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.
Alternatively, if someone in authority demanded the use of two seismic transducers per bearing housing and two-out-of-two voting logic, the reliability engineer could plan to install these probes in the vertical (X) and horizontal (Y) directions. He should consider implementing two-out-of-two trip voting logic (Figure 1) and install the two probes in readily accessible locations on the bearing housings. Each probe may be placed at a convenient angle or at the traditional 12 and 3 o’clock locations. Encountering vibration velocity excursions should trigger an alarm if one of the two readings were to exceed 7 mm/sec. Automatic trip activation should be linked to both probes measuring an activity exceeding 7 mm/sec.
A special caveat was illustrated in a so-called “orbital plot” generated by the reliability engineer's rather sophisticated vibration monitoring system. His plot depicted an elliptical “squashed orbit” with the X-vibration probe showing much less amplitude than the Y-probe. Therefore, it’s possible that during a vibration excursion, the Y-probe could be in a trip state (i.e., "HiHi") even as the X-probe showed normal. Over the years, we’ve found that if one end of a rotor is in distress, the other end should show at least some change from normal. While the output might not be in a "HiHi" alarm state on the non-distressed end, that end should at least be showing a "Hi" alarm on one of its vibration probes. Of course, more cards would have to be installed in the electronic monitoring rack for this kind of exploration.
Experience with different lubrication technologies
It's understandable that a facility would make the case for more vibration monitoring; however, we thought this large ammonia/urea plant would do well making its priorities reliable bearings and lubrication. Vibration monitoring (hopefully) lets us know when there’s a problem; sound best-in-class lubrication and lubricant application strategies prevent problems from developing in the first place.
It should be self-evident that lack of lubrication will cause bearings to fail. Periodic and frequent re-lubrication is needed on plants interested in high electric-motor reliability. However, manual re-lubrication is expensive when done properly – and even more expensive when it involves neglect. Reliability professionals should study and adopt only best-available bearing selection, and, if cost-justified, partial or plantwide automated lubrication strategies.
In this instance, the plant’s top and mid-level managers should be briefed on why dry sump (“pure”) oil mist has been successfully used on rolling element bearings by best-in-class companies since about 1965. An estimated 27,000 electric motors are equipped with dry sump (“pure”) oil mist as of 2016. When we last asked one experienced oil mist user (in 2014), we were told that some of the motors so lubricated have not needed bearing replacement in the time period from 1977 until 2014 – at least 37 years.
A major electric motor manufacturer (Siemens1) allows oil mist in motors up to 3,000 kW. Unless the bearings are lubricated by oil mist, best-in-class companies usually disallow rolling element bearings for electric motors above 400 kW. But this rule-of-thumb should be applied only on rolling element bearings being re-lubricated with traditional greases.
Lifetime-lubricated rolling element bearings with sealed-in perfluoropolyether (PFPE) greases offer an important lubrication alternative for plants that are unable to guarantee proper re-lubrication of their open or shielded motor bearings.2 Facilities insisting on mineral oil based greases must obey re-lubrication rules and schedules. Here, details on automatically or manually applied grease lubrication are important, and these details will differ with the location and orientation of shields (if any) and drain ports.2,3
There’s considerable reliability impact depending on the type of grease. In addition, certain grease application methods sometimes result in wrong fill volume, excessive grease pressure (which can deflect bearing shields), rust or dust in bearing element paths, and bearing flat spots (in an installed spare pump set) due to shafts not being rotated, to name just a few. Again, proper greasing procedures and lubrication management are far more important than placing, mounting, and maintaining more monitors on a rolling element-equipped motor bearing housing.
The 3,600-rpm electric motor in this case study was equipped with 85-mm bearings. Shaft peripheral speeds are high on this relatively large motor. With grease re-lubrication, an 85-mm bearing becomes maintenance-intensive because it will require grease replenishment at least six and, in some cases 16, times per year. However, the reliability engineer had reported that no re-lubrication had been performed in the 10 months before the motor’s massive failure. A massive failure often is explained by rivet heads popping off in a riveted-cage bearing. Should this happen, the motor can grind to a halt in mere seconds.
We referred the reliability engineer to an article4 describing how oil mist can be used on many electric motor bearings – an article that was obviously written decades ago and still the best available technology today. Why this company wasn’t availing itself of oil mist lubrication1 or pump-around circulating oil lubrication (see Figure 2) is difficult to comprehend. The one sure thing we know about achieving reliability is that it can’t be obtained with business-as-usual mindsets and lots of manual labor.
Evidence of outdated lubrication technology in this instance
We were now ready to zero-in on the real problems as we saw them. First, the reliability engineer was probably only responsible for vibration monitoring and analysis tasks. His assignment may be limited in scope, and he may not be able to tell higher management that we believe his company is vulnerable in its use of old lubrication technology. However, it’s our belief that the plant team must identify the sources and initiators of persistent deviations from long and reliable operational performance of machines. They also must adopt a stance that refuses merely to treat the symptoms of premature equipment distress. Instead, the team members must make it their goal to firmly establish the root causes of deviations.
In this instance, the root causes of bearing failures were not addressed by vibration monitoring; indeed, failure was inevitable when the re-lubrication needs of these rolling element bearings were disregarded. Good asset management pays attention to lubrication strategies that reduce reliance on the human element.
Perhaps someone at this ammonia/urea plant will give thought to the matter by thinking about what happened at this facility:
- Using mineral oil-based grease and not observing the required replenishing intervals for this grease leaves the plant vulnerable
- Using application methods that were discontinued by best-in-class companies in the early- to mid-1960s leaves room for upgrade opportunities
Finally, reliability professionals must become agents for permanent change, not purveyors of temporary solutions or advocates of more and more monitoring. Proper motor lubrication vastly reduces the need for vibration monitoring, and failure elimination is always better than hoping to catch component degradation at just the right time.