Motor Generator Fluid Drive Oil Analysis

Hi rflagg

As far as I understand, no major OEM gives specific rejection limits for discarding the oil -n service based on RPVOT on particular applications.

As a thumb rule, Turbine oils in service are advised to be replaced with fresh charge once they attain 25 % of their original RPVOT of the fresh oil value. Some manufacturers put this value at 100.

However, RPVOT value alone should not form as the basis of initiating replacement decision. It should be viewed in accrodance with the TAN value of the oil also. In fact its better to increase the frequancy of testing and maintain a data base to track the depletion levels and if the tests indicate that the oil is able to sustain the parameters without significant deteroriation levels in the readings over the previous test results, the oil can be continued to be used. however, the control on sample frequancy and the test result authencity should be adequatly maintained. That is precisely the reason a close encounter is avoided and since the cost of bearings is pretty high in comparison to the cost of a fresh charge, the thumb rule of replacement i.e 25% of the original value of fresh value of RPVOT has mostly been followed world wide by Turbine oil users. It can be extended a bit by either readditizing (very critical issue needs expert handling) or by monitoring closely the results of TAN, RPVOT, Viscosity, Foam, air release values etc.

BTW, you have not indicated as to in exactly how many hours of running (in use) the oil reached this level i.e. RPVOT of 85 Minutes against 435 on new oil? what is the maximum sump temperature and the bearing temperature and the temp at oil inlet and outlet? If the new oil has attained the level of 85 in a fairly short span of time, u may not have been using the right grade..I mean if the temperature levels are high, it will be worth spending a little more for High initial RPVOT value oils with Gr II base stocks.

While surfing the web I came across a wonderful article on "Performace evolution of New and In-Service Turbine oils" and am forwrding to your mail ID. it might prove useful to u.

regards

Sandy deeds

As best I can tell the oil in these MG sets is the original fill with occasional additions and a filtering evolution about every 2 years. That would make the base oil 25 years old. Sump temperatures are in the "normal" band with no cooler leaks. I ahve seen a increase in AN from 0.17 to 0.23 in the last 2 samples. Sample frequency is 1/year.
rgf

Dear rgf

You mean to say that your Oils are In-service for the past 25 Years???

Gr8 Opearting conditions u have rgf..Let the oil people live...

on a serious note...

Now is the time to enhance the sample frequancy for RPVOT and TAN more specifically..

Use the same LAB and draw the samples under identical conditions for a proper comparison..

Maintain proper record of the values and plot time (Hrs.) V/s TAN and RPVOT value to get the curves.

Rapid increase in TAN take place when the oil is nearing at the end of its useful life.. and can be identified clearly as a sharp rise in the curves of neut value plotted on time.

Fresh Turbine oils usually have a TAN value of not more than 0.15-0.2 mg KOH/gm.

In certain Turbine oils it is obsreved that initial acidity is high due to presence of additves but this acidity drops down slowly and then starts growing up with rate of deterioration.

ASTM D 4378-1997 offers guidelines of 0.3 to 0.4 mg KOH/gm above the intial value as an upper warning level but generally it is probable that once the acidity level attains a value of 0.4 mg KOH/gm further oxidation of the oil would be rapid and a change may be inevitable.

In your case the TAn value is 0.23 in the latest sample..do u have the data on TAN value of the fresh oil when it was charged with u? In any case assuming that it was 0.10-0.15 the current value of 0.23 is still below the general limit of 0.4 mg KOH/gm.

BTW, how long back did u get the latest analysis done? If it was done past 2-3 months, its better that u get it done immediately once
again and maintain the frequancy of sampling and getting TAN done at every 450 Hours.

This should give you a fair idea of the direction the oil is heading to..

what other parameters are u getting the oil tested for? Are the other parameters like Viscosity, VI, Flash point, Pour point,
Air release value, rust test, Cu strip corrosion, foam within specified limits?


Personally I feel 25 years is a good period for the oil to have yielded the service and this time should u plan for a change over u may like to try out High Initial RPVOT value oils with GR II base oils.

Regards

Sandy Deeds