PRESSURE RELIEF VALVE

A pressure relief valve is a safety device designed to protect a pressurized vessel or system during an overpressure event.
An overpressure event refers to any condition which would cause pressure in a vessel or system to increase beyond the specified design pressure or maximum allowable working pressure (MAWP).
The primary purpose of a pressure relief valve is protection of life and property by venting fluid from an overpressurized vessel.
Many electronic, pneumatic and hydraulic systems exist today to control fluid system variables, such as pressure, temperature and flow. Each of these systems requires a power source of some type, such as electricity or compressed air in order to operate. A pressure relief valve must be capable of operating at all times, especially during a period of power failure when system controls are nonfunctional.
The sole source of power for the pressure relief valve, therefore, is the process fluid.
Once a condition occurs that causes the pressure in a system or vessel to increase to a dangerous level, the pressure relief valve may be the only device remaining to prevent a catastrophic failure. Since reliability is directly related to the complexity of the device, it is important that the design of the pressure relief valve be as simple as possible.
The pressure relief valve must open at a predetermined set pressure, flow a rated capacity at a specified overpressure, and close when the system pressure has returned to a safe level. Pressure relief valves must be designed with materials compatible with many process fluids from simple air and water to the most corrosive media. They must also be designed to operate in a consistently smooth and stable manner on a variety of fluids and fluid phases.

SPRING LOADED PRESSURE RELIEF VALVE

The basic spring loaded pressure relief valve has been developed to meet the need for a simple, reliable, system actuated device to provide overpressure protection.
The image on the right shows the construction of a spring loaded pressure relief valve.
The valve consists of a valve inlet or nozzle mounted on the pressurized system, a disc held against the nozzle to prevent flow under normal system operating conditions, a spring to hold the disc closed, and a body/bonnet to contain the operating elements. The spring load is adjustable to vary the pressure at which the valve will open.
When a pressure relief valve begins to lift, the spring force increases. Thus system pressure must increase if lift is to continue. For this reason pressure relief valves are allowed an overpressure allowance to reach full lift. This allowable overpressure is generally 10% for valves on unfired systems. This margin is relatively small and some means must be provided to assist in the lift effort.
Most pressure relief valves, therefore, have a secondary control chamber or huddling chamber to enhance lift. As the disc begins to lift, fluid enters the control chamber exposing a larger area of the disc to system pressure.
This causes an incremental change in force which overcompensates for the increase in spring force and causes the valve to open at a rapid rate. At the same time, the direction of the fluid flow is reversed and the momentum effect resulting from the change in flow direction further enhances lift. These effects combine to allow the valve to achieve maximum lift and maximum flow within the allowable overpressure limits. Because of the larger disc area exposed to system pressure after the valve achieves lift, the valve will not close until system pressure has been reduced to some level below the set pressure. The design of the control chamber determines where the closing point will occur.
The difference between the set pressure and the closing point pressure is called blowdown and is usually expressed as a percentage of set pressure.

BALANCED BELLOWS VALVES AND BALANCED PISTON VALVES

When superimposed back pressure is variable, a balanced bellows or balanced piston design is recommended. A typical balanced bellow is shown on the right. The bellows or piston is designed with an effective pressure area equal to the seat area of the disc. The bonnet is vented to ensure that the pressure area of the bellows or piston will always be exposed to atmospheric pressure and to provide a telltale sign should the bellows or piston begin to leak. Variations in back pressure, therefore, will have no effect on set pressure. Back pressure may, however, affect flow.
Image of a balanced-bellows, spring-loaded PRV.

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OTHER DESIGNS OF RELIEF VALVES

Safety Valve. A safety valve is a pressure relief valve actuated by inlet static pressure and characterized by rapid opening or pop action. (It is normally used for steam and air services.)

Low-Lift Safety Valve. A low-lift safety valve is a safety valve in which the disc lifts automatically such that the actual discharge area is determined by the position of the disc.
Full-Lift Safety Valve. A full-lift safety valve is a safety valve in which the disc lifts automatically such that the actual discharge area is not determined by the position of the disc.

Relief Valve. A relief valve is a pressure relief device actuated by inlet static pressure having a gradual lift generally proportional to the increase in pressure over opening pressure. It may be provided with an enclosed spring housing suitable for closed discharge system application and is primarily used for liquid service.
Safety Relief Valve. A safety relief valve is a pressure relief valve characterized by rapid opening or pop action, or by opening in proportion to the increase in pressure over the opening pressure, depending on the application and may be used either for liquid or compressible fluid.

Conventional Safety Relief Valve. A conventional safety relief valve is a pressure relief valve which has its spring housing vented to the discharge side of the valve. The operational characteristics (opening pressure, closing pressure, and relieving capacity) are directly affected by changes of the back pressure on the valve.
Balanced Safety Relief Valve. A balanced safety relief valve is a pressure relief valve which incorporates means of minimizing the effect of back pressure on the operational characteristics (opening pressure, closing pressure, and relieving capacity).

Pilot-Operated Pressure Relief Valve. A pilotoperated pressure relief valve is a pressure relief valve in which the major relieving device is combined with and is controlled by a self-actuated auxiliary pressure relief valve.
Power-Actuated Pressure Relief Valve. A poweractuated pressure relief valve is a pressure relief valve in which the major relieving device is combined with and controlled by a device requiring an external source of energy.
Temperature-Actuated Pressure Relief Valve. A temperature-actuated pressure relief valve is a pressure relief valve which may be actuated by external or internal temperature or by pressure on the inlet side.
Vacuum Relief Valve. A vacuum relief valve is a pressure relief device designed to admit fluid to prevent an excessive internal vacuum; it is designed to reclose and prevent further flow of fluid after normal conditions have been restored.

CODES, STANDARDS AND RECOMMENDED PRACTICES

Many Codes and Standards are published throughout the world which address the design and application of pressure relief valves. The most widely used and recognized of these is the ASME Boiler and Pressure Vessel Code, commonly called the ASME Code.
Most Codes and Standards are voluntary, which means that they are available for use by manufacturers and users and may be written into purchasing and construction specifications. The ASME Code is unique in the United States and Canada, having been adopted by the majority of state and provincial legislatures and mandated by law.
The ASME Code provides rules for the design and construction of pressure vessels. Various sections of the Code cover fired vessels, nuclear vessels, unfired vessels and additional subjects, such as welding and nondestructive examination. Vessels manufactured in accordance with the ASME Code are required to have overpressure protection. The type and design of allowable overpressure protection devices is spelled out in detail in the Code.

Terminology

The following definitions are taken from DIN 3320 but it should be noted that many of the terms and associated definitions used are universal and appear in many other standards. Where commonly used terms are not defined in DIN 3320 then ASME PTC25.3 has been used as the source of reference. This list is not exhaustive and is intended as a guide only; it should not be used in place of the relevant current issue standard:

Operating pressure (working pressure) is the gauge pressure existing at normal operating conditions within the system to be protected.
Set pressure is the gauge pressure at which under operating conditions direct loaded safety valves commence to lift.
Test pressure is the gauge pressure at which under test stand conditions (atmospheric backpressure) direct loaded safety valves commence to lift.
Opening pressure is the gauge pressure at which the lift is sufficient to discharge the predetermined flowing capacity. It is equal to the set pressure plus opening pressure difference.
Reseating pressure is the gauge pressure at which the direct loaded safety valve is re-closed.
Built-up backpressure is the gauge pressure built up at the outlet side by blowing.
Superimposed backpressure is the gauge pressure on the outlet side of the closed valve.
Backpressure is the gauge pressure built up on the outlet side during blowing (built-up backpressure + superimposed backpressure).
Accumulation is the increase in pressure over the maximum allowable working gauge pressure of the system to be protected.
Opening pressure difference is the pressure rise over the set pressure necessary for a lift suitable to permit the predetermined flowing capacity.
Reseating pressure difference is the difference between set pressure and reseating pressure.
Functional pressure difference is the sum of opening pressure difference and reseating pressure difference.
Operating pressure difference is the pressure difference between set pressure and operating pressure.
Lift is the travel of the disc away from the closed position.
Commencement of lift (opening) is the first measurable movement of the disc or the perception of discharge noise.
Flow area is the cross sectional area upstream or downstream of the body seat calculated from the minimum diameter which is used to calculate the flow capacity without any deduction for obstructions.
Flow diameter is the minimum geometrical diameter upstream or downstream of the body seat.
Nominal size designation of a safety valve is the nominal size of the inlet.
Theoretical flowing capacity is the calculated mass flow from an orifice having a cross sectional area equal to the flow area of the safety valve without regard to flow losses of the valve.
Actual flowing capacity is the flowing capacity determined by measurement.
Certified flowing capacity is actual flowing capacity reduced by 10%.
Coefficient of discharge is the ratio of actual to the theoretical discharge capacity.
Certified coefficient of discharge is the coefficient of discharge reduced by 10% (also known as derated coefficient of discharge).

The following terms are not defined in DIN 3320 and are taken from ASME PTC25.3:

Blowdown (reseating pressure difference) - difference between actual popping pressure and actual reseating pressure, usually expressed as a percentage of set pressure or in pressure units.
Cold differential test pressure the pressure at which a valve is set on a test rig using a test fluid at ambient temperature. This test pressure includes corrections for service conditions e.g. backpressure or high temperatures.
Flow rating pressure is the inlet static pressure at which the relieving capacity of a pressure relief device is measured.
Leak test pressure is the specified inlet static pressure at which a quantitative seat leakage test is performed in accordance with a standard procedure.
Measured relieving capacity is the relieving capacity of a pressure relief device measured at the flow rating pressure.
Rated relieving capacity is that portion of the measured relieving capacity permitted by the applicable code or regulation to be used as a basis for the application of a pressure relieving device.
Overpressure is a pressure increase over the set pressure of a pressure relief valve, usually expressed as a percentage of set pressure.
Popping pressure is the value of increasing static inlet pressure of a pressure relief valve at which there is a measurable lift, or at which the discharge becomes continuous as determined by seeing, feeling or hearing.
Relieving pressure is set pressure plus overpressure.
Simmer is the pressure zone between the set pressure and popping pressure.
Maximum operating pressure is the maximum pressure expected during system operation.
Maximum allowable working pressure (MAWP) is the maximum gauge pressure permissible at the top of a completed vessel in its operating position for a designated temperature.
Maximum allowable accumulated pressure (MAAP) is the maximum allowable working pressure plus the accumulation as established by reference to the applicable codes for operating or fire contingencies.

Source and images for this page:
Crosby®  - Pressure Relief Valve Engineering Handbook -
Anderson Greenwood Crosby  - Technical Seminar Manual -
Spirax Sarco  - Alternative Plant Protection Devices and Terminology -