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No matter the application or operating environment in which they are used, lubricants will eventually become contaminated with substances from the environment. The lubricant’s reaction with these substances causes the formation of degradation by-products such as sludge, varnish and other insolubles that can cause a host of system performance problems and shorten the life of the lubricant. To lessen the effects of these degradation by-products, it is important to analyze the lubricants using the test slate that is most appropriate for the lubricant and its application.
To learn more about the causes of degradation by-products and the most effective ways to deal with them, we asked some industry experts to share their knowledge about removing sludge, carbon and oxide insolubles, and varnish from industrial lubricants. Following are their comments.
1. The terms sludge, carbon and oxide insolubles, and varnish are often used to describe oil degradation by-products. What exactly are these?
All are contaminants entering from the environment or formed by the oil reacting with the environment. Most significant to the oil degradation process is oxygen, heat and water. Sludge is a soft mud-like deposit that settles out of the oil. Carbon and oxide insoluble are results of changes to the oil molecules. They tend to be dispersed in the oil. Varnish is a resinous matter formed as a result of high oil temperature or sludge. It adheres more or less to all oil-wetted surfaces. In a turbine or hydraulic system, it is normally a thick sticky substance and in engines it cures to a hard glossy coating called lacquer.
Lubricant degradation by-products are decomposed hydrocarbon molecules and components from depleted additive systems. Their production is a result of permanent damage and alteration of the hydrocarbon or additive molecule. For example, an oxidized hydrocarbon molecule will transform to an aldehyde and ketone before they end up as a carboxylic acid molecule. Eventually, the degradation by-products precipitate out of the lubricant creating sludge, varnish and other insolubles that can cause a host of system performance problems and shorten the life of the lubricant.
2. What are the best oil analysis tests to indicate the presence of degradation by-products and how should they be used in a routine oil analysis program?
If the primary mechanism of degradation is oxidation, the best tests are Fourier Transform infrared (FTIR) and specifically the oxidation peak at 1740 cm-1, acid number (AN), viscosity, color and patch test. For detecting thermal degradation, you should use FTIR and look for signs of nitration around 1630 cm-1, patch test, ultracentrifuge, color and odor.
Oil analysis must be supplemented with machine inspection to get the full picture. In routine oil analysis, the person pulling the sample can judge the color and the odor, which can be good indicators. The Millipore patch test can also give valuable information about all contaminants, as well as the oil’s filterability.
Traditional oil analysis tests cannot detect the molecular changes in hydrocarbons and additives. Nor can traditional tests identify the accumulation of degradation by-products because they are so small. As a result, lubricants that appear to be in excellent condition may still have a high potential to produce varnish and may even cause significant performance problems. FTIR is a useful test to monitor chemical changes on a molecular level and can detect the formation of new molecules that lead to sludge and varnish. Analytical tests such as the ultracentrifuge, gravimetric patch, interfacial tension and colorimetric are sensitive enough to recognize increases in degradation by-products. An easy, on-site test can be conducted by mixing equal parts of the lubricant and petroleum ether and performing a blotter test. Dark brown hues will signify degradation by-products, indicating a varnish potential.
AN and FTIR-oxidation will give you an indication of the oil’s status and can reveal oil that is in a critical oxidation condition. However, these tests will not indicate when the oil begins to produce thermal degradation by-products that result in varnish.
Rotating Pressure Vessel Oxidation Test (RPVOT) is often used to indicate remaining useful life based on an oil’s resistance to oxidation or antioxidant capacity. However, this test is both expensive and time-consuming. RPVOT was developed for turbine oils and is not typically used for other lubricants like paper machine oils, gear oils or hydraulic oils.
The ultracentrifuge test can be used to indicate if an oil is beginning to thermally degrade to the point where varnish is starting to form. A sample of the oil is spun in a test tube at 13,000 to 15,000 rpms and submicron particles are deposited on the bottom of the test tube. The results are then rated on a 1 to 10 scale with the heaviest deposits registering 10. This test can indicate when an oil is thermally degrading even when a particle count shows an extremely clean oil.
In addition, an oil’s conductivity will change with increased levels of degradation byproducts, but it can also be affected by other contaminants.
3. What is the difference between oxidation and thermal degradation, and how can oil analysis be used to differentiate between the two?
Oxidation is the product of a chemical reaction with oxygen and increases with temperature. Contaminants act as catalysts. Thermal degradation happens at temperatures greater than 400°F without the presence of oxygen. It can be caused by hot spots, micro-dieseling or compression. As indicated in question No. 2, FTIR can be a sensitive tool for differentiating the two, by looking at the relative rates of oxidation and nitration.
Unlike thermal degradation, oxidation occurs at lower temperatures and in the presence of oxygen. The intense heat required to initiate thermal degradation is generally caused by the implosion of air bubbles, or a localized hot spot. FTIR is generally the best method for distinguishing the degradation mechanism. Thermal degradation produces by-products in the nitro-nitrate spectral region (1630cm-1), while oxidation results in carboxylic acids with characteristic peaks around 1714 cm-1. The RULER® can also be used to differentiate between oxidation and thermal degradation by analyzing the difference in depletion rates between phenolic and amine-based, antioxidants.
Oxidation occurs in the presence of oxygen, while oxygen is not required for thermal degradation to occur. An elevated AN or oxidation number will indicate oxidation has occurred. If severe, thermal degradation will show up in the ISO Cleanliness Code level. However, an ultracentrifuge test would normally be required.
4. Why are some systems, such as high-pressure hydraulics and wet sump air compressors, more prone to varnish buildup than other systems?
It is the nature of the beast. Systems with reservoirs that are too small in which low-solvency oils are used will be more prone to varnish buildup than other systems.
Some lubrication systems are more susceptible to varnish formation than others. High-pressure and high-temperature systems that are prone to water contamination are especially vulnerable because both are major contributing factors to sludge and varnish build up. Although the presence of sludge and varnish are abundant in such systems, the two do not always cause performance concerns. For example, varnish can be found in most hydroelectric turbine governor systems, but oftentimes the varnish does not cause a performance problem until mechanical controllers are replaced with tight-clearance digital controllers.
High-pressure hydraulics create extremely high temperatures inside valves. The oil in most systems does not get circulated throughout the system to be conditioned (cooled and filtered), but moves only incrementally as the valve is actuated. This results in ideal thermal degradation and varnish-forming conditions.
5. Have you noticed differences between conventionally refined Group II base stocks and highly refined Group III or PAO synthetic fluids when it comes to their tendency to form sludge and varnish?
No. Except in applications as mentioned above where oil life is overexploited.
Group II and Group III basestocks have a lower solvency as measured by their higher aniline point compared with conventional Group I oils. This translates into a greater propensity to form varnish because, generally, the lower the solvency, the lower the tolerance for degradation by-products. This phenomenon makes predicting the onset of varnish more challenging in Group II and III basestocks. For example, we have seen severe varnishing problems in Group II turbine oils where the RPVOT value is high, the particle count is low and there is no change in the AN. If one is using a Group II or III basestock, we recommend incorporating additional analytical tests as part of the predictive maintenance program.
PAO synthetics tend to have a higher resistance to varnish formation due to the fact that every molecule is consistent and identical. These types of fluids do not have the wide cut that the base stocks of conventional oils have, so high temperature has less of an effect on them.
Similarly, a highly refined Group III base stock will perform better than a conventionally refined Group II base stock for the same reasons.
6. Is there a good way to clean a circulating system to remove sludge and varnish buildup from tank walls and piping?
Clean up the oil and the oil will clean up the system. Using an offline filter system with a high amount of cellulose and a depth-type filter insert is the simplest and the least expensive method. Oil samples should be sent to the oil supplier and, if recommended, the oil should be changed.
Chemical flushing (the addition of an oil with a highly additized mixture of detergents and emulsifiers) is the most effective method of removing contamination from the internals of a system. Unfortunately, residual flushing fluid in the system will have an adverse impact on the performance of many lubricants. (A small amount of the additive package can affect a lubricant’s demulsibility and foaming properties.) Mechanically removing the sludge and varnish through confined space tank cleaning is an excellent solution, but addresses only the easily accessible parts of the system such as the reservoir. High-velocity, hot oil flushing is an excellent tool to remove particulate contamination. The tenacity and tackiness of varnish, however, requires an extremely high Reynolds number for successful removal, meaning that flushing provides only limited success. There have been several case studies and scientific papers indicating that the use of electrostatic oil cleaners will reabsorb and remove varnish from the internals of a system. Although electrostatic oil cleaning requires many system circulations at a low flow rate taking several weeks to accomplish, it is an effective means of cleaning system internals.
7. What is the best way to protect a system that is prone to oil degradation due to environmental stressing factors like high temperatures and aeration?
If the oil reservoir can be redesigned, that might be the best remedy. If that is not possible, adding an offline cleaning system with automatic air release will be a great help. Adding coolers should also be considered to lower the overall oil temperature.
Unfortunately, many lubrication systems are poorly designed, providing insufficient settling time for contaminants and inadequate cooling periods. Systems that have unsatisfactory aeration can be aided by products that remove bubbles in the reservoir, preventing thermal degradation. Analytically monitoring the varnish potential of the fluid (refer to answers in question No. 2) is recommended in all suspect systems, especially those using Group II or Group III basestocks. This can alert the user to hot spots in the system, allowing corrective actions to be taken. If it is not feasible to redesign aspects of the lubricant system, incorporating a technology such as an electrostatic oil cleaner to remove the degradation by-products as they form is the best method of protecting a system.
Try to solve the system conditions first. Effective solutions to specific applications in the field include the addition of an oil cooler or a system air blower, and checking pumps and other equipment for air leaks. The use of higher quality fluids, including changing to synthetics, can also be an effective solution.
8. Is it possible to remove sludge and carbon and oxide insolubles from an in-service oil, and if so, what is the best method for doing so?
Yes, it is possible to remove all contaminants including oxidation by product solids and water with one off-line system. Using compressed cellulose media to adsorb, filter and absorb is the simplest and least expensive method.
Standard mechanical filtration is incapable of removing the majority of degradation by-products because the particles are typically smaller than one micron. Luckily, a couple of options exist. There is evidence supporting the use of certain high-density cellulose filter medias as the high molecular weight of the degradation by-products has an affinity to the filter media. In this case, the filter media absorbs the degradation by-products rather than filters the contaminants. Some technologies electrically charge the degradation by-products, agglomerating them together. Once the particles become large enough, they can be removed through the system’s mechanical filters. Unfortunately, this technology results in a sharp, temporary increase in the fluid’s particle count. Electrostatic oil cleaning is an industry-tested technology that removes submicron contaminants. As a result, electrostatic oil cleaning will remove the harmful degradation by-products before they accumulate to form varnish.
Theoretically, it may be effective to use a unit that puts a small electrical charge on the oil to agglomerate submicron contaminants so that they can then be filtered out. This could help minimize varnish formation. The downside to these units is that it is usually a long time before they begin to work - typically six months. In addition, large contamination issues will severely limit their effectiveness (that is, a large ingress of water). These units may be effective after a varnish flush is performed.