NPSH Calculator (Cavitation)
Calculate the available net positive suction head (NPSHa) at the suction port of a centrifugal pump and verify cavitation safety. From ambient or tank pressure, the vapour pressure of the liquid, the geodetic suction lift and the suction-line losses the tool derives NPSHa; the comparison with the required NPSHr from the pump datasheet plus a safety margin is evaluated live with a traffic light.
NPSH Calculator (Cavitation Protection)
Model: steady operating point, bubble-free liquid, absolute ambient pressure p_amb. NPSHr is taken from the pump datasheet (curve-dependent, usually at 3 % head drop); H_v,s is the previously determined head loss of the suction line. Engineering sizing tool, not a substitute for the pump manufacturer's design.
Results
Calculating …
Formulas and fundamentals
The available suction head follows from the energy balance of the suction side up to the pump inlet: NPSHa = (p_amb − p_v)/(ρ·g) − H_z − H_v,s. Here p_amb is the absolute ambient or tank pressure, p_v the vapour pressure of the liquid at pumping temperature, ρ the density, g = 9.81 m/s², H_z the geodetic suction lift (pump above the liquid level positive, flooded suction negative) and H_v,s the head loss of the suction line. The term (p_amb − p_v)/(ρ·g) is the pressure head that, after subtracting vapour pressure, elevation and friction, remains as NPSHa at the impeller inlet.
The vapour pressure p_v rises steeply with temperature: for water it grows from about 611 Pa at 0 °C through 2339 Pa at 20 °C to 101325 Pa at 100 °C. The tool stores a small water vapour-pressure table (0 … 100 °C, published saturation values, linearly interpolated) and alternatively allows p_v to be entered directly for other media. Hot or easily boiling media shrink the pressure head considerably, which makes suction lift and suction-line losses especially critical.
The operating point is cavitation-free when NPSHa ≥ NPSHr + margin. NPSHr is the value given by the pump manufacturer in the datasheet (curve-dependent, usually at 3 % head drop). The traffic light rates: green when the margin (default 0.5 m) is met, amber when NPSHa exceeds NPSHr but the margin is not reached, and red when NPSHa < NPSHr, i.e. imminent cavitation. If NPSHa drops below zero the pressure head is already used up and the suction lift is no longer physically possible.
Worked example
A centrifugal pump handles water at 20 °C (vapour pressure p_v = 2340 Pa) at atmospheric pressure p_amb = 101300 Pa and a density ρ = 998 kg/m³. The pump stands 3 m above the open suction tank (H_z = 3 m), the suction line causes H_v,s = 0.5 m of head loss. The datasheet gives NPSHr = 4 m.
The pressure head is (p_amb − p_v)/(ρ·g) = (101300 − 2340)/(998·9.81) = 10.108 m. After subtracting suction lift and suction loss, NPSHa = 10.108 − 3 − 0.5 = 6.608 m remains.
The comparison yields a margin of NPSHa − NPSHr = 6.608 − 4 = 2.608 m, well above the required 0.5 m. The operating point is therefore cavitation-safe (green). Raising the pump or heating the medium would quickly push NPSHa into the amber or red range.
Frequently asked questions
What does NPSH mean and how do NPSHa and NPSHr differ?
NPSH stands for Net Positive Suction Head at the suction port. NPSHa (available) results from the system: ambient pressure, vapour pressure, suction lift and suction-line losses. NPSHr (required) is a pump property from the datasheet. Operation is cavitation-free only when NPSHa exceeds NPSHr with a margin.
How does the suction lift H_z enter the calculation?
H_z is the geodetic height difference between the liquid level and the pump suction port. If the pump sits above the level (suction lift), H_z is positive and reduces NPSHa. If the level is above the pump (flooded suction), enter H_z as negative, which increases NPSHa. Flooded suction is therefore common for hot or easily boiling media.
Why is hot water especially prone to cavitation?
Because the vapour pressure p_v rises steeply with temperature: from about 2340 Pa at 20 °C to over 100000 Pa at 100 °C. This shrinks the pressure head (p_amb − p_v)/(ρ·g) drastically, and in the limit the liquid already boils at the inlet. Boiler-feed and hot-water pumps are therefore almost always run with flooded suction and low suction losses.
What safety margin is reasonable?
Typical values are 0.5 m to 1 m above NPSHr, more for critical or fluctuating operating points. The reason: datasheet NPSHr usually applies at 3 % head drop, but incipient cavitation starts earlier. The tool uses an adjustable margin (default 0.5 m) and rates amber as soon as it is not met.
Where does the suction-line loss H_v,s come from?
H_v,s is the sum of all friction and local losses of the suction line from the tank to the pump inlet, expressed as a head. It can be determined with a pressure-loss calculator (Darcy-Weisbach for straight pipe plus ζ-values for bends, valves, foot valve) and entered here as a finished head loss. A suction line that is too narrow or convoluted is a common cause of cavitation.
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