Affinity Laws

The performance of a centrifugal pump is affected by a change in speed or impeller diameter.

Q = Capacity (GPM)

D = Impeller Diameter

H = Total Dynamic Head(Feet)

BHP = Brake Horsepower

N= Speed(RPM)

• The affinity law for a centrifugal pump with the impeller diameter held constant and the speed changed:

Flow:

Q1 / Q2 = N1 / N2

Example:   100 / Q2 = 1750/3500

Q2 = 200 GPM

Head:

H1/H2 = (N1) x (N1) / (N2) x (N2)

Example:    100 /H2 = 1750 x 1750 / 3500 x 3500

H2 = 400 Ft

Horsepower(BHP):

BHP1 / BHP2 = (N1) x (N1) x (N1) / (N2) x (N2) x (N2)

Example:   5/BHP2 = 1750 x 1750 x 1750 / 3500 x 3500 x 3500

BHP2 = 40

• The affinity law for a centrifugal pump with the speed held constant and the impeller diameter changed:

Flow:

Q1 / Q2 = D1 / D2

Example: 100 / Q2 = 8/6

Q2 = 75 GPM

Head:

H1/H2 = (D1) x (D1) / (D2) x (D2)

Example: 100 /H2 = 8 x 8 / 6 x 6

H2 = 56.25 Ft

Horsepower(BHP):

BHP1 / BHP2 = (D1) x (D1) x (D1) / (D2) x (D2) x (D2)

Example: 5/BHP2 = 8 x 8 x 8 / 6 x 6 x 6

BHP2 = 2.1