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Application Notes on Lubricating Oil Analyses In service oil analysis, also known as condition monitoring through oil analysis, has gained wide acceptance as a predictive maintenance and cost-cutting strategy in modern industry. In order to be effective, an in-house oil analysis program must monitor the condition of the mechanical integrity of the machine from which the oil sample is taken. Concurrently, it must also monitor the condition of the lubricant. A modern condition monitoring program based on oil analysis takes the following form :
1. Fourier-Transform Infra-Red Infra-Red Spectroscopy (FT-IR) is routinely used to assess the degradation and/or contamination of Lubricating Oils. This test enables both petroleum-based and synthetic oils to be detected for: a) Contaminants in lubricants b) Cross-contamination Cross-contamination of lubricants c) Degradation of Lubricants Organic compounds present in lubricating oils will absorb infrared light at specific frequencies. The most common frequencies measured in oil analysis indicate fuel soot, oxidation, nitration, water and glycol. Reference samples, usually new oil, are required for effective determination and interpretation. 2. Total Acid Number (TAN) is a key analytical test to determine the degree of deterioration of in-service lubricants. The higher the acidic value of the lubricant, the higher the degree of degradation of the lubricant. The TAN of a lubricant is expressed as milligrams of Potassium hydroxide per gram of oil (mgKOH/g). Analysis of lubricant for TAN is performed per formed electrometrically using an Autotitrator. 3. Kinematic Viscosity is basically a measurement of a fluid’s resistance to flow. Kinematic Viscosity is usually expressed as Centistokes ( cSt). Testing is typically o
o
performed at two temperatures: 40 Celcius and 100 Celcius. These temperatures embrace the bulk of o operating perating temperatures of machinery. The measurement is performed using a Brookfield Digital Rotary Viscometer with a full range of spindles, which are selected on the basis of Viscosity range. The method is c ommonly referred to as the Brookfield method and is detailed in Standard ASTM D2983. Monitoring and trending of Viscosity is one of the most important components of any oil analysis program. Small changes in viscosity can be magnified at operating
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temperatures to the extent that a lubricant is no longer able to provide sufficient or adequate lubrication. Reduction of Viscosity can lead to: a) Loss of oil film resulting in excessive wear. b) Increased mechanical friction causing excessive energy consumption. c) Increased sensitivity to particle contamination due to reduced oil film. d) Oil film failure at high temperature, high loads or during start-ups or coastdowns. Increase of Viscosity can lead to: a) Excessive heat generation resulting in oil oxidation, sludge and varnish build-up. b) Gaseous cavitation due to inadequate oil flow to pumps and bearings. c) Lubrication starvation due to inadequate oil flow. d) Oil whip in journal bearings. e) Excess energy consumption to overcome fluid friction. f) Poor air detrainment or demulsibility. g) Poor cold-start pump ability.
4. Viscosity Index is a test which may prove useful in lubricant analysis. Viscosity Index (VI) is the difference in viscosity at two different temperatures. It is commonly known that Viscosity decreases with increasing temperature but it is not generally known that the amount of change in viscosity is not linear. 5. Fuel Dilution is the measure of a fuel present in a lubricant. Excess fuel in oil reduces the oil film strength due to decrease of viscosity, thereby increasing metal-to-metal contact and wear. Excessive fuel will also cause premature oil oxidation. High Fuel Dilution is generally caused by excessive idling, improper adjustment, and/or faulty components within the fuel delivery system. 6. Fuel Soot (% mass) may be accurately measured by Light Extinction Measurement (LEM) technique. Fuel soot levels are indicative of air/fuel ratios, fuel delivery and valve settings and combustion/exhaust efficiency. The state of the fuel soot depicts dispersant additive effectiveness.
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7. Spectrometric Oil Analysis is the measure of trace levels of wear metals present in a lubricant. These metals may originate from moving parts wear, as well as from external contamination. Metals may also be present due to additive treat of oil. Table 1 showing metals typically present in a lubricating system. Wear Metals
External Contaminants
Additives
Aluminium as Al Cadmium as Cd Chromium as Cr Copper as Cu Iron as Fe Lead as Pb Magnesium as Mg Manganese as Mn Nickel as Ni Silver as Ag Tin as Sn Titanium as Ti Vanadium as V Zinc as Zn
Boron as B Calcium as Ca Potassium as K Silicon as Si Sodium as Na
Barium as Ba Boron as B Calcium as Ca Chromium as Cr Copper as Cu Magnesium as Mg Molybdenum as Mo Phosphorus as P Silicon as Si Zinc as Zn
8. Particle Shape is determined using high-power Trinocular Microscopy which captures the silhouette image of particles in oil. The image of the particles in the sample is captured by a USB camera and stored in computer memory using a specific software package. The objects are then analysed for size and shape characteristics which are then used to classify particles into wear classes. Air bubbles are recognised and eliminated from the count and water droplets are recognised, classified and quantified.
If we examine an ISO code of a LUBRICANT of, say 16/14/11, this basically means that : a. 16 is the number of particles greater than 4 microns in diameter. b. 14 is referring to the number of particles equal to or greater than 6 microns. c. 11 is referring to the number of particles equal to or greater than 14 microns. These particle diameters are derived from the ISO Standard 11171 and are used in all modern analytical laboratories. Particle sizing is measured using a Malvern Particle Sizing Laser Counting Analyser.
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9. Water ( % Volume) . The amount of water suspended in lubricant is measured by the Karl Fischer Electrometric titration method and expressed in parts per million (ppm). This method measures water levels to as low as 0.1 ppm and is generally applied to fluids from systems which have a low water tolerance or low water requirements.
10. Total Base Number (TBN) It is the measure of the Alkalinity remaining in a lubricant. A relatively low TBN or a decrease in TBN compared to the new product, indicates low acid-neutralizing characteristics or a depleted additive package. 11. Ferrography is used to assess overall levels of metal contamination. The method is based on a Microscopy technique after the ferrous wear particles are magnetically sorted and the particle density of each range is optically measured.
Big Problems often start small! Spectrochemical analysis allows us to analyse an oil sample and search for the presence of minute metallic elements. Due to circulation and function within a mechanical system, every lubricant that is in-service will contain microscopic particles of metallic elements. Spectrochemical analysis indentifies and measures these particles in parts per million (ppm) by weight. The analysed elements are grouped into 3 main categories :
a) Wear Metals b) Contaminants c) Additives
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Wear Metals and Additives
Iron
Indicates wear originating from rings, shafts, gears, valve train, Cylinder walls and pistons in some engines
Chromium
Primary sources are chromed parts such as rings, liners etc. and some coolant additives
Nickel
Secondary indicator of wear from bearings, shafts and valves
Aluminium
Indicates piston wear, rod bearings and bushings
Lead
In diesel engines, overlay of most ma/rod bearings. In Petrol engines, mostly from Tetraethyl Lead contamination.
Copper
Wear from bearings, rocker arm bushings, wrist-pin bushings, thrust washers, bronze and brass parts. In some transmission, wear from discs and clutch plates. Oil additive or anti-seize compound.
Tin
Indicates wear from bearings when babbit overlays are used. Tin is also an indicator of piston wear in some engines.
Silver
Bearings wear, secondary indicator of oil cooler problems, especially if coolant sample is detected.
Titanium 5
Alloy in high quality steel for gears and bearings.
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Wear Metals and Additives
Silicon
Airborne dust and dirt contamination.
Boron
Coolant additive, used as an additive in some oils.
Sodium
Coolant additive, used as an additive in some oils.
Potassium
Coolant additive
Zinc
Antioxidants, corrosion inhibitors, anti-wear additives, detergents, extreme pressure additives.
Molybdenum
Indicates ring wear. Used as an additive in some oils.
Phosphorus
Antirust agents, spark-plug and combustion chamber deposits
Calcium.
Detergents, Dispersants, acid neutralizers.
Barium
Corrosion inhibitors, Detergents, Rust Inhibitors
Magnesium
Dispersant, Detergent additive, alloying metal.
Antimony
Bearing overlay alloy or oil additive.
Vanadium
Heavy fuel contaminant
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Lubricating Oil Testing Laboratory All methods listed are American Society for Testing and Materials (ASTM)
Method Reference
Test
D-2007
Aromatics by isolation
D-2007
Asphaltenes
D-445
Kinematic Viscosity cSt
D-664
Total Acid Number TAN
D-287
API Gravity
D-482
Ash Content
D-2896
Total Base Number TBN
D-808
Chlorine in new and used oils
D-2500
Cloud Point
D-189
Carbon Residues
D-130
Copper Strip corrosion
D-322
Fuel Dilution % volume
D-86
Distillation of Petroleum products
D-93
Flash Point Determination
D-2982
Glycols estimation
D-3228
Total Nitrogen Content
D-4047
Additive Content of Lubricating Oils
D-97
Pour Point
D-473
Sediment Content
D-1552
Sulphur Content
D-893
Toluene Insolubles/Pentane Insolubles
D-96
Water Content
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