In order to be ready for operation, hydraulic systems normally need the pump to continue running even if its flow rate is not needed at that time, e.g. in the loading phase of a working cycle. The pump is then switched to circulation, i.e. the flow rate is circulated back to the container on the shortest route possible with the smallest possible pressure loss. Either the centre position of a directional valve (Figure U 15 a) or a separate idle circulation valve is used for this (b), with the full flow rate always circulated - even in the case of variable pumps if they do not have p/Q feedback control - which is not always without losses, as in addition to the internal idling losses in the pump, there are also flow and pressure losses, which the flow rate causes on its way through the idle circulation valve (directional valve) and the filter into the container.
These circulation losses can be almost completely avoided by using a pressure/flow control for the pump.
Figure U 15: Pump circulation using directional valves (a) and idle circulation valves (b)