Pumps & Systems, November 2007
The benefits of predictive maintenance are becoming readily accepted throughout industry in Europe as well as North America. Guy Mertens, who is responsible for predictive maintenance at the Belgian Refinery Corporation (BRC), added an extra dimension to his predictive maintenance program by using it to identify common machine faults for the maintenance staff to watch for.
Since the early 1990s, BRC has employed predictive maintenance to get longer life out of refinery machinery while avoiding unexpected downtime. Mertens now monitors 515 machines (motors, pumps, gearboxes, compressors, turbines, fans, etc.) as a part of this predictive maintenance program. With support from Belgian distributor Coservices, Mertens relies on vibration data from the field and periodic analysis of machine oil samples to determine if and when a machine should be removed from service for repairs.
After any piece of rotating equipment is removed to repair a problem identified as a result of vibration monitoring, a root cause analysis is always performed. When certain issues occur too frequently, Mertens issues advisories to help refinery personnel avoid them in the future to prevent breakdowns and improve machine life. Results achieved through these root cause studies are often cited in Mertens' annual predictive maintenance report to upper management.
Anyone responsible for the upkeep and longevity of rotating machinery may want to implement a similar root cause analysis program. An awareness of frequently occurring conditions, along with suggested corrective actions, can be a useful addition to any predictive maintenance program.
Brinelling (Damage Occurring When a Machine is Not Running)
Most processes in refineries have spare machines installed for redundancy. If an operating machine breaks down or needs to be taken off-line for any reason, the spare can be activated to take over the load so no production time is lost. This means many machines are kept on standby, sitting idle for extended periods. If these machines are in an environment where a lot of vibration is common, a phenomenon known as brinelling may occur.
When a bearing is idle, the grease at the bottom of the bearing between the outer race and the rolling elements (also between the rolling elements and the inner race) will be slowly pushed away due to the gravity and whatever vibration may be present. At some point, all of the grease between the lower rolling element and the outer race will disappear, resulting in metal-to-metal contact.
General vibration occurring in an industrial setting can cause the rolling element to hammer at the same spot on the outer race until it is damaged (see Figure 1).
Figure 1, left. Brinelling of bearing outer race.
Even if the machine is brand new but has not been run for two years, severe vibration soon after start-up may indicate that bearing damage has occurred. To avoid this condition with equipment that sits unused for extended periods:
- Fill holes under the fundation plates (grounding).
- Replace cylinder bearings in motors with double ball bearings. It appears that these cylinder bearings are much more sensitive to brinelling then ball bearings.
Brinelling Due To Corrosion
Corrosion is much more likely to occur when machines are not running. This too can lead to brinelling (see Figure 2).
Figure 2. Bearing corrosion due to moisture ingress
To prevent corrosion in bearings due to the entry of moisture as well as condensation:
- Frequently alternate the use of one motor and then the other. Establish an operating schedule so that no machine is left idle for extended periods.
- If this problem reoccurs on a number of machines, an improvement can be made by adding an extra seal specially designed to keep water out of the bearings (see Figure 3).
Figure 3. Effective sealing arrangement for grease lubrication. V-ring seals are suitable for both grease and oil-lubricated applications. For sealing grease-lubricated bearing arrangements against dust or water spray, the V-ring should be arranged outside the housing cover or housing wall. Dust, water spray and other impurities can be excluded in this position. The V-ring will also act as a grease valve. Used grease or excess new grease can escape between the counterface and the sealing lip.
Ball Bearings Versus Cylinder Bearings
A survey on 120 motors in the refinery revealed that the vibration impact level on NU-bearings on the coupling side is much greater than with ball bearings (see Figures 4a, 4b).
Figure 4a , above
Figure 4b, above
This can occur when the bearings are not running under minimum load as specified by the bearing supplier (2 percent of the nominal load of a bearing). In this situation, ball bearings fit the application very well and there is no need for cylinder bearings. Well-aligned machine trains, such as motor-pump groups, exert no major force on the bearing coupling side, so ball bearings are sufficient. A recommendation was given to replace the cylinder bearings with ball bearings in these motors.
Moisture Penetration in Pump Bearings
Several pumps in the refinery are equipped with steam tubes on the seals. If the seal fails, allowing product to escape, the risk of spontaneous combustion is reduced. However, machines equipped with steam tubes have a much higher risk of water penetration, and for that reason, the steam tube method is no longer recommended.
Mean Time Between Repairs (MTBR)
Predictive maintenance corrections implemented as a consequence of vibration monitoring and analysis have resulted in substantially longer life cycles for the 515 monitored machines in the refinery. This machine longevity is expressed as Mean Time Between Repairs (MTBR). See Figure 5.
Figure 5
The MTBR calculation has been done the same way very consistently over the years, providing a reliable trend indicator. The equation for MTBR (expressed in years) is simply: MTBR = # machines / # repairs during the year. All interventions related to electrical and mechanical problems initiated as the result of vibration monitoring and analysis were taken into account in the MTBR calculations, including:
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All bearing changes on motors, pumps, fans, gearboxes, mixers, etc. because of bearing damage or bearing clearance
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Problems with couplings, including the replacement of gear couplings by disk couplings
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Revisions of gearboxes due to gear wear
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Electrical repairs (rotorbar defects and stator problems)
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Belt replacement
Minor maintenance without replacing parts is not considered to be a "repair." Those are not included in MTBR calculations. For example, replacing a seal without replacing the bearing may not constitute an intervention resulting from data developed through condition monitoring. Other examples include checking of clearances, belt tension, extra lubrication, oil levels, etc.
Conclusion
Figure 5 provides ample evidence that moving from preventive maintenance to predictive maintenance, combined with the extra dimension of root cause analysis, had a positive and growing effect on the mean time between repairs for monitored machines in the BRC Refinery. The slight dip in the MTBR-plot in 2001 was due to a lengthy refinery shutdown during which many planned preventive revisions were implemented.
Without question, periodic vibration measurements in conjunction with effective analysis of the root cause of machinery problems have resulted in an enviable predictive maintenance program.