Check valve is a kind of automatic opening and closing valve that does not need manual or other operation. It depends on the flow of the pipeline medium itself to open or close the valve. It is mainly used to prevent the reverse flow control of the pipeline medium.Check valve only one direction of flow, that is, can only flow along the inlet can not reverse, medium reverse flow when the disc is closed.Check valves are generally installed to prevent backflow of media when the valve is not closed.Like common water pump outlet will be installed check valve, prevent high pressure water pouring into the pump, pump impeller or other parts due to hydraulic impact loose or damaged, resulting in accidents.In general, check valves are used in piping or equipment to prevent backflow of media. The principle of the check valve is very simple, as shown in the figure, the medium flows in the specified direction, when the flow pressure exceeds the opening pressure of the check valve, the disc is pushed open under the action of the fluid pressure, thus connecting the flow of the pipeline medium.On the contrary, when the pipeline medium is reversed to the backflow, under the pressure of the fluid, the disc is pressed on the check valve body, at this point the valve is closed, thereby cutting off the pipeline medium flow.And the greater the pressure of the backflow medium, the valve disc closed more tightly, the better the sealing effect, completely put an end to the possibility of media backflow. Check valve according to the structure of the different forms, can be divided into lifting type, swing and butterfly three, lifting type can be divided into vertical and horizontal two, its movement is along the axis of the movement.Swing can be divided into single-valve, double-valve and multi-valve, which work by rotation of the center of gravity.And thin butterfly structure for straight flow, by the spring on the hinge pressure in the seat to seal.