Check valves are valves that are constructed to allow fluids to flow in one direction. Any fluid pressure counter to the orientation of the check valve causes the valve to shut, and flow to stop. Some varieties of check valves require the pressure to reach a certain threshold before forward flow is allowed. A common check valve consists of a ball next to a small hole. When fluid pressure reverses, the ball fits into the hole, restricting flow.
There are many types of check valves including silent, wafer, swing, horizontal, tilting disc, piston and lift.
Silent check valves are used to control water hammer and other pressure surges. They include a helical spring which closes the valve when flow drops to zero
Lift check valves have relatively high pressure drops because of the flow path through the valve.
Spring valves contain a spring that provides force to close the valve when the pressure drops below a certain value. These valves can wear out over time. Other valves, called swing checks use gravity to force them closed. Because gravity only works in one direction, these valves must be installed with the appropriate orientation or they won't work.
Check valves are available in many formats and materials. True union ball check valves allow the body to be removed for inspection, maintenance and repair.