Unloading valves are used to unload the pump to tank when the pressure in a separate part of the circuit reaches a pre-set level. The most common application is to maintain pressure in an accumulator which may be used in an emergency to operate an essential hydraulic function.
Inlet pressure acts on Port 1 and the system pressure in port 3 acts on the pilot port of the cartridge. When the pressure rises to the valve setting, the relief section opens and the system pressure acts on the pilot piston to hold the valve in the open position. The ratio between the pilot piston and seat diameter to the relief pilot section ensures that the valve will be maintained in the fully open position until the system pressure drops to approximately 85% of the unload pressure
Valve bodies are supplied as line bodies for the use in accumulator circuits. These include a built in check valve which provides with an internal connection to the valve pilot port.