Pressure relief valves are designed to protect a pressure system against excessive normal or subnormal pressure in the event of positive or negative excursion of the system pressure. They are required to open at a predetermined system pressure, to discharge or let enter a specified amount of fluid so as to prevent the system pressure from exceeding a specified normal or subnormal pressure limit, and to reclose after the normal system pressure has been restored. Pressure relief valves must also be self-actuated for maximum reliability except where permitted by the applicable Code of Practice for specific applications.
The most commonly employed pressure relief valve type is the direct-loaded pressure relief valve in which the fluid pressure acting on the valve disc in the opening direction is opposed by a direct mechanical-loading device such as a weight or spring. Direct-loaded pressure relief valves may also be provided with an auxiliary actuator, which may be designed to introduce a supplementary seating load on valve closing and/or to assist valve opening. The power for actuating the auxiliary actuator is commonly of an extraneous source. Should the external power source fail, the valves will open and close without hindrance in the manner of direct-loaded pressure relief valves.
The second type of pressure relief valve is the pilot-operated pressure relief valve that consists of a main valve and a pilot. The main valve is the actual pressure-relieving device while the pilot positions the disc of the main valve disc...
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