Valve flow characteristics and actuator settings—those who understand it, please share their thoughts
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For example, if a valve has an equal percentage flow characteristic, during tuning it is common to observe that at an output of 25% the valve position is 50%, and at an output of 50% the valve position is 70%. Can it be assumed then that the output of the positioner should also be set as a percentage? Since the valve stem’s operation is already based on percentages, isn’t it incorrect to set the valve positioner to use percentages as well?
The flow characteristics and the output of the locator have no relation to the valve opening; when the locator output is 25%, the valve position is 50%, and it is necessary to adjust the valve’s travel distance.
Yes, it’s a mistake, but there is a prerequisite: the flow characteristics of the locator are currently not very useful, and you can simply set the locator to linear mode
Refer to the figure below: control valves need to have different flow characteristics in order to adapt to the properties of the process variable being controlled. That is, the non-linearity of the control valve is utilized to correct the non-linearity in the characteristics of the process equipment, thereby making the entire control system linear. The corrective effect of control valve characteristics on the characteristics of process equipment also applies to the relationship between positioners and control valves. In other words, by using the locator for the characteristic in the lower left corner of the diagram below, it is possible to make the relationship between the flow characteristic of the control valve for the characteristic in the upper right corner and the signal follow a linear pattern, and vice versa. Nonlinear positioners can also be used to endow control valves with linear characteristics with certain nonlinear flow characteristics. The characteristics of a valve positioner refer to the relationship curve between the positioner’s input and output, which can be observed directly through the positioner’s input and output signals ； The characteristics of a control valve refer to the relationship between the travel position of the valve (or in other words, the input signal, which is the output of the actuator) and the flow rate passing through the valve; these characteristics cannot be observed without specialized equipment. ◆ In most cases, control valves with certain flow characteristics are used, in combination with positioners that have a linear characteristic. In this case, the flow characteristic of the control valve + actuator is the flow characteristic of the control valve itself ； ◆ But! There are also a few control valve manufacturers that design the flow characteristics of their control valves to be linear, and then equip them with positioners featuring different characteristics according to the customer’s requirements. In this case, the flow characteristics of the control valve + actuator are determined by the actuator. (Rarely used; only in very few imported valves, or when special flow characteristics are required.) ◆ Of course, when the flow characteristics of the control valve itself do not meet the requirements, it is possible to modify these characteristics by using a positioner – thereby altering the flow characteristics of the control valve together with the positioner. Attention! Use a control valve with certain flow characteristics, in combination with a positioner featuring linear characteristics. It is the approach taken in the vast majority of cases. If no control valve with a specific characteristic can be found, set the positioner to that characteristic.
You’re right; if the valve already has percent characteristics, then a linear actuator will suffice, and together they result in equal percentage performance. However, some valves cannot be easily set to percent-based operation due to their structural characteristics; for example, eccentric valves, hemispherical valves, ball valves, butterfly valves, etc. Their inherent behavior is approximately linear, which requires that the positioner be set to percent-based operation, resulting in percent-based control overall. The locator can have any desired properties set, which is achieved through the handheld operator.