The primary purpose of a pressure relief device is protecting 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.
A pressure relief valve is a pressure relief device designed to open and relieve excess pressure and to reclose and prevent further flow of fluid after normal conditions have been restored. A pressure relief valve opens when its upstream pressure reaches the set pressure. It then allows fluid to flow until its upstream pressure falls to the closing pressure (The value of the closing pressure is less than that of the set pressure and will be reached after the blow down phase is completed). It then closes, preventing further flow.
Examples of specific types of pressure relief valves include; safety valve, relief valve, conventional safety relief valve, balanced safety relief valve and pilot-operated pressure relief valve.
1.1.1. Dimensional Characteristics of Pressure Relief Devices
- Curtain area: The area of the cylindrical or conical discharge opening between the seating surfaces above the nozzle scat created by the lift of the disc.
- Discharge area:see actual discharge area.
- Effective discharge area: A nominal or computed area used with an effective discharge coefficient to calculate the minimum required relieving capacity for a pressure relief valve per the preliminary sizing equations contained in this practice. API Standard 526 provides effective discharge areas for a range of sizes in terms of letter designations, ‘”U” through “T”.
- Bore area: The minimum cross-sectional flow area of a nozzle. Also referred to as nozzle area, nozzle throat area and throat area.
- Bore diameter: the minimum diameter of a nozzle
- Huddling chamber: An annular chamber located downstream of the seat of a pressure relief valve for the purpose of assisting the valve to achieve lift.
- Inlet size: The nominal pipe size (NPS) of the valve at the inlet connection, unless otherwise designated.
- Outlet size: The nominal pipe size (NPS) of the valve at the discharge connection, unless otherwise designated.
- Lift: The actual travel of the disc from the closed position when a valve is relieving.
- Developed lift: the actual travel of the disc from closed position to the position reached when the valve is at flow-rating pressure.
1.1.2. Operational Characteristics of Pressure Relief Devices
Coefficient of discharge: The ratio of the mass flow rate in a valve to that of an ideal nozzle. It is used for calculating flow through a pressure relief device.
- The effective coefficient of discharge is a nominal value used with an effective discharge area to calculate the minimum required relieving capacity of a pressure relief valve This capacity is determined in accordance with the applicable per the preliminary sizing equations given in this Recommended Practice.
- The rated coefficient of discharge is determined in accordance with the applicable code or regulation and is used with the actual discharge area to calculate the rated flow capacity of a pressure relief valve.
1.1.3. System Pressures and Temperatures
- The maximum operating pressure is the maximum pressure expected during normal system operation.
- The maximum allowable working pressure (MAWP) is the maximum gauge pressure permissible at the top of a completed vessel in its normal operating position at the designated coincident temperature specified for that pressure.
The pressure is the least of the values for the internal or external pressure as determined by the vessel design rules for each element of the vessel using actual nominal thickness, exclusive of additional metal thickness allowed for corrosion and loadings other than pressure. The maximum allowable working pressure is the basis for the pressure setting of the pressure relief devices that protect the vessel. The MAWP is normally greater than the design pressure but must be equal to the design pressure when the design rules are used only to calculate the minimum thickness for each element and calculations are not made to determine the value of the MAWP.
- The design pressure of the vessel along with the design temperature is used to determine the minimum permissible thickness or physical characteristic of each vessel component as determined by the vessel design rules. The design pressure is selected by the user to provide a suitable margin above the most severe pressure expected during normal operation at a coincident temperature. It is the pressure specified on the purchase order. This pressure may be used in place of the maximum allowable working pressure in all cases where the
MAWP has not been established. The design pressure is equal to or less than the MAWP.
- Accumulation is the pressure increase over the maximum allowable working pressure of the vessel allowed during discharge through the pressure relief device, expressed in pressure units or as a percentage of MAWP or design pressure. Maximum allowable accumulations are established by applicable codes for emergency operating and fire contingencies.
- Overpressure is the pressure increase over the set pressure of the relieving device allowed to achieve rated flow. Overpressure is expressed in pressure units or as a percentage of set pressure. It is the same as accumulation only when the relieving device is set to open at the maximum allowable working pressure of the vessel.
- Blowdown: the difference between actual popping pressure of a pressure relief valve and actual reseating pressure expressed as a percentage of set pressure or in pressure units.
- Blowdown pressure: the value of decreasing inlet static pressure at which no further discharge is detected at the outlet of a pressure relief valve after the valve has been subjected to a pressure equal to or above the popping pressure.
- The set pressure is the inlet gauge pressure at which the pressure relief device is set to open under service conditions.
- relieving pressure: set pressure plus overpressure
- The cold differential test pressure (CDTP) is the pressure at which a pressure relief valve is adjusted to open on the test stand. The cold differential test pressure includes corrections for the service conditions of back pressure or temperature or both.
- Opening pressure is the value of increasing inlet static pressure at which there is a measurable lift of the disc or at which discharge of the fluid becomes continuous, as determined by seeing, feeling or hearing.
- Closing Pressure is the value of decreasing inlet static pressure at which the valve disc reestablishes contact with the seat or at which lift becomes zero as determined by seeing, feeling or hearing.
- Simmer is the audible or visible escape of compressible fluid between the seat and disc which may occur at an inlet static pressure below the set pressure prior to opening.
- Back pressure: the static pressure existing at the outlet of a pressure relief device due to pressure in the discharge system.
- Built-up back pressure: pressure existing at the outlet of a pressure relief device caused by the flow through that particular device into a discharge system.
- Superimposed back pressure: the static pressure existing at the outlet of a pressure relief device at the time the device is required to operate. It is the result of pressure in the discharge system from other sources. This back pressure can be constant if the valve outlet is connected to a process vessel or system which is held at a constant pressure. In most cases, however, the superimposed back pressure will be variable as a result of changing conditions existing in the discharge system.
- Chatter: abnormal rapid reciprocating motion of the movable parts of a pressure relief valve in which the disc contacts the seat.
Codes and Standards
- ASME Section II, Materials
- ASME Section VIII, Pressure Vessels (which covers safety relief valves)
- ASME/ANSI B 16.34, Valves – Flanged, Threaded and Welding End.
- ASME/ANSI B 16.5. Pipe flanges and flanged fittings
- API RP 520 Part I, Sizing and Selection of Pressure Relieving Devices in Refineries
- API RP 520 Part II, Installation of Pressure Relieving Devices in Refineries
- API RP 521, Guide for Pressure Relief and De-pressurizing Systems
- API STD 526, Flanged Steel Safety/Relief Valves for use in the Petroleum Industry
- API STD 527, Commercial Seat Tightness of Safety/Relief Valves with Metal to Metal and Soft Seals
- DIN EN ISO 4126-1, Safety Devices for Protection against Excessive Pressure—Part 1: Safety Valves
- DIN EN ISO 4126-4, Safety Devices for Protection against Excessive Pressure—Part 4: Pilot-