Pumps - Specific Speed
Characterizing of impeller types in pumps in a unique and coherent manner.
Specific Speed is a number characterizing the type of impeller in pump in a unique and coherent manner. Specific speed is determined independent of the pump size and can be useful when comparing different pump designs.
- Specific Speed identifies the geometrically similarity of pumps
Specific speed can be expressed as
Ns = n q1/2 / h3/4 (1)
where
Ns = specific speed
n = 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)
Online Specific Speed Pump Calculator
The calculator below can used to calculate the 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)
- radial flow pump design - 500 < Ns < 4000 - typical for centrifugal impeller pumps with radial vanes - double and single suction. Francis vane impellers in the upper range
- mixed flow pump design - 2000 < Ns < 8000 - more typical for mixed impeller single suction pumps
- axial flow pump design - 7000 < Ns < 20000 - typical for propellers and axial fans
Convert Specific Speed between US units (gpm) and Metric units (m3/h)
- Ns(US gpm, ft) = 0.861 Ns(m3/h, m)
- Ns(British gpm, ft) = 0.787 Ns(m3/h, m)
when
- 1 US gpm = 0.2271 m3/h
- 1 British gpm = 0.2728 m3/h
- 1 ft = 0.3048 m
Example - Specific Speed for a Pump with different units
A pump has the capacity 1500 US gal/min (1249 British gpm, 340 m3/h, 5667 liters/min) at 100 ft (30.5 m) of head and is rotating at 1760 rev/min.
US gpm, ft
Ns(US gpm, ft) = (1760 rev/min) (1500 gal/min)1/2 / (100 ft)3/4
= 2156
British gpm, ft
Ns(British gpm, ft) = (1760 rev/min) (1249 gal/min)1/2 / (100 ft)3/4
= 1967
m3/h, m
Ns(m3/h, m) = (1760 rev/min) (340 m3/h)1/2 / (30.5 m)3/4
= 2500
liter/min, m
Ns(liters/min, m) = (1760 rev/min) (5667 liters/min)1/2 / (30.5 m)3/4
= 10209
Specific Speed Hydro-power Turbines
Specific speed is also used when evaluating hydro-power turbines. Typical range of turbine specific speeds:
- Francis turbine : 70 - 500
- Kaplan or propeller turbine : 100 - 350
- Pelton, Turgo or Cross-flow turbines : 10 - 80
