This article was printed in CONTROL's May 2009 edition.
Greg McMillan and Stan Weiner bring their wits and more than 66 years of process control experience to bear on your questions, comments, and problems. Write to them at firstname.lastname@example.org.
By Greg McMillan and Stan Weiner, PE
Greg: It is best to have lots of field experience to provide the reality check and confidence to go beyond the rote duplication of designs. About 40 years ago, I was sent immediately out into the field after an intensive 12-week instrument engineering course to lead the construction and checkout for the automation systems of five rubber chemical plants. These were difficult batch processes with nasty chemicals. I was lucky to be learning so much so fast, but felt like the three-legged pig named “Lucky.” I graduated to helping rebuild, check out and start up the replacement of a reactor that blew up, and with the help of Stan to become the lead design and construction (D&C) engineer for project where the catalyst would explode if the bag was dropped, and the lead D&C again for a project for a Texas City plant with the world’s largest hydrogen cyanide production rate as a byproduct.
Stan: We all learned the hard way what worked and didn’t work. When it comes to sensor speed, we got sensitized by compressor surge control, furnace pressure control, exothermic reactor temperature control and neutralizer pH control.
Greg: The fastest automation requirements were compressor surge because going into surge was like falling off a cliff. There can be a precipitous drop in flow in less than 0.03 seconds. Once into surge, there were flow reversals every 2 seconds, too fast for any feedback loop to deal with. You can get an overview of such super-fast applications in the “Analog Control Holdouts” entry on April 2, 2007, and the fastest upsets in the “Stuff That Comes at You Too Fast” entry on Jan. 22, 2007, on my site www.modelingandcontrol.com. To see the effect of transmitter speed on damping settings on surge cycles, check out Figure 7-3 on page 84 of the E-book Centrifugal and Axial Compressor Control posted on April 3, 2009, on the same website.
Stan: In the days of pneumatic controllers, transmitters and tubing, the measurement and control system could be as slow as the valve. For fast pressure control, we used field regulators. For compressor control, we used hydraulic controls and shut down if things were not right. When analog electronic controllers and transmitters came on the scene, we could get rid of the field regulators and hydraulics. The slowest components in the loop were the control valves that had pneumatic actuators and positioners. Unless you had a variable-speed drive, the inherent speed of the sensor and control system were not the limiting factors. When digital controllers came on the scene, some were designed to sample at 0.1 and 0.25 seconds. Users of big modern DCSs typically ran the execution time at once per second, which was fine for the flow and noncritical pressure loops. They didn’t realize they could go with 5 to 10 seconds for level, composition and temperature control. The downside of a faster-than- necessary DCS scan/execution time is controller loading and A/D noise, but with the processor speeds and A/D resolutions used today, these should no longer be issues. For wireless, we can go with a variable update time that is as fast as 1 second (see Greg’s "Is Wireless Process Control Ready for Prime Time?")I was ready to take a nap, confident in the future of process control until I heard a rumor that the proposed design for a next-generation control system doesn’t understand the importance of speed. The question in that case is, “Do you feel lucky?”
Greg: Let’s hope that’s just a bad joke and concentrate on making some sense out of sensor speed requirements. Sensor speed can be set in a lot of places. The sensor design may have some inherent lag time (time constant). Usually this is 0.2 seconds or less for flow, pressure and level sensors—fast enough for all but the most extreme applications. The default filter time setting in the analog input and PID blocks is 0 seconds. Signal filtering exists in the transmitter in terms of a damping adjustment. In some cases, the default filter time is 2 to 3 seconds. This high default setting, combined with reduced visibility and knowledge of a damping setting and the insidious nature of signal smoothing, has caused unexpected deterioration when the field measurements were upgraded. A user describes the possible dramatic consequences.
User: In the past few years, I have twice seen major equipment accepted into service and then malfunction. One was the lube oil system of an FCCU wet gas compressor where the standby pump didn’t start quickly enough. The other was when two CNG gas export compressors had serious problems with the anti-surge system and load-sharing functions. In both cases, the problem came down to pressure transmitters that had been installed with factory defaults for the transmitter damping of a generous 2 seconds. My control and instrumentation colleagues were quick to point fingers in all other directions, as they had tuned the loops personally (or instructed their reports to do so) and could see no problems.
I have twice found damping left at factory presets to be the major or only cause of problems. None of the data relating to the instruments makes a big deal about this. Is this where the problem lies? I wish I knew where all the similar problems in the world were because I am sure I could be a very wealthy person based just on this knowledge alone. Based on my findings, the savings are probably in excess of $2 million per annum.