Author: John Knox
This is the first of a three–part discussion about the results shown in a Lubricant Analysis report. Our company provides technical assistance with lubricant analysis for multiple maritime vessels. We will look at the reported values by the classifications they belong to (Properties, Additives, Contaminants or Wear Metals) and cover the possible sources and effects of each.
Properties describe the physical characteristics of the lubricant.
The properties tested are:
Monitoring and trending viscosity is one of the most important components of any oil analysis program. Even small changes in viscosity can be magnified at operating temperatures to the extent that an oil is no longer able to provide adequate lubrication.
A significant reduction in viscosity can result in:
- Loss of oil film causing excessive wear
- Increased mechanical friction causing excessive energy consumption & heat generation due to mechanical friction & internal or external leakage
- Increased sensitivity to particle contamination due to reduced oil film
- Oil film failure at high temperatures, high loads or during start–ups or coast–downs
High viscosity levels can cause:
- Excessive heat generation resulting in oil oxidation, sludge, and varnish build–up
- Gaseous cavitation due to inadequate oil flow to pumps and bearings
- Lubrication starvation due to inadequate oil flow
- Oil whip in journal bearings
- Excess energy consumption to overcome fluid friction
- Poor air detrainment or demulsibility (foaming)
- Poor cold–start pumpability
Whenever a significant change in viscosity is observed, the root cause of the problem should always be investigated and corrected. Viscosity is the first indicator of a misidentified sample or of having an incorrect oil in service. These faults combined are, by far, the most frequent failures in the oil analysis program.
Total Acid Number (TAN)
TAN is the measure of acid concentration in a nonaqueous solution. It is determined by the amount of potassium hydroxide (KOH) base required to neutralize the acid in one gram of an oil sample. The TAN measurement detects both weak organic acids and strong inorganic acids. As the lubricant becomes acidic, its ability to protect surfaces from corrosion turns into an agent that attacks the surfaces, causing corrosion.
A change in the acid concentration of an oil can originate from multiple sources, including acidic contaminants, wrong oil, alkaline–reserve depletion, and oxidation by–products, which can all cause an increase in acid concentration.
Understanding the extent of additive depletion is key in determining the Remaining Useful Life (RUL) of an oil. Some additives are weakly acidic and can elevate the oil’s initial TAN. As the lubricant ages these additives deplete, thereby reducing the acidity created by the additives. The common anti–wear additive, Zinc Dialkyl Di–Thiophosphate (ZDDP) can cause an initially elevated TAN indication which will decrease to a “steady state” level as the ZDDP depletes.
Total Base Number (TBN)
TBN is a measurement of basicity that is expressed in terms of the number of milligrams of potassium hydroxide per gram of oil sample (mg KOH/g). The higher an oil’s TBN, the better its ability to neutralize contaminants such as combustion by–products and acidic materials. TBN is a measure of (alkaline) additives in the oil. Higher TBN oils can neutralize a greater amount of acidic materials, which results in improved protection against corrosive reactions and longer oil life.
Because of the presence of reserve alkalinity in engine oils, TAN would be zero until the alkalinity reserve is depleted, which is why TBN rather than TAN is used for engine oils.
TBN levels are targeted for the intended application. For example, gasoline motor oils typically display lower TBN numbers, while diesel oils must manage the high contaminant–loading from soot and sulfur and typically have a higher TBN.
TBN levels decrease as the oil remains in service. When the level reaches a point where it can no longer protect against corrosion, the oil must be changed. A “rule of thumb” often used is that TBN should not go below one–half the TBN value of the oil when new.
Oils that are formulated specifically for extended drain intervals (synthetics for example) typically display elevated TBN to ensure proper corrosion protection for the duration of the extended interval.
Oxydation Number is a measure of the lubricants’ ability to resist oxidation. Oxydation is the most predominant reaction of a lubricant in service. It is responsible for numerous lubricant problems including viscosity increase, varnish, sludge and sediment formation, additive depletion, base oil breakdown, filter plugging, loss in foam control, Acid Number (AN) increase, rust formation, and corrosion.
When an oil formulation is designed, a series of tests is required to validate it meets the target formulation. The test series always includes several oxidation tests, and in most cases, the remaining oxidation reserve is measured by testing the lubricant’s behavior under an oxidation experiment.
While oxidation is very useful for most mineral oils, it can be misleading when using synthetic oils. Some synthetic oils show extremely high oxidation levels right out of the box.
The flash point is the lowest temperature at which the vapor above the oil sample will momentarily ignite or flash when an ignition source is passed over it. The flash point is an indication of the safety hazards of a lubricant with respect to fire and explosion.
Because of the low flash points of most fuels, a sudden drop in flash temperature, along with a decrease in viscosity in a crankcase oil can usually be relied upon as an indication of fuel dilution.