Page 113 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 2 .21 Pump Best Practices

Fig 2.21.2 Performance change vs. impeller diameter change (Courtesy of Union Pump Company)

exceed that of existing pumps (refer to Figure 2.21.1 for affinity
law relationships).
Once a pump has been selected and the impeller diameter
to deliver a defined flow rate for a required level of head
(energy) has been determined, the affinity laws can be used
to determinewhatnew speedorimpellerdiameterisre-
quired to satisfy the alternative operating conditions. A re-
vised flow versus head (energy) versus horsepower curve can
be developed and plotted from these relationships. Refer to
Figure 2.21.2 for an example of an impeller diameter
change.
The affinity law relationships for changing impeller diameter
usually work pretty well for relatively small changes; of the
order of 10%. If the change exceeds 10%, the relationship
between the impeller and casing can change significantly, which
potentially can alter the design configuration of the pump. It is
always good practice to check the pump rating curves to
determine whether the pump has been tested with that par-
ticular impeller diameter.
The variation of pump performance with changes in speed
also follows the affinity laws, but with a higher level of accuracy
than do changes in impeller diameter.
When the performance is known for a pump operating at
a lower speed (e.g. 1150 rpm) and it is desired to operate at
a higher speed (e.g. 1750 rpm), the affinity laws can be used to
Fig 2.21.3 Performance change versus speed change calculate performance at the new speed, and the converse is also

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