A check valve, clack valve,
non-return valve or one-way valve
is a mechanical device, a valve, which normally allows fluid
(liquid or gas) to flow through it in only one 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. 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
An 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.
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