# 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,

N

is given by;
_{s}N_{s} = Q^{0.5}·n/(g·H)^{0.75}

Specific speed can also be calculated as follows where

φ

andψ

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

_{s}

### Medium Specific Speed N_{s}=0.10

_{s}

### High Specific Speed N_{s}=0.20

_{s}

## Flow Parameter

The flow parameter,

φ

is given by;
φ = Q/n/D^{3}

## Pressure Parameter

The pressure parameter,

ψ

is given by;
ψ = g·H/n^{2}/D^{2}

## Power Parameter

The power parameter,

Π

is given by;
Π = φ·ψ·η

## Diameter Parameter

The diameter parameter,

Δ

is given by;
Δ = ψ^{0.25}/φ^{0.5}