By Saurabh Lawate and Michelle Graf, The Lubrizol Corp.
In the 1600s, the author Miguel Cervantes’ hero, Don Quixote, charged at windmills in Spain. Fancying himself a knight, Quixote was under the illusion that the stubby, multi-bladed giants were evil-doers. Wind technology has come a long way since then. Currently, about 74,223 MW of electricity is generated by wind power worldwide.
Thanks to design changes, wind turbines have substantially increased power generating capabilities in the past few years. These design changes have included longer blades, taller towers and improved gearboxes. Improved mechanical designs and technology, lubricants and maintenance have also enabled growth.
Don Quixote would be thunderstruck if he wandered into a turbine farm today. If your business is dealing with these modern-day giants, however, you could likely be more overwhelmed than thunderstruck by the mechanical and operational issues presented by these large machines.
Mechanical and Operational Issues
Wind turbine maintenance experts are charged with two main issues: unusual stresses and limited accessibility to major components.
Because the turbine blades are connected directly to the gearbox, the wind-driven blade-rotor assembly subjects the turbine’s internal components, such as the gears, bearings and shaft, to unusual stresses. These stresses are taken into account during the design process of the turbine and its components. Unfortunately, the stresses encountered in the field have often been higher than those anticipated during design, resulting in gearboxes with shorter-than-expected lifetimes. In an effort to further improve wind turbine design, researchers are continually trying to better understand the loads to which turbines are subjected. The gearbox is both an expensive and a heavy part of the turbine, so replacing or repairing it are challenges. The cost of the gearbox, its installation and the associated downtime can be expensive and has a direct affect on the turbine’s return on investment.
Typically, the gearbox is not easily accessible. It’s in the turbine’s nacelle, which houses all of the mechanics. On the largest machines the nacelle is hundreds of feet in the air, at the top of the turbine tower. To make matters worse, turbines can be located in remote areas, including offshore, making access difficult and costly. Offshore repairs and maintenance, of course, are even more difficult and costly than on-shore installations.
The bottom line is: Operators need to keep wind turbines running. Relying on both preventive and predictive maintenance practices that include lubrication analysis can be a major factor in accomplishing that goal.
To keep equipment running, maintenance experts recommend that operators obtain oil samples periodically so that the oil’s properties can be analyzed and the results used to develop a maintenance plan. Typically, oil properties such as viscosity, additive concentration and the presence of metal particulates are good indictors of the oil’s (and by extension the gearbox’s) condition. For example, an oil change may be recommended if analysis uncovers anything unusual, such as a significant presence of metal particles. Significant metal particles in the oil indicate that wear has occurred in the gearbox components. Oil analysis allows the operator to catch abnormal wear problems early and keep the gearbox working, avoiding the need for costly replacement.
Fortunately for wind turbine operators, industry standards have been created to assist in lubricant selection and analysis. One such standard is ANSI/AGMA/AWEA 6006-A03. This standard is a result of collaboration between the American Wind Energy Association (AWEA) and the American Gear Manufacturers Association (AGMA). The American National Standards Institute (ANSI) adopted it. The standard was specifically written for wind turbine gearboxes and essentially includes information on every aspect of the gearbox-designing, manufacturing, procuring, operating and maintaining these devices.
The standard’s so-called “Annex F” contains information on lubricant selection and condition monitoring. It includes information on how to obtain lubricant samples and what analyses to conduct on the drain sample. Table 1, adapted from Annex F, includes recommended condemning limits for each test to aid in interpreting results. The best practice is to conduct oil analysis over time and trend the results because every turbine is different and each situation must be assessed based on the individual turbine’s history and circumstances.
Predictive maintenance entails real-time lubricant monitoring using sensors. The necessary lubrication sensors are in addition to any sensors designed to evaluate vibration and other parameters. Predictive maintenance lubrication monitoring requires different sensors to measure characteristics of the gear oil such as viscosity, metal particles, moisture and acid number. Advances in metallurgy and application-specific gearbox design have significantly helped reduce maintenance issues. But using today’s advanced lubricants, coupled with proper preventive or predictive maintenance, also go a long way in reducing overall operational costs.
There are generally five areas where lubricants play a role in preventing problems. (The first two can more immediately affect equipment performance because they are more likely to lead to catastrophic failure.)
- Protecting against gear micropitting
- Preventing bearing failure
- Preventing loss in viscosity grade
- Inhibiting foam
- Preventing or cleaning up dirty parts.
Micropitting damages the gear surface, which is a serious and well-documented failure in wind turbines. Lubricants that provide good micropitting resistance and bearing protection reduce risk of catastrophic failure and therefore reduce operating cost. The typical failure can cost in the neighborhood of $200,000 to $400,000.
The lubricant must also maintain viscosity, not form sludge and provide oxidative and thermal stability. Well-formulated lubricants are shear stable, which means they do not lose viscosity and therefore ensure that the proper lubricating film is maintained. When oil stays in viscosity grade, it reduces the number of oil changes or top-offs to bring the oil into grade. When the lubricant’s viscosity is reduced, it is less able to protect moving parts because the parts have a thinner film protecting them.
Oxidative stability is another issue. Whereas shear stability issues can lead to loss of viscosity, oxidation generally leads to an increase in viscosity and varnish deposits. Proper formulation of the lubricant-the balance of various components-that leads to high oxidative stability, results in a stable and cleaner system. Overall, these factors lead to longer oil lifetimes and better protection of equipment.
Beyond these lubrication challenges, operators must choose between using mineral oil- or synthetic oil-based lubricants. Top formulators understand the properties of both. Although synthetics cost more, they are used in many turbine applications because they generally last longer, provide better oxidative stability and perform better in temperature extremes. Extending the lubricant change interval results in less downtime and labor cost. (Information on lubricant selection is contained in ANSI/AGMA/AWEA 6006-A03, previously mentioned.)
Formulation efforts also must be directed so that key original equipment manufacturer and industry specifications are met. Following are the three most applicable specifications, ordered based on the extensiveness of their performance requirements:
- US Steel 224 is a basic industrial gear oil specification. Oil that meets this specification provides basic protection for gears. Performance aspects include extreme pressure, antiwear, metal corrosion, demulsibility (separation from water) and thermal stability.
- DIN 51517 Part 3 is a more rigorous industrial gear oil specification. In addition to meeting the requirements of US Steel 224, the oil needs to perform well with respect to foam and seal compatibility and bearing protection.
- Flender (which refers to a gear manufacturer) requirements are also often specified for wind turbine gearboxes. Oil must meet DIN 51517 Part 3 and be tested extensively to assure it is compatible with gearbox components, including seals, epoxies and paint used during gearbox construction. Other performance parameters include micropitting resistance and a specialized foam test. Flender approval indicates top-tier industrial gear oil. In addition to the Flender approval, other original equipment manufacturers such as Winergy and Hansen, have their own specifications.
As wind turbines continue to grow in number and scale and as more and more are built offshore, those responsible for keeping them operating will have a variety of solutions in their toolkits. Preventive and predictive maintenance each play a role, as does the use of the proper lubricant and oil analysis.
Authors: Saurabh Lawate is the global commercial manager for industrial gear oils at The Lubrizol Corp. His responsibility includes wind turbine gear oil products and their development. He has held various positions at the company, first in technology and lubricant formulation and then on the commercial side of the business. He also has global commercial responsibility for such environmentally driven products as biodegradable lubricants and non-CFC or HFC lubricants.
Michelle Graf is a project manager for industrial gear and hydraulic oils at The Lubrizol Corp. She has held a number of positions at Lubrizol, most of which have been in research and development where she uses both traditional oil analysis techniques and online oil quality sensors to characterize how oil additives change during use.