Pressure Relief Valve: A pressure relief device designed to close once normal conditions are restored, preventing further fluid flow.
Safety Valve: A spring-loaded pressure relief valve actuated by the static upstream pressure of the valve, characterized by rapid opening or popping action. Typically used for compressible fluids.
Relief Valve: A spring-loaded pressure relief valve actuated by static upstream pressure, opening proportionally to pressure increase above the set point. Commonly used for incompressible fluids.
Safety Relief Valve: A spring-loaded pressure relief valve that can function as either a safety or relief valve depending on the application.
Conventional Safety Relief Valve: A spring-loaded pressure relief valve whose operational characteristics are directly affected by changes in back pressure.
Bellows Safety Relief Valve: A spring-loaded pressure relief valve equipped with a bellows or similar device to minimize the effect of back pressure on valve performance.
Valve Trim: Refers to the internal components of a pressure relief valve, typically including the nozzle and disc.
Set Pressure: The inlet pressure at which the pressure relief valve is set to open under service conditions. For gas, steam, or vapor service, it is the pressure at which the valve pops. For liquid service, it is the pressure at which a steady stream of liquid begins to discharge vertically from the valve.
Overpressure: The pressure increase above the set pressure of the relief device. It may exceed the allowable accumulation if the valve is set below the vessel’s maximum allowable working pressure (MAWP).
Maximum Allowable Working Pressure (MAWP): Defined by pressure vessel codes, MAWP is the highest pressure at which the vessel may be operated at a specific metal temperature, based on material type, thickness, and design.
Blowdown: The difference between the actual popping pressure and the actual reseating pressure of a pressure relief valve, expressed as a percentage of the set pressure or in bar/kPa.
Lift: The vertical movement of the disc in a pressure relief valve.
Back Pressure: Pressure existing at the outlet of a safety relief valve.
Built-up Back Pressure: The pressure developed in the discharge header as a result of flow after the valve opens.
Superimposed Back Pressure: The pressure present in the discharge header before the valve opens.
Nozzle: A pressure-containing component in continuous contact with the process medium, whether the valve is open or closed.
