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,

N_{s}

is given by;

N_{s} = Q^{0.5}·n/(g·H)^{0.75}

Specific speed can also be calculated as follows where

φ

and

ψ

aredefined below

N_{s} = φ^{0.5}/ψ^{0.75}

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