The approach of detecting and analyzing sludge and varnish problems in machinery is not the same as used oil analysis. In many instances this is because the evidence is not always in the oil. The sludge and varnish should be analyzed directly, using a completely different set of tests and evaluation parameters. Still, used oil analysis plays an important diagnostic role to help reveal candidate causes as well as to rule-out others.

The conditions that commonly lead to sludge and varnish problems vary, which complicates the process of identifying the root cause analytically. There are at least 25 unique lubricant degradation mechanisms leading to sludge or varnish formation.

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Hot oil has typically received the most press, even in this publication. After all, oil is not like fine wines that get better over time. In fact, even the very best wine will stress-out when exposed to too much heat. For instance, most wines will age roughly twice as fast at 77°F compared to 55°F, which is why connoisseurs and collectors prefer cellar temperature to store their liquid assets.

In 1903, Svante Arrhenius won a Nobel Prize when he figured out the relationship between temperature and most chemical reaction rates. Often called the Arrhenius Rate Rule, it relates to the fact that lubricants, once they’ve exceeded their base activation temperature, will degrade or oxidize twice as fast for every 10°C (18°F) increase in temperature.

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There has been much said in recent years about the increased propensity of some modern oil formulations to produce sludge and varnish. Ironically, many of the oils accused of contributing to varnish are the same oils endowed with exceptionally high oxidation stability due to advanced refining technology and formulation science. To better understand this paradox, let’s begin by defining a new term.Lubricants have an intrinsic Impurity- holding Capacity (IHC) relating to their ability to retain very small (submicron) impurities in solution. These impurities are largely oil-degradation products, but include other oil suspensions as well.

This holding capacity is like an imaginary silo or cup within the oil. If the cup is half empty, it has unutilized or reserve capacity. However, conditions can occur that can cause an overfill, leading to insoluble suspensions in the oil and varnish (sometimes called the “dumping point”).

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In recent years, there has been an unusually large number of reported cases associated with varnish and sludge formation in turbine-generator and compressor applications using Group II turbine oil formulations. Explanations for these problems have varied but typically include Group I/Group II incompatibility, additive instability, bulk oil oxidation, adiabatic compressive heating, electrostatic discharge, among others. This paper reviews these failure pathways and discusses actual case history including root cause analysis. Analytical methods for the sludge/varnish and the degraded oll are also reviewed.

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The products of lubricant degradation are called sludge and varnish. These products start in the dissolved form and accumulate until the lubricant reaches its capacity, referred to as the saturating point, forcing any excess to convert into insoluble degradation products. In certain instances, deposits form on machine surfaces at the exact location where the oil has degraded. In other cases, the oil degrades in one location but, the insoluble degradation products are carried elsewhere by the moving fluid forming deposits on surfaces.

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Most oil-related problems in machinery lead to wear or corrosion. However, there is one very serious problem – the presence of sludge and varnish – that may cause neither. Instead, these substances plug flow passages and cause valve sticking. Sludge and varnish can occur even In the most well maintained machines. Surprisingly, they may also appear when oils are not particularly old or contaminated. And they can occur in even though most thermally robust synthetic hydraulic fluids. While there a:re many well-known reasons why oils will throw sludge, there are an equal number that are unknown or misunderstood.

First, what is the distinction between varnish and sludge? Both can form directly or through n sequence of intermediate steps. For simplify lion, we will define varnish as tough, adherent oxide or carbonaceous material that coats internal machine surfaces. Heat and/or time will cure varnish to a hard, brittle consistency. On the other hand, sludge, which is sometimes a precursor to varnish, is soft and sticky in form. Sludge can move around a system until it finally comes to rest at sump bottoms, troughs, strainers, filters, and narrow fluid passages. Other common names for varnish and sludge include deposits, lacquer, tars, pigments, gums, and resins.

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