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What is the difference between relief and safety valve?

Relief or Safety valves perform the following basic functions :

They protect a system against excessive pressure or sudden increases in pressure. When the pressure in a line reaches a pre-set high pressure, the valve opens and allows the pressure to escape either to atmosphere or to another part of the system. Safety valves are generally used for steam, air or other gases. Relief valves are usually used for liquids.

Relief valves versus Safety valves
Relief ValveSafety Valve
Purpose is to limit pressure to a specific pressure.Fail-safe purpose, protect people, property and environment.
Opens proportional to the pressure increase.Starts opening before the set pressure and pops open at the setpoint or at a defined point.
Opens when the pressure reaches a set pressure limit.Starts opening before the set pressure limit.
Can not be used at maximal overpressure values of 3% maximum allowable operating pressure (MAWP)Can be used at maximal overpressure values of 3% MAWP