Technical Terms
In the petroleum refining, petrochemical and chemical manufacturing, natural gas processing and power generation industries, the term relief valve is associated with the terms pressure relief valve (PRV), pressure safety valve (PSV) and safety valve:
- Pressure relief valve (PRV) or pressure safety valve (PSV): The difference is that PSVs have a manual lever to activate the valve in case of emergency. Most PRV are spring operated. At lower pressures some use a diaphragm in place of a spring. The oldest PRV designs use a weight to seal the valve.
- Set pressure: When the system pressure increases to this value, the PRV opens. The accuracy of the set pressure often follows guidelines set by the ASME.
- Relief valve (RV): A valve used on a liquid service, which opens proportionally as the increasing pressure overcomes the spring pressure.
- Safety valve (SV): Used in gas service. Most SVs are full lift or snap acting, in that they pop completely open.
- Safety relief valve (SRV): A relief valve that can be used for gas or liquid service. However, the set pressure will usually only be accurate for one type of fluid at a time.
- Pilot-operated relief valve (POSRV, PORV, POPRV): A device that relieves by remote command from a pilot valve which is connected to the upstream system pressure.
- Low-pressure safety valve (LPSV): An automatic system that relieves by the static pressure of a gas. The relieving pressure is small and near the atmospheric pressure.
- Vacuum pressure safety valve (VPSV): An automatic system that relieves by the static pressure of a gas. The relieving pressure is small, negative and near the atmospheric pressure.
- Low and vacuum pressure safety valve (LVPSV): An automatic system that relieves by the static pressure of a gas. The relieving pressure is small, negative or positive, and near the atmospheric pressure.
- Snap acting: The opposite of modulating, refers to a valve that "pops" open. It snaps into full lift in milliseconds. Usually accomplished with a skirt on the disc so that the fluid passing the seat suddenly affects a larger area and creates more lifting force.
- Modulating: Opens in proportion to the overpressure.
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