Process Puzzler: Kayo a Cascade Control Complication

Readers suggest how to properly regulate an exothermic reaction.

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Moving the temperature transmitter to the pump discharge is tempting but you want a little lag in the system afforded by the tank volume. Otherwise, the secondary loop will wind up and overshoot.

Another issue may be the orifice plate. The response by the secondary flow loop will be to the flow squared. Ideally, you want the response to be linear. Then, there is the startup state. Orifice plates typically are accurate to only ±1% at full scale and become more inaccurate at low flow. The operators probably start this process in manual and never put it in automatic.
Dirk Willard, senior process engineer
International Steel Services, Inc., Pittsburgh

We’re in the midst of investigating a fatal accident caused by a fire that occurred at a petrochemical storage facility. A farmer living nearby reportedly smelled gasoline for several days before the accident. He says he called the refinery several times but wasn’t taken seriously. The Fluid Catalytic Unit (FCU) that produces much of the refinery gasoline was having an emergency turnaround during this period, so staff were stretched thin. They didn’t see much urgency because of the tank farm’s safety system, which relies on redundant controls with an automatic gauging system and high/high alarms on each tank. Finally, two operators drove out to the tank farm and died in the fire. How should we approach this investigation? Do you have any suggestions for improvements in the tank farm? What do you think killed the two operators?

Send us your comments, suggestions or solutions for this question by June 12. We’ll include as many of them as possible in the July 2009 issue and all on Send visuals — a sketch is fine. E-mail us at or mail to Process Puzzler, Chemical Processing, 555 W. Pierce Road, Suite 301, Itasca, IL 60143. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.

And, of course, if you have a process problem you’d like to pose to our readers, send it along and we’ll be pleased to consider it for publication.

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