Pump cavitation: what it is, causes and how to prevent it

Every liquid has a vapor pressure: the threshold below which it stops being liquid and turns to vapor, even at room temperature. Inside a centrifugal pump, the lowest-pressure region sits at the impeller inlet — the impeller eye. If pressure there drops below the liquid's vapor pressure, thousands of vapor bubbles form.

These bubbles are dragged by the flow into higher-pressure regions inside the impeller — and implode. Each collapse produces a liquid microjet with extremely high localized pressure, enough to tear material off the metal surface. Multiplied by millions of implosions per minute, this process drills through the impeller like accelerated corrosion — the well-known cavitation pitting.

An important distinction: cavitation is vapor of the pumped liquid itself, generated by pressure drop. Air entrained at the suction (through a vortex in the reservoir, a leaking flange or packing) produces similar symptoms but is a different problem, with different fixes.

Cavitation follows simple math. Every installation offers the pump a certain suction energy, the available NPSH (NPSHa) — which depends on the reservoir level, the pressure above it, the liquid temperature and the friction losses in the suction piping. And every pump demands a minimum suction energy, the required NPSH (NPSHr) — printed on the manufacturer's curve and growing with flow. Near that minimum, the pump may already be cavitating.

There is one rule: the installation's NPSHa must exceed the pump's NPSHr, with margin. ANSI/HI 9.6.1, from the Hydraulic Institute, recommends that minimum margin by application, measured from the manufacturer's NPSHr. That is why the design value comes from the application's table or from the manufacturer, not from a single rule.

When that equation fails — because the reservoir level dropped, the suction strainer clogged, the water warmed up or the pump is running above design flow — cavitation begins.

Cavitation is rarely silent. The signs appear together and evolve over time:

Each row links a symptom to its likely cause, the check and the fix, with the source in brackets. Symptoms show up together: confirm with more than one check before acting.

The most obvious damage is the eroded impeller — which gradually loses performance until it needs restoration or replacement. But the real cost usually sits in the collateral damage: mechanical seals that don't last three months, bearings with impact marks, worn shaft sleeves and, ultimately, a shaft cracked by fatigue.

There is also the invisible cost: a cavitating pump operates away from its best efficiency point, consuming more energy per cubic meter pumped. In continuous operation, that bill quickly exceeds the cost of the repair.

Hence the practical rule: cavitation is not an acoustic nuisance, it is a failure countdown running. The sooner the cause is corrected, the smaller the total bill.

The right fix depends on where the NPSH equation is failing. These are the corrections we apply in the field, from simplest to most structural:

When discharge pressure drops, these are the suction and operation causes that have a verified technical source. Check them before dismantling the pump.

If the impeller is already damaged, the correct sequence is: restore or replace the impeller, balance it, and — before returning the pump to operation — fix the cause of the cavitation. Restoring the impeller without addressing NPSH means paying for the same repair twice.

Hydro Pumps executes both ends: impeller restoration (welding, thermal spray and machining, with bench balancing to grade G2.5) and the installation diagnosis, with vibration measurement to confirm cavitation and a suction-condition analysis to eliminate it.