Regulating inlet fluid temperature

In this installment of Ask the Experts, Béla Lipták and his cadre of leading experts in process automation, share tips on regulating inlet fluid temperature and controlling the stroking time of a control valve.

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ANSWER:

A booster can increase valve stem speed where the positioner air flow capacity is not adequate. You might find a simple solution by asking the valve vendor what can be done with standard products.

I agree totally that the booster bypass valve (RS) has to be set properly, or the booster will create a valve stem cycle. I have seen some real problems there. We usually did not see the need for a volume booster until the valve was larger than maybe 8 inches or so, and the valve response had to be faster than standard.

I could not find that specific model number in the online Fairchild catalog to see the specifications. The sketch showed mostly ¼-in. tubing, and I have to ask if ¼-in. connections imply any possible real increase in air flow. In rare cases, we have installed larger connections on the valve actuator. The valve vendor might have an alternate actuator that might help.

Once in a while we discovered that the valve stem speed was being limited by the positioner supply air capacity. A plugged or undersized air filter can restrict response severely. We could make that judgment based on watching the three positioner gauges while the valve is being stroked. If a large change in signal pulls the air supply down very much, you have found the problem. If the positioner output pressure does not react quickly, take a close look at the positioner and find that problem. They can be maladjusted.
The valve stem cycle magnitude is a function of valve packing friction, and this can be reduced by changing to modern low-friction materials.

Cullen Langford, President, Cullen G. Langford Inc.


ANSWER:
The combination of a booster in series with a positioner will result in limit cycle unless some of the input to the booster is bypassed around it. Some boosters have a built-in bypass adjustment. If not, one must be added (e.g. tubing and a needle valve). The bypass is opened until the limit cycle ceases now and in future operation.

The positioner output is normally looking into a relatively large actuator volume. When the positioner output goes into the small booster volume with its high inlet sensitivity, you end up with a limit cycle, unless you bypass enough of the flow around the booster so the positioner again effectively also sees some of the actuator volume. New digital positioners may be able to be tuned to help deal with this situation, so that not so much air needs to be bypassed. Bypassing air around the booster reduces its performance, so it is desirable to have it at the minimum needed to assure stability.

Greg McMillan, Principal Consultant, Emerson Process Management


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Bela LiptakAsk the Experts,” on ControlGlobal.com, is moderated by noted process control authority Béla Lipták. Béla and his cadre of leading experts in process automation, recruited from among the co-authors of the Instrument Engineer’s Handbook 4th Edition, answer process control questions from all comers.
 

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