Non-return valves, also called check valves, uniflow valves or check valves*, are intended to completely close the passage of a circulating fluid - whether gaseous or liquid - in one direction and leave free passage in the opposite direction. It has the advantage of a minimum travel of the disc or shutter to the fully open position.[1]​.

They are used when it is intended to keep a pipe in service under pressure and discharge the feed. The fluid flow that is directed from the inlet hole to the use hole has a free passage, while in the opposite direction it is blocked. They are also often called one-way valves.

Non-return valves often operate without the intervention of automation processes, since their process is linked to changes in fluids to open and close, that is, their operation is automatically mechanical. This type of valve detects the flow of a process by opening at a certain pressure in the same direction and closing in the opposite direction when the flow decreases or loses pressure.[2]​.

Non-return valves are widely used in pipes connected to pumping systems to avoid water hammer, mainly in the pump discharge line.

There are several types of check valve: