Sizing, selection and installation of pressure-relieving devices in refineries
Part I Sizing and selection
1 Introduction
1.1 Scope
This recommended practice 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 103.4kPa (15psig) or greater. The pressure relief devices covered in this recommended practice are intended to protect unfired pressure vessels and related equipment against overpressure from operating and fire contingencies.
This recommended practice 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 a steady state of flow and Newtonian fluids.
Pressure relief devices protect a vessel against overpressure only; they do not protect against structural failure when the vessel is exposed to fire. See SY/T 10043-2002 Guide for pressure-relieving and depressuring systems for information about appropriate ways of reducing pressure and restricting heat input.
Atmospheric and low-pressure storage tanks covered in API Standard 2000 and pressure vessels used for the transportation of products in bulk or shipping containers are not within the scope of this recommended practice.
Fired vessels in accordance with ASME Boiler and Pressure Vessel Code, Section 1 and ASME B31.1 are also not within the scope of this recommended practice.
1.2 Definition of terms
Terms used in this recommended practice relating to pressure relief devices and their dimensional and operational characteristics are defined in 1.2.1 through 1.2.3. The terms are covered more specifically in the applicable clauses, subclauses and of text and accompanying illustrations.
1.2.1 Pressure relief devices
1.2.1.1
pressure relief device
device, such as pressure relief valve, nonreclosing pressure relief device, or vacuum relief valve, actuated by inlet static pressure and designed to open during an emergency or abnormal condition to prevent a rise of internal fluid pressure in excess of a specified value, which may also be designed to prevent excessive internal vacuum
1.2.1.2
spring-loaded pressure relief valve
pressure relief device designed to automatically reclose and prevent the further flow of fluid
1.2.1.2.1
relief valve
spring-loaded pressure relief valve actuated by the static pressure upstream of the valve, which opens normally in proportion to the pressure increase over the opening pressure and is used primarily with incompressible fluids
1.2.1.2.2
safety valve
spring-loaded pressure relief valve actuated by the static pressure upstream of the valve and characterized by rapid opening or pop action, which is normally used with compressible fluids
1.2.1.2.3
safety relief valve
spring-loaded pressure relief valve that may be used as either a safety or relief valve depending on the application
1.2.1.2.4
conventional pressure relief valve
spring-loaded pressure relief valve whose performance characteristics are directly affected by changes in the back pressure on the valve
1.2.1.2.5
balanced pressure relief valve
spring-loaded pressure relief valve that incorporates a means for minimizing the effect of back pressure on the performance characteristics
1.2.1.3
pilot-operated pressure relief valve
pressure relief valve in which the main valve is combined with and controlled by an auxiliary pressure relief valve
1.2.1.4
rupture disk device
nonreclosing differential pressure relief device actuated by inlet static pressure and designed to function by bursting the pressure-containing rupture disk, which includes a rupture disk and a rupture disk holder
1.2.2 Dimensional characteristics of pressure relief devices
1.2.2.1
actual discharge area
measured minimum net cross-sectional area that determines the flow through a valve
1.2.2.2
curtain area
area of the cylindrical or conical discharge opening between the seating surfaces above the nozzle seat created by the lift of the disk
1.2.2.3
effective discharge area or equivalent flow area
nominal or computed area of a pressure relief valve used in recognized flow formulas to determine the size of the valve, which will be less than the actual discharge area
1.2.2.4
nozzle area
cross-sectional flow area of a nozzle at the minimum nozzle diameter
1.2.2.5
huddling chamber
annular pressure chamber in a pressure relief valve located beyond the seat for the purpose of generating a rapid opening
1.2.2.6
inlet size
nominal pipe size (NPS) of the valve at the inlet connection, unless otherwise designated
1.2.2.7
outlet size
nominal pipe size (NPS) of the valve at the discharge connection, unless otherwise designated
1.2.2.8
lift
actual travel of the disk away from the closed position when a valve is relieving
1.2.3 Operational characteristics
1.2.3.1 System pressures
1.2.3.1.1
maximum operating pressure
maximum pressure expected during system operation
1.2.3.1.2
maximum allowable working pressure (MAWP)
maximum gauge pressure permissible at the top of a completed vessel in its operating position for a designated temperature, which is based on calculations for each element in a vessel using nominal thicknesses, exclusive of additional metal thicknesses allowed for corrosion and loadings other than pressure, and serves as the basis for the pressure setting of the pressure relief devices that protect the vessel
1.2.3.1.3
design gauge pressure
pressure under the most severe conditions of coincident temperature and pressure expected during operation, which may be used in place of the maximum allowable working pressure in all cases where the MAWP has not been established and is equal to or less than the MAWP
Foreword i
API Foreword iii
Policy Statement iv
Part I Sizing and selection
1 Introduction
1.1 Scope
1.2 Definition of terms
1.2.1 Pressure relief devices
1.2.2 Dimensional characteristics of pressure relief devices
1.2.3 Operational characteristics
2 Pressure relief devices
2.1 General
2.2 Spring-loaded pressure relief valve
2.2.1 Safety valves
2.2.2 Relief valves
2.2.3 Safety relief valves
2.2.4 Pressure relief valves
2.3 Pilot-operated pressure relief valves
2.4 Rupture disk devices
2.5 Rupture disks
2.5.1 General
2.5.2 Type of rupture disks
2.5.3 Application of rupture disks
2.5.4 Terminology for rupture disk device
2.6 Rupture disk devices in combination with pressure relief valves
2.6.1 General
2.6.2 Rupture disk devices at the inlet of a pressure relief valve
2.6.3 Rupture disk devices at the outlet of a pressure relief valve
2.7 Other types of devices
3 Causes of Overpressure
3.1 General
3.2 Process causes (excluding fire)
3.3 Fire
3.3.1 General
3.3.2 Effect of fire on the wetted surface of a vessel
3.3.3 Effect of fire on the unwetted surface of a vessel
4 Procedures for sizing
4.1 Determination of relief requirements
4.2 Relieving pressure
4.2.1 General
4.2.2 Operation contingencies
4.2.3 Fire contingencies
4.2.4 Steam service
4.3 Sizing for gas or vapor relief
4.3.1 Critical flow behavior
4.3.2 Sizing for critical flow
4.3.3 Sizing for subcritical flow: gas or vapor other than steam
4.3.4 Alternate sizing procedure for subcritical flow
4.4 Sizing for steam relief
4.4.1 General
4.4.2 Example
4.5 Sizing for liquid relief: relief valves requiring liquid capacity certification
4.5.1 General
4.5.2 Example
4.6 Sizing for liquid relief: relief valves not requiring liquid capacity certification
4.7 Sizing for two-phase liquid/vapor relief
4.8 Sizing for rupture disk devices
4.8.1 Rupture disk devices used independently
4.8.2 Rupture disk devices used in combination with pressure relief valves
Appendix A (Normative) Rupture disk device specification sheet
Appendix B (Normative) Special system design considerations
Appendix C (Normative) Principal causes of overpressure
Appendix D (Normative) Determination of fire-relief requirements
Appendix E (Normative) Review of flow equations used in sizing pressure relief valves
Appendix F (Normative) Design of relief manifolds
Appendix G (Normative) Pressure relief valve specification sheet
Part II Installation
1 General
1.1 Scope
1.2 Definition of terms
2 Inlet piping to pressure relief devices
2.1 General requirements
2.1.1 Flow and stress considerations
2.1.2 Vibration considerations
2.2 Pressure-drop limitations and piping configurations
2.2.1 Pressure loss at the pressure relief valve inlet
2.2.2 Size and length of inlet piping to pressure relief valves
2.2.3 Remote sensing for pilot-operated pressure relief valves
2.2.4 Configuration of inlet piping for pressure relief valves
2.3 Inlet stresses that originate from static loads in the discharge piping
2.3.1 Thermal stresses
2.3.2 Mechanical stresses
2.4 Inlet stresses that originate from discharge reaction forces
2.4.1 Determining reaction forces in an open discharge system
2.4.2 Determining reaction forces in a closed discharge system
2.5 Isolation valves in inlet piping
2.6 Rupture disk devices in combination with pressure relief valves
2.7 Process laterals connected to inlet piping of pressure relief valves
2.8 Turbulence in pressure relief device inlets
3 Discharge piping from pressure relief devices
3.1 General requirements
3.2 Safe disposal of relieving fluids
3.3 Back pressure limitations and sizing of pipe
3.4 Considerations for pilot-operated pressure relief valves
3.5 Stresses that originate from discharge piping
3.6 Isolation valves in the discharge piping
4 Isolation valves in pressure relief piping
4.1 General
4.2 Application
4.3 Isolation valve requirements
4.4 Examples of isolation valve installations
4.5 Management procedures related to isolation valves
5 Bonnet or pilot vent piping
5.1 Conventional pressure relief valves
5.2 Balanced bellows pressure relief valves
5.3 Balanced piston pressure relief valve
5.4 Pilot-operated pressure relief valves
6 Drain piping
6.1 Installation conditions that require drain piping
6.2 Safe practice for installation of drain piping
7 Pressure relief device location and position
7.1 Inspection and maintenance
7.2 Proximity to pressure source
7.3 Proximity to other equipment
7.3.1 Reducing stations
7.3.2 Orifice plates and flow nozzles
7.3.3 Other valves and fittings
7.4 Mounting position
7.5 Test or lifting levers
7.6 Heat tracing and insulation
8 Bolting and gasketing
8.1 Care in installation
8.2 Proper gasketing and bolting for service requirements
9 Multiple pressure relief valves with staggered settings
10 Pre-installation handling and inspection
10.1 Storage and handling of pressure relief devices
10.2 Inspection and testing of pressure relief valves
10.3 Inspection of rupture disk devices
10.4 Inspection and cleaning of systems before installation
