Page 227 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Pumping of Liquids 197

fixed condition of suction lift, and relates speed, head
' I ii and capacity. This index is a valuable guide in establishing
"'1" I the maximum suction lifts and minimum suction heads to
1e t-r avoid cavitation of the impeller with resultant unstable
I
hydraulic performance and physical damage. For a given
--;;�icSl'f.f.0
� - - set of conditions on the suction and discharge of a pump,
01,-- I a slow rotative speed will operate safer at a higher suction
lift than a pump of higher rotative speed.

IIO - Rotative Speed
HEAD-CAPACITY ,,. ,........,
llO ' \\""
The rotative speed of a pump is dependent upon the
--p< r--,.....
.., l40 .>./ \ impeller characteristics, type fluid, NPSH available and
�
...
...
l'OIIIT �
i,.,,-- '�
:! L "' ,.� #-- IIAill- � � I \ other factors for its final determination. The most direct
["1CI[..,
,,.,.....
;,::." ·-- 4J, ' \ method is by reference to manufacturer's performance
d
.., J ,,,,, ..., K"' "'" '\ curves. When a seemingly reasonable selection has been
I
c
I
..,
:r:. IO L.- I I " \ 40 made, the effect of this selected speed on the factors such
e eo I I I L as NPSH required, suction head or lift, fluid erosion and
c
40 J 11 I ZO.:- corrosion, etc., must be evaluated. For many systems these
z
to I I I 10• factors are of no concern or conseqt,ence.
I I Normal electric motor speeds run from the standard
0 I 0 induction speeds for direct connection of 3600, 1800 and
0 ! 4 I (! 10 12 14 II 111 ZO 22
1200 rpm to the lower speed standards of the synchro-
CAPACITY, !N 100 CiPM
nous motors, and then lo the somewhat arbitrary speeds
Figure 3-50. Typical centrifugal pump characteristic curve with aux- established by V-belt or gear drives. For some cases, the
iliary specific speed curve. Double-suction, single-stage, 6-in. pump speed is set by the type of drivers available, such as
pump, operating at 1760 rpm constant speed. (By permission, a gasoline engine.
Karassik, I. and Carter, R., Centrifugal Pumps, McGraw-Hill Book Electric motors in pump application never run at the
Co., inc., 1960, p. 197.)
"standard" rotative design speeds noted above, but rotate
at about (with some deviation) 3450, 1750, and 1150 rpm,
ciency. Figure 3-4 7 illustrates the range of typical specific which are the speeds that most pump manufacturers use
speed index numbers for particular types of impellers. for their performance curves. If the higher numbers were
used (motor designated or name plate) for pump perfor-
Example 3-12: "Type Specific Speed" mance rating, the pumps would not meet the expected
performance, because the motors would not be actually
In Figure 3-50, where the pump operates at 1760 rpm rotating fast enough to provide the characteristic perfor-
(a standard motor speed under load) and has maximum mance curves for the specific size of impeller.
efficiency at 1480 GPM and 132 feet head, the "type" spe-
cific speed is Pumping Systems and Performance
It is important to recognize that a centrifugal pump will
(3- 11) operate only along its performance curve [10, 11). External
conditions will adjust themselves, or must be adjusted in
order to obtain stable operation. Each pump operates within
[1760'\ � ] a system, and the conditions can be anticipated if each
N 1740
s
(132)0,o component part is properly examined. The system con-
sists of the friction losses of the suction and the discharge
piping plus the total static head from suction to final dis-
Figure 3-47 indicates the general type of impeller charge point. Figure 3--51 represents a typical system head
installed.
curve superimposed on the characteristic curve for a 10
The specific speed of a given type pump must not by 8-inch pump with a 12-inch diameter impeller.
exceed the specific speed values presented by the Depending upon the corrosive or scaling nature of the
Hydraulic Institute [17]. This is based on a known or liquid in the pipe, it may be necessary to take this condi-