diferent vibration values over time 1

[Esbirinai]

I checked a ventilator (100 kw - 960rpm) and obtained diferent values of vibration. Values go from 4 mm/s to 11 mm/s. The ventilator has 8 blades (2*4 blades, two on 0º, two on 90º, two on 180º and two more on 270º)
The spectrum returns only a peak near 64Hz which i presume to be from the blades (960 / 60 * 4 = 64 Hz) Actualy, this peak goes from 4 to 11 mm/s in separate measures.
I do not understand. Nothing changed between the measures.

I can garantee that there is nothing wrong with the data colector and probes.
Thank's for any help.

 

[vanstoja]

Probably nothing you could see changed between measurements. But how about changes you couldn't see or didn't measure like flow velocity profile into the inlet of the ventilator or the position of the rotor shaft relative to a "cutwater" structure downstream of the impeller blades?
The impeller blade passing (IBP) peak you are seeing in the noise spectrum is caused by either ingestion and blade inlet edge cutting of vortices in the entering flowfield or interaction between wakes being shed from the impeller blade trailing edges and some downstream vane, strut, casing cutwater tongue, etc. The increase in sound pressure level you are seeing amounts to 8.79 decibels (20*Log10(11/4))which is more than one would expect (3 to 6 max dB) for random fluid fluctuations. Perhaps your inlet flow is varying in quality, ie, ingested vortices are more numerous or more intense(higher rotational velocities) with time or the rotor shaft experiences varying radial hydraulic thrust loading which varies the downstream cutwater clearance(and impulse intensity) with time. The fact that you are experiencing vibration velocities straddling the 0.3 ips "rough running" criterion indicates a need to determine the real cause of the variations you are experiencing and their level variation over long time periods.

 

[fredt]

I presume that this is mechanical vibration that you are measuring on some part of the machine. The motor housing?

I am unclear what type of fan it is. The paired blade arrangement that you describe sounds like an axial flow fan. The blade arrangement is probably intended to reduce the blade passing tone. The running speed for a fan of this size is more likely to be around 980 RPM which means a slightly higher frequency than you are measuring.

It may be a structural resonance that is close to 4 times running speed and could be excited by either fan out of balance or blade passing. It may be sometimes excited and sometimes not depending on fan operating conditions.

 

[Esbirinai]

Actually, There were two fans on site, they are exactly the same (axial flow fan). One of them is ok (rms 3 to 5mm/s) and the other has symptoms like tho one i described.
The inlet flow is not varying on any of them.
However, with a close inspection i found that the blades on this fan were slightly bended on one end (the blades on the other fan were not bended) and that a small part of one blade was missing.
Could this be the reason? The bended ends are enought to make this happen?
Thank you,