<P> A relief valve or pressure relief valve (PRV) is a type of safety valve used to control or limit the pressure in a system; pressure might otherwise build up and create a process upset, instrument or equipment failure, or fire . The pressure is relieved by allowing the pressurised fluid to flow from an auxiliary passage out of the system . The relief valve is designed or set to open at a predetermined set pressure to protect pressure vessels and other equipment from being subjected to pressures that exceed their design limits . When the set pressure is exceeded, the relief valve becomes the "path of least resistance" as the valve is forced open and a portion of the fluid is diverted through the auxiliary route . The diverted fluid (liquid, gas or liquid--gas mixture) is usually routed through a piping system known as a flare header or relief header to a central, elevated gas flare where it is usually burned and the resulting combustion gases are released to the atmosphere . As the fluid is diverted, the pressure inside the vessel will stop rising . Once it reaches the valve's reseating pressure, the valve will close . The blowdown is usually stated as a percentage of set pressure and refers to how much the pressure needs to drop before the valve reseats . The blowdown can vary from roughly 2--20%, and some valves have adjustable blowdowns . </P> <P> In high - pressure gas systems, it is recommended that the outlet of the relief valve is in the open air . In systems where the outlet is connected to piping, the opening of a relief valve will give a pressure build up in the piping system downstream of the relief valve . This often means that the relief valve will not re-seat once the set pressure is reached . For these systems often so called "differential" relief valves are used . This means that the pressure is only working on an area that is much smaller than the openings area of the valve . If the valve is opened the pressure has to decrease enormously before the valve closes and also the outlet pressure of the valve can easily keep the valve open . Another consideration is that if other relief valves are connected to the outlet pipe system, they may open as the pressure in exhaust pipe system increases . This may cause undesired operation . </P>

What's the difference between a safety valve and relief valve
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