The Sugar Engineers

Specific Speed of Pumps

Pumps (and fans) can be characterised by various dimensionless parameters.

• Specific Speed,
  N_s
• Flow parameter,
  φ
• Pressure parameter,
  ψ
• Power parameter,
  Π
• Diameter parameter
  Δ

The most important of these is the specific speed

Pump Selection

These dimensionless parameters can be used calculate how similar pumps operate under differing conditions (the similarty laws). These similarty laws (detailed below) can be used to select a pump given a duty point.

Specific Speed

The specific speed,

is given by;

N_s = Q^0.5·n/(g·H)^0.75

Specific speed can also be calculated as follows where

and aredefined below

N_s = φ^0.5/ψ^0.75

The specific speed of a pump is associated with the impeller shape

Low Specific Speed

N_s

=0.05

Medium Specific Speed

N_s

=0.10

High Specific Speed

N_s

=0.20

Flow Parameter

The flow parameter,

is given by;

φ = Q/n/D³

Pressure Parameter

The pressure parameter,

is given by;

ψ = g·H/n²/D²

Power Parameter

The power parameter,

is given by;

Π = φ·ψ·η

Diameter Parameter

The diameter parameter,

is given by;

Δ = ψ^0.25/φ^0.5