Four Wishes

Our new column has a little fun with process engineering. What demented the RTD? Greg imagines a perfect 2003, and a New Puzzler: Reluctant Valves

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Greg McMillan (gkmcmi@msn.com) and Stan Weiner, PE, (sweiner@swfla.rr.com) bring their wit and more than 66 years of process control experience to bear on your questions, comments, and problems.

Greg: You can tell when CONTROL magazine finally makes it to the mailboxes of the readers. We get a burst of replies.

Stan: Its nice to know engineers are interested in technical problems. We would hate to have to start posing puzzlers on management situations or provide tips on stuff really important for a career like "fool-proof goals document verbiage" and "impressive graphics for average results."

Greg: Were uniquely qualified to go this route if necessary since we have posed a record number of creative management opportunities in our illustrious careers.

Stan: Fortunately, based on the number of replies, we can keep our minds reeling in the years of technical challenges. For the answer to Novembers problem, the demented RTD that made the process engineer boiling mad when the measured temperature locked in at 100° C while the actual reactor temperature went literally through the roof, we quote Mark Aspergooey:

Mark: "After reading the batch reaction problem posed I had three possible solutions. Without knowing the background of the maintenance it is hard to say which is the correct answer. I like this one the best: From the clues such as 100° C and boiling mad, one would think the temperature probe was not reading the actual reactor fluid temperature. Since 100° C just happens to be the boiling point of water, I would deduce that the temperature probe/thermal well somehow got filled with water or was in contact with water at atmospheric pressure. This water was boiling off, thus reading 100° C. Once the water all boiled off the probe, the temperature indication should go back to reading the correct higher temperature.

...Other cheap worthless solutions could be some new guy messing with the instrument range in the control system sets the max engineering units to 100. I thought this because this happens all the time with new installations. 100 is the default-look configuration for most signals for level or percent.

...Or a 100 base ohm value RTD was replaced with a 1,000 base ohm value. I know this is dismissed by the fact that the measurement displayed a normal profile and was correctly calibrated. I was reaching but it went through my brain and into this letter for some reason. I'm not sure why I'm even responding to this. I guess it's because I can never get those Car Talk questions right."

Greg: We are not sure why we are writing this column, but so far it seems to be fun and we sure wouldn't be qualified to write a column on car repair, especially since the manufacturers keep the information essential for maintenance buried in a proprietary computer program. Wait, this sounds similar to the problems we have with some new control systems.

Stan: Mark had the right answer. Water was trapped in the thermowell. There could also have been wash water mistakenly left in the reactor. We also got correct replies from Douglas Buck, Bill Clemons, Jerry Ferneding, Robert Frey, Jack Funderburk, Michael McFadden, Tim Kustka, Brian Mork, Tim Shearer, and Steven Warakomski.

Greg: As part of a training exercise for a New Year's celebration, wherein numerous trial quantities of beer were thoroughly tested, I contemplated what would be my wishes for the new year:

If I Had Just Four Wishes for the New Year

If I had just four wishes for the New Year, first I would wish that all the manufacturers and users of field instrumentation and control valves in the world would hold hands from shore to shore and sing a song of unity for developing a common fieldbus standard that would allow all measurements and final elements to achieve their full intellectual potential regardless of heritage, culture, or color.

Wait...first I would wish that I had enough money on my advanced control projects to buy smart positioners and transmitters for all of the critical process variables. Second, I would wish that all the manufacturers and users of field instrumentation and control valves in the world would hold hands from shore to shore and sing a song of unity for developing a common fieldbus standard that would allow all measurements and final elements to achieve their full intellectual potential regardless of heritage, culture, or color.

Wait...first I would wish that sensitivity and resolution of all of my sensors and control valves would exceed my requirements. Second, I would wish that I had enough money on my advanced control projects to buy smart positioners and transmitters for all of the critical process variables. Third, I would wish that all the manufacturers and users of field instrumentation and control valves in the world would hold hands from shore to shore and sing a song of unity for developing a common fieldbus standard that would allow all measurements and final elements to achieve their full intellectual potential regardless of heritage, culture, or color.

Wait...first I would wish that all project managers would approve all of my requests. Second, I would wish that the sensitivity and resolution of all of my sensors and control valves would exceed my requirements. Third, I would wish that I had enough money on my advanced control projects to buy smart positioners and smart transmitters for all of the critical variables. Fourth, I would wish that all the manufacturers and users of field instrumentation and control valves in the world would hold hands from shore to shore and sing a song of unity for developing a common fieldbus standard that would allow all measurements and final elements to achieve their full intellectual potential regardless of heritage, culture, or color.

Wait...first I would wish that all of the instrumentation and control valves in the world would always maintain a precision of at least 0.1% despite age and changes in operating conditions and process composition. Hmm...I don't need the other wishes. Wait, maybe I should add some wishes for sports cars, tennis racquets, and golf clubs with smart sensors, or a spouse who will approve all of my requests, or maybe some just some wishes for a smarter retirement.

Stan: While Greg sorts out his imaginary new year, heres this month's disclaimer: "We actually thought technical specialists got paid as much as management at the same grade level."

This Months Puzzler:

Reluctant Valves

An engineer reads in a control textbook that an equal-percentage trim characteristic should be used to manipulate the cooling water for temperature control of a heat exchanger because it compensates for the decrease in process gain with load. Since the valves only come with linear trim, the engineer specifies equal-percentage signal characterization in all of the cooling water valve positioners. Unfortunately, the technicians lift off their seats when the valves will only lift off their seats when the controller output reaches 40%. Wanted posters for source of the bright idea are posted in the shop. What is making a sticky situation worse?

Send an e-mail with your answer to the Puzzler, control questions, or comments to controltalk@putman.net.

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