Vanquish Vacuum Pump Vexations

Readers suggest how to pinpoint problems

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Bill Kubik, project manager
Graham Corporation, Batavia, N.Y.


DO A PERFORMANCE TEST

The first step is collect data. You will need: 1) trend data for the temperature, pressure and, if possible, the flow of the inlet stream; 2) trip points for the pressure and flow; 3) the inlet and outlet temperatures and flow rate of the seal water; 4) inlet and outlet samples of the seal water (if a closed loop); 5) humidity readings of the inlet vapor stream; 6) an inspection of the inlet and outlet piping, including a leak test; and 7) a review of the vendor piping and electrical drawings and a comparison to what was done during the commissioning. Next, call the vendor. Chances are that if you installed a vacuum pump backwards, there's something else wrong.

Next, run a performance test. Measure the inlet flow to the pump while monitoring the motor draw and rotational speed. You'll want this information before waiting for the vendor. If the pump is off its curve it's usually because the energy is going elsewhere, like the seals or the fluid.

Don't assume that the design was correct. You could have a broken vane caused by an initial surge of flow. The report of trips during startup tells me that this could be a problem. This could indicate that the initial pump design layout is faulty. The solution is to decrease the inlet vapor stream by reducing its initial volume flow rate and its temperature.

Don't assume that the pump assembly is correct. You could have a reversed impeller. Maybe that's why the pump could run in reverse. If you have a spare pump, consider re-running the performance test with the new pump. If all is well, Operations is happy. Then, you can pull the original pump apart and inspect it for damage and construction. Obviously, you violated your warranty by operating the pump in reverse, but if there's an error in assembly you can blame the vendor for some of the problem.
 Dirk Willard, process engineer
Fluor Global Services, Inver Grove Heights, Minn.

AUGUST'S PUZZLER
We want to increase the recycle flow from our sulfuric acid alkylation wash tank. The mixture going through the recycle is mostly alkylate (butanes, iso-butanes and propanes) along with 98% sulfuric acid. We're having trouble linking the percent open of the control valve with the actual flow measured across an orifice plate (Figure 1). Why are we having problems? Do we need a larger valve?

Send us your comments, suggestions or solutions for this question by July 6, 2012. We'll include as many of them as possible in the August 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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