In non-combustion situations, we don't have the byproducts of combustion to cause acids to form; however to a lesser extent, we experience acid generation as the additive package of the oil is depleted due to use and the effect of moisture.
In gear and hydraulic systems, the Sulphurs and Chlorines in the oil will become available to combine with water and oxygen which can lead to the formation of Hydrochloric and Sulphuric acids.
These acids although usually at low levels can destroy metal surfaces if left unchecked, hence we need to monitor the acid levels, particularly in long drain interval situations such as Turbines, Hydraulic systems and gearboxes.
Any rapid rise in TAN should never be left unchecked. A TAN of 0.5 mgKOH/g should be watched, however this level will depend on the system application and limits set by OEMs which should always be known to the operator before any assessment is acted on.
Watch for sudden movements in TAN levels in non-combustion compartments and take action if it moves suddenly.