NPSH and cavitation: keep your pump from failing

Executive summary

Cavitation is the silent cause of most premature damage in centrifugal pumps: pitting on the impeller, a "gravel" noise, vibration and loss of flow. It is prevented by a single design condition: that the NPSH available of your installation exceeds, with margin, the NPSH required of the pump. In this article you will see how NPSHa is calculated, why altitude and temperature erode it, how much margin the Hydraulic Institute demands, and a worked example verified with our calculator.

Who this is for

For plant, maintenance and project engineers who select or troubleshoot pumps: whoever decides where to mount the pump relative to the liquid level, whoever specifies the suction pipe, or whoever investigates why a "good" pump is noisy and delivers no flow. If you work at altitude (Bogotá, Quito, Mexico City, La Paz) or with hot water, this affects you more than you think.

The real plant problem

A pump cavitates when the pressure at the impeller inlet drops below the vapor pressure of the liquid: vapor bubbles form and then collapse violently against the metal. In the field the result shows up as a stony rattle, vibration, failing seals and an impeller pitted as if it had been bitten. What is deceptive is that the pump can be perfectly selected for flow and head and still cavitate: the problem is not in the H-Q curve, but in the suction.

Engineering fundamentals

The NPSH available (NPSHa) is the net energy, referred to the vapor pressure, that the installation delivers to the suction. For an open tank:

NPSHa = (Patm − Pv) / (ρ·g) + h_s − h_f

where Patm is the atmospheric pressure (falls with altitude), Pv the vapor pressure of the liquid (rises with temperature), ρ the density, h_s the height of the liquid above the pump (positive if the pump is flooded, negative if it draws from below) and h_f the friction losses in the suction pipe.

The NPSH required (NPSHr) is provided by the manufacturer on the pump curve. Per the Hydraulic Institute definition, NPSHr (or NPSH3) is the suction value at which the pump head drops 3% due to cavitation; that is, it is not the "zero cavitation" point, but the threshold of already perceptible cavitation. That is why a margin above it is needed.

Step by step

• 1. Atmospheric pressure by altitude. Expressed as water column: ~10.3 m at sea level; ~8.4 m at 1,500 m above sea level; ~7.5 m at 2,600 m above sea level.
• 2. Vapor pressure by temperature. At 20 °C it is ~0.24 m; at 60 °C ~2.0 m; at 80 °C ~4.8 m of water column.
• 3. Suction geometry. Add the height of the liquid above the pump (or subtract it if the pump draws from below) and subtract the friction losses of the suction run.
• 4. Pump NPSHr. Read it on the manufacturer's curve at the operating flow (not the design flow: if the pump runs past its flow, its NPSHr rises).
• 5. Compare with margin. Require NPSHa ≥ NPSHr + 1.0 m (or ratio ≥ 1.1), per ANSI/HI 9.6.1.

Worked example with numbers

Pump in Bogotá (2,600 m above sea level), water at 20 °C, at operating flow, with the pump 2 m above the well level (drawing suction) and 0.5 m of suction losses. Computed with the SEMHYS pump calculator:

If the pump has an NPSHr of 3.5 m at that point, the margin is 4.8 − 3.5 = 1.3 m: above the Hydraulic Institute minimum of 1.0 m, but tight. The same installation at sea level would have an atmosphere of ~10.3 m and an NPSHa close to 7.6 m: altitude alone ate almost 3 meters of margin. Lowering the pump to flood it, shortening and enlarging the suction, or choosing a pump with lower NPSHr are the levers to recover that margin.

When it applies and when it does not

NPSH analysis is critical whenever the pump draws suction (sits above the liquid), when you pump hot water or volatile liquids, at altitude, and when the suction is long or barely sized. It matters less when the pump is clearly flooded with a generous column of cold liquid above it and a short, wide suction; even so, verifying it takes minutes and prevents costly failures.

Common mistakes

• Ignoring altitude: using 10.3 m of atmosphere at an Andean plant overestimates NPSHa by several meters.
• Forgetting temperature: the same system with water at 70–80 °C loses meters of margin to vapor pressure.
• Reading NPSHr at design flow and not at the real flow: an oversized pump runs past its flow, where NPSHr is higher.
• Undersized suction: reducing the suction diameter spikes the h_f losses exactly where there is least margin.
• Elbows and fittings hard against the suction flange: they distort the flow and raise the effective NPSHr.

Decision checklist

• Did you compute atmospheric pressure using the real site altitude?
• Did you use the vapor pressure at operating temperature, not 20 °C by default?
• Did you read NPSHr at the real operating flow on the manufacturer's curve?
• Is the NPSHa − NPSHr margin ≥ 1.0 m (or ratio ≥ 1.1)?
• Is the suction pipe short, wide and free of elbows hard against the pump?

Frequently asked questions

What is the difference between NPSH available and NPSH required?

NPSH available (NPSHa) is set by your installation: the net pressure reaching the suction above the vapor pressure. NPSH required (NPSHr) is set by the pump: the test-rig value at which its head drops 3%. You need NPSHa greater than NPSHr, with margin.

Why does my pump cavitate at altitude but not at sea level?

Because NPSHa depends on atmospheric pressure and that falls with altitude: ~10.3 m at sea level versus ~7.5 m at 2,600 m above sea level. Those nearly 3 meters are subtracted from NPSHa, and the same installation that works at sea level can cavitate in an Andean city.

How much NPSH margin do I need?

The Hydraulic Institute (ANSI/HI 9.6.1) recommends a minimum of 1.0 m (3.3 ft) or an NPSHa/NPSHr ratio of 1.1, whichever is greater. For hot water, volatile liquids or large pumps a larger margin is advisable.

SEMHYS tools

Our free pump calculator computes the NPSH available corrected for altitude and temperature along with the operating point, the power and the speed: enter your altitude, the fluid temperature and the suction geometry, and you get the NPSHa instantly to compare against the NPSHr of your pump. If you need a professional report or a tailored analysis of a critical case, see the options in the SEMHYS store.

References

1. Hydraulic Institute (2012). ANSI/HI 9.6.1 — Rotodynamic Pumps Guideline for NPSH Margin. Parsippany, NJ.
2. Hydraulic Institute. Definition of NPSH3 (3% head drop due to cavitation), Pump Standards.
3. Mott, R. L. (2015). Applied Fluid Mechanics (7th ed.). Pearson.