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At the first Practicing Oil Analysis Conference, a presentation featured X-ray fluorescence (XRF) as an improved method for machine condition monitoring. Eaton Corporation has adopted this technique and taken it to another level in the condition monitoring of hydraulic fluids. Eaton’s proactive maintenance group is dedicated to preventive maintenance as a way of prolonging equipment life. The Vickers® fluids laboratory is a fully equipped, ISO accredited laboratory that offers its services to the hydraulics industry for routine oil analysis as well as failure related analysis. At Eaton, whenever a hydraulic system failure occurs, it is standard practice to request not just routine oil analysis, but to perform in-depth analysis to help diagnose the root cause of the failure.
The X-ray fluorescence spectrometer (XRF) has been in use for many years in the analysis of chemical composition.
The methods employed have consistently advanced in the same stride as the instrument technology itself. Today, XRF analysis is a reliable method of predicting the onset of abnormal wear or resolving failure analysis issues in hydraulic systems.
In condition monitoring, XRF should be routinely employed and specified for basic oil analysis tests, with tests such as viscosity and particle counts. Today’s technology presents XRF analysis as a routine laboratory procedure, as opposed to its former status as a complicated tool used exclusively for research purposes. The results from this type of analysis should be made available to the average technician who is interested in routine condition monitoring or needs to resolve a failure-related problem in his machine. When compared to a simple particle count, XRF reveals information about the rate of wear in the machine and the possible source of contamination.
A common misconception in condition monitoring is the adoption of inductively coupled plasma atomic emission spectroscopy (ICP-AES) results, now popularly known as wear metals, as an absolute measurement of wear and other contaminants. Despite the high resolution and low detection limits available today with inductively coupled plasma (ICP) instrument, its major drawback in wear analysis is that this instrument is limited by particle size. It is generally accepted that this instrument will not resolve contaminants in excess of 10 microns in size. If wear analysis is limited to ICP techniques, the presence of large particles - a strong indicator of abnormal wear modes - may not be detected. The rotating disc electrode spectrometer (RDE), which is fast becoming outdated, is also limited by particle size, among other issues although to a lesser degree than ICP.
The use of XRF to determine wear metal composition in lubricating oils does have some of its own drawbacks. As stated in the applicable ASTM standards, interference does occur with the oil additives, and this affects the measured intensities from the elements of interest. One other prime factor that affects the measured intensity is the oil. The oil tends to absorb some of the X-radiation. The Vickers® laboratory has overcome this obstacle by isolating the wear contaminants from the oil and then subjecting these contaminants to the radiation. The resulting intensities far exceed those acquired when the contaminants remain suspended in the oil.
The wear contaminants are separated using a 0.8-micron filter and a solvent of choice to remove traces of the oil as in a patch test. The filter is then placed in the path of the bulk X-rays. The nature of the bulk X-rays causes the filter material to be analyzed with the contaminants. Programs supplied with the instrument will permit the resolution of the filter and other matrices that may interfere with the desired results. Automatic peak identification is also a common software feature that helps the laboratory technician to identify the elements that make up the contaminants found in the system. The complete test process can be done in a fraction of the time taken for the same test in the past.
Having identified the elemental composition of the contaminants, the next step is to qualify the results by identifying likely sources within the system that generate the observed oil contaminant. The Eaton fluids laboratory has a database of common elements found in machines and their sources. The XRF analysis results are merged with information from this database to assist the technician by supplying references on the test report to the likely contaminant sources within the plant. The importance of contamination data cannot be overemphasized, because experience shows that the majority of machine failures are contamination related.
Because contamination in a machine could be intrinsic or extrinsic to the system, it is important to identify the source and take corrective action. Generally, there is a failure chain reaction that begins with small size particles, which generate other larger size particles over time. The XRF analysis results can be used to monitor contaminant sources before they can cause catastrophic failure of the system.
XRF results have been used to identify coolant leakage into the system through broken gaskets, cracked cylinder heads or oil cooler leakage, when components of the coolant are found in the oil. XRF analysis will reveal the encroachment of dust or sand particles into the system from broken vent filters. Generally, the elements found in the oil can be used to determine wear characteristics of some specific components within the system.
A simple particle count test alone may reveal a large number of contaminants; in turn this should trigger the laboratory to perform an XRF analysis to determine the identity and source of these contaminants. The ease and availability of this technology make it a convenient and powerful tool in condition monitoring. The cost savings achieved through the use of XRF test results in preventing a failure can be very significant.