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# Q.  What is specific speed in relation to centrifugal pumps?

A. Specific speed is a dimensionless index which numerically describes the geometry of a pump's impeller and classifies the pump as to its type. The equation for specific speed (designated Ns ) is:

 $N_{s}=\frac{N\cdot \sqrt{Q}}{H^{0.75}}$

Where:    N = pump rotational speed (rpm)

Q = pump capacity at BEP, full diameter (gpm)

H = pump head per stage at BEP, full diameter (ft)