What is specific speed of a pump and how is it used for pump selection?

Specific speed (Ns) is a dimensionless parameter characterizing pump type and impeller geometry: Ns = N√Q/H^(3/4), where N is RPM, Q is flow (m³/s or GPM), and H is head (m or ft). Different Ns ranges correspond to different impeller types: low Ns (500-1000 US units): radial flow, high head; medium Ns (1000-4000): mixed flow; high Ns (4000-15000): axial flow, high flow/low head. Specific speed helps select appropriate pump type for given duty, predict efficiency, and scale performance. Pumps with similar Ns have similar efficiency curves and hydraulic characteristics.