Is there someone has an article that can explain how water can cause TBN depletion or drop down
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Water all by itself won't cause measurable TBN reduction. Water chemically reacting with sulfur residues from fuel combustion will form sulfuric acid which does reduce TBN. If the oil stays warm the water vaporizes away before it can chemically react to form acids.
I don't think so because the engine has been stopped but TBN keep going lower below 50% from it TBN reserve time by time. Meanwhile water content in the engine oil still 0.27 %Volume So What happen ?
It appears to me you are not telling the full story. Like, what engine or application you are talking about, which brand and viscosity of oil you use, what are conditions of application, did you measured TBN yourself or send it to a lab, how you collected representative sample for testing TBN and water if "engine" is off, etc. etc. However, with the information you did provide, I think refrig guy gave you the right answer.
The Engine is Diesel Power Generating Set,with capacity is 22 MW. And use Mobilgard 430 with TBN reserve is 30 mgKOH/gr. Then 2 years a go the TBN of the Oil start going down rapidly .Then from UOA result show TBN drop until 16 when water content reach 0.3%volume. Besides that soot level also increasing until
The TBN going down when the soot level is going up with moisture present is exactly what I described above. The sulfur from the fuel is making soot in the oil. There is moisture present (0.3% according to your data) allowing the sulfur to react to form acids.
There will be a lag time for the soot to get into the oil and for the moisture to react with the sulfur in the soot. The intial high TBN values will not drop until you get both moisture and soot in the oil. The chemical reaction between the moisture and the soot will go slower while the engine oil is cool, and will go faster when the oil is warmer.
This is one reason you should change engine oil on a time period basis (every 12 months for example) even if there are only very few operating hours on the engine. The additives will be depleted even if the engine is not running.
There will be a lag time for the soot to get into the oil and for the moisture to react with the sulfur in the soot. The intial high TBN values will not drop until you get both moisture and soot in the oil. The chemical reaction between the moisture and the soot will go slower while the engine oil is cool, and will go faster when the oil is warmer.
This is one reason you should change engine oil on a time period basis (every 12 months for example) even if there are only very few operating hours on the engine. The additives will be depleted even if the engine is not running.
And as long as I know soot is just carbon deposit (in solid form) why you said the soot can transform into sulfur.
Soot is from incomplete combustion of the fuel. Diesel fuel has sulfur in it. Soot has both carbon and sulfur in it because the sulfur containing components do not get completely combusted either. Some of the sulfur from the fuel ends up in the oil where it can be converted into sulphuric acid by chemical reaction with moisture.
High levels of sulfur cause more particulates to be formed as the fuel is combusted. I believe this is one of the main reasons for the introduction of low sulfur fuels. If you are burning ultra low sulfur fuel, the acidity formed from soot reacting with moisture should be less.
I suggest you do some reading:
http://en.wikipedia.org/wiki/Ultra-low_sulfur_diesel
High levels of sulfur cause more particulates to be formed as the fuel is combusted. I believe this is one of the main reasons for the introduction of low sulfur fuels. If you are burning ultra low sulfur fuel, the acidity formed from soot reacting with moisture should be less.
I suggest you do some reading:
http://en.wikipedia.org/wiki/Ultra-low_sulfur_diesel
So that's mean Soot and SOx it quitely same? So SOx is also solid form