Hydraulic & Aerodynamic Forces
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.
Flow turbulence often occurs in blowers due to variations in pressure or velocity of the air passing through the fan or connected linework. This flow disruption causes turbulence which will generate random, low frequency vibration, typically in the range of 20 to 2000 CPM.
Cavitation normally generates random, higher frequency broadband energy which is sometimes superimposed with blade pass frequency harmonics. Normally indicates insufficient suction pressure (starvation). Cavitation can be quite destructive to pump internals if left uncorrected. It can particularly erode impeller vanes. When present, it often sounds as if "gravel" is passing through the pump.
-::- This site is designed and authored by David Stevens IEng MIET FIDiagE MICML -::- |
