I work in an oil analysis lab and we often analyze Mobil DTE 832/846 turbine oil for TAN using ASTM D664. Knowing its initial TAN value (which is low, around 0.1), we are observing that it often increases to 0.4-0.5 (which would be an alert level for a "normal" lube) in 3000-4000 hrs of gas turbine service and then it remains quite stable for several thousands of hours. We do not observe any IR oxi increase. Mobil DTE 800 series are turbine oils which contain Phosphorous (about 1000 ppm) as TCP antiwear additive, but this should not interfere with TAN reading, since new oil value is low.
Does someone know what this TAN increase is about? Do you think it's alarming? Do we have to review our TAN limits from the classic "0.25+new oil value"?
Thank you

mecoil,

I don’t have answers on your question, but I will offer you my thoughts on how to (possibly) get to them. IMHO, it would be worthwhile to check RPVOT of new oil, and every time when you check AN. If there is no change in RPVOT, and you noticed again such high increase in AN, then there must be some sort of interference that gives you fault-positive readings. Also, run AN on used oil, and then filter it (a 3-micron cellulose fiber filter would capture a bunch of particles and free and emulsified water if it’s there), and then run AN again and see if there is any difference (phosphorus may form salts with heavy metals and positively interfere with titrant, whether these salts are hydrolysed or not). It would be helpful in this equation to have available results of the analytical spectroscopy (new oil, and filtered and unfiltered in-service oil). This can give you some ideas of possible interferences. I wish I had a free lab access for a few days so I can run various analysis and crunch the data. .

If you are confident that measured AN of 0.4 and 0.5 is not result of fault-positive interferences, and the RPVOT value has been found significantly decreased, then it is alarming. However, IMO, the classic "0.25+new oil value" is just a general guide. I would not condemn an oil solely on the AN value without the collaboration of other parameters, unless the oil is found to be contaminated with acid from within the system or externally.

Good luck.

Mecoil
Tan (mg KOH/g ASTM D664 warning limit for DTE 846 is 0.6 (new oil is 0.1)
RBOT (mins)ASTM D2272 is used to monitor remaining antioxidant capacity of the oil and the warning limit for DTE 846 is 300 or 25% of new oil which is 1100-1200
I expect the phenolic oxidation inhibitor will be doing its job in slowing down the AN this will depend on system operating conditions.

When you say you don't see anything abnormal by FTIR, how did you set the FTIR limits? What type of FTIR are you using?

Have you tried RULER on the same samples that have the high TANs?

Andy

I currently use a Bio-Rad FTS175C JOAP Oil analyzer. I haver never tried the RULER. Our FTIR alarm limits are close or equal to those recommended by JOAP, at least for mineral turbine oils MIL-L-17335. The abnormally high TAN samples show oxi values equal or 1 unit higher than the new oil sample.
However, thank you for your reply.