Oil analysis methods and lubrication monitoring
An oil analysis and lubrication monitoring program should addresses a wide variety of effects and should involve contamination control methods and established oil cleanliness procedures. Friction, lubrication oil, wear, and wear particles are interactive and cannot be separated.
The friction within machineries directly translates into power loss. Lubrication oil costs constitute significant parts of machinery operation costs in any plant. The wear is one of the primary characteristics defining the end of life for plant machinery and leads to costs of maintenance, replacement, and production outage. An effective oil analysis and lubrication oil monitoring program can increase efficiencies of machineries and reduce the operating costs. The value of this program can only be realized if it can be collected and analyzed in a timely and organized manner. The oil analysis can reveal important information about the condition of machinery, oils, and contaminations in the oils.
In order to reduce the operating costs of any plant, the focus should be on its machineries. It’s necessary to improve machinery availability and reduce the operational and maintenance costs associated with them. The reliability and availability of operating machinery depend largely on the protective properties of their lubrication oil. The machinery industry currently uses lubrication oil analysis for monitoring machineries, particularly lubrication oil performance, bearing wear, and gear system operation.
One of the purposes of lubrication oil condition monitoring is to determine whether the lubrication oil has deteriorated to such a degree that it no longer fulfills its functions. However, this is not the only purpose. Different lubrication oil condition monitoring techniques can be used to monitor lubrication oil wetted components and detect their degradation rates and possible developing damage. Many moving components are actually lubrication oil wetted parts such as bearings, gear units, or piston/cylinder in reciprocating machines. Viscosity and dielectric constant could be used as the performance parameters to identify the degradation of lubrication oils. Particles, as the result of wear of wetted machinery components, could also be detected and classified to identify the wear rates and health situations of such components.
Water and impurities
The oil contamination analysis can reveal deterioration or breakdown of the oil, contamination of the system with water or particulate debris, and wear of the lubricated machinery.
Water or impurities in the lubrication oil can clearly be seen at the visual inspection of samples. Water can be seen either in the form of emulsification or as a distinct water layer. The general cleanliness level of the lubrication oil may also be determined by a visual test of samples. Once filtered, the debris should be visually examined prior to microscopic examination. The presence of water within the lubrication oil can also be detected from the filter paper used for oil analysis. This is seen in the form of light circular areas on the filter paper. Water also sometimes oxidizes the ferrous material, and the presence of rust could indicate the ingress of water. Water affects the viscosity of the lubrication oil, considerably reducing the effect of the lubrication oil and increasing wear rates.
Frequently gear units become contaminated with mineral particles such as silica, coal, and shale. These produce fine abrasive wear particles normally only observed under the microscope. The unchecked presence of mineral particles, specifically quartzite with its high hardness, should be avoided in the lubrication oil system. The mineral particles in suspension act as a grinding medium and can produce excessive bearing wear, which leads to loss of shaft location and further accelerates the wearing process.
Wear debris analysis
Wear debris analysis is a technique for analyzing the debris, or particles, present in lubrication oil that could indicate wear, particularly mechanical wear. This method provides microscopic examination and analysis of debris/particles found in a lubrication oil. These particles consist of metallic and nonmetallic matters. The metallic particles usually indicate a wear condition that separates different sizes and shapes of metallic dust from components like bearings, gears, and generally any components that can be wetted by lubrication oil. Nonmetallic particles may consist of dirt, sand, or corroded metallic particle. Analytical ferrography is one of the methods used in wear debris analysis; it’s among the most powerful diagnostic tools for condition monitoring.
When implemented correctly, the wear debris analysis provides very useful information on machinery under operation. It’s not still in common use for all machineries because of its comparatively high price and a general misunderstanding of its value. Wear debris analysis can also help with improving lubrication oil filtration efficiency and frequency for the lubrication oil cleaning and changeover. Machinery performance may be improved through proper filtration of oil. Clean oil lubrication is always more effective.
The wear debris analysis procedure in its comprehensive form is lengthy and requires the skill of trained analysts and experts. As such, there are significant costs for a comprehensive wear-debris-analysis procedure. However, most machinery experts agree that the benefits significantly outweigh the costs and elect to automatically incorporate such a method when abnormal wear is encountered.
The most important aspect of wear debris analysis is its capability to identify developing damages and malfunctions in their initial stages. This is the main reason why such an expensive and complicated condition monitoring method should be used instead of or in addition to simpler and cheaper condition monitoring tools such as online vibration monitoring.
An oil sample contains particles that have been produced at various times. This makes the wear debris analysis difficult. Root causes of the wear-out failures in machineries could include a combination of mechanical loads such as stresses or strains, the process environment, and the surface oil chemistry. The wear process environment could include heat, dust, and water contamination. The surface oil chemistry for oil wetted surfaces can be benign or under chemical attack, depending on the condition of the oil, presence of corrosive oil contamination, and other process details.
Particle analysis methods
The main aspects of wear debris analysis are quantitative figures and qualitative facts. Regarding the qualitative aspects of debris, color, shape, and texture is important. As the first step, the morphology of wear particles should be examined visually by a trained expert. After that the computer scanning and image recognition should be applied. Advances in computers and image recognition make automatic evaluation of the particle morphology possible. It may be characterized by a set of numerical features, and then appropriate classification methods can be used for wear particle identification.
Color and characteristics of particles
Color of particles and debris is an important feature in the wear debris analysis. If the shape and texture allow one to differentiate the wear particles according to their prehistory of formation, color may help to define debris composition or other useful data. Composition of wear particles is determined by the materials of worn surfaces, contaminants, and products of chemical reactions. In lubricated metallic contacts, steel, copper, lead, tin, chromium, and silver often can be generated as wear particles. Ferrous oxides found in the lubrication oils usually can be divided into two groups: red or black oxides. Examination of color allows one to define the source of particle generation and the severity.
White nonferrous particles, often aluminum or chromium, appear as bright white particles. They are deposited randomly across the slide surface with larger particles getting collected against the chains of ferrous particles. The chains of ferrous particles might act as a filter, collecting contaminants, copper particles, and Babbitt particles. Copper particles usually appear as bright yellow particles, but the surface may change to verdigris after heat treatment. Babbitt particles usually consist of tin and lead; Babbitt particles appear gray, sometimes with speckling before the heat treatment. After heat treatment of the slide, these particles still appear mostly gray, but with spots of blue and red on the mottled surface of the object. Also, after heat treatment, these particles tend to decrease in size. These nonferrous particles usually appear randomly on the slide, often not in chains with ferrous particles. Contaminants are usually dirt (silica), and other particulates that don’t change in appearance after heat treatment. They can appear as white crystals and are easily identified by the transmitted light source — that is, they might be somewhat transparent.
Fibers, typically from filters or outside contamination, are long strings that allow the transmitted light to shine through. Sometimes these particles can act as a filter, collecting other particles.
Increased quantities of iron are common, since many machinery parts are composed of iron (different grades of steel), while an increase in content of less common metals such as silver can often indicate precisely which component is being worn abnormally. Visual and microscopic examination of the sample is as important of a source of information as the regular testing of the debris samples. Prior to filtering the sample, examination of the sample visually can give useful information. The size and shape of wear material usually differentiate between wear mechanisms.
Particle size and shape
The sizing is one of the important aspects of the wear debris study. Three parameters — average particle size, maximum particle size, and the particle size distribution — are important. As a rough indication, the damage state of machinery could be proportional to the size of the particles. This is definitely true for some machinery items such as gear units. As an indication, the following size classification is presented for wear debris analysis:
- fine: less than 5 microns
- small: less than 20 microns
- medium: 20-50 microns
- large: above 50 microns.
As a very rough indication, particles over 20 microns could indicate a potentially dangerous damage state for machineries. The wear particle shape could give an indication as to the damage mechanism by which that particle was removed. Platelets, two-dimensional particles, are usually produced by metal-to-metal sliding. Spherical, or 3D, particles are produced by bearing fatigue or by lubrication oil failure resulting in the local overheating. Spirals or similar are most often produced by a harder surface abrading into a softer one. Chunky particles are usually produced by a fatigue mechanism.
SOA
Spectrometric oil analysis (SOA) reveals the chemical composition of metal particles suspended in the oil samples. By comparing the results to the known chemical composition of various machinery parts, abnormal wear of machinery parts can be identified, and servicing of the machinery can be initiated, thus sometimes avoiding further costly repairs or even catastrophic failure.
It’s been used for many machineries particularly aero-derivative gas turbines and small and medium critical machineries using rolling-element bearings. This method is useful to take into account the loss of wear particles with oil usage or by the drainage and replacement of oil. These factors are of particular importance because of the high rate of lubrication oil used in aero-derivative gas turbines or others and the small particle sizes involved; this is the size to which spectrometers are most sensitive.
Wear particles
Sliding adhesive wear particles are found in most lubrication oils. They are usually an indication of normal wear. They are produced in large numbers when one metal surface moves across another. The particles are seen as thin asymmetrical flakes of metals with highly polished surfaces. Cutting abrasive wear produces other particle types, for instance, spiral, loops and threads. The presence of a few of these particles might not be significant, but, if there are several hundred, it could be an indication of serious cutting wear. A sudden dramatic increase in the quantity of cutting particles indicates that a breakdown could be expected.
Online oil monitoring
The basic idea of some failure-detection lubrication oil condition-monitoring methods is the early identification of chemical aging of the lubrication oil and its additives under the influence of high dynamic loadings in the wetted components such as bearings or gears. This can use online methods and could offer extremely important benefits. The online diagnostics system measures components of the specific complex impedance of lubrication oil. For instance, broken oil molecules, forming acids or oil soaps, result in an increase of the electrical conductivity, which directly correlates with the degree of contamination of the lubrication oil. For lubrication oils with additives, the stage of degradation of additives can also be derived from changes in online measurements such as the dielectric constant. The determination of the reduction in the lubrication oil quality by contaminations and the quasi-continuous evaluation of wear and chemical aging can be combined by the holistic approach of a real-time online monitoring. Another concept is the online monitoring of wear debris in lubrication oil. Online sensors can effectively control the proper operation conditions of many critical machinery parts, for instance, bearings and gears.
Multi-method monitoring
Amin Almasi is a rotating equipment consultant in Australia. He is a chartered professional engineer of Engineers Australia (MIEAust CPEng – Mechanical) and IMechE (CEng MIMechE), and he holds a master’s degree in mechanical engineering. Almasi also is a registered professional engineer in Queensland (RPEQ). He specializes in rotating machines including centrifugal, screw, and reciprocating compressors, gas turbines, steam turbines, engines, pumps, offshore rotating machines, LNG units, condition monitoring, and reliability. Almasi is an active member of Engineers Australia, IMechE, ASME, and SPE. He has authored more than 100 papers and articles.
If the rates of wear debris particles are high, it could indicate that machinery is not in good condition and might require maintenance work. On the other hand, the measurement of total wear particles could be misleading, and it may give wrong signals about a problem. Total wear debris measurement is most common, but it usually does not reflect the state of operating machinery change and machinery health.
When only one method or one sensor is utilized, the prediction is not usually reliable. The key for proper lubrication oil analysis is to use different independent techniques to achieve early and reliable monitoring results. For example, when three sensors and methods, such as viscosity online monitoring, dielectric sensor and on-site wear debris analysis, are used simultaneously, the monitoring quality is significantly improved and an accuracy prediction can be obtained.
Further improvements can be obtained by combining different oil and wear debris analysis methods with other condition monitoring systems such as an online vibration monitoring system. If all these methods indicate a developing malfunction in a component — for instance, a bearing — such a prediction could be reliable.
To avoid mischaracterizing data, different monitoring methods should be used. They could be factored using different key data points to allow for correct identifications of changes over time. For instance, different oil analysis methods, wear debris sensors, contaminant details in lubrication oil, and others such as vibration monitoring should be considered together, and, if all point to the same problem, such monitoring is usually accurate.
Conclusions
Machineries are the heart of plants. Proactive maintenance and proper monitoring are the most important factors for increasing machinery life and avoiding machinery shutdowns or damages, which in turn increase the life and profit of the plant. Machinery performance and reliability directly depend upon the health of its moving components. Machinery performance also depends upon its lubrication oil. The oil analysis can reveal important information about the condition of machinery, oils, and contaminations in the oils. Substantial savings can be achievable through an effective oil analysis program.