mechanical engineering

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 is dimensionless and are expressed as
Ns = ω q1/2 / h3/4 (1)
where
Ns = specific speed
ω = pump shaft rotational speed (rpm)
q = flow rate (m3/h, l/s, m3/min, US gpm, British gpm) at Best Efficiency Point (BEP)
h = head rise (m, ft)
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 (gpm)

• radial flow - 500 <>
• mixed flow - 2000 <>
• axial flow - 7000 < Ns < 20000 - typical for propellers and axial fans
  

Convert between US units (gpm) and Metric units (m3/h)

• Ns (US) = 1.63 Ns (metric)
• Ns (Metric) = 0.614 Ns (US)
• Ns (Metric) = 0.67 Ns (British)
  

Example - Specific Speed for a Pump
A pump has a capacity of 1500 gal/min at 100 ft of head and is rotating at 1760 rev/min. Specific speed can be expressed as:
Nsd = (1760 rev/min) (1500 gal/min)1/2 / (100 ft)3/4
= 2156 - a typical centrifugal pump