Page 171 - Valve Selection Handbook

5

PRESSURE RELIEF

VALVES

PRINCIPAL TYPES OF PRESSURE RELIEF VALVES

Pressure relief valves are designed to protect a pressure system against
excessive normal or subnormal pressure in the event of positive or nega-
tive excursion of the system pressure. They are required to open at a pre-
determined 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 sys-
tem pressure has been restored. Pressure relief valves must also be self-
actuated for maximum reliability except where permitted by the applica-
ble 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 man-
ner of direct-loaded pressure relief valves.

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