Concept reference

The Affinity Laws

C.1Plain-language definition

The affinity laws move a pump curve; they do not move the system’s static head. A slower geometrically similar impeller produces proportionally less flow, head falls with speed squared, and power with speed cubed—at corresponding points. Real efficiency, trimming and control limits keep these from being exact everywhere.

C.2Governing relation or decision logic

For speed ratio r = N2/N1, corresponding points follow Q2 = rQ1, H2 = r²H1, and P2 ≈ r³P1. Diameter affinity forms require homologous geometry; a trimmed impeller is not perfectly similar to the original.

C.3The failure to name

Cubing speed reduction and declaring the same energy saving confuses pump-curve power at corresponding points with actual operating power at the new system intersection. Static-dominated systems may reach a minimum speed below which the pump develops too little shutoff head to move any water.

C.4Field review checklist

• Apply the laws to the pump curve, then intersect with an unchanged system curve.
• Confirm speed and trim ranges allowed by the manufacturer.
• Re-read efficiency, power and NPSH at the new operating point.
• Check minimum useful speed, motor cooling, resonance and control constraints.

C.5Where it fits in the course

§6.6 demonstrates the static-head limit; §7.2 finds the new intersection.