Check valves are mechanical valves that permit gases or liquids to flow in only one direction, preventing flow from reversing down the line. They are classified as one-way directional valves. Fluid flow in the desired direction opens the valve, while backflow forces the check valve to close.
Most check valves contain a flat disc that is slightly larger in diameter than that of the port. Flow indicators on the side of the body indicate the direction of the flow for correct installation.