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Maintaining clean oil is one of the best investments maintenance departments can make. If there is any doubt about this statement, the following statistics should remove it.
For instance, according to a National Research Council of Canada study on tribology, more than $5 billion is lost each year in energy and maintenance costs due to friction and wear. Dr. Rabinowicz of MIT reported that almost $200 billion annually is wasted in the United States from tribological losses.
In addition, numerous case studies have been published in Practicing Oil Analysis and Machinery Lubrication magazines about organizations that saved thousands, even millions of dollars by incorporating contamination control.
As companies allocate larger budgets for contamination control technologies, the marketplace has been inundated with new products and services to satisfy this growing market. Global competitive pressures drive companies to search for new methods of increasing productivity while reducing operating costs.
Equipment manufacturers increase the speed and reduce the size of their equipment, thereby decreasing component clearances. Companies attempt to reduce maintenance costs by squeezing every ounce of life from their lubricants. As a result, contamination control is more relevant and valuable today and will continue to be so in the future.
Extending the lifecycle of equipment and maximizing lubricant performance are on-going objectives in a contamination control program. New technologies are now available to meet these objectives and maintain lubricants with lower levels of contaminants then ever before.
The trendsetting technologies highlighted in this article provide superior contamination removal over conventional technologies, offer cost savings to companies that employ the technology and show promise in a range of base oils and applications.
Contamination control requirements depend on both the environment and the application. Hydraulic power system lubricants and bearing lubrication have some of the most critical demands for contaminant-free fluids, and are where trendsetting technologies are initially employed.
Mechanical filtration is the primary technology in most contamination control programs and will continue to be so for the foreseeable future. However, the pore sizes of mechanical filters are quickly approaching their limits and it is evident that this technology does not solve all contamination control problems.
One of the biggest limitations of mechanical filtration is its inability to remove soluble contaminants, as well as insoluble contaminants under 1 micron.
As the demand for cleaner lubricants increases, technologies beyond mechanical filtration will be required. Figure 1 depicts the range of contamination control tools and highlights the trendsetting technologies for today’s most demanding contamination control requirements.
Figure 1. Trendsetting Contamination Control Technologies
The contamination control technology of choice should to some extent depend upon the kind of lubricant being employed, because each base stock has unique requirements. The new demand for Group I, II and III lubricants is the ability to remove extremely small insoluble particles. Synthesized ester base stocks, on the other hand, require technology to remove the buildup of insoluble contaminants.
The majority of hydraulic and bearing lubricants in service today are manufactured with an API Group I base stock, also known as a conventional oil. Higher demands are being placed on these lubricants, as the applications become more severe and users try to maximize their life.
Group I lubricants do not have the robust oxidation life of Group II through IV fluids and therefore are more susceptible to oxidation and thermal degradation. As a result, Group I lubricants have a higher potential to produce degradation by-products that eventually transform into varnish and sludge.
The advantage of Group I lubricants over other base stocks is their high solvency, translating into a high tolerance for degradation by-products. Usually, hints will become visible in routine analysis before varnish and sludge become issues.
Technologies that can remove fluid degradation by-products will be setting the trends for contamination control in the future. Removing fluid degradation by-products will allow the user to fully exploit the life of the lubricant while capitalizing on performance improvements and cost reductions.
The performance advantages of Group II and III lubricants over Group I lubricants are undeniable. Group II and III base stocks have higher purity, strong oxidation stability, lower volatility, higher viscosity index and lower pour points.
Lubricants made from these base stocks typically have a longer service life. Over the last eight years, the percentage of Group II lubricants has risen from 6 percent in 1995 to 49 percent in 2003. There is every reason to believe that this growth will continue.
New processing technologies and higher demands are driving down the price of Group IIIs, making them even more available. In the future, base oils with even higher levels of purity are expected.
Higher purity translates into longer lubricant life. It also translates into lower solvency. Group II and III lubricants have a low tolerance for degradation by-products and, as a result, it is extremely difficult to predict when they will begin to produce varnish and sludge.
The contamination control trend with these products is a technology able to remove fluid degradation by-products. The more highly-refined and pure the base stock, the more the lubricant can benefit from a contamination control technology able to remove degradation by-products.
Phosphate esters (PEs) are used in many hydraulic applications where fire hazards are a concern. PE hydraulic fluids rarely contain additives and as such are a pure chemical. PEs are hydrolytic in nature and will decompose in the presence of heat and water to form acids, which can be detected by an increase in acid number.
Once the acid number in a phosphate ester fluid reaches approximately 0.18, the degradation cycle transforms from linear to exponential. These degradation by-products have an adverse effect on the air retention properties of the PE, leading to more oxidation. This self-propagating cycle will continue indefinitely until interrupted by an acid-scavenging system, as shown in Figure 2.
Figure 2. Phosphate Ester Degradation Process
The primary contamination control concerns are particulate contamination, water and acid. Vacuum dehydrators and microfiberglass filters are commonly used to control dirt and water contaminants. Traditionally, fuller’s earth or activated alumina have been used to scavenge acids.
Unfortunately, both of these products shed dissolved metals into the fluid, creating metal salts or pyrophosphates. Fuller’s earth contributes calcium, magnesium and iron phosphate while activated alumina adds sodium phosphate.
Pyrophosphates are particularly insidious because they act as a catalyst for further fluid degradation. In addition, pyrophosphates lower the fluid’s resistivity, increasing the potential for electrokinetic erosion and electrolytic etching.
Additional problems associated with dissolved metals are servovalve stiction and seizure, foaming, gelling and deposits. Once the dissolved metal level reaches 50 ppm, a system “dump and recharge” or a “bleed and feed” are the traditional solutions. The primary contamination control requirement for PEs is an acid-scavenging system that does not contribute metals to the fluid or have other adverse effects.
Polyol esters (POEs) are widely used in aero derivative turbines and in hydraulic applications demanding less flammable fluids than mineral oils. They have a lower coefficient of friction than mineral oils, good seal compatibility, biodegradable composition and a naturally high viscosity index.
Oils for aero derivative turbines that meet Mil-L-23699 Type II and newer specifications are manufactured with POE base stocks with high oxidation stability. The oil in this application comes in direct contact with metal surfaces ranging in temperature from 200oC to 320oC. The fluid provides cooling and lubrication to the turbine.
Contamination control is accomplished in most aero derivative turbines with in-line, absolute mechanical filters that maintain low particle counts. Contamination control trends for this base stock will be in technologies that have the ability to remove degradation by-products and technologies that can scavenge acids.
The most promising contamination control technologies are currently by-pass systems that provide a host of benefits by slip-stream filtration. Table 1 highlights the evolution of trendsetting contamination control technologies.
Table 1. Evolution of Contamination Control Technologies
Modern contamination control programs challenge technologies to remove smaller and smaller contaminants, and to keep the oil as contaminant-free as possible. As Figure 3 illustrates, most contaminants in lubricants are smaller than 1 micron and cannot be removed with mechanical filters.
Electrostatic oil cleaning removes the majority of insoluble contaminants in oils. As opposed to mechanical filters that can remove particles only down to 1 micron, electrostatic oil cleaners can remove particles as small as 0.01 micron (Figure 3).
Analyses of used hydraulic and turbine oils have shown that more than 70 percent of their contaminants are smaller than 1 micron. The ability of electrostatic separators to remove contaminants smaller than 1 micron provides several performance advantages over mechanical filters.
Electrostatic oil cleaning is based primarily on the principle of Coulomb’s force. Dielectric contamination collectors are strategically sandwiched between electrodes. High, direct current voltage is applied as the oil flows through the media, electrically charging the insoluble particles. The charged particles are then attracted to either the anode or cathode, depending upon their charge, and are captured in the collector.
A secondary influence called the gradient force assists in agglomerating the particles prior to collection. The combination of forces allows extremely fine metallic, nonmetallic, organic or inorganic particles to be captured.
In practice, electrostatic oil cleaners are comprised of a cleaning chamber, pump and electrical control console. The oil is pumped from the lubricant reservoir up through the cleaning chamber, requiring several passes through the device before it is thoroughly cleaned. The media used in the collectors is pleated with sharp angles and a rough finish to help maximize the effective surface area.
It typically takes between 20 and 40 passes of the reservoir volume to remove most insoluble contaminants. After 40 passes, the electrostatic oil cleaner will begin to scrub the machine’s internals, removing degradation by-products.
Electrostatic oil cleaners can also be used in a number of applications and a wide range of base stocks. Table 2 highlights the technology’s effectiveness on a range of base stocks.
Table 2. Electrostatic Oil Cleaning Effectiveness on Various Base Oils
To understand why small contaminants must be removed from lubricants, it is important to understand the different types of insoluble contaminants. Particles can be categorized as either hard or soft. Table 3 compares hard and soft contaminants.
Table 3. Hard Contaminants versus Soft Contaminants
Soft particles are responsible for a host of lubricant problems, such as stiction in hydraulic valves and fouling of critical oil clearances and bearing orifices. Valve seizures can cause power plants to trip off-line. Erratic hydraulic performance will impact product quality with plastic injection molders.
Plugged orifices can starve critical areas of lubrication. Each of these problems can be traced back to a high level of soft contaminants in the oil. Figures 4 and 5 are examples of varnish in gas turbines.
Figure 4. Varnish Formation on a Gas Turbine Bearing
Figure 5. Varnish Formation on Turbine Load Gears
Mechanical filters are unable to remove most soft contaminants. Electrostatic oil cleaning is the most effective technology at eliminating the problems associated with high levels of soft contaminants.
Electrostatic oil cleaning will meet the demands of tomorrow’s contamination control requirements for several reasons. Varnish-related problems are increasing at an alarming trend across several industries. Electrostatic oil cleaners provide a solution to this problem.
Electrostatic oil cleaning will attain lower cleanliness codes than mechanical filters or other contamination control technologies. The cleaner the lubricant, the longer the life of the equipment and the lower the maintenance costs.
The technology has been industry-tested and is cost-effective and easily justifiable. Table 4 illustrates the benefits and drawbacks of electrostatic oil cleaning.
Table 4. Pros and Cons of Electrostatic Oil Cleaning
Although ion exchange is a well-understood science in other fluid treatment industries, applying this technology in lubricant remediation is less widely comprehended.
The benefits of ion exchange are realized primarily with PE hydraulic fluids, although much research has been invested to applying this technology in other applications. Many PE fluids are pure chemicals that operate without additives. In these applications, ion exchange can effectively extend the life of the fluid indefinitely.
Traditional acid-scavenging technologies such as fuller’s earth or activated alumina may have detrimental performance consequences on the fluid. Ion exchange provides two significant advantages over traditional technologies.
First, it is the only acid-scavenging technology available that does not have additional side effects. Second, the process also removes dissolved metals generated from fuller’s earth and activated alumina.
Ion exchange uses specially processed weak-based anion resins that do not shed sodium or other metals. The resin beads adsorb entire acid molecules. Part of the acid adsorption process discards water from the resins and into the fluid, temporarily increasing the fluid’s water content.
This water can be removed through vacuum dehydration, or will be reabsorbed back into the resin beads.
In practice, the ion exchange filter’s performance is dependent upon a specified dwell time and efficient post-filtration. Therefore, a flow meter and regulator may be required in some systems. Ion exchange filters are designed to replace OEM acid scavenging filters and require an absolute 4-micron or better post-filter.
Figure 6 shows a process flow diagram of the ion exchange process for phosphate esters.
Figure 6. Ion Exchange Process
Ion exchange technology requires high uniformity in the base oil. Group I base stocks have too many impurities for ion exchange to be effective. Long-term, ion exchange will be of significant value to lubricants formulated with Group II, III and POE base stocks.
A recent case study by an ion exchange manufacturer demonstrated that this process can lower the acid number in a POE from more than 8.0 to less than 0.1. Tests on Group II and III turbine oils not only lowered acid numbers, but also had a positive impact on the fluid’s RPVOT value.
Table 5 highlights the technology’s effectiveness on a range of base stocks, while Table 6 portrays the pros and cons of ion exchange technology.
Table 5. Ion Exchange Effectiveness on Various Base Stocks
Table 6. Pros and Cons of Ion Exchange
Ion exchange will continue to be a contamination control trendsetting technology for years to come. As the price of resins decline, ion exchange will become mainstream technology in many other markets and for several base stocks.
Magnets have been used to remove ferrous contaminants from fluids for years with mixed results. Innovations in magnetic filters, however, put this technology back in the spotlight.
Magnetic field effect systems can remove particulate down to one micron, have a high dirt-holding capacity, and can be used over and over again, eliminating disposal costs. Furthermore, new filter designs perform well with viscous fluids, are not temperature-dependent and experience negligible pressure drops.
|Figure 7. Magnetic Filtration Canister|
The filter is composed of rare earth magnets combined with steel plates in an annular configuration. The fluid flows through large channels subjected to a strong magnetic force field attracting ferrous insoluble particles (Figure 8).
|Fluid passes through the flow channels and contaminant is drawn into the collection zones by the focused magnetic flux away from the fluid flow|
Figure 8. How a Magnetic Filter Works
Nonferrous particles can also be trapped by the force field. On a time interval determined by contaminant ingression and generation, the magnetic disks are cleaned and the filters put back into service.
Although magnetic filters work well in a wide range of applications, their real advantages over mechanical filters are most apparent in the following conditions:
As can be seen in Table 7, magnetic filters are effective with a range of base stocks and fluid types.
Table 7. Magnetic Filtration Effectiveness on Various Base Stocks
Magnetic filters are currently used in niche applications, where their benefits over standard mechanical filters are unequivocal. Environmental and economic pressures have the potential to bring about widespread application adoption.
Electrostatic oil cleaning, ion exchange and magnetic filtration are among the most advanced contamination control technologies in the marketplace today. What does the future hold? Certainly, one can expect the trendsetting technologies to continue evolving by becoming more efficient and higher performing. What other technologies will become available? Following are some potential products for the future.
There are significant research and development dollars behind the market leaders in mechanical filtration, so product evolutions are expected. However, the filters’ pore sizes are approaching the lower threshold. Synthetic microglass manufacturing techniques may provide slightly smaller pore sizes in the future, but the biggest improvements will be in dirt-holding capacity and filter efficiency. Mechanical filters will continue to be used in high-pressure, in-line filter zones.
There are four different kinds of membrane filtration: microfiltration, ultrafiltration, nanofiltration and reverse osmosis. Although membrane filtration has been successful at separating free water from oil, it has not been commercially applied to removing other contaminants.
Of the four various kinds of membrane filtration, ceramic ultrafiltration holds the greatest promise for use in lubricants as it is designed to separate materials with high molecular weight, such as lubricant degradation by-products. This technology is currently in the research phase and may not be economically and commercially viable for several more years.
Contamination control programs demand superior filtration systems. Group I, II and III lubricants crave technologies that have the ability to remove degradation by-products. Synthesized ester-based fluids yearn for acid-scavenging technologies that remove soluble contaminants, while not contributing dissolved metals in the process.
High-viscosity lubricants in ferrous environments wish for filter technologies that can remove small wear particles, yet are not affected by high pressure. The three technologies discussed here, electrostatic oil cleaning, ion exchange and a new generation of magnetic filtration, will meet the new demands of contamination control programs.
In addition, these three technologies are superior to other conventional tools in contamination removal, are economically attractive and are applicable in a wide range of lubricant base stocks.
Electrostatic oil cleaning, ion exchange and a new generation of magnetic filters are setting the trends for contamination control. These technologies are currently being used in the most advanced contamination control programs and show excellent promise for widespread adoption as future programs have more intense expectations.