PRODUCT OPERATION

The supply pressure (SIG) connected to the relay is sensed by a spring loaded diaphragm and the thrust developed enables a pilot valve to open. The out-put from the pilot valve acts on a piston and the thrust developed by it act upon the plunger, which open the inlet valve to outlet. when malfunction in the control signal circuit occurs pressure holding above the piston down- wards is lost through the exhaust, inlet valva closes with the spring return and the circuit is sealed this closed condition will continue until the signal pressure is established to the original lovel

A manual relief valve is provided in the unit for vent actuator pressure to fail lock drift close or fail lock drift open position of control valves within a time period, also for vent actuator pres-sure when control valve hand wheel is operated. to avoid possible damage to the actuator dia phrogm.