How Pump Affinity Laws Change Flow, Head, and Power
When this applies
Use this guide for rapid what-if checks on variable-speed operation, while being explicit that affinity laws are approximations around similar hydraulic conditions.
Tool to use
Scale flow, head, and power with pump speed using affinity laws.
Open Pump Affinity Laws Calculator →Steps
- 1Start from a known operating point (Q1, H1, P1, N1) on the same pump and impeller.
- 2Set target speed N2 and compute speed ratio r = N2/N1.
- 3Scale flow, head, and power with Q2 = Q1*r, H2 = H1*r^2, P2 = P1*r^3.
- 4Compare scaled point against system curve and motor limits.
- 5Treat the result as screening, then verify with vendor data.
Examples
- Estimating energy impact of reducing speed from 50 Hz to 45 Hz.
- Checking whether a small speed increase could recover required head.
What to avoid
- Applying affinity laws across very large speed changes without validation.
- Ignoring NPSH and motor current constraints.
- Assuming efficiency stays constant for all operating points.
Related tools
On the blog
More in Generators & Calculators
- How to Calculate Pressure Drop in Pipes
- What Reynolds Number Means in Pipe Flow
- How to Calculate Voltage Drop for Cable Runs
- Single-Phase vs Three-Phase Power Basics
- How to Estimate Dew Point
- How to Estimate Beam Deflection Quickly
Browse all task guides or see the full list on the Generators & Calculators hub.
FAQ
Do affinity laws replace pump curves?
No. They provide first-order scaling and should be checked against manufacturer performance data.
Can I use this for impeller trim and speed changes together?
Use caution; combined effects need proper vendor-supported methods.