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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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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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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