Pilot-operated relief valves use the pressure in the vessel rather than a spring to seal the valve and a pilot to activate the mechanism. Figure 13-6 is a schematic of a typical pilot-operated valve. A piece of tubing communicates pressure between the relief valve inlet and pilot. When this pressure is below the set pressure of the pilot, the pilot valve is in the position shown, and there is pressure communication between the inlet pressure and the top of the disc. Since the disc has approximately 25% greater area on the top than in the throat of the nozzle, there is a net closing force on the disc equal to the difference in magnitude of the areas times the vessel pressure. The closer the vessel pressure gets to the set point, the greater the closing force, and thus “simmer,” which can occur in spring loaded valves near the set point, is eliminated. When the set point is reached, the pilot shifts to the right, blocking the pressure from the vessel, venting pressure from above the disc, and allowing the disc to rise.
Pilot-operated valves have the advantage of allowing operations near the set point with no leakage, and the set position is not affected by backpressure. However, they will not function if the pilot fails. If the sensing tine tills with hydrates or solids, the valve will open at 25% over the pressure trapped above the disc (usually the normal operating pressure of the vessel). For this reason they should be used with care in dirty gas service and liquid service. They are used extensively offshore where all the platform relief valves are tied into a single header because up to 50% back-pressure will not affect the valve capacity.
A disadvantage of pilot operated valves is that, if there is no pressure in the vessel, back-pressure could cause the disc to lift. This could occur if a vessel was shut-in and depressured for maintenance, the relief valve was installed in a header, and another valve in the header was opened, building back-pressure. Figure 13-7 shows an arrangement of two check valves in the sensing system to assure that the higher of the vessel pressure or the header pressure is always present above the disc. This is called “backflow protection.” A backflow preventer should be specified if the vessel could be subject to vacuum, such as a compressor suction scrubber, or where the back-pressure in a relief header can exceed the relief valve set pressure when other valves are relieving.