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Specific speed (Ns) is a non-dimensional design index used to classify pump impellers as to their type and proportions.
It is defined as the speed in revolutions per minite at which a geometrically similar impeller would operate if it were of such a size as to deliver one gallon per minute against one foot of head.
The formula used to determine specific speed is
Where
N = Speed of Pump in RPM
Q = Capacity in gpm at the best efficiency point
H = Head per stage in feet at the best efficiency point
The specific speed determines the general shape or class of the impeller. As the specific speed increases, the ratio of the impeller outlet diameter, D2, to the inlet or eye diameter, Di, decreases. This ratio becomes 1.0 for a true axial flow impeller.
Low specific speed - Radial flow Impeller.
Medium specific speed - Mixed flow Impeller.
High specific speed - Axial Flow Impeller.
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