CENTRIFUGAL PUMPS (NPSH, AFFINITY)

NPSH (Net Positive Suction Head) prevents cavitation: NPSHa must exceed NPSHr (from the pump curve) by a margin (typically 1 m or 1.5×). NPSHa = (P_atm - P_vap)/ρg + z_static - h_friction. Affinity laws scale flow Q∝N, head H∝N², power P∝N³ with impeller speed N (or diameter for trim). Specific speed Ns characterizes pump type (radial < 2000 < mixed < 5000 < axial in US units).

• NPSHa = (Patm - Pvap)/ρg + zs - h_fs
• Q₂ = Q₁ · (N₂/N₁)
• H₂ = H₁ · (N₂/N₁)²
• P₂ = P₁ · (N₂/N₁)³
• Ns = N · sqrt(Q) / H^0.75

References & StandardsANSI/HI 1.6, ANSI/HI 9.6.1 (NPSH), API 610

Use for centrifugal pump selection and operating-point analysis. Compute NPSHa for the suction-side piping and verify the margin over NPSHr at design flow. Use affinity laws to predict performance after a speed change (VFD) or impeller trim. Pair with the fluids module to develop the system curve and find the intersection with the pump curve. Always operate near best-efficiency point (BEP) to maximize life.