The formation of varnishes in lubricated systems is a widespread problem in various industrial environments. Usually, “varnishes” are those reddish-brown carbon deposits (see picture), which originate from lubricant degradation byproducts and can cause a series of operational problems, which are particularly serious in turbines and hydraulic systems. If not monitored with sufficiently sensitive and accurate instruments and controlled with appropriate corrective actions, the deposit of such insoluble substances in vital parts of the machine can cause problems of various nature.
Among these, the most common are the increased stress on the servo valve, the reduction and progressive obstruction of the lubrication passages and the increase of operating temperatures. The end result can be increased wear, with potential malfunctions and unplanned downtime.
The problem of varnish formation arose from the first moment an oil was used as a lubricant. As a result of oxidation and aging, deposits are inevitable. However, recently the varnish problem has captured the “limelight” in critical lubrication systems. This is because of the combination of many factors: more demanding operating conditions, lower tolerance, new base oil formulations, prolonged time of operation of the loads.
The impact following the formation of varnishes is transversal and appears in different types of industrial machinery. In paper mills it is reflected on a worse quality of paper as a result of a lessened control of its sensitive rollers; the plastic injection molding industry is equally affected, as well as the large ships that heavily rely on hydraulic systems with servo valves for safe navigation.
These products have the strongest impact in the world of energy production. Accidents are more frequently occurring in the management and difficult ignition of large gas turbines due to the accumulation of such byproducts. Fossil fuel plants have frequent and expensive downtimes because their control valves get stuck. Even some hydroelectric plants have recently experienced (following the introduction of automated steering systems) problems related to the accumulation of varnishes. Globally, we’re talking about an annual loss of tens of millions of euros because of problems directly or indirectly related to the accumulation of varnishes. Therefore, it is no coincidence that the energy industry is desperate for solutions to the problems associated with varnishes.
The first step must be to understand the root causes of this phenomenon. The main form of aging of a lubricant is called oxidation and the chain of reactions responsible for the formation of varnishes is usually defined as the varnish life cycle.
The first moment of the varnish life cycle begins from oxidation. The byproducts of oxidative degradation are soluble and are formed when the presence of specific additives (phenolic or amino products) is lacking in the lubricant. If the process continues (in the absence of corrective actions), the oxidized (and oxidizing) substances aggregate, undergoing phases of condensation and polymerization in which we witness the creation of heavy macro-molecules. Such aggregations (though smaller than 0.1 microns) appear less soluble. Depending on the temperature of the fluid, they tend to precipitate, originating particles generically defined as “soft contamination”.
These contaminants tend to agglomerate and grow in size to form deposits on servomechanisms and other critical components. These aggregations are the starting point for further precipitates formation.
We are witnessing a bi-univocal behavior at the level of solubilization, precipitation, agglomeration and varnish formation, which means that we are in the presence of reversible phenomena. So if the temperature of the fluid rises, the large insoluble precipitated molecules can re-enter the solution. Within this cycle the oxidation and condensation phases are the only ones that cannot be reversed.
Reversal of the process
The reversal of the process within the varnish life cycle emphasizes the effects, whether they’re good or bad, that changing operating conditions or applied technologies can have on the same system.
For example, the integration of cleaning through electrostatic oil cleaning systems (to remove insoluble substances) and a phase of ionization of the fluid (to remove soluble contaminants) can prevent or reverse the formation of varnishes. The relationships between each of the four reversible phases (solubilization, precipitation, agglomeration, varnish formation) are affected by Le Chatelier’s law. According to this law, the balance of reversible reactions can be modified through interventions on the reagents at one of the extremes of the reaction itself. In other words, if the products of an equilibrium reaction are removed from one side, this process tends to shift the new balance in exactly that direction. This means that the removal of the agglomerates through electrostatic filtration pushes the already formed varnishes back into solution.
Since in this situation the agglomerations are also linked to the precipitates, their removal will favor the removal of the precipitates themselves. In fact, the presence of varnishes acts as a catalytic factor responsible for increasing the level of oxidation of the oil. The strong link between preexisting varnishes and new oxidative bonds is a fact that must be reflected in the proactive management of lubricant loads, with measured and targeted monitoring.
Unfortunately, even at its most in-depth level, the traditional approach of oil analysis for predictive maintenance purposes does not offer the possibility to detect the presence of varnish precursors in the oil with sufficient effectiveness. In fact, emission spectrometric techniques (RDE-AES or ICP-AES) are insensitive to carbonaceous contaminants, while infrared spectrometry (FT-IR) only allows these substances to be detected when in alarming concentrations, preventing an effective “predictive” management of the varnish problem. The viscosity of the lubricant is affected by the presence of oil degrading issues, but significant changes in viscosity are usually late signals of this phenomenon. The determination of TAN (Total Acid Number) is not effective in this case either, because many of the varnish precursors are chemically neutral. Finally, normal particle counting techniques (manual, laser, flow decay) are not effective in detecting the presence of varnish precursors, as they are insensitive to particles smaller than 4 µm.
In this context, the only currently reliable approach to detect the formation of varnishes is the visual inspection of mechanical organs: a practice that intervenes when the problem already exists, forcing the machine to stop, with very high management costs.
Mecoil Diagnosi Meccaniche Srl, serving Maintenance Engineering of the most advanced industrial environments in Italy for almost twenty years, has developed a technology for the quantitative evaluation of the varnish formation trends, specifically designed for turbine lubricants and hydraulic fluids (VTI). Since May 2013, we have introduced the new analysis procedure in accordance with the new standard (ASTM D7843). For a long time the laboratory was performing correlation tests between the VTI method, developed internally with the collaboration of international laboratories, and the new method, based on the CIE LAB color space, obtaining excellent results.
According to the new standard, the tendency of the oil to form deposits is estimated with the “DeltaE” index, which measures the perceived difference in the color of the membrane compared to the virgin membrane, with a spectrophotometric measurement. Mecoil, the first and so far only laboratory in Italy, is now able to perform both determinations with the same efficiency and speed, to give you an increasingly complete and updated report.