Sizing, Selection, and Installation of Pressure-relieving Devices in Refineries

Part I—Sizing and Selection

API STANDARD 520

EIGHTH EDITION, DECEMBER 2008

Sizing, Selection, and Installation of Pressure-relieving Devices in Refineries

Part I—Sizing and Selection

Downstream Segment

API STANDARD 520

EIGHTH EDITION, DECEMBER 2008

Special Notes

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Copyright © 2008 American Petroleum Institute

Foreword

API Standard 520, Sizing, Selection, and Installation of Pressure-relieving Devices in Refineries, 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 1st Edition of this standard was initially released as a recommended practice in 1955. The 2nd Edition was published in two parts: Part I, Design, in 1960 and Part II, Installation, in 1963. The 3rd Edition of Part I was issued in November 1967 and reaffirmed in 1973. The 4th edition was issued in December 1976, the 5th Edition was issued in July 1990, the 6th Edition was issued in March 1993, and the 7th Edition was issued in January 2000.

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 of the 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.

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Suggested revisions are invited and should be submitted to the Downstream Segment, API, 1220 L Street, NW, Washington, D.C. 20005, standards@api.org.

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Contents

Page

1. Scope 1

2. Normative References 1

3. Terms and Definitions 2

4. Pressure Relief Devices (PRDs). 7

   1. General 7

   2. Pressure Relief Valves (PRVs) 7

   3. Rupture Disk Devices 26

   4. Pin-actuated Devices 40

   5. Other Types of Devices 43

5. Procedures for Sizing 44

   1. Determination of Relief Requirements 44

   2. API Effective Area and Effective Coefficient of Discharge 44

   3. Backpressure 45

   4. Relieving Pressure 49

   5. Development of Sizing Equations 52

   6. Sizing for Gas or Vapor Relief 53

   7. Sizing for Steam Relief 67

   8. Sizing for Liquid Relief: PRVs Requiring Capacity Certification 69

   9. Sizing for Liquid Relief: PRVs Not Requiring Capacity Certification 74

   10. Sizing for Two-phase Liquid/Vapor Relief 75

   11. Sizing for Rupture Disk Devices 76

Annex A (informative) Rupture Disk Device Specification Sheet 78

Annex B (informative) Review of Flow Equations Used in Sizing Pressure Relief Devices 82

Annex C (informative) Sizing for Two-phase Liquid/Vapor Relief 101

Annex D (informative) Pressure Relief Valve Specification Sheets 120

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

and Constant Pipe Diameter 128

Bibliography 134

Figures

1. Conventional PRV with a Single Adjusting Ring for Blowdown Control 8

2. Balanced-bellows PRV 9

3. Balanced-bellows PRV with an Auxiliary Balanced Piston 10

4. Conventional PRV with Threaded Connections 11

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 16

9. Typical Effects of Backpressure on the Set Pressure of Balanced PRVs 17

10. Pop-action Pilot-operated Valve (Flowing-type) 18

11. Pop-action Pilot-operated Valve (Non-flowing-type) 19

12. Modulating Pilot-operated Valve (Flowing-type) 20

13. Pilot-operated Relief Valve with a Non-flowing Modulating Pilot Valve 21

14. Low-pressure Pilot-operated Valve (Diaphragm-type) 22

    v

15. Pressure Level Relationships for PRVs 23

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

    Pilot-operated PRV 24

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

    Modulating-action Pilot-operated PRV 24

18. Pressure Level Relationships for Rupture Disk Devices 28

19. Rupture Disk Device in Combination with a PRV 29

20. Forward-acting Solid Metal Rupture Disk 31

21. Forward-acting Scored Rupture Disk 32

22. Forward-acting Composite Rupture Disk 33

23. Reverse-acting Rupture Disk with Knife Blades 34

24. Reverse-acting Scored Rupture Disk 35

25. Graphite Rupture Disk 36

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 41

29. Buckling Pin Valve 42

30. Backpressure Correction Factor, Kb, for Balanced-bellows PRV

    (Vapors and Gases) 47

31. Capacity Correction Factor, Kw, due to Backpressure on Balanced-bellows PRVs

    in Liquid Service 48

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

    Assuming Ideal Gas Behavior (USC Units) 58

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

    Assuming Ideal Gas Behavior (SI Units) 59

34. Sample of Completed PRV Specification Sheet 62

35. Values for F2 for Subcritical Flow 63

36. Constant Backpressure Correction Factor, Kb, for Conventional PRVs (Vapors and

    Gases Only) 66

37. Capacity Correction Factor, Kv, Due to Viscosity 72

38. Capacity Correction Factors Due to Overpressure for Noncertified PRVs in Liquid Service 75

1. Correlation for Nozzle Critical Flow of Flashing and Nonflashing Systems. 109

2. Correlation for Nozzle Critical Flow of Inlet Subcooled Liquid 114

3. Backpressure Correction Factor, Kb, for Balanced-bellows PRVs (Liquids) 116

1. Spring-loaded PRV Specification Sheet 126

2. Pilot-operated PRV Specification Sheet 127

Tables

1. Set Pressure and Accumulation Limits for Pressure Relief Devices 49

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

Single Relief Device Installation 50

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

Device Installation 51

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

Relief Device Installation. 51

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

   Relief Device Installation. 52

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

   Valve Installation 53

3. Properties of Gases 55

4. Values of Coefficient C 57

5. Superheat Correction Factors, KSH 70

1. Rupture Disk Device Specification Sheet Instructions 78

2. Rupture Disk Device Specification Sheet 81

1. Results for Supercritical Fluid Example Problem B.1.3 86

2. Results for Subcooled Liquid Example Problem B.2.2 89

3. Results for Gas Example Problem B.3.3 96

1. Two-phase Liquid/Vapor Relief Scenarios for PRVs 101

2. Results for Direct Integration Example C.2.1.2 108

1. Instructions for Spring-loaded PRV Specification Sheet 120

2. Instructions for Pilot-operated PRV Specification Sheet 123

E.1 Determination of Overall Piping Resistance Factor, K 129

1. Pressure Relief System for Example Problem 132

2. Curve Fit for Cp/Cv = 1.4 (Crane 410, Chart A-22) 133

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

1. Scope

   This standard applies to the sizing and selection of pressure relief devices used in refineries and related industries for equipment that has a maximum allowable working pressure 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 Section I and ASME B31.1, and are not within the scope of this standard.

   
   

2. Normative References

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

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

API Std 521/ISO 23251, Guide for Pressure-relieving and Depressuring Systems

API Std 526, Flanged Steel Pressure Relief Valves

API Std 527, Seat Tightness of Pressure Relief Valves

API Std 2000, Venting Atmospheric and Low-pressure Storage Tanks: Nonrefrigerated and Refrigerated

ASME Boiler and Pressure Vessel Code 1, Section I—Power Boilers

ASME Boiler and Pressure Vessel Code, Section VIII—Pressure Vessels, Division 1

ASME BPVC Code Case 2091-3 2, Nonreclosing Pin Pressure Relief Devices

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

ASME BPVC Code Case 2487, Breaking Pin 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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