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One of the most fundamental truths that many oil analysis users learn is that elevated silicon levels from elemental spectroscopic data equate to dirt ingression. The logic is simple: common dirt from road dust and other sources contain high levels of silica. Therefore, elevated silicon from spectrometric analysis means dirt ingression. But is this true in every situation? What other possible sources of silicon exist in a lubricating oil environment that may cause silicon levels to rise?
Depending on the environment in which the equipment is operating, silicon may appear in the lubricant for various reasons.
Possible Ingressed Contaminants
Possible Internal Contaminants
In addition, silicon, in the form of polymeric methyl silicone, is a common antifoam additive used in many types of oils. How can the educated oil analysis user differentiate between silicon in all these different forms, without expensive and elaborate chemical analysis? The key to identifying the silicon’s form and source is lock-step trending. Lock-step trending refers to the process of observing multiple parameters rise and fall simultaneously. This type of trend pattern often indicates a correlation between the two parameters, pointing toward the real root cause. An example of lock-step trending can be found in the following case.
Landfill Gas Cogeneration
A local municipality installed a cogeneration facility designed to predominantly operate on methane gas, which is produced from the city landfill, to generate electricity. After installation, oil analysis revealed rapidly rising levels of silicon in the oil, suggesting that the engine was ingesting dirt. Perhaps more disconcerting was the resultant increase in antimony, a common wear metal used in some sleeve-type bearings. The most obvious explanation was dirt ingression causing abrasive wear, but how could dirt be ingested if the air filter was in good shape and the fuel gas was prefiltered?
The clue to this mystery was the observation that the rise in silicon was accompanied by a rise in acid number as shown in Figure 1.
Figure 1. Sample Report Indicating
Acid number is usually measured to indicate oil degradation due to oxidation. While prolonged dirt ingression can lead to oil degradation as a result of increased friction and the associated increase in temperature, it seemed unlikely that was the cause for rapid oxidation in this case. After a meeting in which individuals were invited to share their views, they decided that some silicon-containing contaminant, other than dirt, was entering the engine and causing acids to build up in the oil. What was the contaminant?
The team decided to analyze a fuel gas sample to help identify the type of silicon. The fuel gas showed high levels of siloxanes, an acidic gas, which is commonly found in North American landfills. The question about the source of the silicon-containing contaminant was revealed; the siloxane gas in the fuel was accumulating in the oil, resulting in an increase in silicon levels, and the resulting acid was corroding the bearing. The solution was to scrub the fuel gas with activated charcoal to remove the siloxanes, which resulted in more normal acid numbers and a decreased antimony levels, suggesting that bearing wear rates were also back to normal.
Although this case study illustrates a rather unique situation (unless of course you’re maintaining a waste gas cogeneration facility); it does illustrate the idea that lock- step trending, together with lateral thinking can lead to problem solving and root cause analysis.
Elevated silicon levels should always raise concern. However, just like every other oil analysis alarm, the root cause of the problem should be investigated in order to formulate an appropriate maintenance response to address the root cause of the problem, without jumping to the obvious, but sometimes wrong assumption. Get into the habit of using all the data from an oil analysis report.