After a complete oil change out and turbine overhaul, our RPVOT number was 800 minutes 3-1/2 years ago. Now the RPVOT is 320 minutes. We went for a Chevron to a Shell product and we discovered excessive varnishing on the turbine bearings at the overhaul. The RPVOT on the Chevron product was 285 minutes before the change and the oil was in service since the mid 70's, with some sweeting over the years. What could have caused this rapid decline in the RPVOT number? What have others done to bring up RPVOT numbers? Sweeting, filtering? What other tests should be run to help dertime action?
Any other suggestions?

Rick,

Considering the offered info, it is obvious you went from Group-1 oil to Group-2 turbine oil. In my opinion, before any change-out or adding make up oil, it is absolutely essential to run compatibility test between the in-service oil and new oil, because their additive packages may not be fully compatible. Incompatibility may cause a serious additives dropout, and other headaches such as increased foaming, poor water separation characteristic of oil, excessive varnishing, sludge buildup, sticking of proportional valves, and plugging of in-line filters. I suggest you run varnish tendencies of your oil. Filtering and “sweetening” approaches might be helpful on the short run, but, in my opinion, you should start considering and making plans for a scheduled replacement of this oil.

Rick

If you have problems with varnishing, you could add to your test package some test for determining and trending the varnish potential rating. There are technology for removing and preventing varnishing problems in Gas Turbines. You can find out more of this in GE TIL 1528-03.

Also, you have to take care about the mix between different oils (like John comments).

Regards,

Agree with everything said above - what is RBOT of new oils you are sweetening with? RBOT of in-service oil seems very low.
RULER can be be used to check antioxidant type and levels in your new and used oils.
Patch tests can be used to check varnish potential/type of varnishing going on - oxidation due to low antioxidants, electrostatic arcing increased by lower polarity of newer oils, etc.
Particle counting can be done to see if varnish particles are suspended if so several companies with electrostatic or other fine filtration techniques to remove suspended particles, but doesn't stop degradation process producing particles just minimizes effects of particles on your system
There was an ASTM symposium on varnishing problem last December don't know if papers are available yet.

Just remembered an article that answers many of your questions is in May - June 2006 edition of Practicing Oil Analysis PP 28 - 34. Also available on-line at www.practicingoilanalysis.com

Rick - Our paper presented at ASTM Norfolk last December showed our results of 3 samples, two used turbine oils and the new oil, sent to 4 different labs, in four different parts of the world. The samples to three of the labs were unmarked to try get get some real "blind" testing results.

Of the blind labs, two of them could not even tell the difference between used oils and the new oil (by RPVOT values, of course the color would have been a dead give-away, but they must of neglected to look at that)

One blind lab maybe looked at the color of the oils, and gave results that sort of made sense, at least the new oil was higher in RPVOT than the used oils. However, they were still way off in the realive RPVOT values.

The only lab to get the results right, was the same lab than blended the oil!!And you guessed it, they were not blind, and knew it was their oil. Lo and behold, they got the new oil value right!

What does that tell you? It tells me that RPVOT does not work with true unknown samples, in true blind testing, and that most RPVOT results are made-up by random number generators or dart-boards!!
(rememeber the accountant job-applicant joke..how much do you want it to be, boss?)

email me at andy.sitton@focuslab.co.th if you want to see the data

We have 2 customers who had a similar problem. To confirm the RPVOT value, we use the lenear sweep voltometry. Plus using the QSA results, we have the potential of varnish in the oil. When the potential of varnish is over 35-40, we recommand a filtration with electrostatic technology. But if the RPVOT is less than 30%, there is not mutch that can be done. If the sweetning didn't work, I gess the only thing that can be done is to replace the oil. And implement a good oil analysis program, taking samples regularly and choosing the righ tests for turbine oil. The ASTM 4378, can guide you.