NPSH and cavitation: why a centrifugal pump cavitates and how to avoid it
FZ INOX technical guide · November 2026 · from the people who build centrifugal pumps, in Thiene, Italy
In short. A centrifugal pump cavitates when the pressure at the impeller inlet drops below the vapour pressure of the liquid: the liquid boils, and the bubbles collapse shortly after, wearing the impeller away. The NPSH available is the margin, in metres, that the system leaves above that point; the NPSH required is the margin the pump asks for. The pump runs well if the available value is higher than the required one.
What cavitation is
Every liquid boils when its pressure drops below its vapour pressure, which depends on temperature. At the impeller inlet the liquid speeds up and the pressure falls: it is the lowest-pressure point of the whole system. If the pressure there goes below the vapour pressure, vapour bubbles form.
The bubbles do no harm when they form, but when they disappear: a few millimetres further on the impeller raises the pressure again and the bubbles collapse against the metal. Each collapse is a tiny impact; millions of impacts pit the surface.
How to recognise it
- The noise: a crackling sound, as if gravel were going through the pump.
- The performance: flow and head drop and become unstable.
- Vibration, which strains the mechanical seal and the motor bearings.
- The impeller: when the pump is opened, the surface near the inlet is dull and pitted.
NPSH available and NPSH required
NPSH stands for Net Positive Suction Head. It is measured in metres of column of the liquid being pumped. There are two of them, and they must not be confused:
- NPSH available: set by the system. It is the pressure the liquid has at the pump suction, minus the vapour pressure.
- NPSH required: set by the pump. It is the minimum the pump needs in order not to cavitate; it changes with flow and generally rises as the flow increases. The manufacturer measures it: by convention it is the value at which the head drops by 3%, that is, when cavitation has already started.
That is why a margin is needed. As a rule of thumb, the NPSH available should exceed the NPSH required by at least half a metre; with hot products or duty points that change, more is taken.
The NPSH required is not on the H–Q curves we publish: if you need it for your pump, ask us for the data sheet of the exact configuration.
The NPSH available formula
NPSH available = (ptank − pvapour) / (ρ · g) ± static head − suction losses
- ptank: the absolute pressure on the liquid surface. With an open tank it is atmospheric pressure, which at sea level is about 10.35 m of water column at 20 °C.
- pvapour: the vapour pressure of the liquid at its temperature.
- ρ · g: liquid density times gravity. It turns pressures into metres.
- Static head: the height between the liquid level and the pump centreline. It is added if the level is above the pump (flooded suction) and subtracted if the pump has to draw from lower down (suction lift).
- Suction losses: friction in the pipe, bends, valves and filters between the tank and the pump. They grow with the square of the flow.
An example: the same system, from cold to hot
Take an open tank at sea level, full of water, and a suction line that loses 0.5 m at the working flow. We change only the temperature and the position of the pump.
| Case | Atmospheric pressure | Vapour pressure | Static head | Suction losses | NPSH available |
|---|---|---|---|---|---|
| Water at 20 °C, pump 1 m below the level | 10.35 m | −0.24 m | +1.00 m | −0.50 m | 10.61 m |
| Water at 60 °C, pump 1 m below the level | 10.51 m | −2.07 m | +1.00 m | −0.50 m | 8.94 m |
| Water at 80 °C, pump 1 m below the level | 10.63 m | −4.98 m | +1.00 m | −0.50 m | 6.15 m |
| Water at 80 °C, pump 2 m above the level | 10.63 m | −4.98 m | −2.00 m | −0.50 m | 3.15 m |
Atmospheric pressure is always the same: in metres it grows slightly with temperature because hot water is less dense.
Between 20 and 80 °C the vapour pressure of water grows about 20 times: from 0.24 to 4.98 m. Without touching the system, the NPSH available goes from 10.61 to 6.15 m. If the pump then sits 2 m above the level instead of 1 m below it, 3.15 m are left.
A system calculated cold and run hot is the most common way to end up with a cavitating pump: it happens with hot products and with high-temperature CIP cleaning.
Calculate the NPSH available in your system
Enter your suction data: the result updates by itself. The calculation is the formula above and is valid for water; for milk, whey and water-based products it is a first estimate.
The result is a first indication, useful to get your bearings. Before ordering or building, check the figures with an engineer against the real data of your system.
Typical causes in food plants
- Hot product or hot cleaning: the case in the example. The closer the liquid is to boiling, the less margin is left.
- Tank under vacuum: a tank at −0.3 bar takes away 3.1 m of NPSH compared with an open one. It happens with deaerators, evaporators and balance tanks.
- Long or narrow suction line: small pipe, many bends, partly closed valves.
- Dirty suction strainer: losses grow little by little, and the pump starts cavitating halfway through the shift.
- Low level: at the end of emptying the static head drops, and a vortex can form in the tank and draw air in.
- Pump higher than the liquid: every metre of height is a metre of NPSH lost.
- Higher flow than planned: a pump running further right on its curve asks for more NPSH, while suction losses grow.
How to avoid it
- Lower the pump or raise the tank: the most direct remedy, one metre of height is one metre of NPSH.
- A short, wide suction line with few bends. Control valves go on the discharge, not on the suction.
- Calculate for the worst condition: highest temperature, lowest level, highest flow.
- Reduce the flow, with a discharge valve or an inverter: the pump asks for less NPSH and the suction loses less.
- Pressurise the tank, when the process allows it.
- Choose the pump on real data: between two pumps that give the same duty point, the one that runs slower or works further left on its curve generally asks for less NPSH.
How to find the duty point is explained in the guide How to read a centrifugal pump curve.
Cavitation or air? They are not the same thing
A pump that is noisy and losing flow is not always cavitating. Often it is drawing in air: from a vortex in the tank, a leaking fitting, or a line that runs empty, as in a CIP return. Cavitation bubbles are vapour and vanish by themselves as soon as the pressure rises again; air stays in the liquid and shows up at the discharge.
The remedies are different. Against cavitation you work on pressure, temperature and suction losses. Against air you fix the suction side, or you use a pump made to carry it away: our FZ-SP is a liquid-ring pump, which keeps pumping with air and foam. When you need one or the other is explained in the guide Centrifugal or self-priming pump.
The values in the example are calculated for water. For milk, whey and water-based products they are a first estimate; with alcohol, syrups or liquids with dissolved gas the vapour pressure is different and must be taken from the product data.
Frequently asked questions
What is NPSH in simple terms?
It is how much pressure, expressed in metres of liquid, is left above the boiling point when the liquid reaches the impeller. If that margin is not enough, the liquid boils inside the pump: that is cavitation.
What is the difference between NPSH available and NPSH required?
NPSH available depends on the system: pressure on the tank, liquid temperature, height difference and suction losses. NPSH required depends on the pump and on the flow, and is given by the manufacturer. The pump does not cavitate if the available value exceeds the required one with a margin.
How much margin is needed between NPSH available and required?
As a rule of thumb at least half a metre, more if the product is hot or the duty point changes during the cycle. It is not a standard: it is caution, and critical services take more.
How do you increase the NPSH available?
By lowering the pump or raising the tank level, making the suction pipe larger and shorter, removing bends and valves, cooling the product or pressurising the tank.
Why does the pump cavitate only in summer or when the tank is almost empty?
A warmer product has a higher vapour pressure, and a low tank gives less static head: in both cases the NPSH available drops. The calculation must be done for the worst condition, not the average one.
Are cavitation and air in the pump the same thing?
No. In cavitation the bubbles are vapour of the liquid itself and form inside the pump for lack of pressure. Air comes from outside: a vortex in the tank, a leaking fitting, a line that runs empty. The noise is similar, the remedies are different.
Got a difficult suction? Tell us what you pump, at what temperature, from which tank and with what height difference: we choose the pump on those data and build it in our workshop in Thiene, Italy.