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Vane pass frequency (VPF), also called blade pass frequency (BPF), is the rate at which impeller blades pass a fixed point such as the volute cutwater or diffuser vane. It equals the number of impeller blades multiplied by the shaft rotational frequency:

BPF = Z × RPM / 60

Elevated BPF amplitude indicates flow disturbances, recirculation, cavitation, or excessive gap between impeller and cutwater.

Cavitation produces a characteristic broadband high-frequency noise typically in the 1–100 kHz range due to the collapse of vapor bubbles. Key indicators:

• Broadband noise increase in the ultrasonic range
• Elevated BPF amplitude and harmonics
• Random, crackling noise (audible in severe cases)
• Increased overall acceleration levels (gE or HFD)

The combination of elevated broadband noise and increased BPF harmonics is a strong indicator of active cavitation.

NPSH margin is the difference between NPSH available (NPSHa) and NPSH required (NPSHr). A positive margin means sufficient suction pressure to prevent cavitation.

Industry guidelines:

• NPSHa ≥ 2× NPSHr — recommended for critical or high-energy pumps
• NPSHa ≥ 1.3× NPSHr — minimum for general service
• NPSHa < NPSHr — cavitation is certain

Cavitation detection methods:

• High-frequency acceleration — use accelerometers with >10 kHz range, look for broadband noise increase
• Ultrasonic sensors — 20–100 kHz range, most sensitive to cavitation onset
• Envelope/demodulation analysis — reveals cavitation impacts modulating BPF
• Acoustic emission — detects stress waves from bubble collapse
• Audible noise — severe cavitation sounds like gravel or marbles

Common causes include:

• Insufficient suction head (NPSHa < NPSHr)
• High fluid temperature reducing vapor pressure
• Excessive flow rate (operating far right of BEP)
• Suction line restrictions (clogged strainer, partially closed valve)
• Excessive suction lift
• Air entrainment or vortexing at suction
• Operating far from best efficiency point (BEP)
• Recirculation cavitation at very low flows