How it works
When a liquid accelerates, its static pressure drops. If it drops below the vapour pressure at the operating temperature, vapour cavities form. Carried into a region of higher pressure, they collapse in microseconds; collapses near a wall produce micro-jets and pressure shocks that hammer the surface. Each impact is tiny, but millions of cycles fatigue and tear out material, leaving a rough, cratered, sponge-like texture with sharp edges. Where the fluid is also corrosive, the freshly exposed metal corrodes between impacts and the two mechanisms reinforce each other.
Cavitation is therefore a hydraulic problem first. Harder or more resilient surfaces slow the damage; only a change in pressure conditions stops it.
NPSH available versus NPSH required
In a centrifugal pump, the lowest pressure occurs at the impeller eye, just behind the leading edge of the vanes. Two quantities decide whether bubbles form there:
- NPSH available (NPSHa) is a property of the system: the pressure head at the pump suction above the liquid's vapour pressure, set by suction vessel level and pressure, line losses and liquid temperature.
- NPSH required (NPSHr) is a property of the pump at a given flow, determined by the manufacturer's test against a defined performance-loss criterion.
Because NPSHr is defined at a point where cavitation is already measurably affecting performance, operating with NPSHa merely equal to NPSHr is not cavitation-free. A margin is needed, and its size depends on the pump, the liquid and the duty; the pump manufacturer and the applicable pump standard govern it. Hot liquids, blocked suction strainers, falling tank levels and higher flow rates all erode the margin.
Operating far below the best efficiency point causes a different pattern, suction or discharge recirculation, with damage on the pressure side of the vanes or at the outlet tips rather than at the eye. The location of the damage is the first diagnostic clue.
Where else it shows up
Control valves with a high pressure drop, orifice plates, propellers, hydro turbine runners and the cooling-water side of engine cylinder liners all cavitate under the right conditions. Cavitation erosion resistance of materials is ranked in the laboratory with ASTM G32, the vibratory apparatus method.
What to do about it
Correct the cause — raise NPSHa, reduce suction losses, lower liquid temperature, or return the pump to its intended operating range — then restore the surface. Resilient, elastomeric coatings absorb bubble collapse better than rigid ones, but a rebuild without a hydraulic correction only resets the clock. Distinguish it from erosion-corrosion, which leaves smooth, flow-aligned metal loss and calls for a different repair.