What does this worksheet calculate?

It calculates NPSH available at a defined pump datum for one documented steady operating point. It does not calculate the pump's NPSH required or decide an acceptable margin.

Can suction pressure, static elevation and suction losses be entered together?

Not in one route. A pressure measured at the pump suction already reflects the upstream elevation and losses. The worksheet separates a pump-suction measurement route from a supply-reservoir energy-balance route to prevent double counting.

Does NPSHA above NPSH3 guarantee no cavitation?

No. NPSH3 is associated with a 3 percent first-stage head drop, and cavitation-related effects can occur with margin above it. Select margin using the pump, liquid, service and current application standard.

What NPSH margin should be used?

There is no universal 0.5 metre or 1.3 multiplier rule. ANSI/HI 9.6.1-2024 provides application-specific guidance; a licensed source and the pump manufacturer's data are required for a project decision.

Engineering determination worksheet

NPSH Available (NPSHA)

Determine NPSHA for one documented steady operating point without double-counting suction pressure, elevation or losses. This worksheet does not calculate NPSHR or approve a cavitation margin.

Two exclusive routesSI unitsANSI/HI boundary

Determination route

Liquid vapor pressure (kPa absolute at operating temperature; ≥ 0)

Liquid density (kg/m³ at operating condition; > 0)

Acceleration used for head conversion (m/s²; > 0)

Operating-point / source record

I documented one steady operating point, pressure reference (absolute or gauge), liquid temperature/density/vapor pressure, measurement or reservoir plane, NPSH datum and all inputs used. I am not applying this steady rotodynamic-pump worksheet to pulsating/reciprocating flow, flashing mixtures or an unverified instrument reading.

Calculated NPSHA

NPSH available at stated datum

—

Reference absolute pressure

—

Pressure-above-vapor head

—

Velocity-head term

—

Elevation / loss term

—

NPSHR, NPSH3, margin adequacy, cavitation damage and fitness for service are not assessed.

Route 1 — supply reservoir or vessel

NPSHA = (p_surface,abs − p_v)/(ρg) + (z_surface − z_datum) − h_loss

This steady energy-balance route uses the absolute pressure acting on the supply liquid surface. The surface velocity is assumed negligible; h_loss must include entrance, pipe, fitting, valve, strainer and other suction losses at the documented flow.

Route 2 — measurement at pump suction

NPSHA = (p_s,abs − p_v)/(ρg) + v_s²/(2g) + (z_gauge − z_datum)

For a gauge-pressure instrument, p_s,abs = p_ambient,abs + p_s,gauge. The upstream static elevation and friction are not added because their effect is already contained in the measured suction pressure.

What the result means

NPSHA is the total suction head absolute above the vapor-pressure head, expressed as metres of the pumped liquid at the stated condition and datum. Pressure, density and vapor pressure must describe the same operating state.

No universal margin: the removed rules “NPSHA ≥ NPSHR + 0.5 m” and “NPSHA ≥ 1.3 × NPSHR” are not general acceptance criteria. NPSH3 is associated with a 3% first-stage head drop; cavitation effects can occur even with margin above it. Use current ANSI/HI 9.6.1, the manufacturer’s curve at the same flow/speed/liquid, and the controlling project specification.

Excluded cases

Transient, pulsating or reciprocating-pump acceleration head.
Flashing or two-phase mixtures, gas entrainment, uncertain vapor pressure or incompatible density data.
Instrument elevation/velocity corrections or suction losses that have not been established.
NPSHR/NPSH3 derivation, margin selection, cavitation-free guarantee, erosion/life prediction or pump acceptance testing.

Publicly verified sources

Hydraulic Institute Data Tool — Pump Principles: public NPSHA definition; updated 19 July 2024.
Hydraulic Institute Pump FAQs: system inputs, suction losses and NPSH3 limitation.
Hydraulic Institute Pump System Certification formula sheet: pump-suction measurement equation and velocity/elevation terms.
ISO 17769-1:2012: current pump vocabulary, symbols and units standard; confirmed 2023.
ISO 9906:2012: current rotodynamic-pump hydraulic acceptance-test scope. This worksheet is not an ISO 9906 acceptance test.

Licensed-source boundary: ANSI/HI 9.6.1-2024 is the current NPSH-margin guideline. Its public table of contents confirms an informative NPSHA appendix and application-specific margin sections; exact closed recommendations are NEEDS_LICENSED_SOURCE and are not reproduced or guessed here.

NPSHA Determination Worksheet

Calculated NPSHA

Route 1 — supply reservoir or vessel

Route 2 — measurement at pump suction

What the result means

Excluded cases

Publicly verified sources

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