WHAT IS TBN, TAN & SAN

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Alkalinity and acidity

TBN-TOTAL BASE NUMBER- measure of alkaline additives available for the neutralisation of acids from combustion products and oxidation. Level governed by type of fuel.

For crosshead engines the TBN will tend to rise due to contamination by liner lubrication, it should not be allowed to raise more than twice that of the new charge.

As a guide, the TBN of fresh oil should be at least:

·         10 x fuel sulphur content (%) for trunk piston engines (10mgKOH/g)

·         20 x fuel sulphur content (%) for cyl oil in x-head engines (20mgKOH/g)

A simple shipboard go,no-go test is available for measuring the TBN, it involves the addition of an indicator and acid reagent to a 30ml sample. The quantify of acid reagent added is determined by the required level of TBN, for TBN2.5 0.5ml are added, for TBN20 4ml is added. After three minutes the colour is checked against a chart

·         Purple:Good level of TBN

·         Green:Borderline

·         Yellow:Low level of TBN

TAN-TOTAL ACID NUMBER-measure of organic acid and strong acid content of oil. Where SAN is nil, the TAN represents the acidity in the oil due to both the acids in the additives and the oxidation of the hydrocarbons in the oil. The TAN of fresh oils varies with oil type, and tends to climb with age. A high TAN may indicate that an oil should be changed or freshened by top up. A high TAN may be accompanied with increased viscosity.

SAN-STRONG ACID NUMBER-indicates the prescience of strong, highly corrosive (inorganic) acids, usually formed from combustion products. If SAN is non zero the oil should be changed immediately