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Strong Acid Number

What is Strong Acid Number?

We have the oil in our biogas motors analysed on a regular basis. Studying the trends, we have noticed that in some analysis results a SAN (Strong Acid Number) is indicated in the laboratory report alongside the AN or TAN we normally expect to see. Why is this value given and what is its significance?

OELCHECK answers:

In all laboratory reports relating to engine oil analyses we state the base number (BN) and/or the acid number (AN), depending on the scope of analysis required. These are supported by an initial pH value, the i-pH value, for the oil from gas motors. The determination of these values is generally sufficient for a determination of the optimum oil change time, which is heavily dependent on the composition of the gas. If, however, the gas includes aggressive acids, which get into the oil from landfill gas, bio gases, or digester gas, the method for the determination of the AN is extended. The pH value is then less than pH 4 at the start of titration. Only then can an SAN (Strong Acid Number) be determined at all. The consumption of potassium hydroxide required to reach a pH of 4 is stated as the SAN. Titration then continues up to the neutral point (around pH 7) and the consumption to this point is stated as the AN. The SAN, a measure of very strong acids in engine oil, is only stated in the laboratory report if these aggressive acids are present in the oil. The AN always appears in the report, as ever.
Oils age during their use. Oxidation, in which the oxygen molecules change the hydrocarbon compounds in the oil, is one of the consequences of this ageing process. This gives rise to organic products of oxidation which are acid in their reaction. These reactions take place very slowly at room temperature and only have a minor influence on the oil‘s condition. The speed of the reaction is, however, considerably greater at the elevated temperatures found inside an engine. Products of combustion and wear particles in the oil also act as catalysts. If residues from the combustion of gases containing harmful products such as are found in bio gases, landfill gas or digester gas then condense in the engine oil, this has an additional influence on the increase in the acid products created. This ultimately damages the engine to the extent that it needs to be repaired.
One of the tasks of the engine oil is to neutralize acid constituents. The engine components that are vulnerable to attack are thereby protected against corrosive attack from the free acids. To ensure that this acid neutralization succeeds over the longest possible period, the oils contain alkaline additives the modification of which is recorded in the oil analysis in the form of the BN. If these base additives have been used up or are present in the oil in insufficient volumes, the strong, extremely aggressive acids directly attack the vulnerable engine components such as the bearing metals. At the same time, an increase in oil viscosity and the formation of lacquer-like deposits on hot surfaces (e.g. the inside of the piston crown) may be observed.
Monitoring of the oil in respect of its alkaline reserve and the acids present in the oil is therefore decisive in determining the optimum time for an oil change to protect the engine. Such oil values are also taken into consideration alongside wear values and contamination in the diagnosis of the oil. Thus the BN or base number is an important criterion for the quantity of acid constituents that can still be neutralized and rendered harmless by the oil. This BN potential is compared with the acid AN potential and the i-pH value in the analysis of the old oil. At the OELCHECK laboratory, unlike the laboratories operated by oil and engine manufacturers, we use an extended analysis method for the titration curves to distinguish between the Strong Acid Number (SAN) and the Acid Number (AN) to trace the very aggressive acids that occur in particular in special gas-powered engines.
In the laboratory report we only state the SAN if it is at all possible to determine a value because of the presence of strong acids. This is fortunately generally not the case. However. as soon as a SAN (stated in the laboratory report with values > 0.01 mgKOH/g) is measured, the oil must be changed without delay. At the same time it is recommends that the oil change interval should be shortened so that no SAN occurs the next time. A measurable value means that there is an acute risk of corrosion for all engine parts from the oil that has become too acid.

Article from "ÖlChecker Summer 2010", page 8
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