What is Varnish? Varnish formation Lubricant varnish is defined per ASTM D02.C01 WK27308 as a thin, hard, lustrous, oil-insoluble deposit, composed primarily of organic residue, and most readily definable by color intensity. It is not easily removed by wiping with a clean, dry, soft, lint-free wiping material and is resistant to saturated (light hydrocarbon) solvents. Its color may vary, but it usually appears in gray, brown, or amber hues. Varnish begins its life as a soluble degradation product before converting to an insoluble particulate form. The process responsible for the deposition of particulate varnish is reversible. Lubricant solvency Under normal operating conditions, turbine lubricants are subjected to oxidation, which produces polar molecules, the varnish precursors, from lubricant mineral-oil base stocks. These polar species represent the starting point of the varnish life cycle. As a result, lubricants in service are a complex combination of base stocks, additives, and contaminants. A lubricant’s solvency is defined as its ability to dissolve these distinct components. Everything in the oil has a finite solubility which is affected by numerous variables (molecular polarity, contaminant levels, temperature, etc). When the solubility of a molecule is low, the lubricant cannot dissolve those components which then release from the fluid to form deposits. However, when the solubility of a molecule is high, the lubricant will have a high capacity to dissolve it, avoiding the formation of varnish deposits. Contaminant levels As the oil degrades and oxidation products accumulate, the solvency of the fluid decreases accordingly. Beyond the saturation point, the fluid can no longer dissolve additional varnish precursors formed by continuing oxidation and varnish will begin to precipitate from the solution. Temperature Oil temperature directly affects the solubilities of all the species dissolved within it. As temperature decreases, so too does the solubility of varnish and its precursors. Because metals are more polar than the lubricant’s base stock, the precipitated polar varnishes prefer to adhere to the metal and form potentially damaging deposits. When the level of varnish precursors in a lubricant is at (or near) the fluid’s saturation point, varnishing in cooler regions is very likely to occur. Types of varnish The images below depict four different formations of varnish as they can appear in different types and locations throughout a lube system. While this list is not comprehensive, the types listed below are among the most commonly seen. Varnish can be soft and gooey (Sludge) Varnish can be hard and brittle (Lacquer) Varnish on reservoir ceiling (Stalactites) Varnish deposits on reservoir floor (Plated) Testing for varnish Varnishing can cause costly turbine downtime. An easy solution to combat this is to determine the lubricant’s potential for varnish formation. Two of the most widely adopted techniques are QSA® (quantitative spectrophotometric analysis) and the standardized MPC (membrane patch colorimetry, ASTM 7843). Both methods can produce results which vary significantly depending upon the length of time during which the oil sample was “aged.” Indeed, longer sample aging periods produce higher MPC values, suggesting that degradation of lubricants continues in the sample bottle. For this reason, the ASTM MPC method suggests all samples be incubated at room temperature for 72 hours after being heated to 140°F (60°C) for 24 hours. This well-defined and standardized aging time has provided inter-laboratory consistency and improved testing repeatability. hyprofiltration.com/ 96
