CYLINDER OIL , SULFUR CONTENT IN FUEL AND COLD CORROSION
Now a days the average sulfur content of HFO used for marine diesel engines is about 2.7%. This will further reduce with forthcoming stricter emission legislation. With such varying content of sulfur in fuel forces us to use appropriate cylinder lub oil in the engines.
With the enforcement of new legislation regarding SOx emission, the engine maker's declared without giving any thought that their engines are compatible with fuel having low sulfur including distillate fuel.
But there was deep research and study about the use of appropriate cylinder oil in relation to varying sulfur content of fuel.This study was required because of the presence of cold corrosion in cylinder liners. The cold corrosion is considered to be the most influencing cause of wear in the liners, which is basically due to the condensation of SO3 in exhaust gases.
In order to neutralize sulfuric acid the cylinder lub oil must contain appropriate alkaline components. The BN is actually a measure of cylinder lub oil's ability to neutralize acid. The Base number is therefore an important parameter in controlling corrosion on the cylinder liner surface. Controlled corrosion is therefore important to ensure creation of adequate lub oil film on the cylinder liner surface. If neutralization is too efficient, the liner surface may get polished i.e. lub oil film may get damaged and thus risk of scuffing increases.
In other words, operating the engine with unmatched BN/ fuel sulfur content for long time could result either in scuffing or excessive corrosion wear.
The total alkaline content of the cylinder oil has to match the sulfur content in the fuel in accordance with the equation:-
Dosage= F*S% where F= 0.2 gm/kwh for the new large bore engines based on BN 70 cylinder oil.
The minimum feed rate for proper oil distribution and oil film thickness has so far been set at down to 0.6gm/Kwh. This means that the theoretical limit, for using an ordinary BN 70 oil is 3% sulfur content fuel at minimum feed rate. In other words an engine using 1% sulfur fuel at dosage of 0.6gm/Kwh would therefore is over lubricated.
A fuel with sulfur content as low as 0.1% may thus need a combination of a low cylinder oil dosage and a low BN oil.
As per MAN B&W, cylinder oil feed rate should also be changed as per BN of the oil. For example, Cylinder feed rate is 1.1gm/Kwh for BN 70 oil and new BN 60 oil has to be used, the new cylinder feed rate would correspond to 1.28gm/Kwh basis the formula:-
Old BN
New feed rate = Old feed rate X ------------------- = 1.1*70/60= 1.28gm/Kwh
New BN
Since engines are operated at different loads, hence cylinder oil feed rate should be adjusted manually or automatically. Some of the engines adjust adjust cylinder feed rate automatically using various control modes like
With the enforcement of new legislation regarding SOx emission, the engine maker's declared without giving any thought that their engines are compatible with fuel having low sulfur including distillate fuel.
But there was deep research and study about the use of appropriate cylinder oil in relation to varying sulfur content of fuel.This study was required because of the presence of cold corrosion in cylinder liners. The cold corrosion is considered to be the most influencing cause of wear in the liners, which is basically due to the condensation of SO3 in exhaust gases.
In order to neutralize sulfuric acid the cylinder lub oil must contain appropriate alkaline components. The BN is actually a measure of cylinder lub oil's ability to neutralize acid. The Base number is therefore an important parameter in controlling corrosion on the cylinder liner surface. Controlled corrosion is therefore important to ensure creation of adequate lub oil film on the cylinder liner surface. If neutralization is too efficient, the liner surface may get polished i.e. lub oil film may get damaged and thus risk of scuffing increases.
In other words, operating the engine with unmatched BN/ fuel sulfur content for long time could result either in scuffing or excessive corrosion wear.
The total alkaline content of the cylinder oil has to match the sulfur content in the fuel in accordance with the equation:-
Dosage= F*S% where F= 0.2 gm/kwh for the new large bore engines based on BN 70 cylinder oil.
The minimum feed rate for proper oil distribution and oil film thickness has so far been set at down to 0.6gm/Kwh. This means that the theoretical limit, for using an ordinary BN 70 oil is 3% sulfur content fuel at minimum feed rate. In other words an engine using 1% sulfur fuel at dosage of 0.6gm/Kwh would therefore is over lubricated.
A fuel with sulfur content as low as 0.1% may thus need a combination of a low cylinder oil dosage and a low BN oil.
As per MAN B&W, cylinder oil feed rate should also be changed as per BN of the oil. For example, Cylinder feed rate is 1.1gm/Kwh for BN 70 oil and new BN 60 oil has to be used, the new cylinder feed rate would correspond to 1.28gm/Kwh basis the formula:-
Old BN
New feed rate = Old feed rate X ------------------- = 1.1*70/60= 1.28gm/Kwh
New BN
Since engines are operated at different loads, hence cylinder oil feed rate should be adjusted manually or automatically. Some of the engines adjust adjust cylinder feed rate automatically using various control modes like
- RPM Dependent
- LOAD Dependent
- MEP Dependent
Since some corrosion is beneficial to the engine condition as this keeps an open graphite lamella structure of the liner surface from where the cylinder lubricant can spread The purpose is therefore not to avoid corrosion but to control corrosion. This is done by adjusting the amount of Base, or by optimizing the feed rate to the actual fuel sulfur level or a combination of both.
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