What cavitation is
Every liquid has a vapor pressure, the pressure below which it starts to boil at a given temperature. Water at room temperature boils if its pressure falls low enough, and hot water boils at a pressure well above that. Cavitation happens when the pressure inside a pump drops below the liquid’s vapor pressure at some point, so small pockets of vapor form in the flow.
The bubbles themselves are not the main problem. As the liquid moves on into a region of higher pressure, such as the outer part of an impeller, the bubbles collapse rapidly. Each collapse produces a tiny, intense shock, and when millions of them happen against a metal surface, the surface is gradually damaged. The result is pitting, roughening and eventually loss of material.
This is different from air getting into the suction line, which also causes noise and poor performance but has a different cause and fix. It is also a separate issue from water hammer, which is a pressure surge in the pipework when flow is stopped or changed quickly.
Where the low pressure comes from
In a centrifugal pump, liquid enters the eye of the impeller where the pressure is at its lowest point in the pump. Losses in the suction pipe, plus the local acceleration of the liquid as it enters the vanes, reduce the pressure further. If what remains is not comfortably above the vapor pressure, bubbles form.
Pump engineers compare two quantities. The net positive suction head available (NPSHa) is what the system provides at the pump inlet above the vapor pressure. The net positive suction head required (NPSHr) is what the pump needs, and it is given on the manufacturer’s curve. As a rule, NPSHa should exceed NPSHr with a margin. How to check this when comparing pumps is part of the pump selection steps, so here the focus is on what lowers NPSHa in practice.
Common causes
Most cavitation traces back to the suction side of the system:
- Low liquid level or a high pump position. The less static head pushing liquid toward the pump, the less margin remains. A pump sitting well above the source has to lift the liquid in.
- Restricted suction. A partly closed suction valve, a blocked inlet screen or a fouled strainer reduces inlet pressure. Keeping line strainers clean is one of the cheapest forms of protection.
- Long, narrow or complicated suction piping. Small diameter pipe, many fittings and sharp bends all add friction loss.
- Hot liquid. As temperature rises, vapor pressure rises, so the same system that works with cold water may cavitate with warm water.
- Running far from the pump’s best point. At very high flows, the pump asks for more inlet pressure and NPSHr climbs. At very low flows, internal recirculation can create similar damage, which is why a pump should not be run against a closed discharge.
- High speed. Faster impellers need more inlet pressure, so increasing speed can bring on cavitation in a system that was fine before.
- Dissolved gas or volatile liquids. Some process liquids give off vapor easily and leave little margin.
Signs to listen and look for
Cavitation tends to announce itself, though early on the signs may be mild:
- Noise. The classic description is crackling, rattling or the sound of gravel passing through the pump. It is usually loudest near the suction side.
- Vibration. The collapsing bubbles make the flow uneven, and bearings, couplings and pipework can start to shake. Regular readings, as practiced in vibration analysis, can reveal a rising trend.
- Falling performance. Flow, discharge pressure or both may drop, and gauges may fluctuate. Heavy cavitation can cause the pump to lose prime or stop delivering altogether.
- Motor load changes. Current may swing or fall, depending on the pump type.
- Damage on inspection. A worn impeller with a spongy, pitted look, usually on the low-pressure side of the vanes, is strong evidence. Eroded casing surfaces near the inlet are another.
Because these symptoms can also come from air leaks, blocked impellers or worn parts, a careful look at the whole system is better than assuming a cause.
Why it matters beyond the impeller
Pitting weakens the impeller and reduces efficiency, but the vibration also loads bearings and shaft seals, shortening their life. Bearing wear patterns and recurring seal leaks on a pump with no obvious defect are sometimes a clue that cavitation is happening unnoticed. Left alone it can lead to an unplanned shutdown and an expensive repair.
General ways to reduce it
The right fix depends on the cause, so confirm the cause first. Typical approaches include:
- Increasing the liquid level in the supply tank or lowering the pump relative to the source.
- Cleaning strainers and fully opening suction valves. Throttling on the suction side is not a good way to control flow.
- Shortening and enlarging suction piping, or removing unnecessary fittings.
- Cooling the liquid or otherwise lowering its vapor pressure where the process permits.
- Reducing the flow demand or the speed. If a variable frequency drive is fitted, small speed reductions can help, as covered under variable speed drives.
- Choosing a pump with a lower NPSHr, such as one with a larger impeller eye, when the system cannot change.
- Using a booster or inducer arrangement when the manufacturer recommends it.
Any change to a running system should be made by trained staff, following the manufacturer’s guidance, your site’s lockout tagout rules and local regulations. This article is general information, not engineering advice, and a specific cavitation problem is best confirmed by someone who can measure the actual suction conditions.