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Old March 20th, 2017, 02:35 PM   #1
Epy
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PID control of differential pressure across a control valve

Second guessing myself here, need someone to either tell me I'm wrong or confirm that my thinking is correct please.

I have an equal-percentage characteristic control valve and I need to control differential pressure across it. This is a secondary control mode. The primary mode is control based on flow rate (equal-percentage -> linear installed characteristic, so flow rate is linear with valve position, ideal for PID). When I have to do dP control, PV should be the square root of dP, correct? Because dP is proportional to flow rate squared, so I should sqrt dP in order for it to work with a PID loop properly.
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Old March 20th, 2017, 04:35 PM   #2
Tom Jenkins
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If you are trying to use a valve as both control device and flow meter you will probably have poor results.

If you are trying to control both flow and differential pressure you will definitely have poor results.

Look at the equations here: http://www.engineeringtoolbox.com/fl...nts-d_277.html

For a given valve position Cv is fixed. But, unless your upstream pressure is a constant, the flow for a given valve position isn't fixed. You can have many flow rates for a given valve position as the upstream pressure varies.

If you want to control flow you should use a flow meter, and use your control loop to vary valve position until you get the flow you need.
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Old March 20th, 2017, 04:40 PM   #3
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Was about to come and post to disregard, stress-related brainfart. Thanks for replying though Tom.

There is a flow meter. Primary control mode is based on flow rate from flow meter, changing valve position to control flow rate. Secondary control mode is differential pressure control based on upstream/downstream pressure transducers. Won't be doing both modes at the same time, one or the other. Will use the square root of dP as PV for the secondary mode.
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Old March 20th, 2017, 05:11 PM   #4
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Just a comment...

Equal percentage characteristic is based on the pressure drop across the valve NOT being constant. It achieves a near linear installed characteristic only if the pressure at low opening is much higher than at full opening. If you control the pressure across an equal percentage valve, then the installed flow characteristic becomes parabolic.
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