Specific Speed - Pumps

Specific speed may be determined independent of the pump size

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)

where

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)

Online Specific Speed Pump Calculator

The calculator below can used to calculate pump specific speed:

ω - 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 - 500 < Ns < 4000 - typical for centrifugal impeller pumps with radial vanes - double and single suction. Francis vane impellers in the upper range
  • mixed flow - 2000 < Ns < 8000 - more typical for mixed impeller single suction pumps
  • axial flow - 7000 < Ns < 20000 - typical for propellers and axial fans

Convert 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 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:

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) (94.4 m3/h)1/2 / (30.5 m)3/4

    = 1317

Related Topics

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

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