The non-return valves or check valves are used to block the reverse flow of oil or gas in a fluid power circuit. Additionally this valve may be used for pressure control or directional control. The check valve consists of a valve body with inlet and outlet ports and an internal movable member biased by the spring. The movable member can be a flapper or a plunger but most often in valves of hydraulic systems, it is a ball or poppet. They are built to provide free flow in one direction and a check in the other direction and prevent flow-back in hydraulic systems.

     When system pressure at the check valve inlet port is high enough to overcome the biasing spring force on the poppet, the poppet is pushed off its seat allowing oil to flow through the valve. This is known as the check valve's free flow direction. When the fluid attempts reverse direction of flow, the spring pushes the poppet back on the seat to block flow through the valve. Check valves can be direct acting or pilot operated. A pilot operated version is used where the no-flow characteristic of the valve is desired only for a portion of the system cycle. A pilot operated check valve also allows free flow from its inlet port to its outlet port just as in an in-line check valve.