Where Contaminants Come From

Moisture and dust often enter bearing housings (Fig. 1) through old-style labyrinth seals or lip seals as airborne water vapor, or via a stream of water from hose-down operations. Contaminants can also enter through a breather vent, or from the widely used non-pressure balanced constant level lubricators (Fig. 2 and Ref.1). An often-overlooked source of oil contamination is abraded oil ring material, which will be discussed later in this article.

How to Stop the Contamination

Unless the rotating equipment is provided with suitable bearing housing seals, an interchange of internal and external air (called "breathing") takes place during alternating periods of operation and shutdown. Bearing housings "breathe" because rising temperatures during operation cause gas volume expansion, and dropping temperatures at night or after shutdown cause gas volume contraction (Ref. 2). Open or inadequately sealed bearing housings promote this back-and-forth movement of moisture-laden, contaminated air.

To stop this breathing and resulting contamination, there should be no interchange between the housing interior air and the surrounding ambient air. Breather vents (Fig. 1) should be removed and plugged.

Instead of the widely used (non-pressure-balanced) constant level lubricators (Fig. 2), which allow the oil to come in contact with dirty air, a pressure-compensated (or "balanced") constant level lubricator should be installed (Fig. 3 and Ref. 3). Both devices will be explained more fully a bit later.

Finally, full sealing of the bearing housing requires the use of face seals. An API-610 compliant magnetically-activated dual-face seal used on oil mist-lubricated rolling element bearings is shown in Fig. 4; a similar device is used to seal conventional oil-splash lubricated bearings. The use of a face seal, along with the other recommendations above (plugging the vent and using balanced oilers), will prevent the entry of all EXTERNAL contamination into the housing.

Beware of Non-Obvious Sources of Contamination and Insufficient Lubrication

Regardless of whether the bearing housing is sealed or not, the serious limitations of the oil rings ("slinger rings") shown in Fig. 1 also need to be addressed, as they can be a source of INTERNAL contamination. Operating oil rings on rotating shaft systems that are not horizontal will cause the bronze slinger ring to spin and rub against the low side of the housing, resulting in severe wear on the ring. The resulting bronze particles can clearly damage the bearings.

Beware of oil sumps with incorrect oil viscosity, or with varying depths of oil ring immersion, or incorrect roundness or rough surface finish of the slinger ring. All of these conditions can result in insufficient lubrication from the oil ring.

Not all versions of "constant level" oilers will serve the reliability-focused user well. The oil level below the reservoir bottle of most constant level lubricators is contacted by ambient air (Fig. 2). In these devices, a wing nut adjustment sets the height of the transparent bottle. The oil level near the tip of the wing nut (Point "B" in Fig. 2) is contacted by the surrounding (ambient) air. An increasing gas temperature (usually air, or an air-oil mixture) in the bearing housing ("A" in Fig. 2) tends to elevate pressure in the bearing housing. This elevated pressure drives down the oil level in the housing (arrows in Fig. 2), increases the oil level in the narrow annular space above the tip of the wing nut, and can result in overflow of oil from the annular space onto the ground. When this happens, bearings starve for oil and will be quickly and permanently damaged.

Pressure-balanced oilers (Fig. 3) decrease downtime risk (Ref. 3). They differ from the non-balanced type by incorporating an external pressure balance pipe so as to make sure that the pressure inside the bearing housing and the pressure at the tip of the wing nut in the constant level lubricator are always identical. Consequently, the oil in the bearing housing is pushed downward by the hot gas (air) with the SAME pressure that is pushing downward on the oil in the oiler, and there is no change in the oil level.

Bearing protector seals can greatly improve the cleanliness of the lubricating oil and extend the life and reliability of the rotating equipment. However, bearing protector seals serve no purpose if oil contamination originates with oil ring inadequacies or if used with unbalanced oilers, or if the oil is not kept at the proper oil level.

On the other hand, bearing protector seals clearly WOULD have helped prevent the large amount of water ingress into the oil of the bearing housing being drained below in Fig. 5.

Lip Seals vs. Rotating Labyrinth Seals

In today's economy, many people ask "How can you justify spending $150 on a rotating labyrinth seal, when a lip seal costs only $5?" The answer to this question requires us to look at the Total Cost of Ownership, not just the cost of the seal.

Lip seals will seal only while the elastomer material (the lip) makes full sliding contact with the shaft (Fig. 6, upper portion). Operating at typical shaft speeds on process pumps, lip seals show leakage after about 2,000 operating hours (Ref. 4). To prevent contaminant intrusion, one would have to replace lip seals just before they fail --- four times per year, to be safe. In sharp contrast, modern rotating labyrinth seals incorporating the features seen in the lower portion of Fig. 6 (and also in Fig. 8) have been available and operating since 2004. As of this publication date, not one of the many thousands now running has been reported to have failed in operation. Comprehensive statistical and probabilistic assessments using Weibull and WeiBayes analyses have been conducted. One analysis included the few "failures" that occurred during installation, yet still predicted a component life in excess of ten years. It would thus be very conservative to assume a four-year life before opting for an early and purely precautionary change-out of the various O-rings in the labyrinth seal. It should be noted that, in modern bearing protector seals, these O-rings are fieldreplaceable, whereas in the old-style seals of Fig. 7 they cannot be replaced by the user.

A comparison of the Total Cost of Ownership between a lip seal and a labyrinth seal will prove revealing. Our comparison assumes a cost of $10 for two lip seals (with 4 changes per year) vs. $300 for two rotating labyrinth seals (with one change every 4 years); in each case, the cost of maintenance labor would be $500 per event. Lip seal replacements would cost ($10+$500)*4 =$2,040 per year, or $8,160 over 4 years. Rotating labyrinth seals would cost ($300+$500)/4=$200 per year, or $800 over 4 years. The cost of ownership of the rotating equipment with lip seals is about TEN TIMES the cost of ownership with modern rotating labyrinth seals! By using the available O-ring replacement kit (about $30 each), the cost of ownership of the labyrinth seals can be reduced even further.

Rotating Labyrinth Seals: How They Work and How They Differ

Findings of rapid payback and quantifiable failure reductions are supported both by industry statistics and the failure rate plots issued by several lip seal manufacturers. For decades, lip seals have been out of compliance with the minimum requirements stated in the widely accepted API-610 industry standard for centrifugal process pumps (Ref. 5). Indeed, most rotating labyrinth seals are a good choice for bearing protection and will generally outperform lip seals by wide margins.

It must be realized, however, that there are many types and designs of rotating labyrinth seals. Different configurations will allow anything from a minimal amount of "breathing" and virtually zero oil leakage, to a rather significant amount of breathing and worrisome leakage. The amounts of breathing and leakage depend very much on the design and construction features of a given brand and must be compared against the construction features of another design or brand.

Before deciding on a brand, a value-focused buyer will require potential vendors to provide test data and cross-sectional views that disclose the operating principles of different versions of bearing protector seals. Reliability professionals are not asking for the disclosure of proprietary manufacturing drawings; however, they are entitled to see exactly what they are about to purchase. Some vendors may refuse or are unable to provide data other than marketing claims and anecdotal references, and they deserve neither one's time nor active consideration.

When looking at different labyrinth seal designs, note that certain old-style designs incorporate specially contoured rotating lip seals which slide on a stationary component (Fig. 7, right).

Other seal designs are fitted with an O-ring that moves radially in and out of a groove (Fig. 7, left). Some manufacturers use these O-rings only to make the seal into a cartridge-style assembly and have ignored the consequences of an O-ring contacting simultaneously the sharp edges of the rotating and the stationary elements of the seal.

Available area of contact is important in O-ring devices; basic engineering principles tell us that pressure equals force divided by area of contact. When we apply a given force to a large area vs. applying the same force to a small area, the resulting contact pressures will differ in proportion to the ratio of the contact areas. Sliding one's finger over the sharp edge (small contact area) of a knife will have more pressure (and do more damage!) than sliding one's finger over the dull back of the knife (large contact area). The sharp area of contact of the old-style labyrinth protector design shown above in Fig. 7 (left diagram) is much more likely to damage the O-ring seal than the large, smooth area of contact shown in the design in Fig. 8

The rotating labyrinth bearing protector seal of Fig. 8 uses two O-rings to clamp the rotor to the shaft. This makes it considerably more stable than seal designs that use a single O-ring as a clamp for the rotor (Fig. 7). There is more stability with two clamping O-rings and the risk of rotor skewing or "walking" is reduced. If the dynamic O-ring of Fig. 7 were to make contact with the grooves in the stator, undesirable frictional heat would be generated, and O-ring degradation would take place. O-ring degradation (wear) is sometimes observed as "black oil."

Certain modern bearing protector seals are engineered hybrids (Fig. 9) that incorporate both the face-contacting features of a lip seal with the generous wide-contact and shut-off valve features of the modern rotating labyrinth seal in the lower portion of Fig. 6. The lip seal shown in Fig. 9 provides excellent oil retention while sealing on an internal shaft sleeve to prevent damage (fretting) to the equipment shaft. Meanwhile, the outboard labyrinth and shut-off valve keep out water and airborne particulates, which are a lip seal's worst enemies. This design gives the user the "best of both worlds".

Best Lube Application Practices Examined Plant-wide oil mist lubrication systems have proven their superiority since the late 1960's (Fig. 10 and Ref. 6), with demonstrated reductions of pump bearing failures from 80 to 90% (Ref. 7).

The advantages and disadvantages of oil-mist lubrication as compared to wet sump lubrication may be summarized as follows:

Advantages:

•Reduced bearing failures of 80 to 90%.

•Lower bearing operating temperatures of 10 to 20 F.

•Flushing-off bearing wear particles.

•Slight positive system pressure eliminates contaminant entry.

•Reduced Energy costs of 3 to 5%.

•Reduced oil consumption of about 40%.

•No moving parts

Disadvantages:

•Capital investment

•Cost of compressed air

We had earlier commented on the demonstrated vulnerabilities of oil application methods that depend on oil rings. While it is acknowledged that oil rings are satisfactory as long as shaft peripheral speeds are neither too slow nor too fast (Ref. 3) and as long as shaft horizontality, ring immersion, ring eccentricity, bore surface finish and lube oil viscosity are well controlled, reliability-focused thinking has led to a re-examination of the vulnerabilities of oil rings.

Elastomeric oil slinger DISKS (Fig. 11) are being used by many equipment owners to replace the oil slinger RINGS. The disk is attached to the shaft with set screws, and eliminates ALL of the previously discussed problems with oil rings involving the shaft being out-of-level, too much or too little oil in the sump, oil viscosity, out-of-roundness, and surface finish roughness.

Summary

Rolling element bearings are precision components which require a very clean film of lubricant in the appropriate amount (neither too much nor too little) in order to provide rotating equipment reliability and long life. Modern bearing protectors can both prevent the entry of contaminants, as well as the loss of lubricant. Two types of modern bearing protectors are now available: contacting face-seals, and rotating labyrinth seals. The payback period for a modern bearing protector, as compared to a lip seal, can be as fast as 4 or 5 weeks, after which it begins saving the equipment owner asmuch as $1,000 to $2,000 per year in avoided maintenance costs.

References

1. TRICO Manufacturing Corporation, Pewaukee, WI, Commercial Literature Also, see Ref. 3, pp. 118, 144, 232, 234

2. Charles, Jacques; Les Ancient Papiers de l'Academie Francaise, ~1787

3. Bloch, Heinz P. and Allen Budris; "Pump User's Handbook--Life Extension," 2006, Fairmont Press, Lilburn, GA 30047; ISBN 088173-517-5

4. Brink, R. V., Gernik, D. E. and Horve, L. A. "Handbook of Fluid Sealing, "1993 (McGraw-Hill, New York).

5. American Petroleum Institute, Alexandria, VA, API-610, "Centrifugal Pumps", 10th Edition, 2009

6. Bloch, Heinz P.; "Practical Lubrication for Industrial Facilities," 2nd Edition (2009), Fairmont Press, Lilburn, GA, 30047 (ISBN 088173-579-5)

7. Bloch, Heinz P. and Abdus Shamim; "Oil Mist Lubrication--Practical Applications" (1998), Fairmont Press, Lilburn, GA, 30047 (ISBN 088173-256-7)

8. TRICO Manufacturing Corporation, Pewaukee, WI, Commercial Literature, Also, see Ref. 3, pp. 126, 232, 238

Heinz P. Bloch is a professional engineer with offices in West Des Moines, Iowa. He advises process and power plants worldwide on reliability improvement and maintenance cost reduction opportunities. Heinz is the author of 17 full-length texts and over 400 papers and technical articles. His most recent texts include "A Practical Guide to Compressor Technology" (2006, John Wiley & Sons, NY, ISBN 0-471-727930-8); "Pump User's Handbook: Life Extension," (2006, Fairmont Publishing Company, Lilburn, ISBN 0-88173-517-5) and "Machinery Uptime Improvement," (2006, Elsevier-Butterworth-Heinemann, Stoneham, MA, ISBN 0-7506-7725-2)

Chris Rehmann is Marketing Manager for AESSEAL's North American operations. He holds a BS in Electrical Engineering from the University of Notre Dame. Prior to that, Chris worked for Schlumberger, an oilfield engineering firm, for 15 years, holding positions in field engineering, technical sales, and management in the USA, Middle East, and Asia-Pacific. He joined AESSEAL in 2002, moving his family from Saudi Arabia to Knoxville, Tennessee. Chris has taught several courses and authored a number of technical papers dealing with bearing protection on pumps, electric motors, oil mist, and gear boxes. He can be reached at: AESSEAL, Inc., 355 Dunavant Dr., Rockford, TN, 37853, http://www.aesseal.com  or 865-531-0192.