Premature Bearing Failures In Nuclear Bus Duct Cooling Fans

A nuclear power plant had changed bus duct cooling fan bearings approximately five times in two years. The plant wanted to find the root cause of the repetitive bearing failure so they could address the main problem instead of changing out bearings more frequently.  The increased replacement frequency brought increased cost and unscheduled maintenance.

• Problem Statement (Challenge)
  • What (type of machine): Bus Duct Cooling Fan in Nuclear Service
  • Where (State/ Country): Tennessee
  • Why (problem/reason): A nuclear power plant had changed bus duct cooling fan bearings approximately five times in two years. The fan bearings were replaced due to detected fatigue failure, high harmonics of the running speed, and eventually the locking ring at the outboard bearing was discovered to be chipped/ cracked.  The plant wanted to find the root cause of the failures so they could address the main problem instead of changing out bearings more frequently.  The increased frequency brought increased cost and unscheduled maintenance.
• Work Performed: MSI's Troubleshooting Approach
  
  • Methods: Experimental Modal Analysis (EMA), Condition Monitoring, and Operating Deflection Shape (ODS)
• Results/ Solution
  • Findings: The root cause of the high vibration and repetitive failure of the bearings was due to fatigue cracks present at the welds along the radial vanes and the inner can structure. The initial cracks were predicted to have initiated from unintended forces/impacts during the impeller extraction for bearing replacements.  Cracks of the welding of the inner can were present at the lower section of the can where the “A” frame is supported (at 4 and 8 O’clock positions).
  • Impact: The plant discovered the locations where cracks existed and fully repaired the welds.  The plant was able to extend its maintenance schedule of the equipment and no longer has to worry about costly, unscheduled bearing replacements.

                                     X-section view of the duct cooling fan                                            Front view showing can and radial support vanes                                                              Picture of weld cracks found