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Pump Affinity Law — Flow vs Speed

Also known as pump laws · affinity laws · VFD flow change

Q2Q1=N2N1\frac{Q_2}{Q_1} = \frac{N_2}{N_1}

Worked example: 1450 rpm gives 1200 L/min; 900 L/min needs 1087.5 rpm — press Try an example to run it live, then adjust anything.

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Pump Affinity Law — Flow vs Speed explained

N2N1Q1Q2

A centrifugal impeller is a volumetric scoop: every revolution flings out roughly the same slug of liquid, so turn it faster and the capacity climbs in lockstep. Slow a 1750 rpm pump to 1150 rpm and a 500 gpm duty becomes 500 × 1150/1750 ≈ 329 gpm. This is the whole economic case for variable-frequency drives — but the flow law is the gentlest of the three; head follows the square and power the cube, which is where the savings actually come from.

The trap is applying it across a duty the pump cannot reach. Affinity laws slide a point along a parabola through the origin, not along the system curve; if your system has 40 ft of static lift, dropping speed eventually drives the pump head below that lift and flow collapses to zero rather than scaling smoothly. Always plot the new curve against the real system curve before promising a customer a linear turndown.

Pump Affinity Law — Flow vs Speed formula

Q2Q1=N2N1\frac{Q_2}{Q_1} = \frac{N_2}{N_1}
Where
  • Q1Q_1= Flow at speed 1 (L/min)
  • N1N_1= Speed 1 (rpm)
  • Q2Q_2= Flow at speed 2 (L/min)
  • N2N_2= Speed 2 (rpm)

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Part of this formula set

The pump and fan affinity laws

7 formulasHow flow, head and power scale with impeller speed and diameter — the first, second and third power relationships behind every VFD retrofit.