The specific speed determines the general shape of a centrifugal pump impeller. As the specific speed increases, the ratio of the impeller outlet diameter to the inlet of the eye diameter decreases. This ration becomes 1.0 for an axial flow pump. Radial flow impellers develop head through centrifugal force. Radial impellers are low flow high head designs. Pump of higher specific speeds develop head partly by centrifugal force and partly by axial force. A higher specific speed indicates a pump design with head generation ore by axial forces and less by centrifugal forces. An axial flow or propeller pump with a specific speed of 10,000 or greater generates its head exclusively through axial forces. Axial flow impellers are high flow low head designs.
Specific speed (Ns) identifies the approximate acceptable ration of the impeller eye diameter (D1) to the impeller maximum diameter (D2) in designing an impeller:
Ns 500 to 5000 D1/D2 > 1.5 - radial flow pump
Ns 5000 to 10000 D1/D2 < 1.5 - mixed flow pump
Ns 10000 to 15000 D1/D2 = 1 - axial flow pump