Multivariable Control on Distillation ColumnsWhats the Best Way?
In the October issue of Control, Bela Liptak described multivariable controllers used on distillation columns. Yet the structures he showed underlying the MVCs have reflux and boilup flows manipulated for composition control, with product flows under level control.
This, the least stable of structures for column control, and the MVC system will not be able to compensate for it. These structures typically have relative gains of 10-100 or more, and even very small decoupler errors can result in instability. For example, a two-loop system having a relative gain of 16 will go completely unstable with a decoupler error as little as 3 percent.
This may explain the poor success rate of some MVC systems.
Process Control Consultant
Wolfeboro, NH 03894
Safety Is Everybodys Job
I read with interest your editorial on Whats It Going to Take? in the October 2007 edition, and it compels a response. The answer to the question is leadership from top management all the way down to the janitor. Everyoneyes, everyonemust take responsibility for safety. Further, we all need to lead the effort to improve the safety of our operations.
You mention in the ICL example that a co-op student performed an inspection of the pipeline that failed, leading to deaths and injuries. Where was the maintenance department when this inspection occurred? If the managing directors son was unqualified to do this inspection, who spoke up? Better still, who trained the individual to do the job right?
Another example cited is the Olympic pipeline. This pipeline apparently failed due to undetected damage caused by inadequate inspection and/or inaccurate evaluation, and the situation was exacerbated by operational issues. Who developed the inspection protocol and what was their qualification? Were operators aware of how their actions shortened the life of the pipeline? Better still, is it reasonable to expect the individuals who developed the inspection protocol or the operators who made process changes to know the consequences of their actions?
Past articles from Jim Montague have a similar theme to this editorial. It seems to me you are trying to provoke action from regulatory agencies or non-government organizations focused on industry oversight. I agree that someone should be watching over the process industries to make sure the bad actors are dealt with appropriately. But true improvement will come if we all work together and all take responsibility for process safety and the well-being of those who work in our organizations. Trade magazines can help by publishing best practices and work to inform readers on how practical solutions are available that can be applied in various situations.
I think Control magazine does a fine job with this, and I will continue to read your publication with interest. I hope you focus on how you can make a difference, and focus less on editorial and criticism. We all come to work each day hoping to make a difference, hoping to improve the status quo the productivity, profitability, and the safety of our organization. We derive job satisfaction by making that difference. Help us find new, creative and effective ways to make an even bigger difference!
King Industries, Inc.
Editors Note: Peter Montagnas article, Leadership and Process Safety which arose out of this letter to the editor, will be published in a future issue of Control. He makes a fundamental point: safety is the responsibility of the plant, from operator to enterprise, and is a job that cannot be shirked.