In the 1930s, the butterfly valve was invented in the United States. It was introduced into Japan in the 1950s. It was not widely used in Japan until the 1960s, but it was popularized in China after the 1970s. The main features of butterfly valve are small operating moment, small installation space and light weight. Take DN1000 as an example, butterfly valve is about 2T, while gate valve is about 3.5T, and butterfly valve is easy to be combined with various driving devices, so it has good durability and reliability. The disadvantage of rubber sealed butterfly valve is that when it is used as throttle, the improper use will cause cavitation, which will lead to rubber seat peeling and damage, so how to choose it correctly depends on the working conditions. The relationship between the opening of butterfly valve and the flow rate basically shows a linear proportional change. If it is used to control flow, its flow characteristics are closely related to the flow resistance of piping. For example, the caliber and form of valves installed in two pipes are identical, and the flow rate of valves varies greatly with different loss coefficients of pipes. If the valve is in a larger throttling range, the back of the valve plate is prone to cavitation, which may damage the valve, generally used outside 15 degrees. When the butterfly valve is in the middle opening, the shape of the opening formed by the valve body and the front end of the butterfly plate is centered on the valve shaft, and different states are formed on both sides. The front end of the butterfly plate on one side moves in the direction of downstream water and the other side moves in the direction of countercurrent water. Therefore, one side of the valve body and the valve plate forms a nozzle-like opening, the other side is similar to the orifice-shaped opening, and the flow speed of the nozzle side is faster than that of the throttle side. Negative pressure will be generated under the valve, and rubber seals will often fall off. The operating moment of butterfly valve varies with the opening and opening direction of the valve. Horizontal butterfly valve, especially large-caliber valve, can not be ignored because of the depth of water and the difference between the shaft and the head of the valve. In addition, when the elbow of the valve inlet side device forms a bias flow, the moment will increase. When the valve is in the middle opening, the operating mechanism needs self-locking due to the action of the hydrodynamic moment.