Wind Turbine Gearbox Oil Cleanliness
Proper and long term functioning of the Gearbox is very critical and depends mainly on the quality of oil cleanliness. Gearbox failures are primarily a result of oil contamination due to ingress of dust, metallic wear on account of friction, water contamination resulting in the change of oil parameters, oil degradation, and excess vibrations leading to probable surface cracks, misalignment of pump+motor shaft. Wind turbines being placed in remote locations, regular monitoring of oil condition assumes importance. Choosing the right amount to invest in contamination control to optimize your return on investment should be the goal. The ideal ISO Cleanliness Code for oil in gearbox is 16/15/12 and water content < 500 ppm. This ensures a very long life of the gearbox which is typically 15 years to 20 years. The ISO Code 16/15/12 means the following:
|Particles/ml as per Size||Range number 16||Range number 15||Range number 12|
|4 micron||> 320 and < 640||–||–|
|6 micron||–||> 160 and <320||–|
|14 micron||–||–||> 20 and < 40|
Usually the online oil filtration system has a 50 + 10-micron cartridge in the lube circuit. The Gearbox oil is checked every 6 months with a complete change every 24 months.
In a wind-turbine gearbox, the most common gear-oil viscosity is ISO 320 which is calculated at 40°C and reported as centistokes (CST). If the viscosity decreases beyond permissible limit, the oil is shearing and there is potential for reduced oil-film thickness, resulting in increased wear. If the viscosity increases, it may be due to oxidation by-products which can be associated with oil degradation and shortened filter and oil life.
It is a well-known fact that it costs at least 10 times more to remove a gram of dirt than it would exclude it in the first place Even in a very tiny amount, contaminants can increase operating costs associated with maintenance expenses or gearbox replacement. The main contaminants affecting oil and gearbox life are water, wear particles, and dirt. Along with additive depletion, water can cause filter plugging, sludge formation, reduced oil-film thickness, and be difficult and expensive to remove.
As a general rule, it is accepted that water is detrimental in wind-turbine gear oil. A 0.05% presence of water in oil equates to a 58% reduction in bearing life. However, filtration from 10 microns to 3 microns will increase bearing life by 50%. This allowable presence of water in the oil limit ( ppm ) varies with acceptable levels from 0 to <500 ppm.
Each oil type has a different starting point for total acid number (TAN) which is directly associated with additive content. A general rule is to condemn the oil when the TAN increases more than two points higher than the initial TAN. In this case it is best to consult your oil supplier for the new oil-TAN value and used the oil-condemning limit. If the TAN increases and exceeds the condemning limit it may be an indication the oil is turning corrosive and may contribute to depleted additives and increased wear.
Filtek has recently introduced a 1-micron microglass filter which can be used in offline mode to maintain the required ISO cleanliness of gearbox oil. Additionally, a high powered rare earth magnetic chamber captures all the metal particles in oil as shown in the below picture.