Greg: Normally we associate these sharp responses with backlash and stiction in control valves but they can be caused by poor resolution of speed input cards for variable frequency drives, extended at-line analyzer cycle times, poor resolution thermocouple cards in a 1980s vintage DCS, improper wireless settings, data historian update time and compression settings that are too large, and actuator designs meant for on-off valves. Don't get me started.
Stan: We don't want to get Greg started so let's get back to how do you gain additional knowledge?
George: With so much data history available, we can use engineering rules to find more opportunities. We look for naturally occurring bump tests and automatically develop tuning. We notify operations only of the there is a big change in the tuning settings. Some users have no experience so you need protections against common mistakes like using data from a bump test during a load upset.
Greg: If the settings used are slower (much lower gain or larger reset time) than identified, the problem could be an unidentified nonlinearity or someone messing with the tuning. At any rate, the slower tuning settings can be readily translated into an increase in peak and integrated errors. If the settings used are faster than identified, it could be due to some degradation of catalyst, unit operations, and sensors. Fouling of heat transfer surfaces and column trays can considerably increase process lags in series creating a large amount of extra dead time. The 86% response time of a pH electrode can go from 6 seconds to 6 minutes due coating or aging of the glass.
Stan: Are there some easy pickings?
George: There is a whole bunch of low hanging fruit. We find instruments that are completely dead. Some were never put back in service after maintenance. The faked number and red tag was never removed. The operator loves the faked number because it is rock solid often close to exactly what he wants, which was purposely done to keep the operator happy during maintenance. A simple check to see if the measurement ever moves will find these "dead" instruments.
Greg: Sometimes noise is a clue to the problem. Terry Tolliver, a longtime friend and Fellow Hall of Famer, found out the poor level control upsetting the triple effect evaporator he was working on was due to a an unsecured level capillary system of a recent differential pressure level transmitter dangling and blowing in the wind.
Stan: What can you say to put a damper on this before we get Greg all worked up? He does love stories about dampers about as much as on-off valves as final control elements.
George: In one plant, we wanted to do bump tests on a hot air damper. When we asked to move the damper, the plant said no because the damper was wide open, and the loss in efficiency would be too much, because the damper was a source of free energy. Later we noticed the temperature increased when the damper closed. It turns out the damper was configured increase to open and the DCS was setup to be increase to close. The plant was running with the damper fully closed. Correcting the valve action in the DCS resulted in millions of dollars in savings and increased production.
Greg: Since we are running out of space and time, let's take the big step forward and say we have made sure the automation system is not the limitation, how do we make the big decision on how fast to tune a loop given there is always tradeoff between robustness and performance. The tuning settings for minimum peak and integrated error are nice to know but due to the inevitable operating point and run time nonlinearities and unknown, we have to make the tuning settings slower. The question in my mind is how do you know how much slower? You need to recognize the goal. For example, the purpose of a level loop on a surge tank is to maximize the absorption of variability so flow changes coming into the tank are minimized in terms of manipulated flow changes out of the tank that is inevitably feeding downstream operations. For liquid column and vessel and temperature control, the goal is to minimize the integrated error from setpoint and the peak error particularly if undesirable side reactions can be triggered or exothermic reactions are occurring. For pressure control loops operating near the relief or shutdown point, minimizing peak errors are critical. How do you deal with all the different goals?