Valve position control ( how to determine ratio of Cv values of the coarse and fine valve )

[Yobbo]

Dear Readers,

I am aware that my question may be on the threshold of control engineering and mechanical engineering, but I give it a try anyway. For a flow control of natural gas I need to create a situation, where the controllability is high. With that in mind I am opting for a valve position control solution, where a fine and a coarse valve are parallely connected and where the attached control scheme is being applied. The principle is that the fine valve will be able to control small deviations from the set point, whereas the coarse valve will lag behind with a integrator-only controller when deviations persist and cannot be corrected by the fine valve alone. This way the coarse valve will provide the base load and the fine valve will take care of small disturbances. The integrator-only controller must be provided with anti-windup facility, so that when the output of the integrator-only controller will stop integrating when the output has either reached 0% or 100%. For manual control it should be available as well.
The question is how I should determine the ratio of the size or capacity ( Cv or Kv value) over the coarse and the fine valve. Intuitively I would think of resp. 75% and 25%, but there may be criteria or guide lines, that I do not know of now, but that might help me make this choice.
I would be thankfull for any suggestions about either where to search for them or how to actually determine this ratio.

With best regards I remain,

Karel Postulart

Karel Postulart, The Netherlands
Nuon Power Generation

 

[xnuke]

The "Instrument Engineers' Handbook Volume 2: Process Control and Optimization" has a section on control valve sequencing that shows how to do this.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
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[Yobbo]

Thanks Xnuke,
I will try to get a hold on this book. I did look at the index, but I couldn't make out whether this subject is in there.


Karel Postulart, The Netherlands
Nuon Power Generation

 

[roydm]

The small valve must be able to control the flow at the minimum setpoint.
The large valve has to be big enough to handle all the flow, say 125%
The controller for the large valve is arranged as a position controller, it opens or closes to maintain the small valve at around mid travel.

I would guess a 10:1 ratio would be a starting point.

 

[controlnovice]

Note that the you may want the I-only controller (ZC) to have Gap control, such that when the small valve is between 20-80% open, the large valve doesn't move. This 20-80% could be 30-70% ...or whatever range you feel best. When the small valve opens > 80%, then the large valve will start to open...when the small valve closes < 20%, the large valve starts to close. Once the small valve output comes within the 20-80% again, the large valve stops at it's last position.

Be careful on trim selection of the valves. I've seen brand new valves have no flow between 0 and 15-20%, then all of a sudden flow hits at 15-20% open. Probably more of wrong process data dictating valve Cv or something...


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[controlnovice]

Attached is some technical information from Foxboro circa 1970 regarding sequencing control valves.

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