Page 206 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Pumping of Liquids 177

head is twice that of the rated pressure of one pump at the
designated flow rate (Figure 3-35). The pump casing of
Atmosphere each stage (particularly the last) must be of sufficient pres-
Side sure rating to withstand the developed pressure.

Pumps in Parallel
Figure 3-31C. Double mechanical seal, two rotary elements against
common stationary. (By permission, Fischer, E. E., Chem Process- Pumps are operated in parallel ta divide the load
ing, Oct. 1983 [24].) between two (or more) smaller pumps rather than a single
large one, or to provide additional capacity in a system on
short notice, or for many other related reasons. Figure 3-
, , (- 35 illustrates the operational curve of two identical pumps
Barrier Fluid In
in parallel, each pump handling one half the capacity at
Atmosphere the system head conditions. In the parallel arrangement of
Side two or more pumps of the same or different characteristic
curves, the capacities of each pump are added, at the head
of the system, to obtain the delivery flow of the pump sys-
tem. Each pump does not have to c?JTY the same flow; but
Figure 3-310. Tandem double seal. (By permission, Fischer, E. E., it will operate on its own characteristic curve, and must
Chem. Processing, Oct. 1983 [24].) deliver the required head. At a common tie point on the
discharge of all the pumps, the head will be the same for
the sum of the individual discharge pressures of the indi- each pump, regardless of its flow.
vidual pumps. For identical pumps, the capacity is that of The characteristic curves of each pump must be con-
one pump, and the discharge pressure of the last pump is tinuously rising (right to left) as shown for the single
the sum of the individual heads of each pump acting as a pump of Figure 3-35, otherwise with drooping or looped
single unit. Thus, for two identical pumps the discharge curves they may be two flow conditions for any one head,

l !

\
"{ .- __ -· - ·-..... ._tfficlency
_c
-
.....
Q) ,d I',._ ,Head, ft. a f Liquid ,,./ ... '!- -- -.-1 ...........
r-,
"C / :.,..-- -,
"'
:i: ......... I f'.....r--L -: / ... 7" A= Axial
>- M =Mixed
u ' ......... :----_ -; / '
c: l(._ ./ -.,L. C =Centrifugal
Q)
·.::; " i- M / -
.,--
iE -·- ·- ·- /;; -·- �
�
UJ �� ?2 -· - ·- :::,... _
c.: ·.-
I//
:i: ./ // Rating I ;:;:
co � Point � ·-. �
--- r-- � � / ira ke Horsepower t \ [\ I
,/
A
i-- u, :M:_
"7
/ � � _ ..... c -- - I
" /- -·-
0 -·--· r-.; I
� � r---....... I
'.// I I
Gallons per minute
Figure 3-32. Comparison of impeller types for centrifugal pump performance. (Adapted by permission from Pie-a-Pump, Allis-Chalmers Mfg.
Co.)