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Aug
02

Tips-N-Techniques (TNT) – Tuning Level Loops

I thought of naming this series “Truth or Consequences” or the “The End of the Innocence.”

Level loops look so simple, they must be easy to tune. Also, what is so important about level? Don’t you just need to keep the vessel from running dry or overflowing?

Did you know that fast surge or feed tank level control is the most common source of variability in a plant? Did you know that slow distillate receiver or recycle tank level control frequently undermines the performance of important unit operations? Did you know that all of these loops are probably oscillating?

You might not know you can have too low of a controller gain for a level loop and that if you decrease the controller gain, the oscillations get worse. The common quick fix for getting rid of slow rolling level oscillations is to increase the controller gain for distillation columns and recycle tanks and increase the reset time for surge and feed tanks by an order of magnitude (10x) or more. If you want to know why or want to do a better job of tuning, read on but be aware you might experience “The End of the Innocence” or at least the inspirational influence of this album by Don Henley that I am listening to.

There is a window of allowable controller gains for loops that do not line out (reach a steady state) when the controller is in manual. These processes are called non-self-regulating. The most common example is level. If you put a level loop in manual, it will eventually if not immediately ramp away. Level is an integrating process, where the process output (level) is the integral of the process inputs (inlet and outlet flows). The change in % level ramp rate per % change in controller output is the integrating process gain. For 99% of the applications, the integrating gain is low (e.g. < 0.01% PV per sec per % delta OUT). For big tanks (e.g. storage tanks), and some horizontal tanks (e.g. distillate receivers), the ramp rate is very slow and consequently the integrating process gain is incredibly small (e.g. < 0.0001% PV per sec per % delta OUT). Since the controller gain is inversely proportional to this integrating process gain, it is the rare bird that has a level loop has a controller gain that is too high. You would be hard pressed to make a level loop go unstable due to too high of controller gain. You would be way beyond your comfort level (e.g. controller gains > 10), the controller output would be spiking due to measurement noise, and if you ever made a set point change, the controller output would most likely step to its output limit.

This brings to mind an example of how far below the upper controller gain stability limit we operate. Around 1980, analog controllers on a distillation column were replaced with DCS controllers on a test basis to see if there was any benefit to this new technology. The analog level controller setting of 100% proportional band was used in the DCS level controller. Fortunately, the configuration person didn’t know how to convert from proportional band to controller gain. The DCS level controller gain was set to 100 on the distillate receiver and the column performed better than ever due to a tight enforcement of the column’s material balance. The DCS was deemed truly wonderful technology and the rest is history.

Many distillation columns show slow rolling oscillations that are not readily evident unless you look over several shifts. These oscillations are always damped but never die out because the chance of a disturbance once per day even if it is just the day to night temperature change is high.

The product of the controller gain and reset time must be greater than 4 divided by the integrating process gain. Thus, you can increase the controller gain or the reset time to get rid of the slow rolling oscillations. For surge and feed tanks, the level just needs to be kept within the alarm limits and sudden changes in the manipulated flow show up as disruptive changes in feed to important unit operations (e.g. columns, crystallizers, evaporators, extruders, and reactors). For these applications, the reset time should be increased in most cases. For recycle tanks where the change in make-up reactant feed manipulated by a level controller matches the change in recycle reactant flow, tighter level controller generally helps maintain the correct material balance of reactant in the recycle tank. Here the level controller gain should be increased to get rid of the slow oscillations. This is also the case for column distillate receivers. For fed-batch reactors where one of the byproducts is an off-gas, tight level control by manipulation of reactant feed helps match the reactant feed to the reaction rate. This brings to mind that many reactor pressure loops have an integrating response and that tight pressure control by manipulation of a gaseous reactant feed helps keep the reactant concentration and hence the reaction rate constant. Many furnaces and incinerators have an integrating response but here the integrating process gain can be quite high since the allowable pressure range is in inches of water column. In these applications it is possible to encounter a controller gain that is too high in terms of instability or excessive amplification of noise.

There are many types of integrating loops. Nearly every fed-batch reactor has a non-self regulating response for concentration and temperature control. There are even the extreme examples of runaway reactors whose divergence may start out as a ramp but then accelerates. The tuning of these reactor temperature controllers and the point of no return is the subject for next week.

“People don’t run out of dreams, they just run out of time” in “River of Dreams” by Glen Frey. I have this dream that process design and configuration engineers and users and suppliers understand and communicate about controller tuning and loop performance, but in this hectic work place with expertise attrition, we are all running out of time.