A check valve is a device that permits the flow of fluids (liquids and gases) in one direction only, intercepting the flow of liquids/gases to reverse. Check valves are commonly referred to as two- port valves, implying they have two openings. One opening is for the gas to enter, and the other one is for the gas to depart.
A check valve, also called a non-return valve, depends on the differential pressure for its operation. Higher pressure is needed on the inlet side of the valve as compared to the outlet side to open the valve. The valve will close if the output side pressure is more than the input side pressure.