Tap the Best Tie-in

Readers consider where to get compressed air for a pump.

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Demurrage costs are prompting our refinery to install air diaphragm pumps as a backup for loading decanter oil on barges. We plan to use two pumps in tandem, each running at 200 gpm. Pump operation will require approximately 300 scfm of air at a minimum of 70 psig. The question is where to tie into the compressed air system (Figure 1). Refinery management is leery of tapping into nearby lines because of the variety of loads they serve. What's the best place to get the compressed air?


DO AN ANALYSIS
This is a complicated problem without knowing more about the air demand in the different units of the refinery. The best approach is to do a survey of usage rates by each unit. This could be difficult and time-consuming if meters aren't in place. If not, consider installing orifice plates at each unit. The idea is that 300 scfm is a lot of air. Using an estimate of about $250/year/scfm, this is an additional cost of about $75,000/year; so the demurrage costs will have to be substantial. By doing a load capacity study at normal flow and at peak demand, it will be possible to determine how far you need to go back to the main header. This affects the overall viability of the project.

Another approach to this survey is to count the number of control valves and automatic valves in service. A crude estimate of demand is 1 scfm/valve. This assumes that compressed air isn't used for other applications — for such applications, it might be possible to get the maximum design rates from vendors. Of course, time of service can only be estimated roughly.

There's another way to look at this problem: Consider an analysis of equivalent length (see Figure 2). It clearly shows that in most cases a 2-in. pipe is sufficient for 300 scfm, assuming the run length to the tie point is less than a mile. Based on this analysis, I would tie into the 3-in. blind flange located on the FCC header.

One simple solution might be to link across one of the 2-in. lines allowing the burden to be shared. This is a common solution where intermittent demands can be larger than a single draw alone.

Once the loads are established, it will be necessary to confirm if the 3-in. tie-point offers the least disruption of the pressure available.
Dirk Willard, lead process engineer
Fluor Global Services, Inver Grove Heights, Minn.


LOOK AT THE ECONOMICS
I would approach this problem in the following manner: 1) carry out a cost/benefit analysis of demurrage versus upgrading the compressor facility; 2) if the profitability index favors facility upgrading, compare the cost of adding one more compressor stage (if technically feasible in view of the discharge temperature limitations) to installing a single-stage compressor on-site in order not to disrupt the existing process arrangement; and 3) optimize the existing facilities to see if the current capacity of the compressor can serve the arrangement without going stone cold.
Dennis Omenka, MS student, chemical engineering
University of Lagos, Akoka, Nigeria

SEPTEMBER'S PUZZLER

We overhauled our isobutene/n-butane fractionating column during a recent turnaround. We cleaned its sieve trays, replaced the reboiler (a circulating thermosyphon) and slightly lowered the suction nozzle to avoid splashing the bottom tray. The bottom-most thermocouple failed during start-up total reflux; it went full-scale. We switched reboiler control to an available thermocouple higher in the column. We replaced the failed thermocouple after the column was dehydrated. Now, we're having trouble with the temperature profile below the feed tray. The reboiler differential temperature is sporadic. What's causing this and what can we do?

Send us your comments, suggestions or solutions for this question by August 10, 2012. We'll include as many of them as possible in the September 2012 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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