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Pump Affinity Laws

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

 

Head or pressure

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, ..)

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