Specific speed

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Specific Speed is an index that, in mechanical terms, represents the impeller speed of a pump necessary to generate 1 gallon per minute at 1 foot of head.

Ns = (n*sqrt(Q)) / (gH^(3/4))

Ns = specific speed (unitless) n = pump rotational speed in rpm Q = flowrate (GPM) H = total head per stage (feet) g = acceleration due to gravity

The specific speed is not a dimensionless quantity.

Standard impellers can have values from 500 to 10,000, with the highest efficiency at values between 2,000 and 3,000. It is effectively an index used to predict certain pump characteristics.