Cavitation and What It Tells You
Why this matters
Cavitation is a symptom, not a root cause, and it is shouting a specific message: the liquid is flashing to vapor inside a pump or line because the local pressure dropped below the liquid's vapor pressure, then those vapor bubbles collapse violently. The noise (gravel rattling, marbles in the housing) and the slow erosion of metal are the consequences. If you only quiet the noise, you have not fixed anything. Read what cavitation is telling you and you find the real fault: a starved suction, a restriction, a temperature problem, or a system asked to do something it cannot.
What cavitation actually is
Every liquid boils at a temperature that depends on pressure. Lower the pressure enough and the liquid boils at the temperature it is already at. Inside a pump, the fastest, lowest-pressure spot is right at the inlet of the impeller. If pressure there falls below the liquid's vapor pressure, tiny vapor bubbles form. Microseconds later, as the liquid moves into the higher-pressure part of the pump, those bubbles implode. Each collapse is a pinpoint shockwave. Millions per minute pit the metal, chew the impeller, and rattle the housing.
The key idea: cavitation is a pressure problem at the suction, not a problem with the pump itself. The pump is the victim.
The two failure modes it points to
Cavitation comes in two flavors, and each names a different fault family.
- Suction-side starvation (the common one). The pump cannot get enough liquid in at enough pressure. Causes: a clogged inlet strainer, a closed or partly closed suction valve, a suction line that is too long, too small, or has too many fittings, a low fluid level, or a suction lift that is too high.
- Discharge or recirculation issues. Less common, but a pump run far off its design point, or with a near-closed discharge forcing internal recirculation, can cavitate too.
When you hear cavitation, your first move is almost always to look upstream of the pump, at the suction.
Reading the signs
| Sign | What it suggests |
|---|---|
| Gravel or rattling noise that rises with flow demand | Classic suction starvation; the harder it pulls, the worse it gets |
| Pitted, eroded impeller or housing | Sustained cavitation over time; confirm and fix the cause, do not just replace the part |
| Fluctuating discharge pressure and flow | Vapor pockets forming and collapsing, interrupting steady flow |
| Noise worse when fluid is hot | Higher temperature means higher vapor pressure, so cavitation starts more easily |
| Noise worse at high lift or with a dirty strainer | Suction pressure is being dropped before the liquid reaches the pump |
What it tells you about temperature
Hot liquid cavitates more easily, because warming a liquid raises its vapor pressure, which means it flashes to vapor at a lower pressure drop. A system that ran fine cold and starts rattling once it is up to temperature may be cavitating purely because the fluid got hot. That is a heat problem masquerading as a pump problem. The cure is to manage the temperature or raise the suction pressure, not to swap the pump.
How to act on it
Cavitation is a diagnostic gift: it localizes the fault for you. Work the suction side first.
- Check the inlet: strainer, screen, and filter for clogging. Restore a clean, open path.
- Confirm every suction valve is fully open and no fitting is throttling the flow.
- Verify the fluid level and the suction lift are within the pump's accepted range.
- Consider temperature: if the fluid is hotter than design, the suction pressure margin shrank.
- Only after the suction is proven good should you suspect the pump itself or a discharge-side recirculation.
Never "fix" cavitation by throttling the discharge or by ignoring the noise. Running a cavitating pump destroys it from the inside and can eventually fail in a way that floods or strands the system.
References
- Hydraulic Institute guidance on pump suction conditions and net positive suction head
- Manufacturer pump curves and minimum suction-pressure requirements
- Trade-standard pump troubleshooting practice
- See related: The Pressure Differential Tells the Story; Airlock vs Blockage: Decision Tree