In the 1930s, the United States invented the butterfly valve. When it was introduced to Japan in the 1950s and was generally used in Japan in the 1960s, promotion in China was after the 1970s. The main features of the butterfly valve are: small operating torque, small installation space and light weight. Take DN1000 as an example, the butterfly valve is about 2T, while the gate valve is about 3.5T, and the butterfly valve is easy to combine with various driving devices, and has good durability and reliability. The disadvantage of rubber seal butterfly valve is used for throttling, due to improper use will produce cavitation, rubber peel off, damage and so on, so how to correct selection should be based on working conditions. The relationship between the opening of the butterfly valve and the flow rate changes substantially linearly. If it is used to control the flow, its flow characteristics are closely related to the flow resistance of the pipe. For example, the diameters and forms of the valves installed in the two pipes are all the same, while the pipe loss coefficient is different, and the flow difference of the valve will also be very large. If the valve is in a state of large throttling, the back of the valve plate is prone to cavitation, the possibility of damage to the valve, are generally used at 15 °. When the butterfly valve is at the opening degree, the shape of the opening formed by the valve body and the front end of the butterfly valve takes the valve shaft as the center and the two sides form different states. The front end of the butterfly plate on one side moves in the direction of flowing water while the other side flows in the direction of flowing water Therefore, one side of the valve body and the valve plate is formed like a nozzle-shaped opening, the other side is similar to the throttle-shaped opening, the nozzle side than the throttling side of the flow rate is much faster, and throttling side of the valve will produce negative pressure, often There will be a rubber seal off. Butterfly valve operating torque, due to the opening and opening and closing the valve of different values, the horizontal butterfly valve, especially the large diameter valve, due to the depth of the valve shaft, the difference between the head and the resulting torque can not be ignored. In addition, the valve inlet side of the device elbow, the formation of bias current, the torque will increase. Valve in the middle of the opening, due to the role of water flow torque, the operating mechanism needs self-locking.