<Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (October 2017) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (October 2017) (Learn how and when to remove this template message) </Td> </Tr> <P> A check valve, clack valve, non-return valve, reflux valve, retention valve or one - way valve is a valve that normally allows fluid (liquid or gas) to flow through it in only one direction . </P> <P> 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 are often part of common household items . Although they are available in a wide range of sizes and costs, check valves generally are very small, simple, or inexpensive . 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 . </P>

What is the function of non return valve