A check valve, clack valve or non-return valve :
a check valve allows flow in one direction and automatically prevents back flow (reverse flow) when fluid in the line reverses direction. They are one of the few self-automated valves that do not require assistance to open and close. Unlike other valves, they continue to work even if the plant facility loses air, electricity, or the human being that might manually cycle them.
Check valves are found everywhere, including the home. If you have a sump pump in the basement, a check valve is probably in the discharge line of the pump. Outside the home, they are found in virtually every industry where a pump is located.
An important concept in check valves is the cracking pressure which is the minimum differential upstream pressure between inlet and outlet at which the valve will operate. Typically the check valve is designed for and can therefore be specified for a specific cracking pressure.
How They Operate?
Check valves are flow sensitive and rely on the line fluid to open and close. The internal disc allows flow to pass forward, which opens the valve. The disc begins closing the valve as forward flow decreases or is reversed, depending on the design. Construction is normally simple with only a few components such as the body, seat, disc, and cover. Depending on design, there may be other items such as a stem, hinge pin, disc arm, spring, ball, elastomers, and bearings.