NPSH Explained: NPSHa vs NPSHr and How to Calculate Both

Few abbreviations appear as often on a pump data sheet — and are as poorly understood — as NPSH. Almost everyone knows it has something to do with cavitation; far fewer know how to calculate it, and fewer still take into account that the same tank, with the same pipework, delivers a very different NPSH depending on the liquid it holds.

NPSH is really a simple balance sheet: how much pressure the liquid has to spare, above its vapour pressure, when it reaches the impeller. If that reserve is smaller than what the pump needs, the liquid boils inside the pump and cavitation sets in, with all the damage we describe in our guide What Is Pump Cavitation? Causes, Warning Signs and How to Prevent It.

In this article we explain exactly what NPSH available (NPSHa) and NPSH required (NPSHr) are, how to carry out an NPSH calculation step by step, what role suction lift, altitude, temperature and density play, and how to use the result to decide on the pump configuration. With worked examples and a focus on pumping corrosive fluids, where NPSH mistakes are particularly costly.

NPSH stands for Net Positive Suction Head. It measures the energy per unit weight of the liquid at the pump suction flange — pressure plus velocity — minus the vapour pressure of the liquid itself at the operating temperature.

The key word is ‘net’. It is not enough to know the pressure at the suction: you need to know how much pressure there is above the boiling point. A liquid at 20 °C and another at 80 °C can reach the pump at the same pressure, yet the second will have far less NPSH, because it boils much more easily.

NPSH is expressed in metres of column of the pumped liquid, not in bar or in metres of water column. That choice of units is not arbitrary: in a centrifugal pump, the pressure needed to avoid cavitation behaves like a head. That is why manufacturers give NPSHr in metres, and why NPSHa must be calculated in metres of column of the actual fluid, with its actual density. As a reference, 1 bar is equivalent to about 10.2 m of water column, but only 5.5 m of concentrated sulphuric acid.

NPSHa is a property of the system, not of the pump. It sums up in a single figure everything that happens between the liquid surface and the suction flange:

Because it depends on the installation, NPSHa is not a fixed value: it falls as the tank empties, as the strainer clogs, as the process heats up or as the flow increases, because friction losses rise with the square of the velocity. That is why it must be calculated under the worst expected conditions, not the rated ones.

NPSHr is a property of the pump. It depends on the design of the impeller eye, the inlet geometry of the vanes and the rotational speed, and it is determined by the manufacturer through testing. The usual procedure, set out in standards such as ISO 9906 (published in the UK as BS EN ISO 9906), is to reduce the NPSH at the suction at constant flow until the pump head drops by 3 %: that value is called NPSH3 and is the basis of NPSHr curves. Since the 2024 revision of the ANSI/HI 9.6.1 guideline, the Hydraulic Institute requires the NPSHr declared by the manufacturer to be equal to or greater than the tested NPSH3.

There are two important practical consequences. First, at the NPSH3 point the pump is already cavitating, so matching NPSHa to NPSHr is not enough; a margin is needed. Second, NPSHr is not constant. It rises with flow — gently at first and very steeply to the right of the best efficiency point — and it rises with rotational speed, roughly with its square. A pump with plenty of margin at 1,450 rpm can be very tight at 2,900 rpm.

On the installation side, the NPSH calculation comes down to a single expression, with every term in metres of column of the pumped liquid:

NPSHa = Pa / (ρ · g) ± Hz − Hf − Pv / (ρ · g)

The first term is the starting ‘capital’, and it depends on where the plant is located. Atmospheric pressure falls with altitude, and with it the NPSHa of any open tank:

These are standard atmosphere values; actual pressure varies by a few kPa with the weather, which is another reason not to design with a bare minimum margin. Vapour pressure, for its part, is taken from tables or from the product data sheet: for water it rises from 0.24 m at 20 °C to almost 5 m at 80 °C, as detailed in the table in our pump cavitation guide.

Suction lift is the term the designer controls most directly. Every metre the pump has to lift the liquid is one metre less of NPSHa. In theory, with cold water at sea level, a pump could not draw from more than 10.35 m below it even with no losses and no cavitation; in practice, once friction losses, vapour pressure and NPSHr are taken into account, the usable suction lift is just a few metres.

Density is the most frequently forgotten term when pumping chemicals. Since atmospheric pressure is the same for any liquid, the denser the liquid, the fewer metres of column it represents:

The conclusion is clear-cut: with concentrated sulphuric acid, an open tank provides almost half the NPSH it would with water. An installation that happily draws water from 3 m below the pump can run out of margin with exactly the same layout in a fertiliser plant handling sulphuric or phosphoric acid. The pump’s NPSHr, on the other hand, stays practically the same in metres of column, because it depends on the hydraulics, not on density.

Temperature acts through vapour pressure, which rises exponentially. In hot surface treatment baths or process solutions at 60–80 °C, vapour pressure alone can consume 2 to 5 m of NPSHa. With concentrated acids or products that give off gases, such as hydrochloric acid, you must use the vapour pressure of the actual product, not that of water: the figure is given in the safety data sheet or the supplier’s documentation.

Let us take a typical case: a horizontal process pump transferring a dilute acid solution (density 1,050 kg/m³) at 50 °C from an open tank, in a plant located 250 m above sea level. The pump centreline is 2.5 m above the minimum tank level and the suction friction losses at maximum flow are 1.2 m. The pump curve shows an NPSHr of 3.2 m at the duty point and 4.5 m at maximum flow.

The result illustrates a typical mistake: the pump meets the requirement at the rated point but runs out of margin at maximum flow, precisely when friction losses are highest and NPSHr is at its peak. If the same duty is handled by a vertical pump whose impeller runs half a metre below the minimum level, NPSHa rises to about 8.7 m: the suction lift and almost all the friction losses disappear.

In an existing installation, NPSHa can also be measured. Fit a compound gauge on the suction flange and suppose it reads −0.35 bar with water at 20 °C, in a plant at 250 m and with a velocity of 2 m/s at the flange. The absolute pressure is 98.4 − 35 = 63.4 kPa, i.e. 6.47 m of column; add the velocity head (v²/2g = 0.20 m) and subtract the vapour pressure (0.24 m): measured NPSHa = 6.43 m. If the gauge is not at the height of the pump centreline, that difference in elevation must also be corrected.

With NPSHa calculated under the worst conditions and NPSHr read from the curve at maximum flow, the check is straightforward: NPSHa must exceed NPSHr with a margin. As a practical guide, at least 0.5–1 m and, in relative terms, 10–30 % or more; the ANSI/HI 9.6.1 guideline refines those margins by type of application. With corrosive, hot or volatile fluids, it is advisable to stay at the upper end.

Rearranged, the same formula answers the question every designer asks: how high above the liquid level can I install the pump?

Hz max = Pa / (ρ · g) − Hf − Pv / (ρ · g) − NPSHr − margin

With water at 20 °C at sea level, 1 m of friction losses, an NPSHr of 3 m and a 1 m margin, the maximum suction lift is about 5.1 m. With 98 % sulphuric acid and the same data, it drops to about 0.6 m: in practice, the pump must be installed with flooded suction or submerged.

This is why NPSH determines not only the size of the pump but also its configuration. Within the Tecnium Pumps range of plastic pumps:

Do you need to check the NPSH of a new or existing installation? Send our technical team the fluid, its concentration and maximum temperature, the required flow and head, the plant altitude, the minimum liquid level relative to the pump and a sketch of the suction pipework. We will calculate the NPSH available, compare it with the NPSH required by the right pump and propose the safest configuration. Request your free quote.