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# Pumps - Specific Speed

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:

n - pump shaft rotational speed (rpm)

q -  flow rate (m3/h, l/s, l/min, m3/min, US gpm, British gpm)

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 (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

## Related Topics

### • Pumps

Piping systems and pumps - centrifugal pumps, displacement pumps - cavitation, viscosity, head and pressure, power consumption and more.

## Related Documents

### Centrifugal Pumps

An introduction to Centrifugal Pumps.

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