Justin Martino, Associate Editor, Power Engineering
May 21, 2013 | 6 Comments
Tulsa, OK -- Like any other piece of major equipment in the power industry, wind turbines represent a large investment for the companies that rely on them to generate both electricity and revenue. Unlike gas turbines or boilers in coal-fired power plants, however, wind turbines present some unique challenges.
Repairing wind turbines can often be difficult for several reasons. Most wind farms are located in remote areas, making it difficult to reach the site. Once at the site, workers are faced with making repairs while anywhere from 75 feet to nearly 400 feet in the air. Also unlike natural gas-fired or coal-fired plants, operators can expect to repeat this process multiple times because of the comparatively small capacity of wind turbines.
One of the keys to preventing costly, time-consuming repairs is planned maintenance, said Julie Rushton, marketing category specialist for Petro-Canada Lubricants Inc. Proper turbine lubrication is an important part of that maintenance.
“Wind turbines are very expensive pieces of machinery, and the lubrication system is very important to keep that machine working well,” she said. “There are lots of different parts of the wind turbine that need to be lubricated, using lubrications from greases to gearbox fluids to hydraulic oils.”
Keeping a wind turbine’s gearbox properly lubricated is important in extending the life of a wind turbine, Rushton said. Petro-Canada’s Harnex 320, designed to be used in wind turbine gearboxes, is a fully synthetic oil designed to withstand the conditions wind turbines may be subject to, whether that is extreme temperatures or the potential of corrosion from saltwater for offshore wind turbines.
The type of oil that is used in a turbine’s gearbox — and for all other parts of a wind turbine — is generally designated by the original equipment manufacturer for the units. One of the main differences is whether the oil is a synthetic oil or mineral oil.
Shell Global Solutions US Inc. offers both synthetic and mineral-based oil for wind turbine gearboxes. Shell Global Solutions Product Application Specialist and Team Lead Felix Guerzoni said one thing the company looks at when designing products is making sure customers can rely on the product to last a significant amount of time without needing additional service.
“With the remoteness of these units, they’re really only serviced, as far as the regreasing, every six months at best,” he said. “In terms of the gear oil, customers want to use the gear oil and have that last anywhere from three to five years without change out, because there are some very significant costs involved in changing the gear oil out as well as in the event of failure of a gearbox and having to change the gearbox out. The rental of specialty cranes is a very significant cost as well.”
The gearbox is not the only part of the turbine that requires lubrication, however. The generator also requires lubrication, and there are lubrication points on the blades. Wind tower blades have bearings that will essentially feather the blade so operators can optimize the blade angle to match wind speed. The main shaft bearing also requires grease for lubrication, as well as the drivetrain and yaw and pitch drives. The turbines also use a hydraulic system that is used to provide a braking mechanism for a unit, but can also be used for hydraulic pitch control on the blades.
With all these different parts requiring lubrication, multiple products could be required in order to maintain a single turbine. Guerzoni said Shell attempts to create products that can be used for multiple purposes when it can be done without losing quality.
“We’re trying to optimize that as much as possible, because obviously from a service engineer’s standpoint, the fewer lubricants they have to apply the better,” he said. “Some companies will try to do a single solution, but then you have to look at the overriding reliability of the unit relative to trying to rationalize the number of products. You can get much better performance and reliability out of a product that is really designed for that specific application, especially when we talk about greases. So there’s a bit of a tradeoff there.”
Travis Lail, Americas Industrial Marketing Adviser for ExxonMobil, said his company takes a similar approach to specializing lubricants for different parts of a wind turbine, but also attempts to make multipurpose products when there would be no decline in quality. The company produces Mobilgear SHC XMP 320 for turbine gear boxes as well as several other products for wind turbines, including Mobil SHC Grease 460 WT for use in main, pitch and yaw bearings and Mobil SHC 524 lubricant for use in hydraulic systems.
“Our approach to the industry is providing products that are optimized for specific applications,” he said. “Of course we try to help customers use the fewest number of correct lubricants possible in order to meet their lubrication needs.”
Optimization the number of potential applications while producing the highest quality product isn’t the only area at which companies look in their research and development process. As turbine technology continues to develop and companies produce larger wind turbines, companies producing the lubrication also need to produce lubricants to keep up with the industry.
Lail said the larger turbines being produced by companies require oil that can handle the extra stress created by the extra size.
“The gearbox actually creates additional stress, and the essence is it challenges the oil’s ability to maintain a sufficient film strength,” he said. “You have to improve your additive packages and base oils that you use to make sure you’re able to optimize your oxidation resistance but still maintain low temperature fluidity.”
One way to ensure products are optimized for different wind turbines is to work directly with the original equipment manufacturers, Guerzoni said.
“We’ve got a very well-proven and successful product line at the current time, but as these turbines are getting larger in size with higher towers, longer blade lengths and higher megawatt class, that’s adding new challenges and putting more stress on the unit and more stress on the oil,” he said. “As a result of that the specifications are changing at a very rapid rate, especially on the gear oil side, and so as an oil supplier we’re constantly looking at the updates and changes in trends of the design and bringing out new products to meet those requirements.”