By David Wagman, Chief Editor
North America’s fleet of wind turbines face some of the most demanding operating conditions on earth, with units expected to operate in everything from the brutal cold of a January day in North Dakota to the blistering, dusty heat of an August day in Texas.
Extremes of hot and cold, not to mention frequent changes in pitch and yaw as the winds shift, put enormous strain on operating units, in particular bearings and gearboxes, a wind turbine’s Achilles heel. What’s more, many wind farms are not easily accessible along ridge lines and other remote locations. And once on site maintenance workers face the prospect of ascending a couple of hundred feet to reach the nacelle and begin work.
It’s almost an understatement to say that lubrication issues reach a level of complexity almost a magnitude of order greater than in a conventional fossil-fired generating unit.
“Lubrication affects the bottom line and an operator’s ability to produce electricity on a profitable level,” said Travil Lail, Americas Industrial Marketing Advisor for ExxonMobil.
He said oil drainage intervals can vary widely between operators and is based principally on a turbine’s location and hours in service. He said a fully synthetic wind turbine lubricant, like ExxonMobil’s, can extend the time between oil changes from 18 months to as much as three years. “We have documented proof of up to 40,000 hours,” he said.
“The big challenge” of wind turbines is their remoteness, said Felix Guerzoni, product application specialist with Shell Global Solutions. At best, a wind turbine may be visited once every six months for routine inspection and maintenance and for an opportunity to sample gearbox oil for testing or to relubricate or grease points.
Last May, Shell Lubricants announced a portfolio of lubrication products for wind turbine hydraulic systems, blades, gearboxes, yaw and pitch drives.
A major reliability concern is gearbox micropitting, which result from factors that include case hardening and the surface roughness of the gear teeth. Minimal surface roughness is preferred as it reduces the likelihood of micropitting, he said. Specialized gear oils can further help reduce the potential for mircopitting. Given the gearbox size and potential for air entrainment in the system, anti-foaming is an important gear oil property. Excessive foam can lead to a loss of viscosity, which in turn reduces a lubricant’s ability to guard against micropitting and bearing failure.
To help guard against bearing failure, Guerzoni said there is a tendency for the recommended level of filtration to move from 10 microns down to 3 microns. This enhances oil cleanliness and extends bearing life.
Among the lubrication products Shell offers are:
- Tellus Arctic 32, used as the hydraulic fluid for extreme-climate wind turbines. The product is recommended or listed by Svendborg Brakes and by wind turbine manufacturers including GE Wind, Voith Wind, Vestas, Dongfang Wind Turbines, Sinovel, RePower, Nordex and DHI.
- Shell Rhodina BBZ is designed to provide protection to blade bearings against fretting corrosion, moisture contamination and false brineling at temperatures as low as -55ºC.
- Shell Omala HD 320 synthetic gear oil helps protect against common failure modes, including micropitting and bearing wear.
In addition, Shell Lubricants offers Shell Tivela S 150 & 320 synthetic gear oil for yaw and pitch drives; Shell Albida EMS 2 electric motor bearing synthetic grease; Shell Stamina HDS main bearing grease; and Shell Malleus GL & OGH premium quality open gear grease.
Shell said cost is a challenge any time synthetic oil is discussed, but that its advantages becomes more readily apparent if lifetime operating and maintenance costs are considered. One major advantage is that the longer life of a synthetic reduces the number of times maintenance workers need to climb the turbine tower. Synthetics can also reduce the chances that a gearbox will need to be repaired or replaced due to lubrication issues. And, given the extreme range of temperatures a wind turbines must operate in, synthetics can offer an advantage over conventional fluids, which have a relatively poor fluidity at low temperatures.
Another best practice is to have a continuous oil monitoring program in place, said Lail. Such a service can monitor viscosity, metal wear issues, thermal and oxidation stability and water contamination, among other factors.
“Many customers end up with water contamination that affects the life of the bearings and the oil filter,” he said. “Those can have a significant impact on operations.”
He said over the past year the industry has shown a greater focus on oil cleanliness and particulate counting. He said no industry-wide standards yet exist for particulate counting even though turbine manufacturers require testing as part of their warranty requirements and lubricant suppliers guarantee their oil for a specific cleanliness. He said dilutants used to pretreat an oil can help guard against water infiltration. And ultrasonic treatment can remove oxygen, which can have a material affect on cleanliness.
In general, Lail said a good oil analysis program can offer insights into equipment health and help operators make condition-based rather than calendar-based maintenance decisions.
A new player in the wind turbine lubrication market is Dow Chemical, which introduced a polyalkalene glycol lubricant in May after 18 months of product trials. Initial sales will be focused in Europe and Asia, followed by North America. Rather than offer a range of viscosities the Dow product has just one: 320. The strategy is to offer a high viscosity index, which in turn offers a broader span of viscosity, said Brian Goldstein, global marketing manager for UCON Fluids and Lubricants. “It allows for an all-season product,” he said.
The polyalkalene glycol lubricant can help guard against sludging and varnishing, the gooey film that builds up and causes stickiness, friction and heat transfer, all of which affects efficiency and can lead to failure in equipment with high loads such as wind turbines.
For a practical look at what’s involved in a typical wind turbine oil change, talk to someone like Aaron Sage, chief operating officer with Sage Oil Vacuum, based in Amarillo, Texas. The company has been in the wind turbine maintenance business since the mid-1990s. It generally takes two maintenance workers to perform an oil change, one worker on the ground and the other in the nacelle. A 330-foot-long hose is pulled up into the nacelle’s yaw deck just under the gearbox. The hose is connected to the gearbox through a ball valve. A vacuum is pulled using an air pressurizer on the ground. Draining the rate of four to six gallons a minute it can take around 30 minutes to drain the roughly 80 gallons of oil from the gearbox, Sage said. Refilling the gearbox can take another 30 minutes, meaning the total fluid exchange process from start to finish can take as much as two hours.
Sage Oil’s equipment offers either a 250- or a 390-gallon tank, which means that as many as four oil changes can be done each day by a single two-person crew. Sage Oil has fielded 20 oil exchange systems in the United States since 2008. Most of those are in Texas and Iowa, the two states with the largest installed wind capacity. A system can cost anywhere from $40,000 to $65,000 depending on options. Systems can either be sold outright to an operator or service provider or leased.
The market for lubrication and related services seems likely to grow in the coming years. For one thing, the fleet of win turbines continues to grow. For another, units are coming out of manufacturers’ warranty periods. The opportunity exists for operators to partner with service providers who have the knowledge and experience with lubricants to help maintain equipment for long-term availability and performance, said Lail.
“Customers have a significant opportunity to evaluate products and practices to improve productivity,” he said.
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