Enter suction conditions — get NPSH available and the margin over the pump's NPSH required, before you buy the wrong pump.
Open the free calculator →Net Positive Suction Head (NPSH) is the pressure head available at a pump's suction above the fluid's vapour pressure. If the available head falls below what the pump needs, the liquid boils locally at the impeller eye and the pump cavitates — losing head, eroding metal, and vibrating. Every pump specification lives or dies on this check.
NPSHa = (P_atm − P_vap)/(ρ·g) + H_static,suction − H_friction,suction
where P_atm is the absolute pressure over the suction liquid, P_vap the fluid's vapour pressure at pumping temperature, ρ the density, g gravity, H_static the vertical distance from liquid level to pump centreline (positive if flooded), and H_friction the suction-line losses.
NPSHr comes from the pump manufacturer's curve at your operating flow. The pump is safe only when:
NPSHa ≥ NPSHr + margin
Why it matters: vapour pressure rises fast with temperature — a margin that's fine on a cold morning can vanish on a hot day or when pumping near the bubble point. EnggiBasis flags the shortfall before it becomes a field problem.
| Input | Value |
|---|---|
| Atmospheric pressure | 101.3 kPa |
| Vapour pressure (water, 25 °C) | 3.17 kPa |
| Density | 997 kg/m³ |
| Static suction head (flooded) | +2.0 m |
| Suction friction loss | 0.6 m |
Result: NPSHa = (101.3 − 3.17)×1000 / (997 × 9.81) + 2.0 − 0.6 ≈ 11.4 m. Against a pump NPSHr of 3.5 m, the margin is a healthy 7.9 m. Try it with your own numbers →
NPSHa = (P_atm − P_vap)/(ρg) + static suction head − suction friction, all as head in metres of the pumped fluid.
NPSHa is your system; NPSHr is the pump (from its curve). Keep NPSHa above NPSHr plus a margin to avoid cavitation.
The NPSH check is part of EnggiBasis' pump & hydraulics tools — free to try, full sizing in Pro (₹99/month).
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