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Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

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182 <strong>Centrifugal</strong> <strong>Pumps</strong>: <strong>Design</strong> <strong>and</strong> <strong>Application</strong><br />

Then, if the rotational spe<strong>ed</strong> is chang<strong>ed</strong> by some factor x, we have:<br />

So we see that specific spe<strong>ed</strong> is unchang<strong>ed</strong> with change in rotational<br />

spe<strong>ed</strong>; head <strong>and</strong> flow change in such a way as to keep specific spe<strong>ed</strong> constant,<br />

Mathematically, the specific spe<strong>ed</strong> expression is seen to vary from zero<br />

at zero flow or shutoff, to infinity at zero head or cutoff. By definition,<br />

specific spe<strong>ed</strong> has meaning only at the best efficiency point or flow rate<br />

at which maximum efficiency occurs. Then, the head expression can be<br />

exp<strong>and</strong><strong>ed</strong> to read<br />

Flow is proportional to the product of tip spe<strong>ed</strong> <strong>and</strong> discharge throat area<br />

according to the following expression<br />

In the flow expression, it is seen that the flow coefficient, $, is simply<br />

the ratio of discharge throat velocity to impeller tip spe<strong>ed</strong>. It is at first<br />

surprising to note that

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