Acid and Base Numbers

Acid and base number determination are carried out in non-aqueous solutions and are therefore not directly related to absolute levels of acidity or alkalinity.

Total Acid Number (TAN or Neutralisation Value):

TAN is a measure of the total amount of both weak and strong organic acidity present in the oil. Compounds formed in the early stage of oil oxidation are not in themselves harmful. Further oxidation will however convert initial oxidation products into acids, which attack and corrode metals. Hydraulic oils have a residual total acid number, which, with service, will increase. Twice the original value is usually an actionable level.

Total Base Number (TBN):

TBN is a measurement of the reserves of alkalinity present in the oil. Crankcase oils are continuously monitored for TBN particularly in marine, residual and natural gas engine applications when the quality of the fuel is suspect. Higher sulphur fuels quickly deplete the reserve alkalinity of the crankcase oil, which is then unable to neutralise the harmful acids formed as a result of the combustion process.

The advantage of site testing is that the results are instantly available for action. Laboratory testing, although more comprehensive, has the disadvantage of a delay associated with first taking a sample then dispatching and awaiting the results and the laboratory analysis usually includes lubricant and wear debris analysis in conjunction with the above tests. A major problem of site testing is the 'management' of such a programme. Operators in the present economic climate are often transient and programmes initiated with good intent soon fall into disuse with a change of personnel and loss of experience.

The combination of site testing and laboratory analysis should however be considered the optimum when the results are combined into a fully integrated maintenance management programme.