Specific speed is a number characterizing the type of impeller in a unique and coherent manner. Specific speed are determined independent of pump size and can be useful comparing different pump designs. The specific speed identifies the geometrically similarity of pumps.
Specific speed can be expressed as
Ns = ω q1/2 / h3/4 (1)
Ns = specific speed
ω = pump shaft rotational speed (rpm)
q = flow rate (m3/h, l/s, l/min, m3/min, US gpm, British gpm) at Best Efficiency Point (BEP)
h = head rise (m, ft)
The calculator below can used to calculate pump specific speed:
Note! When comparing pumps and their documentation be aware of the units used.
Typical values for specific speed - Ns - for different designs in US units (US gpm, ft)
A pump has a capacity of 1500 US gal/min (1249 British gpm, 340 m3/h, 94.4 liters/min) at 100 ft (30.5 m) of head and is rotating at 1760 rev/min. Specific speed can be expressed as:
Ns(US gpm, ft) = (1760 rev/min) (1500 gal/min)1/2 / (100 ft)3/4
Ns(British gpm, ft) = (1760 rev/min) (1249 gal/min)1/2 / (100 ft)3/4
Ns(m3/h, m) = (1760 rev/min) (340 m3/h)1/2 / (30.5 m)3/4
Ns(liters/min, m) = (1760 rev/min) (94.4 m3/h)1/2 / (30.5 m)3/4