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The Practical Pumping Handbook ================================= ~:~ .... ~ ...... -~"- ...

The relationships that exist between the pump output and the
peripheral speed of the impeller are identified collectively as the
'Affinity Laws'. In one such relationship, when an impeller diameter is
changed from D 1 to D2, the Capacity developed will change from Q1 to
0.2, in the same proportion.
D2 Q2
D1 Q1

However, the same relative change in impeller diameter from D1 to D2,
will change the Total Head from H1 to H 2 in proportion to the square
of the variation in diameter.

It must be stressed that this relationship only applies to different
diameters of the same impeller, and not to different impellers.

Exactly the same relationships apply when the pump rotational speed
changes instead of the impeller diameter.

Q2 = D2 or N2
Q1 D1 N1

Thus, a change in Capacity from Q1 to Q2, can be achieved by, either
changing the impeller diameter, or by changing the pump speed by the
same ratio.
Similarly a change in Total Head from H1 to H2, can be achieved by
changing the impeller diameter or by changing the pump speed by the
same ratio.

It is important to note that a change in impeller diameter or speed
affects the whole performance range of the pump, and not just one
specific point.
Any reduction in peripheral speed, however caused, will make it appear
as though the performance curve will shrink down to the left on the
graph. Conversely, a larger impeller diameter or faster pump speed will
show the curve extending up to the right.
The relationship of the Powcr Draw to the rotation speed or impeller
diameter may be estimated as being in the same proportion as the
change in speed or diameter raised to the third power.

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