Coking occurs at very high temperatures. Possibly aggravated by moisture and oxgygen.

In contrast, sludging occurs over a longer period of time at milder elevated temperatures, assisted by the same factors of moisture and oxygen.

Is sludging the same as varnishing?

What can cause high temp or overheating of the lube oil? Is it due to insufficient cooling by lube oil cooler fans?

Here's how I would say it (I welcome corrections)

Coking occurs at the highest temperatures.
Varnish occurs at medium temperatures.
Sludging occurs at even lower temperatures.

An example of coking is hard black deposits on the oil-immersed metal reversing contacts of a transformer load tap changer. I'm guessing temperatures 500F+. Copper melts at 2000F.

An example of varnishing - rolling bearing found with brown stains. Perhaps 200F-400F?

Example of sludging - goop that settles from really old oxidized oils and may clog orifices or coolers.

Good explanation, Pete.

For gas turbines, what can cause the turbine oil to be coked? Has anyone experienced this?

I wonder whether it's the same problem discussed at length in this forum:

http://theoildrop.server101.com/forums/showflat.php?Cat...r=754379&an=0&page=0

I researched a bit this subject and found this which I would o share with others:

http://www.practicingoilanalysis.com/article_detail.asp...bookgroup=Hydraulics

I researched a bit this subject and found this which I would o share with others:

http://www.practicingoilanalysis.com/article_detail.asp...bookgroup=Hydraulics

http://www.oilanalysis.com/learning_center/category_art...bookgroup=Hydraulics