Check_Valves

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.

Single disc swing valves are designed with the closure element attached to the top of the cap. The closure element can be pushed aside by the flow, but swings back into the close position upon flow reversal.  These valves are not good choices in services with frequent flow reversal, as the disc is often pounded into the seat and wears quickly.  These valves may be installed vertically or horizontally, although when mounted vertically a lever and counter-balance should be applied to reduce the valves tendency to stay open in situations where flow velocity changes slowly. 

Double disc or wafer check valves consist of two half-circle disks hinged together that fold together upon positive flow and retract to a full-circle to close against reverse flow.  The valve is inserted between two flanges.   These are most commonly used liquid services.   Wafer valves are popular because they are generally compact and have a relatively low cost.   

Lift-check valves have a disc that is guided.   Spring-loaded types can operate in either vertical or horizontal mounting.   The style that lacks the spring must be mounted in such a way as to allow gravity to close disc.   Composition disc valves are available for gritty liquid services.  

Silent or center guide valves are similar to lift check valves, with a center guide extending from inlet to outlet ports. The valve stopper is spring and bushing actuated to keep the movement "quiet.”

Ball-check valves use a free-floating or spring loaded ball resting in a seat ring as the closure element. Upon reverse flow, the ball is forced back into its seat preventing backflow.  These valves are good for most services.  They can even handle fluids that produce gummy deposits.   Since the disc is free to rotate wear is distributed evenly and this all helps to keep the valve seats clean. 

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.