In pressure increases occurring in pressure line, the pumped fluid applies force to the inlet side of the by-pass valve. If this pressure increase exceeds a certain valve, the by-pass spring is compressed by the force applied by the fluid and the by-pass is opened by entering the circuit.
The fluid on the pressure line side is transferred back to the tank via a by-pass. In this way circulation of the fluid in the installation is ensured. If this circulation continues for along time the temperature increases in the pump and in the flow.