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The pressure relief valve is a type of valve used to control or limit the pressure in a system or vessel which can build up by a process upset, instrument or equipment failure, or fire.The pressure is relieved by allowing the pressurized fluid to flow from an auxiliary passage out of the system.ressure Relief Valves. Pressure Relief Valves protect against excessive pressure in pump stations, distribution systems, and transmission mains by opening when the inlet pressure exceeds a preset value and discharges high pressure water to waste pump suction or a zone of lower pressure.
Here i will assume that you are referring Pressure Control Valve as Back Pressure Valve and not Pressure Reducing Valve (Correct me if you are referring to Pressure Reducing Valve) The main difference between Pressure Relief Valve (PRV) and Back Pressure Valve (BPV) is its application. PRV is use for the protection of the Equipment or/and Pipe in the offset condition (If pressure exceed the design pressure).