Oil quality is a decisive factor for gearbox service life, but changing it is time consuming and costly. Thus, turbine operators are always looking for ways to stretch oil-change intervals. At least two companies have developed methods to monitor and maintain oil to extend service life.
Gearbox wear is usually determined by operating conditions and stresses exerted on components. Hence, without a lab analysis, gear-oil life is difficult to estimate. Impurities, the degradation of additives, and oil-aging products generated during operation all contribute to premature aging of gear oil. Most gearbox manufacturers recommend no more than 36 months between oil changes. In some cases, the service life of gear oil is not met due to impurities that cause the main-flow filter to prematurely fail. Research shows that only a small percent of the main-flow filter’s dirt-holding capacity absorbs impurities. This is sometimes due to a deposit of oil-aging products that formed a seal over the main flow filter, causing its failure.
However, Availon engineers found that a recent kidney-loop filtration upgrade (for systems with a fine filtration level of 10µ or higher) improves oil quality and prevents the need to perform some filter and oil changes recommended by the manufacturer. This reduces down times and some costly repairs. As its name implies, kidney-loop filtration provides an oil flow to clean the gear oil independent of the main flow filter. The filter removes oil-aging products and particles larger than 3µ, as well as moisture. The company’s engineers estimate the filtration unit can extend intervals between gear-oil exchanges to 60 months.
Other filter manufacturers have ideas as well. For example, Parker Hannifin Corp’s philosophy about oil is to let it tell when it needs changing and show how equipment in the nacelle is performing. How is this possible? Olli Rantanen with Parker’s Heavy-Duty Lube and Filtration division says magnetic pre-filtration can increase the lifetime of hydraulic filters up to 40%. “Further development in the use of patented media and element design have seen life of filter elements go from a nominal 3 to 6 months, up to 15—a significant reduction in maintenance costs for turbine owners,” he says.
In the past, wind turbine OEMs built fluid filtration systems by assembling components from different manufacturers. “While such systems worked reasonably well, hydraulic-component companies are now designing complete lubrication-oil systems for turbine gearboxes,” Rantanen explains. These systems include filters, pumps, valves, coolers, and heaters, as well as manifolds and piping to connect components. Furthermore, with regard to filtration systems, he says an emerging design trend is to integrate functions and components in a manifold block rather than piping the components individually. “Manifolds minimize leak points, reduce assembly time, and provide a more compact, lighter, and reliable unit,” he says.
For OEMS that must buy subsystems, Rantanen notes another emerging industry trend will assist by providing turbine-specific, 3D-models of the hydraulic equipment, which can be imported directly into the 3D-CAD gearbox models or some preferred location in the CAD model of a nacelle. “This lets the hydraulic supplier take responsibility for the system and saves the OEMs significant time to accelerate product to market, money, and resources.
Companies are also using particle detectors for another level of contamination control. A few features of one include independent monitoring of system contamination trends, and early warning LEDs or digital display indicators that tell of low, medium, and high contamination levels.
“Recent ideas in oil monitoring allow minimizing or eliminating the use of bottle sampling,” Rantanen says. In this older monitoring method, an oil sample was tested in a lab and results relayed later to O&M crews. Results are now available in real time, instead of waiting days for lab results. This also allows planning preventative maintenance for turbines, thus significantly reducing downtime and maintenance cost.”