Sizing, Selection, and Installation of Pressure-relieving Devices

Part I—Sizing and Selection

API STANDARD 520

NINTH EDITION, JULY 2014

Special Notes

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Copyright © 2014 American Petroleum Institute

Foreword

API Standard 520, Sizing, Selection, and Installation of Pressure-relieving Devices, is the result of several years' work by engineers in the petroleum industry. The information in this standard is intended to supplement the information contained in Section VIII—Pressure Vessels, of the ASME Boiler and Pressure Vessel Code. The recommendations presented in this publication are not intended to supersede applicable laws and regulations.

Users of this standard are reminded that no publication of this type can be complete, nor can any written document be substituted for qualified engineering analysis.

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.

The current edition of this standard, published in two parts, has been updated with respect to the practices generally used in the installation of all devices covered in the previous editions; the current edition also contains additional information based on revisions suggested by many individuals and several organizations.

The First Edition of this standard was initially released as a recommended practice in 1955. The Second Edition was published in two parts: Part I, Design, in 1960 and Part II, Installation, in 1963. The Third Edition of Part I was issued in November 1967 and reaffirmed in 1973. The Fourth Edition was issued in December 1976, the Fifth Edition was issued in July 1990, the Sixth Edition was issued in March 1993, the Seventh Edition was issued in January 2000, and the Eighth Edition was issued in December 2008.

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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

3. Terms and Definitions 2

4. Pressure-relief Devices 8

   1. General 8

   2. Pressure-relief Valves (PRVs) 8

   3. Rupture Disk Devices 27

   4. Pin-actuated Devices 41

   5. Open Flow Paths or Vents 44

   6. Other Types of Devices 44

5. Procedures for Sizing 44

   1. Determination of Relief Requirements 44

   2. API Effective Area and Effective Coefficient of Discharge 45

   3. Backpressure 46

   4. Relieving Pressure 50

   5. Development of Sizing Equations 54

   6. Sizing for Gas or Vapor Relief 55

   7. Sizing for Steam Relief 72

   8. Sizing for Liquid Relief: PRVs Requiring Capacity Certification 75

   9. Sizing for Liquid Relief: PRVs Not Requiring Capacity Certification 79

   10. Sizing for Two-phase Liquid/Vapor Relief. 80

   11. Sizing for Rupture Disk Devices 81

   12. Sizing for Open Flow Paths or Vents 82

Annex A (informative) Rupture Disk Device Specification Sheet 83

Annex B (informative) Review of Flow Equations Used in Sizing Pressure-relief Devices 88

Annex C (informative) Sizing for Two-phase Liquid/Vapor Relief 109

Annex D (informative) Pressure-relief Valve Specification Sheets 129

Annex E (informative) Capacity Evaluation of Rupture Disk and Piping System 100 % Vapor Flow and Constant Pipe Diameter 137

Bibliography 143

Figures

1. Conventional PRV with a Single Adjusting Ring for Blowdown Control 9

2. Balanced-bellows PRV 10

3. Balanced-bellows PRV with an Auxiliary Balanced Piston 11

4. Conventional PRV with Threaded Connections 12

5. PRV Operation-Vapor/Gas Service 13

6. Typical Relationship Between Lift of Disk in a PRV and Vessel Pressure 14

7. PRV Operation-Liquid Service 15

8. Typical Effects of Superimposed Backpressure on the Opening Pressure of Conventional PRVs 17

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   Contents

   Page

9. Typical Effects of Backpressure on the Set Pressure of Balanced PRVs 18

10. Pop-action Pilot-operated Valve (Flowing Type) 19

11. Pop-action Pilot-operated Valve (Nonflowing Type) 20

12. Modulating Pilot-operated Valve (Flowing Type) 21

13. Pilot-operated Relief Valve with a Nonflowing Modulating Pilot Valve 32

14. Low-pressure Pilot-operated Valve (Diaphragm Type) 23

15. Pressure Level Relationships for PRVs 24

16. Typical Relationship Between Lift of Disk or Piston and Vessel Pressure in a Pop-action

    Pilot-operated PRV 25

17. Typical Relationship Between Lift of Disk or Piston and Vessel Pressure in a Modulating-action

    Pilot-operated PRV 25

18. Pressure Level Relationships for Rupture Disk Devices 29

19. Rupture Disk Device in Combination with a PRV 30

20. Forward-acting Solid Metal Rupture Disk 32

21. Forward-acting Scored Rupture Disk 33

22. Forward-acting Composite Rupture Disk 34

23. Reverse-acting Rupture Disk with Knife Blades 35

24. Reverse-acting Scored Rupture Disk 36

25. Graphite Rupture Disk 37

26. Rupture Disk Application Parameters Assuming No Superimposed Backpressure 39

27. Common Types of Manufacturing Ranges and Corresponding Burst Pressure Marking 40

28. Rupture Disk Application Parameters with Superimposed Backpressure 42

29. Buckling Pin Valve 43

30. Backpressure Correction Factor, Kb, for Balance Spring-loaded PRV (Vapors and Gases) 48

31. Capacity Correction Factor, Kw, Due to Backpressure on Balanced Spring-loaded PRVs in

    Liquid Service 49

32. Curve for Evaluating Coefficient C in the Flow Equation from the Specific Heat Ratio, Assuming

    Ideal Gas Behavior (USC Units) 61

33. Curve for Evaluating Coefficient C in the Flow Equation from the Specific Heat Ratio, Assuming

    Ideal Gas Behavior (SI Units) 62

34. Sample of Completed PRV Specification Sheet 65

35. Values for F2 for Subcritical Flow 67

36. Constant Backpressure Correction Factor, Kb, for Conventional PRVs (Vapors and Gases Only) 70

37. Capacity Correction Factor, Kv, Due to Viscosity 77

38. Capacity Correction Factors Due to Overpressure for Noncertified PRVs in Liquid Service 79

A.1 Rupture Disk Device Specification Sheet 87

1. Correlation for Nozzle Critical Flow of Flashing and Nonflashing Systems 117

2. Correlation for Nozzle Critical Flow of Inlet Subcooled Liquid 123

3. Backpressure Correction Factor, Kb, for Balanced-bellows PRVs (Liquids) 125

1. Spring-loaded PRV Specification Sheet 135

2. Pilot-operated PRV Specification Sheet 136

1. Pressure-relief System for Example Problem 141

2. Curve Fit for Cp/Cv = 1.4 (Crane 410, Chart A-22) 142

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Contents

Tables

Page

1. Set Pressure and Accumulation Limits for Pressure-relief Devices 51

2. Example Determination of Relieving Pressure for Operating Contingencies for a Single Relief

   Device Installation 51

3. Example Determination of Relieving Pressure for Operating Contingencies for a Multiple Relief

   Device Installation 52

4. Example Determination of Relieving Pressure for Fire Contingencies for a Single Relief

   Device Installation 53

5. Example Determination of Relieving Pressure for Fire Contingencies for a Multiple Relief

   Device Installation 54

6. Example Determination of Relieving Pressure for Fire Contingencies for a Supplemental

   Valve Installation 55

7. Properties of Gases 57

8. Values of Coefficient C 60

9. Superheat Correction Factors, KSH 73

A.1 Rupture Disk Device Specification Sheet Instructions 83

1. Results for Supercritical Fluid Example Problem B.1.3 92

2. Results for Subcooled Liquid Example Problem B.2.2 95

3. Results for Gas Example Problem B.3.3 103

1. Two-phase Liquid/Vapor Relief Scenarios for PRVs 109

2. Results for Direct Integration Example C.2.1.2 115

1. Instructions for Spring-loaded PRV Specification Sheet 129

2. Instructions for Pilot-operated PRV Specification Sheet 132

E.1 Determination of Overall Piping Resistance Factor, K 138

vii

Sizing, Selection, and Installation of Pressure-relieving Devices Part I—Sizing and Selection

1. Scope

   This standard applies to the sizing and selection of pressure-relief devices used in refineries, chemical facilities, and related industries for equipment that has a maximum allowable working pressure (MAWP) of 15 psig (103 kPag) or greater. The pressure-relief devices covered in this standard are intended to protect unfired pressure vessels and related equipment against overpressure from operating and fire contingencies.

   
   

   This standard includes basic definitions and information about the operational characteristics and applications of various pressure-relief devices. It also includes sizing procedures and methods based on steady state flow of Newtonian fluids.

   
   

   Pressure-relief devices protect a vessel against overpressure only; they do not protect against structural failure when the vessel is exposed to extremely high temperatures such as during a fire. See API 521 for information about appropriate ways of reducing pressure and restricting heat input.

   
   

   Atmospheric and low-pressure storage tanks covered in API 2000 and pressure vessels used for the transportation of products in bulk or shipping containers are not within the scope of this standard.

   
   

   The rules for overpressure protection of fired vessels are provided in ASME BPVC, Section I and are not within the scope of this standard.

   
   

2. Normative References

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 520, Sizing, Selection, and Installation of Pressure-relieving Devices in Refineries, Part II— Installation

API Standard 521, Pressure-relieving and Depressuring Systems

API Standard 526, Flanged Steel Pressure-relief Valves

API Standard 527, Seat Tightness of Pressure-relief Valves

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

ASME Boiler and Pressure Vessel Code (BPVC) 1, Section I: Power Boilers

ASME Boiler and Pressure Vessel Code (BPVC), Section VIII: Pressure Vessels; Division 1

ASME Boiler and Pressure Vessel Code (BPVC) Code Case 2203 2, Omission of Lifting Device Requirements for Pressure Relief Valves on Air, Water Over 140°F, or Steam Service

ASME B31.1, Power Piping

ASME B31.3, Process Piping

ASME PTC 25, Pressure Relief Devices

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

2 Code Cases are temporary in nature and may not be acceptable in all jurisdictions. The user should verify the current applicability of the referenced Code Cases.

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