# Specific Speed of Pumps

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

• Specific Speed,
Ns
• 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,

Ns
is given by;

Ns = Q0.5·n/(g·H)0.75

Specific speed can also be calculated as follows where

φ
and
ψ
aredefined below

Ns = φ0.50.75

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

### Low Specific Speed Ns=0.05 ### Medium Specific Speed Ns=0.10 ### High Specific Speed Ns=0.20 ## Flow Parameter

The flow parameter,

φ
is given by;

φ = Q/n/D3

## Pressure Parameter

The pressure parameter,

ψ
is given by;

ψ = g·H/n2/D2

## Power Parameter

The power parameter,

Π
is given by;

Π = φ·ψ·η

## Diameter Parameter

The diameter parameter,

Δ
is given by;

Δ = ψ0.250.5