By Michelle Froese | Senior editor
Windpower Engineering & Development
Oil has been called the “lifeblood” of a wind turbine, and for good reason. Proper lubrication mitigates friction between key components, such as the bearings in a gearbox, and keeps a turbine running reliably. What is less well known is that lubricants also help control equipment temperature and transport dirt and other debris away from the friction surface.
To endure the variable load, speed, and temperature conditions under which gearboxes operate, regular oil analysis and the occasional change are critical to a long operating life.
However, maintenance checks and oil changes are costly and labor intensive. Turbine downtime halts production, but a trip up tower also poses a safety risk to any wind tech. So the wind industry has developed certain mitigating strategies to lengthen lubricant performance. Some are more effective than others.
Extending drain intervals
The cost of a lubricant alone for a complete “drain, flush, and fill” for a mid-sized turbine can exceed $5,000. Then, there are the overhead charges for the O&M crew, transport, and potential crane call-out. It is no wonder that extended oil drain intervals are of interest to wind-farm operators looking to lower high maintenance costs.
“A typical wind-turbine lubricant will have an oil drain interval of 36 months, but more advanced synthetic lubricants formulated specifically for wind can help extend those intervals,” shares Gary Hennigan, a National Account Executive with ExxonMobil. The key to maximum performance is choosing a gearbox lubricant that is correctly specified for a wind-turbine’s operating conditions. A poor lube choice or ignoring oil cleanliness because of a desire to extent drain intervals means a turbine’s gearbox will likely fail to function properly or with much longevity.
In fact, Hennigan says that when selecting gear oil, one of the least considered factors is a balanced formulation. “Equipment performance depends on using lubricants developed with a balanced formulation approach, which means using optimal base stocks and a tailored additive package that meets the specific needs of the wind turbine.”
Take micropitting, for example, which can form on surface-hardened gears within the first several hours of operation if a gearbox is not properly lubricated. The result is reduced gear tooth accuracy.
“To mitigate this effect, operators should look for oils formulated with a micropitting additive package, such as conventional extreme pressure additives.” Hennigan says it is also important to employ a gear finish (a surface finish on gear teeth), as specified by American Gear Manufacturers Association’s AGMA 6006 standard.
“Furthermore, an oil formulated with advanced base fluids that provide a high viscosity index – typically 160 or higher – and lower traction co-efficient, also helps. The higher viscosity index provides a thicker lubricant film at operating temperature while the lower traction co-efficient helps increase energy efficiency,” he adds.
Top treating oil
Additive top treating gearbox oil is another trend that has developed in the wind industry to mitigate lubricant degradation, extend oil life, and potentially save costs. Wind operators identify when an oil’s additives start to deplete and then “re-additize” that oil with the addition of an after-market formula.
The concern with this approach is that additive top treating may introduce new contaminants that could impact equipment performance. For example, surface active additives, such as anti-wear additives and rust inhibitors, compete for space on the metal surfaces in a gearbox. Formulating an oil so that both of these additives are present in the correct amounts is a delicate balance not typically achieved by top treating.
Hennigan adds that topping up oil with different ratios or types of these additives could cause more harm than good in the long run. “Top treating may lead to an uneven distribution of additives, rendering the lubricant less effective than originally formulated,” he said. Baking a cake provides an analogy.
“It is like forgetting to include vanilla extract in the mix, and then deciding to add it after the cake is baked. Even if you tried to douse the cake in extract, it would not taste right and result in an uneven distribution of flavor.” He says if you want to bake a quality cake, it is necessary to include all the ingredients in the correct order from the beginning. Quality lubrication is a similar process.
“Granted, while topping up with additives may not be as invasive as flushing and completely replacing gear oil, regular top treating actually requires operators to increase site visits and equipment interaction — which in turn increases the potential for safety issues,” explains Hennigan. “The longer you can rely on your lubricant to perform reliably in-service without the need for maintenance, the better for your staff and operation.”
This means wind operators should instead use a lubricant with a balanced formulation, including the right mix of advanced base oils and additives, to ensure long-lasting performance. “High-quality formulations are designed to protect equipment from common issues such as scuffing, micropitting fatigue, and rust and corrosion. A proper formulation also ensures protection at extremely high temperatures and good reliability at low temperatures.”
Some oils today have been proven to perform reliably for up to seven years, says Hennigan, demonstrating the capability to protect the machine even after 60,000 hours in service.
Analyzing for contaminants & debris
Lubricant performance can deteriorate over time, particularly under the harsh and variable conditions to which wind turbines are typically exposed. If the lubricant is inadequate to protect turbine gears, such as when gusts produce load spikes, the oil may lack the consistency to do its job and lead to component failure and turbine downtime.
To prevent such events, effective and regular oil-analysis monitoring is essential because it allows tracking the condition of equipment and lubricants, and may lead to longer intervals between oil changes. An oil analysis assesses the condition of the lubricant to ensure it has maintained the correct viscosity and necessary additives for its application. Importantly, it also checks for contaminants in the oil.
For example, water is one of the most critical contaminants in a wind-turbine gearbox. Even small amounts can significantly shorten gear, bearing, and oil life.
“To obtain the greatest benefit from oil analysis, it is imperative to work closely with an expert lubricant manufacturer and conduct an oil analysis typically every six months,” says Hennigan. Although, he points out, particle detectors are now available to continuously monitor gearbox oil for contaminants.
“These sensors look for debris in oil. However, some wear debris sensors are limited to the size of the particles they can detect, so it is important to work with an expert that knows your equipment,” he says. The most damaging particles are less than 10 µm in size, which are small enough the pass through standard oil filtration.
Fortunately, improvements in analysis techniques and particle sensors can help mitigate potential damage to gearbox components and keep wind turbines running smoothly.
“Instead of cutting corners, say by extending oil analysis intervals or top treating oil, take the proper steps to mitigate potential problems from the start. This means finding the right oil formulation for your wind turbine and the conditions it must endure and analyzing oil on a regular basis.”
When choosing a wind-turbine lubricant, formulation matters. To optimize gearbox performance, look for advanced synthetic lubricants that are formulated with base stocks and additives that offer:
- High performance in extreme temperatures
- Enhanced oxidation and water resistance
- Protection against wear and micropitting
- Long equipment life, and
- Energy efficiency benefits