Tuning to Meet Process Objectives

McMillan, Weiner and James Beall Talk About Tuning the Controller

By Greg McMillan, Stan Weiner

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Stan: Where have you used this method beside surge tanks?

James: I used the same calculation for a large distillate receiver where the level controller manipulates distillate flow to take full advantage of the receiver size to smooth out the flow to downstream operations. Surprisingly, it can be beneficial for the level controller on the base of a distillation column where it manipulates the bottoms flow that feeds the next column. Ironically, you can also use the same equation, with a small "allowable error," to provide "tight," yet not oscillatory control of an integrating process such as a reactor level.

Greg: If a temperature controller was manipulating distillate flow, and the receiver level controller was manipulating reflux flow, very tight level control would be needed because the effect of temperature control actions do not get into the column until the level controller makes the corresponding correction in reflux flow. Other cases where you need tight level control are recycle tank material balance control and continuous liquid reactor residence time control.

James: These are great examples to think about variability pathways and maximizing process performance! In the case of the distillation example, feed-forward (with a negative 1 lb/lb gain) from the distillate flow to reflux flow (via the level controller) and a Lambda to provide maximum absorption of variability provides an immediate correction in the reflux flow without passing the variability of the condensing system and receiver level into the column.

Greg: I agree, feed-forward control offers a significant advantage. We normally extend feed-forward control to account for feed flow changes by ratioing the distillate flow to the feed flow. Feed-forward summers provide correction of the distillate flow by the temperature controller and reflux flow by the level controller. Tight temperature and level control correct for flow measurement errors and unknowns. Tight level control offers an additional advantage of helping provide internal reflux control. For example, a cold rainstorm can be a sudden severe upset that reduces the column vapor traffic and condensing rate even though the feed rate is constant. Tight level control will reduce the reflux to the column, helping to maintain the internal reflux to vapor ratio inside the column. Temperature correction of the distillate flow is late because of the significant dead time in the composition response. If there are operating or control problems with the condensing system, these need to be fixed. Measurements should have a high threshold sensitivity and minimal noise for tight control. A resistance temperature detector (RTD) and guided wave radar (GWR) offer this capability. 

Top Ten Signs Your Loop Tuning Is a Mess
 
(10) Tuning software is nonexistent.
(9) Lots of trials and errors.
(8) Don't know if the proportional setting is PB or gain.
(7) Units of reset setting are unknown.
(6) Lambda is zero.
(5) Trend charts look like a tempest.
(4) Valves are worn out.
(3) Tempers are worn thin.
(2) Low and high output limits are set equal.
(1) Preferred mode is manual.

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