Posted by Don Snowden on October 13, 1999 at 18:59:24:
In Reply to: Tech Tip #1 - Q Controllers on DCS System posted by Mike McCarty on October 07, 1999 at 15:05:14:
I agree with your assessment of Q controllers on Crude units and also on their application for their use on column reboilers.
In addition, the use of Q controllers for steam reboilers must also be approached cautiously. The most significant effect that changes the duty on steam reboilers is the effect of changes in flow due to changes in pressure (ie flow compensation). For example, small pressure changes on a 20# reboiler represents a significant change in density thus flow. The real issue in whether you can compensate this flow is "how independent is the flow with the pressure". If this is a large reboiler (relative to the steam system) then you can exasperate the problem if you start pulling hard on the system when the pressure changes.
I believe the more succesful applications for Q controllers would be on furnace fuel gas applications and "hot-water" belts (i.e. other heat mediums). Since fuel gas flows have the same issue with pressure changes as steam you must be careful in the pressure compensation term, but correction for heating value is appropriate. (The measure of heating values is another topic probably worth discussing!).
I have seen several "other" heating mediums properly compensated for temperature changes. Another good application for handling "unmeasured" changes in heat input is to tie the reboiler to something that can be measured. For example, on some distillation columns (e.g. C3 Splitters) you can tie the reboiler to the bottoms level. Therefore, you get quick feedback when the heat input changes.