Pump cavitation: what it is, symptoms and how to eliminate it
Cavitation is the formation and violent collapse of vapor bubbles inside a pump, occurring when the liquid pressure at the suction drops below its vapor pressure. The result is that characteristic 'pumping gravel' noise, vibration, falling performance and progressive impeller erosion. This guide explains why it happens and how to eliminate it — in practice.
Updated on July 8, 2026 · Reviewed by Hydro Pumps engineering
What cavitation actually is
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.
The root cause: available NPSH below required
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 to operate without vaporizing the liquid, the required NPSH (NPSHr) — printed on the manufacturer's curve and growing with flow.
There is one rule: the installation's NPSHa must exceed the pump's NPSHr, with margin. The practice recommended by the Hydraulic Institute (ANSI/HI 9.6.1) is to keep a minimum margin — typically 10% or 0.6 meter above NPSHr, whichever is greater; critical services call for larger margins.
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.
The 6 classic symptoms
Cavitation is rarely silent. The signs appear together and evolve over time:
Gravel or jackhammer noise. The 'pumping rocks' sound is the acoustic signature of bubble collapse — audible from meters away in severe cases.
Broadband vibration. Unlike unbalance (which vibrates at running speed), cavitation generates random high-frequency vibration, visible in the spectrum as a raised 'carpet'.
Falling flow and pressure. Vapor bubbles occupy space in the impeller and 'steal' hydraulic capacity — the pump curve collapses.
Gauge and ammeter oscillation. Discharge pulses because flow inside the impeller is unstable.
Visible impeller erosion. At disassembly: spongy surface, craters and holes at the vane inlets — unmistakable.
Repeated seal and bearing failures. Vibration and pressure shocks destroy the mechanical seal film and hammer the bearings — cavitation kills the pump collaterally.
What cavitation destroys (and what it costs)
The most obvious damage is the eroded impeller — which gradually loses performance until it needs recovery 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.
How to eliminate it: 7 practical corrections
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:
1. Restore the suction level. Reservoir level below design is the most common and cheapest cause to fix — review level control and interlocks.
2. Clear the suction line. Clogged strainers and foot valves create head loss that consumes NPSHa. Cleaning and an inspection routine solve it.
3. Reduce piping losses. A long, narrow, elbow-riddled suction is bad design: increase the diameter, shorten the run, replace elbows with long-radius bends.
4. Control liquid temperature. Warmer liquid has higher vapor pressure — sometimes cavitation only appears in summer or when the process heats up. Re-check NPSHa at the worst thermal condition.
5. Bring flow back to design. NPSHr grows with flow. A pump running wide-open beyond its design point cavitates even in a correct installation — restrict flow or review the selection.
6. Reduce speed. Where a VFD exists, lowering speed drops NPSHr — often the fastest correction while the structural cause is addressed.
7. Review the pump selection. If the pump was selected wrong for the available suction, no workaround fixes it: it is a case for re-selection or an inducer-equipped impeller.
It cavitated. Now what?
If the impeller is already damaged, the correct sequence is: recover or replace the impeller, balance it, and — before returning the pump to operation — fix the cause of the cavitation. Recovering the impeller without addressing NPSH means paying for the same repair twice.
Hydro Pumps executes both ends: impeller recovery (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.
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Cavitation FAQ
A damaged bearing usually produces continuous metallic noise or squealing that follows rotation; cavitation sounds like gravel or rocks being pumped, and typically changes when you alter the flow (partially closing the discharge often reduces recirculation cavitation noise, and vaporization noise changes with suction conditions). The definitive diagnosis is the vibration spectrum: bearings show at characteristic frequencies; cavitation as broadband energy.
Yes — and it is common. Cavitation is a problem of the installation and the operation, not of equipment age. A new pump selected with NPSHr above what the suction offers, or operated far beyond its design flow, cavitates on day one.
It is the pressure headroom that keeps the liquid from boiling inside the pump. The installation offers a headroom (available NPSH) and the pump demands a minimum (required NPSH, from the manufacturer's curve). As long as available exceeds required with margin, there is no vaporization cavitation.
In most cases it can be repaired: the erosion is recovered by welding and thermal spray coating, followed by machining and dynamic balancing. Recovery stops being worthwhile when erosion structurally compromises the vanes or the hub — in that case we manufacture a new impeller to drawing, via reverse engineering if needed.
Yes, and it is one of the most frequent causes of premature seal failure. Cavitation's vibration and pressure pulses break the lubrication film between the seal faces and hammer the secondary seals. If your pump's seals don't last, investigate cavitation before changing the seal model.
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