15 case-in-points of common control myths

Greg: In our time-proven tradition of subjecting everything to scrutiny and ridicule, where I am scrutiny and Stan is ridicule, we offer the following control myths.

Stan: This should take no time at all, or less, if you prefer.

Greg’s List of Control Myths

Auto tuners can compute controller tuning settings with an accuracy of more than one digit. Act surprised when unmeasured disturbances, load changes, valve stick-slip, and noise cause each result to be different. Look forward to the opportunity to play bingo with the second digit.

You can just dump all your historical data into an artificial neural network and get wonderful results. Forget about the same stuff that causes auto tuners to have problems, and use variables that draw straight lines because anything that smooth or well controlled must be important. Use the controlled variables (process variables) instead of the manipulated variables (controller outputs). Don’t try to avoid extraneous inputs or identification of the control algorithm instead of the process. If you want to purse a career in data processing, use every input you can find.

Models can predict a process variable that isn’t measured in the field or lab. This is a great way to spur creativity in training an artificial neural network (ANN) and validate a first-principal model, plus it has the added bonus of the model never being wrong. Wait until your customers figure out something is wrong with the composition of your product. Discount as hearsay any suggestions that even the best models need periodic correction.

To reduce variability in process outputs (temperatures and compositions), keep all the process inputs (flows) constant. Keep believing that you can fix both the process inputs and outputs, and don’t accept the notion that process control must transfer variability from process outputs to process inputs to compensate for disturbances.

Process control doesn’t apply to batch processes. Use that time-tested, fixed sequence. After all, batch cycle time is a tradition, and the golden batch sure looks shiny.

Positioners shouldn’t be used on fast loops. This was true in the good old days of pneumatic positioners and analog controllers. Surely, digital positioners with tuning settings and digital, control-system scan times can’t make the original theoretical concerns less important than the practical issues of real valves. If you’d rather believe the controller outputs are the actual valve positions, and just want valve problems to slip by, save some bucks on your project, and only put positioners on slow loops. Just don’t stick around for start up.

Stan: Don’t call me Shirley. “If you really want to save money on positioners, then don’t put them on any valves smaller than 3 in. For even more fun, forget about rotary valves with weird spring ranges such as; 7.5 to 21 psig. Then stand in the control room and guess what the valve is doing.”[

Pressure and temperature-compensated orifice flow measurements provide mass flow measurements. If your favorite vendors believe the composition in the pipeline is always exactly the composition stated in the process flow diagram and the equations are accurate for all operating conditions, then give them an achievement award for imaginative marketing.

Stan’s List of Control Myths

If you believe the vendor in Myth #5 was really looking out for you and your company’s best
interests in measuring mass flow, you probably slept through that class in instrument school. This is about the nicest thing that we could say about you and the vendor. (Please note: when Greg wants to get me really upset, he begins talking or writing about orifice plates and other differential flow measuring devices.)

If you’re using orifice plates for flow measurement and suspect there’s an unacceptable error, there’s no need to check the orifice plate. They’re made of stainless-steel, which means their holes’ sizes never change. Besides, it’s s a real pain to have to remove them from the piping.

If your flow requirements increase, replace the control valve with one that has a larger Cv.

Make sure you tune every level-control loop, so the level in the vessel stays at exactly 50%.

Never use a three-element boiler feed water system. When designing the level control system for a steam drum, keep it simple. Just make sure the operators understand that the load can’t be changed quickly, and chalk it up to a lack of sophistication.

We need to keep repeating this one every few years. As a director once told me; “If you’re going to buy instruments that don’t work, make sure they’re cheap”.

If your company decides to modernize a unit by replacing the pneumatic instruments with a distributed control system (DCS), the controller settings can be transferred without any problems.

Integrating your basic process controllers and the plant safety system can reduce costs by using the same hardware, configuration, and maintenance. Make sure management understands that it almost always takes more than two or three failures to produce a spectacular explosion, fire, or release of toxic materials.

Greg: From time to time, our column may have a quote from special guest(s). Last month it was Groucho Marx. This month, it’s Wally and the Beave from the classic TV sitcom, Leave it to Beaver.

This Month's Puzzler

A vortex meter was working fine during water batching of a new process area. Suddenly it begins reading very erratically. When the sensor voltage was displayed on an oscilloscope, it was a squiggly mess. What happened? Send an e-mail with your answer, questions, or comments to The Puzzler.

Wally: Any executive with half a brain would realize that without experienced control people, myths rule.

Beave: I hope it’s not the dumb half.

Greg: On a personal note, I was glad to hear that there is a Hell, Minnesota, which means I was correct last summer in noting that Austin was “hotter than Hell.” I plan to go there some day, so I can brag that I have been “to hell and back.” And, if I take an airport limo, aptly named “hand basket,” I also can say “I’m going to hell in a hand basket.”

About the Authors

Greg McMillanandStan Weiner, PEbring their wits and more than 68 years of process control experience to bear on your questions, comments, and problems. Write to them at [email protected].