Specific speed may be determined independent of the pump size

[Esam]

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

N_s = ω q^1/2 / h^3/4 (1)
where
N_s = 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)


Note! When comparing pumps and their documentation be aware of the units used.
Typical values for specific speed - N_s - for different designs in US units (US gpm, ft)

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

[h=Convert between US units (gpm) and Metric units (m³/h)]3[/h]

• N_{s(US gpm, ft)} = 0.861 N_{s(m³/h, m)}
• N_{s(British gpm, ft)} = 0.787 N_{s(m³/h, m)}

when

• 1 US gpm = 0.2271 m³/h
• 1 British gpm = 0.2728 m³/h
• 1 ft = 0.3048 m
  

[h=Example - Specific Speed for a Pump - with different units]3[/h] 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:
[h=US gpm, ft]4[/h] N_{s(US gpm, ft)} = (1760 rev/min) (1500 gal/min)^1/2 / (100 ft)^3/4
= 2156

[h=British gpm, ft]4[/h] N_{s(British gpm, ft)} = (1760 rev/min) (1249 gal/min)^1/2 / (100 ft)^3/4
= 1967
[h=m³/h, m]4[/h] N_{s(m3/h, m)} = (1760 rev/min) (340 m³/h)^1/2 / (30.5 m)^3/4
= 2500
[h=liter/min, m]4[/h] N_{s(liters/min, m)} = (1760 rev/min) (94.4 m³/h)^1/2 / (30.5 m)^3/4
= 1317

[Esam]

Centrifugal pumps and maximum - or shut-off - head



The maximum or "shut-off" head of a centrifugal pump can be expressed as

h_s = (d n / 1840)² (1)
where
h = head (feet)
d = outside diameter impeller (inches)
n = wheel velocity - revolution per minute - (rpm)