Diagnosing Cavitation

1. Definition: What is Cavitation?

Cavitation is a destructive phenomenon that occurs in pumps and other hydraulic systems. It is the rapid formation and violent collapse (implosion) of vapor bubbles in a liquid. This happens when the static pressure of the liquid falls below its vapor pressure.

While often described as a “hissing” or “marbles rattling” sound, cavitation is a significant source of vibration and can cause severe, erosive damage to pump impellers and casings. It is a sign of a hydraulic problem, not a mechanical one, but it is readily detectable with vibration analysis.

2. The Two Types of Cavitation

a) Suction Cavitation

This is the most common form of cavitation. It occurs when the pump is “starved” of fluid, meaning the Net Positive Suction Head Available (NPSHa) is less than 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 vapor bubbles. As these bubbles are carried into the higher-pressure regions of the impeller vanes, they collapse violently.
• Causes: A clogged suction filter, a partially closed suction valve, a suction line that is too long or too small in diameter, or the pump being required to lift the fluid from too great a height.

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 is trapped between the impeller vanes and recirculates at high velocity, creating a low-pressure vacuum zone where bubbles form. These 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).

3. The Vibration Signature of Cavitation

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

• High-Frequency Broadband Noise: The primary indicator of cavitation is a significant increase in the “noise floor” of the FFT spectrum, particularly at high frequencies (typically above 2,000 Hz). It appears as a “hump” of random, broadband energy rather than as distinct peaks.
• Random and Unsteady: The vibration is random and non-periodic, which is why it doesn’t create sharp peaks. The overall amplitude can fluctuate significantly.
• Potential Harmonics of Blade Pass Frequency: In some cases, the random energy from cavitation can excite the Blade Pass Frequency (BPF = Number of Vanes × Running Speed) and its harmonics, but the dominant feature will still be the broadband noise floor.

If cavitation is severe, it can cause secondary damage, such as erosion of the impeller, which can then lead to a true mechanical unbalance, which would show up as a high 1X peak.

4. Confirmation

Because the vibration signature is one of random noise, it can sometimes be confused with other sources of turbulence or flow-related issues. Confirmation can be achieved by:

• Listening: Cavitation often produces a distinct audible sound like gravel or marbles rolling around inside the pump.
• Process Changes: For suspected suction cavitation, carefully and slowly opening the suction valve (if partially closed) or cleaning the suction strainer should immediately reduce or eliminate the high-frequency vibration noise. This is a very effective diagnostic test.

It is critical to address cavitation quickly, as the implosions act like a micro-jet hammer, chipping away at and eroding the impeller vanes and pump volute, leading to premature failure.

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