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:
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
Design of pumping systems and pipelines. With centrifugal pumps, displacement pumps, cavitation, fluid viscosity, head and pressure, power consumption and more.
Related Documents
BEP - the Best Efficiency Point of a Pump
BEP is where the pump is most efficient.
Centrifugal Pumps
An introduction to Centrifugal Pumps.
Centrifugal Pumps - Capacity Modulation
Modulating pumps to adapt capacities to variable process demands.
Centrifugal Pumps - Minimum Flow
Minimum continuous flow to prevent flashing in centrifugal pumps.
Centrifugal Pumps - Stable and Unstable Flow
The characteristics of stable and unstable flow.
Efficiency in Pumps or Fans
The overall pump and fan efficiency is the ratio power gained by the fluid to the shaft power supplied.
Fans - Calculate Air and Brake Horsepower
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Pumps - Affinity Laws
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Pumps - Suction Specific Speed
Suction Specific Speed can be used to determine stable and reliable operations for pumps with max efficiency without cavitation.
Pumps, Fans and Turbines - Horsepower
British Horse Power as used for pumps, fans and turbines - and how to convert to other units.
Turbo Machines - Specific Work done by Pumps, Compressors or Fans
Calculate specific work done by pumps, fans, compressors or turbines.