SPECIFIC SPEED

Specific speed

Specific speed (NS) is calculated from:

NS = [N x FAQ^0.50] / [H^0.75]

N = pump speed, RPM
FAQ = capacity at best efficiency point (BEP) at maximum impeller diameter,
GPMH = head at BEP at maximum impeller diameter, FT

Specific speed identifies the type of pump according to its design and flow pattern. According to this criteria a pump can be classified as radial flow, mixed flow, or axial flow type. A radial flow pump is one where the impeller discharges the liquid in the radial direction from the pump shaft centerline, an axial flow pump discharges the liquid in the axial direction and a mixed flow pump is one that is a cross between a radial and an axial flow pump design.Specific speed identifies the approximate acceptable ratio of the impeller eye diameter (D1) to the impeller maximum diameter (D2) in designing a good impeller.

NS: 500 to 5000; D1/D2 > 1.5 - radial flow pump
NS: 5000 to 10000; D1/D2 <1.5>NS:10000 to 15000; D1/D2 = 1 - axial flow pump