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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A wide range of fluid degradation and contamination-related issues can affect turbine lube oil systems. One serious and growing concern is the presence of sludge and varnish. This condition can occur in even the most well-maintained machines. Surprisingly, it can also happen when oils are not particularly old or contaminated. And it can occur with even the most thermally robust synthetic lubricants and hydraulic fluids.

In turbine systems, there are few failure conditions that can disrupt operation as quickly and completely as a varnished and seized-up control valve operation. This can be the cause of a tripped turbine forced outage or other production losses. So too, sludge in many circulating lube oil systems can gum up flow controls, strainers and critical oil ways. In recent years, there has been a large number of reported cases associated with varnish and sludge formation in turbine-generator applications. Explanations for these problems have varied but typically include Group II mineral oil solubility issues, additive instability, bulk oil oxidation, adiabatic compressive heating and electrostatic discharge, among others. This paper will review precursor conditions that lead to sludge formation, some of the common lubricant degradation methods and the role of oil analysis in recognizing the potential risk well ahead of failure.

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Anybody who has been in the lubrication field for at least a couple of years has likely been badgered by problems relating to air contamination. Indeed, aeration and foam present many serious reliability concerns which merit immediate attention by machinery maintainers. However, for most people, the causes are too complex to troubleshoot, leaving the root-cause mystery unsolved. Sometimes an oil change provides a suitable remedy, but too often it serves as only a temporary fix. In other words, there is a need to find why it happened.

Laboratory researchers have conducted numerous studies over the years relating to air contamination. From this body of work, we’ve learned many interesting new facts that help us both diagnose the problem and prescribe corrective actions. Some of these new insights into the causes and effects of air entrainment and foaming problems were discussed in the article “The Perils of Aerated Oil – Let Your Machine Burp”, published in the January 2005 issue of Practicing Oil Analysis.

In this column, I want to share more snippets of information that can be useful in the control of air contamination. I’m guessing that even seasoned oil analysis and lubrication professionals will pick up a pointer or two.

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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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In all of my years in oil analysis, I cannot recall a single time when data from a surface tension (ST) or interfacial tension test (IFT) appeared on a routine analytical report, aside from transformer oil analysis, for which the test is considered routine.

The reason this is so surprising is that many studies have reported that changes in an oil’s surface tension to be the earliest sign of contamination, sludge potential and oxidation. Quoting from the book “Lubrication of Industrial and Marine Machinery” by William G. Forbes, “The interfacial tension test is the most valuable single test that can be used to evaluate a turbine oil.

It is generally agreed that when the interfacial tension is between 15 and 20 dynes/cm, deposits may or may not be forming in the system. Safe practice would call for an oil change in this range.”

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