Editing Check valve (section)

{{short description|Flow control device}}
{{more citations needed|date=October 2017}}
[[Image:Inconel-Tilting disc check valve--The-Alloy-Valve-Stockist.JPG|right|thumb|Tilting-disc [[inconel]] check valve]]
[[Image:Check Valve.svg|thumb|right|140px|Check valve symbol on [[piping and instrumentation diagram]]s. The arrow shows the flow direction.]]
[[File:Vertical check valve.jpg|thumb|right|Vertical lift check valve]]
A '''check valve''', '''non-return valve''', '''reflux valve''', '''retention valve''', '''foot valve''', or '''one-way valve''' is a [[valve]] that normally allows [[fluid]] ([[liquid]] or [[gas]]) to flow through it in only one direction.<ref>{{Cite book|title=Valves, piping, and pipelines handbook|last=Christopher.|first=Dickenson, T.|date=1999|publisher=Elsevier Advanced Technology|isbn=9781856172523|edition=3rd|location=Oxford, UK|oclc=41137607}}</ref>

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, and 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.

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.