Page 224 - Valve Selection Handbook

Pressure Relief Valves 211

(beyond 5000 SSU) and in services in which coking, polymerization or
corrosion of the wetted parts of pilot and main valve can occur, unless the
valve is designed for this purpose. Pilot-operated pressure relief valves
used for this purpose are speciality valves in which the critical internals of
main valve and pilot are protected from the process medium by a clean
external medium, even though the pilot is actuated by the process fluid.

Pilot-Operated Pressure Relief Valves with
Indirect-Acting Pilot

Pilot-operated pressure relief valves with indirect acting pilots may be
of two principal types, one in which the main valve is being deenergized
to open and one in which the main valve is being energized to open.
The pilot shown in Figure 5-42b is a spring-loaded safety valve
designed in this case for steam. The main valve shown is of the deener-
gize-to-open type. When the pilot opens on rising overpressure, the dis-
charging fluid acts on a check valve, which in turn vents the dome of the
main valve shown in Figure 5-42a. The system pressure acting on the

Figure 5-42. Main Valve Designed to Open on Being Deenergized and Associated
Direct-Acting Pilot. (Courtesy of Sempell A.G.)