Net positive suction head available at the pump: elevation-adjusted atmospheric pressure, static suction head or lift, vapor pressure at water temperature, and suction-side friction — compared against the pump's NPSHr with margin. Runs entirely offline; inputs persist in this browser via localStorage.
Use limits: Clean water assumed (vapor pressure table is for water; specific gravity ≈ 1). Entrained air, dissolved gases coming out of solution, and vortexing at the intake all rob suction performance in ways NPSH arithmetic doesn't capture — a passing NPSHa with a bad intake still cavitates. NPSHr comes from the manufacturer's curve at the actual operating flow, not the rated point.
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Atmospheric pressure at elevation
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Static suction head
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Vapor pressure at temperature
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Suction friction
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NPSH available
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NPSHr + margin
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Margin actually available
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How the Numbers Are Derived
The NPSHa equation
NPSHa = h_atm ± h_static − h_vp − h_friction (all in feet of water)
Atmospheric pressure falls with elevation per the standard atmosphere: P(psia) = 14.696 × (1 − 6.8754×10⁻⁶ × elev)^5.2559, converted to feet at 2.31 ft/psi. Static head is positive for flooded suction, negative for lift. Vapor pressure is interpolated from the water table below and subtracted because water flashes to vapor once local pressure falls to it — that flashing is cavitation.
Linear interpolation between points. Note how fast this climbs with temperature — the same suction that works on 60°F water can cavitate on 130°F digester recirculation.
Field diagnosis notes
Cavitation sounds like pumping gravel and shows as pitting on the impeller eye, falling flow/head, and fluctuating amps.
NPSHr rises steeply with flow. A pump that runs out to the right of its curve (low system head, e.g. during a bypass with a short discharge) can cavitate purely from over-flow — throttle the discharge to walk it back up the curve.
The classic marginal case: self-priming trash pump, hot day, long suction hose, plugged strainer, and the wet well drawn low. Every one of those terms moves the same direction.
NPSHa margin is not a substitute for good intake hydraulics — submergence to prevent vortexing is a separate check (Hydraulic Institute intake design).
References
ANSI/HI 9.6.1 — Rotodynamic Pumps: Guideline for NPSH Margin. https://www.pumps.org/
Karassik et al., Pump Handbook — NPSH fundamentals.
Manufacturer's certified pump curve — NPSHr at the actual operating flow (governing input).