Cavitation and NPSH
How water can boil at room temperature inside a pump — and eat solid metal.
⏱ 7 min
- Everyday comparisonStep 1 of 6
Boiling tea on a mountain
On top of Mount Everest, water boils at only about 70 °C. The air pressure is so low that the water turns to vapour more easily.
So boiling isn’t only about heat — it is about pressure. Lower the pressure enough and even cold water boils.
- ExplainedStep 2 of 6
The weak spot: the eye
The lowest pressure in a pump is at the eye, just as the water meets the vanes. If it falls below the liquid’s , tiny bubbles of vapour form — the water boils, cold.
The vanes carry the bubbles outward into higher pressure, where they collapse in a split second. That is .
- Try itStep 3 of 6
Make the pump cavitate
The pump draws water from a tank. Move the tank lower than the pump, make the water hotter, or open up the flow — and watch the bars on the right.
Bubbles appear when NPSH available drops below NPSH required.
Challenge: At 20 °C and 40 m³/h, how far below the pump can the tank be before it cavitates? Now try it with 80 °C water.
- ExplainedStep 4 of 6
Why it destroys pumps
Each collapsing bubble fires a microscopic jet of water at the metal, like a tiny hammer. Millions per second slowly chip the vanes into a sponge-like mess.
Cavitation sounds like gravel rattling through the pump. It also shakes the bearings and seals and drops the head.
- ExplainedStep 5 of 6
NPSH: the scorecard
is how much “pressure to spare” the installation gives at the inlet, in metres. is how much this pump needs, from the maker’s tests.
Rule: available must beat required, with a safety margin. A tank above the pump, short fat suction pipes and cool liquid all raise the available NPSH.
- The impeller connectionStep 6 of 6
The 10-metre limit
Here is a surprise: no pump can pull cold water up from more than about 10 m below it — and in practice only 6–7 m. The air pushing on the water surface can only hold up a 10 m column. Above that, the water simply boils.
That is why deep wells use pumps that sit down in the water and push it up instead.
📌 Remember this
If the pressure at the eye falls below the vapour pressure, bubbles form and then collapse violently: cavitation. Keep NPSH available above NPSH required — raise the tank, shorten the suction pipe, cool the liquid or reduce the flow.