Centrifugal Pump Selection & Affinity Laws

Selecting the right pump requires understanding the relationship between the system curve and the pump curve. The Affinity Laws predict how changes in speed or impeller diameter affect performance.

The Affinity Laws (Speed Change)

  • Flow (Q): Varies linearly with speed (Q2 = Q1 * (N2/N1))
  • Head (H): Varies as the square of speed (H2 = H1 * (N2/N1)²)
  • Power (P): Varies as the cube of speed (P2 = P1 * (N2/N1)³)

Key Terms

NPSHa: Net Positive Suction Head Available. Must be greater than NPSHr to prevent cavitation.
BEP: Best Efficiency Point. The flow rate where the pump operates most efficiently.
TDH: Total Dynamic Head. The total resistance the pump must overcome.