Flow characteristics of butterfly valves
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I’m seeking advice from experts: is the flow characteristic of a butterfly valve equal percentage or linear? Can it be done in equal percentages? We have a valve, and its data table is shown in the figure below. Is this equal percentage for the valve determined by the valve itself or set through a positioner? This valve is a butterfly valve. Currently, equal percentage is set for this locator. The corresponding valve opening degrees are: 4mA at 0%, 8mA at 5.5%, 12mA at 14.3%, 16mA at 38.1%, and 20mA at 100%. Is this positioner set to operate in a proportional manner or linearly?
The flow characteristic of butterfly valves is usually nonlinear. This is because there is a nonlinear relationship between the opening degree of the butterfly valve and the flow rate. At a given opening degree, the flow rate of the butterfly valve does not change strictly in an equal percentage manner. Rather, it is influenced by various factors, including the design of the valve itself and its operating conditions. Therefore, the flow characteristics of butterfly valves are generally described as a nonlinear relationship. Regarding the data sheets for the valves and positioners you mentioned, it can be seen that within the current range of 4mA to 20mA, the opening degree of the valve does not change in a strictly proportional manner. Based on the given data, it can be inferred that the locator is set to a non-linear equal percentage. In summary, the flow characteristics of butterfly valves are usually nonlinear, and depending on the design of the valve and actuator, it is possible to set them to adjust the valve opening in a proportional manner or in other ways. .
The flow characteristic of a butterfly valve is generally such that it approaches full flow up to a closure degree of 50%, after which there is a rapid decrease in flow.
The flow characteristic curve generally has the valve opening on the horizontal axis and flow rate on the vertical axis; currently, the flow rate data is missing. Butterfly valves are generally of the quick-opening type. The linear type is basically y=x; the fast-opening curve is above y=x, the parabolic type is below y=x, and the equal-percentage type is below the parabolic type
The flow characteristics of a butterfly valve are determined by its design structure; they are neither linear nor proportional. When a proportional control is desired, a linear controller is generally used, while in special cases a nonlinear controller is chosen. The selection of the controller is related to the valve’s stroke – the signal sent corresponds to the stroke, and this signal is converted into a stroke output that in turn controls the actuator to adjust the position of the valve core and thus regulate the flow rate. As for why a nonlinear controller is chosen in special cases, I’m not sure either
A positioner enables the valve to exhibit the desired flow characteristics by adjusting its opening degree, without changing the valve itself. Under normal conditions, the flow characteristic of the positioner should be set to provide a linear output