Cla-Val Models 50-90 and 650-90 Pressure Sustaining Valves are hydraulically operated, pilot-controlled, modulating valves designed to maintain constant upstream pressure within close limits. These valves can be used for pressure relief, pressure sustaining, back pressure, or unloading functions in a by-pass system. In a typical application, 50-90 and 650-90 valves are installed between an upper zone and a lower area of heavy demand. The valves act to maintain desired upstream pressure to prevent “robbing” pressure from the upper zone while maintaining smooth control and positive pressure regulation.