Calculate specific speed.
Determine nq and Ns from speed, flow and head, with unit conversion and allowance for stages and impeller inlets.
What does specific speed describe?
Specific speed combines speed, flow and head at the best efficiency point. It helps compare geometrically similar impellers and initially classify a suitable design type.
nq = n · √Qinlet / Hstage0,75
Metric convention: n in min⁻¹, Q in m³/s and H in m. The US convention Ns uses the same equation with US gpm and ft. The numerical values therefore differ by a factor of about 51.6 and must not be compared directly.
For equally loaded stages, Hstage = H / number of stages. For a double-suction impeller, half the total flow is used per inlet.
Classifying the design type
Low values tend to indicate radial designs; higher values increasingly indicate mixed-flow and finally axial designs. The result bands below 35, 35–80, 80–160 and 160 or above are rough metric guidelines, not binding design limits.
Specific speed alone determines neither efficiency nor impeller geometry. Assess it together with duty point, suction behaviour, size and other design parameters.