Inputs
Absolute pressure above the liquid surface, vapour pressure at pumping temperature, density, static head with its sign and the suction line head loss; optionally the pump's NPSHr from its curve.
Hydraulics · Cavitation
NPSH of a pump installation
Calculate the available NPSH of your installation and check whether it exceeds the pump's required NPSH. The calculator breaks out all four terms separately – pressure head, vapour pressure head, static head and suction line loss – showing directly which item consumes the cavitation margin and which one can be improved by design.
Steady-state balance at the pump inlet using absolute pressures. The NPSHr comes from the curve of the specific pump at the operating point considered and is not estimated here. Start-up, pressure transients, dissolved and entrained gas and the scatter of NPSHr are not covered.
NPSH≥ NPSHr + 0,5 m
02
NPSH and cavitation margin
Set the suction side and calculate.
Inputs and method
Available NPSH of a pump installation
Calculate the cavitation margin at the pump inlet and check it against the pump's required NPSH.
Calculation
NPSHa = (p − pv)/(ρ·g) + hstat − Hloss. All pressures are absolute, because the vapour pressure is absolute by definition. The sign of the static head carries the real message: positive for a flooded suction, negative for a suction lift. The installation is free of cavitation when NPSHa exceeds the pump's NPSHr with a margin; at least 0.5 m, or a ratio of about 1.3, is conventional.
Example
Water at 20 °C from an open tank, pump 3 m above the level, 1.2 m of suction line loss: pressure head 10.35 m, vapour pressure head 0.24 m, so NPSHa = 10.35 − 0.24 − 3 − 1.2 = 5.91 m. Against an NPSHr of 3 m that leaves a margin of 2.91 m.
Sources and limits: Classical head balance at the pump inlet; the 0.5 m minimum margin as the common convention. NPSHr is a flow-dependent property of the specific pump and is not estimated here. Start-up, pressure transients, entrained gas and the scatter of NPSHr are not covered.
Available NPSH of a pump installation in detail
Calculate the cavitation margin at the pump inlet and check it against the pump's required NPSH.
What is NPSH and why does cavitation occur?
Cavitation occurs when the pressure at the impeller inlet falls below the vapour pressure of the liquid: vapour bubbles form and then collapse abruptly further into the impeller, destroying the material. The available NPSH is precisely the distance to that point – the head by which the inlet pressure exceeds the vapour pressure. It is decided on the suction side and cannot be improved by the pump itself.
Formula and variables
NPSHa = (p − pv)/(ρ·g) + hstat − Hloss
All pressures absolute; open tank: p = atmospheric pressurehstat > 0 for flooded suction, hstat < 0 for suction liftFree of cavitation: NPSHa ≥ NPSHr + 0.5 mAlternative rule of thumb: NPSHa / NPSHr ≥ 1.3
| Symbol / input | Meaning |
|---|---|
| p, pv | Absolute pressure above the surface and vapour pressure at pumping temperature. |
| hstat | Static head with its sign; the single most important item. |
| Hloss | Suction line head loss at the operating flow rate. |
| NPSHa, NPSHr | What the installation offers and what the pump demands at the operating point. |
Choose the inputs correctly
Absolute pressure above the liquid surface, vapour pressure at pumping temperature, density, static head with its sign and the suction line head loss; optionally the pump's NPSHr from its curve.
How to use the calculator
Enter the absolute pressure above the liquid surface – for an open tank that is atmospheric pressure, not zero. Add the vapour pressure at the actual pumping temperature and the density. The sign of the static head is then decisive: positive when the liquid stands above the pump, negative for a suction lift. Finally the suction line head loss at the operating flow and, if known, the NPSHr from the pump curve.
Worked example
Water at 20 °C from an open tank, pump 3 m above the level, 1.2 m of suction line loss: pressure head 10.35 m, vapour pressure head 0.24 m, so NPSHa = 10.35 − 0.24 − 3 − 1.2 = 5.91 m. Against an NPSHr of 3 m that leaves a margin of 2.91 m.
How should the result be read?
The available NPSH alone says little – only the comparison with the pump's NPSHr gives an answer, and it has to be with margin, not marginal. The separate breakdown of the four terms is the real value: it shows whether the margin is lost to vapour pressure, to installation height or to the suction line, and therefore which change would actually help. The static head limit answers that directly for the installation position.
Pressures absolute in bar, density in kg/m³, all heads and NPSH values in metres of head. g = 9.80665 m/s² is used.
Typical applications
Checking whether a centrifugal pump can draw at its installed position, fixing the permissible suction lift, diagnosing an existing installation with cavitation noise or impeller damage, sizing suction lines for hot or volatile media, and judging whether a conversion to flooded suction is necessary.
Assumptions, limits and common mistakes
Classical head balance at the pump inlet; the 0.5 m minimum margin as the common convention. NPSHr is a flow-dependent property of the specific pump and is not estimated here. Start-up, pressure transients, entrained gas and the scatter of NPSHr are not covered.
Common mistake: The most common error is gauge instead of absolute pressure: an open tank is 1.013 bar absolute, not 0 bar. The second is the sign of the static head – a suction lift is entered as negative. The third is a room-temperature vapour pressure on a hot medium: it rises by a factor of twenty between 20 °C and 80 °C and then dominates everything else. And NPSHr is not a fixed pump value; it applies only at the flow rate considered.
Frequently asked questions
Why does my pump cavitate on hot water but not cold?
Because vapour pressure rises extremely steeply with temperature. Water is about 0.023 bar at 20 °C but roughly 0.47 bar at 80 °C – almost 4.8 m of head deducted from the margin instead of 0.24 m. At boiling point the vapour pressure consumes the entire pressure head, which is why hot media practically always require a flooded suction.
Why can no pump draw from more than about 10 m?
Because the pressure head of atmospheric pressure on water is roughly 10.3 m and physically no more suction work is available. After deducting vapour pressure, line losses and the pump's own NPSHr, rarely more than six or seven metres remain in practice. Mounting the pump higher cannot be solved with a more powerful pump – on the contrary, larger pumps usually have a higher NPSHr.
What is the difference between NPSHa and NPSHr?
NPSHa is what the installation supplies: it follows from vessel pressure, temperature, installation height and suction line, and it is what this calculator determines. NPSHr is what the pump demands: a measured property of the impeller at a given flow rate, published in the manufacturer's curve. Operation is free of cavitation only when NPSHa exceeds NPSHr with a margin.
Why is 0.1 m of margin not enough?
Because both values move. NPSHr rises considerably with flow, while NPSHa falls with rising temperature, an increasingly fouled suction strainer and a dropping tank level. A margin of 0.5 m is the usual minimum requirement; on large pumps a ratio of about 1.3 is demanded in addition.
Which term is easiest to improve?
Usually the suction line loss and the installation height. A shorter, larger-bore suction line with fewer fittings often gains several tenths of a metre, and every metre the pump is lowered goes one-for-one into the margin. Vessel pressure and vapour pressure, by contrast, are mostly dictated by the process.