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: