Gearbox breathers are the first line of defense against airborne contaminants. The devices let the gearbox take in air as it cools while filtering water vapor and solid contaminants before they enter the fluid system. When gearboxes warm up, breathers should let expanding air escape while keeping oil mist and splash inside. When a breather blocks up, the pressure differentials in a warming gearbox can push oil out its labyrinth seals and onto nacelle floors.
Filters, of course, remove particulate matter from the lubricating oil, to a degree. On a two-stage filter, the first stage removes debris 10 µm and larger while the second catches particles 50 µm and larger. On cold startups when the oil is thick and slow to move, a valve lets the oil bypass the 10 µm filter. Generally, the flow rate and oil temperature, often 10 to 30°C, governs when the bypass valve begins closing for finer filtering. The main filter, the outer portion with the pass protection, handles the bulk of the oil flow.
Filters and breathers on wind-turbine gearboxes must work under conditions found in no other industrial setting. Gearbox oil, for instance, undergoes large viscosity changes due to a wide temperature span and operating conditions. And then there is potential for a high water content in the oil from humidity and condensation. Recent detailed analysis of fluid flow through a range of filter materials has led to a better understanding of conditions inside a working hydraulic-fluid filter. The work has let one filter manufacturer identify factors responsible for pressure loss in the folded material. The result is a special web technology for production of a new hybrid fabric that maintains an optimal opening of the fold channels. Thus pressure loss in the folds drops as much as 50%.
To improve filter performance, it is necessary to reduce its pressure loss. Calculations show that specific flow resistance depends on the filter materials, as well as on the structure and length of the intermediate fold gaps, the so-called fold channels. The longer a fold gap, the greater the specific flow resistance in the fold. This is because the hydraulic fluid cannot flow unobstructed through the fold gap.
The filter manufacturer says it has been able to implement the findings and confirm them in numerous trials. Reducing a pressure loss in the filter element by as much as 40% at constant flow rate means it can increase up to 65% at a specified pressure loss. This also means, depending on application, smaller filters can be used to trim weight, resources, and costs. Also, reducing the pressure loss in existing systems means the bypass circuit (it protects the fine filter on cold startups) opens less often and for shorter periods. Consequently fewer particles get through the bypass to the clean-oil side and the danger of malfunction due to non-filtered oil significantly reduces.
The low differential pressure and the high dirt holding capacity of the filter elements allow longer periods between service and improved cold-start characteristics.
Depending on application, filter elements are subject to strong flexural-fatigue stresses induced by flow-rate fluctuations. These come from rpm fluctuations of drive motors, cylinder ratios, as well as the increasing use of variable displacement pumps in modern machines.
Conventional filter elements use a metal or plastic support fabric on the out-flow or clean-oil side. A support of metal also brings the advantage of electric conductivity, but has the danger of fatigue failure. Wire fatigue then leads to wire pieces in the hydraulic fluid. Some strength comes with plastic fabrics insensitive to flexural fatigue stresses. On their downside, such fabrics have extremely low electrical conductivity.
To counter the material-specific disadvantages, the manufacturer uses a hybrid fabric which has proven effective in years of application to support the filter material. The patented fabric consists of a mix of stainless steel and polyester fibers. This combination exploits all the advantages of metal and plastic fabrics and avoids the disadvantages of pure metal or plastic-only versions. Stainless-steel wire arranged longitudinally ensures complete dissipation of electrostatic charges, which prevents damage to the filter material and dirtier oil. The polyester fibers arranged transverse to the metal threads ensure optimal flexural fatigue strength and avoids of fatigue failure.
The structure of the filter material has been completely redesigned to contain multiple interlaminated filter and support layers. These use suitable laminating agents to improve the characteristics of the materials. In fact, it improves the differential-pressure stability of the filter material by a factor of three, relative to non-laminated materials.
In addition, a plastic sheathing shrink-fit onto the filter bellows ensures that they fit tightly on the perforated frame. This makes the filter element even more resistant to flexural fatigue-stress than conventionally manufactured filters. Improved fatigue characteristics, differential pressure stability, as well as safe dissipation of electrostatic charges significantly contribute to the long service life of the filter elements. WPE
Filed Under: Filters, News, O&M
