Control Notes

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.

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 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.

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.

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