Cavitation, vapour pressure & aeration
Where the pressure drops too low, the oil boils or sucks in air. The bubbles that form then collapse — and they can hammer a pump to pieces.
Source: Rabie, Fluid Power Engineering, Ch. 2.
Before you start
What you need first
- Pressure (Topic 2) and the idea of pump suction / head (Topic 8).
What you'll be able to do
- Explain vapour pressure and cavitation.
- Tell cavitation apart from aeration.
- Name where it happens and how to prevent it.
Start here · the idea
Vapour pressure — oil can boil cold
A liquid boils whenever its pressure falls to its vapour pressure — not only when it is hot. Drop the pressure low enough (at the pump inlet, or at a sharp restriction) and the oil "boils" at ordinary temperature, forming vapour bubbles.
Cavitation — bubbles that form, then implode
In a low-pressure zone, vapour bubbles appear. Carried into the next high-pressure zone, they collapse violently — each implosion fires a microscopic jet at the metal.
Aeration is not the same thing
Both fill the oil with bubbles and make it noisy and spongy — but they are different faults with different cures:
| Cavitation | Aeration | |
|---|---|---|
| Bubble contains | oil vapour | air drawn in from outside |
| Cause | pressure falls below vapour pressure | air leaks in (loose suction fitting, low oil level, splashing return) |
| Worst damage | metal erosion / pitting | sponginess, oxidation, noise (less erosion) |
| Cure | raise inlet pressure / lower suction velocity | seal air leaks, raise oil level, calm the return |
Where it strikes, and how to prevent it
The danger spot is the pump suction (and sharp restrictions). To keep the inlet pressure up:
- Use a flooded suction — tank above the pump where possible.
- Keep the suction line short and fat → low velocity (Topic 15) → less pressure drop.
- Keep the suction filter clean; keep the oil level up; seal all inlet joints (stops aeration too).
✏️ Try it yourself
A pump runs noisy and the oil looks foamy. Give one quick test (and what each result means) to decide whether the cause is cavitation or aeration — and name a fix for each.
Recap — the whole topic on one screen
| Idea | What you own now |
|---|---|
| Vapour pressure | Oil boils when pressure falls to it — even cold |
| Cavitation | Vapour bubbles form (low p) then implode (high p) → pitting |
| Aeration | Air drawn in → spongy & noisy, but less erosion |
| Prevention | Flooded suction, short/fat inlet, sealed joints, oil level up |