Vibration problem

Excessive vibration in a fan
Centrifugal fan on foundation used to force heated air over drying malt
Specification:
• 2.5 m dia impeller
• spherical roller bearings
• multiple v-belt drive
Operation problems:
• Pulsations, on closure of inlet vanes (flow control device)
• Excessive vibration levels in spite of careful impeller balancing.
• Prominent second and third harmonics in the frequency spectrum
• When run down, irregular knocking noises produced.
Also damage to machine identified on inspection:
•
Failure of two 24mm bolts holding steel pedestal to concrete foundation block
• Partial disintegration of grouting between pedestal & foundation
• Some damage to shaft surfaces where bearings attached.

Using accelerometers, vibration amplitude was measured at various heights down the pedestal and foundation:

Questions
1. What are the likely causes of the vibration problems?
2. What would you, as plant engineer, recommend to prevent recurrence?