Check valve

A check valve is a type of valve which normally allows fluid (liquid or gas) to flow through it in one direction but not the opposite direction. Check valves are two-port valves, meaning 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. Checks valves are often part of common household items. Although they are available in a wide range of sizes and costs, many check valves are very small, simple, and/or cheap. Check valves work automatically and most are not controlled by a person or any external control; accordingly, most do not have any valve handle or stem. The bodies (external shells) of most check valves are made of plastic or metal.

An ball valves important concept in check valves is the cracking pressure which is the minimum upstream pressure at which the valve will operate. Typically the check valve is designed for and can therefore be specified for a specific cracking pressure.

Because of the force of gravity on the disc, various types of check valves need to be placed horizontally in an upright position to work properly, especially swing and lift check valves which are not spring loaded. Ball valves, particularly very small ones, are often less susceptible to this problem. Spring loading may work against the force of gravity, forcing the disc against the seat to help keep the valve shut in other orientations. The drawback is that there can be more resistance to forward flow