Conrad J. Horvath, rotating machinery engineer
Syncrude Canada Limited, Fort McMurray, Alta.
Reduce the pipe pressure drop
Toluene has a vapor pressure of 22 torr (0.425 psia) at 20oC (68oF) and is obviously causing NPSHA problems by flashing and subsequently causing cavitation. It can be deduced from Figure 1 that the suction piping is the cause of the problem. Tanks 1, 2, and 5 can supply fluid without a problem because the suction route from those tanks only has 2 elbows to flow through. However, tanks 3 and 4 have to supply fluid to the pumps by flowing suction fluid through a minimum of four 90° turns just to get to pump 1. An additional two 90° turns are required to get to pump 2. All these 90° turns (whether elbow or branch tees) are the culprits leading up to the problem. Pump 2 suffers because of all the 90° turns mentioned and also because it has no suction calming distance in its proximate entry suction piping as does pump 1.
A better installation is shown in Figure 3:
- Note that I try to space the pumps so that they receive an equitable suction route from each tank as much as is humanly possible, given the odd number of tanks;
- The suction entry to each pump should have an eccentric reducer prior to entering each pump, and the suction line to each pump flange should be approximately 10 to 15-diameters long (and straight).
Art Montemayor, senior process engineer
Universal Ensco, Inc., Houston, Texas
Check the pump selection, change the suction pipe
Reviewing the piping drawing it would appear that the No. 2 pump suction is limited or restricted to some extent. The additional elbow so close to the suction will also cause a problem. The cause for the seal failures is the lack of pump suction. The lack of adequate suction will cause heat to generate in the pump stuffing box and contribute to premature seal failure. As for the cavitation caused by the seasonal temperature the specific gravity of the fluid will change and this should be considered in the selection of the style of pump required to perform in the intended function required for year around service. Figure 4 would be an improvement. It is always a very good practice to design the suction of a pump without any turns in the piping. The lower set of pumps with the dark lines would be the best design.
George A. Easom, maintenance training supervisor
Georgia Pacific, Cedar Springs, Ga.
The research department is introducing a new color additive. This additive is blended with a 2-hp high-sheer mixer prior to addition to a weigh-cell feeder (Figure 2). The feeder drains to a soap plodder (mixer) and eventually product is extruded as bar soap. The specific gravity of the new liquid is 20% higher and viscosity is 10-times greater than the additive used in the past. Calculations show that the power draw by the high-shear agitator will exceed the motor limits. Because the new product will roll-out in a few weeks, we need a short-term solution quick. What alternatives will allow production to continue?
Send us your comments, suggestions or solutions for this question by May 5, 2006. Well include as many of them as possible in the June 2006 issue. Send visuals --- a sketch is fine. E-mail us at ProcessPuzzler@putman.net or mail to ProcessPuzzler, Chemical Processing, 555 W. Pierce Rd., 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 youd like to pose to our readers, send it along and well be pleased to consider it for publication.