Chemical Processing - May 2008 - (Page 43)

PROCESS PUZZLER < > THIS MONTH’S PUZZLER A self-priming slurry pump circulates hot slurry through a salt mine several hundred feet below the surface of the plant. Steam is injected into the discharge line to help dissolve the salt — the solution being pumped is above saturation; the pump operates with a low NPSHA/NPSHR. The pump is designed to run dry for long periods of time. Unlike its predecessor, it hasn’t reliably performed. With the discharge diaphragm valve throttled full open, the pump produces a little noise. The pressure gauge bounces wildly but settles in. Throttling back the valve to its operating point causes the pump to scream. We frequently replace the seals and bearings. During the pump’s installation, the suction diameter was increased to provide more NPSH; new expansion joints were put in, as was a new partial-open impeller to replace the fully-open impeller. Operating conditions also have changed. To boost the profitability of the plant, the total dissolved solids level has been increased by 10%. How can we improve the performance of this pump? Replace seals with packing For a pump designed to run dry, running half full can be destructive. During the transition between dry and liquid full, the pump can experience cavitation, flashing, extreme pressure fluctuations, high radial shaft and bearing loads, and high vibration. All lead to shortened seal and bearing life. Replacing the mechanical seals with graphite/Kevlar packing (not PTFE) and adding an external flush to a lantern ring could reduce the impact of the service conditions. The external flush is the key. The graphite packing would act as a steady bearing on the shaft, reducing bearing load, dampening vibration and stiffening the shaft. This won’t solve the noise problem but could reduce wear on the impeller and casing, bringing reliability to a tolerable level. Van Richard, engineer Georgia Gulf Corp., Plaquemine, La. Use recirculation The problem is the noise. The solution is a recirculation line of suitable diameter between the suction of the pump and its discharge. The line should be provided with a control valve adjusted for the desired feed to the plant. Also, consider replacing the diaphragm valves; they seem to cause a high drop. B. S. Gadodia, consultant engineer Indore, India www.chemicalprocessing.com >> JULY’S PUZZLER Our spray dryer is fed from two booms. These booms have a dozen tiny spray nozzles each. Each set of nozzles is set into a head, one to each boom. It’s a new design that our management hopes will increase throughput. The result has been completely the opposite. We think the droplets collide where the streams meet forming large droplets. Our vendor suggests going with only one boom but this will cut our rate and spray won’t be centered. The droplets must fall at a specific rate to crystallize properly. Is there anything we can do that will allow us to work with the existing heads? Send us your comments, suggestions or solutions for this question by May 16, 2008. We’ll include as many of them as possible in the July 2008 issue and all on CP.com. Send visuals — a sketch is fine. E-mail us at ProcessPuzzler@putman.net or mail to ProcessPuzzler, 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. May 2008 • 43 http://CP.com http://www.chemicalprocessing.com

Table of Contents Feed for the Digital Edition of Chemical Processing - May 2008

From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor WirelessHART signals a change at plants Avoid costly fabrication mistakes Watch out with variable speed pumping Hot cutover boosts control system migration Plant InSites Process Puzzler Equipment & Services Product Spotlight/Classifieds Ad Index End Point

Chemical Processing - May 2008

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