As cavitation commences, it reduces the performance of the pump rapidly. Continued operation can also lead to erosion damage or surface pitting. Cavitation can be avoided if the pressure everywhere in the machine is kept above the vapor pressure of the operating liquid. At constant speed, this requires that a pressure somewhat greater than the vapor pressure of the liquid be maintained at the pump inlet (suction). It is important to carefully limit the pressure drop in the inlet piping system.
A pressure gauge on the suction line before the pump will help you identify the flow condition of the liquid. You can also use this to test at what suction pressure the cavitation occurs and adjust your process accordingly.
There’s a useful web video on cavitation at: http://web.mit.edu/hml/ncfmf.html.
(Since this condition probably occurs during startup and shutdown it may be necessary to run the evaporator in manual mode, not automatic, during these times.)
Errol Williams, chemical process engineer
OPT FOR A RECIRCULATION LINE
The solution to the puzzler is a recirculation line. I would pump a portion of the liquid back into the receiver. (All of the liquid should be sent back to the receiver during startup.)
Dr. V. Ravichandran, technical director
Sartime Horological (P) Ltd., Perungudi Chennai, India
CONSIDER A BLEEDER
Could the level control issue be eliminated if you put a bleeder valve on the pump to get rid of the cavitation problem?
Jim Deary, shift supervisor
Bayer Films, Berlin, Conn.
PUT IN A RESTRICTIVE ORIFICE
This is an all too common problem. With the equipment on order, you could be looking at a serious hiccup in your project: shipment delay and unexpected costs for an unanticipated nozzle addition. Or you could recirculate flow to pipe —preferably one going back to the feed. But I am getting ahead of myself. First, you must review the operations manual and the programmable-logic-controller requirements because a new flow control loop will be required for startup and shutdown of the evaporators.
There are three ways to protect the pump from cavitation: flow, pressure and level. The least expensive, since the level is already part of the control, is to use the level control to turn off the pump. I’m assuming that an agitator stirs the bottom of the receiver. Because the impeller is ineffective when there’s too low a liquid level above it, the pump may be needed to provide what little agitation it can; pumps make poor agitators but crystallized solids can kill the whole startup. This probably rules out shutting off the pump.
So, with few options left, we probably need the pump circulating at all times via a recirculation line. We could get fancy with flow meter and flow control valve but, remembering Occam’s razor — i.e., don’t make any more assumptions than the minimum needed, the best way may be to size a restrictive orifice for this purpose.
Size the orifice for twice the pump minimum flow and check its capacity after including this loss; a larger impeller may be needed. Next, install a nozzle in the evaporator feed line because this will have the least impact on the project schedule. Finally, review the entire process for mistakes as this type of error is really serious.
Dirk Willard, senior process engineer
Swenson Technologies, Monee, Ill.
We replaced a thermocouple in a continuous hydrocarbon fluid-bed reactor during a turnaround. It’s located in a duct where high erosive flow occurs because of high velocity catalyst beads. We also replaced the carbon-steel thermowell with a high-nickel-alloy one because of a report of a slight corrosion problem. Since then, the thermocouple has failed every six months when previous thermowells lasted a year until the turnaround. What do you think the cause is and what can we do about it?
Send us your comments, suggestions or solutions for this question by July 10. We’ll include as many of them as possible in the August 2009 issue and all on CP.com. Send visuals — a sketch is fine. E-mail us at mailto: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.