API-RP-576

4.8 Pilot-operated Pressure-relief Valve
4.8.1 General
A pilot-operated safety-relief valve is a pressure-relief valve in which the major
relieving device or main valve is combined with and controlled by a self-actuated
auxiliary pressure-relief valve (pilot).
Depending on the design, the pilot valve (control unit) and the main valve may be
mounted on either the same connection or separately. The pilot is a spring-loaded valve
that operates when its inlet static pressure exceeds its set pressure. This causes the main
valve to open and close according to the pressure. Process pressure is either vented off
by the pilot valve to open the main valve or applied to the top of the unbalanced piston,
diaphragm, or bellows of the main valve to close it. Figure 7 illustrates a low-pressure
diaphragm-type pilot-operated valve. Figure 8 and Figure 9 show a high-pressure pilotoperated
valve that uses an unbalanced piston with an integrally-mounted pilot. Figure
9 also illustrates optional remote pressure sensing from the vessel and optional dual
outlets to equalize thrust.
Fion Zhnag/ Charlie Chong