Cut open a pump that failed young and you sometimes find an impeller that looks eaten, pitted as if sandblasted. That is cavitation, and it is one of the few failures that is never bad luck. It is always a suction-side mistake.
What it is
When pressure at the impeller inlet drops low enough, water boils at room temperature. Vapour bubbles form, travel a fraction of a rotation into higher pressure, and collapse violently. Each collapse is microscopic, but a collapsing bubble next to metal hits like a tiny hammer, thousands of times a second. Weeks of that erodes even stainless steel.
How it sounds
Like gravel rattling inside the pump, or a harsh crackling under the normal hum. If your pump suddenly sounds like it is pumping marbles, stop it and check the suction side before you check anything else.
The usual causes
- Suction lift too high for the pump's NPSH requirement.
- Suction pipe too small, too long, or clogged.
- Valve half-closed on the suction side, the most self-inflicted cause of all.
- Foot valve or strainer fouled with debris.
- Hot water, which boils easier and demands more suction margin.
The fix is always upstream
Never throttle the suction line; throttle the discharge if you must reduce flow. Give the pump a shorter, fatter, cleaner path to the water, or lower the pump. The impeller the cavitation destroyed was the victim, not the cause.