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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 Spectroscopy (FT-IR) is routinely used to assess thedegradation 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 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 fuelsoot, 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 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
performed at two temperatures: 40o
Celcius and 100o Celcius. These temperatures
embrace the bulk of operating 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 commonly 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 coast-
downs.
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 Boron as B Barium as Ba
Cadmium as Cd Calcium as Ca Boron as B
Chromium as Cr Potassium as K Calcium as Ca
Copper as Cu Silicon as Si Chromium as Cr
Iron as Fe Sodium as Na Copper as Cu
Lead as Pb Magnesium as Mg
Magnesium as Mg Molybdenum as Mo
Manganese as Mn Phosphorus as P
Nickel as Ni Silicon as Si
Silver as Ag Zinc as Zn
Tin as Sn
Titanium as Ti
Vanadium as V
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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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 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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