Understanding the nomenclature is crucial to understanding the various factors affecting pressure relief valves and the operations that they, in turn, effect. Here are some of the key terms to know in the relief valve world.

Pressure Relief Valve

A pressure relief valve (PRV) opens when pressure in the vessel exceeds a specific limit, then closes when the pressure is normalized. Safety valves, safety relief valves, and relief valves are all types of pressure relief valves.

Safety Relief Valve

A safety relief valve can have two functions: as a safety valve or a relief valve. A safety valve opens quickly and fully, often “popping” open, due to static upstream pressure, and is often used with compressible fluids. A relief valve opens when static upstream pressure is higher than the opening pressure, and it opens in proportion to the pressure increase. This type of valve is more often used with incompressible fluids.

Operating Pressure

The expected pressure of a vessel while in operation. The operating pressure is generally at least 10 percent lower than the MAWP.

Set Pressure

Set pressure or nameplate set pressure is the inlet pressure at which the valve must open, required by code.

Maximum Allowable Working Pressure

Maximum Allowable Working Pressure, or MAWP, is the maximum pressure allowed at a specified temperature in the vessel. This is used to determine the set pressure of the pressure relief valves.

Opening Pressure

The pressure at which the valve begins to open.

Design Pressure

The maximum pressure the valve is designed to withstand while maintaining function.


Overpressure is presure that is over the set pressure for the valve.

Back Pressure

Constant or variable pressure at the outlet of a pressure relief valve. Backpressure can build up as a result of flow after the valve opens or can be superimposed, occurring at the discharge header before the valve opens.

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