Check valve is a kind of valve whose opening and closing parts are circular discs and whose action is produced by its own weight and medium pressure to prevent medium backflow. It is an automatic valve, also called check valve, check valve, return valve or isolation valve. The valve flap can be divided into lifting and rotary motion. The lift check valve is similar to the globe valve, and only lacks the stem that drives the disc. The medium flows from the inlet end (bottom side) and flows from the outlet side (upper side). When the inlet pressure is greater than the sum of the disc weight and flow resistance, the valve is opened. Conversely, when the medium is reversed, the valve is closed. Swing check valves have a diagonal disc that rotates around the shaft and work in a similar way to lift check valves. Check valves are commonly used as bottom valves for pumping units to prevent backflow of water. The combination of check valve and globe valve can play a role of safety isolation. The disadvantage is that the resistance is large and the sealing is poor when closed.