Cavitation

Definition

Cavitation is a hydraulic condition in pumps where local fluid pressure drops below the vapor pressure of the liquid, causing vapor bubble formation and subsequent collapse as pressure recovers.

Physical Mechanism

When fluid enters a low-pressure region (typically at the pump suction or impeller eye), vapor cavities form. As these bubbles move into higher-pressure regions, they collapse violently.

This collapse produces:

• Localized shock waves
• High-energy micro-impacts on metal surfaces
• Pressure fluctuations transmitted through the casing and structure

Signal Signature

Cavitation typically appears in vibration data as:

• Broadband high-frequency vibration increase
• Random, non-periodic impact content in waveform
• Elevated noise floor across spectrum
• Possible modulation of blade pass frequency (in centrifugal pumps)
• Unstable and fluctuating amplitude behavior

Unlike imbalance or misalignment, cavitation is not a discrete frequency fault, but a random impact phenomenon.

Diagnostic Relevance

Cavitation is commonly associated with:

• Insufficient suction pressure (low NPSH available)
• Restricted inlet flow
• High fluid temperature (increased vapor pressure)
• Blocked strainers or suction piping issues
• Excessive pump speed for system conditions

It is significant because it can lead to:

• Impeller pitting and erosion
• Rapid efficiency loss
• Seal and bearing damage from vibration transmission
• Severe long-term hydraulic degradation

Interpretation Notes

• Cavitation is primarily identified through broadband energy increase, not a single frequency peak.
• It often intensifies with load reduction or off-design operation.
• It may coexist with blade pass frequency but is not caused by it.
• Audible noise (“gravel” or “marbles” sound) is a common field indicator.
• Differentiation from mechanical looseness is important—cavitation has a hydraulic origin and is flow-dependent.

Summary

Cavitation is a hydraulic failure condition caused by vapor bubble formation and collapse in low-pressure regions of a pump. It produces broadband, high-frequency vibration and can rapidly damage impellers and degrade pump performance if not corrected.