Butterfly valve opening and flow between basically a linear proportional change. If it is used to control the flow, the flow characteristic is also closely related to the flow resistance of the pipes. For example, the diameters and forms of the valves installed in the two pipes are exactly the same, while the flow rate of the valves will vary greatly with different loss coefficients of the pipes.
When the butterfly valve is at a medium opening, the opening shape formed by the front end of the valve body and the valve plate is centered on the valve shaft, and different states are formed on both sides: the front end of the butterfly valve on one side moves in the direction of downstream water, and the other side moves in the direction of upstream water. Therefore, one side of the valve body and the valve plate form an opening similar to the nozzle shape, the nozzle side is much faster than the flow rate measured by throttle, and the throttle side valve will produce negative pressure below, often rubber seals fall off.