Above: Kittiwake TAN Test in mineral and synthetic oils.
Combustion in gas engines creates high temperatures and rapid oxidation of the lubricant. Unless operated on a lean burn principle.
A rise in TAN is generally associated with oil oxidation due to time and/or operating temperature. Trend as well as absolute values should be used to monitor TAN.
Info Centre >> Fuel and Lube Oil Technical Manual >> 7. Lube Oil - Test Results >> 7.1 Key Condition Indicators for Lube Oils >> 7.1.6 Total & Strong Acid Number (TAN & SAN)
Total and Strong Acid Number (TAN & SAN) Testing
Oils continually react with atmospheric oxygen to produce organic oxidation products that are acidic in nature. At ambient temperature, this reaction is very slow and has little effect upon oil conditions. At the elevated temperatures that exist within an operating engine, reaction rates are much higher. This is especially true when localised heating occurs on sliding surfaces, as the temperatures can be very much higher than that of the surrounding metal. Combustion and wear products in solution in the oil can additionally catalyse the reaction between the oil and oxygen.
The degradation products are organic acids, they do not react easily with the alkaline TBN additives and tend to co-exist. They cause increased oil viscosity and can deposit as lacquers on hot surfaces.
Strong Acid Number or SAN is a measure of just the strong acids present in oil. It is an indication that the alkaline additives in a diesel engine oil have been exhausted and there is a risk of severe engine corrosion. Testing for SAN in engine oils is quite simple but the results are of little direct use; comparable to closing the stable door once the horse has bolted. It is more advisable to closely monitor TBN depletion.
Total Acid Number or TAN is the measure of both the weak organic and strong inorganic acids present in oil. It is applicable to gearbox, gas engine, gas turbine and hydraulic lubricants.
TAN is not generally associated with engine crankcase lubricants unless they are severely contaminated. The singular exception to this may be crankcase lubricants for gas engines. In this instance, TBN can deplete very rapidly and elevated operating temperatures can generate high levels of weak organic acids.
High combustion temperatures in gas engines (especially ones using a stoichiometric (1:1) Air/Fuel ratio) will lead to rapid TBN depletion. This is further exaggerated when using landfill gas where typical oil life is less than 500 hours. Under these circumstances, high TAN and viscosity usually become the limiting parameters for oil life.
Lubricants used in applications other than engines are often slightly acidic for a number of reasons, including improved water shedding ability. Some oils are supplied with an initial ‘high’ TAN number. This can drop as the additives are depleted with use, then slowly rise again as the effects of ageing become apparent. It is therefore important to monitor TAN by trend rather than single test results.
High TAN will cause:
