How does a positioner change the flow characteristics of a control valve?
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Theoretically, the flow characteristics of a control valve depend only on the structure of its valve element. However, the manual for the positioner states that it is possible to change the flow characteristics of the control valve using the positioner. How is this change achieved? For example, on the ABB positioner, it is possible to set the working characteristic curve of the positioner: linear, 1/25, 1/50, 25/1, 50/1. What do these parameter characteristics mean? Based on my verification during on-site calibration, only a very limited conclusion can be drawn: the relationship is linear, with the valve position changing proportionally to the given signal ； 1/50 represents a slow opening and fast closing characteristic ； 50/1 has a fast opening and slow closing characteristic; that is, with a small signal applied, the valve can open to a large degree, and the change in valve position is not proportional to the applied signal.
The theory is that the flow characteristics of a control valve depend solely on the structure of its valve element; however, today it is possible to use inexpensive ball valves/disk valves together with intelligent valve positioners to achieve both the functions of a shut-off valve and a control valve.
Is there a 1/1 configuration? If so, go ahead and use it. Based on the description of the characteristic curve (stroke = f{signal}), I believe it represents a linear proportionality constant.
Flow characteristics of control valves: The relationship between the relative flow rate of the medium passing through the valve and the relative opening degree of the valve. 1:1 linear, 1:25 parabolic linear, 1:50 equal percentage (logarithmic), 25:1, 50:1 fast opening, ?
I haven’t read the manual, but my understanding is that 1/25 and 1/50 indicate positive action with different gains, while 25/1 and 50/1 indicate negative action with different gains
Let’s discuss how positioners change the flow characteristics
The locator should support the HART protocol, and the linearity can be changed using a handheld operator!
It’s not clear enough; could you be more detailed? Explain the principle by which it can bring about changes.
A positioner converts electrical signals into pneumatic signals (output); the flow characteristics are determined by the relationship between the relative travel of the valve and the relative flow rate resulting from those pneumatic signals
By changing the geometric dimensions of the feedback cam, it is possible to alter the amount of feedback from the control valve to the positioner, thereby changing the flow characteristics
It is not the flow characteristics of the valve that are changed, but rather the flow characteristics of the control mechanism, including the positioner.