Chemical Engineering November 2010 - 58

Recirculating entrainment plot
Engineering Practice
full " coverage. Only by continuing to
increase the liquid level can gas entrainment
be avoided.
One other data set from the literature
has not been discussed. Patterson
[1] showed limited data for the critical
submergence if the tank was recirculating
its flow rather than just emptying
(draining) its contents. The author
stated that an emptying tank needs
about three times the critical submergence
to avoid gas entrainment
compared to a recirculating tank. It
is reasonable to assume the returning
flow disrupts the formation of a steady
vortex of whirlpool and thus allows the
free-surface level to be lower (closer to
the exit nozzle). However, trying to add
his data to the master design chart in
Fr versus H/D notation showed confusing
trends (Figure 10), suggesting
that his actual experimental setup
should be investigated further. A recirculating
tank is worthy of further
study as it could be a frequently encountered
operating condition.
Design application
Recently, Eastman Chemical needed
to have the bottom nozzle of a new distillation
column evaluated for possible
gas entrainment. The column was a
retrofit with a higher expected capacity
compared to the previous column.
The bottom nozzle supplies both the
thermosiphon reboiler and the bottom
draw-off. Operation at previous rates
had not exhibited any flow problems.
References
1. Patterson, F.M., Vortexing can be prevented, Oil &
Gas J., August 1969, pp. 118-120.
2. Process Industries Practices, http://www.pip.org/
3. Megyesy, E.F., " Pressure Vessel Handbook, " 6th
Ed., 1983, p. 300.
4. Rousseau, R.W., " Handbook of Separation Process
Technology, " 1987, p.151.
5. Waliullah, S., Do-it-yourself vortex breakers,
Chem. Eng., May 1988, pp.108-109.
6. Kister, H.Z., " Distillation Operation, " 1990, pp.
90-94.
7. McGuire, J.T., " Pumps for Chemical Processing, "
1990, p. 268.
8. McKetta, J.J., " Encyclopedia of Chemical Processing
and Design, " 32, 1993, p.113.
9. Voss, J., and others, " Cleaning and Cleaning Validation:
A Biotechnology Perspective, " 1995, pp.
19-23.
10. Arnold, K., Stewart, M., " Surface Production Operations, "
Vol. 1, 2nd Ed., 1998, p. 112.
11. Borghei, S.M., Partial Reduction of Vortex in
the Vertical Intake Pipe, 4th International Conf.
Hydro-Sci & -Engr, Korea, 2000.
12. Silla, H., " Chemical Process Engineering, Design
and Economics, " 2003, pp. 273-280.
13. Goulds Pumps, On-line Piping Design Sec10,000
1,000
100
10
1
Recirculating
0.1
Patterson's
data
0.01
0.001
0.01
Gravity drain
Souders
Simpson
Harleman
McDuffie
0.1
Lift
Palgrave-min
Palgrave-max
Kocabas
Labour (high D)
Labour (low D)
Lang (high D)
H/D
1
Pumps
Lang (low D)
Gould (low rate, high D)
Gould (high rate, low D)
Self-vent
FIGURE 10. Shown is an additional master chart plotted with an approximate depiction
of the data provided by Patterson [1]. Notice the left-end of the new data has
a slope similar to the pump-driven curve discussed previously. No description of the
piping arrangement was provided (so it is not clear whether the recirculating piping
re-enters the tank on the side or from the top)
The maximum size of the bottom nozzle
was limited due to building structural
supports that could not be modified.
This maximum nozzle size was
below that required for the self-venting
flow correlation, per Kister [6].
In this application, the use of a
vortex breaker could be problematic
due to possible flow restriction
or plugging. Using the gravity-drain
master chart of Figure 5 created in
a spreadsheet, operational data for
a given design can be converted into
the non-dimensional format and plottion
02, accessed at: www.gouldspumps.
com/pag_0006.html.
14. Rotonics Manufacturing's online products list,
accessed at http://www.rotonics.com/tanks/accessories/RMI%20Tank%20Accessories.pdf
.
15. Perry's " Chemical Engineering Handbook, " 6th
Ed., 1984, McGraw-Hill, pp. 5-44.
16. Kalinske, A., Hydraulics of Vertical Drain and
Over-Flow Pipes, University of Iowa Studies in
Engineering, Bulletin No. 26, 1941, pp. 26-40.
17. McDuffie, N.D., Vortex free downflow in vertical
drains, AIChE J, Vol. 23 (1), Jan 1977, pp. 37-40.
18. Simpson, L., Sizing piping for process plants,
Chem. Eng., June 17, 1968, pp. 192-214.
19. Souders, M., Huntington, R.L., Corneil, H.G.,
Emert, F.L., Performance of bubble-plate columns,
froth heights and pressure differentials, Ind. Eng.
Chem., Vol. 30 (86), 1938.
20. Harleman, D.R.F., Morgan, R.L., Purple, R.A.,
Selective Withdrawal from a Vertically Stratified
Fluid, 8th Congress International Assn. for Hydraulic
Research, 10-C-1, Montreal Canada, Aug.
1959.
21. Anderson, A.G., Vaidyaraman, P.P., Chu, C.S., " Hydraulics
of Long Vertical Conduits and Associated
Cavitation " , U.S. E.P.A. Water Pollution Control
Research Series, 11034 FLU 06/71, 1971.
22. Simpson, L., Process piping: Functional design,
42 ChemiCal engineering www.Che.Com november 2010
ted on the regime chart (Figure 11).
The recirculation rate, potential
nozzle size, and operating level were
varied to generate the three design
operating ranges. Operation in these
ranges (as shown in Figure 11) could
be problematic because all except operation
at low level indicate the potential
for vapor entrainment, and in
no situation does it run full. However,
since this design uses recirculating
flow, Patterson's data [1] shown in Figure
10 suggest the actual H/D may be
equivalent to three times that shown
Chem. Eng./Deskbook Issue, April 14, 1969,
pp.167-181.
23. Simpson, L., Weirick, M., Designing Plant Piping,
Chem. Eng./Deskbook Issue, April 3, 1978, pp.
35-48.
24. Stepanyants, Y.A., Yeoh, G.Y., Stationary bathtub
vortices and a critical regime of liquid discharge,
J. Fluid Mech, V604, 2008, pp. 77-98.
25. Lubin, B.T., Springer, G.S., The formation of a dip
on the surface of a liquid draining from a tank,
J Fluid Mech, Vol. 29 Part 2, 1967, pp. 385-390.
26. Lang Engineering White Paper by Flint Evans,
entitled Rules to Follow to Avoid Pump Problems,
no date.
27. Goulds Pumps: Pump Care Manual, ITT Industries
Ed, 9-2002, accessed at: http://www.gouldspumps.com/download_files/literature_misc/
PumpCareManual.pdf
.
28. Palgrave, R., " Troubleshooting Centrifugal Pumps
& Their Systems, " Elsevier, 2002.
29. Labour Taber white paper by Larry Bachus,
reproducing information from " Know and Understand
Centrifugal Pumps, " by L Bachus,
A Custodio, Elsevier 2003.
30. Kocabas, F., Unal, S., Unal, B., A neural network
approach for prediction of critical submergence
of an intake in still water and open channel flow,
Computers & Fluids, 37, 2008, pp. 1040-1046.
10
Gravity
drain
Emptying
Pump
driven
Froude number
http://www.gouldspumps http://www.rotonics.com/tanks/acces http://www.pip.org/ http://www.gould http://www.spumps.com/download_files/literature_misc/ http://www.Che.Com
Chemical Engineering November 2010

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