The relationships derived above help to establish the characteristics of the pump at
different speeds, once the Q–H curve at the normal speed N is known.
The relationships derived are based on a major assumption that the efficiency remains
constant when transferring a point on one characteristic to the homologous point on
another characteristic.
The results obtained by these equations often differ from the values obtained in practical
application. The difference becomes larger when the speed change is more than ±25%.
Let us consider a point P
2
on the Q–H curve-c in Figure 5.8. The homologous points
P and P
1
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