Discussion
If the impeller speed of a centrifugal pump is doubled, the power consumption will be?
Answer: D
The power consumption becomes eight times.
The affinity laws for a centrifugal pump of fixed impeller diameter give
\[ \begin{aligned} Q &\propto N \\ H &\propto N^{2} \\ P &\propto N^{3} \end{aligned} \]
Doubling the speed N doubles the discharge, multiplies the head by four and the power by \(2^{3} = 8\).
'Doubled' is the change in discharge and 'Four times' the change in head, so both mix up the laws. Power cannot stay the same when the pump is doing more work at a higher speed.
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