When the pump affinity laws apply
The speed relationships are a first estimate for the same centrifugal pump handling the same fluid, with similar hydraulic behavior. They predict how points on the pump curve shift with rotational speed; the actual operating point is where the new pump curve intersects the system curve.
Static head changes the result
A system with substantial static head does not reduce flow in direct proportion to speed at the installed operating point. At low speed the pump may produce too little head to overcome the static component, so the calculated Q₂ is not a guaranteed delivered flow.
Efficiency and NPSH still matter
Pump and motor efficiency change with duty. Check the manufacturer’s speed-specific curves, best-efficiency region, minimum continuous stable flow, NPSH available/required, motor current, bearing/seal limits and VFD minimum/maximum frequency.
Power is not automatically energy saved
The cube relationship shows ideal pump shaft-power scaling. A savings project needs measured baseline power, real operating profile, control method, static head, VFD losses and production requirements. Use the site’s currency and energy tariff for a financial model.