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Several obstacles may be encountered when attempting to measure varnish in turbine or rust and oxidation (R&O) inhibited lubricating oils. Despite the recently published varnish potential test standard (membrane patch colorimetry, ASTM D7842), there are still real challenges to determine an acceptable level of varnish measurement that can provide trouble-free operation as well as an unacceptable level that would warrant taking measures to prevent and remove varnish from lubrication systems. Another obstacle to successful monitoring and establishing proper alarm levels is the constantly changing combination of oil and equipment types.
Varnish is a submicron-sized soft contaminant that is polar in nature. It is thought to be approximately 0.01 micron in size. This extremely small size has led to a whole new area of research called submicron particle counting. As a soft, insoluble contaminant, it is not clearly definable as liquid or solid. In fact, temperature alone can move it from soluble to insoluble and back again. This makes physical trapping of varnish by mechanical means difficult. However, since it is polar in nature and can accept a positive or negative electrical charge, this polarity can be used in attempts to capture varnish.
Electrostatic sparking in oil (Ref. Pall Corp.)
The majority of root causes for varnish formation in R&O oils are foaming/air entrainment and bubble implosion, electrostatic spark discharges, and additive depletion/incompatibilities. Adiabatic decompression is the technical term for the bubble implosion that takes place when oil bubbles do not completely rise out of the lubricant (from oil pressure) upon returning to the lubrication reservoir. These suspended bubbles get sent back into active lubrication duty before they have had time to rise and disperse at the surface. They become highly compressed in between gear and bearing surfaces. This can also lead to microdieseling.
Samples with different levels of varnish potential
To understand the dieseling term, think of the main difference between a spark-ignition (gasoline) engine and a compression-ignition (diesel) engine. One uses a spark plug, while the other uses a high pressure/temperature rise as the source of ignition. Adiabatic compression causes a high heat source of the air/hydrocarbon vapor in the bubble to combust (during implosion), resulting in thermal/oxidative damage of the oil to form varnish.
Electrostatic energy is generated by turbulent oil movement, especially by pushing oil through the pore of a mechanical filter. When too much oil is forced through a small pore space, too much static charge is produced to be dissipated by normal means. This static charge will find a point of least resistance for discharge. This point is usually inside a mechanical filter housing or at the tip of the lubricant return pipe to the reservoir. The sparking creates an oxidation byproduct known as varnish.
Additive depletion can be a source of varnish with certain types of antioxidant additives such as phenyl-alpha-naphthylamine (PANA). This synergistic additive is good at rejuvenating itself when it depletes, but it does so by creating a soft and polar byproduct. This byproduct, which has also been lumped into the varnish category, is easy to identify since its solubility is very sensitive to temperature. It will cause oil to appear hazy at ambient temperatures and clear at higher operating temperatures. Additive incompatibilities can be major or minor and will almost always be present, even when mixing brands or types of oils and within the same base-stock category.
Varnish detection has been difficult in the past not only due to a lack of standardized testing but also because of a lack of understanding of the best type of testing. It was common to see “free oil analysis reports” from oil companies showing normal ranges for the rotating pressure vessel oxidation test (RPVOT), acid number and viscosity, and thus no reason could be given for varnish-related problems. Once the existence of varnish was proven, the search began for an acceptable test method.
ASTM D7843 was developed to create a standardized test, titled the Membrane Patch Colorimetric Method. This has been dubbed the MPC test. MPC patch testing involves diluting the oil sample with a strong solvent and vacuum-filtering the mixture through a fine-pore, ultra-white-colored membrane. The color of the residue that is left on the white membrane is measured by the international CIELAB color scale. The CIELAB value between pure white and the residue color is given as the test result.
Correlation of the iMPC/MPC average to the water-separation characteristics of turbine oils
The basic problems with the MPC test method include controlling the temperature of the oil/solvent during preparation and filtration, resulting in a loss of data during filtration and difficulty in differentiating between soluble and insoluble. By measuring the patch residue’s weight in some repeatability testing, you can get an idea of just how challenging the MPC method’s repeatability can be.
An improved version of the MPC test method has been developed based on many of the same principles, including the same color scale and membrane. The iMPC method represents a measurement of the total varnish present in the oil - both the soluble and insoluble varnish. It uses the capillary pressure of the membrane and the straight oil, i.e., no dilution with a solvent. The pure oil sample is adsorbed by the membrane and allowed to dry into a stain, not a residue. The color of the stain is measured and calculated in the same manner as the MPC patch residue. The CIELAB value between pure white and the residue color is then given. As a result, no color data is lost during the vacuum filtration where soluble color bodies are carried away by the solvent.
The simplest way to establish if an oil is functioning normally or is depositing insoluble varnish onto a system’s metallic surfaces is to compare the ratio of the iMPC to the MPC. Since the iMPC value represents the total varnish (soluble and insoluble), it should always be higher than the MPC value. The best situation is a 3-to-1 or 2-to-1 ratio (iMPC to MPC). As the values become closer to 1-to-1, the higher the chance of varnish deposits in the system. An inverse value of a higher MPC to a lower iMPC is a clear signal that significant varnish deposits exist inside the system.
Correlation of air-release time with days of varnish-removal filtration
An alternative to charting two different values is to add the iMPC and MPC values and divide by 2 for an average value. This practice is useful for trending and monitoring systems that employ a varnish-removal filtration system.
The most important advantage of the iMPC/MPC ratio method is that it does not depend upon a statistically derived absolute value. By using a ratio system, it is more universally applicable to a wide variety of lubricant types and equipment types. It does not rely on a massive amount of sample data from the same oil type and equipment type. The iMPC/MPC ratio method also correlates well with other ASTM tests, including water-separation characteristics (ASTM D1401), air-release properties (ASTM D3427), and foaming tendency and stability (ASTM D892).
The various types of equipment using R&O oils are constantly being tweaked by their design engineers for perceived improvements, such as smaller lubricant reservoirs, combined lubricant functions and increased machine output. However, these changes also place higher stress on the lubricant.
As these equipment modifications are taking place, the lubricants also are undergoing changes. Unlike the equipment changes, which are well-documented, the lubricant adjustments go by relatively unnoticed. Lubricants can be modified and sold under the same trade name even though radical changes may have been made to their formulation, such as in the base stock or additive type. In the lubricant business, this is called reformulation. This reformulation is sometimes due to refinery upgrading, which makes legacy base stocks unavailable, or a newly discovered undesirable additive characteristic, which leads to its replacement. While reformulation is fine if equipment designs are stagnant, in the constantly changing world of equipment design, reformulation can sometimes lead to unwanted consequences, including varnish.
An agglomeration type of varnish-removal system (Ref. ISOPur Inc.)
Over the past 10 to 15 years, industrial lubricants with the same trade names have silently become composed of Group II and III base stocks, which are better suited to the automotive lubricant market. Group I base stocks are more soluble of varnish contaminants, making them better industrial lubricants for many applications, but industrial lube sales volumes pale in comparison to automotive lube sales. This quiet disappearance of Group I base stocks began to have a pronounced negative effect in certain machines and applications, causing “varnish buildup.” The category reformulations coupled with the expanded use of PANA-type additives led to worldwide reliability issues. At the time, the root cause of this varnish problem was unknown. Was it the changes in additives, base stocks or equipment designs? In reality, a combination of all of these factors played a role.
A wide variety of varnish-removal systems have been introduced during the last 10 years. Most have been aimed at removal (reactive), although some have been designed for varnish prevention (proactive). The three main categories are agglomeration electric-charge filters, depth-media filters and precipitation electric-charge filters.
Agglomeration-type filters are effective with submicron particles and soluble and insoluble varnish. They can use conventional filters of almost any type. The proprietary electrical-charge component is fixed and not changed, which yields low operating costs. These types of filters offer the advantage of high flow rates for large reservoirs. They are best used as a preventive measure.
Depth-media filters can quickly remove larger, more insoluble varnish like PANA. They are not usually affected by water content and require a relatively low upfront cost and little maintenance. These filters are best used as a removal tool for insoluble varnish.
Precipitation electric-charge filters are well-suited for smaller reservoirs. Like agglomeration-type filters, they are effective with submicron particles and soluble and insoluble varnish. They also have a relatively low upfront cost and are best used to remove insoluble varnish.
Combining these technologies could be very beneficial, particularly the use of an agglomeration type of charging system with a depth-media filter to collect agglomerated varnish instead of using conventional filtration. An ideal system might consist of all three technologies - an agglomerator, a precipitator and a depth-media filter for collection.
Several recent trends may make the varnish issue a moot point. These include the phasing out of PANA-type antioxidant additives, the introduction of varnish-solubility additives that work in conjunction with electric-charge technology, the use of Group I base stocks as “boutique oil blends,” and the development of submicron particle counting, which likely will lead to better ways of monitoring soft contaminant issues. In addition, original equipment manufacturers and oil companies are communicating more frequently in all-volunteer organizations like ASTM.
ASTM’s work on automatic particle counting of submicron particles less than 4 microns may offer a more credible approach for varnish measurement. It does not measure a specific color range but rather a specific size range and likely will not discriminate what is detected by the root cause of the contaminant (color). While the proposed new standard is still in its infant stages, many hope it will overcome the problems inherent in the MPC patch.
All this attention on the varnish issue has served as a reminder of how prevention is much less costly than the cure. Indeed, it is easier to keep things clean from the beginning than to clean them up after they are dirty. Most conservative research estimates place the relative cost savings at nearly 80 to 90 percent. In other words, it only costs about 10 to 20 percent more to implement a contaminant prevention program than it costs to remove the contaminants after they are in the system.