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Big Valve Small valve - how to size the small valve

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Justice100

Justice100

Chemical
Jun 18, 2008
42
Hi,

We are in a position where we are about to install a pressure control valve but new requirements have come in for a much lower turndown so the intention is to install a small control valve in parallel to provide the turndown. There needs to be controllability all the way from the normal flow rate to the turndown rate. I have looked through the Liptak Instrument Engineers Handbook Vol 2, which explains some different control techniques for this. The 1st technique uses the small valve at the low range and then switches to the large valve once outside the small valve range; only one valve is to be open at a time so there is a switch point where it closes and the other opens and vice versa. Sizing the small valve seems straight forward.

The other technique is "floating" where the big valve provides coarse control and the small valve provides fine control. It seems this technique has a slower response but to me seems a better way of doing it as there is no switching point. Also, I am not sure how to size the small valve for this technique - does anyone have a good reference for this? Also, any experience or learning on these techniques would be appreciated.
 
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OP,
Why is the lower turn down required? Is it two distinct processes such as filling a tank or just maintaining level? Do you need controllability across the entire range or just at the top and low ends? Could this be accomplished with two valves the same size in parallel? How precisely do you need the flow regulated and at which flow rates? Edit: a few more considerations. What valve types (globe, butterfly, vball..etc.) are being considered? Valve characteristics (% open vs CV)? Are you married to the valve you originally size?

There are multiple ways of skinning this proverbial cat and which one you choose should be based on the current processes and any future processes these valves might be involved with controlling.
 
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For all split flow / pressure control schemes I've set up or come across for control applications similar to this, the small valve is full open when the big valve starts to open, and it remains full open all time when the big valve is modulating. Why do you ask for this little valve to be closed ?
 
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To Heaviside1925:
People change minds! No, it is simply pressure control based on an upstream separator; the Separator is fed from multiple wells/choke valves. Precise control is not necessary. Control is required across the entire range. Again, precise control is not necessary, just need to maintain a stable pressure. Globe valve, linear characteristic for existing big valve. The big valve is fixed and can't be changed.

georgeverghese:
I was just describing what is in the Liptak book. The problem with what you describe is control in the transition region - when the small valve is going from say 90% to 100% open and initial opening of the big valve 0-10%. This could be solved by moving to floating control during this transition and this maybe the way we go.

Thanks to both of you to replying
 
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OP,
Thank you for the additional information. The big valve is fixed, this unfortunate because it somewhat limits your options. I agree with georgeverghese's split flow approach for control. Now onto your concern about the transition region. This is where the valve's characteristic (fast opening, linear, equal percent) come into play. Since you have no options to adjust the big valves characteristics then compare the of change of CV vs %open of the lower end 0-10% of the big valve to that of the different option for a smaller valve's higher end 90-100%. You will want the best matching rates (won't be perfect but it will help in tuning). The second part will be to make sure your loops are tuned properly. Your controls engineer should be able to model this, so minimal if any adjustment should be required upon commissioning.
 
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There is usually a small deadband from when the little valve has reached 100% open to the start of opening of the big valve. That usually takes care of any potential instability during the transition. DCS controls engineers have other methods also of dealing with the transition, which I have vague recollection of.
 
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