The purpose of this type of valve is to allow only the medium to flow in one direction and to prevent the flow in the direction. Usually this kind of valve is automatic working, the valve flap opens under the pressure of the fluid flowing in one direction; when the fluid flows in the opposite direction, the valve flap is acted upon by the fluid pressure and the self-weight of the valve flap to cut off the flow. Which check valve belongs to this type of valve, which includes the swing check valve and lift check valve. Swing check valves have a hinged hinge mechanism and a flap-like flap freely rests against the slanted seat surface. To ensure that the valve flap reaches the proper seating surface every time, the valve flap is designed on the hinge mechanism so that the valve flap has enough swivel-open space for true and complete valve flap contact. The valve flap can be made entirely of metal, or it can be fitted with a leather, rubber or synthetic cover over the metal, depending on the performance requirements. Swing check valve in the fully open condition, the fluid pressure is almost unimpeded, so the pressure drop through the valve is relatively small. Lift check valve seat is located on the valve seat sealing surface. In addition to the valve flap valve can be free to rise and fall, the rest of the same as the cut-off valve, the fluid pressure so that the valve flap from the valve seat lifting surface, the media back to cause the valve flap down to the seat, and cut off the flow. Depending on the conditions of use, the valve disc can be of all-metal construction or it can be in the form of a rubber gasket or rubber ring inlaid on the valve disc holder. Like the stop valve, the passage of fluid through the lift check valve is also narrow, so the pressure drop through the lift check valve is larger than the swing check valve and the flow through the swing check valve is limited Few.