<P> In other cases, equipment must be protected against being subjected to an internal vacuum (i.e., low pressure) that is lower than the equipment can withstand . In such cases, vacuum relief valves are used to open at a predetermined low pressure limit and to admit air or an inert gas into the equipment so as control the amount of vacuum . </P> <P> 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: </P> <Ul> <Li> Pressure relief valve (PRV) or Pressure Release 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 PRVs 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 . </Li> <Li> Set pressure: When the system pressure increases to this value, the PRV opens . The accuracy of the set pressure may follow guidelines set by the American Society of Mechanical Engineers (ASME). </Li> <Li> Relief valve (RV): A valve used on a liquid service, which opens proportionally as the increasing pressure overcomes the spring pressure . </Li> <Li> Safety valve (SV): Used in gas service . Most SVs are full lift or snap acting, in that they pop completely open . </Li> <Li> 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 . </Li> <Li> 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 . </Li> <Li> 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 . </Li> <Li> 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 . </Li> <Li> 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 . </Li> <Li> Pressure vacuum release valve (PVRV): A combination of a vacuum pressure and a relief valve in one housing . Used on storage tanks for liquids to prevent implosion or over pressure . </Li> <Li> 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 . </Li> <Li> Modulating: Opens in proportion to the overpressure . </Li> </Ul> <Li> Pressure relief valve (PRV) or Pressure Release 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 PRVs 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 . </Li>

Difference between safety relief valve and pressure relief valve
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