What is cavitation?
Cavitation is what happens when a flowing liquid is pulled apart by a drop in pressure. Instead of boiling because it is hot, the liquid boils because the pressure around it has fallen below its vapour pressure. Vapour bubbles appear in the low-pressure zone, then collapse the moment the pressure rises again — a small but violent event that can destroy metal.
Reading the diagram
Liquid enters the pipe at the left under high pressure. As the pipe narrows into the throat, the flow speeds up and its pressure falls below the vapour pressure, so vapour bubbles form. Further right the pipe widens, the flow slows and the pressure recovers at the outlet, so the bubbles implode. The chart below plots pressure along the pipe: its dip crosses the red vapour-pressure line only at the throat, and it is there and just downstream that the metal suffers pitting damage.
Why it matters
Cavitation is a classic failure mode in pumps, propellers, valves, injectors and hydraulic machinery. It is why a pump must be fed enough inlet pressure, why propellers are shaped to spread their thrust, and why cavitation is studied wherever liquid has to move fast.
1. Liquid arrives at high pressure
The liquid enters the pipe steady and under pressure, with no bubbles in it. As long as its pressure stays well above the vapour pressure, the liquid remains a single, unbroken stream.
2. The throat speeds the flow up
The pipe narrows into a throat. The same amount of liquid has to pass through a smaller opening, so it moves faster — and, by Bernoulli's principle, wherever the flow is faster its pressure is lower.
3. The pressure falls below the vapour pressure
If the throat is tight enough, the pressure there drops below the liquid's vapour pressure. At that point the liquid can no longer stay liquid: it tears apart and pockets of vapour — bubbles — open up inside the flow.
4. Bubbles travel downstream
The bubbles are swept along with the stream. While the pressure stays low they survive; they even grow as more vapour joins them. Nothing is damaged yet — the trouble is what comes next.
5. The bubbles implode and pit the metal
As the pipe widens the flow slows and the pressure recovers, so the bubbles collapse almost instantly. Each collapse focuses the surrounding liquid into a tiny fast jet and a pressure wave. Repeated thousands of times a second, they fatigue the surface and chip away small pits.
What is cavitation?
Cavitation is the forming and violent collapse of vapour bubbles in a liquid. When the local pressure falls below the liquid's vapour pressure, the liquid boils into bubbles; when the pressure recovers, those bubbles implode almost instantly.
What causes the pressure to drop?
Fast-moving liquid has lower pressure than slow-moving liquid. Anywhere the flow is forced to speed up — a narrow throat, a valve, or a pump impeller — the pressure can fall below the vapour pressure and start cavitation.
Why do bubbles damage metal?
Each bubble collapse concentrates the surrounding liquid into a tiny, fast micro-jet and sends out a pressure wave. Repeated thousands of times a second on the same spot, these hammer the surface until small pits break away.
Is cavitation boiling?
It is boiling, but caused by a drop in pressure rather than by heat. The liquid is not hot; it turns to vapour simply because the pressure around it has fallen far enough.
Why is cavitation a problem in pumps?
It eats away the impeller, makes the pump noisy and vibrate, and cuts performance. Because it starts whenever the inlet pressure gets too low, pumps are given a minimum pressure they must be fed at.