The Shand & Jurs 94645 Pilot Operated Relief Valve is designed to meet very specific vacuum needs. The high accuracy of the valve allows the vacuum to be set between -.11 psig to -5 psig using sensitive springs for discrete ranges. The valve operates as follows.
As the internal vacuum reaches the valve’s 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 vacuum 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 vacuum reduces to the point its resulting upward force is less than the spring force, the pilot seat begins to close. This, 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.
The valve’s set point and blowdown can both be adjusted externally. The blowdown adjustment allows the valve to be operated in a snap action mode or a modulating mode. The snap action mode provides full opening at set point while the modulating mode provides a proportional opening with respect to over pressure.