How to get the most out of your software for loop tuning

Control loop performance is more than just about tuning

By Greg McMillan and Stan Weiner

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Greg: I had the opportunity to talk with George Buckbee, the General Manager at ExperTune Inc., the supplier of software for loop tuning and analysis. I was always impressed with George's knowledge and his dedication to helping ISA. George worked many years at Proctor & Gamble followed by a career Sanofi-Aventis before ending up at ExperTune. George brings a lot of the user perspective and experience to his job of helping plants improve their loops.

Stan: What are you seeing in the field?

George: Control loop performance is more than just about tuning. There are huge opportunities. About 20% of control loops are in manual. This is an incredible waste of investment. The cost of a loop is about the same as the cost of a car if you consider the entire design and installation cost. Having the loop in manual is like having the car parked in the driveway. You need to get the car out on the road to deliver results. The same is true for a loop. It is frustrating for management to see an investment not leveraged.

Also, we are seeing a big shift in the needs of plants.  Retirements, downsizing, and skill losses have changed what is required.  In the past, engineers wanted more and more sophisticated technical tools.  Today, there is a higher demand for turn-key services to improve plant performance.  ExperTune's acquisition by Metso Automation has made it very easy for us to shift into a services mode, and helps us to have more of a global presence.

I have seen many Distributed Control Systems (DCS) replaced and yet the performance of the plant is the same. Many companies spend millions on upgrading control systems and don't use readily available tools to measure if  automation system is doing its job.

Control loops in manual can be safety issue such as those making sure the process does not exceed the operating limits of equipment. A level, pressure, or temperature loop in manual can cause a catastrophic equipment failure and hazardous release.

As an industry, we have kind of accepted 20% as the norm. How comfortable would you be if a pilot was only using 20% of an airplane's control systems? What if the air fuel ratio or hydraulic pressure controllers were in manual?

Read Also: Don't Tune These Four Loops

Greg: We put extensive effort into the necessary job of meeting project budgets and schedules often at the price of simply doing what has been done even repeating mistakes neglecting the real goal of improving process control. Since this is fall and football is on my mind, maybe this is like a football team with new uniforms and a new stadium that has the same old plays and doesn't improve the yardage gained or points scored.

Stan: Given that the loop is not in manual due to legitimate reasons such as shutdowns, startups, transitions and sequences of the unit operation, what are some of the reasons loops are being inappropriately put in manual?

George: Great point.  A controller in manual is usually the symptom of some other, underlying problem.  Operators will not run a loop in auto if the operator sees the plant as worse off. The operator knows the plant suffers but not how much or why. The goal is not just to get loops in auto but rather to find out what was the cause. Loops in auto may have caused oscillations upsetting other loops due to noise, interactions, backlash and stiction, and improper control strategies besides tuning.

Stan: How do you find the culprit?

George: We have a target rich environment.  We have to automate the diagnostic analysis, and also to prioritize among them to find the most important root causes. We do a power spectrum analysis and take a closer look at the 3 highest peaks and see what they have in common in terms of frequencies. In the process we  clean-up the data and eliminate misleading information. We look at the wave shapes. Sharp corners such as square waves and sawtooth cycles (ramps up and down) are symptomatic of discontinuous responses from hardware issues. The further downstream you go, the more these oscillations are smoothed out and attenuated by tanks. So the sharpest oscillations are in many ways a good clue as to the source of the problem.

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  • <p>If you will allow me to say so, I think the article missed the most important contribution that PlantTriage can provide for improving plant profitability. What follows is some personal impressions from my experience on the job.</p> <p>George is certainly right in saying that loop performance is about more than tuning, and your article presents some specific examples: - loops in manual - root causes &amp; interaction (which has a strong tuning component in its solution) - valve stiction and hysteresis/backlash - faulty instrumentation and/or actuators</p> <p>No argument with these, as far as they go, but they miss a lot. In the case of controllers in manual, this is not as much of an issue as the article would suggest. Certainly a controller in manual is wasting the cost of its installation, as the article makes clear. But this is a one time, sunk cost - water under the bridge. As the article says, the important questions are - why is it in manual and what will be the benefit of getting it into auto? In my experience, a controller is most often in manual either because it has become obsolete (out of service), or the control it could provide is not really necessary - the process is stable enough without it. Sometimes, a process is more stable with the controller in manual than in auto. This last happens either because the tuning is so tight that the loop is unstable with the controller in auto, or in auto the controller reciprocally interacts with another loop in auto to the point of oscillation. Both of these things happen, but rarely. Much more often, on a difficult-to-control loop, the controllers are in auto, but tuned sluggishly, to keep the loop stable under all operating conditions and hold steady state in the absence of upsets, which may not be that far from being in manual. Manual/operator control is used to handle occasional upsets. So, a more revealing metric is the frequency of auto/manual transfers and the frequency of output changes in manual, which PlantTriage also tracks. Retuning such loops does improve control performance during an upset, and reduce operator load. However, such improvement is not likely to have significant economic benefit, absent other operational changes to the average operating point.</p> <p>The other examples mentioned certainly exist, and when they are found and corrected the benefits can be dramatic and very welcome. Hysteresis and stiction are almost pervasive. Still, the oscillations they cause are self limiting and often more of a nuisance than a significant economic problem (unless they affects one of the variables mentioned in the next paragraph). They do create a maintenance issue because of the associated wear and tear on the valve; however, such cases can go on for months and even years without being addressed because their economic and operational impact is not large enough to demand a solution. Instances of faulty instrumentation that have significant effects such as described provide impressive stories and surprising solutions. However, they are usually singular situations, not really part of a long term/continuous improvement concept, except as gateway events.</p> <p>Such a process of Continuous Improvement is the real money maker enabled by PlantTriage. The article barely hints at this most beneficial reason for using PlantTriage to improve control performance in auto - i.e. minimizing variation (reducing standard deviation) in the PV's that affect plant economic performance through their effect on production rate, product quality, yield, energy efficiency, safety, maintenance costs, and regulatory costs. Higher stability often allows the operating point for these key economic variables to be moved to values that provide increased production rates, higher yields, lower energy costs per unit of production, longer equipment life, higher uptime, and avoid emissions violations and fines. These improvements are less dramatic but they are where the big money is, because their return steadily accumulates over time. An improvement in any of these areas of just $150/hr will return over $1.25 million/yr. Any plant is a "target rich environment" for such opportunities, to borrow George's phrase. A plant of any size will make millions of dollars worth of product a year, and consume millions of dollars worth of energy. Even small percentage improvements in either of these factors will generate huge ROI.</p> <p>This concept deserves an equal discussion in Control Talk.</p> <p>Regards,</p>


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