the intersection, making the minimum length of examination 6 in. on the horizontal weld and 3 in. on the vertical weld. 7.15.6.3 Retest radiographs prescribed in 7.17. 4, when required, shall comply wi th the st andards of acceptability given in 7.15.1.3. Spot radiographs or ultrasonic records may be discarded af ter the tank has been accepted by the inspector, unless previously requested by the Purchaser. 

### 7.16 Examination of Welds NOTE Annex P summarizes the requirements by method of examination and provides the acceptance st andards, Examiner qualifications, and procedure requirements. Annex P is not intended to be used alone to determine the examination requirements for work covered by this document. The spec ific requirements as listed in Sections 1 through 9, and Annexes Q, R, S, and X shal l be followed in all cases.7-6 API S TANDARD 620
7.16.1 Butt-welds Complete penetration and complete fusion is required (to the degree mandated by the acceptance criteria for examination method utilized) for welds join ing tank wall plates to tank wall plates. Examination for quality of welds shall be made using either the radiogr aphic method specified in 7.15.1 and app lied in 7.17, or alternatively, by agreement between the Purchaser and the Manufacturer, usi ng the ultrasonic method s pecified in 7.15.3.1. In addition to the radiographic or ultrasonic examination, these welds shall also be visually examined as specified in 7.15.5. Furthermore, the Purchaser’s Ins pector may visually inspect all butt-weld s for cracks, arc strikes, excessive undercuts, surface porosity, incomplete fusion, and other defects . Acceptance and repair criteria for the visual method are specified in 7.15.5. 7.16.2 Fillet Welds Fillet welds shall be examined by the visual method. Ac ceptance and repair criteria are specified in 7.15.5. 7.16.3 Permanent and Temporary Attachment Welds 7.16.3.1 Permanent attachments are items welded to the tank wall that will remain while the tank is in its intended service. This does not include openings such as nozzles, manholes, and flus h type cleanouts. It does include items such as wind girders, stairs, gauging systems, davits, ri ser pipe supports, tank anchors, walkways, supports for internal items such as heating coils or other piping, l adders, floating roof supports welded to the shell wall, and electrical conduit and fixtures. Items installed above the maximum liquid le vel of the tank are not permanent attachments. The weld connecting the permanent attachment to the tank surface shall be examined visually and by the magnetic particle method (or at the option of the Purchaser, by the liquid penetrant method). Refer to Section 7 for the appropriate examination criteria. 7.16.3.2 Temporary attachments are items welded to the tank wall that will be removed prior to the tank being utilized in its intended service. These are usually construction items such as alignment clips, scaffolding clips, stabilizers, fitting equipment, and lifting clips. The area from which a te mporary attachment is removed shall be examined visually for any indication of flaws requiring repa ir. Additionally, on any tank material listed in Table 4-1 at –20 °C (5 °F) and greater than 12 mm ( 1 / 2 in.) thick, and on all materials listed in Table 4-1 at –37 °C (–35 °F), shall be examined by the magnetic particle me thod (or at the option of the Purcha ser, by the liquid penetrant method). Refer to Section 7 for the appropriate examination criteria. 7.16.4 Examination of Welds Following Stress Relieving After any stress relieving, but before hydrostatic testi ng of the tank, welds attach ing nozzles, manholes, and cleanout openings shall be examined visually and by the magnetic particle method (or at t he option of the Purchaser, the liquid penetrant method). Refer to 7.15.2, 7.15.4, or 7.15.5 for the appropriate examination and repair criteria. 7.16.5 Responsibility The Manufacturer shall be responsible for examinations and any necessary repairs; however, if the Purchaser’s inspector requires examinations in exce ss of the number specified in 7.17 or requires chip-outs of fillet welds in excess of one per 100 ft of weld and no defect is disclosed, the additional ex amination and related work shall be the responsibility of the Purchaser. 

### 7.17 Radiographic/Ultrasonic Examination Requirements 7.17.1 Application 7.17.1.1 Any butt-welded joint in the wall of any tank to whic h these rules apply, and for which complete examination is mandatory under 5.26, shall be examined throughout its ent ire length by the radiographic or ultrasonic method as prescribed by the following sections. Any butt-welded join t for which complete examination would not be mandatory under 5.26.3 shall be similarly examined if the procedur e becomes mandatory in the application of 5.26.4.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 7-7

7.17.1.2 If radiographic or ultrasonic examinati on is considered impractical for the final (or closing-up) joint because of the location or construction of t hat joint, magnetic-particle examination may be substituted for radiographic or ultrasonic examination of the joint if the substitute procedure is applied at stages of the weld ing acceptable to the inspector and it indicates that the joint is sound. In no case shall this exception be interpreted to apply because equipment suitable for making the radiograph ic or ultrasonic examination is not available, or is not in a usable condition. 7.17.1.3 All such welded joints on which backing strips are to remain shall be exam ined by the magnetic-p a rticle method after the first two layers, or beads, of weld metal have been deposited and again after the joint has been completed. 7.17.2 Spot Examination of Welded Joints 7.17.2.1 For all butt-welded main joints (see 5.26.5.2) that are not co mpletely exami ned, spot examination is mandatory an d shall be done according to the pr ocedure and st andards of 7.15.6 and 7.17.4, except roof joints exempted by Table 5-2. 7.17.2.2 Spot examination need not be made of welds in structur al steel m em be rs unless s pecifically requested by the inspector. The method used shall be subject to agreement between the Manufacturer and the inspector. 7.17.3 Number and Location of Spot Examination 7.17.3.1 In all cases in which spot examination is mandat ory under 7.17.2.1, the number and location of spot s examined in longitudinal or meri dional joint s and in equivalent circumferential or latitudinal joints as defined in Table 5- 2, Note 5, shall conform to the r equirements of 7.17.3.2 through 7.17.3.4. 7.17.3.2 At least one spot shall be examined from the first 10 f t of completed joint of each type and thickness 36 welded by each welder or welding operator. Thereafter, without regard to the number of welders or welding operators involved, one additional spot shall be ex amined for each additional 50 ft—or remain ing fractional part of this length— of each type and thickness of welded l ongitudinal, meridional, or equivalent jo int subject to examination. The inspector shall designate the locations of all spot s that are to be examined, of which at least 25 % of the selected spot s shall be at junctions of m e ridional and latitudinal joints with a minimum of two such intersections per tank (see 7.15.6.2), both under the foregoing provisi ons and the provisions of 7.17.3.4. Su ch spots need not have any regularity of spacing. 7.17.3.3 If more than one welding procedure is used or if more than one welder or welding oper ator does the welding, at least one spot shall be examined for each procedure and for each welder or welding operator. Any spot examined may coincidentally represent one procedure, one welder or welding operator, and one interval of 50 ft of joint length. The same welder or welding operator may or may not weld both sides of the same butt joints; therefore, it is permissible to test the work of two welders or weld ing operators with one spot exam ination if they weld opposite sides of the same butt joint. When a spot of this type is rejected, further tests shall determine whether one or both welders or welding operators were at fault. 7.17.3.4 Whenever spot examination is requir ed for circumferential or latitudinal joint s other than those considered in 7.17.3.2 and 7.17.3.3, one spot shall be examined from the first 10 ft of completed jo int of each type and thickness (see Footnote 36) welded by each welder or welding operator if not already done on other joints for the same welder or welding operator on the same structure. Thereafter, wi thout regard to the number of welders or welding operators working, one additional spot shall be examined in each a dditional 100 ft (approximately) and any remaining fraction thereof of each type and thickness of welded circumferentia l or latitudinal joints of the kind considered in 7.17.2.

36 This is based on the thickness of the thinner plate at the joint. For the purpose of th is application, plates shall be consider ed of the same thickness when the difference in the specified or design thickness does not exceed 1 / 8 in.7-8 API S TANDARD 620

7.17.4 Spot-examination Retests 7.17.4.1 When a spot has been examined at any location select ed in accordance with 7.17.2 and the welding doe s not comply with the st andards prescribed in 7.15.1.3 fo r radiographic examination, or Annex U for ultrasonic examination, two additional spots shall be examined in the same seam at locations to be selected by the inspector— one on each side of the original spot—to determine the limits of the potentially deficient welding. If any welding is found at either spot that fails to co mply with the minimum quality requirement s in 7.15.1.3 radiographic examination, or Annex U for ultrasonic examination, additional spots nearby shall be exam ined until the limits of unacceptable welding are determined. In addition, the inspector may re quire that an additional spot be examined at one location selected by the inspector in each seam not previously examined on which t he same operator has welded. If any additional spot fails to comply with the minimum quality requirements, the limits of unacceptable welding shall be determined as in the original examination. 7.17.4.2 All welding within the limit for s pot examination found to be below t he standards required in 7.15.1.3 for radiographic examination, or Annex U for ultrasonic exam ination, shall be rejected. The rejected weld shall be removed and the joint shall be rewelded, or at the Manufacturer’s option, the entire unit of weld represented shall be completely examined and only the de fective welding need be corrected. 

### 7.18 Standard Hydrostatic and Pneumatic Tests 7.18.1 General After erection is completed and stress relieving, radiogr aphic examinations, or other similar operations, as may be req uired, are performed, each tank shall satisfactorily pass a series of hydrostatic and pneumatic tests as prescribed in 7.18.2 through 7.18.6. Whenever a solution film is specifi ed in this section to be applied to welding, linseed oil or another equivalent material for disclosing air leakage may be s ubstituted. In freezing weather, linseed oil or a similarly suitable material shall be used. 7.18.2 Test Preliminaries 7.18.2.1 Before water is introduced into the tank, the prelim inar y operations described in 7.18.2.2 thr ough 7.18.2.6 shall be performed. 7.18.2.2 The attachment welding around all openings and their re inforcement s in the walls of the t ank shall be examined by the magnetic-particle method both inside and outside the tank. When the underside of a tank bottom rests directly on the tank grade (and is not accessible after erection), such examination of welding on the underside of the bottom and subsequent air testing may be omitted. However, such examinat ion and testing of any openings in the bottom plates shall be done before these plat es are placed in position on the tank grade. 7.18.2.3 Following the examination specified in 7. 18.2.2, air at a pressure of 15 lbf/in. 2 gauge (or, if the parts involved cannot safely withstand this pressure, as near this pressure as the parts should safely withstand) shall be introduced between the tank wall and the reinforcing plate, saddle flange, or integral re inforcing pad on each opening, using the telltale holes specified in 5.16.10. While each space is subject to the pressure, a solution film shall be applied to all attachment welding around the re inforcement, both inside and outside the tank. 7.18.2.4 In cases in which the bottom of the tank rests di re ctly on the tan k grade (preventing access to the underside of the bottom of the tank), all joints between the bo ttom plates shall be tested on the inside of the tank by applying a solution film to the joints and pulli ng a partial vacuum of at least 3 lbf/in. 2 gauge by means of a vacuum box with a transparent top. As an alternate to vacuum box te sting, a suitable tracer gas and compatible detector can be used to test the integrity of welded bottom joints for their entire length if an appropriate tracer gas testing procedure has been reviewed and approved by the Purchaser. 7.18.2.5 Tanks with anchors shall be grouted (if required by design) and anchor retainers shall be attached.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 7-9
7.18.2.6 After all the welding has been examined and tested and all defective welding disclosed by such examination and testing has been repaired and retested, the t ank shall be filled wi th air to a pressure of 2 lbf/in. 2 gauge or one-half the pressure P g

for which the vapor space at the top of t he tank is designed, whichever pressure is smaller. A solution film shall be applied to all joints in t he tank wall above the high liquid (c apacity) design level. If any leaks appear, the defects shall be removed and rewelded, and the applicable preliminary tightness tests specified shall be repeated. When anchors are not provided near the bounda ry of contact to hold down a dished tank bottom resting directly on the t ank grade, the bottom at this boundary may be ri se slightly off the foundation during the tightness test when air pressure is in the tank. In this case, sand shall be tamped firmly under the bottom to fill the gap formed while the tank is under pressure (see 7.18.8). 7.18.3 Combination Hydrostatic-pneumatic Tests 7.18.3.1 Tanks that have not been designed to be filled with liquid to a test level higher than their specified cap acity level (see 5.3.1.2) shall be subjected to combination hydr ostatic-pneumatic pr essure tests in accordance with the procedure described in 7.18.3.2 through 7.18.3.5. 7.18.3.2 After the preliminary tightness tests specified in 7. 18.4 have been completed, the pr essur e-vacuum relief valve or valves shall be blinded off. With the top vented to the atmosphere to prevent accumulation of pressure, the tank shall be filled with water to its high liquid (capacity) desi gn level (see 7.18.7). Tank anchor retainers shall be adjusted to a uniform tightness after the t ank is filled with water. If the pr essure-vacuum valve or valves are not available at the time of the test, the tank connections may be blinded off and the test procedure continued by agreement between the Purchaser and the Manufacturer. With the vents at the top of t he tank closed, air shall be injected slowly into the top of the tank until the pr essure in the vapor space is about one-half the pressure P g , for which this space is designed. The air pressure shall be increased slowly until the pressure in the vapor space is 1.25 times the pressure, P g , for which the space is designed. 7.18.3.3 An air test introduces some hazard. In view of the large amount of air that will be present in the t ank during this test, no one should be permitted to go near the tank while pressure is being applied for the first time during this test. While the pressure in the tank exceeds the pressu re for which the vapor space is designed, the inspections should be made at a reasonable distance from the tank usi ng field glasses as required for close-up observation of particular areas. 7.18.3.4 As the pressure is being increased, the tank shall be inspected for signs of di st ress. The maximum test pressure of 1.25 times the vapor space design pressure s hall be held for at least one hour, after which the pressure shall be released slowly and the blinds s hall be removed from the pressure-vacuum relief valves. The operation of the relief valves shall then be checked by inje cting air into the top of the tank until the pressure in the vapor space equals the pressure, P g , for which this space is designe d, at which time the relief valves shall start to release air. 7.18.3.5 While this latter pressure is held, a solution film shall be applied to all of the welding involved above the high liquid (capacity) design level for which the tank is designed. A prior vac uum box check may be substituted for the close visual with solution-film examination. The solution-fi lm examination shall still be ma de, above the liquid level, on all welds around openings, all piping joints, and the compression ring welds to the roof and shell, except the prior vacuum box is permitted for any listed below.

— Continuous double lap roof to compression ring welds.

— Shell to compression ring welds, continuous inside and out side, and applying a thickened upper shell ring detail similar to Figure 5-6 details f or f-1. T he thickened upper shell ring shall be greater than 1 / 2 of the conical compression ring thickness and greater than tw o times the adjacent shell ring thickness.

— Full fusion butt-welded connections.7-10 API S TANDARD 620
7.18.4 Complete Hydrostatic Tests 7.18.4.1 Tanks that have been designed and constructed to be filled with liquid to the top of the roof (see 5.3.1.2) shall be subjected to full hydrostatic tests in accordance wi th the procedure prescribed in 7.18.4.2 and 7.18.4.4, in lieu of the procedure specified in 7.18.3. 7.18.4.2 Following the test preliminaries called for in 7.18.2, the p ressure-vacuum r elief valve or valves shall be blinded off; with the top of the tank vented to the atmosphere, the tank shall be filled with water to the top of the roof (see 7.18.7) while allowing all air to es cape to prevent the accumulation of pressure. If the pressure-vacuum relief valve or valves are not available at the time of the test, the tank connections may be blinded off and the test procedure continued by agreement between the Purchaser and the Manufacturer. The vents used during water filling of the tank shall then be closed, and the pressure in the tank shall be increased slowly until the hydrostatic pressure under the topmost point in the r oof is 1.25 times the pressure, P g , which the vapor space is designed to withstand when in operation with the tank filled to its specified high liquid (capacity) level. 7.18.4.3 This test procedure shall be held for at least one hour. The hydr ostatic pr essur e under the topmost point in the roof shall then be reduced to the pressure, P g , for which the vapor space is designed and shall be held at this level for a sufficient time to permit close visual inspection of all joints in the walls of the tank and all welding around manways, nozzles and other connections. 7.18.4.4 The tank shall then be vented to atmosphere, the wate r level shall be lowered below the inlet s to the pressure-relief valves, and the blinds shall be removed from t he relief valves. The operation of the relief valves shall then be checked by injecting air into the top of the tank until the pressure in the vapor space equals the pressure, P g , for which this space is designed, at which time the relief valves shall start to release air. 7.18.5 Partial-vacuum Tests 7.18.5.1 Following the tests specified in 7.18.3 (or in 7. 18.4) whe re this latter procedure has been used), the pressure in the vapor space of the tank shall be rel eased and a manometer shall be connected to this space. The ability of the upper part of the tank to withstand the pa rtial vacuum for which it is designed and the operation of the vacuum-relief valve or valves on the tank shall then be che cked by withdrawing water from the tank, with all vents closed, until the design partial vacuum is developed at the top of the tank and by observing the differential pressure at which the valve or valves start to open. The vacuum-relief va lve or valves must be of a size and be set to open at a partial vacuum closer to the external atmospheric pressure than the partial vacuum for which the tank is designed. The partial vacuum in the tank should ne ver exceed the design value (see Annex K). 7.18.5.2 After completing 7.18.5.1, the withdrawal of water fr om the tan k shall be continued, with the vents closed and without exceeding the specified maximum partial vacuum in the top of the tank, until the level in the tank reaches one-half the high liquid (capacity) level for which the tank is designed. Alternatively, to speed up the withdrawal of water to the degree thought expedient, the vents may ei ther be kept closed and air pressure not exceeding P g

at the top of the tank applied, or the vents may be opened during most of this interval if in ei ther procedure they are closed long enough before the level in the tank reaches half height for the specified partial vacuum to be developed by the time the level of the water reaches half height. 37 Air shall then be again injected into the tank until the pressure above the water level equals the pressure, P g , for which the vapor space at t he top of the tank is designed. 7.18.5.3 Careful observation shall be made under all of the spec ified conditions of loadi ng, as well as with atmospheric pressure above the surfac e of the water when the level is at half height, to determine whether any appreciable changes occur in the shape of the tank (see 7.18. 8). In the case of a vertical tank with cylindrical

37 These provisions presuppose that an ejec tor or vacuum pump is not available fo r drawing a partial vacuum on the tank. However, if such equipment is available, it may be used; vents may be opened during the entire period while the water level is being lowered; and the sequence of the vacuum and pressure test may be reversed if either the tank Manufacturer or the Purchaser so selects.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 7-11 sidewalls, no tests are required with the wa ter level at half height; in this case, the tests specified in 7.18.5.4 shall be applied immediately after the first va cuum test specified in 7.18.5. 7.18.5.4 The water remaining in the tank shall then be withdr awn and when the tank is substantially empty, a vacuum test comparable to that specifi ed in 7.18.5.1, except with regard to t he level of water in tank, shall be applied to the tank. After this, air s hall again be injected into the tank until t he pressure in the tank equals the pressure, P g , for which the vapor space at the top of the tank is designed. Observations shall be made, both with the specified partial vacuum and with the vapor space design pressure above the surface of the water, to determine whether any appreciable changes in the shape of the t ank occur under either condition of load ing. In the case of a tank whose dished bottom rests directly on the tank grade, if the bottom rises slightly off the foundation during the pressure test, sand shall be tamped firmly under the botto m to fill the gap formed while the tank is under pressure (see 7.18.2.6 and 7.18.8). 7.18.6 Visual Inspection Upon completion of all the foregoing tests, the pressure in the t ank shall be released and a thorough visual inspection shall be made of both the inside and outside of the tank, giving particular attention to all internal ties, braces, trusses, and their attachments to the walls of the tank. Anchors s hall be checked for snug tight ness and adjusted if required. Anchor threads shall be fouled by peening or tack welding to prevent loosening. In lieu of thread fouling, double nuts may be used. 7.18.7 Rate of Water Filling and Water Temperature The rate at which water is introduced into a tank for a hy drost atic test shall not exceed 3 f t of depth per hour. The foundation, venting equipment, or other c onditions may limit the water filling to a lower rate. Pressure shall not be applied above the surface of the water before the tank and its contents are at about the same temperature. The temperature of the water used in the tests should be not less than 60 °F whenever practicable. 7.18.8 Changes in Tank Shape If in any of the foregoing tests there is an excessive rise of the b ottom of the tank around the boundary of contact with grade, or off its foundations, or if an y of the specified conditions of test loading c ause other appreciable changes in the shape of the tank, the design shal l be reviewed and means shall be provi ded in the tank for holding the shape within permissible limits under all conditions of loading. 7.18.9 Additional Tests The tests prescribed in 7.18 are believed to be sufficient for most t anks constructed accordin g to these rules; if, in the opinion of the designer, additional tests are needed to investigate the safety of a tank under certain other conditions of loading, as determined from the design computations, these tests shall be m ade on the tank involved in addition to the tests specified in this standard. 7.18.10 Tanks Subject to Corrosion In the case of tanks that are subject to corrosion on some or all of their wall pl ates or on internal ties, braces, or other members that carry pressure-imposed loads, the test specified in 7.18.3 (or the test specif ied in 7.18.4, if applicable) should be repeated periodically during the lives of the t anks as the metal added for corrosion allowance disappears. 

### 7.19 Proof Tests for Establishing Allowable Working Pressures 7.19.1 General Because pressures in liquid storage tanks built according to these rules vary quite markedly fr om the tops to the bottoms of the tanks, proof testing of these tanks presents problems not us ually encountered in the construction of7-12 API S TANDARD 620 unfired pressure vessels–especially where the parts under investigation are located near the bottoms of the tanks. The principal difficulty is devis ing a test or series of tests that will reli ably establish the working pressure that can be permitted on the part of the unproven desi gn without, at the same time, imposi ng hazardous conditions on other parts located at higher levels in the tank. A nother possible complication is that, because of the large volumetric capacities of these tanks, it may not be practicable to completely remove all pressure loading from the part under investigation in order to obtain strain-gauge readings under no-load conditi ons after successive incr ements of pressure have been applied in the test procedure. Also, in the case of tan ks designed for storing only gases or vapors, water cannot be used as a testing medium. 7.19.2 Use of Design Rules The design rules and formulas given in t he design se ction of these rules will be found to cover all of the more common designs of ver tical tanks, shapes of openings, and so forth. The absence of a standard proof-test procedure will not greatly affect the usefulnes s of these rules. Whether a standard proof-t est procedure can be devised that will be applicable to all shapes, sizes and types of tanks that might be constructed under t hese rules is not known, but it is recognized that in special cases a Manufacturer may be able to propose a proof-test procedure that would be satisfactory for a particular tank (see 7.19.3). 7.19.3 Developing Proof Tests 7.19.3.1 Pending development of an approved standard proof-test procedure, whenever a M anufacturer desires to construct a t ank that will be marked as specified in 8.1 and embodies any features that s hould be proof-tested because of the provisions of 5. 1.5 and 5.13.5.3 or any other prov isions of these rules that ca ll for proof test or straingauge surveys, the Manufacturer shall dev elop specifications for an appropriate proof-test procedure for the tank and obtain approval from the Purchaser or the Purchaser’s agent, preferably before st arting fabrication of the tank. Such specifications shall cover all important details of the proposed proof-test procedure, including but not necessarily limited to a description of how the tank would be prepared fo r the test, how the test l oadings would be applied, what medium would be used for the test, the increments in which the loadings would be applied, what kind of data would be taken, how the test results would be interpreted, and the basis upon which allowable working pressures would be established for the part or parts under investigation. In seek ing approval of such a test, the Manufacturer shall furnish full information concerning the general construction of the proposed tank, the design and location of the part or parts of uncertain strength, the conditions of loading to which t he tank would be subjected in service, and other pertinent matters. 7.19.3.2 In case the Purchaser or the Purchaser’s agent does not approve a special proof-t est proce dure, the tank in question shall not be marked as specified in 8.1, nor shall a tank be so marked after failing to satisfactorily pass a special proof test that has been so approved, unless the tank is strengthened in a manner acceptable to the inspector and is then retested and satisfactorily passes the second proof test. 

### 7.20 Test Gauges 7.20.1 An indicating gauge shall be connected directly to the topm ost part of the roof on t he tank under test. In the case of a tank which is designed for the storage of gases or vapors alone and is to be tested only with air, the gauge may be connected to the tank at some lower level. If the in dicating gauge is not readily visible to the operator who is controlling the pressure applied, an additional indicating gauge shall be provided where it will be visible to the operator throughout the test. Means shall be provided to ensur e that the required test pr essure will not be exceeded. 7.20.2 A recording gauge shall also be used on each tank, and a record shall be kept of the pressures dur ing al l st ages of the tests. This gauge shall be connected either to the piping that leads to the i ndicating gauge or directly to the tank at a point near the indicating gauge connection. 7.20.3 Indicating gauges used during the tests shall be calib rated against a stand ard deadw eight tester before the tests are started.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 7-13
7.20.4 If at any time during a test there is reason to believe that a gauge is in error, it s calibration shall be checked. If the gauge is in error, it should preferably be adjusted to read correctly, or a calibration curve may be made to indicate the correct pressures for the readings indicated by the gauge. 7.20.5 In all cases in which a gauge is mounted at a level lo wer than it s connection to the tank or lower tha n some p art of the piping that leads to the gauge, suitable precauti ons shall be taken to prevent accumulation of any static head of condensed moisture (or water from other sources) in the piping leads above the level of the gauge. Not preventing this would result in erroneous readings.8-1 

### SECTION 8—MARKING
8.1 Nameplates 8.1.1 A tank made in accordance with this standard shall be identified by a nameplate si milar to that shown in Figure 8-1. The nameplate shall indicate, by means of letters and numerals not less tha n 5 / 32 in. high, the following information. a) API Standard 620. b) Applicable Annex. c) Year completed. d) Applicable edition and revision number of this publication. e) Nominal diameter and nominal height, in ft and in. 38 f) Nominal capacity, in barrels of 42 gallons per barrel. 38 g) Design liquid level, in ft and in. 38 h) Design specific gravity of liquid. i) Maximum test level for hydrostatic test with water, in ft and in. 38 j) Design pressure for gas or vapor spac e at the top of the tank, in lbf/in. 2 gauge. 38 k) Design metal temperature, in °F. 38 Use the lower of the following temperatures: 1) the temperature described in 4.2.1, or 2) the minimum design temperature of product storage gi ven by the Purchaser for refrigerated product t anks. l) Purchaser’s tank number. m) Maximum operating temperature, which shall not exceed 250 °F. 38 n) The name of the Manufacturer with a serial number or contract number to identify the specific tank. o) If thermal stress relief is applied to a part in acco rdance with 5.25 or R.7.3, t he nameplate shall be mar k ed “SR,” and the part shall be identified on the Manufacturer’s certificate. p) The material specification number for each shell course. 8.1.2 On request by the Purchaser or at the discretion of the Manufacturer, additional pertinent info rmation may be shown on the nameplate. The size of t he nameplate may be incr eased accordingly. 8.1.3 The nameplate shall be attached to t he t ank shell adjacent to a manhole or to a manhole reinforcing plate immediately above the manhole. A nameplate that is placed directly on the she ll plate or reinforcing plate shall be attached by continuous welding or brazi ng all around the plate. A nameplate that is riveted or otherwise permanently

38 Unless other units are specified by the Purchaser.8-2 API S TANDARD 620 attached to an auxiliary plate of ferrous material shall be at tached to the tank shell plate or reinforcing plate by continuous welding. The nameplate shall be of corrosion-resistant metal. 8.1.4 When a tank is fabricated and erected by a single or ganization, that organizatio n ’s name shall appe ar on the nameplate as both fabricator and erector. 8.1.5 When a tank is fabricated by one organization and erect ed by another , the names of both organizations sha l l appear on the nameplate, or separate nameplates shall be applied by each. 

### 8.2 Division of Responsibility Unless otherwise agreed upon, when a tank is fabricated by one organization and erected by another, the erection Manufacturer shall be considered as havi ng the primary responsibility. The Manuf acturer shall make certain that the materials used in the fabrication of the components and in the construction of the tank are in accordance with all applicable requirements. 

### 8.3 Manufacturer’s Report and Certificate 8.3.1 Upon completion of all tests, exami nations, and inspections on ea ch t ank, the Manufacturer shall prepare a report summarizing all the data on the tank, including foundat ions (if they are within the Manufacturer’s scope of responsibility) and shall attach to the report all drawings and charts as required by other paragraphs in this section of the rules (see 7.13 and Annex M). 8.3.2 The Manufacturer shall furnish and fill out a certificate fo r each t ank ( such as that s hown in Annex M), attesting that the tank has been constructed according to the rules in this standard. This certificate shall be signed by the Manufacturer and the Purchaser’s inspector. This certificat e, together with the nameplate or markings placed on the tank, shall guarantee that the Manufacturer has comp lied with all applicable requirements of these rules. 8.3.3 If the Purchaser so requests, the Manufacturer shall at t ach to the report copies of the records of the qualification test of welding pr ocedures, of welders, and/or of welding operators (see 6.7 and 6.8). 

### 8.4 Multiple Assemblies In the case of assemblies that consist of two or more t anks or compartments designed and built according to the rules of this standard, each tank or compartment in the asse mbly shall be marked separately, or the markings may be grouped at one location and arranged so that the data for t he separate compartments can be identified. Removable pressure parts shall be marked to identify them with the tank to which they belong.Figure 8-1—Nameplate API STANDARD 620

ANNEX EDITION NOMINAL DIAMETER NOMINAL CAPACITY DESIGN SPECIFIC GRAVITY DESIGN PRESSURE PURCHASER’S TANK NO. MANUFACTURER’S SERIAL NO. MANUFACTURER YEAR COMPLETED ADDENDUM NUMBER NOMINAL HEIGHT DESIGN LIQUID LEVEL MAXIMUM TEST LEVEL DESIGN METAL TEMP. MAXIMUM OPERATING TEMP. PARTIAL STRESS RELIEF

SHELL COURSE MATERIAL D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS 8-39-1 

### SECTION 9—PRESSURE- AND VACUUM-RELIEVING DEVICES
9.1 Scope The Manufacturer or Purchaser shall equip tanks constructed within the pressure limits of these rules with pressurerelieving and emergency vacuum-relieving valves, or other equivalent permissible devices, as a means of safeguarding the storage and adjacent equipment involved (see 9.6.1.2 and Annex N). 

### 9.2 Pressure-relieving Devices 9.2.1 Tanks shall be protected by automatic pressure-relieving devices that will prevent t he pressure at the top of the tank from rising more than 10 % above the maximum positive gauge pressure except as prov ided in 9.2.2 (see Annex K). 9.2.2 Where an additional hazard can be cr eated by the exposure of the t ank to accident al fire or another unexpected source of heat external to the tank, supplemental pressure-relievi ng devices shall be installed. These devices shall be capable of preventing the pressure from rising more than 20 % above the maximum positive gauge pressure. A single pressur e-relieving valve may be used if it satisfies the requirements of this paragraph and 9.2.1. 9.2.3 Vacuum-relieving devices shall be installed to permit the entr y of air (or o ther gas or vapor is so designed) to avoid collapse of the tank wall if this could occur under natural operating conditions. T hese devices shall be located on the tank so that they will never be sealed off by the contents of the t ank. Their size and pressure (or vacuum) setting shall be such that the partial vacuum developed in the tank at the maximum specified rate of air (or gas) inflow will not exceed the partial vacuum for which t he tank is required to be designed (see 5.10.5). 

### 9.3 Construction of Devices Pressure- and vacuum-relieving valves shall be constructed of materials that are not subject to excessive corrosion for the intended service or subject to sticking at the seat or moving parts under any climatic conditions for which they are supplied. 

### 9.4 Means of Venting The applicable rules of 5.4 in API 2000 shall govern. 

### 9.5 Liquid Relief Valves A tank, which is likely to operate completely filled with liqui d, shall be equipped with one or more liquid relief valves at the top of the roof, unless ot herwise protected against overpressure. When su ch valves are, in effect, supplementary relief devices, they may be set at a pressure not greater t han 1.25 times the design pressure . Because the relief valve at the pump, which provides the inflow of liquid to the tank, is set at a pressure great er than 1.25 times the design pressure of any tank that may be built under these rules, provision should be made for pr eventing overfilling of the tank by a self-closing float valve, by some practicable pilot-valve cont rol, or by any other proven device. 

### 9.6 Marking 9.6.1 Safety and Relief Valves 9.6.1.1 Each safety and relief valve 1 / 2 -in. pipe size and larger shall be plainly marked by the manufacturer with the required data in such a way that the marking will not be obliterated in service. Smaller valves are exempted from marking requirements. The marking may be placed on the valve or on a plate or plates securely fastened to the valve.9-2 API S TANDARD 620 Valves may be marked with the required data stamped, etched, impressed, or cast on the valve or nameplate. The marking shall include the following: a) name or identifying trademark of the manufacturer; b) Manufacturer’s design or type number; c) size of valve (pipe size of the valve inlet); d) set pressure, in lbf/in. 2 gauge; e) full open pressure, in lbf/in. 2 gauge; f) capacity of valve, in ft 3 or air 39 per minute (60°F and 14.7 lbf/in. 2 absolute), see 9.6.1.2. 9.6.1.2 In many installations of tanks constructed according to t hese rules, the safety- or relief-valve inlet pressure is so low in relation to the outlet pr essure that valve capa cities predicted on acoustic ve locity of flow through the discharge area of the valve (the usual basis for establishing safety-valve ratings) are not attainable. For valves that handle light hydrocarbons or vapors, the condi tion described will exist if the ratio of the absolute pressure at the valve outlet to the absolute pressure at the valve inlet (set pressure in lbf/in. 2 gauge times 1.10, plus atmospheric pressure) exceeds a value of approximately 0.6: in such cases, formu las of the type given in Sect ion VIII, Appendix 11, of the ASME Code are not appropriate for computing safety- or relief -valve capacity conversi ons. Where this condition exists, the valve manufacturer should be consulted concerning the size of the va lve or valves required for the desired capacity in terms of the spec ific gas or vapor to be handled, the set pres sure to be employed, and the pressure to be imposed on the outlet of the valve. If atmospheric pressure in the locality where the valve is to be used differs materially from 14.7 lbf/in. 2 absolute, its normal value should be giv en in the inquiry to the manufacturer. 9.6.2 Liquid Relief Valves Each liquid relief valve shall be marked with the following dat a: a) name or identifying trademark of the manufacturer; b) Manufacturer’s design or type number; c) size of valve, in inc hes (pipe size of inlet); d) set pressure, in lbf/in. 2 gauge; e) full open pressure, in lbf/in. 2 gauge; f) relieving capacity, in ft 3 of water (see Footnote 16) per minute at 70 °F. 

### 9.7 Pressure Setting of Safety Devices 9.7.1 Except as provided in 9.5 for certain liquid relief valv es, the pressur e setting of a pressure-relieving device shall in no case exceed the maximum pressure that can exist at the level where the device is located when the pressure at the top of the tank equals the nominal pressure rating for the tank (see 5.3. 1) and the liquid contained in the tank is at the maximum design level. 9.7.2 Vacuum-relieving devices shall be set to open at such a pres sure or pa rtial vacuum that the p artial vacuum in the tank cannot exceed that for which t he tank is designed when the inflow of air (or other gas or vapor) through the device is at its maximum specified rate.

39 In addition, the manufacturer may indicate the corresponding capacity in other fluids.A-1 

## Annex A (informative) Inquiries and Suggest ions for Change A.1 Introduction This Annex describes the process est ablished by API for 1) submitting inqui ries to API, and 2) for submitting suggestions for changes to this standard. Inquiries and suggestions for change are welcome and encouraged, because they provide useful reader feedback to the responsible API Committee regarding technical accuracy, current technology use, clarity, consistency, and completeness of the st andard. API will attempt to answer all valid inquiries. Submittals not complying with th is Annex will be returned unanswered. Sections A.2 throu gh A.8 below cover the submitting of i nquiries. See Section 9 for in str uctions about submitting suggestions for change. 

### A.2 Inquiry References A.2.1 API maintains several webs ites that provide information that shoul d be reviewed before submitting an inquiry. A.2.2 Your inquiry may have been previous ly addressed by the Subc ommittee a nd the resulting interpretation posted on the API web site as follows: For all standards: http://mycommittees.api.org/st andards/techinterp/default.aspx For Refining Standards: http://mycommittees.api.org/standar ds/techinterp/refequip/default.aspx For both links, click on the standard in question to download the file. A.2.3 In addition, an addendum or errata, which may have addres sed your issue, can be found on the API web site as follows: For all standards: http://www.api.org/standards/addenda/ For Refining Standards: http://www.api.org/s tandards/addenda/add-ref.cfm 

### A.3 Definitions A.3.1 inquiry A question that asks what is the meaning of a specific paragraph, figure, or t able in the standard; i.e. what do the words say. It is not a question that asks about the intention of the standard. A.3.2 interpretation The answer to the inquiry

. Typically, the answer is simply a “Yes” or “No” response, with a brief clarification if needed. This term is also used to refer to the combined question and answer. 

### A.4 API Policy Regarding Inquiries A.4.1 API has established the following limits on it s activity in the handling of inquiries. a) API does not approve, certify, rate , or endorse any item, construction, proprietary device, or activity.A-2 API S TANDARD 620 b) API does not act as a consult ant on specific engineering problems. c) API does not provide information on the general understanding or application of the standard. A.4.2 All inquiries and resulting interpretations will be made available to the public on the API website. 

### A.5 Submission of Inquiries A.5.1 An electronic form for submitting a reques t can be found on the API Web site at http://rfi.api.org/ . Please use this means to submit your inquiry. A.5.2 All inquiries must comply with the following. a) Current standard: If an inquiry refers to a version or addendum that is not the la test, the Subcomm ittee will develop the interpretation based on the r equirements stated in the current version. b) Specific Reference: The applicable paragraph number, figur e number , or table number must be cited in the in quiry. c) Sentence Structure: Inquiries must be written such that the answer can be a YES or NO, with tech nical details added if necessary. The inquiry statement should be te chnically and editorially correct, and written in understandable English. d) Background: Providing a background explanation is optional, but is encour aged to assist the committee in underst andin g the query. e) Single Subject: The scope of an inquiry shall be limited to a single subject or a group of closely related subjects. f) General Format: 1) The general format of the inquiry should be as follows: “Does Paragra ph XXX of API-6XX require that ....?” 2) The inquirer shall state what is required in his or her option, as the answer to the query. 3) If a revision to the standard is believed to also be needed, provide recommended wording. g) The Inquirer should not use the inquiry pr ocess to improve his general understanding, design skills, and usage of the standard. Consultants not affiliated wit h API are available for this purpose. h) It is important that the Inquirer understand the di f ference between an inquiry and a suggestion for change. API encourages both, but the submittal and commi ttee handling procedures are different. A.5.3 General guidelines for submission can also be found on the API web site at: http://www.api.org/Publicat ions-Standards-and-Statistics/ FAQs-and-Inquiries/FAQs/Technical-Question/Guidelinesfor-submission.aspx 

### A.6 Typical Inquiry Procedure A.6.1 The typical procedure of an inquiry is as follows. a) The Inquirer must prepare the inquiry , including any necessary backgrou nd info rmation, in full co mpliance with this Annex and submit to the API Standards Coordinator. b) API Standards Coordinator checks the i nquiry to verify compliance with the requirements of submitting an inquiry.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS A-3 c) If the inquiry cannot be answered for any reason, the Coordinator will issue a response to the inquirer advising the reason(s) for not answering the inquiry. A form or c hecklist will typically be used for this response. d) If the Coordinator believes the inquiry is valid, it will be forwarded to the Subcom mittee for study , and the inquirer will be so advised using the form letter. e) The Subcommittee will evaluate the inquiry and either develop a response or determine that the inquiry cannot be answered, and advise the Coordinator accordingly. The Subcommittee will consider the need for modifying the standard to resolve technical issues, add new requirements, make editorial corrections, improve clarity, remove conflicts, etcetera. f) The interpretation will be published on the API website when approved b y the Subcommittee. A.6.2 The time required to process a valid inquiry as described in 6.1 may ta ke as long as a year. 

### A.7 Interpretations R esponding to Inquiries A.7.1 An interpretation is written by the Subcommittee to provi de the specific answer to an i nquiry. It typically will not state the intent of the standard, nor give reasons for the requirements, nor give historical bases, nor provide overall understanding of the standard. If the inquiry is properly phr ased, the interpretation can be a one-word response. With many inquiries, there may be a need to provide clarifying statements, such as the limits on the applicability. A.7.2 Although it is not possible to develop interpretations quickly to remedy immedi ate needs, the industry benefit s as a whole when inquiries are utilized as a means of trying to understand the te chnical requirements in the standard. 

### A.8 Form Response Sent to Inquirer A.8.1 A form letter or email will be used to reply to inquirers indicating the action taken by API, and, if applicable, the reason(s) for not being able to accept the inquiry. A.8.2 Reasons for not being able to accept an inquiry may include: a) Not Current Standard b) No Specific Reference c) Unclear Sentences d) Inadequate Background Information e) Unrelated Subjects f) Format g) Application to a Specific Tank h) Consultant Question 

### A.9 Suggestions for Changes A.9.1 A “Suggestion for Change” is not an inquiry; it is simp ly a communication (email preferred) from a reader to API proposing that a specific change be made to the standard.A-4 API S TANDARD 620
A.9.2 Any format is acceptable, as long as the content is clear. A.9.3 The m ost effective means to submit suggestions is to send an em a il to the API Coordinator (standards@api.org). A.9.4 The content of a suggestion must include the st andard number , ed ition, and addendum in question. The relevant paragraph numbers, table number, figure number, etc mu st also be stated. Provide as much explanation as necessary to be sure the Subcommittee understands the tec hnical issues. Provide specif ic language that you think is needed to implement the change. Last, include your name, company affiliation if any, and your return email or mailing address. A.9.5 API will forward all suggestions t hat are suitably written to the Subcommittee for cons ideration. The Subcommittee will evaluate each suggestion and determine if a change is needed. Suggestions that are accepted by the Subcommittee will be reflected in a future edition or addenda, but a reply advisi ng the submitter of the Subcommittee’s decisi on may not be issued.B-1 

## Annex B (normative) Use of Materials That are Not Iden tified with Listed Specifications B.1 General Plates and lengths of seamless or welded pipe that are not completely identified with any listed specification may, under the conditions described in B.2 through B.7, be used in the construction of tanks covered by this standard. Whenever the term listed specifications appear s in this annex, it shall refer to a ma terial specificati on that is listed as being approved for this standard. 

### B.2 Materials with Au thentic Test Records If an authentic test record for each heat or heat-treating lot of material is available that proves it to have chemical requirements and mechanical properties within the permissible range of an ASTM specification listed in this standard, the material may be used. If the test requirements of the listed specification are more re strictive than any of the specifications or authentic tests that have been reported for the material, the more restrictive tests shall be made in accordance with the requirements of a comparative listed specification, and the result s shall be submitted to the Purchaser for approval. 

### B.3 Materials without Au thentic Test Records B.3.1 General If an authentic test record is not available or if all of the material cannot be positively identified with the test record by legible stamping or marking, the material s hall be tested as described in B.3.2 and B.3.3. 

### B.3.2 Plate Each plate shall be subjected to the chemical check analysis and physi cal tests required in the designated specification, with the following modifications: The carbon and manganese cont ents shall be determined in all check analyses. The Purchaser shall decide w hether these contents are acceptable when the designated specification does not specify carbon and manganese limits. When the direction of rolling is not def initely known, two tension specimens shall be taken at right angles to each other to form a corn er of each plate, and one tension specimen shall meet the specification requirements. 

### B.3.3 Pipe Each length of pipe shall be subjected to a chemical check analysis and physical tests which satisfy the Purchaser that all of the material is properly identified with a giv en heat or heat-treatment lot and that the chemical and physical requirements of the designated specification are compiled wi th. Material specified as suitable for welding, cold bending, close coiling, and so forth shall be given check tests which satisfy the Purchaser that each length of material is suitable for the fabrication procedure to be used. 

### B.4 Marking of Identified Material After material has been properly identif ied with a designated specification and the Purchaser has been satisfied that the material complies with the specif ication in all respects, the testing agenc y shall stencil or otherwise mark, as permitted by the material specificati on, a serial S-number on each plate or each length of pipe (o r as alternately provided for small sizes in the specificat ion) in the presence of the Purchaser.B-2 API S TANDARD 620 

### B.5 Report on Tests of Non-identified Materials Suitable report forms that are clearly marked as being a r eport on tests of non identified materials shall be furnished by the tank Manufacturer or testing agency, properly fill ed out, certified by the testing agency, and approved by the Purchaser. 

### B.6 Acceptance or Rejection The Purchaser shall have the right to accept or reject the testing agency or the test results. 

### B.7 Requirements for Fabrication The requirements for fabrication that are applicable to the designated specification to wh ich the nonidentif ied material corresponds shall be followed, and the a llowable design stress values shall be those specified elsewhere in this standard for that corresponding specification.C-1 

## Annex C (informative) Suggested Practice Regarding Foundations C.1 Introduction The practices suggested in this section are intended only to supply information to those who are not fully conversant with the foundation problems of important structures. These pr actices are in no sense to be taken literally in providing the best design for any particular site. The experienced judgment of a competent engineer is needed to pass on any but well-proven sites in any locality, barring only the possibility of spot variations. For this reason, the mi nimum checks of subgrade included in this section will usually prove worthwhi le. Such checks may even be superfluous when a qualified geologist has passed on the general area or where the measured settlement of existing structures around the proposed sites, which produce a similar type of loading, confir ms the load-bearing capacity to be selected. No set of rules can cover all possibl e combinations of subgrade loading conditions. Types of subgrade structures and the final design of the finished in stallation may be affected by groundw ater or local climatic changes. Many large vertical storage t anks have been built with cylindrical shells and flat bottoms that re st directly on simply prepared subgrade. In the case of unequal settlement, a rele veling of the tank and subgrade has forestalled failure. However, for tanks that have formed bottom plates, such as may be built according to this standard, uniformity of support and avoidance of excessive settlement are much more important than they are in the case of flat-bottom, vertical storage tanks. Hence, sites fo r erection of tanks construc ted according to the rules of this standard shall be chosen only after careful consideration and evaluation of the bearing properties of the soil at the locations involved. 

### C.2 General For a low-pressure tank in the large sizes covered by this standard, the nature of the subgrade can be of prime importance. Many industrial plants that require such stor age are located near large streams, where the areas to be built on are alluvial deposits. These deposits are usually interspersed with gr avel and sand, all affected by previous changes in the course of the stream, so that both the character and depth of t he composite layers have no uniformity. The recommendations made in this secti on will therefore omit any referenc e to rock or even shales and hardpan (cemented gravel) for direct support of masonry footings . Long-standing practices for such conditions are well known. For large tanks that will rest on or near grade level, proper grade preparation c an have an important bearing on bottom corrosion. Tanks erected on poorly drained grades in direct contact with corrosive soils or on heterogeneous mixtures of different types of soils are subject to electrol ytic attack on the bottom side. Assuming that soil-bearing conditions have been determined to be adequate, the simplest form of foundation is a sand pad laid directly on the earth. All loam or organic material shall be removed and replaced with suitable well compacted material. Often a satisfactory fi ll material is available at the site. If not, bank run gravel is excellent and is readily compacted. The grade for the tank shall be elev ated slightly above the surrounding terr ain to ensure complete drainage from beneath the entire bottom of the tank. Sufficient berm shall be provided to prevent washing away and weathering under the tank bottom. The berm width shall be at least 5 ft. Weathering can be minimize d if the berm is subsequently protected with trap rock, gr avel, or an asphaltic flashing. The nature of predictable settlement may determine the choice of the kind of support for large field-assembled tanks that rest directly on a prepared grade, sometimes retained with in curb walls, as well as for those tanks that shall be supported on ring walls, pedestals and columns, skirts, or ring girders.C-2 API S TANDARD 620 Except in the case of tanks founded on solid rock, hardpan, or similar substances, some amount of settlement is bound to take place. Every reasonable pr ecaution shall be taken to ensure that the settlement will be kept to an acceptable minimum and that any settl ement which does occur will be as uniform as possible. Large, and perhaps even moderate, irregularities in settlement may lead to an unbalancing of the loading conditions assumed in the design and possibly to serious distortion of important elements of the tank. For those locations where the use of piling is the only logi cal procedure, the piling design factor s would be well known to the engineer charged with making the dec ision for the contractor or owner. Competent guidance may be needed in choosing between a pile that depends on skin friction alone or mainly on end bearing with small credit for lateral support in combination with skin friction for part of the length. 

### C.3 Design The designer of these large tanks shall supply the data on superimposed loadings to be assumed for the foundation design or, if there are no foundations, fo r direct loading on the subgrade. A slab or mat may be provided to support the superstructure and shall be considered to distribute the load more evenly over a lower natural subgrade compared with merely stripping, leveling, and rolling the existing grade. Foundations and subgrade shall safely carry the weight of the t ank and its content s when the tank is filled with water to the highest level required for a hydr ostatic test or other water-loading operations, even though the tank itself may be designed for some lesser density of liquid. However, in cases where the character of the soil justifies it and a competent soil expert advises it, allowance may be made for the relatively shor t duration and intermitt ent nature of the water loadings if suitable account is taken of all such l oadings that may be expected to occur during the life of the tank, including not only those loadings that are incidental to periodic repetitions of the hydrostatic test in accordance with 7.18 but also water-filling operations for gas-freeing purposes. For simple spheroids or tanks of similar design, in whic h the distribution of the impos ed weight s shif ts because the liquid level in combination with the vapor pressure may c hange the shape of the tank, the designer shall consider the possibility of such a change. 

### C.4 Soil-bearing Values The bearing values selected shall be conservative on the assu mption that suitable field tests will be made if borings or test pits, or both, do not give satisf actory information on the depth required. Determination of the allow able maximum soil-bear ing value shall be the responsibility of the Purchaser. 

### C.5 Investigation of Subgrade On actual tank sites to be used, test borings or test pits, or both, may be made at the direction of a competent engineer who will specify the number and location. They need not be equally spaced but should be laid out to uncover possible weak spots. Test borings, where required, shall be carried to sufficient depths to disclose any deep -lying sof tor insufficiently consolidated strata beneath the surface. If such strata is discovered, its effects on the bearing properties at the surface of the grade shall be carefully evaluated, giving due consideration to the loaded-area size effect of the total area loaded by the tank. In general, test loadings of subgrade at the bottom of test pits need be reso rted to only when such heavy loading as may be imposed by footings for major column supports for spheres or similarly elevated low-pressure tanks is specified. Results may be deceptive if average load-bearing capacity over a c onsiderable area is wanted. All field data shall be recorded with maps, and copies shall be su pplied to all engineers concerne d with design, erection, and later operation.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS C-3 

### C.6 Minimum Depth of Footings The depth of the bottom of footings shall be determined by lo cal subgrade conditions. The bas e of these footings shall be placed below the expected frost line, away from any near by excavations, and below any nearby sewers or piping which, if leaky, could cause se rious impairment of the foundation. 

### C.7 Concrete in Foundations ACI Standard 318 shall govern the design of all concrete and the specifications for the cement, the aggregate, and the mixing and placing thereof, unless ot herwise specified in the contract. 

### C.8 Installation of Foundations C.8.1 Except for what is standard practice to be specified on the plans, the limitations described in C.8.2 thr ough C.8.7 are suggested. C.8.2 The lowest footing course shall be bedded directly against the sides of the excavati on when the sides are selfsupporting. Before the concrete is poured, adjacent dry so ils shall be thoroughly moistened by sprinkling with water. Likewise, all loose material from cave-in, plus any soft rain-soaked soil, shall be removed from the bottom of the excavation. C.8.3 The tops of all concrete slabs or mats shall be at l east 6 in. above the final grade to be provided, and the tops of the pedestals and other foundations to support steelwork sh all be at least 12 in. above the final grade or any mats or paving surfaces, if built adjacent. C.8.4 The tops of foundations shall be large enough to project at least 3 in. out side of any steel baseplates of the superstructure. C.8.5 The exposed surfaces, other than the tops of c oncrete pedest al and wall foundation s , shall be smooth finished down to 6 in. below the proposed final grade. Any sma ll holes left in the faces of pedestals, down to the first footing top, shall be troweled over with 1:3 mortar as soon as possible after forms have been removed. C.8.6 Under column-type superstructures, base plates shall be provided, and allowance shall be made for 1-in

. minimum grout. C.8.7 Concrete ring walls or slab foundations for flat-bottom t anks, where the foundations specified are nominally true to the horizontal plane, shall be level within ± 1 / 8 inches in any 30 ft of circumference and within ± 1 / 4 inches in the total circumference measured from the average elevation. 

### C.9 Anchorage Anchor bolts or straps and reinforcing steel for foundati ons may be supplied by the contractor or Purchaser, as specified in the contract. 

### C.10 Backfill and Grading All backfill around and over foundations shall be carefully deposited and rammed where it is next to concrete. No water streams shall be used to compact t he backfill, except where no clay is pr esent and quick drainage is assured by the general contours. Bulldozers, scrapers, and crane-bucket di scharge may be used if they are kept completely clear of the pedestals and walls. If special adverse conditions are met, a foundation enginee r sh all be cons ulted regarding compaction control.C-4 API S TANDARD 620 Particular attention shall be given to surface regrading around the finished structure to permi t efficient erection of the superstructure and to provide proper dr ainage that is consistent with the records of local weather conditions. The finished grade under a flat-bottom tank shall be crowned from the pe riphery to the center . A slope of 1 inch in 10 ft is suggested as a minimum. This crown will partly compensate for slight settle ment, which is likely to be greater at the center; it will also aid in draining and cleaning the tank. 

### C.11 Inspection during the Hydrostatic Test As a final check on the adequacy of the foundations and s ubgrade, the Purchaser shall take level readings with surveyor’s instruments around the entire periphery of the tank before water is introduced into the tank for the hydrostatic test. The readings shall be continued at reasonable intervals during the entire filling operation and shall be plotted promptly in suitable form to indicate whether any undue or uneven settlement is occurring. The results of these observations shall be reported to t he tank erector and the Purchaser’s engineering representative. If at any time any questionable amount or rate of settlement does occur, furt her filling of the tank sha ll be stopped until a decision is reached as to what, if any, corrective measures are needed. Reference points on a tank or its foundations for use in making such observations shall be selected with care to ensure that the readings accurately reflect settlement of the subgrade and are not affected by possible c hanges in the shape of the tank walls. If a minor amount of settlement is observed during the course of the filling operation and sti ll continues after a t ank is filled to the highest level required in t he hydrostatic test, the water level in the tank shall not be lowered until further settlement has substantially ceased or a decision is reached that it might be unsafe to hold the water at that level any longer. In no event, however, shall the water test be used as a planned means of soil comp action. 

### C.12 References 1) Standard for Welded Steel Elevated Tank, St andpipes, and Reservoirs of W ater Storage , AWS-AWWA D5.2. 2) “Oil Storage Tank Foundations,” Technical Bulletin, Chicago Bridge and Iron Co., March 1951. 3) K. T erzaghi and R.B. Peck, Soil Mechanics in Engineer ing Pr actice , John Wiley and Sons, Inc., New York, 1948.D-1 

## Annex D (informative) Suggested Practice Regard ing Supporting Structures D.1 General When a tank is supported on columns, a supporting ring or skirt, brackets, or comparable members, it will have concentrated loads imposed on its walls in the region wher e the supports are attached. When tanks of certain shapes are subject to internal pressure, secondar y stresses may exist in th e wall adjacent to the attachment of such supports that are lower than when the tank is fi lled with liquid before any pressure is imposed other than that caused by the static head. Methods for calculating the forces involved ar e not given in this standard because they involve so many variables that depend on the size, shape, and weight of the tank; the temperature of service; the internal pressure; the arrangement of the supporting structure; and the piping attached to the tank as installed. 

### D.2 Details of S upporting Structures D.2.1 The details of supports shall confo rm to good structural practice, bearing in mind the considerations described in D.2.2 through D.2.5 (see 5.13 and the Steel Construction Manual ). D.2.2 All supports shall be designed to prevent excessive localized stresses by temperature changes in the tank or deformations produced by variations in the pressure and li quid-level conditions within the tank. Any arrangement of the structure that does not permit a r easonably free expansion and contraction of the tank walls will tend to weaken the tank. D.2.3 External stays and ring girders or certain internal frami ng may exert a stiffening effect on the tank wall where exterior supporting members of the tank are to be attac hed. This stiffening effect may be beneficial or harmful, depending on the operating temperature and t he location of the stiffening members. D.2.4 In many cases it is preferable to use details that permit continuous welds which extend completely around the periphery of the attachment and avoid intermittent or dead-end welds at which there may be local stress concentration. A thicker wall plate at the support may serve to reduce se condary stresses, and if desired, a complete ring of thicker wall plates may be installed. D.2.5 When forces acting on a tank wall at the attachment areas for supports of any kind can produce high bending stresses, and thicker wall plates do not seem appropriate, an oval or circular reinforcing plate may be used. The attachment of such reinforcing plates shall be designed to minimize flexing of the plate under forces normal to the surface of the tank wall.E-1 

## Annex E (informative) Suggested Practice Regarding Attached Structures (Internal and External) E.1 General Some tanks constructed according to the rules of this st andard may have internal structural bracing. Should these or their attachments fail, severe damage to the tank would result. The designer shall keep this possible hazard in mind and shall design such members and their attachments with sufficient strength and due allowance for corrosion. 

### E.2 Cautionary Suggestions E.2.1 Cautionary suggestions, which shall be considered in t he design of internal and external structures, are described in E.2.2 through E.2.5. E.2.2 Where the structures are connected to the tank wall, details shall be provided th at will prevent excessive localized tensile stress outward from the wall face because of the connection. E.2.3 If platforms or stairways have separate supports, they shall preferably rest on top of the supports instead of hanging by bolts or rods. E.2.4 If corrosion is expected, additional metal shall be provided. The corrosion allowance does not have to be the same as in the tank wall if the supports and structures can be readily and economically replaced without replacing the entire tank. E.2.5 Corrosion-resistant metals may be used in the fabric ation of the structural supports, but whenever the supports are attached by welding, the parts joined shall be weldable. These we lds shall not introduce any objectionable conditions at or near the attachment, including har d or brittle zones, or both, or differences in electrical potential that might result in electrolytic corrosion.F-1 

## Annex F (informative) Examples Illustrating Application of Rules to Various Design Problems
F.1 Determination of Allowable Stress Valu es for Biaxial Tension and Compression F.1.1 Example 1 F.1.1.1 Given Conditions In this example, an area of tank wall is constr ucted of ASTM A131, Grade B, steel plate that is 3 / 4 in. thick and has fully radiographed, double-welded butt joints. The wall is subject to tension in a la titudinal direction and to compression in a meridional direction. The values of R 1 and R 2 at the point under consideration are 60 in. and 315 in., respectively. A corrosion allowance of 1 / 16 in. is required. The computed (meridional) compressive stress, s cc , in the net thickness after deduction of the corrosion allowance is 3400 lbf/in. 2

. F.1.1.2 Problem The problem is this example is to find the maximum allowable (latitudinal) tensile stress value for the given conditions, in conformance with the provisions of 5.5.3.3. F.1.1.3 Solution Since the compressive stress is meridional, the governing value of R in this situation is R 2 , or 315 in. Then,

Figure 5-1 shall be entered in the text at a value of ( t – c )/ R = 0.00218. The ordinate shall be proceeded along vertically from this point to its in tersection with the horizontal line for s c = 3400 lbf/in. 2 , and the value of N should be read at this intersection. In the case under consideration, N = 0.867. As determined from Table 5-1, the maximum allowable tensile stress value, S ts , for ASTM A131, Grade B, Steel plat e in simple tension is 16,000 lbf/in. 2 . Therefore, the maximum allowabl e tensile stress, s ta , for the conditions cited in this example is as follows:

s ta = NS ts = (0.867) (16,000) = 13,870 lbf/in. 2 An efficiency factor need not be applied to this value because E for fully radiographed, double-welded butt joints exceeds the value of N as determined in the foregoing procedure. Howeve r, if the joints were spot radiographed—not fully radiographed—butt joints, E would have a value of only 85 %. In this case, the net allowable tensile stress, ES ts , would be only 0.85 × 16,000 = 13,600 lbf/in. 2 determined from Figure 5-1, would govern. Alternatively, s ta could be determined by enter ing the computed value of M in Figure F-1 and obtaining the allowable coexistent value of N . For this particular example, the value of S cs would be 15,000 lbf/in. 2 . Hence, t c

– R --------- 0.75 0.0625

– 315 -------------------------------- 0.00218 = =

M S cc S cs ------ 3 400 , 15 000

, ----------------- 0.227 == =F-2 API S TANDARD 620

NOTE An initial check shall be made to assu re that the actual compressiv e stress, s cc , equaling 3400 lbf/in. 2 does not exceed 1,800,000 [( t – c )/ R ], which is calculated as follows: 1,800,000 × 0.00218 = 3920 lbf/in. 2

. Entering this value of M in Figure F-1 obtains the value N = 0.867. Therefore,
S

ta = NSt s = (0.867) (16,000) = 13,870 lbf/in. 2 

### F.1.2 Example 2 F.1.2.1 Given Conditions In this example, the area of tank wall used is of the same construction, material, and geometry as that described in F.1.1.1 except that the th ickness of the plate is 9 / 16 in. The wall is stressed in the same manner as is described in F.1.1.1. A corrosion allowance of 1 / 16 in. is required. The computed (meridional) compressive stress, s cc , in the net thickness after deduction of the co rrosion allowance is 4600 lbf/in. 2

. F.1.2.2 Problem The problem in this example is to fi nd the maximum allowable (lat itudinal) tensile str ess val ue for the given conditions, in conformance with the provisions of 5.5.3.3. F.1.2.3 Solution Since the compressive stress is mer idional, the governing value of R in this situation is R 2 , or 315 in. Then,

t c

– R --------- 0.5625 0.0625

– 315 -------------------------------------- 0.00158 = = Figure 5-1 shall be entered in the text at a value of ( t – c )/ R = 0.00158. The ordinate shall be proceeded along vertically from this point, noting that this line intersects the line with the value s c = 4600 lbf/in. 2 on the chart to the left of line 0-A and that an extrapolation of the N curves would be required to determine the value of N . Since such an extension or extrapolation of the N curves is not permissible, no coexistent tensile stress is permissible under the conditions cited in this example. In fa ct, the compressive stress of 4600 lbf/in. 2 greatly exceeds t he allowable stress, S

cs , of 2840 lbf/in. 2 for simple compression for the thickness-to-r adius ratio involved. Consequently, either the thickness must be increased or t he shape of the wall must be changed. 

### F.1.3 Example 3 F.1.3.1 Given Conditions In this example, an area of tank wall is constructed of ASTM A2 8 5, Grade C, steel plate that is 5 / 8 -in. thick and has spot-radiographed, double-welded butt joints. The wall is subject to tension in a meridional direction and to compression in a latitudinal direction. The values of R 1 and R 2 at the point under consideration are 75 in. and 300 in. respectively. A corrosion allowance of 1 / 16 in. is required. The comput ed (meridional) t ensile stress, s tc , in the net thickness after deduction of the co rrosion allowance is 6000 lbf/in. 2

. F.1.3.2 Problem The problem in this example is to find the maximum allo wable (latitudinal) compressiv e str ess value for the given conditions, in conformance with the provisions of 5.5.4.5.Figure F-1—Reduction of Design Stresses Required to Allow for Biaxial Stress of the Opposite Sign 1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0Compression factor,M

Tension factor, N
N 2 + MN + M 2 = 1 or ( S t /S ts ) 2 + ( S c /S cs ) ( S t /S ts ) + ( S c /S ts ) 2 =1 ( S t /S ts ); tensile stress, in pounds per square inch, at the point under consideration; maximum allowable stress for simple tension, in pounds per square inch, as given in Table 5-1; (S c /S cs ); compressive stress, in pounds per square inch, at the point under consideration; maximum allowable longitudinal compressive stress, in pounds per square inch, for a cylindrical wall acted upon by an axial load with neither a tensile nor a compressive force acting concurrently in a circumferential direction. N S t S ts

M S c S cs =

=

=

=

=

= where D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS F-3

F.1.3.3 Solution As determined from Table 5-1, the ma ximum allowable tensile stress value , S ts , for ASTM A285, Grade C, steel plate in simple tension is 16,500 lbf/in. 2 . Since the compressive stress is latitudinal, the governing value of R in this situation is R 1 , or 75 in. Then,

t c

– R --------- 0.626 0.0625

– 75 ----------------------------------- 0.0075 = =F-4 API S TANDARD 620
The value N = s tc / S

ts = 6000/16,500 = 0.364 shall be computed. The ( t – c )/ R value of 0.0075 in Figure 5-1 shall be entered in the text, and the ordinate shall be proceeded along ve rtically at this value until it intersects with an N curve that represents the value N = 0.364; proceeding horizontally fr om this point to the left-side ordinate scale, the value s c = 11,500 lbf/in. 2 should be read. In this case, the value r epresents the allowable compressive stress, s ca . Alternatively, s ca could be determined by entering the computed value N = 0.364 in Figure F-1 and obtaining the corresponding allowable value M = 0.767. The allowable compressive stress, s ca , could be calculated by substituting this value of M in the equation s ca = 15,000M. Thus, s ca = 15,000 × 0.767 = 11,500 lbf/in. 2

. NOTE A check shall be made to ensure that the compressive stress does not exceed 1,800,000 [( t – c )/ R ] which is calculated as fol lows: 1,800,000 × 0.0075 = 13,500 lbf/in. 2

. 

### F.1.4 Example 4 F.1.4.1 Given Conditions In this example, the area of tank wall used is of the same construction, material, and geometry as that described in F.1.3.1 except that the th ickness of the plate is 3 / 8 in. The wall is stressed in the same manner as is described in F.1.3.1. A corrosion allowance of 1 / 16 in. is required. The comput ed (meridional) tensile stress, s tc , in the net thickness after deduction of the corrosion allowance is 8000 lbf/in. 2

. F.1.4.2 Problem The problem in this example is to find the maximum allo wable (latitudinal) compressiv e str ess value for the given conditions, in conformance with the provisions of 5.5.4.5. F. 1.4.3 Sol ution As determined from Table 5-1, the ma ximum allowable tensile stress value , S ts , for ASTM A285, Grade C, steel plate in simple tension is 16,500 lbf/in. 2 . Since the compressive stress is latitudinal, the governing value of R in this situation is R 1 , or 75 in. Then,

t c

– R --------- 0.375 0.0625

– 75 ----------------------------------- 0.00415 = =
The value N = s tc / S

ts = 8000/16,500 = 0.485 shall be computed. The ( t – c )/ R value of 0.00415 shall be entered at the bottom of Figure 5-1 in the text. The ordinate shall be proceeded along vertically at this value, noting that the N curves would have to be extrapolated to the left of line 0-A to intersect with the vertical line that represents the ( t – c )/ R value of 0.00415. Since no extrapolation is permitted to the left of line 0-A, the intersection of this vertical line with line 0-A yields a value on the left ordinate scale of 7500 lbf/in. 2 , which represents the maximum allowable compressive stress, s ca , for this particular value of ( t – c )/ R . A higher value of tensile stress is permi ssible, since the allowable coexistent value of N equals 0.65. Thus, in this particular example, t he allowable compressive st ress is governed by the ( t – c )/ R value rather than by the coexistent tensile stress. 

### F.2 Determination of Minimum Re quired Thicknesses for Walls S ubject to Biaxial Te nsion and Compression F.2.1 Example 1 F.2.1.1 Given Conditions In this example, an elemental area of t ank wall used is constructed of ASTM A442, Grade 5 5, steel plate subjected to a meridional unit force, T 1 , of 4000 lbf/in. 2 tension and a latit udinal unit force, T 2 , of 5060 lbf/in. 2 compression. The meridional radius of curvature, R 1 , is 75 in., and the length of the normal from the surface to the axis of revolution, R 2 ,D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS F-5 is 300 in. The joints in the wall are of double-welded butt-joint construction with a tensile efficiency of 85 %. A corrosion allowance of 1 / 16 in. is required. F.2.1.2 Problem The problem in this example is to graphically find the minimu m thickness of t ank wall r equired for the given condition s (see 5.10.3.3).

Table F-1—Computed Values of ( t – c ) R , s c , s t , and N for the Assumed Thicknesses: Example 1 (See F2.1.3) Assumed Thickness, t (in.) t – c R 1 s t T 2 t c

– --------- = s t T 1 t c

– --------- = N s t S ts ----- =
1 0.0125 5.400 4270 0.258 3 /

4 0.0092 7.360 5820 0.353 5 / 8 0.0075 9.000 7110 0.431 9 / 16 0.0066 10.120 8000 0.485 1 / 2 0.0058 11.570 9140 0.554
F.2.1.3 Solution As determined from Table 5-1, the maximum allowable tensile stress value, S ts , for ASTM A442, Grade 55, steel plate in simple tension is 16,500 lbf/in. 2 . Since the compressive stress is latitudinal, the governing value of R in this situation is R 1 , or 75 in. A series of four or five different plate thicknesses should be assumed, coveri ng the range in which the required thickness will probably be found. The values of ( t – c )/ R , s c , s t , and N shall be computed and tabulated for each of the assumed thicknesses, as shown in Table F-1. The values of s c shall be plotted on Figure F-2 40 at the respective values of ( t – c )/ R associated with them, and a smooth curve U-U shall be drawn that connects the point s located in this manner. The N and ( t –

c )/ R points shall also be plotted, and a smooth curve V-V shall be drawn to connect them. The intersection of these two cu rves represent s the minimum ( t – c )/ R value that w ill s atisfy both the compressive stress and tensile stress li mitations involved in this example. At this point s c (which is equivalent to s ca in this problem) equals 10,000 lbf/in. 2 ; N equals approximately 0.480; and ( t – c )/ R equals 0.0067. The efficiency, E , for the type of joints involved is 85 %. Since this is greater than N , the value of s t (or s ta ) for the conditions under consideration is equal to the value of NS ts , or 0.480 × 16,500 = 7920 lbf/in. 2

.
t T 1 S ta ----- c + 4000 7920 ----------- 0.063 + 0.505 0.063+ == =
= 0.568 in.

or
t T 2 S ca ------ c + 5060 10 000

, ----------------- 0.063 + 0.506 0.063+ == =
= 0.569 in.

40 Figure F-3 is a copy of this chart without the illustrative ex ample lines. It may be reproduced by the designer for sure in gra phical solutions.F-6 API S TANDARD 620

or
t t c

– R ---------     R 1 c + 0.0067 ( ) 75 ( ) 0.063 + = =
= 0.503 + 0.063 + 0.566 in. 

### F.2.2 Example 2 F.2.2.1 Given Conditions In this example, an elemental area of tank wall is constr ucted of ASTM A516, Grade 55, steel plate subjected to a meridional u nit force, T 1 , of 2620 lbf/in. tension and a latitudinal unit force, T 2 , of 2880 lbf/in. compression. The meridional radius of curvature, R 1 , is 132 in., and the length of the normal from the surface to the axis of revolution, R 2 , is 409 in. The joints in the wall are spot-radiographed, double-welded butt joints, and no corrosion allowance is required. F.2.2.2 Problem The problem in this example is to graphically find the minimu m thickness of t ank wall r equired for the given condition s (see 5.10.3.3 ). F.2.2.3 Solution As determined from Table 5-1, the maximum, allowable tens il e stress value, S ts , for ASTM A516, Grade 55, steel plate in simple tension is 16,500 lbf/in. 2 . Since the compressive stress is latitudinal, the governing value of R for this situation is R 1 , or 132 in. A series of four or five different plate thicknesses shall be assumed, covering the range in which the required thickness will probably be found. The values of ( t – c )/ R , s c , s t , and N shall be computed and tabulated for each assumed thickness, as shown in Table F-2. The value of s c shall be plotted on Figure F-2 at the respective values of ( t – c )/ R associated with them, and a smooth curve W-W shall be drawn between the points located in this manner. The N and ( t – c )/ R points shall also be plotted, and a smooth curve X-X shall be drawn to connect them. These two curves intersect each other on the left-hand side of line A-A. The use of values represented by point s in this ar ea is prohibited. All values of N in the vicinity of these two cu rves are well below the efficiency, E , of the type of joints involved; thus, the allowable compre ssive stress is obviously the critical factor in this problem. A point must be found where the computed compressive stress, represent ed by points on the curve W-W, does not exceed the allowable compressive stress. This will be at the intersection of curve W-W and line A-A, where s c = 6300. This value is the allowable compressive stress, s ca , for the conditions given in this example. Therefore,

t T 2 S ca ------ c + 2880 6300 ----------- 0 + 0.457 in. == = This value is the minimum required th ickness. The computed tensile stress fo r this thickness is only 2620 ÷ 0.457 = 5730 lbf/in. 2 , whereas the values of N at the intersection of curve W-W and line A-A indicates that a tensile stress of 16,500 × 0.72 = 11,880 lbf/in. 2 would have been permissible. Thus, the plate at this level will not be stressed to its fullest potential for tensile loading.1.00.90.80.70.60.50.40.30.20.101.00.90.80.70.60.50.40.30.20.1001,0002,0003,0004,0005,0006,0007,0008,0009,00010,00011,00012,00013,00014,00015,0000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018 0.020 0.022

Factor N - (tensile stress, S t )/(maximum allowable tensile stress, S ts )(t-c)/Rratio (see Note 1)XXWWUUVVNOTE 1 If compressive stress is latitudinal, useR=R1, ifcompressive stress is meridional, useR=R2.NOTE 2 The sc curves on this chart may be extended into thearea to the left of line A-A merely for convenience in makinggraphical solutions of problems such as the one illustrated in F.2.2(Example 2). None of the points in this area represent validallowable stress values.Example 2 (F.2.2)Example 1 (F.2.1) Factor N

Compressive stress, s c 1 , in psiAAFigure F-2—Examples Illustrating the Use of aBiaxial Stress Chart for Combined Tension andCompression, 30,000 to 38,000 Pounds per Square Inch Yield Strength Steels D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS F-7Factor N1.00.90.80.70.60.50.40.30.20.101.00.90.80.70.60.50.40.30.20.1001,0002,0003,0004,0005,0006,0007,0008,0009,00010,00011,00012,00013,00014,00015,0000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018 0.020 0.022(t-c)/Rratio (see Note 1)NOTE 1 If compressive stress is latitudinal, useR=R1, ifcompressive stress is meridional, useR=R2.NOTE 2 The sc curves on this chart may be extended into thearea to the left of line A-A merely for convenience in makinggraphical solutions of problems such as the one illustrated in F.2.2(Example 2). None of the points in this area represent validallowable stress values. Compressive stress, s c 1 , in psiAA

Factor N - (tensile stress, S t )/(maximum allowable tensile stress, S ts )Figure F-3—Form for Use in Graphical Solutionsof Problems Involving Biaxial Tension andCompression, 30,000 to 38,000 Pounds per Square Inch Yield Strength Steels F-8 API S TANDARD 620Table F-2—Computed Values of ( t – c ) R , s c , s t , and N for the Assumed Thicknesses: Example 2 (See F.2.2.3) Assumed Thickness, t (in.) t – c R 1 s t T 2 t c

– --------- = s t T 1 t c

– --------- = N s t S ts ----- =

3 /

4 0.05680 3.840 3.490 0.212 1 / 2 0.00378 5.760 5.240 0.318 3 / 8 0.00284 7.680 6.990 0.424 5 / 16 0.00237 9.200 8.380 0.507 1 /

4 0.00189 11.520 10.480 0.635 D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS F-9



### F.3 Determination of Minimu m Required Thicknesses for Walls Subject to Biaxial Compression from Meridional and Latitudinal Unit Forces F.3.1 Given Conditions In this example, the tank used to store liquid has a dome-shaped, self-suppor ting roof with varying values for R 1 and R 2 . The size and vacuum settings of t he vacuum-relieving devices are such that the partial vacuum developed in the tank at the maximum air inflow is 0.40 lbf/in. 2 gauge (see 5.3.1). The roof is covered with insulation weighing 2 lb/ft 2

. The design requirements include a live snow load of 25 lb/ft 2 on the horizontal projection of the surface of the roof, which has a slope of 30° or le ss with the horizontal and a 1 / 16 in. corrosion allowance. 

### F.3.2 Problem The problem in this example is to find the required plate thicknesses for the vacuum and external loading (a) at the center of the roof, where R 1 = R 2 = 1200 in. and (b) at a radial distance of 12. 5 ft from the center of the roof, where R 1 = 1117 in. and R 2 = 1172 in. 

### F.3.3 Solution F.3.3.1 General Figure F-4 is a free-body sketch of the roof above the plane of the level under cons ider ation. S pecific values for the variables used in this figure are as follows (see Figure 5-4 for typical free-body diagrams and 5.10.1 for definitions of the other variables):
P equals –0.40 lbf/in. 2 gauge, a negative value because of the internal vacuum;
W is the sum of the weights of the steel plate, insulation load, and snow load. W must be g iven the same sign as P in this case because it acts in the same dire ction as the pressure on the plane of the level under consideration; therefore, W is negative (see 5.10.1 of the definition of W );
F equals zero because no ties, braces, supports, or other similar members are cut by the plane of the level under consideration. F.3.3.2 Finding the Thickness at the Center of the Roof As a trial, a plate thickness of 27 / 32 in. (0.844 in.) at the center of the roof is assumed, including a 1 / 16 in. corrosion allowance, which is equivalent to a unit weight of 34.4 lb/ft 2

.Figure F-4—Free-body Sketch (See F.3) T 1 T

1

R 1 - R 2 R 1

R 2 P Insulation load Snow load

Axis of revolution (b) (a) F-10 API S TANDARD 620

On a 1 in. 2 area at the corner of the roof, W A ---- 2 25 34.4 + + 144 ------------------------------- 0.426 lb/in. 2 = = From 5.10.2.5, using Equations 4 and 5,
T 1 1200 2 ----------- 0.40 – 0.426

– ( ) [ ] =

= –495.6 lbf/in.
T

2 = 1200 (–0.40 – 0.426) – (–495.6)

= –495.6 lbf/in. From 5.10.3.4, using Equation 17,

t 495.6 s ca ------------- = where

s ca equals 1,000,000[( t – c )/ R ]

.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS F-11 Substituting s ca = 1,000,000 [( t – c )/ R ] for s ca in Equation 17 yiel ds the following:

t c

– ( ) 2 495.6 R 1 000 000

, , -------------------------- =

t 495.6 ( ) 1200 ( ) 1000 --------------------------------------- 0.063 + 0.834 in. = =
This value is slightly less than the assumed thickness. A more exact solution could be worked out using a second thickness whose value is between the first assumption and the calculated value.
F. 3.3.3 Finding t he Thickness at a Radial Distance of 12.5 ft As a trial, a plate thickness of 13 / 16 in. (0.813 in.) at a radial distance of 12.5 ft is assumed, including a 1 / 16 in. corrosion allowance, which is equival ent to a unit weight of 33.2 lb/ft 2

. W

=( π )(12.5) 2 (2 + 25 + 33.2) = 29,550 lbs sin θ = 12.5 ( ) 12 ( ) 1172 ------------------------- 0.1280 =

cos θ = 0.9918 θ = 7.35°
NOTE Technically, the surface area of the roof above the level under con sideration shall be used in the preceding calculation of W

; however, from a practical standpoint, in this example the difference between t he actual surface area and the area of the horizontal plane bounded by the free-body is relative ly small and can be ignored in the calculation of W . The designer is cautioned that in many cases a more exact calculation of the roof area and weight will be necessary. Normal-to-the-surface components of the metal, insulation, and snow loads are given per unit area of plate surface as follows: a) For metal, 33.2 lb/ft 2 ÷ 144;

cos θ = 0.229 lb/in. b) For insulation, 2.0 lb/f t 2 ÷ 144;

cos θ = 0.014 lb/in. 2 c) For snow, 25.0 lb/ft 2 ÷ 144; cos2 θ = 0.171 lb/in. 2

. The total of the normal components of load is 0.414 lbf/in. 2

.F-12 API S TANDARD 620 From 5.10.2.1, using Equations 1 and 2 with the foregoing normal components of load,

T 1 1172 2 ----------- 0.40

– 29 550 , – π ( ) 12.5 ( ) 2 144 ( ) --------------------------------------- + =
= –479 lbf/in.

T

2 1172 0.40 – 0.414

– ( ) 479

– 1117 -----------    

– =
= –451 lbf/in. From 5.10.3.5, using Equation 18 and 19 where T  ́ = T 1 , T  ́ ́ = T 2 , R  ́ = R 2 , and R  ́ ́ = R 1 : In this first step, according to Equation 18,

t 479 0.8 ( ) 451 ( ) 1172 ( ) + 1342 ---------------------------------------------------------- 0.063in. + =
= 0.802 in. According to Equation 19,

t 451 ( ) 1117 ( ) 1000 ---------------------------------- 0.063 in. 0.773 in. = + = In the second step, for the th ickness determined by Equation 18,

t c

– R ′ --------- 0.802 0.063

– 1172 -------------------------------- 0.000631 = = For the thickness determined by Equation 19,

t c

– R ′′ --------- 0.773 0.063

– 1117 -------------------------------- 0.000636 = = Since both ( t – c )/ R ratios are less than 0.0067, the larger of the thicknesses calculated in the first step is the required thickness if it is consistent with the assumed thickness. Further calculations using Steps 3 through 6 are unnecessary. The calculated thickness of 0.802 in. is sli ghtly smaller than the assumed thickness of 13 / 16 in. (0.813 in.) and is thus consistent from a practical standpoint with the assumed ro of loading. A recalculation us ing a new assumed thickness shall be made whenever the calculated thickness is appreciably greater t han the thickness assumed for the determination of the total roof load. 

### F.4 Design of Compression-ring Regions F.4.1 Example 1 F.4.1.1 Given Conditions In this example, a cylindrical tank 30 ft in diamet er is designed for an internal pr essur e of 5 lbf/in. 2 gauge in the vapor space. The plate material is ASTM A131, Grade A, for thicknesses of 1 / 2 in. and less. The top course of the buttwelded cylindrical sidewall is 1 / 4 in. thick, including a 1 / 16 in. corrosion allowance. The roof is a butt-welded sphericalD ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS F-13
dome with an internal radius of 30 ft and a thickness of 1 / 4 in., including a 1 / 16 in. corrosion allowance. The maximum design liquid level is 6 in. below the plane of t he juncture of the roof and sidewall. F.4.1.2 Problem The problem in this example is to design the compression- ring regio n at the juncture of the roof and cylindrical sidewall. F

.4.1.3 Solution From Figure 5-5, cos = 15/30 = 0.5. Hence, θ = 60 degrees and sin θ = 0.866. Equations 7 and 13 in 5.10.2.5 gov ern the design of the roof an d sidewall because the term ( W + F ) ÷ A t is negligible compared with P g . T

1 = T 2 = (0.5) (5) (360) = 900 lbf/in.
T

2 s = (5) (180) = 900 lbf/in. From 5.12.4.2 and 5.12.4.3, using Equations 24 through 27,

w h = 0.6 360 0.25 0.0625

– ( ) 4.9 in. =

w c = 0.6 180 0.25 0.0625

– ( ) 3.5 in. = Q

= (900) (4.9) + (900) (3 .5) – (900) (180) (0.866) = – 133,000 lb
A c = 133,000/15,000 = 8.86 in. 2 The area of participating width of t he roof plate is determined as follows: 4.9 (0.25 – 0.0625) = 0.92 in. 2 The area of participating width of the sidewall plate is determined as follows: 3.5 (0.25 – 0.0625) = 0.66 in. 2 The total area provided is 1.58 in. 2

. From 5.12.5.3, the required additional area is 8.86 – 1.58 = 7.2 8 in. 2

. From 5.12.5.1, the required horiz ontal projection of the ef fectiv e compre ssion-ring region is 0.015 R c = 0.015 × 180 = 2.7 in. The horizontal projection of the roof plate within the compr ession-ring region is 4.9 × 0.866 = 4.25 in., which fulfills the requirement of 5.12.5.1. The required area and horizontal projection can be provi ded by any of the st andard angles listed in Table F-3. Any of these angles may be used in accordance with the details of Figure 5-6, detail a or b, but if details c, h, or i in Figure 5-6 were intended, the net area of the angle must be calculated by deducting the area expected to be lost by corrosion from that p ar t of the angle surf ace that is exposed to the interior of the tank. The net area of the angle mustF-14 API S TANDARD 620 equal or exceed the calculated required additional area. The area can also be provided with a bar or channel section as illustrated in Figure 5-6, details d through g, with proper consideration given to the 0.015 R c minimum width, the 16 t maximum width, and the net area after deduction of the corroded thickness. No bracing is required for any of the previously listed angles because in no case does the widt h of any leg exceed 16 times its thickness (see 5.12.5.8). The centroid of the compression region shall be checked to meet the conditions of 5.12.5.2. T

able F- 3—Cross-Sectional Area of Standard Angles: Example 1 (See F.4.1.3) Angle Dimensions (in.) Cross-Sectional Area (in. 2 ) 6 ×6 × 3 / 4 8.44 5 ×5 × 7 / 8 7.98 9 ×4 × 5 / 8 7.73 8 ×6 × 9 / 16 7.56 8 ×4 × 3 / 4 8.44 7 ×4 × 3 / 4 7.69 6 ×4 × 7 / 8 7.98 

### F.4.2 Example 2 F.4.2.1 Given Conditions In this example, a cylindrical tank 75 ft in diameter is designed for an internal pressure of 0.5 lbf/in. 2 gauge in the vapor space. The plate material is ASTM A131, Grade B, steel for thicknesses of 1 / 2 in. and less. The top course of the butt-we lded cylindrical sidewall is 1 / 4 in. thick. The roof is a c onical shape with a 2:12 slope and a 1 / 4 in. thickness with single fu ll-fillet welded lap joints. The maximum desi gn liquid level is 6 in. below the plane of juncture of the roof and sidewall. F.4.2.2 Problem The problem in this example is to design the compression- ring regio n at the juncture of the roof and cylindrical sidewall. F

.4.2.3 Solution From Figure 5-5, tan α = 12/2 = 6.0. Hence, α = 80.54°, sin α = 0.9864, and cos α = 0.1643. R c = R 3 = 37.5 ft = 450 in. At the edge of the roof, R 2 = 450/0.1643 = 2740 in. Because of the relatively low pressure, the weight of the roof plate is a practical factor. In view of the small dif ference between the conical area of the roof and th e projected area on a horizontal plane, W , can be calculated with sufficient accuracy by using the unit weight of 10.2 lb/ft 2 for the 1 / 4 -in. roof plate and the cross-sectional area of the tank at the roof-sidewall juncture. F is zero because no internal or external ties, braces, diaphragms, trusses, columns, skirts, or other structural supports are attached to the roof. For practical purposes, in this example ( W + F )/

A t

= 10.2/144. W must be given a negativ e sign in this case because it acts in the direction opposite from P , and P is positive (see the definition of W in 5.10.1).D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS F-15 From 5.10.2.5, using Equations 8 and 9,
T 1 450 2 ( ) 0.1643 ( ) ---------------------------- 0.5 ( ) 10.2

– 144 ------------- + = = 588 lbf/in.
T

2 0.5 ( ) 450 ( ) 0.1643 ------------------------- 1370 lbf/in. = = T

2 s = (0.5) (450) = 225 lbf/in. From 5.12.4.2 and 5.12.4.3, using Equations 24 – 27,

w h = 0.6 2740 ( ) 0.25 ( ) 15.7 in. =

w c = 0.6 450 ( ) 0.25 ( ) 6.4 in. = Q = (1370) (15.7) + (225) (6 .4) – (588) (450) (0.9864)

= 240,000 lb
A c = 240,000/15,000 = 16.0 in. 2
NOTE The width of lap-welded roof plate, w h , must be used in calculating the force Q , but the lap-welded roof plates cannot be given credit for contributing to the area required for resisting th e compressive force or providing a width of horizontal proje ction of the compressive-ring region (see 5.12.2). The area of the participating width of the roof plate (lap- welded) is 0.00 in. 2

. The area of the participating width of the shell plate is 6.4 × 0.25 = 1.6 in. 2

. The required additional area (see 5.12.5.3, Item a) is 16.0 – 1.6 = 14.4 in. 2

. The required horizontal projection of the effectiv e compression-ring region (see 5.12.5.1) is 0.015 R c = 0.015 × 450 = 6.75 in. Because the lap-welded construction of the roof may not be used to satisfy the horizontal wid t h requirement, the horizontal projection of th e roof plate with the compression-ring region must be provided by the added member. Angles may not be a practical method of providing the r equir ed additional area and horizont al projection. Bars or channels may be furnished as illustrated in Figure 5-6, details d through g , with proper consideration given to the 0.015 R c minimum width requirement and the requirements conc erning the bracing of the compression ring, where applicable. 

### F.4.3 Example 3 F.4.3.1 Given Conditions In this example, a cylindrical tank 62 ft 6 in. in diameter is designed for an internal pr essu re of 4 lbf/in. 2 gauge in the vapor space. The plate material is ASTM A131 steel with appropriate grades for different plate thicknesses in accordance with Table 4-1 for a des ign metal temperature less than 65 °F but not less than 25 °F. No corrosion allowance is required for any portion of the t ank. T he top course of the butt-welded cylindrical sidewall is 1 / 4 in. thick. The roof is a single lap-welded spherical dome with an internal r adius of 50 ft and a thickness of 1 / 4 in. The maximum design liquid level is 6 in. below the plane of t he juncture of the roof and sidewall.F-16 API S TANDARD 620
F.4.3.2 Problem The problem in this example is to design the compression- ring region at the juncture of the roof and cylindrical sidewall. F

.4.3.3 Solution From Figure 5-5, cos α = 31.25/50 = 0.625. Hence, α = 51.4 degree s and sin α = 0.781. Equations 7 and 13 in 5.10.2.5 govern the desi gn of the roof and sidewall, since the te rm ( W + F )/

A t

is negligible compared with P g . T

1 = T 2 = ( 1 / 2 ) (4) (600) = 1200 lbf/in.
T

2 s = (4) (375) = 1500 lbf/in. From 5.12.4.2 and 5.12.4.3, using Equation (24) through Equation (27),

w h = 0.6 600 ( ) 0.25 ( ) 7.34 in. =

w c = 0.6 375 ( ) 0.25 ( ) 5.80 in. = Q = (1200) (7.34) + (1500) (5.80) – (1200) (375) (0.781)

= –334,000 lb
A c = 334,000/15,000 = 22.3 in. 2
NOTE The width of the lap-welded roof plate, w h , must be used in calculating the force Q , but the lap-welded roof plates cannot be given credit for contributing to the area required for resist ing the compressive force or providing a width of horizontal pr ojection of the compression-ring region (see 5.12.2). The area of the participating width of the roof plate (lap-welded) is 0.00 in. 2 The area of the partic ipating width of the sidewall plate is 5.80 × 0.25 = 1.45 in. 2 The total area provided is 1.45 in. 2

. From 5.12.5.3, the required additional area is 22.3 – 1.45 = 20.85 in. 2 . Since this area cannot be provided by standard angles, a detail employing a bar, ring girder, or channel must be used. A bar 1-in. thick should be assumed, as illustrated in Figure 5-6, det ail e. From 5.12.4.2 and 5.12.4.3, using Equations 24 through 27,

w h = 0.6 600 ( ) 1 ( ) 14.7 in. =

w c = 0.6 375 ( ) 0.25 ( ) 5.80 in. = Q = (1200) (14.7) + (1500) (5.8) – (1200) (375) (0.781)

= – 325,160 lb
A c = 325,160/15,000 = 21.68 in. 2 The area of the participating widt h of the compression ring is 14.7 × 1 = 14.70 in. 2 The area of the participating width of the sidewall plate is 5.8 × 0.25 = 1.45 in. 2 . The total area provided is 16.15 in. 2

.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS F-17 Since the required area, A c , is larger than the total area provided, the area of the compression ring must be increased. This can be accomplished by extending the bar outside the sidewall. The additional width required is computed as follows: 21.68 15.15

– ( ) 1.00 ------------------------------------- 5.53 in. = The bar extension is less than 16 t maximum for projecting parts of a compression ring that is not braced (see 5.12.5.8). The total width of the compression bar is 14.7 + 5.53 = 20.23 in. From 5.12.5.1, the required horiz ont al projection of the effectiv e compression-ring region is 0.015 R c = 0.015 × 375 = 5.62 in. The horizontal projection of the compression ring is 14.7 × 0.781 = 11.5 in., which fulfills the r equir ement of 5.12.5.1. The centroid of the compression region shall be checked to meet the conditions of 5.12.5.2. 

### F.5 Design of Reinforcement for Si ngle Openings in Tank Walls F.5.1 Example 1 The 20 in. × 29 in. obround manhole shown in Figure F-5 is located in solid plate in the sidewall of a cylindrical storage tank 45 ft in diameter in an area where the thickness of the wall plate, t w , is 1 / 2 in. No corrosion allowance is required. The total internal pressure, P l + P g , at the horizontal centerline of the opening is 27.5 lbf/in. 2 gauge. The thickness of the wall plate, t , required by 5.10.3 for t he latitudinal unit forces, T 2 , acting at this level is 0.485 in. The manhole neck is fabricated by welding from 3 / 8 in. plate. The materials in the tank wall, the manhole neck, and the reinforcing pad conform to ASTM A516, Grade 60. The joints in the tank wall and the longitudinal joint or joints in the manhole neck are double-welded butt joints , spot radiographed in accordance with 7.16 and 7.17. The adequacy of the reinforcement and attachment welds shown in Figure F-5 shall be determined. The net thickness required for a seamless t ank wall at the horizontal center line of the opening is calculated as follows:

t r 0.485 ( ) 0.85 ( ) 0.412 in. = = The thickness required for the semici rcular ends of the manhole neck and for the roundabout stresses in the fla t portions of the neck for the pressure at the horizon tal centerline of the opening is calculated as follows:

t rn 27.5 ( ) 10 ( ) 18.000 ( ) 0.85 ( ) ------------------------------------ 0.018 in. = = NOTE A thickness of 0.17 in. is required in the flat portions of th e neck for the stresses that are parallel to the axis of the manhole; these stresses result from the beam action of the elem ents calculated as simple beams supported by the tank wall and the manhole flange. A thickness of not less than 3 / 8 in. must be provided in the entire neck to satisfy the provisions of 5.19.2, Item b. Neither of these requirements affects the value of t rn used for reinforcement. To determine the length of the manhole neck within the limits of reinforcement, the smaller of the following calculated values shall be used: (2.5)(0.5) = 1.25 in. (2.5)(0.375) + 0.5 = 1.438 in.Figure F-5—Example of a Reinforced Opening (See F.5.1) 10 in. 10 in. 9 in.10 in. 10 in.20 in.20 in.

1 / 2 in.

1 / 2 in.2 ft, 6 in.16 in.16 in. 5 in.3/8in. 3 / 8 in.3/16in.

diameter

45 ftLimits ofreinforcement

5 in. Assumed critical section for computing

attachment welds A

A Section A-A F-18 API S TANDARD 620

The minimum size of the outer fillet weld permitted by Figure 5-8, Panel k, is calculated as follows: 0.5 ( ) 0.5 ( ) 0.707 ------------------------ 0.35 in. = The minimum size of the inner fillet weld permitted by Figure 5-8, Panel k, is calculated as follows: 0.7 ( ) 0.375 ( ) 0.707 ------------------------------ 0.37 in. = Therefore, the weld sizes used meet the mi nimum requirements of Figure 5-8, Panel k. The area of reinforcement requirement at and between the vert ical centerlines of the semi circular ends is calculated as follows : A r

= (20)(0.485)(0.85) = 8.25 in. 2D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS F-19 The areas of reinforcement provided are as follows: a) From excess thickness in the tank wall,
A 1 = 20 × (0.5 – 0.412) = 1.76 in. 2 b) From excess thick ness in the manhole neck,
A 2 = 2 × 1.25 × (0.375 – 0.018) = 0.89 in. 2 c) In fillet welds,
A 3 = 4 × 0.5 × (0.375)2 = 0.28 in. 2 d) In the reinforcing pad,
A 4 = (32 – 20.75) × 0.5 = 5.62 in. 2 The total area of reinforc ement provided is 8.55 in. 2 , which is adequate. The allowable unit stress values for t he attachment elements ar e as follows: a) For the outer and inner fillet welds, 18,000 × 0.6 0 = 10,800 lbf/in. 2 b) For shear across the groove weld, 18,000 × 0.8 × 0.87 5 × 0.75 = 9450 lbf/in. 2 c) For tension across the groove weld, 18,000 × 0.875 × 0.75 = 11 ,81 0 lbf/in. 2 d) For shear in the manhole neck, 18,000 × 0.8 × 0.87 5 = 12,6 00 lbf/in. 2 The strengths of the attachment elements beyond the critical section shown in Figure F-5 are as follows: a) Element 1 . For the outer fillet weld,

π 2 --     32 ( ) 2 ( ) 5 ( ) – 0.375 ( ) 0.707 ( ) 10.800 ( ) = 115,300 lb b) Element 2 . For the inner fillet weld,

π 3 --     20.75 ( ) 0.375 ( ) 0.707 ( ) 10.800 ( ) = 62,200 lb c) Element 3 . For the groove weld in shear , π 3 --     20.75 ( ) 0.1875 ( ) 9.450 ( ) 38.600 lb =F-20 API S TANDARD 620 d) Element 4 . For the groove weld in tension,

π 3 --     20.75 0.375 + ( ) 0.5 ( ) 11.810 ( ) 131.000 lb =

e. Element 5 . For the manhole neck in shear,

π 3 --     20.375 ( ) 0.375 ( ) 12.600 ( ) 101.100 lb = For the investigation of possible paths of failure through the attachment elements, the following loads and strengths shall be compared. The combined load on Elements 1, 3, and 5, which attach the add ed reinforcement to the t ank wall, is calculated as follows: (8.25 – 1.76)(18,000) = 116,900 lb The combined strength of Element s 1, 3, and 5 is as follows: 115,300 + 3 8,600 + 101,100 = 255,000 lb This value is more than adequate. The combined load on Elements 1 and 4, which attach to the tank wall the added reinforcement plus that section o f the manhole neck which coincides with the thickness of the tank wall, is calculated as follows: [8.25 – 1.76 + (2)(0.5)(0.375)](18,000) = 123,600 lb The combined strength of El ements 1 and 4 is a follows:
1 15, 300 + 131,000 = 246,300 lb This value is more than adequate. The combined load on Elements 2 and 5, from the standpoint of developing the strength of the reinforce m ent in the manhole neck, is as follows: (0.89) (18,000) = 16,000 lb The strength of either of these el ements alone exceeds this requirement. 

### F.5.2 Example 2 The 20-in. inside-diameter nozzle shown in Figure F-6 is located in solid plate in the sidewall of a cylindrical storage tank 148 ft in diameter in an area w here the thickness of the wall plate, t w , is 1 1 / 2 in. A corrosion allowance of 0.10 in. is required on all surfaces of the tank exposed to the stored liquid. The total internal pressure, P l + P g , at the center of the opening is 24.9 lbf/in. 2 gauge. The thickness of the wall plate, t , required by 5.10.3 for t he latitudinal unit forces, T

2 , acting at this level is 1.44 in. The noz zle neck is fabricated by welding from 1 / 2 -in. plate. The materials in the tank wall, the nozzle neck, and the reinforcing pad conform to AST M A442, Grade 55. The main joints in the tank wall are fully radiographed, double-welded butt joints. The longitudinal joint in the nozzle neck is of the same type but is not radiographed; however, the longitudinal joint and all other parts of the nozzle-and-wall-plate assembly have been shop stress relieved after fabrication, as required by 5.25. The adequacy of the reinforcement and attachment welds shown in Figure F-6 shall be determined.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS F-21 The net thickness required for a seamless tank wall at the horizontal centerline of the opening, exclusive of the corrosion allowance, is calculated as follows:

t r = (1.44 – 0.1)(1.00) = 1.34 in. The net thickness required for the nozzle neck, exclusive of the corrosion allowance, is calculated as follows:

t rn 24.9 ( ) 10 0.1 + ( ) 16,500 ( ) 0.85 ( ) --------------------------------------- 0.018 in. = =
NOTE A net thickness of not less than 3 / 8 in., exclusive of the corrosion allowance, must be provided in the nozzle neck to satisfy the provisions of 5.19.2, Item b, but th is requirement does not affect the value of t rn used for reinforcement computations. To determine the length of the nozzle neck within the limits of reinforcement, the smaller of the following calculated values shall be used: (2.5)(1.5 – 0.1) = 3.5 in. (2.5)(0.5 – 0.1) + 1.5 = 2.5 in. The minimum sizes of the attachment welds permitted by Figure 5-8, Panel l, are calculate d as follows: a) For the outer fillet weld, 0.5 ( ) 0.75 ( ) 0.707 --------------------------- 0.53 in. = b) For the inner fillet weld, 0.25 ( ) 0.707 -------------- 0.35 in. = c) For the groove weld between t he tank wall and the nozzle neck, (0.7)(0.5 – 0.1) + 0.1 = 0.38 in. d) For the groove weld between the p ad and the nozzle neck, (0.7)( 0.5 – 0.1) = 0.28 in. Therefore, the weld sizes used meet the mi nimum req uirements of Figur e 5-8, Pa nel l. The area of reinforcement required at the vertical centerline of the opening is calculated as follows:
A r

= [20 + (2) (0.1)] (1.44 – 0.1) (1.00) = 27.07 in. 2 The areas of reinforcement provided are as follows: a) From excess thickness in the tank wall,
A 1 = (20 + 0.2) × (1.5 – 0.1 – 1.34) = 1.21 in. 2 b) From excess thickness in the nozzle neck,
A c = 2 × 2.5 × (0.5 – 0.1 – 0.018) = 1.91 in. 2Figure F-6—Example of a Reinforced Opening (See F.5.2) 5.05 in.0.4 in.10.1 in.10.1 in.0.1 in.1/2in.

5 in.

1 1 / 2 in. 3 / 8 in. 5 / 8 in. 1 1 / 2 in.Limits ofreinforcement 10.1 in. 20 in.365/8in.20.2 in.20.2 in. 5 / 8 in. 5 / 18 in.

0.1 in. corrosion3 ft, 4 in. 148 ft diameter See note

Assumed critical section for computing attachment welds A A

Section A-A NOTE Upon computation, a 5 / 8 in. outer fillet weld was found to be inadequate for strength requirements. The outer weld should be a 1 in. fillet weld. F-22 API S TANDARD 620

c) In fillet welds,
A 3 = 2 × 0.5 × [(0.625)2 + (0.375)2] = 0.53 in. 2 d) In the reinforcing pad,
A 4 = 1.5 × (36.625 – 21) = 23.44 in. 2 The total area of reinforc ement provided is 27.09 in. 2 which is adequate. The allowable unit stress values for t he attachment elements ar e as follows: a) For the outer and inner fillet welds, 16,500 × 0 .60 = 9900 lbf/in. 2D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS F-23 b) For tension across the groove welds, 16,500 × 0.875 × 0.70 = 10,10 0 lbf/in. 2 c) For shear in the nozzle neck, 16,500 × 0.8 × 0.87 5 = 1 1,500 lbf/in. 2 The strengths of the attachment elements beyond the critical section shown in Figure F-6 are as follows: a) Element 1. For the outer fillet weld,
π 2 --     36.625 ( ) 2 ( ) 5.05 ( )

– 0.625 ( ) 0.707 ( ) 9900 ( ) = 207,400 lb b) Element 2. For the inner fillet weld,
π 3 --     21 ( ) 0.375 ( ) 0.707 ( ) 9900 ( ) 57 9000 lb , = c) Element 3. For the groove weld between the t ank wall and the nozzle neck, in tension,
π 3 --     21 ( ) 0.625 0.1

– ( ) 10.100 ( ) 116.900 lb = d) Element 4. For the groove weld between the pad and the nozzle neck, in tension,
π 3 --     21 ( ) 0.3125 ( ) 10.100 ( ) 69 400 lb , = e) Element 5. For the nozzl e neck in shear,
π 3 --     20.6 ( ) 0.4 ( ) 11.500 ( ) 99.500 lb = For the investigation of possible paths of failure through the attachment elements, the following loads and strengths shall be compared. The combined load on Elements 1 and 5, which attach the added reinfo rcement to the t ank wall, is calculated as follows: (27.07 – 1.21)(16,500) = 426,700 lb The combined strength of Elem ent s 1 and 5 is as follows: 207,400 + 99,500 = 306,900 lb This value is inadequate. If the size of the outer fillet weld were increased to 1 in. instead of 5 / 8 in., the strength of El ement 1 would become the following:
π 2 --     36.625 ( ) 2 ( ) 5.05 ( ) – 1

. 0 ( ) 0.707 ( ) 9900 ( ) = 331,800 lb The combined strength of Elements 1 and 5 would now become the follow ing:
3 31,800 + 99,500 = 431,300 lbF-24 API S TANDARD 620 This value would be adequate. Hence, the size of the outer fillet weld shall be increased to 1 in. The combined load on Elements 1 and 3, which attach to the tank wall the added reinforcement plus that section o f the nozzle neck which coincides with the thickne ss of the tank wall, is calculated as follows: [(27.07 – 1.21) + (2) (1.4) (0.4)](16,500) = 445,670 l b The combined strength of Element s 1 and 3, based on the size of the outer fillet weld being increased to 1 in., is as fol lows: 331,800 + 116,900 = 448,700 lb This value is adequate. The combined load on Elements 2, 4, and 5, from the st andpoint of developing the strength of the r e inforcement in the nozzle neck, is calc ulated as follows: (1.91) (16,500) = 31,500 lb The strength of any one of these three el ement s alone exceeds this r equirement. 

### F.5.3 Example 3 A cylindrical nozzle with a 12-in. inside diameter is located in solid plate in the sidewall of a cylindrical storage tank 60 ft in diameter so that its axis lies in a horizontal plane and forms an angle of 55° with a perpendicular to the sidewall at the point of intersecti on, as shown in Figure F-7. The thickness of the sidewall plate, t w , in this area is 5 / 8 in., and no corrosion allowance is required. The total internal pressure, P l + P g , at the center of the opening is 26.1 lbf/ in. 2 gauge. The thickness of the wall plate, t , required by 5.10.3 for the latitudinal unit forces, T 2 , acting at this level is 0.57 in. The nozzle neck is seam less steel pipe and conforms to ASTM A53, Grade A; the materials in the tank wall and reinforcing pad conform to ASTM A442, Grade 55. The main joints in the tank walls are fully radiographed, double-welded butt joints. The adequacy of the reinforcement and attachment welds shown in Figure F-7 shall be determined. The net thickness required for a seamless t ank wall at the horizont al center line of the opening is calculated as follows:

t r = (0.57)(1.00) = 0.57 in. The thickness required for the nozzl e neck is calculated as follows:

t rn 26.1 ( ) 6 ( ) 1.00 ( ) 14.400 ( ) 1.00 ( ) -------------------------------------- 0.011 in. = =
NOTE A thickness of 3 / 8 in. must be provided in the nozzle neck to satisfy t he provisions of 5. 19.2, Item b, but this requirement does not affect the value of t rn used for reinforcement computations. To determine the length of the nozzle neck within the limits of reinforcement, the smalle r of the following calculated values shall be used: (2.5)(0.625) = 1.56 in. (2.5)(0.375) + 0.75 = 1.69 in.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS F-25 The minimum size of the outer fillet weld permitted by Figure 5-8, Panel m, is calculated as follows: 0.5 ( ) 0.625 ( ) 0.707 ------------------------------ 0.44 in. = The minimum size of the inner fillet weld permitted by Figure 5-8, Panel m, is calculated as follows: 0.25 0.707 ------------- 0.35 in. = Therefore, the weld sizes used meet the mi nimum requirements of Figure 5-8, Panel m. The area of reinforcement required at the vertical centerline of the opening is calculated as follows:
A r = (12)(0.57)(1.00) = 6.84 in. 2 The areas of reinforcement provided are as follows: a) From excess thickness in the tank wall,
A 1 = (12) (0.625 – 0.57) = 0.66 in. 2 b) From excess thickness in the nozzle neck,

A 2 2 ( ) 1.56 ( ) 0.375 0.011

– ( ) 14.400 ( ) 16.500 ---------------------------------------------------------------------------------- = = 0.99 in. 2 c) In fillet welds,
A 3 = (2)(0.5)[(0.5)2 + (0.375)2] 0.39 in. 2 d) n the reinforcing pad,
A 3 = (19.5 – 12.75)(0.75) = 5.06 in. 2 The total area of reinforc ement provided is 7.10 in. 2 , which is adequate. The allowable unit stress values for t he attachment elements ar e as follows: a) For the outer fillet weld, 16,500 × 0.60 = 9900 lbf/in. 2 b) For the inner fillet weld, 14,400 × 0.60 = 8640 lbf/in. 2 c) For groove welds in tens ion against the nozzle neck, 14,400 × 0.75 × 0.875 = 9320 lbf/ in. 2 d) For groove welds in tension against the tank wall, 16,500 × 0.75 × 0.875 = 10,66 0 lbf/in. 2F-26 API S TANDARD 620 e) For the groove weld in shear, 16,500 × 0.8 × 0.75 × 0.8 75 = 85 30 lbf/in. 2 f) For the nozzle neck in shear, 14,400 × 0.8 × 0.87 5 = 10,0 80 lbf/in. 2 The strengths of the attachment elements beyond the crit ical section shown in Figure F-7 are as follows: a) Element 1. For the outer fillet weld,

2.22 14.25 ( ) 2 9.75 ( ) 2 + 2 ( ) 5.1 ( )

– [ ] × (0.500)(0.707)(9900) = 98,500 lb b) Element 2. For the inner fillet weld,

2.22 10.77 ( ) 2 6.56 ( ) 2 + 2 ( ) 5.1 ( )

– × (0.375)(0.707)(8640) = 39,400 lb c) Element 3. For the groove weld between the tank wall and the nozzle neck, in tension, (17.2)(0.625)(10,660) = 114,500 lb d) Element 4. For the groove weld between the pad and the nozzle neck, in tension,
2.22 10.58 ( ) 2 6.375 ( ) 2 + 2 ( ) 5.1 ( )

– [ ] × (0.75) (9320) = 120,000 lb e) Element 5. For the nozzle neck in shear , 2.22 10.39 ( ) 2 6.19 ( ) 2 – 2 ( ) 5.1 ( )

– [ ] × (0.375)(10,080) = 62,900 lb f) Element 6. For groove welds in shear , (1 7.2)(0.1875)(8530) = 27,500 lb For the investigation of possible paths of failure through the att achment elements, the following loads and strength s shall be compared. The combined load on Elements 1, 5, and 6, which attach the add ed reinforcement to the t ank wall, is calculated as follows: (6.84 – 0.66)(16,500) = 102,000 lb The combined strength of Element s 1, 5, and 6 is as follows: 98,50 0 + 62,900 + 27,500 = 188,900 lb This value is adequate.Figure F-7—Example of a Reinforced Opening (See F.5.3)3/8in.4 14 1 /

4 in.6 in. 6 in. 14 1 /

2 in.

10.2 in. 10.4 in.

5.1 in. 6 in. 6 in.

5 / 8

in. 3 / 4

in. A

A 1 /

4 in. min.

55° 1 / 2 in.

3 / 8 in.3/8in.93/4in.93/4in.191/2in.12 in. 12 in.6 in.6 in.3/8in.

3 / 16 in. at root1/2in. 60 f diame Line of reinforcement

Section A-A Horizontal section through opening

Assume critical section for computing attachment welds D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS F-27

The combined load on Elements 1 and 3, which attach to the tank wall the added reinforcement plus that section of the nozzle neck which coincides with the thickne ss of the tank wall, is calculated as follows:

6.84 0.66

– ( ) 2 ( ) 0.625 ( ) 0.375 ( ) 14.400 16.500 ----------------     + × (0.500) (0.707) (9 900) = 98,500 lb The combined strength of Elem ents 1 and 3 is as follows: 98,500 + 114,500 = 213,000 lb This value is adequate. The combined load on Elements 2, 4, and 5, from the st andpoint of developing the strength of the r e inforcement in the nozzle wall, is calc ulated as follows: (0.99)(16,500) = 16,400 lb The strength of any one of these three el ement s alone exceeds this r equirement. The combined load on Elements 1 and 6, from the standpoint of developing the strength of the reinfor c ing pad, is calculated as follows: (5.06)(16,500) = 83,490 lbF-28 API S TANDARD 620 The combined strength of Elem ents 1 and 6 is as follows: 98,500 + 27,500 = 126,000 lb This value is adequate. 

### F.5.4 Example 4 The pressed-steel, round manhole with a 20-in. inside diamet er shown in Figure F-8 is located in solid plate in spherical portion of a torispherical r oof on a cylindrical storage tank 72 ft in diameter. The internal pressure, P g , on the underside of the roof is 15 lbf/in. 2 gauge. The thickness, t , of the roof plate required by 5.10.3 for the spherical portion of the roof is 1 / 2 in., which is exactly the thickness provided. No corrosion allowance is required. The materials in the roof plates and manhole frame conform to ASTM A283, Grade C; the main joints in the roof are double-welded butt joints, spot radiographed in accordance with 7.16 and 7.17 . The adequacy of the reinforcement and attachment welds shown in Figure F-8 shall be determined. The net thickness required for a seam le ss tank wall at the location o f the manhole is calculated as follows:

t r = (0.5)(0.85) = 0.425 in. The thickness required for the manhol e neck is calculated as follows:

t rn 15 ( ) 10 ( ) 1.00 ( ) 15.200 ------------------------------------ - 0.010 in. = =
NOTE A thickness of not less than 3 / 8 in. must be provided in the manhole neck to sa tisfy the provisions of 5.19.2, Item b, but this requirement does not affect the value of t rn used for reinforcement computations. To determine the length of the manhole neck within the limits of reinforcement, the smaller of the following calculated values shall be used: (2.5)(0.5) = 1.25 in. (2.5)(0.4) + 0.5 = 1.5 in. The minimum size of the outer fillet weld permitted by Figure 5- 8, Panel i, is calculated as follows: 0.5 ( ) 0.5 ( ) 0.707 ------------------------ 0.35 in. = The minimum size of the inner fillet weld permitted by Figure 5-8, Panel i, is calculated as follows: 0.5 ( ) 0.7 ( ) 0.707 ------------------------ 0.50 in. = Therefore, the weld sizes used meet the mi nimum requirements of Figure 5-8, Panel i. The area required at the centerline of the opening is calculated as follows:
A r = (22.25)(0.5)(0.85) = 9.46 in. 2Figure F-8—Example of a Reinforced Opening (See F.5.4) 3 / 8 in. 1 / 2 in. radius

44 1 / 2 in. limits of reinforcement 22 1 / 4 in.

35 in. 20 in. inside diameter

1 / 2 in.1/2in.1/2in. About 0.4 in.

72 ft spherical radius D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS F-29

The areas of reinforcement provided are as follows: a) From excess thickness in the spherical head,
A 1 = (22.25)(0.500 – 0.425) = 1.67 in. 2 b) From excess thickness in the formed manhole neck,

A 2 2 π 1 2 0.5 2

– ( ) 4 ---------------------------- 1.25 1

– ( ) 0.4 ( ) 1.25 ( ) 0.010 ( )

– + = = 1.36 in. 2 c) In fillet welds,
A 3 = 2(0.5)[(0.375)2 + (0.5)2] = 0.39 in. 2 d) In the dished reinforcing collar,
A 4 = (0.5)(35 – 21.8) = 6.60 in. 2 The total area of reinforc ement provided is 10.02 in. 2 , which is adequate. The allowable unit stress value for the outer and inner fillet welds is calculated as follo ws: (15,200)(0.6) = 9120 lbf/in. 2 The strength of the attachment elements bey ond the critical sect ion is as follows: a) Element 1. For the outer fillet weld,

π 2 -- 35

( ) 9120 ( ) 0.707 ( ) 0.375 ( ) 132.000 lb =F-30 API S TANDARD 620

NOTE It should be assumed that the critical section for computing the strength of the att achment is at the centerline of the opening as indicated in 5.16.8.1. b) Element 2. For the inner fillet weld,

π 2 -- 22.25

( ) 9120 ( ) 0.707 ( ) 0.50 ( ) 112.600 lb = For the investigation of possible paths of failure through the attachment elements, the following loads and strengths shall be compared. The combined load on Elements 1 and 2, which attach the added reinforcement to the roof, is calculated as follows: (9 .46 – 1.67)(15,200) = 118,400 lb The combined strength of Elem ents 1 and 2 is as follows:
1 32,9 00 + 112,600 = 245,500 lb This value is more than adequate.G-1 

## Annex G (informative) Considerations Regarding Corrosion Allowance and Hydrogen-induced Cracking G.1 Tank Groups Based on Corrosion Rate All large low-pressure tanks of the type covered by this standard may be classified under one of the following general groups based on corrosion. a) Tanks in which corrosion rates may be definitely establis hed by reason of accurate k nowledge, available to the designer, covering the chemical characte ristics of whatever substances the tanks are to contain. Such knowledge may, in the case of standard commercial products, be obtained from published sources or, whenever special processes are involved, from reliable records compiled from results of previous observations by the user or others under similar conditions of operation. b) Tanks in which corrosion rates, although known to be re latively high, are either variable or indeterminate in magnitude. c) Tanks in which corrosion rates, although indet erminate, are known to be relatively low. d) Tanks in which corrosion effects are known to be negligible or entirely absent. 

### G.2 Corrosion Allowance G.2.1 In cases in which the rate of corrosion is closel y predictable, additional metal thickness over and above that required for the initial operating conditions shall be provided and shall be at least equal to the expected corrosion loss during the desired life of the tank. G.2.2 When corrosion effects are indeterminate before the tank is designed (although they are known to be inherent to some degree in the service for which the tank is to be used), and when corrosion is inci dental, localized, or variable in rate and extent, the best judgement of the designer must be exercised in establis hing reasonable maximum excess tank wall thicknesses. For all tanks that come under th is classification, a minimu m corrosion allowance of 1 / 16 in. shall be provided. This minimum allowance may, of course, be increased according to the judgement of the designer. G.2.3 In all cases in which corrosion effects can be shown to be negligible or entirely absent, no excess thickness need be provided. 

### G.3 Service Conditions an d Hydrogen-induced Cracking When the service conditions might in clude the presence of hydr ogen sulfide or other condi tions that may promote hydrogen-induced cracking effects, particularly near the bot tom of the shell at the shell-to-bottom connections, care shall be taken to ensure that the materials and details of construction of the tank are adequate to resist hydrogeninduced cracking. The Purchaser shall consider restricti ng the sulphur content of the material to be stored through thickness testing of the steel , laboratory surveillance tests, and the use of internal t ank coatings to reduce the possibility of hydrogen-induced cracking. The hardness of the welds contacting these environments, including the heat-affected zones, shall be considered. The weld metal and adjacent heat-affected zone often contain a zone of hardness that is well in excess of a value of 22 on the Rockwell C scale and could be expected to be more susceptible to cracking than would unwelded metal. Any har dness criteria shall be a matter of agreement between the Purchaser and the Manufacturer and shall be based on an eval uation of the expected hydr ogen sulfide concentration in the product, the possibility of mo isture being present on the inside me tal surface, and t he strength and hardness characteristics of base metal and weld metal.H-1 

## Annex H (informative) Recommended Practice for Use of Preh eat, Post-heat, a nd Stress Relief H.1 Introduction The majority of tanks covered by this standard are usually not subjected to a convent ional stress-relief post-heat treatment after erection is completed. However, the very nature of these tanks and their expected service requires that the utmost care be taken to obtain completed tanks that have the highest safety factor possible with regard to notch toughness. The thermal stress-relief treatment performed on pressure vessels is recognized as a means for reducing the probability of brittle failures. Evidence is accumulati ng that shows the benefit of improving notch toughness by metallurgical changes rather than by relief of residual stresses. 

### H.2 Thickness as it Affects Prehea t and Post-heat Requirements Plate with thicknesses below 1 / 2 in. is reasonably notch tough. In most steels, when the plate thickness exceeds 3 / 4 in. to 1 in., notch toughness, particularly as welded, decreases sharply. The decrease in notch toughness can be minimized by the conventional post-heat treatment and, in many steels, by preheat treatment. The benefits of preheat treatment in steels 3 / 4 in. to 1 in. thick have been demonstrated; similar benefits could be expected in thicker steels, but sufficient experimental data are currently lacking. 

### H.3 Post-heat Treatment (Stress Relief) The post-heat treatment now performed on pressure vessel s is of established value, although the mechanism by which the improvement is realized may be open to debate. Po st-heat treatment of tank sections when the plate thickness exceeds 1 1 / 4 in. is required as stated in 5.25. In special ca ses, the possibilities of post-heat treatment after erection should be explored. Post-heating an insulated tank may be possible if an ample source of heat is readily available and if the rigidi ty of the tank is adequate. When the service conditi ons are expected to produce stress corrosion cracking, relief of stresses is necessary. Preheat treatment has not been shown to be an adequate substitute for post-heat treatment when it is applied to avoid stress corrosion cracking. 

### H.4 Preheat Treatment Many laboratory tests have shown preheat treatment of carbon steel to 300 °F to 400 °F to be the equivalent of the post-heat treatment at no less than 1100 °F insofar as the physical properties of the weldment are concerned. 41,42 Some tests have indicated a slight advantage of the post- heat treatment. Most of the tests have been made on plates 3 /

4 in. to 1 in. thick; results must be viewed with caution if preheating is applied appreci ably beyond this thickness range. However, for all practical purposes, improvement re sulting from preheating is sufficiently well established so that preheat should be considered for fi eld fabrication of plates over 3 / 4 -in. thick whenever toughness of the tank is highly desired and the thermal post- heat treatment is impractical. Preheating should be performed by heating and maintaining this heat in appreciable lengths of the joint to be welded, preferably using a strip burner wi th a mild flame rather than a harsh flame such as that from a cutting torch. Electrical strip heaters are available and have been found to be sati sfactory. The preheat of 300 °F should be checked with a

41 Robert D. Stout, “The Preheating and Postheating of Pressu re Vessel Steels,” Welding Journal New York, Research Supplement 32 [1], 14s – 22s, 1 953 (including bibliography). 42 Harry Uldine, “Preheat Versus Postheat,” a paper prepared in connection with the work of Subcommittee 8 of ASME B31.1.H-2 API S TANDARD 620 temperature-sensitive crayon, or similarl y accurate means, so that the steel 4 in. (or four times the plate thickness, whichever is greater) on each side of the joint will be ma intained at the minimum preheat temperature. Ring burners or heaters are recommended for nozzle and manway welds. At no time during the welding should the base metal fall below a temperature of 300 °F.I-1 

## Appendix I (informative) Suggested Practice for Peening I.1 General Peening is used to eliminate distortion in thin plates and to prevent cracking in thick plates when the weld is built up of several layers of weld metal. Peening is intended to r educe the internal stresses introduced in welded structures because the weld shrinks more than the relatively cold adjacent base metal. Proper peening strains the stressed weld metal above its yield point and, in this manner, adjusts t he stresses in proportion to the amount of flow caused by peening. 

### I.2 Effective Peening Effective peening occurs below the red-hot temperature. Peening is wasted when it occurs above a temperature at which the weld metal begins to take on strength. The first two layers and the last layer of weld metal must not be peened. To be effective, peening must move the weld metal. The shape, size, and hardness of peening tools are important. Bruises and surface roughness of the weld metal caused by peening are not objectionable, since these are melted by the deposition of subsequent layers of weld metal. 

### I.3 Peening as an Alternati ve to Thermal Stress Relief When peening occurs as an alternative to thermal stress relief under permissible procedures, it shall be done carefully to minimize distortion of the weldment. Some steels that are weldable have to be peened sufficiently to temporarily create stresses in the reverse direction, whic h will disappear on cooling. T hese are the steels which get so hard when they are cold that the metal is only burnished by the peening tool instead of being cold-worked. When peening is done to avoid the formation of cracks on welds that are subsequently to be stress relieved, underpeening may be satisfactory. 

### I.4 Factors Involved in Peening I.4.1 General For peening to be acceptable or dependable as a means of stress relief, a thorough study must be made of all the factors involved, including the type of st eel, the thickness of the weld, and the thicknesses of successive layers of welding (see 6.19.2). Two guides to satisfac tory results are outlined in I.4.2 and I.4.3. 

### I.4.2 Amount of Peening Necessary An approximation of the amount of peening that is necessary may be obtained by welding two small plates of a given material and thickness, with one plate held rigidly and the other free to move as the weld shrinks. The peening required to overcome the shrinkage gives a fair idea of the degree of peening that will be required in the actual operation. 

### I.4.3 Measurements during Welding and Peening Punch marks shall be made on oppos ite sides of the weld, and the distance be tween these marks shall be kept within 1 / 32 in. by peening during the welding of the seam. The init ial measurement shall be made after two layers of weld metal have been deposited. After the weld has been built up to a depth of 1 1 / 4 – 1 1 / 2 in., the likelihood of deviation in the distance between punch marks is greatly reduced, since the stresses caused by shrinkage of the recently deposited weld metal are more fully resisted by the cooler prior layers. Cracking of unpeened welds in ordinary steel is most likely to occur at this point. If the peening has been done so that t he deviation in distance between punch marks is kept to a minimum up to this point, the same d egree of peening for the remainder of the weld will protect the weld from cracking.J-1 

## Appendix J (Reserved for Future Use)K-1 

## Annex K (informative) Suggested Practice for Determining the Relieving Capacity Required K.1 General This annex attempts to outline a safe and reasonable practice to be used for the usual environment and operating conditions. The many variables that must be consi dered in connection with tank venting problems make it impracticable to set forth defin ite, simple rules. Engineer ing studies for any particular tank may show that it is desirable to use either a larger or smaller venting capa city than that estimated in accordance with these rules. 

### K.2 Determination of Required Capacity The aggregate capacities required for any vent valves, pr essure-relieving valves, and/or vacuum-relieving valves to be provided should be determined as follows: a) For thermal breathing and product movement, the rules gi ven in API 2000 shall be followed. The required capacity as the result of the product movement in to the tank given in API 2000, shall be multiplied by the ra tio of the absolute tank pressure to atmospheric pressure (14.7 lbf/in. 2 absolute). b) For supplemental safety pressure-relie ving devices to take care of extra venting capacity required in case of external fire exposure, the rules in API 2000 shall be followed.L-1 

## Annex L (normative) Seismic Design of API 620 Storage Tanks L.1 Scope This annex provides minimum requirements for the design of welded storage tanks that may be subject to seismic ground motion and are designed and constructed to the API 620 standard. These requirements represent accepted practice for application to welded steel flat-bottom tanks supported at grade. This annex is based on the requirements of API 650, Annex E and uses the sa me notations except as supplemented herein. The design procedures contained in this annex are based on allow able stress design (ASD) methods All tanks designed and constructed to the requirements defined in 1.2 shall meet the requirements of API 650, Annex E unless specifically modified or augmented herein. All of the requirements contai ned in API 650, Annex E are not duplicated here, but are wholly incorporated by referenc e. Special provisions for tanks designed and constructed in accordance with Annexes Q and R are included in this annex. Application to tanks supported on a framework el evated above grade is beyond the scope of this annex. Design procedures are included for the consideration of the increased damping and increase in natural period of vibration due to soil-structure intera ction for mechanically anchored tanks. Tanks located in regions where S 1 is less than or equal to 0.04 and S S less than or equal to 0.15, or the peak ground acceleration for the ground motion defined by the regulatory requirements is less than or equal to 0.05 g , need not be designed for seismic forces; however, in these regions, tanks in SUG III sha ll comply with the freeboard requirements of this annex. Dynamic analysis methods incorporating fl uid-structure and soil-structure intera ction are permitted to be used in lieu of the procedures contained in this annex with Purchaser approval and prov ided the design and construction details are as safe as otherwise provided in this annex. The provisions for outer tanks of double walled tank systems are limited to metallic outer tanks designed and constructed in accordance with API 650 or API 620. Outer ta nks designed and constructed of other materials such as reinforced or prestressed concrete are outside the scope of this standard. 

### L.2 Notations h s is additional shell height required ab ove the sloshing wave height, mm (ft);
W

ns is the effective weight of insulation acting on the tank shell for lateral seismic load, N (lbf);
W

nr is the effective weight of insulation acting on the tank roof for lateral seismic load, N (lbf);
X

ns is the height from the bottom of the shell to the cent er of action for the insulation load on the tank shell, m (ft);
X

nsr is the height from the bottom of the shell to the cent er of action for the insulation load on the tank roof, m (ft);L-2 API S TANDARD 620 

### L.3 Special Provisions for Tanks De signed and Constructed to API 620 , 

### Annex Q and Annex R L.3.1 General For storage tanks required to meet Annex Q and Annex R, the provisions of API 650, Annex E shall be modified as shown in API 620, Annex L. If the require ments of API 650, Annex E or API 620, Annex L conflict wi th those of API 620, Annex Q or Annex R, the more conservative requirements shall apply. Special provisions for refrigerated tanks requiring per formance level designs are contained in Section L.4. 

### L.3.2 Force Reduction Factor The response modification factor for ground supported, li quid storage tanks designed and detailed to these provisions shall be less than or equal to the values shown in Table L-1Q or L-1R, as applicable. Table L-1Q—Force Reduction Factors for ASD Methods, Annex Q Tanks Anchorage System R wi , (impulsive) R wc , (convective) Inner Tank: Steel (nickel, or stainless)

Self-anchored 1.5 1.0 Mechanically-anchored 1.75 1.0 Aluminum

Self-anchored 1.25 1.0 Mechanically-anchored 1.5 1.0 Outer Tank (Empty): Self-anchored 2.0 n/a Mechanically-anchored 2.0 n/a

Table L-1R—Force Reduction Factors for ASD Methods, Annex R Tanks Anchorage System R wi , (impulsive) R wc , (convective) Inner Tank: Self-anchored 2.25 1.5 Mechanically-anchored 2.5 1.5 Outer Tank (Empty): Self-anchored 2.0 n/a Mechanically-anchored 2.0 n/a The inner and outer tanks may be decoupled for seismic design of the tank and anchorage and assumed to act independently. However, if the inner and outer tanks are supported by a comm on foundation, the seismic foundation loading shall be calculated using the less er response modification values for eit her the inner tank or outer tank for both tanks and a dynamic analysis shall be per formed to determine the combined effect. NOTE The above R w factors are applied for CLE (or SSE) event. For OLE (or OBE), the elastic design ( R w = 1.0) is used.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS L-3


### L.3.3 Resistance to Design Loads L.3.3.1 Allowable Stresses For Annex R tanks, design allowable stresse s shall be per API 620, Section R.3.3. For Annex Q t anks, design allowable stre sses shall be per API 620, Section Q.3.3. L.3.3.2 Annular Bot t om Plates Thickness shall be per API 650, Annex E (with loads modi fied by requirement s of this ann ex) plus corr osion allowance unless a thickness increase is required for compli ance with Annex R or Annex Q. Annular plate width shall be per API 650, Annex E (with loads modified by requirement s of this annex) unless a width increase is required for compliance with Annex R or Annex Q. Thickness used for determination of API 650, Annex E width shall be based on the thickness required for seismic product hold down. L.3.3.3 Resistance to Sliding The tank system, whether self-anchored or mechanically -anchored, shall be configu red such that the overall horizontal shear force at the base of t he tank does not exceed the friction ca pacity as defined in API 650, Section E.7.6. Mechanical anchorage shall not be used to resist sliding. L.3.3.4 Insulation Load For tanks designed and constructed with an outer tank conta i ning loose fill in sulation in the annular space between the tanks, the insulation weight shall be divided equally to the inner and outer tank wall for seismic lateral loads unless a more rigorous analysis is performed to determine the distribution. The insulation within the annular space shall not be used to calculate resistance to overturning. Insulation on the roof or suspended deck shall be applied to the tank supporting the load at t he point or center of gravity of attach ment and may be used to resist overturning. For single wall tanks with insulation or double wall tanks wi th the insulation adhered to the plate surface, the additional weight of the insulation shall be included and may be include d in the t ank weight, w t , used to resist overturning. The insulation weight shall also be incl uded in the definition of the terms, W T and W RS . Modify Equation (E.6.1-2) of API 650, Annex E as shown in Equation (L-1):
V

i A i W s W r W f W i W ns W nr ++++ + ( ) = (L-1) Modify Equation (E.6.1.5-1) and Equation (E.6.1.5-2) of API 650, Annex E as shown in Equation (L-2) and Equation (L-3): Ringwall Moment, M rw : M

rw A i W i X

i W s X

s W r X

r W ns

X

ns W nr

X

nr + + + + ( ) [ ] 2 A c W c X

c ( ) [ ] 2 + = (L-2) Slab Moment, M s : M

s A i W i X

is W s X

s W r X

r W ns X ns W nr

X

nr + + + + ( ) [ ] 2 A c W c X

cs ( ) [ ] 2 + = (L-3) L.3.3.5 Additional Roof Loads When S DS

> 0.33 g and the tank is classified as SUG III; and equipment loads such as pumps, platforms, piping platforms supported directly by the roof exceed 25 % of the combined weight of the roof and shell, W r + W s , a dynamicL-4 API S TANDARD 620 analysis shall be performed to determine the effective roof load and the amplified roof acceleration for the design of the roof, roof supports and superstructure supported by or suspended from the roof. L.3.3.6 Alternate Performance Basis Design If the governing regulations or project documents requi re the tank system to be designed for an operating level earthquake or to consider aftershocks, the provisions in L.4 may be used. Adjustment may be required to the definition of the ground motion (i.e. different recurrence interval). If base isolation of the tank system is pe rmitted, the requirement s of L.5.5 shall apply. L.3

. 3.7 Soil Structure Interaction Soil-Structure Interaction (SSI) per API 650 E.6.1.6 may be applied. Application of SS I to self-anchored t anks is allowed without re gard to API 650 E.6.1.6 if the R wvalue applied is not greater than those in Table L-1Q and Table L1R, as applicable. 

### L.4 Special Provisions for Tanks Re quiring Performance Level Designs L.4.1 General This section is applicable to refrigerated tanks built to API 620, Annex Q or Annex R with supplemental seismic design methods addressing an operating level earthquake (OLE), sometimes referred to as OBE, a contingency level earthquake (CLE), sometimes referred to as SSE, and an aftershock level earthquake (ALE) when required by API 625, regulations, or the Purchaser. The performance basis objectives for the gro und motions are a s follows. a) OLE—the tank system will remain ope rational with only minor repair requir ed. The t ank system should be cap able of withstanding multiple events with this ground motion without significant damage. b) CLE—the primary liquid contai ner will survive and contain th e liquid (with only minor leaks permitted) to protect the public but extensive damage may occur and the tank syste m may not be repairable after this event. This is assumed to be a singular event in the design life of the tank system. c) ALE—the primary liquid container is assumed to be damaged by the CLE event and the secondar y containment system is containing the liquid. The secondary containment is intended to survive multiple aftershocks of the ALE ground motion with minor damage and leaks. The requirements of API 650, Annex E, and L. 3 of this st andar d apply unless modified herein. 

### L.4.2 Ground Motions The definition of the ground motions to be used wi th the OLE, CLE and ALE events may vary depending on regulations for the specific location. Within the U.S. federal regulations 49 CFR 193 and NFPA 59A are the primary regulatory and standard documents for LNG storage tanks. T he user is referred to those documents or similar regulatory documents when the tank is located outside the U.S., for ground motion definitions to be used with this annex. Vertical earthquake shall be considered. A site-specific response spectrum is r equir ed for tanks located in regions w here peak ground acceleration is greater than 0.15 g or S s is greater than 0.3 g unless otherwise specified.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS L-5 The exception in API 650, Secti on E.4.6.2 limiting the upper value of the spectral acceleration, S a * under specific tank configurations is not applicable. Current U.S. requirements are based on a response spectrum with 5 % damping. If the site specific re gulations require a dif f erent damping value, and a site-specific spectrum is not required, the following factors may be applied to the impulsive spectral component to adjust the 5 % damped values to other values of damping. The convective multiplier, K , from API 650, Annex E is unchange d and equal to 1.5. Alternative adjustment factors to damping ratios are permitted providing they are based on local geotechnical data and rational analysis. Table L-2—Impulsive Damping Ratio Adjustment Damping Ratio Adjustment Factor, K

i 20 % 0.45 10 % 0.6 5 % 1.0 2 % 1.65 1 % 2.0 0.5 % 2.2 

### L.4.3 Operating Level Earthquake (OLE) L.4.3.1 General Unless otherwise defined by the governing local regulat ions, the operating level earthquake ground motion shall be defined as the motion due to an event with a 10 % probability of exceedence within a 50-year period (a 475-year recurrence interval). L.4.3.2 OLE Definiti on Based on Annex E, ASCE 7 Method To utilize API 650, Section E.4 to define the OLE ground motion, the following modifications shall be made based on the provision s in this annex. 1) Re-define the following terms for OLE only:
S

S is mapped, 10 % PE50 earthquake from the USGS data, 5-percent-damped, spectral response acceleration paramete r at short periods (0.2 sec), % g ; S 1 is mapped, 10 % PE50 earthquake from the USGS data, 5-percent-damped, spectral response acceleration paramete r at a period of one second, % g . 2) The scaling factor, Q , is not applicable. 3) API 650, Annex E, Equa tion (E.4.6.1-2) and Equation (E.4.6.1-3), do not apply. 4) Equation (E.4.6.1-1) shall be modified:
A i K i F

a S

s = 5) Equation (E.4.6.1-4) and Equation (E.4.6.1-5) shall be modified:L-6 API S TANDARD 620 When, T C ≤ T L :

A c KS D 1 1 T

c ----     2.5 KF v S 0 T S T

C -----     A i ≤ = =
When, T C > T L : A c KS D 1 T L T

c 2 -----     2.5 KF v S 0 T S T

L T

C 2 ----------     A i ≤ = =

L.4.3.3 Adjustment Factors Unless specifically permitted by the r egulations, the OLE design forces shall not be adjusted by an importance factor, I

, or force reduction factor, R . Nor shall the forces be reduced by the 0.7 multiplier (1/1.4) commonly applied to convert contingency level events to ASD methods. L.4.3.4 Damping Unless otherwise defined by regulatory r equirement s, the damping ratio for the im pulsive spectral accelerations sha ll be 5 %. L.4.3.5 Soil Structure Interaction Soil structure interaction per API 620, Section L.3.3.7 may be used for OLE design providin g the damping ratio does not exceed 10 %. L.4.3.6 Allowable Stresses Design allowable stresses shall be per AP I 620, Section L.3.3.1, increased by 33 % for load combinations including earthquake loads. L.4 .3.7 Self -anchored Inner Tank The anchorage ratio for a self-anchored inner tank, J , shall not exceed 1.0 for the OL E design combination to limit uplif t and stresses in the annular plate and corner weld. L.4.3.8 Foundation Stability The overturning ratio defined in API 650, Section E.6.2.3, Equ ation (E .6.2.3- 1) shall be equal to or greater than 3.0 for the defined OLE event. L.4.3.9 Inner Tank Freeboard Freeboard shall be provided for the OLE event in accordance wi th the following where the terms are as defin ed in API 650, Annex E δ s

= 0.42 DA f + h s An additional shell height, h s , shall be added to the calculated value abov e the sloshing height as required by the governing regulations. T he minimum value of h s for the OLE event shall be 300 mm (1ft).D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS L-7 If provided, the site-specific response spectrum may be used to determine the effectiv e spectral acceleration, A f

, in lieu of using the T L values in API 650, Annex E. Alternative sloshing height calculation methods may be used if approved by the regu latory body providing the calculated sloshin g height is not less than 80 % of the value required by these provisions. L.4.3.10 Piping Flexibility Piping, piping supports, suppor t foundations and superstructur es supporting piping att ac hed to the t ank shall be designed for the piping displacements in API 650, Table E-8. A 33 % increase in stress is permitted. L.4.3.11 Sliding Resistance The calculated sliding force at the base of the t ank shall not exceed V s . The maximum coefficient of friction, μ , shall be (tan 30 ° /1.5) where 1.5 is the factor of safe ty against sliding. The coefficient of friction selected shall consider the materials underlying the tank bottom. Anchorage may not be us ed to resist sliding. If t he sliding force exceeds the allowable, the tank shall be re-configured. L.4.3.12 Connections with Adjacent Structures The calculated or tabular displacements in API 650, Section E.7.8 shall be amplified b y 1.25 for OLE. L.4.3.13 Bottom and Shell Support The tank under-bottom insulation shall be designed to resi st the combined pressures from the product load, the overturn ing seismic load and the vertical seismic l oad. These seismic pressures may be combined by SRSS. The bearing ring under the shell shall be designed to resist the calculated OLE peak compressive force in the t ank shell due to overturning (see API 650, Section E.6.2.2), including dead and live loads. A 33 % increase in allowable bearing stress is permitted. 

### L.4.4 Contingency Level Earthquake (CLE)
L.4.4.1 General Unless otherwise defined by the gov erning local regulations, the contingency level earthquake g round motion shall be defined as the motion due to an event with a 2 % probability of exceedence within a 50-year period (a 2475-year recurrence interval) which is t he maximum considered earthqua ke in API 650, Annex E and ASCE 7. L.4.4.2 CLE Definition Based on API 650, Annex E, AS CE 7 Method To utilize API 650, Section E.4 to define the CLE ground motion, the following modifi cations shall be made based on the provision s in this annex: 1) the scaling factor, Q , is not applicable and may be set equal to a value of 1.0; 2) the response modification factor shall be as def ined in T able L1-Q or Table L1-R as applicable; 3) the importance factor, I , shall be ta ken as 1.0. L.4.4.3 Inner Tank Freeboard Freeboard shall be provided in accordance with L.4.3.9 except the value of h s shall be taken as zero unless required by the governing regulations.L-8 API S TANDARD 620

L.4.4.4 Sliding Resistance The calculated sliding force at the base of the tank shall not exceed V s . The maximum coefficient of friction, μ, shall be tan 30 ° . The coefficient of friction select ed shall consider the materials underly ing the tank bottom. Anchorage may not be used to resist sliding. If the sliding force ex ceeds the allowable, the tank shall be re-configured. L.4.4.5 Damping The damping ratio for soil-structure interaction shall not exceed 20 %. L.4

. 4.6 Allowable Stresses Design allowable stresses shall be per AP I 620, Section L.3.3.1, increased by 33 % for load combinations including ear thquake loads. 

### L.4.5 Aftershock Level Earthquake (ALE) L.4.5.1 General This design case shall be applicable only when regulations or project documents s pecifically require the t ank system to be designed or evaluated for aftershocks. Unless otherwise defined by the governing local regulat ion s, the af tershock level earthquake (ALE) ground motion shall be defined as the motion due to an event with a 2 % probability of exceedence with in a 50-year period (a 2475- year r ecurr ence interval), which is the maximum considered earthquake in AP I 650, Annex E and ASCE 7, with the spectral values reduced by 50 %. If the outer tank is not designed as a secondary cont ainment (i.e. it serves as vapor barrier and pressure boundary only and is not constructed of API 620 material suitable for the inner tank), then no design or evaluation for ALE is required by these provisions for the inner or outer tank. If the outer tank is designed as the secondary containment (i .e. constructed of API 620 materials suit able for the inner tank and designed for the product hydrostatic pressure), the outer tank, foundation and anchorage shall be designed for the ALE assuming the inner tank no l onger exists and all of the liquid is contained by the outer tank system and the following provisions apply. L.4.5.2 Modification Factors The secondary containment shall be designed for ALE while c ont aining liquid using an importance factor equal to 1.0 and response modification values in Table L-1Q or Table L-1R as applicable for the inner tank. L.4.5.3 Damping Unless otherwise defined by regulatory r equirement s, the damping ratio for the im pulsive spectral accelerations shall be 5 %. L.4.5.4 Soil Structure Interaction Soil structure interaction per API 620, Sectio n L.3.3.7 m ay be used for ALE design. L.4.5.5 Allowable Stresses Design allowable stresses shall be per API 620, Sect ion L.3.3.1 increased by 33 % for load combinations including earthquake loads.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS L-9


### L.4.6 Base isolation Base isolations systems may be used to alter the tank response to the desig n ground motions providing: 1) the inner and outer tanks are both isolated on a co mmon foundation to avoid excessive differential displacement s between the t anks and connecting internals; 2) the anchorage, internal and external piping, insulati on and other attached equipment are designed for the lar ger differential deformations associ ated with an isolated system; 3) a site-specific response spectrum is mandatory and in cludes the long term per iods necessar y to define the system response; 4) all external piping connections to t he isolated system are designed for t he calculated displacements for the actual ground motions (no I or R modifications); 5) the design is peer reviewed for technical adequacy by an indep endent party knowled geable in the design and behavior of base-isolation systems; or, the design is verified by scaled shak e-table tests or 3-dimensional nonlinear analysis applying simultaneous hor izontal and vertical time histor ies fit to the design OLE and CLE spectra and including the s upporting soil and isolators.M-1 

## Annex M (informative) Recommended Scope of the Manufacturer’s Report M.1 General This annex does not set down rigid rules for the preparation of the Manufacturer’s report. The extent of the information contained in the report—with the accompanyi ng supplementary sketches, gr aphs of tests, and possibly special items required by the Purchaser, as shown on purchase orders—cannot possibly be listed here (see 7.13). 

### M.2 Shop Stress Relief When parts of the structure are shop assemblies, which are stress relieved as called for in 5.25 and 6.18, the plans shall indicate this in the customary general notes. 

### M.3 Field Repairs or Changes When more than minor repairs or changes and/or additions are made to the structure in the field for any reason, it is assumed that both the Manufacturer and the Purchaser wil l want to have a record of these repairs or changes attached to the Manufacturer’s report. 

### M.4 Tank Certification A certificate shall be suppli ed for each tank. This practice is intended to simplify keeping the records of future inspection in separate files for convenience. When a group of tanks is being constructed on one order and in one general location some specific form of reporting other than a Manufacturer’s report may be preferred by both parties. It would seem desirable that the details on each contract be settled when the purchas e order is placed; if they are not covered in the proposal, then they shall be given as information in the inquiry. 

### M.5 Tank Certificate Wording The suggested format and wording for a certificate is as follows: WE CERTIFY that the design, material s, construction and workmanship on this low-pressure tank conforms to the requirements of API 620, Design and Construction of Large, We lded, Low-pressure Storage Tanks. Date___________ 20___ Signed _____________________________________________by ________________

I have inspected the tank described in this Manufactu rer’s report dated ___________, and state that to the best of my knowledge, the Manufacturer has constructed this tank in accordance with the applicable sections of API 620. The tank was inspected and subjected to a test of _______ lbf/in. 2 gauge.

Date___________ 20___ Signed _____________________________________________ by _______________N-1 

## Annex N (normative) Installation of Press ure-relieving Devices N.1 General Pressure-relieving devices shall be instal led so that they are readily accessible for inspection and removable for repairs. The practices suggested in API 520, Part 2, for the installation of ASME approved safety relief valves shall generally apply, with due consi deration given to the difference in pressure ranges. 

### N.2 Location If the relieving devices for gases are not located on the roof, they shall be installed on the piping connected to the vapor space, if any, as close to the tank as is practicable; if the relieving devices are v ented to atmosphere, they shall be at a sufficient height to pr event chance ignition (see API 2000). 

### N.3 Size of Tank Opening The opening from the tank leading to the relieving device shall have a size at least as large as the inlet nominal pipe size of the relieving device. 

### N.4 Discharge Pipes N.4.1 When a discharge pipe is used on the outlet side of such re lieving devices, its area s hall be not less than the area of the valve outlet, or if a single pipe provides for di scharge from several relief devices, its area shall be not less than the aggregate area of the valve out lets. The discharge pipe shall be fitted with an open drain to prevent water or other liquids from lodging in th e discharge side of the valves. N.4.2 Discharge pipes shall be supported so that no undue stre ss is placed on the valve body. Open discharges shall be placed and oriented so that the outflow is dire cted away from the tank and will not create a hazard over walkways, stairways, or operating platforms. 

### N.5 Security against Damage The assemblies of relieving devices shall be secured against damage in service, the effects of storms, or mishandling. Access ladders and platforms that meet plant safety rules are suggested. 

### N.6 Vacuum-relieving Devices A vacuum-relieving device, when used, shal l be as direct in inflow as possibl e with no pockets where moisture can collect, and it shall have no piping except a weather hood ahead of the inlet. An adequate vacuum-air inlet shall also be provided. 

### N.7 Stop Valves Stop valves, if used between the reliev ing devices and the tank to help se rvice these devices, shall be locked or sealed open, and an authorized person shall be present if this condition is changed. If the tank is in use, the authorized person shall remain there until the locked or sealed-open position of the affe cted relieving devices is restored.O-1 

## Annex O (informative) Suggested Practice Regarding Install ation of Low-pressure Storage Tanks O.1 Introduction The practices recommended in this annex are intended for genera l guidance only. They are not essentially a part of the construction rules for low-pressure storage tanks because in most instanc es sound engineering principles for safe and efficient operation will dictate t he proper procedure for each installation. 

### O.2 Marking When the owner or operator provides an additional plate to show the current operating pressure range in a tank, the plate shall be securely attached, pr eferably near the Manufacturer’s namepl ate (showing the ma rkings required by these rules). Such markings shall not be covered by the additional plate. 

### O.3 Access All openings and accessories for tanks cons tructed according to this standard shall be installed so that any periodic inspections required can be readily made. 

### O.4 Corrosion When a tank bottom rests directly on the ground, a survey shall be made to es tablish the need for cathodic protection. 

### O.5 Drainage All tanks in which water might accumulate under the hydrocarbon contents shall be provided with adequate drains that are suitably protected from freezing. 

### O.6 Fireproofing O.6.1 Although general fire prevention and fire protection measures are expected to be fully covered by other safety codes, tanks constructed according to these rules, which may be subject to fire exposure resulting from any cause, shall have their supports suitably fireproofed. O.6.2 Special consideration shall be given to provisions for ample drainage faci lities for accidental spills or leakage of flammable contents from such tank s or adjacent piping and other equipment if the contents may become ignited. O.6.3 Subject to special considerations in isolated locations, tanks in which fl ammable liquid products are stored at temperatures well above their average boiling point s hall be suitably fireproofed or otherwise protected.P-1 

## Annex P (informative) NDE and Testing Requirements Summary

Process Welds for which the Inspection and Testing is Required Reference Section

Air Test Through tell-tale holes, all welds of nozzles with single thickness rein forcing plates, saddle flanges, or integral reinforcing pads. Does not include nozzles on the underside of tank bottoms or reinforcements that are too narrow. 5.16.10, 7.18.2.3 Air Test Completed tank 7.18.2.6 Air Test Roofs of tanks not designed for liquid loading. 7.18.3.1 Air Test Annex R tanks: Shell-to-bottom weld s which are not complete penetration. R8.2.3 Air Test Annex R tanks: Completed tank. R.8.4 Air Test Annex R tanks: Outer tank of a double wall refrigerated tank. R.9 Air Test Annex Q: The shell-to-bottom welds that are not full penetration. Q.8.2.2 Air Test Annex Q: Vapor space above hydrostatic test leve l when the tank is subjected to pneumatic pressure. Q.8.5.1 & Q.8.5.4 Hydro Shell only if the roof is not designed for liquid loading. 7.18.3.1 Hydro Complete tank including the roof if so designed. 7.18.4.1 Hydro Annex R tanks. R.8 Hydro Annex Q. Q.8.1.1

MT Flush type shell connections: Nozz le-to-tank shell, Repad welds, shell-to-bottom reinforcing pad welds on the root pass, every 1 / 2 in. of deposited weld, and completed weld. 5.27.11

MT Welds attaching nozzles, manways, and clea n out openings unless given a liquid penetrant test. 7.18.2.2

MT Annex R carbon steel tanks: all butt welds not completely radiographed, cylindrical wall to bottom annular plate weld, all welds of openings that are not completely radiographed (includes progressive MT) attachment welds to primary components, and the second layer of weld on joints with permanent backing strips. R.7.5

MT Permanent and temporary attachments if not examined by PT. 7.16.3, R.7.8

PT Annex R stainless steel tanks: all butt welds not completely radiographed, cylinder wall to bottom annular plate weld, all welds of openings that are not completely radiographed (includes progressive PT) attachment welds to primary components, and the second layer of weld on joints with permanent backing strips. R.7.5

PT Welds attaching nozzles, manways, and clean out openings instead of MT if approved. Not for Annex Q tanks. 7.16.4 PT Annex Q: All longitudinal and circumferential butt-welds not completely radiographed. Q.7.5.a PT Annex Q: Cylindrical wall-to-annular plate welds. Q.7.5.b PT Annex Q: Opening welds that are not radi ographed. PT the root pass and every 1 / 2 in. of deposited weld metal. Q.7.5.c PT Annex Q: Attachment welds of non-pres sure parts to primary components. Q.7.5.d PT Annex Q: The second pass of joints on whic h backing strips ar e to remain. Q.7.5.e PT Annex Q: Base metal repairs for erection lug removal areas on primary components. Q.7.9P-2 API S TANDARD 620

RT 100 % butt welds of joints with plate thicker than 1.25 in. and tension stress greater than 0.1 times the specified minimum tensile stress of the material and where required by joint efficiency. 5.26.2, 5.26.3

RT Spot examination for all butt-welded main joints that are not 100 % radiographed except for roof joints exempted by Table 5-2, tank bo ttoms fully supported, and components designed for compressive stress only (5.26.4). 7.17.2.1 RT Flush-type shell connections. 5.27.11 RT Annex Q: 100 % of butt-welds with operating stress greater than 0.1 times plate tensile strength. Q.7.6.1

RT Annex Q: Spot RT butt-welds with operating stress less than or equal to 0.1 times the plate tensile strength. Q.7.6.2 RT Annex Q: 100 % of butt-welds around thickened insert plates. Q.7.6.3 RT Annex Q: All three plate butt joints except flat bottoms uniformly supported. Q.7.6.4 RT Annex Q: 25 % of butt-welded annular plate radial joints shall have 6-in. radiographs taken at the outside of the selected joints. Q.7.6.5

RT Annex Q: 25 % of butt-welded compression bar radial joints shall have 6-in. radiographs except as required by 5.26.3.3. Q.7.6.6

RT Annex Q: 100 % of longitudinal butt welds in pipes and pipe fittings containing liquids within the limitations of 1.3.2 except for pipe 12 in. diameter or less that has been welded without filler metal and has been hydrotested. Q.7.7.2

RT Annex Q: 100 % of longitudinal butt welds in pipes and pipe fittings containing vapor within the limitations of 1.3.2 except for pipe 18 in. diameter or less that has been welded without filler metal and has been hydrotested. Q.7.7.3 RT Annex Q: 30 % of all circumferential welded pipe joints. Q.7.7.4 RT Annex Q: 100 % of butt-welded joints used to fabricate tank fittings. Q.7.7.5 RT Annex R tanks: Primary-component butt welds. R.7.6 RT Annex R tanks: Butt welds in piping. R.7.7 VB Bottom Welds unless tested by tracer gas. 7.18.2.4

VB Annex R tanks: all bottom welds, full penetration shell-to-bottom welds, and fillet welds around bottom openings that do not receive repad pressure test. R.8.2.1, R.8.2.2, R.8.2.5 VB Annex Q: All bottom welds and full penetration joints between the shell and bottom. Q.6.3.5 VB Annex Q: Welds above hydro test level when the inner tank pneumat ic pressure is equalized on both sides. Q.6.3.6

VB Annex Q: Attachment fillets around bottom openings that cannot be tested by air pressure behind the repads. Q.6.3.7 VE Tack welds left in place. 6.9.1.4 VE All welds. 7.15.5 VE Shell-plate butt welds. 7.16.1 VE Annex Q: Base metal repairs for erection l ug removal areas on secondary components. 7.16.1 VE Welds attaching nozzles, manway s, and clean out openings. 7.16.4 Process Welds for which the Inspection and Testing is Required Reference SectionD ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS P-3

Definitions: MT= Magnetic Particle Examination PT = Liquid Penetrant Examination Pen Oil = Penetrating Oil Test RT = Radiographic Testing VB = Vacuum Box Testing VE = Visual Examination
Acceptance Standard: MT: ASME Section VIII, Annex 6, Paragraphs 6-3, 6-4, and 6-5 PT: ASME Section VIII, Annex 8, Paragraphs 8-3, 8-4, and 8-5 VE: API 620, Sections 7.15.5.2 and 7.15.5.3 RT: ASME Section V III, Paragraph UW-51(b)

Examiner Qualifications: MT: API 620, Section 7.15.2.3 PT: API 620, Section 7.15.4.3 VE: None VB: None RT: ASNT Level II or III
Procedure Requirements: MT: ASME Section V, Article 1, T-150 PT: ASME Section V, Article 6 VE: None VB: None RT: A procedure is not required. However, the examination method must comply with ASME Section V, Article 2. Process Welds for which the Inspection and Testing is Required Reference SectionQ-1 

## Annex Q (normative) Low-pressure Storage Tanks for Liqu efied Gases at –325 °F or Warmer Q.1 Scope Q.1.1 General This annex, together with the basic sections of API 620, provides requirements for the materials, design, and fabrication of the metallic portions of a refrigerated tank system . The requirements for a basic API 620 tank apply to primary and secondary liquid containers, refrigerated tem perature roofs, warm product vapor containers, purge gas containers and their appurtenances except where they are s uperseded by any requirements of this annex. The complete tank system, of which the metallic com ponents are a part, shall be in accordance with API 625. 

### Q.1.2 Piping Limitations Piping limitations given in API 620 1.3.2 are superseded by API 625, Section 1.6. 

### Q.1.3 Pressure The provisions in this annex apply to design pressures from –0.25 psig to +7.00 psig. 

### Q.1.4 Temperature The provisions in this annex apply to desi gn metal temperatures of –325 °F or warmer. 

### Q.1.5 Definitions The definitions of the following specialized terms used in this annex are found in API 625. — refrigerated tank system — single containment tank system — double containment tank system — full containment tank system — primary liquid container — secondary liquid container — warm product vapor container — purge gas container — refrigerated temperature roof — design pressureQ-2 API S TANDARD 620

— annular space

— suspended deck

— design metal temperature 

### Q.2 Materials Q.2.1 General The material requirements are based on the storage of re frigerated products at the design metal temperature. 

### Q.2.2 Product Temperature Materials Q.2.2.1 Materials for the following metallic components (inclu ding their penetrations, pipi ng, anchors, stiffener s, and attachments) shall be selected from Table Q-1 an d shall comply with the requirements of Q.2.3: a) primary liquid containers; b) secondary liquid containers; c) refrigerated temper ature roofs (this includes inner r oofs of double roof tanks, and single roofs of tanks with external roof insulation); d) thermal distance pieces connecting cold piping to warm vapor or purge gas cont ainers ; e) for full containment tank systems ; portions of warm p r oduct vapor containers that may experience cold gas flows in the event of primar y liquid container leakage; f) metallic susp ended decks for ins u lation; g) Liner plates, if required for li quid containm en t, in concrete primary or secondary liquid containers , loaded in tension under cool down, operati ng, or other design conditions. Q.2.2.2 Materials for liner plates, if required for liquid cont ainment, in concrete pr imary or se condary liquid containers , loaded in compression under all design conditions shall be selected from materials explicitly listed in Table 4-1, excluding materials rated for design metal temperature 65 °F and higher. Q.2.2.3 Two stainless steel plates identical in material type may be welded together prior to ere ction in order to form a single shell plate subassembly. Plates we lded together shall have thicknesses within 1 / 16 in. of each other with the maximum plate thickness being 1 / 2 in. No more than two plates shall be used to form one subassembly. Vertical edges of the pair of plates compri sing a subassembly shall be aligned. T he subassembly shall conform to the dimensional tolerances contained in Section 6 and shall be subjected to inspection requirements contained in Section 7, Q.5.5, and Q.5.6. All welding pr ocedur e sp ecifications shall be in accordance with Section 6 and Q.4. 

### Q.2.3 Impact Test Requirements for Product Temperature Materials Q.2.3.1 9 % or 5 % nickel steel shall be impact te sted in accordance with Q.2.3.2 through Q.2.3.4. Impact testing is not required for austenitic st ainless steel , nickel alloy, and aluminum material s. Welds in high-alloy (austenitic) stainless steel shall be impact tested if required by Q.4.4.Table Q-1—ASTM Standards for Product Temperature Materials Plates and Structural Members Piping, Pipe Fittings, and Tubing Forgings Bolting
A353 A553, Type 1 A645, Grade A A645, Grade B A844 A333, Grade 8 (see note 2) A334, Grade 8 (see note 2) B444 (UNS-N06625), Gr. 1 B444 (UNS-N06625), Gr. 2 B619 (UNS-N10276) (see notes 3 & 6) B622 (UNS-N10276) A522

A240, Type 304 A240, Type 304L

A240, Type 316 A240, Type 316L

A240, Type 201LN (UNS-S20153) A213, Grade TP 304 A213, Grade TP 304L A312, Grade TP 304 (see note 3) A312, Grade TP 304L (see note 3) A403, Grade WP304 A403, Grade WP304L A213, Grade TP316 A213, Grade TP316L A312, Grade TP316 (see note 3) A312, Grade TP316L (see note 3) A403, Grade WP316 A403, Grade WP316L A358, Grade 304, Class 1 (see note 4) A182, Grade F304 A182, Grade F304L

A182, Grade F316 A182, Grade F316L A320, Grades B8, B8C , B8M, and B8T

B209, Alloy 3003-0 (se e note 5) B209, Alloy 5052-0 (see note 5) B209, Alloy 5083-0 (see note 5) B209, Alloy 5086-0 (see note 5) B209, Alloy 5154-0 (see note 5) B209, Alloy 5456-0 (see note 5) B221, Alloy 6061-T4 and T6 B308, Alloy 6061-T6 B210, Alloy 3003-0 B210, Alloy 3003-H113 B210, Alloy 5052-0 B210, Alloy 5086-0 B210, Alloy 5154-0 B241, Alloy 5052-0 B241, Alloy 5083-0 B241, Alloy 5086-0 B241, Alloy 5454-0 B241, Alloy 5456-0 B247, Alloy 3003-H112 B247, Alloy 5083-H112 B247, Alloy 6061-T6 F468, Alloy 6061-T6 D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS Q-3

NOTE 1 Note void. NOTE 2 Seamless piping and tubing only. NOTE 3 Purchased welded pipe shall be welded without the addition of filler metal us ing a process permitted by the named ASTM specification and shall be tested hydrostatica lly or by eddy current to ASTM requirements. NOTE 4 Impact test of welds shall be made for the welding procedure when required by Q.4.4. NOTE 5 ASTM B221 structural sections are also permitted. NOTE 6 Pipe conforming to ASTM B619 and note 3 of this table ma y be used in diameters exceeding the 8-in. limit stated in B619 when approved by Purchaser. Further, for this pipe over 8-in. diameter, the addition of filler metal is permitted.Q-4 API S TANDARD 620
Q.2.3.2 Impact testing of plates, including structural members made of plate, shall co mply with the following. a) Impact test specimens shall be taken transve rse to the direction of final plate rolling. b) For ASTM A353, A553, and A844 steels, Charpy V-notch specimens sha ll be cooled to a temperature of –320 °F. NOTE This temperature is selected to be consistent wit h the standard requirements of the ASTM specifications. The temperature of –320 °F also provides a convenient and safe medium (liquid nitrogen) for cooling; for testing techniques, see ASTM A370. c) For ASTM A645 steels, Charpy V-notch specimens shall be cooled to a temperature of –320 °F unless the design metal temperature is –155 °F or warmer, in which case, the specimens may be cooled to the alternate temperature of –220 °F. d) The transverse Charpy V-notch impact values shall conform to Table Q- 2. e) Each test shall consist of three specimens, and each specimen shall have a lateral exp ansion opposite the notch of not less than 0.015 in. (15 mils) as required by ASTM A353, A553, A645, and A844. f) Retests shall be in accordance with ASTM A353, A553, A645, and A844. Q.2.3.3 Impact testing of structural mem bers shall comply wi th the following: a) For each different shape in each heat-treatment lot, one set of three specimens taken in the longitudinal direction from th e thickest p art of each shape shall be tested. If the heat-treatment lot consis ts of shapes from several ingots, tests shall be conducted on the various shapes of each ingot. b) Charpy V-notch specimens shall be cooled to a temperature of –320 °F (see Q.2.3.2, Items b and c) for A353, A553, A645, Grade A or B, and A 844 steels for imp act testing. c) The longitudinal Charpy V-notch impact values shall conform to T able Q-2. d) Each test shall consist of three specimens, and each specimen shall have a later al expansion opposite the notch of not less than 0.015 in. (15 mils) as required by ASTM A353, A553, A645, and A844. e) Retests shall be in accordance with ASTM A353, A553, A645, and A844. Q.2.3.4 Impact testing of forgings, piping, and tubing shall comply with the following: a) Im p act test specimens shall be taken from each heat included in any heat-treatment lot. b) Charpy V-notch specimens shall be cooled to a temperature of –320 °F (see Q.2.3.2, Items b and c) for A522, A333 (Grade 8), and A334 (Grade 8) steels for imp act testing. c) The minimum Charpy V-notch impact values shall conform to the longitudina l values in T able Q-2. d) Each test shall consist of three specimens, and each specimen shall have a later al expansion opposite the notch of not less than 0.015 in. (15 ml) as required by ASTM A 522, A 333, (Grade 8), and A 334 (Grade 8). e) Retests shall be in accordance with ASTM A 522, A 333 (Grade 8), and A 334 (Grade 8). 

### Q.2.4 Atmospheric Temperature Materials Q.2.4.1 The following are considered warm product vapor container co mponents: a) roofs over suspended decks ;Table Q-2—Charpy V-notch Impact Values a Transverse Longitudinal Size of Specimen (mm) Value Required for Acceptance b (ft-lb) Minimum Value Without Requiring Re test c

(ft-lb) Value Required for Acceptance b (ft-lb) Minimum Value Without Requiring Re test c

(ft-lb) 10 × 10.00 20 16 25 20 10 × 7.50 15 12 19 16 10 × 6.67 13 10 17 13 10 × 5.00 10 8 13 10 10 × 3.33 7 5 8 7 10 × 2.50 5 4 6 5 D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS Q-5

b) outer shells of double wall, single containment tank systems ; c) outer bottoms of double wall, single containment tank systems ; d) met allic liners for concrete secondary liquid containers where the liner s are acting as warm product vapor containers , but not required for se condary liquid containment. Q.2.4.2 Material for warm product vapor containers shall conform to on e of the following. a) Table 4-1 for design metal temperatures down to –35 °F (lowest one-day mean ambient temperatur e of –35 °F) without impact tests unless they are requi red by Table 4-1 or by the Purchaser. b) Table R-3 for d esign me tal temperatures down to –60 °F without impact tests unless they are required by Table R-4 or by the Purc haser. c) Paragraph Q.2.2 without impact tests unless they are specified by the Pu rchaser

. d) If approved by the Purchaser, the material may be select ed according to the requirements of 4.2.2. Where wall liner systems are composed of embedded plat es with liner plates less than 3 / 16 in. thick attached by lap welds, 4.2.2 shall not be applied to the liner or embed plate material. Q.2.4.3 The following are considered purge gas container component s: a) outer roofs of double wall, double roof, single containment t ank systems ; b) outer shells of double wall, double roof, single containment t ank system s ; c) outer bottoms of double wall, double roof, single containment ta nk systems ; d) metallic liners functioning with a concrete secondary liquid containe r as a moisture vapor barrier but not acting as a warm product vapor container and not required for secondary liquid containment. a When the alternate flaw acceptance criteria of Table U-2 are applied, the higher impact values of Table U-3 are required for pl ates. b Average of three specimens.

c Only one specimen of a set.Q-6 API S TANDARD 620
Q.2.4.4 Material for purge gas containers shall conform to one of the approv ed materi als listed in Table 4-1. Consideration of the design metal temperature is not required if the actual stre ss does not exceed one-half the allowable tensile design st ress for the material. 

### Q.2.5 Structural Shapes Structural shapes of 9 % and 5 % nickel steel may be furnis hed to the chemical and physical requirements of ASTM A353, A553, A645, or A844. Physic al tests shall be in accordance with the requirements of ASTM A6. 

### Q.2.6 Piping, Tubing, and Forgings Q.2.6.1 In addition to the specific requirement s of this annex, all piping within the li mitations of Q.1.2 shall fulfill the minimum material requirements of ASME B31.3. Q.2.6.2 Except as allowed by Q.2.6.3 and Q.2.6.4, piping, t ubing, and forgings used for openings within a distance of 2 d t n × ( ) × from the tank wall shall be compatible in welding, strength, and thermal expansion coefficient with the tank wall material ( d and t n are defined in Figure 5-7). Q.2.6.3 Nickel alloy material B444 (UNS-N06625), B622 and B6 19 (UNS-N10276) in Table Q-1 may be used for piping and tubing as a substitute for A333, Grade 8 or A334, Grade 8 for openings through 9 % Ni (A353, A553, A844) and 5 % Ni (A645) storage tanks, providi ng these materials meet the ap plicable requirements in this annex and are not used for reinforcement. Q.2.6.4 300 series stainless steel materials in Table Q- 1 m ay be used for piping and tubing for openings through 201LN storage tanks, providing these materials meet the applicable requirements in this annex and are not used for reinforcement. 

### Q.3 Design Q.3.1 General Design considerations shall be as spec ified in API 625, Section 6, “Design and Performance Criteria” together with the additional prov isions of this Section Q.3. 

### Q.3.2 Density of Liquid Stored The density of the liquid stored shall be its maximu m density within the range of design temperatures. 

### Q.3.3 Allowable Design Stresses Q.3.3.1 The maximum allowable design stresses for the mate rials outlined in Q.2.2 shall be in accordance with T

able Q-3. Q.3.3.2 The values for the allowable design tensile stre ss given in Table Q- 3 fo r materials other than bolting steel are the lesser of (a) 33 1 / 3 % of the specified minimum ultimate tens ile strength for the material or (b) 66 2 / 3 % of the specified minimum yield st rength, but they are 75 % of the specified minimum yield strength for the stainless steel, nickel alloy, and aluminum material s. Allowable test stresses are based on the limitati on of Q.6.2.2. Q.3.3.3 For the base materials associated wi th Table Q-3, notes a and b; if: a) the weld filler metal has an unspecified yield strength; or b) the weld filler metal has specified mini mum yield or ultimate tens ile streng th below the specified minimums for the base metal; or whenD ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS Q-7
Table Q-3—Maximum Allowable Stress Values Stress Value (lbf/in. 2 ) Specified Minimum Allowable Stress ASTM Specifications Tensile Strength Yield Strength Design Test Plate and Structural Members A353 100,000 75,000 a a A553, Type 1 100,000 85,000 a a A645, Grade B 100,000 85,000 a a A645, Grade A 95,000 65,000 31,700 b 42,000 b A844 100,000 85,000 a a
A240, Type 304 75,000 30,000 22,500 27,000 A240, Type 304L 70,000 25,000 18,750 22,500 A240, Type 201LN (UNS-S20153) 95,000 45,000 31,700 40,500
A240, Type 316 75,000 30,000 22,500 27,000 A240, Type 316L 70,000 25,000 18,750 22,500
B209, Alloy 3003-0 14,000 5,000 3,750 4,500 B209, Alloy 5052-0 25,000 9,500 7,100 8,550
B209, Alloy 5083-0 40,000 g 18,000 g 13,300 g 16,200 g B209, Alloy 5086-0 35,000 14,000 10,500 12,600
B209, Alloy 5154-0 30,000 11,000 8,250 9,900 B209, Alloy 5456-0 42,000 g 19,000 g 14,000 g 17,100 g
B221, Alloy 3003-0 14,000 5,000 3,750 4,500 B221, Alloy 5052-0 25,000 10,000 7,500 9,000
B221, Alloy 5083-0 39,000 16,000 12,000 14,400 B221, Alloy 5086-0 35,000 14,000 10,500 12,600 B221, Alloy 5154-0 30,000 11,000 8,250 9,900
B221, Alloy 5456-0 41,000 19,000 13,650 17,100 B221, Alloys 6061-T4 and T6 (welded) 24,000 8,000 10,000
B308, Alloys 6061-T4 and T6 (welded) 24,000 8,000 10,000Q-8 API S TANDARD 620

Piping and Tubing A333, Grade 8 100,000 75,000 a a A334, Grade 8 100,000 75,000 a a A213, Grade TP, Type 304 75,000 30,000 22,500 27,000 A213, Grade TP, Type 304L 70,000 25,000 18,750 22,500 A312, Grade TP, Type 304 c 75,000 30,000 22,500 27,000 A312, Grade TP, Type 304L c 70,000 25,000 18,750 22,500
A358, Grade 304, Class I 75,000 30,000 22,500 27,000
A213, Grade TP316 75,000 30,000 22,500 27,000 A213, Grade TP316L 70,000 25,000 18,750 22,500
A312, Grade TP316 75,000 30,000 22,500 27,000 A312, Grade TP316L 70,000 25,000 18,750 22,500 B210, Alloy 3003-0 14,000 5,000 3,750 4,500 B210, Alloy 3003-H113 14,000 5,000 3,750 4,500 B210, Alloy 5052-0 25,000 10,000 7,500 9,000 B210, Alloy 5086-0 35,000 14,000 10,500 12,600 B210, Alloy 5154-0 30,000 11,000 8,250 9,900 B241, Alloy 5052-0 25,000 10,000 7,500 9,000 B241, Alloy 5083-0 39,000 16,000 12,000 14,400 B241, Alloy 5086-0 35,000 14,000 10,500 12,600 B241, Alloy 5454-0 31,000 12,000 9,000 10,800 B241, Alloy 5456-0 41,000 19,000 13,650 17,100 B444 (UNS-N06625), Grade 1 120,000 60,000 40,000 f 54,000 f B444 (UNS-N06625), Grade 2 100,000 40,000 30,000 f 36,000 f B619 (UNS-N10276), Class I c 100,000 41,000 30,750 f 36,900 f B622 (UNS-N10276) 100,000 41,000 30,750 f 36,900 f Table Q-3—Maximum Allowable Stress Values (Continued) Stress Value (lbf/in. 2 ) Specified Minimum Allowable Stress ASTM Specifications Tensile Strength Yield Strength Design TestD ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS Q-9

Forgings A522 100,000 75,000 a a A182, Grade F, Type 304 75,000 30,000 22,500 27,000 A182, Grade F, Type 304L 65,000 25,000 18,750 22,500 A240, Trade F316 75,000 30,000 22,500 27,000 A240, Grade F316L 70,000 25,000 18,750 22,500 B247, Alloy 3003-H112 14,000 5,000 3,750 4,500 B247, Alloy 5083-H112 40,000 18,000 13,300 16,200 B247, Alloy 6061-T6 (unwelded) 38,000 35,000 12,650 20,900 Bolting e F468, Alloy 6061-T6 42,000 35,000 14,000
A320 (strain-hardened: Grade B8, B8C, B8M and B8T) ≤ 3

⁄ 4 in. 125,000 100,000 30,000 > 3

⁄ 4 to 1 in. 115,000 80,000 26,000 > 1 to 1 1 ⁄ 4 in. 105,000 65,000 21,000 > 1 1 ⁄ 4 to 1 1 ⁄ 2 in. 100,000 50,000 16,000 A320 (solution-treated and strain-hardened grades when welded) Grades B8, B8M, and B8T-all sizes 75,000 30,000 15,000

a The allowable stresses for these materials are based on the lower yield and tensile strength of the weld metal or base metal, a s determined by Q.4.2.1 and Q.4.2.2, and the design rules in Q.3.3.2 and Q. 3.3.3. Further, the allowable stresses shall be considered joint by joint as limits on the stress acting across that join t considering the weld metal used at that joint. The minimum measured tensile stren gth shall be 95,000 lbf/in. 2 and minimum measured yield st rength shall be 52,500 lbf/in. 2 , except that for circumferential seams only in the sidewall of a cylindrical tank, the minimu m measured tensile strength shall be 80,000 lbf/in. 2 and the minimum measured yield strength shall be 42,000 lbf/in. 2 . For all seams, the maximum permitted values to be used for determining the allowable stress are 100,000 lbf/in. 2 for tensile strength and 58,000 lbf/in. 2 for yield strength. b Based on the yield and tensile strength of the weld metal, as determined by Q.4.2. The minimum measured tensile strength shall be 95,000 lbf/in. 2 and the minimum measured yield strength shall be 52,500 lbf/in. 2

. c For welding piping or tubing, a joint effici ency of 0.80 shall be applied to the allo wable stresses for longitudinal joints in accordance with 5.23.3. d Not used.

e See 5.6.6. f Not to be used for opening reinforcement when used with A353, A 553, A645, and A844. g These allowable stresses are for thicknesses up to and including 1.5 in. For thicknesses over 1.5 in., determine allowable stre sses per Q.3.3.2 using ASTM minimum tensile strength and minimum yield strength for these thicknesses. h Not to be used for opening reinforcement when used with A353, A553, and A645. Table Q-3—Maximum Allowable Stress Values (Continued) Stress Value (lbf/in. 2 ) Specified Minimum Allowable Stress ASTM Specifications Tensile Strength Yield Strength Design TestQ-10 API S TANDARD 620 c) the welding procedure qualification test shows the deposited weld metal tensile strength is lower than the specified minimum ultimate tensile st rength of the base metal, then allowable stresses should be based on the weld metal and heat affected zone strengths as determined by Q.4.2.1 and Q.4.2.2. Q.3.3.4 Where plates or structural members are used as anchor bars for resisting the s hell uplif t, the allowable design and test stresses for the materi al shall be used for the design and over load test conditi ons, respectively. Q.3.3.5 Allowable compressive stresses shall be in accordance wi th 5.5.4 except that for aluminum alloy plate the allowable compressive stresses sha l l be reduced by the ratio of the modulus of compress ive elasticity to 29,000 for values of ( t – c )/ R less than 0.0175 and by the ratio of the minimum yield strength for the aluminum alloy in question to 30,000 for values of ( t – c )/ R equal to or greater than 0.0175 (see 5.5.2 for def initions). In all ot her equations in this standard where yield strength or modulus of elasticity is used, such as Equations 27 and 28, similar corrections shall be made for aluminum alloys. Q.3.3.6 The maximum allowable tensile stress for design l oadings combined with wind loadings shall not exceed 90 % of the minimum specifi ed yield strength for stain l ess steel or aluminum. Q.3.3.7 For aluminum structural members, determine allowabl e stresses and compressive modulus of elasticity using the Aluminum Association, Aluminum Design Manual , “ Specification for Aluminum Structures”. Materials shall be those listed in Table Q-1. 

### Q.3.4 Piping All process piping within the limitations of Q.1.2 (except pump columns as gov erned by API 625, Section 7.3.3) shall fulfill the minimum design requirements of ASME B31.3, except t hat for the basic allowabl e membrane stresses in tension, the values of Table Q-3 shall be used. 

### Q.3.5 Bottom Plates for Primary and Secondary Liquid Containers Q.3.5.1 Primary liquid containers and secondary liquid containers shall have butt-welded annular bottom plates with a radial width that provides at least 24 in. between the insi de of the shell and any lap-wel ded joint in the remainder of the bottom and at least a 2-in. projection outs ide the shell. A greater radial width ( L min ) of annular plate is required when calculated by t he following equations: For steel, L min 390 t b H ( ) G ( ) ---------------------- in. = For aluminum, L min 255 t b H ( ) G ( ) ---------------------- in. = where
t b is the nominal thickness of the annular plate, in inches; H

is the design height of the liquid, in ft; G is the design specific gravity of the liquid to be stored. Q.3.5.2 The thickness of the annular bottom plates shall be in ac cordance with Table Q-4 (for steel or aluminum, as applicable). The thicknesses shown are minimums.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS Q-11
Q.3.5.3 The ring of annular plates shall have a circular outsi de circumference, but it may have a regular polygonal shape inside the tank shell with the number of sides equal to the number of annular plates. These pieces shall be butt-welded in accordance with Q.5.1.1, Item a. Q.3.5.4 The plates of the first shell course shall be att ached to the annular bottom plates by a weld as required b y 5.9.5 except when a full -penetration weld is used or required (see Q.5.1.1). Q.3.5.5 Butt-welds in annular plates shall be not closer than 12 in. from any vertical weld. Q.3.5.6 Three-plate laps or butt-weld junctions in tank bot toms shall be not closer than 12 in. from each other. Q.3.5.7 Bottom plates, other than annular bottom plates for a 9 % or 5 % nickel steel or stainless steel primary or secondary liquid cont ainer , shall have a minimum thickness of 3 / 16 in. exclusive of any specified corrosion allowance. 

### Q.3.6 Shell Stiffening Rings for Primary and Secondary Liquid Containers Q.3.6.1 Internal or external shell stiffening rings may be r equired to maintain roundne ss when the t ank is subjected to wind, vacuum, or other specified loads. When stiffening ri ngs are required, the stiffener-to-shell weld details shall be in accordance with Figure Q-1 and Q.3.6.2 through Q.3.6.5. Q.3.6.2 The stiffener ring and backing strip (if used) shall comply with the requirement s of Q.2.2. The stiffener rings may be fabricated from plate using an intermittent weld on alternating sides between the web and the flange. Q.3.6.3 One rat hole with a minimum radius of 3 / 4 in. shall be provided at each longitudinal shell joint and ring juncture weld (see Figure Q-1). Q.3.6.4 Except for aluminum or stainless steel tanks, all fillet welds shall cons ist of a minimum of two passes. The ends of the fillet welds shall be 2 in. from the rat hole (see Figure Q-1), and these welds shall be deposited by starting 2 in. from the rat hole and welding away from the rat hole. An acceptable alternative to the detail that includes stopping fillet welds 2 in. short of the rat hole would be to weld continuously through the rat hole from one side of the stiffener to the opposite side. All craters in fillet welds shall be repaired by back welding. Table Q-4A—Minimum Thickness for the Annular Bottom Plate: Steel Tanks Nominal Thickness of

First Shell Course (in.) Design Stress a in First Shell Course (lbf/in. 2 )
≤ 19,000 22,000 25,000 28,000 31,000 34,000

≤ 0.7 5 1 ⁄ 4 1 ⁄ 4 1 ⁄ 4 9 ⁄ 32 11 ⁄ 32 13 ⁄ 32

> 0.75 to 1.00 1 ⁄ 4 1 ⁄ 4 9 ⁄ 32 11 ⁄ 32 7 ⁄

16 17 ⁄ 32

> 1.00 to 1.25 1 ⁄ 4 1 ⁄ 4 11 ⁄ 32 7 ⁄

16 17 ⁄ 32 21 ⁄ 32

> 1.25 to 1.50

— 9 ⁄ 32 13 ⁄ 32 17 ⁄ 32 21 ⁄ 32 25 ⁄ 32 NOTE The thicknesses and widths (see Q.3.5.1) in this table are based on the foundation providing a uniform support under the full width of the annular plate. Unless the foundation is properly compacted, particularly at the inside of a concrete ringwall, settlement will produce additional stresses in the annular plate. The thickness of the annular bottom plates need not exceed the thickness of the first shell cour se. The minimum thicknesses for annular bottom plates were derived based on a fatigue cycle life of 1000 cycles for steel tanks.
a The stress shall be calculated using the formula [(2.6 D ) × ( HG )]/ t , where D = nominal diameter of the tank, in ft; H = maximum filling height of the tank for design, in ft; G = design specific gravity; and t = design thickness of the first shell course, excluding corrosion allowance, in inches.Table Q-4B—Minimum Thickness for the Annular Bottom Plate: Aluminum Tanks Nominal Thickness of

First Shell Course (in.) Design Stress a in First Shell Course (lbf/in. 2 ) 12,000 13,000 14,000 15,000 16,000 17,000

≤ 0.50 1 ⁄ 4 1 ⁄ 4 9 ⁄ 32 9 ⁄ 32 5 ⁄

16 5 ⁄ 16

> 0.50 to 0.75 11 ⁄ 32 3 ⁄ 8 13 ⁄ 32 15 ⁄ 32 1 ⁄ 2 17 ⁄ 32

> 0.75 to 1.00 15 ⁄ 32 17 ⁄ 32 19 ⁄ 32 5 ⁄ 8 11 ⁄

16 23 ⁄ 32

> 1.00 to 1.25 5 ⁄ 8 11 ⁄

16 3 ⁄ 4 13 ⁄

16 7 ⁄ 8 29 ⁄ 32

> 1.25 to 1.50 3 ⁄ 4 13 ⁄

16 29 ⁄ 32 31 ⁄ 32 1 1 ⁄ 32 1 1 ⁄ 8

> 1.50 to 1.75 7 ⁄ 8 1 1 1 ⁄

16 1 5 ⁄ 32 1 1 ⁄ 4 1 5 ⁄ 16

> 1.75 to 2.00 1 1 1 ⁄ 8 1 7 ⁄ 32 1 5 ⁄

16 1 13 ⁄ 32 1 1 ⁄ 2 Q-12 API S TANDARD 620

Q.3.6.5 Any joints between the adjacent sections of stiffening rings, as shown in Figure Q-1, shall be made so that the required moment of inerti a of the combined ring-shell se ction is provided. Weld join ts between adjacent sections shall be made with full-thickness and full -penetration butt-welds. Stiffening-ring butt-welds may employ metal backing strips. Backing strips and the associated welding shall be made in a manner that provides a smooth contour in the rat hole and all other weld joint ends. All weld passes shall be started at the rat hole and other weld joint ends and shall be completed by moving away from these ends. Passes shall be overlapped away from edges to provide a smooth continuous weld. 

### Q.3.7 Tank Anchorage for Primary and Secondary Liquid Containers Q.3.7.1 In addition to the loads in Q.3.8, the anchorage for the primary liquid container and any secondary liquid container shall be designed to meet the requirements of Q.3.7.2 through Q.3.7.5. Q.3.7.2 The anchorage shall accommodate movement of the t ank wall and bottom caused by ther mal changes. Q.3.7.3 For Annex Q tanks, 9 % or 5 % nickel steel, stai nless steel, or aluminum anchorage may be used

. Aluminum anchorage shall not be imbedded in reinforced concre te unless it is suitably protected against corrosion. Q.3.7.4 Anchorage subject to load from internal pressure sh all be designed as described in Q.3.7.4.1 through Q.3.7.4.3. Q.3.7.4.1 When the top shell course is the minimum thickness indicated in T abl e 5-6, the minimum anchorage shall be designed for normal loads as specified by the Purchaser and by this standard. See 5.11.2.3 for the allowable stress. NOTE The thicknesses and widths (see Q.3.5.1) in this table are based on the foundation providing a uniform support under the full width of the annular plate. Unless the foundation is properly compacted, particularly at the inside of a concrete ringwall, settlement will produce additional stresses in the annular plate. The thickness of the annular bottom plates need not exceed the thickness of the first shell cour se. The minimum thicknesses for annular bottom plates were derived based on a fatigue cycle life of 1000 cycles for aluminum tanks.
a The stress shall be calculated using the formula [(2.6 D ) × ( HG )] t , where D = nominal diameter of the tank, in ft; H = maximum filling height of the tank for design, in ft; G = design specific gravity; and t = design thickness of the first shell course, excluding corrosion allowance, in inches.Figure Q-1—Typical Stiffening-ring Weld Details See note 2 See note 1

Tank shell Continuous fillet weld (see Q.3.5)

NOTE 1 See Q.3.5.4 for alternative fillet-weld termination details. NOTE 2 Backing strips are permitted on stiffening-ring junction welds. 2 in. 2 in. D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS Q-13

Q.3.7.4.2 When the top shell course is thickened beyond the minimum thickness pr ovided in Table 5-6 or as in Figure 5-6, details f and g, or a knuckle is used, the minimum anchorage shall be designed for three times the internal design pressure. The allowable stress for this loading is 90 % of the minimum specified yield strength of anchorage material. Q.3.7.4.3 As an alternative to Q.3.7.4.2, the Purc haser m a y specify a combination of normal anchorage design , (see Q.3.7.4.1) and emerg ency venting. The Purchaser shall specify required emergency venting discharge rates considering upset conditions including those addressed in API 2000 (see 9.2 and K.1). Q.3.7.5 The foundation design loading for Q.3.7.4 is described in Q.8. 

### Q.3.8 Combination of Design Loads for Double-Wall Tanks Q.3.8.1 General The inner and outer containers shall be des igned for the most critical load co mbinations per 5.4.2 and per Q.3.8.2 through Q.3.8.6 as applicable. Q.3.8.2 Inner Tank The primary liquid container (inner tank ) shall also be designed for the static insulation pressure, the insulation pressure as the inner t ank expands during warming after an in-service period, and the purging or operating pressure of the space between the inner and outer tank shells, unless the pressure is equalized on bot h sides of the inner tank.Q-14 API S TANDARD 620
Q.3.8.3 Single Containment Outer Wall A metallic warm vap o r or purge gas container for a double wall, single containment tank system shall also be designed for the purging and operating pressure of the sp ace between the inner and outer tank shells and for the loading from the insulation. Q.3.8.4 Double Containment Outer Wall A metallic warm vapor, purge gas , or secondar y liquid container for a double containment tank system shall be designed for the load combinations s pecified for the outer wall of a single containment tank system . A metallic secondary liquid container shall also be designed for t he following upset conditions: a) Dead load and liquid head [ D

L + P L ], b) Dead load, liquid head, and seismic [ D

L + P L + E ], where D L , P L , and E are defined in Q.3.8.6. Q.3.8.5 Full Containment Outer Wall A metallic outer wall for a full cont ainment tank system shall be designed for the load co mbinations specified for the outer wall of a single containment tank system . The metallic outer wall shall also be designed for the following upset conditions: a) Dead load, design pressure and liquid head [ D

L + P g + P L ], b) Dead load, design pressure, liquid head, and seismic [ D

L + P g + P L + E ], where D L , P g , P L , and E are defined in Q.3.8.6. Q.3.8.6 Nomenclature
D

L is the dead load;
P

g is the design pressure of the secondary liquid container ; P

L is the liquid head in the secondary liquid container determined from the maximum normal operating capacity of the primary liquid container ; E is th e ALE seismic as r equired by L.4, including 10 % snow load. 

### Q.3.9 Minimum Wall Requirements Q.3.9.1 Warm Product Vapor and Purge Gas Containers Design of warm vapor and purge gas cont ainers shall be in accordance with Secti on 5 of this standard together with the additional provis ions of this Section Q.3.9.1. Warm vapor and purge gas containers shall have a minimum nominal thickness of 3 ⁄

16 in. except for metallic wall liners for conc rete containers which shall have a minimum nominal thickness of 0.12 in. Q.3.9.2 Primary and Secondary Liquid Containers The sidewall thickness of a metallic primary or secondary liq uid container shall in no case be less than that described in Table Q-5.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS Q-15

NOTE The nominal thickness of cylindrical sidewall plates refers to the tank shell as construc ted. The thicknesses specified are based on erection requirements. Q.3.9.3 Primary and Secondary Liquid Container Tank Tolerances The tolerances of the sidewall of a metallic primary or secondary liquid container shall be in accordance with 6.5.2 , 6.5.3, 6.5.4, and Table Q-6, which supersedes Table 6-1. 

### Q.4 Welding Procedures Q.4.1 General The rules in this section shall apply to primary and secondary liquid containers , refrigerated temperature roofs , and suspended insulation decks . Covered electrodes, bare-wire electrodes, and flux cored electrodes used to weld 9% and 5 % nickel steel shall be limit ed to those listed in AWS 5.11, A W S 5.14, and AWS 5.34. Warm vapor and purge gas containers shall be welded in accordance with the basic rules of this standar d unless the requirements of this annex or Annex R are applicable. Table Q-5—Nominal Thickness of Primary and Secondar y Liquid Container Cylindrical Sidewall Plates Nominal Cylinder Diameter (ft) Nominal Plate Thickness (in.) Stainless steel and nickel steel < 60 3 / 16 60 to 140 1 / 4 > 140 to 220 5 / 16 > 220 3 / 8 Aluminum < 20 3 / 16 20 to 120 1 / 4 > 120 to 200 5 / 16 > 200 3 / 8 Table Q-6—Radius Tolerances for Primary and Secondary LIquid Container Shells Diameter Range (ft) Radius Tolerance (in.) Stainl ess steel and nickel steel < 140 ± 3 / 4 140 to 220 ± 1

> 220 ± 1 1 / 4 Aluminum < 20 ± 1 / 2 20 to 120 ± 3 / 4 > 120 to 200 ± 1 > 200 ± 1 1 / 4Q-16 API S TANDARD 620 Purge gas containers may be of single-welded lap or single-welded partial penetration butt construction when the thickness does not exceed 3 / 8 in. Such single side welds shall be made from the outside to prevent corrosion and the entrance of moisture. At any th ickness, the outer tank may be of single we lded butt construction from either side with full penetration and fusion or double-welded butt constructi on without necessarily havi ng full fusion and penetration. Warm product vapor containers shall conform to the lap- or bu tt-wel ded construction described in this stan dard except as required in Q.5.2.1. 

### Q.4.2 Welding Procedure Qualification Q.4.2.1 Specifications for the standard welding procedure tests and confirmation of the minimum ultimate tensile strength are in 6.7. Q.4.2.2 When required by Q.3.3.3, two all-weld-metal specim ens that conform to the dimensional st andard o f Section 12.1 of AWS A5.11 shall be test ed to determine the minimum yield and ultimate tensile strength required by Table Q-3; or for determining allowable stress values in Q.3.3.2. The yield strength shall be determined by the 0.2 % Offset Metho d

. Q.4.2.3 The 9 % nickel steel specifications A353, A553, and A844 each are considered to have differing heat treatments and each shall require a separate welding procedure qualification (ref. 6.7). 

### Q.4.3 Impact Tests for 9 % and 5 % Nickel Steel Q.4.3.1 Impact tests for components of 9 % and 5 % nickel steel shall be made for each welding procedure as described in Q.4.3.2 through Q.4.3.6. Q.4.3.2 Charpy V- notch specimens shall be taken from t he weld met al and from the heat-af f ected zone of the welding procedure qualificati on test plates or from duplicate test plates. Q.4.3.3 Weld metal impact specimens shall be taken across the weld with the notch in the weld met al. The specimen shall be oriented so that the notch is normal to the surface of the material. One face of the specimen shall be substantially parallel to and within 1 / 16 in. of the surface. Q.4.3.4 Heat-affected zone impact specimens shall be taken across the weld and as near the surface of the material as is practicable. The specim ens shall be of sufficient length to locate the notch in the heat-affected zone after etching. The notch shall be cut approximately normal to the material surface to include as much heat-affected zone material as possible in the resulting fracture. Q.4.3.5 Impact test specimens shall be cooled to the temperature stated in Q.2.3. Q.4.3.6 The required impact values and latera l exp ansion values of the weld metal and the heat-affected zone shall be as given in Q.2.3.2, Items d and e, respectively. Where erra tic impact values are obtained, retests will be allowed if agreed upon by the Purchaser and the Manufacturer. 

### Q.4.4 Impact Tests for High Alloys Q.4.4.1 Impact tests are not required for the high-alloy (austeni tic stainless stee l) base ma terials, nickel alloy based materials, aluminum base materials, and weld deposited for the nonferrous (aluminum) materials. Q.4.4.2 Impact tests are not required for austenitic stainless steel welds d eposited by all the welding processes for services of –200 °F and above.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS Q-17
Q.4.4.3 Austenitic stainless steel weld s deposited for service below –200 °F by all welding processes shall be impact tested in accordance with Q.4.3 except that the required impact values shall be 75 % of the values as given in Q.2.2.2, Item d. Electrodes us ed in th e production welding of the tank shall be tested to meet the above requirements. Q.4.4.4 Impact tests are not required for nickel alloy welds made with electrodes/filler metals cover ed by A WS Specification A5.11 or AWS Specification A5. 14, provided the nominal nickel content is 50 % or greater, and the weld is deposited by the shielded m et al-arc welding (SMAW), gas metal-arc welding (GMAW), gas tungsten-arc welding (GTAW), or plasma-arc welding (PAW) processes. When A5.1 1/A5.14 specifies the nickel content as a “remainder,” the nickel content shall be determined by summing the maximum specified values of the other elements (use the average specified value for elements with specified ranges) and s ubtracting from 100%. 

### Q.4.5 Impact Tests for Warm Product Vapor Container Components When impact tests are required by Q.2.3.2 for warm product vapor container components, they shall conform to the requirements of ASTM A20, Supplementary Requirement, paragraph S 5, this annex, or Annex R, whichever is applicable. Weld material for lap wel ded wall liners shall meet requirements for design metal temperature without exception (i.e., including cases where 4.2.2 is applied for liner selection). 

### Q.4.6 Production Welding Procedures The production welding procedures and the production weldi ng shall conform to the requi rements of the procedure qualification tests within the following limitations. a) Individual weld layer thickness shall not be substantiall y greater than that used in t he procedure qualificat ion test. b) Electrodes shall be of the same AWS classificati on and shall be of the same nominal size or smaller. c) The nominal preheat and interpass temperatures shall be the same. 

### Q.4.7 Production Weld Tests Q.4.7.1 Production weld test plat es shall be welded for primary and secondary liquid container butt-welded shell plates when welding procedure qualificat ions are required to be impact test ed per Q.4.3 and Q.4.4. The number of production weld tests shall be based on the requirements of Q.4.7.3 and Q.4.7.4. The locations impact tested (i.e., HAZ and/or weld deposits) shall likewise be the same as required for weld procedure qualifications per Q.4.3 and Q.4.4. Weld testing shall be in accor dance with Q.4.7.5. Test pl ates shall be made from plates produced only from the heats that are used to produce t he shell plates for the tank. Q.4.7.2 Test plates shall be welded using t he same qualified welding procedu re and electrodes that are required for the t ank shell plate joints. The test plates need not be we lded as an extension of the tank shell joint but shall be welded in the required qualifying positions. Q.4.7.3 One test weld shall be made on a set of plates from each specification and grade of plate material, using a thickness that would qualify for all thicknesses in t he shell. Each test welded of thickness t shall qualify for plate thicknesses from 2 t down to t / 2 , but not less than 5 / 8 in. For plate thicknesses less than 5 / 8 in., a test weld shall be made for the thinnest shell plate to be welded; th is test weld will qualify plate thicknesses from t up to 2 t

. Q.4.7.4 Test welds shall be made for each positi on and for each process used in welding primary and secondary liquid cont ainers’ shell plates except for the following. a) A manual or semi-automatic vertical test weld will qual ify manual or semi-automatic welding using the same weld process in all positions. b) A semi-automatic vertical test weld will qualify machine welding using the same weld process in all positions.Q-18 API S TANDARD 620 c) Test welds are not required for machine welded circumferential joints in cylindrical shells. Q.4.7.5 The impact specimens and testing procedure s hall conform to Q.4.3.2 through Q.4.3.6 for 9 % and 5 % nickel steel. The impact specimens an d testing procedure shall conform to Q.4.4.3 for austenitic stainless steel welds deposited for service below –200 °F. Q.4.7.6 By agreement between the Purchaser and the Manufacturer, pro duction test welds for the first t ank sha ll satisfy the requirements of this paragraph for similar tanks at the same location if the tanks are fabricated within 6 months of the time the impact tests were made and found satisfactory and the same we ld procedure specifications are used. 

### Q.5 Requirements for Fabrication, Op enings, Examination, and Testing Q.5.1 Miscellaneous Requirements for Primary and Secondary Liquid Containers and Refrigerated Temperature Roofs Q.5.1.1 The following shall be joined with double butt-welds that have complete penetration and complete fusion except as noted. a) Longitudi nal and circumferential shell joints and joints that connect the annular bot tom p lates together . When approved by Purchaser, these may be welded from a single side provided temporary non-fusible backing is used with complete penetration and complete fusion. Both sides of the joint shall be 100 % visually examined as specified in 7.15.5. b) Joints that connect sections of co mpression rings and sectio ns of shell stiffeners together. Backup bars may be used for these joints with complete penetration and complete fusion detail. c) Joints around the periphery of a shell insert plate. d) Joints that connect the shell to the bottom, unless a me thod o f leak checking is used ( s ee Q.6.3.3), in which case double fillet welds are acceptable (see Q.6.3.3). e) Joints that connect nozzle necks to flanges. f) Butt-welds in piping nozzles, manw ay necks, and pipe fitting s , including weld neck flang es, shall be made using double butt-welded joints. When accessibility does not permit the use of double butt-welded joints, single buttwelded joints that ensure full penetration through the root of the joint are permitted. Q.5.1.2 Fillet welds shall be made in the manner described in Q.5.1.2.1 through Q.5.1.2.2. Q.5.1.2.1 All fillet welds shall have a minimum of two pass es, except aluminum mate rial and as permitted for stif fening ring attachment to shell (see Q.3.6.4). Q.5.1.2.2 For 9 % nickel material, sandblasting or other a dequate means must be used to remove mill scale from a l l plate edges and surfaces before fillet welds in contac t with the refrigerated liquid and vaporized liquefied gas are welded. Sandblasting, or other adequate means, is required to remove slag from the first welding pass if coated electrodes are used. Q.5.1.3 Connections: Q.5.1.3.1 Slip-on flanges may be used where specif ically approve d by the Purchaser

. Q.5.1.3.2 All connections shall have complete penetration and complete fusion.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS Q-19
Q.5.1.3.3 Acceptable types of welded opening connections are shown in Figure 5-8, Panels a, b, c, g, h, m, and o. Q.5.1.3.4 Flanges for nozzles shall be in accordance with this standard; however, the materi al shall com ply with the requirements of Q.2.2 or Q.2.3. Q.5.1.3.5 Manways shall have welded closures rather than depending on gaskets. 

### Q.5.2 Warm Product Vapor Container Welds Q.5.2.1 Warm product vapor container bottom components joined together by fillet welds shall have a minimum of two passes. Q.5.2.2 Metallic wall liners for concrete secondary liq uid containers where the liners are acting as warm product vapor containers but not required for secondary liquid containmen t shall be partial or full penetration butt welded together or lap welded to embedment pl ates. Fillet welds for lap welded wall liners shall be two pass minimum except that wall liners less than 3 / 16 in. may be single pass. 

### Q.5.3 Postweld Heat Treatment Q.5.3.1 Cold-formed 9 % and 5 % nickel plates shall be postw eld heat treated (or stress relieved) when the extreme f iber strain from cold forming exceeds 3 %. Cold-formed 201LN stainless steel shall be reheat-treated in accordance with ASTM A480 when the extreme fiber strain from cold forming exceeds 4 %. Forming strain shall be as determined by the formula:

s 65 t R

f ------- 1 R f R o ----- –    

= where

s is the strain, in percent;

t is the plate thickness, in inches;
R

f is the final radius, in inches;
R o is the original radius, in inch es (infinity for flat plate). Q.5.3.2 If postweld heat treatment (or stre ss relief) is r equired for 9 % and 5 % nickel, the procedure shall be in accordance with paragraph UCS-56 in Section VIII of the ASME Code (with a holding temperature range from 1025 °F to 1085 °F), but the cooling rate from the postwe ld heat treatment shall be not less than 300 °F per hour down to a temperature of 600 °F . A vessel assembly, or plate that requi res postweld heat treatment, must b e postweld heat treated in its entirety at the same time. Methods fo r local or partial postweld heat treatment cannot be used. Pieces individually cold fo rmed that require postweld heat treatment may be heat treated before being welded into the vessel or assembly. Q.5.3.3 Postweld heat treatment of nonferrous materials is n ormally not necessary or desira ble. No postweld heat treatment shall be performed except by agreement between the Purchaser and the Manufacturer. Q.5.3.4 Postweld heat treatment of austeniti c stainless steel materi als is neither requir ed nor prohibited, but paragraphs UHA-100 through UHA-109 in Section VIII of the ASME Code shall be reviewed if postweld heat treatment is considered by the Purchaser or the Manufacturer.Q-20 API S TANDARD 620 

### Q.5.4 Spacing of Connections and Welds Q.5.4.1 In primary and secondary liquid containers , all opening connectio ns 12 in. or la rger in nominal diameter in a shell plate that exceeds 1 inch in thickness shall conform to the spacing requirements for butt and fillet welds described in Q.5.4.2 through Q.5.4.4. Q.5.4.2 The butt-weld around the periphery of a thickened insert plate, or the fillet weld around the periphery of a reinforcing plate, shall be at least the greate r of 10 times the shel l thickness or 12 in. from any butt-welded seam or the bottom-to-shell or roof-to-shell joint. As an alternative, the insert plate (or the reinforcing plate in an assembly that does not require stress relief) may ex tend to and intersect a flat-bottom-to-s hell corner joint at approximately 90 degrees. Q.5.4.3 In primary or secondary liquid container and steel warm vapor cont ainer tank shells , excluding concrete wall liners, the longitudinal weld joints in adjacent shell courses, including compre ssion ring welds, shall be offset from each other a minimum distance of 12 in. Q.5.4.4 Radial weld joints in a compression ring shall be not closer than 12 in. from any longitudinal we ld in an adjacent shell or roof plate. 

### Q.5.5 Examination of Welds by the Liquid-penetrant Method Q.5.5.1 The following primary and secondary liquid container welds shall be examined by the liquid-penetrant method after stress relieving, if any, and bef ore the hydrostatic test of the tank. a) All longitudinal and circumferential butt-welds not completely radiographed. Examination shall be on both sides of the joint. b) The welded joint that joins the cylindrical wall of the ta nk to the bottom annular plates. c) All welds of opening connections t hat are not completely radiograp hed, including nozzle and manhole neck weld s and neck-to-flange welds. Examination shall also incl ude the root pass and ever y 1 / 2 in. of thickness of deposited weld metal (see 5.27.11) as welding progresses. d) All welds of attachments, such as stiffeners, compr ession ri ngs, clips, and other nonpressure parts. e) All welded joints on which backing strips are to remain shall also be examined by t he liquid-penetr ant method after the first complete layer (normally two beads) of weld metal have been deposited. Q.5.5.2 All longitudinal and circumferential butt-welds in thermal dist ance pieces connecting cold piping to warm vapor or purge gas containers shall also be examined by the liquid-penetrant method. 

### Q.5.6 Radiographic/Ultrasonic Examination of Butt-welds in Plates Q.5.6.1 Primary and secondary liquid cont ainer butt-welds shall be examined by radiographic or ultrasonic methods. When the term “examination” is used in Q.5.6 and its subsections, it shall be understood to refer to radiographic or ultrasonic examination. The extent of the ex amination shall be as des cribed in Q.5.6.2 through Q.5.6.8. When the examinati on is by the ultrasonic method, it shall be done in accordanc e with the requirements of Annex U. Q.5.6.2 Butt-welds in all tank wall courses subjected to a maximum actual operati ng membrane tensile stress, perpendicular to the welded joint, greater than 0.1 times the specified minimum tensile str ength of the plate material shall be completely examined.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS Q-21
Q.5.6.3 Butt-welds in all tank wall courses subjected to maximum actual operating membrane tensile stress, perpendicular to the welded joint, less than or equal to 0. 1 times the specified minimu m tensile strength of plate material shall be spot examined in accordance with Figure Q-2. Q.5.6.4 Butt-welds around the periphery of a thickened insert plate shall be completely examined. This does no t include the weld that joins the insert plate with the bottom plate of a flat-bottom tank. Q.5.6.5 Butt-welds at all three-plate junctions in the tank wall shall be examined ex cept in the case of a flat bottom (wall) supported uniformly by t he foundation. This does not incl ude the shell-to-bottom weld of a flat-bottom tank. See Figure Q-2 for minimum examination dimensions. Q.5.6.6 Twenty-five percent of the butt-welded annular plate r adial joint s shall be spot examined for a minimum length of 6 in. The location shall be at the out er edge of the joint and under the tank shell. Q.5.6.7 Twenty-five percent of the butt-welded compression bar r adial joints shall be spot examined for a minimum length of 6 in., except as required by 5.26.4.3.

Figure Q-2—Radiographic/Ultrasonic Examination Requirement s for Butt -welded Shell Joints in Primary and Secondary Liquid Containers NOTE 1 One circumferential spot examination shall be performed in the first 10 ft for each welding operator of each type and thickness. After the first 10 ft, without regard to the number of welders, one circumferential spot examination shall be performed between each longitudinal joint on the course below. NOTE 2 One longitudinal spot examination shall be performed in the first 10 ft for each welder or welding operator of each type and thickness. After the first 10 ft, without regard to the number of welders, one longitudinal spot examination shall be performed in each longitudinal joint. NOTE 3 Longitudinal joints shall be 100 % examined. NOTE 4 All intersections of joints shall be examined. C L C L 1

3 3

3

3 3 3 3 44 144 1 2

2 2 2 2 2 2

4 4 44

4 4 4 4 4 4

44 44

4 44 44 4 1

1 1

1 1

1 1

1 1 3 in. 3 in. 3 in.

3 in.

3 in. 6 in. 6 in.

6 in. Top of tank wall

Bottom of tank wall Numbers in squares correspond to Notes 1 through 4 below

Tank wall courses with maximum calculated operating membrane stress less than or equal to 0.1 of the specified minimum tensile strength of the material (see Q.7.6.2).

Tank wall courses with maximum calculated operating membrane stress greater than 0.1 of the specified minimum tensile strength of the material (see Q.7.6.1).Q-22 API S TANDARD 620
Q.5.6.8 For aluminum tanks the radiography shall satisfy API 650, Annex AL. 

### Q.5.7 Examination of Butt-welds in Piping Q.5.7.1 Butt-welds in piping and in pipe fitti ngs within the limitations of API 625, Section 1.6 shall be examined in conformance with Q.5.7.2 through Q.5.7.9. Q.5.7.2 Longitudinal welded joints in piping carrying liqu id shall be completely radiographed except for manufactur ed pip e welded without filler metal, 12 in. or less in diameter, which is test ed hydrostatically or by eddy current to ASTM requirements. Q.5.7.3 Longitudinal welded joints in piping carrying v apor shall be completely radiographed except for manufactur ed pip e welded without filler metal, 18 in. or less in diameter, which is test ed hydrostatically or by eddy current to ASTM requirements. Q.5.7.4 Thirty percent of the circumferential welded joints (including weld neck flange to pipe joint s) in liquid and vapor carrying piping shall be 100 % radiographed. Q.5.7.5 Butt-welded joints used to fabricate liquid and vapor carrying built-up p ipe fitti ngs shall be completely rad i ographed. Q.5.7.6 Lines carrying liquid located outside the pr imary liquid cont ainer in double wall tanks shall be hydrostatically or pneumatically pressurized at a minimum pressure of 35 lbf/in. 2 and butt welded joints shall be simultaneously visually examined (hydrostatic) or solution film tested (pneumatic ) for tightness. If manufactured pipe has been hydrostatically tested previously to ASTM requirements, then only circumferential welds need to be examined. Q.5.7.7 Lines carrying product vapor within a pur ge gas cont a iner’s annular space shall be pneumatically pressurized at a minimum pressure of 5 lbf/in. 2 and circumferential butt-welded joints shall be simultaneously solution film tested for tightness. Q.5.7.8 For piping that does not carry liquid or product vapor (e.g., instrument cond uit and purge lines) examination shall satisfy only the applicable requirements of Q.2. Q.5.7.9 Method of examination and acceptance criteria for radiography of butt-welds in piping shall comply with ASME B31.3 Pro cess Piping rules, Norm al Fluid Service conditions. 

### Q.5.8 Permanent Attachments All permanent structural attachments welded directly to 9 % and 5 % nickel steel shall be of the same material or of an austenitic stainless stee l type t hat cannot be hardened by heat treatment. 

### Q.5.9 Non-pressure Parts Welds for pads, lifting lugs, and other nonpressure parts, as well as temporary lugs for alignment and scaffolding attached to primary and secondary liquid containers and refrigerated temperature roofs , shall be made in full compliance with a welding procedure qualified in accordanc e with Q.4.2. Lugs attached fo r erection purposes shall be removed, and any significant projections of weld metal shall be ground to a smooth contour. Plate that is gouged or torn in removing the lugs shall be repaired using a qualified procedure and then ground to a smooth contour. Where such repairs are made in primary and secondary liquid containers , the area shall be examined by the liquid-penetrant method. A visual examination is adequate for repairs in warm vapor and purge gas containers

.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS Q-23 

### Q.5.10 Repairs to Welded Joints When repairs are made to welded joints, including the weld s in Q.5.9, the repair procedure shall be in accordance with a qualified welding procedure. 

### Q.5.11 Marking of Materials Q.5.11.1 Material for primary and seconda ry liquid containers, and refrigerated temperature roofs shall be marked so that the individual components can be related back to the mill test report. For aluminum materials, a certificate of conformance shall be provided in place of a mill test repor t stating that the material has been sampled, tested, and examined in accordance with the specif ications and has met the requirements. Q.5.11.2 All mill markings shall be in accordance with the requirement s of ASTM A20 and ASTM A480 a s applicable. All material markings performed by the tank Manu facturer shall be in accordanc e with the requirements of 7.7 and Q.5.11.1. Q.5.11.3 Under some conditions, marking material that c ont ains carbon or heavy-met al compounds can cause corrosion of aluminum. Chalk, wax-base crayons, or marking inks with organic coloring are usually satisfactory. 

### Q.5.12 Construction Practices Excessive hammering during fabricati on and construction shall be avoided on primary and secondary liquid containers, and refrigerated temperature roofs so that the material is not hardened or severely dented. Any objectionable local thinning caused by hammering can be re paired by welding using a qual ified procedure, followed by grinding. The extent of rework for any repair that is permissible must be agreed to between the Purchaser and the Manufacturer. If the rework is determined to have been e xcessive, the reworked area shall be cut out and replaced. 

### Q.5.13 Protection of Plates during Shipping and Storage Q.5.13.1 Plates shall be adequately protected during shipping and storage to avoid damage to plate surfaces and edges fr om handling (scratches, gouge marks, etc.) and from environmental conditions (cor rosion, pitting, etc.). Q.5.13.2 Plates shall be protected from moisture or stored in inclined position to prevent water fr om collecting and standing on sur face. Q.5.13.3 Nine percent and five percent nickel plates which are exposed to hu m id or corrosive atmosphere shall be sand or grit blasted and coated with a suitable coating. The Purchaser shall specify when plates are exposed to humid or corrosive atmosphere. 

### Q.6 Testing the Primary Liquid an d Primary Vapor Containers Q.6.1 General The provisions stated in Q.6.2 through Q.6.5 are testing requirements for the primary liquid container . Provisions stated in Q.6.6 cover the pneumatic testing of the warm product vapor container (or when inner tank is not open top, the refrigerated temperature roof ). 

### Q.6.2 General Procedure Q.6.2.1 A thorough check for tightness and structural adequacy is essential for the primary liquid container

. Except as permitted by Q.6.6.8 and Q.6.7, the test shall be conducted after the entire tank is complete, before the insulation is applied. The test shall consist of filling the tank with water to a height equal to t he design liquid height times the product design specific gravity times 1.25, but not great er than the design liquid level and applying an overload air pressure of 1.25 times the pressure for which the vapor space is designed.Q-24 API S TANDARD 620
Q.6.2.2 The maximum fill shall not produce a stress in any part of the t ank greater than 85 % (may be 90 % for stainless steel and aluminum materials) of the specified minimum yiel d strength of the material or 55 % of the specified minimum tensile strength of the material. 

### Q.6.3 Test Preliminaries Q.6.3.1 Before the tank is filled with water, the procedures described in Q.6.3.2 through Q.6.3.8 shall be completed. Q.6.3.2 All welded joints in the bottom and complete penetration and complete fusion sidewall-to-bottom welds shall be examined by applying a solutio n film to the welds and pulling a partial vacuum of at least 3 lbf/in. 2 gauge above the welds by means of a vacuum box with a transparent top. Q.6.3.3 When the sidewall-to-bottom weld in Q.6.3.2 does not h ave complete penetration and complete fusion, the initial weld passes, inside and outside of the shell, shall have all slag and nonmet als removed from the surface of the welds and the welds examined visually. After completion of the inside and outside fillet or partial penetration welds, the welds shall be tested by pressurizing the volume between the inside and outside welds with air pressure to 15 lbf/ in. 2 gauge and applying a solution film to both welds. To assure t hat the air pressure reaches all parts of the welds, a sealed blockage in the annular passage bet ween the inside and outside welds must be provided by welding at one or more points. Additionally, a small pipe coupling on the out side weld and communicating wi th the volume between the welds must be welded on each side of and adjacent to the blockages. The air supply must be connected at one end and a pressure gauge connected to a coupling on the other end of the segment under test. Q.6.3.4 The attachment welding around all openings and their rein forcement s in the bottom, shell, and roof shall be examined by magnetic p art icle method and solution film test in accordance with 7.18.2.2 and 7. 18.2.3. Following the solution film test, the telltale hole in warm vapor and pur ge gas container reinforcing plates may be sealed with a threaded plug. Q.6.3.5 For 9 % nickel tanks, all testing surfac es of bottom lap-welds and shell-to-bottom welds shall be cleaned b y sandblasting or other adequate means before the vacuum box te st to prevent slag or dirt from masking leaks. Q.6.3.6 Where the pneumatic pressure to be applied in Q.6.6 will be equalized on both sides of the inner t ank, all welded joints above the test water level shall be checked with a solution film and by a vacuum box examination. Q.6.3.7 The attachment fillet welds around bottom openings, which do not permit the ap plication of air pressure behind their reinforcing plates , shall be examined by applying a solution film and by a vacuum box examination. Q.6.3.8 All welds of bottom, wall, and roof metallic liners of concrete cont ainers shal l be examined by applying a solution film to the welds and applying a partial vacuum of between 3 lbf/in. 2 and 5 lbf/in. 2 gage above the welds by means of a vacuum box with transparent t op. Where single pass lap welds less than 3 / 16 in. are used for wall liners, a second partial vacuum of at least 8 lbf/in. 2 shall be applied. 

### Q.6.4 Quality of Test Water Q.6.4.1 The materials used in the construction of Annex Q tanks may be subject to severe pitting, cracking, or rusting if they are exposed to contaminated test water for extended periods of time. The Purchaser shall specify a minimum quality of test water that conforms to Q.6.4.2 thr ough Q.6.4.8. After the water test is completed, the tank shall be promptly drained, cleaned, and dried. Q.6.4.2 Water shall be substantially clean and clear. Q.6.4.3 Water shall have no objectionable odor (tha t is, no hydrogen sulfide). Q.6.4.4 Water pH shall be between 6 and 8.3.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS Q-25
Q.6.4.5 Water temperature shall be below 120 °F. Q.6.4.6 For austenitic stainless steel tanks, in addition to Q.6.4.2 through Q.6.4 .5, the chloride content of the water shall be below 50 parts per million. Q.6.4.7 For aluminum tanks, in addition to Q.6.4.2 through Q.6 .4.5, the mercur y content of the water shall be less than 0.005 p a rts per million, and the copper content shall be less than 0.02 part s per million. Further, the water used to test the tank shall either: 1) be potable water with a free residual chlorine of at least 0.2 ppm; or 2) have chloride content not exceeding 200 ppm. Q.6.4.8 If the water quality outlined in Q.6.4.1 through Q.6.4.7 cannot be achieved, alternative test methods tha t utilize suit able inhibi tors (for example, Na 2 CO 3 and/or NaO 3 ) may be used if agreed to by the Purchaser and the Manufacturer. 

### Q.6.5 Hydrostatic Test Q.6.5.1 The tank shall be vented to the atmosphere when it is filled with or emptied of water. Q.6.5.2 During water filling, the elevations of at least eight equidistant point s at the botto m of the tank shell and on top of the ringwall or slab shall be c hecked. Differential settlement, or unifo rm settlement of substantial magnitude, requires an immediate stop to water filling. Any further filling with water will depend on an evaluation of the measured settlement. Q.6.5.3 The tank shall be filled with water to the level given in Q.6.2. Q.6.5.4 After the tank is filled with water and before the pneumat ic pressure is applied, anc horage, if provided, sha l l be tightened against the hold-down brackets. Q.6.5.5 All welds in the shell, including the corner weld bet ween th e shell and the bottom, shall be visually checked for tightness. 

### Q.6.6 Pneumatic Pressure Q.6.6.1 An air pressure equal to 1.25 times the pressure fo r which the vapor space is designed shall be applied to the enclosed space above the water level. In the case of a double-wall tank with an open-top inner tank, where the air pressure acts against the outer tank and t he inner tank is thus not stressed by the air pressure, the inner tank may be emptied of water before the pneumatic pressure test begins. Q.6.6.2 The test pressure shall be held for 1 hour. Q.6.6.3 The air pressure shall be reduced until the design pressure is reached. Q.6.6.4 Above the water level, all welded joints shall be chec ked with a solution film. A prior vacuum box check may be substituted for the solution-film ex amination. The solution-film examinat ion shall still be made, above the water level, on all welds around openings, all piping joints, and the compression ring welds to the roof and shell except any listed below.

— Continuous double lap roof to compression ring welds.Q-26 API S TANDARD 620

— Shell to compression ring welds, continuous inside and outside, and applying a thickened upper shell ring detail similar to Figure 5-6 details f or f-1. The thickened upper shell ring shall be greater than half of the conical compression ring thickness and greater than tw o times the adjacent shell ring thickness.

— Full fusion butt welded connections. Q.6.6.5 The opening pressure or vacuum of the pressure reli ef and vacuum re lief valves shall be checked b y pumping air above the water level and releasing the pressure and then partially withdraw ing water from the tank. Q.6.6.6 After the tank has been emptied of water and is at at mospheric pressure, the anchorage, if provided, sha ll be rechecked for tightness. Q.6.6.7 Air pressure, equal to the design pressure, shall be applied to the empty ta nk, a nd the anchorage, if provided, and the foundation shall be checked for uplift. Q.6.6.8 Following the hydrostatic and pneumat ic test, all welded seams in the primary liquid cont ainer bo ttom, and complete penetration and complete fusion sidewall-to-bottom welds in the primary liquid container , shall be examined by means of a vacuum box test as described in Q.6.3. 2. Sidewall-to-bottom welds not having complete penetration and complete fusion shall be examined by means of either a vacuum box test of the inside weld as described in Q.6.3.2, or where approved by t he Purchaser, a direct pressure solution film test as described in Q. 6.3.3. If any leaks are detected, they shall be repaired and the vacuum box test repeated. The primary liquid container hydrostatic test need not be repeated. 

### Q.6.7 Temporary Openings after Hydrostatic Test Q.6.7.1 When approved by the Purchaser in writing, and only in the case of tanks which when complete have no shell penetrations, it is permitted to rest ore by welding up to four temporary shell openings after the hydrostatic test in accordance with the provisions of this section. Q.6.7.2 Each temporary opening shall be restored by the insert ion of a shell plate that matches the thickness and specification of adjacent shell material, and is welded into place with full fusion butt-welds. The insert plate shall be round with diameter no less than 24 in. and no greater than 42 in. Q.6.7.3 The insert plate weld shall not cross any shell seams and sh all be at least the greater of 10 times the shell thickness or 12 in. from any other weld in the shell includi ng shell seams, shell-to-bottom weld or attachment welds. Q.6.7.4 The butt weld around the periphery of the plate shall be examined over 100 % of its length by both liquid penetrant method and radiographic method. The liquid penetran t examination is required on th e root pass, on the back-gouged surface, and on the inside and outside finished weld surfaces. Additionally, the weld shall be vacuum box leak tested. 

### Q.7 Testing a Purge Gas Container Q.7.1 General The tightness test shall be made before insulation is installed. Where the pneumat ic pressure described in Q.6.6 acts against the warm product vapor container , the testing requirements of Q.6.6 will result in a check of the outer tank, and the procedure outlined in Q.7.2. 1 through Q.7.2.5 may be omitted.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS Q-27 

### Q.7.2 Test Procedure Q.7.2.1 The inner tank shall be opened to the atmosphere, and a sufficient amount of water shall be added to the inner t ank to balance the upward pressure against the inner tank bottom produced by the pneumatic test of the outer tank; as an alternative, the pressure bet ween the inner and outer tanks can be equalized. Q.7.2.2 Air pressure shall be applied to the space enclosed by the outer tank equal to at least the de sign gas pressure but not exceeding a pressure that woul d overstress either the inner or outer tank. Q.7.2.3 While the test pressure is being held, all lap welded seams and all welds in connections in the outer she ll and roof shall be thoroughly examined with a solution film unl ess they were previously checked with a vacuum box. Q.7.2.4 The air pressure shall be released. Q.7.2.5 Pressure relief and vacuum relief valves shall be checked by applying th e design gas pressure to the outer t ank, followed by evacuation of the outer space to the vacuum setting of the relief valve. 

### Q.8 Foundations Q.8.1 Foundations shall be in accordanc e with API 625, Section 6.7. Q. 8.2 Uplift on foundation: Q.8.2.1 The increased uplift described in Q.8. 2.2 and Q.8.2.3 is int ended to apply to the size of the ringwall and foundation but not the anchorage. Q.8.2.2 F or tanks with an internal design pressure less than 1 lbf/in. 2 gauge, the uplift shall be taken as the smaller of the maximum uplift values com puted under the following conditions: a) The internal design pressure times 1.5 pl us the design wind load on the shell and r oof. b) The internal design pressure plus 0.25 lbf/in. 2 gauge plus the design wind load on the shell and roof. Q.8.2.3 For tanks with an internal design pressure of 1 lbf/in. 2 gauge and over, the uplift, if any, shall be calculated under the combined conditions of 1.25 times the internal design condition plus the des ign wind load on the shell and roof. Q.8.2.4 When the anchorage is designed to meet the requirement s of Q.3.7.4.2, the foundation should be designed to resist the uplift that results from three times the design pressure with the tank full to the design liquid level. When designing to any of the conditions in this paragraph, it is permissible to utiliz e friction between the soil and the vertical face of the ringwall and all of the effective liquid weight. 

### Q.9 Marking Except for 8.2 on Division of Responsibility, marking requirements of Sect ion 8 are superseded by the requirements of API 625, Section 11. 

### Q.10 Reference Standards For rules and requirements not covered in this annex or in the basic rules of this standard, the following documents shall be referred to for the type of material used in the tank: a) API 625;Q-28 API S TANDARD 620 b) For 9 % and 5 % nickel steels, Part UHT in Section VIII of the ASME Code ; c) For stain less steel, Part UH A in Section VIII of the AS ME Code ; d) For aluminum, Part UNF in Section VIII of the ASME Code and API 650, Annex AL.R-1 

## Annex R (normative) Low-pressure Storage Tanks Op erating Between +40 °F and –60 °F R.1 Scope R.1.1 General This annex together with the basic se ctions of API 620 provides require ments for the materials, design, and fabrication of the metallic portions of a refrigerated tank system . The requirements for a basic API 620 tank apply to primary and secondary liquid containers, refrigerated temperat ure roofs, warm product vapor containers, purge gas containers, and their appurtenances except where they are superseded by any requirements of this annex. The complete tank system, of which the metallic com ponents are a part, shall be in accordance with API 625. 

### R.1.2 Piping Limitations Piping limitations given in API 620, 1.3. 2 are superseded by API 625, Section 1.6. 

### R.1.3 Pressure Range The provisions in this annex apply to design pressures from –0.25 psig to +7.00 psig. 

### R.1.4 Temperature The provisions in this annex apply to design meta l temperatures from +40 °F to –60 °F, inclusive. 

### R.1.5 Definitions The definitions of the following specialized terms used in this annex are found in API 625: — Refrigerated Tank System — Single Containment Tank System — Double Containment Tank System — Full Containment Tank System — Primary Liquid Container — Secondary Liquid Container — Warm product vapor container — Purge gas container — Refrigerated temperature roof — Design Pressure — Annular Space — Suspended deck — Design Metal TemperatureR-2 API S TANDARD 620 

### R.2 Materials R.2.1 General The material requirements are based on the storage of re frigerated products at the design metal temperature. 

### R.2.2 Product Temperature Materials R.2.2.1 General R.2.2.1.1 Materials for the following metallic components (inc luding their penetrations, pi ping, anchors, stiffeners, and attachments) shall be selected from Table R-1 and shall be impact tested in accordance with R.2.2.2 through R.2.2.5. a) Primary Liquid Containers ; b) Secondary Liquid Containers ; c) Refrigerated Temperature Roofs (see R.2.2.1.3 for low stress exception): — this includes inner roofs of double roof tanks, and sing le roofs of tanks with external roof insulation); d) Thermal distance pieces connecting cold piping to warm vapor or purge gas containers ; e) For full containment tank systems : Portions of warm product vapor containers that may experience cold gas flows in the event of primar y liquid container leakage; f) Metallic suspended decks for insulation (see R.2. 2.1.4 for exceptions); g) Liner plates, if required for liquid containment, in concrete primary or secondary liquid containers , loaded in tension under cool down, operati ng, or other design conditions. R.2.2.1.2 Materials for liner plates, if requir ed for liquid containment, in concrete primary or secondary liquid containers , loaded in compression under all design conditions shall be selected from materials explicitly listed in Table 4-1 excluding materials rated for desig n metal temperature 65 °F and over. R.2.2.1.3 Material for refrigerated temperature roofs where combined membrane and primary bending tensile stress does not exceed 6000 lb/in. 2 , may also satisfy the criteria of R.2.3.2 a, b, or c in lieu of the requirements of R.2.2. R.2.2.1.4 Materials for metallic suspended decks supporting insulation where combined membrane and primary bending tensile stress does not exceed 6000 lb/in. 2 require no impact testing and any metallic material is permitted. Alternately, the material may be selected from Table Q-1 without regard to stress level. Material to be welded shall be of weldable quality. R.2.2.2 Impact Test Requirements for Plates R.2.2.2.1 Impact testing of plates , including structural members made of pl ate, shall comply with Table R-1. R.2.2.2.2 Impact test specimens shall be taken transvers e to the direction of final plate rolling. R.2.2.2.3 The Charpy V-notch test shall be used, and the minimum impact value at the design metal temperature shall be as given in Table R-2. For s ubsize specimen acceptance criteria, see AST M A20. An impact test temperature lower than the design metal temperature may be used by the Manufacturer, but in such a case the impact values at the test temperature must comply with Table R-2.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS R-3

R.2.2.2.4 All other impact requirements of ASTM A20, Supplementary Requirement S5, shall apply for all materials listed in Table R-2, including specificat ions that do not refer to ASTM A20. R.2.2.2.5 When as-rolled plate material complies with impact te st requirements as specified here, the material need not be normalized. If, as with ASTM A516, the specification prohibits impact test without normalizing but otherwise permits as-rolled plates, the material may be ordered in accordance with the above prov ision and identified as “MOD” for this API modification. R.2.2.3 Impact Requirements for Pipe, Bolting, and Forgings R.2.2.3.1 Pipe (including structural members made of pipe), bol ting, and forgings shall be impact tested as defined below. Pipe, bolting, and forgings shall also be in accordanc e with ASTM specifications referred to in Table R-1, except as permitted in R.2.2.3.3. R.2.2.3.2 Piping materials made according to t he ASTM specifications referred to in Table R-1 shall not be used at design metal temperatures lower than the impact test temperature required by the ASTM specification for the applicable material grade except as permitted in R.2.2.3.3. Table R-1—Standards for Product Temperature Materials Component Materials Notes Plate Refer to R.2.2.2 1 and 2 Pipe ASTM A333 (seamless only) 2 and 3 Pipe fittings ASTM A420 2 and 3 Structural members Plate or pipe as listed above Structural shapes ASTM A36 ASTM A992 ASTM A131 Grades D and E ASTM A633 Grade A CSA G40.21 Grades 38WT, 44WT and 50WT ISO 630 (1995) E275 and E355 in Quality D EN 10025-2 S275 in Quality J2 EN 10025-2 S355 in Quality J2 and K2 4 5 6

7, 8 7, 8 7, 8 Forgings ASTM A350 2 and 3 Bolts ASTM A320 Grade L7 3 NOTE 1 See R.2.2.4. NOTE 2 Type 304, 304L, 316, or 316L stainless steel material, as permitted in Table Q-1 may be used at the maximum allowable st ress values permitted by Table Q-3. Impact tests of this material are not required. Welding procedures shall be qualified in accorda nce with the more restrictive requirements of R.4.2 and Q.4.4 as applic able to the base materials and welding material. NOTE 3 See R.2.2.3. NOTE 4 Plate or pipe materials to be made into a structural me mber shall conform to the impact testing requirements of R.2.2.5. NOTE 5 Structural shapes shall be normalized, if necessary, to me et the required minimum Charpy V-notch impact values of R.2.2. 5. NOTE 6 The steel shall be made with fine gr ain practice, and manganese content shall be in the range of 0.80 % to 1.20 % by ladle analysis. NOTE 7 Minimum Mn to be 0.80 %. NOTE 8 These grades require supplementary impact testing at the design metal temperature notwi thstanding any impact testing at any other warmer temperature that may be manda ted in the national specification.R-4 API S TANDARD 620
R.2.2.3.3 For Design Metal Temperatures below the impact test temperatures required in Table R-1 specifications, and for seamless pipe, bolting, and forgings to other specific ations listed in Table R-3, one of the following shall be satisfied. a) The impact test temperature shall be at least 30 °F colder than the de-sign metal temperature. b) Materials impact tested at the design metal temperature or lower with Charpy impact test energy value of 25 ft-lb (average), 20 ft-lb (minimum) are acceptable for design metal temperatures above –40 °F. Materials with an energy value of 30 ft-lb (average), 25 ft-lb (minimum) ar e acceptable for design metal temperatures of –40 °F or lower. Table R-2—Minimum Charpy V-notch Impact a Requirements for Product Temperature Material Plate Specimens (Transverse) and Weld Specimens Including the Heat-affected Zone Specification Number Grade Range in Thickness (in.) Plate Impact Value b (ft–lb) Weld Impact Value (ft-lb) Average Individual Average Individual ASTM A516 55 and 60 3 / 16 to 2 25 20 20 15 ASTM A516 65 and 70 3 / 16 to 2 25 20 20 15 ASTM A516 65 and 70 Mod 1 d 3 / 16 to 2 25 20 20 15 ASTM A516 65 and 70 Mod 2 d 3 / 16 to 2 25 20 20 15 ASTM A841 1 3 / 16 to 2 25 20 20 15 ASTM A537 1 3 / 16 to 2 25 20 20 15 ASTM A537 2 3 / 16 to 2 30 25 25 20 ASTM A662 B and C 3 / 16 to 2 25 20 20 15 ASTM A678 A c 3 / 16 to 1 1 / 2 25 20 20 15 ASTM A678 B c 3 / 16 to 2 30 25 25 20 ASTM A737 B 3 / 16 to 2 25 20 20 15 ASTM A841 1 3 / 16 to 2 25 20 20 15
ISO 630 (1995) E 355 Quality D c,d,e 3 / 16 to 2 25 20 20 15 EN 10028 P275 Qualities NH, NL1, and NL2 3 / 16 to 2 25 20 20 15
EN 10028 P355 Qualities N, NH, NL1, and NL2 3 / 16 to 2 25 20 20 15 CSA G40.21 38WT c,d,e 3 / 16 to 2 25 20 20 15 CSA G40.21 44WT c,d,e 3 / 16 to 2 25 20 20 15 CSA G40.21 50wt c,d,e 3 / 16 to 2 25 20 20 15

a See R.2.2.2. b For design metal temperatures of –40 °F and lower, the plate impact values shall be raised 5 ft-lb.
c The frequencies of testing for mechanica l and chemical properties shall be at least equal to those of ASTM A20. d See 4.2.3 for a complete description of this material.

e The steel shall be fully killed and made with fine-grain practice.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS R-5

R.2.2.4 Impact Requirements for Thermo-mechanical Control Process (TMCP) Plates Subject to the approval of the Purchas er, TMCP plates (material produced by a mechanical-thermal rolling process designed to enhance the notch toughness) may alternativ ely be used where heat-treated plates are normally required. In this case, each TMCP plate-as-rolled shall receive Charpy V-notch impact energy testing in accordance with R.2.2.2. R.2.2.5 Impact Requirements for Structural Shapes Impact test for structural shapes listed in Table R-1 shal l be made in accordance with ASTM A673 on a piece-testing frequency. Impact values, in foot-pounds, shall be 25 minimum average of 3 and 20 minimum individual at a temperature no warmer than the design metal temperature

. 

### R.2.3 Atmospheric Temperature Materials R.2.3.1 The following are considered warm product vapor container components: a) roofs over suspended decks ; b) outer shells of double wall, single containment tank systems having open-top inner tanks; c) outer bottoms of double wall, single containment tank systems having open-top inner tanks; d) metallic liners for concrete secondary liquid containers where the liners are acting as warm product vapor containers but not required for secondary liquid containment. Table R-3—Atmospheric Temperature Material Specifications Component Design Metal Temperature Material –60 °F to below –20 °F –20 °F to +40 °F Plate Materials as listed in Table R-4 Materials as listed in Table R-4 Pipe ASTM A106 As listed in 4.3 Piping fittings ASTM A420 As listed in 4.3 Structural members Plate or pipe as listed above ASTM A36 structural shapes (see Note 2) ASTM A131 Grade D and E CSA G40.21 Grades 38W, 44W, and 50W (see Note 1) ISO 630 (1995) E275 and E355 in Quality D EN 10025-2 S275 in Quality J2 EN 10025-2 S355 in Quality J2 and K2 Plate or pipe as listed above Structural shapes as list ed in 4.5 or as listed under the –60 °F to –20 °F temperature heading

Forgings ASTM A105 As listed in 4.3 Bolts ASTM A193 Grade B7 ASTM A320 Grade L7 As listed in 4.4

NOTE 1 The steel shall be fully kill ed and made to fine-grain practice. NOTE 2 The steel shall be made with fine grain practice, and manganese c ontent shall be in the range of 0.80 % to 1.20 % by ladle analysis.R-6 API S TANDARD 620
R.2.3.2 Material for warm product vapor containers shall conform to one of the following: a) Table 4-1 for design metal temperatures down to –35 °F (lowest 1-day mean ambient temperature of –35 °F) without impact test unless they are requi red by Table 4-1 or by the Purchaser. b) Table R-3 for design metal temperatures down to –60 °F wi thout impact tests unless t hey are required by Table R4 or by the Purchaser. c) If approved by the Purchaser, the material may be selected by the requirements of 4.2.2. Where wall liner systems are composed of embedded plates wi th liner plates less than 3 / 16 in. thick attached by lap welds, 4.2.2 shall not be applied to the liner or embedded plate material. Table R-4—Minimum Permissible Design Metal Temperat ure for Atmospheric Temperature Material Plates Used without Impact Testing

Group Specification Number Grade Minimum Design Metal Temperature, °F Plate Thickness Including Corrosion Allowance, in. 3 / 16 to 3 / 8 > 3 / 8 to 1 / 2 > 1 / 2 to 1 > 1 to 1 1 / 2 I (semikilled) A 36 d A 131 CSA G40.21 ISO 630 (1995) EN 10025 B 38W E 275 Quality C b S 275 Quality J0 b –20 –20

0 –20 –20 –10 –10 +10 –10 –10 +5 +5 +25 +5 +5 —

—

— +5 +5 II (fully killed) A 573 A 516 A 516 ISO 630 (1995) EN 10025 CSA G40.21 58 b 55 and 60 55 and 60 c E 275 Quality D S 275 Quality J2 b 38W b –30 –30 –40 –30 –30 –40 –20 –20 –30 –20 –20 –30 –10 –10

— –10 –10 –15 0 0

— 0 0 0 III (fully killed and high strength) A 573 A 516 A 516 A 537 A 662 A 633 A 678 A 737 ISO 630 (1995) EN 10025 CSA G40.21 CSA G40.21 65 and 70 65 and 70 65 and 70 Mod 1 a 1 and 2 B and C C and D A and B B E 355 Quality D S 355 Quality J2 and K2 b 44W b 50W b –30 –30 –40 –60 –40 –60 –60 –60 –30 –30 –40 –30 –20 –20 –30 –50 –30 –50 –50 –50 –20 –20 –30 –10 –10 –10 –15 –35 –15 –35 –35 –35 –10 –10 –15 +5 +5 +5 0 –20

0 –20 –20 –20 +5 +5 0 +20 NOTE When normalized, materials in this table may be used at te mperatures 20°F below those shown (except for A 537 Classes 1 an d 2, A 633 Grades C and D, A 678 Grades A and B, And A 737 Grade B). If impact tests are required for the materials listed in this tab le, they shall be in accordance with Table R-5.
a See 4.2.3 for a complete description of this material. b The steel shall be fully killed and made with fine-grain practice, without normaliz ing, for thicknesses of 3 /16 in. through 1 1 /2 in.

c The manganese content shall be in the range from 0.85 % to 1.20 % by ladle analysis. d The manganese content shall be within the range of 0.80 % to 1.20 %. The steel shall not be rimmed or capped.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS R-7

R.2.3.3 The following are considered purge gas container components: a) outer roofs of double wall, double roof, single containment tank systems ; b) outer shells of double wall, double roof, single containment tank systems ; c) outer bottoms of double wall, double roof, single containment tank systems ; d) metallic liner functioning with a concrete secondary liquid container as a moisture vapor barri er but not acting as a warm product vapor container and not required for secondary liquid containment. Table R-5—Minimum Charpy V-notch Impact Requireme nts for Atmospheric Temperature Material Plate Specimens (Transverse)
Group Specification Number Grade Range in Thickness (in.) Impact Value a (foot-lbs) Average Individual I (semikilled) A 36 e A 131 ISO 630 (1995) EN 10025 B E 275 Quality C S 275 Quality J0 3 / 16 to 1 3 / 16 to 1 3 / 16 to 1 1 / 2 3 / 16 to 1 1 / 2 13 13 13 13 9 9 9 9 II (fully killed) A 573 A 516 A 516 ISO 630 (1995) EN 10025 CSA G40.21 58 c 55 and 60 55 and 60 d E 275 Quality D S 275 Quality J2 38WT 3 / 16 to 1 1 / 2 3 / 16 to 2 3 / 16 to 1 / 2 3 / 16 to 1 1 / 2 3 / 16 to 1 1 / 2

3 / 16 to 2 15 15 15 15 15 15 10 10 10 10 10 10 III (fully killed and high strength) A 573 A 516 A 516 A 516 A 537 A 537 A 633 A 662 A 678 A 678 ISO 630 (1995) EN 10025 CSA G40.21 A 841 65 and 70 65 and 70 65 and 70 Mod 1 b 65 and 70 Mod 2 b 1 2 C and D B A B E 355 Quality D S 355 Quality J2 and K2 44WT 1 3 / 16 to 2 3 / 16 to 2 3 / 16 to 2 3 / 16 to 2 3 / 16 to 2 3 / 16 to 2 3 / 16 to 2 3 / 16 to 2 3 / 16 to 1 1 / 2

3 / 16 to 2 3 / 16 to 2 3 / 16 to 2 3 / 16 to 2 3 / 16 to 2 15 15 15 15 15 20 15 15 20 20 15 15 15 15 10 10 10 10 10 15 10 10 15 15 10 10 10 10

a The stated values apply to full-sized specimens. For sub-size spec imen acceptance criteria, see ASTM A20. An impact test temper ature lower than the design metal temperature may be used by the Manufacturer, but the impact values at the test temperature must comply with Table R-5. When plate is selected, consider ation must be given to the possible degrada tion of the impact properties of the plat e in the weld heat-affected zone. b See 4.2.3 for a complete description of this material.

c The steel shall be fully killed and made with fine-gr ain practice, without norma lizing, for thicknesses of 3 / 16 in. to 1 1 / 2 in. d The manganese content shall be in the range from 0.85 % to 1.20 % by ladle analysis.

e The manganese content shall be within the range of 0.80 % to 1.20 %. The steel shall not be rimmed or capped.R-8 API S TANDARD 620
R.2.3.4 Material for purge gas containers shall conform to one of the approv ed materials listed in Table 4-1. Consideration of the design metal temperature is not required if the actual stre ss does not exceed one-half the allowable tensile design st ress for the material. 

### R.3 Design R.3.1 General Design considerations shall be as specif ied in API 625, Section 6, “Design and Performance Criteria,” together with the additional prov isions of this Section R.3. 

### R.3.2 Density of Liquid Stored The density of the liquid stored shall be its maximum density within the range of design temperatures, but not less than 36 lbf/ft 3

. 

### R.3.3 Design Allowable Stress The maximum allowable tensile stress shall be taken from Table 5-1 or Table Q-3. For the maximum allowable stresses for design loadings combined with wind or earth quake loads, see 5.5.6 for carbon steel and Q.3.3.6 for stainless steel and aluminum. 

### R.3.4 Piping All process piping within the limitations of R.1.2 (except pump columns as gove rned by API 625 7.3.3) shall fulfill the minim um design requirements of ASME B31.3 but using the allowable stresses of Table 5-1. 

### R.3.5 Bottom Plates for Primary and Secondary Liquid Containers R.3.5.1 Primary liquid containers and secondary liquid containers shall have butt-we lded annul ar bottom plates with a radial width that provides at least 24 in. between the insi de of the shell and any lap-wel ded joint in the remainder of the bottom and at least a 2-in. projection outs ide the shell. A greater radial width ( L min ) of annular plate is required when calculated by the following equation:

L min 390 t b H ( ) G ( ) ---------------------- =
where

t b is the nominal thickness of the annular plate, in inches;
H is the design height of the liquid, in ft;
G is the design specific gravity of the liquid to be stored (see R.3.2). R.3.5.2 The thickness of the annular bottom plates shall be not less than the thicknesses listed in T able R-6. R.3.5.3 The ring of annular plates shall hav e a circular outside circumference , but may have a regular polygonal shape inside the tank shell with the number of sides equal to the number of annular plates. These pieces shall be butt-welded in accordance with R.5.1.1, Item a. R.3.5.4 The plates of the first shell course shall be attached to the annular bottom plates by welds as required by 5.9.5 except whe n a full penetration we ld is used or required (see R.5.1.1).Table R-6—Thickness Requirements a for the Annular Bottom Plate (in.)
Nominal Thickness of First Shell Course

(in.) Design Stress b in First Shell Course (lbf/in. 2 )

≤ 20,000 22,000 24,000 26,000

≤ 0.75
> 0.75 to 1.00
> 1.00 to 1.25
> 1.25 to 1.50 1 / 4
1 /

4
1 /

4
1 /

4 1 / 4
1 /

4
1 /

4
9 / 32 1 / 4
1 / 4
5 / 16
3 / 8 1 / 4
5 / 16
3 / 8

7 / 16 D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS R-9

R.3.5.5 Butt-welds in annular plates shall be not closer t han 12 in. from any vertical weld in the t ank shell. R.3.5.6 Three-plate laps or butt-weld junctions in the tank bottom shall be not closer than 12 in. from e ach other. 

### R.3.6 Shell Stiffening Rings for Primary and Secondary Liquid Containers R.3.6.1 Internal or external shell stiffening rings may be r equired to maintain roundness when the t ank is subjected to wind, vacuum, or other specified loads. When stiffening ri ngs are required, the stiffener-to-shell weld details shall be in accordance with Figure R-1 and R.3.6.2 through R.3.6.5. R.3.6.2 The stiffener ring and backing st r ip, if used, shall comply with the r equirements of R.2.2. The stiffener ring may be fabricated from plate using an intermittent weld on alternating sides between the web and the flange. R.3.6.3 One rat hole with a minimum radius of 3 / 4 in. shall be provided at each longitudinal shell joint and ring juncture weld (see Figure R-1). R.3.6.4 All fillet welds shall consist of a minimum of two pass es. The ends of the fillet welds shall be 2 in. from the rat hole (see Figure R-1), and these welds shall be deposited by starting 2 in. from the rat hole and welding away from the rat hole. An acceptable alternative to stopping fillet welds 2 in. short of th e rat hole would be to weld continuously through the rat hole from one side of the stiffener to the oppos ite side. All craters in fillet welds shall be repaired by back welding. R.3.6.5 Any joints between the adjacent sections of stif fening rings, as shown in Figure R-1, shall be made so tha t the required moment of inerti a of the combined ring-shell se ction is provided. Weld join ts between adjacent sections shall be made with full-thickness and full -penetration butt-welds. Stiffening-ri ng butt-welds may employ metal backing strips. Backing strips and the associated welding shall be ma de in a manner that provides a smooth contour in the rat hole and all other weld joints ends. All weld passes shall be started at the rat hole and other weld joint ends and shall be completed by moving away from these ends. Passes sh all be overlapped away from the edge to provide a smooth continuous weld. a The thicknesses and width (see R.3.4.1) are based on th e foundation providing a uniform support under the full width of the annular plate. Unless the foundation is properly compacted, particularly at the inside of a concrete ringwall, settlement will produce addi tional stresses in the annular plate. b The stress shall be calcul ated using the formula (2.6 D )( HG )/ t , where D = nominal diameter of the tank, in ft; H = maximum filling height of the tank for design, in ft; G = design specific gravity; and t = design thickness of the first shell course, excluding corro sion allowance, in inches.Figure R-1—Typical Stiffening-ring Weld Details See note 2 See note 1

Tank shell Continuous fillet weld (see R.3.5)

NOTE 1 See R.3.5.4 for alternative fillet-weld termination details. NOTE 2 Backing strips are permitted on stiffening-ring junction welds. 2 in. 2 in. R-10 API S TANDARD 620



### R.3.7 Tank Anchorage for Primary and Secondary Liquid Containers R.3.7.1 In addition to the loads in R.3.8, the anchorage for the primary liquid container and any secondary liquid cont ainer shall be designed to meet the require ments of R.3.7.2 through R.3.7.5. R.3.7.2 The anchorage shall accommodate move m ent of the t ank wall and bottom caused by thermal changes. R.3.7.3 The Manufacturer and the Purchaser shall either use st ainless steel anchorage mate rials, or provide for corrosion allowance when carbon ste els are used. Materi al for tank anchorage shall meet the requirements for primary components given in R.2.2. R.3.7.4 Anchorage subject to load from internal pressure sh all be designed as described in R.3.7.4.1 through R.3.7.4.3. R.3.7.4.1 When the top s hell course is the mi nimum thickness indicated in Tabl e 5-6, the minimum anchorage shall be designed for normal loads as specified by the Purchaser and by this standard. See 5.11.2.3 for the allowable stress. R.3.7.4.2 When the topshell course is thickened beyond minimum thickness pro vided in Table 5-6 or as in Figure 5-6, details f and g, or when a knuckle is used, the minimum anchorage shall be designed for three times the internal design pressure. The allowable stress for this loading is 90% of the minimum specified yield strength of the anchorage material.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS R-11
R.3.7.4.3 As an alternative to R.3. 7.4.2, the Purchaser may specify a co mbination of normal anchorage design (see R.3.7.4.1) and emergency venting. The Purchaser s hall specify required emergency venting discharge rates considering upset conditions including those addressed in API 2000 (see 9.2 and K.1). R.3.7.5 The foundation design loading for R.3.7.4 is described in R.8. 

### R.3.8 Combination of Design Loads for Double-wall Tanks
R.3.8.1 General The inner and outer containers shall be designed for the most critical load combinations per 5.4.2 and per R.3.8.2 through R.3.8

. 6 as applicable. R.3.8.2 Inner Tank The primary liquid container (inner tank) shall also be designed for the static insulation pressure, the insulation pressure as the inner tank expands during warming after an in-service period, and the purging or operating pressure of the space between the inner and outer tank shells, unless the pressure is equalized on both sides of the inner tank. R.3.8.3 Single Containment Outer Wall A metallic warm vapor , or purge gas container for a double wall, single containment tank system shall also be designed for the purging and operating pressure of the sp ace between the inner and outer tank shells and for the loading from the insulation. R.3.8.4 Double Containment Outer Wall A metallic warm vapor, purge gas, or secondar y liquid container for a double containment tank system shall be designed for the load combinations s pecified for the outer wall of a single containment tank system . A metallic secondary liquid container shall also be designed for t he following upset conditions: a) Dead load and liquid head [ D

L + P L ] b) Dead load, liquid head, and seismic [ D

L + P L + E ] where
D

L , P L , and E are defined in R.3.8.6.
R.3.8.5 Full Containment Outer Wall A metallic outer wall for a full containment tank system shall be designed for the load co mbinations specified for the outer wall of a single containment tank system . The metallic outer wall shall also be designed for the following upset conditions: a) Dead load, design pressure, and liquid head [ D

L + P g + P L ] b) Dead load, design pressure, liquid head, and seismic [ D

L + P g + P L + E ] where
D

L , P g , P L , and E are defined in R.3.8.6.R-12 API S TANDARD 620
R.3.8.6 Nomenclature
D

L is the Dead load;
P

g is the Design pressure of the secondary liquid container ; P

L is the Liquid head in the secondary liquid container determined from the maximum normal operating capacity of the primary liquid container ; E is the ALE seismic as required by L.4. including 10 % snow load. R.3.9 W arm Product Vapor and Purge Gas Containers R.3.9.1 Design of war m vapor and pur ge gas co ntainers shall be in accordance with Section 5 of this standard together with the additional provis ions of this Section R.3.9. R.3.9.2 Warm vapor and purge gas container s

shall have a minimum nominal thickness of 3 ⁄

16 in. except for metallic wall liners for concrete containers which shal l have a minimum nominal thickness of 0.12 in. R.3.9.3 Purge gas containers may be of single-welded lap or of singl e- welded partial penetration butt construction when the thickness does not exceed 3 / 8 in. Such single side welds shall be made from the outside to prevent corrosion and the entrance of moisture. At any thickness, it may be of single-we lded butt construction from either side with full penetration and fusion or double-welded butt construction without necessar ily having full fusion and penetration. R.3.9.4 Warm product vapor containers shall conform to the la p- or butt-we lded construction described elsewhere in this standard

. 

### R.4 Welding Procedures R.4.1 General These rules shall apply to primary and secondary liquid containers , refrigerated temperature roofs , and suspended insulation decks of the tank. Warm vapor and purge gas containers shall be welded in accordance with the basic rules of this standard. 

### R.4.2 Welding Procedure Qualification
R.4.2.1 The qualification of welding procedures shall conform to 6.7. Fo r product temperature materials (see R.2.2), impact tests are also required for each welding procedur e (with exceptions for Type 304 or 304L stainless steel described in Table R-1, Note 2). Charpy V-notch specimens that conform to AS TM E23 shall be taken from the weld metal and from the heat-affected zone of the welding procedur e qualification test plates or duplicate test plates. R.4.2.2 Weld metal impact specimens shall be t aken across the weld with th e notch in the weld metal. The specimen shall be oriented so that the notch is normal to t he surface of the material. One face of the specimen shall be substantially parallel to and within 1 / 16 in. of the surface. R.4.2.3 Heat-affected-zone impact specimens shall be taken across the weld and as near the surface of the material as is pr acticable. The specim ens shall be of sufficient length to loca te, after etching, the notch in the heataffected zone. The notch shall be cut approximately normal to the material surface to include as much heat-affected zone material as possible in the resulting fracture. R.4.2.4 Impact test specimens shall be tested at the design metal temperature or at a lower temperature, as agreed upon by the Purchaser and the Manufacturer.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS R-13
R.4.2.5 The required impact values of the weld and heat -affected zone shal l be as given in Table R-2. 

### R.4.3 Production Welding Procedures The production welding procedures and the production weldi ng shall conform to the requi rements of the procedure qualification tests within the following limitations. a) Individual weld layer thickness shall not be substantiall y greater than that used in t he procedure qualificat ion test. b) Electrodes shall be of the same size and American Welding Society (A WS) classification. c) The nominal preheat and interpass temperatures shall be the same. 

### R.4.4 Production Weld Tests R.4.4.1 Production weld test plates shall be welded and tested for primary and secondary liquid container, buttwelded shell pla tes when welding proc edure qualifications are required to be impact tested per paragraph R.4.2.1. The number of production weld tests shall be based on the requirements of R.4.4.3 and R.4.4.4. Weld testing shall be in accordance with R.4.4.5. Test pl ates shall be made from plates produced only from the heats used to produce the shell plates for the tank. R.4.4.2 Test plates shall be welded using the same qualified welding procedure and electrodes as req u ired for the tank shell plate joints. The test plates need not be welded as an extension of the tank shel l joint but shall be welded in the required qualifying positions. R.4.4.3 One test weld shall be made on a set of plates from ea ch specification and grade of plate mater ial, using a thickness that would qualify for all thicknesses in t he shell. Each test welded of thickness t shall qualify for plate thicknesses from 2 t down to t / 2 , but not less than 5 / 8 in. For plate thicknesses less than 5 / 8 in., a test weld shall be made for the thinnest shell plate to be welded; th is test weld will qualify the plate thickness from t up to 2 t

. R.4.4.4 Test welds shall be made for each position and for each process used in welding primary and secondar y liquid containers’ shell plates except for the following. a) A manual or semi-automatic vertical test weld will qual ify manual or semi-automatic welding using the same weld process in all positions. b) A semi-automatic vertical test weld will qualify machine welding using the same weld process in all positions. c) Test welds are not required for machine welded circumferential joints in cylindr ical shells. R.4.4.5 The impact specimens and testing procedure shall conform to R.4.2.2 through R.4.2.5. R.4.4.6 By agr eement between the Purchaser and the Manufacturer, production weld test plates for the fir st tank shall satisfy the requirements of this paragraph for similar t anks at the same location if t he tanks are fabricated within six months of the time the impact tests were made and f ound satisfactory and the same we ld procedure specifications are used. 

### R.4.5 Impact Tests for Warm Product Vapor Container Components When impact tests are required by R.2.3.2 for warm product vapor container components, they shall conform to the requirements of ASTM A20, Supplement ary Requirement, paragraph S 5, or this annex, whichever is applicable. Weld material for lap welded wall liners shall meet requirem ents for design metal temperature without exception, (i.e., including cases where 4.2.2 is applied for liner selection).R-14 API S TANDARD 620 

### R.5 Requirements for Fabrication, Open ings, Examination, and Testing R.5.1 Miscellaneous Requirements for Primary and Secondary Liquid Containers and Refrigerated Temperature Roofs R.5.1.1 The following shall be joined with double butt-welds that have comple te penetr ation and complete fusion except as noted. a) Longitudinal and circumferential shell joints and joints that connect the annular bottom plates to gether. When approved by Purchaser, these may be welded from a singl e side provided temporary non-fusible backing is used with complete penetration and complete fusion. Both sides of the joint shall be 100% visually examined as specified in 7.15.5. b) Joints that connect sections of co mpression rings and sections of she ll stif fener s together. Backup bars may be used for these joints with complete penetration and complete fusion details. c) Joints around the periphery of a shell insert plate. d) Joints that connect the shell to the bottom, unless a method of leak checking is used (s ee R.6.3.4); in that case , double fillet welds are acceptable. e) Joints that connect nozzle necks to flanges. f) Butt-welds in piping nozzles, manw ay necks, and pipe fitting s , including weld neck flang es, shall be made using double butt-welded joints. When accessibility does not permit the use of double butt-welded joints, single buttwelded joints that ensure full penetration through the root of the joint are permitted. R.5.1.2 All fillet welds shall have a minimum of two passes. R.5.1.3 Connections: R.5.1.3.1 Slip-on flanges may be used where spec ifically approved by the Purc hase r. R.5.1.3.2 All connections shall have comple te penetration and complete fusion. R.5.1.3.3 Accept able types of welded opening connec tions are shown in Fi gure 5-8, panels a, b, c, g, h, m, and o. R.5.1.3.4 Flanges for nozzles shall be in accor dance with this standar d; however , the material shall co mply with the requirements of R.2.2. R.5.1.3.5 Manways shall have welded closur es rather than depending on gaskets

. 

### R.5.2 Warm Product Vapor Container Welds Metallic wall liners for concrete secondary liquid containers where the liners are acting as warm product vapor containers but not required for secondary liquid containment shall be partial or full penetration butt welded together or lap welded to embedment plates. Fillet welds for lap welded wall liners shall be two pass minimum except that wall liners less than 3 / 16 in. may be single pass.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS R-15


### R.5.3 Postweld Heat Treatment R.5.3.1 All primary and secondary liquid container opening connections shall be welded into the shell plate or a thickened insert plate, and the welded asse mbly shall be stress relieved prior to installation in the tank unless one of the following exceptions is fulfilled: a) The stress level in the plate, under the d esign conditions, does not exceed 10 % of the minimum tensile strength of the plate material. The opening shal l be reinforced for the low stress. b) The impact tests on the material and welding fulfill the requirement s of R.2.2.2 and Tabl e R-2 , and the thickness of the material is less than 5 / 8 in. for any diameter of connection or less than 1 1 / 4 in. for connections that have a nominal diameter less than 12 in. The thickness of the noz zle neck without stress relief shall be limited to the value of ( D + 50)/120, as described in 5.25.3. c) Opening reinforcement is made from forgings similar in co n figuration to Figure 5-8, Panels o-1, o-2, o-3, and o-4. R.5.3.2 The stress-relieving requirements of 5.25 shall still be mandator y for both primary and secondary component s. R.5.3.3 When used in stress relieved assemblies, the material of TMCP steel A 841 sha l l be represented by test specimens that have been subjected to the same heat treat ment as that used for the stress relieved assembly. 

### R.5.4 Spacing of Connections and Welds R.5.4.1 In primary and secondary liquid containers , all opening connections in a shel l plate shall conform to the requirements of R.5.4.2 through R.5.4.4 for the spacing of butt and fillet welds. R.5.4.2 The butt-weld around the periphery of a thickened insert plate or the fillet weld around the periphery of a reinforcing plate shall be at least the gr eater of 10 times the shell thickness or 12 in. from any butt-welded shell seams except where the completed periphery weld has been stress relieved prior to the welding of the adjacent butt-welded shell seams. Where stress relief has been performed, the spacing from the periphery weld to a shell butt-weld shall be at least 6 in. from the longit udinal or meridional joints or 3 in. from the ci rcumferential or latitudi nal joints if in eith er case the spacing is not less than 3 times the shell thickness. These rules shall also apply to the bottom-to-shell joint; however, as an alternative, the insert plate or reinforcing plate may extend to and intersect the bottom-to-shell joint at approximately 90°. The stress-relieving requirements do not apply to the weld to the bottom or annular plate. R.5.4.3 In primary or secondary liquid container and steel warm v apor cont ainer tank shells , excluding concrete wa ll liners, the longitudinal weld joint s in adjacent shell courses, including compre ssion ring welds, shall be offset from each other a minimum distance of 12 in. R.5.4.4 Radial weld joints in a compression ring shall not be closer than 12 in. fr om any vertical weld. 

### R.5.5 Examination of Welds by Magnetic-particle or Liquid-penetrant Methods R.5.5.1 The following primary and secondary liquid cont ainer welds shall be examined, using the magnetic-particle method (see 7.15) for carbon steel and the liquid-penetrant method (see 7.15) for stainless steel, after stress relieving, if any, and before the hy drostatic test of the tank. a) All longitudinal and circumferential butt-welds th at are not com p letely radiographed. Examination shall be on both sides of the joint. b) The welded joint that joins the cylindrical wall of the tank to the bottom annular plates.R-16 API S TANDARD 620 c) All welds of opening connections that are not complete ly radiographed, including nozzle and manhole neck welds and neck-to-flange welds. Examination sha ll also include the root pass and every 1 / 2 in. of thickness of deposited weld metal (see 5.27.11) as welding progresses. d) All welds of attachments such as stiffeners, co mpression rings, clip s, and other nonpressure parts. e) All welded joints on which backing strips are to remain shall also be examined after the first complete layer (normally two beads) of weld metal have been deposited. R.5.5.2 All longitudinal and circumferential butt-welds in thermal dist ance piec es connecting cold pipin g to warm product vapor or purge gas containers shall also be examined by the magnet ic-particle method for carbon steel and the liquid-penetrant method for stainless steel. 

### R.5.6 Radiographic/Ultrasonic Examination of Butt-welds in Plates R.5.6.1 Primary and secondary liquid cont ainer butt-welds shall be examined by radiographic methods or by ultrasonic methods. When the term “examinat ion” is used in R.5.6 and its subsec tions, it shall be understood to refer to radiographic or ultrasonic examination. The extent of the examination shall be as listed in R.5.6.2 through R.5.6.7. When the examination is by the ultrasonic method, it s hall be done in accordance with the requirements of Annex U. R.5.6.2 Butt-welds in all tank wall courses subjected to a maximum actual operating me mbrane tensile stress per pendicular to the welded joint that is greater than 0.1 times t he specified minimum tensil e strength of the plate material shall be completely examined. R.5.6.3 Butt-welds in all tank wall courses subjected to a maximum actual operating me mbrane tensile stress per pendicular to the welded joint that is less than or equal to 0.1 times the specified minimum tensile strength of the plate material shall be examined in accordance with Figure R-2. R.5.6.4 Butt-welds around the periphery of a thickened insert plate shall be completely examined. This does not include the weld that joins the insert plate with the bottom plate of a flat-bottom tank. R.5.6.5 Butt-welds at all three-plate junctions in the tank wall shall be examined except in the case of a flat bottom (wall) suppor ted uniformly by t he foundation. This does not incl ude the shell-to-bottom weld of a flat-bottom tank. See Figure R-2 for minimum ex amination dimensions. R.5.6.6 Twenty-five percent of the butt-we lded annular plate radial joint s s hall be spot examined for a minimum length of 6 in. The location shall be under the tank shell at the outer edge of the joint. R.5.6.7 Twenty-five percent of the butt-wel ded compression bar r adial joints shall be spot examined for a minimum length of 6 in. except as required by 5.26.4.3. 

### R.5.7 Examination of Butt-welds in Piping R.5.7.1 Butt-welds in piping and in pipe fittings within the limitations of API 625, Section 1.6, shall be examined in conformance with R.5. 7.2 through R.5.7.9. R.5.7.2 Longitudinal welded joints in piping carrying liquid shall be completely radiograp hed except for welds in manufactur ed pip e welded without filler metal, 12 in. or less in diameter, which is test ed hydrostatically or by eddy current to ASTM requirements. R.5.7.3 Longitudinal welded joints in piping carrying vapor shall be completely radiographed except for welds in manufactured pipe welded without filler metal, 18 in. or less in diameter, which is test ed hydrostatically or by eddy current to ASTM requirements.Figure R-2—Radiographic/Ultrasonic Examination Requirements for Butt-welded Shell Joints in Primary and Secondary Liquid Containers NOTE 1 One circumferential spot examination shall be performed in the first 10 ft for each welding operator of each type and thickness. After the first 10 ft, without regard to the number of welders, one circumferential spot examination shall be performed between each longitudinal joint on the course below. NOTE 2 One longitudinal spot examination shall be performed in the first 10 ft for each welder or welding operator of each type and thickness. After the first 10 ft, without regard to the number of welders, one longitudinal spot examination shall be performed in each longitudinal joint. NOTE 3 Longitudinal joints shall be 100 % examined. NOTE 4 All intersections of joints shall be examined. C L C L 1

3 3

3

3 3 3 3 44 144 1 2

2 2 2 2 2 2

4 4 44

4 4 4 4 4 4

44 44

4 44 44 4 1

1 1

1 1

1 1

1 1 3 in. 3 in. 3 in.

3 in.

3 in. 6 in. 6 in.

6 in. Top of tank wall

Bottom of tank wall Numbers in squares correspond to Notes 1 through 4 below

Tank wall courses with maximum calculated operating membrane stress less than or equal to 0.1 of the specified minimum tensile strength of the material (see R.7.6.2).

Tank wall courses with maximum calculated operating membrane stress greater than 0.1 of the specified minimum tensile strength of the material (see R.7.6.1). D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS R-17

R.5.7.4 Ten percent of the circumferential welded joints (including weld neck fl ange to pipe joint s) in liquid and vapor carrying piping shall be completely radiographed. R.5.7.5 Butt-welded joints used to fabricate liquid and vapor carrying built- up pipe fittings shall be completely radiogra phed. R.5.7.6 Lines carrying liquid located outside the primary liquid cont ainer in double wall t ank s shall be hydrostatically or pneumatically pressurized at a minimum pressure of 35 lbf/in. 2 and butt welded joints shall be simultaneously visually examined (hydrostatic) or solution film test ed (pneumatic) for tightness. If manufactured pipe has been hydrostatically tested previously to ASTM requirem ents, then only circumferential welds need to be examined. R.5.7.7 Lines carrying product vapor within a purge gas cont ainer’s annular sp ace shall be pneumatically pressurized at a minimum pressure of 5 lbf/in. 2 and circumferential butt-welded join ts shall be simultaneously solution film tested for tightness.R-18 API S TANDARD 620
R.5.7.8 For piping that does not carry liqui d or product vapor (e.g., instrum ent conduit and purge lines) examination needs to satisfy only the applicable requirements of R.2. R.5.7.9 Radiography of butt-welds in piping shall comply with ASME B31.3, Process Piping rules, Nor mal Fluid Service conditions. 

### R.5.8 Nonpressure Parts Welds for pads, lifting lugs, and other nonpressure parts, as well as temporary lugs for alignment and scaffolding attached to primary and secondary liquid containers , shall be made in full compli ance with a welding procedure qualified in accordance with R.4.2. Lugs attached for erection purposes shall be removed by the grinding of all remaining welds followed by magnetic-particle examination. Plate that is gouged or torn in removing the lugs shall be repaired using a qualified procedure, followed by grinding. Where such repairs are made in primary and secondary liquid containers , the area shall be examined using the magnetic-par ticle method. A visual examination is adequate for repaired areas in warm product vapor and purge gas containers

. 

### R.6 Testing the Primary Liquid and Primary Vapor Containers R.6.1 General The provisions stated in R.6.2 through R.6.4 are testing requirements for the primary liquid container . Provisions stated in R.6.5 cover the pneumatic testing of the warm product vapor container (or when inner tank is not open top, the refrigerated temperature roof ). 

### R.6.2 General Procedure A thorough check for tightness and stru ctural adequacy is es sential for the primary liquid container . Except as permitted by R.6.6, the test shall be conducted after the entire tank is co mplete, but before the insulation is applied. The hydrostatic test shall be performed by filling the tank with water to the design liquid level and applying an overload air pressure of 1.25 times the pressure for which the vapor space is designed. The hydrostatic test shall not produce a membrane tensile stress in any part of the tank exceeding 85 % of the minimum specified yield strength or 55% of the minimum specified tensile strength of the m aterial. 

### R.6.3 Test Preliminaries R.6.3.1 Before the tank is filled with water, the procedures described in R.6. 2.2 through R.6.2.7 shall be completed. R.6.3.2 All welded joints in the bottom of the tank shall be examined by applying a so lution film to the welds and pulling a partial vacuum of at least 3 lbf/in. 2 gauge above the welds by means of a va cuum box with a transparent top. R.6.3.3 Complete penetration and complete fusion welds that join the cylindrical wall to the t ank bottom shall be examined by applying a solution film to the welds and pulling a partial vacuum of at least 3 lbf/in. 2 gauge above the welds by means of a vacuum box with a transparent top. R.6.3.4 When the weld in R.6.2.3 does not have complete penetration and complete fusion, the initial weld p asses, inside and outside of the shell, shall have all slag and non-me tals removed from the surface of the welds and the welds examined visually. After completion of the inside and outside fillet or partial penetration welds, the welds shall be tested by pressurizing the volume between the inside and outside welds with air pressure to 15 lbf/in. 2 gauge and applying a solution film to both welds. To assure that the ai r pressure reaches all parts of the welds, a sealed blockage in the annular passage between the inside and outside welds must be provi ded by welding at one or more points. Additionally, a small pipe coupling comm unicating with the volume between the welds must be welded on each side of and adjacent to the blockages. The air supply must be connected at one end and a pressure gauge connected to a coupling on the other end of the segment under test.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS R-19
R.6.3.5 The attachment welding around all openings and their rein forcements in the bottom, shell, and ro of shall be examined by magnetic particle method and solution film test in accordance with 7.18.2.2 and 7.18.2.3. Following the solution film test, the telltale hole in warm vapor and pur ge gas container reinforcing plates may be sealed with a threaded plug. R.6.3.6 The attachment fillet welds around bottom openings, whic h do not permit the application of air pressure behind the re inforcing plate, shall be examined by applying a solution film and by a vacuum box examination. R.6.3.7 All welds of bottom, wall, and roof met allic liners of concrete cont ainers shall be examined by applying a solution film to the welds and applying a partial vacuum of between 3 lbf/in. 2 and 5 lbf/in. 2 gauge above the welds by means of a vacuum box with transparent t op. Where single pass lap welds less than 3 / 16 in. are used for wall liners, a second partial vacuum of at least 8 lbf/in. 2 shall be applied. 

### R.6.4 Hydrostatic Test R.6.4.1 The provisions described in R.6.3.2 through R.6.3.6 shall apply during and after water filling for the hydrostatic test. R.6.4.2 The tank shall be vented to the atmosphere w hen it is filled with or emptied of water

. R.6.4.3 Dur ing water filling, the elevations of at least eight equidistant points at the bot tom of the tank shell and on top of the ringwall or slab shall be c hecked. Differential settlement, or unifo rm settlement of substantial magnitude, requires an immediate stop to water filling. Any further filling with water will depend on an evaluation of the measured settlement. R.6.4.4 The tank shall be filled with water to the design liquid level. In the case of settlement, as stated in R.6.4.3, an appropriate corrective action shall be taken before further fi lling or, alternatively the design liquid level shall be reduced to the actual maximum test water level. R.6.4.5 After the tank is filled with water and before the pneumat ic test pressure is appli ed, anchor bolt s or anchor straps, if provided, shall be tightened against the hold-down brackets. R.6.4.6 All welds in the shell, including the corner weld bet ween the shell and the bottom , shall be visually checked for tightness. 

### R.6.5 Pneumatic Pressure R.6.5.1 An air pressure equal to 1.25 times the pressure fo r which the vapor space is designed shall be applied to the enclosed space above the water level. In the case of a double-wall tank with an open-top inner tank, where the air pressure acts against the outer tank and t he inner tank is thus not stressed by the air pressure, the inner tank may be emptied of water before the pneumatic pressure testing begins. R.6.5.2 The test pressure shall be held for 1 hour. R.6.5.3 The air pressure shall be reduced unt il the design pressure is reached. R.6.5.4 Above the water level, all welded joints shall be checked with a solution film. A prior vacuum box check may be substituted for the solution -film examination of the welded joints. Above the water level, the solution-film examination shall be made, of all welds around openings, all piping joints, and the compression ring welds to the roof and shell except any listed below.

— Continuous double lap roof to compression ring welds.R-20 API S TANDARD 620

— Shell to compression ring welds, continuous inside and outside, and applying a thickened upper shell ring detail similar to Figure 5-6, details f or f-1. The thickened u pper shell ring shall be greater than half of the conical compression ring thickness and greater than tw o times the adjacent shell ring thickness.

— Full fusion butt welded connections. R.6.5.5 The opening pressure or vacuum of the pressure re lief and vacuum relief valves shall be checked b y pumping air above the water level and releasing the pressu re, then partially withdrawing water from the tank. R.6.5.6 After the tank has been emptied of wa ter and is at atmospher ic pr essur e, the anchorage, if provided, shall be rechecked for tightness. R.6.5.7 Air pressure equal to the design pressure shall be applied to the empty t ank, and the anchorage, if provided, and the foundation shall be checked for uplift. 

### R.6.6 Temporary Openings after Hydrostatic Test R.6.6.1 When approved by the Purchaser in writing, and only in the case of tanks which when complete have no shell penetrations, it is permitted to rest ore by welding up to four temporary shell openings after the hydrostatic test in accordance with the provisions of this section. R.6.6.2 Each temporary opening shall be restored by the inserti on of a shell plate that matches the thickness and specification of adjacent shell material, and is welded into place with full fusion butt-welds. The insert plate shall be round with diameter no less than 24 in. and no greater than 42 in. R.6.6.3 The insert plate weld shall not cross any shell seams and shall be at least the greater of 10 times the she ll thickness or 12 in. from any other weld in the shell includi ng shell seams, shell-to-bottom weld or attachment welds. R.6.6.4 The butt weld around the periphery of the plate shall be examined over 100 % of its length by the magnetic p article method, by the radiographic me thod and by the ultr asonic method. Ultras onic examination acceptance criteria shall be as given in Table U-1. The magnetic particle exami nation is required on the root pass, on the back-gouged surface, and on the inside and outside fini shed weld surfaces. Additionally, the we ld shall be vacuum box leak tested. 

### R.7 Testing a Purge Gas Container R.7.1 The tightness test shall be made before insulation is inst alled. R.7.2 The inner tank shall be opened to the atmosphere, and a sufficient amount of water shall be added to the inner tank to balance the upward pressure against the inner tank bottom produced by the pneumatic te st of the outer t ank; as an alternative, the pressure bet ween the inner and outer tanks can be equalized. R.7.3 Air pressure shall be applied to the sp ace enclosed by the outer t ank equal to at least the design gas pressure but not exceeding a pressure that would ov erstress either the inner or outer tank. R.7.4 While the test pressure is being held, all lap welded seams and all welds in connections in the outer shell and roof shall be thoroughly examined with solution film unles s they were previously c hecked with a vacuum box. R.7.5 The air pressure shall be released. R.7.6 Pressure relief and vacuum relief valves shall be che cke d by applying the d esign gas pressure to the outer tank, followed by evacuation of the outer space to the vacuum setting of the relief valve.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS R-21 

### R.8 Foundations R.8.1 Foundations shall be in accor dance with API 625, Section 6.7. 

### R.8.2 Uplift on Foundation R.8.2.1 The increased uplift described in R.8.2.2 and R.8.2.3 is intended to apply to the size o f the ringwall and foundation but not to the anchorage. R.8.2.2 For tanks with an internal design pressure less than 1 lbf/in. 2 gauge, the uplift shall be taken as the smaller of the maximum uplift values com puted under the following conditions: a) The internal design pressure times 1.5 pl us the design wind load on the shell and r oof. b) The internal design pressure plus 0.25 lbf/in. 2 gauge plus the design wind load on the shell and roof. R.8.2.3 For tanks with an internal design pressure of 1 lbf/in. 2 gauge and over, the uplift, if any, shall be calculated under the combined conditions of 1.25 times the internal design condition plus the des ign wind load on the shell and roof. R.8.2.4 When the anchorage is designed to meet the requirem ent s of R.3.7.4.2, the foundation should be designed to r esist the uplift that results from three times the design pressure with the tank full to the design liquid level. When designing to any of the conditions of this paragraph, it is permissib le to utilize friction betw een the soil and the vertical face of the ringwall and all of the effective liquid weight. 

### R.9 Marking Except for 8.2 on Division of Responsibility, marking requirements of Sect ion 8 are superseded by the requirements of API 625, Section 11.S-1 

## Annex S (normative) Austenitic Stainless Steel Storage Tanks S.1 Scope S.1.1 This annex covers materials, desi gn, fabrication, erection, and testi ng requirements for aboveground, welded, austenitic stainless steel storage t anks constructed of material grades 304, 304L, 316, 316L, 317, and 317L. This annex does not cover stainless steel cl ad plate or strip lined construction. S.1.2 This annex applies only to tanks in non-refrigerated servic e. For stainless steel tanks in refrigerated service, refer to Annex Q of this standard. Minimum design metal tem perature of the non-refrigerated tanks in the scope of the annex is not limited. Maximum design me tal temperature shall be lim ited as given in 1.2.2. For the purposes of this annex, the design temperature shall be the maximum operating temperature as specified by the Purchaser. Ambient temperature tanks (non-heated) shall have a design temperat ure of 40 °C (100 °F). It is cautioned that exothermic reactions occurring inside unheated storage tanks can produce temperatures exceeding 40 °C (100 °F). S.1.3 The minimum thicknesses in this annex do not contain any allowance for corrosion. S.1.4 This annex states only the requirement s that differ from the basic rules in this standard. For requirements not stated, the basic ru les must be followed. 

### S.2 Materials S.2.1 Selection and Ordering S.2.1.1 Materials shall be in accordance with Table S-1. S.2.1.2 Selection of the type/grade of stainless steel depends on the service and environment to which it will be exposed and the effects of fabric ation processes. The Purchaser shall specify the type/grade. S.2.1.3 External structural attachments may be carbon steels meeting the requirements of Section 2 of this standard, providing they are protected from corrosion and t he design and details consider the dissimilar properties of the materials used. (This does not include shell, roof , or bottom openings and their reinforcement.) Carbon steel attachments (e.g., clips for scaffolding) shall not be we lded directly to any internal surface of the tank. 

### S.2.2 Packaging Packaging stainless steel for shipment is important to its corrosion resistance. Precautions to protect the surface of the material depend on the surface finish supplied and may vary among manufacturers. Normal packaging methods may not be sufficient to protect the material from norma l shipping damage. If the int ended service requires special precautions, special instructions s hall be specified by the Purchaser. 

### S.2.3 Impact Testing Impact tests are not required for aus tenitic stainless steel base metals. 

### S.3 Design S.3.1 Operating Temperature The Purchaser shall specify the maxi mum operating temperature of the tank, not to exceed 120°C (250°F) given in 5.2.Table S-1a—ASTM Materials for Stainless Steel Components (SI Units)
Plates and Structural

Members (Note 1) Piping and Tubing Seam less or Welded (Note 2) Forgings (Notes 2 , 3) Bolting and Bars (Notes 4, 5) A240M, Type 304 A240M, Type 304L A240M, Type 316 A240M, Type 316L A240M, Type 317 A240M, Type 317L A213M, Grade TP 304 A213M, Grade TP 304L A213M, Grade TP 316 A213M, Grade TP 316L A213M, Grade TP 317 A213M, Grade TP 317L A312M, Grade TP 304 A312M, Grade TP 304L A312M, Grade TP 316 A312M, Grade TP 316L A312M, Grade TP 317 A312M, Grade TP 317L A358M, Grade 304 A358M, Grade 304L A358M, Grade 316 A358M, Grade 316L A403M, Class WP 304 A403M, Class WP 304L A403M, Class WP 316 A403M, Class WP 316L A403M, Class WP 317 A403M, Class WP 317L A182M, Grade F 304 A182M, Grade F 304L A182M, Grade F 316 A182M, Grade F 316L A182M, Grade F 317 A182M, Grade F 317L A193M, Class 1, Grades B8, B8A, and B8M A194M, Grades B8, B8A, B8M, and B8MA A320M, Grades B8, B8A, B8M, and B8MA A479M, Type 304 A479M, Type 304L A479M, Type 316 A479M, Type 316L A479M, Type 317 S-2 API S TANDARD 620



### S.3.2 Maximum Tensile Stress The maximum tensile stress shall be in accordance with 5.5.3 except Table S-2 shall be used to determine S ts . 

### S.3.3 Maximum Compressive Stress Allowable compressive stresses shall be in accordance with 5.5.4, except the allowable co mpressive stress shall be reduced by the ratio material modulus of elasticity at the design temperature to 200,000 Mpa (29,000,000 lbf/in. 2 ) for values ( t
– c )/ R less than 0.0175 and by the ratio of the materials minimum yield strength at the design temperature to 205 Mpa (30,000 lbf/in. 2 ) for values of ( t
– c )/ R equal to or greater than 0.0175. 

### S.3.4 Maximum Allowable Stress for Structural Members and Bolts The maximum allowable stress values for structural mem bers shall be in accordance with Table 3-3 except the allowable stresses for compression shall be reduced by the ratio of the materials yield st rength at design temperature to 205 Mpa (30,000 lbf/in. 2 ).

NOTE 1 Unless otherwise specified by the Purchaser, plate, sheet, or strip shall be furnished with a No. 1 finish and shall be hot-rolled, annealed, and descaled. NOTE 2 Carbon steel flanges and/or stub end s may be used by agreement between the Pu rchaser and Manufacturer, providing the de sign and details consider the di ssimilar properties of the materials used and are suitable for the intended service. NOTE 3 Castings shall not be used unless specified by the Purchaser. If specified, castings shall meet ASTM A351 and shall be inspected in accordance with ASME Section VIII, Division 1, Appendix 7. NOTE 4 All bars in contact with t he product shall be furnished in the hot-r olled, annealed, and descaled condition. NOTE 5 Other bolting materials may be used by agreement between the Purchaser and Manufacturer.Table S-1b—ASTM Materials for Stainless Steel Components (US Customary Units) Plates and Structural Members (Note 1) Piping and Tubing Seamless or Welded (Note 2) Forgings ( Notes 2, 3) Bolting and Bars (Notes 4, 5) A240, Type 304 A240, Type 304L A240, Type 316 A240, Type 316L A240, Type 317 A240, Type 317L A213, Grade TP 304 A213, Grade TP 304L A213, Grade TP 316 A213, Grade TP 316L A213, Grade TP 317 A213, Grade TP 317L A312, Grade TP 304 A312, Grade TP 304L A312, Grade TP 316 A312, Grade TP 316L A312, Grade TP 317 A312, Grade TP 317L A358, Grade 304 A358, Grade 304L A358, Grade 316 A358, Grade 316L A403, Class WP 304 A403, Class WP 304L A403, Class WP 316 A403, Class WP 316L A403, Class WP 317 A403, Class WP 317L A182, Grade F 304 A182, Grade F 304L A182, Grade F 316 A182, Grade F 316L A182, Grade F 317 A182, Grade F 317L A193, Class 1, Grades B8, B8A, and B8M A194, Grades B8, B8A, B8M, and B8MA A320, Grades B8, B8A, B8M, and B8MA A479, Type 304 A479, Type 304L A479, Type 316 A479, Type 316L A479, Type 317

Table S-2—Maximum Allowable Stress Values for Simple Tension

Material

Type Min Yield Min Tensile Allowable Stress for Design Temperature Not Exceeding ( S

ts ) 40 °C 100 °F 90 °C 200 °F 120 °C 250 °F MPa lbf/in. 2 MPa lbf/in. 2 MPa lbf/in. 2 MPa lbf/in. 2 MPa lbf/in. 2 304 205 30,000 515 75,000 155 22,500 129 18,750 122 17,800 304L 170 25,000 485 70,000 129 18,750 110 16,050 104 15,200 316 205 30,000 515 75,000 155 22,500 133 19,350 126 18,400 316L 170 25,000 485 70,000 129 18,750 109 15,825 103 15,000 317 205 30,000 515 75,000 155 22,500 134 19,425 127 18,500 317L 205 30,000 515 75,000 155 22,500 134 19,425 127 18,500 D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS S-3

NOTE 1 Unless otherwise specified by the Purchaser, plate, sheet, or strip shall be furnished with a No. 1 finish and shall be hot-rolled, annealed, and descaled. NOTE 2 Carbon steel flanges and/or stub ends may be used by agreement between the Purchaser and Manufacturer, providing the de sign and details consider the dissimilar properties of the materials used and are suitable for the intended service. NOTE 3 Castings shall not be used unless specified by the Purchaser. If specified, castings shall meet ASTM A351 and shall be inspected in accordance with ASME Section VIII, Division 1, Appendix 7. NOTE 4 All bars in contact with t he product shall be furnished in the hot-rol led, annealed, and de scaled condition. NOTE 5 Other bolting materials may be used by agr eement between the Purchaser and Manufacturer.

NOTE 1 S ts may be interpolated between temperatures. NOTE 2 The design stress corresponds to the lesser of 0.33 of the minimum tensile strength or 0.75 of the minimum yield streng th. NOTE 3 For dual certified materials (e.g. ASTM A182M/A182 Type 304L/304), use the allowable stress of the grade specified by t he Purchaser.Table S-3—Allowable Stresses for Plate Ring Flanges

Material Type Allowable Stress for Design Temperatures Not Exceeding 40 °C 100 °F 90 °C 200 °F 120 °C 250 °F MPa lbf/in. 2 MPa lbf/in. 2 MPa lbf/in. 2 304 140 20,000 115 16,700 108 15,800 304L 117 16,700 99 14,300 93 13,500 316 140 20,000 119 17,200 112 16,300 316L 117 16,700 97 14,100 92 13,300 317 140 20,000 119 17,300 114 16,500 317L 140 20,000 119 17,300 114 16,500

Table S-4—Yield Strength Values

Material Type Yield Strength for Design Temperature Not Exceeding 40 °C 100 °F 90 °C 200 °F 120 °C 250 °F MPa lbf/in. 2 MPa lbf/in. 2 MPa lbf/in. 2 304 205 30,000 170 25,000 163 23,700 304L 170 25,000 148 21,400 140 20,300 316 205 30,000 178 25,800 170 24,500 316L 170 25,000 145 21,100 138 20,000 317 205 30,000 179 25,900 170 24,700 317L 205 30,000 179 25,900 170 24,700 Table S-5—Modulus of Elasticity at the Design Temperature Modulus of Elasticity for Design Temperature Not Exceeding °C °F MPa lbf/in. 2 40 100 193,000 28,000,000 90 200 189,000 27,400,000 120 250 186,000 27,000,000 S-4 API S TANDARD 620



### S.3.5 Flat Bottoms of Cylindrical Tanks The minimum thickness for bottom plates shall be 5 mm ( 3 / 16 in.), exclusive of any corrosion allowance specified by the Purchaser. 

### S.3.6 Intermediate Wind Girders for Cylindrical Sidewalls S.3.6.1 The value H 1 in 5.10.6.1 shall be reduced by the ratio of the materials modulus of elasticity at the design temperature to 200,000 Mpa (29,000,000 lbf/in. 2 ). S.3.6.2 The value W tr

in 5.10.6.2 shall be reduced by the ratio of the materials modulus of elasticity at the design temperature to 200,000 Mpa (29,000,000 lbf/in. 2 ). NOTE 1 Allowable stresses may be interpolated between temperatures. NOTE 2 The allowable stresses are based on a lower level of permanent strain. NOTE 3 The design stress shall be the lesser of 0.3 of the minimum tensile strength, or 2 / 3 of the minimum yield strength. NOTE 4 For dual certified materials (e.g. ASTM A182M/A 182 Type 3 04L/304), use the allowable stress of the grade specified by t he Purchaser.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS S-5


### S.3.7 Compression Rings The value of 15,000 in equation 27 in 5.12.4. 3 shall be reduced by the ra tio of the material yield strength at the design temperature to 205 Mpa (30,000 lbf/in. 2 ). 

### S.3.8 Flat Cover Plates and Blind Flanges The value s in 5.21 shall be in accordance with Table S-3. 

### S.3.9 Stress Relieving The stress relieving requirements of 5.25 need not be performed unless specified by the Purchaser. 

### S.3.10 Flush-type Shell Connection The value t b in 5.27.4.5 shall be reduced by the ratio of the ma terials yield stress at the design temperature to 205 Mpa (30,000 lbf/in. 2 ). 

### S.4 Fabrication S.4.1 General Special precautions must be observed to minimize the risk of damage to the corrosion resistance of stainless steel. Stainless steel shall be handled in a manner that minimizes contact with iron or other types of steel during all phases of fabrication, shipping, and construction. The following sections descr ibe the major precautions that shall be observed during fabr ication and handling. 

### S.4.2 Storage Storage should be under cover and removed from shop dirt , fumes, and pickling operations. If outside storage is necessary, provisions shall be made for rainwater to drain and allow the material to dry. Stainless steel shall not be stored in contact with carbon steel. Materials containing ch lorides, including foods, bever ages, oils, and greases, shall not come in contact with stainless steel. Inadvertent co ntamination shall be removed by methods described in S.4.5. 

### S.4.3 Thermal Cutting S.4.3.1 Thermal cutting of stainless steel shall be by the ir on powder burning carbon arc or the plasma-arc method. S.4.3.2 Thermal cutting of stainless steel may leave a heat- affected zone and intergranular carbide precipit ates. This heat-af fected zone may have reduced corrosion resistance unless removed by machining, grinding, or solution annealing and quenching. The Purchaser shall specify if the heat-affected zone is to be removed. 

### S.4.4 Forming S.4.4.1 Stainless steels shall be formed by a cold, warm, or hot forming procedure that is non-injurious to the mat erial. S.4.4.2 Stainless steels may be cold formed, providing the maximum strain produce d by su ch forming does not exceed 10% and control of forming spring-bac k is provided in the forming procedure. S.4.4.3 Warm forming at 540 °C (1000 °F) to 650 °C (1200 °F) may cause intergranular carbide precipitation in 304, 316, and 317 grades of stainless steel. Unless stainless steel in this sens itized condition is acc eptable for the service of the equipment, it will be necessa ry to use 304L, 316L, or 317L grades or to solution anneal and quench after forming. Warm forming shall be performed only with agreement of the Purchaser.S-6 API S TANDARD 620
S.4.4.4 Hot forming, if required, may be performed within a temperature range of 900 °C (1650 °F) to 1200 °C (2200 °F). S.4.4.5 Forming at temperatures between 650 °C (1200 °F) and 900 °C (1650 °F) is not permitted. 

### S.4.5 Cleaning S.4.5.1 When the Purchaser requires cleaning to remove su rface contaminant s that may impair the normal corrosion resistance, it shall be done in accordance with ASTM A380, unles s otherwise specified. Any additional cleanliness requirements for the intended serv ice shall be specified by the Purchaser. S.4.5.2 When welding is completed, flux residues and weld spatter shall be re moved mechanically using stainless steel tools. S.4.5.3 Removal of excess weld metal, if required, shall be done with a grinding wheel or belt that has not been previously used on other metals. S.4.5.4 Chemical cleaners used shall not have a detrimental effe ct on the st ainless steel and welded joints and shall be disposed of in accordance with laws and regulations gov erning the disposal of such chemicals. The use of chemical cleaners shall always be followed by t horough rinsing with water and drying (see S.4.9). 

### S.4.6 Blast Cleaning If blast cleaning is necessary, it shall be done with sharp acicular grains of sand or grit containing not more than 2 % by weight iron as free iron or iron oxide. Steel shot or s and used previously to clean nonstainless steel is not permitted. 

### S.4.7 Pickling If pickling of a sensitized stainless steel is necessary, an acid mixture of nitric and hydrofluoric acids shall not be used. After pickling, the stainl ess steel shall be thoroughly rinsed with water and dried. 

### S.4.8 Passivation or Iron Freeing When passivation or iron freeing is specified by the Purchaser, it ma y be achieved by treatment with nitric or citric acid. The use of hydrofluoric acid mixtures for passivation purposes is prohibited for sensitized stainless. 

### S.4.9 Rinsing S.4.9.1 When cleaning and pickling or passivati on is required, these operations shall be followed immediately by rinsing, not allowing the surf aces to dry between operations. S.4.9.2 Rinse water shall be potable and shall not contain more than 200 part s per million chloride at temperatures below 40 °C (100 °F), or no more than 100 parts per mil lion chloride at temperatures above 40 °C (100 °F) and below 65 °C ( 150 °F), unless specified other wise by the Purchaser. S.4.9.3 Following final rinsi ng, the material shall be completely dried. 

### S.4.10 Welding S.4.10.1 Welding shall be by any of the pr ocesses permitted in 6.6.2. Galvanized components or component s with zinc-bearing coatings shall not be we lded to austenitic stainless steel.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS S-7
S.4.10.2 Filler metal chemistry shall match the type of base meta ls joined. Dissimilar welds to carbon steels sha ll use filler metals of E 309 or higher alloy content. S.4.10.3 Two stainless steel plates identical in m aterial type may be welded together prior to e rection in order to form a single shell plate subassembly. Plates we lded together shall have thicknesses within 1 / 16 in. of each other with the maximum plate thickness being 1 / 2 in. No more than two plates shall be used to form one subassembly. Vertical edges of the pair of plates compri sing a subassembly shall be aligned. T he subassembly shall conform to the dimensional tolerances contained in Section 6 and shall be subjected to inspection requirements contained in Section 7. At least 25 % of vertical spot radiographs shall be made at t he subassem bly ho rizontal weld to field vertical weld intersection. All welding procedure specific ations shall be in accordance with Section 6. 

### S.4.11 Welding Procedure and Welder Qualifications Impact tests are not required for austenitic stai nless steel weld metal and heat-affected zones. 

### S.4.12 Postweld Heat Treatment Postweld heat treatment of austeniti c stainless steel materi als need not be performed unl ess specified by the Purchaser. 

### S.5 Inspection and Testing S.5.1 Weld Examination Where specified, the magnetic-particle method of examination shall be replaced by the liquid- penetrate examination method. 

### S.5.2 Hydrostatic Test Considerations—Quality of Test Water S.5.2.1 The materials used in the construction of stainless steel t anks may be subjec t to severe pitting, cracking, or rusting if they are exposed to contaminated test water for extended periods of time. T he Purchaser shall specify a minimum quality of test wa ter that conforms to the following requirements. a) Unless otherwise specified by the Purchaser, water us ed for hydrost atic testing of t anks shall be potable and treated, containing at least 0.2 parts per million free chlorine. b) Water shall be substantially clean and clear. c) Water shall have no objectionable odor (that is, no hydrogen sulfide). d) Water pH shall be between 6 and 8.3. e) Water temperature shall be below 50 °C (120 °F). f) The chloride content of the water shall be below 50 parts per million, unless specifie d otherwise by the Purchase r. S.5.2.2 When testing with potable water, the exposure time s hall not exceed 21 days, unless specified otherwise by the Purchaser

. S.5.2.3 Wh en testing with other fresh waters, the exposur e time shall not exceed seven days. S.5.2.4 Upon completion of hydrostatic test, water shall be completely drained. Wetted surfaces shall be washed with potable water when non-potable water is used for the te st and completely dried. Particular attention shall be given to low spots, crevices, and similar areas. Hot air drying is not permitted.S-8 API S TANDARD 620 

### S.6 Marking Brazing shall be deleted from 6.1.3 

### S.7 Annexes The annexes are applicable for use with austenitic stainless steels as follows: a) Annex D is applicable; however see S.2.1.3 for specia l requirements when att aching to carbon steel supports. b) Annex E is applicable; however s ee S.2.13 for special requir ement s w hen attaching external carbon steel supports. Internal supports shall meet the material requirements of Annex S. c) Annex H is not applicable; stress re lieving is only required when spec ified by the Purchaser and must be performed with care and in such a manner that does not damage or alter the properties of the stainless steel. d) Annex Q is not applicable. e) Annex R is not applicable. f) All other annexes are applic able without m odifications.S-1 

## Annex SC (normative) Stainless and Carbon Steel Mi xed Materials Storage Tanks SC.1 Scope SC.1.1 This annex covers materials, design, fabrication, erection, and testing requirements for aboveground, closed-top, welded, storage tanks constr ucted with austenitic stai nless steel, duplex stainl ess steel and carbon steel. In this annex, the term stainless steel includes austenitic or duplex unless not ed otherwise. Stainless steel and carbon steel may be used in the same tank for tank wall plates, bottom plates, roof structure and other parts of a tank to provide product storage for conditions that require only certain portions of the tank to provide added corrosion resistance. These tanks are mixed material tanks. Stai nless steel and carbon steel plates may be mixed in the bottom, roof or within the tank wall. This annex does not cover stainless steel clad plat e or strip lined construction. SC.1.2 This annex applies only to tanks in non-refrigerated services with a maximum design temperature not exceeding that specified in 1.2.2 and a minimum design metal temperature limited to –40 °C (–40 °F). For the purposes of this annex, the design temperature shall be the maxi-mum design temperature as specified by the Purchaser. Ambient temperature tanks (non-heated) s hall have a design temperature of 40 °C (100 °F). It is cautioned that exothermic reactions occurring inside unheated storage tanks can produce temperatures exceeding 40 °C (100 °F). SC.1.3 This annex states only the requirements that differ from the basic rules in this standard. For requirements not stated, the basic rules must be foll owed including Annex S and Annex X as appl icable. References to paragraphs in this Annex are to this standard unless stated otherwise. SC.1.4 For limitations due to thermal effects see paragraph S.3.1 and X.3.1 SC.1.5 The nameplate of the tank shall indicate that the tank is in accordance with this annex by the addition of Annex SC to the information required by 8.1. In addition, the nameplate sha ll be marked with the maximum design temperature in the space indicated in Figure 8-1. SC.1.6 Annexes: These annexes are applicable for use wi th stainless steels as follows. a) Annex D is applicable; however, s ee S.2.1.3 and X.2.1.3 for special requirements when attaching to carbon steel supports. b) Annex E is applicable; however, see S.2.1.3 and X.2.1.3 for special requirements when attaching external carbon steel supports. If internal supports are stainless steel, they shall m eet the material requirements of Annex S or Annex X as applicable. c) Annex H is not applicable; stress relieving is only required when spec ified by the purchaser and must be performed with care and in such a manner that does not damage or alter the properties of the stainless steel (reference X.3.9.1) d) Annex Q is not applicable. e) Annex R is not applicable. f) All other annexes are applicable.S-2 API S TANDARD 620 

### SC.2 Materials SC.2.1 Materials shall be in accordance with Secti on 4 or Annex S or Annex X as applicable. SC.2.2 Selection of the type/grade of st ainless steel and carbon steel for mixed material t anks depends on the service and environment to which it will be exposed and the effects of fabricati on processes. (See paragraphs S.4.3.2 and S.4.4.3, X.2.1.1, X.4.3.2. ) The Purchaser shall select the type/grade. The Purchaser shall also specify which components shall be stainless steel. SC.2.3 Components of a tank including the tank wall, roof, bottom or bottom openings and their re inforcement ma y be car bon steels meeting the requirements of Section 4 provided they are protected from corrosion and the design and details consider the dissimilar properties of the ma terials used. Carbon steel attachments (e.g., clips for scaffolding) shall not be welded directly to any internal stainless steel tank surface. SC.2.4 Impact tests are not required for austenitic stainles s steel base m e tals. See X.2.3.2 for impact testing requirements for duplex stainless steel. Carbon steels in a mixed material tank shall require impact testing in accordance with the basic document. SC.2.5 Welding of stainless steel to carbon steel shall use st ainless steel filler metal appropr iate for the type/grade of st ainless steel used and t he welding process employed. 

### SC.3 Design SC.3.1 General A structural analysis of the entire tank structure is required to adequately predict stresses due to differential movements when austenitic stainless steel is joined to either carbon steel or duplex stainless steel components such as bottom to tank wall, adjacent tank wall courses and roof to the top of the tank wall. The design shall account for differential component expansion. The ma terial combination of this paragraph applies to all other sub-paragraphs in Section SC.3. No analysis of stresses from differential movements is requir ed for duplex stainless steel joined to carbon steel. 

### SC.3.2 Tank Wall Design SC.3.2.1 Austenitic stainless steel insert plates shall not be used in ca rbon steel or duplex stainless steel plates and carbon steel or duplex stainless steel inse rt plates shall not be used in austeni tic stainless steel plates except when an evaluation for differential movement due to temperature is performed. 

### SC.3.3 Nozzles and Manways S.3.3.1 Reinforcement requirements of 5.16 and 5.17 must be main tained except insert plates shall comply with SC 3.2.1. S.3.3.2 No zzles and Manways in the tank wall shall be of t he same material as the t ank wall unless otherwise specified by the Pur c haser. S.3.3.3 Reinforcing plates for the tank wall shall be carbon steel to carbo n steel and st ainless steel to st ainless steel even if the nozzle material differs from the tank wall material. 

### SC.4 Miscellaneous Requirements SC.4.1 Chemical cleaners and pickling solutions used shall not have a detrimental effect on the stainless steel or carbon steel in mixed material tank s and their welded joints. Chemical cleaners and pickling solutions shall beD ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS S-3 disposed of in accordance with laws and regulations governing the disposal of su ch chemicals. The use of chemical cleaners shall always be followed by thorough rinsing with potable water and drying (see S.4.9 and X 4.9). SC.4.2 Impact tests are not required for aus tenitic st ainless steel weld met a ls and heat-affected zones. Impact tests of the carbon steel or duplex stainless steel heat affected zone shall be performed when required by this standard or Annex X. SC.4.3 Postweld heat treatment of austeni tic st ainless steel and d uplex st ainless steel materials need not be performed unless specified by the purchaser. PWHT of carbon steel components shall be performed when required by the basic document. For mixed material nozzle assemb lies, the PWHT requirement s of 5.25 and 6.18 are not mandatory except when specifi ed by the purchaser. The purchaser is caut ioned that mixed material nozzles with duplex stainless steel should not be PWHT due to the pot ential damaging effects of high temperature on the duplex material. The Purchaser is advised to di scuss with a materials consultant or mill representative to determine what PWHT can be done for the specific material/chemistry/configuration. SC.4.4 Surfaces of carbon steel plates shall be free of rust, foreign materials, and scale p r ior to welding to stainless steel plates. SC.4.5 In order to prevent excessive weld metal dilution at butt welds between stainles s and car bon steel, at least one side of the joint shall be bev eled with land not to exceed t / 3

. SC.4.6 Internal galvanic corrosion will occu r by using mixed materi al constr uction and additional mitigation such as appropriate localized coatings shall be considered. SC.4.7 Where substantial quantities of unc oated stainless steel ar e we lded to coated carbon steel, accelerated corrosion rates are possible at holid ays in the carbon steel coating.U-1 

## Annex U (normative) Ultrasonic Examination in Lieu of Radiography U.1 Purpose and Scope This annex provides detailed rules for the use of the ultr asonic examination (UT) method for the examination of tank seams as permitted by 5.26, R.7.6, and Q.7.6. This alternative is limited to joints where t he thickness of the thinner of the two members joined is greater than or equal to 10 mm ( 3 / 8 in.). 

### U.2 Definitions The following definitions apply. U.2.1 documenting Preparation of text and/or and figures. U.2.2 evaluation All activities required in U.6.3 through U.6. 6 to determine the acceptability of a flaw. U.2.3 flaw A reflector that is not geometric or meta llurgical in origin that may be detecta ble by nondestructive testing but is not necessarily rejectable. U.2.4 flaw categorization Whether a flaw is a surface flaw or is a subsurface flaw (see U.6.4). Note that a flaw need not be surface breaking to be categorized as a surface flaw. U.2.5 flaw characterization The process of quantifying the size, location, and shape of a flaw. See U.6.3 for size and location. The only shape characterization required by this annex is applied to the results of supplemental surface examination by MT or PT (see U.6.6.2). U.2.6 indication That which marks or denotes the presence of a reflector. U.2.7 interpretation The determination of whether an indication is relevant or nonrelevant, i.e., whether it originates from a geometric or metallurgical feature or conversely originates from a flaw (see U.6.2). U.2.8 investigation Activities required to determine the interpre tation of an indication (see U.6.1 and U.6.2).U-2 API S TANDARD 620
U.2.9 recording The writing of ultrasonic data o nto an appropriate electronic medium. U.2.10 reflector An inter face at which an ultrasonic beam encounters a c hange in acoustic impedance and at wh ich at least part of the energy is reflected.


### U.3 Technique
U.3.1 The ultrasonic examination volume shall include the we ld metal, plus the lesser of 25 mm (1 in.) or t of adjoining base metal on each side of the weld unless otherwise agreed upon by the Purchaser and the Manufacturer.
U.3.2 Ultrasonic examination for the detec tion of flaws shall be performed us ing automated, computer-based data acquisition except that scanning of adj acent base metal for flaws that can in terfere with the examination may be performed manually. Ultrasonic examination for sizing of flaws shall be performed as described in U.6.3.1.
U.3.3 A documented examination strategy or scan plan shall be provided showing transducer placement, movement, and component coverage that provides a standardized and repeatable methodology for weld acceptance. The scan plan shall also include ultrasonic beam angle to be us ed, beam directions with respect to weld centerline, and tank material volume examined for each weld. The documentation shall be made available to the Owner upon request.
U.3.4 Data from the examination volume, per U.3.1, shall be recorded and/or documented as follows: a) For automated computer-based scans, data shall be re cor ded us ing the same sys tem essential variables, specified value or range of values, used for t he demonstration of the procedure per U.4.3 below. b) For manual scans, results shall be documented in a written report.
U.3.5 The ultrasonic examination sh all be per formed in accordance with a writt en procedure which has been reviewed and approved by the Purchaser and conforms to the requirements of Se ction V, Article 4, except that: a) The calibration block shown in Figure T-434.2.1 of Section V , Articl e 4 shall be used, and; b) For examination techniques that provi de plate quality information (e.g., time of flight diff r action), the initial base material straight-beam exam ination need not be performed.
U.3.6 The examination methodology (including U.6.6) shall be demonstrated to be effective over the full weld volume. It is recognized that time of f light diffraction (TOFD) may have limitati ons in detection of flaws at the surface such that it may be necessary to suppl ement TOFD with pulse-echo techniques suitable for the detection of near-field and far-field flaws. The variety of surf ace and sub-surface category flaws in t he test plate mandated by U.4.3.a are intended to ensure that any such limitations are adequately addressed.
U.3.7 It is recognized that in the ultrasoni c examination of joints with austeni tic weld metals, initial screening for defects may be done by methods that determine flaw lengths but give only limited information on flaw height. In these cases a length and upper bound height acceptance criteria is applied (see Table U-2).D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS U-3 

### U.4 Personnel Qualificat ions and Training U.4.1 Personnel Qualifications Personnel performing and evaluating UT examinations shall be qual ified and certified in accordance with their employer's written practice. ASNT SNT-TC-IA or CP-189 s hall be used as a guideline. Only Level II or III personnel shall perform UT examinations, analyze the data, or interpret the results. 

### U.4.2 Qualification Records Qualification records of certified personnel shall be approv ed by the Manufacturer and maintained by their employer. 

### U.4.3 Personnel Testing Personnel who acquire and analyze UT data shall be trai ned using the equipment of U. 3.2, and the procedure of U.3.5 above. Additionally, they shall pass a practical examination based on the technique on a blind test plate. The testing program details shall be by agreement between the Purchaser and the inspection company, but shall in any case include the following elements as a minimum. a) The test plate shall contain a variety of surface and sub-surf ace category flaws including multiple flaws described in section U.6.5. Some of the flaws shall be acceptable and others unacceptable per the applicable criteria of Tables U-1 or U-2. b) The practical examination should cove r detection, interpret ation, sizing, plotting, categoriz ation, grouping, and characterization that is sufficient to cover the cases outlined in U.6. c) Criteria for passing the test shall include limits on t he number of miscalls, both of rejectable flaws, missed or accepted, and acceptable regions rejected. d) Testing shall be facilitated by a third-party or by the Purchaser

. 

### U.5 Level III Review U.5.1 The final data package shall be reviewed by a UT Level III individual qualified in accordance with U.4.1 and U.4.3 above. The review shall include: a) The ultrasonic data record. b) Data interpretations. c) Evaluations of indications perform ed by another qualified L evel II or II I individual. The data review may be performed by another individual from the same organization. U.5.2 Alternatively, the review may be achieved by arranging for a dat a acquisiti on and initial interpretation by a Level II individual qualified in accordance with. U.4.1 and U.4.3 above, and a final interpretation and evaluation shall be performed by a Level III individual qualified per U.5.1. 

### U.6 Interpretation and Evaluation U.6.1 Investigation Criteria Reflectors that produce a response greater than 20 % of the reference level shall be investigated. Alternatively, for methods or techniq ues that do not use amplitude recording levels, sized reflectors longer than 40 % of the acceptable surface or subsurface flaws in T able U-1 or U-2 as applic able shall be investigated. The investigation shall interpretU-4 API S TANDARD 620 whether the indication originates from a flaw or is a geom etric indication in accordance with U.6.2 below. When the reflector is determined to be a flaw, the flaw shall be ev aluated and acceptance criteria of Table U-1 or U-2 as applicable shall apply. 

### U.6.2 Interpretation as Geometric/Metallurgical Ultrasonic indications of geometric and meta llurgical origin shall be interpreted as described in U.6.2.1, U.6.2.2, and U.6.2.3. U.6.2.1 Indications that are determined to originate from the su rfa ce configurations ( s uch as weld reinforcement or root geometry) or variations in metallurgical structure of materials may be interpreted as geometric indications, and: a) need not be sized or categorized in accordance with U.6.3 and U.6.4; b) need not be compared to the allowable flaw acceptance criteria of Table U-1 or U-2; c) the maximum indication amplitude (i f applicable) and location shall be doc umented, for example: internal attachments, 200 % DAC maximum amplitude, 25 mm (1 in.) above the weld centerline, on the inside su rface, from 90 to 95 degrees. U.6.2.2 The following steps shall be taken to cla ssify an indication as geometric: a) interpret the area containing the indication in ac cordance with the applicable examination procedur e; b) plot and verify the indication's coor dinates; provide a cross-sectional disp lay showing the indication's position and any sur face conditions such as root or counterbore; and c) review fabrication or weld prep drawings. U.6.2.3 Alternatively, other NDE methods or techniques may be applied to interpret an indication as geometric (e.g., alter native UT beam angles, radiography, ID and/or OD profiling). 

### U.6.3 Flaw Sizing U.6.3.1 Flaws shall be sized using automated, computer- based data acquisition or by a supplemental manual technique that has been demonstrated to perform acceptably per U.4.3. U.6.3.2 The dimensions of the flaw shall be defined by the rect angle that fully co nt ains the area of the flaw. The length ( l ) of the flaw shall be drawn parallel to the inside pressure-retaining surface of the component. The height ( h ) of the flaw shall be drawn normal to the inside pressure retaining surface. 

### U.6.4 Flaw Categorization If the space between the surface and the flaw in the thru -thickness direction is less than one-half the measured height of the flaw, then the flaw shall be cat egorized as a surface flaw with flaw height extending to the surface of the material. 

### U.6.5 Grouping of Multiple Flaws U.6.5.1 Discontinuous flaws that are oriented primarily in parall el planes shall be considered to lie in a single plane if the distance between the adjacent planes is equal to or less than 13 mm ( 1 / 2 in.). U.6.5.2 If the space between two flaws, aligned along the axis of weld, is less than the l ength of the longer of the two, the two flaws shall be considered a singular flaw.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS U-5
U.6.5.3 If the space between two flaws aligned in the through-thic kness direction is less t han the height of the flaw of greater height, the two flaws s hall be considered a singular flaw. 

### U.6.6 Flaw Acceptance Criteria U.6.6.1 Acceptance Criteria Tables Flaw dimensions resulting after the application of the rules of U.6.3, U.6.4, and U. 6.5 shall be evaluated for accept ance using the applicable criteria of Table U-1 or U-2. U.6.6.2 Surface Examination Flaws characterized as surface flaws du ring the UT examination may or may not be surface con nected. Therefore , unless the UT data analysis conf irms that the flaw is not surface connected, a supplemental surf ace examination (MT or PT) shall be performed in accordance with 5.15.2 or 5.15. 4 as applicable for all surface flaws. Any flaws detected by MT or PT, and characterized as planar, are unacceptable, regardless of length. 

### U.7 Repairs All repaired areas, plus the lesser of 25 mm (1 in.) or t of the adjoining weld on each side of the repair, shall be reinspected per this annex. 

### U.8 Flaw Documentation In addition to the data record prescribed by U.3.4, written documentation shall be produced for each unacceptable flaw and those acceptable flaw s that either exceed 50 % of reference level for amplit ude based techniques or exceed 75 % of the acceptable length fo r non-amplitude techniques. Table U-1—Flaw Accept ance Criteria for UT Indications (May be Used for All Materials and 201LN)

Thickness at Weld ( t ) a mm (in.) Acceptable Flaw Lengths, ( l ) mm (in.) For Surface Flaw b with Height, ( h ) mm (in.) For Sub-surface Flaw with Height, ( h ) mm (in.) 2 (0.08) 2.5 (0.10) 3 (0.12) 2 (0.08) 3 (0.12) 4 (0.16) 5 (0.2) 6 (0.24) 10 (0.375) to < 13 (0.50) 8 (0.30) 8 (0.30) 4 (0.15) 14 (0.55) 5 (0.20) 4 (0.15) Not allo wed Not allowed 13 (0.50) to < 19 (0.75) 8 (0.30) 8 (0.30) 4 (0.15) 38 (1.50) 8 (0.30) 5 (0.20) 4 (0.15) 3 (0.10) 19 (0.75) to < 25 (1.0) 8 (0.30) 8 (0.30) 4 (0.15) 75 (3.00) 13 (0.50) 8 (0.30) 6 (0.25) 5 (0.20) 25 (1.0) to < 32 (1.25) 9 (0.35) 8 (0.30) 4 (0.15) 100 (4.00) 20 (0.80) 9 (0.35) 8 (0.30) 6 (0.25) 32 (1.25) to < 38 (1.50) 9 (0.35) 8 (0.30) 4 (0.15) 125 (5.00) 30 (1.20) 10 (0.40) 8 (0.30) 8 (0.30) 38 (1.50) to < 44 (1.75) 9 (0.35) 8 (0.30) 4 (0.15) 150 (6.00) 38 (1.50) 10 (0.40) 9 (0.35) 8 (0.30)

a t = thickness of the weld excluding any allo wable reinforcement. For a butt weld joini ng members having differ ent thickness at t he weld, t is the thinner of the two. b Any surface flaw, to be deemed acceptable, must satisfy both the size limitations of this tabl e and additionally satisfy the MT /PT characterization limitations of U.6.6.2.Table U-2—Alternate Flaw Acceptance Criteria for UT Indications a (Applicable to 3XX and 201LN stainless steel and alum inum with toughness meeting Annex Q, and to 9% nickel steel with Table U-3 Toughness.) Thickness at

weld b ( t ) mm (in.) Limit on flaw length ( l ) mm (in.) Limit on surfaced flaw height c ( h ) mm (in.) Limit on sub-surface flaw height ( h ) mm (in.) Based on Length and Upper Bound Height All t 12.0 (0.4 5) 50 % of t 50 % of t Based on Flaw Length and Sized He ight All t 18.0 (0. 70) Smaller of 6.5 (0.25) or 2 5 % of t Smaller of 9. 0 (0.35) or 50 % of t

Table U-3—Charpy V-notch Impact Values Required to Use Table U-2 for 9% Nickel Steel

Size of Specimen (mm) Transverse Value Required for Acceptan ce a J (ft-lbs) Minimum Value without Requiring Retest b J (ft-lbs) 10 × 10.00 50 (37) 40 (30) 10 × 7.50 38 (28) 30 (22) U-6 API S TANDARD 620

a In order to use this acceptance criteria for 9 % nickel steel, it is required that t he Charpy V-notch impact results for base m etal (transverse) as required in Q.2.3.2, and for weld metal and heat affected zones as required in Q.4.3.3 and Q.4.3.4 conform to t he higher energy levels given in Table U-3 in lieu of Table Q-2. b t = thickness of the weld excluding any allowable reinforcement. Fo r a butt weld joining two members having different thickness a t the weld, t is the thinner of these two thicknesses.

c Any surface flaw, to be deemed acceptable, must satisfy both the size limitations of this table and additionally satisfy the MT /PT characterization limitations of U.6.6.2. d When Table U.2 is applicable, any flaw is acceptable if it satisfies either of the two lines of Table U-2.

a Average of three specimens. b Only one specimen of a set.X-1 

## Annex X (normative) Duplex Stainless Steel Storage Tanks X.1 Scope
X.1.1 This annex covers materials, desi gn, fabrication, erection, and testi ng requirements for aboveground, welded, duplex stainless steel storage tanks constructed of material grades 2205 (UNS S31803), 2003 (UNS S32003), 2101 (UNS S32101), 2202 (UNS S32202), 2205 (UNS S32205) , 2304 (UNS S32304), 255 (UNS S32550), 255+ (UNS S32520), 2507 (UNS S32750), and Z100 (UNS S32760).This annex does not cover stainless st eel clad plate or strip lined construction.
X.1.2 This annex applies only to tanks in non-refrigerated services with a maximum desi gn temperature limited as given in 1.2.2 and a minimum design metal temperatur e equal to or higher than –40 °C (–40 °F). Ambient temperature tanks (non-heated) shall have a design temperat ure of 40 °C (100 °F). It is cautioned that exothermic reactions occurring inside un heated storage tanks can produce temperatur es exceeding 40 °C (100 °F). For the purposes of this annex, the design temperature shall be t he maximum operating temperature as specified by the Purchaser.
X.1.3 The minimum thicknesses in this annex do not contain any allowance for corrosion.
X.1.4 This annex states only the requirement s that differ from the basic rules in this standard. For requirements not stated, the basic ru les must be followed. 

### X.2 Materials X.2.1 Selection and Ordering
X.2.1.1 Materials shall be in accordance with Table X-1.
X.2.1.2 Specification of the type/grade of duplex stainless steel depends on the service and environment to which it will be exposed and the effects of fabrication proces ses. The Purchaser shall select the type/grade.
X.2.1.3 External structural attachments may be carbon steels meeting the requirements of Section 4 of this standard, providing any permanent attachments are protected from corrosion. (This does not include shell, roof, or bottom openings and their reinforcement.) Carbon steel attachments (e.g., clip s for scaffolding) shall not be welded directly to any internal surface of the tank. 

### X.2.2 Packaging Packaging duplex stainless steel for shipment is important to maintain its corrosion resist ance. Precautions to protect the surface of the material depend on the surface finish supplied and may vary among manufacturers. Standard packaging methods may not be sufficient to protect the mate rial from normal shipping damage. If the intended service requires special precautions, the Purchas er shall specify s pecial instructions. 

### •X-2 API S TANDARD 620



### X.2.3 Qualification Testing X.2.3.1 Tests for detecting detrimental intermetallic phases for ASTM A923 are required from one plate per heat treat lot as follows: UNS S32205/S31803 Methods B & C UNS S32202/S32304 Method B 1 UNS S32101 Method B 2 UNS S32003 Method B 1 UNS S32750 Method B 1 & C UNS S32550/S32520 Method B 1 & C UNS S32760 Method B 1 & C 3
1 B test values to be agreed upon between Purchaser a nd manufacturer but not less than 54 J (40 ft-lbf). 2 B test values to be agreed upon between Purchaser a nd manfacturer, but not le ss than 27 J (20 ft-lbf). 3 C test values to be agreed upon between Purchaser and manufacturer. Table X-1—ASTM Materials for Duplex Stainless Steel Components UNS S31803 UNS S32003 UNS S32101 UNS S32202 UNS S32205 UNS S32304 UNS S32550 UNS S32520 UNS S32750 UNS S32760 2205 2003 2101 2202 2205 2304 255 255+ 2507 Z100 Plates and Structural Members A240 X X X X X X X X X X A276 X X X X X X X Tube or Pipe Seamless and Welded A789 X X X X X X X A790 X X X X X X X A928 X XXXXXXX Forgings and Fittings A182 X X X X X A815 X X X X Bolting and Bars A

4 7 9 X XXX X XX NOTE 1 Unless otherwise specified by the Purchaser, plate, sheet, or strip shall be furnished with a No. 1 finish and shall be hot-rolled, annealed, and descaled. NOTE 2 Carbon steel flanges and/or stub ends may be used by agr eement between the Purchaser and Manufacturer, providing the des ign and details consider the dissimilar properties of the ma terials used and are suitable for the intended service. NOTE 3 Castings shall not be used unless spec ified by the Purchaser. If s pecified, castings shall meet ASTM A890 and shall be i nspected in accordance with ASME Boiler and Pressure Vessel Code , Section VIII, Division 1, Appendix 7. NOTE 4 All bars in contact with the product shall be fu rnished in the hot-rolled, annealed, and descaled condition. NOTE 5 Other bolting materials may be used by agreement between the Purchaser and Manufacturer.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS X-3
X.2.3.2 Charpy Impact testing per ASME UHA-51 at minimum des ign metal temperature is required for shell plates, shell reinforcing plates, shell insert plates, bottom plates welded to the s hell, plates used for manhole and nozzle necks, plate-ring shell-nozzle flanges, blind flanges, and manhole cover plates: a) in all thicknesses, when the minimum design temperat ure is between –29 °C and –40 °C (–20 °F and –40 °F), and b) for those that have thickness greater than 10 mm ( 3 / 8 in.) for all temperatures. ASTM A923 Practice B test results may be used to fulf ill these requirements provided the lateral expansion is measured and reported. 

### X.3 Design X.3.1 Operating Temperature The Purchaser shall specify the maximum operating temperat ure of the tank, not to exceed 120 °C (250 °F) given in 5.2. 

### X.3.2 Maximum Tensile Stress The maximum tensile stress shall be in accordance with 5.5.3, except Table X-2 shall be used to determine S ts . 

### X.3.3 Maximum Compressive Stress Allowable compressive stresses shall be in accordance with 5.5.4, except the allowable compressive stress shall be modified by the ratio of the material’s modulus of elasticity at the design temperature to 200,000 MPa (29,000,000 lbf/ in. 2 ) for values ( t – c )/ R less than 0.0175 and by the ratio of the material’s minimum yield strength at the design temperature to 205 MPa (30,000 lbf/in 2 ) for values of ( t – c )/ R equal to or greater than 0.0 175. Table X-2a—Allowable Stresses for Tank Shells (SI)

Alloy Min Yld Min Ten Allowable Stress MPa for Design Temp Not Exceeding ( Sts ) MPa MPa 40 °C 90 °C 120 °C S31803 450 620 186 186 183 S32003 450 655 197 174 168 S32101 450 650 194 176 172 S32202 450 650 196 194 187 S32205 450 655 197 176 172 S32304 400 600 180 174 168 S32550 550 760 228 227 221 S32520 550 770 232 203 201 S32750 550 795 240 239 232 S32760 550 750 223 203 199 NOTE 1 Sts may be interpolated between temperatures. NOTE 2 The design stress shall be the lesser of 0.3 of the mi nimum tensile strength or 0.6 of the minimum yield strength. NOTE 3 For dual certified materials, S31803/S32205 and S32550/S32520, use the allowable stress of the grade specified by the Pu rchaser. NOTE 4 The hydrotest stress allowable shall be as shown in 5.5.7.Table X-2b—Allowable Stresses for Tank Shells (USC)

Alloy Min Yld Min Ten Allowable Stress lbf/in. 2 for Design Temp Not Exceeding ( Sts ) lbf/in. 2 lbf/in. 2 100 °F 200 °F 250 °F S31803 65,000 90,000 27,000 27,000 26,500 S32003 65,000 95,000 28,500 25,200 24,400 S32101 65,000 94,000 28,200 25,500 24,900 S32202 65,000 94,000 28,500 28,300 27,300 S32205 65,000 95,000 28,500 25,500 25,000 S32304 58,000 87,000 26,100 25,200 24,400 S32550 80,000 110,000 33,000 32,900 32,000 S32520 80,000 112,000 33,600 29,400 29,100 S32750 80,000 116,000 34,800 34,700 33,700 S32760 80,000 108,000 32,400 29,400 28,800 X-4 API S TANDARD 620



### X.3.4 Maximum Allowable Stress for Structural Members and Bolts The maximum allowable stress values for structural mem bers shall be in accordance with Table 5-3 except the allowable stresses for compression shall be modified by th e ratio of the material’s yield strength at design temperature to 205 MPa (30,000 lbf/in 2 ). 

### X.3.5 Flat Bottoms of Cylindrical Tanks The minimum thickness for bottom plates shall be 5 mm ( 3 / 16 in.), exclusive of any corrosion allowance specified by the Purchaser. 

### X.3.6 Intermediate Wind Girders for Cylindrical Sidewalls X.3.6.1 The value H 1 in 5.10.6.1 shall be modified by the ratio of th e materials modulus of elasticity at th e design temperature to 200,000 MPa (29,000,000 lbf/in. 2 ). X.3.6.2 The value Wtr in 5.10.6.2 shall be modi fied by the ratio of the materials m odulus of elastici ty at the design temperature to 200,000 MPa (29,000,000 lbf/in. 2 ). 

### X.3.7 Compression Rings The value of 15,000 in equation (27) in 5. 12.4.3 shall be modified by the ratio of the material yiel d strength at the design temperature to 205 MPa (30,000 lbf/in. 2 ). 

### X.3.8 Flat Cover Plates and Blind Flanges The value s in 5.21 shall be in accordance with Table X-2.

NOTE 1 Sts may be interpolated between temperatures. NOTE 2 The design stress shall be the lesser of 0.3 of the mi nimum tensile strength or 0.6 of the minimum yield strength. NOTE 3 For dual certified materials, S31803/S32205 and S32550/S32520, use the allowable stress of the grade specified by the P urchaser. NOTE 4 The hydrotest stress allowable shall be as shown in 5.5.7.Table X-3a—Yield Strength Values (SI)

Alloy Yield Strength MPa for Design Temp Not Exceeding 40 °C 90 °C 120 °C S31803 450 396 383 S32003 450 386 369 S32101 450 379 365 S32202 448 377 355 S32205 450 358 348 S32304 400 343 331 S32550 550 484 464 S32520 550 448 434 S32750 550 486 466 S32760 550 455 441

Table X-3b—Yield Strength Values (USC)

Alloy Yield Strength lbf/in. 2 for Design Temp Not Exceeding 100 °F 200 °F 250 °F S31803 65,000 57,500 55,600 S32003 65,000 56,000 53,500 S32101 65,000 55,000 53,000 S32202 65,000 55,500 52,400 S32205 65,000 52,000 50,500 S32304 58,000 49,800 48,050 S32550 80,000 70,200 67,250 S32520 80,000 65,000 63,000 S32750 80,000 70,500 67,600 S32760 80,000 66,000 64,000 D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS X-5

NOTE 1 Interpolate between temperatures. NOTE 2 Reference: Table Y-1 of ASME Sectio n II, Part D, or manufacturer’s data sheets.

NOTE 1 Interpolate between temperatures. NOTE 2 Refer ence: Table Y-1 of ASME Section II, Part D, or manufacturer’s data sheets.Table X-4a—Modulus of Elasticity at the Maximum Operating Temperature (SI)

Alloy Modulus of Elasticity in MPa for Design Temp Not Exceeding 40 °C 90 °C 120 °C S31803 198,000 190,000 187,500 S32003 209,000 205,000 203,000 S32101 198,000 194,000 192,000 S32202 198,000 195,000 193,000 S32205 198,000 190,000 187,500 S32304 198,000 190,000 187,500 S32550 209,000 206,000 204,000 S32520 209,000 206,000 204,000 S32750 202,000 194,000 191,000 S32760 199,000 193,000 191,500

Table X-4b—Modulus of Elasticity at th e Maximum Operating Temperature (USC)

Alloy Modulus of Elasticity in lbf/in. 2 for Design Temp Not Exceeding 100 °F 200 °F 250 °F S31803 28,700,000 27,600,000 27,200,000 S32003 30,300,000 29,800,000 29,500,000 S32101 28,700,000 28,100,000 27,800,000 S32202 28,800,000 28,200,000 27,800,000 S32205 28,700,000 27,600,000 27,200,000 S32304 28,700,000 27,600,000 27,200,000 S32550 30,300,000 29,900,000 29,600,000 S32520 30,300,000 29,900,000 29,600,000 S32750 29,300,000 28,100,000 27,650,000 S32760 28,800,000 28,000,000 27,800,000 X-6 API S TANDARD 620



### X.3.9 Stress Relieving The stress relieving requirements of 5.25 shall not be performed unless specified by the Purchaser. 

### X.3.10 Flush-type Shell Connection The value tb in 5.27.4.5 shall be modified by the ratio of the material’s yield stress at the design temperature to 205 MPa (30,000 lbf/in. 2 ). NOTE 1 Interpolate between temperatures.

NOTE 1 Interpolate between temperatures.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS X-7 

### X.4 Fabrication X.4.1 General Special precautions must be observed to minimize the risk of loss of the corrosion resistance and toughness of duplex stainless steel. Duplex stainless steel shall be handled so as to minimize contact with free iron or other types of carbon steels during all phases of fabricat ion, shipping and construction. The therma l history of the material must also be controlled. The following sections describe the major precautions th at shall be observed during handling. 

### X.4.2 Storage Storage shall be under cover and well removed from shop dirt and fumes from pickling operations. If outside storage is necessary, provisions shall be made for rainwater to drain and allow the material to dry. Duplex stainless steel shall not be stored in contact with carbon steel. Materials c ontaining chlorides, including foods, beverages, oils, cleaners, and greases, shall not come in contact with duplex stainless steel . Inadvertent contamination shall be removed by methods described in X.4.5. 

### X.4.3 Thermal Cutting X.4.3.1 Thermal cutting of duplex stainles s steel shall be by the plasma-a rc method or by laser cutting. X.4.3.2 

### • Thermal cutting of duplex stainless steel may leave a heat -affected zone with intermetallic precipitates. This heat-affected zone may have reduced corrosion resistance a nd toughness unless removed by machining or grinding. Normally the HAZ from thermal cutting is thin enoug h to be removed by edge preparation machining and adjacent base metal melting during welding. T he Purchaser shall specify if the heat-affected zone is to be removed. 

### X.4.4 Forming X.4.4.1 Duplex stainless steels shall be formed by a cold or hot forming procedure that is not injurious to the mat erial. X.4.4.2 Duplex stainless steels may be cold formed. The maxi mum strain produced by such cold forming shall no t exceed 10% and control of formi ng spring-back shall be provided in the forming procedure. X.4.4.3 Hot forming, shall be performed within a temperature range shown in Table X-5. X.4.4.4 The minimum soaking temperature shall be achieved before commenc ing any hot forming. X.4.4.5 Forming at temperatures between 315 °C (600 °F ) and the minimum temperature shown in Table X-5 is not permitted. X.4.4.6 Following hot forming an anneal and water quench shall be performed. 

### X.4.5 Cleaning X.4.5.1 

### • When the Purchaser requires cleaning to remove su rface contaminant s that may imp air the normal

corrosion resistance, it shall be done in accordance with ASTM A380, unless otherwise specified. The Purchaser shall specify any additional cleanliness requirement s for the intended service. X.4.5.2 When welding is completed, flux residues and weld spatter shall be re moved mechanica lly using st ainless steel tools.Table X-5a—Minimum and Maximum Hot Forming Temperatures and Minimum Soaking Temperature (SI) Alloy °C Max °C Min °C Min Soaking Temperature S31803 1 230 950 1 040 S32003 1100 950 1010 S32101 1100 900 980 S32202 1100 1000 1080 S32205 1230 950 1040 S32304 1100 950 980 S32550 1230 1000 1080 S32520 1230 1000 1080 S32750 1230 1025 1050 S32760 1230 1000 1100

Table X-5b—Minimum and Maximum Hot Forming Temp eratures and Minimum Soaking Temperature (USC) Alloy °F Max °F Min °F Min Soaking T

emperature S31803 2 250 174 0 1900 S32003 2010 1740 1850 S32101 2010 1650 1800 S32202 2010 1830 1975 S32205 2250 1740 1900 S32304 2010 1740 1800 S32550 2250 1830 1975 S32520 2250 1830 1975 S32750 2250 1875 1920 S32760 2250 1830 2010 X-8 API S TANDARD 620

X.4.5.3 Removal of excess weld metal, if required by Purchaser , shall be done with a grinding wheel or belt that has not been previously used on other metals. X.4.5.4 Removal of weld heat tint, if required by Purc haser, shall be done using an appropriate pickling pr oduct and pickling procedure. X.4.5.5 Chemical cleaners and pickling solutions used shall not have a detriment al effect on the du plex stainless steel or welded joints and shall be di sposed of in accordance with laws and regulations governing the disposal of such chemicals. Thorough rinsing with water and drying shall always follow the use of any chemical cleaners or pickling solutions (see X.4.9).D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS X-9


### X.4.6 Blast Cleaning If blast cleaning is necessary, it shall be done with sharp acicular grains of sand or grit containing not more than 1 % by weight iron as free iron or iron oxide. Steel shot or sand previously used to clean non stainless steel materials is not permitted. 

### X.4.7 Pickling If pickling of a duplex stainless steel is necessary, an acid mixture of nitric and hydrofluoric acids shall be used. After pickling, the duplex stainless steel shall be thoroughly rinsed with water and dried. 

### X.4.8 

### • Passivation or Surface Iron Removal

When the Purchaser specifies passivation or surface iron removal, cleaning shall be achieved by treatment with nitric or citric acid. Nitric hydrofluoric acid shall be used to remove embedded iron. 

### X.4.9 Rinsing X.4.9.1 When cleaning, pickling or pa ssivation is required, these operati ons shall be followed immediately by rinsing, not allowing the surf aces to dr y between operations. Manufacturer’s instructions will require a neutralization treatment before rinsing for some pickling products. X.4.9.2 

### • Rinse water shall be potable and shall not contain more than 200 parts per million chloride at temperatures below 40 °C (100 °F), or no more than 100 parts per mil lion chloride at temperatures above 40 °C (100 °F) and below 65 °C ( 150 °F). X.4.9.3 Following final rinsi ng, the equipment shall be completely dried. 

### X.4.10 Welding X.4.10.1 Welding shall be by any of the processes permitted in 6.6.2. Galvanized component s or components with zinc-bearing coatings shall not be welded to duplex stainless steel. X.4.10.2 

### • Filler metal chemistry shall be as specified by the Purchaser. Proper filler metal selection may be

discussed with the materials ma nufacturer. Dissimilar welds to carbon steels shall use filler metals of E309L or higher alloy content. 

### X.4.11 Welding Procedure and Welder Qualifications
X.4.11.1 

### • Welding Procedure and Welder Qualification requirements shall be as specified in Section 6.7 and 6.8. In addition, procedures shall meet the requirements of AS TM A923 Method B and when specified by Purchaser also Method C. Welding Procedure Qualificat ion Records shall document the results of tests required both by Section 6.7 and by ASTM A923. X.4.11.2 For any material that has not been assigned a P- number in T able QW-422 of Section IX of the ASME code the Welding Procedure and the Welder Qualification shall be developed for that specific material. 

### X.4.12 Postweld Heat Treatment Post weld heat treatment of duplex stai nless steel materials shall not be performed.X-10 API S TANDARD 620 

### X.5 Inspection and Testing X.5.1 Inspection of Welds X.5.1.1 Radiographic Inspection of Butt-Welds X.5.1.1.1 Radiographic examination of butt-welds sha ll be in accordance with 7.15.1 and 7.17 .1.1. X.5.1.1.2 When shell designs use joint efficiency = 0.85, spot radiographs of vert ical joint s shall conform to 7.17.2, 7.17.3, and 7.17.4. X.5.1.2 Inspection of Welds by Liquid Penetrant Method The following component welds shall be examined by the liqui d penetrant method before the hydrost a tic test of the tank. a) The shell-to-bottom inside attachment weld. b) All welds of opening connections in t ank shell that are not completely radiographed, including nozzle and manhole neck welds and neck-to-fla nge welds. c) All welds of attachments to shells, such as stiffener s, compr ession rings, clips, and othe r nonpressure p arts for which the thickness of both part s joined is greater than 19 mm ( 3 / 4 in.). d) All butt-welded joints in tank annular plat es on which backing strips are to remain. 

### X.5.2 Hydrostatic Testing X.5.2.1 The rules of 7.18.3 and 7.18.4 apply to hydrostatic testing. X.5.2.2 

### • The materials used in the constructi on of duplex stainless steel tanks ma y be subject to pitting, or general corrosion if they are exposed to contaminated test water for extended periods of time. T he Purchaser shall specify a minimum quality of te st water that conforms to the following requirements. a) Unless otherwise specifi ed by the Purchaser, water used for hydr osta tic testing of tanks shall be potable and treated, containing at least 0.2 parts per million free chlorine. b) Water shall be substantially clean and clear. c) Water shall have no objectionable odor (that is, no hydrogen sulfide). d) Water pH shall be between 6 and 8.3. e) Water temperature shall be below 50 °C (120 °F). f) The chloride content of the water shall b e below 50 p arts per million. X.5.2.3 

### • When testing with potable water, the exposure time s hall not exceed 21 days unles s otherwise specified by the Purchaser. X.5.2.4 When testing with other fresh waters, the exposure time shall not exceed 7 days.D ESIGN AND C ONSTRUCTION OF L ARGE , W ELDED , L OW - PRESSURE S TORAGE T ANKS X-11
X.5.2.5 Upon completion of the hydrostatic test, water s hall be completely drained. Wetted surfaces shall be washed with potable + water when non-potable water is used fo r the test, and completely dried. Particular attention shall be given to low spots, crevices, and si milar areas. Hot air drying is not permitted. 

### X.6 Marking Brazing shall be deleted from 8.1.3. 

### X.7 Annexes The annexes are applicable for use with duplex stainless steels as follows:

— Annex D is applicable; howev er see X.2.1.3 for special r equirements when attaching to carbon steel supports.

— Annex E is applicable; however see X.2.1.3 for special requirement s w hen attaching external carbon steel supports. Internal supports shall meet the material requirements of Annex X.

— Annex H is not applicable; stress relieving is only required when spec ified by the Purchaser and must be performed with care and in such a manner that does not dam age or alter the properties of the stainless steel.(Ref X.3.9.1.)

— Annex Q is not applicable.

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