Greg McMillan ([email protected]) and Stan Weiner, PE, bring their wit and more than 66 years of process control experience to bear on your questions, comments, and problems.
Greg: The response to our December "Depressing Decoupler" furnace pressure control problem was depressing.
Stan: Our readers must have been busy opening their presents.
Greg: Maybe their mother-in-law scarfed up their issue and is cutting out the pictures of men in hard hats.
Stan: I know I would go into withdrawal if I didn't get my monthly fix of technical magazines.
Greg: That reminds me that it is time for our meeting of Control Engineers Anonymous. Stan, how about you kick it off with the opening prayer?
Stan: I am a Control Engineer. I have been since 1964. I can change, if I have to, I guess.
Greg: I think I had a breakthrough last night. My wife comes home and she is sighing and stomping. I astutely recognize something is wrong, I mute the TV, and say "WHAT?" So she goes on and on about how the fabric store doesn't have a pink border for the quilt that matches the dust ruffle. I realize while she is talking to me I can be thinking about other stuff so I start jamming in my head to Led Zeppelin. I am nodding my head and she thinks I am agreeing with her, when really I am figuring that large, low-inertia dampers probably no longer require restrictions in the outputs from positioners to prevent oscillations since the new smart positioners have travel rate feedback and tuning settings. Then, I am thinking about the hockey game and how much beer is left while I keep time to a riff by Jimmy Page. She finally leaves the room satisfied that I am interested in her viewpoint and sympathetic to her problems. I can't wait till she talks to me again because I get some of my best jamming and thinking done.
Stan: Back to the depressing problem where the furnace pressure dipped from an increase in air flow and then got worse by the addition of a classic decoupler to the pressure loop. One of our readers, Lyle Mariam, posed an interesting possibility that it could be caused by a pressure wave originating from the exhaust damper. Another reader, Michael McFadden, correctly reasoned it could also be caused by an excessively high decoupler gain.
Greg: But our answer is in the simple relationship between pressure and temperature shown in the ideal gas law (PV = nRT where P is the absolute pressure, V is volume, n is the number of moles, R is the universal gas constant, and T is the absolute temperature). Basically, the cold air causes a drop in furnace temperature, which causes a drop in furnace pressure until the number of moles of fresh air accumulated in the furnace volume increases enough to increase the pressure. Opening the exhaust damper only aggravates the inverse response by letting more of these moles out.
Stan: Well, now its time to pass on our secrets to success.
Greg: It is our legacy; or at least our fricassee.
Stan: It is most important to be experienced, yet conceptual. In this profession more than any other, an engineer learns the most from mistakes. Yet, except for our columns last year, these experiences are rarely documented. Vendors and consultants prefer to tout new features and successes, users are too busy solving the problems, plus nobody likes to advertise a failure. Making mistakes can be a good experience if you are there to help solve it. Since you may not encounter exactly the same situation, it is important to conceptualize the problem and solution to extrapolate to other scenarios. The real worth of an engineer can be measured by the amount of time spent in the field where the ideal ideas meet the harsh reality of the plant.
Greg: Yeah, reality reeks. Another key is to be open, yet focused. You need to be receptive to new ideas and technologies, especially when they are in conflict with your own view. I have seen consultants who will not let loose of an invalid idea even when posed with obvious evidence. Unfortunately, the bigger the consultant, the bigger the ego. We need someone at work to function like our wife and kids to help keep our egos in check. Technologies are in a constant state of advancement and old rules are ripe to be broken. However, there are so many new ideas and advances, an engineer can wander forever pursuing the best technology rather than focusing on the simplest solution that makes money for the plant. There is a balance here, as in most of what we suggest.
Stan: Next, one needs to be skeptical yet positive. Don't believe the sales hype. Test as much as possible before commissioning, using calibration and simulation parameters as close as possible to the actual plant conditions. The brochure was designed to sell the instrument or control system and gives specs mostly based on laboratory conditions. Expect problems with new technologies, but also tout the successes and advantages to pave the way for further opportunities to try better tools.
Greg: Finally, one should be an individual, yet a team. These days, with a shortage of experienced people, there is a greater need than ever to take responsibility and do the work. Even when you are a leader or big-shot consultant, getting your hands dirty may be the only way the job gets done. Everybody knows about the importance of teams these days since it is more difficult than ever to find the time or have all the skills to put together all the pieces of a successful application. On-stream time depends upon technology, engineering, operations, and maintenance working together. The concept of the team should also extend to the rest of the corporation, the vendor, the manufacturer, college professors, and in fact, the entire technical community. I have always sought out the right person to answer a question no matter where that person resided. Some of the best engineers spend too much time spinning their wheels or reinventing the wheel since they think they can do it all or don't want to reveal they don't know something.
Stan: One person who has exemplified these qualities is John Berra. And I am not saying this just because, in his acceptance of the ISA Life Achievement Award, he acknowledged the value of my early influence as one of the most experienced instrument engineers.
Greg: I was just walking in the engineering building of Washington University in St. Louis and there is plaque honoring John as the distinguished alumni for 2002. I would have passed him the ball more on our basketball team if I knew he was going to be so important. I might not have given him the gag going-away gift of elevator music.
Stan: Before we wax too philosophic and end up in a wax museum, let's throw out another puzzler.
Greg: Each one of 3 pH electrodes in slurry service is installed in each of 3 parallel lines. The electrode in the bottom line shows the smoothest trend. This is the operator's favorite electrode anyway, so he selects it for control. Why does the downstream process go nuts?
Stan: Now for this month's disclaimer:The authors tried to add a new dimension to their humor by explaining the supergravity of supersized egos but got all tied up in string theory.
This Months Puzzler:
Smooth-Talking Sensor
Each of three pH electrodes in slurry service is installed in one of three parallel lines. The electrode in the bottom line shows the smoothest trend. This is the operator's favorite electrode anyway, so he selects it for control. Why does the downstream process go nuts?
