Cavitation and NPSH: protecting the impeller, seals and bearings
Cavitation damages the impeller, adds noise and vibration, and shortens seal and bearing life. It is controlled by the margin between the suction pressure and the vapour pressure of the liquid.
What cavitation is
Cavitation starts when the pressure inside the pump falls below the vapour pressure of the liquid at its temperature. Small vapour bubbles form, for example where the liquid speeds up at the impeller eye. The bubbles themselves are not the harm. The harm comes when they collapse as the pressure rises again further along the impeller.
Three effects
- Erosion: the collapsing bubbles pit the vane surface, especially with water-based liquids.
- Noise and vibration: a crackling or gravel sound, higher vibration, and shorter seal and bearing life.
- Loss of performance: the vapour blocks part of the impeller passage and cuts flow and head. In severe cases the pump loses its head completely.
NPSH: the margin that prevents it
The margin by which the suction pressure exceeds the vapour pressure, expressed as a head of liquid, is the net positive suction head available (NPSHA). The pump needs a certain value to work without cavitation (NPSHR), and the maker gives it on the pump curve. NPSHR rises with flow. The rule is to keep NPSHA above NPSHR with a margin.
| Item | Raises NPSHA | Lowers NPSHA |
|---|---|---|
| Liquid level | Higher level above the pump inlet | Lower level, or a pump above the liquid |
| Liquid temperature | Cooler liquid, with a lower vapour pressure | Hotter liquid, with a higher vapour pressure |
| Suction pipe | Short, large, with few fittings | Long, small, with bends, a partly closed valve or a blocked strainer |
| Surface pressure | Pressurised vessel | Open tank at low atmospheric pressure, high altitude |
What to check when you suspect cavitation
- Is the pump running far to the right of its curve, at a flow higher than the duty?
- Is the suction strainer blocked, or the suction valve partly closed?
- Has the liquid temperature risen, or the tank level fallen?
- Has an impeller been trimmed too far, or a speed increased, so NPSHR has gone up?
- Are the vibration and noise worse at one flow than at another?
Link to energy saving
Several energy fixes change the suction conditions. Increasing the speed raises NPSHR sharply, and trimming too deeply can hurt NPSH. Check NPSH before changing speed or diameter, and use the maker's curves for the final numbers.