control pilot relieves is the functional set pressure of the PORV. When the pilot valve reaches set pressure it opens and releases the pressure from the dome. The piston is then free to open and the main valve exhausts the system fluid. The control pilot opens either to the main valve exhaust pipe or to atmosphere. Snap acting At set pressure the valve snaps to full lift. This can be quite violent on large pipes with significant pressure. The pressure has to drop below the set pressure in order for the piston to reseat (see blowdown in relief valve article). Modulating The pilot is designed to open gradually, so that less of the system fluid is lost during each relief event. The piston lifts in proportion to the overpressure. Blowdown is typically short. Comparison to Conventional Pressure Relief Valves Advantages Smaller package on the larger pipe sizes. More options for control. Seals more tightly as the system pressure approaches but does not reach set pressure. Control pilot can be mounted remotely. Some designs allow for changes in orifice size within the main valve. Can be used in engines. Higher Operating-to-Set Pressure ratio. Disadvantages More complex, resulting in various fail-open failure modes. More expensive at smaller sizes (starts to even out as pipe size increases). Small parts in pilot valve are sensitive to contaminant particles. Soft goods typically installed in pilot valves cannot withstand higher temperature fluids. See also Three Mile Island accident Relief valve Safety valve References Safety valves