Check Valves:

Check valves are designed to avoid the reversal of flow within a piping system. These valves are activated through the flowing material within the pipeline. The pressure of the fluid passing by the system opens the valve, although any reversal of flow will close the valve. The Closure is accomplished through the weight of the check mechanism, through back pressure, through a spring, or through a combination of these means. The common categories of check valves are swing, piston, tilting-disk, butterfly, and stop.