Tank Inspection, Repair, Alteration, 

and Reconstruction

API STANDARD 653

FOURTH EDITION, APRIL 2009

ADDENDUM 1, AUGUST 2010

ADDENDUM 2, JANUARY 2012

ADDENDUM 3, NOVEMBER 2013

Tank Inspection, Repair, Alteration, 

and Reconstruction

Downstream Segment

API STANDARD 653

FOURTH EDITION, APRIL 2009

ADDENDUM 1, AUGUST 2010

ADDENDUM 2, JANUARY 2012

ADDENDUM 3, NOVEMBER 2013
Special Notes

API publications necessarily address problems of a general nature. With respect to particular circumstances, local, 

state, and federal laws and regulations should be reviewed.

Neither API nor any of API's employees, subcontractors, consultants, committees, or other assignees make any 

warranty or representation, either express or implied, with respect to the accuracy, completeness, or usefulness of the 

information contained herein, or assume any liability or responsibility for any use, or the results of such use, of any 

information or process disclosed in this publication. Neither API nor any of API's employees, subcontractors, 

consultants, or other assignees represent that use of this publication would not infringe upon privately owned rights.

Classified areas may vary depending on the location, conditions, equipment, and substances involved in any given 

situation. Users of this standard should consult with the appropriate authorities having jurisdiction.

Users of this standard should not rely exclusively on the information contained in this document. Sound business, 

scientific, engineering, and safety judgment should be used in employing the information contained herein. 

API publications may be used by anyone desiring to do so. Every effort has been made by the Institute to assure the 

accuracy and reliability of the data contained in them; however, the Institute makes no representation, warranty, or 

guarantee in connection with this publication and hereby expressly disclaims any liability or responsibility for loss or 

damage resulting from its use or for the violation of any authorities having jurisdiction with which this publication may 

conflict.

API publications are published to facilitate the broad availability of proven, sound engineering and operating 

practices. These publications are not intended to obviate the need for applying sound engineering judgment 

regarding when and where these publications should be utilized. The formulation and publication of API publications 

is not intended in any way to inhibit anyone from using any other practices.

Any manufacturer marking equipment or materials in conformance with the marking requirements of an API standard 

is solely responsible for complying with all the applicable requirements of that standard. API does not represent, 

warrant, or guarantee that such products do in fact conform to the applicable API standard.

10

All rights reserved. No part of this work may be reproduced, translated, stored in a retrieval system, or transmitted by any means, 

electronic, mechanical, photocopying, recording, or otherwise, without prior written permission from the publisher. Contact the

Publisher, API Publishing Services, 1220 L Street, N.W., Washington, D.C. 20005. 

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

10

13
Foreword

[text deleted]

Nothing contained in any API publication is to be construed as granting any right, by implication or otherwise, for the 

manufacture, sale, or use of any method, apparatus, or product covered by letters patent. Neither should anything 

contained in the publication be construed as insuring anyone against liability for infringement of letters patent.

Shall: As used in a standard, “shall” denotes a minimum requirement in order to conform to the specification.

Should: As used in a standard, “should” denotes a recommendation or that which is advised but not required in order 

to conform to the specification. 

Each edition, revision, or addenda to this API standard may be used beginning with the date of issuance shown on 

the cover page for that edition, revision, or addenda. Each edition, revision, or addenda to this API standard becomes 

effective six months after the date of issuance for equipment that is certified as being rerated, reconstructed, 

relocated, repaired, modified (altered), inspected, and tested per this standard. During the six-month time between 

the date of issuance of the edition, revision, or addenda and the effective date, the purchaser and manufacturer shall 

specify to which edition, revision, or addenda the equipment is to be rated, reconstructed, relocated, repaired, 

modified (altered), inspected, and tested.

Portions of this publication have been changed from the previous edition. The locations of changes have been 

marked with a bar in the margin, as shown to the left of this paragraph. In some cases the changes are significant, 

while in other cases the changes reflect minor editorial adjustments such as renumbering of a section/subsection or 

figure/table (the references within the text that have been subsequently updated are not marked). The bar notations in 

the margins are provided as an aid to users, but API makes no warranty as to the accuracy of such bar notations.

This document was produced under API standardization procedures that ensure appropriate notification and 

participation in the developmental process and is designated as an API standard. Questions concerning the 

interpretation of the content of this publication or comments and questions concerning the procedures under which 

this publication was developed should be directed in writing to the Director of Standards, American Petroleum 

Institute, 1220 L Street, N.W., Washington, D.C. 20005. Requests for permission to reproduce or translate all or any 

part of the material published herein should also be addressed to the director.

Generally, API standards are reviewed and revised, reaffirmed, or withdrawn at least every five years. A one-time 

extension of up to two years may be added to this review cycle. Status of the publication can be ascertained from the 

API Standards Department, telephone (202) 682-8000. A catalog of API publications and materials is published 

annually by API, 1220 L Street, N.W., Washington, D.C. 20005.

Suggested revisions are invited and should be submitted to the Standards Department, API, 1220 L Street, NW, 

Washington, D.C. 20005, standards@api.org.

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Contents

Page

1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

1.2 Compliance with This Standard. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

1.3 Jurisdiction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

1.4 Safe Working Practices. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

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

2.1 Referenced Publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

2.2 Other References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2

3 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

4 Suitability for Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1

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

4.2 Tank Roof Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1

4.3 Tank Shell Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-2

4.4 Tank Bottom Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-11

4.5 Tank Foundation Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-15

5 Brittle Fracture Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1

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

5.2 Basic Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1

5.3 Assessment Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1

6 Inspection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1

6.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1

6.2 Inspection Frequency Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1

6.3 Inspections from the Outside of the Tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1

6.4 Internal Inspection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2

6.5 Alternative to Internal Inspection to Determine Bottom Thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-6

6.6 Preparatory Work for Internal Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-6

6.7 Inspection Checklists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-6

6.8 Records. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-6

6.9 Reports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-7

6.10 Nondestructive Examination (NDE). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-8

7 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1

7.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1

7.2 New Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1

7.3 Original Materials for Reconstructed Tanks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1

7.4 Welding Consumables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1

8 Design Considerations for Reconstructed Tanks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1

8.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1

8.2 New Weld Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1

8.3 Existing Weld Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1

8.4 Shell Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1

8.5 Shell Penetrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1

8.6 Windgirders and Shell Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-2

8.7 Roofs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-2

8.8 Seismic Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-2

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9 Tank Repair and Alteration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1

9.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1

9.2 Removal and Replacement of Shell Plate Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1

9.3 Shell Repairs Using Lap-welded Patch Plates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-6

9.4 Repair of Defects in Shell Plate Material. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-8

9.5 Alteration of Tank Shells to Change Shell Height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-8

9.6 Repair of Defective Welds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-8

9.7 Repair of Shell Penetrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-8

9.8 Addition or Replacement of Shell Penetrations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-10

9.9 Alteration of Existing Shell Penetrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-10

9.10 Repair of Tank Bottoms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-13

9.11 Repair of Fixed Roofs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-19

9.12 Repair of Floating Roofs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-20

9.13 Repair or Replacement of Floating Roof Perimeter Seals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-20

9.14 Hot Taps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-21

10 Dismantling and Reconstruction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-1

10.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-1

10.2 Cleaning and Gas Freeing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-1

10.3 Dismantling Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-1

10.4 Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-3

10.5 Dimensional Tolerances10-5

11 Welding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-1

11.1 Welding Qualifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-1

11.2 Identification and Records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-1

11.3 Preheat or Controlled Deposition Welding Methods as Alternatives to Post-weld Heat 

Treatment (PWHT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-1

11.4 Welding Safety. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-3

12 Examination and Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12-1

12.1 NDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12-1

12.2 Radiographs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12-3

12.3 Hydrostatic Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12-5

12.4 Leak Tests. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12-7

12.5 Measured Settlement During Hydrostatic Testing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12-8

13 Marking and Recordkeeping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-1

13.1 Nameplates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-1

13.2 Recordkeeping. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-2

13.3 Certification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-2

Annex A Background on Past Editions of API Welded Storage Tank Standards. . . . . . . . . . . . . . . . . . . . . . . A-1

Annex B Evaluation of Tank Bottom Settlement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-1

Annex C Checklists for Tank Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-1

Annex D Authorized Inspector Certification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D-1

Annex E (Intentionally Left Blank) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E-1

Annex F NDE Requirements Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F-1

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Annex G Qualification of Tank Bottom Examination Procedures and Personnel . . . . . . . . . . . . . . . . . . . . . G-1

Annex H Similar Service Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . H-1

Annex I Inquiries and Suggestions for Change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-1

Annex S Austenitic Stainless Steel Storage Tanks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S-1

Annex SC Stainless and Carbon Steel Mixed Materials Storage Tanks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SC-1

Annex X Duplex Stainless Steel Storage Tanks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . X-1

Figures

4.1 Inspection of Corrosion Areas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-3

4.2 Pit Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-5

5.1 Brittle Fracture Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-2

5.2 Exemption Curve for Tanks Constructed from Carbon Steel of Unknown Material Specification . . . . 5-3

9.1 Acceptable Details for Replacement of Shell Plate Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2

9.2 Details for Door Sheets in Riveted Seam Tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-4

9.3 Details for Door Sheets in Lap Weld Seam Tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-4

9.4 Details for Door Sheet in Butt-Weld Shell Seam Tank—No Vertical Seam Offset . . . . . . . . . . . . . . . . . . 9-5

9.5 Details for Door Sheet in Bull-Weld Shell Seam tank—Vertical Seam Offset. . . . . . . . . . . . . . . . . . . . . . 9-5

9.6 Lapped Patch Repair Plates at the External Shell-to-bottom Joint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-7

9.7 Typical Details for Addition of Reinforcing Plate to Existing Shell Penetration . . . . . . . . . . . . . . . . . . . 9-9

9.8 Typical Details for Addition of “Tombstone” Shape Reinforcing Plate to Existing Shell Penetration . 9-9

9.9 Method for Raising Shell Nozzles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9-11

9.10 Details for Installing a New Bottom Through an Existing Tombstone Reinforcing Plate . . . . . . . . . . . 9-12

9.11 Details for Installing a New Bottom Through an Existing Tombstone Reinforcing Plate . . . . . . . . . . . 9-13

9.12 Details for Installing a New Bottom Through an Existing Tombstone Reinforcing Plate . . . . . . . . . . . 9-14

9.13 Typical Welded-on Patch Plates on Tank Bottom Plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-16

9.14 Hot Tap for Tanks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-23

10.1 Tank Shell and Bottom Cut Locations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-2

13.1 Nameplate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-3

13.2 Certification Forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-4

B.1 Measurements of Shell Settlement (External) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-2

B.2 Measurements of Bottom Settlement (Internal) Tank Out-of-service . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-2

B.3 Graphical Representation of Tank Shell Settlement per B.2.2.4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-3

B.4 Graphical Representation of Shell Settlement per B.2.2.5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-6

B.5 Graphical Representation of Shell Settlement per B.2.2.5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-6

B.6 Edge Settlement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-7

B.7 Correction for Measured Edge Settlement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-8

B.8 Bottom Settlement Near Shell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-9

B.9 Localized Bottom Depressions or Bulges Remote from Shell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-10

B.10 Localized Tank Bottom Settlement Limits for Single Pass Welds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-12

B.11 Maximum Allowable Edge Settlement for Areas with Bottom Lap Welds

Approximately Parallel to the Shell. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-14

B.12 Maximum Allowable Edge Settlement for Areas with Bottom Lap Welds

Approximately Perpendicular to the Shell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-15

B.13 Edge Settlement with a Lap Weld at an Arbitrary Angle to the Shell . . . . . . . . . . . . . . . . . . . . . . . . . . . B-15

H.1 Steps in Conducting Similar Service Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . H-6

H.2 Example Corrosion Rate Curves for Bottom of Storage Tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . H-7

H.3 Example Corrosion Rate Curves for Top Course of Storage Tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . H-8

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Page

Tables

4.1 Maximum Allowable Shell Stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-7

4.2 Joint Efficiencies for Welded Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8

4.3 Joint Efficiencies for Riveted Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-10

4.4 Bottom Plate Minimum Thickness. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-15

4.5 Annular Bottom Plate Thicknesses (in.). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-15

6.1 Tank Safeguard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-4

9.1 Hot Tap Connection Sizes and Shell Plate Thicknesses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-21

10.1 Maximum Thicknesses on New Welds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-4

10.2 Radii Tolerances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-5

11.1 Welding Methods as Alternatives to Post-weld Heat Treatment (PWHT) Qualification Thicknesses 

for Test Plates and Repair Grooves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-2

A.1 Editions of API Standard 650 and its Precursor, API Standard 12C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-1

G.1 Suggested Essential Variables for Qualification Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-6

H.1 Similar Service Product Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . H-5

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1-1

Tank Inspection, Repair, Alteration, and Reconstruction

1 Scope

1.1 Introduction

1.1.1 This standard covers steel storage tanks built to API 650 and its predecessor API 12C. It provides minimum 

requirements for maintaining the integrity of such tanks after they have been placed in service and addresses 

inspection, repair, alteration, relocation, and reconstruction.

1.1.2 The scope is limited to the tank foundation, bottom, shell, structure, roof, attached appurtenances, and 

nozzles to the face of the first flange, first threaded joint, or first welding-end connection. Many of the design, welding, 

examination, and material requirements of API 650 can be applied in the maintenance inspection, rating, repair, and 

alteration of in-service tanks. In the case of apparent conflicts between the requirements of this standard and API 650 

or its predecessor API 12C, this standard shall govern for tanks that have been placed in service.

1.1.3 This standard employs the principles of API 650; however, storage tank owner/operators, based on 

consideration of specific construction and operating details, may apply this standard to any steel tank constructed in 

accordance with a tank specification.

1.1.4 This standard is intended for use by organizations that maintain or have access to engineering and inspection 

personnel technically trained and experienced in tank design, fabrication, repair, construction, and inspection.

1.1.5 This standard does not contain rules or guidelines to cover all the varied conditions which may occur in an 

existing tank. When design and construction details are not given, and are not available in the as-built standard, 

details that will provide a level of integrity equal to the level provided by the current edition of API 650 must be used.

1.1.6 This standard recognizes fitness-for-service assessment concepts for evaluating in-service degradation of 

pressure containing components. API 579-1/ASME FFS-1, Fitness-For-Service, provides detailed assessment 

procedures or acceptance criteria for specific types of degradation referenced in this standard. When this standard 

does not provide specific evaluation procedures or acceptance criteria for a specific type of degradation or when this 

standard explicitly allows the use of fitness-for-service criteria, API 579-1/ASME FFS-1 may be used to evaluate the 

various types of degradation or test requirements addressed in this standard.

1.2 Compliance with This Standard

The owner/operator has ultimate responsibility for complying with the provisions of this standard. The application 

of this standard is restricted to organizations that employ or have access to an authorized inspection agency as 

defined in 3.3. Should a party other than the owner/operator be assigned certain tasks, such as relocating and 

reconstructing a tank, the limits of responsibility for each party shall be defined by the owner/operator prior to 

commencing work.

1.3 Jurisdiction

If any provision of this standard presents a direct or implied conflict with any statutory regulation, the regulation shall 

govern. However, if the requirements of this standard are more stringent than the requirements of the regulation, then 

the requirements of this standard shall govern.

1.4 Safe Working Practices

An assessment shall be made of the potential hazards to which personnel may be exposed when conducting 

internal tank inspections, making repairs, or dismantling tanks. Procedures shall be developed according to the 

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1-2 API STANDARD 653

guidelines given in API 2015, API 2016, and API 2217A that will include safeguard for personnel health and 

safety, prevention of accidental fires and explosions, and the prevention of property damage. Conformance to 

permit procedures is an essential safe work practice for protection of personnel and property.Where welding and 

hot work are involved, API 2009 states “Except in areas specifically designated as safe for hot work, a hot work 

permit shall be obtained before starting any work that can involve a source of ignition.”

Special procedures may need to be developed for certain activities described in this standard that are not fully 

covered by the referenced API publications; e.g. safety precautions for personnel accessing floating roof tanks that 

are in service, or gas freeing the bottom side of a tank. Appendix B of API 2009 provides brief information on inerting 

tanks. Use of inerting as a safety precaution should address personnel hazards introduced when using inert gas in 

the workplace and implementation should be done in consultation with specialists that are familiar with such 

processes. Finally, procedures must comply with any federal or state safety regulations pertaining to “confined 

spaces” or any other relevant provisions.

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2-1

Section 2—References

2.1 Referenced Publications

The following standards, codes, publications, and specifications are cited in this standard. The latest edition or 

revision shall be used unless otherwise noted.

API Recommended Practice 579-1/ASME FFS-1, Fitness-For-Service

API Recommended Practice 580, Risk Based Inspection

API Standard 620, Design and Construction of Large, Welded, Low-pressure Storage Tanks

API Standard 650, Welded Tanks for Oil Storage

API Recommended Practice 651, Cathodic Protection of Aboveground Storage Tanks

API Recommended Practice 652, Lining of Aboveground Petroleum Storage Tank Bottoms

API Standard 2000, Venting Atmospheric and Low-pressure Storage Tanks: Nonrefrigerated and Refrigerated

API Recommended Practice 2003, Protection Against Ignitions Arising Out of Static, Lightning, and Stray Currents

API Recommended Practice 2009, Safe Welding, Cutting, and Hot Work Practices in the Petroleum and 

Petrochemical Industries

API Standard 2015, Requirements for Safe Entry and Cleaning of Petroleum Storage Tanks

API Recommended Practice 2016, Guidelines and Procedures for Entering and Cleaning Petroleum Storage Tanks

API Recommended Practice 2201, Safe Hot Tapping Practices in the Petroleum and Petrochemical Industries

API Recommended Practice 2207, Preparing Tank Bottoms for Hot Work

API Standard 2217A, Guidelines for Safe Work in Inert Confined Spaces in the Petroleum and Petrochemical Industries

ASME Boiler and Pressure Vessel Code (BPVC) 1, Section V: Nondestructive Examination

ASME BPVC, Section VIII: Pressure Vessels; Division 2: Alternative Rules

ASME BPVC, Section IX: Welding and Brazing Qualifications

ASNT SNT-TC-1A 2, Personnel Qualification and Certification in Nondestructive Testing

ASTM A6 3, 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

1 ASME International, 3 Park Avenue, New York, New York 10016-5990, www.asme.org.

2 American Society for Nondestructive Testing, 1711 Arlingate Lane, Columbus, Ohio, 43228-0518, www.asnt.org.

3 ASTM International, 100 Barr Harbor Drive, West Conshohocken, Pennsylvania 19428-2959, www.astm.org.

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2-2 API STANDARD 653

ASTM A36, Standard Specification for Carbon Structural Steel

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

ASTM A992, Standard Specification for Structural Steel Shapes

AWS D1.1 4, Structural Welding Code—Steel

AWS D1.6, Structural Welding Code—Stainless Steel

NACE Standard RP 0205-2005 5, Recommended Practice for the Design, Fabrication, and Inspection of Tanks for the 

Storage of Petroleum Refining Alkylation Unit Spent Sulfuric Acid at Ambient Temperatures

2.2 Other References

Although not cited in this standard, the following publication may be of interest.

API Standard 2610, Design, Construction, Operation, Maintenance, and Inspection of Terminal and Tank Facilities

ANSI/AWS Z49.1, Safety in Welding and Cutting and Allied Processes

4 American Welding Society, 550 N.W. LeJeune Road, Miami, Florida 33135, www.aws.org.

5 NACE International (formerly the National Association of Corrosion Engineers), 1440 South Creek Drive, Houston, Texas 

77218-8340, www.nace.org.

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3-1

Section 3—Definitions

For the purposes of this standard, the following definitions apply.

3.1

alteration

Any work on a tank that changes its physical dimensions or configuration.

3.2

as-built standard

The standard (such as API standard or UL 5 standard) used for the construction of the tank component in question. If 

this standard is not known, the as-built standard is the standard that was in effect at the date of the installation of the 

component. If the date of the installation of the component is unknown, then the current applicable standard shall be 

considered to be the as-built standard. See Annex A for a list of API welded storage tank standards. The standard 

used for repairs or alterations made after original construction is the as-built standard only for those repairs or 

alterations, so there may be more than one as-built standard for a tank. 

3.3

authorized inspection agency

One of the following organizations that employ an aboveground storage tank inspector certified by API.

a) The inspection organization of the jurisdiction in which the aboveground storage tank is operated.

b) The inspection organization of an insurance company which is licensed or registered to and does write 

aboveground storage tank insurance.

c) An owner/operator of one or more aboveground storage tank(s) who maintains an inspection organization for 

activities relating only to his/her equipment and not for aboveground storage tanks intended for sale or resale.

d) An independent organization or individual under contract to and under the direction of an owner/operator and 

recognized or otherwise not prohibited by the jurisdiction in which the aboveground storage tank is operated. The 

owner/operator’s inspection program shall provide the controls necessary for use by authorized inspectors 

contracted to inspect aboveground storage tanks.

3.4

authorized inspector

An employee of an authorized inspection agency who is qualified and certified to perform inspections under this 

inspection standard. Whenever the term inspector is used in API 653, it refers to an authorized API Standard 653 

inspector.

3.5

breakover point

The area on a tank bottom where settlement begins.

3.6

candidate tank

The tank(s) for which corrosion rates are not known. 

3.7

change in service

A change from previous operating conditions involving different properties of the stored product such as specific 

gravity or corrosivity and/or different service conditions of temperature and/or pressure.

5 Underwriters Laboratories, 333 Pfingsten Road, Northbrook, Illinois, 60062-2096, www.ul.com.

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3-2 API STANDARD 653

3.8

control tank

The tank(s) for which corrosion rates and service history are known and documented.

3.9

corrosion rate

The total metal loss divided by the period of time over which the metal loss occurred.

3.10

critical zone

The portion of the tank bottom or annular plate within 3 in. of the inside edge of the shell, measured radially inward.

3.11

current applicable standard

The current edition of the standard (such as API standard or UL standard) that applies if the tank were built today.

3.12

door sheet

A plate (or plates) cut from an existing tank shell to create a temporary access opening. After planned work is 

completed, the door sheet(s) shall be reinstalled or replaced.

3.13

examiner

A person who assists the inspector by performing specific nondestructive examination (NDE) on aboveground 

storage tanks and evaluates to the applicable acceptance criteria, but does not interpret the results of those 

examinations in accordance with API 653, unless specifically trained and authorized to do so by the owner/user.

3.14

external inspection

A formal visual inspection, conducted or supervised by an authorized inspector, to assess all aspects of the tank as 

possible without suspending operations or requiring tank shutdown (see 6.3.2).

3.15

fitness-for-service assessment

A methodology whereby flaws contained within a structure are assessed in order to determine the adequacy of the 

flawed structure for continued service without imminent failure.

3.16

hot tap

Identifies a procedure for installing a nozzle in the shell of a tank that is in service.

3.17

hydrotest

A test performed with water, in which static fluid head is used to produce test loads. 

3.18

inspector

A shortened title for an authorized tank inspector qualified and certified in accordance with this standard.

3.19

internal inspection

A formal, complete inspection, as supervised by an authorized inspector, of all accessible internal tank surfaces (see 

6.4.1).

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TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 3-3

3.20

major alteration/or major repair

An alteration or repair that includes any of the following:

a) installing a shell penetration larger than NPS 12 beneath the design liquid level;

b) installing a bottom penetration within 12 in. of the shell;

c) removing and replacing or adding a shell plate beneath the design liquid level where the longest dimension of the 

replacement plate exceeds 12 in.; 

d) removing or replacing annular plate ring material where the longest dimension of the replacement plate exceeds 

12 in.;

e) complete or partial (more than one-half of the weld thickness) removal and replacement of more than 12 in. of 

vertical weld joining shell plates or radial weld joining the annular plate ring;

f) installing a new bottom;

NOTE Installation of a portion of a new bottom as described in 12.3.3.3 is not defined as a major repair.

g) removing and replacing part of the weld attaching the shell to the bottom, or to the annular plate ring, in excess of 

the amounts listed in 12.3.2.5.1 a); 

h) jacking a tank shell.

3.21

owner/operator

The legal entity having both control of and/or responsibility for the operation and maintenance of an existing storage 

tank.

3.22

product-side

The side of the tank that is in contact with the stored liquid product. 

3.23

recognized toughness

A condition that exists when the material of a component is deemed acceptable for use by the provisions of any of the 

following sections of this standard:

a) Section 5.3.2 (based on edition of standard of tank’s original construction, or by coupon testing);

b) Section 5.3.5 (based on thickness);

c) Section 5.3.6 (based on lowest design metal temperature);

d) Section 5.3.8 (based on exemption curves).

3.24

reconstruction

Any work necessary to reassemble a tank that has been dismantled and relocated to a new site.

3.25

reconstruction organization

The organization having assigned responsibility by the owner/operator to design and/or reconstruct a tank.

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3-4 API STANDARD 653

3.26

repair

Work necessary to maintain or restore a tank to a condition suitable for safe operation. Repairs include both major 

repairs (see 3.20) and repairs that are not major repairs. Examples of repairs include:

a) removal and replacement of material (such as roof, shell, or bottom material, including weld metal) to maintain tank 

integrity;

b) re-leveling and/or jacking of a tank shell, bottom, or roof;

c) adding or replacing reinforcing plates (or portions thereof) to existing shell penetrations;

d) repair of flaws, such as tears or gouges, by grinding and/or gouging followed by welding.

3.27

repair organization

An organization that meets any of the following:

a) an owner/operator of aboveground storage tanks who repairs or alters his/her own equipment in accordance with 

this standard;

b) a contractor whose qualifications are acceptable to the owner/operator of aboveground storage tanks and who 

makes repairs or alterations in accordance with this standard;

c) one who is authorized by, acceptable to, or otherwise not prohibited by the jurisdiction, and who makes repairs in 

accordance with this standard.

3.28

similar service assessment

The process by which corrosion rates and inspection intervals are established for a candidate tank using corrosion 

rates and service history from a control tank for the purpose of establishing the next inspection date. 

3.29

soil-side

The side of the tank bottom that is in contact with the ground.

3.30

storage tank engineer

One or more persons or organizations acceptable to the owner/operator who are knowledgeable and experienced in 

the engineering disciplines associated with evaluating mechanical and material characteristics that affect the integrity 

and reliability of aboveground storage tanks. The storage tank engineer, by consulting with appropriate specialists, 

should be regarded as a composite of all entities needed to properly assess the technical requirements. 

3.31

unknown toughness

A condition that exists when it cannot be demonstrated that the material of a component satisfies the definition of 

recognized toughness.

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4-1

Section 4—Suitability for Service

4.1 General 

4.1.1 When the results of a tank inspection show that a change has occurred from the original physical condition of 

that tank, an evaluation shall be made to determine its suitability for continued use.

4.1.2 This section provides an evaluation of the suitability of an existing tank for continued service, or for a change 

of service, or when making decisions involving repairs, alterations, dismantling, relocating, or reconstructing an 

existing tank.

4.1.3 The following list of factors for consideration is not all-inclusive for all situations, nor is it intended to be a 

substitute for the engineering analysis and judgment required for each situation:

a) internal corrosion due to the product stored or water bottoms;

b) external corrosion due to environmental exposure;

c) stress levels and allowable stress levels;

d) properties of the stored product such as specific gravity, temperature, and corrosivity;

e) metal design temperatures at the service location of the tank;

f) external roof live load, wind, and seismic loadings;

g) tank foundation, soil, and settlement conditions;

h) chemical analysis and mechanical properties of the materials of construction;

i) distortions of the existing tank;

j) operating conditions such as filling/emptying rates and frequency.

4.2 Tank Roof Evaluation

4.2.1 General

4.2.1.1 The structural integrity of the roof and roof support system shall be verified.

4.2.1.2 Roof plates corroded to an average thickness of less than 0.09 in. in any 100 in.2 area or roof plates with any 

holes through the roof plate shall be repaired or replaced.

4.2.2 Fixed Roofs

4.2.2.1 Roof support members (rafters, girders, columns, and bases) shall be inspected for soundness by a method 

acceptable to the responsible inspector. Distorted (such as out-of-plumb columns), corroded, and damaged members 

shall be evaluated and repaired or replaced if necessary. Particular attention must be given to the possibility of severe 

internal corrosion of pipe columns (corrosion may not be evidenced by external visual inspection).

4.2.2.2 When a frangible roof-to-shell joint is required, evaluate for items impacting compliance with requirements 

under API 650, Section 5.10.2.6. Examples of some items to evaluate include tank bottom-to-shell joint corrosion or 12
4-2 API STANDARD 653

tank roof-to-shell joint modification (such as reinforcement of the joint, attachment of handrail, or other frangible joint 

area change). 

4.2.3 Floating Roofs

4.2.3.1 Areas of roof plates and pontoons exhibiting cracks or punctures shall be repaired or the affected sections 

replaced. Holes through roof plates shall be repaired or replaced.

4.2.3.2 Areas that are pitted shall be evaluated to determine the likelihood of through-pitting occurring prior to the 

next scheduled internal inspection. If so, the affected areas shall be repaired or replaced.

4.2.3.3 Roof support systems, perimeter seal systems, appurtenances such as a roof rolling ladder, anti-rotation 

devices, water drain systems, and venting systems shall be evaluated for needed repairs or replacements.

4.2.3.4 Guidance for the evaluation of existing floating roofs shall be based on the criteria of API 650, Annex C, for 

external floating roofs, and Annex H for internal floating roofs. However, upgrading to meet this standard is not 

mandatory.

4.2.4 Change of Service

4.2.4.1 Internal Pressure

All requirements of the current applicable standard (e.g. API 650, Annex F) shall be considered in the evaluation and 

subsequent alterations to the tank roof and roof-to-shell junction.

4.2.4.2 External Pressure

As applicable, the roof support structure (if any), and the roof-to-shell junction shall be evaluated for the effects of a 

design partial vacuum. The criteria outlined in API 650, Annex V shall be used.

4.2.4.3 Operation at Elevated Temperature

All requirements of API 650, Annex M, shall be considered before changing the service of a tank to operation at 

temperatures above 200 °F. 

4.2.4.4 Operation at Lower Temperature Than Original Design

If the operating temperature is changed to a lower temperature than the original design, the requirements of the 

current applicable standard for the lower temperature shall be met.

4.2.4.5 Normal and Emergency Venting

4.2.4.5.1 Effects of change in operating conditions (including product service and pumping rates) on normal and 

emergency venting shall be considered.

4.2.4.5.2 Vents shall be inspected for proper operation and screens shall be verified to be clear of obstruction.

4.3 Tank Shell Evaluation

4.3.1 General

4.3.1.1 Flaws, deterioration, or other conditions (e.g. change of service, relocation, corrosion greater than the 

original corrosion allowance) that might adversely affect the performance or structural integrity of the shell of an 

existing tank must be evaluated and a determination made regarding suitability for intended service.

13

13

13

13
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 4-3

4.3.1.2 The evaluation of the existing tank shell shall be conducted by a storage tank engineer and shall include an 

analysis of the shell for the intended design conditions, based on existing shell plate thickness and material. The 

analysis shall take into consideration all anticipated loading conditions and combinations, including pressure due to 

fluid static head, internal and external pressure, wind loads, seismic loads, roof live loads, nozzle loads, settlement, 

and attachment loads.

4.3.1.3 Shell corrosion occurs in many forms and varying degrees of severity and may result in a generally uniform 

loss of metal over a large surface area or in localized areas. Pitting may also occur. Each case must be treated as a 

unique situation and a thorough inspection conducted to determine the nature and extent of corrosion prior to 

developing a repair procedure. Pitting does not normally represent a significant threat to the overall structural integrity 

of a shell unless present in a severe form with pits in close proximity to one another. Criteria for evaluating both 

general corrosion and pitting are defined below.

4.3.1.4 Methods for determining the minimum shell thickness suitable for continued operation are given in 4.3.2, 

4.3.3, and 4.3.4 (see Section 6 for frequency of inspection).

4.3.1.5 If the requirements of 4.3.3 (welded) or 4.3.4 (riveted) cannot be satisfied, the corroded or damaged areas 

shall be repaired, or the allowable liquid level of the tank reduced, or the tank retired. The allowable liquid level for the 

continued use of a tank may be established by using the equations for a minimum acceptable thickness (see 4.3.3.1 

and 4.3.4.1) and solving for height, H. The actual thickness, as determined by inspection, minus the corrosion 

allowance shall be used to establish the liquid level limit. The maximum design liquid level shall not be exceeded.

Figure 4.1—Inspection of Corrosion Areas

A

L

A

t2

tnom

tavg

tank diameter D

a bcde

An area of

corrosion

Key

Procedure

SECTION A-A

a – e are inspection planes

selected by inspector.

t2 = least min. thickness

in entire area, exclusive

of pits.

1) Determine t2.

2) Calculate L = 3.7 Dt2, but not more than 40 in.

3) Locate L to get minimum tavg, which is t1.

Profile along Plane c , the

plane having the lowest

average thickness, t1.
4-4 API STANDARD 653

4.3.2 Actual Thickness Determination

4.3.2.1 For determining the controlling thicknesses in each shell course when there are corroded areas of 

considerable size, measured thicknesses shall be averaged in accordance with the following procedure (see 

Figure 4.1).

a) For each area, the authorized inspector shall determine the minimum thickness, t2, at any point in the corroded 

area, excluding widely scattered pits (see 4.3.2.2).

b) Calculate the critical length, L:

, but not more than 40 in.

where

L is the maximum vertical length, in inches, over which hoop stresses are assumed to “average out” around 

local discontinuities;

NOTE The actual vertical length of the corroded area may exceed L.

D is the tank diameter, in feet;

t2 is the least thickness, in inches, in an area of corrosion, exclusive of pits.

c) The authorized inspector shall visually or otherwise decide which vertical plane(s) in the area is likely to be the 

most affected by corrosion. Profile measurements shall be taken along each vertical plane for a distance, L. In the 

plane(s), determine the lowest average thickness, t1, averaged over a length of L, using at least five equally 

spaced measurements over length L.

d) See 4.3.3.1 for minimum permitted values for t1 and t2. The additional loads in 4.3.3.4 shall also be considered.

e) The criteria for continued operation is as follows:

i) the value t1 shall be greater than or equal to tmin (see 4.3.3 or 4.3.4), subject to verification of all other loadings 

listed in 4.3.3.5; 

ii) the value t2 shall be greater than or equal to 60 % of tmin; and

iii) any corrosion allowance required for service until the time of the next inspection shall be added to tmin and 

60 % of tmin.

4.3.2.2 Widely scattered pits may be ignored provided that:

a) no pit depth results in the remaining shell thickness being less than one-half the minimum acceptable tank shell 

thickness exclusive of the corrosion allowance; and

b) the sum of their dimensions along any vertical line does not exceed 2 in. in an 8-in. length (see Figure 4.2).

4.3.3 Minimum Thickness Calculation for Welded Tank Shell

NOTE In general, the minimum acceptable thickness (tmin) for an entire shell course is determined using 4.3.3.1 a) with H

determined to the bottom of each shell course and the results used as a basis for judging the suitability for continued service for the 

tank. If locally thinned areas are identified or if specific areas are investigated (such as for a shell nozzle installation), the method of 

4.3.3.1 b) may be used to complete the evaluation with H determined for that particular location.

L 3.7 Dt = 2
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 4-5

4.3.3.1 The minimum acceptable shell plate thickness for continued service shall be determined by one or more of 

the methods noted herein. These methods are limited to tanks with diameters equal to 200 ft or less.

a) When determining the minimum acceptable thickness for an entire shell course, tmin is calculated as follows:

b) When determining the minimum acceptable thickness for any other portions of a shell course (such as a locally 

thinned area or any other location of interest), tmin is calculated as follows:

where

tmin is the minimum acceptable thickness, in inches for each course as calculated from the above equation; 

however, tmin shall not be less than 0.1 in. for any tank course;

D is the nominal diameter of tank, in feet (ft);

H is the height from the bottom of the shell course under consideration to the maximum liquid level when 

evaluating an entire shell course, in feet (ft); or

is the height from the bottom of the length L (see 4.3.2.1) from the lowest point of the bottom of L of the 

locally thinned area to the maximum liquid level, in feet (ft); or

is the height from the lowest point within any location of interest to the maximum liquid level, in feet (ft);

G is the highest specific gravity of the contents;

Figure 4.2—Pit Measurement

d1

d2

d3

8 in.

d1 + d2 + d3 ... ≤ 2 in.

tmin

2.6 ( ) H – 1 DG

SE = ------------------------------------

tmin

2.6 HDG

SE = ----------------------
4-6 API STANDARD 653

S is the maximum allowable stress in pound force per square inch (lbf/in.2); use the smaller of 0.80Y or 0.429T

for bottom and second course; use the smaller of 0.88Y or 0.472T for all other courses. Allowable shell 

stresses are shown Table 4.1 for materials listed in the current and previous editions of API 12C and API 650;

NOTE for reconstructed tanks, S shall be in accordance with the current applicable standard;

Y is the specified minimum yield strength of the plate; use 30,000 lbf/in.2 if not known;

T is the smaller of the specified minimum tensile strength of the plate or 80,000 lbf/in.2; use 55,000 lbf/in.2 if 

not known;

E is the original joint efficiency for the tank. Use Table 4.2 if original E is unknown. E = 1.0 when evaluating the 

retirement thickness in a corroded plate, when away from welds or joints by at least the greater of 1 in. or 

twice the plate thickness. 

4.3.3.2 If the tank will be hydrostatically tested, the hydrostatic test height, Ht, shall be limited by one or more of the 

following methods. The tank shall not be filled above the level determined by the lesser value of Ht determined below.

a) After determining the controlling thickness of an entire shell course, Ht calculated as follows:

b) After determining the controlling thickness by 4.3.2.1 for a locally thinned area, or at any other location of interest 

within a shell course, Ht is calculated as follows:

where

Ht is the height from the bottom of the shell course under consideration to the hydrostatic test height when 

evaluating an entire shell course in feet; or

is the height from the bottom of the length, L, (see 4.3.2.1) for the most severely thinned area in each shell 

course to the hydrostatic test height in feet; or

is the height from the lowest point within any other location of interest to the hydrostatic test height in feet;

St is the maximum allowable hydrostatic test stress in pound force per square inch (lbf/in.2); use the smaller of 

0.88Y or 0.472T for bottom and second courses; use the smaller of 0.9Y or 0.519T for all other courses.

NOTE 1 Depending on the specific gravity of the content used to determine tmin, Ht may be less than H. Testing the tank to H

may yield the corroded area.

NOTE 2 If Ht is less than H, owner/operator shall determine the consequence and acceptability of operating the tank to H, its 

maximum design liquid level. Repairs to shell sections above Ht shall comply with the requirements of 12.3.2.

NOTE 3 For reconstructed tanks, St shall be per the current applicable standard.

4.3.3.3 Alternatively, the minimum acceptable shell plate thickness for tanks with diameters equal to or less than 

200 ft may be calculated in accordance with the variable design point method in API 650, 5.6.4, substituting “S × E” for 

“S ”; E and S may be defined as in 4.3.3.1.

4.3.3.4 The variable design point method shall be used for tanks greater than 200 ft in diameter, with all variables 

defined as in 4.3.3.1.

Ht

StEtmin

2.6D = --------------- + 1

Ht

StEtmin

2.6D = ---------------
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 4-7

Table 4.1—Maximum Allowable Shell Stresses (Not for Use for Reconstructed Tanks, See Note 6)

 

NOTE 1 ASTM A7, ASTM A9, ASTM A10 and ASTM A442 are obsolete ASTM material specifications previously listed in API 12C and API 650.

NOTE 2 The yield stress and tensile strength values shown are per API 653 for welded ASTM material of unknown origin.

NOTE 3 This provision is for riveted tanks, constructed of any grade of material, evaluated per 4.3.4.1 of this standard.

NOTE 4 This provision is for riveted tanks, constructed of known grades of material, evaluated per 4.3.4.2 of this standard. For all courses, the 

maximum allowable shell stress for both product and hydrostatic test conditions are listed under column for allowable product stress, S.

NOTE 5 This provision is for riveted tanks, constructed of unknown grades of material, evaluated per 4.3.4.2 of this standard.

NOTE 6 The allowable stresses for reconstructed tanks are tabulated in API 650, Table 5-2a or 5-2b or calculated per 8.4 of this standard.

NOTE 7 The allowable stresses are calculated per 4.3.3.1 and 4.3.3.2 of this standard, unless otherwise noted. The calculated allowable 

stresses are rounded to the nearest 100 lbf/in.2.

Material Specification 

and Grade

Minimum 

Specified 

Yield Stress, 

Y (lbf/in.2)

Minimum 

Specified 

Tensile 

 Strength, 

T (lbf/in.2)

Lower Two 

Courses Upper Courses Lower Two 

Courses Upper Courses

A 283-C 30,000 55,000 23,600 26,000 26,000 27,000

A285-C 30,000 55,000 23,600 26,000 26,000 27,000

A36 36,000 58,000 24,900 27,400 27,400 30,100

A131-A, B, CS 34,000 58,000 24,900 27,400 27,400 30,100

A131-EH 36 51,000 71,000 30,500 33,500 33,500 36,800

A573-58 32,000 58,000 24,900 27,400 27,400 28,800

A573-65 35,000 65,000 27,900 30,700 30,700 31,500

A573-70 42,000 70,000 30,000 33,000 33,000 36,300

A516-55 30,000 55,000 23,600 26,000 26,000 27,000

A516-60 32,000 60,000 25,600 28,200 28,200 28,800

A516-65 35,000 65,000 27,900 30,700 30,700 31,500

A516-70 38,000 70,000 30,000 33,000 33,000 34,200

A662-B 40,000 65,000 27,900 30,700 30,700 33,700

A662-C 43,000 70,000 30,000 33,000 33,000 36,300

A537-Class 1 50,000 70,000 30,000 33,000 33,000 36,300

A537-Class 2 60,000 80,000 34,300 37,800 37,800 41,500

A633-C, D 50,000 70,000 30,000 33,000 33,000 36,300

A678-A 50,000 70,000 30,000 33,000 33,000 36,300

A678-B 60,000 80,000 34,300 37,800 37,800 41,500

A737-B 50,000 70,000 30,000 33,000 33,000 36,300

A841 50,000 70,000 30,000 33,000 33,000 36,300

A10 (Note 1) 30,000 55,000 23,600 26,000 26,000 27,000

A7 (Note 1) 33,000 60,000 25,700 28,300 28,300 29,700

A442-55 (Note 1) 30,000 55,000 23,600 26,000 26,000 27,000

A442-60 (Note 1) 32,000 60,000 25,600 28,200 28,200 28,800

G40.21, 38W 38,000 60,000 25,700 28,300 28,300 31,100

G40.21, 44W 44,000 65,000 27,900 30,700 30,700 33,700

G40.21, 50W 50,000 65,000 27,900 30,700 30,700 33,700

G40.21, 50WT 50,000 70,000 30,000 33,000 33,000 36,300

Unknown (Note 2) 30,000 55,000 23,600 26,000 26,000 27,000

Riveted Tanks:

A7, A9 or A10 (Note 1, Note 3) NA NA 21,000 21,000 21,000 21,000

Known (Note 4) Y T Note 4 Note 4 Note 4 Note 4

Unknown (Note 5) NA NA 21,000 21,000 21,000 21,000

ASTM Specifications

CSA Specifications

Stress, S (lbf/in.2) (Note 7) Test Stress, St (lbf/in.2) (Note 7)

Allowable Product Allowable Hydrostatic

12
4-8 API STANDARD 653

4.3.3.5 The thickness determinations of 4.3.3.1, 4.3.3.2, and 4.3.3.3 consider liquid loading only. All other loads 

shall also be evaluated according to the original standard of construction; and engineering judgment shall be used to 

evaluate different conditions or new information. As applicable, the following loadings shall be taken into account:

a) wind-induced buckling;

b) seismic loads;

c) operation at temperatures over 200 °F;

d) vacuum-induced external pressure;

e) external loads caused by piping, tank-mounted equipment, hold down lugs, etc.;

f) wind-induced overturning;

g) loads due to settlement.

Table 4.2—Joint Efficiencies for Welded Joints

Standard Edition

and Year

Type

of Joint

Joint

Efficiency E

Applicability

or Limits

API 650

Seventh and Later

(1980 to Present)

Butt 1.00 Basic Standard

Butt 0.85 Annex A

Spot RT

Butt 0.70 Annex A

No RT

First to Sixth

(1961 to 1978)

Butt 0.85 Basic Standard

Butt 1.00 Annexes 

D and G

API 12C

14th and 15th

(1957 to 1958) Butt 0.85

3rd to 13th

(1940 to 1956)

Lap a 0.75 3/8 in. max. t

Butt c 0.85

First and Second

(1936 to 1939)

Lap a 0.70 7/16 in. max. t

Lap b 0.50 + k/5 1/4 in. max. t

Butt c 0.85

Unknown

Lap a 0.70 7/16 in. max. t

Lap b 0.50 + k/5 1/4 in. max. t

Butt 0.70

Lap d 0.35

a Full double lap-welded.

b Full fillet weld with at least 25 % intermittent full fillet opposite side; k = percent of intermittent weld 

expressed in decimal form. c Single butt-welded joints with a back-up bar were permitted from the years of 1936 to 1940 and 1948 to 

1954.

d Single lap-welded only.

13
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 4-9

4.3.3.6 As an alternative to the procedures described above, any thinning of the tank shell below minimum required 

wall thickness due to corrosion or other wastage may be evaluated to determine the adequacy for continued service 

by employing the design by analysis methods defined in Section VIII, Division 2, Appendix 4 of the ASME Code; or 

API 579-1/ASME FFS-1, Section 4, Section 5 or Section 6, as applicable. When using the ASME criteria, the stress 

value used in the original tank design shall be substituted for the Sm value of Division 2, if the design stress is less 

than or equal to the lesser of 2/3Y (specified minimum yield strength) or 1/3T (specified minimum tensile strength). If 

the original design stress is greater than 2/3Y or 1/3T, then the lesser of 2/3Y or 1/3T shall be substituted for Sm.

4.3.4 Minimum Thickness Calculation for Riveted Tank Shell

4.3.4.1 The minimum acceptable thickness for riveted tank shells shall be calculated using the equation in 4.3.3.1 

except that the following allowable stress criteria and joint efficiencies shall be used:

S is 21,000 lbf/in.2;

E is 1.0 for shell plate 6 in. or more away from rivets. See Table 4.3 for joint efficiencies for locations within 

6 in. of rivets.

4.3.4.2 The rivet joint efficiencies given in Table 4.3 are conservative minimums for riveted tank construction details 

and are included to simplify riveted tank evaluations. However, in some cases it may be advantageous to calculate 

the actual rivet joint efficiencies using computational methods applicable to lap and butt type riveted joints. When this 

alternative of calculated joint efficiencies is used, the following maximum allowable stresses shall apply:

a) for the maximum tensile stress in net section of plate, use the lesser of 0.80Y or 0.429T; use 21,000 lbf/in.2 if T or Y

is unknown;

b) for the maximum shear in net section of rivet, use 16,000 lbf/in.2;

c) for the maximum bearing stress on plates or rivets, use 32,000 lbf/in.2 for rivets in single shear, and 35,000 lbf/in.2

for rivets in double shear.

4.3.4.3 For tanks with riveted joints, consideration shall be given to whether, and to what extent, corrosion affects 

such joints. If calculations show that excess thickness exists, this excess may be taken as corrosion allowance.

4.3.4.4 Non-liquid loads (see 4.3.3.5) shall also be considered in the analysis of riveted tanks.

4.3.5 Distortions

4.3.5.1 Shell distortions include out-of-roundness, buckled areas, flat spots, and peaking and banding at welded 

joints.

4.3.5.2 Shell distortions can be caused by many conditions such as foundation settlement, over- or underpressuring, high wind, poor shell fabrication, or repair techniques, and so forth.

4.3.5.3 Shell distortions shall be evaluated on an individual basis to determine if specific conditions are considered 

acceptable for continuing tank service and/or the extent of corrective action.

4.3.6 Flaws

Flaws such as cracks or laminations shall be thoroughly examined and evaluated to determine their nature and extent 

and need for repair. If a repair is needed, a repair procedure shall be developed and implemented. The requirement 

for repairing scars such as arc strikes, gouges, or tears from temporary attachment welds must be evaluated on a 

case-by-case basis. Cracks in the shell-to-bottom weld shall be removed.

10

10
4-10 API STANDARD 653

4.3.7 Wind Girders and Shell Stiffeners

The evaluation of an existing tank shell for suitability for service must also consider the details and condition of any 

wind girders or shell stiffeners. Degradation by corrosion of these structural elements or their attachment welds to the 

shell may render these elements inadequate for the design conditions.

4.3.8 Shell Welds

The condition of the tank shell welds shall be evaluated for suitability for service using criteria from this standard, 

the as-built standard, or fitness-for-service assessment. Any flaws or deterioration such as corrosion or pitting of 

the existing welds shall be evaluated. If necessary, appropriate repair procedures shall be established or the tank 

safe fill height reassessed. Some typical shell butt-welded flaws and recommended procedures for repairs are 

given in 9.6. 

4.3.9 Shell Penetrations

4.3.9.1 The condition and details of existing shell penetrations (nozzles, manways, cleanout openings, etc.) shall be 

reviewed when assessing the integrity of an existing tank shell. Details such as type and extent of reinforcement, weld 

spacing, and thickness of components (reinforcing plate, nozzle neck, bolting flange, and cover plate), are important 

considerations and shall be reviewed for structural adequacy and compliance with the as-built standard. Existing 

welds on the tank shell that are not to be modified or affected by repairs and are closer than required by API 650 

(Seventh Edition or later) are acceptable for continued service if the welds are examined by the magnetic particle 

method and have no rejectable defects or indications. Grinding to eliminate weld defects is permissible if the resulting 

profile satisfies base thickness and weld size requirements. Weld repairs may not be used to accept weld spacings 

closer than permitted by API 650 (Seventh Edition or later) except as permitted by 9.10.2.7. Any other 

noncompliance, or deterioration due to corrosion, must be assessed and repair procedures established where 

appropriate or the tank re-rated, as necessary.

4.3.9.2 Nozzle wall thickness shall be evaluated for pressure and all other loads.

4.3.10 Operation at Elevated Temperatures

Tanks of welded construction that operate at elevated temperatures (exceeding 200 °F, but less than 500 °F) shall be 

evaluated for suitability of service. The requirements of this section are based in part on the requirements of API 650, 

Annex M.

Table 4.3—Joint Efficiencies for Riveted Joints

Type of 

Joint

Number of 

Rivet Rows

Joint Efficiency 

E

Lap 1 0.45

Lap 2 0.60

Lap 3 0.70

Lap 4 0.75

Butt a 2 b 0.75

Butt 3 b 0.85

Butt 4 b 0.90

Butt 5 b 0.91

Butt 6 b 0.92

a All butt joints listed have butt straps both inside and outside. b Number of row on each side of joint center line.

10

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TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 4-11

4.3.10.1 Continued Operation at Elevated Temperatures

4.3.10.1.1 Existing tanks that were originally designed and constructed to the requirements of API 650, Annex M1a or M-1b, shall be evaluated for continued service, as follows.

a) The tank shell shall be evaluated in conformance with 4.3.3, except that the allowable stress (S) for all shell 

courses shall not exceed 0.80Y. The value of Y shall be taken as the minimum specified yield strength of the shell 

material multiplied by the yield strength reduction factor in of API 650, Table M-1. When the minimum specified 

yield strength of the shell material is not known, the evaluation shall be based upon an assumed value of 

30,000 lbf/in.2.

b) If the bottom plate material in the critical zone has been reduced in thickness beyond the provisions of the original 

tank bottom corrosion allowance, if any, the shell-to-bottom joint shall be evaluated for elevated temperature, liquid 

head and thermal cycles. The simplified analysis technique recommended in API 650, Section M.4, may be used 

to satisfy this requirement.

4.3.10.1.2 Existing elevated temperature service tanks that were not originally designed and constructed to the 

requirements of API 650, Annex M, but have a successful service history of operation shall be evaluated for continued 

service as noted in 4.3.10.1.1. If the tank diameter exceeds 100 ft and the tank was not constructed with a buttwelded annular ring, an analysis of the critical zone is required [see 4.3.10.1.1 b)]. In addition, the maximum operating 

temperature shall not exceed the temperatures at which the tank has operated successfully in the past.

4.3.10.2 Conversion to Operation at Elevated Temperatures

Existing tanks that were not originally designed and constructed to the requirements of API 650, Annex M shall be 

evaluated for a change to service to elevated temperatures as follows.

a) The tank shell shall be evaluated in conformance with API 650, Annex M. The allowable shell stresses of this 

standard (API 653) shall not be used.

b) The need for a butt-welded annular ring shall be determined in conformance with API 650, Annex M and installed if 

required.

c) The shell-to-bottom joint shall be evaluated for fatigue conditions. In addition, the adequacy of the bottom plate 

material in the critical zone shall be based upon the requirements of this standard.

4.4 Tank Bottom Evaluation

4.4.1 General

Tank bottom inspection strategies shall provide suitable data which, when used with the procedures in this 

standard, will determine the tank bottom integrity necessary to prevent leakage of fluids that may cause 

environmental damage. Each aspect of corrosion phenomena, and other potential leak or failure mechanism must 

be examined. Periodic assessment of tank bottom integrity shall be performed in addition to the internal 

inspections specified in 6.4. The assessment period shall be less than or equal to the appropriate internal 

inspection interval given in 6.4.2. The use of leak detection tests or monitoring systems (such as double bottoms or 

liners under tank bottoms with leak detection pipes) will satisfy the requirement for periodic assessment between 

internal inspections.

Excessive foundation settlement of storage tanks can affect the integrity of tank shells and bottoms. Therefore, 

monitoring the settlement behavior of tanks is a recognized practice to assess the integrity of tank bottoms. See 

Annex B for techniques for evaluating tank bottom settlement.

13

12

13

13
4-12 API STANDARD 653

4.4.2 Causes of Bottom Failure

The following list gives some historical causes of tank bottom leakage or failure that shall be considered in the 

decision to line, repair, or replace a tank bottom:

a) internal pitting and pitting rates in the anticipated service;

b) corrosion of weld joints (weld and heat affected zone);

c) weld joint cracking history;

d) stresses placed on the bottom plates by roof support loads and shell settlement;

e) underside corrosion (normally in the form of pitting);

f) inadequate drainage resulting in surface water flowing under the tank bottom;

g) the lack of an annular plate ring when required;

h) uneven settlement that results in high localized stresses in the bottom plates;

i) roof support columns or other supports welded to the tank bottom where adequate allowance for movement was 

not made;

j) rock or gravel foundation pads with inadequately filled-in surface voids;

k) nonhomogeneous fill under the tank bottom (e.g. a lump of clay in a sand foundation pad);

l) inadequately supported sumps.

4.4.3 Tank Bottom Release Prevention Systems (RPSs)

API supports the use of a release prevention system (RPS) to maintain the integrity of tank bottoms. The term RPS 

refers to the suite of API standards and recommended practices that are designed to maintain tank integrity and thus 

protect the environment. With respect to tank bottoms, these include: internal inspection of the tank bottom; leak 

detection systems and leak testing of the tank; installing cathodic protection for the underside of the tank bottom; 

lining the bottom of the tank interior; providing a release prevention barrier (RPB) under the tank bottom; or some 

combination of these measures, depending on the operating environment and service of the tank.

4.4.3.1 Internal Inspection

Internal inspection of the tank bottom is intended to assess the current bottom integrity and identify problem 

conditions that may lead to future loss of integrity. Internal inspection techniques, such as bottom settlement 

monitoring, and considerations for determining appropriate inspection frequency, are found in 4.4.6, Section 6, Annex 

B, Annex C, and elsewhere.

4.4.3.2 Leak Detection Systems and Leak Testing

Tank leak detection systems and leak testing are intended to identify, quantify, and/or locate a tank bottom integrity 

failure that is not detectable visually or through inventory reconciliation. Leak detection may be integral to the tank 

design, either as constructed or as modified (e.g. RPB with interstitial monitoring) or may operate separately (e.g. soil 

vapor monitoring and chemical marker); may be operated by the tank owner or as a third party test or service; and 

may detect leaks continuously or on a periodic basis. Tank leak detection systems and testing methods are listed and 

discussed in API 575.
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 4-13

4.4.3.3 Cathodic Protection

Cathodic protection systems are intended to mitigate corrosion of steel surfaces in contact with soil, such as the 

underside of tank bottoms. A selection basis for cathodic protection systems is covered by API 651.

4.4.3.4 Internal Lining Protection

Internal linings and coatings for the top side of the tank bottom are intended to mitigate corrosion by providing a 

barrier between the tank bottom and corrosion sources. Applied linings and coatings for internal surfaces of tank 

bottoms are covered by API 652.

4.4.3.5 Release Prevention Barriers (RPBs)

An RPB includes steel bottoms, synthetic materials, clay liners, concrete pads, and all other barriers or combinations 

of barriers placed in the bottom of or under a tank, which have the function of: 

1) preventing the escape of released material, and 

2) containing or channeling released material for leak detection. 

RPB design is covered in detail in Annex I of API 650. Replacement of tank bottoms is covered in 9.10.2. 

If a decision is made to replace an existing bottom, API supports the evaluation of installing an RPB or continued use 

of an RPS. The evaluation should consider the effectiveness of other RPS controls, the product stored, the location of 

the tank, and environmental sensitivities.

4.4.4 Bottom Plate Thickness Measurements

Various methods for determining tank bottom plate soilside corrosion are available. The methods vary to the extent by 

which they can reliably measure general corrosion and pitting. A combination of these methods may be required along 

with extrapolation techniques and analysis to establish the probable conditions of the entire tank bottom. Magnetic flux 

leakage (MFL) tools are commonly used, along with ultrasonic (UT) thickness measurement tools, to examine tank 

bottoms. Ultrasonic thickness measurement techniques are often used to confirm and further quantify data obtained 

by MFL examination, but these techniques may not be required depending on the specific procedure and application. 

The quality of data obtained from both MFL and ultrasonic thickness techniques is dependent on personnel, 

equipment and procedures. Annex G may be used to provide guidance in qualifying personnel and procedures for 

obtaining thickness data.

4.4.5 Minimum Thickness for Tank Bottom Plate

Quantifying the minimum remaining thickness of tank bottoms based on the results of measurement can be done by 

the method outlined in 4.4.5.1. Other approaches such as the probabilistic method in 4.4.5.2 may be used.

4.4.5.1 An acceptable method for calculating the minimum acceptable bottom thickness for the entire bottom or 

portions thereof is as follows:

MRT = (Minimum of RTbc or RTip) – Or (StPr + UPr)

where

MRT is the minimum remaining thickness at the end of interval Or. This value must meet the requirements of 

Table 4.4 and 4.4.5.4 and 4.4.6;

Or is the in-service interval of operation (years to next internal inspection) not to exceed that allowed by 6.4.2;

13
4-14 API STANDARD 653

RTbc is the minimum remaining thickness from bottom side corrosion after repairs;

RTip is the minimum remaining thickness from internal corrosion after repairs;

StPr is the maximum rate of corrosion not repaired on the top side. StPr = 0 for coated areas of the bottom. The 

expected life of the coating must equal or exceed Or to use StPr = 0;

UPr is the maximum rate of corrosion on the bottom side. To calculate the corrosion rate, use the minimum 

remaining thickness after repairs. Assume a linear rate based on the age of the tanks. UPr = 0 for areas 

that have effective cathodic protection.

NOTE 1 For areas of a bottom that have been scanned by the magnetic flux leakage (or exclusion) process, and do 

not have effective cathodic protection, the thickness used for calculating UPr must be the lesser of the MFL threshold 

or the minimum thickness of corrosion areas that are not repaired. The MFL threshold is defined as the minimum 

remaining thickness to be detected in the areas examined. This value should be predetermined by the tank owner 

based on the desired inspection interval. 

Areas of bottom side corrosion that are repaired should be evaluated with the corrosion rate for the repaired area 

unless the cause of corrosion has been removed. The evaluation is done by using the corrosion rate of the repaired 

area for UPr, and adding the patch plate (if used) thickness to the term “minimum of RTbc or RTip.”

NOTE 2 Corrosion of the bottom plate includes loss of metal from isolated or general corrosion.

4.4.5.2 For the probabilistic method, a statistical analysis is made of thickness data from measurements (see 4.4.6) 

projecting remaining thickness, based on sample scanning of the bottom.

4.4.5.3 If the minimum bottom thicknesses, at the end of the in-service period of operation, are calculated to be less 

than the minimum bottom renewal thicknesses given in Table 4.4, or less than the minimum bottom renewal 

thicknesses providing acceptable risk as determined by an RBI assessment per 6.4.2.4, the bottom shall be lined, 

repaired, replaced, or the interval to the next internal inspection shortened.

4.4.5.4 Unless a stress analysis is performed, the minimum bottom plate thickness in the critical zone of the tank 

bottom defined in 9.10.1.2 shall be the smaller of one-half the original bottom plate thickness (not including the 

original corrosion allowance) or 50 % of tmin of the lower shell course per 4.3.3.1 but not less than 0.1 in. Isolated 

pitting will not appreciably affect the strength of the plate.

4.4.5.5 The repair of internal pitting, when performed to extend the in-service period of operation, shall be by pit 

welding, overlay welding, or lap patching, followed by inspection and testing. The extent of weld repairs is limited in 

the critical zone in accordance with 9.10.1.2. 

4.4.5.6 The treatment of bottom pitting by the use of non-welded repairs (e.g. coatings, caulking) can not be used to 

increase RTip for calculating MRT.

4.4.5.7 The thickness of the projection of the bottom plate beyond the shell as measured at the toe of the outside 

bottom-to-shell fillet weld shall not be less than 0.1 in. The projection of the bottom plate beyond the outside toe of the 

shell-to-bottom weld shell shall be at least 3/8 in. 

4.4.6 Minimum Thickness for Annular Plate Ring

4.4.6.1 Due to strength requirements, the minimum thickness of annular plate ring is usually greater than 

0.10 in. Isolated pitting will not appreciably affect the strength of the plate. Unless a stress analysis is performed, 

the annular plate thickness shall be in accordance with 4.4.6.2 or 4.4.6.3, as applicable.

4.4.6.2 For tanks in service with a product specific gravity less than 1.0, which require annular plates for other than 

seismic loading considerations, the thickness of the annular plates shall be not less than the thicknesses given in 

Table 4.5, plus any specified corrosion allowance. Interpolation is allowed within Table 4.5 based on shell stress 

determined per Note b of Table 4.5.

13

11
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 4-15

4.4.6.3 For tanks in service with a product specific gravity of 1.0 or greater, which require annular plates for other than 

seismic loading considerations, the thickness of the annular plates shall be in accordance with API 650, Table 5-1a or 5-

1b, plus any specified corrosion allowance. Interpolation is allowed within API 650, Table 5-1a or 5-1b based on shell 

stress determined per Note b of API 650, Table 5-1.

4.4.6.4 For tanks that utilize thickened annular plates for seismic considerations, a seismic evaluation shall be 

performed in accordance with the requirements of the as built standard, using the actual thickness of the existing 

annular plate.

4.4.6.5 For the thickness and projection of the annular plate beyond the shell, see 4.4.5.7. 

4.5 Tank Foundation Evaluation

4.5.1 General

4.5.1.1 The principal causes of foundation deterioration are settlement, erosion, cracking, and deterioration of 

concrete initiated by: calcining, attack by underground water, attack by frost, and attack by alkalies and acids. To 

ensure suitability for service, all tank foundations shall be inspected periodically (see 6.3).

Table 4.4—Bottom Plate Minimum Thickness

Minimum Bottom 

Plate Thickness at 

Next Inspection

(in.)

 Tank Bottom/

Foundation Design

0.10 Tank bottom/foundation design with no means 

for detection and containment of a bottom leak.

0.05

Tank bottom/foundation design with means to 

provide detection and containment of a bottom 

leak.

0.05

Applied tank bottom reinforced lining, 

> 0.05 in. thick, in accordance with 

API 652.

Table 4.5—Annular Bottom Plate Thicknesses (in.) (Product Specific Gravity < 1.0)

Plate Thickness a

of First Shell

Course 

(in.)

Stress b in First Shell Course

(lbf/in.2)

< 24,300 < 27,000 < 29,700 < 32,400

t ≤ 0.75 0.17 0.20 0.23 0.30

0.75 < t ≤ 1.00 0.17 0.22 0.31 0.38

1.00 < t ≤ 1.25 0.17 0.26 0.38 0.48

1.25 < t ≤ 1.50 0.22 0.34 0.47 0.59

t > 1.50 0.27 0.40 0.53 0.68

NOTE The thicknesses specified in the 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.

a Plate thickness refers to the tank shell as constructed.

b Stresses are calculated from [2.34D (H – 1)]/t.

12

10
4-16 API STANDARD 653

4.5.1.2 Some mechanisms of concrete deterioration are briefly described below.

a) calcining (loss of water of hydration) can occur when concrete has been exposed to sufficiently high temperature 

for a period of time. During intermediate cooling periods, the concrete can absorb moisture, swell, lose its strength, 

and crack;

b) deterioration of concrete exposed to underground water can be caused by chemical attack, by cyclic changes in 

temperature, and by freezing moisture;

c) expansion of freezing moisture in porous concrete, or in concrete with minor settlement cracks or temperature 

cracks, can result in spalling and/or the development of serious structural cracks;

d) sulfate-type alkalies, and to a lesser extent, chlorides, can act corrosively to destroy the bond of the concrete;

e) temperature cracks (hairline cracks of uniform width) do not seriously affect the strength of the concrete foundation 

structure; however, these cracks can be potential access points for moisture or water seepage that could 

eventually result in corrosion of the reinforcing steel.

4.5.1.3 When a tank is to be used in elevated temperature [> 93 °C (200 °F)] service, the provisions of API 650, 

Section B.6 shall be considered in the evaluation of the suitability for service of the tank foundation.

4.5.2 Foundation Repair or Replacement

4.5.2.1 If there is a need for foundation replacement or installation, the new foundation elevation profile must 

meet the tolerance in 10.5.6. Alternatively, if the new foundation is to be constructed up to the bottom, changing 

the levelness of the tank is not required if reviewed and approved by a storage tank engineer considering the 

plumbness of the shell, presence, or absence of shell distortion, and original construction levelness which warrant 

leaving the tank at the current state of levelness.

4.5.2.2 Concrete pads, ringwalls, and piers, showing evidence of spalling, structural cracks, or general deterioration, 

shall be repaired to prevent water from entering the concrete structure and corroding the reinforcing steel.

4.5.3 Anchor Bolts

Distortion of anchor bolts and excessive cracking of the concrete structures in which they are embedded may be 

indications of either serious foundation settlement or a tank overpressure uplift condition.

10

13
5-1

Section 5—Brittle Fracture Considerations

5.1 General

This section provides a procedure for the assessment of existing tanks for suitability for continued operation or 

change of service with respect to the risk of brittle fracture and does not supplement or replace the requirements of 

Section 12 for the examination and testing for the hydrostatic testing of repaired, modified, or reconstructed tanks. 

The procedure applies to both welded and riveted tanks; however, the procedure is based primarily on experience 

and data obtained from welded tanks.

5.2 Basic Considerations

5.2.1 A decision tree (see Figure 5.1) is used to present the assessment procedure for failure due to brittle fracture. 

The decision tree is based on the following principles.

5.2.2 In all reported incidents of tank failure due to brittle fracture, failure occurred either shortly after erection during 

hydrostatic testing or on the first filling in cold weather, after a change to lower temperature service, or after a repair/

alteration. This experience shows that once a tank has demonstrated the ability to withstand the combined effects of 

maximum liquid level (highest stresses) and lowest operating temperature without failing, the risk of failure due to 

brittle fracture with continued service is minimal.

5.2.3 Any change in service must be evaluated to determine if it increases the risk of failure due to brittle fracture. In 

the event of a change to a more severe service (such as operating at a lower temperature or handling product at a 

higher specific gravity) it is necessary to consider the need for a hydrostatic test to demonstrate fitness for a new 

more severe service. The following aspects should be considered:

a) the likelihood of repairs/alterations since the original hydrostatic test not meeting requirements of this standard;

b) deterioration of the tank since original hydrostatic test.

5.3 Assessment Procedure

5.3.1 The assessment procedure illustrated in Figure 5.1 shall be used. Each of the key steps, numbered 1 through 

11 on the decision tree, correspond sequentially to the explanations provided below.

5.3.2 Step 1—The tanks meet the requirements of API 650 (Seventh Edition or later) or API 650, Appendix G (Fifth 

and Sixth Editions) to minimize the risk of failure due to brittle fracture. Alternatively, tanks may also be shown to meet 

the toughness requirements of API 650 (Seventh Edition or later) by impact testing coupon samples from a 

representative number of shell plates.

5.3.3 Step 2—Many tanks that continue to operate successfully in the same service were not built to the 

requirements of API 650 (see editions and appendices named in 5.3.2). These tanks are potentially susceptible to 

failure due to brittle fracture and require an assessment as illustrated by the decision tree.

5.3.4 Step 3—For the purpose of this assessment, hydrostatic testing demonstrates fitness for continued service 

with minimal risk of failure due to brittle fracture, provided that all governing requirements for repairs, alterations, 

reconstruction, or change in service are in accordance with this standard (including a need for hydrostatic testing 

after major repairs, major alterations or reconstruction). The effectiveness of the hydrostatic test in demonstrating 

fitness for continued service is shown by industry experience.

12
5-2 API STANDARD 653 Tank meets API 650 (7th Edition or later) or API 650, Appendix G (5th or 6th Edition). Tank continues to operate in same service ? Alterations, repairs and reconstruction Meets API 653? Tank does not meet API 650 (1980) or API 650, Appendix G (5th or 6th Edition). Prior hydro demonstrates fitness for continued service ? Tank thickness 0.5 in. or below ?

Change in service

1

2

11

3 4 5 6 7 8

9 3

10

Is service

more severe?

Operates

at

metal

temperature

above

60 oF

?

Membrane

stress

below

7 ksi

?

Tank

full

at lowest

one day

temperature per

API 650

Fig. 4-2?

Does tank

meet the

requirements

of API 650

or equal?

Tank

continues

to operate

in same

service

?

Hydrotest to

demonstrate

fitness for

continued

service

Yes

Yes

Yes

Yes

Yes

Yes

Yes Yes

Yes

Yes Yes

Use Use

Use

Use Use Use

Use

Use

No

No

No

(See note)

No

Continue to use

Continue to use

Continue to use

No No No

No

No

Rerate tank based

on prior operating

history

	
	
	


	
		

metal temperature

		


NOTE This does not

exempt tanks from other

hydrotest requirements

stipulated in this standard.

Tank 

steel exempt

 from impact

testing API 650

 Table 4-1a or 4-1b

or API 653 

Fig. 5.2?

Figure 5.1—Brittle Fracture Considerations

12

12
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 5-3

5.3.5 Step 4—If a tank shell thickness is no greater than 0.5 in., the risk of failure due to brittle fracture is minimal, 

provided that an evaluation for suitability of service in accordance with Section 4 has been performed. The original 

nominal thickness for the thickest tank shell plate shall be used for this assessment.

5.3.6 Step 5—No known tank failures due to brittle fracture have occurred at shell metal temperatures of 60 °F or 

above. Similar assurance against brittle fracture can be gained by increasing the metal temperature by heating the 

tank contents.

5.3.7 Step 6—Industry experience and laboratory tests have shown that a membrane stress in tank shell plates of at 

least 7 ksi is required to cause failure due to brittle fracture.

5.3.8 Step 7—Tanks constructed from steels listed in Figure 4-1 of API 650 can be used in accordance with their 

exemption curves, provided that an evaluation for suitability of service in conformance with Section 4 of this standard 

has been performed. Additionally, tanks constructed in accordance with another nationally recognized code or 

standard containing toughness rules (such as API 620) may be used in accordance with the current toughness rules 

of that standard. Tanks fabricated from steels of unknown material specifications, thicker than 1/2 in. and operating at a 

shell metal temperature below 60 °F, can be used if the tank meets the requirements of Figure 5.2. The original 

nominal thickness for thickest tank shell plate shall be used for the assessment. For unheated tanks, the shell metal 

temperature shall be the design metal temperature as defined in API 650, Section 3.4.

5.3.9 Step 8—The risk of failure due to brittle fracture is minimal once a tank has demonstrated that it can operate at 

a specified maximum liquid level at the lowest expected temperature without failing. For the purpose of this 

assessment, the lowest expected temperature is defined as the lowest one-day mean temperature as shown in API 

650, Figure 4-2. It is necessary to check tank log records and meteorological records to ensure that the tank had 

operated at the specified maximum liquid level when the 1-day mean temperature was as low as shown in API 650, 

Figure 4-2.

Figure 5.2—Exemption Curve for Tanks Constructed from Carbon Steel of Unknown Material Specification

12

60

70

50

40

30

20

10

0

0.0 1.0 1.5 2.0 0.5

Shell thickness (in.)

Shell metal temperature, °F

NOTE For unheated tanks, the shell metal temperature shall equal the design 

metal temperature as defined in API 650, Section 3.4. The original nominal 

thickness for the thickest tank shell plate shall be used for the assessment.

Safe for use

Safe for use

Additional assessment required

12
5-4 API STANDARD 653

5.3.10 Step 9—An evaluation can be performed to establish a safe operating envelope for a tank based on the 

operating history. This evaluation shall be based on the most severe combination of temperature and liquid level 

experienced by the tank during its life. The evaluation may show that the tank needs to be re-rated or operated 

differently; several options exist:

a) restrict the liquid level.

b) restrict the minimum metal temperature.

c) change the service to a stored product with a lower specific gravity.

d) combinations of Items a), b), and c), above.

The owner/operator can also make a more rigorous analysis to determine the risk of failure due to brittle fracture by 

performing a fracture mechanics analysis based on established principles and practices. The procedures and 

acceptance criteria for conducting an alternative analysis are not included in this standard.

5.3.11 Step 10—All repairs, alterations, and relocations shall be made in compliance with this standard.

5.3.12 Step 11—An assessment shall be made to determine if the change in service places the tank at greater risk of 

failure due to brittle fracture. The service can be considered more severe and create a greater risk of brittle fracture if the

service temperature is reduced (e.g. changing from heated oil service to ambient temperature product), or the product is 

changed to one with a greater specific gravity and thus increased stresses.
6-1

Section 6—Inspection

6.1 General

Periodic in-service inspection of tanks shall be performed as defined herein. The purpose of this inspection is to assure

continued tank integrity. Inspections, other than those defined in 6.3 shall be directed by an authorized inspector.

6.2 Inspection Frequency Considerations

6.2.1 Several factors must be considered to determine inspection intervals for storage tanks. These include, but are

not limited to, the following:

a) the nature of the product stored;

b) the results of visual maintenance checks;

c) corrosion allowances and corrosion rates;

d) corrosion prevention systems;

e) conditions at previous inspections;

f) the methods and materials of construction and repair;

g) the location of tanks, such as those in isolated or high risk areas;

h) the potential risk of air or water pollution;

i) leak detection systems;

j) change in operating mode (e.g. frequency of fill cycling, frequent grounding of floating roof support legs);

k) jurisdictional requirements;

l) changes in service (including changes in water bottoms);

m)the existence of a double bottom or a release prevention barrier.

6.2.2 The interval between inspections of a tank (both internal and external) should be determined by its service

history unless special reasons indicate that an earlier inspection must be made. A history of the service of a given

tank or a tank in similar service (preferably at the same site) should be available so that complete inspections can be

scheduled with a frequency commensurate with the corrosion rate of the tank. On-stream, nondestructive

examination methods shall be considered when establishing inspection frequencies.

6.2.3 Jurisdictional regulations, in some cases, control the frequency and interval of the inspections. These

regulations may include vapor loss requirements, seal condition, leakage, proper diking, and repair procedures.

Knowledge of such regulations is necessary to ensure compliance with scheduling and inspection requirements.

6.3 Inspections from the Outside of the Tank

6.3.1 Routine In-service Inspections

6.3.1.1 The external condition of the tank shall be monitored by close visual inspection from the ground on a routine

basis. This inspection may be done by owner/operator personnel, and can be done by other than authorized

13
6-2 API STANDARD 653

inspectors as defined in 3.4. Personnel performing this inspection should be knowledgeable of the storage facility

operations, the tank, and the characteristics of the product stored.

6.3.1.2 The interval of such inspections shall be consistent with conditions at the particular site, but shall not exceed

one month.

6.3.1.3 This routine in-service inspection shall include a visual inspection of the tank’s exterior surfaces. Evidence of

leaks; shell distortions; signs of settlement; corrosion; and condition of the foundation, paint coatings, insulation

systems, and appurtenances should be documented for follow-up action by an authorized inspector.

6.3.2 External Inspection

6.3.2.1 All tanks shall be given a visual external inspection by an authorized inspector. This inspection shall be

called the external inspection and must be conducted at least every five years or RCA/4N years (where RCA is the

difference between the measured shell thickness and the minimum required thickness in mils, and N is the shell

corrosion rate in mils per year) whichever is less. Tanks may be in operation during this inspection.

6.3.2.2 Insulated tanks need to have insulation removed only to the extent necessary to determine the condition of

the exterior wall of the tank or the roof.

6.3.2.3 Tank grounding system components such as shunts or mechanical connections of cables shall be visually

checked. Recommended practices dealing with the prevention of hydrocarbon ignition are covered by API 2003.

6.3.3 Ultrasonic Thickness Inspection 

6.3.3.1 External, ultrasonic thickness measurements of the shell can be a means of determining a rate of uniform

general corrosion while the tank is in service, and can provide an indication of the integrity of the shell. The extent of

such measurements shall be determined by the owner/operator.

6.3.3.2 When used, the ultrasonic thickness measurements shall be made at intervals not to exceed the following.

a) When the corrosion rate is not known, the maximum interval shall be five years. Corrosion rates may be estimated

from tanks in similar service based on thickness measurements taken at an interval not exceeding five years.

b) When the corrosion rate is known, the maximum interval shall be the smaller of RCA/2N years (where RCA is the

difference between the measured shell thickness and the minimum required thickness in mils, and N is the shell

corrosion rate in mils per year) or 15 years. 

6.3.3.3 Internal inspection of the tank shell, when the tank is out of service, can be substituted for a program of

external ultrasonic thickness measurement if the internal inspection interval is equal to or less than the interval

required in 6.3.3.2 b).

6.3.4 Cathodic Protection Surveys

6.3.4.1 Where exterior tank bottom corrosion is controlled by a cathodic protection system, periodic surveys of the

system shall be conducted in accordance with API 651. The owner/operator shall review the survey results.

6.3.4.2 The owner/operator shall assure competency of personnel performing surveys.

6.4 Internal Inspection

6.4.1 General

6.4.1.1 Internal inspection is primarily required to do as follows.

a) Ensure that the bottom is not severely corroded and leaking.
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 6-3

b) Gather the data necessary for the minimum bottom and shell thickness assessments detailed in Section 6. As

applicable, these data shall also take into account external ultrasonic thickness measurements made during inservice inspections (see 6.3.3).

c) Identify and evaluate any tank bottom settlement.

6.4.1.2 All tanks shall have a formal internal inspection conducted at the intervals defined by 6.4.2. The

authorized inspector shall supervise or conduct a visual examination and assure the quality and completeness of

the nondestructive examination (NDE) results. If the internal inspection is required solely for the purpose of

determining the condition and integrity of the tank bottom, the internal inspection may be accomplished with the

tank in-service utilizing various ultrasonic robotic thickness measurement and other on-stream inspection

methods capable of assessing the thickness of the tank bottom, in combination with methods capable of

assessing tank bottom integrity as described in 4.4.1. Electromagnetic methods may be used to supplement the

on-stream ultrasonic inspection. If an in-service inspection is selected, the data and information collected shall be

sufficient to evaluate the thickness, corrosion rate, and integrity of the tank bottom and establish the internal

inspection interval, based on tank bottom thickness, corrosion rate, and integrity, utilizing the methods included in

this standard.

6.4.2 Inspection Intervals

Initial and subsequent inspection intervals shall be in compliance with the requirements of 6.4.2.1 and 6.4.2.2. 

For existing tanks, tank owner/operators shall review the internal inspection interval and be in compliance with this

section within 5 years from date of first publication of API Standard 653, Fourth Edition, Addendum 3.

6.4.2.1 Initial Internal Inspection Interval

The initial internal inspection intervals for newly constructed tanks and/or refurbished tanks shall be established either

per 6.4.2.1.1 or 6.4.2.1.2. 

6.4.2.1.1 The interval from initial service date until the first internal inspection shall not exceed 10 years unless a

tank has one or more of the leak prevention, detection, corrosion mitigation, or containment safeguards listed in Table

6.1. The initial internal inspection date shall be based on incremental credits for the additional safeguards in Table 6.1

which are cumulative.

For example, the maximum interval for a 1⁄4 in. bottom that has a release prevention barrier and a fiberglass lining

would be determined as follows:

10 years (initial) + 5 years (fiberglass lining) + 10 years (release prevention barrier) = 25 years.

The initial inspection interval shall not exceed 20 years for tanks without a Release Prevention Barrier, or 30 years for

tanks with a Release Prevention Barrier.

Table 6.1—Tank Safeguard

Tank Safeguard Add to Initial Interval

i. Fiberglass-reinforced lining of the product-side of the tank bottom installed per API RP 652. 5 yrs

i Installation of an internal thin-film coating as installed per API RP 652. 2 yrs

iii. Cathodic protection of the soil-side of the tank bottom installed, maintained, and inspected 

per API RP 651. 5 yrs

iv. Release prevention barrier installed per API Std 650, Annex I. 10 yrs

v. Bottom corrosion allowance greater than 0.150 in. (Actual corrosion allowance -

150 mils) / 15 mpy

13

13
6-4 API STANDARD 653

6.4.2.1.2 As an alternative to establishing the initial interval in accordance with Section 6.4.2.1 and Table 6.1, the

initial internal inspection date and reassessment can be established using Risk Based Inspection (RBI) assessment

per 6.4.2.2.2.

These assessments may establish an initial inspection interval exceeding 10 years but shall not exceed 20 years for

tanks without a Release Prevention Barrier, or 30 years for tanks with a Release Prevention Barrier except as follows.

If an RBI assessment has been performed, the maximum initial internal inspection interval does not apply to tanks

storing the following.

1) Highly viscous substances which solidify at temperatures below 110 °F, (some examples of these substances

are: asphalt, roofing flux, residuum, vacuum bottoms and reduced crude), or;

2) Any substance or mixture that is:

a) not identified or regulated either as a hazardous chemical or material under the applicable laws of the

jurisdiction; and 

b) that the owner/operator has determined will not adversely impact surface or groundwater beyond the facility

or affect human health or the environment.

6.4.2.2 Subsequent Internal Inspection Interval

The interval between subsequent internal inspections shall be determined in accordance with either the corrosion rate

procedures of 6.4.2.2.1 or the risk based inspection procedures as outlined in 6.4.2.2.2.

6.4.2.2.1 The subsequent inspection interval (beyond the initial inspection) can be determined using the measured

tank bottom corrosion rate and the minimum remaining thickness in accordance with 4.4.5. During any examination to

determine corrosion rates the owner/operator should ensure they understand the effectiveness of the inspection

techniques employed for detecting and measuring potential damage mechanisms.

When changing service, an owner/operator may decide to use internal corrosion rates obtained from similar service

assessment (performed per Annex H of API 653) when setting subsequent internal inspection dates. 

When using the corrosion rate procedures of 6.4.2.2.1 the maximum subsequent internal inspection interval shall be

20 years for tanks without a Release Prevention Barrier, or 30 years for tanks with a Release Prevention Barrier.

6.4.2.2.2 An owner/operator can establish the subsequent internal inspection interval using risk based inspection

(RBI) procedures in accordance with API RP 580 and the additional requirements of this section.

The results of the RBI assessment shall be used to establish a tank inspection strategy that defines the most

appropriate inspection methods, appropriate frequency for internal, external and in-service inspections, and

prevention and mitigation steps to reduce the likelihood and consequence of tank leakage or failure. 

An RBI assessment shall consist of a systematic evaluation of both the likelihood of failure and the associated

consequences of failure, in accordance with API RP 580. The RBI assessment shall be thoroughly documented,

clearly defining all factors contributing to both likelihood and consequence of tank leakage or failure.

The RBI assessment shall be performed by a team including inspection and engineering expertise knowledgeable in

the proper application of API RP 580 principles, tank design, construction, and modes of deterioration. The RBI

assessment shall be reviewed and approved by a team as above at intervals not to exceed 10 years or more often if

warranted by process, equipment, or consequence changes. 

13
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 6-5

The applied RBI methodology (not every individual assessment) shall have a documented validation review to

demonstrate that it has all the key elements defined in API 580 and this section. The validation should be performed

by an entity external to the RBI assessment team.

If corrosion rates are based on prior inspections, they shall be derived from either high or medium inspection

effectiveness as defined by the owner-user procedures. Refer to API RP 581 for examples of high and medium

inspection effectiveness. Corrosion rates from low inspection effectiveness such as spot UT shall not be used in the

RBI process.

A tank shall be removed from service when the risk exceeds the acceptable risk criteria established per the owneruser procedure.

NOTE API does not recommend running tank bottoms to failure, or operating tanks indefinitely with known or suspected bottom

leaks.

6.4.2.2.2.1 Likelihood factors that shall be evaluated in tank RBI assessments, in addition to the likelihood factors in

API RP 580 include, but are not limited to, the following:

a) original thickness, weld type, and age of bottom plates;

b) analysis methods used to determine the product-side, soil-side and external corrosion rates for both shell and

bottom and the accuracy of the methods used;

c) inspection history, including tank failure data;

d) soil resistivity;

e) type and design quality of tank pad/cushion including quality control at construction;

f) water drainage from berm area;

g) type/effectiveness of cathodic protection system and maintenance history;

h) operating temperatures;

i) effects on internal corrosion rates due to product service;

j) internal coating/lining/liner type, age and condition;

k) use of steam coils and water draw-off details;

l) quality of tank maintenance, including previous repairs and alterations;

m) design codes and standards and the details utilized in the tank construction, repair, and alteration (including tank

bottoms);

n) materials of construction;

o) effectiveness of an inspection includes examination methods and scope which are to be determined by the

inspector;

p) functional failures, such as floating roof seals, roof drain systems, etc.;

q) settlement data;

r) quality assurance/control during tank construction, including pad cleanliness, slope of bottom, foundation

installation, document/records to show how the tank was built, etc.

13
6-6 API STANDARD 653

6.4.2.2.2.2 Consequence factors that shall be evaluated in tank RBI assessments include, but are not limited to, the

following:

a) tank bottom with a Release Prevention Barrier (RPB) details (single, double, RPB, internal reinforced linings,

etc.);

b) product type and volume;

c) mode of failure, (i.e. slow leak to the environment, tank bottom rupture or tank shell brittle fracture);

d) identification of environmental receptors such as wetlands, surface waters, ground waters, drinking water

aquifers, and bedrock;

e) distance to environmental receptors;

f) effectiveness of leak detection systems and time to detection;

g) mobility of the product in the environment, including, for releases to soil, product viscosity and soil permeability;

h) sensitivity characteristics of the environmental receptors to the product;

i) cost to remediate potential contamination;

j) cost to clean tank and repair;

k) cost associated with loss of use;

l) impact on public safety and health;

m) dike containment capabilities (volume and leak tightness).

6.5 Alternative to Internal Inspection to Determine Bottom Thickness

In cases where construction, size, or other aspects allow external access to the tank bottom to determine bottom

thickness, an external inspection in lieu of an internal inspection is allowed to meet the data requirements of

Table 4.4. However, in these cases, consideration of other maintenance items may dictate internal inspection

intervals. This alternative approach shall be documented and made part of the permanent record of the tank.

6.6 Preparatory Work for Internal Inspection

Specific work procedures shall be prepared and followed when conducting inspections that will assure personnel

safety and health and prevent property damage in the workplace (see 1.4).

6.7 Inspection Checklists 

Annex C provides sample checklists of items for consideration when conducting in-service and out-of-service

inspections.

6.8 Records

6.8.1 General

Inspection records form the basis of a scheduled inspection/maintenance program. (It is recognized that records may

not exist for older tanks, and judgments must be based on experience with tanks in similar services.) The owner/

operator shall maintain a complete record file consisting of three types of records, namely: construction records,

inspection history, and repair/alteration history.

13
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 6-7

6.8.2 Construction Records

Construction records may include nameplate information, drawings, specifications, construction completion report,

and any results of material tests and analyses.

6.8.3 Inspection History

The inspection history includes all measurements taken, the condition of all parts inspected, and a record of all

examinations and tests. A complete description of any unusual conditions with recommendations for correction of

details which caused the conditions shall also be included. This file will also contain corrosion rate and inspection

interval calculations.

6.8.4 Repair/Alteration History

The repair/alteration history includes all data accumulated on a tank from the time of its construction with regard to

repairs, alterations, replacements, and service changes (recorded with service conditions such as stored product

temperature and pressure). These records should include the results of any experiences with coatings and linings.

6.9 Reports

6.9.1 General

For each external inspection performed per 6.3.2 and each internal inspection performed per 6.4, the authorized

inspector shall prepare a written report. These inspection reports along with inspector recommendations and

documentation of disposition shall be maintained by the owner/operator for the life of the tank. Local jurisdictions

may have additional reporting and record keeping requirements for tank inspections.

6.9.2 Report Contents

Reports shall include at a minimum the following information:

a) date(s) of inspection;

b) type of inspection (external or internal);

c) scope of inspection, including any areas that were not inspected, with reasons given (e.g. limited scope of

inspection, limited physical access;

d) description of the tank (number, size, capacity, year constructed, materials of construction, service history, roof and

bottom design, etc.), if available;

e) list of components inspected and conditions found (a general checklist such as found in Annex C may be used to

identify the scope of the inspection) and deficiencies found;

f) inspection methods and tests used (visual, MFL, UT, etc.) and results of each inspection method or test;

g) corrosion rates of the bottom and shell;

h) settlement survey measurements and analysis (if performed);

i) recommendations per 6.9.3.1;
6-8 API STANDARD 653

j) name, company, API 653 certification number and signature of the authorized inspector responsible for the

inspection;

k) drawings, photographs, NDE reports and other pertinent information shall be appended to the report.

6.9.3 Recommendations

6.9.3.1 Reports shall include recommendations for repairs and monitoring necessary to restore the integrity of the

tank per this standard and/or maintain integrity until the next inspection, together with reasons for the

recommendations. The recommended maximum inspection interval and basis for calculation that interval shall also

be stated. Additionally, reports may include other less critical observations, suggestions and recommendations.

6.9.3.2 It is the responsibility of the owner/operator to review the inspection findings and recommendations,

establish a repair scope, if needed, and determine the appropriate timing for repairs, monitoring, and/or

maintenance activities. Typical timing considerations and examples of repairs are:

a) prior to returning the tank to service—repairs critical to the integrity of the tank (e.g. bottom or shell repairs);

b) after the tank is returned to service—minor repairs and maintenance activity (e.g. drainage improvement, painting,

gauge repairs, grouting, etc.);

c) at the next scheduled internal inspection—predicted or anticipated repairs and maintenance (e.g. coating renewal,

planned bottom repairs, etc.);

d) monitor condition for continued deterioration—(e.g. roof and/or shell plate corrosion, settlement, etc.).

The owner/operator shall ensure that the disposition of all recommended repairs and monitoring is documented in

writing and that reasons are given if recommended actions are delayed or deemed unnecessary.

6.10 Nondestructive Examination (NDE)

Personnel performing NDE shall meet the qualifications identified in 12.1.1.2, but need not be certified in accordance

with Annex D. The results of any NDE work, however, must be considered in the evaluation of the tank by an

authorized inspector.
Section 7—Materials

7.1 General

This section provides general requirements for the selection of materials for the repair, alteration, and reconstruction 

of existing tanks. Specific requirements for repairs and alterations are covered in Section 9.

7.2 New Materials

All new materials used for repair, alterations, or reconstruction shall conform to the current applicable standard.

7.3 Original Materials for Reconstructed Tanks

7.3.1 Shell and Bottom Plates Welded to the Shell

7.3.1.1 All shell plate materials and bottom plates welded to the shell shall be identified. Materials identified by 

original contract drawings, API nameplates, or other suitable documentation do not require further identification. 

Material not identified shall be tested and identified by the requirements as outlined in 7.3.1.2. After identification, 

determination shall be made as to suitability of the material for intended service.

7.3.1.2 Each individual plate for which adequate identification does not exist shall be subjected to chemical 

analysis and mechanical tests as required in ASTM A6 and ASTM A370 including Charpy V-notch. Impact values 

shall satisfy the requirements of API 650, Section 4.2.9, Section 4.2.10, Section 4.2.11, and Table 4-4a or Table 4-

4b. When the direction of rolling is not definitely known, two tension specimens shall be taken at right angles to 

each other from a corner of each plate, and one of those test specimens must meet the specification requirements.

7.3.1.3 For known materials, all shell plates and bottom plates welded to the shell shall meet, as a minimum, the 

chemistry and mechanical properties of material specified for the application with regard to thickness and design 

metal temperature given in API 650, Figure 4-1a or Figure 4-1b.

7.3.2 Structural

Existing rolled structural shapes that are to be reused shall meet the requirement of ASTM A7 as a minimum. New 

structural material shall meet the requirements of ASTM A36 or ASTM A992 as a minimum.

NOTE ASTM A7 was a steel specification that was discontinued in the Fourth Edition of API 650, 1970.

7.3.3 Flanges and Fasteners

7.3.3.1 Flange material shall meet the minimum requirements of the material specifications in the as-built standard.

7.3.3.2 Fasteners shall meet the material specifications of the current applicable standard.

7.3.4 Roof, Bottom, and Plate Windgirders

If existing plates are to be used to reconstruct the tank, they shall be checked for excessive corrosion and pitting (see 

Section 4 and Section 6).

7.4 Welding Consumables

Welding consumables shall conform to the AWS classification that is applicable to the intended use.

13

12

12

12

8-1

Section 8—Design Considerations for Reconstructed Tanks

8.1 General

Any specific design considerations other than normal product loading shall be specified by the owner/operator. See 

4.4.3 for release prevention systems and release prevention barrier definition.

8.2 New Weld Joints

8.2.1 Weld joint details shall meet the welding requirements of the current applicable standard.

8.2.2 All new shell joints shall be butt-welded joints with complete penetration and complete fusion.

8.3 Existing Weld Joints

Existing weld joints shall meet the requirements of the as-built standard.

8.4 Shell Design

8.4.1 Thickness to be used for each shell course when checking tank design shall be based on measurements 

taken within 180 days prior to relocation. (See 4.3.2 for measuring procedure, number, and locations of measured 

thicknesses.)

8.4.2 The maximum design liquid level for product shall be determined by calculating the maximum design liquid 

level for each shell course based on the specific gravity of the product, the actual thickness measured for each shell 

course, the allowable stress for the material in each course, and the design method to be used. The allowable stress 

for the material shall be determined using API 650, Table 5-2a or Table 5-2b. For material not listed in Table 5-2a or 

Table 5-2b, an allowable stress value of the lesser of 2/3 yield strength or 2/5 tensile strength shall be used.

8.4.3 The maximum liquid level for hydrostatic test shall be determined by using the actual thickness measured for 

each shell course, the allowable stress for the material in each course, and the design method to be used. The 

allowable stress for the material shall be determined using API 650, Table 5-2a or Table 5-2b. For material not listed in 

Table 5-2a or Table 5-2b, an allowable stress value of the lesser of 3/4 yield strength or 3/7 tensile strength shall be 

used.

8.4.4 If a corrosion allowance is required for the reconstructed tank, the required corrosion allowance shall be 

deducted from the actual thickness before calculating the maximum liquid level. If the actual thickness is greater than 

that necessary to allow the liquid level required, the extra thickness can be considered as corrosion allowance.

8.4.5 The joint efficiency and allowable stress levels used for the design liquid level calculations shall be consistent 

with the design method used and with the degree and type of examination made on welded joints. The joint efficiency 

and allowable stress levels for existing welded joints that are not to be removed and replaced shall be based on the 

original degree and type of examination.

8.5 Shell Penetrations

8.5.1 Replacement and new penetrations shall be designed, detailed, welded, and examined to meet the 

requirements of the current applicable standard.

8.5.2 Existing penetrations shall be evaluated for compliance with the as-built standard.

12

12

13
8-2 API STANDARD 653

8.6 Windgirders and Shell Stability

8.6.1 Top and intermediate windgirders for open top tanks shall meet the requirements of the current applicable 

standard.

8.6.2 Tanks to be reconstructed shall be checked for wind-induced buckling in accordance with the procedures of 

the current applicable standard, using the wind requirements for the location where the tank will be reconstructed.

8.7 Roofs

8.7.1 Roof designs shall meet the requirements of the as-built standard.

8.7.2 If the new site requires a larger design load than the original site, the adequacy of the existing roof shall be 

evaluated using the current applicable standard.

8.8 Seismic Design

Tanks that will be reconstructed shall be checked for seismic stability based on the rules of the current applicable 

standard using the dimensions and thicknesses of the reconstructed tank. Reconstructed tanks shall be built to meet 

the stability requirements of the current applicable standard. Thickened bottom plates under the bottom shell course 

or anchoring of the tank may be required even if not used on the original tank.
9-1

Section 9—Tank Repair and Alteration

9.1 General

9.1.1 The basis for repairs and alterations shall be an API 650 equivalence.

9.1.2 Hydrostatic testing requirements, NDE requirements, acceptance criteria for the welds, and repairs to shell 

plate and existing welds are specified in Section 12.

9.1.3 All repair work must be authorized by the authorized inspector or an engineer experienced in storage tank 

design, before commencement of the work by a repair organization. Authorization for alterations to storage tanks 

that comply with API 650 may not be given without prior consultation with, and approved by, an engineer 

experienced in storage tank design. The authorized inspector will designate inspection hold points required during 

the repair or alteration sequence and minimum documentation to be submitted upon job completion. The 

authorized inspector may give prior general authorization for limited or routine repairs as long as the authorized 

inspector is sure that the repairs will not require hydrostatic testing or do not require an engineering evaluation.

9.1.4 All proposed design, work execution, materials, welding procedures, examination, and testing methods 

must be approved by the authorized inspector or by an engineer experienced in storage tank design. The 

authorized inspector or an engineer experienced in storage tank design shall approve all specified repair and 

alteration work at the designated hold points and after repairs and alterations have been completed in accordance 

with the requirements of this standard.

9.1.5 Annex F summarizes the requirements by method of examination and provides the acceptance standards, 

examiner qualifications, and procedure requirements. Annex F is not intended to be used alone to determine the 

examination requirements for work covered by this document. The specific requirements as listed in Section 1 

through Section 12 shall be followed in all cases.

9.2 Removal and Replacement of Shell Plate Material

9.2.1 Minimum Thickness of Replacement Shell Plate

The minimum thickness of the replacement shell plate material shall be calculated in accordance with the as-built 

standard. The thickness of the replacement shell plate shall not be less than the greatest nominal thickness of any 

plate in the same course adjoining the replacement plate except where the adjoining plate is a thickened insert 

plate. Any changes from the original design conditions, such as specific gravity, design pressure, liquid level, and 

shell height, shall be considered.

9.2.2 Minimum Dimensions of Replacement Shell Plate

9.2.2.1 The minimum dimension for a replacement shell plate is 12 in. or 12 times the thickness of the replacement 

plate, whichever is greater. The replacement plate may be circular, oblong, square with rounded corners, or 

rectangular with rounded corners except when an entire shell plate is replaced. See Figure 9.1 for typical details of 

acceptable replacement shell plates.

9.2.2.2 Where one or more entire shell plates or full height segments of shell plates are to be removed and replaced, 

the minimum spacing requirements specified in Figure 9.1 for vertical weld joints shall be maintained. It is acceptable 

to remove and replace entire shell plates or full height segments of shell plates by cutting and rewelding along the 

existing horizontal weld joints. Prior to welding the new vertical joints, the existing horizontal welds shall be cut for a 

minimum distance of 12 in. beyond the new vertical joints. The vertical joints shall be welded prior to welding the 

horizontal joints.
9-2 API STANDARD 653

9.2.3 Weld Joint Design

9.2.3.1 Shell replacement plates shall be welded with butt joints with complete penetration and complete fusion, 

except as permitted for lapped patch shell repairs.

9.2.3.2 Weld joint design for replacement shell plates shall be in accordance with API 650, Section 5.1.5.1 through 

Section 5.1.5.3. Joints in lap-welded shell tanks may be repaired according to the as-built standard. Lap-welded joint 

design for lapped patch shell repairs shall meet the requirements of 9.3. Details of welding shall be in accordance with 

7.2 of API 650, and Section 9 of this standard.

9.2.3.3 For existing shell plates over 1/2-in. thick, the outer edge of the butt weld attaching the replacement shell 

plate shall be at least the greater of 8 times the weld thickness or 10 in. from the outer edge of any existing buttwelded shell joints. For existing shell plates 1/2-in. thick and less, the spacing may be reduced to 6 in. from the outer 

edge of vertical joints or 3 in. from the outer edge of horizontal joints. See Figure 9.1 for minimum dimensions.

Figure 9.1—Acceptable Details for Replacement of Shell Plate Material

R

A

V

V

V

R

V

A

Note 2

R

R

R

B

H

H B

Weld joint

(annular or

bottom plates)

C

* See Note 3

Existing welds

Existing

welds

Greater of

R or H

Bottom plate

R

B

H

V

A

6 in.

6 in.

3 in.

6 in.

12 in.

Dimension t ≤ 0.5 in. t > 0.5 in.

Greater of 6 in. or 6t

Greater of 10 in. or 8t

Greater of 10 in. or 8t

Greater of 10 in. or 8t

Greater of 12 in. or 12t

Minimum Weld Spacing Between

Edges (Toes) of Welds for Thickness

of Replacement Shell Plate, t, (inches)

NOTE 1 All weld intersections shall be at approximately 90o.

NOTE 2 Prior to welding new vertical joints, cut existing horizontal weld for a minimum of 12 in. beyond the new vertical 

 joints. Weld the horizontal joint last.

NOTE 3 Prior to welding new vertical joints, cut existing shell-to-bottom weld for a minimum of 12 in. beyond the new 

vertical joints. The cut shall extend past or stop short of existing bottom plate welds by at least 3 in. or 5t. Weld the

shell-to-bottom weld last.

C Greater of 3 in. or 5t

* * *

See
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 9-3

For existing shell plates over 1/2-in. thick, the outer edge of the butt weld attaching the replacement shell plate shall 

be at least the greater of 8 times the weld size or 10 in. from the edge (toe) of the fillet weld attaching the bottom 

shell course to the bottom except when the replacement shell plate extends to and intersects the bottom-to-shell 

joint at approximately 90°. For existing shell plates 1/2-in. thick and less, this spacing may be reduced to 6 in. For 

shell plates of unknown toughness not meeting the exemption criteria of Figure 5.2, the edge of any vertical weld 

joint attaching a replacement plate shall be at 3 in. or 5t. from the edge of a weld joint in the bottom annular ring or 

weld joints in bottom plates under the tank shell. Figure 9.1 has minimum dimensions.

9.2.3.4 To reduce the potential for distortion of an existing tank due to welding a replacement plate into an 

existing tank shell, fit-up, heat input, and welding sequence must be considered.

9.2.4 Door Sheet Installation

This section describes the requirements for reinstallation or replacement of a door sheet. The requirements of Figure 

9-1, Figure 9-2, Figure 9-3, Figure 9-4 and Figure 9-5 shall be used to locate door sheets relative to existing seams, 

unless an alternative design is designed by a Storage Tank Engineer and the Owner/Operator approves the 

alternative in writing.

9.2.4.1 Door sheet installation shall meet the requirements of Section 9.2.1, Section 9.2.2, Section 9.2.3 and 

Section 12.2.1.6.

9.2.4.2 The removed shell plate section of a door sheet in a butt-welded tank may be re-installed in its original 

location or the section may be replaced with new shell plate material. In either case the door sheet installation shall 

utilize joints with complete penetration and complete fusion.

9.2.4.3 For lap welded and riveted tanks, reinstallation of an original plate section that crosses an existing horizontal 

seam is not permitted. Door sheets that cross vertical riveted or lap welded seams are not permitted in any case.

9.2.4.4 If a door sheet vertical cut-line crosses an existing seam in a butt-welded tank without an offset and the 

removed section is reinstalled then additional weld examination shall be required at the intersection of the new 

vertical weld seam and existing horizontal weld seam. In addition to the examination requirements of 12.1.5.1 the 

back gouged surface of the root pass and the final pass (each side) of the new welds shall be examined by magnetic 

particle or liquid penetrant methods. The existing horizontal weld seam intersected by the new vertical weld shall also 

be examined by magnetic particle or liquid penetrant methods for a 6 in. distance on both sides.

NOTE “Offset” is the horizontal distance between the vertical welds above and below a horizontal seam, as shown in Figure 9.2, 

Figure 9.3, and Figure 9.5.

9.2.4.5 If a door sheet vertical cut-line crosses an existing seam in a butt-welded tank with an offset, the minimum 

offset must equal dimension ‘V’ as shown in Figure 9.5. Prior to welding the new vertical seams, cut existing 

horizontal seam weld for a minimum of 12 in. beyond the new vertical joints. Weld the horizontal seam last. 

9.2.4.6 If a door sheet cut line crosses a lap-welded or a riveted horizontal seam, the replacement assembly shall 

be constructed of two separate plates, with the lower section butt-welded to the adjacent shell course by means of full 

penetration, full fusion vertical welds. The upper section shall be lapped over or under the lower section and the upper 

plate shall be butt welded to the existing shell plate. After the butt welds are completed, the horizontal lap shall be fillet 

welded along both inside and outside edges. 

9.2.4.7 New weld seams in riveted tanks shall be located a minimum of 12 in. from existing rivet seams to minimize 

potential for rivet and rivet seam leaks or the rivets and existing lap rivet seams shall be seal welded or sealed by the 

application of caulk or coating that is compatible with the specified stored product.

NOTE The heat created by welding may cause nearby rivets and rivet seams to leak.

12
9-4 API STANDARD 653

Figure 9-2—Details for Door Sheets in Riveted Seam Tank

Figure 9-3—Details for Door Sheets in Lap Weld Seam Tank

Seal rivets

and rivet

seams

Greater of R or H See Note 4

See

Note 4

See Note 4

12 in. minimum (see Note 5)

3 in. or 5t minimum (see Note 5)

See

Note 4

R

V

Plug weld rivet

holes (see Note 6)

NOTES:

1. See table in Figure 9-1 for minimum weld spacing and dimensions H, R, and V.

2. When a door sheet crosses the horizontal joint between two courses, a replacement door sheet assembly shall be 

constructed of two separate plates. The upper door sheet thickness shall be the same as the higher shell course 

thickness. The lower door sheet thickness shall be the same as the lower shell course thickness.

3. Fillet Weld size shall be equal to thickness of thinner of two plates.

4. Rivets and existing lap rivet seams located within 12 in. of a weld may need to be sealed with calk/coating or 

seal-welded to prevent product seepage.

5. Prior to welding new vertical seam which intersect the bottom plate, cut existing shell-to-bottom weld for a minimum of 

12 in. beyond the new vertical weld seam. The cut shall extend past or stop short of existing bottom plate welds by at 

least 3 in. or 5t. Weld the shell-to-bottom weld last.

6. Plug weld all rivet holes in the shell plate where rivets are removed. Filler weld to full plate thickness and grind flush.

7. Door sheets need not extend to shell-to-bottom weld provided that weld spacing and corner radii are in accordance 

with Figure 9-1.

12

Greater of R or H 6 in. minimum

See

Note 3

12 in. minimum (see Note 4)

Greater of 3 in. or 5t (see Note 4)

R V

Existing weld

V Existing

weld

NOTES:

1. See table in Figure 9-1 for minimum weld spacing and dimensions H, R, and V.

2. When a door sheet crosses the horizontal joint between two courses, a replacement door sheet assembly shall be 

comprised of two separate plates. The upper door sheet thickness shall be the same as the higher shell course 

thickness. The lower door sheet thickness shall be the same as the lower shell course thickness.

3. Fillet Weld size shall be equal to thickness of thinner of two plates.

4. Prior to welding new vertical joints which intersect the bottom plate, cut existing shell-to-bottom weld for a minimum of 

12 in. beyond the new vertical weld joint. The cut shall extend past or stop short of existing bottom plate welds by at 

least 3 in. or 5t. Weld the shell-to-bottom weld last.

5. Door sheets need not extend to shell-to-bottom weld provided that weld spacing and corner radii are in accordance 

with Figure 9-1.
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 9-5

Figure 9-4—Details for Door Sheet in Butt Weld Shell Seam Tank—No Vertical Seam Offset

Figure 9-5—Details for Door Sheet in Butt Welds Shell Seam Tank—Tank-Vertical Seam Offset

Greater of R or H

12 in. minimum (see Note 3)

Greater of 3 in. or 5t (see Note 3)

H

V

Door sheet seam

R

V Existing

weld

NOTES:

1. See table in Figure 9-1 for minimum weld spacing and dimensions H, R, and V.

2. Detail shown for door sheet that utilizes removed section of tank shell. If new material is utilized, see Figure 9-1 for 

requirements.

3. Prior to welding new vertical weld seam which intersects the bottom plate, cut existing shell-to-bottom weld for a 

minimum of 12 in. beyond the new vertical weld seam. The cut shall extend past or stop short of existing bottom plate 

welds by at least 3 in. or 5t. Weld the shell-to-bottom weld last.

4. Door sheets need not extend to shell-to-bottom weld provided that weld spacing and corner radii are in accordance 

with Figure 9-1.

12

Greater of R or H Door sheet seams

12 in. minimum (see Note 3)

Greater of 3 in. or 5t (see Note 3)

R

V

Existing weld V

H

See

See Note 2

Note 2 Existing

weld

NOTES:

1. See table in Figure 9-1 for minimum weld spacing and dimensions H, R, and V.

2. Prior to welding new vertical joints, cut the existing horizontal weld for a minimum of 12 in. beyond the new vertical 

weld seam. Weld the horizontal seam last.

3. Prior to welding new vertical joints which intersect the bottom plate, cut the existing shell-to-bottom weld for a minimum

of 12 in. beyond the new vertical weld joint. The cut shall extend past or stop short of existing bottom plate welds by at 

least 3 in. or 5t. Weld the shell-to-bottom weld last.

4. Door sheets need not extend to shell-to-bottom weld provided that weld spacing and corner radii are in accordance 

with Figure 9-1.
9-6 API STANDARD 653

9.3 Shell Repairs Using Lap-welded Patch Plates

9.3.1 Lapped patch shell repairs are an acceptable form of repair for butt-welded, lap-welded, and riveted tank 

shells, under the conditions outlined in 9.3.2, 9.3.3, and 9.3.4; only when specified by the owner. In addition, the 

repair details shall comply with the requirements of 9.3.1.1 through 9.3.1.10. These repairs are permanent repairs 

subject to an ongoing inspection and maintenance program. These requirements may be used to evaluate 

existing lapped patch shell repairs; however, the plate thickness limits need not apply.

9.3.1.1 All repair material shall comply with the requirements of the current applicable standard of construction 

and API 653.

9.3.1.2 Lapped patch shell repairs shall not be used on any shell course thickness (original construction) that 

exceeds 1/2 in., nor to replace doorsheets or shell plates.

9.3.1.3 Except as permitted in 9.3.3.2 and 9.3.4.3, the repair plate material shall be the smaller of 1/2 in. or the 

thickness of the shell plate adjacent to the repairs, but not less than 3/16 in.

9.3.1.4 The shape of the repair plate may be circular, oblong, square, or rectangular. All corners, except at the shellto-bottom joint, shall be rounded to a minimum radius of 2 in. The nozzle reinforcing plate shapes of API 650, Figure 

5-8, are also acceptable.

9.3.1.5 The repair plate may cross any butt-welded vertical or horizontal shell seams that have been ground flush, 

but must overlap a minimum of 6 in. beyond the shell seam. The weld spacing requirements of Figure 9.1 shall be 

used as a basis for locating repair plates relative to butt-welded, fillet-welded, and riveted seams and other repair 

plates.

9.3.1.6 Repair plates may extend to and intersect with the external shell-to-bottom joint if the vertical sides intersect 

the tank bottom at a 90° angle and the shell-to-bottom weld is in conformance with Figure 9.6. Repair plates 

positioned on the shell interior shall be located such that the toe-to-toe weld clearances are a minimum of 6 in. to the 

shell-to-bottom weld.

9.3.1.7 The maximum vertical and horizontal dimension of the repair plate is 48 in. and 72 in., respectively. The 

minimum repair plate dimension is 4 in. The repair plate shall be formed to the shell radius.

9.3.1.8 Shell openings and their reinforcements shall not be positioned within a lapped patch shell repair.

9.3.1.9 Prior to application of a lapped patch shell repair, the areas to be welded shall be ultrasonically examined for 

plate defects and remaining thickness.

9.3.1.10 Repair plates shall not be lapped onto lap-welded shell seams, riveted shell seams, other lapped patch 

repair plates, distorted areas, or unrepaired cracks or defects.

9.3.2 Lapped patch repair plates may be used for the closure of holes caused by the removal of existing shell 

openings or the removal of severely corroded or eroded areas. In addition, the following requirements shall be 

satisfied.

9.3.2.1 The welding shall be continuous on the outer perimeter of the repair plate and the inner perimeter of the hole 

in the shell plate. The minimum hole diameter is 2 in. Shell openings due to plate removal shall have a minimum 

corner radius of 2 in.

9.3.2.2 Nozzle necks and reinforcing plates shall be entirely removed prior to installation of a repair plate.

9.3.2.3 The repair plate thickness selection shall be based on a design that conforms to the as-built standard and 

API 653, using a joint efficiency not exceeding 0.70. The welds of the repair plate shall be full fillet welds. The 

minimum repair plate dimension shall be 4 in. with a minimum overlap of 1 in. and a maximum overlap of 8 times the 

shell thickness (8t).

9.3.2.4 The repair plate thickness shall not exceed the nominal thickness of the shell plate adjacent to the repair.

12

13
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 9-7

9.3.3 Lapped patch repair plates may be used to reinforce areas of severely deteriorated shell plates that are not 

able to resist the service loads to which the tank is to be subjected. Lapped patch repair plates may also be used for 

shell plates that are below the retirement thickness, providing the following additional requirements are satisfied.

9.3.3.1 The selection of the repair plate thickness shall be based on a design that conforms to the as-built standard 

and API 653, using a joint efficiency not exceeding 0.35. The perimeter weld shall be a full fillet weld.

9.3.3.2 The repair plate thickness shall not exceed the shell plate thickness at the perimeter of the repair plate by 

more than one-third, but no more than 1/8 in. The repair plate thickness shall not exceed 1/2 in.

9.3.3.3 The remaining strength of the deteriorated areas under the repair plate shall not be considered as effective 

in carrying the calculated service or hydrotest loads.

9.3.4 Lapped patch repair plates may be used to repair small shell leaks, or minimize the potential from leaks from 

severely isolated or widely scattered pitting if the following requirements are satisfied.

9.3.4.1 The existing shell thickness, excluding the holes and pitting, meets the minimum acceptable shell thickness 

as determined by 4.3.2 and 4.3.3.

9.3.4.2 The repair plate is designed to withstand the hydrostatic pressure load between the repair plate and the 

shell assuming a hole exists in the shell using a joint efficiency of 0.35.

9.3.4.3 The repair plate thickness shall not exceed the shell plate thickness at the perimeter of the repair plate by 

more than one-third, but no more than 1/8 in. The repair plate thickness shall be no thinner than 3/16 in. nor thicker 

than 1/2 in. A full fillet perimeter weld is required.

Figure 9.6—Lapped Patch Repair Plates at the External Shell-to-bottom Joint

T

Tank shell

Repair Plate

W 3/16

Trim repair

plate to clear

shell-to-bottom

weld

T

Detail A

For T > existing

shell-to-bottom

weld size

Detail B

For T ≤ existing

shell-to-bottom

weld size

W = lesser of repair plate thickness

or bottom plate thickness

12
9-8 API STANDARD 653

9.3.4.4 This repair method shall not be used if exposure of the fillet welds to the product will produce crevice 

corrosion or if a corrosion cell between the shell plate and repair plate is likely to occur.

9.3.4.5 This repair method shall not be used to repair shell leaks if the presence of product between the shell plate 

and repair plate will prevent gas freeing from the tank to perform hot work.

9.3.4.6 The existing shell plate under the repair plate shall be evaluated at each future inspection to ensure it 

satisfies the requirements of 9.3.4.1. If the existing shell plate thickness does not satisfy 9.3.4.1 or the repair plate does 

not satisfy 9.3.3, the area is to be repaired in accordance with 9.2 or 9.3.2.

9.4 Repair of Defects in Shell Plate Material

The need for repairing indications such as cracks, gouges or tears (such as those often remaining after the removal 

of temporary attachments), widely scattered pits, and corroded areas discovered during an inspection of the tank 

shell shall be determined on an individual case basis in accordance with Section 4. In areas where the shell plate 

thickness exceeds that required by design conditions, it is permissible to grind surface irregularities to a smooth 

contour so long as the remaining thickness is adequate for the design conditions. Where grinding to a smoothly 

contoured surface will result in unacceptable shell plate metal thickness, the shell plate may be repaired by 

deposition of weld metal, followed by examination and testing in accordance with 12.1.8. If more extensive areas of 

shell plate require repair, use of butt welded shell replacement plate or lap-welded patch plate shall be considered.

9.5 Alteration of Tank Shells to Change Shell Height

Tank shells may be altered by adding new plate material to increase the height of the tank shell. The modified shell 

height shall be in accordance with the requirements of the current applicable standard and shall take into 

consideration all anticipated loadings such as wind and seismic.

9.6 Repair of Defective Welds

Types of weld flaws and nonconformances that need repair are described in 9.6.1 through 9.6.4. 

9.6.1 Cracks, lack of fusion, and rejectable slag and porosity that need repair shall be removed completely by 

gouging and/or grinding and the resulting cavity properly prepared for welding.

9.6.2 Generally, it is not necessary to remove existing weld reinforcement in excess of that allowed by API 650 

when discovered on an existing tank with a satisfactory service history. However, if operating conditions are such that 

the excessive weld reinforcement may be deleterious (such as for a floating roof with flexible seals), consideration 

shall be given to repairing the welds by grinding.

9.6.3 Existing weld undercut deemed unacceptable based on suitability for service considerations shall be repaired 

by additional weld metal, or grinding, as appropriate.

9.6.4 Welded joints that have experienced loss of metal due to corrosion may be repaired by welding.

9.6.5 Arc strikes discovered in or adjacent to welded joints shall be repaired by grinding and/or welding. Arc strikes 

repaired by welding shall be ground flush with the plate.

9.7 Repair of Shell Penetrations

9.7.1 Repairs to existing shell penetrations shall be in compliance with API 650, Section 5.7.

9.7.2 Reinforcing plates may be added to existing unreinforced nozzles when deemed appropriate. The reinforcing 

plate shall meet all dimensional and weld spacing requirements of API 650, Section 5.7. See Figure 9.7 and Figure 

9.8 for acceptable details.

12
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 9-9

Figure 9.7—Typical Details for Addition of Reinforcing Plate to Existing Shell Penetration

Figure 9.8—Typical Details for Addition of “Tombstone” Shape Reinforcing Plate to Existing Shell Penetration

A

A

A

T

t

T or t

(1.5 in. max) Tank

shell

1/4 in. tell tale hole, drill and

tap for 1/8-in. pipe thread

Greater

of

2 in. or 6T

1/16 in.

SECTION A-A

9/16 in.

3/4 in.

15/16 in.

1 1/8 in.

1 5/16 in.

1 9/16 in.

1 3/4 in.

1/4 in.

5/16 in.

3/8 in.

7/16 in.

1/2 in.

9/16 in.

5/8 in.

t and T

(maximum) A

NOTE All details, dimensions, and weld spacing shall be in accordance

with the requirements of API 650.

12

A

A

A

B

T

t

T or t

(1.5 in. max) Tank

shell

1/4-in. telltale hole, drill and

tap for 1/8-in. pipe thread

Greater

of

2 in. or 6T

SECTION A-A

9/16 in.

3/4 in.

15/16 in.

1 1/8 in.

1 5/16 in.

1 9/16 in.

1 3/4 in.

1/4 in.

5/16 in.

3/8 in.

7/16 in.

1/2 in.

9/16 in.

5/8 in.

t and T

(maximum) A

1/4 in.

1/4 in.

5/16 in.

5/16 in.

3/8 in.

3/8 in.

3/8 in.

B

NOTE All details, dimensions, and weld spacing shall be in accordance

with the requirements of API 650.

1/16 in.

12
9-10 API STANDARD 653

9.7.3 As an alternative, the reinforcing plates may be added to the inside of the tank provided that sufficient nozzle 

projection exists.

9.8 Addition or Replacement of Shell Penetrations

9.8.1 New shell penetrations (addition or replacement) shall be in accordance with material, design, and stress relief 

requirements of API 650, Section 5.7 and in accordance with 9.8.2 through 9.8.6 of this standard. 

9.8.2 The required penetration reinforcement area of API 650, Section 5.7.2, shall be determined using the required 

shell thickness calculated by the equation in 4.3.3.1 b) of this standard except the variable S shall be the allowable 

design stress from API 650, Table 5-2a or Table 5-2b for the existing shell plate; use 20,000 lbf/in.2 if of unknown 

material. A joint efficiency of 1.0 may be used (see 9.8.5). The variable H shall be the height from the centerline of the 

penetration to the maximum liquid level, in ft.

9.8.3 Penetrations shall be prefabricated in thermally stress relieved insert assemblies when required by API 650, 

Section 5.7.4. API 650, Section 4.1.5, may be used when reinforcing material is from API 650 Group-IV through 

Group-VI and the existing shell is a Group-I through Group-IIIA material.

9.8.4 The following erection requirements shall be met:

a) if an integral reinforcement design is used, the insert plate at its periphery shall have a 1:4 reduction taper to match 

the shell plate thickness when the insert plate exceeds the shell plate thickness by more than 1/8 in.; 

b) spacing of welds shall be in accordance with Figure 9.1;

c) the new insert plate shall be joined to existing shell plate with full penetration and full fusion butt welds.

9.8.5 Examinations shall be per Section 12, except penetrations located on a shell joint shall receive additional shell 

radiography in accordance with API 650, Section 5.7.3. 

9.8.6 Penetrations larger than 2 in. NPS shall be installed with the use of an insert plate if the shell plate thickness is 

greater than 1/2 in. and the shell plate material does not meet the current design metal temperature criteria. In 

addition, the following requirement shall be met:

a) the minimum diameter of the insert plate shall be at least twice the diameter of the penetration or the diameter plus 

12 in., whichever is greater;

b) when reinforcing plates are used, the minimum diameter of the insert plate shall equal the diameter of the 

reinforcing plate plus 12 in.

9.9 Alteration of Existing Shell Penetrations

9.9.1 Existing shell penetrations may be altered if the altered details comply with the requirements of API 650, 

Section 5.7 including the requirements for minimum reinforcing area and the requirements for spacing of welds 

around connections.

9.9.2 When installing a new tank bottom above the existing bottom, it may be necessary to alter existing shell 

penetrations in the bottom course of a tank shell. If the new bottom is slotted through the tank shell several inches 

above the existing bottom, the spacing between existing welds around penetrations and the new bottom-to-shell weld 

may not comply with API 650 requirements. Options for altering the penetrations and/or reinforcing plates are given in 

9.9.2.1 through 9.9.2.3. 

9.9.2.1 The existing reinforcing plate may be trimmed to increase the spacing between the welds provided that the 

altered detail complies with the requirements of API 650, Section 5.7. Care must be exercised during the trimming 

12

12

12

12
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 9-11

operation to avoid damaging the shell material beneath the reinforcing plate. The existing weld attaching the portion 

of the reinforcing plate to be removed shall be completely removed by gouging and grinding.

9.9.2.2 The existing reinforcing plate may be removed and a new reinforcing plate added except that reinforcing 

plate replacement is not permitted in existing stress relieved assemblies unless the requirements of 11.3 are met. If it 

is not known whether the assembly was thermally stressed relieved, then the alteration shall meet the requirements 

of API 650, Section 5.7.4. Care must be exercised when removing the existing reinforcing plate to avoid damaging 

the shell plate beneath the reinforcing plate. When the upper half of the existing reinforcing plate meets all 

requirements of API 650, it can be left in place with approval of the purchaser. In this case, only the lower half of the 

existing reinforcing plate need be removed and replaced with the new one. The existing upper half of the reinforcing 

plate and the new lower section shall be provided with new a telltale hole, if needed, or drilled hole, and a welded pipe 

coupling for the pneumatic test. The shell plate thickness under the telltale hole or drilled hole shall be checked after 

drilling and the thickness shall not be less than 1/2 tmin, as calculated in 4.3.3.1, plus any required corrosion 

allowance. The welds to be replaced around the perimeter of the reinforcing plate and between the reinforcing plate 

and neck of the penetration shall be completely removed by gouging and grinding. The new reinforcing plate shall be 

in accordance with Figure 9.7. If required to maintain weld spacing, a tombstone shaped reinforcing plate may be 

used (see Figure 9.8).

9.9.2.3 The existing penetration may be moved by cutting the section of the shell containing the fitting and 

reinforcing plate, and raising the entire assembly to the correct elevation (see Figure 9.9). 

Figure 9.9—Method for Raising Shell Nozzles

10

Filler plate

same thickness

as shell

Old bottom

R—see Figure 9-1 +

+

Spacing per Section 

5.7.3 of API 650, 

or as permitted

per 9.10.2.4

New bottom

R—see Figure 9-1

Old bottom Reinforcing plate

Top of

proposed

new bottom

AFTER RAISING

BEFORE RAISING

Cut lines in shell plate

12
9-12 API STANDARD 653

9.9.3 Any components of the penetration (neck, flange, and reinforcing plate) that are in serviceable condition after 

removal may be reused.

9.9.4 A new bottom may be installed through an existing tombstone reinforcing plate, provided all weld spacing and 

reinforcement requirements, as specified in API 650, are met. One of the following methods shall be used.

a) Remove only that portion of the existing reinforcing plate necessary to weld and test the new bottom-to-shell weld. 

The lower edge of the reinforcing plate shall be cut reasonably straight and horizontal and beveled to facilitate 

welding. See Figure 9.10 for weld joint details.

b) Bevel the shell from the inside to allow for a full penetration weld between the bottom and shell. This method 

shall only be used on tanks where the annular plate or bottom sketch plate thickness is equal to or greater 

than 10 mm (3/8 in.). This weld detail shall be used along the full width of the reinforcing plate and shall extend 

a minimum of 25 mm (1 in.) beyond the edges of the reinforcing plate. Once beyond the reinforcing plate, the 

full penetration weld shall tie in to the outside shell-to-bottom fillet weld to create a “water stop” and then 

transition to the typical shell-to-bottom weld detail. See Figure 9.11 for weld joint details. 

c) The bottom portion of the reinforcing plate may be removed using a horizontal cut between the bottom invert 

of the nozzle neck and the new bottom per requirements of Figure 9.12. The removed (or new) reinforcing 

plate shall be prepared for a full fusion splice weld with telltale hole added (see Figure 9.12). The removed (or 

new) reinforcing plate shall be re-installed after the shell-to-bottom weld has been completed, inspected, and 

tested. The splice weld shall be made prior to the reinforcing plate weld to bottom plate weld. The completed 

splice weld shall be magnetic particle examined.

Figure 9.10—Details for Installing a New Bottom Through an Existing Tombstone Reinforcing Plate

10

12

10

Centerline of nozzle

Tank shell

Fillet weld sizes per

API 650, Section 5.1.5.7

Not less than the greater of T/2

or the fillet size determined

from API 650, Section 5.1.5.7

New bottom or

annular plate

Existing tombstone

reinforcing plate

See Note 1

T

t

Existing shell penetration

(existing welds not shown)

NOTE 1 In using this detail, it must be confirmed that the distance from top of 

new bottom to center of nozzle meets API 650, Table 5-6a or Table 5-6b.

NOTE 2 All welds shown shall be individually examined to API 650, Section 

7.2.4.

12
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 9-13

d) The lower portion of the existing reinforcing plate may be removed and re-installed after the new shell-to-bottom weld 

is complete. The existing reinforcing plate shall be cut at the horizontal centerline of the nozzle. Telltale holes are 

required in both parts of the reinforcing plate (see Figure 9.8).

e) The existing reinforcing plate may be removed, modified and re-installed after the new shell-to-bottom weld is 

complete (see Figure 9.8).

NOTE 1 Options d) and e) are not permitted on existing post-weld heat treated nozzles unless the requirements of 11.3 are met. 

NOTE 2 To minimize damage to the shell plate such that repairs can be made, care must be exercised when removing the 

existing reinforcing plate. 

9.10 Repair of Tank Bottoms

9.10.1 Repairing a Portion of Tank Bottoms

9.10.1.1 General Repair Requirements

The use of welded-on patch plates for repairing a portion of uniformly supported tank bottoms is permitted within the 

limitations given in this section and 9.10.1.2. See Figure 9.13 for acceptable details for welded-on patch plates.

a) The minimum dimension for a welded-on patch plate that overlaps a bottom seam or existing patch is 12 in. The 

welded-on patch plate may be circular, oblong, or polygonal with rounded corners.

b) A welded-on patch plate smaller than 12 in. in diameter is permitted if: it is equal to or exceeds 6 in. in diameter; it 

does not overlap a bottom seam; it is not placed fully or partially over an existing patch; and it extends beyond the 

corroded bottom area, if any, by at least 2 in.

Figure 9.11—Details for Installing a New Bottom Through an Existing Tombstone Reinforcing Plate

Centerline of nozzle

Tank shell

Full penetration

weld Fillet weld sizes per

API 650, Section 5.1.5.7

New bottom or

	

3/8 in.)

Existing tombstone

reinforcing plate

See Note 1

Existing shell penetration





NOTE 1 In using this detail, it must be confirmed that the distance from top of 

new bottom to center of nozzle meets API 650, Table 5-6a or Table 5-6b.

NOTE 2 All welds shown shall be individually examined to API 650, Section 

7.2.4.

12

10

12

10
9-14 API STANDARD 653

Figure 9.12—Details for Installing a New Bottom Through an Existing Tombstone Reinforcing Plate

NOTE 1 In using this detail, it must be confirmed that the distance from top of new bottom to

center of nozzle meets API 650, Table 5-6a or Table 5-6b and that additionally there is sufficient space 

to provide the minimum H and J spacings.

NOTE 2 All welds shown shall be individually examined to API 650, Section 7.2.4.

Fillet weld sizes per

API 650, Section 5.1.5.7

Telltale

Existing tombstone

reinforcing plate

Re-installed existing

or new reinforcing plate

Full penetration weld

Tank shell

Existing welds

Centerline of nozzle

 H

(min.)

 J

(min.)

New bottom

or annular plate

See Note 1

Existing shell penetration

Material Thickness H

(the larger of)

 J

(the larger of)

Case 1, 2, and 3

Case 1:

Case 2:

Case 3:

t2 in. 1

t2 in. 1

t2 in. 1

t2 in. 1

3 in.

2t

4 in. or 4t

5 in. or 4t

1

3 in.

3 in. or 4tw

3 in. or 4tw

5 in. or 4tw

Case 1—Shell material meets API 650, Seventh Edition or later toughness requirements and opening was PWHT’d.

Case 2—Shell material meets API 650, Seventh Edition or later toughness requirements.

Case 3—Shell material does not meet API 650, Seventh Edition or later toughness requirements.

where tw is the required weld size from API 650, Section 5.1.5.7.

T

t

10

12
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 9-15

c) Welded-on patch plates shall not be placed over areas of the tank bottom that have global dishing, local dishing 

[except as allowed by 9.10.1.1 d)], settlement, or distortion greater than the limits of Annex B.

NOTE If the tank is still undergoing settlement, the addition of welded-on patch plate may not be advisable.

d) A welded-on patch plate may be placed over a mechanical dent or local dishing if: its unsupported dimension does 

not exceed 12 in. in any direction; it is at least 1/4 in. thick; it is at least as thick as the existing bottom; and does not 

overlap seams nor other patches, except for tanks designed in accordance with API 650, Annex M, which shall 

have welded-on patch plates at least 3/8 in. thick.

e) These repairs are permanent repairs subject to an on-going inspection and maintenance program. 

f) Installation of a new sump shall conform to API Standard 650 Section 5, paragraph 5.8.7 (Water Drawoff Sumps), 

Tables 5-16a and 5-16b and Figure 5-21.

9.10.1.2 Repairs within the Critical Zone

The use of welded-on patch plates is permitted for repairing a portion of tank bottoms within the critical zone (see 3.10 

for definition) provided 9.10.1.1 requirements and the following additional requirements are met.

a) Maximum plate thickness for welded-on patch plates within the critical zone is 1/4-in. and must meet the toughness 

requirements of API 650, Section 4.2.10. 

b) When a welded-on patch plate is within 6 in. of the shell, the welded-on patch plate shall be tombstone shaped. 

The sides of the tombstone shaped welded-on patch plate shall intersect the shell-to-bottom joint at approximately 

90°.

c) Perimeter welds on welded-on patch plates within the critical zone shall be two-pass, minimum, and examined per 

12.1.1.3 and 12.1.7.2. 

d) Installation of a welded-on patch plate by butt-welding to an adjacent existing patch is not permitted in the critical 

zone.

e) Welded-on patch plates over existing patches are not allowed in the critical zone.

f) The bottom plate under the perimeter of a welded-on patch plate shall meet the thickness requirements in 4.4.

g) For tanks with shell plate of unknown toughness as defined in Section 3, new fillet welds utilized to install a 

tombstone patch plate in the critical zone shall be spaced at least the greater of 3 in. or 5t from existing vertical 

weld joints in the bottom shell course, where t is the thickness of the bottom shell course, in inches. See Figure 

9.13 for further guidance on weld spacing.

NOTE The bottom plate thickness at the attachment weld must be at least 0.1-in. thick before welding the welded-on patch plate 

to the bottom plate. Refer to API 2207 for further information.

9.10.1.2.1 No welding or weld overlays are permitted within the critical zone except for the welding of: widely 

scattered pits (see 4.3.2.2), pinholes, cracks in the bottom plates, the shell-to-bottom weld, welded-on patch plates, or 

where the bottom plate welded to the shell is being replaced.

9.10.1.2.2 A welded-on patch plate shall not be used if the covered bottom plate minimum remaining thickness at 

the toe of the internal shell-to-bottom weld will be less than the minimum thickness required by 4.4.5 and 4.4.6 at the 

next internal inspection.

9.10.1.2.3 Welded-on patch plates are not permitted in the critical zone on a tank bottom with an operating 

temperature exceeding 200 °F for carbon steel or 100 °F for stainless steel.

9.10.1.2.4 If more extensive repairs are required within the critical zone than those listed in 9.10.1.2, the bottom 

plate welded to the shell shall be cut out and a new plate shall be installed. Weld spacing requirements shall be in 

accordance with 9.10.2.4, and API 650, Section 5.1.5.4 and Section 5.1.5.5. The shell-to-bottom weld shall be 

removed and replaced for a minimum distance of 12 in. on each side of the new bottom plate.

13

13

13

12

12

10
9-16 API STANDARD 653 Typical “tombstone-shaped” welded-on plates 2 in. minimum radius

AA

L1 L2

t

Note 3 Note 3

Vertical

shell welds The greater of 3 in.

or 5t (see Note 7)

Note 2

2 in.

(min.)

6 in.

(min.)

2 in. (min.) 2 in. (min.)

12 in. (min.)

12 in.

(min.)

2 in. (min.)see Note 4 Bottom seam Bottom seam

Three-plate lap

(see Note 5)

12 in. (min.)

6 in. (min.)

45° (min.)

Shell plate

3 in.

Critical

zone Welded-on patch plate

(1/4 in. maximum

thickness in thecritical zone)

Shell

Bottom plate or annular plate ring

Section 

A-A

Plan View

NOTE 1 Dimensions given are from toe of fillet welds or to centerline of butt-weld and also apply to new-to-existing 

welds. 

NOTE 2 Minimum distance between two welded-on patch plates in the critical zone shall be one half of the lesser of 

L1 or L2.

NOTE 3 The maximum dimension along the shell for welded-on patch plates in the critical zone is 24 in. 

NOTE 4 When the edge of a welded-on patch plate is approximately parallel to a bottom seam, the edge shall be 

held at least 2 in. from weld seam.

NOTE 5 Patches over three-plate laps shall extend a minimum of 12 in. in all directions along all laps beyond the 

three-plate lap.

NOTE 6 These rules apply to butt-welded bottoms, where applicable.

NOTE 7 Applies to shells of unknown toughness.

12 in.

(min)

Three-plate lap

(see Note 5)

2 in. (min)

12 in. (min)

Bottom corrodedarea, if any

For welded-on patch plates less than12 in. dimension, see 9.10.1.1 b)

2 in.

(min.)

Three-plate lap

Figure 9.13—Typical Welded-on Patch Plates on Tank Bottom Plates

12
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 9-17

9.10.1.3 The use of welded-on patch plates that do not meet the requirements of 9.10.1.1 or 9.10.1.2 is permitted if 

the repair method has been reviewed and approved by an engineer experienced in storage tank design in 

accordance with API 650. The review shall consider brittle fracture, stress due to settlement, stress due to shellbottom discontinuity, metal temperature, fracture mechanics, and the extent and quality of NDE.

9.10.1.4 Unacceptable indications such as cracks, gouges, tears, and corroded areas discovered in bottom 

plates, located outside the critical zone, may be repaired by deposition of weld metal followed by examination and 

testing in accordance with 12.1.7.3. Surface irregularities and contamination within the area to be repaired shall be 

removed before welding.

9.10.1.5 The repair of sumps located within the critical zone shall be in accordance with 9.10.1.2.

9.10.1.6 The repair of corroded plates in the critical zone is limited to pit welding or overlay welding as noted in this 

section. The weld repair of bottom plate corrosion is permitted if all of the following conditions are satisfied.

a) The sum of the pit dimensions along an arc parallel to the shell-to-bottom joint does not exceed 2 in. in an 8-in. 

length.

b) There must be sufficient remaining bottom plate thickness for completion of a sound weld and to avoid burnthrough. The minimum acceptable bottom plate thickness for weld repairs is 0.10 in. A lesser thickness is permitted 

for weld repairs only if reviewed and approved by an engineer experienced in storage tank design and repair.

c) All weld repairs shall be ground flush with the surrounding plate material and be examined in accordance with 

12.3.2.4.

9.10.2 Replacement of Tank Bottom Plates

9.10.2.1 Requirements governing the installation of a replacement bottom over an existing bottom are given in 

9.10.2.1.1 through 9.10.2.1.5. 

9.10.2.1.1 Suitable noncorrosive material cushion such as sand, gravel, or concrete shall be used between the old 

bottom and the new bottom.

9.10.2.1.2 The shell shall be slotted with a uniform cut made parallel to the tank bottom. The cut edges in the slot 

shall be ground to remove all slag and burrs from cutting operations. The new bottom plate shall extend outside the 

shell as required by API 650, Section 5.4.2. All rules for weld spacing shall be followed.

9.10.2.1.3 Voids in the foundation below the old bottom shall be filled with sand, crushed limestone, grout, or 

concrete.

9.10.2.1.4 Except as permitted in 9.10.2.7, existing shell penetrations shall be raised or their reinforcing plates 

modified if the elevation of the new bottom results in inadequate nozzle reinforcement details (see Figure 9.8 and API 

650, Section 5.7.2) or if the weld space requirements given in API 650, Section 5.7.3 are not met.

9.10.2.1.5 For floating roof tanks, the new bottom profile must keep the roof level when it is resting on its support 

legs. The levelness of the floating roof can be adjusted by changing the length of the support legs. The support legs 

can either remain the same length to maintain the original height above the bottom or be shortened by the same 

amount as the thickness of the cushion and new bottom plate.

9.10.2.2 New bearing plates for fixed roof support columns shall be installed. For steel floating roof legs, steel pads or 

other means shall be used to distribute the loads on the bottom of the tank and provide a wear surface. If pads are used, 

they shall be continuously welded to the tank bottom. For aluminum floating roofs, the pads may be omitted if the owner/

operator approves and new austenitic stainless steel or acceptable non-metallic (e.g., Teflon) spacers are installed to 

isolate legs from the carbon steel bottom. For aluminum floating roofs, austenitic stainless steel or acceptable non12

10
9-18 API STANDARD 653

metallic (e.g., Teflon) spacers may installed to isolate legs from the carbon steel bottom instead of welded pads if the 

spacers will not damage bottom coatings, there is no evidence of corrosion damage from such spacers on the previous 

bottom, and if the owner/operator approves.

9.10.2.3 When removing an existing tank bottom, the tank shell shall be separated from tank bottom either by:

a) cutting the shell parallel to the tank bottom a minimum of 1/2 in. above the bottom-to-shell weld (cut line B-B as 

shown in Figure 10.1), or

b) removing the entire shell-to-bottom attachment weld, including any penetration and heat affected zone by suitable 

methods such as arc gouging and/or grinding.

All arc-gouged areas of the tank shell-to-bottom weld shall be magnetic particle examined, and defective areas 

repaired and re-examined.

9.10.2.4 Installation of a new tank bottom, after removal of the existing tank bottom, shall meet all requirements of 

API 650. Except as permitted in 9.10.2.7, existing shell penetrations shall be raised or their penetration reinforcing 

plates modified if the elevation of the new bottom results in inadequate nozzle reinforcement (see Figure 9.8 and API 

650, Section 5.7.2) or if the weld spacing requirements given in API 650, Section 5.7.3 are not met. For tanks with shell 

plate of unknown toughness as defined in Section 3, new weld joints in the bottom or annular ring shall be spaced at 

least the greater of 3 in. or 5t from existing vertical weld joints in the bottom shell course, where t is the thickness of the 

bottom shell course, in inches.

9.10.2.5 Replacement of portions of an existing tank bottom (entire rectangular plates or large segments of plates) 

not within the critical zone (see 3.10 for definition) are permitted under the same rules that govern installation of 

bottoms in new tank construction per API 650, Sections 5.4 and 5.5.

9.10.2.6 The following shall be considered for tanks with cathodic protection and under-bottom leak detection.

a) For tanks having cathodic protection (CP) installed under the existing bottom, consideration shall be given to 

removal of the entire bottom and unused dead shell to prevent shielding of CP current to the new bottom. Removal 

of the old bottom is also important in preventing galvanic corrosion (refer to API 651). Where this is possible, 

removal of the entire old bottom, except the unused dead shell and not more than 18 in. of bottom annulus 

attached to the shell, shall be considered. 

b) Consideration shall be given to installing under-bottom leak detection at this time (such as a RPB) to contain and 

channel any bottom leak to a location where it can readily be observed from outside of the tank. See 4.4.3.5 and 

Annex I.

9.10.2.7 For tanks constructed from materials having 50,000 lbf/in.2 yield strength or less, existing shell 

penetrations need not be raised if the following conditions are met.

a) For reinforced penetrations, including low-types, a minimum of 4 in. shall be maintained between the shell-tobottom weld toe and the nearest penetration attachment weld toe (reinforcing plate periphery weld, or nozzle neck 

weld to low type reinforcing plate and shell welds).

b) For self-reinforced penetrations, the greater of 3 in. or 21/2t shall be maintained between the shell-to-bottom weld 

toe and the nearest penetration attachment weld toe.

c) The shell-to-bottom weld is to be welded with low hydrogen electrodes and with welding procedures that are 

designed to limit distortion and residual stress.

d) The toes of the welds shall be blend-ground to minimize stress concentrations as follows. 

10

12

13
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 9-19

i) For circular reinforcing plates, blend-grind the periphery attachment weld from the “four o'clock” position to the 

“eight o'clock” position. Blend-grind the inside and outside of the shell-to-bottom weld a minimum of one penetration diameter length on either side of the penetration centerline.

ii) For diamond-shaped reinforcing plates, blend-grind the lower horizontal length of the diamond shaped attachment weld. Blend-grind the inside and outside of the shell-to-bottom weld a minimum of one penetration 

diameter length on either side of the penetration centerline.

iii) For low-type penetrations, blend-grind the nozzle attachment weld (shell and reinforcing plate) from the “four 

o'clock” position to the “eight o'clock” position. Blend-grind the inside and outside of the shell-to-bottom weld a 

minimum of one penetration diameter length on either side of the penetration centerline.

e) The blend-ground lengths of welds listed in 9.10.2.1.7 d) shall be magnetic particle examined before and after 

hydrostatic test.

9.10.3 Additional Welded-on Plates

9.10.3.1 If other welded-on plates such as wear, isolation, striker, and bearing plates, are to be added to tank 

bottoms, they shall be installed in accordance with 9.10.1, and examined in accordance with 12.1.7. For these 

additional welded-on plates, if the lap weld spacing requirements as set forth in Figure 9.13 are not met, magnetic 

particle (MT) or liquid penetrant (PT) examination is required for the exposed welds or portions of welds failing to 

meet minimum spacing criteria. See Section 12 for acceptance requirements.

9.10.3.2 Welded-on plates that fall within the critical zone (see 3.10 for definition) shall be installed in accordance 

with 9.10.1.2 and comply with all of its requirements.

9.11 Repair of Fixed Roofs

9.11.1 Repairs

9.11.1.1 Roof repairs involving tank venting shall be made such that normal and emergency venting meet the 

requirements of API 650, Section 5.8.5. 

9.11.1.2 Roof repairs involving modification of the roof structure and the frangible joint (if applicable) shall be in 

compliance with the requirements of API 650, Section 5.10.

9.11.2 Supported Cone Roofs

9.11.2.1 The minimum thickness of new roof plates shall be 3/16 in. plus any corrosion allowance as specified in the 

repair specifications. In the event roof live loads in excess of 25 lbf/ft2 are specified (such as insulation, operating 

vacuum, high snow loads), the plate thickness shall be based on analysis using the allowable stresses in 

conformance with API 650, Section 5.10.3 (see 9.11.2.2).

9.11.2.2 The roof supports (rafters, girders, columns, and bases) shall be repaired or altered such that under design 

conditions the resulting stresses do not exceed the stress levels given in API 650, Section 5.10.3. 

9.11.3 Self-supporting Roofs

9.11.3.1 The nominal thickness of new roof plate shall be 3/16 in. or the required plate thickness given in API 650, 

Section 5.10.5 or Section 5.10.6, plus any specified corrosion allowance, whichever is greater.

9.11.3.2 The details of the roof-to-shell junction shall meet the requirements of API 650, Section 5.10.5, Section 

5.10.6, or Annex F of this standard, as applicable, for the intended service.
9-20 API STANDARD 653

9.12 Repair of Floating Roofs

9.12.1 External Floating Roofs

Any method of repair is acceptable that will restore the roof to a condition enabling it to perform as required.

9.12.2 Internal Floating Roofs

Repairs to internal floating roofs shall be made in accordance with the original construction drawings, if available. If 

the original construction drawings are not available, the roof repairs shall be in compliance with the requirements of 

API 650, Annex H.

9.12.3 Repair of Leaks in Pontoons

All leaks in pontoons or compartments of double deck floating roofs shall be repaired by rewelding the leaking joints 

and/or use of patch plates.

9.13 Repair or Replacement of Floating Roof Perimeter Seals

9.13.1 Primary Seals

Rim-mounted primary shoe seals and toroidal seal systems can be removed, repaired, or replaced. To minimize 

evaporation losses and reduce potential hazard to the workers, no more than one-fourth of the roof seal system 

should be out of an in-service tank at one time. Temporary spacers to keep the roof centered shall be used during the 

repairs. Primary seal systems mounted partly or fully below the bolting bar or top of the rim usually cannot be reached 

to allow removal in service. In this case, in-service repairs are limited to replacement of the primary seal fabric.

9.13.2 Secondary Seals

Rim-mounted and shoe-mounted secondary seals may be readily installed, repaired, or replaced while the tank is 

in service.

9.13.3 Seal-to-shell Gap

Repair and other corrective actions to maintain seal-to-shell gap requirements, include the following.

a) Adjusting the hanger system on primary shoe seals, and adding foam filler in toroidal seals.

b) Increasing the length of rim mounted secondary seals in the problem area.

c) Replacing all or part of the primary seal system along with possible installation of a rim extension for a secondary 

seal. This step shall be taken only after checking the annular space variation at several levels from low pump out to 

high liquid level.

9.13.4 Mechanical Damage

Damaged parts shall be repaired or replaced. Prior to taking this action, the cause of the damage shall be identified 

and corrected. Buckled parts shall be replaced, not straightened. Torn seal fabric shall be replaced.

9.13.5 Deterioration of Seal Material

Material deterioration results from wear and corrosion on metallic elements, and chemical and weather 

deterioration of seal fabric. The service life and inspection information shall be used to determine whether a 

change of material is warranted.

13
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 9-21

9.13.6 Installation of Primary and Secondary Seals

9.13.6.1 The replacement or addition of primary and secondary seals shall be in accordance with the 

recommendations of the seal manufacturer. In addition, the final installation shall comply with all applicable 

jurisdictions.

9.13.6.2 If the roof rim thickness is less than 0.10-in. thick, it shall be replaced. The new roof rim shall be 3/16-in. 

thickness, minimum.

9.14 Hot Taps

9.14.1 General

9.14.1.1 The requirements given herein cover the installation of radial hot tap connections on existing in-service tanks. 

Hot taps are not permitted on shell material requiring thermal stress relief as specified in API 650, Section 5.7.4.

a) For tank shell plates of recognized toughness, as defined in Section 3, the connection size and shell thickness 

limitations are shown in Table 9.1.

b) For tank shell plates of unknown toughness, as defined in Section 3, the following limitations apply.

1) Nozzles shall be limited to a maximum diameter of 4 in. NPS.

2) The shell plate temperature shall be at or above the minimum shell design metal temperature for the entire hot 

tapping operation.

3) All nozzles shall be reinforced. The reinforcement shall be calculated per API 650, Section 5.7.2. The minimum 

thickness of the reinforcing plate shall be equal to the shell plate thickness, and the minimum reinforcing plate 

diameter shall not be less than the diameter of the shell cutout plus 2 in.

4) The maximum height of tank liquid above the hot tap location during the hot tapping operation shall be such that 

the hydrostatic tank shell stress is less than 7,000 lbf/in.2 at the elevation of the hot tap.

9.14.1.2 The minimum height of tank liquid above the hot tap location shall be at least 3 ft during the hot tapping 

operation.

9.14.1.3 Welding shall be done with low hydrogen electrodes.

9.14.1.4 Hot taps are not permitted on the roof of a tank or within the gas/vapor space of the tank.

Table 9.1—Hot Tap Connection Sizes and Shell Plate Thicknesses

Connection Size, 

NPS

(in.)

Minimum Shell

Plate Thickness

(in.)

≤ 6 3/16 

≤ 8 1/4

≤ 14 3/8

≤ 18 1/2
9-22 API STANDARD 653

9.14.1.5 Hot taps shall not be installed on laminated or severely pitted shell plate.

9.14.1.6 Hot taps are not permitted on tanks where the heat of welding may cause environmental cracking (such as 

caustic cracking or stress corrosion cracking).

9.14.2 Hot Tap Procedures

A hot tap procedure specific to carrying out the work shall be developed and documented. The procedure shall 

include the practices given in API 2201.

9.14.3 Preparatory Work

9.14.3.1 Minimum spacing in any direction (toe-to-toe of welds) between the hot tap and adjacent nozzles shall be 

equivalent to the square root of RT where R is the tank shell radius, in inches, and T is the shell plate thickness, in 

inches.

9.14.3.2 Shell plate thickness measurements shall be taken at a minimum of four places along the circumference of 

the proposed nozzle location.

9.14.4 Material Limitations

Only hot tap steels of recognized toughness, as defined in Section 3, unless the additional requirements of 9.14.1.1 b) 

are met.

9.14.5 Installation Procedure

9.14.5.1 Pipe nozzles shall be cut to the contour of the shell and beveled from the outside for a full penetration weld 

(see Figure 9.14). The nozzle neck-to-shell weld shall be examined in accordance with 12.1.2.3.

9.14.5.2 After the pipe is welded, the reinforcing plate shall be installed either in one piece or two pieces with 

horizontal weld. The reinforcing plate to nozzle shall be installed with a full penetration weld. Care shall be taken to 

limit the heat input to the welds. 

9.14.5.3 After the reinforcing plate has been welded to the shell and NDE performed, the pad shall be pneumatically 

tested by the procedure described in API 650, Section 7.3.4. After the valve has been installed on the flange, a 

pressure test at least 1.5 times the hydrostatic head shall be performed on the nozzle prior to mounting the hot tap 

machine, which shall be bolted to the valve. The required pressure for the pressure test shall be at least the value 

computed by the following equation: 

where

H2 is the height of tank shell in inches (in.); 

G is the specific gravity of product stored, as specified by purchaser. The specific gravity shall not be less than 1.0;

γw is the density of water in pound force for cubic inch (lbf/in.3).

9.14.5.4 A qualified operator shall operate the hot tap machine and cut the hole in the tank following the hot tap 

machine manufacturer’s procedures.

P( ) psi = 1.5H2Gγw
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 9-23

Figure 9.14—Hot Tap for Tanks (See Note 1)

Flange, Class 150

W.N. or S.O.

Reinforcing plate (T = t)

rolled to outside radius

of tank shell

Pipe nozzle, 

Schd extra strong

See Detail a

t

T

T

Grind out root and

seal weld for

nozzle size 10 in. NPS

and larger

1/4 in.

min

HOT TAP ASSEMBLY

DETAIL a

REINFORCING PLATE DETAIL

(See Note 1 and Note 2)

12 in. min. Do

Vertical

weld

Horizontal weld if

reinforcing plate

is in two pieces

Spacing per

API 650, 

Section 5.7.3

Horizontal weld

EqualEqual

NOTE 1 See 12.1 for NDE requirements.

NOTE 2 Reinforcing plate dimensions are as given in API 650, Table 5-6a or Table 5-6b.

Greater of

2 in. or 6T

1/4 in. telltale hole,

drill and tap for

1/8-in. pipe

thread

12

10-1

Section 10—Dismantling and Reconstruction

10.1 General

10.1.1 This section provides procedures for dismantling and reconstructing existing welded tanks that are to be 

relocated from their original site.

10.1.2 Hydrostatic testing requirements, NDE requirements, and acceptance criteria for the quality of welds for 

reconstructed tanks are specified in Section 12.

10.1.3 All reconstruction work must be authorized by the authorized inspector or an engineer experienced in 

storage tank design, prior to commencement of the work by a reconstruction organization (see 3.23). The authorized 

inspector will designate inspection hold points required during the reconstruction process and minimum 

documentation to be submitted upon job completion.

10.1.4 The authorized inspector or an engineer experienced in storage tank design shall approve all reconstruction 

work at the designated hold points and after reconstruction has been completed in accordance with the requirements 

of this standard.

10.2 Cleaning and Gas Freeing

The tank shall be cleaned and gas-freed prior to commencement of dismantling.

10.3 Dismantling Methods

10.3.1 General

Roof, shell, and bottom plates may be cut into any size pieces that are readily transportable to the new site for 

reconstruction.

10.3.2 Bottoms

10.3.2.1 Bottom plates that will be reused shall be cut by deseaming of lapwelds; or by cutting alongside of the 

remaining welds at a minimum of 2 in. away from existing welds, except where cuts cross existing weld seams.

10.3.2.2 If the bottom is to be used, one of the following methods is acceptable:

a) the bottom plates may be cut from the shell along a line A-A and line B-B shown in Figure 10.1, scrapping the welds 

and the bottom plate directly attached to the shell;

b) if the entire bottom is to be reused, the bottom may be cut from the shell on the line C-C leaving the shell with part 

of the bottom attached;

c. if the tank has an existing butt-welded annular ring, this ring can be left attached to the shell or removed from the 

shell by cutting out along line B-B or otherwise removing the existing shell to annular ring welds. 

10.3.3 Shells

10.3.3.1 Tank shell plates may be dismantled using one of the following methods or a combination thereof.

a) Any shell ring may be dismantled by cutting out existing weld seams and the heat affected zone (HAZ) of the weld. 

For the purpose of this method, the minimum HAZ to be removed will be 1/2 of the weld metal width or 1/4 in., 

whichever is less, on both sides of the weld seam.
10-2 API STANDARD 653

b) Any shell ring 1/2-in. thick or thinner may be dismantled by cutting through the weld without removing the HAZ.

c) Shell rings may be dismantled by cutting vertical and/or horizontal cuts through the shell a minimum of 6 in. away 

from existing welds, except where cuts cross existing welds.

10.3.3.2 Shell stiffening rings, including wind girders and top angles, may be left attached to the shell plates or may 

be removed by cutting at the attachment welds. The area where temporary attachments are removed shall be ground 

flush with the shell plate.

10.3.3.3 The shell shall be cut from the bottom plate along line B-B as shown in Figure 10.1. The existing shell-tobottom weld connection shall not be reused unless the entire bottom is to be reused intact.

10.3.4 Roofs

10.3.4.1 Roof plates shall be cut by deseaming of lap welds, or by cutting alongside of the remaining welds at a 

minimum of 2 in. away from existing welds, except where cuts cross existing welds.

10.3.4.2 Roof supporting structures shall be dismantled by removing bolts (if bolted) or deseaming the structural 

attachment welds.

10.3.5 Piece Marking

10.3.5.1 Shell, bottom, and roof plates shall be marked prior to dismantling for ready identification and placement 

when the tank is reconstructed. Marking material shall be of a durable type. Drawings showing piece mark locations 

are also a useful adjunct.

Figure 10.1—Tank Shell and Bottom Cut Locations

Shell

B B

C

Bottom C

plate

1/2 in. min.

1/2 in. min.

A

A

12 in. min.
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 10-3

10.3.5.2 A minimum of two sets of matching center punch marks shall be located on the top and bottom edges of 

each shell plate to facilitate proper alignment during reconstruction.

10.4 Reconstruction

10.4.1 General

10.4.1.1 The foundation for the reconstructed tank shall meet the construction tolerances given in 10.5.6.

10.4.1.2 Temporary attachments shall be removed, and the attachment area ground flush with the plate surface.

10.4.2 Welding

10.4.2.1 Provisions shall be made during the reconstruction of a tank to ensure that weld spacing requirements of 

Figure 9.1 are maintained. New vertical joints in adjacent shell courses, made in accordance with 10.3.3.1, shall not 

be aligned but shall be offset from each other a minimum distance of 5t, where t is the plate thickness of the thicker 

course at the point of the offset.

10.4.2.2 Tanks and their structural attachments shall be welded in accordance with the processes specified in 

API 650, Section 5.8 and the requirements of 10.4.2.3 through 10.4.2.11.

10.4.2.3 No welding of any kind shall be performed when the surfaces of the parts to be welded are wet from rain, 

snow, or ice; when rain or snow is falling on such surfaces; or during periods of high winds unless the welder and the 

work are properly shielded. No welding of any kind shall be performed when the temperature of the base metal is less 

than 0 °F. When the temperature of the base metal is between 0 °F and 32 °F or the thickness is in excess of 1 in., the 

base metal within 3 in. of the place where welding is to be started shall be heated to a temperature warm to the hand 

(approximately 140 °F) before welding. (See 10.4.4.3 for preheat requirements for shell plates over 1 1/2-in. thick.)

10.4.2.4 Each layer of weld metal of multilayer welding shall be cleaned of slag and other deposits before the next 

layer is applied.

10.4.2.5 The edges of all welds shall merge with the surface of the plate without a sharp angle. Maximum 

permissible weld undercut shall be in accordance with API 650, Section 7.2.1.4 and Section 8.5.1 b).

10.4.2.6 The reinforcement of the new welds on all butt joints on each side of the plate shall not exceed the 

thicknesses shown in Table 10.1.

10.4.2.7 Tack welds used in the assembly of vertical joints of tank shells shall be removed and shall not remain in 

the finished joint when the joints are welded manually. When such joints are welded by the submerged-arc process, 

the tack welds shall be thoroughly cleaned of all welding slag but need not be removed provided that they are sound 

and are thoroughly fused into the subsequently applied weld beads. Tack welds 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 qualified welders.

10.4.2.8 If weldable primer coatings have been applied on surfaces to be welded, they shall be included in welding 

procedure qualification tests for the brand, formulation, and maximum thickness of primer applied. All other coatings 

shall be completely removed from the weld area prior to welding.

10.4.2.9 Low-hydrogen electrodes shall be used for manual metal-arc welds, including the attachment of the first 

shell course to the bottom plates or annular plate ring, as follows:

a) for all welds in shell courses over 1/2-in. thick of API 650 Group I – III materials;

b) for all welds in all shell courses of API 650 Group IV – VI materials.

12
10-4 API STANDARD 653

10.4.2.10 Low-hydrogen electrodes shall be used for welding temporary and new permanent attachments to 

shells of API 650 Group IV, IVA, V, or VI materials. The welding procedure selected shall not cause underbead 

cracking; also, the need for preheat of thick plates and the effect of low ambient temperature during welding shall 

be considered.

10.4.2.11 If existing welds are found to be unsatisfactory by the as-built standard, they shall be repaired in 

accordance with 9.6.

10.4.3 Bottoms

10.4.3.1 After the bottom plates are laid out and tacked, they shall be joined by welding the joints in a sequence that 

results in the least distortion from shrinkage and provides, as nearly as possible, a plane surface.

10.4.3.2 The welding of the shell to the bottom (except for door sheets) shall be completed prior to the welding of 

bottom joints.

10.4.3.3 Plates shall be held in close contact at all lap joints during welding.

10.4.4 Shells

10.4.4.1 Plates to be joined by butt welding shall be matched accurately and retained in position during welding. 

Misalignment in completed vertical joints over 5/8-in. thick shall not exceed 10 % of the plate thickness, with a 

maximum of 1/8 in. Misalignment in joints 5/8-in. thick or less shall not exceed 1/16 in. Vertical joints shall be completed 

before the lower horizontal weld is made.

10.4.4.2 In completed horizontal butt joints, the upper plate shall not project beyond the face of the lower plate at 

any point by more than 20 % of the thickness of the upper plate, with a maximum projection of 1/8 in., except that a 

projection of 1/16 in. is acceptable for upper plates less than 5/16-in. thick.

10.4.4.3 For horizontal and vertical joints in tank shell courses constructed of material over 1 1/2-in. thick (based 

on the thickness of the thicker plate at the joint), multi-pass weld procedures are required, with no pass more than 

3/4-in. thick permitted. A minimum preheat of 200 °F is required of these welds.

10.4.5 Roofs

This standard does not include special stipulations for reconstruction thereof, except that the structural framing (such 

as rafters and girders) must be reasonably true to line and surface. Other requirements shall be in accordance with 

the as-built standard.

Table 10.1—Maximum Thicknesses on New Welds

Plate Thickness

(in.)

Maximum Reinforcement Thickness

(in.)

Vertical Joints Horizontal Joints

≤ 1/2 3/32 1/8

> 1/2 through 1 1/8 3/16

> 1 3/16 1/4
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 10-5

10.5 Dimensional Tolerances

10.5.1 General

10.5.1.1 The tolerances given in this section have been established to produce a reconstructed tank of acceptable 

appearance and structural integrity and to permit proper functioning of floating roofs and seals.

10.5.1.2 Measurements to verify these tolerances shall be taken before the hydrostatic test of the reconstructed 

tank.

10.5.2 Plumbness

10.5.2.1 The maximum out-of-plumbness of the top of the shell relative to the bottom of the shell shall not exceed 1/100

of the total tank height, with a maximum of 5 in. The 1/100 criteria, with a maximum of 5 in., shall also apply to fixed roof 

columns. For tanks with internal floating roofs, apply the criteria of this section or API 650, Section 7.5.2 and Annex H, 

Section H.4.1.1, whichever is more stringent.

10.5.2.2 The out-of-plumbness in one shell course shall not exceed the values specified for mill tolerances in 

ASTM A6 or ASTM A20, whichever is applicable.

10.5.3 Roundness

Radii measured at 1 ft above the shell-to-bottom weld shall not exceed the tolerances shown in Table 10.2.

Radius tolerances measured higher than one foot above the shell-to-bottom weld shall not exceed three times the 

tolerances given in Table 10.2.

10.5.4 Peaking

With a horizontal sweep board 36-in. long, peaking shall not exceed 1/2 in. The sweep board shall be made to the true 

outside radius of the tank.

10.5.5 Banding

With a vertical sweep board 36-in. long, banding shall not exceed 1 in.

Table 10.2—Radii Tolerances

Tank Diameter

(ft)

Radius Tolerances

(in.)

< 40 ±1/2

40 to < 150 ±3/4

150 to < 250 ±1

≥ 250 ±1 1/4

13
10-6 API STANDARD 653

10.5.6 Foundations

10.5.6.1 To achieve the tolerances specified in 10.5.1 through 10.5.5, it is essential that foundations true to a plane 

be provided for the tank reconstruction. The foundation shall have adequate bearing capacity to maintain the trueness 

of the foundation.

10.5.6.2 Where foundations true to a horizontal plane are specified, tolerances shall be as follows:

a) where concrete ringwalls are provided under the shell, the top of the ringwall shall be level within ±1/8 inches in any 

30 ft of the circumference and within ±1/4 inches in the total circumference measured from the average elevation;

b) where concrete ringwalls are not provided, the foundation under the shell shall be level within ±1/8 in. in any 10 ft of 

circumference and within ±1/2 in. in the total circumference measured from the average elevation.

10.5.6.3 For foundations specified to be sloped from a horizontal plan, elevation differences about the 

circumference shall be calculated from the specified high point. Actual elevation differences about the 

circumference shall be determined from the actual elevation of the specified high point. The actual elevation 

differences shall not deviate from the calculated differences by more than the following tolerances:

a) where concrete ringwalls are provided ±1/8 inches in any 30 ft of circumference and ±1/4 inches in the total 

circumference;

b) where concrete ringwalls are not provided, ±1/8 inches in any 10 ft of circumference and ±1/2 inches in the total 

circumference.
11-1

Section 11—Welding

11.1 Welding Qualifications

11.1.1 Welding procedure specifications (WPSs) and welders and welding operators shall be qualified in 

accordance with Section IX of the ASME Code, the additional requirements of API 650, Section 9, and this standard. 

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.

11.1.2 Weldability of steel from existing tanks shall be verified. If the material specification for the steel from an 

existing tank is unknown or obsolete, test coupons for the welding procedure qualification shall be taken from the 

actual plate to be used.

11.2 Identification and Records

11.2.1 Each welder and welding operator shall be assigned an identifying number, letter, or symbol. Records of this 

identification, along with the date and results of the welder’s qualification tests shall be accessible to the inspector.

11.2.2 The welder or welding operator’s identification mark shall be hand- or machine-stamped adjacent to and at 

intervals not exceeding 3 ft along the completed welds. In lieu of stamping, a record may be kept that identifies the 

welder or welding operator employed for each welded joint; these records shall be accessible to the inspector. Roof 

plate welds and flange-to-nozzle-neck welds do not require welder identification.

11.3 Preheat or Controlled Deposition Welding Methods as Alternatives to Post-weld Heat 

Treatment (PWHT)

Preheat and controlled deposition welding, as described in 11.3.1 and 11.3.2, may be used in lieu of PWHT for repairs 

to existing nozzles where PWHT is required by API 653 or was performed in the original construction but is 

inadvisable or mechanically unnecessary for the repair. Prior to using any alternative method, a metallurgical review 

conducted by a storage tank engineer shall be performed to assess whether the proposed alternative is suitable for 

the application. The review shall consider the reason for the original PWHT of the equipment, susceptibility of the 

service to promote stress corrosion cracking, stresses in or near the weld, etc. 

If materials are of unknown toughness and fall under the Figure 5.2 exemption curve, follow the requirements of 

11.3.2. The storage tank engineer must concur in writing with the PWHT exemption. Also, the tank owner/operator 

must authorize the exemption in writing.

Selection of the welding method used shall be based on technical consideration of the adequacy of the weld in the aswelded condition at operating and hydrotest conditions.

11.3.1 Preheating Method (Impact Testing Not Required)

If impact testing is not required, the following additional preheat requirements apply.

a) This method is limited to use on P-1 materials that were not required to be impact tested as part of the original 

construction or under current requirements of API 650, Section 9.2.

b) The welding shall be limited to the shielded-metal-arc welding (SMAW), gas-metal-arc welding (GMAW), flux-cored 

arc welding (FCAW) and gas-tungsten-arc welding (GTAW) processes.

c) The welders and welding procedures shall be qualified in accordance with the applicable rules of the original code 

of construction, except that the PWHT of the test coupon used to qualify the procedure shall be omitted. 

12

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11-2 API STANDARD 653

d) The weld area shall be preheated and maintained at a minimum temperature of 150 °C (300 °F) during welding. 

The 150 °C (300 °F) temperature shall be checked to assure that 100 mm (4 in.) of the material or four times the 

material thickness (whichever is greater) on each side of the groove is maintained at the minimum temperature 

during welding. The maximum inter-pass temperature shall not exceed 315 °C (600 °F). When the weld does not 

penetrate through the full thickness of the material, the minimum preheat and maximum inter-pass temperatures 

need only be maintained at a distance of 100 mm (4 in.) or four times the depth of the repair weld (whichever is 

greater) on each side of the joint.

11.3.2 Controlled-deposition Welding Method (Impact Testing Required)

If impact testing is required, the following welding requirements apply.

a) This method may be used when welding is to be performed on materials that were required to be impact tested per 

4.2.9 and 4.2.10 of API 650 as part of the original construction or under current requirements of API 650, Section 

9.2, and is limited to P-1, P-3 and P-4 steels.

b) The welding shall be limited to the shielded-metal-arc welding (SMAW), gas-metal-arc welding (GMAW), flux-cored 

arc welding (FCAW) and gas-tungsten-arc welding (GTAW) processes.

c) A weld procedure specification shall be developed and qualified for each application. The welding procedure 

shall define the preheat temperature, the inter-pass temperature and the post heating temperature requirement 

in Item e), 8) below. The qualification thickness for the test plates and repair grooves shall be in accordance with 

Table 11.1.

The test material for the welding procedure qualification shall be of the same material specification (including 

specification type, grade, class and condition of heat treatment) as the original material specification for the repair. 

If the original material specification is obsolete, the test material used should conform as much as possible to the 

material used for construction, but in no case shall the material be lower in strength or have a carbon content of 

more than 0.35 %.

d) When impact tests are required by the construction code applicable to the work planned, the PQR shall include 

sufficient tests to determine if the toughness of the weld metal and the heat-affected zone of the base metal in the 

as-welded condition are adequate at the minimum design metal temperature. If special hardness limits are 

necessary for corrosion resistance (e.g. those set forth in NACE RP 0472, NACE MR 0103 and NACE MR 0175), 

the PQR shall include hardness test results. 

e) The WPS shall include the following additional requirements. 

1) The supplementary essential variables of ASME Code, Section IX, Paragraph QW-250, shall be required.

2) The maximum weld heat input for each layer shall not exceed that used in the procedure qualification test.

Table 11.1—Welding Methods as Alternatives to Post-weld Heat Treatment (PWHT) 

Qualification Thicknesses for Test Plates and Repair Grooves

Depth t of Test 

Groove Welded a

Repair Groove 

Depth Qualified

Thickness T of 

Test Coupon Welded

Thickness of Base 

Metal Qualified

t < t < 50 mm (2 in.) < T

t < t ≥ 50 mm (2 in.) 50 mm (2 in.) to unlimited 

a The depth of the groove used for procedure qualification must be deep enough to allow removal of the required test specimen.

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TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 11-3

3) The minimum preheat temperature for welding shall not be less than that used in the procedure qualification 

test.

4) The maximum inter-pass temperature for welding shall not be greater than that used in the procedure 

qualification test.

5) The preheat temperature shall be checked to assure that 100 mm (4 in.) of the material or four times the 

material thickness (whichever is greater) on each side of the weld joint will be maintained at the minimum 

temperature during welding. When the weld does not penetrate through the full thickness of the material, the 

minimum preheat temperature need only be maintained at a distance of 100 mm (4 in.) or four times the depth 

of the repair weld, whichever is greater, from the edge of each weld.

6) For the welding processes in 11.3.2 b), use only electrodes and filler metals that are classified by the filler metal 

specification with an optional supplemental diffusible-hydrogen designator of H8 or lower. When shielding gases 

are used with this process, the gas shall exhibit a dew point that is no higher than –50 °C (–60 °F). Surfaces on 

which welding is to performed shall be maintained in a dry condition during the welding and free of rust, mill 

scale and hydrogen-producing contaminants such as oil, grease and other organic materials.

7) The welding technique shall be a controlled-deposition, temper-bead or half-bead technique. The specific 

technique shall be used in the procedure qualification test.

8) For welds made by SMAW, after completion of welding and without allowing the weldment to cool below the 

minimum preheat temperature, the temperature of the weldment shall be raised to a temperature of 260 °C ± 

30 °C (500 °F ± 50 °F) for a minimum period of two to four hours to assist out-gassing diffusion of any weldmetal hydrogen picked up during welding. This hydrogen bake-out treatment may be omitted provided the 

electrode used is classified by the filler metal specification with an optional supplemental diffusible-hydrogen 

designator of H4 (such as E7018-H4).

9) After the finished repair weld has cooled to ambient temperature, the final temper bead reinforcement layer 

shall be removed substantially flush with the surface of the base material.

11.4 Welding Safety

Welding shall conform to the permit and safety precautions of Section 1.4 of API 2009. Permits shall consider tank 

conditions in the hot-work areas which might release flammable vapors (such as perforation resulting from corrosion).

10

12-1

Section 12—Examination and Testing

12.1 NDE

12.1.1 General

12.1.1.1 NDE shall be performed in accordance with API 650, Section 8, and any supplemental requirements given 

herein.

12.1.1.2 Personnel performing NDE shall be qualified in accordance with API 650, Section 8, and any supplemental 

requirements given herein.

12.1.1.3 Acceptance criteria shall be in accordance with API 650, Section 8, and any supplemental requirements 

given herein.

12.1.1.4 Each newly deposited weld or any cavity resulting from gouging or grinding operations shall be visually 

examined over its full length. Additional NDE of these welds may be required as described in the following relevant 

sections.

12.1.1.5 Annex G may be used to provide additional guidance in qualifying personnel and procedures when 

magnetic flux leakage (MFL) tools are used to examine tank bottoms. Owner/operators should determine specific 

requirements to meet their tank bottom integrity needs.

12.1.2 Shell Penetrations

12.1.2.1 Ultrasonic examination of shell plate for laminations shall be made in the immediate area affected when:

a) adding a reinforcing plate to an existing unreinforced penetration,

b) adding a hot tap connection.

12.1.2.2 Cavities resulting from gouging or grinding operations to remove attachment welds of existing reinforcing 

plates shall be examined by magnetic particle or liquid penetrant methods.

12.1.2.3 Completed welds attaching nozzle neck to shell, and reinforcing plate to shell and to nozzle neck, shall be 

examined by the magnetic particle or liquid penetrant methods. Consider additional examination (e.g. fluorescent 

magnetic particle examination and/or ultrasonic examination) for hot tap connections to shell plates of unknown 

toughness as defined in Section 3.

12.1.2.4 Completed welds of stress-relieved assemblies shall be examined by the magnetic particle or liquid 

penetrant methods after stress relief, but before hydrostatic testing.

12.1.3 Repaired Weld Flaws

12.1.3.1 Cavities resulting from gouging or grinding operations to remove weld defects shall be examined by the 

magnetic particle or liquid penetrant methods.

12.1.3.2 Completed repairs of butt-welds shall be examined over their full length by radiographic or ultrasonic 

methods.

12.1.3.3 Completed repairs of fillet welds shall be examined over their full length by the appropriate NDE method 

listed herein.

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12-2 API STANDARD 653

12.1.4 Temporary and Permanent Attachments to Shell Plates

12.1.4.1 The welds of permanent attachments (not including shell-to-bottom welds) and, areas where temporary 

attachments are removed and the remaining weld projections have been removed, shall be examined visually.

12.1.4.2 Completed welds of new permanent attachments (not including shell-to-bottom welds) and areas where 

temporary attachments have been removed (API 650 tank shell materials of Groups IV, IVA, V, or VI), shall be 

examined by either the magnetic particle method (or, at the option of the purchaser, by the liquid penetrant method).

12.1.5 Shell Plate to Shell Plate Welds

12.1.5.1 New welds attaching existing shell plate to existing or new shell plate shall be examined by radiographic 

methods (see 12.2). In addition, for plate thicknesses greater than 1 in., the back-gouged surface of the root pass and 

final pass (each side) shall be examined for its complete length by magnetic particle or liquid penetrant methods.

12.1.5.2 New welds joining new shell plate material to new shell plate material (partial or full shell course 

replacement or addition) need only be examined radiographically in accordance with API 650, Section 8.1.

12.1.6 Shell-to-bottom Weld

12.1.6.1 New welding on the shell-to-bottom joint shall be examined for its entire length by using a right-angle 

vacuum box and a solution film, or by applying light diesel oil. Additionally, the first weld pass shall be examined by 

applying light diesel oil to the side opposite the first weld pass made. The oil shall be allowed to stand at least 4 hours 

(preferably overnight) and then the weld examined for wicking action. The oil shall be removed before the weld is 

completed.

12.1.6.2 As an alternative to 12.1.6.1, 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 examined visually. Additionally, 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 psig 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.

12.1.6.3 The existing weld at the shell-to-bottom joint shall be examined by visual, as well as by magnetic particle or 

liquid penetrant methods, for the full length under a welded-on patch plate. An additional 6 in. of the shell-to-bottom 

joint on each side of the welded-on patch plate shall be examined similarly before placement of the repair plate to 

assure weld integrity and to confirm the absence of weld cracks.

12.1.7 Bottoms

12.1.7.1 Upon completion of welding on a tank bottom, the plates and the entire length of new welds for tank bottom 

plates shall be examined visually for any potential defects and leaks. Particular attention shall apply to areas such as 

sumps, dents, gouges, three-plate laps, bottom plate breakdowns, arc strikes, temporary attachment removal areas, 

and welding lead arc burns. Visual examination acceptance and repair criteria are specified in API 650, Section 8.5. In 

addition, all new welds, including the weld attaching a patch plate to the bottom, the areas of bottom plate restored by 

welding, and the restoration of welds found with defects during an internal inspection shall be examined by one of the 

methods specified in API 650, Section 7.3.3. Leaking areas shall be repaired by grinding and rewelding as required, 

and the repaired area shall be retested.

12.1.7.2 In addition to the requirements in 12.1.7.1, the root and final pass of a welded-on patch plate weld in the 

critical zone (see 3.10 for definition) shall be visually examined and examined by either magnetic particle or liquid 

penetrant method over its full length.

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TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 12-3

12.1.7.3 In addition to the requirements in 12.1.7.1, areas of bottom plate repaired by welding shall be examined by 

the magnetic particle method or the liquid penetrant method. In addition, the repaired area shall also be tested using 

a vacuum box and solution or a tracer gas and detector.

12.1.8 Shell Plate 

12.1.8.1 Shell Plate Repairs by Weld Metal Deposit

Areas of shell plate to be repaired by welding shall be examined visually. In addition, shell plate areas repaired by 

welding shall be examined by the magnetic particle method (or the liquid penetrant method).

12.1.8.2 Shell Plate Repairs by Lap-welded Patches

The attachment welds of new lap-welded shell patches shall be visually examined, and shall be examined by either 

the magnetic particle or liquid penetrant methods.

12.1.9 Fixed Roofs

Newly welded roof joints and repairs shall be examined in accordance with API 650, Section 7.3.2.2 and Section 7.3.7.

12.1.10 Floating Roofs

12.1.10.1 Repair Work to Steel Floating Roofs

After repair work is complete: 

a) perform a visual examination from the top and bottom side of the floating roof;

b) perform an air leak, vacuum box, penetrating oil, tracer gas, or other applicable non destructive test of the repaired 

welds (see Annex F).

As an alternative to Item b), conduct a flotation test of the repaired roof.

Examination and acceptance criteria for NDT shall be in accordance with 12.1.

12.2 Radiographs

12.2.1 Number and Location of Radiographs

The number and location of radiographs shall be in accordance with API 650, Section 8.1.2 and the following 

additional requirements:

12.2.1.1 For vertical joints:

a) new replacement shell plates to new shell plates, no additional radiographs required, other than those required by 

API 650, Section 8.1.2.2 and Figure 8-1 for new construction;

b) new replacement shell plates to existing shell plates, one additional radiograph shall be taken in each joint;

c) repaired joints in existing shell plates shall have one additional radiograph taken in each joint.

12.2.1.2 For horizontal joints:

a) new replacement shell plates to new shell plates, no additional radiographs required, other than those required by 

API 650, Section 8.1.2.3 and Figure 8-11 for new construction;

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12-4 API STANDARD 653

b) new replacement shell plates to existing shell plates, one additional radiograph for each 50 ft of repaired horizontal 

weld;

c) repaired joints in existing shell plates shall have one additional radiograph taken for each 50 ft of repaired 

horizontal weld.

12.2.1.3 For intersections of vertical and horizontal joints:

a) new replacement shell plates to new shell plates, no additional radiographs required, other than those required by 

API 650, Section 8.1.2 and Figure 8-1 for new construction;

b) new replacement shell plates to existing shell plates, each intersection shall be radiographed;

c) all repaired intersections in existing shell plates shall be radiographed.

12.2.1.4 For reconstructed tanks, each butt-welded annular plate joint shall be radiographed in accordance with 

API 650, Section 8.1.2.9.

12.2.1.5 For reconstructed tanks, radiographic examination is required for 25 % of all junctions of new welds over 

existing seams.

The owner/operator shall, with the consent of the contractor, determine the extent of further examination and repair 

that may be required. 

Any further examination or repair of existing welds will be handled by contractual agreement between the owner/

operator and tank reconstruction contractor.

12.2.1.6 New and replaced shell plate and door sheet welds shall be radiographed. All junctions between repair and 

existing welds shall be radiographed. If defects are found, 100 % radiography shall be performed on the repaired weld.

12.2.1.6.1 For circular replacement plates, a minimum of one radiograph shall be taken regardless of thickness. 

When the circular replacement plate is located in a shell plate with thickness exceeding 1 in., the weld shall be fully 

radiographed.

12.2.1.6.2 For square and rectangular replacement plates, at least one radiograph shall be taken in a vertical joint, 

and at least one in a horizontal joint, and one in each corner. When the square or rectangular replacement plate is 

located in a shell plate with thickness exceeding 1 in., the vertical joints shall be fully radiographed.

12.2.1.7 The minimum diagnostic length of each radiograph shall be 6 in.

12.2.1.8 For penetrations installed using insert plates as described in 9.8.6, the completed butt welds between the 

insert plate and the shell plate shall be fully radiographed. 

12.2.2 Acceptance Criteria for Existing Shell Plate to Shell Plate Welds

If the radiograph of an intersection between a new and old weld detects unacceptable welds by the current applicable 

standard, the existing welds shall be:

a) evaluated according to the as-built standard, or

b) evaluated using fitness-for-service assessment, or

c) repaired in accordance with 9.6.

12.2.3 Marking and Identification of Radiographs

12.2.3.1 Each film shall show an identification of the welder(s) making the weld. A weld map showing location of 

welds, weld number, radiograph number, welder identification, and grading of each weld is an acceptable alternative 

to this requirement.

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TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 12-5

12.2.3.2 Radiographs and radiograph records of all repaired welds shall be marked with the letter “R.”

12.3 Hydrostatic Testing

12.3.1 When Hydrostatic Testing is Required

A full hydrostatic test, held for 24 hours, shall be performed on the following.

a) A reconstructed tank.

b) Any tank that has undergone major repairs or major alterations (see Section 3) unless exempted by 12.3.2 for the 

applicable combination of materials, design, and construction features.

c) A tank where an engineering evaluation indicates the need for the hydrostatic test due to an increase in the 

severity of service. Examples of increased service severity are an increase in operating pressure (such as storing 

a product with a higher specific gravity), lowering the service temperature (see Figure 5.2), and using tanks that 

have been damaged.

12.3.2 When Hydrostatic Testing is Not Required

12.3.2.1 General

A full hydrostatic test of the tank is not required for major repairs and major alterations if 12.3.2.2 is satisfied plus 

either of the following:

a) appropriate parts of 12.3.2.3 through 12.3.2.6, or alternatively;

b) fitness-for-service evaluation per 12.3.2.7. 

12.3.2.2 Review/Approval/Authorization Requirements

Items a) and b) below must be satisfied. 

a) The repair has been reviewed and approved by an engineer experienced in storage tank design in accordance 

with API 650. The engineer must concur in writing with taking the hydrostatic testing exemption.

b) The tank owner/operator has authorized the exemption in writing.

12.3.2.3 Shell Repair

12.3.2.3.1 For welds to existing metal, develop welding procedure qualifications based on existing material 

chemistry, including strength requirements. Welding procedures shall be qualified with existing or similar materials, 

and shall include impact testing. Impact testing requirements shall follow appropriate portions of API 650, Section 

9.2.2 and shall be specified in the repair procedure.

12.3.2.3.2 New materials used for the repair shall meet the current edition of API 650, Section 4 requirements.

12.3.2.3.3 Existing tank materials in the repair area shall meet at least one of the following requirements.

a) API 650 requirements (Seventh Edition or later).

b) Fall within the “safe for use” area on Figure 5.2.

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12-6 API STANDARD 653

c) Stress in the repair area shall not exceed 7000 lbf/in.2. This limiting stress shall be calculated as follows:

where

S is the shell stress in pound force per square inch (lbf/in.2);

H is the tank fill height above the bottom of repair or alteration in feet (ft);

t is the shell thickness at area of interest in inches (in.);

D is the tank mean diameter in feet (ft);

G is the specific gravity of product.

12.3.2.3.4 New vertical and horizontal shell butt-welds shall have complete penetration and fusion.

12.3.2.3.5 The root pass and final pass examination shall be in accordance with 12.1.5. In addition, the finished 

weld shall be fully radiographed.

12.3.2.3.6 Shell welds for the reinforcing plate-to-nozzle neck and nozzle neck-to-shell joints shall have complete 

penetration and fusion. The root pass of the nozzle attachment weld shall be back-gouged and examined by 

magnetic particle or liquid penetrant methods. The completed weld shall be examined by magnetic particle or liquid 

penetrant methods and by the ultrasonic method. Examination and acceptance criteria for NDE shall be in 

accordance with 12.1.

12.3.2.3.7 See 12.3.2.5 for shell-to-bottom weld restrictions.

12.3.2.3.8 Door sheets shall comply with the requirements of this standard for shell plate installation, except they 

shall not extend to or intersect the bottom-to-shell joint.

12.3.2.4 Bottom Repair within the Critical Zone

12.3.2.4.1 Repairs to the annular ring or bottom plates, within the critical zone (see 3.10) shall comply with the 

following.

a) Meet the requirements of 12.3.2.3.1 through 12.3.2.3.3.

b) Be examined visually prior to welding, and examined after the root pass and the final pass by the magnetic particle or 

liquid penetrant methods. Annular plate butt-welds shall also be examined by ultrasonic methods after the final pass. 

Examination and acceptance criteria for NDE shall be in accordance with 12.1.

12.3.2.5 Shell-to-bottom Weld Repair

12.3.2.5.1 Repair of the weld attaching the shell to the annular ring or the shell to the bottom plate shall meet one of 

the following requirements.

a) A portion of the weld (of any length) may be removed and replaced as long as the replaced weld meets the size 

requirements of API 650, Section 5.1.5.7, and the portion replaced does not represent more than 50 % of the 

required weld cross-sectional area.

b) The weld on one side of the shell may be completely removed and replaced for a length not exceeding 12 in. 

Shell-to-bottom weld repairs replacing more than 50 % of the required weld cross-sectional area shall not be 

closer than 12 in. to each other, including repairs on the opposite side of the shell.

S 2.6 HDG

t = -------------------------
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 12-7

12.3.2.5.2 Repairs shall be examined prior to welding, after the root pass, and after the final pass by visual, as well 

as magnetic particle or liquid penetrant methods. Examination and acceptance criteria for NDE shall be in accordance 

with 12.1.

12.3.2.6 Minor Shell Jacking

12.3.2.6.1 Tank shell and critical zone materials shall meet one of the requirements of 12.3.2.3.3.

12.3.2.6.2 The engineer shall consider all pertinent variables when exempting a minor shell jacking repair from 

hydrostatic testing, including but not limited to: the magnitude of jacking required; material; toughness; quality control; 

inspection before and after repair; material temperature; future foundation stability; and jacking techniques (including 

controls and measurement). Careful consideration shall be given to potential stresses and damage that may result 

from jacking.

12.3.2.7 Fitness-for-service Evaluation

The owner/operator may utilize a fitness-for-service or other appropriate evaluation methodology based on 

established principles and practices to exempt a repair from hydrostatic testing. The procedures and acceptance 

criteria for conducting an alternative analysis are not included in this standard. This evaluation shall be performed by 

an engineer experienced in storage tank design and the evaluation methodologies used.

12.3.3 Other Cases Not Requiring Hydrostatic Testing

12.3.3.1 General

For clarity, the situations of 12.3.3.2 and 12.3.3.3 do not in themselves require a hydrostatic test because they are not 

major repairs or major alterations.

12.3.3.2 Repair or Alteration Made to a Floating Roof

No hydrotest is required.

12.3.3.3 Bottom Repair or Replacement Outside the Critical Zone

Portions of new bottoms (any or all rectangular plates or large segments of plates) in tanks may be replaced without a 

hydrotest when the subgrade under the new plates is found to be in a condition acceptable to the authorized inspector 

or is restored to such condition and either of the following conditions is met. 

1) For tanks with annular rings, the annular ring and the area of support under the annular ring (concrete 

foundation or grade material) remains intact.

2) For tanks without annular rings, the bottom repair or replacement does not result in welding on the remaining 

bottom within the critical zone and the shell and bottom support in the critical zone remains intact. See 3.10 for a 

definition of the critical zone.

12.4 Leak Tests

New or altered reinforcing plates of shell penetrations shall be given an air leak test in accordance with API 650, 

Section 7.3.4. 12
12-8 API STANDARD 653

12.5 Settlement Survey During Hydrostatic Testing

12.5.1 When Settlement Survey is Required

A settlement survey shall be conducted for all existing tanks that undergo a hydrostatic test, except for tanks that 

have a documented service history of acceptable settlement values, and no settlement is anticipated to occur during 

the hydrotest.

12.5.2 Initial Settlement Survey

When a settlement survey is required in accordance with 12.5.1, the tank settlement shall initially be surveyed with 

the tank empty, using an even number of elevation measurement points, N, uniformly distributed around the 

circumference. An initial settlement survey, prior to the first hydrostatic test, provides baseline readings for future 

settlement evaluation. In the absence of this initial survey, the tank shall be assumed to be initially level.

The minimum number of elevation points shall be as indicated by the following equation:

N = D/10

where

D is the tank diameter, in feet (ft).

And

N is the minimum required number of settlement measurement points, but no less than eight. All values of N 

shall be rounded to the next higher even whole number. The maximum spacing between settlement 

measurement points shall be 32 ft.

12.5.3 Settlement Survey During Hydrostatic Testing

When a settlement survey is required in accordance with 12.5.1, tank settlement shall be measured during filling and 

when the test water reaches 100 % of the test level.

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13-1

Section 13—Marking and Recordkeeping

13.1 Nameplates

13.1.1 Reconstructed Tanks

13.1.1.1 Tanks reconstructed in accordance with this standard shall be identified by a corrosion-resistant metal 

nameplate similar to that shown in Figure 13.1. Letters and numerals not less than 5/32 in. high shall be embossed, 

engraved, or stamped in the plate to indicate information as follows:

a) reconstructed to API 653;

b) edition and revision number;

c) year reconstruction was completed;

d) if known, the as-built standard and the year of original construction;

e) nominal diameter;

f) nominal shell height;

g) design specific gravity;

h) maximum permissible operating liquid level;

i) the name of the reconstruction contractor and the assigned serial number or contract number;

j) the owner/operator’s tank number;

k) shell material for each shell course;

l) maximum operating temperature;

m)allowable stress used in calculations of each shell course.

13.1.1.2 The new nameplate shall be attached to the tank shell adjacent to the existing nameplate, if any. An 

existing nameplate shall be left attached to the tank. Nameplates shall be attached as specified in API 650, Section 

10.1 and Figure 10-1.10

13.1.2 Tanks Without Nameplates

13.1.2.1 At the owner’s request a nameplate may be attached to a tank meeting the requirements in 13.1.2.2 

through 13.1.2.4.

13.1.2.2 If information required to complete the nameplate as required by the as-built standard is available and 

traceable to the tank, a new Replacement Nameplate, similar to that shown in Figure 10-1 in API 650, may be 

attached under the direction of the Authorized inspector. The new nameplate shall contain all of the information 

required by the as-built standard and be marked ‘Replacement Nameplate.’

13.1.2.3 If information required to complete the nameplate as required by the as-built standard is not available, an 

‘Assessment Nameplate’ may be attached under the direction of the Authorized inspector, provided a suitability for 

service assessment is performed per API 653, Sections 4 and 5. The new nameplate shall contain the following 

information:

a) API Standard 653, Assessment Nameplate;

10

10

12

13

10
13-2 API STANDARD 653

b) Owner's tank number;

c) the company performing the assessment;

d) the date the assessment was performed;

e) the date of the edition and the addendum number of API 653 used to perform the assessment;

f) the nominal diameter and nominal height, in meters (ft and in.);

g) the maximum capacity in m3 (42-gallon barrels);

h) the liquid level in meters (ft. and in.) used to perform the assessment;

i) the specific gravity of the liquid used to perform the assessment;

j) the design metal temperature in °C (°F) used to perform the assessment;

k) the pressure and vacuum used to perform the assessment;

l) the maximum design temperature in °C (°F) used to perform the assessment;

m) the material specification, if known, for each shell course;

n) the allowable stress values in MPa (psi) used to perform the assessment;

o) the joint efficiency used to perform the assessment (see 4.3.3. or 4.3.4).

13.1.2.4 The nameplate shall be made of a corrosion resistant metal embossed, engraved, or stamped with letters 

and numerals not less than 4 mm (5/32 in.) high. Nameplates shall be attached as specified in API Std. 650. In 

addition, the nameplate shall be clearly marked as an API 653, Assessment Nameplate. Refer to Figure 13-2. 

13.2 Recordkeeping

When a tank is evaluated, repaired, altered, or reconstructed in accordance with this standard, the following 

information, as applicable, shall be made a part of the owner/operator’s records for the tank (see 6.8).

13.2.1 Calculations for:

a) component evaluation for integrity, including brittle fracture considerations (see Section 5);

b) re-rating (including liquid level);

c) repair and alteration considerations.

13.2.2 Construction and repair drawings.

13.2.3 Additional support data including, but not limited to, information pertaining to:

a) examinations (including thicknesses);

b) material test reports/certifications;

c) tests;

10

13
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION 13-3

d) radiographs (radiographs shall be retained for at least one year);

e) brittle fracture considerations;

f) original tank construction data (date, as-built standard, etc.);

g) location and identification (owner/operator’s number, serial number);

h) description of the tank (diameter, height, service);

i) design conditions (liquid level, specific gravity, allowable stress, unusual design loadings, etc.);

j) shell material and thickness by course;

k) tank perimeter elevations;

l) construction completion record;

m)basis for hydrostatic test exemption.

13.3 Certification

Tanks reconstructed in accordance with this standard shall require documentation of such reconstruction, and 

certification that the design, reconstruction, inspection, and testing was performed in compliance with this standard. 

The certification shall contain information as shown in Figure 13.2 for design and/or reconstruction as applicable.

Figure 13.1—Nameplate

RECONSTRUCTED TO API 653 EDITION

Original Standard

Tank No.

Tank Diam. Height

Specific Gravity

Design Pressure

Orig. Const. Date

Year Reconstructed

Liquid Level Max.

Capacity

Max. Operating Temp.

Date Completed

Serial No.

Shell

Course

Allowable

Stress Material

Reconstructed by:
13-4 API STANDARD 653

Figure 13.2—Certification Forms

CERTIFICATION FOR TANK

RECONSTRUCTED TO API 653

 We hereby certify that the tank reconstructed at ___________________________________________ and described as follows:

Location

Serial No. Owner's No. Height Capacity Floating or Fixed Roof

Reconstruction Organization

Authorized Representative

Date

Reconstruction Organization

Authorized Representative

Date

was reconstructed, inspected, and tested in accordance with all applicable requirements of API Standard 653,

________________ Edition, ________________ Revision, Dated ________________ (including all material supplied by the 

reconstruction organization).

CERTIFICATION FOR TANK

DESIGNED TO API 653

 We hereby certify that the design evaluation of the tank reconstructed at _______________________________________________ 

 and described as follows: Location

Serial No. Owner's No. Height Capacity Floating or Fixed Roof

was performed by the undersigned organization in accordance with all design requirements of API Standard 653,

________________ Edition, ________________ Revision, Dated ________________.
A-1

Annex A

Background on Past Editions of API Welded Storage Tank Standards

API published a specification for welded steel storage tanks in 1936 entitled API Standard 12C, All-Welded Oil 

Storage Tanks. Fifteen editions and seven supplements to API 12C were published between 1936 and 1958. API 12C 

was replaced by API Standard 650, Welded Tanks for Oil Storage; 11 editions and 19 supplements, revisions or 

addenda to API 650 have been issued. The current edition of API 650 is the 11th Edition, Addendum 1, published in 

November 2008. 

The table below provides a list of editions, supplements, and revisions to API 12C and API 650.

Table A.1—Editions of API Standard 650 and its Precursor, API Standard 12C

API Standard 12C, All-Welded Oil Storage Tanks API Standard 650, Welded Tanks for Oil Storage

Edition Date Edition Date

First July 1936 First December 1961

Second October 1937 Supplement 1963

Supplement 1 April 1938 Second April 1964

Supplement 2 September 1938 Third July 1966

Supplement 3 April 1939 Supplement 1 December 1967

Third April 1940 Fourth June 1970

Fourth March 1941 Supplement 1 April 1971

Fifth May 1942 Fifth July 1973

Sixth August 1944 Supplement 1 October 1973

Seventh August 1946 Supplement 2 April 1974

Supplement 1 September 1947 Supplement 3 March 1975

Eighth September 1948 Sixth April 1977

Supplement 1 December 1949 Revision 1 May 1978

Ninth October 1950 Revision 2 December 1978

Tenth September 1951 Revision 3 October 1979

Eleventh September 1952 Seventh November 1980

Supplement 1 September 1953 Revision 1 February 1984

Twelfth October 1954 Eighth November 1988

Thirteenth September 1955 Ninth July 1993

Supplement 1 October 1956 Addendum 1 December 1994

Fourteenth October 1957 Addendum 2 December 1995
A-2 API STANDARD 653

Fifteenth 1958 Addendum 3 December 1996

Addendum 4 December 1997

Tenth November 1998

Addendum 1 March 2000

Addendum 2 November 2001

Addendum 3 August 2003

Addendum 4 December 2005

Eleventh June 2007

Addendum 1 November 2008

NOTE Ordering information for past editions of API 12C and API 650 is available by calling IHS Inc. at 1-800-854-7179 (www.global.ihs.com).

Table A.1—Editions of API Standard 650 and its Precursor, API Standard 12C (Continued)

API Standard 12C, All-Welded Oil Storage Tanks API Standard 650, Welded Tanks for Oil Storage

Edition Date Edition Date
B-1

Annex B

Evaluation of Tank Bottom Settlement

B.1 Introduction

B.1.1 In determining the effects of soil settlement on storage tanks, it is common practice to monitor settlement of

the tank bottom. In most cases, such a monitoring program is initiated during the construction and continued during

hydrostatic testing and operations. During operations, settlement measurements should be taken at a planned

frequency, based on an assessment of soil settlement predictions. For existing tanks that do not have initial

settlement data, a program of settlement monitoring should be based on prior service history.

B.1.2 If at any time settlement is deemed excessive, the tank should be emptied and re-leveled. Re-leveling of a

sizable tank is expensive and rather difficult to achieve. Thus, a decision to re-level a tank is a crucial one, and

relies very much on the proper interpretation and evaluation of the monitored settlement data.

B.1.3 Approaches used to correct tank shell and bottom settlement include techniques such as localized repairs

of the bottom plates, partial re-leveling of the tank periphery, and major re-leveling of the entire tank bottom. Major

re-leveling of the tank, involving total lifting of the tank shell and bottom at one time, can introduce highly localized

stresses in the structure and impair its integrity. Therefore, when choosing techniques for correcting settlement

problems, an alternative to total lifting of the tank shell and bottom should be considered as a first choice. If it is

decided to lift the entire tank shell and bottom at one time, it should be done by personnel with demonstrated

experience in this technique.

B.2 Types of Settlement

B.2.1 Settlement Measurements

Measurements of tank settlement should be performed by personnel experienced in the types of measurement

procedures being performed, using equipment capable of sufficient accuracy to distinguish settlement differences.

The principle types of tank settlement consist of settlements that relate to the tank shell and bottom plate. These

settlements can be recorded by taking elevation measurements around the tank circumference and across the tank

diameter. Figure B.1 and Figure B.2 show minimum recommended locations on a tank shell and bottom plate for

settlement measurements. Data obtained from such measurements should be used to evaluate the tank structure.

Additional settlement readings may be required to better define local bottom depressions or edge settlements, to

refine shell settlement measurements in areas suspected to have local out-of-plane settlements, or to otherwise

improve bottom or shell settlement evaluation. Settlement measurement locations should be re-used in any future

settlement surveys and evaluations.

In cases of distortion or corrosion of the tank bottom extending beyond the shell, shell settlement measurements

taken near lap welds in the tank bottom can result in significant errors in measured elevation. Repaired or replaced

bottom plates, or new slotted-in bottoms may not have been installed parallel to the bottom shell course. In some

cases, more consistent and accurate results may be obtained by surveying the elevation of the weld between the first

and second courses.

Measure bottom and edge settlement carefully, taking into account that measurements taken when the bottom is not

in contact with the soil or foundation under the tank can overestimate or underestimate edge or bottom settlement

significantly. If the measured settlement is near the maximum allowable settlement, consider repeating the

measurement with the bottom forced down to the soil, e.g. standing on it, or take an additional set of measurements in

the same area, where the bottom is in firm contact with the soil. 
B-2 API STANDARD 653

Figure B.1—Measurements of Shell Settlement (External)

Figure B.2—Measurements of Bottom Settlement (Internal) Tank Out-of-service

0

11

10

9 3

1

7

8

6

5

4

2

Tank shell

Maximum spacing of 32 ft around circumference. 

There must be at least 4 equally spaced diametrical 

measurement lines.

NOTE 1 There must be at least eight settlement points. The maximum spacing 

of the settlement points is 32 ft around the circumference.

NOTE 2 Points shall be equally spaced around the tank shell. See 12.5.1.2 for

method of determining the number of measurement points.

Maximum spacing

of 32 ft around

circumference.

There must be at

least 4 equally

spaced diametrical

measurement lines.

Tank shell

Maximum spacing

of 10 ft across

diameter.

0

5 1

4 2

3 3

2 4

1 5

0

NOTE See 12.5.1.2 for method of determining the number of 

measurement points.
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION B-3

B.2.2 Shell Settlement Evaluation

Settlement of a tank is the result of either one, or a combination of the following three settlement components.

B.2.2.1 Uniform settlement. This component often can be predicted in advance, with sufficient accuracy from soil

tests. It may vary in magnitude, depending on the soil characteristics. Uniform settlement of a tank does not induce

stresses in the tank structure. However, piping, tank nozzles, and attachments must be given adequate consideration

to prevent problems caused by such settlement.

B.2.2.2 Rigid body tilting of a tank (planar tilt). This component rotates the tank in a tilted plane. The tilt will cause an

increase in the liquid level and, therefore, an increase in the hoop stress in the tank shell. Also, excessive tilting can

cause binding of peripheral seals in a floating roof and inhibit roof travel. This type of settlement could affect tank

nozzles that have piping attached to them. Figure B.3 shows that the settled location of the tank shell, after rigid body

tilt, can be represented by either a cosine or sine wave with respect to its original position in a horizontal plane.

Figure B.3—Graphical Representation of Tank Shell Settlement per B.2.2.4

U10

U11 U12

U13

Max. settlement at

(Pt. 11) Min.

settlement

(Pt. 22)

Plane of rigid tilt

represented by 

optimum cosine curve

Actual edge

settlement

Uniform or minimum settlement

Datum line for adjusted settlement

Datum line for net settlement

N (23) data points, equally (L) spaced apart.

Tank circumference envelope (πD)

13 5 7

2 46

9

8

11

10

13

12

15

14

17

16

19

18

21

20

23

22 1

Out-of-plane deflection for Point “i” is

Si

 = Ui − (1/2 Ui − 1+ 1/2 Ui + 1), for example

S11 = U11 − (1/2U10 + 1/2U12)

Ui

 = out-of-plane settlement of Point, “i”

(+) when above cos. curve;

(−) when below cos. curve, for example:

U11 = (+)

U13 = (−)

NOTE See 12.5.1.2 for definition of “N.”
B-4 API STANDARD 653

B.2.2.3 Due to the fact that a tank is a rather flexible structure, the tank may settle in a non-planar configuration,

inducing additional stresses in the tank shell. The out-of-plane settlements of the shell can lead to out-of-roundness at

the top of the shell, and depending on the extent of the induced out-of-roundness, may impede the proper functioning

of the floating roof in such a way that re-leveling is required. The out-of-roundness caused by settlement may also

affect internal roof support structures such as columns, rafters, and girders. Also, such settlements may cause flat

spots to develop in the tank shell. This type of settlement could affect tank nozzles that have piping attached to them.

B.2.2.4 While uniform settlement and rigid body tilt of a tank may cause problems as described in B.2.2.1 and

B.2.2.2, the out-of-plane settlement is the important component to determine and evaluate in order to ensure the

structural integrity of the shell and bottom. Based on this principle, a common approach is to determine the

magnitudes of the uniform settlement and rigid body tilt (if a rigid tilt plane exists or can be identified) for each data

point on the tank periphery. If a plane of rigid tilt can be distinguished, it becomes important as a datum from which to

measure the magnitudes of the out-of-plane settlements. When the out-of-plane settlement pattern of a tank has an

easily distinguishable plane of rigid tilt, the methodology in this paragraph can be used to evaluate the acceptability of

the tank’s out-of-plane settlement. If a rigid tilt plane can not be readily determined, the methodology in B.2.2.5 can be

used to evaluate the acceptability of the tank’s out-of-plane settlement.

A graphical representation illustrating tank shell settlement with a rigid tilt plane well–defined by a cosine curve fit is

shown in Figure B.3. The construction of this settlement plot has been developed in accordance with the following.

a) The actual settlement (in most cases an irregular curve) is plotted using points around the tank circumference as

the abscissa.

b) The vertical distance between the abscissa and the lowest point on this curve (Point 22) is the minimum

settlement, and it is called the uniform settlement component. A line through this point, parallel to the abscissa,

provides a new base or datum line for settlement measurements called adjusted settlements.

c) The plane of rigid tilt settlement, if well-defined, is represented by the optimum cosine curve. Several methods exist

for determining the optimum cosine curve. The least accurate method is by free hand drawing techniques, a kind of

trial and error procedure to fit the best cosine curve through the data. A better method is to use the mathematical

and graphical capabilities of a computer.

d) The vertical distances between the irregular curve and the cosine curve represent the magnitudes of the out-ofplane settlements (Ui at Data Point i). 

e) A commonly used and accepted method is to use a computer to solve for constants a, b, and c, to find the optimum

cosine curve of the form:

Elevpred = a + b × cos (θ + c)

Where Elevpred is the elevation predicted by the cosine curve at angle theta. A typical starting point for a computer

best-fit cosine curve is a least-square fit where a, b, and c are chosen to minimize the sum of the square of the

differences between measured and predicted elevations. The optimum cosine curve is only considered valid (i.e.

accurately fits the measured data) if the value R2 is greater than or equal to 0.9.

where

Syy is the sum of the squares of the differences between average measured elevation and the measured 

elevations;

SSE is the sum of the square of the differences between the measured and predicted elevations.

R2 ( ) Syy – SSE

Syy

= ---------------------------
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION B-5

Linear least square fitting and the R2 method of curve fitting are basic statistical tools. The use of a more rigorous

statistical method to determine the optimum cosine curve, such as non-linear or iterative procedures, may be used by

those experienced in their use.

Obtaining a statistically valid cosine curve may require taking more measurements than the minimums shown in

Figure B.1. In many cases, the out-of-plane settlement may be concentrated in one or more areas. In such cases, the

least-squares fit approach may under predict the local out-of-plane settlement and is not conservative. In these

cases, R2 will typically be less than 0.9, and more rigorous curve-fitting procedures should be considered.

Alternatively, the settlement may not indicate a well-defined rigid tilt plane and the procedure in B.2.2.5 should be

considered.

f) The vertical distances between the irregular curve and the optimum curve represent the magnitudes of the out-ofplane settlements (Ui at Data Point i). Si is the out-of-plane deflection at Point i (see Figure B.3).

NOTE When determining the optimum cosine curve described in B.2.2.4 e), taking additional measurements around the shell

will result in a more accurate cosine curve fit. However, using all of the measurement points in the equation shown in B.3.2.1 will

result in very small allowable out-of-plane settlements, Smax since the arc length L between measurement points is small. It is

acceptable to all measurement points to develop the optimum cosine curve, but only use a subset of these points spaced no further

than 32 ft (8 minimum) when calculating Si and Smax.The points used must include the points furthest from the optimum cosine

curve. For example, if 8 points are required, but 16 measurements are taken, and the arc length between measurement points is

only 15 ft, calculate the optimum cosine curve using all 16 points, but use only 8 points to calculate Si . The equations in Figure B.3

would be revised to read:

Si = Ui – (1/2 Ui – 2 + 1/2 Ui + 2)

S11 = U11 – (1/2 U9 + 1/2 U13)

B.2.2.5 If a well-defined rigid tilt plane can not be determined or the maximum out-of-plane settlement determined in

accordance with B.3.2.1 is exceeded, the procedures given in this section may be used in lieu of more rigorous

analysis or repair. 

B.2.2.5.1 For settlement profiles without a well-defined rigid tilt plane, the settlement arc length, Sarc, and out-ofplane settlement at the point under consideration, Si, must be determined from a plot of the measurement data. Figure

B.4 is a graphical illustration of the various measurement terms and procedures for determining estimates of the

settlement arc length and the corresponding out-of-plane settlement, including the refinement of measurements,

when needed. 

a) The actual settlement is plotted using points around the tank circumference as the abscissa. 

b) An initial settlement arc length and maximum settlement is determined from the points on the plotted data that

indicate a change in direction of settlement slope (see Figure B.4).

c) Additional settlement measurement points may be needed halfway between the points indicating a change in

direction of the settlement slope to further refine the settlement arc length and location and magnitude of maximum

settlement.

d) Step c) may need to be repeated. The best estimate of the settlement arc length and maximum settlement shall be

considered in the procedure given in B.3.2.2.

B.2.2.5.2 If a valid cosine fit of the rigid tilt plane can be determined, but the maximum out-of-plane settlement

determined in accordance with B.3.2.1 is exceeded, the procedure in B.3.2.2 may be used to evaluate the settlement.

In this case, see Figure B.5 for a graphical illustration of the determination of the settlement arc length and the

corresponding out-of-plane settlement. 
B-6 API STANDARD 653

Figure B.4—Graphical Representation of Shell Settlement per B.2.2.5

(Tilt Plane Not Described by an Optimal Cosine Curve)

Figure B.5—Graphical Representation of Shell Settlement per B.2.2.5

(Tilt Plane Described by an Optimal Cosine Curve)

Si,1

Sarci ,1

P O IN T 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Sunif has been 

eliminated from all 

measurements

Sj,1

Sarcj,1

Sk,1

Sarck,1

Si,2

Sarci,2

 initial 16 measurements

 additional measurements to better define settlement arc and maximum settlement

Si,N = maximum out-of-plane settlement measured from indicated plane, Nth estimate

Sarci,N = settlement arc corresponding to S i, N

Si

Sj

Sarci

Sarcj

Si = maximim out-of-plane settlement measured from tilt plane for i th arc

Sarci = settlement arc corresponding to Si

Sunif has been eliminated from all measurements

P O IN T 1 2 ... 16
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION B-7

B.2.2.5.3 If an examination of the measured settlement plot indicates a fold pattern about a diameter of the tank, the

maximum out-of-plane settlement should be determined using a settlement arc length of 50 % of the tank’s

circumference.

B.2.3 Edge Settlement

B.2.3.1 Edge settlement occurs when the tank shell settles sharply around the periphery, resulting in deformation of

the bottom plate near the shell-to-bottom corner junction. Figure B.6 illustrates this settlement. 

B.2.3.2 The equation given in B.3.4 can be used to evaluate edge settlement. Alternatively, a rigorous stress

analysis can be carried out for the deformed profile. The determination of the deformed profile should take into

consideration the following.

[text moved to B.2.1]

a) Locating the breakover point where the settled area begins requires some judgment. Placing a straight edge on the

unsettled bottom as shown in Figure B.6, and observing where the bottom separates from the straight edge will

help define the breakover point.

b) If the tank bottom is cone up or cone down, the settlement B, should be measured from a projection of the

unsettled bottom, not from level. See Figure B.7.

B.2.3.3 The measured edge settlement B is defined as shown in Figure B.6. Bew is defined as the allowable edge

settlement in an area where there is a bottom lap weld in the settled area that is essentially parallel (±20°) to the shell.

Be is defined as the allowable settlement in an area with no bottom welds, or only butt-welds in the bottom, or lap

welds in the bottom that are essentially perpendicular (±20°) to the shell.

B.2.3.4 Section B.3.4 provides methods for evaluation of measured edge settlement B against allowable edge

settlement Bew and Be. Since Bew is more conservative than Be, the simplest approach is to initially evaluate measured

Figure B.6—Edge Settlement

Breakover point

Tank

shell

Tank

shell

Original

bottom

position

Original

bottom

position

R

R

Tank bottom

Tank bottom

B

B

R = Radius width of settled area

B = Settlement

Use straight edge

to determine the

breakover point

Breakover point
B-8 API STANDARD 653

settlement B against Bew for all settled areas. If all areas meet this criterion, the settlement is acceptable and no

further evaluation is necessary. If necessary, different settled areas can be evaluated separately against Bew and Be.

For areas containing lap welds at an arbitrary angle to the shell, interpolation to find an allowable settlement between

Bew and Be based on the angle of the weld to the shell is allowed.

B.2.4 Bottom Settlement Near the Tank Shell

B.2.4.1 Figure B.8 illustrates bottom settlement near the tank shell. 

B.2.4.2 The equation given in B.3.3 can be used to evaluate settlement near the tank shell. Alternatively, a rigorous

stress analysis can be carried out for the deformed profile.

B.2.5 Localized Bottom Settlement Remote from the Tank Shell

B.2.5.1 Localized bottom settlement remote from tank shell are depressions (or bulges) that occur in a random

manner, remote from the shell (see Figure B.9). 

Figure B.7—Correction for Measured Edge Settlement

Cone down

bottom

Cone up

bottom

Shell

Original

bottom

position

Settled bottom

position

Shell

Original bottom

Breakover

Settled bottom

B — Measured from original

bottom position

B — Incorrect!

Measured from level line from

breakover point to shell
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION B-9

B.2.5.2 Acceptability of these localized settlements is dependent on localized stresses in the bottom plate, design

and quality of the lap welds (single-pass or multi-pass), and voids below the bottom plate. The equation given in B.3.3

can be used to evaluate localized settlement remote from the tank shell. These limits are applicable to tank bottoms

that have single-pass lap-welded joints.

B.3 Determination of Acceptable Settlement

B.3.1 General

For existing tanks with history of successful service, it may be possible to accept greater settlement and distortion of the

foundation from a true plane than new tank construction standards allow. Each tank must be evaluated based on service

conditions, materials of construction, soil characteristics, tank foundation design, and tank service history. The methods

discussed in following sections are not mandatory and approximate the maximum permissible settlement. However,

experience has shown that if settlements exceed the following requirements, further assessment or repair is required.

B.3.2 Permissible Out-of-plane Settlement

From the measurements procedures described in B.2.2.4 and B.2.2.5, determine the maximum out-of-plane

settlement.The magnitude (absolute value) of the maximum settlement shall be compared to the permissible values

given in B.3.2.1 or B.3.2.2, as applicable. The permissible out-of-plane settlement given in B.3.2.1 and B.3.2.2 do not

Figure B.8—Bottom Settlement Near Shell

R R

B

B

Tank shell

A A

SECTION A-A

R = Radius of inscribed circle in bulged or depressed area

B = Settlement or depression depth or height of bulge

R

R R

R

R ≤ 2R
B-10 API STANDARD 653

take into consideration abrupt changes in shell elevation (ridges) or discontinuities near the bottom of the tank in the

settled region, such as low nozzles. They also do not consider fold patterns in cone roof tanks when the fold line is

adjacent to or through a line of one or more roof columns, or to patterns of settlement that include combined shell and

edge settlements. The permissible settlement criteria in B.3.2.2 are applicable to API 650 carbon steel and stainless

steel tanks and diameter ranges given in B.3.2.2. Out-of-plane settlement that does not meet these limitations should

be further examined by a more rigorous engineering assessment to determine the need for repairs, see B.3.2.4

B.3.2.1 When using the procedure with an optimal cosine curve approach defined in B.2.2.4 to determine out-ofplane settlement, the permissible out-of-plane settlement is given by the following equation (see Note):

Figure B.9—Localized Bottom Depressions or Bulges Remote from Shell

Localized depressions

or bulges in tank

bottom plate

R R

B

B

Tank shell

A A

R

R

R

R = Radius of inscribed circle in

 bulged or depressed area

B = Settlement depth or

 height of bulge

SECTION A-A

Smax, ft

L2 ( ) × Y × 11

2[ ] ( ) E H× = -------------------------------
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION B-11

where

Smax, ft is permissible out-of-plane settlement, in feet (ft);

L is arc length between measurement points, in feet (ft);

Y is yield strength of the shell material, in pound force per square inch (lbf/in.2);

E is Young’s Modulus, in pound force per square inch (lbf/in.2);

H is tank height, in feet (ft).

NOTE This equation is based on “Criteria for Settlement of Tanks,” W. Allen Marr, M. ASCE, Jose A. Ramos, and T. William

Lambe, F. ASCE, Journal of Geotechnical Engineering Division, Proceedings of the American Society of Civil Engineers, Vol. 108,

August, 1982.

B.3.2.2 When using the procedure in B.2.2.5 to determine out-of-plane settlement, the permissible out-of-plane

settlement is given by the following equation (see Note): 

where

Smax, in is permissible out-of-plane settlement, in inches (in.);

Sarc is effective settlement arc, see B.2.2.5.1, in feet (ft);

D is tank diameter, in feet (ft);

Y is yield strength of the shell material, in pound force per square inch (lbf/in.2); 

E is Young’s Modulus, in pound force per square inch (lbf/in.2);

H is tank height, in feet (ft).

NOTE This equation is based on “Final Report on the Study of Out-of-Plane Tank Settlement,” J. Andreani, N. Carr, Report to

API SCAST, May, 2007.

B.3.2.3 Serviceability may also be a concern for tanks with significant out-of-plane settlement. Out-of-roundness

can impede floating roof operation and also affect internal roof support structures. The out-of-roundness that a tank

experiences with out-of-plane settlement is fairly sensitive to the actual pattern of settlement. The owner may wish to

specify additional inspection or a more rigorous assessment of the tank’s out-of-roundness.

Tank Diameter

ft K, Open Top Tanks K, Fixed Roof Tanks

D 50 28.7 10.5

50 < D 80 7.8 5.8

80 < D 120 6.5 3.9

120 < D 180 4.0 2.3

180 < D 240 3.6 Not applicable

240 < D 300 2.4 Not applicable

300 < D Not applicable Not applicable

Smax, in min K S × arc

D

H

---     Y

E

---     = × × , 4.0

≤ 5

≤ 5

≤ 5

≤ 5

≤ 5

≤ 5
B-12 API STANDARD 653

B.3.2.4 If measured out-of-plane settlement exceeds the applicable limits described in B.3.2.1 or B.3.2.2, a more

rigorous evaluation may be performed to determine the need for repairs. This evaluation should be done by an

engineer experienced in tank settlement analysis.

B.3.3 Internal Bottom Settlements or Bulges

Measure the bulge or depression. The permissible bulge or depression is given by the following equation (see Note).

BB = 0.37R

where

BB is maximum height of bulge or depth of local depression, in inches;

R is radius of inscribed circle in bulged area or local depression, in feet.

Figure B.10 is a graphical representation of this equation. 

NOTE This equation is based on “Criteria for Settlement of Tanks,” W. Allen Marr, M. ASCE, Jose A. Ramos, and T. William

Lambe, F. ASCE, Journal of Geotechnical Engineering Division, Proceedings of the American Society of Civil Engineers, Vol. 108,

August 1982.

B.3.4 Edge Settlement

B.3.4.1 Maximum allowable settlement Bew is shown in Figure B.11 for settled areas that include bottom lap welds

essentially parallel to the shell (±20°). In settled areas where the measured settlement B exceeds 75 % of allowed

settlement Bew, all shell-to-bottom welds and bottom welds should be examined visually and with magnetic particle

Figure B.10—Localized Tank Bottom Settlement Limits for Single Pass Welds

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

0 5 10 15 20 25 30 35 40 45

Repairs or a rigorous assessment

should be conducted if the depth

of the observed bulge or

depression is above this line

B

R

R

R R

R

RR RR B

Partial ringtype depression

or bulge

(at edge only)

where R < 2R

Depression

or bulge

(anywhere)

B = Depth of depression or bulge at center (in.)

R = Radius of inscribed circle in depression or bulge (ft)

13
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION B-13

or liquid penetrant methods. All indications should be repaired, or evaluated for risk of brittle fracture, and/or fatigue

failure prior to returning the tank to service.

B.3.4.2 For settled areas where measured settlement B exceeds 75 % of Bew , any welds within 12 in. of either side

of the breakover area (see Figure B.6) should be examined visually. Any suspect areas should be examined with

either magnetic particle examination or liquid penetrant examination. All indications should be repaired or evaluated

for risk of fatigue prior to returning the tank to service.

B.3.4.3 Maximum allowable settlement Be is shown in Figure B.12 for areas of edge settlement with no welds, butt

welds, or lap welds in the bottom that are essentially perpendicular to the shell (±20°). In settled areas where the

measured settlement exceeds 75 % of the allowed settlement, all shell-to-bottom welds and bottom welds should be

examined visually and with magnetic particle or liquid penetrant methods. All indications should be repaired or

evaluated for risk of brittle fracture and/or fatigue prior to returning the tank to service.

B.3.4.4 Maximum allowable settlement for areas of edge settlement with a lap weld at an arbitrary angle to the shell

may be interpolated from Be and Bew from Figure B.11 and Figure B.12, and the following equation:

Bα = Be – (Be – Bew) × sin α

Where α is the angle of the weld to a tank centerline and Bα is the allowable settlement for an area with a weld at that

angle (see Figure B.13).

B.3.4.5 In general, settlement occurs slowly, and for most existing tanks, the majority of settlement is presumed to

have occurred in the first few years of service. Significant additional settlement will not be expected after the initial

inspections. Therefore, typical practice is to compare the measured edge settlement with the maximum allowable

edge settlement Bew and Be, and not include allowance for additional settlement during subsequent operation. Note

that erosion of the pad adjacent to the tank may cause local settlement. In this case the settlement will continue

unless the pad is repaired and future erosion prevented. For cases where significant additional settlement is

expected, an engineer experienced in tank settlement evaluation should evaluate the settlement expected at the next

inspection with the limits in B.3.4. This is analogous to a corrosion allowance for components expected to corrode.

B.3.4.6 The edge settlement limits described in B.2.3.1 through B.2.3.4 were developed for typical 1/4-in. thick tank

bottoms with minimal corrosion.

a) Edge settlement limits can be applied with reasonable accuracy to 5/16- and 3/8-in. thick tank bottoms.

b) Edge settlement limits can be applied with reasonable accuracy to bottoms with general corrosion, as long as the

areas near all welds are thicker than 3/16 in.

c) Edge settlement limits can be applied with reasonable accuracy to bottoms with local corrosion, if all locally thin

areas in the settled area (closer than “R” to the shell) thinner than 3/16 in. are smaller than 12 in. in diameter and the

thin area does not include a weld.

d) Settlement is presumed to be slow, and a small amount of additional settlement is expected to occur prior to the

next inspection.

B.3.4.7 Edge settlement increases secondary stress at the bottom-to-shell weld. If weld repairs are made to the

bottom-to-shell weld in an area where settlement exceeds 1/2 of Be, these additional stresses should be evaluated by

an engineer experienced in tank settlement evaluation before waiving a hydrostatic test per 12.3.

B.4 Repairs

B.4.1 If it is determined that settlements have occurred which are beyond the permissible limits established in the

previous sections, then consideration should be given to making repairs or a rigorous stress analysis should be

13

12

13

12
B-14 API STANDARD 653

performed to evaluate the deformed profile. Various repair techniques have been discussed above. The judgment

on repairs should be tempered with knowledge of tank service history, previous repairs, previous inspections, tank

foundation conditions, soil characteristics, the material of construction, and estimates of future settlement. See

9.10 for suggested repair details.

B.4.2 For tanks with edge settlement exceeding the limits and assumptions given in B.2.3.1 through B.2.3.7,

the tank should be repaired. Any plate exceeding acceptable strains (typically 2 % to 3 %) should be replaced.

Re-leveling the tank will not remove the plastic strain, so leveling the tank without replacing the strain may not be

a sufficient repair. Welds in the area of the high strains should be removed and replaced, or be subjected to a

fitness-for-service evaluation by an engineer experienced in tank settlement evaluation. The condition leading to

the unacceptable settlement should be corrected. Depending on the severity and location of the settlement,

required repairs may include regrading the soil under the tank bottom, and/or repairing the foundation. Jacking

and re-leveling the shell may be required to prevent additional settlement damage. Jacking and leveling are

usually done in conjunction with, not instead of, replacing damaged plate and welds. In lieu of repairs, a detailed

analysis of the settled area may be performed by an engineer experienced in tank design and settlement

evaluation. The analysis should consider primary and secondary stress and the risk of brittle fracture.

Figure B.11—Maximum Allowable Edge Settlement for Areas with Bottom Lap Welds 

Approximately Parallel to the Shell

10.00

9.00

8.00

7.00

6.00

5.00

4.00

3.00

2.00

1.00

0.00

0 123456

Tanks with larger edge settlements 

are to be repaired, or have detailed 

analysis of bottom, and bottom-toshell junction

Welds in tanks with settlement 

greater than or equal 75 % of Bew, 

and larger than 2 in., are to be 

examined with magnetic particle or 

liquid penetrant methods

Diameter, ft

Tanks with settlement less than 75 % of 

Bew may be returned to service

160 & Larger

120

100

80

60

50

40

30

20

R = Radius of settled area, ft

Bew = Maximum allowable settlement, (in.)

13
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION B-15

Figure B.12—Maximum Allowable Edge Settlement for Areas with Bottom Lap Welds 

Approximately Perpendicular to the Shell

Figure B.13—Edge Settlement with a Lap Weld at an Arbitrary Angle to the Shell

10.00

9.00

8.00

7.00

6.00

5.00

4.00

3.00

2.00

1.00

0.00

0 123456

Tanks with larger edge settlements 

are to be repaired, or have detailed 

analysis of bottom, and bottom-toshell junction

Welds in tanks with settlement 

greater than or equal 75 % of Bew, 

and larger than 2 in., are to be 

examined with magnetic particle or 

liquid penetrant methods

Diameter, ft

Tanks with settlement less than 75 % of 

Bew may be returned to service

160 & Larger

120

100

80

60

50

40

30

20

R = Radius of settled area, ft

Be = Maximum allowable settlement, (in.)

13

Angle

alpha

Settled

area

Tank

centerline

Bottom plate

layout

C-1

Annex C

Checklists for Tank Inspection

Annex C contains sample checklists illustrating tank components and auxiliary items that should be considered for

internal and external inspection of tanks. This information is provided as guidance to the owner/operator for

developing an inspection assessment schedule for any specific tank installation. The checklist format facilitates the

recording of inspection findings. 

NOTE 1 Users of checklists should not rely exclusively on the information contained in this document. Sound business,

scientific, engineering, and safety judgment should be used in employing the information contained herein. 

NOTE 2 Where applicable, authorities having jurisdiction should be consulted.

TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION C-3

Tank In-service Inspection Checklist

Item Completed

√ Comments

C.1.1 Foundation

Measure foundation levelness and bottom elevations (see Annex B for extent of 

measurements).

C.1.1.1 Concrete Ring

a) Inspect for broken concrete, spalling, and cracks, particularly under backup bars used 

in welding butt-welded annular rings under the shell.

b) Inspect drain openings in ring, back of waterdraw basins and top surface of ring for 

indications of bottom leakage.

c) Inspect for cavities under foundation and vegetation against bottom of tank.

d) Check that runoff rainwater from the shell drains away from tank.

e) Check for settlement around perimeter of tank.

C.1.1.2 Asphalt

a) Check for settling of tank into asphalt base which would direct runoff rain water under 

the tank instead of away from it.

b) Look for areas where leaching of oil has left rock filler exposed, which indicates 

hydrocarbon leakage.

C.1.1.3 Oiled Dirt or Sand

Check for settlement into the base which would direct runoff rain water under the tank 

rather than away from it.

C.1.1.4 Rock

Presence of crushed rock under the steel bottom usually results in severe underside 

corrosion. Make a note to do additional bottom plate examination (ultrasonic, hammer 

testing, or turning of coupons) when the tank is out of service.

C.1.1.5 Site Drainage

a) Check site for drainage away from the tank and associated piping and manifolds.

b) Check operating condition of the dike drains.

C.1.1.6 Housekeeping

Inspect the area for buildup of trash, vegetation, and other inflammables buildup.

C.1.1.7 Cathodic Protection

Review cathodic protection potential readings.

C.1.2 Shells

C.1.2.1 External Visual Inspection

a) Visually inspect for paint failures, pitting, and corrosion.

b) Clean off the bottom angle area and inspect for corrosion and thinning on plate and 

weld.

c) Inspect the bottom-to-foundation seal, if any.

C.1.2.2 Internal (Floating Roof Tank)

Visually inspect for grooving, corrosion, pitting, and coating failures.

C.1.2.3 Riveted Shell Inspection

a) Inspect external surface for rivet and seam leaks.

b) Locate leaks by sketch or photo (location will be lost when shell is abrasive cleaned 

for painting).

c) Inspect rivets for corrosion loss and wear.

d) Inspect vertical seams to see if they have been full fillet lap-welded to increase joint 

efficiency.
C-4 API STANDARD 653

Tank In-service Inspection Checklist (Continued)

Item Completed

√ Comments

e) If no record exists of vertical riveted seams, dimension and sketch (or photograph) 

the rivet pattern: number of rows, rivet size, pitch length, and note whether the joint is 

butt-riveted or lap-riveted.

C.1.2.4 Wind Girder (Floating Roof Tanks)

a) Inspect wind girder and handrail for corrosion damage (paint failure, pitting, corrosion 

product buildup), especially where it occurs at tack-welded junction, and for broken 

welds.

b) Check support welds to shell for pitting, especially on shell plates.

c) Note whether supports have reinforcing pads welded to shell.

C.1.3 Shell Appurtenances

C.1.3.1 Manways and Nozzles

a) Inspect for cracks or signs of leakage on weld joint at nozzles, manways, and 

reinforcing plates.

b) Inspect for shell plate dimpling around nozzles, caused by excessive pipe deflection.

c) Inspect for flange leaks and leaks around bolting.

d) Inspect sealing of insulation around manways and nozzles.

e) Check for inadequate manway flange and cover thickness on mixer manways.

C.1.3.2 Tank Piping Manifolds

a) Inspect manifold piping, flanges, and valves for leaks.

b) Inspect fire fighting system components.

c) Check for anchored piping which would be hazardous to the tank shell or bottom 

connections during earth movement.

d) Check for adequate thermal pressure relief of piping to the tank.

e) Check operation of regulators for tanks with purge gas systems.

f) Check sample connections for leaks and for proper valve operation.

g) Check for damage and test the accuracy of temperature indicators.

h) Check welds on shell-mounted davit clips above valves 6 in. and larger.

C.1.3.3 Autogauge System

a) Inspect autogauge tape guide and lower sheave housing (floating swings) for leaks.

b) Inspect autogauge head for damage.

c) Bump the checker on autogauge head for proper movement of tape.

d) Identify size and construction material of autogauge tape guide (floating roof tanks).

e) Ask operator if tape tends to hang up during tank roof movement (floating roof tanks).

f) Compare actual product level to the reading on the autogauge (maximum variation is 

2 in.).

g) On floating roof tanks, when the roof is in the lowest position, check that no more than 

two ft of tape are exposed at the end of the tape guide.

h) Inspect condition of board and legibility of board-type autogauges.

i) Test freedom of movement of marker and float.

C.1.3.4 Shell-mounted Sample Station

a) Inspect sample lines for function of valves and plugging of lines, including drain or 

return-to-tank line.

b) Check circulation pump for leaks and operating problems.
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION C-5

Tank In-service Inspection Checklist (Continued)

Item Completed

√ Comments

c) Test bracing and supports for sample lines and equipment.

C.1.3.5 Heater (Shell Manway Mounted)

Inspect condensate drain for presence of oil indicating leakage.

C.1.3.6 Mixer

a) Inspect for proper mounting flange and support.

b) Inspect for leakage.

c) Inspect condition of power lines and connections to mixer.

C.1.3.7 Swing Lines: Winch Operation

a) Nonfloating. Raise, then lower the swing line with the winch, and check for cable 

tightness to confirm that swing line lowered properly.

b) Floating. With tank half full or more, lower the swing line, then let out cable and check 

if swing has pulled cable tight, indicating that the winch is operating properly.

c) Indicator. Check that the indicator moves in the proper direction: Floating swing line 

indicators show a lower level as cable is wound up on the winch. Non-floating swing 

line indicators show the opposite.

C.1.3.8 Swing Lines: External Guide System

Check for leaks at threaded and flanged joints.

C.1.3.9 Swing Lines: Identify Ballast Varying Need

Check for significant difference in stock specific gravity.

C.1.3.10 Swing Lines: Cable Material and Condition

a) For nonstainless steel cable, check for corrosion over entire length.

b) All cable: check for wear or fraying.

C.1.3.11 Swing Lines: Product Sample Comparison

Check for water or gravity differences that would indicate a leaking swing joint.

C.1.3.12 Swing Lines: Target

Target should indicate direction of swing opening (up or down) and height above bottom 

where suction will be lost with swing on bottom support.

C.1.4 Roofs

C.1.4.1 Deck Plate Internal Corrosion

For safety, before accessing the roof, check with ultrasonic instrument or lightly use a ball 

peen hammer to test the deck plate near the edge of the roof for thinning. (Corrosion 

normally attacks the deck plate at the edge of a fixed roof and at the rafters in the center 

of the roof first.)

C.1.4.2 Deck Plate External Corrosion

Visually inspect for paint failure, holes, pitting, and corrosion product on the roof deck.

C.1.4.3 Roof Deck Drainage

Look for indication of standing water. (Significant sagging of fixed roof deck indicates 

potential rafter failure. Large standing water areas on a floating roof indicate inadequate 

drainage design or, if to one side, a nonlevel roof with possible leaking pontoons.)

C.1.4.4 Level of Floating Roof

At several locations, measure distance from roof rim to a horizontal weld seam above the 

roof. A variance in the readings indicates a nonlevel roof with possible shell out-of-round, 

out-of-plumb, leaking pontoons, or hang-up. On small diameter tanks, an unlevel 

condition can indicate unequal loading at that level.
C-6 API STANDARD 653

Tank In-service Inspection Checklist (Continued)

Item Completed

√ Comments

C.1.4.5 Gas Test Internal Floating Roof

Test for explosive gas on top of the internal floating roof. Readings could indicate a 

leaking roof, leaking seal system, or inadequate ventilation of the area above the internal 

floating roof.

C.1.4.6 Roof Insulation

a) Visually inspect for cracks or leaks in the insulation weather coat where runoff rain 

water could penetrate the insulation.

b) Inspect for wet insulation under the weather coat.

c) Remove small test sections of insulation and check roof plate for corrosion and holes 

near the edge of the insulated area.

C.1.4.7 Floating Roof Seal Systems

a) Inspect the condition of the seal, measure and record maximum rim spaces and sealto-shell gaps around the full roof circumference at the level of inspection. 

NOTE Inspection of the seal and measurement of the rim spaces and seal-to-shell 

gaps at more than one level may be necessary to more fully determine if any 

problems exist at other levels of tank operation).

b) Measure and record annular space at 30-ft spacing (minimum of four quadrants) 

around roof and record. Measurements should be taken in directly opposite pairs.

1) _______ _______ Opposite pair 1.

2) _______ _______ Opposite pair 2.

c) Check if seal fabric on primary shoe seals is pulling shoes away from shell (fabric not 

wide enough).

d) Inspect fabric for deterioration, holes, tears, and cracks.

e) Inspect visible metallic parts for corrosion and wear.

f) Inspect for openings in seals that would permit vapor emissions.

g) Inspect for protruding bolt or rivet heads against the shell.

h) Pull both primary and secondary seal systems back all around the shell to check their 

operation.

i) Inspect secondary seals for signs of buckling or indications that their angle with the 

shell is too shallow.

j) Inspect wedge-type wiper seals for flexibility, resilience, cracks, and tears.

C.1.5 Roof Appurtenances

C.1.5.1 Sample Hatch

a) Inspect condition and functioning of sample hatch cover.

b) On tanks governed by Air Quality Monitoring District rules, check for the condition of 

seal inside hatch cover.

c) Check for corrosion and plugging on thief and gauge hatch cover.

d) Where sample hatch is used to reel gauge stock level, check for marker and tab 

stating hold-off distance.

e) Check for reinforcing pad where sample hatch pipe penetrates the roof deck.

f) On floating roof sample hatch and recoil systems, inspect operation of recoil reel and 

condition of rope.

g) Test operation of system.

h) On ultra clean stocks such as JP4, check for presence and condition of protective 

coating or liner inside sample hatch (preventing rust from pipe getting into sample).

12
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION C-7

Tank In-service Inspection Checklist (Continued)

Item Completed

√ Comments

C.1.5.2 Gauge Well

a) Inspect visible portion of the gauge well for thinning, size of slots, and cover condition.

b) Check for a hold-off distance marker and tab with hold-off distance (legible).

c) On floating roofs, inspect condition of roof guide for gauge well, particularly the 

condition of the rollers for grooving.

d) If accessible, check the distance from the gauge well pipe to the tank shell at different 

levels.

e) If tank has a gauge well washer, check valve for leakage and for presence of a bull 

plug or blind flange.

C.1.5.3 Fixed Roof Scaffold Support

Inspect scaffold support for corrosion, wear, and structural soundness.

C.1.5.4 Autogauge: Inspection Hatch and Guides (Fixed Roof)

a) Check the hatch for corrosion and missing bolts.

b) Look for corrosion on the tape guide’s and float guide’s wire anchors.

C.1.5.5 Autogauge: Float Well Cover

a) Inspect for corrosion.

b) Check tape cable for wear or fraying caused by rubbing on the cover.

C.1.5.6 Sample Hatch (Internal Floating Roof)

a) Check overall conditions.

b) When equipped with a fabric seal, check for automatic sealing after sampling.

c) When equipped with a recoil reel opening device, check for proper operations.

C.1.5.7 Roof-mounted Vents (Internal Floating Roof)

Check condition of screens, locking and pivot pins.

C.1.5.8 Gauging Platform Drip Ring

On fixed roof tanks with drip rings under the gauging platform or sampling area, inspect 

for plugged drain return to the tank.

C.1.5.9 Emergency Roof Drains

Inspect vapor plugs for emergency drain: that seal fabric discs are slightly smaller than 

the pipe ID and that fabric seal is above the liquid level.

C.1.5.10 Removable Roof Leg Racks

Check for leg racks on roof.

C.1.5.11 Vacuum Breakers

Report size, number, and type of vacuum breakers. Inspect vacuum breakers. If high legs 

are set, check for setting of mechanical breaker in high leg position.

C.1.5.12 Rim Vents

a) Check condition of the screen on the rim vent cover.

b) Check for plating off or removal of rim vents where jurisdictional rules do not permit 

removal.

C.1.5.13 Pontoon Inspection Hatches

a) Open pontoon inspection hatch covers and visually check inside for pontoon leakage.

b) Test for explosive gas (an indicator of vapor space leaks).
C-8 API STANDARD 653

Tank In-service Inspection Checklist (Continued)

Item Completed

√ Comments

c) If pontoon hatches are equipped with locked down coves, check for vent tubes. 

Check that vent tubes are not plugged up. Inspect lock-down devices for condition 

and operation.

C.1.6 Accessways

See Tank Out-of-service Inspection Checklist, Item C.2.12.

NOTES 
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION C-9

Tank Out-of-service Inspection Checklist (Continued)

Item Completed

√ Comments

C.2.1 Overview

a) Check that tank has been cleaned, is gas free, and safe for entry.

b) Check that the tank is completely isolated from product lines, all electrical power, and 

steam lines.

c) Check that roof is adequately supported, including fixed roof structure and floating 

roof legs.

d) Check for presence of falling object hazards, such as corroded-through roof rafters, 

asphalt stalactites, and trapped hydrocarbons in unopened or plugged equipment or 

appurtenances, ledges, etc.

e) Inspect for slipping hazards on the bottom and roof decks.

f) Inspect structural welds on accessways and clips.

g) Check surfaces needing inspection for a heavy-scale buildup and check weld seams 

and oily surfaces where welding is to be done. Note areas needing more cleaning, 

including blasting.

h) Review cathodic protection potential readings.

C.2.2 Tank Exterior

a) Inspect appurtenances opened during cleaning such as lower floating swing sheave 

assemblies, nozzle interiors (after removal of valves).

b) Hammer test or ultrasonically test the roof.

c) Enter and inspect the floating roof pontoon compartments.

C.2.3 Bottom Interior Surface

a) Using a flashlight held close to and parallel to the bottom plates, and using the bottom 

plate layout as a guide, visually inspect and hammer test the entire bottom.

b) Measure the depth of pitting and describe the pitting appearance (sharp edged, lake 

type, dense, scattered, etc.)

c) Mark areas requiring patching or further inspection.

d) Mark locations for turning coupons for inspection.

e) Inspect all welds for corrosion and leaks, particularly the shell-to-bottom weld.

f) Inspect sketch plates for corrosion.

g) Check condition of internal sump, if applicable. Standing liquid should be removed 

from the sump to allow for complete inspection and vacuum testing of weld seams as 

appropriate. Sump bottom and sidewall plate and seams need to be evaluated 

for both product-side and soil-side corrosion. 

h) Locate and mark voids under the bottom.

i) Record bottom data on a layout sketch using the existing bottom plates as a grid. 

List the number and sizes of patches required.

j) Vacuum test the bottom lap welds.

k) Hammer test or ultrasonically examine any slightly discolored spots or damp areas.

l) Check for reinforcing pads under all bottom attached clips, brackets, and supports.

m) Inspect floating roof leg pads for pitting or cutting, and excessive dimpling (indicating 

excessive loading).

n) Check the column bases of fixed roof supports for adequate pads and restraining clips.

o) In earthquake Zones 3 and 4, check that roof supports are not welded down to the 

tank bottom, but are only restrained from horizontal movement.

p) Check area beneath swing line cable for indications of cable cutting or dragging.

q) Mark old oil and air test connection for removal and patching.

r) Identify and report low areas on the bottom that do not drain adequately.

s) Inspect coating for holes, disbonding, deterioration, and discoloration.
C-10 API STANDARD 653

Tank Out-of-service Inspection Checklist (Continued)

Item Completed

√ Comments

C.2.4 Shell Seams and Plate

a) On cone up bottoms, closely inspect and gauge the depth of metal loss on the lower 

2 in. to 4 in. of the shell (area of standing water).

b) Measure the depth of pitting on each course.

c) Inspect and estimate the amount of metal loss on the heads of rivets and bolts.

d) Inspect shell-to-bottom riveted lap joints.

e) Inspect for vertical grooving damage from seal assembly protrusions.

f) Inspect existing protective coatings for damage, deterioration, and disbonding.

g) Check for areas of rubbing (indicating too much pressure by the seal assembly shoes 

or inadequate annular space).

h) Visually inspect the shell plates and seams for indications of leakage.

i) If the shell has riveted or bolted seams, record the leak locations by film or chart in 

case the locations are lost during surface preparation for painting.

j) Measure annular space at 40-ft intervals.

k) Survey the shell to check for roundness and plumb.

C.2.5 Shell-mounted Overflows

a) Inspect overflow for corrosion and adequate screening.

b) Check location of overflow that it is not above any tank valves or equipment.

C.2.6 Roof Interior Surface

C.2.6.1 General

a) Visually inspect the underside surface of the roof plates for holes, scale buildup, and 

pitting.

b) Hammer test or ultrasonically examine to check for thin areas, particularly in the vapor 

space of floating roofs and at edge of roof on cone roof tank.

c) Check all clips, brackets, braces, etc., welded to the roof deck plate for welded

reinforcing pads and see that they have not broken free.

d) If no pad is present, penetrant test for cracking of the weld or deck plate.

e) Inspect for protective coating for breaks, disbondment, and deterioration.

f) Spark test the interior surface coating if recoating is not planned.

C.2.6.2 Fixed Roof Support Structure

a) Inspect the support columns for thinning in the upper 2 ft.

b) On API columns (two channels welded together) check for corrosion scale breaking 

the tack welds, unless the joint between the channels is completely seal welded.

c) Check that the reinforcing pad on the bottom is seal-welded to the tank bottom with 

horizontal movement restraining clips welded to the pad.

d) Determine if pipe column supports are concrete filled or open pipe. If open pipe, check 

for a drain opening in the bottom of the pipe.

e) Inspect and gauge rafters for thinning, particularly near the center of the roof. Report 

metal loss.

f) Check for loose or twisted rafters.

g) Inspect girders for thinning and check that they are attached securely to the top of the 

columns.
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION C-11

Tank Out-of-service Inspection Checklist (Continued)

Item Completed

√ Comments

h) Report if the columns have cross bracing in the area between the low pump out of the 

top of the shell (for future internal floating roof installation).

i) Inspect and report presence of any roof-mounted swing line bumpers.

j) Photograph the roof structure if no rafter layout drawing exists.

C.2.7 Fixed Roof Appurtenances

C.2.7.1 Inspection and Light Hatches

a) Inspect the hatches for corrosion, paint and coating failures, holes, and cover sealing.

b) On loose covers, check for a safety chain in good condition.

c) On light hatches over 30 in. across, check for safety rods.

d) Inspect the condition of the gaskets on bold or latched down hatch covers.

C.2.7.2 Staging Support Connection

Inspect the condition of the staging support for corrosion.

C.2.7.3 Breathers and Vents

a) Inspect and service the breather.

b) Inspect screens on vents and breathers.

C.2.7.4 Emergency P/V Hatches

a) Inspect and service pressure/vacuum hatches. (Setting should be high enough to prevent chattering of breather during normal operation. See breather manufacturer’s 

guide.)

b) Inspect liquid seal hatches for corrosion and proper liquid level in the seal.

C.2.7.5 Sample Hatch

a) Inspect sample hatch for corrosion.

b) Check that the cover operates properly.

c) If the tank has no gauge well, check for a hold-off distance marker and check measurement.

C.2.8 Floating Roof

C.2.8.1 Roof Deck

a) Hammer test the area between roof rim and shell. (If access for hammer testing is 

inadequate, measure the distance from the bottom edge of the roof to the corroded 

area and then hammer test from inside the pontoon.)

b) In sour water service, clean and test all deck plate weld seams for cracking unless the 

lower laps have been seal-welded.

c) Check that either the roof drain is open or the drain plug in the roof is open in case of 

unexpected rain.

d) On flat bottomed and cone bottom roof decks, check for a vapor dam around the 

periphery of the roof. The dam should be continuous without break to prevent escape 

of vapors to the seal area from under the center of the roof.

C.2.8.2 Floating Roof Pontoons

a) Visually inspect each pontoon for liquid leakage.

b) Run a light wire through the gooseneck vents on locked down inspection hatch covers 

to make sure they are open.

c) Inspect lockdown latches on each cover.
C-12 API STANDARD 653

Tank Out-of-service Inspection Checklist (Continued)

Item Completed

√ Comments

d) Check and report if each pontoon is:

1) vapor tight (bulkhead seal welded on one side on bottom, sides, and top),

2) liquid tight (seal-welded on bottom and sides only), or

3) unacceptable (minimum acceptable condition is liquid tight).

C.2.8.3 Floating Roof Cutouts

a) Inspect underside of cutouts for mechanical damage.

b) Inspect welds for cracks.

c) Inspect plate for thinning, pitting, and erosion.

d) Measure mixer cutouts and record plate thickness for future mixer installation or 

replacement. 

Plate thickness ______ .

C.2.8.4 Floating Roof Supports

a) Inspect fixed low and removable high floating roof legs for thinning.

b) Inspect for notching at bottom of legs for drainage.

c) Inspect for leg buckling or felling at bottom.

d) Inspect pin hole in roof guide for tears.

e) Check plumb of all legs.

f) Inspect for adequate reinforcing gussets on all legs through a single portion of the 

roof.

g) Inspect the area around the roof legs for cracking if there is no internal reinforcing pad 

or if the topside pad is not welded to the deck plate on the underside.

h) Inspect the sealing system on the two-position legs and the vapor plugs in the fixed 

low leg for deterioration of the gaskets.

i) On shell-mounted roof supports, check for adequate clearance based on the maximum floating roof movement as determined by the position of the roof relative to the 

gauge well and/or counter-rotational device.

C.2.9 Floating Roof Seal Assemblies

C.2.9.1 Primary Shoe Assembly

a) Remove four sections of foam log (foam-filled seals) for inspection on 90° locations.

b) Inspect hanger attachment to roof rim for thinning, bending, broken welds, and wear 

of pin holes.

c) Inspect clips welded to roof rim for thinning.

d) Shoes—inspect for thinning and holes in shoes.

e) Inspect for bit-metal bolts, clips, and attachments.

f) Seal fabric—inspect for deterioration, stiffening, holes, and tears in fabric.

g) Measure length of fabric from top of shoe to roof rim, and check against maximum 

anticipated annular space as roof operates.

h) Inspect any modification of shoes over shell nozzles, mixers, etc., for clearance.

i) Inspect shoes for damage caused by striking shell nozzles, mixers, etc.

C.2.9.2 Primary Toroidal Assembly

a) Inspect seal fabric for wear, deterioration, holes, and tears.
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION C-13

Tank Out-of-service Inspection Checklist (Continued)

Item Completed

√ Comments

b) Inspect hold-down system for buckling or bending.

c) Inspect foam for liquid absorption and deterioration.

C.2.9.3 Rim-mounted Secondaries

a) Inspect the rim-mounted bolting bar for corrosion and broken welds.

b) Measure and chart seal-to-shell gaps.

c) Visually inspect seam from below, looking for holes as evidenced by light.

d) Inspect fabric for deterioration and stiffness.

e) Inspect for mechanical damage, corrosion, and wear on tip in contact with shell.

f) Inspect for contact with obstructions above top of shell.

C.2.10 Floating Roof Appurtenances

C.2.10.1 Roof Manways

a) Inspect walls of manways for pitting and thinning.

b) On tanks with interface autogauges, check seal around gauge tape cable and guide 

wires through manway cover.

c) Inspect cover gasket and bolts.

C.2.10.2 Rim Vent

a) Check rim vent for pitting and holes.

b) Check vent for condition of screen.

c) On floating roof tanks where the environmental rules require closing off the vent, 

check the vent pipe for corrosion at the pipe-to-rim joint and check that the blinding is 

adequate.

C.2.10.3 Vacuum Breaker, Breather Type

a) Service and check operation of breather valve.

b) Check that nozzle pipe projects no more than 1/2 in. below roof deck.

C.2.10.4 Vacuum Breaker, Mechanical Type

Inspect the stem for thinning. Measure how far the vacuum breaker cover is raised off the 

pipe when the roof is resting on high or low legs.

a) On high legs: _________ .

b) On low legs: _________ .

C.2.10.5 Roof Drains: Open Systems, Including Emergency Drains

a) Check liquid level inside open roof drains for adequate freeboard. Report if there is 

insufficient distance between liquid level and top of drain.

b) If tank comes under Air Quality Monitoring District rules, inspect the roof drain vapor 

plug.

c) If emergency drain is not at the center of the roof, check that there are at least three 

emergency drains.

C.2.10.6 Closed Drain Systems: Drain Basins

a) Inspect for thinning and pitting.

b) Inspect protective coating (topside).

c) Inspect basin cover or screen for corrosion.

d) Test operation of check valve.
C-14 API STANDARD 653

Tank Out-of-service Inspection Checklist (Continued)

Item Completed

√ Comments

e) Check for presence of check valve where bottom of basin is below product level.

f) Inspect drain basin(s) to roof deck welds for cracking.

g) Check drain basin(s) outlet pipe for adequate reinforcement to roof deck (including 

reinforcing pad).

C.2.10.7 Closed Drain Systems: Fixed Drain Line on Tank Bottom

a) Hammer test fixed drain line on tank bottom for thinning and scale/debris plugging.

b) Inspect supports and reinforcing pads for weld failures and corrosion.

c) Check that pipe is guided, not rigidly locked to support, to avoid tearing of tank bottom 

plate.

C.2.10.8 Closed Drain Systems: Flexible Pipe Drain

a) Inspect for damage to exterior of pipe.

b) Check for obstructions that pipe could catch on.

c) Inspect shields to protect pipe from snagging.

d) Inspect results of hydrostatic test on flexible roof drain system.

C.2.10.9 Closed Drain Systems: Articulated Joint Drain

a) Hammer test rigid pipe in flexible joint systems for thinning and scale/debris plugging.

b) Inspect system for signs of bending or strain.

c) Inspect results of system hydrostatic test.

d) Inspect landing leg and pad.

C.2.10.10 Autogauge System and Alarms

a) Check freedom of movement of tape through autogauge tape guide.

b) Inspect sheaves for freedom of movement.

c) Test operation checker.

d) Inspect tape and tape cable for twisting and fraying.

e) Test the tape’s freedom of movement through guide sheaves and tape guide pipe.

f) On open-top tanks, check that gate tapes with cables have no more than one foot of 

tape exposed with float at lowest point.

g) Check float for leakage.

h) Test float guide wire anchors for spring action by pulling on wire and releasing.

i) Inspect floatwells in floating roofs for thinning and pitting of walls just above the liquid 

level.

j) Check that the autogauge tape is firmly attached to the float.

k) Inspect the tape cable and float guide wire fabric seals through the float well cover.

l) Inspect the bottom guide wire attachment clip: inspect for a temporary weighted bar 

instead of a permanent welded down clip.

m) Inspect board-type autogauge indicators for legibility and freedom of movement of 

indicator.

n) Measure and record these distances to determine if seal damage will occur if tank is 

run over from:

1) Shell top angle to underside of tape guide system.

2) Liquid level on floating top to top of secondary seal.
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION C-15

Tank Out-of-service Inspection Checklist (Continued)

Item Completed

√ Comments

o) Identify floating roofs where the tape is connected directly to the roof.

p) Overfill alarm: Inspect tank overfill prevention alarm switches for proper operation.

C.2.11 Common Tank Appurtenances

C.2.11.1 Gauge Well

a) Inspect gate well pipe for thinning at about two-thirds distance above the bottom: look 

for thinning at the edge of the slots.

b) Check for corrosion on the pipe joint. Check that sample cords, weights, thermometers, etc., have been removed from the pipe.

c) Check for cone at bottom end of pipe about one foot above the bottom.

d) Check condition of well washer pipe and that its flared end is directed at the near side 

of the hold off pad.

e) Check that supports for gauge well are welded to pad or to shell and not directly to 

bottom plate.

f) Check operation of gauge well cover.

g) Check presence of a hold-off distance marker in well pipe and record hold-off 

distance.

Hold-off distance _________ .

h) Identify and report size and pipe schedule, and whether pipe is solid or slotted. Report 

slot size.

i) Check that the hold-off distance plate is seal-welded to the bottom and that any gauge 

well supports are welded to the plate and not directly to the bottom.

j) Inspect vapor control float and cable.

k) Check for presence and condition of gauge well washer.

l) Check for bull plug or plate blind on gauge well washer valve.

m) Inspect gauge well guide in floating roof for pitting and thinning.

n) Inspect the guide rollers and sliding plates for freedom of movement.

o) Inspect condition of gauge well pipe seal system.

p) On black oil and diesel services: if gauge well is also used for sampling, check for 

presence of a thief- and gauge-type hatch to avoid spillage.

q) Visually inspect inside of pipe for pipe weld protrusions which could catch or damage 

vapor control float.

C.2.11.2 Sampling Systems: Roof Sample Hatches

a) Inspect roof-mounted sample hatches for reinforcing pads and cracking.

b) Inspect cover for operation.

c) For tanks complying with Air Quality Monitoring District rules, inspect sample hatch 

covers for adequate sealing.

d) Check horizontal alignment of internal floating roof sample hatches under fixed roof 

hatches.

e) Inspect the sealing system on the internal floating roof sample hatch cover.

f) Inspect floating roof sample hatch cover recoil reel and rope.

C.2.11.3 Shell Nozzles

a) Inspect shell nozzles for thinning and pitting.

b) Inspect hot tap nozzles for trimming of holes.
C-16 API STANDARD 653

Tank Out-of-service Inspection Checklist (Continued)

Item Completed

√ Comments

c) Identify type of shell nozzles.

d) Identify and describe internal piping, including elbow-up and elbow-down types.

C.2.11.4 For Nozzles Extended Into the Tank

a) Inspect pipe support pads welded to tank bottom.

b) Inspect to see that pipe is free to move along support without strain or tearing action 

on bottom plate.

c) Inspect nozzle valves for packing leaks and damaged flange faces.

d) Inspect heater stream nozzle flanges and valves for wire cutting.

e) Report which nozzles have thermal pressure relief bosses and valves.

f) In internal elbow-down fill line nozzles, inspect the wear plate on the tank bottom.

g) On elbow-up fill lines in floating roof tanks, check that opening is directed against 

underside of roof, not against vapor space. Inspect impact are for erosion.

C.2.11.5 Diffusers and Air Rolling Systems

a) Inspect diffuser pipe for erosion and thinning.

b) Check holes in diffuser for excessive wear and enlargement.

c) Inspect diffuser supports for damage and corrosion.

d) Check that diffuser supports restrain, not anchor, longitudinal line movement.

e) Inspect air spiders on bottom of lube oil tanks for plugging and damaged or broken 

threaded joints.

C.2.11.6 Swing Lines

a) Inspect flexible joint for cracks and leaks.

b) Scribe the flexible joint across the two moving faces and raise end of swing line to 

check the joint’s freedom of movement, indicated by separation of scribe marks.

c) Check that flexible joints over 6 in. are supported.

d) Inspect the swing pipe for deep pitting and weld corrosion.

e) Loosen the vent plugs in the pontoons and listen for a vacuum. Lack of a vacuum 

indicates a leaking pontoon.

f) Check the results of air test on pontoons during repairs.

g) Inspect the pontoons for pitting.

h) Inspect the pull-down cable connections to the swing.

i) Inspect the condition of the bottom-mounted support, fixed roof limiting bumper, or 

shell-mounted limiting bumper for wood condition, weld and bolt corrosion, and seal 

welding to bottom or shell.

j) Inspect safety hold-down chain for corrosion and weak links.

k) Check that there is a welded reinforcing pad where the chain connects to the bottom. 

l) If the floating swing in a floating or internal floating roof tank does not have a limiting 

device preventing the swing from exceeding 60 degrees, measure and calculate the 

maximum angle possible with the roof on overflow.

Max. angle on overflow _____________ .

(If the calculated angle exceeds 65 degrees, recommended installation of a limiting 

bracket.)

m) Inspect pull-down cable for fraying.
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION C-17

Tank Out-of-service Inspection Checklist (Continued)

Item Completed

√ Comments

n) Inspect for three cable clamps where cable attaches to end of swing line (singlereeved) or to roof assembly (double-reeved). Inspect sheaves for freedom of movement.

o.) Inspect winch operation and check the height indicator for legibility and accuracy.

p) Inspect bottom-mounted sheave assembly at end of pontoon for freedom of rotation 

of sheave.

q) Inspect shell-mounted lower sheave assembly for freedom of rotation of sheave, 

corrosion thinning, and pitting of sheave housing.

r) Inspect upper sheave assembly for freedom of movement of sheave.

s) Inspect the cable counterbalance assembly for corrosion and freedom of operation.

C.2.11.7 Manway Heater Racks

a) Inspect the manway heater racks for broken welds and bending of the sliding rails.

b) Measure and record the length of the heater and length of the track.

C.2.11.8 Mixer Wear Plates and Deflector Stands

a) Inspect bottom and shell plates and deflector stands.

b) Inspect for erosion and corrosion on the wear plates. Inspect for rigidity, structural 

soundness, corrosion, and erosion of deck plates and reinforcing pads that are sealwelded to the bottom under the deflector stand legs.

c) Measure for propeller clearance between the bottom of deflector stand and roof when 

the roof is on low legs.

C.2.12 Access Structures

C.2.12.1 Handrails

a) Identify and report type (steel pipe, galvanized pipe, square tube, angle) and size of 

handrails.

b) Inspect for pitting and holes, paint failure.

c) Inspect attachment welds.

d) Identify cold joints and sharp edges. Inspect the handrails and midrails.

e) Inspect safety drop bar (or safety chain) for corrosion, functioning, and length.

f) Inspect the handrail between the rolling ladder and the gaging platform for a hazardous opening when the floating roof is at its lowest level.

C.2.12.2 Platform Frame

a) Inspect frame for corrosion and paint failure.

b) Inspect the attachment of frame to supports and supports to tank for corrosion and 

weld failure.

c) Check reinforcing pads where supports are attached to shell or roof.

d) Inspect the surface that deck plate or grating rests on, for thinning and holes.

e) Check that flat-surface-to-flat-surface junctures are seal-welded.

C.2.12.3 Deck Plate and Grating

a) Inspect deck plate for corrosion-caused thinning or holes (not drain holes) and paint 

failure.

b) Inspect plate-to-frame weld for rust scale buildup.

c) Inspect grating for corrosion-caused thinning of bars and failure of welds.
C-18 API STANDARD 653

Tank Out-of-service Inspection Checklist (Continued)

Item Completed

√ Comments

d) Check grating tie down clips. Where grating has been retrofitted to replace plate, 

measure the rise of the step below and above the grating surface and compare with 

other risers on the stairway.

C.2.12.4 Stairway Stringers

a) Inspect spiral stairway stringers for corrosion, paint failure, and weld failure. Inspect 

attachment of stairway treads to stringer.

b) Inspect stairway supports to shell welds and reinforcing pads.

c) Inspect steel support attachment to concrete base for corrosion.

C.2.12.5 Rolling Ladder

a) Inspect rolling ladder stringers for corrosion.

b) Identify and inspect ladder fixed rungs (square bar, round bar, angles) for weld 

attachment to stringers and corrosion, particularly where angle rungs are welded to 

stringers.

c) Check for wear and corrosion where rolling ladder attaches to gaging platform.

d) Inspect pivot bar for wear and secureness.

e) Inspect operation of self-leveling stairway treads.

f) Inspect for corrosion and wear on moving parts.

g) Inspect rolling ladder wheels for freedom of movement, flat spots, and wear on axle.

h) Inspect alignment of rolling ladder with roof rack.

i) Inspect top surface of rolling ladder track for wear by wheels to assure at least 18 in. 

of unworn track (track long enough).

j) Inspect rolling ladder track welds for corrosion.

k) Inspect track supports on roof for reinforcing pads seal-welded to deck plate.

l) Check by dimensioning, the maximum angle of the rolling ladder when the roof is on 

low legs. 

Max. angle ____________ .

m) If rolling ladder track extends to within 5 ft of the edge of the roof on the far side, check 

for a handrail on the top of the shell on that side.

NOTES
D-1

Annex D

Authorized Inspector Certification

D.1 Examination

A written examination to certify an authorized inspector within the scope of API 653 shall be administered by a third party 

designated by API. The examination shall be based on the current API 653 body of knowledge as published by API.

D.2 Certification

D.2.1 An API 653 authorized inspector certificate will be issued when an applicant has successfully passed the API 

653 certification examination, and satisfies the criteria for education and experience. Education and experience, when 

combined, shall be equal to at least one of the following.

a) A bachelor of science degree in engineering or technology plus one year of experience in supervision or 

performance of inspection activities as described in API 653.

b) A two-year degree or certificate in engineering or technology, plus two years of experience in the design, 

construction, repair, inspection, or operation of aboveground storage tanks, of which one year must be in 

supervision or performance of inspection activities as described in API 653.

c) A high school diploma or equivalent, plus three years of experience in the design, construction, repair, inspection, 

or operation of aboveground storage tanks, of which one year must be in supervision or performance of inspection 

activities as described in API 653.

d) A minimum of five years of experience in the design, construction, repair, inspection, or operation of aboveground 

storage tanks, of which one year must be in supervision or performance of inspection activities as described in 

API 653.

D.2.2 An API certificate for an authorized inspector is valid for three years from its date of issuance.

D.2.3 An API 653 authorized inspector certificate is valid in all jurisdictions and any other location that accepts or 

other wise does not prohibit the use of API 653.

D.3 Certification Agency

API shall be the certifying agency.

D.4 Retroactivity

The certification requirements of API 653 shall not be retroactive or interpreted as applying before 12 months after 

the date of publication of this edition or addendum of API 653. The recertification requirements of D.5 shall not be 

retroactive or interpreted as applying before three years after the date of publication of this edition or addendum of 

API 653.

D.5 Recertification

D.5.1 Recertification is required three years from the date of issuance of the API 653 authorized inspector 

certificate. Recertification by written examination will be required for authorized inspectors who have not been actively 

engaged as authorized inspectors within the most recent three-year certification period. Recertification exams will be 

in accordance with all of the provisions contained in API 653.
D-2 API STANDARD 653

D.5.2 Actively engaged as an authorized inspector shall be defined as one of the following provisions:

a) a minimum of 20 % of the time spent performing inspection activities, or supervision of inspection activities, or 

engineering support of inspection activities as described in API 653 over the most recent three-year certification 

period;

b) performance of inspection activities or supervision of inspection activities or engineering support of inspection 

activities on 75 aboveground storage tanks as described in API 653 over the most recent three-year certification 

period

NOTE Inspection activities common to other API inspection documents (NDE, record keeping, review of welding documents, 

etc.) may be considered here.

D.5.3 Once every other recertification period (every six years), inspectors actively engaged as an authorized inspector 

shall demonstrate knowledge of revisions to API 653 that were instituted during the previous six years. This requirement 

shall be effective six years from the inspector’s initial certification date. Inspectors who have not been actively engaged 

as an authorized inspector within the most recent three-year period shall recertify as required in D.5.1.
E-1

Annex E

(Intentionally Left Blank)

F-1

Annex F

NDE Requirements Summary

F.1 Introduction

This annex is a summary of all NDE requirements for repairs and reconstruction of tanks. It is provided only as a 

guide to assure that the proper examinations are performed and that the acceptance standards, examiner 

qualifications, and procedure requirements are followed.

F.2 Visual Examination

F.2.1 Visual examinations are required for:

a) cavities from removal of repads (see API 653, 12.1.2.2);

b) completed welds of stress-relieved assemblies after stress relief but before hydrostatic testing (see API 653, 

12.1.2.4);

c) all fillet welds and completed repairs of fillet welds (see API 650, Section 7.3.2.2 and API 653, 12.1.3.3);

d) completed welds of new permanent attachments and areas of removed temporary attachments (see API 653, 

12.1.4.2);

e) new shell plate-to-shell plate welds (see API 653, 12.1.5);

f) tack welds left in place (see API 650, Section 7.2.1.8);

g) bottom plate and all welds, including the weld attaching a patch plate to the bottom, for new bottom plates (see API 

653, 12.1.7.1);

h) root and final weld pass of patch plate to bottom in the critical zone (see API 653, 12.1.7.2);

i) areas of a bottom plate repaired by welding (see API 653, 12.1.7.3);

j) areas of a shell plate to be repaired by welding (see API 653, 12.1.8);

k) cavities from removal of weld defects (see API 653, 12.1.3.1);

l) annular plate butt-welds root pass and final pass [see API 653, 12.3.2.4.1 b)];

m)repaired areas of the shell-to-bottom weld (see API 653, 12.3.2.5.2).

F.2.2 The examination acceptance standard is API 650, Section 8.5.

F.2.3 The examiner’s employer shall determine and certify that each visual examiner meets the requirements of API 

Standard 650, Section 8.5.1.

F.3 Magnetic Particle and Liquid Penetrant Examination

F.3.1 Magnetic particle or liquid penetrant examinations are required for:

a) cavities from removing existing reinforcing pad welds (see API 653, 12.1.2.2);

12
F-2 API STANDARD 653

b) new welds of nozzle neck-to-shell, nozzle neck-to-repad, and repad-to-shell (see API 653, 12.1.2.3);

c) completed welds of stress-relieved assemblies after stress relief, before hydrostatic testing (see API 653, 

12.1.2.4);

d) cavities from removal of weld defects (see API 653, 12.1.3.1);

e) completed welds of new permanent attachments and areas of removed temporary attachments on API 650 

material Groups IV, IVA, V, or VI (see API 653, 12.1.4.2);

f) the back-gouged surface of the root pass and the final surface of new shell plate welds where the shell is thicker 

than 1 in. (see API 653, 12.1.5);

g) existing shell-to-bottom welds that will be under a patch plate, plus 6 in. on each side (see API 653, 12.1.6.3);

h) root and final weld pass of patch plate to bottom in the critical zone (see API 653, 12.1.7.2);

i) bottom plate restored by welding (see API 653, 12.1.7.3);

j) areas of a shell plate repaired by welding (see API 653, 12.1.8);

k) MT or PT repairs to the annular plate or bottom plates within the critical zone after root and final pass [see API 653, 

12.3.2.4.1 b)].

l) MT or PT repairs to the shell-to-bottom welds before and after root pass, and after final pass (see API 653, 

12.3.2.5.2);

m)the back-gouged surface of the root pass of full penetration nozzle neck-to-shell and repad welds as required by 

API 653, 12.3.2.3.6, specific hydrostatic test exemption requirement;

n) the back-gouged surface of the root pass and final surface of new vertical and horizontal shell joints as required by 

API 653, 12.3.2.3.5, specific hydrostatic test exemption requirement.

F.3.2 Magnetic particle examination alone is required for weld removal areas of the bottom-to-shell welds when 

removing a bottom [see API 653, 9.10.2.2 b)].

F.3.3 The magnetic particle examination acceptance standard is ASME Section V, Article 7. The acceptance 

standards for the removal and repair of defects shall be in accordance with ASME Section VIII, Annex 6, Paragraphs 

6-3, 6-4, and 6-5.

F.3.4 The liquid penetrant examination acceptance standards is ASME Section V, Article 6. The acceptance 

standards for the removal and repair of defects shall be in accordance with ASME Section VIII, Appendix 8, 

Paragraphs 8-3, 8-4, and 8-5.

F.3.5 The requirements for examiner qualifications shall follow API 650, Section 8.2.3, requiring an examiner with 

adequate vision and competent in the examination, interpretation, and evaluation of results.

F.3.6 Procedure requirements shall follow ASME Section V.

F.4 Ultrasonic Examination

F.4.1 Ultrasonic examinations are required for:

a) shell areas over which lap patch plates are to be welded (see API 653, 9.3.1.9);

13
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION F-3

b) shell areas over which new reinforcing or hot tap nozzles are to be welded (see API 653, 12.1.2.1);

c) completed repairs of butt-welds unless radiographed (see API 653, 12.1.3.2);

d) the full penetration nozzle neck-to-shell and repad welds as required by API 653, 12.3.2.3.6, specific hydrostatic 

test exemption requirement;

e) repairs to annular plate butt-welds after final pass [see API 653, 12.3.2.4.1 b)].

F.4.2 The ultrasonic examination acceptance standard shall be agreed upon by the owner/operator and contractor 

(see API 650, Section 8.3.2.5).

F.4.3 The requirements for examiner qualifications shall follow ASNT Level II or III, according to ASNT SNT-TC-1A. 

Level I personnel may be used if they are given written acceptance/rejection criteria prepared by Level II or Level III 

personnel, and are under the direct supervision of Level II or Level III personnel.

F.4.4 Procedure requirements shall follow ASME Section V.

F.5 Vacuum Box Testing

F.5.1 Vacuum box testing is required for:

a) new shell-to-bottom welds, unless diesel tested (see API 653, 12.1.6);

b) new bottom welds, unless tracer gas tested (see API 653, 12.1.7);

c) new roof plate welds for tanks designed to be gas tight (see API 650, Section 7.3.7);

d) potential bottom plate leak paths, (see API 653, 12.1.7.1);

e) patch plates welded on to the bottom, (see API 653, 12.1.7.1 and 12.1.7.2);

f) bottom plate restored by welding (see API 653, 12.1.7.3).

F.5.2 The requirements for examiner qualifications shall follow API 650, Section 8.6.4, requiring an examiner with 

adequate vision and competence in the examination method, interpretation, and evaluation of results.

F.5.3 Procedure requirements shall follow API 650, Section 8.6 (3 lbf/in.2 gauge minimum).

F.6 Tracer Gas Testing

F.6.1 Tracer gas testing is required for new bottom welds, unless vacuum box tested (see API 653, 12.1.7).

F.6.2 There are no requirements for examiner qualifications.

F.6.3 Test must be performed to a procedure that has been reviewed and approved by the tank owner/operator.
F-4 API STANDARD 653

F.7 Diesel Oil Test

F.7.1 Diesel oil testing is required for:

a) the first pass of new shell-to-bottom welds and the final new shell-to-bottom weld, unless the final weld is vacuum 

box tested (see API 653, 12.1.6);

b) floating roof deck seams and other joints required to be liquid tight or vapor tight (see API 650, Section H.6.2 and 

Section C.4.2).

F.7.2 There are no requirements for examiner qualifications.

F.7.3 There are no procedure requirements.

F.8 Air Leak Testing

F.8.1 Air leak testing is required for: 

a) repad-to-shell, repad-to-nozzle, and nozzle-to-shell welds of new or altered shell nozzles (see API 653, 12.4);

b) initial pass of the shell-to-bottom welds and inside and outside of the shell, if the weld is not tested by vacuum box 

or diesel oil (see API 653, 12.1.6.2).

F.8.2 There are no requirements for examiner qualifications.

F.8.3 Procedure requirements shall follow API 650, Section 7.3.5.

F.9 Radiographic Examination

F.9.1 Radiographic examinations are required for:

a) insert plates for installing penetration butt-welds shall be fully radiographed (see API 653, 12.2.1.8);

b) repairs to butt welds, unless ultrasonically examined (see API 653, 12.1.3.2);

c) vertical and horizontal joints and junctions of new shell plates welded to new shell plates and new plates welded to 

existing plates (see API 653, 12.2) This section covers shell replacement plates and door sheets;

d) tank shell butt welds on reconstructed tanks (see API 653, 12.2.1.5);

e) new annular plate joints (see API 650, Section 8.1.2.9);

f) new and replaced shell insert plate and door sheet welds (see API 653, 12.2.1.6);

g) vertical and horizontal shell joints as required by API 653, 12.3.2.3.5, specific hydrostatic test exemption 

requirement.

F.9.2 The examination acceptance standard is ASME Section VIII, Paragraph UW-51(b).

F.9.3 The requirements for examiner qualifications shall follow ASNT Level II or III, according to ASNT SNT-TC-1A. 

Level I personnel may be used if they are given written acceptance/rejection criteria prepared by Level II or Level III 

personnel. These procedures must contain the applicable requirements of ASME Section V, Article 2. All Level I 

personnel must be under the direct supervision of Level II or Level III personnel.

F.9.4 Procedure requirements shall follow ASME Section V, Article 2.

12
G-1

Annex G

Qualification of Tank Bottom Examination Procedures and Personnel

G.1 Introduction

G.1.1 This annex provides guidance for qualifying both tank bottom examination procedures and individuals that 

perform tank bottom examinations. Owner/operators may elect to either apply this annex as written or modify it to meet 

their own applications and needs. Tank bottom examinations are an important factor in providing the owner/operator 

increased assurance of tank integrity. As a result, it is important that qualified examination procedures and personnel 

are used in these examinations. Specific agreements and requirements for qualification of tank bottom examination 

procedures and tank bottom examiners should be established between the owner/operator and the authorized 

inspection agency. 

G.1.2 There have been many NDE tools developed for examining tank bottoms. Most of these tools are complex 

and require the operator to have a high level of knowledge and skill. The effectiveness of these examinations may 

vary greatly depending on the equipment used, the examination procedure, and the skill of the examiner. 

Often the owner/operator will not have the ability to easily determine if the tank bottom examination has been effective 

in assessing the actual condition of the tank bottom. The requirements in this annex will provide the owner/operator 

additional assurance that the tank bottom examination will find significant metal loss.

G.2 Definitions

G.2.1

authorized inspection agency

Organizations that employ an aboveground storage tank inspector certified by API (see 3.3).

G.2.2

bottom scan

The use of equipment over large portions of the tank bottom to detect corrosion in a tank bottom. One common type 

of bottom-scanning equipment is the magnetic flux leakage (MFL) scanner.

G.2.3

essential variables

Variables in the procedure that cannot be changed without the procedure and scanning operators being re-qualified.

G.2.4

examiners

Scanning operators and NDE technicians who prove-up bottom indications.

G.2.5

non-essential variables

Variables in the procedure that can be changed without having to re-qualify the procedure and/or scanning operators.

G.2.6

qualification test

The demonstration test that is used to prove that a procedure or examiner can successfully find and prove-up tank 

bottom metal loss.

13
G-2 API STANDARD 653

G.2.7

scanning operator 

operator

The individual that operates bottom-scanning equipment. 

G.2.8

sizing 

prove-up

The activity that is used to accurately determine the remaining bottom thickness in areas where indications are found 

by the bottom scanning equipment. This is often accomplished using the UT method.

G.2.9

tank bottom examination

The examination of a tank bottom using special equipment to determine the remaining thickness of the tank bottom. It 

includes both the detection and prove-up of the indications. It does not include the visual examination that is included 

in the internal inspection.

G.2.10

tank bottom examination procedure

TBP

A qualified written procedure that addresses the essential and non-essential variables for the tank bottom 

examination. The procedure can include multiple methods and tools, i.e. bottom scanner, hand scanner, and UT 

prove-up. 

G.2.11

tank bottom examiner qualification record

TBEQ

A record of the qualification test for a specific scanning operator. This record must contain the data for all essential 

variables and the results of the qualification test. 

G.2.12

tank bottom procedure qualification record 

TBPQ

A record of the qualification test for a tank bottom examination procedure. This record must contain the data for all 

essential variables and the results of the qualification test. 

G.2.13

variables or procedure variables

The specific data in a procedure that provides direction and limitations to the scanning operator. Examples include: 

plate thickness, overlap of adjacent bottom scans, scanning speed, equipment settings, etc.

G.3 Tank Bottom Examination Procedures

G.3.1 Each authorized inspection agency performing tank bottom examinations is responsible to have and use 

TBPs. These procedures provide direction for examiners performing tank bottom examinations. A procedure also 

allows the owner/operator or authorized inspector to verify whether the examiners are correctly performing the 

examinations.

G.3.2 The authorized inspection agency that performs the tank bottom examinations should develop the TBPs. 

G.3.3 Each TBP shall address essential and non-essential variables. Section G.5.4 provides guidance for 

determining appropriate TBP essential and non-essential variables. Each procedure should specify limits on 

appropriate variables, e.g. plate thickness range.
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION G-3

G.4 Tank Bottom Examiners 

G.4.1 Examiners need only to be qualified for the work they do in the field. For example, scanning operators who 

only use the bottom scanning equipment and do not prove-up the flaw with a follow-up method need only to be 

qualified for the scanning operation.

G.4.2 The purpose of qualifying the tank bottom examiner is to determine if the examiner is capable of satisfactorily 

using a qualified procedure to determine the condition of the tank bottom. 

G.4.3 Each authorized inspection agency is responsible to train, test and qualify the scanning operators and 

examiners they employ using follow-up techniques. Qualifications gained through one authorized inspection agency 

are not necessarily valid for any other authorized inspection agency [see G.4.4 and G.4.9 f)].

G.4.4 The authorized inspection agency is responsible for training each scanning operator they employ. Each 

scanning operator should receive a minimum of 40 hours of training. This training should include:

a) instruction on the NDE principles/methods used by the bottom scanner, limitations and application of the specific 

scanning equipment and procedure, scanning equipment calibration and operation, key scanning equipment 

operating variables, etc.; 

b) hands-on operation of the bottom scanner under the direct supervision of a qualified scanning examiner.

When hiring experienced examiners, the authorized inspection agency should verify and document previous 

examiner training and provide any additional training if necessary. Experienced examiners should be provided 

training regarding specific procedural requirements and test equipment to be utilized by the new employer.

G.4.5 The authorized inspection agency is responsible for testing each scanning operator by written examination. 

The test questions should be appropriate for the scanning method to be used. The authorized inspection agency 

should establish the passing score for the written examination. 

G.4.6 The authorized inspection agency is responsible for qualifying all examiners they employ. All examiners 

(scanning operators and examiners performing prove-up on the indications) shall be qualified by performing an 

examination on test plates as specified in G.5. Only third-party companies, having no conflict of interest in tank bottom 

examination applications, or owner/operator companies may facilitate qualification tests. The examiner shall be 

considered qualified if the acceptance criteria specified in G.5.3 has been met.

Examiners performing prove-up of indications using ultrasonic testing methods should be qualified in accordance 

with API 650, Section 8.3.2 and supplemental requirements given in this annex.

G.4.7 During the qualification test, a TBEQ must be completed for each examiner. The TBEQ is a record of the 

variables used during the qualification test. On the TBEQ, the qualifying company must record: 

a) the essential variables from the qualification test;

b) the qualification test results;

c) number of hours the individual has been trained;

d) test score from the written training examination.

The TBEQ shall be certified (signed) as accurate by a representative of the authorized inspection agency and a 

representative of the company facilitating the test. 

G.4.8 The TBEQ may be written in any format that contains all the required information. 

12
G-4 API STANDARD 653

G.4.9 The bottom-scanning examiners (operators and/or UT examiners) should be re-qualified when any of the 

following apply:

a) when the examiner is not qualified to the TBP that is to be used at the owner/operator facility; 

b) when the authorized inspection agency changes the TBP and that change requires the procedure to be requalified;

c) when the operator has not performed a tank bottom scan in six months;

d) when the operator has not used the specific procedure (TBP) for 12 months;

e) when the authorized inspection agency has reason to question the ability of the examiner;

f) when an examiner changes to a new employing authorized inspection agency that uses procedures with essential 

variables that are different from the previous employer’s procedures.

G.5 Qualification Testing

G.5.1 Qualification Test Plates

G.5.1.1 The qualification test will be performed on a sample tank bottom with designed flaws. The sample tank bottom 

should be a minimum of 70 ft2 to provide space for the designed flaws. The plate material used to fabricate sample 

plates may be either new steel or used steel. It should be noted that the results obtained during qualification tests might 

not be indicative of the results of examinations performed on other plates of differing quality or permeability. When used 

steel is utilized for qualification purposes, the qualification test acceptance standards recommended in G.5.2 may not be 

appropriate. The owner/operator should establish its own acceptance standards in such cases.

G.5.1.2 The minimum number and types of underside test pits located on the test plates are described below:

where

T is nominal bottom thickness;

t is the remaining bottom thickness at test plate flaws.

NOTE Test pits should generally be hemispherical having a depth-to-diameter ratio of from 20 % to 50 %. Test pits should 

not be flat bottom holes since examiners may interpret these as a lamination. Also, machined conical holes should not be 

used since they are difficult to size with UT methods. 

The owner/operator may consider placing additional flaws near the plate edge, i.e. less than 6 in. from the edge, to 

determine if such flaws can be detected by authorized inspection agency procedures. Any flaws placed closer than 

6 in. to the plate edge should be in addition to those shown above and should not be included in determining 

qualification unless specifically required by an owner/operator and such defects are stated as being detectable in 

authorized inspection agency procedures.

Remaining Bottom 

Thickness (t) 

(in.)

Minimum Number of Pits

t < 0.050 2

0.050 < t < 1/2T 5

1/2T < t < 2/3T 4
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION G-5

G.5.1.3 The minimum number and types of product side test pits located on the test plates are described below.

G.5.1.4 There should also be at least one area representing general soilside corrosion. This area should be at least 

10 in.2 (64.52 cm2) and have a remaining bottom thickness of about 1/2T (nominal plate thickness). 

G.5.2 Qualification Test Acceptance Standards

G.5.2.1 The following acceptance criteria must be met when qualifying either an examination procedure or an 

examiner. If all the acceptance criteria are met, the procedure or examiner shall be considered qualified. Owner/

operators may substitute alternative acceptance criteria, either more or less conservative, based on their specific 

needs and requirements. 

G.5.2.2 When qualifying either a procedure or a scanning operator, the operator must be able to detect the following 

flaws.

G.5.2.3 When qualifying either a procedure or an examiner, who proves up the indications, the examiner must be 

able to determine the flaw depth as follows:

The owner/operator should determine if additional flaw dimensions must be addressed in the qualification process.

Remaining Bottom 

Thickness (t) 

(in.)

Minimum Number 

of Pits

0.050 < t < 1/2T 2

1/2T < t < 2/3T 2

Remaining Bottom 

Thickness (t)

(in.)

Flaws That Must

be Found

t < 0.050 90 % to 100 %

0.050 in. < t < 1/2T 70 % to 90 %

1/2T < t < 2/3T 40 % to 60 %

Area of general corrosion 100 %

Type of Tank Bottom Prove-up (Flaw Depth)

Not coated ±0.020 in.

Thin coating < 0.030 in. ±0.030 in.

Thick coating > 0.030 in. Per agreement with 

owner/operator
G-6 API STANDARD 653

G.5.2.4 While false calls, also referred to as over-calls, tend to be more of an examination efficiency issue than a 

tank bottom integrity issue, the owner/operator should determine if they should be addressed in the qualification 

process.

G.5.3 Qualification Test Variables

G.5.3.1 Essential variables are those items that may have a significant effect on the quality of the examination if they 

are changed from those used during the qualification test.

G.5.3.2 Table G.1 lists suggested items that may be considered as essential variables for the qualification test when 

qualifying either a tank bottom examination procedure or a tank bottom examiner. Essential variables may be different 

for different types of tank bottom scanners. Authorized inspection agencies are responsible for determining what 

additional variables should be considered essential variables for each tank bottom scanner.

G.5.3.3 Essential variables and the values must be recorded on the TBP and on the TBEQ. 

G.5.3.4 Non-essential variables are those items that will have a lesser affect on the quality of the examination. Nonessential variables may be different for different types of tank bottom scanners. 

G.5.3.5 Non-essential variables must be listed on the TBP but need not be addressed on the TBPQ or the TBEQ. 

The following is a list of examples of items that might be considered as non-essential variables. Equipment 

Table G.1—Suggested Essential Variables for Qualification Tests

Essential Variable Used During Test Qualified

Scanner Equipment As tested Same as tested

Prove-up Equipment As tested Same as tested

Prove-up Procedure As tested Same as tested

Plate Thickness (T) T T +0.005 in. – T –0.130 in.

Coating Thickness (tc) No coating used 0.000 in.

0.001 in. < tc < 0.030 in. 0.001 in. to 0.030 in.

0.031 in. < tc < 0.080 in. 0.031 in. to 0.080 in.

tc > 0.080 0.081 in. to tc

Distance from Shell (ds) ds Lesser of 8 in. or ds

Critical Equipment Settings As tested Per manufacturer

Threshold Settings (Th) Th < 10 %Th

Calibration or Functional Check As tested Same as tested

13
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION G-7

manufacturers and authorized inspection agencies are responsible to determine what addition factors should be 

considered non-essential variables for each tank bottom scanner:

a) scanner speed;

b) scanning pattern;

c) height limitations;

d) overlap between scans;

e) plate cleanliness;

f) non-critical equipment settings.

NOTE Some of the listed non-essential variables may actually be essential variables for specific types of scanners. 

H-1

Annex H

Similar Service Assessment

This annex is provided as guidance for performing a similar service assessment. This annex is not all inclusive and is 

not mandatory. It is not intended to prevent the use of other similar service assessment methods. This annex contains 

sample datasheets illustrating items that should be considered when conducting a similar service assessment. The 

datasheet format facilitates the recording of assessment findings.

NOTE 1 The following examples are merely examples for illustration purposes only. Each company should develop its own 

approach. They are not to be considered exclusive or exhaustive in nature. API makes no warranties, express or implied for 

reliance on or any omissions from the information contained in this document.

NOTE 2 Where applicable, authorities having jurisdiction should be consulted.
H-2 API STANDARD 653

H.1 Scope

This annex provides guidance for conducting a similar service assessment to establish inspection intervals for tanks 

for which corrosion rates have not been directly measured as referenced in 6.3.2 and 6.4.2. This annex is intended for 

use by an authorized inspector, storage tank engineer and other person(s) having knowledge and experience in 

performing similar service assessments.

H.2 Similar Service Assessment

Several criteria must be evaluated to determine whether the candidate tank is in similar service with the control tank. 

Similar service assessment requires a sufficient amount of data collection and analysis and needs to be performed in a 

comprehensive and thorough fashion in accordance with an established risk management program. Similar service 

assessment is conducted using design, construction, operation, maintenance and inspection data. Data should be 

obtained by using direct and indirect examination procedures such as MT and UT, evaluating product corrosivity, 

measuring CP levels, determining soil properties, and other factors. Refer to the “Similar Service Assessment—

Datasheet,” which provides a place to record the required data. Data should be collected for each of the tank 

characteristics listed on the datasheet for both the control tank and candidate tanks and an assessment made to 

determine if the services are similar. Typically, there will not be an exact match of all data, or some of the required data 

will not even be known. When there is not an exact match between one or more criteria, additional evaluation is 

necessary to determine whether the tanks can be considered to be in similar service. Figure H.1 illustrates the steps in 

conducting a similar service assessment. The “Similar Service Assessment—Datasheet” is to facilitate the comparison 

of data for the two tanks. If the criteria for the control tank and candidate tank match, the candidate tank may be 

considered in similar service as the control tank for that particular criterion. 

H.2.1 Additional Assessment 

When additional assessment is required because an individual basic criterion does not match, the table references 

the section describing additional factors that must be assessed. If the additional factors in the specified reference 

section are assessed to be sufficiently similar, the tanks are considered in similar service for that factor. If all additional

provisions are satisfied, the tanks are considered in similar service. Additional assessment(s) must be documented 

and maintained in the record file as per 6.8. 

H.2.1.1 Year Tank Erected: If the criteria for the control tank and candidate tank do not match, the following 

additional provisions must be satisfied to consider both tanks in similar service:

a) the difference in ages of the tanks must be considered in the corrosion rate calculations, and

b) any substantive differences in the design and/or construction standards to which the tanks were constructed must 

be considered in the similar service evaluation.

H.2.1.2 Bottom Material: If the criteria for the control tank and candidate tank do not match, the following additional 

provisions must be satisfied to consider both tanks in similar service:

a) the bottom material of the candidate tank must have similar corrosion-resistance properties as the bottom material 

of the control tank,

b) the candidate tank, or both the candidate and the control tanks, utilize an effective lining to prevent corrosion of the 

product-side of the bottom, and

c) the potential for corrosion of the soil-side of the bottom is assessed to be similar for both tank bottom materials.
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION H-3

H.2.1.3 Shell Material: If the criteria for the control tank and candidate tank do not match, the following additional 

provisions must be satisfied to consider both tanks in similar service:

a) the shell material of the candidate tank must have similar corrosion-resistance properties as the shell material of 

the control tank, 

b) the candidate tank, or both the candidate and the control tanks, utilize a suitable lining to prevent corrosion of the 

product-side of the shell, and

c) the candidate tank, or both the candidate and the control tanks, utilize a suitable paint or coating to prevent 

corrosion of the external side of the shell.

H.2.1.4 Corrosion Allowance, Bottom/Shell: If the criteria for the control tank and candidate tank do not match, the 

difference in corrosion allowance should be accounted for in the remaining life and inspection interval calculations to 

consider both tanks in similar service.

H.2.1.5 Bottom Lining Type/Thickness/Age: If the criteria for the control tank and candidate tank do not match, the 

differences in the bottom lining systems must be assessed. The provisions of API 652 should be used to assess the 

relative corrosion protection provided by the different lining systems.

H.2.1.6 Cathodic Protection: If the criteria for the control tank and candidate tank do not match, the following 

additional provisions shall apply. The provisions of API 651 should be used to assess the relative corrosion protection 

provided by the cathodic protection systems:

a) if the candidate tank is protected with a properly designed and functional cathodic protection system, and the 

control tank is not, the candidate tank may be considered to be in similar service with respect to cathodic 

protection; 

b) if the control tank is protected with a properly designed and functional cathodic protection system, and the 

candidate tank is not, the candidate tank may not be considered to be in similar service with respect to cathodic 

protection;

c) if the control tank and the candidate tank are protected with properly designed and functional cathodic protection 

systems, the tanks may be considered to be in similar service with respect to cathodic protection.

H.2.1.7 Double Bottom: If the candidate tank and/or the control tank has multiple bottoms, the similar service 

assessment of soil-side corrosion should be based on the material that is in contact with the underside of the primary 

(upper) bottom plate

H.2.1.8 Soil/Material in Contact with Bottom Plate: Any differences in the following factors between the control and 

the candidate tank must be assessed in determining whether the candidate tank is in similar service as the control 

tank:

a) soil or material type;

b) pH;

c) alkalinity;

d) moisture;

e) salinity;

f) resistivity;
H-4 API STANDARD 653

g) oil type (if oiled sand foundation);

h) soil/material cleanliness;

i) soil gradation;

j) chlorides;

k) sulfates.

H.2.1.9 Ambient Conditions: Any differences in the following factors between the control tank and the candidate 

tank must be assessed in determining whether the candidate tank is in similar service as the control tank:

a) low one day mean temperature;

b) exposure to salt air or other corrosive elements.

H.2.1.10 Current Service Conditions: Any differences in the following factors between the control tank and the 

candidate tank must be assessed in determining whether the candidate tank is in similar service as the control tank:

a) product classification (see Table H.1);

b) specific gravity of liquid;

c) Reid vapor pressure at 60 °F;

d) normal operating temperature;

e) inert gas blanket, if used;

f) water bottom, if used;

g) sulfur content;

h) length of time in service;

i) product corrosivity.

H.2.1.11 Previous Service Conditions: If the control tank and/or candidate tank have previously been used for 

different services than the current service, the same factors described in H.2.1.10 should be evaluated for the 

previous service conditions.

H.2.1.12 Product Classification: Table H.1 classifies a wide variety of liquids commonly stored in aboveground 

storage tanks. This table serves as guidance in assessing current or previous service conditions.

H.2.1.13 Additional Considerations: In addition to the factors discussed above, the following data, if available for the 

control tank and the candidate tank, should be assessed in determining whether the candidate tank is in similar 

service as the control tank:

a) MFL data for the tank bottom;

b) ultrasonic thickness (UT) measurement data;

c) fiber optic monitoring system data;
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION H-5

d) cathodic protection monitoring tube data;

e) tank bottom integrity testing data;

f) maintenance procedures, including frequency and methods of tank cleaning. 

H.3 Example of Remaining Life Determination

H.3.1 Tank Bottom

Figure H.2 illustrates one method of determining the time interval in which a tank bottom will reach its minimum 

bottom plate thickness, beyond which the tank should be repaired or removed from service. In this example, the 

original metal thickness was 1/4 in. when the tank was constructed in 1970. The minimum bottom plate thickness at 

the next inspection interval was 0.05 in. (See also Table 4.4.) At the time of this evaluation (June 10, 1990), the tank 

was in sour crude service. Previous service included 20 years in sour crude service. Based on thicknesses measured 

and the calculated corrosion rate, the remaining life, or time to reach the minimum bottom plate thickness of 0.050 in. 

is projected to be approximately 20 years, or June 10, 2010. See 4.4.5 for the minimum thickness calculation for a 

tank bottom.

H.3.2 Tank Shell

Figure H.3 illustrates one method of determining the time interval in which a tank shell course will reach its limit of 

metal loss, beyond which the tank should be repaired or removed from service. In this example, the original metal 

thickness was 1/2 in. when the tank was constructed in 1990. The limit of metal loss of the top shell course was 

calculated to be 1/4 in. At the time of this evaluation (November 15, 2002), the tank was in sweet gasoline service. 

Previous services included nearly seven years in sweet crude and nearly three years in sour crude service. Based 

on thicknesses measured at periodic inspections and corrosion rates calculated from them, the remaining life, or 

time to reach the metal loss limit of 1/4 in. is projected to be approximately four years, or August 1, 2006. See 4.3.3 

for the minimum thickness calculation for an entire shell course.

Table H.1—Similar Service Product Classification 

Class Description Example

A Low sulfur light oil (< 1 % sulfur) No. 2 fuel oil, diesel, kerosene, jet fuel, gasoline

B High sulfur light oil (> 1 % sulfur) Unfinished heating oil, distillate

C Sweet sulfur heavy oil (< 1 % sulfur) Heavy gas oil and sweet residual

D Sour sulfur heavy oil (> 1 % sulfur) Sour residual

E Slop and process water See description

F Finished lube oil Automotive, diesel and aviation oil

G Sludge Acidic, non-acidic

H Crude oils

Light volatile oil (Class 1)

Non-sticky oil (Class 2)

Heavy sticky oil (Class 3)

Non-fluid (heavy crude, high paraffin) (Class 4)

I Additive Gasoline performance additives

J Solvent Ketones, alcohol, toluene, xylene, glycols, glycol ethers

K Chemicals Phosphoric, sulfuric, hydrochloric, formic, and nitric acids

NOTE This table is not all inclusive. Additionally, there may be significant variability in the chemical composition of 

products within a listed classification.
H-6 API STANDARD 653

Figure H.1—Steps in Conducting Similar Service Assessment

Collect Tank Data for

the Control Tank and the

Candidate Tank

Collect Similar Service

Assessment Using the

Datasheet

Determine if Similar

Service Can be Applied to

the Candidate Tank

Determine the Corrosion

Rates to Apply to the

Candidate Tank

Establish the Next Internal

Inspection Date for

the Candidate Tank

Document the Revised

Internal Inspection Date for

the Candidate Tank

As Additional Inspection Data Becomes

Available, Validate the Corrosion Rate

Established for Candidate Tank

Determine if the Additional Inspection

Data Changes the Inspection Schedule

for the Candidate Tank
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION H-7

Figure H.2—Example Corrosion Rate Curves for Bottom of Storage Tank

2010

0 1/32 1/16 3/32 3/16 7/32

1970

2000

1/8 5/32

METAL THICKNESS, IN INCHES

1/4

1980

1990

Min. Bottom Plate Thickness at Next Inspection (See Table 4.4)

Tank New January 21, 1970

YEAR

First Internal Inspection June 10, 1990

Sour Crude

Sour Crude

Extended

Corrosion 

Curve

Limit of metal loss June 10, 2010

Tank Bottom Repaired to 1/4-in.

1/16
H-8 API STANDARD 653

Figure H.3—Example Corrosion Rate Curves for Top Course of Storage Tank

2010

0 1/16 1/8 3/16 3/8 7/16

1990

2005

1/4 5/16

METAL THICKNESS, IN INCHES

1/2

1995

2000

Limit of thickness for top course

Limit of metal loss August 1, 2006

Fourth Inspection November 15, 2002

Third Inspection July 20, 1999 - Service Changed

Second Inspection December 2, 1996 - Service Changed

First Inspection June 10, 1992

Extended 

Corrosion 

Curve

Tank New January 21, 1990

YEAR

Sour Crude

Sweet Crude

Gasoline
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION H-9

CONTROL TANK ID: LOCATION:

DIAMETER (FT): HEIGHT (FT): CAPACITY: BBLS

CANDIDATE TANK ID: LOCATION:

DIAMETER (FT): HEIGHT (FT): CAPACITY: BBLS

SECTION 1—TANK BOTTOM (PRODUCT-SIDE) ASSESSMENT

H.1.1 TANK CHARACTERISTICS

a) YEAR TANK ERECTED

b) BOTTOM MATERIAL

c) CORROSION ALLOWANCE

d) BOTTOM LINING TYPE

e) BOTTOM LINING THICKNESS

f) BOTTOM LINING AGE

H.1.2 CURRENT SERVICE CONDITIONS

a) CURRENT PRODUCT NAME

b) PRODUCT CLASS. (TABLE H.1)

c) SPECIFIC GRAVITY OF PRODUCT

d) NORMAL OPERATING TEMP.

e) WATER BOTTOM?

f) SULFUR CONTENT

g) TIME IN THIS SERVICE

h) PRODUCT CORROSIVITY

H.1.3 PREVIOUS SERVICE CONDITIONS

a) PREVIOUS PRODUCT NAME

b) PRODUCT CLASS. (TABLE H.1)

c) SPECIFIC GRAVITY OF PRODUCT

d) NORMAL OPERATING TEMP.

e) WATER BOTTOM?

f) SULFUR CONTENT

g) TIME IN THIS SERVICE

h) PRODUCT CORROSIVITY

H.1.4 CONCLUSIONS

a) Does this assessment include additional assessment documentation (see H.2.1), YES or NO

b) Based on the criteria reviewed in this similar service evaluation, 

similar service is OR is NOT recommended for this tank (check appropriate box)

c) The corrosion rate to be applied to the product side of the tank bottom is: ___________mpy.

COMMENTS:

NOTE THE DATASHEET SHALL BE MAINTAINED IN THE RECORD FILE AS PER 6.8.

ASSESSED BY: DATE:

APPROVED BY: DATE:

NOTE 1 The control tank is the tank for which service conditions and corrosion rates are well known.

NOTE 2 The candidate tank is the tank to be compared to the control tank to determine if similar service concepts apply.

NOTE 3 “Y” or “Yes” indicates that the candidate tank criterion essentially matches the control tank .

NOTE 4 If the candidate tank criterion does not match the control tank criterion, see H.2.1.

(tank owner/operator)

H.2.1.5

MATCH 3

Control Tank Yes No 1

MATCH 3

Control Tank No 1 Candidate Tank 2 Yes

Candidate Tank 2

Control Tank 1

MATCH 3

Candidate Tank Yes No 2

H.2.1.10

IF NO,

H.2.1.10

H.2.1.10

H.2.1.10

H.2.1.10

H.2.1.10

H.2.1.10

H.2.1.10

H.2.1.10

SEE SEC. 4

IF NO,

SIMILAR SERVICE ASSESSMENT—DATASHEET

SEE SEC. 4

SEE SEC. 4

IF NO,

H.2.1.5

H.2.1.1

H.2.1.2

H.2.1.4

H.2.1.5

H.2.1.10

H.2.1.10

H.2.1.10

H.2.1.10

H.2.1.10

H.2.1.10

H.2.1.10
H-10 API STANDARD 653

CONTROL TANK ID: LOCATION:

DIAMETER (FT): HEIGHT (FT): CAPACITY: BBLS

CANDIDATE TANK ID: LOCATION:

DIAMETER (FT): HEIGHT (FT): CAPACITY: BBLS

SECTION 2—TANK BOTTOM (SOIL-SIDE) ASSESSMENT

H.2.1 TANK CHARACTERISTICS

a) YEAR TANK ERECTED

b) BOTTOM MATERIAL

c) CORROSION ALLOWANCE

d) DOUBLE BOTTOM

H.2.2 SOIL/MATERIAL IN CONTACT WITH OR AROUND BOTTOM PLATE

 5

a) SOIL TYPE

b) SOIL pH

c) SOIL ALKALINITY

d) SOIL MOISTURE

e) SOIL SALINITY

f) SOIL CONDUCTIVITY

g) OIL TYPE (IF OIL SAND FOUNDATION)

h) SOIL CLEANLINESS

H.2.3 CURRENT OPERATING CONDITIONS

a) NORMAL OPERATING TEMP.

b) CATHODIC PROTECTION 5

c) PONDING/WATER

H.2.4 PREVIOUS OPERATING CONDITIONS

a) NORMAL OPERATING TEMP.

b) CATHODIC PROTECTION 5

c) PONDING/WATER

H.2.5 CONCLUSIONS

a) Does this assessment include additional assessment documentation (see H.2.1), YES or NO

b) Based on the criteria reviewed in this similar service evaluation, 

similar service is OR is NOT recommended for this tank (check appropriate box)

c) The corrosion rate to be applied to the soil side of the tank bottom is: ___________mpy.

COMMENTS:

NOTE THE DATASHEET SHALL BE MAINTAINED IN THE RECORD FILE AS PER 6.8.

ASSESSED BY: DATE:

APPROVED BY: DATE:

NOTE 1 The control tank is the tank for which service conditions and corrosion rates are well known.

NOTE 2 The candidate tank is the tank to be compared to the control tank to determine if similar service concepts apply.

NOTE 3 “Y” or “Yes” indicates that the candidate tank criterion essentially matches the control tank .

NOTE 4 If the candidate tank criterion does not match the control tank criterion, see H.2.1.

NOTE 5 See API 651, Section 5.3.

(tank owner/operator)

H.2.1.8

H.2.1.8

MATCH IF NO, 3

H.2.1.6

Yes No SEE SEC. 4

H.2.1.10

Yes No

Control Tank 1 Candidate Tank 2

Control Tank 1

SIMILAR SERVICE ASSESSMENT—DATASHEET

MATCH IF NO, 3

No

IF NO,

SEE SEC. 4

H.2.1.10

SEE SEC. 4

H.2.1.8

H.2.1.8

H.2.1.8

H.2.1.8

H.2.1.8

H.2.1.6

H.2.1.1

H.2.1.2

H.2.1.4

H.2.1.7

H.2.1.8

H.2.1.8

H.2.1.8

SEE SEC. 4

IF NO,

MATCH 3

Candidate Tank Yes No 2

Control Tank 1 Candidate Tank 2 Yes

Control Tank 1 Candidate Tank 2

MATCH 3
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION H-11

CONTROL TANK ID: LOCATION:

DIAMETER (FT): HEIGHT (FT): CAPACITY: BBLS

CANDIDATE TANK ID: LOCATION:

DIAMETER (FT): HEIGHT (FT): CAPACITY: BBLS

SECTION 3—TANK SHELL (PRODUCT-SIDE) ASSESSMENT

H.3.1 TANK CHARACTERISTICS

a) YEAR TANK ERECTED

b) SHELL MATERIAL

c) CORROSION ALLOWANCE

d) SHELL LINING TYPE

e) SHELL LINING THICKNESS

f) SHELL LINING AGE

H.3.2 AMBIENT CONDITIONS

a) LOW ONE DAY MEAN TEMP.

b) EXPOSURE TO SALT AIR

H.3.3 CURRENT SERVICE CONDITIONS:

a) CURRENT PRODUCT NAME

b) PRODUCT CLASS. (TABLE H.1)

c) SPECIFIC GRAVITY OF PRODUCT

d) REID VAPOR PRESSURE @ 60 o

F

e) NORMAL OPERATING TEMP.

f) INERT GAS BLANKET?

g) WATER BOTTOM?

h) SULFUR CONTENT

i) TIME IN THIS SERVICE

j) PRODUCT CORROSIVITY

H.3.4 PREVIOUS SERVICE CONDITIONS

a) PREVIOUS PRODUCT NAME

b) PRODUCT CLASS. (TABLE H.1)

c) SPECIFIC GRAVITY OF PRODUCT

d) REID VAPOR PRESSURE @ 60 o

F

e) NORMAL OPERATING TEMP.

f) INERT GAS BLANKET?

g) WATER BOTTOM?

h) SULFUR CONTENT

i) TIME IN THIS SERVICE

j) PRODUCT CORROSIVITY

H.3.5 CONCLUSIONS

a) Does this assessment include additional assessment documentation (see H.2.1), YES or NO

b) Based on the criteria reviewed in this similar service evaluation, 

similar service is OR is NOT recommended for this tank (check appropriate box)

c) The corrosion rate to be applied to the product side of the tank shell is: ___________mpy.

COMMENTS:

H.2.1.5

H.2.1.5

H.2.1.9

SEE SEC. 4

H.2.1.10

SEE SEC. 4

H.2.1.10

MATCH 3 IF NO,

Control Tank 1

H.2.1.5

H.2.1.1

H.2.1.3

H.2.1.4

SIMILAR SERVICE ASSESSMENT—DATASHEET

H.2.1.10

H.2.1.10

H.2.1.9

IF NO,

H.2.1.10

H.2.1.10

Candidate Tank 2 Yes No SEE SEC. 4

H.2.1.10

H.2.1.10

H.2.1.10

H.2.1.10

MATCH 3 IF NO,

Control Tank 1 Candidate Tank 2 Yes No SEE SEC. 4

MATCH 3 IF NO,

Control Tank 1 Candidate Tank 2 Yes No

MATCH 3

Control Tank Yes No 1 Candidate Tank 2

H.2.1.10

H.2.1.10

H.2.1.10

H.2.1.10

H.2.1.10

H.2.1.10

H.2.1.10

H.2.1.10

H.2.1.10

H.2.1.10
H-12 API STANDARD 653

NOTE THE DATASHEET SHALL BE MAINTAINED IN THE RECORD FILE AS PER 6.8.

SUBMITTED BY: DATE:

APPROVED BY: DATE:

NOTE 1 The control tank is the tank for which service conditions and corrosion rates are well known.

NOTE 2 The candidate tank is the tank to be compared to the control tank to determine if similar service concepts apply.

NOTE 3 “Y” or “Yes” indicates that the candidate tank criterion essentially matches the control tank .

NOTE 4 If the candidate tank criterion does not match the control tank criterion, see H.2.1.

SIMILAR SERVICE ASSESSMENT—DATASHEET
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION H-13

CONTROL TANK ID: LOCATION:

DIAMETER (FT): HEIGHT (FT): CAPACITY: BBLS

CANDIDATE TANK ID: LOCATION:

DIAMETER (FT): HEIGHT (FT): CAPACITY: BBLS

SECTION 4—TANK SHELL (EXTERNAL SIDE) ASSESSMENT

H.4.1 TANK CHARACTERISTICS

a) YEAR TANK ERECTED

b) SHELL MATERIAL

c) CORROSION ALLOWANCE

d) INSULATION TYPE

e) INSULATION THICKNESS

f) INSULATION AGE

g) COATING TYPE

h) COATING THICKNESS

i) COATING AGE

H.4.2 AMBIENT CONDITIONS

a) LOW ONE DAY MEAN TEMP.

b) EXPOSURE TO SALT AIR

H.4.3 CURRENT OPERATING CONDITIONS

a) PONDING/WATER

b) NORMAL OPERATING TEMP.

H.4.4 PREVIOUS OPERATING CONDITIONS

a) PONDING/WATER

b) NORMAL OPERATING TEMP.

H.4.5 CONCLUSIONS

a) Does this assessment include additional assessment documentation (see H.2.1), YES or NO

b) Based on the criteria reviewed in this similar service evaluation, 

similar service is OR is NOT recommended for this tank (check appropriate box)

c) The corrosion rate to be applied to the ambient side of the tank shell is: ___________mpy.

COMMENTS:

NOTE THE DATASHEET SHALL BE MAINTAINED IN THE RECORD FILE AS PER 6.8.

ASSESSED BY: DATE:

APPROVED BY: DATE:

NOTE 1 The control tank is the tank for which service conditions and corrosion rates are well known.

NOTE 2 The candidate tank is the tank to be compared to the control tank to determine if similar service concepts apply.

NOTE 3 “Y” or “Yes” indicates that the candidate tank criterion essentially matches the control tank .

NOTE 4 If the candidate tank criterion does not match the control tank criterion, see H.2.1.

(tank owner/operator)

H.2.1.10

H.2.1.10

Candidate Tank 2

MATCH 3 IF NO,

Control Tank 1 Candidate Tank 2 Yes No SEE SEC. 4

Yes No

MATCH 3

Yes No

Control Tank Yes No 1 Candidate Tank 2

Candidate Tank 2

SIMILAR SERVICE ASSESSMENT—DATASHEET

H.2.1.9

MATCH 3

MATCH 3 IF NO,

SEE SEC. 4

H.2.1.1

H.2.1.3

H.2.1.4

H.2.1.4

H.2.1.4

H.2.1.4

IF NO,

H.2.1.4

H.2.1.4

H.2.1.4

H.2.1.10

IF NO,

H.2.1.10

SEE SEC. 4

H.2.1.9

SEE SEC. 4

Control Tank 1

Control Tank 1 
H-14 API STANDARD 653

CONTROL TANK ID: LOCATION:

DIAMETER (FT): HEIGHT (FT): CAPACITY: BBLS

CANDIDATE TANK ID: LOCATION:

DIAMETER (FT): HEIGHT (FT): CAPACITY: BBLS

SECTION 5—CONCLUSION SUMMARY

a) From Section 1, the corrosion rate to be applied to the product-side of the tank bottom is: ___________mpy.

b) From Section 2, the corrosion rate to be applied to the soil-side of the tank bottom is: ___________mpy.

c) From Section 3, the corrosion rate to be applied to the product-side of the tank shell is: ___________mpy.

d) From Section 4, the corrosion rate to be applied to the external side of the t ank shell i s: ___________mpy.

e) Based on the corrosion rates applied, the next internal inspection for this tank will be completed in Year _________.

COMMENTS:

NOTE THE DATASHEET SHALL BE MAINTAINED IN THE RECORD FILE AS PER 6.8.

ASSESSED BY: DATE:

APPROVED BY: DATE:

(tank owner/operator)

SIMILAR SERVICE ASSESSMENT—DATASHEET
I-1

Annex I

(normative)

Inquiries and Suggestions for Change

I.1 Introduction

This annex describes the process established by API for submitting inquiries to API and for submitting suggestions for 

changes to this standard. Inquiries and suggestions for change are welcome and encouraged, however, submittals 

not complying with this annex and additional API policies will be returned unanswered. These submittals 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. 

I.2 Inquiry References

I.2.1 API maintains several websites that provide information that should be considered when considering 

submitting an inquiry.

I.2.2 Your inquiry may have been previously formally addressed by the Subcommittee and the resulting 

interpretation posted on the API web site at: 

— For all Standards: http://mycommittees.api.org/standards/techinterp/default.aspx

— Refining only: http://mycommittees.api.org/standards/techinterp/refequip/default.aspx

For both links, click on the standard in question to download the file.

I.2.3 In addition, an addendum or errata, which may have addressed your issue, can be found on the API web site 

at:

— For all Standards: http://www.api.org/Standards/addenda/

— Refining only: http://www.api.org/Standards/addenda/add-ref.cfm

I.3 Definitions of Terms

I.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.

I.3.2

interpretation

The subcommittee’s 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. 

I.3.3

suggestion for change

As opposed to an inquiry, a suggestion for change is a proposal to API for modifying the standard to make additions 

or to resolve apparent conflicts or omissions. API encourages both; however the submittal and handling procedures 

are different.

13
I-2 API STANDARD 653

I.4 API Policy Regarding Inquiries

I.4.1 API has established the following limits on its activity in the handling of inquiries. 

— API does not approve, certify, rate, or endorse any item, construction, proprietary device, or activity. 

— API does not act as a consultant on specific engineering problems.

— API does not provide information on the general understanding, rationale or application of the Standard.

I.4.2 All inquiries that result in interpretations will therefore be made available to the general public on the API 

website. 

I.5 Submission of Inquiries

I.5.1 All background and policies for submitting inquiries are stated in the following API web pages:

1) Click on: http://www.api.org/publications-standards-and-statistics/faqs-and-inquiries.aspx

2) Then click on: “Technical Questions and Requests for Interpretation”

3) Then click on: “Click here for detailed information on Technical Questions and Requests for Interpretation”

4) Then click on: “RFI website here” to access the required electronic form for submitting an inquiry. 

I.5.2 All inquiries must comply with the following:

a) be limited to a single subject or closely related subjects;

b) be stated as briefly and precisely as possible, including the applicable paragraph, figure, or table numbers;

c) providing a background explanation is optional, but is encouraged to assist the subcommittee in understanding the 

query;

d) be in the form of a “yes” or “no” question;

e) be technically and editorially correct and written in understandable English;

f) be related to the current edition/addendum of the document in question (Note: API will not issue interpretations of 

superseded or withdrawn documents);

g) The general format of the inquiry should be as follows: “Does Paragraph XXX of API-6XX require that ....?”. The 

inquirer shall also state the potential answer to the inquiry. In addition, if the inquirer believes a revision to the 

Standard is also needed, he or she shall provide recommended wording.

I.6 Typical Inquiry Process

I.6.1 The typical process of an inquiry is:

a) The Inquirer must prepare the inquiry, including any necessary background information, in full compliance with this 

Annex and submit via the API web page link.

b) API Standards Coordinator checks the inquiry to verify compliance with the requirements of submitting an inquiry.

13
TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION I-3

c) If the inquiry cannot be answered for any reason, the Coordinator will issue the form letter to the inquirer advising 

the reason(s) for not answering the inquiry.

d) If the inquiry is valid, it will be forwarded to the Subcommittee for study, and the inquirer will be 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. 

I.6.2 The time required to process a valid inquiry as described in I.6.1 may take as long as a year.

I.7 Responses to Inquiries

I.7.1 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 limits on the applicability. 

I.7.2 The industry benefits as a whole when inquiries are utilized as a means to understand the technical 

requirements of the standard. 

I.7.3 It is not possible to develop interpretations quickly to resolve immediate needs.

I.7.4 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.

I.8 Suggestions for Changes

I.8.1 A Suggestion for Change is a communication (email preferred) from a reader to API proposing that a specific 

change be made to the standard.

I.8.2 Any format is acceptable, as long as the content is clear.

I.8.3 Submit suggestions to: pubweb@api.org.

I.8.4 The content of a suggestion should include the Standard number, edition, and addendum, and the relevant 

paragraph, table or figure numbers, etc. Provide as much explanation as necessary to be sure the reader 

understands the technical issues. Provide specific language that you think is needed to implement the change. 

I.8.5 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. The change may be included in a 

future edition or addendum. 

13

S-1

Annex S

Austenitic Stainless Steel Storage Tanks

S.1 Scope

S.1.1 This annex covers the inspection, repair, alteration, and reconstruction of stainless steel tanks that were 

constructed in accordance with API 650, Annex S.

S.1.2 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 References

No changes to Section 2.

S.3 Definitions

No changes to Section 3.

S.4 Suitability for Service

S.4.1 In 4.2.4.1, the requirements of API 650, Section S.3.5 shall also be satisfied.

S.4.2 In 4.2.4.3, Annex M requirements shall be met for stainless steel tanks with design temperatures over 40 °C 

(100 °F) as modified by API 650, Sections S.3.6.2 thru S.3.6.7. 

S.4.3 In 4.3.3.1, the maximum allowable stress S shall be modified as follows, for the design condition (Sd) and the 

hydrostatic test condition (St) the maximum allowable stress for all shell courses shall be the lesser of 0.95Y or 0.4T.

S.4.4 Table 4.2 shall be in accordance with API 650, Table S-4. When the radiography schedule applied to the 

existing weld is unknown, the joint efficiency of 0.7 shall be used.

S.4.5 Section 4.3.3.5 c), shall be changed to read “Operation at temperatures over 40 °C (100 °F).”

S.4.6 In 4.3.3.6, the factor 2/3Y shall be replaced with 3/4Y.

S.4.7 In 4.3.4, these rules do not cover stainless steel tanks.

S.5 Brittle Fracture

S.5.1 The tank is suitable for continued use in ambient temperature service.

S.6 Inspection

No changes to Section 6.

S.7 Materials

S.7.1 In 7.3.1.2, add reference to ASTM A480.

S.7.2 Structural may be shapes fabricated from plate. Plate and structural material shall meet API 650, Section S.2.

13

12

10

10
S-2 API STANDARD 653

S.8 Design Considerations for Reconstructed Tanks

S.8.1 In 8.4.3, the allowable stress shall be revised to meet the allowable stresses of API 650, Section S.3.2.2.1.

S.9 Tank Repair and Alteration

S.9.1 In applying 9.1.1 to fabrication and construction requirements, API 650, Sections S.4.1 through S.4.9 shall be 

met as applicable.

S.9.2 Hot taps for stainless steels (reference Section 9.14) are not addressed by this annex.

S.10 Dismantling and Reconstruction

S.10.1 In 10.4.2, welding shall also meet the requirements of API 650, Section S.4.11.

S.10.2 Thermal cutting of stainless steel shall be by the iron powder burning, carbon arc, plasma-arc, water jet, or 

laser cutting methods.

S.10.3 The storage requirements of API 650, Section S.4.2 shall be met.

S.10.4 If specified by the owner/operator, the requirements of API 650, Section S.4.5 shall be met.

S.11 Welding

S.11.1 Welding shall also meet the requirements of API 650, Sections S.4.11 and S.4.12.

S.12 Examination and Testing

S.12.1 Any reference to magnetic particle method shall be replaced with the liquid penetrant method.

S.12.2 In 12.3, the quality of test water shall meet API 650, Section S.4.10.2.

S.13 Annexes

S.13.1 Annex F (NDE Requirements Summary)—any references to magnetic particle examination shall be 

disregarded.

12

12

12

10

12

12
SC-1

Annex SC

Stainless and Carbon Steel Mixed Storage Tanks

SC.1 Scope

SC.1.1 This annex covers the inspection, repair, alteration and reconstruction of mixed material tanks constructed in 

accordance with API 650, Annex SC.

SC.1.2 This annex states only the requirements that differ from the basic rules in this standard, Annex S of this 

standard, Annex X of this standard and API 650, Annex SC. For requirements not stated, the basic rules shall be 

followed.

SC.1.3 In this annex the term “stainless steel” includes austenitic or duplex stainless steel unless noted otherwise.

SC.2 References 

No changes to Section 2.

SC.3 Definitions

No changes to Section 3.

SC.4 Suitability for Service

SC.4.1 Add to 4.2.4.1: The requirements of API 650, Sections S.3.5 and API 650, and X.3.6, shall also be satisfied 

for the stainless steel components of the tank.

SC.4.2 Add to 4.2.4.3: This annex applies only to tanks in non-refrigerated services with a maximum design 

temperature not exceeding 93 °C (200 °F). Mixed material tanks operating at temperatures greater than 93 °C 

(200 °F) are not addressed in this annex except for tanks covered in SC.4.3 below. For the purposes of this annex, 

the design temperature shall be the maximum design temperature as specified by the owner/operator.

NOTE Exothermic reactions occurring inside unheated storage tanks can produce temperatures exceeding 40 °C (100 °F). 

SC.4.3 Tanks containing mixed materials which do not meet the temperature limitations specified in SC.4.2, but 

have a successful service history of operation, shall be evaluated for thermal differential expansion at mixed material 

interface in order to remain in continued service. This analysis shall be performed in accordance with API 650 Annex 

SC, Section SC.3 and by an engineer experienced in storage tank design and evaluation methodologies. If no 

accounting for differential expansion effects in prior design work is documented, then such effects shall be evaluated 

at the time of existing tank assessment.

SC.4.4 Add to 4.3.3.1: The maximum allowable stress S shall be modified as follows, for the design condition (Sd) 

and the hydrotest condition (St) the maximum allowable stress for austenitic stainless steel shell courses shall be the 

smaller of 0.95Y or 0.4T.

SC.4.5 Table 4-2—Joint efficiencies for welded joints shall be in accordance with API 650, Table S-4, or API 650, 

Table X-3. When the radiography schedule applied to the existing weld is unknown, then the joint efficiency of 0.7 

shall be used.

SC.4.6 Revise 4.3.3.5.c to read ‘Operation at temperatures over 40 °C (100 °F).’

SC.4.7 Revise 4.3.3.6 by replacing the 2/3Y factor with 3/4Y for austenitic stainless steel components.

10

13

10

13

10
SC-2 API STANDARD 653

SC.4.8 The rules in 4.3.4 for riveted tanks do not cover mixed material tanks.

SC.5 Brittle Fracture Considerations

Evaluation of brittle fracture shall be done according to Section 5 of this standard for carbon steel, Section S.5 of this 

standard for austenitic stainless steel, and Section X.5 of this standard for duplex stainless steel components.

SC.6 Inspection 

No Changes to Section 6.

SC.7 Materials

Materials requirements for mixed materials situations are unchanged from the base document except as modified by 

API 653 Appendices S and X (S.7 and X.7) for stainless steels.

SC.8 Design Considerations for Reconstructed Tanks

The allowable stress in 8.4.2 and 8.4.3 for stainless steel components shall be revised to meet the allowable stresses 

of API 650 Annex S or API 650 Annex X.

SC.9 Tank Repair and Alteration

SC.9.1 Revise 9.2 to read: Shell insert plates shall be made in accordance with API 650, Annex SC 3.2.2.

SC.9.2 Revise 9.3 to read: Lap patches shall be made carbon steel to carbon steel and stainless steel to stainless 

steel

SC.9.3 Revise 9.8 to read: Shell penetrations and reinforcing shall be made in accordance with API 650, Annex SC 

3.4.

SC.9.4 Revise 9.10 to read: Repair of tank bottoms shall be made in accordance with API 650, Annex SC 3.1.

SC.9.5 Revise 9.14 to read: Hot taps in stainless steel are not addressed by this annex.

SC.10 Other 

For Dismantling and Reconstruction, Welding, Examination and Testing, and Annexes see the following sections of 

the basic document: S.10 through S.13 for austenitic stainless steel, and X.10 through X.13 for duplex stainless steel 

components.

10

10

13
X-1

Annex X

Duplex Stainless Steel Storage Tanks

X.1 Scope

X.1.1 This annex covers the inspection, repair, alteration and reconstruction of duplex stainless steel tanks that 

were constructed in accordance with API 650, Annex X.

X.1.2 This annex states only the requirements that differ from the basic rules in this Standard. For requirements not 

stated, the basic rules shall be followed.

X.2 References 

No changes to Section 2.

X.3 Definitions 

No changes to Section 3.

X.4 Suitability for Service

X.4.1 In 4.2.4.1, the requirements of API 650, Section X.3.6 shall also be satisfied.

X.4.2 In 4.2.4.3, the requirements of API 650, Annex M requirements shall be satisfied for duplex stainless steel 

tanks with design temperatures over 40 °C (100 °F) as modified by API 650, X.3.7.2 thru X.3.7.5.

X.4.3 In 4.3.3.1, the maximum allowable stress S shall be calculated the same way as for carbon steel.

X.4.3.1 Y = specified minimum yield strength of the plate at design temperature; use material S32304 properties if 

duplex material/specification is not known.

X.4.3.2 T= specified minimum tensile strength of the plate at design temperature; use material S32304 properties if 

duplex material/specification is not known.

X.4.4 Table 4-2 shall be in accordance with API 650, Table X-3. When the radiography schedule applied to the 

existing weld is unknown; the joint efficiency of 0.7 shall be used.

X.4.5 4.3.3.5c, shall be changed to read “Operation at temperatures over 40 °C (100 °F).”

X.4.6 The rules of 4.3.4 for riveted tanks do not cover duplex stainless steel tanks.

X.5 Brittle Fracture Considerations

X.5.1 In 5.3.2 the applicable API 650 edition and addendum for duplex stainless steel tanks is 11th edition, 

Addendum 1 or later.

X.5.2 5.3.5 does not apply to duplex stainless steel tanks.

X.5.3 The rules of 5.3.8 shall be replaced with the following: Tanks constructed of duplex stainless steels whose 

toughness testing or testing exemption conformed to API 650, X.2.3.2 may be considered to be adequately tough for 

continued operation.

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X-2 API STANDARD 653

X.6 Inspection 

No Changes to Section 6.

X.7 Materials

X.7.1 In 7.3.1.2, add reference to ASTM A480 and A240.

X.7.2 Structural sections may be shapes fabricated from plate. Plate and structural material shall meet API 650, X.2.

X.8 Design Considerations for Reconstructed Tanks

In 8.4.2 and 8.4.3 the allowable stress shall be revised to meet the allowable stresses of API 650, Annex X.

X.9 Tank Repair and Alteration

X.9.1 In applying 9.1.1 to fabrication and construction requirements, API Standard 650, Sections X.4.1 through 

X.4.9 shall be met as applicable except as permitted in X.10.2 of this annex.

X.9.2 Hot taps for duplex stainless steels (reference section 9.14) are not addressed by this annex.

X.10 Dismantling and Reconstruction

X.10.1 In 10.4.2, welding shall also meet the requirements of API 650, Section X.4.11.

X.10.2 Carbon arc cutting shall not be used except when agreed to, in writing, by Purchaser as an exception to 

X.9.1, for certain dismantling operations.

X.10.3 The storage requirements of API 650, Section X.4.2 shall be met.

X.10.4 When specified by the Purchaser the requirements of API 650, Section X.4.5 shall be met.

X.11 Welding

Welding shall also meet the requirements of API 650, Sections X.4.11 and X.4.12.

X.12 Examination and Testing

X.12.1 Any reference to magnetic particle method shall be replaced with the liquid penetrant method.

X.12.2 In 12.3, the quality of test water shall meet API 650, Section X.4.10.

X.13 Annexes

Annex F, NDE Requirements Summary; any reference to magnetic particle method shall be replaced with the liquid 

penetrant method.

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Product No. C65304