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Engineering explained · CAVITATION

The pump that boils water at room temperature.

Pressure falls. Vapour forms. Bubbles collapse. Here is the suction-side check behind the reel, with a worked example you can change.

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01 / WHY

Not hot. Just low pressure.

Liquid accelerates into the impeller eye and the local static pressure falls. If it reaches the liquid’s vapour pressure, vapour cavities can form. They collapse when the flow carries them into higher pressure. Repeated collapses near a surface can erode the metal.

Illustrative cutaway of a centrifugal pump, showing the inlet eye, curved impeller blades and volute
Illustrative pump cutaway. Geometry is schematic, not a vendor design.
Pressure along a pump flow pathLocal static pressure falls below vapour pressure at the impeller eye, then recovers. Vapour bubbles can form in the low pressure region and collapse as pressure rises. This schematic is not a measured pressure trace.vapour pressuresuctionimpeller eyepressure recoverylocal absolute pressure · schematic

For water at 20 °C, vapour pressure is about 2.34 kPa absolute; at 25 °C it is about 3.17 kPa. The reel’s “about 2 kPa” is rounded, and the pressure must be absolute, not gauge. Hotter liquid usually leaves less suction margin.

Sources: NIST water properties; Grundfos, cavitation and its effects on pumps.

02 / NPSH

“Required” needs a definition.

NPSHA
Net positive suction head available: a system property. Total absolute suction head at the impeller datum minus the liquid’s vapour-pressure head.
NPSH3
The tested NPSH at a 3% reduction in pump total head due to cavitation, at the specified operating point. It is a performance criterion, not a measured metal-damage threshold.
NPSHR
The manufacturer’s required NPSH for specified performance. It may exceed NPSH3. Ask which criterion is on the curve.

A clarification to the reel: NPSHR is not universally the 3% point. Hydraulic Institute’s 2024 guidance uses manufacturer-supplied NPSHR, which must be at least NPSH3. “Already damaged” is too literal: head loss does not establish the extent of erosion. The useful takeaway is that meeting the published requirement alone is not a guarantee of cavitation-free or damage-free operation.

Use the vendor’s margin recommendations and the applicable guidance for your service. The reel’s 0.5–1 m and 20% are illustrative rules of thumb, not interchangeable or universal acceptance limits.

Source: Hydraulic Institute, 2024 ANSI/HI 9.6.1 update.

03 / CHECK

One line from the tank surface.

NPSHA = (psurface,abs − pvapour) / (ρg) + z − hloss

For a large supply tank with negligible surface velocity, steady incompressible single-phase flow to the suction flange, and all inlet losses included. z is the liquid surface elevation above the impeller datum; a suction lift is negative. Use metres of the pumped liquid throughout.

Try the suction-side worksheet

Defaults: illustrative open tank, water near 20 °C. Edit values from your case, in the units shown.

NPSHA10.61 m

NPSHA − NPSHR7.61 m

NPSHA / NPSHR3.54

Exceeds the entered 1.00 m extra margin by 6.61 m.

The entered target is a comparison input, not a recommended design margin. This worksheet does not model cavitation, gas entrainment, transients, inlet recirculation, multicomponent flashing or pump damage.

The default calculation is (101.325 − 2.34) × 1000 / (998.2 × 9.80665) + 2 − 1.5 ≈ 10.61 m. With a vendor NPSHR of 3 m, the available extra head is about 7.61 m. A higher tank level adds NPSHA; more suction losses or a higher vapour pressure removes it.

04 / VERIFY

Check the worst credible suction condition.

  • Lowest vessel pressure and liquid level; highest liquid temperature.
  • Losses at the actual flow, including strainers, valves and fouling.
  • Vendor curves at the actual speed, impeller and fluid conditions.
  • Margin over the full operating range, including start-up and transients.
  • Absolute and gauge pressure, elevation datum and liquid-head units.

A pump/system-curve intersection answers the operating-flow question. It does not replace this suction check. For the earlier selection reel, see the pump-curve page.

Size the pump. Check the suction side.

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Nathan Ruberto · Co‑Founder, CEO

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