Total Base Number (TBN)

 This test is to monitor the alkalinity of engine oil, primarily in trunk piston engines, to ensure it is above the engine manufacturer's recommended minimum. The value is measured in units of mgKOH/g.

 (1) Crosshead Engines

 There are two types of system oils for slow speed crosshead engines.

Alkaline oils have a new TBN of 5-12, therefore TBN depletion is a factor in the monitoring of used oil analysis. TBN in the used oil should not be allowed to fall below 50% of the new value. 
Probable causes for the change in TBN;

Recommended actions :

A TBN value below the minimum allowable level can normally be corrected by taking the following action.


(2) Trunk-Piston Engines

TBN of an oil for a specific engine type is mainly dependent on the sulfur level of the fuel being burned.
Different engine manufacturers require different TBN levels in the new lubricant for the same sulfur content of the fuel. These TBN requirements may be summarized as below.

Sulfur content of fuel, wt% TBN of unused lubricant
Under  10-12
1-2 12-20
2-3 20-30
Over 3 40

The TBN value of the lubricant falls in use as the alkalinity is needed to neutralize the acids formed during combustion.
The rate of TBN depletion depends on the following factors.

The TBN will drop over an initial engine operating period. It will then stabilize at a normal operating value.
The minimum acceptable TBN value is dependent on several factors.

When determining whether the minimum allowable TBN has been reached, it is essential to refer to the operating manual for the engine in question. The following table may be used as a guide for minimum TBN values of the used lubricant. TBN in the used oil should not be allowed to fall below 50% of the new value.

Fuel sulfur, wt% TBN new Used oil TBN min.
Under  10-12 5
1-2 12-20 5-10
2-3 20-30 12-22
Over 3 40 30

Recommended actions :

The remedial measures are dependent on the system capacity, engine condition, lubricant and fuel used. The following general guidance can be followed.

  

   

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