Process Puzzler: Spike Tank Spikes

Readers suggest how to avoid column feed tank problems.

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Of course, it is possible that there is water in the instrument air. Most dryers are designed for a dew point of -20°F. There's a remote chance the valves could not fight the spring because the ambient temperature fell below this point, the dryer failed or oil was allowed into the instrument air.

There are some process workarounds that may help. Some of the heat in the furnace can be reduced by changing the shutdown plan to include a cooldown using the combustion air blower. But this won't completely eliminate the risk of blowing the relief valve if the feed valve is closed. Perhaps it would be better in this case to change the actuator in the feed valve to a fail-open orientation. This, too, should be decided by a new HAZOP.
Dirk Willard, senior process engineer
Middough Consultants, Holland, Ohio

AUGUST'S PUZZLER
As part of our refinery expansion, we must confirm the capacity of a schedule-80 crude-oil-desalter wastewater line. It's a 2,500-ft pipeline consisting of two heat exchangers and a mix of 3-in. and 4-in. pipe emptying into a slop-oil decanter operating at 5 psig. We're hoping that capacity can be increased by 25%. We collected some data using existing pressure gauges. An initial hydraulic study with software using equations from Crane Technical Paper No. 410 indicated there is an "extra" 65 psi when 125 gal/min of brine flows at 160 psig. However, another study done using different Crane-based software gave about-50-psig extra pressure. How could two programs produce such different results?

Send us your comments, suggestions or solutions for this question by July 8, 2011. We'll include as many of them as possible in the August 2011 issue and all on chemicalprocessing.com. Send visuals — a sketch is fine. E-mail us at ProcessPuzzler@putman.net 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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