Control Notes

Earlier this year I did a controls optimization project on a process unit in a large chemical company. They had a complex and interactive process, but their controls were mostly simple control loops with no cascade, feedforward, gain scheduling etc. I am a firm believer in keeping things simple, but no simpler than they should be – which brings me to the story.

The unit had a huge tank to collect effluent from various parts of the process. The mixture was then pumped back to the front end of the process for reprocessing. The tank had a discharge pump and a valve for level control. Because of the chemical composition of the various effluents, small flakes of solids formed in the liquid. The flakes would often plug up the control valve. As a result, the discharge flow will get blocked and the level in the tank would slowly increase. The increasing level caused the valve to slowly open, but the solids kept on blocking the valve as fast as it would open. This would carry on until the high-level alarm on the tank triggered. Then the operator would stroke the valve open and closed a few times to clear out the solids.

Before long, the plant discovered that the control valve will not get blocked if it moves often enough. In an effort to make the system more reliable, the plant installed a flow control loop and gave the operator a signal selector to switch between level control and flow control (Figure 1). Because the control valve was much more active when driven by the flow controller, it seldom got blocked in flow-control mode. Consequently, most operators then ran the system using flow-control mode.

Figure 1. Control strategy allowed operator to select between flow control and level control. (Click to enlarge)

You might have realized it already, but in flow-control mode there is no level control. So the tank is either slowly filling up or draining down. This required the operator to check the level every few hours and adjust the flow controller’s set point based on the tank level at the time. If the level was too high, the operator would increase the flow set point, and if the level got too low, the operator would decrease it (Figure 2).

Figure 2.  Control performance as found. Blue = tank level; Magenta = discharge flow rate; Green = level set point.

Although they operated like this for years, the plant did put the problem on my list of loops to look at because it remained a burden to the operators. The problem was that both level and flow control had their advantages but also their disadvantages.

The solution was quite simple; it drew upon the advantages of both level and flow control; and it eliminated the disadvantages of both modes. You guessed it… we implemented cascade control. We simply let the output of the level controller drive the set point of the flow controller (Figure 3). In this way the level controller kept the level in check, and adjusted the flow set point to do so.

Figure 3. Cascade control had all the benefits, but no disadvantages.

The flow controller was active enough to respond to deviations from set point and clear out blockages before the level was affected. We also tuned both controllers for optimal performance. Compared to what they had before the changes, the new control performance was quite remarkable (Figure 4).

Figure 4. Control performance after implementing cascade control and tuning the two loops. Blue = tank level; Magenta = discharge flow rate.

Learn more about advanced control strategies and controller tuning from the book Process Control for Practitioners.

Stay Tuned!

Jacques Smuts
Founder and Principal Consultant
OptiControls Inc.