The Shand & Jurs Pilot Operated Relief Valve is designed to meet very specific pressure needs. The high accuracy of the valve allows the pressure to be set between 3” of water column pressure and 15 psig using sensitive springs for discrete ranges. The valve operates as follows.
As the internal pressure reaches the valves set point, the upward force on the sensing diaphragm overcomes the downward spring force. This causes the pilot seat to slightly lift, resulting in a small release of flow and marginal pressure relief. This enables the large diaphragm cavity to create a large, upward force which fully opens the pilot seat. This causes a large pressure reduction on the upper side of the main valve diaphragm which results in a full lift of the main valve seat.
When the pressure reduces to the point its resulting upward force is less than the spring force, the pilot seat begins to close. Thus, in turn, builds up pressure on the upper side of the main valve diaphragm which closes the main valve seat. The pressures are again returned to their normal operating levels.
Adapted from press release by Joe Green