Water/Wastewater / Instrumentation

Stop Loops Driving You Round the Bend

Practical pointers trump educators' edicts for real-world process control.

By Dirk Willard, Contributing Editor

Process control was my forte in college. I scored in the top 10% on the final. Ask me to solve a Laplace transform and it was done. Did that prepare me to develop my first process and instrumentation diagram? Not one bit.

Suppose you walked into your control room for the first time.

Our professors meant well. They taught us to solve the most difficult problems, not the simple ones.

Let's start with the basics, the analog feedback control loop. A controller compares a process variable to the desired value or setpoint. A control valve forces the variable to match the setpoint. The controller then monitors new process measurements — simple, right? Not really. There're several potential problems -- the controller, the valve, the instrument and all the connections in-between -- that never were mentioned in school but that certainly can figure in real-world troubleshooting.

So, let's look at practical everyday process control. The first question should be: Is there a problem?

Suppose you walked into your control room for the first time. Is everything running smoothly? Let's check for telltale signs of impending trouble. Start by counting the number of loops in manual mode. More than 10% per operator indicates a problem. Manual control is risky for safety, quality and smooth operations. It requires constant attention by operators -- they've got only so much of that to go around. It's a particular problem during shutdowns and startups.

Other problems to look for include: linchpin loops, i.e., ones at the center of everything that thus generate an avalanche of alarms; chattering alarms; and loops relying on poor control elements or instruments.

In school we learned about proportional (P), integral (I) and derivative (D) control terms. I was taught PI control is best. With proportional-only control you must live with offset. For example, with a 25°C offset for a 400°C setpoint, the controller ranges between 375°C and 425°C. With PI control the worst thing you need to worry about is overshoot caused by the integral term. The professors dismissed use of a derivative term because, they said, it makes a loop too responsive. Try tuning a burner control for a vaporizer without it.

What you're controlling affects your choice of terms. Pressure and flow change quickly and require fast response. Temperature, level and analytical measurements like pH generally change slowly. The P and I terms usually are fast; D is slow. P and I often are linked because the I term quickly can dampen an oscillation; D balances a fast response in PI loops, keeping a lag from causing a wide swing in the response.

Let's consider simple storage-tank level control first. With large storage tanks there's a lot of dead time. A proportional controller may suffice because accuracy isn't crucial.

Other stand-alone level loops are more challenging. One of the worst is where tank flow, inward or outward, varies a lot. If the controller handles different batch processes, the solution is simple -- use unique control parameters; usually PI works best. When the recipe solution won't work, you must add another hierarchy of control, a cascade.

Now, let's look at a couple of more-complex feedback loops. These fall into two general categories -- cascade and ratio.

We were taught in school that cascade control consists of primary and secondary controllers. Most control loops are part of a greater web, interacting with or linked directly as a secondary or primary controller. Some common examples of cascade control are: reactor jacket temperature control with subordinate flow controls of steam or cooling water; level control with slave flow control of makeup; and distillation column top temperature with secondary reflux flow back to column. In each case the slow controller is in charge; this stabilizes the fast loop. One caution with flow control: watch the instability of the flow measurement with differential pressure transmitters.

Ratio control is typical where a small flow is added to a large one. Examples include ozone-to-wastewater and fuel-to-air in burners. A less-well-known application is for a preheater for heat-sensitive material; ratio control allows a portion of one of the streams, usually the feed, to bypass the preheater to maintain the desired downstream temperature.

While what they taught us in school is useful for especially tough applications, process control at the plant level is fairly simple.

Dirk Willard is a Chemical Processing contributing editor. You can e-mail him at dwillard@putman.net.