VALVES

CHECK VALVE

Swing Check Valves

The most common type of check valve operates by gravity. The velocity pressure of the fluid exerts a force over the area of the disk and overcomes the force of gravity, unseating the disk. It is imperative that these valves be oriented in a position in which the disk seats itself when no fluid flows.
As might be expected, there is a minimum flow required to unseat the disk. Further, and less obvious, there is a minimum velocity that must be maintained in order to prevent the valve from chattering, or opening and closing rapidly. This is an unstable condition that at best will result in noise, and may lead to valve component failure. Therefore, the valve must sometimes be downsized in order to keep the velocity high enough to keep the disk lifted. For steam, the recommended velocity is 14,000 to 15,000 FPM (71 to 76 m/sec).The conventional swing check valve pivots on a pin attached to the top of the disk, rotating the disk up and toward the edge of the flow stream when pressure is applied in the intended flow direction by the velocity of the fluid. Should the flow direction happen to be reversed due to a pressure imbalance, the disk swings back down against the valve seat, sealing the valve closed.Some manufacturers offer a disk whose pin is located closer to the center of the disk. This balances the disk and is alleged to provide better flow characteristics and non-slam closing