Most of the wind turbine gearbox manufacturers have compiled or are in the process of compiling new lubrication specifications. These specifications are more stringent than those for industrial gear applications, and more accurately reflect true operating conditions, including low-temperature conditions. Performance expectations for lubricants used in offshore wind turbines are higher due to demand for extended life. Some new trends and measures include:
- The FAG FE 8 bearing test, which is part of the standard DIN 51517 Part III specification, is required by all gearbox manufacturers. Other bearing tests are under evaluation for inclusion in the new oil specifications as well.
- There is a move toward synthetic formulations (PAO/ester, ester and PAG-based). The synthetic oils offer longer life and so reduce the need for oil changes.
- PAOs (Poly Alpha Olefin) provide excellent viscosity index and low pour point. These properties make them a fluid of choice for applications characterized by wide ranges of operating temperatures.
- PAO/ester Blend. There have been hydrolysis issues (breakdown in the presence of water), making selection of hydrolytically stable products a critical issue.
- PAGs (Polyalkalene Glycol) offer improved resistance to micropitting but have compatibility problems with coatings and seal material.
Dr. Helen Ryan Head of Global Industrial Development at Ethyl Petroleum Additives said, “The tests specified by the gearbox manufacturers are known entities and technology is already in place to meet these requirements. It is the inclusion of the new bearing tests, which are evaluating not only wear on the bearing and bearing cages but also corrosive pitting and staining of the bearings, that will cause a paradigm shift in how industrial gear lubricants are formulated. Preventing this type of bearing damage will require a move away from very active and aggressive EP additives. The ultimate gearbox oil for wind turbine application should have the thermal stability of a top-tier hydraulic oil combined with the EP properties of current gear oils. In addition, the components added to prevent micropitting need to be carefully selected to ensure that surface activity is balanced.”
Implications for the Future
With the latest trend of offshore wind turbine parks, accessibility is even more difficult than on land, so proactive prediction of the useful life of lubricants becomes the new maintenance strategy rather than the reactive strategy based on measuring acid number and viscosity.
Turbine operators, analysis labs and component manufacturers are collaborating in the development of methods to characterize the conditions of in-service lubricants to meet the new challenges. For instance, bearing manufacturer SKF solicited the expertise of Fluitec to develop a test procedure for in-service bearing lubricants and greases, which could be recommended to the (SKF) customers, in order to predict the remaining lubricant life.
Another collaborative effort has produced a monitoring system for in-service wind turbine lubricants that detect and trend the remaining concentration of antioxidants. As a quick check for fluid condition, at a minimum the turbine operators should be measuring: cleanliness (contamination by ISO class), oxidation, water and viscosity.
By trending these four major parameters, 90 percent of the lubricant and component information is accessible onsite in a short time. Also of key importance is quality control of incoming oil batches. When refilling with new greases and lubricants, it is important to control the quality and make sure the right oil is added to the reservoir to avoid mixing and creating deposits into the gearbox.
“To date, we have seen very poor maintenance procedures in the field. These will have to change dramatically, especially for the larger sizes of wind turbines and gearboxes where exposure to higher oxidative and wear stresses will occur quickly,” said Jo Ameye, global sales and marketing manager of Fluitec International.