# PUMP AFFINITY LAWS

2nd Oct 2014

The ‘Affinity Law’ associated with centrifugal pumps describe the impact of changes in speed or impeller diameter on a pump flow, head and Horse Power.

Whether you are an industry expert or a beginner in the industry, the Pump Affinity Law is necessity tool for you to understand.

… Don’t worry its very straight forward!.

The law is a productive way of predicting a pumps performance change when the speed or impeller diameter is changed. This in turn affects the head and flow.

Be aware that there are two sets of affinity laws;

• Affinity laws for a specific centrifugal pump, to approximate the head, capacity and power curves for different motor speeds and/or different diameters of impellers
• Affinity laws for a family of geometrically similar centrifugal pumps – to approximate head, capacity and power curves for different motor speeds and/or different diameters of impellers

Pump Affinity Law for a specific Centrifugal Pump

Volume Capacity

The volume capacity of a centrifugal pump can be expressed as follows;

q1 / q2 = (n1 / n2) (d1 / d2)         (1)

where

q = volume flow capacity (m3/s, gpm, cfm, ..)

n = wheel velocity – revolution per minute – (rpm)

d = wheel diameter

The head or pressure of a centrifugal pump can be expressed like

dp1 / dp2 = (n1 / n2)2 (d1 / d2)2         (2)

where

dp = head or pressure  (m, ft, Pa, psi, ..)

#### Power

The power consumption of a centrifugal pump can be expressed as

P1 / P2 = (n1 / n2)3 (d1 / d2)3         (3)

where

P = power (W, bhp, ..)

We often post articles and other news on our LinkedIn page. Go and follow to keep up to date, or for any pump queries you may have please contact us.