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Vane/Blade Pass 3x+

Symptoms: 3X+ radial (V & H)

Blade Pass Frequency (BPF) = number of blades (or vanes) x RPM. This frequency is inherent in pumps, fans and compressors and normally does not present a problem. However, large amplitude BPF (and harmonics) can be generated in the pump if the gap between the rotating vanes and the stationary diffusers is not kept equal all the way round. Also, BPF (or harmonics) sometimes coincide with with a system natural frequency causing high vibration. High BPF can be generated if the wear ring seizes on the shaft or if welds fastening diffuesers fail. Also, high BPF can be caused by abrupt bends in linework (or duct), obstructions which disturb the flow path, or if the pump or fan rotor is positioned eccentrically within the housing.

Vane/Blade Pass is characterised by:


Spectrum:
  • Vane pass = Number of vanes x RPM
  • Blade pass = Number of blades x RPM
  • often higher in the horizontal direction compared with the vertical direction due to stiffness
We expect to see a high peak in the spectrum at 1X turning speed of the shaft. It is often higher in the horizontal direction compared with the vertical direction due to stiffness.
Waveform:
  • Emersons autocorelated rotated waveform should show the balde pattern. ie a 5 point star = 5 vanes
Phase:
  • Not used

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