A check valve or non-return valve which normally allows to flow through it in only one direction.
Check valves are two-port valves, means they have two openings in the body, one for fluid to enter and the other for fluid to leave. There are various types of check valves used in a wide variety of applications. Although they are available in a wide range of sizes and costs, Check valves work automatically and most are not controlled An important concept in check valves is the cracking pressure which is the minimum upstream pressure at which the valve will operate.
A swing check valve or tilting disc check valve is check valve in which the disc, the movable part to block the flow, swings on a hinge or trunnion, either onto the seat to block reverse flow or off the seat to allow forward flow.
A lift-check valve is a check valve in which the disc, can be lifted up off its seat by higher pressure of inlet or upstream fluid to allow flow to the outlet or downstream side. A guide keeps motion of the disc on a vertical line, so the valve can later reseat properly. When the pressure is no longer higher, gravity or higher downstream pressure will cause the disc to lower onto its seat, shutting the valve to stop reverse flow.