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How do I get a PID control valve to react quicker?

Hi DCS experts,

I have a PID control valve controlling the level of pressure in our reactor, the controller seems to react slowly to changes in the set point, this ends up with us having too much or too little pressure for longer than intended while it opens and closes before settling eventually at a % that controls the pressure at our intended set point.

The controller/module is a PID Light Controller V1.2 sub type PID_LT_DY

there is an analogue control ramp but this is set to false with setting of ramp rate 0.1 ramp duration 0.1

there is also a block valve that opens when the difference between set point and CV is too great, because the controller is slow to control this block valve is used much too frequently.

could it be the actual control valve is old and slow and not the DCS control?

any help is greatly appreciated

• pid control controller vacuum pressure

Lee W.

• 29 Apr 2015 12:53 PM
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2 Replies

• Bruce Brandt

  

  • 29 Apr 2015 3:17 PM

  What have you done to tune the loop? Your tuning constants may be causing the issue. If you think that the valve is the issue then put the loop in manual and make a step change in the output of the loop and time how long it takes the process to react and to settle. If the response time of making a manual output step change indicates that the process is capable of reacting quickly then the problem is the tuning. If the response time is very slow then you should investigate the valve starting with how long it takes to stroke and then with its sizing, If it strokes quickly then the valve is probably undersized, if it strokes slowly then you need to investigate what's causing it to do that. Hope this helps.

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• Lou Heavner

  

  • 29 Apr 2015 6:16 PM

  I replied earlier but apparently the system didn't take my reply. I agree pretty much with Bruce and said so in the lost reply. Pressure loops, especially vacuum loops can be challenging because they are fast and they are often integrating processes. It doesn't take much deadtime to destabilize the control.  In any case, you need to ensure the valve is capable and making some small steps with the valve in MANual is the best way to to do that.  Make some small steps in the same direction and then some samall steps in the opposite direction. If the valve is not capable, you know where to begin. It could be the valve or the positioner or both. If there isn't a positioner, you may need one. If the valve introduces deadtime, it is going to be difficult to tune for non-oscillatory response until you get that fixed. When it comes time to tune, I recommend lambda tuning. Integrating or self regulating, you can select a lambda value that gives the fastest response without introducing oscillation. I would recommend choosing a slightly larger lambda to ensure stable control over time as valve wear may lead to changing dynamic response. If you can't achieve control with the current design, you might consider adding another smaller control valve in parallel for fine control and use the existing valve for course control. You will also want to make sure there isn't anything else interacting with the pressure control that will become more apparent when the loop is speeded up.

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