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Check valves are mechanical valves that permit gases and liquids to flow in only one direction, preventing process flow from reversing. They are classified as one-way directional valves. Fluid flow in the desired direction opens the valve, while backflow forces the valve closed. The mechanics of check valve operation are not complicated. Most check valves contain a ball that sits freely above the seat, which has only one through hole. The ball has a slightly larger diameter than that of the through hole. When the pressure behind the seat exceeds that above the ball, liquid is allowed to flow through the valve. But once the pressure above the ball exceeds the pressure below the seat, the ball returns to rest in the seat, forming a seal that prevents backflow.

Check valves use a variety of technologies to allow and stem the flow of liquids and gases. They are as follows: single disc swing valves, double disc swing valves, lift-check, silent, ball-check and cone-check.

Cone check valves use a free-floating or spring loaded cone resting in the seat ring as the closure element. Upon reverse flow, the cone is forced back into its seat preventing backflow.