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work at a plant that was constructed in the 1940s. I’ve been there for 29 years and have seen the slow evolution from pneumatic controls to electronic controls to centralized control systems that were the state-of-the art in 1970. From there I witnessed the change from distributed control systems using network architectures that were not too far removed from cable TV systems, to the systems we are now deploying based on PLCs, fieldbuses, Ethernet and yes, WiFi.
Some observers might say we’ve just put lipstick on a pig, but while the processing equipment is old, we have been successful in teaching it to dance to the new drummer of advanced automation technology. Notwithstanding these success stories, there’s a tendency by some employees to assign fault where it’s not deserved and therein lies opportunity for education.
The “call” came around 3:30 in the afternoon. This kind usually does because it was approaching shift change.
“We’re having a problem with the slurry tank level,” the control room chief informed me. “The instrument technician is here and he says it’s in the PLC.” I’d heard that before. “Tell him to wait. I’ll come down,” I said.
The drive to the production facility took about 10 minutes, during which I played a few scenarios in my mindnone of them had to do with the PLC. I exchanged pleasantries with the chief and the technician and turned my attention to the HMI. The trend for the past hour was clearly indicating that the measurement and the setpoint were having a hard time staying together. The measurement variations were a bit square-looking. This was a loop that rarely caused problems. I had an idea what was wrong.
To the instrument tech, I said, “Let’s take a radio and go out to the production floor. I have a theory and I want to see if we can validate it.”
He took a quick glance at his watch and followed me out. We went to the level control valve (sans positioner), which was sitting at 30% open. We called the chief on the radio and asked him to put the loop in manual. He complied. We asked him to set the valve position to 31%, which he said he did.
“See any change?” I said to the technician. “Nope,” he replied.
“Go to 32% please.” Again, there was no change to the valve position.
We continued this exercise one percent at a time until we hit 40% at that point the valve moved from its 30% position to 40% open.
“Voila!” I said to the technician, “what did you just see?”
“Looks like the valve was stuck,” said the observant technician.
"I opted instead to seize the 'teachable moment' to show my young technician something that he was likely to see many more times in an old plant like ours."
“Did this problem fit the trend in the control room?” I asked. “Yes, I would say that it did,” came the technician’s reply.“OK,” I said, “they can live with this until the next down day. That will give you guys time to make sure the spare valve is in good working order. Let’s plan on installing it on Monday.”Point made. The incident could have been used to show how inept the technician was (he wasn’t) or how utterly reliable PLC-based process control was (it is) but neither discussion would have been beneficial or, more importantly, instructive. I opted instead to seize the “teachable moment” to show my young technician something that he was likely to see many more times in an old plant like ours. Further, after getting back to my office, I hit the web and found an excellent article on, what else, “valve stiction” which I promptly printed and copied for each instrument technician in the shop. I checked with the shop supervisor a few days later and he told me that the article was well received and the story of the stuck valve had made the rounds among the group. As control professionals we have an obligation to the greater good, and when opportunities such as the one I described arise, we need to find the hidden lesson and capitalize on the teachable moment. There’s always something that can be learned from any problematic process situation. I challenge every engineer who might find him or herself in a similar situation to capitalize on it. These are the lessons that aren’t in the textbooks. These are the lessons that stick with us. These are the lessons that turn mediocre employees into superstars and companies into profit centers. D.L. Carlyle is an eEngineer with Alliant Techsystems.
