What are non return valves? 

Non return valve is also known as check valve, one way valve, cut off valve, and back pressure valve. Non return refers to a valve that automatically opens and closes the valve plate by replying on the flow of the medium itself to prevent the back flow of the medium.

How non return valve works? 

Non return valve only allows the mediums to flow in one direction, and it can prevent the medium from flowing in the opposite direction. Usually, non return valve is one kind of automatic valve. The valve disc opens under the action of the fluid pressure flowing in one direction; when the fluid flows in the opposite direction, the fluid pressure and the self-weight of the valve plate act on the disc to force it close to valve seat, thereby to cut off the flow. 

Why non return valve is used? 

Non return valve is one kind of automatic valve whose main function is to prevent the back flow of the medium, the reverse rotation of the pump, and the driving motor, and the discharge of the container medium. Non return valve may also be used on lines supplying auxiliary systems where the pressure may rise above the system pressure. 

Non return valve can be mainly divided into swing type non return valve and lift type non return valve.