A multitude of different types of Check Valves have been used to pass fluid in one direction through a conduit and block fluid flow in another. A Check Valves should be reliable in operation, should be useful for controlling upward and downward as well as horizontal flow, and should be compact. The valve should result in a low-pressure drop. The valve should not chatter in operation and should close at zero flow, rather than slamming shut after flow reversal. The valve should have a long service life, should be easily repairable, and should be relatively simple and economical to construct.

Stem-guided piston-type CHECK VALVES are often biased closed by a spring to prevent slamming after a flow reversal. However, these valves typically have a high-pressure drop due to the turbulence resulting from the spider supporting the stem and the fact that the valve member remains in the flow path. Further, these valves must be relatively large to accommodate this structure. Butterfly and split check valves may be made more compact than piston-type valves, but are usually found to be unreliable when handling fluids carrying sand or fibrous material. Swinging-type check valves have heretofore been generally large and bulky, and normally are useful only in horizontal runs. In addition, swinging check valves usually promote turbulence and result in somewhat high pressure drops.

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