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Pump Hydraulics

Figure 2.10: Effect of speed or diameter change

.P2 =(02/3 or (.2/3
HP~ k-~-~ ] k-~-~ ]

It must be noted however, that this is only an approximation as the
pump efficiency may change with the change in peripheral velocity,
however it may have been accomplished.

2.3.1 Change of electric motor for performance change
As a change of an electric motor to a higher speed unit is not an
uncommon way to effect an increase in pump output, please take very
careful note of the results of this example. Consider a process pump
being driven by a 25 HP, 1800 rpm. motor, and the motor is replaced
by one running at 3600 rpm.

HP2 = (360013
25 U8oo)

From the Affinity Laws we can calculate that we would have to replace
the 25 HP, 1800 rpm motor with a 200 HP motor running at 3600
rpm. Unfortunately, most pumps purchased to operate on 25 HP arc
unlikely to be strong enough to accept 200 HP on the same shaft and
beatings, thus resulting in premature failure.

2.3.2 Other changes
A change in the NPSH Required can also bc estimated by the Affinity
Laws, but only when the pump speed is changed. Any change brought
about by a change in impeller diameter does not need the Affinity

25 !