Pilot-operated pressure relief valves are characterized by the presence of a pilot valve in addition to the main valve, which encloses a floating piston. The floating piston is designed with larger effective area on its top than on its bottom. A sensing line is used to allow the process pressure to be sensed by the pilot and to act on both the top and bottom surfaces of the floating piston. At the set pressure, the pilot opens and vents the pressure from the top of the piston, allowing it to open to relieve overpressure in the protected equipment. Many pilot valve designs are available, resulting in different operating characteristics. The pilot design may result in pop action or modulating action of the piston. In addition, the process fluid may or may not actually flow through the pilot (flowing or non-flowing). Due to the variety of available designs, the manufacturer should be consulted for particular applications. Figure 1200-15 shows a schematic diagram of a typical pilot-operated pressure relief valve.
Pilot-operated valves have a few primary advantages that may warrant selective application. The required margin between operating and set pressure is smaller than for either conventional or balanced valves, because the process pressure actually holds the floating piston tightly closed until the pilot vents. Designs are available that will tolerate higher levels of back pressure than conventional or balanced pressure relief valves. The effect of inlet line flowing pressure drop (see “Inlet Piping” on page 1200-43) can be avoided by connecting the pressure sensing line directly to the protected equipment (remote sensing).