Synthetic lubricants have a reputation for improved performance over conventional mineral based oils. This study of one confirms the reputation by closely examining several key characteristics.
Kevin Harrington, Sandra Legay, Rick Russo
ExxonMobil Fuels, Lubricants & Specialties Marketing
www.exxonmobil.com
Wind turbines are sophisticated machines that labor in demanding environments. As such, selecting the right lubricant can improve wind turbine availability. This article focuses on the challenges in wind turbine lubrication, specifically addressing the use of an advanced synthetic gear oil in wind turbine gearboxes, in this case, Mobilgear SHC XMP 320.
ExxonMobil began tracking wind turbine gear box lubrication in 2000, and over the last 12 years collected more than 38,000 oil samples. To describe the performance of the lubricant, we will look at system wear, oil oxidation stability, viscosity retention, and water contamination because these are most relevant in determining proper gearbox operation.
Wear indicated by the presence of iron
Results from more than 38,000 oil samples showed that 99.5% were below the alert level for iron.
This study used inductively coupled plasma (ICP) spectroscopy to determine the presence and concentration of wear metals in the oil samples. Iron, copper, chrome, aluminum, lead, and tin define this category, with iron the predominant wear metal in wind turbine gearboxes. As shown in Iron content by ICP, when ExxonMobil examined 38,680 iron samples, it found 99.5% below the alert level for iron and that over 30,000 results were under 20 ppm – 10% of the limit. Furthermore, the illustration Reported oil age versus iron content shows that of the 25,680 samples examined, iron content does not increase with the age of the oil, verifying the long term wear protection provided by the lubricant.
Oil oxidation as determined by Total Acid Number
The Total Acid Number (TAN) is the amount of potassium hydroxide in milligrams needed to neutralize the acids in one gram of oil. It is an important quality measurement of the lubricant because it reflects the oxidative state of the oil. As the TAN value of an oil increases, viscosity rises, compromising the oil’s lubricating potential, leading to increased wear. In addition, the corrosive tendencies of the oil will increase, further exacerbating component wear. In the illustration Total Acid Number, ExxonMobil technicians examined 30,778 samples and found that 99.8% of the results were below alert levels, and that 25,123 results showed little if any increase in TAN over time. This means that the life of the advanced synthetic lubricant was not impacted by turbine gearbox operation.
The reported oil age for the sample tested for iron contents showed this distribution.
Viscosity retention as an indicator of film strength
Viscosity is a measure of a fluid’s resistance to flow. Most used-oil analysis laboratories report it as kinematic viscosity in centistokes (cSt) at either 40°C or 100°C. The technicians examined over 38,600 data points and found that 96% of the readings, reported in Kinematic viscosity (cSt) were within viscosity range for the fluid.
In addition, a more focused look at 25,674 samples, the plot of Reported oil age versus kinematic viscosity, found almost no oxidative thickening or shear thinning over time, suggesting the lubricant stayed in viscosity grade throughout the reported service. This is important because it confirms that the oil can maintain film strength providing excellent wear protection throughout its service life.
In-service water levels and wear potential
Water as a contaminant is most relevant because its presence may cause additive depletion, viscosity drop, and accelerated wear of components. Bearing manufacturers indicate that bearing fatigue life significantly drops when their lubricant contains high levels of moisture. The illustration Water content by Karl Fischer, reports on just over 22,000 samples, and the plot of Reported oil age versus water content, shows the water concentration in the oil over time. The samples revealed exceedingly low levels of water in the oil and that the levels present did not facilitate wear, suggesting the lubricant lends itself to prolonged service performance.
The synthetic lube with five years’ service
The total acid number showed that 99.8% of the samples below the alert level.
As a follow-up to this study, a company team examined the performance of Mobilgear SHC XMP 320 in 74 wind turbines where the oil is known to have over five years of service life. This step was taken to confirm the findings of the general data analyzed thus far. As shown in Study of 74 wind turbines, this data from a smaller number of turbines mimics the findings of the much larger study. It found minimal component wear, that oxidation by TAN is insignificant, in-service oil viscosity was maintained, and water levels in the oil were low and non-impactful.
What we learned
The use of Mobilgear SHC XMP 320 in the wind turbine main gearbox provides benefits such as:
- Reduced levels of component wear → longer gearbox life
- Tendencies of the oil will increase → extended lubricant life
- Retention of oil viscosity → longer gearbox and lubricant life
- Maintenance of low-level water contamination → longer gearbox and lubricant life.
Why is this important? The cost to generate wind energy is above that of fossil fuel: 4.5¢/kWh for coal versus for 7.5¢/kWh for wind. To make wind energy sustainable, it is important to control all aspects of the generation. Clearly, through this study, two parameters which can be improved are oil longevity and gearbox life.
In wind turbines, extending oil-drain intervals means reducing oil maintenance and extending the length of time the lubricant is in service. Not long ago, oil in a wind turbine gearbox had an expected service life of 18 months. Today, that projection has increased 3 to 5 years and the synthetic lubricant has shown that five continuous years of lubricant service can be reality.
The reported oil age versus TAN showed this distribution.
What does this mean? Over the expected 20 year life of a wind turbine, the owner who increases gearbox oil life from 3 to 5 years, will save about $15,000 per turbine. In more meaningful terms, the operator of an 80 MW wind farm with 40, 2-MW turbines, will save $600,000 over the life cycle of the wind farm.
The cost of a utility scale wind turbine is about $1.75 million per MW with replacement of the main wind turbine gearbox about 10% of the overall wind turbine costs. For a 2 MW wind turbine, replacing the gearbox would run about $350,000. Over the 20 years expected life of a wind turbine, the main gearbox is expected to be replaced 2.2 times. If through the use of synthetic gear oil, gearbox life can be extended one year, the replacement costs associated with the wind turbine life cycle will be reduced by $77,000. Apply this savings again to the 40-turbine wind farm saves $3.08 million over the life of the turbines.
Many companies recognize the need to help make wind energy more sustainable. Mobilgear SHC XMP 320 wind turbine main-gearbox lubricant does just that with the advantages of longer oil and equipment life and a lower cost to produce wind energy.
The kinematic viscosity versus oil age stayed in the grade throughout service.
The water content versus oil age shows that the levels present did not facilitate wear, suggesting that the lubricants lends itself to prolonged service performance.
The plots derived from oil samples from 74 turbines track time versus (upper left and cw) Iron parts per million, Total acid number, Kinematic viscosity, and Water parts per million. The findings reflect those of the larger study.
The water content in more than 22,000 sample, as measured by the Karl Fischer method, was exceedingly low.
More than 38,600 data points showed 96% of the reading within the lube’s viscosity range.
Filed Under: Lubricants, News, O&M
