Oil filter debris analysis’ (FDA) origins lay mainly in the aviation industry for many years. A program developed by Air Research in the 1980s claimed a 95-percent success rate of predicting catastrophic failures. It has gained recognition in the last decade in many other industries as a powerful diagnostic tool. This analysis can give maintenance personnel additional protection beyond normal external service inspections and greatly reduce the possibility of premature mechanical failure. When used with wet chemical testing and modern technologies such as the scanning electron microscope (SEM) or X-ray Fluorescence, exact determination of any type metallurgy or contaminate can be achieved.
Detection Limitations of Oil Analysis
The most common technique used in oil analysis to measure wear metals and contaminates is atomic emission spectroscopy (AES). While this works well for a wide range of applications it is limited in its detection range. It typically only measures particles from 2-10 microns in size. There is a variation of this method called LPS or RFS which can measure particles greater than 10 microns however, once you get over 100 microns it is not very effective. Another technique used is analytical ferrography where particles are captured from the oil sample and deposited on a slide which is viewed through a microscope. This method is also limited to only the particles which can pass through the filtration system. The only way to capture these larger particles is to look for them where they reside which is the filter.
Oil Filter Debris Analysis Test Methods
The testing process begins by removing the debris from the filter. This is typically done by placing the filter or filter media in a beaker which is filled with a solvent and placed in an ultrasonic bath for a specified period of time. The material that is extracted from is then back-flushed through a filter membrane with a specified pore size. The filter membrane is then dried for inspection. At this point the material can either by viewed by microscope for the presence of ferrous and non-ferrous wear particles along with various other contaminates. When significant amounts and/or types of material is observed, the material is then identified by a variety of methods. The first and most economical way is by wet chemistry testing. Non-ferrous metals such as Aluminum, Silver, and Magnesium can be identified by a simple spot test using one chemical which provides a positive or negative response. Ferrous debris such as carbon steels, tool steels, alloy steels, and stainless steels must first be digested in acid for the testing process. The presence of certain alloys such as nickel can be quickly identified using the digested solution. To identify the overall alloy composition the digested solution is then analyzed on an atomic emission spectrometer. In cases where there is very little material to work with or the material is a non-metallic contaminate which you need to identify, the debris would need to be analyzed by either SEM or X-ray for elemental composition.
Identifying the Source
Oil filter debris analysis when performed by SEM or X-ray is pretty straight forward when it comes to taking the test results and matching them up to possible sources. This is due to the fact that the instrumentation is equipped with software that matches the alloy composition with library profiles stored in the computer. This usually translated as a AMS designation number which specifies exact alloy composition and percentage of alloy. When performing the analysis atomic emission spectrometer the alloys present have to manually identified and calculated to reflect the percentage of each alloy. This is then used to consult with the equipment manufacturer (OEM) to determine the source of wear.
This article was previously published in the Reliable Plant 2014 Conference Proceedings.
By Scott Shoemaker, Analysts, Inc.
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