What is vane pass frequency (BPF)?

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. It equals the number of impeller vanes multiplied by shaft rotational frequency: BPF = Z × RPM / 60. Elevated BPF amplitude indicates flow disturbances, recirculation, or cavitation.

What frequencies indicate pump cavitation?

Cavitation produces broadband high-frequency noise typically in the 1–100 kHz range due to the collapse of vapor bubbles. In addition, vane pass frequency (BPF) and its harmonics often increase in amplitude during cavitation. The combination of elevated broadband noise and increased BPF is a strong indicator of cavitation.

What is NPSH margin and why does it matter?

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 typically recommend NPSHa ≥ 1.3× NPSHr for general service, and NPSHa ≥ 2× NPSHr for high-energy or critical pumps.

How do I detect cavitation using vibration analysis?

Cavitation detection via vibration: (1) High-frequency broadband noise (ultrasonic range 20–100 kHz), (2) Increased BPF harmonics amplitude, (3) Raised overall vibration levels, (4) Acoustic emission monitoring. Enveloping or demodulation techniques on acceleration data are effective for early cavitation detection.

What causes pump cavitation?

Common causes include: insufficient suction head (NPSHa < NPSHr), high fluid temperature reducing vapor pressure, excessive flow rate, suction line restrictions (clogged strainer, partially closed valve), excessive suction lift, air entrainment, and operating far from best efficiency point (BEP). Recirculation cavitation occurs at very low flows.

免費工程工具

泵浦空蝕頻率估算器

計算離心泵的葉片通過頻率 (BPF)、軸頻率、預期空蝕頻率範圍，以及帶有風險等級指示器的 NPSH 餘量。

葉片通過頻率 / BPF 空蝕 NPSH 餘量

結果

葉片通過頻率 (BPF)

—

軸頻率 (1×)

—

預期空蝕範圍

—

葉片數量

—

BPF 諧波

諧波	頻率 (Hz)	CPM	注意

空蝕特徵： 寬頻高頻噪音（通常為 1–100 kHz）伴隨 BPF 諧波升高。使用超聲波或高頻加速度測量進行早期檢測。

葉片通過頻率 (BPF)

葉片通過頻率（也稱為葉片通過頻率，BPF）是葉輪葉片通過固定點（如蝸殼切口）的速率：

Z — 葉輪葉片（葉片）數量
n — 泵軸轉速 (RPM)

空蝕頻率特性

離心泵中的空蝕會因蒸氣氣泡的內爆而產生特徵性的寬頻高頻噪音：

除了寬頻噪音外，空蝕通常會導致 BPF 振幅及其諧波（2×BPF、3×BPF）增加。

NPSH 餘量

NPSH（淨正吸入壓頭）餘量表示防止空蝕的安全係數：

NPSH 比率 (NPSHa/NPSHr)	風險等級	建議
> 2.0	低	餘量充足 — 預期無空蝕
1.3 – 2.0	中等	一般服務可接受，需監控
1.0 – 1.3	高	餘量不足 — 可能發生空蝕
< 1.0	嚴重	NPSHa 低於 NPSHr — 必定發生空蝕

範例 — 轉速 2950 RPM 的 6 葉片離心泵

已知： Z = 6, n = 2950 RPM, NPSHa = 8 m, NPSHr = 3 m

軸頻率 = 2950 / 60 = 49.17 Hz

BPF = 6 × 2950 / 60 = 295.0 Hz

NPSH 餘量 = 8 − 3 = 5.0 m

NPSH 比率 = 8 / 3 = 2.67 — 低風險

⚠️ 注意： 空蝕噪音為寬頻且主要位於超聲波範圍（>20 kHz）。標準振動感測器（加速規上限為 10–15 kHz）可能無法捕捉。使用高頻加速規、超聲波感測器或聲發射換能器進行可靠的空蝕檢測。