Check valves, also known as nonreturn or one-way valves, enable fluid to flow one way in a pipeline. A clapper hangs from a hinge, clapper shaft, or pin mounted to the underside of the bonnet. This design inhibits backflow in a line.
Because of their simple design, check valves generally operate without automation or human interaction and instead rely on the flow velocity of the fluid to open and close. This means they do not have a method of outside operation like a handle or lever. The minimum upstream pressure required to operate the valve is called the cracking pressure, generally between 1 and 5 psi. Check valves are designed specifically with this number in mind.
The degree of opening on a check valve is determined by the flow rate. The higher the flow rate, the more open the valve will be until it reaches its maximum, full-open position. On many check valves, the full open position is approximately 85°.