Check Valve Closing

If the upstream pressure ever falls below the cracking pressure or there is a back pressure (flow attempting to move from the outlet to the inlet), then the valve will close. Depending on the check valve design, the closing mechanism can change. In short, the back pressure pushes a gate, ball, diaphragm, or disc against the orifice and seals it. Depending on the design the closing process can be assisted by a spring or gravity.