Setting Acid Number and Base Number Limits
There is a rule-of-thumb that oil should be condemned at double its new oil Acid Number, or at half of its new oil Base Number. The problem with this rule is that is has too many exceptions to be valid. Rather than identify the valid instances, it is better to understand how to derive a correct limit for any application.
Acid Number (AN) is a measure of the acidic constituents in a lubricant, some of which are present from the additives, and increases attributed to degradation products formed in service, such as oxidation products. The AN increases over time and the rate is accelerated by water contamination and increases in temperature; some fluids may increase fairly linearly, while other may increase exponentially over time.
To that end, ASTM recommends that any condemning limits be established empirically (from experience). Therefore, trending of AN will identify when the rate of formation increases, suggesting a depletion of the oxidation inhibitor additive; this value can be used to derive a limit. Typically, the limit would be set just below the AN that deviates from the normal trend, or could be set at or slightly above the last AN that was on trend. However, the use of linear regression negates the need for limits, as deviations from trend are easily identified using this technique.
Base Number (BN) is a measure of the basic constituents in a lubricant and can be entirely attributed to the additives. BN decreases over time, and the rate is dependent on the rate of formation of acids. Since BN is a more direct measure of the additives, the absolute value is of far greater importance than the trend. ASTM D6224 recommends a warning limit be set at 20% of the new oil value, whereas the ASTM methods for determining BN reiterate determining condemning limits empirically.
Most end-users will find oils remaining well above 20% of the new oil value during regular service intervals, even under extreme conditions, suggesting the use of such a limit will only highlight abnormalities. For those seeking to perform condition-based oil changes, this limit would still provide some buffer before the additives are completely depleted.
With either AN or BN, the Original Equipment Manufacturers’ and/or lubricant suppliers’ recommendations should always be adhered to, but in their absence AN limits should be based on historical performance and BN limits can be safely set as low as 20% of the new oil value.
