Inspection Practices for Atmospheric and Low-Pressure Storage Tanks

API RECOMMENDED PRACTICE 575 THIRD EDITION, APRIL 2014

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Copyright © 2014 American Petroleum Institute

Foreword

This recommended practice is based on the accumulated knowledge and experience of engineers, inspectors and contractors that work with tanks in the oil, gas, petroleum refining, and chemical process industries.

The information presented in this recommended practice does not constitute and should not be construed as a code of rules, regulations, or minimum safe practices. The guidelines described in this publication are not intended to supplant other practices that have proven satisfactory, nor is this publication intended to discourage innovation and originality in the inspection and maintenance of storage tanks. Users of this recommended practice are reminded that no book or manual is a substitute for the judgment of a responsible, qualified person.

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.

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, NW, Washington, DC 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Generally, API standards are reviewed and revised, reaffirmed, or withdrawn at least every five years.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, NW, Washington, DC 20005.

Suggested revisions are invited and should be submitted to the Standards Department, API, 1220 L Street, NW, Washington, DC 20005, standards@api.org.

iii

Contents

Page

1. Scope 1

2. Normative References 1

   1. Codes, Standards, and Related Publications 1

   2. Other References 4

3. Terms and Definitions 4

4. Types of Storage Tanks 6

   1. General 6

   2. Atmospheric Storage Tanks 7

   3. Low-Pressure Storage Tanks 12

5. Reasons for Inspection and Causes of Deterioration 21

   1. Reasons for Inspection 21

   2. Deterioration of Tanks 22

   3. Deterioration of Other than Flat Bottom and Non-steel Tanks 23

   4. Leaks, Cracks, and Mechanical Deterioration 24

   5. Deterioration and Failure of Auxiliary Equipment 27

6. Inspection Plans 27

   1. General 27

   2. Developing an Inspection Plan 28

7. Frequency and Extent of Inspection 30

   1. Frequency of Inspection 30

   2. Condition-based Inspection Scheduling and Minimum Acceptable Thickness 31

   3. Similar Service Methodology for Establishing Tank Corrosion Rates 35

   4. Fitness-For-Service Evaluation 36

8. Methods of Inspection 36

   1. Preparation for Inspections 36

   2. External Inspection of an In-service Tank 38

   3. External Inspection of Out-of-Service Tanks 50

   4. Internal Inspection 54

   5. Testing of Tanks 65

   6. Inspection Checklists 66

9. Leak Testing and Hydraulic Integrity of the Bottom 66

   1. General 66

   2. Leak Integrity Methods Available During Out-of-Service Periods 67

   3. Leak Detection Methods Available During In-Service Periods 71

10. Integrity of Repairs and Alterations 75

    1. General 75

    2. Repairs 75

    3. Special Repair Methods 78

11. Records 80

    1. General 80

    2. Records and Reports 80

    3. Form and Organization 81

v

Contents

Page

Annex A (normative) Selected Non-destructive Examination (NDE) Methods 82

Annex B (normative) Similar Service Evaluation Tables 86

Annex C (normative) Qualification of Tank Bottom Examination Procedures and Personnel 89

Selected Bibliography 95

Figures

1. Cone Roof Tank 8

2. Umbrella Roof Tank 8

3. Geodesic Dome Roof Tank 9

4. Self-supporting Dome Roof Tank 9

5. Pan Type Floating-roof Tank 9

6. Annular-pontoon Floating-roof Tank 10

7. Double-deck Floating-roof Tank 10

8. Cross-section Sketches of Floating-roof Tanks Showing the Most Important Features 11

9. Floating-roof Shoe Seal 12

10. Floating-roof Log Seal 13

11. Floating Roof Using Counterweights to Maintain Seal 14

12. Floating Roof Using Resilient Tube-type Seal 14

13. Cable-supported Internal Floating Roof Tank 14

14. Typical Internal Floating-roof Components 15

15. Typical Arrangement for Metallic Float Internal Floating-roof Seals 16

16. Plain Breather Roof Tanks 17

17. Tank with Vapor Dome Roof 17

19 Cutaway View of Vapor Dome Roof 17

18 Balloon Roof Tank 17

1. Welded Horizontal Tank Supported on Saddles 18

2. Plain Hemispheroids 19

3. Noded Hemispheroid 19

4. Drawing of Hemispheroid 19

5. Plain Spheroid 19

6. Plain Hemispheroid with Knuckle Radius 20

7. Noded Spheroid 20

8. Drawing of Noded Spheroid 20

9. Foundation Seal 23

10. Cracks in Tank Shell Plate 25

11. Extensive Destruction from Instantaneous Failure 25

12. Cracks in Bottom Plate Welds Near the Shell-to-bottom Joint 26

13. Cracks in Tank at Riveted Lap Joint to Tank Shell 26

14. Hypothetical Corrosion Rate Curve for Top Course of Storage Tank 32

15. Failure of Concrete Ringwall 40

16. Anchor Bolt 40

17. Corrosion of Anchor Bolts 40

18. Corrosion Under Insulation 42

19. Close-up of Corrosion Under Insulation 42

20. Corrosion (External) at Grade 43

21. Caustic Stress Corrosion Cracks 44

22. Small Hydrogen Blisters on Shell Interior 46

    vi

    Contents

    Page

23. Large Hydrogen Blisters on Shell Interior 46

24. Tank Failure Caused by Inadequate Vacuum Venting 47

25. Roof Overpressure 47

26. Example of Sever Corrosion of Tank Roof 51

27. Deterioration of Floating-roof Seal 52

28. Collapse of Pan-type Roof from Excessive Weight of Water While the Roof was Resting

    on its Supports 52

29. Pontoon Floating-roof Failure 53

30. Tank Buggy Used for Inspection and Repairs Inside of Tank 55

31. Remote Control Automated Crawler 55

32. Example of Vapor-liquid Line Corrosion 56

33. Corrosion Behind Floating-roof Seal 57

34. Localized Corrosion-erosion at Riveted Seam in a Tank Bottom 59

35. Example of Extensive Corrosion of a Tank Bottom 59

36. Shell-to-bottom Weld Corrosion 61

37. External View of Erosion-corrosion Completely Penetrating a Tank Shell 61

38. Deterioration of Lining on Roof of Tank Caused by Leaks in Lining 62

39. Internal Corrosion on Rafters and Roof Plates 63

40. Failure of Roof Supports 64

41. Fin-tube Type of Heaters Commonly Used in Storage Tanks 64

42. Example of Corrosion of Steam Heating Coil 65

43. Hydraulic Integrity Test Procedures 67

44. Vacuum Box Used for Testing Leaks 68

45. Vacuum Test Box Arrangement for Detection of Leaks in Vacuum Seals 69

46. Helium Tester 71

47. Method of Repairing Tank Bottoms 76

48. Temporary “Soft Patch” Over Leak in Tank Roof 78

69 Tank Jacked Up for Repairing Pad 79

68 Mastic Roof Coating 79

1. Automatic UT 84

2. MFL Scanner 84

3. UT Scrub 85

4. Robotic Inspection Tool 85

Tables

1. Tools for Tank Inspection 37

2. Useful Supplemental Tools 37

1. Selected Factors for Using Similar Service Principles in Estimating Corrosion Rates

   for Tank Bottoms 86

2. Similar Service Example for Product-side Corrosion 88

C.1 Suggested Items that May Be Considered as Essential Variables for the Qualification Test 94

vii

Inspection Practices for Atmospheric and Low-Pressure Storage Tanks

1. Scope

   
   

   This document provides useful information and recommended practices for the maintenance and inspection of atmospheric and low-pressure storage tanks. While these maintenance and inspection guidelines may apply to other types of tanks, these practices are intended primarily for existing tanks which were constructed to one of the following four standards: API 12A, API 12C, API 620, or API 650. This document addresses the following:

   
   

   1. descriptions and illustrations of the various types of storage tanks;

      
      

   2. new tank construction standards;

      
      

   3. maintenance practices;

      
      

   4. reasons for inspection;

      
      

   5. causes of deterioration;

      
      

   6. frequency of inspection;

      
      

   7. methods of inspection;

      
      

   8. inspection of repairs;

      
      

   9. preparation of records and reports;

      
      

   10. safe and efficient operation;

       
       

   11. leak prevention methods.

   
   

   This Recommended Practice (RP) is intended to supplement API 653, which provides minimum requirements for maintaining the integrity of storage tanks after they have been placed in service.

   
   

2. Normative References

2.1 Codes, Standards, and Related Publications

The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

API Specification 12A, Specification for Oil Storage Tanks with Riveted Shells (withdrawn) API Specification 12B, Bolted Tanks for Storage of Production Liquids

API Specification 12C, API Specification for Welded Oil Storage Tanks (withdrawn) API Specification 12D, Field Welded Tanks for Storage of Production Liquids

API Specification 12E, Specification for Wooden Production Tanks (withdrawn)

1