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Jul-2011

Cost-effective condition monitoring

The elemental analysis of used lubricating oil has become an essential part of “condition monitoring”, the use of physical and chemical techniques to measure the condition of plant and equipment, with the objective of preventing equipment failure and optimising maintenance programs.

Olaf Schulz
SPECTRO Analytical Instruments
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Article Summary
Specialist service laboratories and major plant operators analyse hundreds of oil samples per day for a wide range of elements, to detect component wear and the presence of foreign matter that may accelerate wear. The systematic analysis of lubricating oils in service can result in lower operating costs, reduced downtimes, extended plant and equipment lifetimes, and more effective maintenance programs.

There are many techniques for elemental analysis, but Inductively Coupled Plasma — Optical Emission Spectrometry (ICP-OES) has become the technique of choice in most service laboratories. There are two main types of ICP-OES instrument: sequential ICP-OES spectrometers measure each element in turn by “scanning” from one element to the next, whereas simultaneous instruments measure all the programmed elements at one time. The latter approach offers considerable speed advantages and hence faster sample throughput, but simultaneous instruments have traditionally been significantly more expensive than sequential systems.

The Spectro Genesis is a simultaneous instrument that offers a real economic alternative to sequential ICP and Atomic Absorption spectrometers. The latest detector technology combined with factory-installed applications packages and remarkably low running costs provide an effortless introduction to simultaneous ICP-OES for those unfamiliar with the technique and a powerful and efficient tool for routine used oil analysis. The Spectro Genesis is the first and only ICP-OES spectrometer available with a complete set of factory calibrated methods for used oil analysis — truly “plug & analyse” without needing to first develop a method.

This paper describes the Spectro Genesis in some detail and illustrates its suitability for elemental analysis in condition monitoring.

Condition analysis: introduction
Lubricating oil analysis has been used to monitor the condition of engines and other machinery for over 50 years. It has been likened to the use of blood tests by a clinician in determining the condition of a patient. This analogy is quite useful, as it suggests the main object of the exercise: to assess the condition of the mechanical system that the oil is lubricating, rather than that of the oil itself. It can be applied to most lubricated mechanical systems, such as engines, gear transmissions, hydraulics and the like, and has wide application in areas such as construction machinery, power generation and transportation, including aviation, fleet operations and public transport. One of the most powerful arguments for condition analysis is that it can trigger preventive maintenance before component wear leads to potentially catastrophic failure. Early detection of foreign matter in the oil, perhaps due to an air filter failure, can prevent wear and costly repairs.

There are several causes of wear, such as friction between moving surfaces, abrasion by contaminants such as grit, corrosion processes and so on, but most give rise to the presence of microscopic metallic particles in the lubricant as components wear away. Quantitative measurement of metallic elements in the oil can therefore be a useful indicator of wear. Furthermore, as different metals are used to manufacture different components, elemental analysis can often provide a clue as to which components are subject to wear. Analysis can also detect the presence and possibly the origin of foreign matter in the oil, such as dust that may have entered an engine via a defective filter.

Many other changes can occur in oils under fault conditions, such as dilution by fuels, or contamination by water or anti-freeze. Processes such as oxidation can lead to changes in lubricant properties like viscosity, leading to accelerated wear rates. Not all these processes can be detected by elemental analysis, so several different physical and chemical measurement techniques are necessary for comprehensive condition monitoring, but elemental analysis has become the essential tool for wear detection.

Interpretation of the analytical results from oil analysis is itself a complex and specialised task. Many lubricating oils contain additives to improve their properties that are themselves metallic compounds, and some of these metals may occur in the wearing components themselves. Therefore the presence of a particular element does not necessarily indicate wear. Indeed, these additives are used to improve or extend the lubricant properties of the oil, and may be consumed over time. This is known as “additive depletion”, and unless the oil is changed or the additives replenished, the oil itself may lead to increased wear, so the level of additives needs to be monitored. Mechanical systems and engines are often subjected to a running in period, during which wear can be quite rapid but is actually beneficial. For these reasons, system management decisions are rarely made on the basis of single oil analysis measurements or against predetermined limit values, but by following trends established by regular sampling regimes.

Software packages designed to assist in the interpretation of measurement data are commercially available. Unless wear is severe, metallic particles entering the lubricant are usually very finely divided (10 microns or less) and remain largely suspended in the oil without settling out. Oil samples like this can be treated essentially as solutions and are amenable to analysis by several well-established laboratory techniques. In more severe wear, larger particles can be produced that settle out and require a different approach.

Large metallic particles in any lubricating oil are a cause for concern, and one popular technique is the use of magnetic sump plugs to harvest such particles for subsequent analysis aimed at identification of their origin. Table 1 indicates the possible significance of some elements found in used oil: it is by no means exhaustive.

Analytical considerations and techniques
As mentioned above, fine wear metal particles remain suspended in the oil. Additive elements are usually in solution. Under these circumstances the oil sample can be regarded as homogeneous and analysed by solution techniques. Typical concentration levels for wear metals lie in the range from 1 to 500 parts per million, and some additive elements can be at several thousand ppm. For most elements, these concentrations are well within the scope of spectroscopic techniques such as ICP-OES, Atomic Absorption Spectrometry (AAS) and Energy Dispersive X-ray Fluorescence (EDXRF). The requirement for speed and the need to measure many elements in each sample means that sequential techniques (including sequential ICP-OES) are usually considered too slow for high-throughput applications. So when high sample throughput is required the relatively high speed of simultaneous ICP-OES has made it the technique of choice in service laboratories, particularly as the sample preparation required is usually limited to a simple dilution with a solvent such as kerosene.
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