By Allan R. Budris, P.E.
In past columns, I've addressed various issues related to pump reliability, with lubricant contamination often taken for granted; however, being the second leading cause of pump failures (with mechanical seal contagion being the first), it can have a marked impact on bearing life.
Lubricant contamination by air, solid particles and moisture is often overlooked and must be addressed by reliability-focused users. Fewer than 10 percent of all ball bearings run long enough to succumb to normal fatigue. Most bearings fail at an early age because of static overload, wear, corrosion, lubricant failure, contamination, and/or overheating, with lubricant contamination being one of the major factors. Even in small concentrations, air, wear particles and moisture can cause extremely serious oil contamination.
The potential sources of lubricant contamination are many. Except for the occasional dislocated oil ring, forced spray or sandblasting action directed against pumps, both moisture and airborne dirt typically enter the bearing housings by the breathing effect of the air existing above the lubricating oil. Atmospheric air contacts not only the oil surface in most bearing housings, but also the oil in the surge chamber of unbalanced constant level lubricators.
TYPES OF LUBRICANT CONTAMINATION
Lubricant contamination originates from a number of possible sources and can be a factor in "unexplained" repeated failures. Lubricant contaminants are everywhere, from the oil purchase and storage to the exchange of internal and external air ("breathing") to contaminants created within the pump bearing housing. Typical contaminants can be in the form of:
Oil cleanliness deserves our utmost attention. We should learn to focus not only on initial water content but on oil cleanliness in general. Clean oil greatly extends the life of bearings and equipment, but most oils are not clean. They may not look dirty to the unaided eye but will often contain a profusion of particles in the 2-30 micron (0.0001-0.0015 inch) range of diameters. The particles may consist of fibrous, metallic or sand-like contaminants. With water as the catalyst, these contaminants form sludge, which is harmful for pump bearings in many facilities. Bearing and pump equipment manufacturers believe that purchasing ultra-clean oils will increase bearing life from 300 to 500 percent over oils with barely-passing cleanliness values.
AIR CONTAMINATION/AIR ENTRAINMENT
Air entrainment (bubbles) in lubricating oil can create layers of foam leading to the overflow of the reservoir. This can also cause excessive bearing wear by decreasing the load-carrying ability of the lubricant film, which can lead to shorter bearing life. Further, it can also cause premature oxidation and/or lower lubricant flow to the bearings.
Solutions to these air-related problems include:
- Increasing reservoir temperature (However, use caution not to impact oil life; keep oil temperature below 140°F, unless synthetic oil is used.)
- Maintaining proper oil level in the bearing housing
- Inspecting and tightening all joints and fittings
- Carefully adding silicon antifoam (only in turbulent systems with supplier assistance)
- Filtering the oil, changing out the oil at regular intervals and considering decreasing viscosity one grade
- Checking the proper operation of vents
- Replacing clogged filters
- Not using synthetic fiber filters less than 2 microns
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Unless pumps are provided with suitable bearing housing seals, breathing takes place during alternating periods of operation and shutdown. Bearing housings "breathe" in the sense that rising temperatures during operation cause air volume expansion, and decreasing temperatures at night or after shutdown cause air volume contraction. Open or inadequately-sealed bearing housings promote this back-and-forth movement of moisture-laden and dust- containing ambient air.
This introduction of moisture-laden ambient air can lead to:
- Premature failure or excessive wear due to reduced lubricating ability, as shown in Figure 1 (which shows the impact of a relatively small amount of water contamination on basic bearing life)
- Corrosion of bearings or other machine elements
- Premature oxidation of lubricant
Actions preventing water contamination include these actions:
- If possible, hermetically seal the bearing housing.
- Install a moisture detection unit (see Figure 2).
- Inspect heat exchangers, steam coils and/or packing for leaks.
- Do not allow water to fall onto equipment; build shelters.
- Use desiccant air-filter breathers on vents to reduce condensation.
- Train operators in the proper use of cleanup hoses.
- Keep hatches and covers on reservoirs closed.
Moisture Detection Unit
DIRT: THE MOST DESTRUCTIVE CONTAMINANT
Abrasive dirt (introduced with the oil fill, sucked in with the ambient air or generated within the bearing housing - such as from oil ring wear or rust) causes increased wear. The best way to stop dirt particle contamination is to prevent it from entering the lubricant through these actions:
- Install proper storage and handling.
- Keep reservoir hatches closed.
- Use air filter-breathers on all reservoirs.
- Clean reservoirs before installing new oil.
- Filter all oil introduced in reservoirs/sumps with the use of a filter cart or off-line filtration.
ARGUMENT FOR SEALING BEARING HOUSINGS
As discussed above, when the volume of air inside the bearing housing warms up and expands, some air is expelled along the shaft protrusion and through the housing vent into the atmosphere. Later, the housing temperature declines, and the remaining air within the bearing housing contracts; this causes unclean atmospheric air to re-enter the housing along the same two pathways.
Ideally, housings should not invite breathing and the resulting contamination. There should be little or no interchange between the housing interior air and the surrounding ambient air. The breather vents can often be removed and plugged.
Modern and technically-advantageous versions of bearing housing protector seals should be used for both the inboard and outboard bearings. Lip seals are not good enough, and neither are outdated rotating labyrinth seal designs. Instead, consider hermetically sealing the bearing housings with magnetic shaft seals and closed system (balanced) oilers with no vent plugs; this will keep out moisture and dirt, which can greatly extend bearing life.
OTHER MEANS TO PREVENT BEARING LUBRICANT CONTAMINATION
Another (generally superior) way to preclude the introduction of contaminated ambient air is to install an "oil mist" system (see "http://www.waterworld.com/articles/print/volume-24/issue-12/departments/pump-tips-techniques/how-to-get-more-from-pump-preventive-maintenance.html">How to Get More From Pump Preventive Maintenance," Feb. 2008). Oil mist provides fine droplets of clean, fresh and cool lubricant to the bearings. Contaminants are excluded from the bearings by the oil mist pressure inside the bearing housing. The mist can also be supplied to the bearings when the pump is idle for maximum bearing protection from contamination and condensation. Oil-jet bearing lubrication may also be an ideal option to avoid oil contamination on some pumps, when available.
Unless the lubrication oil in a pump's bearing housing is kept clean, optimum reliability cannot be achieved. Even the most outstanding lubricant cannot save a bearing unless the oil is kept clean. Likewise, bearings lubricated with "clean" oil will live at least twice as long as bearings that are lubricated with oil of basic cleanliness.
About the Author: Allan R. Budris, P.E., is an independent consulting engineer who specializes in training, failure analysis, troubleshooting, reliability, efficiency audits, and litigation support on pumps and pumping systems. With offices in Washington, N.J., he can be contacted via email at firstname.lastname@example.org.