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lubricating oils, with the rest being greases. It is difficult to overestimate the influence of the quality of lubricating oil on the wear resistance of operating mechanisms. This article substantiates the need for a comprehensive approach to solving the problems of condition monitoring in order to timely replace used lubricants. The principles of oil condition monitoring and the key analytical parameters are discussed. Examples are given to illustrate the application of used oil analysis to saving the company resources.
The life of machinery is impossible without lubricants. Any mechanism which has contacting parts requires lubrication. Millions of tons of lubricants for various purposes are produced annually in the world. Over 95% of this quantity is represented by lubricating oils, with the rest being greases. It is difficult to overestimate the influence of the quality of lubricating oil on the wear resistance of operating mechanisms. This article substantiates the need for a comprehensive approach to solving condition monitoring problems in order to assess a machine's current health and remaining useful life. The principles of oil condition monitoring and the key analytical parameters are discussed. Examples are given to illustrate the application of used oil analysis to saving the corporate resources.
Much attention is paid in Russia to lubricant manufacturing. A variety of reference books [1, 2] have been published in Russian, international scientific and practical conferences and symposiums are held on a regular basis, universities produce lubricant manufacturing specialists. However, it is not always and not everywhere possible to find true understanding of how to use lubricants in a correct way.
What happens to the lubricating oil after it has been produced and shipped to a consumer? It starts its operating life and functioning as a machine component. The oil circulates inside the machine and forms a film which separates contacting surfaces to prevent their wear and destruction.
But nothing lasts forever. Mechanisms do wear over time, and wear debris inevitably penetrate the oil. At the same time, the oil itself deteriorates. Its oxidation products build up, the antiwear and antioxidant additives deplete, the polymer molecules of viscosity improvers break into smaller molecules. Besides, without adequate protection, water and dirt from the outside can enter the lubrication system. As a result, losing its original properties and accumulating particulate, oil begins to work as an abrasive, causing further wear. Practice shows that over 80% of machine failures are caused by oil contamination.
An engineer operating a machine faces a number of questions. How long will the machine run once it has started to deteriorate? Is it possible to predict what exactly will break in it? How to ensure its maximum possible service life? The simplest thing is to follow the operation recommendations provided by the machine manufacturer, for example, to change oil once a year. This is the most common practice. When a machine breaks down, people just shut it down for repair. Those people who do not care about their money.
Those who do have an opportunity to significantly extend their machinery service life and reduce unplanned downtime. For this purpose, oil condition monitoring is used to assess the machine health. Simple economic calculations show that, for example, monitoring the condition of a turbine is dozens of thousands of times cheaper than shutting the turbine down and repairing it after a catastrophic failure.
Machinery Health Monitoring by Assessing the Lubricating Oil Condition
Table 1 demonstrates an example of an analytical report for a turbine oil sampled in April, May and July 2011. The oil is relatively new, it has only worked for 4,487 hours. However, the analysts have noticed a problem. The oil is contaminated with particulate. A low content of metals suggests that this particulate has nothing to do with turbine component wear and has a different nature. Obviously, it is dirt ingested from the environment. This conclusion is indirectly confirmed by an increased content of silicon, which is one of basic elements of sand, clay and soil. The laboratory made recommendations to use portable filtration system and replace the filter.
Along with particulate ingression, other unfortunate things may happen to a machine: bearing failures accompanied by local overheating, oil pump failures, fuel dilution, water and coolant glycol contamination of oil, accumulation of soot and other harmful products. If the problem is detected and fixed in time, it is possible to save the machine operability with minimum losses (replacement of a bearing, a pump, a filter etc.).
Oil condition monitoring is specifically used for detecting faults at the earliest stages. This widely recognized and highly efficient approach still has not received due consideration in Russia. It is only large international companies that have their own oil analysis laboratories, carrying over their experience from abroad. There are some independent oil analysis laboratories in Russia, but they can be counted on the fingers of one hand.
The Principles of Oil Condition Monitoring
The concept of used oil analysis radically differs from the principles of oil analysis during the oil manufacturing process. When formulating a new oil, it is important to make sure that its quality parameters fall within the known limits which are stipulated in the standards and technical specifications. When monitoring an in-service oil, it is important to monitor the changes of the oil parameters over time rather than their values. This approach is often referred to as trend analysis. Collecting and analyzing statistical data allows one to make important conclusions about the health of the oil and the equipment.
Along with the traditional quality parameters (viscosity, flash point, additive content), condition monitoring uses additional diagnostic indicators (ferrous debris and wear metals content, soot content, nitration, sulphation, varnish potential etc.). At the same time, such a parameter as density is hardly ever used in condition monitoring. Besides, the precision requirements are less strict.
There are basically three levels of oil condition monitoring: (1) in-field express oil analysis, (2) in-house laboratory-based oil analysis, and (3) outsourcing the oil analysis to a specialized independent laboratory. In the first case, a minimum set of easily measurable parameters are determined using express techniques. In the second case, these parameters are determined with higher accuracy using standard laboratory methods. In the third case, the machine user receives a qualified independent report based on the comprehensive analysis of a number of characteristics, including those which require the use of special expensive analytical instruments.
Another feature of oil condition monitoring is the lack of standards prescribing universal diagnostic procedures. There is a wide variety of machines which work in different operational conditions and therefore require individually tailored condition monitoring programs. Thus, the task of condition monitoring program development is the responsibility of the enterprise maintenance personnel.
Briefly described below are the key parameters used in condition monitoring. For more details, see book [3] by Dr. Jim Fitch, one of the condition monitoring ideologists.
Key Diagnostic Indicators
The viscosity of any lubricating oil should be sufficient to form a film that separates the contacting surfaces. A change in viscosity indicates the oil degradation or contamination and is a signal that urgent action is needed. Viscosity monitoring does not require high accuracy, like in standard ASTM D445, when manufacturing base stocks. It is important to monitor the trend in order to detect a dramatic increase or decrease in viscosity. For quick in-field monitoring, a simple viscosity comparator is suitable (Fig.1). The used oil flow rate is compared with the new oil flow rate, and the change in viscosity is evaluated in percentage.
Large fast-turnaround laboratories use high-throughput automatic viscometers, e. g. with Huillon tubes or accelerated flow tubes (Fig.2). Their accuracy is lower compared to the classical method.
Particle counting. Particulate content is one of the most critical oil condition indicators. Particles that have a size comparable with the size of clearances between contacting surfaces can cause extensive abrasion of the machine components.
There are two methods of particulate content evaluation: gravimetric analysis and particle counting. In terms of oil condition monitoring, the latter is more preferable. It provides information not only about the general content of particles, but also on their size distribution. This information can be obtained in three ways: by counting the number of particles in different size ranges using a microscope, an optical particle counter, or a pore blockage particle counter.
The first technique is the most readily available, but also the most time-consuming. It only requires a measuring microscope, but the sample processing can take hours. The second technique uses the principle of light blockage, usually with a laser light source (Fig.3). The shadow of a particle falling into the laser beam is detected by a photocell sensor, and the resulting signal is automatically converted into the particle size. By counting shadows of different sizes, it is possible to easily discriminate the particles passing through the light channel by their sizes. This is the most popular method. A number of companies manufacture such instruments, and an example is shown in Fig.4. The result is obtained within a few minutes. Its disadvantage consists in possible false positive results, caused by air bubbles, water microdroplets, and soft varnish particles, which are non-dangerous for contacting surfaces, but can be mistaken for solid particles.
In the third method, the oil is forced through a rigid calibrated filter with pores of specific size and shape. Solid particles block the pores of the filter and create a measurable flow resistance (Fig.5). The advantage of this method is that it is only sensitive to solid particles. However, this method is less popular, and it is relatively difficult to find instruments for its implementation.
Whichever method is used, the result is usually presented as cumulative values exceeding specified thresholds (see Table 1). The minimum requirement is the number of particles greater than 4, 6 and 14 microns. For simplifying the report, these three values are converted into a three-number code according to ISO 4406:1999. For example, the code 24/21/17 (see Table 1) indicates that 1 ml of the oil contains 80,000 to 160,000 particles greater than 4 microns, 10,000 to 20,000 particles greater than 6 microns, and 640 to 1,300 particles greater than 14 microns (very dirty oil!). The Russian standard GOST 17216-2001 specifies an alternative system of fluid cleanliness codes with the use of particle size ranges. The overall contamination of the oil is determined in this system as the highest code among all the particle size ranges (Fig.6).
Ferrous debris content. This indicator is used as a quantitative characteristic of the metal components wear degree. It is determined by disturbance of a balanced magnetic field between two induction coils. Depending on the hardware design, the result may be obtained either in parts per million (ppm) of ferrous material, or in the form of so-called PQ-index, a conventional dimensionless value proportional to the mass of the ferrous particles in the analyzed oil sample. The corresponding instruments are depicted in Fig.7.
Acid number (AN) is a measure of the total content of acidic substances accumulated in the oil during its service life.
Base number (BN), on the contrary, is a measure of the ability of oil to neutralize external acidic contaminants and oil oxidation products.
AN and BN are determined by means of titration methods using classical glass burettes or state-of-the-art automatic titrators (Fig.8). Both of these parameters are given in milligrams of potassium hydroxide per 1 g of oil. Sometimes, it causes confusion, as AN is determined by titration with an alkali, and BN is determined by titration with an acid). While oil is in service (engine oil in particular), the AN increases and the BN decreases. The balance between these indicators is a good criterion for the oil remaining useful life.
Flash point is the temperature at which oil flashes when ignited by flame or electricaly. The open Cleveland cup or the closed Pensky-Martens cup can be used to determine the flash point (Fig.9).
Gaining more and more popularity is a safe flash point test method using portable instruments with a continuously closed cup (Fig.10). This test takes less time and provides better reproducibility. The reduction of oil's flash point indicates the accumulation of light flammable products. This may be caused by both thermal decomposition of the oil (e.g., on an overheated bearing) and fuel dilution of the oil in the crankcase.
Water content is another critical indicator of oil condition. Once water has entered the oil, it causes corrosion of metal parts, hydrolysis and washout of additives, and leads to disruption of the oil film integrity and subsequent wear of the machine components. Timely detecting water ingression and taking measures to remove it from oil can substantially extend the machine's life.
There are two techniques used in oil condition monitoring to determine water content: the crackle test by heating the oil and traditional Karl Fischer titration (Fig.11). In the second case, in the order to avoid additive-related interferences, special heaters are often used, allowing one to evaporate water from the oil and channel it into the titration cell.
Analytical ferrography is a technique for examing ferrous particles deposited from the oil onto a glass slide by means of a magnet. The particle distribution pattern obtained is referred to as a ferrogram (Fig.12). Microscopic inspection of the ferrogram can give information on specific wear products: babbits, copper or aluminum alloys, high- or low-alloy steels. The shape, size and appearance of particles can often be indicative of a specific wear mechanism. The material of particles can be identified by a change in their color, when heating the ferrogram to 330°C. This information often helps to localize the faulty component of the machine.
Elemental analysis by spectral methods. Ferrous debris analyzers only provide information on the contamination of oil with such metals as iron and nickel. To gain a deeper insight into the wear and contamination processes taking place in the machine, it is important to have maximum information on the elemental composition of the oil sample. This information can be obtained using two basic techniques: inductively coupled plasma atomic emission spectroscopy (ICP-AES) or rotating disc electrode atomic emission spectroscopy (RDE-AES). Each of these methods provides simultaneous determination of the content of over 20 chemical elements in the sample. The operation principles of ICP and RDE atomic emission spectrometers are illustrated in Fig.13.
The first method is more popular with analysts, for example, when determining metals in environmental objects. The sample is atomized in argon plasma, and the intensity of the characteristic spectral bands of the desired elements is measured. The disadvantage of this technique in terms of oil analysis is the size limitations for solid particles in the sample. If they are too big (greater than 3 to 5 microns), they burn in the plasma incompletely, and the concentration of the elements will be underestimated.
The second method allows for analyzing samples with particle sizes of up to 8 to 10 microns. In this technique, the elements are atomized by spark discharge at the graphite disk electrode that carries the film of the oil sample while rotating (Fig.13).
The oxidation stability test. Virtually all lubricating oils contain antioxidant additives (antioxidants) aimed at protecting the oil from oxidation by atmospheric oxygen. They include, among others, zinc dialkyl dithiophosphates, metal sulfonates and phenates, sterilically hindered phenols and aromatic amines. Antioxidants oxidize more easily than the base stock components, thus neutralizing the detrimental effect of oxygen. As these additives oxidize, their concentration in the oil decreases.
The oxidation stability of an oil can be checked in several ways. One of the most common techniques is the Rotating Pressure Vessel Oxidation Test (RPVOT). In this method, the oil sample is placed into a sealed high-pressure cylinder ("bomb"), with a catalyst added and oxygen charged up to a pressure of 620 kPa. The cylinder is placed into a liquid or dry block bath at 150°C at an angle of 30° (Fig.14). Then it is rotated at a constant speed, and the inside pressure is monitored. A rapid drop in the bomb pressure indicates the onset of extensive oxidation of the base stock components. The time elapsed from the beginning of the test to the pressure drop is referred to as the oxidation induction period and is used as an indicator of oil's oxidation stability. The RPVOT method is utilized both to check oil during its acceptance, and to assess the oil's remaining useful life. The disadvantage of this method is the relatively long test time. Oils having high oxidation resistivity can withstand RPVOT tests for dozens of hours. Moreover, such additives as corrosion inhibitors and metal passivators can poison the oxidation catalyst in the RPVOT, which can lead to erroneous results.
In the early 2000s, a new method was developed to assess the oxidation stability, which is known as the remaining useful life evaluation routine (RULER). This technique is based on direct determination of the antioxidant content. One of the reasons for developing this method was the appearance of group II and III based types of turbine oils with complex antioxidant packages designed for the use under harsh operating conditions.
The method is based on the property of the antioxidants, being reducing agents by nature, to oxidize electrochemically. When a linearly increasing potential is applied, the additives generate electric current signals in the form of waves (peaks) on the voltammogram (Fig.15). This analytical technique is referred to as linear sweep voltammetry (LSV).
By comparison of the peak areas for the new and used oil samples, the remaining antioxidant content is determined, and subsequently the remaining useful life of the oil is evaluated. The method is applicable not only to turbine oils, but also to any oil containing antioxidants.
Infrared spectrometry. The mid-infrared spectrum of oil (600 ... 4,000 cm-1) contains abundant information on the oil composition. By analyzing specific absorption bands in the spectrum, one can determine the content of certain additives and contaminants. Subtracting the new oil spectrum from the used oil spectrum yields a difference spectrum representing the substances which accumulated and depleted during the oil service. This technique allows for estimation of the content of water, soot, glycols, fuel, as well as oxidation, nitration, sulphation products (Fig.16).
Interpretation of the oil analysis results
To properly use the information obtained by different oil analysis methods, it is necessary to understand which oil properties relate to specific machine condition parameters. Table 2 shows the characteristics of oil and machine condition, resulting from oil analysis. The primary (P) indicators are the first signals of a possible problem. The secondary (S) indicators are used for its confirmation [3].
See book [3] for more information on interpreting the results of oil analysis.
The importance of proper sampling
Determination of the oil condition parameters and machine health control may become totally useless, if the oil was sampled in an improper manner. Samples taken, for example, from the oil tank carry no information on the actual processes taking place in the machine. Samples should be extracted from the points where the oil carries the maximum amount of information about the running machine component. To achieve that, a set of important rules should be followed.
Samples should be taken from a running machine, i. e. when it is under its habitual operating conditions with normal operating parameters.
Samples should be taken UPSTREAM of filters and DOWNSTREAM of machine components (bearings, gears, pistons, shafts and so on).
Oil samples should be taken on a regular basis, with a frequency sufficient for early detection of faults or abnormal condition of the system.
All sampling devices, including valves, tubes, fittings, as well as sample bottles must be clean.
Samples must be extracted each time from the same points of the system, preferably from the live zones, where the oil flow is turbulent.
The new oil sample should be stored for further investigations. Some analytical techniques, such as IR spectroscopy or voltammetry, are based on the comparison of the in-service oil with the new oil, which is used as a reference material.
These are just the most basic rules.
Machinery Diagnostics by Means of Oil Condition Monitoring
To fulfil the main task, machinery operational cost minimization, oil condition monitoring should become part and parcel of the diagnostics program for all the equipment used at the facility. A properly designed program should take account of all the nuances related to the criticality of specific equipment, the manufacturer’s recommendations, the operational modes and conditions, maintenance and consumables costs, the type of the oil used, etc. The introduction of such a program can improve the equipment reliability and help save a lot of money by minimizing operational risks and reducing inefficient expenses.
Below are a few examples.
A hydraulic system running a fluid with ISO cleanliness 18/15/12 will operate 3 times longer if the fluid cleanliness is improved to ISO 15/12/9. Conversely, if contamination control of the fluid in the same machine is lost and the ISO code changes from 15/12/9 to 18/15/12, one can expect that the wear rate will triple over the same period [3]. In order to monitor fluid cleanliness, the fluid should regularly be analyzed with a particle counter.
Fuel dilution of oil in the engine crankcase can lead to a decrease in the oil viscosity, which will result in extensive wear of the engine. Failure to monitor fuel dilution of the oil may eventually lead to an irretrievable engine breakdown. Monitoring the fuel content by identifying flash point or viscosity changes can be dozens or hundreds of times cheaper than repairing the system repair.
Changing used oil circulating in large frame units with an oil tank volume of about 10 to 15 thousand liters is a challenge associated with substantial costs of oil drain and disposal, system flushing, purchasing new oil and refilling the system. In many cases, oil change after a specific interval is unreasonable, since the drained oil is often "healthy" and quite suitable for further operation. Monitoring the antioxidant content by means of the RULER technique, along with such parameters as viscosity and AN, one can optimize oil drain intervals and save huge amounts of money and resources.
Global Condition Monitoring Practice
The world’s leading expert on machinery lubrication is Noria Co. (www.noria.com), an American company founded in 1997 by Dr. Jim Fitch. Noria Co. is the publisher of regular magazines Machinery Lubrication, Practicing Oil Analysis and Reliable Plant, which are dedicated to lubricant application and analysis.
Another scope of the company’s business is helping industrial corporations to adopt modern lubrication and condition monitoring technologies. Noria’s website, www.noria.com, contains case studies by such large companies as Cargill, Boeing, and GoodYear, which share their experience of reducing maintenance costs and saving money through the implementation of oil analysis programs.
Noria Co. also offers specialists a variety of training courses in machinery lubrication and practical oil analysis, including the courses provided by Noria’s authorized partners. The Estonian company AA Inlube (www.inlube.eu) regularly delivers Noria’s training courses in Russian.
There is also the Society of Tribologists and Lubrication Engineers (STLE) (www.stle.org). It was founded in 1944 and has more than 10,000 members from over 150 countries. This organization provides fundamental education in tribology and lubrication sciences and publishes books related to the topic.
Another enterprise that should be mentioned is the International Council for Machinery Lubrication (ICML) (www.lubecouncil.org). ICML is an independent non-profit organization that unites machinery lubrication professionals. One of its main objectives is certification of lubrication specialists after relevant training. The certificates issued by ICML, unlike, for example, certificates awarded to lawyers and doctors, are not mandatory for getting a job, but are indicative of high qualification of the specialists. Meanwhile, there are two ISO standards based on the ICML certification system:
ISO 18436-4:2014. Condition monitoring and diagnostics of machines. Requirements for qualification and assessment of personnel. Part 4: Field lubricant analysis.
ISO 18436-5:2012. Condition monitoring and diagnostics of machines. Requirements for qualification and assessment of personnel. Part 5: Lubricant laboratory technician/analyst.
In future, these requirements are going to become mandatory in the ISO certification system.
Conclusion
There are four stages that can be identified in the global evolution of machinery maintenance approaches.
Reactive maintenance. This means repairing or replacing equipment upon its failure. It does not involve any diagnostics and failure prevention costs, but is accompanied by vast capital expenses related to the repair of the damaged equipment.
Preventive maintenance. This is routine maintenance based on scheduled intervals. The probability of a sudden failure is reduced (but not eliminated), but there are expenses associated with the repair of equipment which may still be in good condition. Preventive maintenance costs can be excessive.
Condition-based proactive maintenance. This approach is underpinned by the principle "Do not repair, eliminate the root cause". It includes regular monitoring and is aimed at reducing maintenance activity and extending the equipment service life.
Asset optimization. This approach is aimed at optimization of all the processes and the pursuit of conditions under which nothing fails.
Many large international corporations went through all the four levels to adopt maintenance strategies which are based on oil condition monitoring.
In Russia, the majority of companies use an approach combining the first and second levels. Fully proactive maintenance based on oil condition monitoring is only utilized by large international companies, some of which have their own oil analysis laboratories, while others send their oil samples for analysis to laboratories abroad.
There is no doubt that the need for diagnostic analytics will continue to grow, as technical specialists in Russia gain deeper understanding of the economic benefits of oil condition monitoring. This is what has been the focus of this article.
References
Combustibles and lubricants. Encyclopedic Dictionary. Edited by V.M.Shkolnikov. "Tekhnoinform". Moscow. 2010.
T.Mang, W.Dresel (ed.). Lubricants and Lubrication, 2nd, Completely Revised and Extended Edition. Wiley-VCH. 2007. / Translation into Russian. Edited by V.M.Shkolnikov. EPC "Professiya". St. Petersburg. 2010.
J.Fitch, D.Troyer. Oil Analysis Basics. Noria Co. Tulsa, USA. 2010. / Translation into Russian. Edited by E. A. Novikov, M. V. Kiryukhin. Published by EPC "Professiya". St. Petersburg. 2014.
Much attention is paid in Russia to lubricant manufacturing. A variety of reference books [1, 2] have been published in Russian, international scientific and practical conferences and symposiums are held on a regular basis, universities produce lubricant manufacturing specialists. However, it is not always and not everywhere possible to find true understanding of how to use lubricants in a correct way.
What happens to the lubricating oil after it has been produced and shipped to a consumer? It starts its operating life and functioning as a machine component. The oil circulates inside the machine and forms a film which separates contacting surfaces to prevent their wear and destruction.
But nothing lasts forever. Mechanisms do wear over time, and wear debris inevitably penetrate the oil. At the same time, the oil itself deteriorates. Its oxidation products build up, the antiwear and antioxidant additives deplete, the polymer molecules of viscosity improvers break into smaller molecules. Besides, without adequate protection, water and dirt from the outside can enter the lubrication system. As a result, losing its original properties and accumulating particulate, oil begins to work as an abrasive, causing further wear. Practice shows that over 80% of machine failures are caused by oil contamination.
An engineer operating a machine faces a number of questions. How long will the machine run once it has started to deteriorate? Is it possible to predict what exactly will break in it? How to ensure its maximum possible service life? The simplest thing is to follow the operation recommendations provided by the machine manufacturer, for example, to change oil once a year. This is the most common practice. When a machine breaks down, people just shut it down for repair. Those people who do not care about their money.
Those who do have an opportunity to significantly extend their machinery service life and reduce unplanned downtime. For this purpose, oil condition monitoring is used to assess the machine health. Simple economic calculations show that, for example, monitoring the condition of a turbine is dozens of thousands of times cheaper than shutting the turbine down and repairing it after a catastrophic failure.
Machinery Health Monitoring by Assessing the Lubricating Oil Condition
Table 1 demonstrates an example of an analytical report for a turbine oil sampled in April, May and July 2011. The oil is relatively new, it has only worked for 4,487 hours. However, the analysts have noticed a problem. The oil is contaminated with particulate. A low content of metals suggests that this particulate has nothing to do with turbine component wear and has a different nature. Obviously, it is dirt ingested from the environment. This conclusion is indirectly confirmed by an increased content of silicon, which is one of basic elements of sand, clay and soil. The laboratory made recommendations to use portable filtration system and replace the filter.
Along with particulate ingression, other unfortunate things may happen to a machine: bearing failures accompanied by local overheating, oil pump failures, fuel dilution, water and coolant glycol contamination of oil, accumulation of soot and other harmful products. If the problem is detected and fixed in time, it is possible to save the machine operability with minimum losses (replacement of a bearing, a pump, a filter etc.).
Oil condition monitoring is specifically used for detecting faults at the earliest stages. This widely recognized and highly efficient approach still has not received due consideration in Russia. It is only large international companies that have their own oil analysis laboratories, carrying over their experience from abroad. There are some independent oil analysis laboratories in Russia, but they can be counted on the fingers of one hand.
The Principles of Oil Condition Monitoring
The concept of used oil analysis radically differs from the principles of oil analysis during the oil manufacturing process. When formulating a new oil, it is important to make sure that its quality parameters fall within the known limits which are stipulated in the standards and technical specifications. When monitoring an in-service oil, it is important to monitor the changes of the oil parameters over time rather than their values. This approach is often referred to as trend analysis. Collecting and analyzing statistical data allows one to make important conclusions about the health of the oil and the equipment.
Along with the traditional quality parameters (viscosity, flash point, additive content), condition monitoring uses additional diagnostic indicators (ferrous debris and wear metals content, soot content, nitration, sulphation, varnish potential etc.). At the same time, such a parameter as density is hardly ever used in condition monitoring. Besides, the precision requirements are less strict.
There are basically three levels of oil condition monitoring: (1) in-field express oil analysis, (2) in-house laboratory-based oil analysis, and (3) outsourcing the oil analysis to a specialized independent laboratory. In the first case, a minimum set of easily measurable parameters are determined using express techniques. In the second case, these parameters are determined with higher accuracy using standard laboratory methods. In the third case, the machine user receives a qualified independent report based on the comprehensive analysis of a number of characteristics, including those which require the use of special expensive analytical instruments.
Another feature of oil condition monitoring is the lack of standards prescribing universal diagnostic procedures. There is a wide variety of machines which work in different operational conditions and therefore require individually tailored condition monitoring programs. Thus, the task of condition monitoring program development is the responsibility of the enterprise maintenance personnel.
Briefly described below are the key parameters used in condition monitoring. For more details, see book [3] by Dr. Jim Fitch, one of the condition monitoring ideologists.
Key Diagnostic Indicators
The viscosity of any lubricating oil should be sufficient to form a film that separates the contacting surfaces. A change in viscosity indicates the oil degradation or contamination and is a signal that urgent action is needed. Viscosity monitoring does not require high accuracy, like in standard ASTM D445, when manufacturing base stocks. It is important to monitor the trend in order to detect a dramatic increase or decrease in viscosity. For quick in-field monitoring, a simple viscosity comparator is suitable (Fig.1). The used oil flow rate is compared with the new oil flow rate, and the change in viscosity is evaluated in percentage.
Large fast-turnaround laboratories use high-throughput automatic viscometers, e. g. with Huillon tubes or accelerated flow tubes (Fig.2). Their accuracy is lower compared to the classical method.
Particle counting. Particulate content is one of the most critical oil condition indicators. Particles that have a size comparable with the size of clearances between contacting surfaces can cause extensive abrasion of the machine components.
There are two methods of particulate content evaluation: gravimetric analysis and particle counting. In terms of oil condition monitoring, the latter is more preferable. It provides information not only about the general content of particles, but also on their size distribution. This information can be obtained in three ways: by counting the number of particles in different size ranges using a microscope, an optical particle counter, or a pore blockage particle counter.
The first technique is the most readily available, but also the most time-consuming. It only requires a measuring microscope, but the sample processing can take hours. The second technique uses the principle of light blockage, usually with a laser light source (Fig.3). The shadow of a particle falling into the laser beam is detected by a photocell sensor, and the resulting signal is automatically converted into the particle size. By counting shadows of different sizes, it is possible to easily discriminate the particles passing through the light channel by their sizes. This is the most popular method. A number of companies manufacture such instruments, and an example is shown in Fig.4. The result is obtained within a few minutes. Its disadvantage consists in possible false positive results, caused by air bubbles, water microdroplets, and soft varnish particles, which are non-dangerous for contacting surfaces, but can be mistaken for solid particles.
In the third method, the oil is forced through a rigid calibrated filter with pores of specific size and shape. Solid particles block the pores of the filter and create a measurable flow resistance (Fig.5). The advantage of this method is that it is only sensitive to solid particles. However, this method is less popular, and it is relatively difficult to find instruments for its implementation.
Whichever method is used, the result is usually presented as cumulative values exceeding specified thresholds (see Table 1). The minimum requirement is the number of particles greater than 4, 6 and 14 microns. For simplifying the report, these three values are converted into a three-number code according to ISO 4406:1999. For example, the code 24/21/17 (see Table 1) indicates that 1 ml of the oil contains 80,000 to 160,000 particles greater than 4 microns, 10,000 to 20,000 particles greater than 6 microns, and 640 to 1,300 particles greater than 14 microns (very dirty oil!). The Russian standard GOST 17216-2001 specifies an alternative system of fluid cleanliness codes with the use of particle size ranges. The overall contamination of the oil is determined in this system as the highest code among all the particle size ranges (Fig.6).
Ferrous debris content. This indicator is used as a quantitative characteristic of the metal components wear degree. It is determined by disturbance of a balanced magnetic field between two induction coils. Depending on the hardware design, the result may be obtained either in parts per million (ppm) of ferrous material, or in the form of so-called PQ-index, a conventional dimensionless value proportional to the mass of the ferrous particles in the analyzed oil sample. The corresponding instruments are depicted in Fig.7.
Acid number (AN) is a measure of the total content of acidic substances accumulated in the oil during its service life.
Base number (BN), on the contrary, is a measure of the ability of oil to neutralize external acidic contaminants and oil oxidation products.
AN and BN are determined by means of titration methods using classical glass burettes or state-of-the-art automatic titrators (Fig.8). Both of these parameters are given in milligrams of potassium hydroxide per 1 g of oil. Sometimes, it causes confusion, as AN is determined by titration with an alkali, and BN is determined by titration with an acid). While oil is in service (engine oil in particular), the AN increases and the BN decreases. The balance between these indicators is a good criterion for the oil remaining useful life.
Flash point is the temperature at which oil flashes when ignited by flame or electricaly. The open Cleveland cup or the closed Pensky-Martens cup can be used to determine the flash point (Fig.9).
Gaining more and more popularity is a safe flash point test method using portable instruments with a continuously closed cup (Fig.10). This test takes less time and provides better reproducibility. The reduction of oil's flash point indicates the accumulation of light flammable products. This may be caused by both thermal decomposition of the oil (e.g., on an overheated bearing) and fuel dilution of the oil in the crankcase.
Water content is another critical indicator of oil condition. Once water has entered the oil, it causes corrosion of metal parts, hydrolysis and washout of additives, and leads to disruption of the oil film integrity and subsequent wear of the machine components. Timely detecting water ingression and taking measures to remove it from oil can substantially extend the machine's life.
There are two techniques used in oil condition monitoring to determine water content: the crackle test by heating the oil and traditional Karl Fischer titration (Fig.11). In the second case, in the order to avoid additive-related interferences, special heaters are often used, allowing one to evaporate water from the oil and channel it into the titration cell.
Analytical ferrography is a technique for examing ferrous particles deposited from the oil onto a glass slide by means of a magnet. The particle distribution pattern obtained is referred to as a ferrogram (Fig.12). Microscopic inspection of the ferrogram can give information on specific wear products: babbits, copper or aluminum alloys, high- or low-alloy steels. The shape, size and appearance of particles can often be indicative of a specific wear mechanism. The material of particles can be identified by a change in their color, when heating the ferrogram to 330°C. This information often helps to localize the faulty component of the machine.
Elemental analysis by spectral methods. Ferrous debris analyzers only provide information on the contamination of oil with such metals as iron and nickel. To gain a deeper insight into the wear and contamination processes taking place in the machine, it is important to have maximum information on the elemental composition of the oil sample. This information can be obtained using two basic techniques: inductively coupled plasma atomic emission spectroscopy (ICP-AES) or rotating disc electrode atomic emission spectroscopy (RDE-AES). Each of these methods provides simultaneous determination of the content of over 20 chemical elements in the sample. The operation principles of ICP and RDE atomic emission spectrometers are illustrated in Fig.13.
The first method is more popular with analysts, for example, when determining metals in environmental objects. The sample is atomized in argon plasma, and the intensity of the characteristic spectral bands of the desired elements is measured. The disadvantage of this technique in terms of oil analysis is the size limitations for solid particles in the sample. If they are too big (greater than 3 to 5 microns), they burn in the plasma incompletely, and the concentration of the elements will be underestimated.
The second method allows for analyzing samples with particle sizes of up to 8 to 10 microns. In this technique, the elements are atomized by spark discharge at the graphite disk electrode that carries the film of the oil sample while rotating (Fig.13).
The oxidation stability test. Virtually all lubricating oils contain antioxidant additives (antioxidants) aimed at protecting the oil from oxidation by atmospheric oxygen. They include, among others, zinc dialkyl dithiophosphates, metal sulfonates and phenates, sterilically hindered phenols and aromatic amines. Antioxidants oxidize more easily than the base stock components, thus neutralizing the detrimental effect of oxygen. As these additives oxidize, their concentration in the oil decreases.
The oxidation stability of an oil can be checked in several ways. One of the most common techniques is the Rotating Pressure Vessel Oxidation Test (RPVOT). In this method, the oil sample is placed into a sealed high-pressure cylinder ("bomb"), with a catalyst added and oxygen charged up to a pressure of 620 kPa. The cylinder is placed into a liquid or dry block bath at 150°C at an angle of 30° (Fig.14). Then it is rotated at a constant speed, and the inside pressure is monitored. A rapid drop in the bomb pressure indicates the onset of extensive oxidation of the base stock components. The time elapsed from the beginning of the test to the pressure drop is referred to as the oxidation induction period and is used as an indicator of oil's oxidation stability. The RPVOT method is utilized both to check oil during its acceptance, and to assess the oil's remaining useful life. The disadvantage of this method is the relatively long test time. Oils having high oxidation resistivity can withstand RPVOT tests for dozens of hours. Moreover, such additives as corrosion inhibitors and metal passivators can poison the oxidation catalyst in the RPVOT, which can lead to erroneous results.
In the early 2000s, a new method was developed to assess the oxidation stability, which is known as the remaining useful life evaluation routine (RULER). This technique is based on direct determination of the antioxidant content. One of the reasons for developing this method was the appearance of group II and III based types of turbine oils with complex antioxidant packages designed for the use under harsh operating conditions.
The method is based on the property of the antioxidants, being reducing agents by nature, to oxidize electrochemically. When a linearly increasing potential is applied, the additives generate electric current signals in the form of waves (peaks) on the voltammogram (Fig.15). This analytical technique is referred to as linear sweep voltammetry (LSV).
By comparison of the peak areas for the new and used oil samples, the remaining antioxidant content is determined, and subsequently the remaining useful life of the oil is evaluated. The method is applicable not only to turbine oils, but also to any oil containing antioxidants.
Infrared spectrometry. The mid-infrared spectrum of oil (600 ... 4,000 cm-1) contains abundant information on the oil composition. By analyzing specific absorption bands in the spectrum, one can determine the content of certain additives and contaminants. Subtracting the new oil spectrum from the used oil spectrum yields a difference spectrum representing the substances which accumulated and depleted during the oil service. This technique allows for estimation of the content of water, soot, glycols, fuel, as well as oxidation, nitration, sulphation products (Fig.16).
Interpretation of the oil analysis results
To properly use the information obtained by different oil analysis methods, it is necessary to understand which oil properties relate to specific machine condition parameters. Table 2 shows the characteristics of oil and machine condition, resulting from oil analysis. The primary (P) indicators are the first signals of a possible problem. The secondary (S) indicators are used for its confirmation [3].
See book [3] for more information on interpreting the results of oil analysis.
The importance of proper sampling
Determination of the oil condition parameters and machine health control may become totally useless, if the oil was sampled in an improper manner. Samples taken, for example, from the oil tank carry no information on the actual processes taking place in the machine. Samples should be extracted from the points where the oil carries the maximum amount of information about the running machine component. To achieve that, a set of important rules should be followed.
Samples should be taken from a running machine, i. e. when it is under its habitual operating conditions with normal operating parameters.
Samples should be taken UPSTREAM of filters and DOWNSTREAM of machine components (bearings, gears, pistons, shafts and so on).
Oil samples should be taken on a regular basis, with a frequency sufficient for early detection of faults or abnormal condition of the system.
All sampling devices, including valves, tubes, fittings, as well as sample bottles must be clean.
Samples must be extracted each time from the same points of the system, preferably from the live zones, where the oil flow is turbulent.
The new oil sample should be stored for further investigations. Some analytical techniques, such as IR spectroscopy or voltammetry, are based on the comparison of the in-service oil with the new oil, which is used as a reference material.
These are just the most basic rules.
Machinery Diagnostics by Means of Oil Condition Monitoring
To fulfil the main task, machinery operational cost minimization, oil condition monitoring should become part and parcel of the diagnostics program for all the equipment used at the facility. A properly designed program should take account of all the nuances related to the criticality of specific equipment, the manufacturer’s recommendations, the operational modes and conditions, maintenance and consumables costs, the type of the oil used, etc. The introduction of such a program can improve the equipment reliability and help save a lot of money by minimizing operational risks and reducing inefficient expenses.
Below are a few examples.
A hydraulic system running a fluid with ISO cleanliness 18/15/12 will operate 3 times longer if the fluid cleanliness is improved to ISO 15/12/9. Conversely, if contamination control of the fluid in the same machine is lost and the ISO code changes from 15/12/9 to 18/15/12, one can expect that the wear rate will triple over the same period [3]. In order to monitor fluid cleanliness, the fluid should regularly be analyzed with a particle counter.
Fuel dilution of oil in the engine crankcase can lead to a decrease in the oil viscosity, which will result in extensive wear of the engine. Failure to monitor fuel dilution of the oil may eventually lead to an irretrievable engine breakdown. Monitoring the fuel content by identifying flash point or viscosity changes can be dozens or hundreds of times cheaper than repairing the system repair.
Changing used oil circulating in large frame units with an oil tank volume of about 10 to 15 thousand liters is a challenge associated with substantial costs of oil drain and disposal, system flushing, purchasing new oil and refilling the system. In many cases, oil change after a specific interval is unreasonable, since the drained oil is often "healthy" and quite suitable for further operation. Monitoring the antioxidant content by means of the RULER technique, along with such parameters as viscosity and AN, one can optimize oil drain intervals and save huge amounts of money and resources.
Global Condition Monitoring Practice
The world’s leading expert on machinery lubrication is Noria Co. (www.noria.com), an American company founded in 1997 by Dr. Jim Fitch. Noria Co. is the publisher of regular magazines Machinery Lubrication, Practicing Oil Analysis and Reliable Plant, which are dedicated to lubricant application and analysis.
Another scope of the company’s business is helping industrial corporations to adopt modern lubrication and condition monitoring technologies. Noria’s website, www.noria.com, contains case studies by such large companies as Cargill, Boeing, and GoodYear, which share their experience of reducing maintenance costs and saving money through the implementation of oil analysis programs.
Noria Co. also offers specialists a variety of training courses in machinery lubrication and practical oil analysis, including the courses provided by Noria’s authorized partners. The Estonian company AA Inlube (www.inlube.eu) regularly delivers Noria’s training courses in Russian.
There is also the Society of Tribologists and Lubrication Engineers (STLE) (www.stle.org). It was founded in 1944 and has more than 10,000 members from over 150 countries. This organization provides fundamental education in tribology and lubrication sciences and publishes books related to the topic.
Another enterprise that should be mentioned is the International Council for Machinery Lubrication (ICML) (www.lubecouncil.org). ICML is an independent non-profit organization that unites machinery lubrication professionals. One of its main objectives is certification of lubrication specialists after relevant training. The certificates issued by ICML, unlike, for example, certificates awarded to lawyers and doctors, are not mandatory for getting a job, but are indicative of high qualification of the specialists. Meanwhile, there are two ISO standards based on the ICML certification system:
ISO 18436-4:2014. Condition monitoring and diagnostics of machines. Requirements for qualification and assessment of personnel. Part 4: Field lubricant analysis.
ISO 18436-5:2012. Condition monitoring and diagnostics of machines. Requirements for qualification and assessment of personnel. Part 5: Lubricant laboratory technician/analyst.
In future, these requirements are going to become mandatory in the ISO certification system.
Conclusion
There are four stages that can be identified in the global evolution of machinery maintenance approaches.
Reactive maintenance. This means repairing or replacing equipment upon its failure. It does not involve any diagnostics and failure prevention costs, but is accompanied by vast capital expenses related to the repair of the damaged equipment.
Preventive maintenance. This is routine maintenance based on scheduled intervals. The probability of a sudden failure is reduced (but not eliminated), but there are expenses associated with the repair of equipment which may still be in good condition. Preventive maintenance costs can be excessive.
Condition-based proactive maintenance. This approach is underpinned by the principle "Do not repair, eliminate the root cause". It includes regular monitoring and is aimed at reducing maintenance activity and extending the equipment service life.
Asset optimization. This approach is aimed at optimization of all the processes and the pursuit of conditions under which nothing fails.
Many large international corporations went through all the four levels to adopt maintenance strategies which are based on oil condition monitoring.
In Russia, the majority of companies use an approach combining the first and second levels. Fully proactive maintenance based on oil condition monitoring is only utilized by large international companies, some of which have their own oil analysis laboratories, while others send their oil samples for analysis to laboratories abroad.
There is no doubt that the need for diagnostic analytics will continue to grow, as technical specialists in Russia gain deeper understanding of the economic benefits of oil condition monitoring. This is what has been the focus of this article.
References
Combustibles and lubricants. Encyclopedic Dictionary. Edited by V.M.Shkolnikov. "Tekhnoinform". Moscow. 2010.
T.Mang, W.Dresel (ed.). Lubricants and Lubrication, 2nd, Completely Revised and Extended Edition. Wiley-VCH. 2007. / Translation into Russian. Edited by V.M.Shkolnikov. EPC "Professiya". St. Petersburg. 2010.
J.Fitch, D.Troyer. Oil Analysis Basics. Noria Co. Tulsa, USA. 2010. / Translation into Russian. Edited by E. A. Novikov, M. V. Kiryukhin. Published by EPC "Professiya". St. Petersburg. 2014.
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