Sizing, Selection, and Installation of Pressure-relieving Devices

Part II—Installation

API STANDARD 520, PART II

SEVENTH EDITION, OCTOBER 2020

Special Notes

API publications necessarily address problems of a general nature. With respect to particular circumstances, local, state, and federal laws and regulations should be reviewed. The use of API publications is voluntary. In some cases, third parties or authorities having jurisdiction may choose to incorporate API standards by reference and may mandate compliance.

Neither API nor any of API's employees, subcontractors, consultants, committees, or other assignees make any warranty or representation, either express or implied, with respect to the accuracy, completeness, or usefulness of the information contained herein, or assume any liability or responsibility for any use, or the results of such use, of any information or process disclosed in this publication. Neither API nor any of API’s employees, subcontractors, consultants, or other assignees represent that use of this publication would not infringe upon privately owned rights.

Classified areas may vary depending on the location, conditions, equipment, and substances involved in any given situation. Users of this standard should consult with the appropriate authorities having jurisdiction.

Users of this standard should not rely exclusively on the information contained in this document. Sound business, scientific, engineering, and safety judgment should be used in employing the information contained herein.

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 authorities having jurisdiction with which this publication may conflict.

API publications are published to facilitate the broad availability of proven, sound engineering and operating practices. These publications are not intended to obviate the need for applying sound engineering judgment regarding when and where these publications should be utilized. The formulation and publication of API publications is not intended in any way to inhibit anyone from using any other practices.

API is not undertaking to meet the duties of employers, manufacturers, or suppliers to warn and properly train and equip their employees, and others exposed, concerning health and safety risks and precautions, nor undertaking their obligations to comply with authorities having jurisdiction.

Information concerning safety and health risks and proper precautions with respect to particular materials and conditions should be obtained from the employer, the manufacturer or supplier of that material, or the material safety data sheet.

Any manufacturer marking equipment or materials in conformance with the marking requirements of an API standard is solely responsible for complying with all the applicable requirements of that standard. API does not represent, warrant, or guarantee that such products do in fact conform to the applicable API standard.

All rights reserved. No part of this work may be reproduced, translated, stored in a retrieval system, or transmitted by any means, electronic, mechanical, photocopying, recording, or otherwise, without prior written permission from the publisher. Contact the Publisher, API Publishing Services, 200 Massachusetts Avenue, NW, Suite 1100, Washington, DC 20001.

Copyright © 2020 American Petroleum Institute

Foreword

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.

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.

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, 200 Massachusetts Avenue, Suite 1100, Washington, DC 20001. 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, 200 Massachusetts Avenue, Suite 1100, Washington, DC 20001.

Suggested revisions are invited and should be submitted to the Standards Department, API, 200 Massachusetts Avenue, Suite 1100, Washington, DC 20001, standards@api.org.

iii

Contents

Page

1. Scope 1

2. Normative References 1

3. Terms and Definitions 1

4. PRD Location 1

   1. General 1

   2. Proximity to Protected Equipment 1

   3. Pressure Fluctuations 1

   4. Vibration 3

   5. Operating Environment 3

   6. Free-draining 3

   7. Maintainability 3

5. Inlet Piping Requirements 3

   1. General 3

   2. Inlet Piping Diameter Requirements 4

   3. Layout 4

   4. Isolation Valves in Inlet Piping 6

   5. Process Laterals Connected to Inlet Piping of PRVs 7

   6. PRV Inlet Line Length and Pressure Loss 7

   7. Inlet Stresses that Originate from Static Loads in the Discharge Piping 8

   8. Inlet Stresses that Originate from Discharge Reaction Forces 8

6. Discharge Piping 11

   1. General 11

   2. Safe Disposal of Relieving Fluids 12

   3. Backpressure Limitations and Sizing of Pipe 12

   4. Considerations for Pilot-Operated PRVs 12

   5. Stresses in Discharge Piping During Release 13

   6. Isolation Valves in the Discharge Piping 13

   7. Rupture Disks Installed at the Outlet of a PRV 13

7. PRV Stability 13

   1. General 13

   2. Potential Causes of PRV Instability 14

   3. PRV Inlet Pressure Drop Limitations. 16

8. PRD Isolation (Stop) Valves 25

   1. General 25

   2. Application 25

   3. Isolation Valve Requirements 25

   4. Examples of Isolation Valve Installations 32

   5. Administrative Controls Related to Isolation Valves 33

9. Rupture Disk Installations 35

   1. Rupture Disk Devices in Combination with PRVs 35

   2. Rupture Disks In Series 37

10. Bonnet or Pilot Vent Piping 37

    1. General 37

    2. Conventional Valves 37

       v

       Contents

       Page

    3. Balanced Bellows Valves 37

    4. Balanced Piston Valves 42

    5. Pilot-Operated Valves 42

11. Drain Piping 43

    1. Installation Conditions that Require Drain Piping 43

    2. Safe Practice for Installation of Drain Piping 43

12. Pre-installation Handling and Inspection 43

    1. General 43

    2. Storage and Handling of PRDs 43

    3. Inspection and Cleaning of Systems Before Installation 44

13. Pressure-relief Device Installation and Maintenance 44

    1. Mounting Position 44

    2. Care in Installation 44

    3. PRVs 44

    4. Rupture Disk Devices 44

    5. Pin-actuated Devices 45

    6. Proper Gasketing and Bolting for Service Requirements 45

    7. Inspection and Maintenance 45

    8. Test or Lifting Levers 45

    9. Heat Tracing and Insulation 45

Annex A (informative) Rupture Disk Installation Guidelines 46

Annex B (informative) Installation and Maintenance of Pin-actuated Non-reclosing PRDs 52

Annex C (informative) PRV Acoustic Interaction 54

Bibliography 57

Figures

1. Typical Installation Avoiding Unstable Flow Patterns at PRV Inlet 2

2. Typical Pressure-relief Valve Installation: Atmospheric (Open) Discharge 4

3. Typical Pressure-relief Valve Installation: Closed System Discharge 5

4a Typical Rupture Disk Device Installation: Atmospheric (Open) Discharge 6

4b Typical Rupture Disk Device Installation: Atmospheric (Open) Discharge 6

1. Avoiding Process Laterals Connected to Pressure-relief Valve Inlet Piping 7

2. Typical Pressure-relief Valve Installation with Vent Pipe 9

3. Typical Pressure-relief Valve Mounted on Process Line. 21

4. Typical Pressure-relief Valve Mounted on Long Inlet Pipe 22

5. Typical Pilot-Operated Pressure-relief Valve Installation 24

6. Typical PRD Installation with an Isolation Valve 28

7. Typical PRD Installation for 100 % Spare Relieving Capacity 29

8. Alternate PRD Installation for 100 % Spare Relieving Capacity 30

9. Three-Way Changeover Valve—Shuttle Type. 31

10. Three-Way Changeover Valve—Rotor Type 31

11. Three-Way Changeover Valve—Ball Types 33

12. Typical Flare Header Block Valves 34

13. Typical Isolation Block Valves for Spare Compressor 35

14. Typical Rupture Disk Device in Combination With Relief Valve: Inlet Side Installation 36

    vi

    Contents

    Page

15. Bonnet Vent for Bellows Valves with Vent Located at the PRV 39

16. Bonnet Vent for Bellows Valves Handling Vapor with Remote Vent Location 40

17. Bonnet Vent for Bellows Valves Handling Liquids Where a Leak Needs to Be Routed Away

    from the PRV 41

18. Bonnet Vent for Bellows Valves Handling Liquids or Flashing Liquid and Vapor

with Remote Vent Location 42

1. Typical Configuration of Companion Flanges, Gaskets, and Rupture Disk Assembly 47

2. Proper Handling of a Rupture Disk 49

3. Improper Handling of a Rupture Disk 49

4. Proper Alignment of Rupture Disk Indicated by Tag Arrows 50

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

1. Scope

   
   

   This standard covers methods of installation for pressure-relief devices (PRDs) for equipment that has a maximum allowable working pressure (MAWP) of 15 psig (1.03 barg or 103 kPAg) or greater. Pressure-relief valves (PRVs) or rupture disks (RDs) may be used independently or in combination with each other to provide the required protection against excessive pressure accumulation. As used in this standard, the term pressure-relief valve includes safety-relief valves used in either compressible or incompressible fluid service, and relief valves used in incompressible fluid service. This standard covers gas, vapor, steam, two-phase, and incompressible fluid service; it does not cover special applications that require unusual installation considerations.

   
   

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 I–Sizing and Selection

   
   

3. Terms and Definitions

   
   

   The terminology for PRDs that is used in this standard is in general agreement with the definitions given in API 520, Part I.

   
   

4. PRD Location

   
   

   1. General

      
      

      There are a number of design factors that should be considered when determining the location of the PRD.

      
      

   2. Proximity to Protected Equipment

      
      

      If other factors permit, the PRD should normally be placed close to the protected equipment or system of equipment so that the pressure in the protected equipment stays within code allowable limits and to avoid PRV instability (see Section 7).

      
      

      See 7.3.3 for guidance on establishing set pressure if PRD will be remote from the protected equipment.

      
      

   3. Pressure Fluctuations

The PRD should not be located where there are pressure fluctuations large enough to result in relief valve simmering/activation or rupture disk fatigue. On installations that have pressure fluctuations that peak close to the set pressure of the PRV or burst pressure of a rupture disk, the PRD should be located farther from the source and in a more stable pressure region.

Examples of areas that may have pressure fluctuations include: