Pump cavitation is the formation and violent collapse of vapor bubbles inside a pump when the liquid's local pressure falls to its boiling point and then recovers. It is the failure mode that adequate NPSH exists to prevent, and it is destructive: the collapsing bubbles hammer the impeller, erode metal, rob the pump of performance, and shake the whole machine. This guide describes the bubble-formation-and-collapse mechanism, the unmistakable gravel-in-the-pump sound, the pitting it leaves on the impeller, and the flow, pressure, and vibration symptoms an operator actually sees.
Pump Cavitation in one line: Cavitation occurs when the pressure of the liquid inside a pump drops to its vapor pressure, so the liquid boils into vapor bubbles, and then those bubbles are swept into a higher-pressure region where they implode. Each implosion releases a tiny, intense shockwave, and countless collapses against the impeller pit and erode the metal while making the pump sound as if it is pumping gravel. It degrades flow and head, spikes vibration, and if left running, destroys the impeller.
Cavitation is a two-step process. First, somewhere inside the pump, usually at the impeller eye where velocity is highest and pressure lowest, the liquid's local pressure falls to its vapor pressure and the liquid flashes into vapor, forming bubbles or cavities in the flow. This is the cavitation that a shortfall in NPSH produces: the suction supply simply cannot keep the pressure at the eye above the liquid's boiling point.
Second, those vapor bubbles are carried by the flow into the impeller passages, where pressure rises sharply. Back above vapor pressure, the bubbles can no longer exist, and they collapse, imploding almost instantly. The surrounding liquid rushes in to fill each collapsing void, and where the collapse happens against a solid surface it drives a needle-like microjet and a shockwave into the metal. Multiply that by millions of bubbles and it becomes a continuous, punishing bombardment of the impeller and casing.
It is the collapse, not the formation, that does the damage. The vapor bubbles themselves are harmless; the energy released when they implode against metal is what erodes it. That distinction explains why cavitation damage clusters just downstream of where the bubbles form, on the impeller vanes and shrouds a little past the eye, rather than at the very lowest-pressure point.
Cavitation has a signature no experienced operator forgets. The classic description is that the pump sounds as if it is pumping gravel or marbles, a harsh crackling rattle from the countless bubble collapses, sometimes with a lower rumble on top. Vibration rises and turns rough and random rather than the smooth signature of a healthy pump, and the whole machine can shake. On the panel, discharge pressure and flow become unsteady and typically sag, because vapor in the impeller passages is spoiling the pump's ability to develop head.
The physical evidence is impeller erosion. Cavitation pitting looks like the metal has been eaten away in a rough, spongy, orange-peel texture on the vanes and shrouds, and severe cases bore holes clean through the impeller. This is distinct from ordinary wear or from abrasive erosion by solids; cavitation damage is a localized, cratered pattern from repeated implosions. Left unchecked it also chews at seals and bearings through the constant vibration.
It helps to know the two common forms. Suction cavitation, the type NPSH addresses, comes from too little pressure at the inlet, from a starved suction, a low level, a clogged strainer, or too-high flow, and it damages the impeller near the eye. Discharge cavitation is different: it comes from running a pump against a nearly closed discharge, so liquid recirculates violently at the impeller tip and cavitates there, damaging the outer edge of the impeller and the casing cutwater. Both sound similar but have opposite causes, so the fix depends on which one it is.
Cavitation rarely appears without warning in the process data. Its early fingerprints, an erratic or sagging discharge pressure, an unstable flow, a rising and roughening vibration, and a dropping suction pressure, are all quantities that are already instrumented on important pumps. Because the onset ties directly to suction conditions, watching suction pressure against flow is often the earliest tell that a pump is slipping toward cavitation.
A cloud SCADA platform trends these signals together, so the pattern is visible rather than something an operator has to happen to hear on a site walk. A discharge pressure that has grown noisy at steady speed, a suction pressure sagging as a tank drains or a strainer fouls, or a vibration trend that has climbed and become erratic all point to cavitation, and a low-suction-pressure or high-vibration alarm turns that into an immediate notification.
For the unattended pump stations common in oil and gas, that remote visibility is decisive, because cavitation destroys an impeller over hours to days of running, not instantly. Merobix historizes suction and discharge pressure, flow, and vibration across the fleet and alarms on-call staff when the signature appears, so the pump can be throttled or its suction restored before the damage is done. Diagnosing whether it is suction or discharge cavitation from those trends then points to the right corrective action.
Cavitation classically sounds as if the pump is pumping gravel, marbles, or sand, a harsh crackling rattle produced by countless vapor bubbles imploding inside the pump, sometimes with a deeper rumble layered on. The sound is usually accompanied by rough, elevated vibration and unsteady discharge pressure. It is distinctive enough that experienced operators can often diagnose cavitation by ear before looking at any instrument.
Suction cavitation comes from too little pressure at the pump inlet, from a starved suction, low level, clogged strainer, or excessive flow, and it damages the impeller near the eye where the bubbles form. Discharge cavitation comes from running the pump against a nearly closed discharge, causing violent recirculation at the impeller tip that cavitates there and damages the outer edge and the casing cutwater. They sound similar but have opposite causes, so the correct fix depends on which one is occurring.
When vapor bubbles collapse against the impeller metal, each implosion drives a microjet and shockwave into the surface, and the repeated bombardment erodes the metal into a rough, cratered, orange-peel texture. Severe cavitation can bore holes clean through the vanes or shrouds. The damage clusters just downstream of where the bubbles form, which is why it typically appears on the vanes a little past the impeller eye rather than at the lowest-pressure point.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.