CHECK VALVE - NON-RETURN The non-return valves, also called non-return valves, single-flow valves or check valves, have the objective of completely closing the flow of a circulating fluid -whether gaseous or liquid- in one direction and leaving free passage in the opposite direction . It has the advantage of minimal travel of the disc or shutter to the fully open position. They are used when it is intended to keep a pipeline in service under pressure and to discharge the supply. The flow of the fluid that is directed from the inlet port towards the use port has a free passage, while in the opposite direction it is blocked. They are also often called one-way valves.