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: Normally we associate these sharp responses with backlash and stiction in control valves but they can be caused by poor resolution of speed input cards for variable frequency drives, extended at-line analyzer cycle times, poor resolution thermocouple cards in a 1980s vintage DCS, improper wireless settings, data historian update time and compression settings that are too large, and actuator designs meant for on-off valves. Don't get me started.

Stan: We don't want to get Greg started so let's get back to how do you gain additional knowledge?

George: With so much data history available, we can use engineering rules to find more opportunities.  We look for naturally occurring bump tests and automatically develop tuning. We notify operations only of the there is a big change in the tuning settings. Some users have no experience so you need protections against common mistakes like using data from a bump test during a load upset.

Greg: If the settings used are slower (much lower gain or larger reset time) than identified, the problem could be an unidentified nonlinearity or someone messing with the tuning. At any rate, the slower tuning settings can be readily translated into an increase in peak and integrated errors. If the settings used are faster than identified, it could be due to some degradation of catalyst, unit operations, and sensors. Fouling of heat transfer surfaces and column trays can considerably increase process lags in series creating a large amount of extra dead time. The 86% response time of a pH electrode can go from 6 seconds to 6 minutes due coating or aging of the glass.

Stan: Are there some easy pickings?

George: There is a whole bunch of low hanging fruit. We find instruments that are completely dead. Some were never put back in service after maintenance. The faked number and red tag was never removed. The operator loves the faked number because it is rock solid often close to exactly what he wants, which was purposely done to keep the operator happy during maintenance.  A simple check to see if the measurement ever moves will find these "dead" instruments.

Greg: Sometimes noise is a clue to the problem. Terry Tolliver, a longtime friend and Fellow Hall of Famer, found out the poor level control upsetting the triple effect evaporator he was working on was due to a an unsecured level capillary system of a recent differential pressure level transmitter dangling and blowing in the wind.

Stan: What can you say to put a damper on this before we get Greg all worked up? He does love stories about dampers about as much as on-off valves as final control elements.

George: In one plant, we wanted to do bump tests on a hot air damper.  When we asked to move the damper, the plant said no because the damper was wide open, and the loss in efficiency would be too much, because the damper was a source of free energy.  Later we noticed the temperature increased when the damper closed. It turns out the damper was configured increase to open and the DCS was setup to be increase to close. The plant was running with the damper fully closed. Correcting the valve action in the DCS resulted in millions of dollars in savings and increased production.

Greg: Since we are running out of space and time, let's take the big step forward and say we have made sure the automation system is not the limitation, how do we make the big decision on how fast to tune a loop given there is always tradeoff between robustness and performance. The tuning settings for minimum peak and integrated error are nice to know but due to the inevitable operating point and run time nonlinearities and unknown, we have to make the tuning settings slower. The question in my mind is how do you know how much slower? You need to recognize the goal. For example, the purpose of a level loop on a surge tank is to maximize the absorption of variability so flow changes coming into the tank are minimized in terms of manipulated flow changes out of the tank that is inevitably feeding downstream operations. For liquid column and vessel and temperature control, the goal is to minimize the integrated error from setpoint and the peak error particularly if undesirable side reactions can be triggered or exothermic reactions are occurring. For pressure control loops operating near the relief or shutdown point, minimizing peak errors are critical. How do you deal with all the different goals?

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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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