Smart positioners can change the flow characteristics of valves
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All positioners come with this function; it can be set in the menu. The positioner’s flow characteristics can be adjusted using options such as equal percentage, linear, and quick open! The flow characteristics are determined based on the flow curve. Then I have a question: is it okay to make such changes? If it’s really that convenient, does that mean we need to consider many fewer factors when selecting valves? Do any of you have practical experience in modifying the flow characteristics of valves? Besides valves that allow the modification of flow characteristics, what other factors must be taken into account when selecting valves? Process medium? Valve response time? Sealability? What is the impact of valves on flow rate? :P
Will modifying the valve characteristics not affect its stability performance (MTBF) and service life?
This is a characteristic of the positioner control; it has little to do with the properties of the valve itself – they are not the same!
Setting the flow characteristics on the positioner can change the valve’s flow characteristics, but it is not as accurate as those of the valve itself
The flow characteristics of the control valve are still selected based on the control system; if the actual control quality does not meet the requirements, corrections are made using a positioner.
Request an expert to provide an answer; please give examples of cases in Hart where the flow characteristics differ from those of the actual valve:)
The flow characteristics of the control valve itself are determined by the structure of its valve core; I think the flow characteristics of the positioner are the result of modifications to the signal control method. The flow characteristics of the valve itself are inherent; as long as its structure is not damaged, they should remain unchanged.
Valve positioners have the ability to modify the flow characteristics of control valves. However, in practical use, especially during design, this feature should be avoided as much as possible. A valve positioner alters the flow characteristics of a control valve by changing the relationship between the signal and the stroke. In other words, after using the function of the positioner to modify the flow characteristics of the control valve, there will no longer be a linear relationship between the signal of the control valve and its stroke. This will cause certain difficulties in maintenance, adjustment, and management. So it’s best not to use it if possible. Changing the flow characteristic of a control valve using a valve positioner is merely a contingency measure when no other option is available. In fact, merely determining whether the flow characteristics of a control valve meet the requirements of the process equipment demands a high level of theoretical knowledge and experience. It is usually considered for use only when the flow characteristics and design of the control valve differ.
The control valve operates on a proportional basis, and the machining characteristics of its valve element have been largely determined. If these meet the process requirements, it would seem appropriate to choose a linear output for the intelligent valve positioner’s output characteristics. I’m a bit puzzled as to why it isn’t set to the same proportional setting as the valve itself; anyone who understands this could please explain it to me.
Personally, I think the valve should originally be set at EQ%, with corrections made through a positioner. I encountered many such situations during the selection process. I consulted the locator supplier, but they also couldn’t explain why this happened