May 2014System Integration & Control Systems Design
SA Instrumentation and Control contributing editor, Michael Brown, recently received some feedback on a loop optimisation article we published in the August 2013 edition. Since it is such a topical experience from a C&I engineer, we decided to publish it.
Mike,
I was reading your article in the August 2013 edition of SA Instrumentation and Control about strangely behaving valves and it reminded me of an experience I had when I was tuning a loop in a DMS plant. It was a simple loop controlling density with water flow, but I did not have a flowmeter to check the flow rates.
An I/P valve manufacturer had been issued with an order to provide a valve of a certain flow range that they did not have in stock, but they could modify the range of another valve by exchanging the seat. What they failed to do, however, was to recalibrate the valve for 4-20 mA: valve fully closed to fully open.
The valve only started opening above 8 mA and, since the loop was quite slow, this caused some problems as you will appreciate. Subsequently the project leader accused me of not knowing what I was doing, since the loop was not controlling the density satisfactorily. I stuck to my guns and said that the valve was not opening when it should.
I checked the density control later when I could open an outlet to see if there was water flowing, and sure enough there was no flow. Now it was my turn to make accusations about who had not done their job. Needless to say there were no apologies.
Case History 195: Unstable reboiler steam flow Michael Brown Control Engineering
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