Diagnosing Cavitation

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Dynamic balancer “Balanset-1A” OEM

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Cavitation is a destructive phenomenon that occurs in pumps and other hydraulic systems: the rapid formation and violent collapse (implosion) of vapour bubbles in a liquid. It happens when the local static pressure of the liquid falls below its vapour pressure, so the liquid momentarily boils at ambient temperature and then re-condenses as the pressure recovers. Although it is often described as a “hissing” or “marbles rattling” sound, cavitation is a significant source of vibration and can cause severe, erosive damage to impellers and casings. Crucially, it is a sign of a hydraulic problem rather than a mechanical one — yet it is readily detectable with vibration analysis, which makes it a classic example of using vibration to diagnose a process fault.

1. Definition: What is Cavitation?

The physics of cavitation hinges on the relationship between local pressure and vapour pressure. Inside a pump, the liquid accelerates as it enters the impeller eye, and by Bernoulli’s principle that acceleration drops the local pressure. If the pressure dips below the liquid’s vapour pressure, tiny vapour cavities nucleate. They survive only until the flow carries them into a higher-pressure region — typically a few millimetres further along the vane — where they collapse almost instantaneously. Each collapse is a microscopic implosion that releases a sharp pressure spike and a burst of high-frequency energy. Multiply that by the thousands of bubbles formed every second and the cumulative effect is both audible noise and measurable vibration, alongside the slow, relentless pitting of metal surfaces.

2. The Two Types of Cavitation

a) Suction Cavitation

This is the most common form. It occurs when the pump is “starved” of fluid — that is, when the Net Positive Suction Head Available (NPSHa) falls below the Net Positive Suction Head Required (NPSHr) by the pump.

• Mechanism: the low pressure at the eye of the impeller causes the liquid to boil, forming vapour bubbles. As these bubbles are carried into the higher-pressure regions along the impeller vanes, they collapse violently.
• Causes: a clogged suction filter or strainer, a partially closed suction valve, a suction line that is too long or too small in diameter, or a pump required to lift the fluid from too great a height.

Suction-side margin is fundamentally an NPSH problem, so when designing or troubleshooting an installation it helps to check the numbers explicitly; our NPSH Calculator works out the available head and shows how close a system is sailing to the cavitation threshold.

b) Discharge Cavitation

This is less common and occurs when the pump’s discharge pressure is extremely high, preventing fluid from flowing out of the pump.

• Mechanism: the fluid becomes trapped between the impeller vanes and recirculates at high velocity, creating a low-pressure vacuum zone where bubbles form. Those bubbles then implode as they move out of the low-pressure area.
• Causes: a blocked or closed discharge valve, or pumping against a “dead head” — a completely blocked discharge line.

The high-velocity internal recirculation behind discharge cavitation is closely related to flow recirculation, another low-flow instability that shares some of the same symptoms and is one of several centrifugal pump defects an analyst learns to distinguish.

3. The Vibration Signature of Cavitation

The violent implosion of thousands of tiny vapour bubbles does not produce a single, neat frequency. Instead it creates a very distinctive vibration signature:

• High-frequency broadband noise: the primary indicator is a significant rise in the “noise floor” of the FFT বর্ণালী, particularly at high frequencies (typically above 2,000 Hz). It appears as a broad “hump” of random energy rather than as discrete peaks.
• Random and unsteady: the vibration is random and non-periodic — which is exactly why it produces no sharp lines — and the overall amplitude can fluctuate noticeably from moment to moment. This randomness is what separates cavitation from ordinary flow turbulence, which tends to be milder and lower in frequency.
• Potential harmonics of blade pass frequency: in some cases the random energy can excite the blade pass frequency (BPF = number of vanes × running speed) and its harmonics, but the dominant feature remains the broadband noise floor. On pumps this same component is often called vane passing frequency.

Because the energy is broadband and impulsive, techniques tuned to repetitive impacts can sharpen the diagnosis: envelope analysis and metrics such as crest factor respond strongly to the rapid bubble-collapse transients. If cavitation is left to progress it can cause secondary damage — erosion of the impeller — which then introduces a genuine mechanical unbalance that shows up as a high 1× peak, a useful reminder that one fault can breed another.

4. Confirmation

Because the signature is one of random noise, it can be confused with other turbulence- or flow-related sources, so confirmation is worthwhile before committing to a repair:

• Listening: cavitation often produces a distinct audible sound, like gravel or marbles rolling around inside the pump — frequently the first clue an operator notices on the floor.
• Process changes: for suspected suction cavitation, carefully and slowly opening a partially closed suction valve, or cleaning the suction strainer, should immediately reduce or eliminate the high-frequency noise. This deliberate change-and-observe test is one of the most effective confirmations available, because it directly manipulates the hydraulic cause.

It is critical to address cavitation quickly. Each implosion acts like a microscopic jet hammer, chipping away at the impeller vanes and pump volute and leading to premature failure. In the field, the practical workflow is to confirm the broadband signature on a vibration analyser, eliminate the hydraulic cause, and then verify the machine has returned to a clean mechanical state. A portable two-channel instrument such as the ব্যালানসেট-১এ is well suited to that final step: once the process fault is cured, it measures the 1× amplitude and phase in the pump’s own bearings at operating speed, so any residual imbalance left behind by erosion can be quantified and corrected by in-place ভারসাম্য.

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