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
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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