The disadvantage of the rubber-sealed butterfly valve is that when it is used for throttling, cavitation may occur due to improper use, causing the rubber seat to peel and damage. To this end, the international development of metal seal butterfly valve, cavitation area decreases in recent years, China has also developed a metal seal butterfly valve in Japan in recent years also developed cavitation-resistant, low vibration, low noise comb-shaped butterfly valve. General seal seat life under normal circumstances, rubber 15 years - 20 years, 80 years of metal - 90 years. But how to choose correctly according to 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 the valve opening and closing direction of different values, horizontal butterfly valve, especially 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.