A Check Valve (also known as a Non-Return Valve or One-Way Valve) is a mechanical device that allows fluid (liquid or gas) to flow through it in only one direction.

Its primary purpose is to prevent backflow, which can contaminate water sources, damage pumps, or cause a system to lose prime. Unlike the other valves we’ve discussed, a check valve is self-acting—it doesn’t need a handle or a motor to work; it responds entirely to the pressure of the fluid.