Power and energy

Centrifugal pump affinity-law calculator

Estimate flow, head and shaft-power changes for the same centrifugal pump and fluid when rotational speed changes.

Calculate

Initial and proposed duty
rpm
rpm
Any flow unit. The new flow is given in the same unit.
Any head unit. The new head is given in the same unit.
kW

Worked example

Shaft power at the new speed, estimated

15.36 kW

Speed ratio, new to initial
0.8
Flow at the new speed (same unit as entered)
80
Head at the new speed (same unit as entered)
25.6
Shaft-power change
-48.8 %

Affinity-law estimate for similar pump operation. Static head, efficiency and the real system curve can materially change the duty point.

Method and assumptions

Formula

Q₂/Q₁ = N₂/N₁; H₂/H₁ = (N₂/N₁)²; P₂/P₁ = (N₂/N₁)³

The affinity laws describe similar pump operation; static head and efficiency changes move the real duty point.

Limits of this result

  • Do not use the cube law alone for static-head-dominated systems; construct the system curve and find the new duty point.
  • The estimate does not check minimum flow, cavitation, NPSH, motor/drive limits, efficiency or control-valve behavior.
  • Use the same pump geometry and fluid; impeller trimming and viscosity require additional corrections.

Technical sources

  1. Adjustable Speed Pumping ApplicationsUS Department of Energy · Pumping Systems Tip Sheet 11
  2. Improving Pumping System PerformanceUS Department of Energy · second edition

Put the value to work

Read the related engineering guidance, or carry this task into System Builder to identify the signal path and EpiSensor products.

Continue in System Builder Read: Pump performance monitoring

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