A well-known technique for increasing pump reliability is sealing the bearing housing with non-contacting Bearing Isolators over less expensive contact seals. Because contact seals use contact as their sealing method, they have a finite life expectancy as they will either wear at the point of contact or grove the shaft. When this occurs, lubricant will escape to atmosphere and contaminants will enter the bearing housing, leading to bearing failure.
Though more expensive, Bearing Isolators effectively retain lubricants and exclude contaminants while having the distinct advantage of a virtually infinite life expectancy due to their non-contacting nature. This is vitally important as the expectation of increased Mean Time Between Repair (MTBR) has made the effectiveness of contact seals problematic at best.
The most common perception of bearing housing seal failure on process pumps is lubricating oil leaking out of the bearing housing. For most operators, the analysis is quite simple: no leaking oil means the seal is fine while leaking oil equates to failure. Though true for contact seals, the presence of leaking oil from a Bearing Isolator is most likely caused by other factors and not seal failure.
In order to enjoy the benefit of increased reliability from Bearing Isolators, reliability professionals must pay particular attention to some of the more common causes of Bearing Isolator lubricant leakage on process pumps.
TOO MUCH OIL
It seems simple, but the greatest cause of Bearing Isolator leakage on process pumps is an over-filled bearing housing. It has become common practice for maintenance professionals to fill up to, if not a bit over, the maximum fill line. The thinking is that if leakage occurs, there will be extra lubricant available. Unfortunately, this practice can contribute to leakage. Fortunately, once returned to the proper level, Bearing Isolators will generally stop leaking and return to normal function. There may be some oil leakage as the seal clears itself of excess lubricant, but that should go away over time.
Most Bearing Isolators have a lubricant return designed into their respective labyrinth patterns. This return needs to be installed at the bottom-dead-center or six o’clock position of the Bearing Isolator for proper function. This allows oil to easily return to the sump. One of the most common causes of improper seal orientation is a lack of training or unclear instructions given to the installers.
OBSTRUCTED LUBRICANT RETURN PATH
Most modern Bearing Isolators are effective at collecting splash lubricant in their respective labyrinth patterns. The issue is that once they have collected the lubricant they need someplace to return it to. Bearing isolators need a clear, unobstructed path to return collected lubricant back to sump. For example, the return patch to sump may be blocked by counter-bores in the housing originally designed to provide a positive stop for pressed-in lip seals. The area between the bearing and the bearing housing seal may lack or have an inadequate drain channel. When this occurs, lubricant will accumulate in this area until the space becomes completely flooded and the seal leaks. To solve this, the area between the bearing and the Bearing Isolator must include an unobstructed return pathway to sump.
Reliance of the lubricant draining to sump only through the bearing will likely result in lubricant leakage.
IMPROPERLY APPLIED EXTERNAL OILERS
External oilers are extremely sensitive to position, and must be installed on the proper side of the housing relative to the direction of shaft rotation. Properly follow the manufacturer’s guidelines. Oilers must also be installed square and straight and not angled in any direction. The pipe connecting the external oiler to the bearing housing must also be sufficiently ridged as to prevent vibration or shaking of the oiler. Questionable installations of external oilers may easily result in over-filling of the bearing housing and subsequent lubricant leakage.
The forceful flow of air over a bearing housing can cause lubricant leakage by creating a pressure differential between the inside and outside of the bearing housing. Some common designs of external oilers are likewise sensitive to air flow, though this is not as big of an issue with newer models. Couplings and external cooling fans attached to pump bearing housings are a potential source of harmful air flow. Gapless, solid coupling guards that completely enclose the bearing housing seals with little or no gap around the bearing housing may induce leakage. While taking all required safety precautions into mind, having some of the coupling and fan guarding accomplished by tight grating, rather than solid surfaces, allows for better air flow and helps prevent pressure from building up.
IMPROPER NON-CONTACT SEAL SELECTION
Some Bearing Isolators are designed specifically for grease lubrication, others for oil or oil mist. There are even some designs that can handle all lubrication types in a single design. In some instances, benefits can be achieved by designing Bearing Isolators for specific applications rather than relying upon standard catalog items. For example, in pump bearing housings with a very high degree of lubricant splash, designing the labyrinth pattern to communicate directly with the lubricant return path can greatly increase effectiveness. Experienced Bearing Isolator providers can design engineer-to-order seals quickly and economically, and assure the seal design addresses any concerns and is applied to provide the best reliability possible. When in doubt, always consult an experienced Bearing Isolator professional. Time spent on those up-front engineering chores is well worth the effort, and assures the Bearing Isolators will perform as intended, freeing you from the chores of subsequent failure analysis.
The advantage of non-wearing Bearing Isolators is that once properly applied, they perform essentially trouble-free for years with no degradation in performance. The challenge is that they require a bit more attention to application details. Taking the time to check a few simple parameters will go a long way towards ensuring trouble-free operation. Properly applied Bearing Isolators are truly an install-and-forget it proposition, so long as they are in fact properly applied.
About The Author
Neil Hoehle is chief engineer for Inpro/Seal, LLC™, a Waukesha Bearings® business, and can be reached at 309.756.5335 or email@example.com. Hoehle has spent over thirty-five years in the design and development of non-contact bearing housing seals, air-purged sealing, and other rotating shaft sealing devises and holds several related patents. During his tenure at Inpro/Seal, Neil has worked in applications engineering, R&D, field service and sales, and operations management. Mr. Hoehle holds a BA from Western Illinois University and an MBA from the University of Iowa. For more information, visit www.inpro-seal.com.
MODERN PUMPING TODAY, October 2017
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