Sizing, Selection, and Installation of Pressure-relieving Devices

Part II—Installation

API STANDARD 520

SIXTH EDITION, MARCH 2015

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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 2

   5. Operating Environment 3

   6. Free-draining 3

   7. Maintainability 3

5. Inlet Piping Requirements 3

   1. General 3

   2. Minimum Diameter 4

   3. Layout 4

   4. Isolation Valves in Inlet Piping 6

   5. Process Laterals Connected to Inlet Piping of PRVs 6

   6. PRV Inlet Line Length and Pressure Loss 6

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

   8. Inlet Stresses that Originate from Discharge Reaction Forces 7

6. Discharge Piping 10

   1. General 10

   2. Safe Disposal of Relieving Fluids 10

   3. Backpressure Limitations and Sizing of Pipe 10

   4. Considerations for Pilot-Operated PRVs 11

   5. Stresses in Discharge Piping During Release 11

   6. Isolation Valves in the Discharge Piping 11

   7. Rupture Disks Installed at the Outlet of a PRV 11

7. PRV Stability 12

   1. General 12

   2. Potential Causes of PRV Instability 13

   3. PRV Inlet Pressure Drop Limitations 14

   4. PRV Trim Selection 22

8P RD Isolation (Stop) Valves 22

1. General 22

2. Application 22

3. Isolation Valve Requirements 22

4. Examples of Isolation Valve Installations 30

5. Administrative Controls Related to Isolation Valves 31

Contents

Page

1. Rupture Disk Installations 32

   1. Rupture Disk Devices in Combination with PRVs 32

   2. Rupture Disks In Series 34

2. Bonnet or Pilot Vent Piping 34

   1. General 34

   2. Conventional Valves 34

   3. Balanced Bellows Valves 34

   4. Balanced Piston Valves 39

   5. Pilot-Operated Valves 39

3. Drain Piping 39

   1. Installation Conditions that Require Drain Piping 39

   2. Safe Practice for Installation of Drain Piping 39

4. Pre-installation Handling and Inspection 39

   1. General 39

   2. Storage and Handling of PRDs 40

   3. Inspection and Cleaning of Systems Before Installation 40

5. Pressure-relief Device Installation and Maintenance 40

   1. Mounting Position 40

   2. Care in Installation 40

   3. PRVs 40

   4. Rupture Disk Devices 41

   5. Pin-actuated Devices 41

   6. Proper Gasketing and Bolting for Service Requirements 41

   7. Inspection and Maintenance 41

   8. Test or Lifting Levers 41

   9. Heat Tracing and Insulation 42

Annex A (informative) Rupture Disk Installation Guidelines 43

Annex B (informative) Installation and Maintenance of Pin-actuated Non-reclosing PRDs 49

Annex C (informative) PRV Acoustic Interaction 51

Bibliography 54

Figures

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

2. Typical Pressure-relief Valve Installation: Atmospheric (Open) Discharge 3

3. Typical Pressure-relief Valve Installation: Closed System Discharge 4

4A Typical Rupture Disk Device Installation: Atmospheric (Open) Discharge 5

4B Typical Rupture Disk Device Installation: Atmospheric (Open) Discharge 5

1. Avoiding Process Laterals Connected to Pressure-relief Valve Inlet Piping 6

2. Typical Pressure-relief Valve Installation with Vent Pipe 8

3. Typical Pressure-relief Valve Mounted on Process Line 18

4. Typical Pressure-relief Valve Mounted on Long Inlet Pipe 19

5. Typical Pilot-Operated Pressure-relief Valve Installation 21

   
   

6. Typical PRD Installation with an Isolation Valve 25

7. Typical PRD Installation for 100 % Spare Relieving Capacity 26

8. Alternate PRD Arrangement for 100 % Spare Relieving Capacity 27

9. Alternate PRD Installation Arrangement for 100 % Spare Relieving Capacity 28

10. Three-Way Changeover Valve—Shuttle Type 29

11. Three-Way Changeover Valve—Rotor Type 29

12. Three-Way Changeover Valve—Ball Types 30

13. Typical Flare Header Block Valves 31

14. Typical Isolation Block Valves for Spare Compressor 32

15. Typical Rupture Disk Device in Combination With Relief Valve: Inlet Side Installation 33

16. Bonnet Vent for Bellows Valves with Vent Located at the PRV 35

17. Bonnet Vent for Bellows Valves Handling Vapor with Remote Vent Location 36

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

    from the PRV 37

19. Bonnet Vent for Bellows Valves Handling Liquids or Flashing Liquid and Vapor with Remote

Vent Location 38

1. Typical Configuration of Companion Flanges, Gaskets and Rupture Disk Assembly 44

2. Proper Handling of a Rupture Disk 46

3. Improper Handling of a Rupture Disk 46

4. Proper Alignment of Rupture Disk indicated by Tag Arrows 47

Tables

1. Performance Differences with PRV Trim 23

2. Design Guidance for PRV Trim Options 23

vii

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:

• locations close to control valves, other valves, and other appurtenances;

  
  

• locations close to orifice plates and flow nozzles;

1