DP Transmitter Sees the Light

Rainy days depress a DP transmitter; E-mailing a prospective engineer; and a new puzzler: Hair-trigger temperature loops

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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

: We had smart answers about the dumb DP transmitter puzzler in the March issue from Bob Cameron and Paul Fielding. Paul also had an interesting anecdote...and antidote for boredom.

Paul: Sounds to me like the dumb transmitter has a zero shift due to ambient temperature change. The smart DP has internal compensation for ambient temperature change. Actually, about 30 years ago, one of our service engineers finally solved a similar problem. A low-range (draft) transmitter would change output on a somewhat regular schedule but after a while return to being OK. Both field and bench calibration checks showed it to be OK. On one of several trips he made to this plant, a rainstorm occurred after lunch on a bright sunny day. The transmitter, which was bathed in sunlight in the morning, suddenly shifted after the rainstorm started. Only then did he notice the window high up in the roof. He had the plant build a sunshade...poof!...problem solved.

Stan: Thanks for shedding some light on dumb transmitters and how to avoid a furnace poof.

Greg: I remember a shifty positioner and some shady operation that caused a cycle in controller output not only from day to night but also with cloud cover. Measurement and positioner shifts are seen in the controller output because this is the degree of freedom the controller has to keep the measurement at setpoint. If the loop is tuned for tight control, the upset is dramatically more noticeable in the manipulated variable than in the controlled variable.

Stan: Speaking of uncontrolled variables, we had the following e-mail dialog with a 17-year-old high school student from Chicago, who's still naive enough to consider studying process control engineering in college. We understand her name is Christine, although we only officially know her by her e-mail name.

Sasscgal@loa.com: hi! wassup guys? Im takin AP calc at my hs (i know, it sux!) & wuz wondrin if u could tell me wassup with bein a control engineer!?! Im leavin 4 college in the fall--what classes do u think I should take next year? Thanx :-)

Greg: If you mean getting a job in process control, you should first go to an engineering school where you will be submitted to endless differential equations, Laplace and Z Transforms, Bode and Nyquist plots, none of which you will use on your job.

Sasscgal@loa.com: k,is that it? Just,math? :-(

Stan: If you do real well in your control classes, you can go to graduate school where you will graduate from linear to nonlinear systems and use Lipanouv plots and Describing Functions. Of course, no one is really looking to hire you for this type of knowledge so you can become a professor and perpetuate the notion that this is important stuff on the verge of a breakthrough. After all, 40 years of academic tradition cannot be wrong. Ten percent of the chemical engineering undergraduates go on to graduate school and 10% of these pursue a degree in advanced control. In my class, this corresponded to 0.2 engineers being glutens for punishment.

Greg: The theory behind the relay auto tuner is based on Describing Functions, and we are starting to see gain and phase margin plots and bode plots in online performance systems, but I find the accompanying time response plot better for getting a real feel for what is going down. However, for every job as a software developer for advanced control there are 10,000 jobs to design, troubleshoot, and improve all types of process automation systems. Most universities sacrifice the needs of the engineer in industry for the elite intent on a more academic future.

Sasscgal@loa.com: Rite,so, ur sayin i jus have 2 hit the math books? :-P

Stan: These are not college text books but manufacturer's catalogs.

Sasscgal@loa.com: Catalogs, like Wet Seal and dELiA's? kool,so, if I read em, im in?

Greg: Yea, but you have to realize some of the product bulletins are designed to dress up a turkey or give wings to a pig. For example, a valve with high seating and packing friction and shaft windup and backlash is called a "high performance" valve. A digital positioner is often added to make it smart but any exact match between the feedback measurement of actuator shaft position used by the positioner and actual ball, disc, or plug position is a mere coincidence.

Sasscgal@loa.com: Gr8! so, u think i can make lots of dough & retire when im old,like, 30?

Stan: You need to work longer unless you start/sell a software company. It took me 33 years.

Greg: Now for this month's Puzzler. A control engineer reads one of my articles and realizes that dead time delays the ability of a controller to see and correct for a load upset. Expecting fame and fortune or at least a good performance review, the engineer reduces the scan times of all the temperature loops from 10 seconds to 1 second and moves all of temperature sensor locations closer to the feed entry into neutralizers, reactors, and columns. One day later, the chatter builds in the control room to a resonating level. The operators cross themselves when he approaches and beg him to undo his improvements. What drove the loops so crazy that the operators wouldn't even accept a peace offering of doughnuts?

Stan: Now for this month's disclaimer: The authors thought professors were really interested in practical problems until they asked us for the state space representation of a typical operator.

This Month's Puzzler:

Hair-Trigger Temperature Loops

Dead time delays the ability of a controller to see and correct for load upsets. So the scan times of all temperature loops are accelerated from 10 seconds to 1 second, and all temperature sensors are moved closer to the feed entry into neutralizers, reactors, and columns. One day later, the chatter builds in the control room to a resonating level. What drove the loops so crazy?

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

 

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