Inspection Practices for Atmospheric and Low-pressure Storage Tanks

API RECOMMENDED PRACTICE 575 FOURTH EDITION, JULY 2020

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iii

Contents

Page

1. Scope 1

2. Normative References 1

   1. Codes, Standards, and Related Publications 1

   2. Other References 1

3. Terms and Definitions 4

4. Types of Storage Tanks 9

   1. General 9

   2. Atmospheric Storage Tanks 10

   3. Low-pressure Storage Tanks 23

5. Reasons for Inspection and Causes of Deterioration 28

   1. Reasons for Inspection 28

   2. Deterioration of Tanks 29

   3. Deterioration of Other than Flat Bottom and Non-steel Tanks 31

   4. Leaks, Cracks, and Mechanical Deterioration 32

   5. Deterioration and Failure of Auxiliary Equipment 35

6. Inspection Plans 36

   1. General 36

   2. Inspections Planning and Reports 37

   3. RBI Plans 39

7. Interval/Frequency and Extent of Inspection 40

   1. Interval of Inspection 40

   2. Condition-based Inspection Scheduling. 41

   3. Inspection Scheduling Based on Minimum Acceptable Thickness 41

   4. Similar Service Methodology for Establishing Tank Corrosion Rates 43

   5. Fitness-For-Service Evaluation 45

8. Inspections 45

   1. Preparation for Inspections 45

   2. External Inspection of an In-service Tank 48

   3. External Inspection of Out-of-service Tanks 62

   4. Internal Inspection 66

   5. Hydrostatic and Pneumatic Testing of Tanks 79

   6. Inspection Checklists 80

9. Leak Testing and Hydraulic Integrity of the Bottom 80

   1. General 80

   2. Leak Integrity Methods Available During Out-of-service Periods 82

   3. Leak Detection Methods Available During In-service Periods 86

10. Integrity of Repairs and Alterations 90

    1. General 90

    2. Repairs 90

    3. Special Repair Methods 93

11. Records 95

    1. General 95

    2. Records and Reports 95

    3. Form and Organization 97

v

Contents

Page

Annex A (informative) Selected NDE Methods 98

Annex B (informative) Similar Service Evaluation Tables 103

Bibliography 106

Figures

1. Cone Roof Tank 11

2. Umbrella Roof Tank 12

3. Self-supporting Aluminum (Geodesic) Dome Roof Tank 12

4. Self-supporting Dome Roof Tank 13

5. Externally Stiffened Pan-type Floating-roof Tank 13

6. Annular-pontoon Floating-roof Tank. 14

7. Double-deck Floating-roof Tank 14

8. Cross-section Sketches of Floating-roof Tanks Showing the Most Important Features 15

9. Floating-roof Shoe Seal 16

10. Floating-roof Log Seal 16

11. Floating Roof Using Counterweights to Maintain Seal 17

12. Floating Roof Using Resilient Tube-type Seal 17

13. Typical Arrangement for Metallic Float Internal Floating-roof Seals 18

14. Typical Internal Floating-roof Components 19

15. Cable-supported Internal Floating-roof Tank 19

16. Plain Breather Roof Tanks 21

17. Tank with Vapor Dome Roof 22

18. Balloon Roof Tank 22

19. Cutaway View of Vapor Dome Roof. 23

20. Plain Hemispheroids 24

21. Noded Hemispheroid 25

22. Drawing of Hemispheroid 25

23. Plain Spheroid 26

24. Plain Hemispheroid with Knuckle Radius. 26

25. Noded Spheroid 27

26. Drawing of Noded Spheroid 27

27. Foundation Seal 30

28. Cracks in Tank Shell Plate 33

29. Extensive Destruction from Instantaneous Failure 34

30. Cracks in Bottom Plate Welds Near the Shell-to-bottom Joint 34

31. Cracks in Tank at Riveted Lap Joint to Tank Shell 35

32. Failure of Concrete Ringwall 49

33. Anchor Bolt 50

34. Corrosion of Anchor Bolts 50

35. Corrosion Under Insulation 51

36. Close-up of Corrosion Under Insulation 52

37. Corrosion (External) at Grade 53

38. Caustic Stress Corrosion Cracks 55

39. Small Hydrogen Blisters on Shell Interior 56

40. Large Hydrogen Blisters on Shell Interior 56

41. Tank Failure Caused by Inadequate/Obstructed Vacuum Venting 58

42. Roof Overpressure 59

43. Example of Severe Corrosion of Tank Roof 64

    vi

    Contents

44. Collapse of Pan-type Roof from Excessive Weight of Water

    Page

    While the Roof Was Resting on Its Supports 65

45. Pontoon Floating-roof Failure 65

46a Rolling Scaffold Used for Inspection and Repairs Inside of Tank 67

46b Tank Buggy Used for Inspection and Repairs on Exterior of Tank 67

1. Remote Control Automated Crawler 68

2. Example of Vapor-Liquid Line Corrosion 70

3. Corrosion Behind Floating-roof Seal 70

4. Localized Corrosion-erosion at Riveted Seam in a Tank Bottom 73

5. Example of Extensive Corrosion of a Tank Bottom 73

6. Shell-to-bottom Weld Corrosion 74

7. External View of Erosion-corrosion Completely Penetrating a Tank Shell 75

8. Internal Corrosion on Rafters and Roof Plates 77

9. Failure of Roof Supports 77

10. Fin-tube Type of Heaters Commonly Used in Storage Tanks 78

11. Example of Corrosion of Steam Heating Coil 78

12. Hydraulic Integrity Test Procedures 81

13. Vacuum Box Used to Test for Leaks 83

14. Vacuum Box Arrangement for Detection of Leaks in Vacuum Seals 84

15. Helium Tester 85

16. Method of Repairing Tank Bottoms 92

17. Temporary “Soft Patch” Over Leak in Tank Roof 94

18. Mastic Roof Coating 94

19. Tank Jacked Up for Repairing Pad 95

1. Automatic Ultrasonic Testing 100

2. MFL Scanner 100

3. Ultrasonic Examination 101

4. Guided Wave Ultrasonic Testing 101

5. Robotic Inspection Tool 102

Tables

1. Tools for Tank Inspection 46

2. Useful Supplemental Tools 47

1. Selected Considerations for Performing Similar Service Assessments 103

2. Similar Service Example for Product-side Corrosion 105

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 that were constructed to one of the following four standards: API Std 12A, API Spec 12C, API Std 620, or API Std 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 is intended to supplement API Std 653, which provides minimum requirements for maintaining the integrity of storage tanks after they have been placed in service.

   
   

2. Normative References

   
   

   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 Standard 653, Tank Inspection, Repair, Alteration, and Reconstruction

      
      

   2. Other References

The following codes and standards are cited in the text of this recommended practice or included in the knowledge base to develop this document. Familiarity with these documents is suggested as they provide additional information pertaining to the inspection and repair of aboveground storage tanks.

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