Pump & Flow Conversions — GPM, m³/h, Head, NPSH

Head (in feet or meters) represents the pump's actual lifting ability regardless of fluid density. A pump that produces 100 ft of head will lift water 100 ft, but only lift a heavy slurry (SG=1.5) about 67 ft. Using head instead of pressure keeps the pump curve independent of the fluid being pumped.

Net Positive Suction Head (NPSH) is the minimum pressure required at the pump inlet to prevent cavitation. NPSHa (available) must exceed NPSHr (required) by the pump manufacturer by at least 1 meter margin. Insufficient NPSH leads to cavitation, vibration, and impeller damage.

Pump affinity laws describe how flow, head, and power change with impeller speed (RPM) or diameter: Flow ∝ RPM, Head ∝ RPM², Power ∝ RPM³. Doubling the impeller speed doubles flow, quadruples head, and octuples power — the last of which can easily overload the motor.

1 US gallon = 231 in³ (3.785 L), 1 Imperial gallon = 277.42 in³ (4.546 L). Imperial GPM is 20% larger. A 100 Imp GPM pump delivers 120 US GPM. This difference has caused costly pump sizing errors on international projects.

For water (SG=1.0): bar = meters head ÷ 10.197. Or bar = feet head × 0.0299. For other fluids, divide by (10.197 × SG). Always confirm with actual fluid density at operating temperature for precision sizing.