Functioning Mode of Pilot-operated Sanitary Relief Valves
The pressure of a pilot-operated sanitary relief valve is provided on the upstream side (the system being protected) at the dome often by a small pilot tube. The downstream side is the pipe or open air where the PORV directs its exhaust. The outlet pipe is usually larger than the inlet. 2 in × 3 in (51 mm × 76 mm), 3 in × 4 in (76 mm × 102 mm), 4 in × 6 in (100 mm × 150 mm), 6 in × 8 in (150 mm × 200 mm), and 8 in × 10 in (200 mm × 250 mm) are some common sizes.
The upstream pressure tries to push the piston but it is opposed by this same pressure because the pressure is directed towards the dome above the piston. The zone of the piston on which the force of the fluid acts is greater in the dome than in the upstream direction; the result is a greater force on the dome side than on the upstream side. This produces a net sealing force.
The pressure from the pilot tube to the dome is routed through the actual control pilot valve. There are many designs but the control driver is basically a conventional PRV with the special job of controlling the pressure at the main valve dome. The pressure at which the pilot is released is the functional operating pressure of the PORV. When the pilot-operated sanitary relief valve reaches the set pressure, it opens and releases the dome pressure. The piston is then free to open and the main valve discharges the fluid from the system. The control pilot opens either on the exhaust pipe of the main valve or on the atmosphere.