By Authority Of THE UNITED STATES OF AMERICA Legally Binding Document By the Authority Vested By Part 5 of the United States Code § 552(a) and Part 1 of the Code of Regulations § 51 the attached document has been duly INCORPORATED BY REFERENCE and shall be considered legally binding upon all citizens and residents of the United States of America. HEED THIS NOTICE: Criminal penalties may apply for noncompliance. e Document Name: CFR Section(s): Standards Body: Official Incorporator: THE EXECUTIVE DIRECTOR OFFICE OF THE FEDERAL REGISTER WASHINGTON, D.C. Design and Construction of Large, Welded, Low-Pressure Storage Tanks Downstream Segment API STANDARD 620 lENTH EDITION, FEBRUARY 2002 American Petroleum Institute Helping You Get The Job Done Right;M SPECIAL NOTES API publications necessarily address problems of a general nature. With respect to partic ular circumstances, local, state, and federal laws and regulations should be reviewed. API is not undertaking to meet the duties of employers, manufacturers, or suppliers to warn and properly train and equip their employees, and others exposed, concerning health and safety risks and precautions, nor lmdertaking their obligations lmder local, state, or fed erallaws. Information concerning safety and health risks and proper precautions with respect to par ticular materials and conditions should be obtained from the employer, the manufacturer or supplier of that material, or the material safety data sheet. 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 prod uct covered by letters patent. Neither should anything contained in the publication be con strued as insuring anyone against liability for infringement of letters patent. Generally, API standards are reviewed and revised, reaffirmed, or withdrawn at least every five years. Sometimes a one-time extension of up to two years will be added to this review cycle. Tllis publication will no longer be in effect five years after its publication date as an operative APr standard or, where an extension has been granted, upon republication. Status of the publication can be ascertained from the API Downstream Segment [telephone (202) 682-8000]. A catalog of API publications and materials is published armually and updated quarterly by API, 1220 L Street, N.W., Washington, D.C. 20005. This document was produced under API standardization procedures that ensure appropri ate notification and participation in the developmental process and is designated as an API standard. Questions concerning the interpretation of the content of this standard or com ments and questions concerning the procedures under which this standard was developed should be directed in writing to the director, Standards Department, American Petroleum Institute, 1220 L Street, N.W., Washington, D.C. 20005. Requests for permission to repro duce or translate all or any part of the material published herein should also be addressed to the general manager. API standards are published to facilitate the broad availability of proven, sOlmd engineer ing and operating practices. These standards are not intended to obviate the need for apply ing sOlmd engineering judgment regarding when and where these standards should be utilized. The fonnulation and publication of API standards 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 prod ucts do in fact conform to the applicable API standard. All rights reserved. No pari oft his work may be reproduced, stored in a retrieval system, or transmitted by any means, electronic, mechanical. photocopying, recording, or otherwise, without prior written pemzissionfinm the publisher. Contact the Publisher, API Publishing Services, 1220 L Street, N W, Washington, D. C. 20005. Copyright '0 2002 American Petroleum Institute FOREWORD This standard is based on the accumulated knowledge and experience of purchasers and manufacturers of welded, low-pressure oil storage tanks of various sizes and capacities for intemal pressures not more than 15 pounds per square inch gauge. The object of this publi cation is to provide a purchase specification to facilitate the manufacture and procurement of such storage tanks. If tanks are pmchased in accordance with the specifications ofthis standard, the purchaser is required to specify certain basic requirements. The purchaser may desire to modify, delete, or amplity sections of this standard, but reference shall not be made to this standard on the nameplate or manufacturer's certification for tanks that do not fulfill the minimum require ments or that exceed the limitations of this standard. It is strongly recommended that such modifications, deletions, or amplifications be made by supplementing this standard, rather than by rewriting or incorporating sections of it into another complete standard. 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 issu ance for equipment that is certified by the manufacturer as being designed, fabricated, con stmcted, examined, 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 man ufacturer shall specify to which edition, revision, or addenda the equipment is to be built. The design mles given in this standard are minimum requirements. More stringent design mles specified by the purchaser or furnished by the manufacturer are acceptable when mutu ally agreed upon by the purchaser and the manufacturer. This standard is not to be inter preted as approving, recommending, or endorsing any specific design, nor as limiting the method of design or construction. This standard is not intended to cover storage tanks that are to be erected in areas subject to regulations more stringent than the specifications of this standard. When this standard is specified for such tanks, it should be followed insofar as it does not conflict with local requirements. After revisions to this standard have been issued, they may be applied to tanks to be com pleted after the date of issue. The tank nameplate shall state the date of the edition and any revision to that edition to which the tank is designed and constmcted. 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 ofthe 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 federal, state, or municipal regulation with which this publication may conflict. Suggested revisions are invited and should be submitted to the standardization manager, American Petrolellm lnstitute, 1220 L Street, N.W., Washington, D.C. 20005. iii CONTENTS Page SCOPE ........................................................................ 1-1 1. 1 General. .................................................................. 1-1 1.2 Coverage ................................................................. 1-1 101 1.3 Limitations ................................................................ 1-2 2 REFERENCES ................................................................. 2-1 101 3 DEFINITIONS ................................................................. 3-1 3.1 Stress and Pressure Tenns .................................................... 3-1 3.2 Capacity Tenns ............................................................ 3-1 3.3 Tank Wall ................................................................. 3-1 3.4 Welding Tenns ............................................................. 3-1 4 MATERIALS .................................................................. 4-1 4.1 General. .................................................................. 4-1 4.2 Plates .................................................................... 4-1 101 4.3 Pipe, Flanges, Forging, and Castings .......................................... .4-5 4.4 Bolting Material. .......................................................... .4-6 4.5 Stnlctural Shapes ........................................................... 4-6 1 01 5 DESIGN ....................................................................... 5-1 5.1 General. .................................................................. 5-1 5.2 Operating Temperature ...................................................... 5-1 5.3 Pressures Used In Design .................................................... 5-1 5.4 Loadings .................................................................. 5-2 5.5 Maximum Allowable Stress for Walls ........................................... 5-2 101 5.6 Maximum Allowable Stress Values for Stnlctural Members and Bolts ................. 5-8 5.7 Corrosion Allowance ........................................................ 5-8 5.8 Linings ................................................................... 5-8 5.9 Procedure for Designing Tank Walls ............................................ 5-8 5.10 Design of Sidewalls, Roofs, and Bottoms ....................................... 5-12 5.11 Special Considerations Applicable to Bottoms That Rest Directly on Foundations ...... 5-18 5.12 Design Of Roof And Bottom Knuckle Regions and Compression-ring Girders ......... 5-20 5.13 Design OfInternal And External Stmctural Members ............................. 5-24 5.14 Shapes, Locations, and Maximum Sizes of Wall Openings ......................... 5-26 5.15 Inspection Openings ....................................................... 5-27 5.16 Reinforcement of Single Openings ............................................ 5-27 5.17 Reinforcement of Multiple Openings .......................................... 5-35 5.18 Design of Large, Centrally Located, Circular Openings in Roofs and Bottoms ......... 5-36 5.19 Nozzle Necks and Their Attachments to the Tank ................................ 5-37 5.20 Bolted Flanged Connections ................................................. 5-38 5.21 Cover Plates .............................................................. 5-38 5.22 Permitted Types of Joints ................................................... 5-42 5.23 Welded Joint Efficiency ..................................................... 5-42 5.24 Plug Welds and Slot Welds .................................................. 5-43 5.25 Stress Relieving ........................................................... 5-43 5.26 Radiography .............................................................. 5-44 5.27 Flush-Type ShellConnectiol1 ................................................ 5-44 101 6 FABRlCATION ............................................... , ................ 6-1 6.1 General. .................................................................. 6-1 6.2 Workmanship .............................................................. 6-1 6.3 Cutting Plates .............................................................. 6-1 6.4 Fonning Sidewall Sections and Roof and Bottom Plates ............................ 6-1 v Page 6.5 Dimensional Tolerances ..................................................... 6-1 6.6 Details of Welding .......................................................... 6-2 6.7 Qualification of Welding Procedme ............................................ 6-2 101 6.8 Qualification of Welders ..................................................... 6-3 6.9 Matching Plates ............................................................ 6-3 6.1 0 Cleaning Smfaces to be Welded ............................................... 6-3 6.11 Weather Conditions for Welding ............................................... 6-3 6.12 Reinforcement on Welds ..................................................... 6-4 6.13 Merging Weld With Plate Surface .............................................. 6-4 6.14 Aligning of Main Joints ......................... " ........................... 6-4 6.15 Repairing Defects in Welds ................................................... 6-4 6.16 Matching Plates of Unequal Thickness .......................................... 6-4 6.17 Fitting Up of Closure Plates .................................................. 6-4 6.18 Thennal Stress Relief ....................................................... 6-4 6.19 Peening Field Welds ........................................................ 6-5 7 INSPECTION AND TESTING .................................................... 7-1 7. 1 Responsibility of Inspector ................................................... 7-1 7.2 Qualifications ofInspectors ...................... " ........................... 7-1 7.3 Access for Inspector ............................................... , ........ 7-1 7.4 Facilities for Inspector ....................................................... 7-1 7.5 Approval of Repairs ......................................................... 7-1 7.6 Inspection of Materials ...................................................... 7-1 7.7 Stamping of Plates ............................. '" ................. , ........ 7-1 7.8 Measuring Thickness of Material ..... , ........... " ........................... 7-1 7.9 Inspection of Surfaces Exposed Dming Fabrication ............................... 7-1 7.10 Surface Inspection of Component Parts .. '" ........... , ........................ 7-2 7.11 Check of Dimensions of Component Parts ....................................... 7-2 7.12 Check of Chemical and Physical Property Data ................................... 7-2 7.13 Data Required From Manufactmer on Completed Tanks ............................ 7-2 7.14 Check of Stress-Relieving Operation .. , ........... " .................. , .... " .. 7-2 7.15 Examination Method and Acceptance Criteria ....... " ....................... " .. 7-2 1 01 7.16 Inspection of Welds ......................................................... 7-4 7.17 Radiographic Examination Requirements ....................................... 7-4 7.18 Standard Hydrostatic and Pneumatic Tests ....................................... 7-5 7.19 Proof Tests for Establishing Allowable Working Pressmes .......................... 7-8 7.20 Test Gauges ............................................................... 7-8 8 MARKING .................................................................... 8-1 8.1 Nameplates ............................................................... 8-1 8.2 Division of Responsibility .................................................... 8-1 8.3 Manufactmer's Report and Certificate ., ........................................ 8-1 8.4 Multiple Assemblies ........................................................ 8-1 9 PRESSURE- AND VACUUM-RELIEVING DEVICES ................................ 9-1 9.1 Scope .................................................................... 9-1 9.2 Pressme Limits ............................................................ 9-1 9.3 Constmction of Devices ..................................................... 9-1 9.4 Means of Venting ........................................................... 9-1 9.5 Liquid Relief Valves ........................................................ 9-1 9.6 Marking ........... " ............ , ........................................ 9-1 9.7 Pressme Setting of Safety Devices ............................................. 9-2 APPENDIX A TECHNlCAL INQUIRY RESPONSES ................................. A-I APPENDIXB USE OF MATERIALS THAT ARE NOT IDENTIFIED WITH LISTED SPECIFIC AT [ONS .................................................. B-1 APPENDIXC SUGGESTED PRACTICE REGARDING FOUNDATIONS ................ C-1 vi Page APPENDIXD SUGGESTED PRACTICE REGARDING SUPPORTING STRUCTURES .... D-l APPENDlXE SUGGESTED PRACTICE REGARDING ATTACHED STRUCTURES (INTERNAL AND EXTERNAL) ....................................... E-I APPENDlXF EXAMPLES ILLUSTRATING APPLICATION OF RULES TO VARIOUS DESIGN PROBLEMS ................................................ F-I APPENDlXG CONSIDERATIONS REGARDlNG CORROSION ALLOWANCE AND HYDROGEN-INDUCED CRACKING ................................. G-l APPENDlXH RECOMMENDED PRACTICE FOR USE OF PREHEAT, POST-HEAT, AND STRESS RELIEF .............................................. H-l APPENDIX I SUGGESTED PRACTICE FOR PEENING ............................... I-I APPENDlXJ (RESERVED FOR FUTURE USE) ...................................... J-I APPENDlXK SUGGESTED PRACTICE FOR DETERMINING THE RELIEVING CAPACITY REQUIRED ............................................. K-J APPENDlXL SEISMJC DESIGN OF STORAGE TANKS ............................... L-I APPENDlXM RECOMMENDED SCOPE OF THE MANUFACTURER'S REPORT ........ M-I APPENDIXN INST ALLA TION OF PRESSURE-RELIEVING DEVICES ................ N-l APPENDlXO SUGGESTED PRACTICE REG ARDING INST ALLAT ION OF LOW-PRESSURE STORAGE TANKS ................................. 0-1 APPENDlXP NDE AND TESTING REQUIREMENTS SUMMARY ..................... P-l APPENDIXQ LOW-PRESSURE STORAGE TANKS FOR LIQUEFIED HYDROCARBON GASES ........................................... Q-I 01 APPENDlXR LOW-PRESSURE STORAGE TANKS FOR REFRIGERATED PRODUCTS ....................................................... R-l Figures 4-1 Isothennal Lines Showing I-Day Mean Ambient Temperature ........................ 4-2 4-2 MinimLU11 Permissible Design Metal Temperature .................................. 4-7 4-3 Governing Thickness for Impact Test Determination of Pipe, Flanges, and Forgings ....... 4-8 5-1 Biaxial Stress Chart for Combined Tension and Compression, 30,000-38,000 psi Yield Strength Steels ......................................................... 5-3 5-2 Method for Preparing Lap-Welded Bottom Plates Under the Tank Sidewall ............ 5-11 5-3 Detail of Double Fillet-Groove Weld for Bottom Plates with a Nominal Thickness Greater than 1/2 in ........................................................... 5-11 5-4 Typical Free-Body Diagrams for Certain Shapes of Tanks .......................... 5-14 5-5 Compression-Ring Region .................................................... 5-22 5-6 Permissible and Nonpermissible Details of Construction for a Compression-Ring Jlmcture ................................................... 5-23 5-7 Reinforcement of Single Openings ............................................. 5-28 101 5-8 Part I-Acceptable Types of Welded Nozzles and Other Connections ................. 5-29 5-8 Part 2-Acceptable Types of Welded Nozzles and Other Connections ................. 5-30 5-8 Part 3-Acceptable Types of Welded Nozzles and Other Connections ................. 5-31 101 5-8 Part 4---Acceptable Types of Welded Nozzles and Other Connections ................. 5-32 5-9 Large Head Openings and Conical Shell-Reducer Sections .......................... 5-36 5-10 Acceptable Types of Flat Heads and Covers ...................................... 5-39 5-11 Spherically Dished Steel Plate Covers with Bolting Flanges ......................... 5-41 5-12 Part 1-FI ush-Type Sidewall Connection ........................................ 5-46 5-12 Part 2-Flush-Type Sidewall Connection ........................................ 5-47 5-13 Design Factors for Flush-Type Connections ...................................... 5-48 5-14 Rotation of Sidewall Connection ............................................... 5-49 6-1 Butt Welding of Plates of Unequal Thickness ...................................... 6-5 8-1 Nameplate ................................................................. 8-2 F-l Reduction of Design Stresses Required to Allow for Biaxial Stress of the Opposite Sign ... F-2 F-2 Examples Illustrating the Use of a Biaxial Stress Chart for Combined Tension and Compression, 30,000-38,000 POlmds per Square Inch Yield Strength Steels ............. F-5 F-3 Fonn for Use in Graphical Solutions of Problems Involving Biaxial Tension and Compression, 30,000-38,000 Pounds per Square Inch Yield Strength Steels ............. F-6 vii Page F-4 Free-Body Sketch ............................................................ F-S F-5 ExampleofaReinforcedOpening ............................................. F-I3 F-6 Example ofa Reinforced Opening ............................................. F-I5 F- 7 Example of a Reinforced Opening ............................................. F -IS F-S Example of aRei nf orced Opening ............................................. F-I9 L-l Seismic Zone Map ........................................................... L-3 L-l Part 2-Seismic Zone Map .................................................... L-4 L-2 Clrrves for Obtaining Factors W\/WT and WzIWTfor the Ratio DIII .................... L-5 L-3 ClITVes for Obtaining FactorsX1IHand XzIHfor the RatioDIH ....................... L-5 L-4 Clrrve for Obtaining Factor k for the Ratio f)IH .................................... L-5 L-5 Clrrve for Obtaining the Value of b when MI[D2(Wt+WrJ] Exceeds 0.7S5 ............... L-7 Q-l Typical Stiffening-Ring Weld Details ........................................... Q-9 Q-2 Radiographic Requirements tor Butt-Welded Shell Joints in Cylindrical Flat-Bottom Tanks .................... , ........... '" .... , .. " , .. , '" .. , ....... Q-14 R-l Typical Stiffening-Ring Weld Details ........................................... R-5 R-2 Radiographic Requirements for Butt-Welded Shell Joints in Cylindrical Flat-Boltom Tanles ....... , ............ , .................................... , ... R-II Tables 4-1 Minimwn Requirements for Plate Specifications to be Used for Design Metal TemperatLrres ......................................................... 4-3 4-2 MaxirmUTI Permissible Alloy Content ........................................... 4-4 5-1 Maximml1 Allowable Stress Values for Simple Tension. , ........................... 5-4 5-2 Maximwn Allowable Efficiencies for Arc-Welded Joints ............................ 5-7 5-3 Maximl11Jl Allowable Stress Values for StructLrral Members ........................ 5-10 5-4 Sidewall-to-Bottom Fillet Weld for Flat-Bottom Cylindrical Tanks ........... , ....... 5-11 5-5 Factors for Determining Values of R I and Rz for Elli psoidal Roofs and Bottoms, ....... 5-15 5-6 Tank Radius Versus Nominal Plate Thickness ........................... , ....... 5-17 5-7 Allowable Tension Stresses for Uplift Pressme Conditions ......................... 5-19 5-S Minimwn Size ofFillet Weld ...................... , .......................... 5-24 5-9 Factors for Detennining Values of kfor Compression-Ring Bracing .................. 5-25 5-10 Dimensions of Flush-Type Shell Connections (Inches) .................... , ....... 5-45 6-1 Diameter Range Versus Radius Tolerance ............................... , ........ 6-1 6-2 Maximmn Thickness of Reinforcement on Welds ................................. 6-4 6-3 Stress-Relievillg Temperatmes and Holding Times .... , ........................... 6-5 7-1 Maximml1 Thickness of Reinforcement on Welds for Radiography Examined Joints ..... 7-2 F-l Computed Values of(t - c)R, So Sf, and N for the Assmned Thicknesses: Example 1. ..... F-4 F-2 Computed Values of(t - c)R, Se, Sf, and N forthe Assmned Thicknesses: Example 2 ...... F-7 F-3 Cross-Sectional Area of Standard Angles: Example 1 .. , ...... , ................... F-l 0 L-l Seismic Zone Tabulation for Some Areas Outside the United States ................... L-l L-2 Seismic Zone Factor (Horizontal Acceleration) ., ..... , ........................... L-2 L-3 Site Coefficients .......................... , ..... , ........................... L-6 Q-l ASTM Materials for Primary Components ........... , .................... , ..... Q-2 Q-2 CharpyV-Notch Impact Values .................... , .......................... Q-3 Q-3 Maximwn Allowable Stress Values ............................................ Q-5 Q-4A MinimlUTI Thickness for the Annular Bottom Plate: Steel Tanks ..................... Q-7 Q-4B Minimum Thickness for the Annular Bottom Plate: Alwninum Tanks ................ Q-7 Q-5 Nominal Thickness of inner Tank Cyli ndrical Sidewall Plates ...................... Q-I0 Q-6 Radi us Tolerances for the Inner Tank Shell ........... , ......................... Q-IO R-l Material for Primary Components .................. , .......................... R-2 R-2 Minimum Charpy V-Notch [mpact Requirements for Primary-Component Plate Specimens (Transverse) and Weld Specimens Including the Heat-Affected Zone ....... R-3 R-3 Material for Secondary Components .......... , . , .............................. R-3 R-4 MinimlUTI Permissible Design Metal Temperatme for Plates Used as Secondary Components Without Impact Testing ............... , ......... , ............. , .. R-4 R-5 Minimllm Charpy V-Notch Impact Requirements for Secondary-Component Plate Specimens (Transverse) .. , .... , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. R- 7 R-6 Thickness Requirements for the Annular Bottom Plate ............................ R-S viii