Page 199 - Valve Selection Handbook

186 Valve Selection Handbook

Pressure relief valves with unrestricted supplementary loading and
lift assistance. Figure 5-24 shows a spring-loaded pressure relief valve
in combination with an auxiliary loading and lift-assist device represent-
ed by a pneumatic actuator. In this particular example, the control system
shown incorporates three identical control lines. Using interlocked valv-
ing, one control line at a time may be taken out of service for testing,
repair, or replacement without disturbing the operation of the pressure
relief valve.
At normal operating conditions, the solenoid vent valves Ml to M3 are
closed while the valve M4 closes against the compressed air system and
vents the chamber H to the atmosphere. With the valves in these posi-
tions, compressed air entering the chamber B introduces a supplementary
seating load.
When the pressure relief valve is called upon to open, the pressure
switches deenergize the vent valves Ml to M3 that open to the atmos-
phere and energize the vent valve M4 that connects the compressed air

Figure 5-24. Schematic of Safety Relief Valve with Supplementary Unrestricted Loading
and Lift Assistance with Associated Control System. (Courtesy of Sempell A.G.)