In reliability organizations, a large emphasis is placed on the application of technologies and the savings associated with discovering a problem and preventing an unplanned outage or catastrophic failure. Many times this procedure is repeated throughout the plant because although the physical problem has been identified, the resulting actions do not always address the root cause. Plants must become proactive - taking the next step when identifying problems with condition monitoring technologies to determine the system or root cause and apply the solution throughout the plant.
Figure 1. Critical Motor
A vibration analyst noted a significant increase in vibration levels on a 450 HP, 1,200 RPM critical motor (Figure 2).
Figure 2. Vibration Levels
The vibration levels increased from less than 0.1 in/s to 0.25 in/s. No others changes were noted on the associated machine train. Analysis of the motor outboard bearing spectrum revealed a high amplitude peak around 7,200 cycles per minute (cpm) and another significant peak around 71 times the motor turning speed (Figure 3).
Figure 3. Bearing Analysis
The vibration analyst suspected an electrically related motor problem, and requested that the motor analysis group evaluate the motor in question. The group performed a current analysis (Figure 4) and power analysis on the motor, and no electrical problems were identified.
Figure 4. Current Analysis
Additional analysis showed a lower frequency, high-resolution spectrum that revealed a peak around two times the line frequency that was actually at 7,239 cpm. Further review of the motor components determined that this frequency was equivalent to the ball pass rate frequency, outer race (BPFO) of the inboard motor bearing. Based on the findings and because previous problems had occurred with this application, a decision was made to replace the motor during an upcoming scheduled PM for the machine train.
Many times, reliability groups stop at this point and claim the savings for preventing an unplanned outage or catastrophic failure. But to obtain greater benefits of condition monitoring technologies, the next step must be taken.
The motor analysis team took the motor to a local motor repair shop to verify the bearing problem and attempt to determine the source of the problem. Analysis of the bearings confirmed the outer race of the bearing was damaged. It was discovered that a spherical roller bearing was used in a belt drive application. A decision was made to change the bearing to a cylindrical roller bearing to increase its radial load capacity.
Additionally, it was noted when the inboard bearing was disassembled that the grease had hardened in the bearing. Upon removing the grease fill and discharge tubes, it was noted that the grease in the fill tube and discharge tube contained different greases. The fill tube had a red, multipurpose inorganic clay grease, which was not the grease recommended for use in the motors. The discharge tube contained a black, electric motor polyurea grease which also differed from the grease specified by the plant (Figure 5). Analysis proved that the two greases were not compatible, resulting in the hardening of the grease.
Figure 5. Problems in Fill and Discharge Tubes
Root Cause Analysis
Correcting the bearing and grease specification on the subject motor is not enough to solve the problem. The next step is to identify the root source of the failure and address these causes to gain the most benefit for the company. How did the wrong greases get into the bearing? Are other motors in this area or in the plant receiving nonspecified greases? Why is the motor repair shop using something other than the specified grease?
Upon talking with the lubrication services group, it was discovered that although this grease was no longer being used on equipment in the plant, many of the lubricators in the plant still had an inventory of this grease in their areas. The lubrication services group changed the grease specification within the plant, but did not communicate this to the affected service providers.
Several actions resulted from this failure:
The inorganic clay grease was removed from all of the lubrication storage areas.
A meeting was held with the motor repair shop to discuss the findings, the result of mixing incompatible greases and future expectations.
A new repair specification was developed in response to this and several other motor failures. The grease specification was now included in the repair specification, which had previously omitted this information.
As a result of the findings on this motor and on several others, a motor repair evaluation process and team were created.
This case study illustrates the benefits of taking condition monitoring programs to the next level. By becoming proactive and taking the next step when identifying problems with condition monitoring technologies, a plant can realize the added gains of determining the system or latent cause and applying the subsequent solution throughout the plant.