CHECK VALVE DEFINITION
Briefly said, a check valve is a protection device that prevents the fluid from flowing in an unwanted direction within the piping system or the pipeline (as backflows may damage upstream equipment). How does a check valve work? The valve lets the fluid flow in the desired direction only (if there is enough pressure), and blocks any flow in the opposite direction. Also, the valve closes automatically when the pressure drops. It is therefore critical that the valve is installed with a proper orientation! Note that this type of valve accomplishes its scope without external forces or actuation. This is a key difference vs. gate or globe valves, that need an external force to work (level, wheel, gear or actuator).