Hello,

recently there were quite a few very informative threads about control valve positioners, especially this one: http://control.com/thread/1026232438#1257611103.

Few days ago, we have mounted positioner on one control valve in our plant. Positioner is very simple, it has two wires for control signal reference (4...20mA), two other inputs for signaling end positions and additional two wires for actual position. It is fail safe design, which means that air (4 bar) is used to close the valve while, springs are used to open it. This way, in case of air is gone, valve will be fully opened.

So far everything seems normal (it is not used in actual application yet, but will when overhaul is finished), but I have one doubt. When going through all parameters, I have noticed that it has an option to choose valve characteristic (linear, equal percentage etc.).
Default option is linear and that is how it is set. However a mechanics guy told me that characteristic of a valve is more likely equal percentage.

My other colleague told me that it doesn't matter, when in closed loop control, it will find optimal position to control the flow. The only difference would be in speed of moving(!?!)

I'm not sure that this doesn't matter (there is an option in the positioner, after all). I wonder what I might expect in case that valve is equal percentage and positioner is set to linear.

Thank you

 

Not a direct answer but there are two things you need to consider the valve trim can be Linear or Equal % also.

If you apply Equal% to the positioner of an Equal% valve you end up over correcting.

Roy

 

Well, I was thinking something similar. If I know that valve has equal percentage characteristic, then I should choose quick opening on the positioner to make the whole loop linear.

I wonder what are other experiences? Did anyone had similar problem?

Thanks

 

Hi,

the experience has shown that it is not necessary to conduct a comprehensive study to determine the best choice of the characteristic of a regulating valve to ensure loop stability, but there are rules allow a safe choice. All depend the application used.

For on-off regulation the valve may have the fast opening characteristic.

For anti-surge system, water and steam systems. In general, linear valves are used for anti-surge, water systems. whilst steam systems tend to operate better with equal percentage valves.

 

The characteristic of the positioner should be set to match the physical response of the valve that you install it on.

Your colleague that told you "it doesn't matter" is only slightly correct. Yes, if you have a PID control loop then the valve will probably eventually find the correct position, as long as that position is somewhere in the middle of the valves operating range. However, you will have difficulty tuning the loop and you will never be able to properly optimise the control loop. If the valve needs to move around much towards the end of the valves control range (i.e. almost closed or almost open) or needs to operate over a wide range, you will get some very undesirable non-linearity in the control.