Lube-oil cleanliness critical to long bearing life – Combined Cycle Journal

Lube-oil cleanliness critical to long bearing life

Special to CCJ ONSite by Brian Hulse, BDHulse Consulting Services LLC

“We have met the enemy and he is us.”

Walt Kelly wrote those words for Pogo to speak in his comic strip, marking the first annual observance of Earth Day back in April 1970. He was making the observation that pollution of the planet was mankind’s self-inflicted wound.

Listening to a presenter at the Western Turbine Users’ 2018 annual conference talk about bearing failures in General Electric LM engines (aeroderivatives) and how root causes are difficult to nail down because of the extensive damage suffered in these events, I couldn’t help but hear Walt’s/Pogo’s prophetic and accurate words in my head. Sadly, about nine times out of 10 we kill our bearings with the same thing Pogo was lamenting that day so long ago—trash.

LM bearings are designed for a service lifespan of approximately 28-billion revolutions (about 50,000 operating hours) in a very harsh environment. To successfully make that journey, they have to be (1) free from manufacturing defects, and (2) operated and maintained as designers intended.

A manufacturing problem usually shows itself within the first 10% of the design service lifespan (about 5000 operating hours in this case). Bearing designers call this L10. If a bearing survives its L10, statistically it has made it out of the “infant mortality” window, is presumed free of manufacturing defects, and should be capable of meeting the full design lifespan.

Assuming an LM bearing fails after the 5000-hr threshold, an operations and/or maintenance oversight is likely the cause. If we make one more assumption—that the engine has not been transported recently—thus eliminating the possibility of mechanical damage to the bearings, we’ve almost guaranteed that.

Important to the discussion is that GE aeros use internal air pressure to help offset shaft thrust in the machine. That air pressure, combined with the mechanical load-carrying capacity of a single ball bearing, balances the thrust loading of the shafts. Air-pressure balance must be monitored and, from time to time, as engine seals degrade through normal wear-and-tear, the air pressure must be adjusted. Failure to perform this maintenance task will usually result in bearing failure.

If we have operated the engine properly and within all limits, are out of the bearing infant- mortality window, haven’t transported the engine recently, have been monitoring recoup curves, and then had a bearing failure, it’s highly likely the cause is a trash problem, or more appropriately in the tribology vernacular, contamination.

How could this have happened? Lessons learned over the years point to these Top 10 ways contamination can kill your bearings:

No. 10, Mixing lubricants

Most plants store their lubricants in one place. Any one or more of the following factors could lead to a mixing event—possibly one with serious repercussions: an inattentive moment, a reshuffling of the drums, similarly painted drums, lack of or poor quality of training, poor lighting at night, colorblindness, the pressure of time.

No. 9, New-oil contamination (Part 1)

Although the number of confirmed events is low, it does happen. Metal wastage from the drum manufacturing process, fill process, and other steps can sometimes leave hard particles in a fresh drum of oil. Most plant facilities do not have filters on their drum pumps or whatever means they use to draw oil from their drums.

No. 8, New-oil contamination (Part 2)

Oils typically are delivered to powerplants in drums. Some facilities have covered drum storage areas, some do not. Even with a cover, rainwater, dust, etc, can collect on the tops of drums. If the drum top is not cleaned prior to opening, when the bung is cracked in it goes. Once a drum is opened and in use, this scenario can play out ever more easily. Keeping the drums and surrounding area clean is an ongoing battle, and the fighting is relentless.

No. 7, Planned/unplanned major maintenance

One of the major strengths of LM machines is their ability to facilitate major maintenance onsite. Whether a combustor change-out, a hot-section exchange, or a TMF (turbine mid frame) replacement, the work can be done at the plant. However, these maintenance activities also expose the bearing cavities and bearings, and can allow contaminants to enter the system. The need for vigilance during maintenance cannot be overstated.

No. 6, Routine maintenance

Filter-element replacement? Leaking hose? Damaged fitting? Checking finger screens? Any one of these routine tasks can introduce contaminants into the system and, ultimately, cause bearing failure. Anytime the system is cracked open, a cleanliness plan should be thought out and implemented to ensure protection against contamination.

No. 5, Filter-element selection (Part 1)

There are several filter-element suppliers in the marketplace, and they offer a wide array of alternative selections that may physically fit in the housings supplied by your packager. Although most supply-chain folks would argue, cheaper is not always better. Understanding the ins and outs of filter specifications and capabilities are a technical minefield, and changes in suppliers or supplier part numbers should be made only under engineering advisement from your team. Trusting the supplier is not enough: Keep those honest folks honest.

No. 4, Filter-element selection (Part 2)

From time to time, gas turbine OEMs (GE for LM aeros) will provide guidance via a service letter or product bulletin, or some other established vehicle, regarding filtration requirements. Keeping abreast of OEM guidance is critical to successful operation and maintenance of these machines. You may think you have all your ducks in a row and are on your “A” game, but the only thing that is constant is change and things can (and do) change quickly with these engines.

No. 3, Poor or no system flush after an event

When a bearing failure does occur, debris is washed into the system. Thus, prior to starting a replacement machine, it is imperative that a thorough flush of the system be performed. To accomplish this, after the tank is opened, cleaned, inspected, and closed, a temporary pump/filter unit is installed in the system and oil is circulated at a velocity higher than that of the system when in normal operation. The oil may or may not be heated. Increasingly fine filter elements are used in the temporary unit until the target cleanliness is achieved. The filter elements usually are removed from the installed housings to focus all of the filtration activity on the temporary filters. 

No. 2, Housekeeping

Poor housekeeping—especially on the tank top and in the surrounding areas of the package— can be a potent contamination source, both during normal operating periods and especially during top-offs. A bit of dirt can be brushed off or blow off a horizontal surface and slip into the tank before anyone sees it.

No. 1, Topping off  

Most plants use one set of tools—funnels, containers, etc—versus a dedicated set for each type of oil for transferring the fluid from its storage location to the point of use. This can lead to oil degradation through mixing over time. Many plants use open containers (buckets) to move oil. It’s uncommon for an oil storage location to be equipped with any kind of proper cleaning station that allows/encourages the technicians to keep the tools clean. With radial bearing clearances as low as 0.0017 in. cold, it does not take a large particle to cause damage when the bearing is at operating conditions.

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