Specific Speed of Pumps

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

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

impeller profile with low specific speed

Medium Specific Speed
Ns
=0.10

impeller profile with medium specific speed

High Specific Speed
Ns
=0.20

impeller profile with high specific speed

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