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Mechanical EngineeringP0

Pump Affinity Law Calculator

Calculate pump performance changes with speed or impeller diameter using affinity laws.

Inputs
Enter your values below
Result
New Power P2 (kW)
—
Enter values and click Calculate
Formula
p1 × (n2 ÷ n1)³ ÷ 1000
Result unit: kW

P₂ = P₁ × (N₂/N₁)³ for variable speed. p1 in kW is converted to W by engine; /1000 returns kW. Power is proportional to speed cubed.

Introduction

Pump affinity laws predict how changing pump speed or impeller diameter affects flow rate (Q), head (H), and power (P). For variable speed drives (VSD), Q∝N, H∝N², P∝N³ — power reduction is cubic with speed.

Calculation Example

Speed increase 1450→1750 RPM (20.7%): Q=120.7, H=72.7m, P=38.7 kW (cubic relationship).

Inputs: q1=100, h1=50, p1=22, n1=1450, n2=1750
Result: 38.7 kW

Engineering Applications

  • •VFD energy savings estimation
  • •Pump speed selection
  • •Impeller trimming
  • •Retrofit evaluation

Frequently Asked Questions

What is the power savings from reducing pump speed?

Power is proportional to N³. Reducing speed 20% reduces power by ~49%. This is why VSDs are so effective for energy savings.

How accurate are affinity laws?

Within ±5% for speed changes up to 20-30%. For larger changes or diameter trimming, efficiency changes and accuracy decreases.

Related Calculators

Pump Power CalculatorPump Flow Calculator

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Disclaimer: Calculations are for reference and educational purposes only. Always verify results independently for engineering design. See full disclaimer.