Photo courtesy of Hydac USA

Filter elements are rated on their ability to remove contaminants of specific targeted sizes from a fluid under specific operating conditions. Filtration ratings can be measured by analyzing three areas of performance:

1. Dirt holding capacity
2. Absolute rating (micron) and percent efficiency (beta ratio)
3. Pressure drop across the element at a specific absolute efficiency

Filters with increased performance provide the benefits of longer filter change intervals, improved oil cleanliness and higher nominal flow rates. Since 2000, the dirt holding capacity of a filter element with a 10 µm(c) media has been increased by more than 100% with design improvements.

Characteristics of a Good Hydraulic Filter
Filter elements are complex hydraulic components. In addition to the three characteristics above, others play an important role. These are flow-fatigue strength, differential-pressure stability, Beta value (efficiency) stability and wide fluid compatibility. This lead to:

• The efficiency and absolute rating of a filter element. These are essential for the system’s oil cleanliness over the entire service life.
• The flow-fatigue strength of the filter material, which ensures the required oil cleanliness also with changing volume flow.
• Long filter-change intervals come from a high dirt holding capacity, and sufficient flow fatigue strength and material resistance.
• Differential-pressure stability guarantees intact and functioning filter elements especially during cold starts which stress filter materials with a low oil viscosity.
• The differences between original and replica filter elements are detectable only in a laboratory on a suitable test bench and by standardized tests.

Today’s Demands
It’s no surprise that user requirements vary. However, the trend is clearly going to higher oil cleanliness levels and longer filter change intervals as well as to an increased media resistance and compatibility with various hydraulic oils.

Special requirements call for special solutions. For example, filter elements for split power transmissions may require a high flow-fatigue strength, water absorbing elements or electrically conductive filter elements when using oil with low conductivity.

Special Problems: Electrostatic Charges
The conductivity of an oil depends on the base oil and additives. Another trend is toward higher refined base oils (group II and III) because of environmental standards. However, these oils display a lower electrical conductivity because they no longer contain heavy metals, and can generate electrostatic charges under certain operating conditions such as high fluid loading.

A non or low-conductive hydraulic oil in a system generates an electrostatic charge at the interfaces between oil and non-conductive surface, such as between the filter fleece and hoses. The charge is a result of the fast separation of two non-conductive surfaces. Thus the charge cannot be balanced thereby producing a charge separation. A large enough charge will produce discharges in the form of flashes. Because of the large non-conductive surface in the filter elements, the effect may occur and grow with increased oil flow.

Conventional filter material may be damaged due to discharge flashes and their related high temperatures. Discharges also produce holes in the filter media through which dirt particles may pass. This results in an increased wear of the hydraulic components, which can end in malfunction and machine failure.

High-temperatures flashes also lead to a deterioration of the oil characteristics, a reduction to oil life, and premature oil aging. The resulting oil-aging products also shorten the filter element life. Adjacent electric components may also be damaged by electric discharges.

A Solution To Electrostatic Charges
Static charges must be balanced to ensure that the electric discharge of the oil does not exceed certain values. For this, a filter element has been developed to ensure charge balancing and prevents the flashes.

System Solutions
The design trend towards system solution continues. With increasing importance of Supply Chain Management among machine manufacturers, there is a clear trend towards integrated solutions in the supply chain. This includes functional and system integration with particular focus on the reduction of interfaces as well as on the production of pre-assembled and tested functional units.

By Meagan Santos, Hydac USA

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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

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The cut away shows a little internal detail of a typical two stage, gearbox-oil filter. Such units weigh about 20 lb empty.

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. “Most current maintenance practices are OEM specified, but as units come out of warranty, their owners are driving for more industry standards and specs,” says HYDAC Technology Corporation’s Meagan Santos. The sales engineer made these comments during the recent WindTech 2012 event in Sweetwater, Texas, and provides them as guidelines for O&M techs and system designers regarding care and maintenance of this lubrication equipment.

For instance, Santos says that gearbox breathers are a 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 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.

The typical lubrication schematic for a multi-megawatt wind turbine is suggested by AGMA 6006. The offline circuit to the right is for a separate pump and filter unit that may be connected to remove smaller particles, or water, or both. AGMA 6006 and ISO 4406 suggest an oil cleanliness requirement of -/16/13. The lube circuit should be capable of maintaining these values.

Filters, of course, remove particulate matter from the lubricating oil, to a degree. “On a two-stage filter from our company, for example, 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 springs on bypass valves vary but a common setting is for about 4 bar (about 59 psi). The main filter, the outer portion with the pass protection, handles the bulk of the oil flow.

Santos points out that the pump motor on the oil line often has two speeds. It runs at low speed till the oil temperature reaches about 30 to 35°C and then increases to high speed. OEMs often adjust the switchover conditions. “On occasion, someone may decide that the oil should run cooler, so they switch the pump speed to the higher setting. But what that can do is increase the pressure and bypass the 10 µm filter when oil should be going through it. That is not good.”

An oil analysis will include an ISO code of three numbers that describe the quantity of particulate contamination, such as 22/18/13. Decoding the middle number 18, for example, tells that there are between 130,000 and 250,000 particles larger than 6 µm.

Most gearboxes include a cooling circuit that typically passes oil through an air-to-oil heat exchanger. “There is always oil in the heat exchanger, even during cold ambient conditions we want some oil passing through the cooler to prevent freezing. This is similar to leaving a faucet dripping in freezing weather. In our example system, normal oil flow is to both the gearbox lube system and the cooler, starting at 45°C, the thermal valve begins to shut off flow to the gearbox, forcing oil flow through the cooler before proceeding to the gearbox lube system.”

Before topping off a gearbox, says Santos, make sure it needs oil. The standard recommended oil level is half way between min and max in the sight glass. So wait about 30 min after shutdown before reading the oil level. It takes time for the oil to settle and pour out of the cooler. It has small passages for oil and passages for air. “You can’t see through it because it’s a convoluted path to generate turbulence and good heat transfer. So it takes a long time for oil to drain out,” she says.

On occasion, the wind tech should remove about 100 to 120 ml of oil from the gearbox for laboratory analysis. A report will come back with a variety of information, one being the amount of particulate in the oil. An ISO code provides a count for three particulate levels: those larger than 4 µm across, larger than 6 µm, and larger than 14 µm. For example, consider a count on a report of 22/18/13. The accompanying table quantifies each value. (AGMA does not quantify this first value in the three, hence the dashes in their figures.) For the example, the 22 means the oil sample held between 2 million and 4 million particles greater than 4 µm across. A complete analysis report also details the chemicals in the oil sample. Companies can request a particular cleanliness level in delivered new oil.

When changing filters, says Santos, you won’t need a big wrench to complete the job. “Just hand tighten the filter-housing lid until it is snug and then back off ¼ turn,” she adds. Some O&M teams do change its O-ring as a matter of course. But there is a tendency to tighten the filter-housing lid as much as possible and that can damage the O-ring.

Water may be a problem and can be removed with temporary offline filters. Santos also cautions that there are small filters and breathers in the brake and hydraulic systems that are often overlooked. Those should be changed on OEM requirements. WPE

A few maintenance strategies

The gearbox breather, from HYDAC includes a desiccant that absorbs water vapor. The material changes color to tell it needs changing.

Although there are more, HYDAC’s Meagan Santos suggests these gearbox maintenance strategies.

• Don’t underestimate the importance of breathers

• Take a proper oil sample of 50 to 100 ml. Labs can provide guidance.

• Use wind-energy specific oil analysis labs and make sure they know the sample comes from a wind turbine. Otherwise, they treat it differently.

• When the lab returns a report, take action guided by the oil’s condition.

• Maintain the correct oil level in the gearbox and hydraulic power unit.

• Follow proper filter changing procedures.

The table comes from the American Gear Manufacturers Assn Standard for Design and Specification of Gearboxes for Wind Turbines. The values in the last line, – /16/13, are also the ISO 4406 targets for cleanliness.

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