Check bearing caps torquing values (or is it the bases only? anyway check them too).
What is the motor speed at each test?
Number of balls in each bearing?
The impeler is the fan, correct?
Can you isolate one fan at a time and check for vibration?
Check which fan is responsible for the vibration.
As mentioned you can tell from the frequency if you have a clearance issue, bearings issue, etc.
Your description of this boiler system is not so clear. What's is the temperature these...
If comparing all three then I agree with what GregLocock said prev, you just can not compare them. They are different requirements. '
You should first perform (3) and based on the data you may decide how to preoceed. Maybe (1) and (2) are not required at all, or maybe both required. Maybe you'll...
Trying to understand something here,
If I understand correctly uglyfatjoe already performed a swept sine test and from the data it is clear that around 13Hz the density of energy is the highest meaning there may be a resonance freq. in that area. Doesn't that mean that test 1 (@ 13Hz) is the...