Design and Construction of

Large, Welded, Low-pressure 

Storage Tanks

API STANDARD 620

TWELFTH EDITION, OCTOBER 2013
Special Notes

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Copyright © 2008, 2009, 2010, 2012, 2013 American Petroleum Institute
Foreword

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suggestions for change.

iii

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 Foundations. . . . . . . . . . . . . . 5-27

5.12 Design of Roof and Bottom Knuckle Regions and Compression-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 Roofs 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

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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

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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 Temperature 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 Construction for a Compression-ring Juncture . . . . . .5-33

vii
Page

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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-57

5-12 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 Stress 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 Problems Involving Biaxial Tension and 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-10

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 R1 and R2 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-29

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

viii
Page

F-1 Computed Values of (t – c)R, sc, st, and N for the Assumed Thicknesses: Example 1. . . . . . . . . . . . . . F-5

F-2 Computed Values of (t – c)R, sc, st, 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 Tanks . . . . . . . . . . . . . . . . . . . . . . . . . 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 Temperature for Atmospheric Temperature Material 

Plates Used without Impact Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . R-6

R-5 Minimum Charpy V-notch Impact Requirements for Atmospheric Temperature 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 for Stainless Steel Components (US Customary Units) . . . . . . . . . . . . . . . . . . . . . . . . S-3

S-2 Maximum Allowable Stress Values for Simple Tension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S-4

S-3 Allowable 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 Stresses 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 and Minimum Soaking Temperature (USC) . . . X-8

ix

1-1

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-assembled 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 because of the variety of tank 

sizes and shapes that may be constructed. Where complete rules for a specific design are not given, the intent is for 

the Manufacturer—subject to the approval of the Purchaser’s authorized 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 shall ensure that the tank is 

constructed in accordance with the requirements of this standard.

The rules presented in this standard are further intended to ensure that the application of the nameplate shall be 

subject to the approval of a qualified inspector who has made the checks and inspections 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 large, 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 

pressures in their gas or vapor spaces not more than 15 lbf/in.2 gauge.

1.2.3 The basic rules in this standard provide for installation in areas where the lowest recorded 1-day mean 

atmospheric temperature is –50 °F. Annex S covers stainless steel low-pressure storage tanks in ambient 

temperature service in all areas, without limit on low temperatures. 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) hold or store liquids 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 horizontal tanks, they are not intended to preclude the 

application of appropriate portions to the design and construction of horizontal tanks designed in accordance with 

good engineering practice. The details for horizontal tanks not covered by these rules 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 suggestions for changes to this standard.

1.2.7 Annex B covers the use of plate and pipe materials that are not completely identified with any of the 

specifications listed in this standard.
1-2 API STANDARD 620

1.2.8 Annex C provides information on subgrade and foundation loading conditions and foundation construction 

practices.

1.2.9 Annex D provides information about imposed loads and stresses from external supports 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 are applied to various design problems.

1.2.12 Annex G provides considerations for service conditions that affect the selection of a corrosion allowance; 

concerns for hydrogen-induced cracking effects are specifically noted.

1.2.13 Annex H covers preheat and post-heat stress-relief practices for improved notch toughness.

1.2.14 Annex I covers a suggested practice for peening weldments to reduce internal stresses.

1.2.15 Annex J is reserved for future use. 

1.2.16 Annex K provides considerations for determining the capacity of tank venting devices.

1.2.17 Annex L covers requirements for the design of storage tanks 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 provides considerations for the safe operation and maintenance of an installed tank, with attention 

given to marking, access, site drainage, fireproofing, water draw-off piping, and cathodic protection of tank bottoms.

1.2.21 Annex P summarizes the requirements for inspection by method of examination 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 alone 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 materials, design, and fabrication 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 fabrication of tanks to be used for the 

storage of refrigerated products.

1.2.24 Annex S covers requirements for stainless steel tanks in non-refrigerated 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 structure, and other parts of a tank requiring 

added corrosion resistance.

1.2.26 Annex U covers detailed rules for the use of the ultrasonic examination (UT) method for the examination of 

tank seams.

1.2.27 Annex X covers materials, design, fabrication, erection, and testing requirements for aboveground, welded, 

duplex stainless steel storage tanks.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 1-3

1.3 Limitations

1.3.1 General

The rules presented in this standard apply to vertical, cylindrical oil storage tanks 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 following locations in piping connected internally or externally 

to the walls1 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 connections that 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, Specification for Line Pipe 

API Recommended Practice 520, Sizing, Selection, and Installation of Pressure-relieving Devices in Refineries—

Part 2, Installation

API Recommended Practice 582, Recommended Practice Welding Guidelines 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, Welded Tanks for Oil Storage

API Standard 2000, Venting Atmospheric 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 Flanged Fittings (ANSI/ASME B16.5)

ASME B31.1, Power Piping

ASME B31.3, Process Piping (ANSI/ASME B31.3)

ASME B36.10M, Welded and Seamless Wrought Steel Pipe (ANSI/ASME B36.10)

ASME B96.1, Welded Aluminum-Alloy Storage Tanks (ANSI/ASME B96.1)

ASME Boiler and Pressure Vessel Code, Section V, “Nondestructive Examination;” Section VIII, “Pressure Vessels, 

Division 1;” and Section IX, “Welding and Brazing Qualifications”

ASNT CP-189 8, Standard for Qualification and Certification of Nondestructive 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 Wacker Drive, Suite 700, Chicago, Illinois 60601, www.aisc.org.

5 American National Standards Institute, 25 West 43rd Street, 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 STANDARD 620

ASNT SNT-TC-IA, Personnel Qualification and Certification in Nondestructive Testing

ASTM A6 9, Standard Specification for General Requirements for Rolled Structural Steel Bars, Plates, Shapes, and 

Sheet Piling

ASTM A20, Standard Specification for General Requirements for Steel Plates for Pressure Vessels

ASTM A27, Standard Specification for Steel Castings, Carbon, for General Application

ASTM A36, Standard Specification for Carbon Structural Steel

ASTM A53, Standard Specification for Pipe, Steel, Black and Hot-Dipped, Zinc-Coated, Welded and Seamless

ASTM A105, Standard Specification for Carbon Steel Forgings for Piping Applications

ASTM A106, Standard Specification for Seamless Carbon Steel Pipe for High-Temperature Service

ASTM A131, Standard Specification for Structural Steel for Ships

ASTM A134, Standard Specification for Pipe, Steel, Electric-Fusion (Arc)-Welded (Sizes NPS 16 and Over)

ASTM A139, Standard Specification for Electric-Fusion (Arc)-Welded Steel Pipe (NPS 4 and Over)

ASTM A181, Standard 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, Standard Specification for Alloy-Steel and Stainless Steel Bolting for High Temperature or High Pressure 

Service and Other Special Purpose Applications

ASTM A194, Standard Specification for Carbon and Alloy Steel Nuts for Bolts for High-Pressure or High-Temperature 

Service, or Both

ASTM A213, Standard Specification for Seamless Ferritic and Austenitic Alloy-Steel Boiler, Superheater, and HeatExchanger Tubes

ASTM A240, Standard Specification for Chromium and Chromium-nickel Stainless Steel Plate, Sheet, and Strip for 

Pressure Vessels and for General Applications

ASTM A283, Standard Specification for Low and Intermediate Tensile Strength Carbon Steel Plates

ASTM A285, Standard Specification for Pressure Vessel Plates, Carbon Steel, Low- and Intermediate-Tensile Strength

ASTM A307, Standard Specification for Carbon Steel Bolts, Studs, and Threaded Rod 60,000 psi Tensile Strength

ASTM A312, Standard Specification for Seamless, Welded, and Heavily Cold Worked Austenitic Stainless Steel Pipes

ASTM A320, Standard Specification for Alloy-Steel and Stainless Steel Bolting for Low-Temperature Service

ASTM A333, Standard 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, Pennsylvania 19428, www.astm.org.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 2-3

ASTM A334, Standard Specification for Seamless and Welded Carbon and Alloy-Steel Tubes for Low-Temperature 

Service

ASTM A350, Standard Specification for Carbon and Low-alloy Steel Forgings, Requiring Notch Toughness Testing for 

Piping Components

ASTM A351, Standard Specification for Castings, Austenitic, Austenitic, for Pressure-Containing Parts 

ASTM A353, Standard Specification for Pressure Vessel Plates, Alloy Steel, Double-Normalized and Tempered 9% 

Nickel

ASTM A358, Standard Specification for Electric-Fusion-Welded Austenitic Chromium-Nickel Stainless Steel Pipe for 

High-Temperature Service and General Applications

ASTM A370, Standard Test Methods and Definitions for Mechanical Testing of Steel Products

ASTM A403, Standard Specification for Wrought Austenitic Stainless Steel Piping Fittings 

ASTM A479, Standard Specification for Stainless Steel Bars and Shapes for Use in Boilers and Other Pressure 

Vessels

ASTM A480, Standard Specification for General Requirements for Flat-Rolled Stainless and Heat-Resisting Steel 

Plate, Sheet, and Strip

ASTM A516, Standard Specification for Pressure Vessel Plates, Carbon Steel, for Moderate- and Lower-Temperature 

Service

ASTM A522, Standard Specification for Forged or Rolled 8 and 9% Nickel Alloy Steel Flanges, Fittings, Valves, and 

Parts for Low-Temperature Service

ASTM A524, Standard Specification for Seamless Carbon Steel Pipe for Atmospheric and Lower Temperatures

ASTM A537, Standard Specification for Pressure Vessel Plates, Heat-Treated, Carbon-Manganese-Silicon Steel

ASTM A553, Standard Specification for Pressure Vessel Plates, Alloy Steel, Quenched and Tempered 8 and 9% 

Nickel

ASTM A573, Standard Specification for Structural Carbon Steel Plates of Improved Toughness

ASTM A633, Standard Specification for Normalized High-Strength Low-Alloy Structural Steel Plates

ASTM A645, Standard Specification for Pressure Vessel Plates, 5% and 5 1/2% Nickel Alloy Steel, Specially Heat 

Treated

ASTM A662, Standard Specification for Pressure Vessel Plates, Carbon-Manganese-Silicon Steel, for Moderate and 

Lower Temperature Service

ASTM A671, Standard Specification for Electric-Fusion-Welded Steel Pipe for Atmospheric and Lower Temperatures

ASTM A673, Standard Specification for Sampling Procedure for Impact Testing of Structural Steel

ASTM A678, Standard Specification for Quenched-and-Tempered Carbon and High-Strength Low-Alloy Structural 

Steel Plates
2-4 API STANDARD 620

ASTM A737, Standard Specification for Pressure Vessel Plates, High-Strength, Low-Alloy Steel

ASTM A841, Standard Specification for Steel Plates for Pressure 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 Aluminum-Alloy Extruded Bars, Rods, Wire, Profiles, and

Tubes

ASTM B241, Standard Specification for Aluminum and Aluminum-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-Alloy 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 Welding Electrodes for Shielded Metal Arc Welding

(ANSI/AWS A5.11)

AWS A5.14, Specification for Nickel and Nickel-Alloy Bare Welding 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 tank is in operation. 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 design formulas given or provided for in this standard for the 

specific kind of material, character 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 capacity of a tank (excluding 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 total volumetric liquid capacity of a tank (excluding 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 atmosphere. Flat bottoms of cylindrical tanks are covered by the 

rules of 5.9.4. As such, the tank walls include the sidewalls (or shell), 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, trusses, structural columns, or other framing;

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

Welding with equipment which performs the welding operation 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—metal, weld metal, 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 STANDARD 620

3.4.3

base metal

The metal or alloy to be welded or cut.

3.4.4

depth of fusion

The distance that fusion extends into the base metal from the surface melted during welding.

3.4.5

filler metal

Metal 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 metal 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 minimum depth a groove weld extends from its face into a joint, exclusive of reinforcement.

3.4.9

lap joint

A joint between two overlapping members. An overlap is the protrusion of weld metal beyond the bond at the toe of 

the weld.

3.4.10

machine welding

Welding with equipment that performs 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

Welding wherein the entire welding operation is performed and controlled by hand.

3.4.12

oxygen cutting

A group of cutting processes wherein the severing of metals is effected by means 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 pockets or voids in metal.

3.4.14

reinforcement of weld

Weld metal on the face of a groove weld in excess of the metal necessary for the specified weld size.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 3-3

3.4.15

semiautomatic arc welding

Arc welding with equipment that controls only the filler metal feed. The advance of the welding is manually controlled.

3.4.16

slag inclusion

Nonmetallic solid material entrapped in weld metal or between weld metal and base metal.

3.4.17

undercut

A groove melted into the base metal 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 melted during welding.

3.4.19

welded joint

A union of two or more members produced by 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 primary responsibility for constructing the tank.

3.5.2

Purchaser

The owner or the owner’s designated agent, such as an engineering contractor.

3.5.3

Examiner

A person who performs nondestructive examinations specified by this standard and is qualified and certified as 

meeting the requirements of ASNT SNT-TC-1A and/or other requirements as determined by the Manufacturer or 

Purchaser for the method employed.

3.5.4

Inspector

The Inspector is the representative of the Purchaser who 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 API 620 tanks shall comply with the specifications in this section (see Annexes 

Q, R, and S for specific material requirements). Material produced to specifications other than those listed in this 

section may be used if the material is certified to meet all the requirements of a material specification 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 according 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 caps, manhole 

frames, and covers, shall be made from materials provided for 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 indicated and is suitable for 

service at the rating indicated. The intent 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 items 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 reports or 

certificates are not customarily furnished) may be used if, in the opinion of the inspector, 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 membrane stress or are otherwise important to the structural 

integrity of a tank, including bottom plates 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 coldest days of record for the locality where the tank is to be 

installed.

4.2.1.2 In all cases, the Purchaser shall specify the design metal temperature, and 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 otherwise provided in the last sentence of this paragraph and in 

4.2.2, the design metal temperature for materials in contact 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 meteorological data shall be used. Where no such data are available, 

the Purchaser shall estimate the temperature from the most reliable information at hand. Where special means, such 

as covering the outside of the tank with insulation or heating 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 ambient temperature, the design 
4-2 API STANDARD 620

metal temperature may be set at a higher level that can be justified by computations or by actual temperature data on 

comparable existing tanks. 

4.2.1.3 Unless exempted per 4.2.2, notch toughness of specially designed plate flanges and cover plates shall be 

evaluated using governing thickness in Table 4-1. (See 4.3.6.3 for definition of governing thickness.)

4.2.2 Low-stress Design

The following design criteria, relative to the use of Table 4-1, apply when the actual stress under design conditions 

does not exceed one-third of the allowable tensile stress.

a) Consideration of the design metal temperature is not required in selecting 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 

components that are exposed to the vapor from the liquid or vapor being stored and are not designed to contain 

the contents of an inner tank.

c) Excluding bottom plates welded to the cylindrical sidewall of flat-bottom tanks, the plates of a non-refrigerated flatbottom tank, counterbalanced in accordance 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 plates, 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 specifications 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 maximum nominal thickness of 3/4 in.).

f) A516, with the following API modifications as required: Mod 1 requires the carbon content to be restricted to a 

maximum of 0.20 % by ladle analysis; a maximum manganese content of 1.50 % shall be permitted. Mod 2 

requires the minimum manganese content 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 manganese content shall be 0.80 % by ladle analysis. The 

maximum manganese content may be increased to 1.60 % by ladle analysis if maximum carbon content is 0.20 % 

by ladle analysis.
Santa Fe

El Paso

Amarillo

San Antonio

Houston

Dallas

OklahomaCity Little Rock

Fort Smith

Shreveport

Jackson

New Orleans

Birmingham

Montgomery

Mobile

Memphis

Nashville

Knoxville

Chattanooga

Springfield Joplin Wichita

Topeka

Keokuk

KansasCity St. Louis

Springfield

Fort

Wayne

Columbus

Louisville

Cleveland Des Moines

North Platte

Minneapolis

Sioux Falls

Sioux City

Aberdeen

Pierre

Moline

Churchill

The Pas

Winnepeg

Kapuskasing

International

Falls

Port Aux Basques

St.

Johns

NEWFOUNDLAND

Buchans

Fargo

Duluth

Edmonton

Medicine Hat

Havre

Billings

Sheridan

Prince

Albert

Saskatoon

Regina

Williston

Helena

Clayoquot

Kamloops

Seattle

Boise

Spokane

Baker

Portland

Penticion

Nelson

Cranbrook

Victoria

Vancouver

Chatham

Bangor Halifax

St. John

Huntsville Ottawa

Pittsburgh

Buffalo

Albany

Raleigh

Norfolk

Columbia

Charleston

Savannah

Jacksonville

Tampa

Miami

Pueblo

Denver

Pocatello

San Diego

Los Angeles

San Francisco

Eureka

Compiled from U.S. Weather Bureau andMeteorological Div. Dept. of Transport ofDominion of Canada Records up to 1952.

Chicago

Detroit

Arvida

35

35

30

25 20

25

20

15

10

5

0

10

15

10

4045

35

25

20

5

–5

0

–15

–10

–25

–20

–10

–5

Ð40

–35

–30

Haileybury

–20

–30

–30 –25

–20

–15

–10

–5

0

5

15

–20

–15

Calgary

30

15

10

5

0

–5

–10

–15

–25

–30

–20

–40

–35

–45

–5 –20 –25

–30

–45–40–35

–55

–50

–45

–20

Richmond Charleston

Cincinnati Washington

Baltimore

New York

Philadelphia

Hartford

Boston

Concord

Charlottestown

Sidney

Quebec

Saranac Lake

Lennoxville

Montpelier

Sault St. Marie

Southhampton

Toronto

London

Ludington Green Bay

Marquette

Port Arthur

Sioux Lookout

Milwaukee

Cheyenne

Lander Red Bluff

Sacramento

Fresno

Reno

Prince

George

Salt Lake

City

Tuscon

Phoenix

Harrisburg

St. Catherine

Portland

Gander

Wilmington

Ashville

Atlanta

Indianapolis

Wythville

Prince

Ruppert

Montreal

Amherst

Providence

–15

–10

Las Vegas

Bismark

Ð45

–25

–25

Grand Canyon

Figure 4-1—Isothermal Lines Showing 1-day Mean Ambient Temperature

DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 4-3
4-4 API STANDARD 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 3
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 approved 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 accordance with Table 4-2. Plates shall have a tensile

strength not more than 20 ksi above the minimum specified for the grade. Fully killed steel made to a fine grain

practice must be specified when required. 

4.2.3.4 ISO Specification

The following ISO specification is approved for plates: 630-1 (Grades E275 and E355 in Qualities C and D only).

Elements added for grain refining or strengthening shall 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 11/2 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 STANDARD 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 the exception of those 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 established 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-process (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 requirements of plates for various grades for given

thickness range and design metal temperature. Any material 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 long dimension of the specimen shall be parallel to the

direction of the expected maximum stress. When the coincident stresses are approximately equal, the specimens

shall be taken transverse to the final direction of the plate rolling. The minimum 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, subject to the approval of the Purchaser. These elements shall be reported when requested by

the Purchaser.

NOTE 2 The material shall conform to these requirements on product analysis subject 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 combined 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. 
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 4-7

Table 4-3—Minimum Charpy V-notch Requirements for Plate Specimens

Group Specification 

Number Grade Range in Thickness (in.)

Impact Valuea

(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 11/2 13 9

EN 10025 S275 J0 3/16 to 11/2 13 9

II (fully killed) A 573 58c 3/16 to 11/2 15 10

A 131 CS 3/16 to 11/2 15 10

A 516 55 and 60 3/16 to 2 15 10

A 516 55 and 60d 3/16 to 1/2 15 10

ISO 630 (1995) E275 Quality D 3/16 to 11/2 15 10

EN 10025 S275 J2 3/16 to 11/2 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 1b 3/16 to 2 15 10

A 516 65 and 70 Mod 2b 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 11/2 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 specimens. For sub-size specimen acceptance criteria, see ASTM A20. An impact test temperature

lower than the design metal temperature may be used by the Manufacturer, but the impact value at the test temperature must comply with

Table 4-3. When plate is selected, consideration must be given to the possible degradation of the impact properties of the plate 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 normalizing, for thicknesses of 3/16 in. to 11/2 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 STANDARD 620

4.2.5.2 All other impact requirements of ASTM A20, Supplementary Requirement S5, shall apply for all materials 

listed in Table 4-3, including specifications that do not refer to ASTM A20.

4.2.5.3 For thickness exceeding the range in Table 4-3, impact test requirements shall be mutually agreed by the 

Manufacturer 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 shall 

conform to applicable requirements of 4.3.2 to 4.3.6 inclusive.

4.3.2 Pipe14

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 helical (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 shall satisfy the requirements of 4.2.3, 4.2.4, and 4.2.5 that are applicable to 

that plate specification.

c) Impact tests for qualifying the welding procedure for the pipe longitudinal welds shall be performed in accordance 

with 4.7.1.

4.3.3 Built-up Fittings

Built-up fittings, such as ells, tees, and return bends, may be fabricated by fusion welding when they are designed 

according to the applicable paragraphs in this standard.

14 For design metal temperatures below –20 °F, the materials shall conform to Tables R-1 and/or R-3.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 4-9

4.3.4 Flanges

4.3.4.1 Hub, slip-on welding neck and long welding neck flanges shall conform to the material requirements of 

ASME B16.5 for forges carbon steel flanges. Plate material used for nozzle flanges shall 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, Series B, may be used, 

subject to the Purchaser’s approval. Particular attention should be given to ensuring that mating flanges of 

appurtenances are compatible.

4.3.5 Castings and Forgings

Large castings and forgings (see Footnote 12 for both materials) not covered in 4.1.3 shall be of welding grade if 

welding is to be done on them, and they shall conform to one of the following 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, the toughness requirements 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 steel flanges used at 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 temperatures, 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 groups 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) Group 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 thicknesses, including corrosion 

allowance, at minimum design metal temperatures no lower 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 welds, it is the thinner 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 performed 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 minimum design metal temperature. Except for the plate 
Figure 4-2—Minimum Permissible Design Metal Temperature for Pipe, Flanges, and Forgings 

without Impact Testing

–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 1.00

Design metal temperature (oF)

Group II

Group I

Thickness, including corrosion allowance (inches)

1.25 1.50

4-10 API STANDARD 620

specified in 4.2.3, the material specified 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 bolts15 may be used if they conform to the following, or to better16, 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.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 shall be made only by the open-hearth, electric-furnace, or basic-oxygen process and

shall conform to one of the 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 Determination of Pipe, Flanges, and Forgings

Tc

tn

Tf

Tc Tf tn

t s

ts

Tc

tn

ts

Tc

tn

Tf

ts

Welding–neck Flange Long Welding–neck Flange

Slip–on Flange Ring–type Flange

CL CL

CL CL

Tf

NOTE 1 Shell reinforcing plate is not included inthe illustration above.

NOTE 2 ts = shell thickness; tn = nozzle neck thickness; Tf

 = flange thickness; Tc = bolted cover thickness.

NOTE 3 The governing thickness for each component shall be as follows:

Components Governing Thickness (whichever is less)

 Nozzle neck tn or ts

 Slip-on flange tn or Tf

 Ring-type flange tn or Tf

 Welding-neck flange tn or Tf

 Long welding-neck flange tn or ts

 Bolted cover 1/4 Tc

4-12 API STANDARD 620
5-1

SECTION 5—DESIGN

5.1 General

5.1.1 Scope of Rules

The rules presented in this standard are intended to establish approved engineering 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 sufficient 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 more independent pressure chambers and have a roof, bottom, or other elements in 

common, each pressure part shall be designed for the most severe combination of pressure or vacuum that can be 

experienced under the specified operating conditions.

5.1.3 Avoidance of Pockets

Tank walls shall be shaped to avoid any pockets on the inside where gases may become trapped 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 design level upon which the nominal capacity is based shall be 

not less than 2 % of the total 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 around 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 stored 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, subjected 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 

understood 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 vacuum) when the inflow of air (or another gas or vapor) 

through the vacuum relief valves is at the tank design maximum in-breathing flow rate. 
5-2 API STANDARD 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 provided 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 space and the relief 

valve’s set pressure. This margin allows 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 pressure accumulation in the tank 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 be the weight per ft3 of the specified liquid contents at 60 °F. When 

the minimum weight is less than 48 lb/ft3, the provisions of 5.5.7 shall be followed. This minimum weight does not 

apply to tanks used for gas storage only, or used for refrigerated liquid storage as discussed in Annexes Q and R.

5.4 Loads

5.4.1 Individual Loads

a) dead load (DL): the weight of the tank or tank component, including any insulation, lining, or corrosion allowance 

unless otherwise noted. 

b) hydrostatic and pneumatic tests (Ht ): the load due to conducting the tests specified in 7.18.

c) loads from connected piping (Lp).

d) loads from platforms and stairways (Ls) (see Annex E).

e) minimum roof live load (Lr): 20 lb/ft2 on the horizontal projected area of the roof.

f) pressure (Pg): the maximum positive gauge pressure given in 5.3.1. 

g) pressure (Pv): the maximum partial vacuum given in 5.3.1. The maximum partial vacuum shall be at least 1 in. wc.

h) seismic (E): seismic loads given in Annex L.

i) snow (S): The ground snow load (lb/ft2) 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 design uniform roof snow load shall be determined from the ground snow load in 

accordance with ASCE 7 or a national standard. 

j) stored liquid (Pl ): gauge pressure (lb/in.2) resulting from the liquid head of the liquid with the density given in 

5.3.3. All liquid levels from empty to the maximum liquid level shall be considered.

k) wind (W ): The 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 specified 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 the 
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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/ft2)

Maximum Pressure 

(lbf/ft2)

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 compression 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 risk category (importance factors) built in, whereas API 620 

uses the working stress. The 0.78 factor applied 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 

results in a more severe condition, that load shall not be included in the combination. 

a) DL + Pg + Pl

b) DL + WL + 0.7Pg

c) DL + WL + 0.4Pv

d) DL + Pv + 0.4(Lr or S)

e) DL + 0.4Pv + (Lr or S)

f) DL + 0.7Pg + Pl + E + 0.1S

g) DL + Ht

h) DL + Ls

i) DL + Lp + Pg + Pl

5.5 Maximum Allowable Stress for Walls17

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 the prescribed test loadings may result in some localized reshaping. This is 

permissible, since localized reshaping is expected as part of a legitimate fabrication operation, if the reshaping is not 

17 See Biaxial Stress Criteria for Large Low-Pressure Tanks, written by J. J. Dvorak and 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 STANDARD 620

so severe that upon release of the test pressure, plastic straining occurs in the opposite direction. This would tend to 

develop continuing plastic straining in subsequent normal operation.

5.5.2 Nomenclature

5.5.2.1 Variables relating to stresses common to the requirements 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;

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

Scs is the maximum allowable longitudinal compressive stress, in lbf/in.2, for a cylindrical wall acted upon by 

an axial load with neither a tensile nor a compressive force acting concurrently in a circumferential 

direction (determined in accordance with 5.5.4.2 for the thickness-to-radius ratio involved);

sta is the allowable tensile stress, in lbf/in.2; sta lower than Sts because of the presence of a coexistent 

compressive stress perpendicular to it;

sca is the allowable compressive stress, in lbf/in.2; sca is lower than Scs because of the presence of a 

coexistent tensile or compressive stress perpendicular to it;

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

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

st is the general variable for indicating 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;

sc 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, st, to the maximum allowable stress for simple tension, Sts;

M is the ratio of the compressive stress sc, to the maximum allowable compressive stress, Scs (see Figure 

F-1).

5.5.2.2 The term tank wall is defined in 3.3. Unless otherwise stipulated in this standard, the stresses in nozzle and 

manway necks, reinforcing pads, flanges, and cover plates shall not exceed the values 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 tank, as determined for any 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 latitudinal unit forces, T1 and T2, are tensile or if one force is tensile and the other 

is zero, the computed tensile stress, Stc, shall not exceed the applicable 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

Scs

= 1,800,000 (t - 

c)/R

Scs = 10,150 - 277,400 (t - c)/R Scs = 15,000 psi

A

B C

N=0

Do not extrapolate

beyond this line

Coexistent 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 Scs 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 = R1

; if compressive stress is meridional, use R = R2.

Values of tensile stress factor N = Stc /Sts

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

N = 0

Scs = MScs -15,000 M

(See Notes 1 and 2 )

DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 5-5

5.5.3.3 If the meridional force, T1, is tensile and the coexistent latitudinal unit force, T2, is compressive or if T2 is 

tensile and T1 is compressive, the computed tensile stress, stc, shall not exceed a value of the allowable tensile 

stress, sta, 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 value of compressive stress (sc = scc) 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 perpendicular to it, the designer has the option of permitting a tensile stress of the magnitude specified in 

5.5.3.2 instead of complying strictly with the provisions of this paragraph, (see F.1 for examples illustrating the 

determination of allowable tensile stress values, sta, in accordance with this paragraph). In no event shall the value of 

sta exceed the product of the applicable joint efficiency for tension 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 compressive stresses in the 

outside walls of a tank, 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 compressive (as in a cylinder acted upon by external pressure). 

However, values of Scs computed as in 5.5.4.2, with R equal R1 when the compressive unit force is latitudinal or to R2

when the compressive unit force is meridional, in some degree form 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 STANDARD 620

Table 5-1—Maximum Allowable Stress Values for Simple Tension

Specified Minimum Maximum Allowable 

Tensile Stress for Tension,

Sts

(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 18,800
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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,

Sts

(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 STANDARD 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 limitations of specifications required by 4.2. through 4.6 shall be complied with.

NOTE 2 Except for those cases where additional factors or limitations are applied as indicated by references to Notes 4, 6, 10, and 12, the

allowable tensile stress values given in this table for materials other than bolting steel are the lesser of (a) 30% of the specified 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 allowable 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 applicable joint efficiency 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 stress values for welds in

tension shall be multiplied by the 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 21/2 in. thickness.

NOTE 8 Less than or equal to 11/2 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 pipe 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,

Sts

(lbf/in.2, See Notes 2 and 3)

Specification

(See Note 1) Grade Notes Tensile Strength

(lbf/in.2)

Yield Point

(lbf/in.2)
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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, 

scc, shall not exceed a value, Scs, established for the applicable thickness-to-radius ratio as follows:

for values of (t – c)/R less than 0.00667,

Scs = 1,800,000 [(t – c)/R]

for values of (t – c)/R between 0.00667 and 0.0175,

Scs = 10,150 + 277,400 [(t – c)/R]

for values of (t – c)/R greater than 0.0175,

Scs = 15,000

5.5.4.3 If both the meridional and latitudinal unit forces, T1 and T2, are compressive and of equal magnitude, the 

computed compressive stress, scc, shall not exceed a value, sca, established for the applicable thickness-to-radius 

ratio as follows: 

for values of (t – c)/R less than 0.00667,

sca = 1,000,000 [(t – c)/R]

for values of (t – c)/R between 0.00667 and 0.0175,

sca = 5650 + 154,200 [(t – c)/R]

for values of (t – c)/R greater than 0.0175,

sca = 8340

5.5.4.4 If both the meridional and latitudinal unit forces, T1 and T2, are compressive but of unequal magnitude, both 

the larger and smaller computed compressive stresses shall be limited to values that satisfy the following 

requirements: 

(S1 + 0.8Ss)/Scs ≤ 1.0 

1.8Ss/Scs ≤ 1.0

where

S1 is the larger stress, in lbf/in.2;

Ss is the small stress, in lbf/in.2;

Scs 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 for the smaller unit force in the second equation.

NOTE In the previous expressions, if the unit force involved is latitudinal, R shall be equal to R1; if the force is meridional, R shall 

be equal to R2.
Table 5-2—Maximum Allowable Efficiencies for Arc-welded Joints

Type of Joint Limitations

Basic 

Joint 

Efficiency

(%)

Radiographed 

(See Note 1)

Maximum 

Joint 

Efficiency 

(%; see 

Note 2)

Butt joints, attained by double-welding or other 

means approved by the Purchaser, 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 above liquid level.

Roofs above liquid level.

85

70

Spot

Full (see 

Note 3)

—

Spot

Full (see Note 3)

85

100

70

85

100

Single-welded butt joint with backing strip or 

equivalent other than those included above. Longitudinal or meridional circumference or 

latitudinal joints between plates not more than 

11/4 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) circumferential 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 

between plates not more than 5/8 in. thick.

70

65

—

—

70

65

Single full-fillet lap joint (see Note 4). Longitudinal or meridional joints and 

circumferential or latitudinal 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 joints for head-to-nozzle 

joints For attachment 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 

reinforcements.

(Included in the 

strength factors in 

5.16.8.3)

Plug welds (see 5.24.5) Attachment welding for nozzle reinforcements 

(see Note 6).

80 — 80

5-10 API STANDARD 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-welded joints between plates with surfaces of double curvature that 

have a compressive stress across the joint from a negative value of Pg or other external loading may be taken as unity; such compressive stress 

shall not exceed 700 lbf/in.2. For all other lap-welded joints, the joint efficiency factor must be applied to the allowable compressive stress, Sca. The 

efficiency for full-penetration butt-welded joints, which are in compression across the entire thickness of the connected plates, may be taken as 

unity.

NOTE 3 All main butt-welded joints (see 5.26.4.2) shall be completely radiographed or ultrasonically examined as specified in 5.26 and nozzle 

and reinforcement attachment welding shall be examined by the magnetic-particle method as specified in 7.15.2.

NOTE 4 Thickness limitations 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 considered subject to the same requirements and limitations 

as are longitudinal or meridional joints when such a circumferential or latitudinal joint is located (a) in a spherical, tori spherical or ellipsoidal shape 

or in any other surface of double curvature, (b) at the junction between a conical or dished roof (or bottom) and cylindrical sidewalls, as considered 

in 5.12.3 or (c) at a similar juncture at either end of a transition section or reducer as shown in Figure 5-9.

NOTE 6 The efficiency factors shown for fillet welds and plug welds are not to be applied to the allowable shearing stress values shown in Table 

5-3 for structural welds.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 5-11

5.5.4.5 If the meridional unit force, T1, is compressive and the coexistent unit force T2, is tensile, except as

otherwise provided in 5.5.4.6, or if T2 is compressive and T1 is tensile the computed compressive stress, scc, shall not

exceed a value of the allowable compressive stress, sca, 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 sc that corresponds to

that point. The value of sc will be the limiting value of sca 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 longitudinal compressive stress Scs or sca, 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 allowable 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 otherwise 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 force 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 there is the need for intermediate wind

girders using the rules of 5.10.6. If the 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, Sts, given in Table 5-1 for the

kind of material involved. Such maximum 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 combined with wind or earthquake loadings shall not exceed

133 % of the stress permitted for the design loading condition; except as allowed in Annex L, this stress shall not

exceed 80 % of the specified minimum yield strength for carbon steel. For stainless steel and aluminum, see Q.3.3.6.

5.5.7 Allowable Stress for Tests

Stresses for hydrostatic and hydrostatic-pneumatic tests shall not exceed 130 % of the allowable design tensile stress

or 125 % of the allowable design compressive stress except as modified in 5.12.4.3. Calculations are required for

product density less than 48 lb/ft3 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 loadings listed in 5.4 or any concurrent combination of such

loadings expected to be encountered in the specified operation, shall not exceed the applicable allowable stresses

given in Table 5-3.
5-12 API STANDARD 620

Table 5-3—Maximum Allowable Stress Values for Structural Members

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)

Tension

Rolled steel, on net section

Butt welds on smaller cross-sectional area, in or at 

edge of 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 columns, structural bracing, 

and structural secondary 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 of weld (crushing)

Plate-girder stiffeners, on gross section

18,000 / [1 + (l

2/18,000r2)]

but not to exceed 15,000

18,000Y / [1 + (l

218,000r2)]

but not to exceed 15,000Y

18,000

18,000

18,000 / [1 + (l

2/18,000r2)]

but not to exceed 15,000

18,000Y / [1 + (l

218,000r2)]

but not to exceed 15,000Y

15,000

15,000

Bending (see Note 2)

Tension on extreme fibres of rolled sections, plate 

girders, and built-up members

Compression on 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

Stress 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 not 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 not 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)

27,000

Per Table 5-1

Same as tens. val. from Table 5-1

[(600) (tension value from Table 5-

1)/[(ld/bt)]

20,000
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 

than 60, or where web 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 section 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 (see 5.16.8.3, Item b)

Plug welds or slot welds, on effective faying-surface 

area 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 + (h2/7200t

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 + (h2/7200t

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 

connected 

Load distributed uniformly, approximately, across 

thickness of the member connected

Unfinished bolts

Load applied to bolt at only one side of the member 

connected

Load distributed uniformly, approximately, across 

thickness of the member connected

24,400

30,000

16,000

20,000

1.33 × tension value from Table 5-1

1.67 × tension value from Table 5-1

0.09 × tension value from Table 5-1

1.1 × tension value from Table 5-1

NOTE 1 The variables in the compressive stress equations are defined as follows: 

l = unbraced length of the column, in in.; r = corresponding least radius of gyration of the column, in inches; 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 are defined as follows: 

l = unsupported length of the member; for a cantilever beam not fully stayed at its outer 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 its compression flange, in inches; t = 

thickness of its compression flange, in inches.

NOTE 3 The variables in the shearing stress equations are defined as follows: 

h = clear distance between 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 STANDARD 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 rods shall not exceed the following 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, 

provided that the member is not ordinarily subject 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/200r).

5.6.4 The gross and net sections of structural members shall be determined as described in 5.6.4.1 through 5.6.4.5.

5.6.4.1 The gross section of a member at any point shall be determined by summing the products of the thickness 

and the gross width 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

4g

-----

where

s is the longitudinal spacing (pitch), in inches, of any two 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 only that part of the thickness of the member that has 

been developed by the welds or other attachments beyond the section considered.

5.6.4.5 In pin-connected tension members other than forged eyebars, 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 across 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 proportioned for unit stresses 25 % greater than those specified in 

Table 5-3 if the required section is not less than that required for all other applicable loads combined on the basis of 
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 are established that recognize possible stressing during initial tightening. 

For flange bolts, these design allowable stresses also recognize additional stressing during overload and testing. 

Where bolts are used as anchorage to resist the shell 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 protecting 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 attached to any element of the tank wall, including 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 direction of the forces that must 

be exerted by the walls of a tank, at the level selected for analysis, to hold in static equilibrium 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 cutting 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 tank for the purpose of 

determining the magnitude and character of the 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 conditions of gas pressure and liquid head for that level. The 

thicknesses required in the main walls of the tank shall 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 for the desired capacity of 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 internal ties, diaphragms, trusses, or 

other members subject to pressure-imposed loads, studies must be made to establish 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 uniformly 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 STANDARD 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 exception 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-welded bottom plates shall be furnished and installed to 

lap over the adjacent plate a minimum of 1 in. Three-plate joints in tank bottoms 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 the outer ends of the joints fitted and lap-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-welded from both sides. Welds shall be full fusion 

through the thickness of the bottom plate. The butt-weld 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 plate 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 each weld shall not be greater than 1/2 in., not less than the nominal thickness of the 

thinner of the two plates joined (that is, the sidewall plate or the bottom plate 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

3/8
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 5-17

5.9.5.2 The plates of the first sidewall course shall be attached to the bottom plates under the sidewall by a fillet 

weld inside and outside as required by 5.9.5.1, but when the sidewall material has a specified 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 the sidewall with a nominal thickness greater than 1/2 in., the attachment welds 

shall be sized so that either the legs of the fillet welds or the groove depth plus the leg of the fillet for a combined weld 

are of a size equivalent to the thickness of the bottom plate under the sidewall (see Figure 5-3).

Figure 5-3—Detail of Double Fillet-groove Weld for Bottom Plates with a Nominal Thickness 

Greater than 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 direction of the meridional tangent, such as the points that 

occur at the juncture between a conical or dished roof (or bottom) and a cylindrical sidewall 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 formulas of 5.10 are defined as follows.

P is the total pressure, in lbf/in.2 gauge, acting at a given level of the tank under a particular condition of 

loading,

= Pl + Pg;

Pl is the gauge pressure resulting from the liquid head at the level under consideration in the tank (see 5.4.1b);

Pg see 5.4.1g, Pg is positive except in computations used to investigate the ability of a tank to withstand a 

partial vacuum; in such computations, its value is negative;
5-18 API STANDARD 620

T1 is the meridional unit force, in lbf/in. of latitudinal arc, in the wall of the tank at the level under consideration. 

T1 is positive when in tension;

T2 is the latitudinal unit force, in lbf/in. of meridional arc, in the wall of the tank at the level under consideration. 

T2 is positive when in tension (in cylindrical sidewalls, the latitudinal unit forces are circumferential unit 

forces);

R1 is the radius of curvature of the tank wall, in inches, in a meridional plane, at the level under consideration. 

R1 is to be considered negative when it is on the side of the tank wall opposite from R2 except as provided in 

5.10.2.6;

R2 is the length, in inches, of the normal to the tank wall at the level under consideration, measured from the 

wall of the tank to its axis of revolution. R2 is always positive except as provided in 5.10.2.6;

W is the total weight, in lb, of that portion of the tank 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 would include the weight of all metal, 

gas, and liquid in the portion of the tank treated as described; however, the gas weight 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 the 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

Pg

T1

W1 W1

T1

T1

T1

Rs

P

W

P

W

W

Rs

Axis of revolution

Supports

Free

body

Panel a

Axis of revolution

Free

body

Level under

consideration

W1

R2 R1

T1

W1

F/n

F/n

Panel b

Supports

Axis of revolution

n = number of

ties cut by this plane

Ties

Free

body

Level under

consideration

Panel c
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 or supports 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 the opposite sign when it acts in the opposite direction.

At is the cross-sectional area, in inches2, of the interior of the tank at the level under consideration;

t is the thickness, in inches, of the sidewalls, roof, or bottom of the tank, including corrosion allowance;

c is the corrosion allowance, in inches;

E is the efficiency, expressed as a decimal, of the weakest joint across which the stress under 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) the lap-welded joints in compression specified in Note 3 of Table 5-2, E

may be taken as unity];

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

sta is the allowable tensile stress, in lbf/in.2, established as prescribed in 5.5.3.3;

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

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

scc 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 analysis as specified in 5.9 (see typical diagrams in 

Figure 5-4) and for each condition of gas and liquid loading that must be investigated at that level, the magnitude of 

the meridional and latitudinal unit 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

T1

R2

2

----- P W F +

At

+ --------------     = (1)

T2 R2 P

T1

R1

– -----     = (2)

T2 R2 P 1 R2

2R1

– --------     R2

2R1

-------- W F +

At

--------------     = – (3)

NOTE Footnote 16 is also applicable to Equation (1), Equation (2), and Equation (3).

18 Equation (2), Equation (5), and Equation (9) have been derived from a summation of the normal-to-surface components of the T1

and T2 forces acting on a unit area of the tank wall subjected only to pressure P. To be technically correct, 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 significant 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 the 

calculated T2 force and resultant thickness. 

Equation (3) and Equation (6) are correct only when P is the free-body pressure without the normal-to-surface components of 

other loads.

The example in F.3 calculates the required roof thicknesses under a small vacuum by considering the metal, insulation and snow 

loads in Equation (1) through Equation (5). The designer should note that if these loads had been omitted, the calculated 

thicknesses would have been much less than the correct values.

In Equation (1), Equation (4), Equation (8), and Equation (10), W is intended to include loads of insignificant value, such as metal 

weight. At points away from the vertical centerline of the roof, the value of T2 is required for the thickness calculations of Equation 

(18), Equation (20), and Equation (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 T2.
5-20 API STANDARD 620

5.10.2.2 Positive values of T1 and T2 indicate tensile forces; negative values indicate compressive forces.

5.10.2.3 Free-body analyses shall be made at the level of each horizontal joint in the sidewalls, roof, and bottom of 

the tank and at any intermediate levels at which the center of curvature changes significantly. The maximum total 

pressure (liquid head plus gas pressure) that can exist at a given level will not necessarily be the governing condition 

for that level. Sufficient analyses shall be made at each level to determine the combination of liquid head and gas 

pressure (or partial vacuum) that, in conjunction with the allowable tensile and compressive 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 sidewalls of double curvature.

5.10.2.4 Mathematically exact instead of approximate values of R1 and R2 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:

R1

b2

a4 ---- a4b2 ---- 1 a2b2 – ----     + x

2 1.5 b2 R2 ( )3

a4 = = -----------------

R2

a4

b2 ---- 1 a2b2 – ----     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 of 

revolution and to any free-body in the tank that is isolated by a horizontal 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 for the respective shapes indicated in Items a through c. 

a) For a spherical tank or a spherical segment of a tank, R1 = R2 = Rs (the spherical radius of the tank or segment), 

and Equation (1) and Equation (2) become the following:

T1

Rs

2

---- P W F +

At

+ --------------     = (4)

T2 RsP T –= 1 (5)

T2

R1

2

----- P W F +

At

– --------------     = (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)/At is negligible compared with Pg, Equation (4) and 

Equation (5) reduce to the following: 

T1 = T2 = 1/2 PgRs (7)

b) For a conical roof or bottom,

R1 = infinity

R2 = R3/cos α
Table 5-5—Factors for Determining Values of R1 and R2 for Ellipsoidal Roofs and Bottoms (See 5.10.2.4)

x/a u = Rt/a v = R2/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

1.911

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

1.998

1.993

1.983

1.970

1.953

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

DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 5-21

where

R3 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 Equation (2) reduce to the following:

T1

R3

2 cosα ----------------     P W F +At

+ --------------     = (8)

T2

PR3

cosα = ----------- (9)

NOTE See Footnote 16 for information applicable to Equation (8) and Equation (9).

c) For cylindrical sidewalls of a vertical tank, R1 = infinity; R2 = Rc, the radius of the cylinder; and Equation (1) and 

Equation (2) become the following:

T1

Rc

2

-----     P W F +

At

+ --------------     = (10)

T2 = PRc (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; R1 = ua; R2 = va.
5-22 API STANDARD 620

Furthermore, if the cylinder is for gas pressure only and (W + F)/At is negligible compared with Pg, Equation (10) and 

Equation (11) reduce to the following:

T1 = 1/2 PgRc (12)

T2 = PgRc (13)

5.10.2.6 Where a horizontal plane that passes through a tank intersects 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 across and are pierced by the axis of revolution. (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 meridional 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 free-body cross section involved. This standard cannot provide formulas for all 

shapes of cross sections and conditions of loading that might be used at these locations; however, for a toroidal 

segment that rests directly on its foundation (see 5.11.1) and has a constant meridional radius, R1, such as is used in 

the outer portion of the bottom of the tanks shown in Figure 5-4, Panel c, applicable equations for the meridional and 

latitudinal unit forces in the walls of the segment are as follows:

T1 PgR1 1

R1

2R2

– --------     = (14)

T2

1

2 = --PgR1 (15)

The variables are defined in 5.10.1; however, in this case, R1 is always positive and R2 is negative when it is on the 

tank wall on the side opposite from R1.

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 determined 

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 sidewall, the portions of the tank near these 

points shall be designed in accordance with the provisions of 5.12.

5.10.3.2 If the units forces T1 and T2 are both positive, indicating tension, for the governing combination of gas 

pressure (or partial vacuum) and liquid head at a given level of the tank, the larger of the two shall be used for 

computing the thickness required at that level, as shown in the following equations:

t

T1

StsE --------- + or c t

T2

StsE = --------- += c (16)

In these equations, Sts and E have the applicable values prescribed in Tables 5-1 and 5-2, respectively.

5.10.3.3 If the unit force T1 is positive, indicating tension, and T2 is negative, indicating compression, for the 

governing combination of gas pressure (or partial vacuum) and liquid head at a given level of the tank or if T2 is 

positive and T1 is negative, the thickness of tank wall required for this condition shall be determined by assuming 

different thicknesses until one is found for which the simultaneous values of the computed tension stress, stc, and the 

computed compressive stress, scc, 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 the 
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 5-23

foregoing provisions, if the unit force acting in compression in the case described does not exceed 5 % of the 

coexistent tensile unit force acting perpendicular 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 complying strictly with the provisions of 

this paragraph. The value of the joint efficiency factor, E, will not enter into this determination unless the magnitude of 

the allowable tensile stress, sta, is governed by the product ESts as provided in 5.5.3.3.

5.10.3.4 If the unit forces T1 and T2 are both negative and of equal magnitude for the governing condition of loading 

at a given level of the tank, the thickness of tank wall required for this condition shall be computed using Equation 

(17):

t

T1

Sca

------ c

T2

Sca

------ +=+= c (17)

In this equation, sca has the appropriate value for the thickness-to-radius ratio involved, as prescribed in 5.5.4.3 and 

5.5.4.6. Lap-welded joints shall be subject to the limitations of 5.5.4.6 and Table 5-2 (including Note 3).

5.10.3.5 If the unit forces T1 and T2 are both negative but of unequal magnitude 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 procedure outlined in Items a through f, that show a proper correlation with the 

respective thickness-to-radius ratios involved in their computation (see Steps 2 and 4).

a) Step 1. The values of Equation (18) and Equation (19) shall be computed as follows:

t

T′ 0.8T′′ ( ) + 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 two 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 R1 and R2, respectively, if the larger unit force is 

latitudinal; conversely, R ́ and R ̋ shall be equal to R2 and R1, 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 Step 1, and the 

thickness-to-radius ratio, (t – c)/R, shall be checked for each thickness 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 Step 2 exceed 0.00667, the values of the following 

equations shall be computed:

t

T′ 0.8T′′ +

15 000 , ----------------------     = + c (20)

t

T′′

8340

----------- += c (21)

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

19See Figure F-3, a copy of a chart used to make graphical solutions.
5-24 API STANDARD 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 thickness 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 Equation (21). If both such thickness-to-radius ratios are greater than 0.0175, 

the larger of the two thicknesses computed in Step 3 will be the required thickness 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 Step 2 or Step 4 fall between 0.00667 and 

0.0175 and the thickness involved was computed 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′ 0.8T′′ += (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 shall 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 the thickness values computed in the 

previous steps (if the value has not been finally established earlier in the procedure). The values of scc shall be 

computed for both T1 and T2 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 these requirements, the necessary adjustments shall be made in the thickness 

to make the values of scc satisfy these requirements.

5.10.3.6 The procedure described in 5.10.3.5 is for the condition in which biaxial compression with unit forces of 

unequal magnitude is governing. In many cases, however, a tentative thickness 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 scc for both T1 and T2 and then check to see that these 

satisfy the requirements of 5.5.4.4, as specified in Step 6. (See F.3 for examples illustrating the application of 

5.10.3.5.)

5.10.4 Least Permissible Thicknesses

5.10.4.1 Tank Wall

The minimum thickness of the tank wall at any level shall be the greatest of the following.

a) A measure of 3/16 in. plus the corrosion allowance.

b) The calculated thickness in accordance with 5.10.3 plus the corrosion allowance.

c) The nominal thickness as shown in Table 5-6. The nominal thickness refers to the tank shell as constructed. The 

thicknesses specified are based on erection requirements. 

5.10.4.2 Nozzle Neck

See 5.19.2 for the minimum thickness 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

DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 Pg is a negative 

value equal to the partial vacuum for which the tank is to be designed, will ensure stability against collapse for tank 

surfaces of double curvature in which the meridional radius, R1, is equal to or less than R2 or does not exceed R2 by 

more than a very small amount. Data on the stability of sidewall surfaces of prolate spheroids are lacking; the 

formulas and procedures are not intended to be used for evaluating 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 sidewalls that are subject to partial 

internal 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 storing 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 level 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 sidewall, in ft, shall not exceed:

H1 6 100 ( )t

100t

D

----------     3

=

where

H1 is the vertical distance between the intermediate wind girder and the top of the sidewall or in the case of 

formed heads the vertical distance between the intermediate 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 ordered condition unless otherwise specified, in inches;

D is the nominal tank 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 imposes a dynamic pressure of 25.6 lbf/ft2. The velocity is increased by 

10 % for either a height above the ground or a gust factor. The pressure is thus increased to 31 lbf/ft2. An additional 5 lbf/ft2 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. H1 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 they are contained within the design wind pressure.
5-26 API STANDARD 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 for the critical uniform external pressure on thin-wall tubes free 

from end loading, subject to the total pressure in Item a.

d) When other factors are specified by the Purchaser which are greater than those in (a) through (c), the total load on the sidewall 

shall be modified accordingly and H1 shall be decreased by the ratio of 36 lbf/ft2 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 Institute, Section III—Refining, American Petroleum Institute, New York, 1963, Vol. 43, 

pp. 458 to 469.

5.10.6.2 To determine the maximum height H1 of the unstiffened sidewall, a calculation shall be made using the 

thickness of the top sidewall course. Next the height of the transformed sidewall shall be calculated as follows.

a) Change the width (W) of each sidewall course into a transposed width (Wtr) of each sidewall course, having the 

top sidewall thickness, by the following relationship:

Wtr w

tuniform

tactual

--------------     5

=

where

tuniform is the thickness of the top sidewall course, as ordered condition in inches, unless otherwise specified;

tactual is the thickness of the sidewall course for which transposed width is being calculated, as ordered 

condition in inches, unless otherwise specified;

W is the actual course width, in ft;

Wtr 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, H1, 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 of

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 sidewall using the foregoing thickness relationship.

b) Other locations for the girder may be used provided the height of the unstiffened sidewall on the transposed 

sidewall does not exceed H1 (see 5.10.6.5).

5.10.6.4 If half the height of the transposed sidewall exceeds the maximum height, H1, a second intermediate girder 

shall be used in order to reduce the height of unstiffened sidewall to a height less than the maximum.

5.10.6.5 Intermediate wind girders shall not be attached 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.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 5-27

5.10.6.6 The required minimum section modulus, in inches cubed, of the intermediate wind girder shall be 

determined by the equation:

Z = 0.0001D2H1

NOTE This equation is based on a 3 sec gust wind velocity of 120 miles per hour. If specified 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 H1 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 actual sidewall, may be substituted for H1 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 upon the properties of the attached 

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 outside of the sidewall are permitted without 

need for an opening in the stiffener when the nominal stairway width is at least 24 in. For greater outward extensions 

of a stiffener, the stairway shall be increased 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 greater 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 combined 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, T1 and T2, in 

the bottom. Under these conditions, the unit forces in the bottom of the tank may be computed considering that P in 

each case is equal to Pg.

5.11.2 Flat-bottom Tanks with Counterbalance

5.11.2.1 General

In tanks that have cylindrical sidewalls and flat bottoms, the uplift that results from the pressure acting on the 

underside of the roof combined with the effect 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 counterbalancing structure such as a concrete ringwall, a slab 

foundation, or another structural system. The means for accomplishing this shall be a matter of agreement between 

the Manufacturer and the Purchaser. Similar 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 materials, 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 internal design pressure plus the wind load on the shell and 
5-28 API STANDARD 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 shall satisfy the following conditions.

a) The design stresses shall satisfy all of the conditions in Table 5-7. 

b) When corrosion is specified for the anchors, thickness 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 corrosion 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 allowable stresses shall be those permitted by Table 5-1.

NOTE The allowable stresses for stainless steel and aluminum 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 tanks without counterbalancing weight shall be a matter of agreement between the

Manufacturer and the Purchaser and shall satisfy the following conditions.

a) The bottom of a flat-bottom tank shall be designed to remain flat during all design and test conditions. When the

flat-bottom tank is designed without anchoring the shell to the counterbalancing weight, the bottom will be

designed to carry all the weight and pressure forces distributed 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 include uplift from design wind velocity. The largest values will be used for

design.

b) The bottom plates in the flat-bottom tank shall be designed as a strength member to span between main structural

members (for example, grillage beams or other structural 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 transfer the uplift forces from the shell to the shear-carrying

elements in the bottom structure.

e) Consideration shall be given to protecting all bottom structural 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 Uplift Pressure Conditions (see 5.11.2.2)

Source of 

Uplift Pressure

Allowable Tension 

Stressa (lbf/in.2)

Tank design pressure

Tank design pressure plus 

wind or earthquake

Tank test pressure

Allowable design stress, Sts (see 

Table 5-1)

Smaller of 1.33 Sts or 80 % of the 

specified minimum yield strength

Smaller of 1.33 Sts or 80 % of the 

specified minimum yield strength

DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 cylindrical sidewalls except 

as indicated on 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 for design of reentrant junctures.)

5.12.2 General

When the roof or bottom of a pressure tank is a cone or partial sphere (or nearly so) and is attached to cylindrical 

sidewalls, the membrane stresses in the roof or bottom pull inward on the periphery of the sidewalls. This pull results 

in circumferential compressive forces at the juncture, which 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 participate20 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 form of compression ring shall not be used in 

connection with it, and there shall 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 shall be not less than 6 %, and preferably not less than 

12 %, of the diameter of the sidewalls. Subject to the provisions of 5.12.3.2, the thickness of the knuckle at all points 

shall satisfy the requirements of 5.10. Use of a knuckle radius as small as 6 % of the sidewall diameter will frequently 

require an excessively heavy thickness for the knuckle region. The thickness requirement for such a region will be 

found more reasonable if a larger knuckle radius is used.

5.12.3.2 The designer should recognize that applying the equations in 5.10.2 to levels immediately above and 

below a point where two surfaces of differing meridional curvature have a common meridional tangent (for example, 

at the juncture between the knuckle region 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 uneconomical or impractical to use butt-welded longitudinal or meridional joints for a distance 

on either side of the juncture as computed 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 Table 5-2, the joints may be lap-welded provided that the plates 

jointed in this way are not given credit for contributing to the net cross-sectional area provided for resisting compressive forces 

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 composite corner compression region (see 5.12.5) shall be made 

as though these plates did actually participate in resisting the compressive force.
5-30 API STANDARD 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 intermediate between the two calculated values, depending on the geometry 

of the tank wall in that area; the designer may adjust the immediately adjacent thicknesses accordingly.

5.12.4 Compression Rings

5.12.4.1 The variables used in Equation (24) through Equation (27) are defined as follows:

wh is the width, in inches, of the roof or bottom plate considered to participate in resisting the circumferential 

force acting on the compression-ring region;

wc is the corresponding width, in inches, of the participating sidewall plate;

th is the thickness, in inches, of the roof or bottom plate at and near the juncture of the roof or bottom and 

sidewalls, including corrosion allowance;

tc is the corresponding thickness, in inches, of the cylindrical sidewalls at and near the juncture of the roof 

bottom and sidewalls;

R2 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;

Rc is the horizontal radius, in inches, of the cylindrical sidewall at its juncture with the roof or bottom of the 

tank;

T1 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;

T2 is the corresponding latitudinal unit force (see 5.10) in the roof or bottom, in lbf/in. of meridian arc;

T2s is the circumferential unit force (see 5.10) in the cylindrical 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 direction of T1 and a vertical line (in a conical surface it is also one-half the total 

vertex angle of the cone);

Q is the total circumferential force, in lb, acting on a vertical cross section through the compression-ring 

region;

Ac is the net area, in inches2, of the vertical cross section of metal required in the compression-ring region, 

exclusive of all corrosion allowances;

Sts 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 the 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 circumferential compressive forces mentioned in 5.12.2 must be 

resisted by other means in the compression-ring region of the tank walls. This region shall be understood to be the 

zone of the tank walls at the juncture between the roof or bottom and the sidewalls, including the width of plate on 

each side of the juncture that is considered to participate in resisting these forces (see Figure 5-5). In no event shall 

the thickness of the wall plate on either side of the juncture be less than the thickness needed to satisfy the 
Figure 5-5—Compression-ring Region

wc = 0.6 Rc (t

c

 - c)

t

c

Rc

R2

t

h

wh

= 0.6 R2

 (t

h - c)

Cylindrical sidewall of tank

Roof of tank Compression-ring region

DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 5-31

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

following equations:

wh 0.6 R2 t = ( ) h – c (24)

wc 0.6 Rc t = ( ) c – c (25)

5.12.4.3 The magnitude of the total circumferential force acting on any vertical cross section through the 

compression-ring region shall be computed as follows:

Q T = 2wh + T2swc – T1Rcsinα (26)

The net cross-sectional area provided in the compression-ring region shall be not less than that found to be required 

by one of the following equations:

Ac = Q/15,000 or Q/StsE (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 shown 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 STANDARD 620

5.12.5 Details of Compression-ring Regions

5.12.5.1 If the force Q is negative, indicating compression, then the horizontal projection of the effective 

compression-ring region shall have a width in a radial direction 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 the circumferential force Q determined in accordance with 5.12.4 is such 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, wh, 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 compression-ring region having the necessary cross-sectional 

area and width as determined on the basis of the thicker plates,22 (b) adding an angle, a rectangular bar, or a 

horizontally disposed ring girder at the juncture of the roof or bottom and sidewalls plates, or (c) using a combination 

of these alternatives. This additional area shall be arranged so that the centroid of the cross-sectional area of the 

composite corner compression region lies ideally in the horizontal plane of the corner formed by the two members. In 

no case shall the centroid be off the plane by more than 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 located either inside or outside the tank (see Figure 5-6) 

and shall have a cross section with dimensions that satisfy the following conditions.

a) The cross-sectional area within the limit of wc or wh makes up the deficiency between the area Ac required by 

Equation (27) and the cross-sectional area provided by the compression-ring region in the walls of the tank.

b) The horizontal width of the angle, bar, or ring girder is not less than 0.015 times the horizontal radius, Rc, 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 projection of the width, wh, of the 

participating roof or bottom plates alone is equal to or greater than 0.015Rc or, with an angle or bar located on the 

outside of a tank, the sum of the projection of the width, wh, and the horizontal width of the added angle or bar is 

equal to or greater than 0.015Rc.

c) When bracing must be provided as specified in 5.12.5.8, the moment of inertia of the cross section around a 

horizontal 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 flange of a ring 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 adjourning wall plates. If this 

construction is not used, the leg, edge, or flange of the compression ring next to the tank shall make good contact with 

the wall of the tank around the entire circumference and shall be attached thereto along both the top and bottom 

edges by continuous fillet welds except as provided in 5.12.5.5. These welds shall 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 width 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 roof 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 T2 and T2s on the resulting increments in width of participating plate may be safely 

neglected, the use of thicker plates involves recomputing not only Th and wc, 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-error procedure.
Figure 5-6—Permissible and Non-permissible Details of Construction for a Compression-ring Juncture

t

16 Wht (max)

16t (max)

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

Wc

t

t

t tt

t t

t

Wh L

Wh

L

Wh

Wh

L

Wc Wc Wc Wc

Wc Wc Wc Wc

Wh

WhWh

Wc Wc Wc

16t (max) 16t (max)

t t

CL

Wh

Wh Wh Wh

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 Detail i Detail i-1 Detail h-1

Detail e Detail f-1 Detail f Detail g

See 6.16

See 6.16 See 6.16

See 6.16

See Note 1

For details e through g, Wh  L  16t and Wh

 32t

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.

DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 5-33
5-34 API STANDARD 620

5.12.5.5 If a continuous weld is not needed for strength or as a seal against corrosive elements, 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 thickness 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 11/4

> 11/4

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 possible 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 tank is shaped in such a way that liquid may be 

trapped, it shall be provided with adequate drain holes uniformly distributed along its length. 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 its 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 vertical flange 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. When bracing is required, the moment of inertia of the cross 

section of the angle, bar, or ring girder about a horizontal axis shall be not less than that computed by the following 

equation:

Il

1.44QpRc

2

29 000 000 , , k -------------------------------- ( ) 0.00000005 QpRc

2

k = = ------------ (28)

where

I1 is the required moment of inertia, in inches to the fourth power, for the cross section of a steel23

compression ring with respect to a horizontal axis through the centroid of the section (not taking 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;

Qp 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;

Rc is the horizontal radius, in inches, of the cylindrical sidewall of the tank at its juncture with the roof or bottom;

23The value of I1 a computed using Equation (28) is not applicable for materials other than steel.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 supports 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 Bracing (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

119.1

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 are limited to a discussion of the basic requirements and principles involved. 

For reasons that will appear obvious, specific 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 tank, or some portion 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 walls themselves to carry the entire loads imposed by all possible combinations 

of gas and liquid loadings that may be encountered 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. Other structural members may be needed on the outside of a tank to support or partly 

support the weight of the tank and its contents, and these shall be provided as required. All such internal and external 

members shall be designed in accordance with good structural engineering practices, using stresses 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 attached to the wall of a tank by welding, the stresses in the 

member at the point of attachment shall be limited to the stress value permitted in the wall of the tank (see Annex D). 

5.13.2.2 In no event shall the nominal thickness, including the corrosion allowance, if any, of any part of any internal 

framing be less than 0.17 in.

5.13.2.3 If any structural members (such as girders at node circles), tank accessories, or other internals are placed 

to form gas pockets inside a tank, adequate and suitably located vent holes shall be provided 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 adequate 
5-36 API STANDARD 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 statically determinate; in other cases, they are statically 

indeterminate. The external columns that are often used for supporting a spherical tank are an example of the 

statically determinate class of members. If the columns are vertical, the force acting on each column is simply the 

combined weight of the tank and its contents 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 vertical 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 

loads (including external pressure load, if any) as may be superimposed upon it, the procedure for designing such 

framing is more or less straightforward, 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 requirements for designing the internal framing in all 

the various shapes of tanks that might be constructed, but an outline of the 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 illustrate 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 girders above the bottom node circles, are determined by static, 

assuming for the purpose of a preliminary analysis that these tension members are replaced by a cylindrical shell if 

the members are vertical or by a conical frustum if the members are inclined.

5.13.4.2 Under these assumed conditions, the vertical components of the T1 (meridional) unit forces in the roof 

plates at their juncture with the cylinder or frustum are transmitted directly to the cylinder or frustum so that an upper 

ring girder is unnecessary in this hypothetical case if (a) the horizontal components of the T1 unit forces in the roof or 

wall plates on opposite sides of the juncture balance each other in the case of the cylindrical tie or (b) the difference 

between them is balanced by the horizontal components of the unit 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 frustum, the summation of the vertical components 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 the top of the cylinder or frustum must equal the total vertical force acting along the edges of 

the bottom of the cylinder or frustum. 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 balanced by the downward gas pressure over a predetermined 

portion of the bottom without undue elastic stressing or straining.

5.13.4.4 If the horizontal forces at the node circles are not otherwise in equilibrium, ring girders must be provided 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 static equilibrium 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 addition 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 variations from the nominal T1

(meridional) roof forces will greatly reduce, if not completely offset, the torsional moments in the girders.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 5-37

5.13.5 Internal Meridional Stiffeners

5.13.5.1 When curved meridional trusses or ribs are fastened to the sidewalls of a tank to prevent the T1

(meridional) compressive forces from buckling the sidewalls, the distribution of meridional forces between the 

sidewalls and trusses or ribs is to a degree indeterminate if the foundation support for the overhanging portions of the 

sidewalls is so uniformly distributed around 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 tank may be computed from Equation (1) in 5.10.2.1, assuming 

for the purposes of these computations only that the cross-sectional area of the trusses or ribs is distributed uniformly 

along the circumference of the sidewalls as an added sidewall 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 meridional forces acting on the 

composite section of sidewalls and trusses or ribs at the level under consideration divided by the circumference of the 

tank at that level.

5.13.5.2 The net cross-sectional area of metal (exclusive of corrosion 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 allowable 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 ribs to enable them to serve their intended function of preventing buckling of the 

sidewalls in a vertical direction (the trusses or ribs must also be proportioned and distributed around the 

circumference of the tank so that they will serve this function) and (b) sufficient thickness is provided in the sidewalls 

to enable them to withstand not only their share of the meridional unit forces but also the entire latitudinal unit force T2

as computed by the following equation:

T2 = R2(P – T1/R1)

In this equation, T1 is the meridional unit force assumed to be actually carried by the sidewalls and is obtained by 

multiplying the hypothetical value of the meridional unit forces acting on the composite section by the ratio of the 

sidewall cross-sectional area to the composite 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 T2 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 composite section of sidewalls and trusses 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 contents 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 sidewall 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 section, 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 tank wall and is attached to it with sufficient weld within the tank wall 

thickness to develop the strength in tension of that section of the wall of the connection which lies within the tank wall 

thickness (that is, the strength of an area equal to twice the product of the nozzle wall thickness and the tank wall 

thickness) in addition to whatever welding is required at this location for reinforcement attachment. In the latter case, 
5-38 API STANDARD 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 the inside surface of the connection and the surface of the tank wall extended.

5.14.2 In all cases, requirements concerning openings shall be understood to refer to dimensions that apply to the 

corroded condition. Unless otherwise specified, dimensions of openings generally refer to measurements taken along 

the chord of the tank wall curvature if the wall is curved in the direction involved; however, when there is more than 

approximately a 2 % difference between the length of chord and the length of the arc that is subtends in the tank wall, 

the measurement shall be taken along the arc of the tank wall curvature.

5.14.3 The rules in this section shall also apply to openings in cylindrical shells that are adjacent to a relatively flat 

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, roofs, or bottoms of tanks 

constructed under these rules shall be circular, elliptical,24 or obround 25 in shape. Where elliptical or obround 

connections are employed, the long dimensions shall not exceed twice the short dimension, as measured along 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 preferably coincide with the direction of the greater stress.

5.14.5 Each opening in the walls of a tank shall be located so that the distance between the outer edge of its 

reinforcement 26 and any line of significant discontinuity in the curvature of the tank walls (such as the juncture 

between two nodes in a noded surface, the juncture between a dished or conical roof or bottom and cylindrical 

sidewalls, or the 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, except as permitted by 5.14.3. No part 

of the attachment for any openings shall be located closer than the larger of these distances 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 the 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 larger 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 (after allowing for corrosion) of any 

opening 28 other than those considered in 5.18 exceed 1.5 times the 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 consideration (see 5.16.7 and 5.18). In the case of large openings 

which have attachments that require shop stress relief (see 5.25.1), shipping clearances, affecting the maximum size 

of assembly that can be shipped, 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 treated 

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 outermost 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 curvature of the tank wall in that direction.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 5-39

5.15 Inspection Openings

Each tank shall be provided with at least two manhole openings to afford access 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 Figure 5-7 and Figure 5-8. See 5.21.1.2, 5.21.1.3, 5.21.2.7, and 

5.21.2.8 for provisions concerning reinforcement of openings in cover plates for nozzles.

5.16.2 Basic Requirements

All openings in the walls of tanks constructed according to these rules and all openings for branch connections 29

from nozzle necks welded to the tank wall shall be fully reinforced 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 inherent in their construction 

for the following conditions:

a) 3 in., or less, pipe size welded connections in tank walls 3/8 in. or less;

b) 2 in., or less, pipe size welded connections in tank walls over 3/8 in.;

c) threaded connections in which the hole in the tank wall is not greater than 2 in. pipe size. 

5.16.2.2 The reinforcement required for openings in tank walls for external pressure conditions need be only 50 % 

of that required in 5.16.5 where t has been determined for external pressure conditions.

5.16.2.3 The requirements for full reinforcement shall not be construed as requiring that a special reinforcing pad be 

provided where the necessary reinforcing metal is available 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 requirements are satisfied for all paths of potential failures through the 

opening extended in either a meridional 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 

principal wall 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 provided along the entire length of the parallel sides of the 

opening between the planes passing through the respective centers of the semicircular ends. However, these planes 

may not be the controlling sections with respect to possible failure through the opening, inasmuch as failure might occur 

along another path (in the case of a cylindrical wall, parallel 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 necks, but the provisions shall 

be understood to apply to openings of this 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

t

2.5(t

w

 - c) 2.5(t

w

 - c)

(d + 2c)

(d + 2c)

t

n

 - c

t

rn

e3

H

d + 2c

d c

Use the smaller

dimension

Use the greater

dimension

or

2.5(tn - c)

G

2 + (t

n

 - c) - (t

w - c)

J K

c

(t - c)

t

r

e1e2

A2

A3

A3

A1

A4

A4

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

e1

e2

e3

A2

A3

A1

A4

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

where

5-40 API STANDARD 620
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 5-41

5.16.3 Size and Shape of Area of Reinforcement

5.16.3.1 The area of reinforcement for a given cross section of an opening shall be understood to be that area in a 

plane normal to the surface of the tank and passing through the section under consideration 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 indicated by lines GH, HK, GJ, and JK 

in Figure 5-7; however, on surfaces that are curved in two directions, 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 shall be the greater of the following limiting distances on 

each side of the axis of the opening, measured along the outside surface of the tank.

a) A distance equal to the diameter of the opening after corrosion; in the case of non circular openings, a distance 

equal to the corresponding clear dimension is substituted for the diameter of the opening.

b) A distance equal to the radius of the opening after corrosion plus the thickness of the nozzle wall plus the 

thickness of the tank wall, all taken in the corroded condition; in the case of non circular openings, a distance 

equal to the corresponding half chord is substituted for the radius of the opening.

5.16.3.3 The maximum width of the area of reinforcement, measured 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 tank wall less the corrosion allowance.

b) A distance equal to 2.5 times the nominal thickness of the nozzle wall less its corrosion allowance plus the 

thickness of any additional reinforcement inside or outside the tank wall 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 provided 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 metal within the limits of the area of reinforcement as described in 

5.16.4.2 and 5.16.4.3 may be considered to act as reinforcement.

5.16.4.2 Metal thickness in the tank wall in excess of that required by 5.10 for 100 % joint efficiency 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 direction30 as the cross section of the opening for which the reinforcement 

requirements are being computed. In no case does this include any metal provided for corrosion allowance. If desired, 

the wall thickness may be arbitrarily increased to make additional amounts of excess thickness available for 

reinforcement in the tank wall instead of adding reinforcement 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 considered to act as 

reinforcement, including those portions of fusion welds and the nozzle wall that remain available for reinforcement of 

the opening after deducting applicable corrosion allowances and allowing for the thickness of nozzle wall needed to 

satisfy minimum thickness and strength requirements for the nozzle wall itself (see 5.19).

30If part of the opening passes through a joint whose direction is approximately perpendicular to the cross section under 

consideration, the presence of the joint may be ignored in the computations for this cross section but must be taken into account 

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 tmin or 

1

/

4 in.

t

1

 + t

2 = 1.25 tmin

Panel a Panel b Panel c

Panel d Panel e-1

Panel e-2 Panel f

5-42 API STANDARD 620
Figure 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 tp

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 j Panel k Panel l

Panel m Panel n Panel o-1

Weld to shell

Weld to shell

DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 5-43
Figure 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

 = 0.2t 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 t + c 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-2a Panel t-1 Panel t-2a Panel u-1 Panel u-2a

Either method of attachment is satisfactory

5-44 API STANDARD 620
Figure 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 3t

h

 but need

not exceed 11

/

2

 in.

Single-fillet lap weld for heads convex to pressure

Panel u-3 Panel va

Panel w

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

tw is the nominal thickness of the tank wall, in inches, including corrosion allowance;

tn is the nominal minimum thickness of the nozzle neck, in inches, including corrosion allowance;

tp is the nominal thickness of the reinforcing pad, in inches, including corrosion allowance if the pad is exposed to 

corrosion;

c is the corrosion allowance, in inches;

tmin is the smaller of 3/4 in. or the thickness less the corrosion allowance of either of the parts joined by a fillet weld or groove 

weld;

t1 or t2 is the a value not less than the smaller of 1/4 in. or 0.7tmin; the sum t1 + t2 shall not be less than 1.25tmin;

t3 is the smaller of 1/4 in. or 0.7(tn – 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);

t4 is a value not less than 0.5tmin;

t5 is a value not less than 0.7tmin;

th is the nominal head thickness, in inches.

NOTE 1 The weld dimensions indicated in this figure are predicated on the assumption that no corrosion is anticipated on the outside 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.

DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 5-45
5-46 API STANDARD 620

5.16.5 Reinforcement Required

5.16.5.1 The total cross-sectional area of the reinforcement provided at any section through 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:

Ar = (d + 2c)(t – c)(E ́)

where

Ar is the area, in square inches, to be provided in the reinforcement of the section under consideration;

d is the inside clear dimension, in inches, across the opening at the section under consideration before 

deducting the applicable corrosion 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 particular area of the tank wall in which the opening is 

located for resisting the unit forces that act in a direction perpendicular to the cross section under 

consideration;

E ́ is the factor whose value shall be equal to the efficiency, E, of the main joints along the edges of the tank 

wall plate containing the opening that are approximately parallel to the cross section under consideration 

where the opening is in solid plate or passes only through a joint that is substantially 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 section under consideration (for values of E, see Table 5-2). The value of 

E ́, when not taken as unity, shall be expressed as a decimal.

5.16.5.2 Consideration must be given to the reinforcement requirements for cross sections in both meridional and 

latitudinal directions, particularly on noncircular openings that have appreciable 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 considered 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 reinforcement 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 material 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 of the reinforcement 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 material provided within the area of reinforcement in that plane by the maximum 

allowable unit stress value for the reinforcement material (this value shall not exceed the allowable unit stress for the 

tank wall). In addition, the reinforcement 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.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 not larger 

than the following sizes.

a) For surfaces that have a radius of curvature of 30 in. or less, the inside diameter (width or length) of the openings 

shall not exceed the radius of curvature of the surface in which the opening is located, nor shall it exceed 20 inches in 

any case.

b) For surfaces whose least radius of curvature is over 30 in., the inside diameter (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 within the limits specified in 5.14.7, shall be given 

special consideration; except as otherwise provided in 5.18, the reinforcement shall meet all of the 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 the maximum thickness of the reinforcement. 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 the opening shall be rounded to a generous radius to reduce stress concentrations. Reinforcement may 

sometimes be more advantageously obtained by inserting a thicker plate or plates in that portion of the wall of the 

tank where the nozzle is located. However, when this is done, consideration shall be given to whether it would 

introduce an objectionable degree of restraint that might affect adjoining plates. The degree to which these and other 

measures should be used will depend on the particular application and the severity 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 the opening and provides adequate protection against failure that might occur in a 

plane (referred to herein as the critical plane) that is somewhat removed from the center of the opening as a result of 

tensile failure of the tank wall in combination with shearing or tensile failure of the reinforcement attachment (see 

5.16.2.4). To this end, the welds and other parts of the assembly that serve as reinforcement attachment shall be 

properly located to transmit the stresses to the reinforcement, 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 center of the opening. Similarly, the 

strength of the attachment between any two parts of attached reinforcement beyond 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 analytically in most cases, it is not essential that it be 

determined analytically for the purposes of this section; the intent 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) sufficient welding and other attachments are provided 

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 made continuous around the 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 according to Item a of this note shall be established from a consideration 

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 other shape where neither of the principal biaxial stresses is less 

than 75 % of the other, the critical plane shall be one that is perpendicular 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 the 

opening is being analyzed or coincides with the lengthwise centerline of the opening if a section in this direction is being 

analyzed.
5-48 API STANDARD 620

2) For an opening in a cylindrical or conical surface or in a surface of some other 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 opening 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 shall govern. Plug welds, wherever applicable, 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 thickness that is sufficient to develop its strength 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 attachment welding given in this standard, the following 

requirements shall be met.

a) The joint efficiencies of butt-welds shall be in accordance 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 tension or shear stress values 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 parallel 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 allowable stress value determined by combining the following factors: 80 % for shear 

strength of weld metal; an efficiency 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 plate or forged steel 

given in Table 5-1;

2) when the load is parallel to the weld, 50 % of the applicable tension stress value 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 Table 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 attachment welds shall comply 

with the following:

a) where the thickness of the thinner of two parts being joined is 3/4 in. or less, exclusive of corrosion allowance, the 

dimensions of the welds shall be not less than the requirements indicated in Figure 5-8;
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 5-49

b) where the thickness of both parts is greater than 3/4 in., exclusive of corrosion allowance, 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 tmin.

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 requirements other than those required by 5.16.8.

5.16.9.3 For fittings attached as shown in Figure 5-8, Panel u-3, the depth of the groove weld, t5, 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 the underside of a tank bottom that rests directly on the tank grade 31 and nozzles with 

reinforcements that are too narrow to permit compliance with the following provisions, single-thickness reinforcing 

plates and saddle flanges or integral reinforcing pads on manholes or nozzles attached to the outside of a tank shall 

be provided with at least one telltale hole 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 welds 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 that the test pressure will extend entirely 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 adjacent 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 openings 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 are reinforced 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 thickness of the fillet weld around the thicker reinforcing plate 

or eight times the nominal thickness of the insert butt-weld 32 for an insert-type connection. In no case shall any 

portion of a cross section be considered to apply to more than one opening, that is, to be evaluated more than 

once in a combined area. Curved sections 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 there be any re-entrant angles therein.

5.17.2 When two or more adjacent openings will be provided with a combined reinforcement, the minimum distance 

between the centers of any two of these openings shall preferably be at least 1.5 times their average diameter, and 

the area of reinforcement between them 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 openings, as considered under 5.17.2, have a distance between centers less than 11/3

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-relieved 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 circumferential or latitudinal joint provided that in either case the 

spacing is not less than 21/2 times the shell thickness.
5-50 API STANDARD 620

5.17.4 Any number of closely spaced adjacent openings, in any arrangement, may be reinforced for an assumed 

opening of a diameter enclosing 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 illustrated in Figure 5-9, which are centrally located in the roof or bottom of 

a tank with the axis of the connected cylindrical 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 opening, 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 specified in 5.12 shall also be used for the region around the 

large end of a conical transition section that connects to the horizontally disposed surfaces of a roof or bottom instead 

of to the sidewalls.

5.18.2 Nomenclature

Variables used in Equation (29) through Equation (32) are defined as follows:

Q is the total circumferential force, in lb, acting on a vertical cross section through the juncture between the 

roof, bottom, or transition section and the neck extending from the opening at one side of the opening;

Ac is the net area, in inches2, of the vertical cross section of metal required to resist Q, exclusive of all 

corrosion allowances;

R2 is the length, in inches, of the normal to the roof, bottom, or transition section 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;

Rn is the horizontal radius, in inches, of the cylindrical neck extending from the opening at the juncture with 

the roof, bottom, or transition section;

T1 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;

T2 is the corresponding latitudinal unit force (see 5.10) in the roof, bottom, or transition section, in lbf/in. of 

meridional arc (if the roof or bottom is of double curvature) or per linear in. along an element of the cone (if 

the surface is that of a conical frustum);

T2n 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. measured along an element of the neck;

α is the angle between the direction of T1 and a vertical line (in a conical surface it is also one-half the vertex 

angle of the cone);

Sts 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 the least efficient joint cutting across the section considered 

(see Table 5-2);

wh is the width, in inches, of the roof, bottom, or transition section plate considered 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

Rn

Rn

Rn

R2

R2

R2

Wh

Wh

Wn

Wn

R1

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

DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 5-51

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

th 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;

tn is the corresponding thickness, in inches, of the cylindrical neck at and near the juncture described for th.

5.18.3 Knuckle Radius

5.18.3.1 A knuckle radius used for the juncture between the roof, bottom, or transition 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 excessively heavy thickness for the knuckle region. The thickness 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 this location, the stress situation at the juncture is the reverse of that

found at the juncture (without a knuckle) between a conical or dished roof and the sidewalls of a cylindrical tank 

because in this case the horizontal components of the T1 meridional unit forces in the roof, bottom, or transition 

section pull outward on the neck extending from the opening and increase the circumferential tensile stresses acting 
5-52 API STANDARD 620

at the juncture. In this case, the walls of the tank and neck of the opening at and near their juncture must be designed 

to withstand a total circumferential load, Q, on each side of the opening, as computed using the following formula:

Q = T2wh + T2nwn + T1Rn sin α (29)

5.18.4 Cross-sectional Area

The total cross-sectional area of metal required to resist the circumferential force is shown by the following equation:

Ac = Q/StsE (30)

The widths of plate available for providing this area and resisting the force Q on each side of the opening shall be 

computed using the following formulas:

wh 0.6 R2 t = ( ) h – c (31)

wn 0.6 Rn 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 wall of another nozzle or with a nozzle cover plate; or, 

subject to the limitations stated in these rules, nozzles may be attached directly to the wall of the tank or another 

nozzle or nozzle cover plate by threading, fusion welding, bearing against the inside 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 as 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 shown in Figure 5-8 or by other methods that 

conform to sound design principles if the nozzle and its attachment 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 allowable stresses as 

specified in 5.5, and to this thickness shall be added the corrosion allowance. The minimum thickness of nozzle neck 

to be used shall be at least equal to the required thickness so obtained; in no case shall the net thickness of the 

nozzle neck, excluding corrosion allowance, be less than the smaller of the following thicknesses:

a) the net thickness, excluding corrosion allowance, of the tank wall adjacent to the nozzle, disregarding any added 

thickness that serves as reinforcement for the opening;

b) the thickness of standard-weight pipe (see ASME B36.10M).
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 5-53

5.19.3 Outer Ends of Nozzles

5.19.3.1 The outer ends of nozzles may be flanged, beveled for welding, or threaded except that threaded ends 

shall not be used unless they are permitted by and meet the requirements of 5.20.4. When bolted flanges are 

provided on nozzle sizes NPS 2 and greater, the minimum distance 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 formed by the back of 

the flange and the nozzle wall shall be provided with a fillet weld. The fillet weld size shall be at least 0.25 times the 

thickness of the nozzle wall, not including corrosion allowance, except that for relatively thick nozzle walls, the fillet 

weld shall be not less than 0.25 times the thickness of standard-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 

attached in accordance with 5.20 in this standard and the provisions of Figure 4-4, Appendix 2, in Section 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 that they are intended to replace.

5.20.2 Bolted flanges for external piping connections other than those meeting the requirements of 5.20.1 shall be 

designed for a pressure of at least 50 lbf/in.2 gauge in accordance with the applicable provisions of Section VIII, 

Appendix 2, of the ASME Code, using for values of Sf and Sn the applicable allowable design stress values given in 

Table 5-1 of this standard (instead of the allowable design stress values specified in Section VIII of the ASME Code) 

and limiting the values for Sh, Sr, and St as follows:

Sh is the longitudinal hub stress, not greater than 1.5Sf, except that for flanges of the types illustrated in Figures 

4-4 (7), (8), (8a), (8b), and (9) in Section VIII, Appendix 2, of the ASME Code, Sh shall not exceed the 

smaller of 1.5Sf or 1.5Sn;

Sr is the radial flange stress, not greater than Sf;

St is the tangential flange stress, not greater Sf.

Also, (Sh + Sr)/2 shall not be greater than Sf, and (Sh + st)/2 shall not be greater than Sf. Design stress values for bolts 

shall not exceed the applicable values 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 conform to ASME B16.5, Class 150 

or shall be designed in accordance with the 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 methods permitted for 

circumferential joints in the walls of the tank; the attachment shall conform to the 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 nozzle neck if the number of full threads 

engaged conforms to or exceeds the 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 STANDARD 620

5.21 Cover Plates

5.21.1 Flat Cover Plates and Blind Flanges

5.21.1.1 The thickness of flat, unstayed cover 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 pressure-temperature ratings and diameters 

given in the standard, or their equivalent, 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 required thickness of flat steel 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

Cd2 =+= ------------------- (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 rigidly 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 ends of nozzles and held in place by some suitable 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 WhG/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 ASME Section VIII, Appendix 2, Paragraph 2-5(e)];

hG is the radial distance from the bolt-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 defined in ASME Section 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, Sts, 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 1.40WhG

HG

d = ( = 

G)

hG

C = 0.30 1.40WhG

HG

+

+

C = 0.30

d

t

d = ( = 

G)

hG

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

DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 5-55
5-56 API STANDARD 620

5.21.1.2 Unreinforced openings up to and including 2-in. pipe size are permissible in flat cover plates without 

increasing the plate thickness if the edges of these openings are not closer to the center of the plate than one-fourth 

the diameter d in Figure 5-10. When this condition is not met, the plate thickness shall be increased by 40 % of the 

thickness required in a solid plate after the loss of corrosion metal. The solid-plate thickness shall 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 Figure 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 removed 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 the 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 after forming, including corrosion allowance, in inches;

tf 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 (torus) 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, Sts, in lbf/in.2, as given in Table 5-1;

Mo is the total moment determined as for loose-type flanges (see ASME Section VIII, Appendix 2, Paragraph 2-

6) except that, for cover plates of the type shown in Figure 5-10, Panel d, a moment Hrhr shall be included 

(that may add or subtract);

Hr is the radial component of the membrane load in the spherical segment = HD cot β1;

hr is the level arm of force Hr 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 width of the skirt 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 2t

and in no case

less than 1

/

2

 in.

Knuckle

radius

Gasket

Knuckle

radius

Gasket Ring

gasket

shown

Preferably

2t minimum

1

/

2

C

1

/

2

B

L L L

1

/

2

A

1

/

4

(A + B)

Point of

HD action

Shown as

welded;

smooth weld

both sides

L

1 HD

Hr

hr

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

2t minimum

Ring

gasket

shown

0.7t 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).

t

t t1

t

1

DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 5-57

5.21.2.4 Cover-plate heads of hemispherical shape need not have an integral 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 cylindrical shell of the same diameter.

5.21.2.5 The thickness of circular dished cover plates with bolting flanges, concave to the pressure 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 Figure 5-11, Panel a, the thickness of the plate, t, shall be determined by 

the following application equation:

for torispherical heads,

t = (0.885PL/s) + c (34)

for 2:1 ellipsoidal heads,

t = (PD/2s) + c (35)
5-58 API STANDARD 620

for hemispherical heads,

t = (PD/4s) + c (36)

The cover plate flange thickness and bolting for these types of cover plates shall comply at least with the applicable 

requirements of Section VIII, Appendix 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 the plate and flanges shall be 

determined by the following applicable equation:

for the thickness of the cover plate,

t = (5PL/6s) + c (37)

for the flange thickness using a ring gasket,

tf

Mo

s

------ A B +

BA B ( ) – = --------------------- (38)

for the flange thickness using a full-face gasket,

tf 0.6 P

s

--- BA B ( ) + ( ) C B–

A B– = ---------------------------------------- (39)

(The radial components of the membrane load in the spherical segment are assumed 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 following equation:

t Q 1 1 7.5Mo

PQBL = ++ --------------- + c (40)

The plate thickness for a full-face gasket 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 using the following equation:

t = (5PL/6s) + c
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 5-59

d) For cover plates of the type shown in Figure 5-11, Panel d, the thickness of the cover plate is determined using the 

following equation:

t = (5PL/6s) + 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 following equation:

tf F 1 1 J

F2 = ++ ----- + c (43)

where

F PB 4L2 B2 –

8s( ) A B– = -------------------------------

J

Mo

sB

------     ( ) A B +

( ) A B– = -----------------

NOTE Inasmuch as Hrhr may add to or subtract from the total moment determined 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 cover plates with bolting flanges, convex to the pressure and conforming 

to the several types illustrated in Figure 5-11, shall be designed in accordance with the 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 1.67Pv, where 

Pv is defined as follows:

Pv is the maximum unbalanced pressure, in lbf/in.2, on the convex side of the plate during operation; however, 

if the pressure is 15 lbf/in.2 or less, Pv shall be 15 lbf/in.2 or 25 % more than the maximum possible 

unbalanced pressure, whichever is the smaller. 

The minimum thickness shall be as calculated plus corrosion allowance or 1/2 in. plus corrosion allowance, whichever 

is greater.

Moreover, if the net plate thickness, t – c, determined previously in this paragraph, is found to be less than or equal to 

0.01 times the inside diameter of the cover-plate flange, a check computation shall 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

4Lc Pv

5350

----------------- += c (44)

where

Lc is the inside crown radius for dished (torispherical) and hemispherical heads, in inches; or 0.9D 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 spherical 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 STANDARD 620

5.21.2.8 Openings greater than 2-in. pipe size may be made in the spherical segment of a dished 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 standard. (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 the 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 semicircular 

ends. Fillet-welded holes should not be construed as a plug or slot weld.

5.22.1.3 The reverse side of a double-welded butt joint shall be prepared by chipping, grinding, or melting-out to 

ensure sound metal at the base of the weld metal first deposited 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.

NOTE The proceeding requirements of this section are not intended to apply to any welding procedure by which proper fusion 

and complete penetration are otherwise obtained 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 strips abutting (including a Tjunction) shall be joined with a full-penetration weld. A backing strip need not be removed after the weld is completed 

unless the joint is to be radiographed and the backing-strip image would interfere with interpretation of the resultant 

radiographs.

5.22.1.5 Double and single lap-joints shall have full-fillet welds, the size of which is equal 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 circumferential joints of diametrical transitions, 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 penetration, which is the depth of chamfering plus the root

penetration when root 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 triangle that can be inscribed within the 

fillet-weld cross section determines the size of the weld. For unequal-leg fillet welds, the leg length of the largest right 

triangle that can be inscribed within the fillet-weld 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 root of the fillet weld to its face. For a convex fillet weld, the 

hypotenuse of the triangle which has the greatest area that can be inscribed within the fillet-weld cross section is 

considered the face.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 Figure 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 used in design computation or in computations that relate the 

strengths of welded structures. The joint 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 factors 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 joints, the joint efficiency factor is assumed to exist between 

the working strength of the joint and the working strength of the solid plate.

In the case of fillet welds evaluated as specified in 5.16.8.3, Item b, plug welds, and other attachment 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 tanks or tank parts fabricated by an arc-welding process 

and the limitations on the use of the various types 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 for welded steel pipe 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 tanks are used will often make the thickness of pipe materials, 

as determined by the cylindrical shell formula, of little significance; the girth joints that are subject to piping strains, 

including even moderate temperature effects, may be the controlling factor. The joint efficiencies for such girth joints 

shall be taken from Table 5-2, but in applying these efficiencies to the 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 

attachments 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 part.

5.24.2 The diameter of plug-weld holes and the width of slot-weld holes in members 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 through which the hole is cut plus 1/4 in. In no case, however, does the dimension need to be 

greater 21/4 in.
5-62 API STANDARD 620

5.24.4 Plug-weld and slot-weld holes shall be completely filled with weld metal when the thickness 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 considered 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. The 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 uniform heating of a structure or portion of a structure to a sufficient 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 field stress relieved after erection because 

its size and weight do not permit adequate support at the temperature 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) will minimize locked-up residual 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 thickness of wall plate greater than 11/4 in.34 at any nozzle or other welded 

attachment and nozzle necks whose thickness at any welded joint therein exceeds (D + 50)/120 shall be thermally 

stress relieved after welding. Thickness of compression rings 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 openings 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 attachments when the welds have 

a size that is a) no greater than 1/2 in. for welds on a flat surface or circumferential 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 radiographic method 

specified in 7.15.1 or alternatively, by agreement between the Purchaser and the Manufacturer, using ultrasonic 

33 Any proposed application of stress-relieving requirements and the 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 11/4 in. thickness may be increased to 11/2 in., provided that a minimum preheat temperature of 200°F is 

maintained during welding.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 5-63

examination in lieu of radiography as specified in 7.15.3.1. When the term “examination” is used in 5.26, it shall be 

understood to refer to radiographic or ultrasonic examination. 

NOTE Ultrasonic testing (UT) and radiography (RT) are complementary methods. UT can detect some types of flaws better 

than RT and vice versa. Therefore, the acceptance criteria provided in API 620 for the two methods differ. In the case of RT, all 

weld flaws require characterization as their type (e.g., crack, incomplete fusion, rounded indication, etc.) and acceptance criteria 

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 required (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 radiation, such as X-rays or Gamma rays through an object 

and obtaining a record of its soundness upon a sensitized film.

Ultrasonic examination is a method of detecting imperfections in material by passing ultrasonic waves through the 

material and interpreting the reflected and/or diffracted waves.

5.26.3 Examination Required by Wall Thickness

Complete examination is required for all butt joints having complete penetration and complete fusion wherever the 

thinner of the plates or the tank-wall thicknesses at the joint exceed 11/4 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 described 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 tank wall or meridional and latitudinal joints in 

walls of double curvature) are of the butt-welded type except for nozzle, manhole, 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 prescribed 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 joints in 

cylindrical or conical surfaces need to be prepared and examined for a distance of only 3 in. on each side of any 

intersection with a longitudinal joint. All joints in a spherical, torispherical, or ellipsoidal shape or in any other 

surface of double curvature shall be considered longitudinal joints. For similar reasons, the juncture without a 

knuckle between a conical or dished roof or bottom, and cylindrical sidewalls and the circumferential joints without 

a knuckle at either or both ends of a transition section 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 complete examination (see 5.26.3). This provision 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-type nozzles and flush-type cleanout doors) are installed using an 

insert style detail, the butt-weld joint around the periphery of the insert plate shall 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 STANDARD 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 (longitudinal and circumferential joints in the tank wall or 

meridional and latitudinal joints in walls of double curvature) for other plates in the same course.

5.26.6 Exemptions

Spot or full examination is not mandatory on tank bottoms that are uniformly supported throughout (for example,

concrete slab or compacted sand) or on components that are not subject to tension stresses 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 bottom under the conditions described 5.27.1.2 through 5.27.1.4.

5.27.1.2 The design pressure for the gas vapor space of the tank 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 5.11.2, in such a manner that no uplift will occur at the cylindrical sidewall, flat-bottom junction.

5.27.1.4 The longitudinal or meridional membrane stress in the cylindrical sidewall 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-type connection opening in the cylindrical sidewall 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.

Table 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, r1

Lower Corner

Radius of

Sidewall

Reinforcing

Plate, r2

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

18
Figure 5-12—Part 1—Flush-type Sidewall Connection

tn = 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 = td

Reinforcing plate = td

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

tb

td

td

11/4 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.

2td + 10 in. 

(See note)

Centerline

of connection

24 in.

min

11/4 in. min

Notch as required to provide

flush joint

Full-penetration

weld

1/4

3/16

r2

r1

b/2

12 in.

min

NOTE The thickness of the thinner plate joined with a maximum of 1/2 inch.

DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 5-65
Figure 5-12—Part 2—Flush-type Sidewall Connection

(See Note 1)

td

(11/2 in. max)

Round corner

when td > 11/2 in.

5 in.

td

td

b/2

tn = 5/8 in. min

Nozzle transition

(See Note 2)

Typical Detail for Connections With b = h

tb ta = 1/2 in. min

2td + 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

11/4 in. min

Bottom

reinforcing

plate

Full-penetration

weld

Flanges per Table 5-10

tn

tb

2td + 10 in.

h

Centerline

of nozzle

Bottom plate

Round corner

td

(11/2 in. max)

t/2 5 in.

td

ta

h

tb

td

Round

corner

when td >

11/2 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

tn

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 11/2 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 STANDARD 620
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 5-67

5.27.2.4 The thickness of the sidewall plate in the clean-out opening assembly shall be at least as thick as the 

adjacent sidewall plate in the lowest sidewall course. 

5.27.2.5 The thickness of the sidewall reinforcing plate shall be of the same thickness as the sidewall plate in the 

flush connection assembly.

5.27.2.6 The thickness, ta, 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 contains the connections, shall be thermally stress relieved at a temperature of 

1100 °F to 1200 °F for a period of 1 hour per in. thickness of sidewall-plate thickness, td.

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 reinforcement over the top of the connection shall be not less than 

the value determined using the following equation:

K1ht/2

where

K1 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 sidewall course in which the connection is located, exclusive of 

corrosion allowance.

5.27.4.3 The reinforcement in the plane of the sidewall shall be provided within a height, L, above the bottom of the 

opening. L shall not exceed 1.5h 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 1.5h, only that portion of the reinforcement within a height of 1.5h 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 greater than the calculated thickness of the lowest side wall 

course, exclusive of corrosion allowance;

c) that portion of the neck plate that has a length equal to the thickness of the reinforcing plate. 
Figure 5-13—Design Factors for Flush-type Connections

Minimum K1

Minimum K1

0 1.0

Key

1.1 1.2

1.25

1.00

0.75

0.50

0.25

1.3 1.4

K1 Coefficient

K1 =

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 STANDARD 620

5.27.4.5 The width of the tank-bottom reinforcing plate at the centerline of the opening shall be 10 in. plus the 

combined thickness of the sidewall plate in the flush connection assembly and the sidewall reinforcing plate. The 

thickness of the bottom reinforcing plate, tb, in inches, shall be calculated using the following equation.

tb

h2

14 000 , ----------------- b280

+= -------- 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. 

The minimum thickness of the bottom reinforcing plate, tb, 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, tn, shall be a minimum of 5/8 in. External 

loads applied to the connection may require that tn be greater than 5/8 in.

5.27.5 Material Requirements

All materials in the flush-type shell connection assembly shall conform to the requirements of Section 4. The plate 

materials of the sidewall containing the assembly, the sidewall reinforcing plate, the nozzle neck attached to the 
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 5-69

sidewall, the transition piece, and the bottom reinforcing plate shall meet the impact test requirements of 4.2.5 at 

design metal temperatures for the respective thickness involved. Notch toughness evaluation for the bolting flange 

and the nozzle neck attached to the bolting flange shall be based on the governing thickness as defined in 4.3.6.3 and 

used in Figure 4-2. Additionally, the yield strength and tensile strength of the sidewall plate in the flush-type shell 

connection and the sidewall reinforcing plate 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 the shell and the circular pipe 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 that 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 uplift, they shall be spaced so that they will be located 

immediately adjacent to each side of the reinforcing plates around the opening, while still 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 and moments applied to 

the sidewall connection. Allowance shall be made for the rotation of the sidewall connection caused by the restraint of 

the tank bottom to the sidewall expansion from stress and temperature as well as for thermal and elastic movement of 

the piping. In double-wall tanks, any insulation or other material shall not restrain or tend to increase the movement of 

the sidewall connection. The rotation of the sidewall connection is illustrated in Figure 5-14.

5.27.9 Foundation

The foundation in the area of a flush-type connection shall be prepared 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. Different methods of supporting the 

bottom reinforcing plate under a flush-type connection are shown in Figure 5-14.

5.27.10 Nozzle Spacing

Flush-type connections may be installed using a common reinforcing pad. However, when this type of construction is 

employed, the minimum distance between nozzle centerlines shall be not less than 1.5(b1 + b2 + 2.5) in. or 2 ft, 

whichever is greater. The dimensions b1 and b2 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 adjacent 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 first circumferential butt-weld in the 

neck closest to sidewall, excluding neck to flange weld shall 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 examination. This magnetic-particle examination 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

Reinforcing plate Bottom

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" min 3 in. 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 STANDARD 620
6-1

SECTION 6—FABRICATION

6.1 General

This section covers details in fabrication practices that are considered essential in constructing 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 

alternatives specified in the Purchaser’s inquiry or order. The workmanship and finish shall be first class in every 

respect and subject to the closest inspection by the Manufacturer’s inspector, whether or not the Purchaser waives 

any part of the inspection.

6.2.2 When material requires straightening, the work shall be done by pressing or another non injurious method 

prior to any layout or shaping. Heating or hammering is not permissible unless the material 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 cutting. After gas or arc cutting, all slag and detrimental 

discoloration of material that has been molten shall be removed by mechanical means before 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 preferably be tool-cut. 

If openings 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 provide criteria for shaping of cylindrical sidewalls to the 

curvature of the tank prior to installation in the tank. Shaping of plates concurrently with installation 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 required shape by any process that will not unduly impair 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 described in 6.5.2 through 6.5.6. The number and frequency of 

measurements are left to the judgement of the Manufacturer 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

ALL

ALL

ALL

m

5

12 18 36

in.

5

/8

1

/2

3

/8

3

/16

Shaping required

prior to installation

Shaping not required

Tank diameter

Side 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

ALL

ALL

ALL

9 15 27

10

13

16

mm

Tank diameter

Side 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 STANDARD 620
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 6-3

6.5.2 Plumbness 

6.5.2.1 For cylindrical sidewalls, the maximum 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 plate shall not exceed the permissible 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 storage tank shall be 

sufficiently true to round so that the difference between the maximum and minimum 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 tanks for which the radii 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 bottoms shall be sufficiently true to round so that the 

difference between the maximum and minimum diameters shall not exceed 1 % of the nominal diameter.

6.5.4 Local Deviations

Local deviations from the theoretical shape, such as weld discontinuities and flat spots, 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 ± 11/4

b) Using a vertical sweep board 36-in. long, banding 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 the appropriate plate flatness and waviness requirements of 

6.5.2.2.

6.5.5 Fitting Attachments

All lugs, brackets, nozzles, manhole frames, reinforcement around openings, and other appurtenances shall fit and 

conform to the curvature of the surface to which they are attached.

6.5.6 Foundation

6.5.6.1 To achieve the tolerances outlined in 6.5, a level foundation must be provided for the tank erection. The 

foundation should have adequate bearing power to maintain the levelness of the foundation.
6-4 API STANDARD 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 outside of the tank radially toward the center, the first foot of the 

foundation (or width of the annular ring) shall comply with the concrete ringwall 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 Purchaser and the Manufacturer, they shall be taken 

before the hydrostatic test. Measurements of local deviations shall be taken during construction. They shall be taken 

with a steel tape—making corrections for temperature, sag, and wind—when the length being measured makes such 

corrections necessary. Deviation measurements shall be taken on the surface of the plate and not on welds.

6.5.8 Double-curvature Roofs, Bottoms, and Sidewalls 

For double-curvature roofs, bottoms and sidewalls, the tolerances shall be as follows: The 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 bottom) under consideration. Such deviations shall be measured 

perpendicular to the design shape and shall not be abrupt but shall merge smoothly into the adjoining surfaces in all 

directions. For a knuckle, D shall be considered to be twice 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 shall 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 welder 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 longitudinal joints of the types permitted 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 metal-arc, gas metal-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 the 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 welding 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 supplementary welding guidelines and practices.

6.7 Qualification of Welding Procedure

6.7.1 Each Welding Procedure Specification (WPS) shall be qualified in accordance with 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 Supplementary 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 grain 
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 6-5

practice for the base metal shall be an additional Supplementary Essential Variable. Plates produced by the 

controlled-rolled process are not to be considered as having received any heat treatment. Welding procedures for 

ladder and platform assemblies, handrails, stairways, and other miscellaneous assemblies but not their attachments 

to the tank, shall comply with either AWS 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 tensile 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 Specification (WPS) shall be conducted by the fabricator. 

6.7.4 The stress-relieving requirements in the procedures 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 arc welding, and welding operators assigned to machine 

welding, shall have successfully passed the tests conducted by the fabricator, or Manufacturer, as prescribed for 

welder qualification in Section IX of the ASME Code. Tests conducted by one Manufacturer 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 welding operator an identifying number, letter, or symbol. 

Except for all lap-welded roof seams and flange-to-neck joints, this identifying mark shall 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 record 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 welders 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 tack 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 are sound and the subsequently 

applied weld beads are thoroughly fused into the tack welds.
6-6 API STANDARD 620

6.9.1.4 Tack welds, whether removed or left in place, shall be made using a fillet-weld or butt-weld procedure 

qualified in accordance with Section IX of the ASME Code. Tack welds to be left in place shall be made by welders 

qualified in accordance with Section IX of the ASME Code and shall be examined visually for defects; if welds are 

found to be defective, they shall be removed.

6.9.2 During assembly of the plates and subject to agreement between the Manufacturer and the Purchaser, the 

welded joints in adjoining segments, which abut at a common transverse joint from opposite sides, need not be 

staggered unless specified by the Purchaser. When specified, the stagger should be at least 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, slag, grease, and any oxide that would lower 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 deposits before the next 

layer is applied.

6.10.3 The reverse side of double welded butt joints shall be prepared by chipping, grinding, or melting out to 

ensure sound metal at the base of the weld metal first deposited 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 procedure by which proper fusion 

and complete penetration are otherwise obtained 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 machined 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 periods of high winds, unless the welder 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 temperature indicated in Table 6-2 within 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

Temperature

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 °F
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 6-7

6.12 Reinforcement on Welds 

6.12.1 Butt joints shall have complete joint penetration and complete 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 left in place (see Table 5-2), the 

requirement 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 smoothly with the surface of the plate without a sharp angle. Maximum permissible 

weld undercut is 1/64 in. for longitudinal or meridional butt joints, similarly oriented permanent attachments, 

attachment welds for nozzles, manholds, flush-type openings, and the inside shell-to-flat bottom welds. For 

circumferential and latitudinal butt joints, similarly oriented permanent attachments, 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 tolerances 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 reached on all sides. Subject to 

the approval of the inspector, the resulting cavity 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 surfaces 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 1/4
6-8 API STANDARD 620

6.17 Fitting Up of Closure Plates 

For the closure of the final joints, plates of extra width and length—not narrow strips or filler bars35—shall be used. 

The fitting up of the closure plates used and the proposed method of installation shall be subject to the inspector’s 

approval before the work is started, and the inspector shall ensure that the closure plates meet all applicable 

requirements.

6.18 Thermal Stress Relief 

6.18.1 General thermal stress relief of an entire tank is not visualized for tanks of this type, but sections of tanks 

shall be stress relieved before erection where required by the provisions of 5.25. 

6.18.2 Parts of a tank that require stress relief according to the rules in 5.25 shall be stress relieved in an enclosed 

furnace before shipment from the fabricators’ shops. 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 

metal thickness, 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 when at the hold temperature not more than 150 °F 

throughout the portion of the tank 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 metal thickness (maximum metal thickness of the tank wall 

plates affected). During the heating and holding periods, the furnace atmosphere 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 furnace or cooling chamber at a rate not 

greater than 500 °F per 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 hour. 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 operation 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 effective as thermal 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. Full consideration should be given to 

radiographic and magnetic-particle methods of examination as well as to the thermal 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 in.

of thickness)

1100 1

1050 2

1000 3

950 5

900 (minimum) 10

DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 6-9

NOTE For intermediate temperatures, the heating time shall be determined 

by straight line interpolation.

7-1

SECTION 7—INSPECTION, EXAMINATION, AND TESTING

7.1 Responsibility of Inspector 

7.1.1 The inspector shall ensure that all materials used in tanks constructed according to the rules in this standard

comply in all respects with the requirements of these rules. This shall be done either by witnessing mill tests or

verifying that the materials to be used are properly identified in the certified mill test reports supplied by the

Manufacturer.

7.1.2 Tanks constructed according to the rules in this standard shall be inspected and tested in accordance with the

sections that follow. The inspector shall 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 rules in this standard shall have had not less than 5 years

experience in design, construction, maintenance and/or repair, or in the responsible supervision of the construction,

maintenance and/or repair of various types of unfired pressure vessels and/or tanks, including at least 1 year of

experience in the construction or supervision 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 help 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 material suppliers who are concerned 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 all tests of the equipment and materials

during fabrication, including all laboratory 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. Defective 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 subjected to pressure-imposed loads shall be inspected before

being incorporated in the tank. Particular attention shall be given to all cut edges to ensure that the material is free

from serious laminations and other defects.
7-2 API STANDARD 620

7.7 Stamping of Plates

Before plates required to be stamped by the steel mill are used, the inspector shall see that they bear the stamp. In 

laying out and cutting the plates, at least one set of the original material identification markings should, if possible, be 

left where it will be plainly visible when the tank is completed. Should the identifying 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 readily 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 cracks 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 determine whether the thickness meets the requirements.

7.9 Inspection of Surfaces Exposed during Fabrication

7.9.1 The edges of plates, openings, and fittings exposed during fabrication shall 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 inspector approves the method and extent of repairs. 

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 record 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 properties as given on mill reports and shall see 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 shall supply marked copies of plans (or a separate sketch) 

showing the location of all plates, with a means of identifying each plate with the heat numbers. 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 operation and shall be satisfied that the temperature readings 

are accurate and that the procedure conforms to the applicable requirements of these rules.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 method employed 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 radiographs shall be submitted to the inspector.

7.15.1.3 The acceptability of welds examined by radiography shall be judged by the standards of Paragraph UW51(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 frequency 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 unless otherwise agreed 

upon by the Purchaser and the Manufacturer. 

7.15.1.6 Personnel who perform and evaluate radiographic examination according to this section 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 Level II or III personnel. 

7.15.2 Magnetic-particle Method Examination

7.15.2.1 When magnetic-particle examination is specified, the method of examination 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 requirements of T-150, Section V, Article I of the ASME Code.

7.15.2.3 The Manufacturer shall determine that each magnetic-particle 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 distinguishing and differentiating contrast between colors used. 

These requirements shall be checked annually. 
7-4 API STANDARD 620

b) The examiner is competent in the techniques of the magnetic-particle examination method for which the examiner

is certified, including making the examinations and interpreting 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.

7.15.2.4 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 applied 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 accordance with Section V, Article 4, of the ASME Code.

Acceptance standards shall be by agreement between Purchaser and Manufacturer.

7.15.3.2.3 Ultrasonic examination shall be performed in accordance with a written procedure that is certified by the

Manufacturer to be in compliance with the applicable requirements of Section V of the ASME Code.

7.15.3.2.4 Examiners who perform ultrasonic examination under this section shall be qualified and certified by the

Manufacturer as meeting the 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 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 applicable 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 distinguishing and differentiating contrast between colors used.

These requirements shall be checked annually. 

b) The examiner is competent in the techniques of the liquid penetrant examination method for which the examiner is

certified, including making the examination and interpreting 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. 
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 requirements.

a) Has vision (with correction, if necessary) 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 color contrast test. Examiners shall be checked annually to 

ensure that they meet this requirement.

b) Is competent in the technique of the visual examination, including performing the examination and interpreting and 

evaluating the results; however, where 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 accordance with 7.15.5.3 and 7.15.5.4. 

7.15.5.3 A weld shall be acceptable by visual examination 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. of

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 reworked 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 resulting thickness is below the minimum 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 followed as closely as is practicable when the spot 

examination is made by radiography. A spot radiograph shall not be considered equivalent to a recheck where 

complete radiography is mandatory and applied. 

7.15.6.2 Spot radiographic or ultrasonic examination 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 prepared 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 with the standards of acceptability 

given in 7.15.1.3. Spot radiographs or ultrasonic records may be discarded after 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 standards, 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 specific requirements as listed in Sections 1 through 9, and Annexes Q, R, S, and X shall 

be followed in all cases.
7-6 API STANDARD 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 joining tank wall plates to tank wall plates. Examination for quality of welds

shall be made using either the radiographic method specified in 7.15.1 and applied in 7.17, or alternatively, by

agreement between the Purchaser and the Manufacturer, using the ultrasonic method specified 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 Inspector may visually inspect all butt-welds 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. Acceptance 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 flush type cleanouts. It does include items

such as wind girders, stairs, gauging systems, davits, riser pipe supports, tank anchors, walkways, supports for

internal items such as heating coils or other piping, ladders, floating roof supports welded to the shell wall, and

electrical conduit and fixtures. Items installed above the maximum liquid level 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 temporary attachment is removed shall be

examined visually for any indication of flaws requiring repair. 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 method (or at the option of the Purchaser, 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 testing of the tank, welds attaching nozzles, manholes, and cleanout

openings shall be examined visually and by the magnetic particle method (or at the 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 excess 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 examination 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 which these rules apply, and for which complete examination

is mandatory under 5.26, shall be examined throughout its entire length by the radiographic or ultrasonic method as

prescribed by the following sections. Any butt-welded joint for which complete examination would not be mandatory

under 5.26.3 shall be similarly examined if the procedure becomes mandatory in the application of 5.26.4. 
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 7-7

7.17.1.2 If radiographic or ultrasonic examination is considered impractical for the final (or closing-up) joint because 

of the location or construction of that 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 welding 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 radiographic 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 examined by the magnetic-particle 

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 completely examined, spot examination is 

mandatory and shall be done according to the procedure and standards 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 structural steel members unless specifically 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 mandatory under 7.17.2.1, the number and location of spots 

examined in longitudinal or meridional joints and in equivalent circumferential or latitudinal joints as defined in Table 5-

2, Note 5, shall conform to the requirements 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 ft 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 examined for each additional 50 ft—or remaining fractional part of this length—

of each type and thickness of welded longitudinal, meridional, or equivalent joint subject to examination. The 

inspector shall designate the locations of all spots that are to be examined, of which at least 25 % of the selected 

spots shall be at junctions of meridional and latitudinal joints with a minimum of two such intersections per tank (see 

7.15.6.2), both under the foregoing provisions and the provisions of 7.17.3.4. Such 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 operator 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 welding operators with one spot examination 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 required for circumferential or latitudinal joints 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 joint 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, without regard to the number of welders or welding operators 

working, one additional spot shall be examined in each additional 100 ft (approximately) and any remaining fraction 

thereof of each type and thickness of welded circumferential 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 this application, plates shall be considered of 

the same thickness when the difference in the specified or design thickness does not exceed 1/8 in.
7-8 API STANDARD 620

7.17.4 Spot-examination Retests

7.17.4.1 When a spot has been examined at any location selected in accordance with 7.17.2 and the welding does 

not comply with the standards prescribed in 7.15.1.3 for 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 comply with the minimum quality requirements in 7.15.1.3 radiographic examination, 

or Annex U for ultrasonic examination, additional spots nearby shall be examined until the limits of unacceptable 

welding are determined. In addition, the inspector may require that an additional spot be examined at one location 

selected by the inspector in each seam not previously examined on which the 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 spot examination found to be below the standards required in 7.15.1.3 for 

radiographic examination, or Annex U for ultrasonic examination, 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 defective welding need be corrected.

7.18 Standard Hydrostatic and Pneumatic Tests 

7.18.1 General

After erection is completed and stress relieving, radiographic examinations, or other similar operations, as may be 

required, 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 specified in this section to be applied to welding, linseed oil or 

another equivalent material for disclosing air leakage may be substituted. 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 preliminary operations described in 7.18.2.2 through 7.18.2.6 

shall be performed.

7.18.2.2 The attachment welding around all openings and their reinforcements in the walls of the tank 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 examination and testing of any openings in the 

bottom plates shall be done before these plates 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 reinforcing 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 reinforcement, both inside and outside the tank.

7.18.2.4 In cases in which the bottom of the tank rests directly on the tank grade (preventing access to the 

underside of the bottom of the tank), all joints between the bottom plates shall be tested on the inside of the tank by 

applying a solution film to the joints and pulling 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 testing, 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. 
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 tank shall be filled with air to a pressure of 2 lbf/in.2

gauge or one-half the pressure Pg for which the vapor space at the top of the tank is designed, whichever pressure is 

smaller. A solution film shall be applied to all joints in the tank wall above the high liquid (capacity) 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 boundary of contact to hold down a dished tank bottom 

resting directly on the tank grade, the bottom at this boundary may be rise 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 capacity 

level (see 5.3.1.2) shall be subjected to combination hydrostatic-pneumatic pressure 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 pressure-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) design level (see 7.18.7). Tank anchor retainers shall be 

adjusted to a uniform tightness after the tank is filled with water. If the pressure-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 the tank closed, air shall be 

injected slowly into the top of the tank until the pressure in the vapor space is about one-half the pressure Pg, 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, Pg, 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 tank 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 pressure for which the vapor space is designed, the inspections 

should be made at a reasonable distance from the tank using 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 distress. The maximum test 

pressure of 1.25 times the vapor space design pressure shall be held for at least one hour, after which the pressure 

shall be released slowly and the blinds shall be removed from the pressure-vacuum 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, Pg, for which this space is designed, 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 vacuum box check may be substituted for the 

close visual with solution-film examination. The solution-film examination shall still be made, 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 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 1/2 of the conical 

compression ring thickness and greater than two times the adjacent shell ring thickness.

— Full fusion butt-welded connections.
7-10 API STANDARD 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 with 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 pressure-vacuum relief 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 escape 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 roof is 1.25 times the pressure, Pg, 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 hydrostatic pressure under the topmost point in 

the roof shall then be reduced to the pressure, Pg, 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 water level shall be lowered below the inlets to the 

pressure-relief valves, and the blinds shall be removed from the 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, Pg, 

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) where this latter procedure has been used), the 

pressure in the vapor space of the tank shall be released and a manometer shall be connected to this space. The 

ability of the upper part of the tank to withstand the partial vacuum for which it is designed and the operation of the 

vacuum-relief valve or valves on the tank shall then be checked 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 valve 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 never exceed the design value (see Annex K).

7.18.5.2 After completing 7.18.5.1, the withdrawal of water from the tank 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 either be kept closed and air pressure not exceeding Pg at the 

top of the tank applied, or the vents may be opened during most of this interval if in either 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, Pg, for which the vapor space at the top of the tank is designed. 

7.18.5.3 Careful observation shall be made under all of the specified conditions of loading, as well as with 

atmospheric pressure above the surface 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 ejector or vacuum pump is not available for 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.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 7-11

sidewalls, no tests are required with the water level at half height; in this case, the tests specified in 7.18.5.4 shall be 

applied immediately after the first vacuum test specified in 7.18.5. 

7.18.5.4 The water remaining in the tank shall then be withdrawn and when the tank is substantially empty, a 

vacuum test comparable to that specified in 7.18.5.1, except with regard to the level of water in tank, shall be applied 

to the tank. After this, air shall again be injected into the tank until the pressure in the tank equals the pressure, Pg, 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 tank occur under either condition of loading. 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 bottom 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 tank 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 shall be checked for snug tightness 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 hydrostatic test shall not exceed 3 ft of depth per hour. The 

foundation, venting equipment, or other conditions 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 bottom of the tank around the boundary of contact with 

grade, or off its foundations, or if any of the specified conditions of test loading cause other appreciable changes in 

the shape of the tank, the design shall be reviewed and means shall be provided 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 tanks constructed according 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 made 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 plates or on internal ties, braces, or other 

members that carry pressure-imposed loads, the test specified in 7.18.3 (or the test specified in 7.18.4, if applicable) 

should be repeated periodically during the lives of the tanks 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 from the tops to the 

bottoms of the tanks, proof testing of these tanks presents problems not usually encountered in the construction of 
7-12 API STANDARD 620

unfired pressure vessels–especially where the parts under investigation are located near the bottoms of the tanks. 

The principal difficulty is devising a test or series of tests that will reliably establish the working pressure that can be 

permitted on the part of the unproven design without, at the same time, imposing hazardous conditions on other parts 

located at higher levels in the tank. Another 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 conditions after successive increments of pressure have been 

applied in the test procedure. Also, in the case of tanks 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 the design section of these rules will be found to cover all of the more common 

designs of vertical tanks, shapes of openings, and so forth. The absence of a standard proof-test procedure will not 

greatly affect the usefulness of these rules. Whether a standard proof-test procedure can be devised that will be 

applicable to all shapes, sizes and types of tanks that might be constructed under these 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 Manufacturer desires to 

construct a tank that will be marked as specified in 8.1 and embodies any features that should be proof-tested 

because of the provisions of 5.1.5 and 5.13.5.3 or any other provisions of these rules that call for proof test or straingauge surveys, the Manufacturer shall develop specifications for an appropriate proof-test procedure for the tank and 

obtain approval from the Purchaser or the Purchaser’s agent, preferably before starting 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 for the test, how the test loadings 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 seeking 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 the 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-test procedure, 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 topmost part of the roof on the 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 indicating 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 ensure that the required test pressure 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 during all 

stages of the tests. This gauge shall be connected either to the piping that leads to the indicating 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 calibrated against a standard deadweight tester before the 

tests are started. 
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 7-13

7.20.4 If at any time during a test there is reason to believe that a gauge is in error, its 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 lower than its connection to the tank or lower than some 

part of the piping that leads to the gauge, suitable precautions 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 similar to that shown in 

Figure 8-1. The nameplate shall indicate, by means of letters and numerals not less than 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 space 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 given by the Purchaser for refrigerated product tanks.

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 accordance with 5.25 or R.7.3, the nameplate shall be marked “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 information may be 

shown on the nameplate. The size of the nameplate may be increased accordingly. 

8.1.3 The nameplate shall be attached to the tank shell adjacent to a manhole or to a manhole reinforcing plate 

immediately above the manhole. A nameplate that is placed directly on the shell plate or reinforcing plate shall be 

attached by continuous welding or brazing all around the plate. A nameplate that is riveted or otherwise permanently 

38 Unless other units are specified by the Purchaser.
8-2 API STANDARD 620

attached to an auxiliary plate of ferrous material shall be attached 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 organization, that organization’s name shall appear on the 

nameplate as both fabricator and erector. 

8.1.5 When a tank is fabricated by one organization and erected by another, the names of both organizations shall 

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 having the primary responsibility. The Manufacturer 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, examinations, and inspections on each tank, the Manufacturer shall prepare a 

report summarizing all the data on the tank, including foundations (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 for each tank (such as that shown 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 certificate, together with the nameplate or markings placed on the 

tank, shall guarantee that the Manufacturer has complied with all applicable requirements of these rules. 

8.3.3 If the Purchaser so requests, the Manufacturer shall attach to the report copies of the records of the 

qualification test of welding procedures, 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 tanks or compartments designed and built according to the rules 

of this standard, each tank or compartment in the assembly shall be marked separately, or the markings may be 

grouped at one location and arranged so that the data for the 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

DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 8-3

9-1

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 the pressure at the top of 

the tank from rising more than 10 % above the maximum positive gauge pressure except as provided in 9.2.2 (see 

Annex K).

9.2.2 Where an additional hazard can be created by the exposure of the tank to accidental fire or another 

unexpected source of heat external to the tank, supplemental pressure-relieving 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 pressure-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 entry of air (or other gas or vapor is so designed) to 

avoid collapse of the tank wall if this could occur under natural operating conditions. These devices shall be located 

on the tank so that they will never be sealed off by the contents of the tank. 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 the 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 liquid, shall be equipped with one or more liquid relief valves at 

the top of the roof, unless otherwise protected against overpressure. When such valves are, in effect, supplementary 

relief devices, they may be set at a pressure not greater than 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 greater than 1.25 times the design 

pressure of any tank that may be built under these rules, provision should be made for preventing overfilling of the 

tank by a self-closing float valve, by some practicable pilot-valve control, 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 STANDARD 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 ft3 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 these rules, the safety- or relief-valve inlet pressure is 

so low in relation to the outlet pressure that valve capacities predicted on acoustic velocity 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 condition 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, formulas of the type given in Section VIII, Appendix 11, of the 

ASME Code are not appropriate for computing safety- or relief-valve capacity conversions. Where this condition 

exists, the valve manufacturer should be consulted concerning the size of the valve or valves required for the desired 

capacity in terms of the specific gas or vapor to be handled, the set pressure 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 given in the inquiry to the manufacturer.

9.6.2 Liquid Relief Valves

Each liquid relief valve shall be marked with the following data:

a) name or identifying trademark of the manufacturer;

b) Manufacturer’s design or type number;

c) size of valve, in inches (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 ft3 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 valves, the pressure 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 pressure or partial vacuum that the partial vacuum in 

the tank cannot exceed that for which the 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 Suggestions for Change

A.1 Introduction

This Annex describes the process established by API for 1) submitting inquiries 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 standard. API will attempt to answer all valid inquiries. 

Submittals not complying with this Annex will be returned unanswered.

Sections A.2 through A.8 below cover the submitting of inquiries. See Section 9 for instructions about submitting 

suggestions for change.

A.2 Inquiry References

A.2.1 API maintains several websites that provide information that should be reviewed before submitting an inquiry.

A.2.2 Your inquiry may have been previously addressed by the Subcommittee and the resulting interpretation 

posted on the API web site as follows: 

For all standards: http://mycommittees.api.org/standards/techinterp/default.aspx

For Refining Standards: http://mycommittees.api.org/standards/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 addressed 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/standards/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 table 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 its 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 STANDARD 620

b) API does not act as a consultant 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 request 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 latest, the Subcommittee will 

develop the interpretation based on the requirements stated in the current version.

b) Specific Reference: The applicable paragraph number, figure number, or table number must be cited in the inquiry.

c) Sentence Structure: Inquiries must be written such that the answer can be a YES or NO, with technical details 

added if necessary. The inquiry statement should be technically and editorially correct, and written in 

understandable English. 

d) Background: Providing a background explanation is optional, but is encouraged to assist the committee in 

understanding 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 Paragraph 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 process to improve his general understanding, design skills, and usage of 

the standard. Consultants not affiliated with API are available for this purpose. 

h) It is important that the Inquirer understand the difference between an inquiry and a suggestion for change. API 

encourages both, but the submittal and committee 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/Publications-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 background information, in full compliance with this 

Annex and submit to the API Standards Coordinator.

b) API Standards Coordinator checks the inquiry to verify compliance with the requirements of submitting an inquiry.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 checklist will typically be used for this response.

d) If the Coordinator believes the inquiry is valid, it will be forwarded to the Subcommittee 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 by the Subcommittee. 

A.6.2 The time required to process a valid inquiry as described in 6.1 may take as long as a year.

A.7 Interpretations Responding to Inquiries

A.7.1 An interpretation is written by the Subcommittee to provide the specific answer to an inquiry. 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 phrased, 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 immediate needs, the industry benefits 

as a whole when inquiries are utilized as a means of trying to understand the technical 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 simply a communication (email preferred) from a reader to 

API proposing that a specific change be made to the standard.
A-4 API STANDARD 620

A.9.2 Any format is acceptable, as long as the content is clear.

A.9.3 The most effective means to submit suggestions is to send an email to the API Coordinator 

(standards@api.org). 

A.9.4 The content of a suggestion must include the standard number, edition, and addendum in question. The 

relevant paragraph numbers, table number, figure number, etc must also be stated. Provide as much explanation as 

necessary to be sure the Subcommittee understands the technical issues. Provide specific 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 that are suitably written to the Subcommittee for consideration. 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 advising the submitter of the 

Subcommittee’s decision may not be issued. 
B-1

Annex B

(normative)

Use of Materials That are Not Identified 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 appears in this annex, it shall refer to a material specification that is listed as

being approved for this standard.

B.2 Materials with Authentic 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 restrictive 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 results shall be submitted to the

Purchaser for approval.

B.3 Materials without Authentic 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 shall 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 physical tests required in the designated

specification, with the following modifications: The carbon and manganese contents shall be determined in all check

analyses. The Purchaser shall decide whether these contents are acceptable when the designated specification does

not specify carbon and manganese limits. When the direction of rolling is not definitely known, two tension specimens

shall be taken at right angles to each other to form a corner 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 given heat or heat-treatment lot and that the chemical and physical

requirements of the designated specification are compiled with. 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 identified with a designated specification and the Purchaser has been satisfied that

the material complies with the specification in all respects, the testing agency shall stencil or otherwise mark, as

permitted by the material specification, a serial S-number on each plate or each length of pipe (or as alternately

provided for small sizes in the specification) in the presence of the Purchaser.
B-2 API STANDARD 620

B.5 Report on Tests of Non-identified Materials

Suitable report forms that are clearly marked as being a report on tests of non identified materials shall be furnished 

by the tank Manufacturer or testing agency, properly filled 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 which the nonidentified material 

corresponds shall be followed, and the allowable 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 practices 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 minimum checks of subgrade included in this

section will usually prove worthwhile. 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, confirms the load-bearing capacity to be selected.

No set of rules can cover all possible combinations of subgrade loading conditions. Types of subgrade structures and

the final design of the finished installation may be affected by groundwater or local climatic changes.

Many large vertical storage tanks have been built with cylindrical shells and flat bottoms that rest directly on simply

prepared subgrade. In the case of unequal settlement, a releveling 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 for erection of tanks constructed 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 storage are located near large streams, where the areas to be

built on are alluvial deposits. These deposits are usually interspersed with gravel and sand, all affected by previous

changes in the course of the stream, so that both the character and depth of the composite layers have no uniformity.

The recommendations made in this section will therefore omit any reference 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 can 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 electrolytic 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 fill material is available at the site. If not, bank run gravel is excellent and is

readily compacted.

The grade for the tank shall be elevated slightly above the surrounding terrain 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 minimized if the berm is subsequently

protected with trap rock, gravel, 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 within 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 STANDARD 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 precaution shall be taken to ensure that the settlement will be kept to an 

acceptable minimum and that any settlement 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 logical procedure, the piling design factors would be well known 

to the engineer charged with making the decision 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, for 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 tank and its contents when the tank is filled with water 

to the highest level required for a hydrostatic 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 short duration and intermittent nature of the 

water loadings if suitable account is taken of all such loadings 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 which the distribution of the imposed weights shifts because the 

liquid level in combination with the vapor pressure may change 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 assumption that suitable field tests will be made if borings or 

test pits, or both, do not give satisfactory information on the depth required.

Determination of the allowable maximum soil-bearing 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 soft or 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 resorted 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 considerable area is wanted. All field 

data shall be recorded with maps, and copies shall be supplied to all engineers concerned with design, erection, and 

later operation.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS C-3

C.6 Minimum Depth of Footings

The depth of the bottom of footings shall be determined by local subgrade conditions. The base of these footings shall 

be placed below the expected frost line, away from any nearby excavations, and below any nearby sewers or piping 

which, if leaky, could cause serious 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 otherwise 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 through 

C.8.7 are suggested.

C.8.2 The lowest footing course shall be bedded directly against the sides of the excavation when the sides are selfsupporting. Before the concrete is poured, adjacent dry soils 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 least 6 in. above the final grade to be provided, and the tops 

of the pedestals and other foundations to support steelwork shall 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. outside of any steel baseplates of the 

superstructure.

C.8.5 The exposed surfaces, other than the tops of concrete pedestal and wall foundations, shall be smooth 

finished down to 6 in. below the proposed final grade. Any small 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 tanks, 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 foundations 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 the backfill, except where no clay is present and quick drainage is assured by 

the general contours. Bulldozers, scrapers, and crane-bucket discharge may be used if they are kept completely clear 

of the pedestals and walls.

If special adverse conditions are met, a foundation engineer shall be consulted regarding compaction control.
C-4 API STANDARD 620

Particular attention shall be given to surface regrading around the finished structure to permit efficient erection of the 

superstructure and to provide proper drainage that is consistent with the records of local weather conditions.

The finished grade under a flat-bottom tank shall be crowned from the periphery to the center. A slope of 1 inch in 10 

ft is suggested as a minimum. This crown will partly compensate for slight settlement, 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 subgrade, 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 the tank erector and the Purchaser’s engineering representative. If at any time 

any questionable amount or rate of settlement does occur, further filling of the tank shall 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 changes in the shape of the tank walls.

If a minor amount of settlement is observed during the course of the filling operation and still continues after a tank is 

filled to the highest level required in the 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 compaction.

C.12 References

1) Standard for Welded Steel Elevated Tank, Standpipes, and Reservoirs of Water Storage, AWS-AWWA D5.2.

2) “Oil Storage Tank Foundations,” Technical Bulletin, Chicago Bridge and Iron Co., March 1951.

3) K. Terzaghi and R.B. Peck, Soil Mechanics in Engineering Practice, John Wiley and Sons, Inc., New York, 

1948.
D-1

Annex D

(informative)

Suggested Practice Regarding 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 where the supports are attached. When tanks of certain shapes

are subject to internal pressure, secondary stresses may exist in the wall adjacent to the attachment of such supports

that are lower than when the tank is filled with liquid before any pressure is imposed other than that caused by the

static head. Methods for calculating the forces involved are 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 Supporting Structures

D.2.1 The details of supports shall conform 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 liquid-level conditions within the tank. Any arrangement of

the structure that does not permit a reasonably 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 framing may exert a stiffening effect on the tank wall where

exterior supporting members of the tank are to be attached. This stiffening effect may be beneficial or harmful,

depending on the operating temperature and the 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 secondary 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 standard 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 the 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 that 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 fabrication of the structural supports, but whenever the

supports are attached by welding, the parts joined shall be weldable. These welds shall not introduce any

objectionable conditions at or near the attachment, including hard 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 Values 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 constructed 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 latitudinal direction and to

compression in a meridional direction. The values of R1 and R2 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, scc, 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 R2, 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 intersection with the horizontal line for sc = 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, Sts, for ASTM A131, Grade B, Steel plate in simple tension is 16,000 lbf/in.2. Therefore,

the maximum allowable tensile stress, sta, for the conditions cited in this example is as follows:

sta = NSts

= (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. However, 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, ESts,

would be only 0.85 × 16,000 = 13,600 lbf/in.2 determined from Figure 5-1, would govern.

Alternatively, sta could be determined by entering the computed value of M in Figure F-1 and obtaining the allowable

coexistent value of N. For this particular example, the value of Scs would be 15,000 lbf/in.2. Hence,

t c –

R

--------- 0.75 0.0625 –

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

M

Scc

Scs

------ 3 400 ,

15 000 , == = ----------------- 0.227
F-2 API STANDARD 620

NOTE An initial check shall be made to assure that the actual compressive stress, scc, 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,

Sta = NSts

= (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 thickness 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, scc, in the net 

thickness after deduction of the corrosion allowance is 4600 lbf/in.2.

F.1.2.2 Problem

The problem in 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.1.2.3 Solution

Since the compressive stress is meridional, the governing value of R in this situation is R2, 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 sc = 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 fact, the compressive stress of 4600 lbf/in.2 greatly exceeds the allowable stress, 

Scs, of 2840 lbf/in.2 for simple compression for the thickness-to-radius ratio involved. Consequently, either the 

thickness must be increased or the 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 A285, 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 R1 and R2 at the point under consideration are 75 in. and 300 in. 

respectively. A corrosion allowance of 1/16 in. is required. The computed (meridional) tensile stress, stc, in the net 

thickness after deduction of the corrosion allowance is 6000 lbf/in.2.

F.1.3.2 Problem

The problem in this example is to find the maximum allowable (latitudinal) compressive stress 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 1.0

Compression factor, 

M

Tension factor, N

N2 + MN + M2 = 1

or

(St /Sts)

2 + (Sc /Scs) (St /Sts) + (Sc /Sts)2 =1

(St /Sts);

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;

(Sc/Scs);

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

St

Sts

M

Sc

Scs

=

=

=

=

=

=

where

DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS F-3

F.1.3.3 Solution

As determined from Table 5-1, the maximum allowable tensile stress value, Sts, 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 R1, or 75 in. Then,

t c –

R

--------- 0.626 0.0625 –

75 = = ----------------------------------- 0.0075
F-4 API STANDARD 620

The value N = stc/Sts = 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 vertically at this value until it intersects with an N curve 

that represents the value N = 0.364; proceeding horizontally from this point to the left-side ordinate scale, the value sc

= 11,500 lbf/in.2 should be read. In this case, the value represents the allowable compressive stress, sca.

Alternatively, sca 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, sca, could be calculated by substituting 

this value of M in the equation sca = 15,000M. Thus, sca = 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 

follows: 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 thickness 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 computed (meridional) tensile stress, stc, 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 allowable (latitudinal) compressive stress value for the given 

conditions, in conformance with the provisions of 5.5.4.5.

F.1.4.3 Solution

As determined from Table 5-1, the maximum allowable tensile stress value, Sts, 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 R1, or 75 in. Then,

t c –

R

--------- 0.375 0.0625 –

75 = = ----------------------------------- 0.00415

The value N = stc/Sts = 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, 

sca, for this particular value of (t – c)/R. A higher value of tensile stress is permissible, since the allowable coexistent 

value of N equals 0.65. Thus, in this particular example, the allowable compressive stress is governed by the (t – c)/R

value rather than by the coexistent tensile stress.

F.2 Determination of Minimum Required Thicknesses for Walls Subject to Biaxial Tension 

and Compression

F.2.1 Example 1

F.2.1.1 Given Conditions

In this example, an elemental area of tank wall used is constructed of ASTM A442, Grade 55, steel plate subjected to 

a meridional unit force, T1, of 4000 lbf/in.2 tension and a latitudinal unit force, T2, of 5060 lbf/in.2 compression. The 

meridional radius of curvature, R1, is 75 in., and the length of the normal from the surface to the axis of revolution, R2, 
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 minimum thickness of tank wall required for the given conditions 

(see 5.10.3.3).

Table F-1—Computed Values of (t – c) R, sc, st, and N for the Assumed Thicknesses: Example 1 (See F2.1.3)

Assumed 

Thickness, 

t (in.)

t – c

R1

st

T2

t c – = --------- st

T1

t c – = --------- N

st

Sts

= -----

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, Sts, 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 R1, or 75 in.

A series of four or five different plate thicknesses should be assumed, covering the range in which the required 

thickness will probably be found. The values of (t – c)/R, sc, st, and N shall be computed and tabulated for each of the 

assumed thicknesses, as shown in Table F-1.

The values of sc 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 points 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 curves represents the minimum (t – c)/R value that will satisfy both the compressive 

stress and tensile stress limitations involved in this example. At this point sc (which is equivalent to sca 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 st (or sta) for the conditions under consideration is 

equal to the value of NSts, or 0.480 × 16,500 = 7920 lbf/in.2.

t

T1

Sta

----- + c

4000

7920 == = ----------- + 0.505 0.063+ 0.063

= 0.568 in.

or

t

T2

Sca

------ + c

5060

10 000 , == = ----------------- + 0.506 0.063+ 0.063

= 0.569 in.

40 Figure F-3 is a copy of this chart without the illustrative example lines. It may be reproduced by the designer for sure in graphical 

solutions.
F-6 API STANDARD 620

or

t

t c –

R

---------     = = R1 + 0.0067 c ( )( ) 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 constructed of ASTM A516, Grade 55, steel plate subjected to a 

meridional unit force, T1, of 2620 lbf/in. tension and a latitudinal unit force, T2, of 2880 lbf/in. compression. The 

meridional radius of curvature, R1, is 132 in., and the length of the normal from the surface to the axis of revolution, 

R2, 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 minimum thickness of tank wall required for the given conditions 

(see 5.10.3.3).

F.2.2.3 Solution

As determined from Table 5-1, the maximum, allowable tensile stress value, Sts, 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 R1, 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, sc, st, and N shall be computed and tabulated for each 

assumed thickness, as shown in Table F-2. 

The value of sc 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 points in 

this area is prohibited. All values of N in the vicinity of these two curves are well below the efficiency, E, of the type of 

joints involved; thus, the allowable compressive stress is obviously the critical factor in this problem. A point must be 

found where the computed compressive stress, represented 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 sc = 6300. This value 

is the allowable compressive stress, sca, for the conditions given in this example. Therefore,

t

T2

Sca

------ + c

2880

6300 == = ----------- + 0.457 in. 0

This value is the minimum required thickness. The computed tensile stress for 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.
1.0

0.90.80.70.60.50.40.30.20.1

0

1.00.90.80.70.60.50.40.30.20.10

0

1,0002,0003,0004,0005,0006,0007,0008,0009,00010,00011,00012,00013,00014,00015,000

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

S Factor N - (tensile stress, t S )/(maximum allowable tensile stress, ts)

(t - c)/

R ratio (see Note 1)

X

X

W

W

U

U

V

V

NOTE 1 If compressive stress is latitudinal, use 

R = 

R1, if 

compressive stress is meridional, use 

R = 

R2

.

NOTE 2 The sc curves on this chart may be extended into the area to the left of line A-A merely for convenience in making graphical solutions of problems such as the one illustrated in F.2.2 (Example 2). None of the points in this area represent valid allowable stress values.

Example 2 (F.2.2)

Example 1 (F.2.1)

Factor N

Compressive stress, sc1, in psi

A

A

Figure F-2—Examples Illustrating the Use of a Biaxial Stress Chart for Combined Tension and 

Compression, 30,000 to 38,000 Pounds per Square Inch Yield Strength Steels

DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS F-7
Factor N

1.00.90.80.70.60.50.40.30.20.1

0

1.00.90.80.70.60.50.40.30.20.10

0

1,0002,0003,0004,0005,0006,0007,0008,0009,00010,00011,00012,00013,00014,00015,000

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

(t - c)/

R ratio (see Note 1)

NOTE 1 If compressive stress is latitudinal, use 

R = 

R1, if 

compressive stress is meridional, use 

R = 

R2

.

NOTE 2 The sc curves on this chart may be extended into the area to the left of line A-A merely for convenience in making graphical solutions of problems such as the one illustrated in F.2.2 (Example 2). None of the points in this area represent valid allowable stress values.

Compressive stress, sc1, in psi

A

A

S Factor N - (tensile stress, t S )/(maximum allowable tensile stress, ts)

Figure F-3—Form for Use in Graphical Solutions of Problems Involving Biaxial Tension and 

Compression, 30,000 to 38,000 Pounds per Square Inch Yield Strength Steels

F-8 API STANDARD 620
Table F-2—Computed Values of (t – c)R, sc, st, and N for the Assumed Thicknesses: Example 2 (See F.2.2.3)

Assumed 

Thickness, 

t (in.)

t – c

R1

st

T2

t c – = --------- st

T1

t c – = --------- N

st

Sts

= -----

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

DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS F-9

F.3 Determination of Minimum 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-supporting roof with varying values for R1 and 

R2. The size and vacuum settings of the 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/ft2. 

The design requirements include a live snow load of 25 lb/ft2 on the horizontal projection of the surface of the roof, 

which has a slope of 30° or less 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 R1 = R2 = 1200 in. and (b) at a radial distance of 12.5 ft from the center of the roof, where R1

= 1117 in. and R2 = 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 consideration. Specific 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 given the same sign 

as P in this case because it acts in the same direction 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/ft2.
Figure F-4—Free-body Sketch (See F.3)

T1

T1

R1 - R2

R1

R2

P

Insulation load

Snow load

Axis of revolution

(b) (a)

F-10 API STANDARD 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,

T1

1200

2 = -----------[ ] ( ) – 0.426 0.40 –

 = –495.6 lbf/in.

T2 = 1200 (–0.40 – 0.426) – (–495.6)

= –495.6 lbf/in.

From 5.10.3.4, using Equation 17,

t

495.6

sca

= -------------

where

sca equals 1,000,000[(t – c)/R].
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS F-11

Substituting sca = 1,000,000 [(t – c)/R] for sca in Equation 17 yields the following:

( ) t c – 2 495.6R

1 000 000 , , = --------------------------

t ( ) 495.6 ( ) 1200

1000 = = --------------------------------------- + 0.834 in. 0.063

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 the 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 equivalent to a unit weight of 33.2 lb/ft2.

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 consideration shall be used in the preceding calculation of 

W; however, from a practical standpoint, in this example the difference between the actual surface area and the area of the 

horizontal plane bounded by the free-body is relatively 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/ft2 ÷ 144; 

cos θ = 0.229 lb/in.

b) For insulation, 2.0 lb/ft

2 ÷ 144; 

cos θ = 0.014 lb/in.2

c) For snow, 25.0 lb/ft2 ÷ 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 STANDARD 620

From 5.10.2.1, using Equations 1 and 2 with the foregoing normal components of load,

T1

1172

2

----------- – 0.40 –29 550 ,

( ) π ( ) 12.5 2

( ) 144 = + ---------------------------------------

= –479 lbf/in.

T2 1172 0.40 ( ) – 0.414 – –479

1117

-----------     = –

= –451 lbf/in.

From 5.10.3.5, using Equation 18 and 19 where T ́ = T1, T ́ ́ = T2, R ́ = R2, and R ́ ́ = R1:

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 thickness 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 slightly smaller than the assumed thickness of 13/16 in. (0.813 in.) and is thus 

consistent from a practical standpoint with the assumed roof loading. A recalculation using a new assumed thickness 

shall be made whenever the calculated thickness is appreciably greater than 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 diameter is designed for an internal pressure 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 spherical 
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 the juncture of the roof and sidewall.

F.4.1.2 Problem

The problem in this example is to design the compression-ring region at the juncture of the roof and cylindrical 

sidewall.

F.4.1.3 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 govern the design of the roof and sidewall because the term (W + F) ÷ At is negligible 

compared with Pg.

T1 = T2 = (0.5) (5) (360) = 900 lbf/in.

T2s = (5) (180) = 900 lbf/in.

 From 5.12.4.2 and 5.12.4.3, using Equations 24 through 27,

wh = 0.6 360 0.25 0.0625 ( ) – = 4.9 in.

wc = 0.6 180 0.25 0.0625 ( ) – = 3.5 in.

Q = (900) (4.9) + (900) (3.5) – (900) (180) (0.866)

= – 133,000 lb

Ac = 133,000/15,000 = 8.86 in.2

The area of participating width of the 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 = 7.28 in.2.

From 5.12.5.1, the required horizontal projection of the effective compression-ring region is 0.015Rc = 0.015 × 180 = 

2.7 in.

The horizontal projection of the roof plate within the compression-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 provided by any of the standard 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 part of the angle surface that is exposed to the interior of the tank. The net area of the angle must 
F-14 API STANDARD 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 0.015Rc minimum width, the 16t

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 width 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. 

Table 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-welded cylindrical sidewall is 1/4 in. thick. The roof is a conical shape with a 2:12 slope and 

a 1/4 in. thickness with single full-fillet welded lap joints. The maximum design 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 region 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. Rc = R3 = 37.5 ft = 450 in.

At the edge of the roof, R2 = 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 difference 

between the conical area of the roof and the projected area on a horizontal plane, W, can be calculated with sufficient 

accuracy by using the unit weight of 10.2 lb/ft2 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)/At = 10.2/144. W must be given a negative 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).
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS F-15

From 5.10.2.5, using Equations 8 and 9,

T1

450

( ) 2 ( ) 0.1643 ---------------------------- ( ) 0.5 –10.2144 = + -------------

= 588 lbf/in.

T2

( ) 0.5 ( ) 450

0.1643 = = ------------------------- 1370 lbf/in.

T2s = (0.5) (450) = 225 lbf/in.

From 5.12.4.2 and 5.12.4.3, using Equations 24 – 27,

wh = 0.6 2740 ( )( ) 0.25 = 15.7 in.

wc = 0.6 450 ( )( ) 0.25 = 6.4 in.

Q = (1370) (15.7) + (225) (6.4) – (588) (450) (0.9864)

= 240,000 lb

Ac = 240,000/15,000 = 16.0 in.2

NOTE The width of lap-welded roof plate, wh, 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 the compressive force or providing a width of horizontal projection 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 effective compression-ring region (see 5.12.5.1) is 0.015Rc = 0.015 × 450 = 

6.75 in.

Because the lap-welded construction of the roof may not be used to satisfy the horizontal width requirement, the 

horizontal projection of the roof plate with the compression-ring region must be provided by the added member.

Angles may not be a practical method of providing the required additional area and horizontal projection. Bars or 

channels may be furnished as illustrated in Figure 5-6, details d through g, with proper consideration given to the 

0.015Rc minimum width requirement and the requirements concerning 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 pressure 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 design metal temperature less than 65 °F but not less than 25 °F. No corrosion 

allowance is required for any portion of the tank. The 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 radius of 50 ft and a thickness of 1/4 in. The maximum 

design liquid level is 6 in. below the plane of the juncture of the roof and sidewall.
F-16 API STANDARD 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 degrees and sin α = 0.781.

Equations 7 and 13 in 5.10.2.5 govern the design of the roof and sidewall, since the term (W + F)/At is negligible 

compared with Pg.

T1 = T2 = (1/2) (4) (600) = 1200 lbf/in.

T2s = (4) (375) = 1500 lbf/in.

 From 5.12.4.2 and 5.12.4.3, using Equation (24) through Equation (27),

wh = 0.6 600 ( )( ) 0.25 = 7.34 in.

wc = 0.6 375 ( )( ) 0.25 = 5.80 in.

Q = (1200) (7.34) + (1500) (5.80) – (1200) (375) (0.781)

= –334,000 lb

Ac = 334,000/15,000 = 22.3 in.2

NOTE The width of the lap-welded roof plate, wh, 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 the compressive force or providing a width of horizontal projection 

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 participating 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, detail e.

From 5.12.4.2 and 5.12.4.3, using Equations 24 through 27,

wh = 0.6 600 ( )( ) 1 = 14.7 in.

wc = 0.6 375 ( )( ) 0.25 = 5.80 in.

Q = (1200) (14.7) + (1500) (5.8) – (1200) (375) (0.781)

= – 325,160 lb

Ac = 325,160/15,000 = 21.68 in.2

The area of the participating width 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.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS F-17

Since the required area, Ac, 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 16t 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 horizontal projection of the effective compression-ring region is 0.015Rc = 0.015 × 375 = 

5.62 in.

The horizontal projection of the compression ring is 14.7 × 0.781 = 11.5 in., which fulfills the requirement 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 Single 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, tw, is 1/2 in. No corrosion allowance is 

required. The total internal pressure, Pl + Pg, 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 the latitudinal unit forces, T2, 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 tank wall at the horizontal centerline of the opening is calculated as follows:

tr = ( ) 0.485 ( ) 0.85 = 0.412 in.

The thickness required for the semicircular ends of the manhole neck and for the roundabout stresses in the flat 

portions of the neck for the pressure at the horizontal centerline of the opening is calculated as follows:

trn

( ) 27.5 ( ) 10

( ) 18.000 ( ) 0.85 = = ------------------------------------ 0.018 in.

NOTE A thickness of 0.17 in. is required in the flat portions of the neck for the stresses that are parallel to the axis of the 

manhole; these stresses result from the beam action of the elements 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 trn 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

/

8 in.

3

/

8

 in.

3

/16 in.

diameter

45 ft

Limits of

reinforcement

5 in.

Assumed critical

section for computing

attachment welds

A

A Section A-A

F-18 API STANDARD 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 minimum requirements of Figure 5-8, Panel k.

The area of reinforcement requirement at and between the vertical centerlines of the semicircular ends is calculated 

as follows:

Ar = (20)(0.485)(0.85) = 8.25 in.2
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS F-19

The areas of reinforcement provided are as follows:

a) From excess thickness in the tank wall,

A1 = 20 × (0.5 – 0.412) = 1.76 in.2

b) From excess thickness in the manhole neck,

A2 = 2 × 1.25 × (0.375 – 0.018) = 0.89 in.2

c) In fillet welds,

A3 = 4 × 0.5 × (0.375)2 = 0.28 in.2

d) In the reinforcing pad,

A4 = (32 – 20.75) × 0.5 = 5.62 in.2

The total area of reinforcement provided is 8.55 in.2, which is adequate.

The allowable unit stress values for the attachment elements are as follows:

a) For the outer and inner fillet welds,

18,000 × 0.60 = 10,800 lbf/in.2

b) For shear across the groove weld,

18,000 × 0.8 × 0.875 × 0.75 = 9450 lbf/in.2

c) For tension across the groove weld,

18,000 × 0.875 × 0.75 = 11,810 lbf/in.2

d) For shear in the manhole neck,

18,000 × 0.8 × 0.875 = 12,600 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 – 0.375 ( ) 2 ( ) 5 ( )( ) 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 STANDARD 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 added reinforcement to the tank wall, is calculated as 

follows:

(8.25 – 1.76)(18,000) = 116,900 lb

The combined strength of Elements 1, 3, and 5 is as follows:

115,300 + 38,600 + 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 of

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 Elements 1 and 4 is a follows:

115,300 + 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 reinforcement in the 

manhole neck, is as follows:

(0.89) (18,000) = 16,000 lb

The strength of either of these elements 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 where the thickness of the wall plate, tw, is 11/2 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, Pl + Pg, 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 the latitudinal unit forces, 

T2, acting at this level is 1.44 in. The nozzle 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 ASTM 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.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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:

tr = (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:

trn

( ) 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 this requirement does not affect the value of trn 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 calculated 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 the tank wall and the nozzle neck,

(0.7)(0.5 – 0.1) + 0.1 = 0.38 in.

d) For the groove weld between the pad and the nozzle neck, 

(0.7)(0.5 – 0.1) = 0.28 in.

Therefore, the weld sizes used meet the minimum requirements of Figure 5-8, Panel l.

The area of reinforcement required at the vertical centerline of the opening is calculated as follows:

Ar = [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,

A1 = (20 + 0.2) × (1.5 – 0.1 – 1.34) = 1.21 in.2

b) From excess thickness in the nozzle neck,

Ac = 2 × 2.5 × (0.5 – 0.1 – 0.018) = 1.91 in.2
Figure 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

/

2 in.

5 in.

11

/

2

 in.

3

/

8

 in.

5

/

8

 in.

11

/

2

 in.

Limits of

reinforcement

10.1 in.

20 in.

365

/

8 in.

20.2 in. 20.2 in.

5

/

8

 in.

5

/

18 in.

0.1 in. corrosion

3 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 STANDARD 620

c) In fillet welds,

A3 = 2 × 0.5 × [(0.625)2 + (0.375)2] = 0.53 in.2

d) In the reinforcing pad,

A4 = 1.5 × (36.625 – 21) = 23.44 in.2

The total area of reinforcement provided is 27.09 in.2 which is adequate.

The allowable unit stress values for the attachment elements are as follows:

a) For the outer and inner fillet welds,

16,500 × 0.60 = 9900 lbf/in.2
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS F-23

b) For tension across the groove welds,

16,500 × 0.875 × 0.70 = 10,100 lbf/in.2

c) For shear in the nozzle neck,

16,500 × 0.8 × 0.875 = 11,500 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 tank 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 nozzle 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 reinforcement to the tank wall, is calculated as 

follows:

(27.07 – 1.21)(16,500) = 426,700 lb

The combined strength of Elements 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 Element 1 would become the 

following:

π

2

--    ( ) 36.625 – 1 ( ) 2 ( ) 5.05 ( ) .0 ( ) 0.707 ( ) 9900 = 331,800 lb

The combined strength of Elements 1 and 5 would now become the following:

331,800 + 99,500 = 431,300 lb
F-24 API STANDARD 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 of

the nozzle neck which coincides with the thickness of the tank wall, is calculated as follows:

[(27.07 – 1.21) + (2) (1.4) (0.4)](16,500) = 445,670 lb

The combined strength of Elements 1 and 3, based on the size of the outer fillet weld being increased to 1 in., is as

follows:

331,800 + 116,900 = 448,700 lb

This value is adequate.

The combined load on Elements 2, 4, and 5, from the standpoint of developing the strength of the reinforcement in the 

nozzle neck, is calculated as follows:

(1.91) (16,500) = 31,500 lb

The strength of any one of these three elements alone exceeds this requirement.

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 intersection, as shown in Figure F-7. The thickness of the sidewall plate, tw, in this area is 5/8

in., and no corrosion allowance is required. The total internal pressure, Pl + Pg, 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, T2, acting at this level is 

0.57 in. The nozzle neck is seamless 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 tank wall at the horizontal centerline of the opening is calculated as follows:

tr = (0.57)(1.00) = 0.57 in.

The thickness required for the nozzle neck is calculated as follows:

trn

( ) 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 the provisions of 5.19.2, Item b, but this requirement 

does not affect the value of trn 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)(0.625) = 1.56 in.

(2.5)(0.375) + 0.75 = 1.69 in.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 minimum requirements of Figure 5-8, Panel m.

 The area of reinforcement required at the vertical centerline of the opening is calculated as follows:

Ar = (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,

A1 = (12) (0.625 – 0.57) = 0.66 in.2

b) From excess thickness in the nozzle neck,

A2

( ) 2 ( ) 1.56 ( ) 0.375 0.011 – ( ) 14.400

16.500 = ---------------------------------------------------------------------------------- = 0.99 in.2

c) In fillet welds,

A3 = (2)(0.5)[(0.5)2 + (0.375)2] 0.39 in.2

d) n the reinforcing pad,

A3 = (19.5 – 12.75)(0.75) = 5.06 in.2

The total area of reinforcement provided is 7.10 in.2, which is adequate.

The allowable unit stress values for the attachment elements are 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 tension 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 = 10,660 lbf/in.2
F-26 API STANDARD 620

e) For the groove weld in shear,

16,500 × 0.8 × 0.75 × 0.875 = 8530 lbf/in.2

f) For the nozzle neck in shear,

14,400 × 0.8 × 0.875 = 10,080 lbf/in.2

The strengths of the attachment elements beyond the critical 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,

(17.2)(0.1875)(8530) = 27,500 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, 5, and 6, which attach the added reinforcement to the tank wall, is calculated as 

follows:

(6.84 – 0.66)(16,500) = 102,000 lb

The combined strength of Elements 1, 5, and 6 is as follows:

98,500 + 62,900 + 27,500 = 188,900 lb

This value is adequate.
Figure F-7—Example of a Reinforced Opening (See F.5.3)

3

/

8 in.

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

/

8 in.

9 3

/

4 in. 9 3

/

4 in.

19 1

/

2 in.

12 in. 12 in.

6 in. 6 in. 3

/

8 in.

3

/

16 in. at root 1

/

2 in.

60 f

diame

Line of

reinforcement

Section A-A Horizontal section

through opening

Assume critical section for

computing attachment welds

DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 thickness 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) (9900) = 98,500 lb

The combined strength of Elements 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 standpoint of developing the strength of the reinforcement in the 

nozzle wall, is calculated as follows:

(0.99)(16,500) = 16,400 lb

The strength of any one of these three elements alone exceeds this requirement.

The combined load on Elements 1 and 6, from the standpoint of developing the strength of the reinforcing pad, is 

calculated as follows:

(5.06)(16,500) = 83,490 lb
F-28 API STANDARD 620

The combined strength of Elements 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 diameter shown in Figure F-8 is located in solid plate in 

spherical portion of a torispherical roof on a cylindrical storage tank 72 ft in diameter. The internal pressure, Pg, 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 seamless tank wall at the location of the manhole is calculated as follows:

tr = (0.5)(0.85) = 0.425 in.

The thickness required for the manhole neck is calculated as follows:

trn

( ) 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 satisfy the provisions of 5.19.2, Item b, but 

this requirement does not affect the value of trn 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 minimum requirements of Figure 5-8, Panel i.

The area required at the centerline of the opening is calculated as follows:

Ar = (22.25)(0.5)(0.85) = 9.46 in.2
Figure F-8—Example of a Reinforced Opening (See F.5.4)

3

/

8

 in. 1

/

2

 in. radius

441

/

2

 in. limits of reinforcement

221

/

4

 in.

35 in.

20 in. inside diameter

1

/

2

 in.

1

/

2 in.

1

/

2 in.

About 0.4 in.

72 ft spherical

radius

DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS F-29

 The areas of reinforcement provided are as follows:

a) From excess thickness in the spherical head,

A1 = (22.25)(0.500 – 0.425) = 1.67 in.2

b) From excess thickness in the formed manhole neck,

A2 2 π 12 0.52 ( ) –

4 = ---------------------------- + ( ) 1.25 1 – ( ) 0.4 – ( ) 1.25 ( ) 0.010 = 1.36 in.2

c) In fillet welds,

A3 = 2(0.5)[(0.375)2 + (0.5)2] = 0.39 in.2

d) In the dished reinforcing collar,

A4 = (0.5)(35 – 21.8) = 6.60 in.2

The total area of reinforcement provided is 10.02 in.2, which is adequate.

The allowable unit stress value for the outer and inner fillet welds is calculated as follows:

(15,200)(0.6) = 9120 lbf/in.2

The strength of the attachment elements beyond the critical section 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 STANDARD 620

NOTE It should be assumed that the critical section for computing the strength of the attachment 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 Elements 1 and 2 is as follows:

132,900 + 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 established by reason of accurate knowledge, available to the

designer, covering the chemical characteristics 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 relatively high, are either variable or indeterminate in

magnitude.

c) Tanks in which corrosion rates, although indeterminate, 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 closely 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 incidental, localized, or variable

in rate and extent, the best judgement of the designer must be exercised in establishing reasonable maximum excess

tank wall thicknesses. For all tanks that come under this classification, a minimum 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 and Hydrogen-induced Cracking

When the service conditions might include the presence of hydrogen sulfide or other conditions that may promote

hydrogen-induced cracking effects, particularly near the bottom 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 restricting the sulphur content of the material to be stored through

thickness testing of the steel, laboratory surveillance tests, and the use of internal tank 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 hardness criteria shall be a matter of agreement between the

Purchaser and the Manufacturer and shall be based on an evaluation of the expected hydrogen sulfide concentration

in the product, the possibility of moisture being present on the inside metal surface, and the strength and hardness

characteristics of base metal and weld metal.

H-1

Annex H

(informative)

Recommended Practice for Use of Preheat, Post-heat, and Stress Relief

H.1 Introduction

The majority of tanks covered by this standard are usually not subjected to a conventional 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 accumulating that shows the benefit of improving notch toughness by

metallurgical changes rather than by relief of residual stresses.

H.2 Thickness as it Affects Preheat 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 vessels is of established value, although the mechanism by

which the improvement is realized may be open to debate. Post-heat treatment of tank sections when the plate

thickness exceeds 11/4 in. is required as stated in 5.25. In special cases, 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 rigidity of the tank is adequate.

When the service conditions 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 appreciably beyond this thickness

range. However, for all practical purposes, improvement resulting from preheating is sufficiently well established so

that preheat should be considered for field 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 with 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 satisfactory. The preheat of 300 °F should be checked with a

41 Robert D. Stout, “The Preheating and Postheating of Pressure Vessel Steels,” Welding Journal New York, Research

Supplement 32 [1], 14s – 22s, 1953 (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 STANDARD 620

temperature-sensitive crayon, or similarly 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 maintained 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 reduce 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 the 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 Alternative 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, which will disappear on cooling. These 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 steel, the thickness of the weld, and the thicknesses of successive layers of

welding (see 6.19.2). Two guides to satisfactory 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 opposite sides of the weld, and the distance between these marks shall be kept within

1/32 in. by peening during the welding of the seam. The initial measurement shall be made after two layers of weld

metal have been deposited. After the weld has been built up to a depth of 11/4 – 11/2 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 the deviation in distance between punch

marks is kept to a minimum up to this point, the same degree 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 considered in connection with tank venting problems make it

impracticable to set forth definite, simple rules. Engineering studies for any particular tank may show that it is

desirable to use either a larger or smaller venting capacity than that estimated in accordance with these rules.

K.2 Determination of Required Capacity

The aggregate capacities required for any vent valves, pressure-relieving valves, and/or vacuum-relieving valves to

be provided should be determined as follows:

a) For thermal breathing and product movement, the rules given in API 2000 shall be followed. The required capacity

as the result of the product movement into the tank given in API 2000, shall be multiplied by the ratio of the absolute

tank pressure to atmospheric pressure (14.7 lbf/in.2 absolute).

b) For supplemental safety pressure-relieving 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 same notations except as supplemented herein. The design procedures contained

in this annex are based on allowable 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 contained in API 650, Annex E are not

duplicated here, but are wholly incorporated by reference. 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 elevated 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 interaction for mechanically anchored tanks.

Tanks located in regions where S1 is less than or equal to 0.04 and SS 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 0.05g, need not be

designed for seismic forces; however, in these regions, tanks in SUG III shall comply with the freeboard requirements

of this annex.

Dynamic analysis methods incorporating fluid-structure and soil-structure interaction are permitted to be used in lieu

of the procedures contained in this annex with Purchaser approval and provided 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 tanks designed and constructed of other materials such as

reinforced or prestressed concrete are outside the scope of this standard.

L.2 Notations

hs is additional shell height required above the sloshing wave height, mm (ft);

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

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

Xns is the height from the bottom of the shell to the center of action for the insulation load on the tank shell, m 

(ft);

Xnsr is the height from the bottom of the shell to the center of action for the insulation load on the tank roof, m 

(ft);
L-2 API STANDARD 620

L.3 Special Provisions for Tanks Designed 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 requirements of API 650, Annex E or API 620, Annex L conflict with those of API 

620, Annex Q or Annex R, the more conservative requirements shall apply.

Special provisions for refrigerated tanks requiring performance level designs are contained in Section L.4.

L.3.2 Force Reduction Factor

The response modification factor for ground supported, liquid 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 Rwi , (impulsive) Rwc , (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 Rwi , (impulsive) Rwc , (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 common foundation, the seismic foundation 

loading shall be calculated using the lesser response modification values for either the inner tank or outer tank for 

both tanks and a dynamic analysis shall be performed to determine the combined effect.

NOTE The above Rw factors are applied for CLE (or SSE) event. For OLE (or OBE), the elastic 

design (Rw = 1.0) is used.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS L-3

L.3.3 Resistance to Design Loads

L.3.3.1 Allowable Stresses

For Annex R tanks, design allowable stresses shall be per API 620, Section R.3.3. 

For Annex Q tanks, design allowable stresses shall be per API 620, Section Q.3.3.

L.3.3.2 Annular Bottom Plates

Thickness shall be per API 650, Annex E (with loads modified by requirements of this annex) plus corrosion 

allowance unless a thickness increase is required for compliance with Annex R or Annex Q. Annular plate width shall 

be per API 650, Annex E (with loads modified by requirements 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 configured such that the overall 

horizontal shear force at the base of the tank does not exceed the friction capacity 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 containing loose fill insulation 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 the point or center of gravity of attachment and may be used to resist overturning.

For single wall tanks with insulation or double wall tanks with the insulation adhered to the plate surface, the additional 

weight of the insulation shall be included and may be included in the tank weight, wt, used to resist overturning. The 

insulation weight shall also be included in the definition of the terms, WT and WRS.

Modify Equation (E.6.1-2) of API 650, Annex E as shown in Equation (L-1):

Vi Ai Ws ++++ + Wr Wf Wi Wns Wnr = ( ) (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, Mrw:

Mrw Ai WiXi + + WsXs WrXr + + WnsXns WnrXnr [ ] ( ) 2 Ac WcXc [ ] ( ) 2 = + (L-2)

Slab Moment, Ms:

Ms Ai WiXis + + WsXs WrXr + + WnsXns WnrXnr [ ] ( ) 2 Ac WcXcs [ ] ( ) 2 = + (L-3)

L.3.3.5 Additional Roof Loads

When SDS > 0.33g 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, Wr + Ws, a dynamic 
L-4 API STANDARD 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 require 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 permitted, the requirements 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 SSI to self-anchored tanks is 

allowed without regard to API 650 E.6.1.6 if the Rw value applied is not greater than those in Table L-1Q and Table L1R, as applicable.

L.4 Special Provisions for Tanks Requiring 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 ground motions are as follows.

a) OLE—the tank system will remain operational with only minor repair required. The tank system should be capable 

of withstanding multiple events with this ground motion without significant damage.

b) CLE—the primary liquid container will survive and contain the liquid (with only minor leaks permitted) to protect the 

public but extensive damage may occur and the tank system 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 secondary 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 standard apply unless modified herein.

L.4.2 Ground Motions

The definition of the ground motions to be used with 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. The 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 required for tanks located in regions where peak ground acceleration is greater 

than 0.15g or Ss is greater than 0.3g unless otherwise specified. 
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS L-5

The exception in API 650, Section E.4.6.2 limiting the upper value of the spectral acceleration, Sa* under specific tank 

configurations is not applicable.

Current U.S. requirements are based on a response spectrum with 5 % damping. If the site specific regulations 

require a different 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 unchanged 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,

Ki

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 regulations, 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 Definition 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 provisions in this annex.

1) Re-define the following terms for OLE only:

SS is mapped, 10 % PE50 earthquake from the USGS data, 5-percent-damped, spectral response 

acceleration parameter at short periods (0.2 sec), % g;

S1 is mapped, 10 % PE50 earthquake from the USGS data, 5-percent-damped, spectral response 

acceleration parameter at a period of one second, % g.

2) The scaling factor, Q, is not applicable.

3) API 650, Annex E, Equation (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:

Ai = KiFaSs

5) Equation (E.4.6.1-4) and Equation (E.4.6.1-5) shall be modified:
L-6 API STANDARD 620

When, TC ≤ TL:

Ac KSD1

1

Tc

----     2.5KFvS0

TS

TC

-----     = = ≤ Ai

When, TC > TL:

Ac KSD1

TL

Tc

2 -----     2.5KFvS0

TSTL

TC

 2 ----------     = = ≤ Ai

L.4.3.3 Adjustment Factors 

Unless specifically permitted by the regulations, 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 requirements, the damping ratio for the impulsive spectral accelerations shall 

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 providing the damping ratio does 

not exceed 10 %.

L.4.3.6 Allowable Stresses

Design allowable stresses shall be per API 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 OLE design combination to limit 

uplift 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, Equation (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 with the following where the terms are as defined in API 

650, Annex E

δs = 0.42DAf + hs

An additional shell height, hs, shall be added to the calculated value above the sloshing height as required by the 

governing regulations. The minimum value of hs for the OLE event shall be 300 mm (1ft).
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS L-7

If provided, the site-specific response spectrum may be used to determine the effective spectral acceleration, Af, in 

lieu of using the TL values in API 650, Annex E. 

Alternative sloshing height calculation methods may be used if approved by the regulatory body providing the 

calculated sloshing height is not less than 80 % of the value required by these provisions. 

L.4.3.10 Piping Flexibility

Piping, piping supports, support foundations and superstructures supporting piping attached to the tank 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 tank shall not exceed Vs. The maximum coefficient of friction, μ, shall be 

(tan 30°/1.5) where 1.5 is the factor of safety against sliding. The coefficient of friction selected shall consider the 

materials underlying the tank bottom. Anchorage may not be used to resist sliding. If the 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 by 1.25 for OLE.

L.4.3.13 Bottom and Shell Support

The tank under-bottom insulation shall be designed to resist the combined pressures from the product load, the 

overturning seismic load and the vertical seismic load. 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 tank

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 governing local regulations, the contingency level earthquake ground 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 the maximum considered earthquake in API 650, Annex E and ASCE 7.

L.4.4.2 CLE Definition Based on API 650, Annex E, ASCE 7 Method

To utilize API 650, Section E.4 to define the CLE ground motion, the following modifications shall be made based on 

the provisions 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 defined in Table L1-Q or Table L1-R as applicable;

3) the importance factor, I, shall be taken 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 hs shall be taken as zero unless required 

by the governing regulations.
L-8 API STANDARD 620

L.4.4.4 Sliding Resistance

The calculated sliding force at the base of the tank shall not exceed Vs. The maximum coefficient of friction, μ, shall be 

tan 30°. The coefficient of friction selected shall consider the materials underlying the tank bottom. Anchorage may 

not be used to resist sliding. If the sliding force exceeds 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 API 620, Section L.3.3.1, increased by 33 % for load combinations including 

earthquake 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 specifically require the tank system 

to be designed or evaluated for aftershocks.

Unless otherwise defined by the governing local regulations, the aftershock level earthquake (ALE) ground 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 the maximum considered earthquake in API 650, Annex E and ASCE 7, with the 

spectral values reduced by 50 %.

If the outer tank is not designed as a secondary containment (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 suitable 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 longer 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 containing 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 requirements, the damping ratio for the impulsive spectral accelerations shall 

be 5 %. 

L.4.5.4 Soil Structure Interaction 

Soil structure interaction per API 620, Section L.3.3.7 may be used for ALE design.

L.4.5.5 Allowable Stresses

Design allowable stresses shall be per API 620, Section L.3.3.1 increased by 33 % for load combinations including 

earthquake loads.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS L-9

L.4.6 Base isolation

Base isolations systems may be used to alter the tank response to the design ground motions providing:

1) the inner and outer tanks are both isolated on a common foundation to avoid excessive differential 

displacements between the tanks and connecting internals;

2) the anchorage, internal and external piping, insulation and other attached equipment are designed for the larger 

differential deformations associated with an isolated system;

3) a site-specific response spectrum is mandatory and includes the long term periods necessary to define the 

system response;

4) all external piping connections to the isolated system are designed for the calculated displacements for the 

actual ground motions (no I or R modifications);

5) the design is peer reviewed for technical adequacy by an independent party knowledgeable in the design and 

behavior of base-isolation systems; or, the design is verified by scaled shake-table tests or 3-dimensional nonlinear analysis applying simultaneous horizontal and vertical time histories fit to the design OLE and CLE 

spectra and including the supporting 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 accompanying supplementary sketches, graphs 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 will want to have a record of these repairs or changes

attached to the Manufacturer’s report.

M.4 Tank Certification

A certificate shall be supplied 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 purchase 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, materials, construction and workmanship on this low-pressure tank conforms to the

requirements of API 620, Design and Construction of Large, Welded, Low-pressure Storage Tanks.

Date___________ 20___ Signed _____________________________________________by ________________

I have inspected the tank described in this Manufacturer’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 Pressure-relieving Devices

N.1 General

Pressure-relieving devices shall be installed 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 consideration 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 vented to atmosphere, they shall

be at a sufficient height to prevent 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 relieving devices, its area shall be not less than the

area of the valve outlet, or if a single pipe provides for discharge from several relief devices, its area shall be not less

than the aggregate area of the valve outlets. The discharge pipe shall be fitted with an open drain to prevent water or

other liquids from lodging in the discharge side of the valves.

N.4.2 Discharge pipes shall be supported so that no undue stress is placed on the valve body. Open discharges

shall be placed and oriented so that the outflow is directed 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, shall be as direct in inflow as possible 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 relieving devices and the tank to help service 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 affected relieving devices is

restored.

O-1

Annex O

(informative)

Suggested Practice Regarding Installation of Low-pressure Storage Tanks

O.1 Introduction

The practices recommended in this annex are intended for general guidance only. They are not essentially a part of

the construction rules for low-pressure storage tanks because in most instances sound engineering principles for safe

and efficient operation will dictate the 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, preferably near the Manufacturer’s nameplate (showing the markings required by

these rules). Such markings shall not be covered by the additional plate.

O.3 Access

All openings and accessories for tanks constructed 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 establish 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 facilities for accidental spills or leakage

of flammable contents from such tanks 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 flammable liquid products are stored at

temperatures well above their average boiling point shall 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 reinforcing 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 welds 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 level 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: Nozzle-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 clean 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 radiographed. PT the root pass and every 1/2 in. of 

deposited weld metal. Q.7.5.c

PT Annex Q: Attachment welds of non-pressure parts to primary components. Q.7.5.d

PT Annex Q: The second pass of joints on which backing strips are to remain. Q.7.5.e

PT Annex Q: Base metal repairs for erection lug removal areas on primary components. Q.7.9
P-2 API STANDARD 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 bottoms 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 pneumatic 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 lug removal areas on secondary components. 7.16.1

VE Welds attaching nozzles, manways, and clean out openings. 7.16.4

Process Welds for which the Inspection and Testing is Required Reference 

Section
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 VIII, 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 

Section

Q-1

Annex Q

(normative)

Low-pressure Storage Tanks for Liquefied 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 temperature 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 components 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 design 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 pressure
Q-2 API STANDARD 620

— annular space

— suspended deck

— design metal temperature

Q.2 Materials

Q.2.1 General

The material requirements are based on the storage of refrigerated products at the design metal temperature.

Q.2.2 Product Temperature Materials

Q.2.2.1 Materials for the following metallic components (including their penetrations, piping, anchors, stiffeners, and 

attachments) shall be selected from Table Q-1 and shall comply with the requirements of Q.2.3:

a) primary liquid containers;

b) secondary liquid containers;

c) refrigerated temperature roofs (this includes inner roofs 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 containers;

e) for full containment tank systems; portions of warm product vapor containers that may experience cold gas flows 

in the event of primary liquid container leakage;

f) metallic suspended decks for insulation;

g) Liner plates, if required for liquid containment, in concrete primary or secondary liquid containers, loaded in 

tension under cool down, operating, or other design conditions.

Q.2.2.2 Materials for liner plates, if required 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 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 erection in order to form 

a single shell plate subassembly. Plates welded 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 comprising a subassembly shall be aligned. The 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 procedure specifications 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 tested in accordance with Q.2.3.2 through Q.2.3.4. Impact testing is 

not required for austenitic stainless steel, nickel alloy, and aluminum materials. 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 (see 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

DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 using a process permitted by the named ASTM 

specification and shall be tested hydrostatically 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 may 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 STANDARD 620

Q.2.3.2 Impact testing of plates, including structural members made of plate, shall comply with the following.

a) Impact test specimens shall be taken transverse to the direction of final plate rolling.

b) For ASTM A353, A553, and A844 steels, Charpy V-notch specimens shall be cooled to a temperature of –320 °F.

NOTE This temperature is selected to be consistent with 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 expansion 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 members shall comply with the following:

a) For each different shape in each heat-treatment lot, one set of three specimens taken in the longitudinal direction 

from the thickest part of each shape shall be tested. If the heat-treatment lot consists 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 A844 steels for impact testing.

c) The longitudinal Charpy V-notch impact values shall conform to Table Q-2.

d) Each test shall consist of three specimens, and each specimen shall have a lateral 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) Impact 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 impact testing.

c) The minimum Charpy V-notch impact values shall conform to the longitudinal values in Table Q-2. 

d) Each test shall consist of three specimens, and each specimen shall have a lateral 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 components:

a) roofs over suspended decks;
Table Q-2—Charpy V-notch Impact Valuesa

Transverse Longitudinal

Size of Specimen

(mm)

Value Required for 

Acceptanceb

(ft-lb)

Minimum Value 

Without Requiring 

Retestc

(ft-lb)

Value Required for 

Acceptanceb

(ft-lb)

Minimum Value 

Without Requiring 

Retestc

(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

DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS Q-5

b) outer shells of double wall, single containment tank systems;

c) outer bottoms of double wall, single containment tank systems;

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.

Q.2.4.2 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 one-day mean ambient temperature of –35 °F) 

without impact tests unless they are required by Table 4-1 or by the Purchaser.

b) Table R-3 for design metal temperatures down to –60 °F without impact tests unless they are required by Table 

R-4 or by the Purchaser.

c) Paragraph Q.2.2 without impact tests unless they are specified by the Purchaser.

d) If approved by the Purchaser, the material may be selected according to the requirements of 4.2.2. Where wall 

liner systems are composed of embedded plates 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 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 liners functioning with a concrete secondary liquid container 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 plates.

b Average of three specimens.

c Only one specimen of a set.
Q-6 API STANDARD 620

Q.2.4.4 Material for purge gas containers shall conform to one of the approved materials listed in Table 4-1. 

Consideration of the design metal temperature is not required if the actual stress does not exceed one-half the 

allowable tensile design stress for the material.

Q.2.5 Structural Shapes

Structural shapes of 9 % and 5 % nickel steel may be furnished to the chemical and physical requirements of ASTM 

A353, A553, A645, or A844. Physical 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 requirements of this annex, all piping within the limitations 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, tubing, 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 tn are defined in Figure 5-7).

Q.2.6.3 Nickel alloy material B444 (UNS-N06625), B622 and B619 (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, providing these materials meet the applicable requirements in this annex and 

are not used for reinforcement.

Q.2.6.4 300 series stainless steel materials in Table Q-1 may 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 specified in API 625, Section 6, “Design and Performance Criteria” together with 

the additional provisions of this Section Q.3.

Q.3.2 Density of Liquid Stored

The density of the liquid stored shall be its maximum density within the range of design temperatures.

Q.3.3 Allowable Design Stresses

Q.3.3.1 The maximum allowable design stresses for the materials outlined in Q.2.2 shall be in accordance with 

Table Q-3.

Q.3.3.2 The values for the allowable design tensile stress given in Table Q-3 for materials other than bolting steel 

are the lesser of (a) 331/3 % of the specified minimum ultimate tensile strength for the material or (b) 662/3 % of the 

specified minimum yield strength, but they are 75 % of the specified minimum yield strength for the stainless steel, 

nickel alloy, and aluminum materials. Allowable test stresses are based on the limitation of Q.6.2.2. 

Q.3.3.3 For the base materials associated with 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 minimum yield or ultimate tensile strength below the specified minimums for the 

base metal; or when 
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 31,700b 42,000b

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 40,000g 18,000g 13,300g 16,200g

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 42,000g 19,000g 14,000g 17,100g

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 10,000
Q-8 API STANDARD 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 304c 75,000 30,000 22,500 27,000

A312, Grade TP, Type 304Lc 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 40,000f 54,000f

B444 (UNS-N06625), Grade 2 100,000 40,000 30,000f 36,000f

B619 (UNS-N10276), Class Ic 100,000 41,000 30,750f 36,900f

B622 (UNS-N10276) 100,000 41,000 30,750f 36,900f

Table Q-3—Maximum Allowable Stress Values (Continued)

Stress Value (lbf/in.2)

Specified Minimum Allowable Stress

ASTM Specifications Tensile Strength Yield Strength Design Test
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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

Boltinge

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 11⁄4 in. 105,000 65,000 21,000

> 11⁄4 to 11⁄2 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, as 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 joint considering the weld metal used at that joint. The minimum measured tensile strength shall be 

95,000 lbf/in.2 and minimum measured yield strength shall be 52,500 lbf/in.2, except that for circumferential seams only in the sidewall of a 

cylindrical tank, the minimum 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 efficiency of 0.80 shall be applied to the allowable 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 stresses 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 Test
Q-10 API STANDARD 620

c) the welding procedure qualification test shows the deposited weld metal tensile strength is lower than the 

specified minimum ultimate tensile strength 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 shell uplift, the allowable 

design and test stresses for the material shall be used for the design and overload test conditions, respectively.

Q.3.3.5 Allowable compressive stresses shall be in accordance with 5.5.4 except that for aluminum alloy plate the 

allowable compressive stresses shall be reduced by the ratio of the modulus of compressive 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 definitions). In all other 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 loadings combined with wind loadings shall not exceed 

90 % of the minimum specified yield strength for stainless steel or aluminum.

Q.3.3.7 For aluminum structural members, determine allowable 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 governed by API 625, Section 7.3.3) shall 

fulfill the minimum design requirements of ASME B31.3, except that for the basic allowable 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 inside of the shell and any lap-welded joint in the remainder of 

the bottom and at least a 2-in. projection outside the shell. A greater radial width (Lmin) of annular plate is required 

when calculated by the following equations:

For steel,

Lmin

390tb

( ) H ( ) G = ---------------------- in.

For aluminum,

Lmin

255tb

( ) H ( ) G = ---------------------- in.

where

tb 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 accordance with Table Q-4 (for steel or aluminum, as 

applicable). The thicknesses shown are minimums. 
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS Q-11

Q.3.5.3 The ring of annular plates shall have a circular outside 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 attached to the annular bottom plates by a weld as required by

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 bottoms 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 container, 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 required to maintain roundness when the tank is subjected 

to wind, vacuum, or other specified loads. When stiffening rings 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 requirements 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 consist 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 Stressa in First Shell Course (lbf/in.2)

≤ 19,000 22,000 25,000 28,000 31,000 34,000

≤ 0.75 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 course. 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 [(2.6D) × (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 Stressa 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 11⁄32 11⁄8

> 1.50 to 1.75 7⁄8 1 11⁄16 15⁄32 11⁄4 15⁄16

> 1.75 to 2.00 1 11⁄8 17⁄32 15⁄16 113⁄32 11⁄2

Q-12 API STANDARD 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 inertia of the combined ring-shell section is provided. Weld joints 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 tank wall and bottom caused by thermal changes.

Q.3.7.3 For Annex Q tanks, 9 % or 5 % nickel steel, stainless steel, or aluminum anchorage may be used. 

Aluminum anchorage shall not be imbedded in reinforced concrete unless it is suitably protected against corrosion.

Q.3.7.4 Anchorage subject to load from internal pressure shall 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 Table 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 course. 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 [(2.6D) × (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.

DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS Q-13

Q.3.7.4.2 When the top shell course is thickened beyond the minimum thickness provided 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 Purchaser may specify a combination of normal anchorage design, 

(see Q.3.7.4.1) and emergency 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 designed for the most critical load combinations 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 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.
Q-14 API STANDARD 620

Q.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 space 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 secondary liquid container for a double containment tank system shall be 

designed for the load combinations specified for the outer wall of a single containment tank system. A metallic 

secondary liquid container shall also be designed for the following upset conditions:

a) Dead load and liquid head [DL + PL],

b) Dead load, liquid head, and seismic [DL + PL + E ],

where DL, PL, and E are defined in Q.3.8.6.

Q.3.8.5 Full Containment Outer Wall

A metallic outer wall for a full containment tank system shall be designed for the load combinations 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 [DL + Pg + PL],

b) Dead load, design pressure, liquid head, and seismic [DL + Pg + PL + E ],

where DL, Pg, PL, and E are defined in Q.3.8.6.

Q.3.8.6 Nomenclature

DL is the dead load;

Pg is the design pressure of the secondary liquid container;

PL 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.

Q.3.9 Minimum Wall Requirements

Q.3.9.1 Warm Product Vapor and Purge Gas Containers

Design of warm vapor and purge gas containers shall be in accordance with Section 5 of this standard together with 

the additional provisions 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 concrete 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 liquid container shall in no case be less than that described 

in Table Q-5.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS Q-15

NOTE The nominal thickness of cylindrical sidewall plates refers to the tank shell as constructed. 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 limited to those listed in AWS 5.11, AWS 5.14, and AWS 5.34. Warm vapor and purge 

gas containers shall be welded in accordance with the basic rules of this standard unless the requirements of this 

annex or Annex R are applicable.

Table Q-5—Nominal Thickness of Primary and Secondary 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.)

Stainless steel and nickel steel

< 140 ± 3/4

140 to 220 ± 1

> 220 ± 11/4

Aluminum

< 20 ± 1/2

20 to 120 ± 3/4

> 120 to 200 ± 1

> 200 ± 11/4
Q-16 API STANDARD 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 thickness, the outer tank may be of single welded butt construction from either side with 

full penetration and fusion or double-welded butt construction without necessarily having full fusion and penetration.

Warm product vapor containers shall conform to the lap- or butt-welded construction described in this standard 

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 specimens that conform to the dimensional standard of 

Section 12.1 of AWS A5.11 shall be tested 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 Method.

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 the weld metal and from the heat-affected zone of the 

welding procedure qualification 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 metal. 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 specimens 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 lateral expansion 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 erratic 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 (austenitic stainless steel) base materials, 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 deposited by all the welding processes for 

services of –200 °F and above.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS Q-17

Q.4.4.3 Austenitic stainless steel welds 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 used in the 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 covered by AWS 

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 metal-arc welding (SMAW), gas metal-arc welding (GMAW), gas tungsten-arc welding 

(GTAW), or plasma-arc welding (PAW) processes. When A5.11/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 subtracting 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 welded 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 welding shall conform to the requirements of the procedure 

qualification tests within the following limitations.

a) Individual weld layer thickness shall not be substantially greater than that used in the procedure qualification test.

b) Electrodes shall be of the same AWS classification 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 plates shall be welded for primary and secondary liquid container butt-welded shell 

plates when welding procedure qualifications are required to be impact tested 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 accordance with Q.4.7.5. Test plates shall be made from plates produced only from the 

heats that are used to produce the shell plates for the tank.

Q.4.7.2 Test plates shall be welded using the same qualified welding procedure and electrodes that are required for 

the tank shell plate joints. The test plates need not be welded 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 the shell. Each test welded of thickness t shall qualify for plate 

thicknesses from 2t 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; this test weld will qualify plate thicknesses from t up to 2t.

Q.4.7.4 Test welds shall be made for each position and for each process used in welding primary and secondary 

liquid containers’ shell plates except for the following.

a) A manual or semi-automatic vertical test weld will qualify 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 STANDARD 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 shall conform to Q.4.3.2 through Q.4.3.6 for 9 % and 5 % 

nickel steel. The impact specimens and 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, production test welds for the first tank shall 

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 weld procedure specifications 

are used. 

Q.5 Requirements for Fabrication, Openings, 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) Longitudinal and circumferential shell joints and joints that connect the annular bottom plates 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 compression rings and sections 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 method of leak checking is used (see 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, manway necks, and pipe fittings, including weld neck flanges, 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 passes, except aluminum material and as permitted for 

stiffening ring attachment to shell (see Q.3.6.4).

Q.5.1.2.2 For 9 % nickel material, sandblasting or other adequate means must be used to remove mill scale from all 

plate edges and surfaces before fillet welds in contact 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 specifically approved by the Purchaser.

Q.5.1.3.2 All connections shall have complete penetration and complete fusion.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 material shall comply 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 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.

Q.5.3 Postweld Heat Treatment

Q.5.3.1 Cold-formed 9 % and 5 % nickel plates shall be postweld heat treated (or stress relieved) when the extreme 

fiber 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

65t

Rf

------- 1 Rf

Ro

– -----     =

where

s is the strain, in percent;

t is the plate thickness, in inches;

Rf is the final radius, in inches;

Ro is the original radius, in inches (infinity for flat plate).

Q.5.3.2 If postweld heat treatment (or stress relief) is required 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 postweld heat treatment shall be not less than 300 °F per hour 

down to a temperature of 600 °F. A vessel assembly, or plate that requires postweld heat treatment, must be postweld 

heat treated in its entirety at the same time. Methods for local or partial postweld heat treatment cannot be used. 

Pieces individually cold formed 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 normally not necessary or desirable. No postweld heat 

treatment shall be performed except by agreement between the Purchaser and the Manufacturer.

Q.5.3.4 Postweld heat treatment of austenitic stainless steel materials is neither required 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 STANDARD 620

Q.5.4 Spacing of Connections and Welds

Q.5.4.1 In primary and secondary liquid containers, all opening connections 12 in. or larger 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 greater of 10 times the shell 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 extend to and intersect a flat-bottom-to-shell corner joint at approximately 90 

degrees.

Q.5.4.3 In primary or secondary liquid container and steel warm vapor container tank shells, excluding concrete wall 

liners, the longitudinal weld joints in adjacent shell courses, including compression 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 weld 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 before 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 tank to the bottom annular plates.

c) All welds of opening connections that are not completely radiographed, including nozzle and manhole neck welds 

and neck-to-flange welds. Examination shall 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, compression rings, clips, and other nonpressure parts.

e) All welded joints on which backing strips are to remain shall also be examined by the liquid-penetrant 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 distance 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 container 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 examination shall be as described in Q.5.6.2 through 

Q.5.6.8. When the examination is by the ultrasonic method, it shall be done in accordance with the requirements of 

Annex U.

Q.5.6.2 Butt-welds in all tank wall courses subjected to a maximum actual operating membrane tensile stress, 

perpendicular to the welded joint, greater than 0.1 times the specified minimum tensile strength of the plate material 

shall be completely examined.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 minimum 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 not 

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 except in the case of a flat bottom 

(wall) supported uniformly by the foundation. This does not include 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 radial joints shall be spot examined for a minimum 

length of 6 in. The location shall be at the outer edge of the joint and under the tank shell.

Q.5.6.7 Twenty-five percent of the butt-welded compression bar radial 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 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.

CL

CL

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

44 44 4

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 STANDARD 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 fittings 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 liquid shall be completely radiographed except for 

manufactured pipe welded without filler metal, 12 in. or less in diameter, which is tested hydrostatically or by eddy 

current to ASTM requirements.

Q.5.7.3 Longitudinal welded joints in piping carrying vapor shall be completely radiographed except for 

manufactured pipe welded without filler metal, 18 in. or less in diameter, which is tested 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 joints) 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 pipe fittings shall be completely 

radiographed.

Q.5.7.6 Lines carrying liquid located outside the primary liquid container 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 purge gas container’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 conduit 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 Process Piping rules, Normal 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 steel type that 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 accordance with Q.4.2. Lugs attached for 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. 
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS Q-23

Q.5.10 Repairs to Welded Joints

When repairs are made to welded joints, including the welds 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 secondary 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 report stating that the material has been sampled, tested, and 

examined in accordance with the specifications and has met the requirements.

Q.5.11.2 All mill markings shall be in accordance with the requirements of ASTM A20 and ASTM A480 as 

applicable. All material markings performed by the tank Manufacturer shall be in accordance with the requirements of 

7.7 and Q.5.11.1.

Q.5.11.3 Under some conditions, marking material that contains carbon or heavy-metal 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 fabrication 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 repaired by welding using a qualified 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 excessive, 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 from handling (scratches, gouge marks, etc.) and from environmental conditions (corrosion, pitting, etc.).

Q.5.13.2 Plates shall be protected from moisture or stored in inclined position to prevent water from collecting and 

standing on surface.

Q.5.13.3 Nine percent and five percent nickel plates which are exposed to humid 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 and 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 the design liquid height times the 

product design specific gravity times 1.25, but not greater 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 STANDARD 620

Q.6.2.2 The maximum fill shall not produce a stress in any part of the tank greater than 85 % (may be 90 % for 

stainless steel and aluminum materials) of the specified minimum yield 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 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.

Q.6.3.3 When the sidewall-to-bottom weld in Q.6.3.2 does not have complete penetration and complete fusion, the 

initial weld passes, inside and outside of the shell, shall have all slag and nonmetals 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 air pressure reaches all parts of the welds, a 

sealed blockage in the annular passage between the inside and outside welds must be provided by welding at one or 

more points. Additionally, a small pipe coupling on the outside weld and communicating 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.

Q.6.3.4 The attachment welding around all openings and their reinforcements in the bottom, shell, and roof 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 purge gas container reinforcing plates may be sealed with a 

threaded plug.

Q.6.3.5 For 9 % nickel tanks, all testing surfaces of bottom lap-welds and shell-to-bottom welds shall be cleaned by

sandblasting or other adequate means before the vacuum box test 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 tank, 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 application 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 containers 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 gage above the welds by 

means of a vacuum box with transparent top. 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 through 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 (that is, no hydrogen sulfide).

Q.6.4.4 Water pH shall be between 6 and 8.3.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 mercury content of the water shall be less 

than 0.005 parts per million, and the copper content shall be less than 0.02 parts 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 that 

utilize suitable inhibitors (for example, Na2CO3 and/or NaO3) 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 points at the bottom of the tank shell and on 

top of the ringwall or slab shall be checked. Differential settlement, or uniform 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 pneumatic pressure is applied, anchorage, if provided, shall 

be tightened against the hold-down brackets.

Q.6.5.5 All welds in the shell, including the corner weld between the 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 for 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 the 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 checked with a solution film. A prior vacuum box check may 

be substituted for the solution-film examination. The solution-film examination 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 STANDARD 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 two times the adjacent shell ring thickness.

— Full fusion butt welded connections.

Q.6.6.5 The opening pressure or vacuum of the pressure relief and vacuum relief valves shall be checked by 

pumping air above the water level and releasing the pressure and then partially withdrawing water from the tank.

Q.6.6.6 After the tank has been emptied of water and is at atmospheric pressure, the anchorage, if provided, shall 

be rechecked for tightness.

Q.6.6.7 Air pressure, equal to the design pressure, shall be applied to the empty tank, and the anchorage, if 

provided, and the foundation shall be checked for uplift.

Q.6.6.8 Following the hydrostatic and pneumatic test, all welded seams in the primary liquid container bottom, 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 the 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 restore 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 insertion 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 shall be at least the greater of 10 times the shell 

thickness or 12 in. from any other weld in the shell including 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 penetrant examination is required on the 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 pneumatic 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.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 tank to balance the upward pressure against the inner tank bottom produced by the pneumatic test of the outer 

tank; as an alternative, the pressure between 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 design gas 

pressure but not exceeding a pressure that would 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 shell 

and roof shall be thoroughly examined with a solution film unless 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 the design gas pressure to the outer 

tank, 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 accordance 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 intended to apply to the size of the ringwall and 

foundation but not the anchorage.

Q.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 computed under the following conditions:

a) The internal design pressure times 1.5 plus the design wind load on the shell and roof.

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 design wind load on the shell and 

roof.

Q.8.2.4 When the anchorage is designed to meet the requirements 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 utilize 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 Section 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 STANDARD 620

b) For 9 % and 5 % nickel steels, Part UHT in Section VIII of the ASME Code;

c) For stainless steel, Part UHA in Section VIII of the ASME 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 Operating Between +40 °F and –60 °F

R.1 Scope

R.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 temperature 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 components 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 metal 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 Temperature
R-2 API STANDARD 620

R.2 Materials

R.2.1 General

The material requirements are based on the storage of refrigerated 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 (including their penetrations, piping, 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 single 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 primary 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, operating, or other design conditions.

R.2.2.1.2 Materials for liner plates, if required 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 design 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 plate, shall comply with Table R-1. 

R.2.2.2.2 Impact test specimens shall be taken transverse 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 subsize specimen acceptance criteria, see ASTM 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.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 specifications that do not refer to ASTM A20.

R.2.2.2.5 When as-rolled plate material complies with impact test 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 provision 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), bolting, and forgings shall be impact tested as defined

below. Pipe, bolting, and forgings shall also be in accordance 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 the 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 stress

values permitted by Table Q-3. Impact tests of this material are not required. Welding procedures shall be qualified in accordance with the more

restrictive requirements of R.4.2 and Q.4.4 as applicable 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 member shall conform to the impact testing requirements of R.2.2.5.

NOTE 5 Structural shapes shall be normalized, if necessary, to meet the required minimum Charpy V-notch impact values of R.2.2.5.

NOTE 6 The steel shall be made with fine grain 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 notwithstanding any impact testing at any other

warmer temperature that may be mandated in the national specification.
R-4 API STANDARD 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 specifications 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) are acceptable for design metal temperatures of –40 °F or

lower. 

Table R-2—Minimum Charpy V-notch Impacta 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 Valueb

(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 1d 3/16 to 2 25 20 20 15

ASTM A516 65 and 70 Mod 2d 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 Ac 3/16 to 11/2 25 20 20 15

ASTM A678 Bc 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 Dc,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 38WTc,d,e 3/16 to 2 25 20 20 15

CSA G40.21 44WTc,d,e 3/16 to 2 25 20 20 15

CSA G40.21 50wtc,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 mechanical 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.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS R-5

R.2.2.4 Impact Requirements for Thermo-mechanical Control Process (TMCP) Plates 

Subject to the approval of the Purchaser, TMCP plates (material produced by a mechanical-thermal rolling process

designed to enhance the notch toughness) may alternatively 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 shall 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 listed 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 killed and made to fine-grain practice.

NOTE 2 The steel shall be made with fine grain practice, and manganese content shall be in the range of 0.80 % to 1.20 % by ladle analysis.
R-6 API STANDARD 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 required by Table 4-1 or by the Purchaser.

b) Table R-3 for design metal temperatures down to –60 °F without impact tests unless they 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 with 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 Temperature 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 11/2

I (semikilled) A 36d

A 131

CSA G40.21

ISO 630 (1995)

EN 10025

B

38W

E 275 Quality Cb

S 275 Quality J0b

–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

58b

55 and 60

55 and 60c

E 275 Quality D

S 275 Quality J2b

38Wb

–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 1a

1 and 2

B and C

C and D

A and B

B

E 355 Quality D

S 355 Quality J2 and K2b

44Wb

50Wb

–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 temperatures 20°F below those shown (except for A 537 Classes 1 and 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 table, 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 normalizing, for thicknesses of 3/16 in. through 11/2 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.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 barrier but not acting as a

warm product vapor container and not required for secondary liquid containment.

Table R-5—Minimum Charpy V-notch Impact Requirements for Atmospheric Temperature Material Plate 

Specimens (Transverse)

Group Specification

Number Grade

Range in 

Thickness

(in.)

Impact Valuea

(foot-lbs)

Average Individual

I (semikilled) A 36e

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 11/2

3/16 to 11/2

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

58c

55 and 60

55 and 60d

E 275 Quality D

S 275 Quality J2

38WT

3/16 to 11/2

3/16 to 2

3/16 to 1/2

3/16 to 11/2

3/16 to 11/2

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 1b

65 and 70 Mod 2b

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 11/2

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 specimen acceptance criteria, see ASTM A20. An impact test temperature

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, consideration must be given to the possible degradation of the impact properties of the plate 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 normalizing, for thicknesses of 3/16 in. to 11/2 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 STANDARD 620

R.2.3.4 Material for purge gas containers shall conform to one of the approved materials listed in Table 4-1. 

Consideration of the design metal temperature is not required if the actual stress does not exceed one-half the 

allowable tensile design stress for the material.

R.3 Design

R.3.1 General

Design considerations shall be as specified in API 625, Section 6, “Design and Performance Criteria,” together with 

the additional provisions 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/ft3.

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 earthquake 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 governed by API 625 7.3.3) shall fulfill the 

minimum 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-welded annular bottom plates with 

a radial width that provides at least 24 in. between the inside of the shell and any lap-welded joint in the remainder of 

the bottom and at least a 2-in. projection outside the shell. A greater radial width (Lmin) of annular plate is required 

when calculated by the following equation:

Lmin

390tb

( ) H ( ) G = ----------------------

where

tb 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 Table R-6. 

R.3.5.3 The ring of annular plates shall have 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 when a full penetration weld is used or required (see R.5.1.1).
Table R-6—Thickness Requirementsa for the Annular Bottom Plate (in.)

Nominal 

Thickness 

of First Shell 

Course

(in.)

Design Stressb 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

DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS R-9

R.3.5.5 Butt-welds in annular plates shall be not closer than 12 in. from any vertical weld in the tank shell.

R.3.5.6 Three-plate laps or butt-weld junctions in the tank bottom shall be not closer than 12 in. from each 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 required to maintain roundness when the tank is subjected 

to wind, vacuum, or other specified loads. When stiffening rings 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 strip, if used, shall comply with the requirements 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 passes. 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 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.

R.3.6.5 Any joints between the adjacent sections of stiffening rings, as shown in Figure R-1, shall be made so that

the required moment of inertia of the combined ring-shell section is provided. Weld joints 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 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 shall 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 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.

b The stress shall be calculated using the formula (2.6D)(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 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 STANDARD 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 

container shall be designed to meet the requirements of R.3.7.2 through R.3.7.5.

R.3.7.2 The anchorage shall accommodate movement of the tank wall and bottom caused by thermal changes.

R.3.7.3 The Manufacturer and the Purchaser shall either use stainless steel anchorage materials, or provide for 

corrosion allowance when carbon steels are used. Material 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 shall be designed as described in R.3.7.4.1 through 

R.3.7.4.3.

R.3.7.4.1 When the topshell course is the minimum thickness indicated in Table 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 provided 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.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS R-11

R.3.7.4.3 As an alternative to R.3.7.4.2, the Purchaser may specify a combination of normal anchorage design (see 

R.3.7.4.1) and emergency 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). 

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 space 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 secondary liquid container for a double containment tank system shall be 

designed for the load combinations specified for the outer wall of a single containment tank system. A metallic 

secondary liquid container shall also be designed for the following upset conditions:

a) Dead load and liquid head [DL + PL]

b) Dead load, liquid head, and seismic [DL + PL + E ]

where 

DL, PL, 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 combinations 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 [DL + Pg + PL ]

b) Dead load, design pressure, liquid head, and seismic [DL + Pg + PL + E ]

where 

DL, Pg, PL, and E are defined in R.3.8.6.
R-12 API STANDARD 620

R.3.8.6 Nomenclature

DL is the Dead load;

Pg is the Design pressure of the secondary liquid container;

PL 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 Warm Product Vapor and Purge Gas Containers

R.3.9.1 Design of warm vapor and purge gas containers shall be in accordance with Section 5 of this standard 

together with the additional provisions of this Section R.3.9.

R.3.9.2 Warm vapor and purge gas containers shall have a minimum nominal thickness of 3⁄16 in. except for metallic 

wall liners for concrete containers which shall have a minimum nominal thickness of 0.12 in.

R.3.9.3 Purge gas containers may be of single-welded lap or of 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 thickness, it may be of single-welded butt construction from either side 

with full penetration and fusion or double-welded butt construction without necessarily having full fusion and 

penetration.

R.3.9.4 Warm product vapor containers shall conform to the lap- or butt-welded 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. For product temperature materials (see R.2.2), 

impact tests are also required for each welding procedure (with exceptions for Type 304 or 304L stainless steel 

described in Table R-1, Note 2). Charpy V-notch specimens that conform to ASTM E23 shall be taken from the weld 

metal and from the heat-affected zone of the welding procedure qualification test plates or duplicate test plates.

R.4.2.2 Weld metal impact specimens shall be taken across the weld with the notch in the weld metal. 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.

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 practicable. The specimens shall be of sufficient length to locate, 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.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS R-13

R.4.2.5 The required impact values of the weld and heat-affected zone shall be as given in Table R-2.

R.4.3 Production Welding Procedures

The production welding procedures and the production welding shall conform to the requirements of the procedure 

qualification tests within the following limitations.

a) Individual weld layer thickness shall not be substantially greater than that used in the procedure qualification test.

b) Electrodes shall be of the same size and American Welding Society (AWS) 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 plates when welding procedure 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 plates 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 required for the 

tank shell plate joints. The test plates need not be welded as an extension of the tank shell 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 each specification and grade of plate material, using a 

thickness that would qualify for all thicknesses in the shell. Each test welded of thickness t shall qualify for plate 

thicknesses from 2t 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; this test weld will qualify the plate thickness from t up to 2t.

R.4.4.4 Test welds shall be made for each position and for each process used in welding primary and secondary 

liquid containers’ shell plates except for the following.

a) A manual or semi-automatic vertical test weld will qualify 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 cylindrical 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 agreement between the Purchaser and the Manufacturer, production weld test plates for the first tank 

shall satisfy the requirements of this paragraph for similar tanks at the same location if the tanks are fabricated within 

six months of the time the impact tests were made and found satisfactory and the same weld 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, Supplementary Requirement, paragraph S 5, or this annex, whichever is applicable. 

Weld material for lap welded wall liners shall meet requirements for design metal temperature without exception, (i.e., 

including cases where 4.2.2 is applied for liner selection).
R-14 API STANDARD 620

R.5 Requirements for Fabrication, Openings, 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 complete penetration and complete fusion 

except as noted.

a) Longitudinal and circumferential shell joints and joints that connect the annular bottom plates 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 compression rings and sections of shell stiffeners 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 (see 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, manway necks, and pipe fittings, including weld neck flanges, 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 specifically approved by the Purchaser.

R.5.1.3.2 All connections shall have complete penetration and complete fusion.

R.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.

R.5.1.3.4 Flanges for nozzles shall be in accordance with this standard; however, the material shall comply with the 

requirements of R.2.2.

R.5.1.3.5 Manways shall have welded closures 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.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 assembly 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 design conditions, does not exceed 10 % of the minimum tensile strength 

of the plate material. The opening shall be reinforced for the low stress.

b) The impact tests on the material and welding fulfill the requirements of R.2.2.2 and Table R-2, and the thickness of 

the material is less than 5/8 in. for any diameter of connection or less than 11/4 in. for connections that have a 

nominal diameter less than 12 in. The thickness of the nozzle 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 configuration 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 mandatory for both primary and secondary 

components.

R.5.3.3 When used in stress relieved assemblies, the material of TMCP steel A 841 shall be represented by test 

specimens that have been subjected to the same heat treatment 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 shell 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 greater 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 longitudinal or meridional joints or 3 in. from the circumferential or latitudinal joints if in either 

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 vapor container tank shells, excluding concrete wall 

liners, the longitudinal weld joints in adjacent shell courses, including compression 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. from 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 container 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 hydrostatic test of the tank.

a) All longitudinal and circumferential butt-welds that are not completely 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 STANDARD 620

c) All welds of opening connections that are not completely radiographed, including nozzle and manhole neck welds 

and neck-to-flange welds. Examination shall 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, compression rings, clips, 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 distance pieces connecting cold piping to warm 

product vapor or purge gas containers shall also be examined by the magnetic-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 container butt-welds shall be examined by radiographic methods or by 

ultrasonic methods. When the term “examination” is used in R.5.6 and its subsections, 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 shall 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 membrane tensile stress 

perpendicular to the welded joint that is greater than 0.1 times the specified minimum tensile 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 membrane tensile stress 

perpendicular 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) supported uniformly by the foundation. This does not include the shell-to-bottom weld of a flat-bottom tank. See 

Figure R-2 for minimum examination dimensions.

R.5.6.6 Twenty-five percent of the butt-welded annular plate radial joints shall 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-welded compression bar radial 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 radiographed except for welds in 

manufactured pipe welded without filler metal, 12 in. or less in diameter, which is tested 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 tested 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.

CL

CL

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

44 44 4

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).

DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS R-17

R.5.7.4 Ten percent of the circumferential welded joints (including weld neck flange to pipe joints) 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 

radiographed.

R.5.7.6 Lines carrying liquid located outside the primary liquid container 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.

R.5.7.7 Lines carrying product vapor within a purge gas container’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.
R-18 API STANDARD 620

R.5.7.8 For piping that does not carry liquid or product vapor (e.g., instrument 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, Normal 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 compliance 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-particle 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 structural adequacy is essential for the primary liquid container. Except as 

permitted by R.6.6, the test shall be conducted after the entire tank is complete, 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 material.

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 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.6.3.3 Complete penetration and complete fusion welds that join the cylindrical wall to the tank 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 passes, 

inside and outside of the shell, shall have all slag and non-metals 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 air pressure reaches all parts of the welds, a sealed blockage 

in the annular passage between the inside and outside welds must be provided by welding at one or more points. 

Additionally, a small pipe coupling communicating 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.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS R-19

R.6.3.5 The attachment welding around all openings and their reinforcements in the bottom, shell, and roof 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 purge gas container reinforcing plates may be sealed with a 

threaded plug.

R.6.3.6 The attachment fillet welds around bottom openings, which do not permit the application of air pressure 

behind the reinforcing 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 metallic liners of concrete containers 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 top. 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 when it is filled with or emptied of water.

R.6.4.3 During water filling, the elevations of at least eight equidistant points at the bottom of the tank shell and on 

top of the ringwall or slab shall be checked. Differential settlement, or uniform 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 filling 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 pneumatic test pressure is applied, anchor bolts 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 between 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 for 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 the 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 until 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 STANDARD 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 two times the adjacent shell ring thickness.

— Full fusion butt welded connections.

R.6.5.5 The opening pressure or vacuum of the pressure relief and vacuum relief valves shall be checked by 

pumping air above the water level and releasing the pressure, then partially withdrawing water from the tank.

R.6.5.6 After the tank has been emptied of water and is at atmospheric pressure, 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 tank, 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 restore 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 insertion 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 shell 

thickness or 12 in. from any other weld in the shell including 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 

particle method, by the radiographic method and by the ultrasonic method. Ultrasonic examination acceptance criteria 

shall be as given in Table U-1. The magnetic particle examination is required on the 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.

R.7 Testing a Purge Gas Container

R.7.1 The tightness test shall be made before insulation is installed.

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 test of the outer 

tank; as an alternative, the pressure between the inner and outer tanks can be equalized.

R.7.3 Air pressure shall be applied to the space enclosed by the outer tank equal to at least the design gas pressure 

but not exceeding a pressure that would overstress 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 unless they were previously checked with a vacuum box.

R.7.5 The air pressure shall be released.

R.7.6 Pressure relief and vacuum relief valves shall be checked by applying the design gas pressure to the outer 

tank, followed by evacuation of the outer space to the vacuum setting of the relief valve.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS R-21

R.8 Foundations

R.8.1 Foundations shall be in accordance 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 of 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 computed under the following conditions:

a) The internal design pressure times 1.5 plus the design wind load on the shell and roof.

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 design wind load on the shell and 

roof.

R.8.2.4 When the anchorage is designed to meet the requirements of R.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 of this paragraph, it is permissible to utilize friction between 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 Section 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, design, fabrication, erection, and testing requirements for aboveground, welded,

austenitic stainless steel storage tanks constructed of material grades 304, 304L, 316, 316L, 317, and 317L. This

annex does not cover stainless steel clad plate or strip lined construction.

S.1.2 This annex applies only to tanks in non-refrigerated service. For stainless steel tanks in refrigerated service,

refer to Annex Q of this standard. Minimum design metal temperature of the non-refrigerated tanks in the scope of the

annex is not limited. Maximum design metal temperature shall be limited 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 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). 

S.1.3 The minimum thicknesses in this annex do not contain any allowance for corrosion. 

S.1.4 This annex states only the requirements that differ from the basic rules in this standard. For requirements not

stated, the basic rules 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 fabrication 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 the 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 welded 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 normal shipping damage. If the intended service requires special

precautions, special instructions shall be specified by the Purchaser.

S.2.3 Impact Testing

Impact tests are not required for austenitic stainless steel base metals.

S.3 Design

S.3.1 Operating Temperature

The Purchaser shall specify the maximum 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 

Seamless 

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 STANDARD 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 Sts.

S.3.3 Maximum Compressive Stress

Allowable compressive stresses shall be in accordance with 5.5.4, except the allowable compressive 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 members shall be in accordance with Table 3-3 except the 

allowable stresses for compression shall be reduced by the ratio of the materials yield strength 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 ends may be used by agreement between the Purchaser and Manufacturer, providing the design 

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 the product shall be furnished in the hot-rolled, 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 (Sts)

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

DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 design 

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 the product shall be furnished in the hot-rolled, annealed, and descaled condition.

NOTE 5 Other bolting materials may be used by agreement between the Purchaser and Manufacturer.

NOTE 1 Sts 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 strength.

NOTE 3 For dual certified materials (e.g. ASTM A182M/A182 Type 304L/304), use the allowable stress of the grade specified by the 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 STANDARD 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 H1 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 Wtr 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 304L/304), use the allowable stress of the grade specified by the Purchaser.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 ratio 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 tb in 5.27.4.5 shall be reduced by the ratio of the materials 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 describe the major precautions that shall be 

observed during fabrication 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 chlorides, including foods, beverages, oils, and greases, shall 

not come in contact with stainless steel. Inadvertent contamination 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 iron 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 precipitates. 

This heat-affected 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 

material.

S.4.4.2 Stainless steels may be cold formed, providing the maximum strain produced by such forming does not 

exceed 10% and control of forming spring-back 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 sensitized condition is acceptable for the service 

of the equipment, it will be necessary 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 STANDARD 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 surface contaminants that may impair the normal 

corrosion resistance, it shall be done in accordance with ASTM A380, unless otherwise specified. Any additional 

cleanliness requirements for the intended service shall be specified by the Purchaser. 

S.4.5.2 When welding is completed, flux residues and weld spatter shall be removed 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 effect on the stainless steel and welded joints and shall 

be disposed of in accordance with laws and regulations governing the disposal of such chemicals. The use of 

chemical cleaners shall always be followed by thorough 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 sand 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 stainless 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 may 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 passivation is required, these operations shall be followed immediately by 

rinsing, not allowing the surfaces to dry between operations.

S.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 million chloride at temperatures above 40 °C (100 °F) and below 

65 °C (150 °F), unless specified otherwise by the Purchaser. 

S.4.9.3 Following final rinsing, the material shall be completely dried.

S.4.10 Welding

S.4.10.1 Welding shall be by any of the processes permitted in 6.6.2. Galvanized components or components with 

zinc-bearing coatings shall not be welded to austenitic stainless steel.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS S-7

S.4.10.2 Filler metal chemistry shall match the type of base metals joined. Dissimilar welds to carbon steels shall 

use filler metals of E 309 or higher alloy content.

S.4.10.3 Two stainless steel plates identical in material type may be welded together prior to erection in order to 

form a single shell plate subassembly. Plates welded 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 comprising a subassembly shall be aligned. The 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 the subassembly horizontal weld to field vertical 

weld intersection. All welding procedure specifications shall be in accordance with Section 6.

S.4.11 Welding Procedure and Welder Qualifications

Impact tests are not required for austenitic stainless steel weld metal and heat-affected zones. 

S.4.12 Postweld Heat Treatment

Postweld heat treatment of austenitic stainless steel materials need not be performed unless 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 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 the following requirements.

a) Unless otherwise specified by the Purchaser, water used for hydrostatic 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 be below 50 parts per million, unless specified otherwise by the Purchaser.

S.5.2.2 When testing with potable water, the exposure time shall not exceed 21 days, unless specified otherwise by 

the Purchaser.

S.5.2.3 When testing with other fresh waters, the exposure 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 test and completely dried. Particular attention shall be 

given to low spots, crevices, and similar areas. Hot air drying is not permitted.
S-8 API STANDARD 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 special requirements when attaching to carbon steel supports.

b) Annex E is applicable; however see S.2.13 for special requirements when attaching external carbon steel 

supports. Internal supports shall meet the material requirements of Annex S.

c) Annex H is not applicable; stress relieving is only required when specified 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 applicable without modifications.
S-1

Annex SC

(normative)

Stainless and Carbon Steel Mixed 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 constructed with austenitic stainless steel, duplex stainless steel and carbon steel.

In this annex, the term stainless steel includes austenitic or duplex unless noted 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. Stainless 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 plate 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) shall 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 followed including Annex S and Annex X as applicable. 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 shall 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 with stainless steels as follows.

a) Annex D is applicable; however, see 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 meet the material requirements of Annex S or Annex X as

applicable.

c) Annex H is not applicable; stress relieving is only required when specified 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 STANDARD 620

SC.2 Materials

SC.2.1 Materials shall be in accordance with Section 4 or Annex S or Annex X as applicable. 

SC.2.2 Selection of the type/grade of stainless steel and carbon steel for mixed material tanks depends on the 

service and environment to which it will be exposed and the effects of fabrication 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 reinforcement may 

be carbon steels meeting the requirements of Section 4 provided they are protected from corrosion and the design 

and details consider the dissimilar properties of the materials 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 stainless steel base metals. 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 stainless steel filler metal appropriate for the type/grade 

of stainless steel used and the 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 material combination of this paragraph applies to all other sub-paragraphs in 

Section SC.3. No analysis of stresses from differential movements is required 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 carbon steel or duplex stainless steel plates and 

carbon steel or duplex stainless steel insert plates shall not be used in austenitic 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 maintained except insert plates shall comply with SC 

3.2.1. 

S.3.3.2 Nozzles and Manways in the tank wall shall be of the same material as the tank wall unless otherwise 

specified by the Purchaser.

S.3.3.3 Reinforcing plates for the tank wall shall be carbon steel to carbon steel and stainless steel to stainless 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 tanks and their welded joints. Chemical cleaners and pickling solutions shall be 
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS S-3

disposed of in accordance with laws and regulations governing the disposal of such 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 austenitic stainless steel weld metals 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 austenitic stainless steel and duplex stainless 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 assemblies, the PWHT requirements of 5.25 and 6.18 are not 

mandatory except when specified by the purchaser. The purchaser is cautioned that mixed material nozzles with 

duplex stainless steel should not be PWHT due to the potential damaging effects of high temperature on the duplex 

material. The Purchaser is advised to discuss 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 prior to welding to stainless 

steel plates.

SC.4.5 In order to prevent excessive weld metal dilution at butt welds between stainless and carbon steel, at least 

one side of the joint shall be beveled with land not to exceed t

/3.

SC.4.6 Internal galvanic corrosion will occur by using mixed material construction and additional mitigation such as 

appropriate localized coatings shall be considered. 

SC.4.7 Where substantial quantities of uncoated stainless steel are welded to coated carbon steel, accelerated 

corrosion rates are possible at holidays 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 ultrasonic 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 the 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 metallurgical in origin that may be detectable 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 interpretation of an indication (see U.6.1 and U.6.2).
U-2 API STANDARD 620

U.2.9

recording

The writing of ultrasonic data onto an appropriate electronic medium.

U.2.10

reflector

An interface at which an ultrasonic beam encounters a change in acoustic impedance and at which at least part of the 

energy is reflected.

U.3 Technique

U.3.1 The ultrasonic examination volume shall include the weld 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 detection of flaws shall be performed using automated, computer-based data 

acquisition except that scanning of adjacent base metal for flaws that can interfere 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 used, 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 recorded using the same system essential variables, 

specified value or range of values, used for the 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 shall be performed in accordance with a written procedure which has been 

reviewed and approved by the Purchaser and conforms to the requirements of Section V, Article 4, except that:

a) The calibration block shown in Figure T-434.2.1 of Section V, Article 4 shall be used, and;

b) For examination techniques that provide plate quality information (e.g., time of flight diffraction), the initial base 

material straight-beam examination 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 flight diffraction (TOFD) may have limitations in detection of flaws at the surface 

such that it may be necessary to supplement TOFD with pulse-echo techniques suitable for the detection of near-field 

and far-field flaws. The variety of surface and sub-surface category flaws in the 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 ultrasonic examination of joints with austenitic 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).
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS U-3

U.4 Personnel Qualifications and Training 

U.4.1 Personnel Qualifications

Personnel performing and evaluating UT examinations shall be qualified and certified in accordance with their 

employer's written practice. ASNT SNT-TC-IA or CP-189 shall 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 approved by the Manufacturer and maintained by their employer.

U.4.3 Personnel Testing

Personnel who acquire and analyze UT data shall be trained 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-surface 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 cover detection, interpretation, sizing, plotting, categorization, grouping, and 

characterization that is sufficient to cover the cases outlined in U.6.

c) Criteria for passing the test shall include limits on the 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 performed by another qualified Level II or III 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 data acquisition 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 techniques that do not use amplitude recording levels, sized reflectors longer than 40 % of the acceptable 

surface or subsurface flaws in Table U-1 or U-2 as applicable shall be investigated. The investigation shall interpret 
U-4 API STANDARD 620

whether the indication originates from a flaw or is a geometric indication in accordance with U.6.2 below. When the 

reflector is determined to be a flaw, the flaw shall be evaluated 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 metallurgical 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 surface configurations (such 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 (if applicable) and location shall be documented, for example: internal 

attachments, 200 % DAC maximum amplitude, 25 mm (1 in.) above the weld centerline, on the inside surface, 

from 90 to 95 degrees. 

U.6.2.2 The following steps shall be taken to classify an indication as geometric:

a) interpret the area containing the indication in accordance with the applicable examination procedure;

b) plot and verify the indication's coordinates; provide a cross-sectional display showing the indication's position and 

any surface 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., 

alternative 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 rectangle that fully contains 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 categorized 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 parallel 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 length of the longer of the 

two, the two flaws shall be considered a singular flaw.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS U-5

U.6.5.3 If the space between two flaws aligned in the through-thickness direction is less than the height of the flaw 

of greater height, the two flaws shall 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 

acceptance using the applicable criteria of Table U-1 or U-2.

U.6.6.2 Surface Examination

Flaws characterized as surface flaws during the UT examination may or may not be surface connected. Therefore, 

unless the UT data analysis confirms that the flaw is not surface connected, a supplemental surface 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 flaws that either exceed 50 % of reference level for amplitude based techniques or exceed 

75 % of the acceptable length for non-amplitude techniques. 

Table U-1—Flaw Acceptance 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 Flawb

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 

allowed

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 allowable reinforcement. For a butt weld joining members having different thickness at the weld, t is 

the thinner of the two.

b 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.
Table U-2—Alternate Flaw Acceptance Criteria for UT Indicationsa

(Applicable to 3XX and 201LN stainless steel and aluminum with toughness meeting Annex Q, and to 9% 

nickel steel with Table U-3 Toughness.)

Thickness at 

weldb (t)

mm (in.)

Limit on flaw 

length (l)

mm (in.)

Limit on surfaced 

flaw heightc (h)

mm (in.)

Limit on sub-surface 

flaw height (h)

mm (in.)

Based on Length and Upper 

Bound Height All t 12.0 (0.45) 50 % of t 50 % of t

Based on Flaw Length and Sized 

Height All t 18.0 (0.70) Smaller of 6.5 (0.25)

or 25 % 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 

Acceptancea

J (ft-lbs)

Minimum Value without 

Requiring Retestb

J (ft-lbs)

10 × 10.00 50 (37) 40 (30)

10 × 7.50 38 (28) 30 (22)

U-6 API STANDARD 620

a In order to use this acceptance criteria for 9 % nickel steel, it is required that the Charpy V-notch impact results for base metal 

(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 the 

higher energy levels given in Table U-3 in lieu of Table Q-2.

b t = thickness of the weld excluding any allowable reinforcement. For a butt weld joining two members having different thickness at 

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, design, fabrication, erection, and testing 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 steel clad plate or strip

lined construction.

X.1.2 This annex applies only to tanks in non-refrigerated services with a maximum design temperature limited as

given in 1.2.2 and a minimum design metal temperature equal to or higher than –40 °C (–40 °F). Ambient

temperature tanks (non-heated) shall 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). For the

purposes of this annex, the design temperature shall be the 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 requirements that differ from the basic rules in this standard. For requirements not

stated, the basic rules 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 processes. 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., clips 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 resistance. 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 material from normal shipping damage. If the intended service

requires special precautions, the Purchaser shall specify special instructions.

•
X-2 API STANDARD 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 B1

UNS S32101 Method B2

UNS S32003 Method B1

UNS S32750 Method B1 & C

UNS S32550/S32520 Method B1 & C

UNS S32760 Method B1 & C3

1 B test values to be agreed upon between Purchaser and manufacturer but not less than 54 J (40 ft-lbf).

2 B test values to be agreed upon between Purchaser and manfacturer, but not less 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

A479 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 agreement between the Purchaser and Manufacturer, providing the design

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 A890 and shall be inspected 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 furnished in the hot-rolled, annealed, and descaled condition.

NOTE 5 Other bolting materials may be used by agreement between the Purchaser and Manufacturer.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS X-3

X.2.3.2 Charpy Impact testing per ASME UHA-51 at minimum design metal temperature is required for shell plates,

shell reinforcing plates, shell insert plates, bottom plates welded to the shell, 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 temperature 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 fulfill 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 temperature 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 Sts.

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/in2) 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 minimum 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 Purchaser.

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 STANDARD 620

X.3.4 Maximum Allowable Stress for Structural Members and Bolts

The maximum allowable stress values for structural members shall be in accordance with Table 5-3 except the 

allowable stresses for compression shall be modified by the ratio of the material’s yield strength at design 

temperature to 205 MPa (30,000 lbf/in2).

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 H1 in 5.10.6.1 shall be modified by the ratio of the materials modulus of elasticity at the 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 modified by the ratio of the materials modulus of elasticity 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 yield 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 minimum 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 Purchaser.

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

DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS X-5

NOTE 1 Interpolate between temperatures.

NOTE 2 Reference: Table Y-1 of ASME Section II, Part D, or manufacturer’s data sheets.

NOTE 1 Interpolate between temperatures.

NOTE 2 Reference: 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 the 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 STANDARD 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.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 fabrication, shipping and construction. The thermal history of the material must also 

be controlled. The following sections describe the major precautions that 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 containing 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 stainless steel shall be by the plasma-arc 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 and toughness unless removed by machining or grinding. 

Normally the HAZ from thermal cutting is thin enough to be removed by edge preparation machining and adjacent 

base metal melting during welding. The 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 

material.

X.4.4.2 Duplex stainless steels may be cold formed. The maximum strain produced by such cold forming shall not 

exceed 10% and control of forming 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 commencing 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 surface contaminants that may impair the normal 

corrosion resistance, it shall be done in accordance with ASTM A380, unless otherwise specified. The Purchaser 

shall specify any additional cleanliness requirements for the intended service.

X.4.5.2 When welding is completed, flux residues and weld spatter shall be removed mechanically using stainless 

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 1230 950 1040

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 Temperatures and Minimum Soaking Temperature (USC)

Alloy °F Max °F Min

°F Min Soaking 

Temperature

S31803 2250 1740 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 STANDARD 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 Purchaser, shall be done using an appropriate pickling product and 

pickling procedure.

X.4.5.5 Chemical cleaners and pickling solutions used shall not have a detrimental effect on the duplex stainless 

steel or welded joints and shall be disposed 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).
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS 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 passivation is required, these operations shall be followed immediately by 

rinsing, not allowing the surfaces to dry 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 million chloride at temperatures above 40 °C (100 °F) and below 

65 °C (150 °F). 

X.4.9.3 Following final rinsing, 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 components 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 manufacturer. 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 ASTM A923 Method B and when specified by Purchaser also 

Method C. Welding Procedure Qualification 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 Table 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 stainless steel materials shall not be performed.
X-10 API STANDARD 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 shall 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 vertical joints 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 liquid penetrant method before the hydrostatic test of the 

tank.

a) The shell-to-bottom inside attachment weld.

b) All welds of opening connections in tank shell that are not completely radiographed, including nozzle and manhole 

neck welds and neck-to-flange welds.

c) All welds of attachments to shells, such as stiffeners, compression rings, clips, and other nonpressure parts for 

which the thickness of both parts joined is greater than 19 mm (3/4 in.).

d) All butt-welded joints in tank annular plates 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 construction of duplex stainless steel tanks may be subject to pitting, or general 

corrosion 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 the following requirements.

a) Unless otherwise specified by the Purchaser, water used for hydrostatic 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 be below 50 parts per million.

• X.5.2.3 When testing with potable water, the exposure time shall not exceed 21 days unless otherwise specified by 

the Purchaser.

X.5.2.4 When testing with other fresh waters, the exposure time shall not exceed 7 days.
DESIGN AND CONSTRUCTION OF LARGE, WELDED, LOW-PRESSURE STORAGE TANKS X-11

X.5.2.5 Upon completion of the hydrostatic test, water shall be completely drained. Wetted surfaces shall be 

washed with potable + water when non-potable water is used for the test, and completely dried. Particular attention 

shall be given to low spots, crevices, and similar 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; however see X.2.1.3 for special requirements when attaching to carbon steel supports.

— Annex E is applicable; however see X.2.1.3 for special requirements when 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 specified 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.(Ref 

X.3.9.1.)

— Annex Q is not applicable.

— Annex R is not applicable.

All other annexes are applicable.

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Product No. C62012