The disadvantage of the rubber-sealed butterfly valve is that when it is used for throttling, it will cause cavitation due to improper use, causing the rubber seat to peel off and damage. For this reason, metal sealing butterfly valves are now being developed internationally, and the cavitation zone is reduced. In recent years, my country has also developed metal sealing butterfly valves.
Generally, the life of the seal seat is 15-20 years for rubber and 80-90 years for metal under normal conditions. However, the correct selection depends on the requirements of the working conditions. The relationship between the opening of the butterfly valve and the flow rate basically changes linearly. If it is used to control flow, its flow characteristics are also closely related to the flow resistance of the piping. For example, two pipelines are installed with the same valve diameter and form, but the pipeline loss coefficient is different, and the flow rate of the valve will also be very different.
If the valve is in a state with a large throttle range, the back of the valve plate is prone to cavitation, which may damage the valve. Generally, it is used outside 15°.
When the butterfly valve is in the middle opening degree, the opening shape formed by the valve body and the front end of the butterfly plate is centered on the valve shaft, and the two sides form different states. The front end of the butterfly plate on one side moves in the direction of the flowing water, and the other side moves against the direction of the flowing water. Therefore, one side of the valve body and the valve plate form a nozzle-like opening, and the other side is similar to a throttle opening. The nozzle side has a much faster flow rate than the throttle side, and negative pressure will be generated under the throttle side valve. The rubber seal may fall off.
The operating torque of the butterfly valve has different values due to the different opening and closing directions of the valve. The horizontal butterfly valve, especially the large-diameter valve, due to the water depth, the torque produced by the difference between the upper and lower water heads of the valve shaft cannot be ignored. In addition, when an elbow is installed on the inlet side of the valve, a bias current 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 the water flow torque.