Vanquish Vacuum Distillation Difficulties

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Steam quality is only part of the problem with the ejector corrosion. You may need to go with high nickel alloys for internal components subject to erosion. Ceramics are another choice; thermal shock normally isn't a problem with modern ceramics, especially with water systems. (Ceramic coatings have been used to protect fired heaters: www.advancedmaterialtechnologies.com/pdf/fmp/C2012-0001687%20(2).pdf.) Coatings such as silicon nitride (Si3N4), silicon carbide (SiC), titanium nitride (TiN) and tantalum nitride (TaN) have been used to protect metals from erosion. A coating is just that, a coating — it may prevent erosion but if the metal behind it is penetrated, corrosion continues. Perhaps, it would be best to build critical components out of ceramic alone; silicon nitride should be the best choice for this type of application.

Another issue is steam operation. You'll want to check the steam trap and the pressure control valve. It may be that the trap is not functioning or has been poorly selected. Perhaps the pressure control valve is being bypassed. Check the condition of the steam line and its history. If the steam separator isn't working well or has been scaled up in the past, it may be because the supply piping is poorly insulated or condensate has built up in the piping. Look at how often the piping has been replaced. Wet steam will cause as much damage as condensing hydrocarbons, line rust or scale.
Dirk Willard, senior process engineer
Ambitech Engineering, Downers Grove, Ill.

MAY'S PUZZLER
Catastrophic pump failures at the tank farm (Figure 1) are plaguing the commissioning of our new plant. The tank farm has an ordinary centrifugal pump (CP) as well as magnetic-drive CPs and a gear pump. The mag-drive pump started failing after the first few days. We got some warning from a pressure switch low (PSL) that flashed for a couple of minutes. We switched to a spare and it did the same thing after a day. The gear pump seemed to be operating fine, even after a few days — but a reading on an infrared gun indicated the casing was hot; the downstream flow meter showed a dropping flow rate. An operator loosened the packing on the pump while I was away at lunch and it now runs fine. The water pump is showing the same symptoms as the mag-drive pump but hasn't failed yet during the first week of commissioning. Are we out of the woods?


Send us your comments, suggestions or solutions for this question by April 11, 2014. We'll include as many of them as possible in the May 2014 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, 1501 E. Woodfield Rd., Suite 400N, Schaumburg, IL 60173. 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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