|by Andre Verlinden, B.Sc.
|Laboratories at Work: Used Oil Analysis at WearCheck, Belgium|
laboratory was originally set up in 1974, under the name
Spectrolab, and in 1987 became part of WearCheck , which
is an international group of laboratories that
concentrates on used oil analysis.
The throughput of the group in 1987 was 2,000 samples a year, which has grown to around 40,000 samples a year today. The Belgian company performs used oil analysis for Belgium, the Netherlands and Luxembourg, France, and other countries within an 8-hour transportation radius, so that results can be produced within a day.
WearCheck Belgium’s clients are mainly oil companies, as would be expected, who make up 64% of the work; the marine sector is 29%, and industry 7%. The distribution of lubricants and fluids that are analysed is: engine oils 62%, hydraulic oils 22%, transmission oils 12%, greases and emulsions 1%, and other oils 3%. The laboratory can handle over 90 different types and standards of test for the following materials: lubricating oils, water-emulsion and solutions, fuels, oils, and disel oil. Tests performed include property tests, such as aniline point, corrosiveness, and flashpoint, performance test, eg. Falex EP, and analytical determinations via spectrometry, microscopy, IR, and emissions ICP.
oil analysis is comparable to a medical analysis with a
blood test. Like blood, lubricating oil contains a good
deal of information about the envelope in which it
circulates. Wear of metallic parts, for example, produces
a lot of minute particles, which are carried by the
lubricant. These small metal particles can give
information about the machine elements that are wearing,
and can be detected by various methods, for example,
Atomic Emission Spectrometry. Determination of larger
particles can be done using optical or electronic
microscopy, or ferrography.
The acidity of an oil shows whether the oil is oxidised as a result of operation at high temperature, if there is a high percentage of moisture, or whether the oil has been in service for too long. The viscosity of the oil is a very important parameter and must be in conformity with the requirements of the machine builder. The alkalinity or the loss of alkalinity of the oil, proves that the oil is in contact with inorganic acids such as sulphuric acid or nitric acid.
|CONTAMINATION OF OIL||The
causes of oil contamination are many, and can be
classified according to source. Thus there is
contamination coming from outside the system -
dust (silica); liquids (mixture with other oils, water,
other contaminated oil). The second is in open
systems - chains, cables, gears in contact with dust,
water, and so on. The third is in closed systems.
Impurities can also come from the settings and processes in which the lubricants work, e.g. manufacturing can produce welding debris, assembling involves dust, perhaps also silicones or polishing powder, while maintenance can introduce impurities via dirty rags or deteriorated joints, and lubrication systems may need or involve aspiration, or open tanks.
The lubricant itself can produce or contain contaminants - wear, sludge (deterioration of the oil), soot, acids (oxidation of the oil, sulphur from fuel), temperature changes or extremes, fuel, anti-freeze, deterioration of packings and seals (e.g. deteriorating through the action of synthetic oils or brake fluids).
The type of contamination can vary according to the source. Thus dust, for example, can arise within a shipyard as sand, i.e. Si, Al. In metallurgy, one can find the oxides or iron. On a ship, there are problems with salt water. Industrial and automotive settings are filled with potential contaminants, for example chemical products, or coal powder. Liquid contaminants can include water, acids, solvents, anti-freeze.
means the loss of solid material due to the effects of
friction of contacting surfaces. It is generally harmful,
although in some cases it can also be beneficial, for
instance during the running-in of an engine. The
deterioration of the surfaces in an engine is generally
due to isolated or simultaneous mechanisms, among which
we can distinguish the following.
Figure 1 Abrasive wear
There are two types of adhesive wear - heavy, liberating relatively large metal particles (50 to 200 microns), called ‘scuffing’, and eventually leads to a failure of the engine; and moderate - the formation of very small metal oxides which is termed ‘soft’ or ‘normal adhesive’ wear (see Figure 1). Adhesive wear can be avoided by the use of an appropriate lubricant containing extreme pressure (EP) additives, and the choice of the correct viscosity oil.
Figure 2 Fretting
Wear by fatigue
Erosion by cavitation
Figure 3 Cavitation
Wear of electrical
|Consequences for the oil||Wear particles
Some metal wear particles, such as iron and copper, play the role of catalysts in the process of oil oxidation.
|Consequences for the engine||Briefly, the implications for the engine of contamination of the lubricant are wear, and vibration, leading to a loss of efficiency, which in turn produces greater energy consumption, a loss of productivity, higher maintenance costs, a reduction in the machine’s life, a loss of oil and, ultimately, fundamental problems.|
|ROUTINE OIL ANALYSIS||Several methods are used to analyze oil condition and contamination. These include spectromtery, viscosity analysis, the blotter test, dilution analysis, water detection, Total Acid Number assessment, Total Base Number assessment, particle counting, microscopy, and sediment analysis.|
|Spectrometry||A spectrometer is an instrument with which one can
measure the quantities and types of metallic elements in a sample of oil.
The operating principle is as follows. A diluted oil sample is pulverised
by an inert gas to form an aerosol, which is magnetically induced to form
a plasma at a temperature of about 9000°C. As a result of this high
temperature the metal ions take on energy, and release new energy in the
form of photons. In this way, a spectrum with different wavelengths is
created for each metallic element. The intensities of the emissions are
measurable for each such element by virtue of its very specific
wavelength, calculated in number of ppm (parts per million). An ICP
spectrometer can detect the very small metal particles in suspension in
the oil, i.e. with a size between 0 and 3 microns.
Those small particles are a good indication of general wear, except in cases of sudden metallic rupture, where there will be relatively more large particles liberated (50 microns and more). The human eye can detect particles of a size starting from 50 microns, which allows them to be visualized using more conventional means. Thus, complementary analysis of such larger particles can be done by spectrometry (after acid attack), by ferrography (or related systems) or by optical or electronic microscopy.
|Viscosity||Engine oils. In the early days of the IC engine there were only monograde oils (e.g., SAE 20, SAE 30, SAE 50). By putting an additive into these oils, called a VI improver, multigrade oils were created. The VI (viscosity index) improver is a flexible molecule, rolled up like a ball at low temperature and stretched out like a string at high temperatures. This allows the oil to remain viscous at high temperatures. One can recognize multigrade oils as being represented by two figures. The first figure, followed by the suffix ‘W’, stands for the viscosity calss at low temperature (W = winter). The second figure is the SAE class at working temperature. Thus, for example, ‘SAE 20W-50’ means that the viscosity of the oil at low temperature corresponds with a SAE 20W, and the oil viscosity at 100°C corresponds with a SAE 50. The table below gives some data on viscosities.|
SAE viscosity grade crankcase oils
|The viscosity of used engine oil is mostly measured
at 100°C, and can drop for reasons of fuel dilution, and/or shearing of
the VI improver. Viscosity can increase as a result of heavy contamination
of the oil by soots, and/or oxidation of the oil.
|The viscosity can be decreased by adding a more fluid oil, or as a result of high water content, of by shearing of the VI-improver. The viscosity can be increased by adding a more viscous oil, and by oil oxidation (e.g. as a result of overheating).|
|Blotter test||This quick and cheap test, which consists in blotting a drop of used engine oil on a filter paper, gives good indications about the dispersancy of the oil, the soot content and the extent of oil oxidation. The spots can also be evaluated by photometer, where a camera scans the surface of the spot and calculates in a few seconds the opacity or the soot content and the percentage of dispersancy. For an average used engine oil, the soot content must be lower than 1 per cent, and the dispersancy higher than 80 per cent.|
|Dilution||Dilution of a use engine oil can be measured
precisely by gas chromatography (GC) or by Fourier Transform Infrared
spectroscopy (FTIR). More common is the use of the SETA-FLASH tester,
where the flash point of oil is tested by a certain temperature. When a
flashpoint is detected, the dilution is heavy (more than 4%), when not,
the dilution is acceptable (less than 4%).
It is evident that heavy dilution of the oil is unfavourable for the engine, since it involves a lower viscosity and reduces the resistance of the oil film. The principal causes of dilution are a defective fuel injection system, a defective air inlet (obstructed air filter), incomplete combustion due to too low a working temperature, and badly regulated valves, or insufficient compression.
|Water detection||The water-content of the oil is usually measured by
the Aquatest or a Karl Fisher apparatus. The possible causes of water
introduction include (a) condensation, due to too low a working
temperature, defective crankcase ventilation, ‘stop and go’ in-service
usage, and obstruction of the exhaust system; or (b) infiltration, due to
leakage at the cylinder head gasket, or damage of the engine block.
Cooling water contains most often an anti-freeze based on glycol. Therefore a glycol test should be performed when water infiltration is suspected. The inhibitor in the anti-freeze agent is usually a sodium borate type.
|TAN (Total Acid Number)||The acidity of the oil is measured by titration
through a base, and expressed in mg KOH/g. The figure below shows this
graphically, showing the evolution of TAN as a function of time.
|TBN (Total Base Number)||The alkalinity of an oil is measured by titration
through an acid, and expressed in mg KOH/g. The comparison between the TBN
volume of the fresh oil and that of the used oil allows the determination
to be made of whether the used oil is still capable of neutralizing acid
residues. These acids are produced by combustion (sulphur in fuel) and
oxidation of the oil and oil additives. When the oil is in service too
long, the TBN will drop significantly.
Too low a TBN volume can be due to: heavy oxidation of the oil, when the oil has been in service for too long, of the oil level was insufficient, or due to a defective cooling system, producing overheating; use of a fuel containing a high sulphur content; use of an inappropriate lubricant; or contamination of the oil by fuel or water.
|Particle couting||This is an especially useful test for a hydraulic
system with high sensitivity (e.g., servo-valves). Insuch a text, a
certain quantity of hydraulic oil flows through a sensor, where all the
insoluble material in the oil is detected and counted using the principle
of light absorption. The particles counted are classified
The results of particle counting can be expressed according to either ISO 4406 or NAS 1638. According to ISO 4406, the results are expressed cumulatively, and the ISO classification is deduced from the two first classes, >5µ and >15µ, following the ISO table.
According to NAS 1638, the results are expressed differentially, in five classes. In each class one can get a NAS quotation, and the NAS code is the figure given to the first class.
example, after a particle count produces the following figures
this is differentially written as
According to ISO 4406, the classification is in this case 19/16. According to NAS 1638 the NAS class is 11.
|Microscopy||After filtration of a certain amount of oil through a
cellulose filter (of 0.8µ), the filter is examined under an optical
microscope (magnitude 100x, 200x), and one is able to distinguish:
|Sediment||A sample of oil is filtered through a 0.8µ filter. After drying and weighing the filter, the total amount of insolubles in the oil greater than 0.8µ is computed and expressed in mg/L oil.|
|CONCLUSION||Oil analysis has proven to be very helpful to maintenance engineers. Besides analyzing the condition of the lubricant itself, oil analysis also tells the engineer a lot about the condition of the equipment, and so allows preventative maintenance. Based on the premise that it is better to prevent that to cure, oil condition monitoring should become a valuable aspect of modern engineering and maintenance.|
|Each test method has limits, within which the oil should be. The following rubrics give some examples of these.|
It is not possible to determine to which API classification an engine oil belongs, only to look at its additive level.
|Viscosity at 100°C|
The viscosity is estimated as a function of the SAE limits.
Gasoline (a) on the road - 0 to 4% = normal, >4% = problematic (b) stop-go service >8% = dangerous
The following table is a guide to the number of wear elements per type of metal found in certain types of industrial equipment.
|Viscosity at 40°C|
If possible, the viscosity at 40°C is estimated as a function of the ISO table given above.
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