Sizing, Selection, and Installation of Pressure-relieving Devices

Part I—Sizing and Selection

API STANDARD 520, PART I

TENTH EDITION, OCTOBER 2020

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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.

The verbal forms used to express the provisions in this document are as follows.

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

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

May: As used in a standard, “may” denotes a course of action permissible within the limits of a standard. Can: As used in a standard, “can” denotes a statement of possibility or capability.

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, the Eighth Edition was issued in December 2008, and the Ninth Edition was issued in July 2014.

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Contents

1. Scope. 1

2. Normative References 1

3. Terms, Definitions, Acronyms, and Abbreviations 2

   1. Terms and Definitions 2

   2. Acronyms and Abbreviations 8

4. Pressure-relief Devices 8

   1. General 8

   2. Pressure-relief Valves. 8

   3. Rupture Disk Devices 34

   4. Pin-actuated Devices 52

   5. Open Flow Paths or Vents 54

   6. Other Types of Devices 54

5. Procedures for Sizing 54

   1. Determination of Relief Requirements 54

   2. API Effective Area and Effective Coefficient of Discharge 55

   3. Backpressure 56

   4. Relieving Pressure 61

   5. Development of Sizing Equations 66

   6. Sizing for Gas or Vapor Relief 67

   7. Sizing for Steam Relief 84

   8. Sizing for Liquid Relief: Pressure-relief Valves Requiring Capacity Certification 91

   9. Sizing for Liquid Relief: Pressure-relief Valves Not Requiring Capacity Certification 95

   10. Special Considerations for Non-Newtonian Fluids 96

   11. Sizing for Two-phase Liquid/Vapor Relief 97

   12. Sizing for Rupture Disk Devices 97

   13. Sizing for Open Flow Paths or Vents 98

Annex A (informative) Rupture Disk Device Specification Sheet 99

Annex B (informative) Review of Flow Equations Used in Sizing Pressure-relief Devices 104

Annex C (informative) Sizing for Two-phase Liquid/Vapor Relief 125

Annex D (informative) Pressure-relief Valve Specification Sheets 147

Annex E (informative) Capacity Evaluation of Rupture Disk and Piping System 100 % Vapor Flow and Constant Pipe Diameter 157

Annex F (informative) Valve Selection Example: Restricted Lift 163

Bibliography 164

Figures

1. Conventional Pressure-relief Valve with a Single Adjusting Ring for Blowdown Control 10

2. Balanced-bellows Pressure-relief Valve 11

3. Balanced-bellows Pressure-relief Valve with an Auxiliary Balanced Piston 12

4. Conventional Pressure-relief Valve with Threaded Connections 13

5. Pressure-relief Valve Operation—Vapor/Gas Service 14

6. Typical Relationship Between Lift of Disk in a Pressure-relief Valve and Vessel Pressure 15

7. Pressure-relief Valve Operation—Liquid Service 16

8. Typical Effects of Superimposed Backpressure on the Opening Pressure of

   Conventional Pressure-relief Valves 18

9. Typical Effects of Backpressure on the Set Pressure of Balanced Pressure-relief Valves 19

10. Pop-action Pilot-operated Valve (Flowing Type) 23

11. Pop-action Pilot-operated Valve (Nonflowing Type) 24

12. Modulating Pilot-operated Valve (Flowing Type) 25

13. Pilot-operated Relief Valve with a Nonflowing Modulating Pilot Valve 26

14. Low-pressure Pilot-operated Valve (Diaphragm Type) 27

15. Pressure Level Relationships for Pressure-relief Valves 28

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

    Pilot-operated Pressure-relief Valve 29

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

    Modulating-action Pilot-operated Pressure-relief Valve 30

18. Different Options for Restricting Lift in Spring-loaded Pressure-relief Valves 33

19. Pressure Level Relationships for Rupture Disk Devices 35

20. Rupture Disk Device in Combination with a Pressure-relief Valve 36

21. Forward-acting Solid Metal Rupture Disk. 39

22. Forward-acting Scored Rupture Disk 40

23. Forward-acting Composite Rupture Disk 41

24. Reverse-acting Rupture Disk with Knife Blades 42

25. Reverse-acting Scored Rupture Disk 44

26. Graphite Rupture Disk 45

27. Rupture Disk Application Parameters Assuming No Superimposed Backpressure 48

28. Common Types of Manufacturing Ranges and Corresponding Burst Pressure

    Marking 49

29. Rupture Disk Application Parameters with Superimposed Backpressure 51

30. Buckling Pin Valve 53

31. Backpressure Correction Factor, Kb, for Balanced Spring-loaded Pressure-relief

    Valves (Vapors and Gases) 59

32. Capacity Correction Factor, Kw, Due to Backpressure on Balanced Spring-loaded

    Pressure-relief Valves in Liquid Service 60

33. Curve for Evaluating Coefficient C in the Flow Equation from the Specific Heat Ratio, Assuming Ideal Gas Behavior (USC Units) 74

34. Curve for Evaluating Coefficient C in the Flow Equation from the Specific Heat Ratio, Assuming Ideal Gas Behavior (SI Units) 75

35. Sample of Completed Pressure-relief Valve Specification Sheet 77

36. Values for F2 for Subcritical Flow 80

37. Constant Backpressure Correction Factor, Kb, for Conventional Pressure-relief Valves 93

38. Capacity Correction Factor, Kv, Due to Viscosity 93

39. Capacity Correction Factors Due to Overpressure for Noncertified Pressure-relief

Valves in Liquid Service 95

A.1 Rupture Disk Device Specification Sheet 103

1. Correlation for Nozzle Critical Flow of Flashing and Nonflashing Systems 134

2. Correlation for Nozzle Critical Flow of Inlet Subcooled Liquid 140

3. Backpressure Correction Factor, Kb, for Balanced-bellows Pressure-relief Valves

(Liquids) 143

1. Spring-loaded Pressure-relief Valve Specification Sheet 151

2. Pilot-operated Pressure-relief Valve Specification Sheet 156

1. Pressure-relief System for Example Problem 161

2. Curve Fit for Cp/Cv = 1.4 (Crane 410, Chart A-22) 162

Tables

1. Spring-loaded Pressure-relief Valve Performance Characteristics as a Function of

   Valve Trim 21

2. Design Guidance for Pressure-relief Valve Trim Options 22

3. Example of Allowable Backpressures for a Multiple Conventional Pressure-relief

   Valve Installation 58

4. Set Pressure and Accumulation Limits for Pressure-relief Devices 62

5. Example Determination of Relieving Pressure for Operating Contingencies for a

   Single Relief Device Installation 63

6. Example Determination of Relieving Pressure for Operating Contingencies for a

   Multiple Relief Device Installation 64

7. Example Determination of Relieving Pressure for Fire Contingencies for a Single Relief Device Installation 65

8. Example Determination of Relieving Pressure for Fire Contingencies for a Multiple

   Relief Device Installation 66

9. Example Determination of Relieving Pressure for Fire Contingencies for a

   Supplemental Valve Installation 67

10. Properties of Gases 70

11. Values of Coefficient C 73

12. Superheat Correction Factors, KSH (USC Units) 86

13. Superheat Correction Factors, KSH (SI Units) 88

A.1 Rupture Disk Device Specification Sheet Instructions 99

1. Results for Supercritical Fluid Example Problem B.1.3 108

2. Results for Subcooled Liquid Example Problem B.2.2 111

3. Results for Gas Example Problem B.3.3. 120

1. Two-phase Liquid/Vapor Relief Scenarios for Pressure-relief Valves 125

2. Results for Direct Integration Example C.2.1.2 132

1. Instructions for Spring-loaded Pressure-relief Valve Specification Sheet 147

2. Instructions for Pilot-operated Pressure-relief Valve Specification Sheet 152

E.1 Determination of Overall Piping Resistance Factor, K 158

s

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 (PRDs) 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, 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: Rules for Construction of Pressure Vessels

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 American Society of Mechanical Engineers, Two 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.