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Butterfly Valves and Control Performance

Because butterfly valves cost less than “real” control valves like globe valves or characterized ball valves, they are sometimes used in place of control valves to save money. This decision is often costly in the long term because of the poor control performance resulting from butterfly valves.

Late last year I optimized several control loops at a mid-sized manufacturer of specialty chemicals. Similar to most plants I have worked at, I found a number of control loops that were oscillating. Many of them oscillated because of valve stiction, incorrect controller settings, or process interactions. One of the loops, a distillation column level control loop, oscillated as a result of using a butterfly valve as the final control element.

Figure 1. Oscillating level control loop.

Figure 1. Oscillating level control loop.

To perform well, a PID control loop needs (among other things) that the process gain remains constant. In other words, the process variable must change linearly with changes in controller output. A small degree of nonlinearity can be tolerated, especially if we apply robust tuning methods, but if the process gain changes by more than a factor of 2, we can expect control problems. And this is why a butterfly valve makes a poor choice for a control valve – it has a very nonlinear, typically S-shaped flow curve, as shown in Figure 2. However, the shape of the curve can also be concave (equal percentage) or convex (quick opening), depending on the process’ flow-pressure characteristic.

Figure 2. Typical butterfly valve flow characteristic.

Figure 2. Typical butterfly valve flow characteristic.

Figure 3 shows how the gain of a typical butterfly valve changes from less than 0.2 to almost 3 over the span of the controller output. The process gain varies by a factor of 15! This large variation in process gain makes it impossible to have consistently good control at all valve positions.

Figure 3. Typical butterfly valve gain.

Figure 3. Typical butterfly valve gain.

At the chemical company the butterfly valve was used to control the bottom level of a distillation column. The distillation column was the last one in a train of three columns, of which each column had a progressively smaller diameter. Moderate increases in feed rate to the first column easily caused high-level alarms when they propagated to the small final column. The level controller originally seemed to be responding too slowly to handle these upsets, so the loop tuner increased the controller gain to achieve fast response at high flow rates. However, at normal flow rates, where the process gain was 15 times higher, the loop was unstable and oscillated continuously as shown in Figure 1.

The correct solution to this problem would have been to replace the butterfly valve with a control valve that has a linear flow characteristic and then retune the control loop. However, this could only be done during the plant’s annual maintenance shutdown. In the mean time we installed a characterizer to linearize the butterfly valve (Figure 4). The characterizer compensated for the butterfly valve’s nonlinearity and made the flow through the valve follow the controller output in a reasonably linear fashion.

Figure 4. Level control loop with characterizer.

Figure 4. Level control loop with characterizer.

With the characterizer in place we retuned the controller. After this the oscillations stopped and the loop performed much better than it did before. However, the control performance was still not as good as what a linear control valve would have provided. The real solution to the problem remained replacing the butterfly valve with control valve, but this had to wait for the next maintenance shutdown.

 

Stay tuned!
Jacques Smuts – Author of the book Process Control for Practitioners

 

7 Responses to “Butterfly Valves and Control Performance”

  • mohamed elsadig:

    Dear Jacques,
    would you please the implementation of the Characterizer is in term of software (functional block or hardware).
    thank you
    mohamed

  • Mohamed, in this case we implemented the characterizer in the DCS using a function block. You could also do the characterization in the valve positioner if the positioner supports it (most digital valve positioners do). My preference is to do it in the control system because if the positioner is replaced, the new positioner might be put in service without the characterizer.

  • H.B.R:

    Dear, Mr. Jacques.
    I’m an I&C engineer working for an EPC contractor. In the site where I worked last year, there was the exactly same problem as described in the figure no.4 of this article.
    The process was distillate water level and it was controlled by the butterfly type control valve.
    The process value was hunted like figure no.1 but with bigger magnitude. I think the problem was because of the actuator or positioner. When PID output sent a demand signal to the valve, the positioner feedback value followed the demand signal after 1~2 seconds.(i,e. there was a deadtime in the actuator.) I couldn’t detect that it was caused by the sticky actuator or positioner problem because the plant was under the commercial operation.(now, I’m working the head office and that problem is still remaining in the site.)
    Can you imagine the status of the process?? It’s oscillated with really big magnitude.(bigger than figure no.1)
    The action I did during the commissioning was only adjusting PID value at that time due to the tight commissioning time. After that, only the oscillation magnitude became smaller and the symptom alarming every minute disappeared but still big oscillation has existed.
    Thank you for your helpful information.

  • Juan:

    Greetings Jacques,

    How did you find the function to put into the characteriser block given there was no flow measurement? Did you derive it from the rate of change of the level or from your experience with butterfly valves?

    Regards and thanks for sharing.

  • Juan – Although my diagram does not indicate it, they did have a flow measurement that we used for the characterization. Your suggestion of using the rate of change of level could work, provided that you can keep the inlet flow to the distillation column constant.

  • Shady:

    Dear Jacques,

    I would like to get your feedback about high performance butterfly valves, which is claimed by valve vendors to have equivalent control performance as globe valves, but with less controllable range.

    Thank you.

  • Shady, Because the valve manufacturers try to get a “bubble-tight” seal with the so-called high-performance control valves, the valves tend to stick in the fully closed position and significant force is needed to open them, causing them jump to 5% or so upon opening. This means that control at the low end is very poor. Also, I have seen many cases of butterfly valves being vastly oversized – achieving maximum flow at 20% to 30% open. This limits your effective control range to be from 5% to 25%. Add to this the valve’s nonlinear flow characteristic and you end up with a very poorly performing final control element.

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