It has nothing to do with the water getting hot. It has everything to do with pressure falling low enough that cold water boils anyway.
Water boils when its vapor pressure exceeds the local pressure around it — normally we get there by raising temperature. But you can just as easily get there by dropping the pressure, at any temperature, including cold tap water. That's cavitation: a liquid boiling locally because the surrounding pressure crashed, not because heat was added. Wherever a flow accelerates hard — a pump impeller eye, a valve throat, a propeller blade tip, a venturi constriction — Bernoulli's principle says pressure has to drop right there. Drop it far enough, below the liquid's vapor pressure, and vapor bubbles form on the spot.
The vapor bubbles that form in a low-pressure zone don't stay there — the flow carries them downstream into a region where pressure has recovered to a higher value. Once a bubble crosses back above the liquid's vapor pressure, it can no longer exist as vapor: it collapses, and it does so violently and almost instantaneously. That collapse focuses enormous energy into a tiny volume — a shock wave and a high-speed micro-jet of liquid slamming into whatever solid surface happens to be nearby.
Bernoulli's equation says that along a streamline, as velocity increases, pressure must decrease to conserve total energy. At an impeller eye, a valve throat, a propeller blade tip, or a venturi constriction, the flow is forced to speed up dramatically over a very short distance — and pressure at that exact spot drops to match. If it drops below the liquid's vapor pressure at the local temperature, the liquid boils right there, cold or not. The bubbles that form are then swept downstream by the same flow into a region where the passage widens or slows, pressure recovers above vapor pressure, and the bubbles can no longer exist as vapor — so they collapse, converting the energy that formed them into a violent, localized implosion.
No — and this distinction is the whole reason cavitation is treated as a serious failure mode instead of a nuisance. Cavitation bubbles are actual water vapor — the liquid itself boiling because local pressure fell below its vapor pressure — not air that leaked in or got entrained by turbulence. Ordinary entrained air bubbles compress gently and cushion themselves as pressure rises; a vapor bubble has essentially nothing inside it, so when pressure recovers, it collapses almost instantaneously with no cushioning, focusing enormous energy into a microscopic point. That's what pits and erodes hardened steel impellers, valve seats, and ship propellers over time, along with the characteristic crackling noise and vibration. It also means the fix is not "keep air out of the system" — it's maintaining adequate Net Positive Suction Head (NPSH)at the pump's suction side, so the local pressure never falls far enough to reach vapor pressure in the first place.
Explains why cavitation is a pressure phenomenon, not a temperature one — local pressure dropping below a liquid's vapor pressure in high-velocity zones causes it to boil at ambient temperature, forming vapor bubbles that later collapse violently when swept into higher-pressure regions.
Because "boiling" is strongly associated with heat, it is easy to assume cavitation bubbles must be caused by the water getting hot, or to mistake them for ordinary air bubbles entrained by turbulent mixing. Neither is correct. Boiling happens whenever local pressure drops to or below a liquid's vapor pressure at whatever temperature it currently is — you can get there by raising temperature (the familiar case) or by lowering pressure (cavitation), and cold water is fully capable of "boiling" this way if the pressure drops far enough.
Bernoulli's principle ties velocity and pressure together along a streamline: where velocity is highest, pressure is lowest. In a pump, that's the impeller eye/inlet; in a control valve, the narrow throat (vena contracta); in a propeller, the blade tip; in a venturi meter, the constriction. If pressure at that point falls below the liquid's vapor pressure, vapor bubbles nucleate and grow. The flow then carries these bubbles into a downstream region where the passage widens and pressure recovers above vapor pressure — at that point the vapor can no longer exist, and the bubble collapses (implodes) in microseconds, generating a shock wave and a high-velocity micro-jet capable of pitting even hardened metal.
Cavitation is prevented, not by removing air, but by ensuring Net Positive Suction Head Available (NPSHA) at a pump's suction stays above the pump's required NPSH (NPSHR) with margin — achieved through adequate suction-side elevation or pressure, minimizing suction-line friction losses, avoiding excessive suction lift, and keeping fluid temperature (and therefore vapor pressure) in mind during design. Left unaddressed, cavitation causes progressive pitting erosion of impellers and valve seats, efficiency loss, noise, vibration, and eventually mechanical failure — a leading cause of unplanned pump and valve maintenance in water and wastewater systems.
No. Cavitation is triggered by a local pressure drop below vapor pressure, which can happen in cold water just as readily as hot water — cold water simply has a lower vapor pressure, so it takes a larger pressure drop to reach it. It is not a heat-driven phenomenon.
A cavitation bubble is water vapor — the liquid itself has locally boiled — with essentially nothing but vapor inside, so it collapses almost instantaneously and violently when pressure recovers. An entrained air bubble contains actual gas that compresses gradually and cushions itself as pressure rises, so it does not produce the same destructive implosion.
Net Positive Suction Head is the margin between the actual pressure (converted to head) available at a pump's suction and the liquid's vapor pressure at that temperature. NPSH Available (a property of the piping system) must exceed NPSH Required (a property of the specific pump, from its performance curve) by an adequate margin, or the pressure at the impeller eye will drop below vapor pressure and cavitation will occur.
A characteristic crackling or "gravel-like" noise and vibration, reduced flow and head output for the same speed and power input, and over time, visible pitting erosion on the impeller vanes and pump casing near the eye and discharge regions.
No — removing entrained air can help overall performance but does not address the actual cause, which is pressure dropping below vapor pressure at a high-velocity point. The real fix is increasing NPSH available (raising suction pressure/elevation, shortening or enlarging suction piping, reducing suction-side losses) or reducing NPSH required (a different pump design or lower flow rate).
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