The relationship between the opening degree and flow rate of a butterfly valve basically varies in a linear‑proportional manner. When used for flow control, its flow characteristic is also closely related to the flow resistance of the connected piping. For example, if two pipelines are fitted with valves of the same nominal size and type but feature different pipeline loss coefficients, there will be significant differences in valve flow rate.
When the valve operates under a large throttling range, cavitation tends to occur on the back of the valve disc, which may cause valve damage. Normally, the valve shall be operated beyond the 15° opening position.
At medium opening of the butterfly valve, the opening profile formed between the valve body and the front edge of the disc is centered on the valve shaft, presenting completely different conditions on both sides. The front edge of the disc moves along the flow direction on one side, and against the flow direction on the other side. As a result, a nozzle‑like opening is formed between the valve body and the disc on one side, while an orifice‑like throttling opening is formed on the opposite side. The flow velocity on the nozzle side is much higher than that on the throttling side. Negative pressure will be generated beneath the valve on the throttling side, which frequently causes the rubber seal to peel off.
The operating torque of a butterfly valve varies with opening degree and valve opening‑closing direction. For horizontal‑mounted butterfly valves, especially large‑size ones, the torque generated by the water head difference above and below the valve shaft due to water depth cannot be ignored. In addition, when an elbow is installed at the valve inlet, biased flow will occur and the torque will increase. When the valve is at intermediate opening, the operating mechanism shall be self‑locking due to the hydraulic torque from flowing water.
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