Electric motors are considered the simplest and most widely used machines. And it would be fair to say that motor bearings and lubrication are less complicated than motor wiring, which is why the former tends to be given less attention than the latter. But both will affect motor reliability and availability; accordingly, both deserve our attention.
Also, vibration and lubricant issues co-mingle, meaning that one leads to the other and the attendant issues are so interwoven that separating them makes little sense. As the case study in this article illustrates, upgrading motor lubrication often is possible, but knowing when and how to upgrade is a different matter.
This case study involved a reliability professional who made it his priority to monitor machine condition on a 3,600-rpm double-ended electric motor. Others were involved in decision-making, but an owner-operator with a number of ammonia and urea plants paid a price for neglecting lubrication matters.
When all is said and done, more emphasis should be given to fundamental failure avoidance. Still, this owner-operator’s reliability engineer deserves much credit for communicating his important observations.
The plant's challenge: Identify the right balance of monitoring and lubrication
The plant's geographic location is hot and humid; yet, blinding sand storms are known to occur at times. In this plant, a high-pressure Carbamate Pump and a Booster Pump were connected to the shaft ends of a double-ended electric motor. After the motor experienced a massive bearing failure, the reliability engineer recorded the following relevant data:
- Each motor bearing housing had one vertical (X) vibration probe and one axial (Y) vibration probe. (There was no horizontal “Y” probe.) “X” and “Y” were seismic probes that resolve acceleration into vibration velocity – in/sec or mm/sec.
- The motor tripped on high vibration at one of its bearings. Initially, only one vertical (X) probe reached the trip value, and the second one didn’t. After 30 seconds, both probes reached the trip value of 7.1 mm/sec, and the motor was shut down, exactly as intended.
- All bearings were deep-groove style 6317, meaning the bearing bore was 85 mm.
- The failed motor bearings showed bluish discoloration on shafts and bearing inner races, pointing to a lubrication issue. Also, there was no trace of lubricant, something which was critically important.
- The original design intent was for automatic grease dispensing devices to lubricate these bearings; however, no such automated process was in place. The bearings were last (manually) lubricated in September, and no regreasing was done until the bearings failed in July or August of the following year after about 10 months of operation. This, too, is critically important information, as will be seen later.
- After rebuilding the motor, the axial (Y) probe was repositioned (relocated) to the horizontal (Y) location.
The reliability engineer at the affected plant inquired about API 670/4th ed. (2000). This American Petroleum Industry (API) Standard mentions dual-voting logic which, the reliability engineer believed, is adopted by a majority of end users. He also noted that the recently released API 670/5th ed. (2014) recommends using single-voting logic for radial vibration.
Based on the reliability engineer's in-house experience, he knew that his company favored either monitoring radial vibration excursions without trip logic or, more recently, two-out-of-two voting logic. He now sought consulting advice on suitable voting logic for radial seismic acceleration/vibration monitoring of electric motors, and asked us to be mindful of management’s ever-present concerns over his facility’s operational availability and machine reliability priorities.
The consultant's advice: Treat root causes, not symptoms
Our advice was experience-based. To have this motor “protected” with one transducer per bearing housing would be cost-justified simply because the plant already had all the associated electronic modules. However, the facility’s managers objected to using just one transducer, because of concerns about spurious trips shutting down a highly profitable plant.
In this situation, we thought it would be appropriate to research the probability of spurious trips in modern installations. Also, the reliability engineer's recollections of failing transducers may no longer pertain and might have to be updated.