Tools & calculators

Calculate NPSHA for centrifugal pumps.

Assess suction conditions for water: account for surface pressure, liquid level, vapour pressure and suction line losses together.

Use a decimal comma or point, without thousands separators. Changing units converts the input value.

Water and installation location

Flow rate Q

Flow rate unit

Water temperature (°C)Calculator scope: pure water from 1 to 95 °C.

Elevation above sea level (m)Standard atmosphere, −500 to 5,000 m; weather-related pressure variations are not included.

Liquid level relative to pump axis z (m)Positive: level above pump. Negative: pump above level. For horizontal pumps.

Tank

Pressure above the liquid surface

Tank pressure (bar)

Pressure reference

Suction line losses

Pressure loss calculation

Known suction line loss

Total suction line loss hL (m)Include inlet and fittings; do not add elevation again.

Pipe and flow data

Pipe length L (m)

Internal pipe diameter D (mm)

Absolute pipe roughness ε (mm)Reference value for new steel pipe; consider actual material and condition.

Kinematic viscosity ν (mm²/s)1.004 approximately applies to water at 20 °C. Enter the appropriate property at other temperatures.

Sum of local loss coefficients ΣKInlet, bends, valves, filters, etc., from geometry or manufacturer data; referenced to the entered pipe velocity.

Other known losses (m)Only losses not already included in pipe friction or ΣK.

Comparison with the pump

Pump NPSH3 (m), optionalFrom the manufacturer's curve at the duty point under consideration.

Selected minimum absolute margin (m)Example value, not a universal safety requirement.

Selected minimum ratio NPSHA / NPSH3Both margins are checked simultaneously. Agree them with the manufacturer.

Distinguishing NPSHA from NPSH3

NPSHA is the head above vapour pressure provided by the system at the pump inlet. NPSH3 is pump-specific: the NPSH at which cavitation causes a 3% head drop. NPSHA exceeding NPSH3 therefore does not automatically mean cavitation-free operation.

NPSHA = (p_surface,abs − p_vapour) / (ρ · g) + z − h_L

The balance starts at the stationary surface of a large tank. z is referenced to a horizontal pump's axis. Surface velocity is neglected. Pump inlet velocity head is not subtracted again in this tank-based balance. All pipe, inlet and fitting losses up to the pump inlet belong to h_L.

Suction line losses

h_L = (f · L/D + ΣK) · v²/(2g) + h_additional
v = 4Q/(πD²) ··· Re = vD/ν

The calculator uses the Darcy friction factor: 64/Re up to Re = 2,000, iteratively solved Colebrook above Re = 4,000, and linear interpolation between the boundary values. The transition region is uncertain. A constant internal pipe diameter is assumed; for multiple diameters, calculate section losses separately and enter their total.

Local loss coefficients are entered as a sum. This accounts for actual fittings and installation conditions rather than deriving generic values solely from the number of components.

Water properties, atmospheric pressure and margins

Vapour pressure and approximate liquid density use the IAPWS SR1-86(1992) saturation equations. Pressure effects on density are neglected. Kinematic viscosity is entered separately. The model is not intended for mixtures, gas-laden or highly viscous media.

Local atmospheric pressure uses p = 101,325 · (1 − 0.0065 · h / 288.15)^5,25588 Pa. For a closed tank, surface pressure can be specified as absolute or relative to this calculated atmospheric pressure.

The example margins of 0.6 m and 1.25 are adjustable. The calculated NPSH3 limit is min(NPSHA − absolute margin; NPSHA / minimum ratio). A negative limit means no positive NPSH3 can satisfy these criteria. Required margins depend on the pump manufacturer, application and operating range.

Calculation references

These calculators support preliminary engineering sizing. Check the suitability of assumptions and results for the specific application. They do not replace complete pump, system or drive design.

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