Relief valves are not new to most people; different types of relief valves are used in our homes and automobiles, as well as many other places. Relief valves are pressure limiting or safety devices commonly used to prevent pressure from building up to a point where it might blow seals or burst or damage the container in which it is installed, etc. In aircraft, relief valves are installed within hydraulic systems to relieve excessive pressurized fluid caused from thermal expansion, pressure surges, and the failure of a hydraulic pumps compensator or other regulating devices.

Main system relief valves are designed to operate within certain specific pressure limits and to relieve complete pump output when in the open position. Relief valves are set to open and close at pressures determined by the system in which they are installed. In systems designed to operate at 3,000 psi normal pressure, the relief valve might be set to be completely open at 3,650 psi and reseat at 3,190 psi. These pressure ranges may vary from one aircraft to another. When the relief valve is in the open position, it directs excessive pressurized fluid to the reservoir return line.

Figure 7-23.Pump compensation. (A) full flow position; (B)reduced flow; (C) minimum flow.

 Figure 7-24 shows atypical main system relief valve and its component parts breakdown. The relief valve consists of a cylindrical housing that contains a poppet valve and piston assembly. Each end of the housing is fabricated to include a wrench-holding surface and a threaded port for installation of a hydraulic fitting, and the housing is stamped to identify the ports as PRESS (pressure) and RET (return).

A coil spring at one end of the piston retains it against a stop on the valve housing; and the poppet valve, which is located just inside the pressure port, is spring seated over a passage through the valve. When fluid pressure at the pressure port reaches 3,650 psi, the pressure forces the piston to depress the coil spring and move clear of the poppet valve. Thus, the passage through the piston is opened, and fluid flows through the valve into the return line. When pressure at the pressure port is reduced to 3,190 psi, the coil spring reseats the piston against the poppet valve, and fluid flow through the relief valve ceases. Should the pressure at the outlet port exceed the pressure at the inlet port, the poppet valve will unseat, and fluid from the return line will flow through the valve into the pressure line.