In the 1930s, the United States invented butterfly valves, which were introduced to Japan in the 1950s and not widely adopted in Japan until the 1960s. However, their promotion in China began after the 1970s. The main characteristics of butterfly valves are: small operating torque, small installation space, and light weight. Taking DN1000 as an example, butterfly valves are about 2T, while gate valves are about 3.5T, and butterfly valves are easy to combine with various driving devices, with good durability and reliability. The disadvantage of rubber sealed butterfly valves is that when used for throttling, improper use can cause cavitation, resulting in rubber seat peeling, damage, and other situations. Therefore, the correct selection should be based on the requirements of the working conditions. The relationship between the opening of butterfly valves and flow rate is basically a linear proportional change. If used to control flow, its flow characteristics are closely related to the flow resistance of the piping. For example, if two pipelines are installed with the same valve diameter and form, but the pipeline loss coefficient is different, the flow rate of the valve will also vary greatly. If the valve is in a state of large throttling amplitude, the back of the valve plate is prone to cavitation, which may damage the valve. Generally, it is used at an angle of 15 °. When the butterfly valve is in the middle opening position, the opening shape 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. One side of the butterfly plate front end moves in the direction of water flow, while the other side moves against the direction of water flow. Therefore, one side of the valve body and the valve plate form a nozzle shaped opening, and the other side forms a throttle hole shaped opening. The nozzle side has a much faster flow rate than the throttle side, and negative pressure is generated below the throttle side valve, often causing rubber seals to fall off. The operating torque of butterfly valves varies due to differences in opening degree and valve opening and closing direction. For horizontal butterfly valves, especially large diameter valves, the torque generated by the difference between the upper and lower water heads of the valve shaft due to water depth cannot be ignored. In addition, when the elbow is installed on the inlet side of the valve, a bias flow is formed, and the torque will increase. When the valve is in the middle opening, the operating mechanism needs to be self-locking due to the action of water flow torque.
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