RPVOT ASTM D2272 Oil Oxidation Tests

I know some lube manufacturers recommend replacing when the value declines by 10%, which is likely too conservative. You'd have to set your own level, but 75% down seems a lot. Sounds from you post that this is the first time you've tested. Do you have historic data? I don't think the RPVOT declines linearly. I think it starts out dropping slowly and then hits a "knee" and drops like a rock.

wd,

It is subjective whether oil should be repalced at 50% of intial RBOT or 25% of initial. At your oil condition there is inadequate reserve oxidation inhibitor for prudence. An oil change is warrented. Your OEM will confirm....or you need a better OEM support resource.

Wd,

Typically once an oils RPVOT has droped below 25% of its inital value you will begin to see a rapid increase in the TAN.

According to ASTM D 4678 (In service monitoring for lubricating oils of steam and gas turbines)a drop to 25% of the oils inital RPVOT accompanied by a increase in TAN of .3 is reason for a change out.

Cheers,
Ash.

WD, the risk to continuing with the fluid is long term sludge, deposit formation, varnish and generally a poor sump condition that will adversly affect your future efforts to maintain fluid health and machine reliability.

Try running a Fluitec cyclic voltammetry (RULER) check to see where the antioxidant levels sit to corroborate your suspicion from the RPVOT.

Mike Johnson, Director
Noria Field Services

Hi Wd,

I would like to know what was/is the effect on your turbine based on this reduced RPVOT. Do you see any rise any temperature of the thrust bearings/ journal bearings etc.

Currently in our plant we are trying to establish the cause for the rise in temperature of our thrust bearing (from about 84 to 89 deg C) that raised big concern within our plant. The vendor specialist was called in and he sailed his way through for about 2 days telling us that they (the vendor - a notable Japanese company) have recently come up with a research etc, and that they fix a "sludge limit" of 100 ppm(wt) with a 1 micron filter and the RPVOT is 25% of initial. WE are still in the process of getting the RPVOT and sludge content analysed by the labs.

Our RPVOT on the used oil is about 420 mins. we are still awaiting results of the new lube oil RPVOT value. Also, in our plant we went ahead with blending of the old oil with new oil (about 35% new oil). WE are awaiting the blended oil RPVOT values as well.

As for the sludge content, you may use ASTM D4055-02. In our case the sludge content was measured with a 0.8 micron filter (the test lab said that at present they dont have 1 micron filter!). The test resulst gave us the follwing values (in PPM):

393 (for New oil)
460 (for old oil)
886 (for blended oil) 340 (for blended oil).

The last two results (886 and 340) for the same blended oil may be due to test errors. And I am not sure whether the test is really reliable or not, because a value of 340 for blended oil (2nd test) is less than the fresh lube oil's sludge content of 393!!

In any case, you can see that an original value of 393 ppm sludge for the new oil is way out of the specified 100 limit by the turbine OEM! But you may point out that we carried out these tests with 0.8 micron filter instead of the specified 1.0 micron filter.. ANy way, to assure ourselves we are going ahead with sludge measurement with 1 micron filter now and hope that the results will be in accordance with the OEM limits.

I shall try to update this post when i have results of the complete tests.

Please note that ASTM 4378 (Table 3) specifies that at 25% of RPVOT the lube oil must be changed. At 50% of RPVOT, a proper trouble shooting must be carried out to ascertain the causes of reduction etc.

One more thing I must point out that, various lube oils (different brands with same grade) may have varying sludge formation rates with respect to the % RPVOT, as was made known to us by the eqpt OEM (according to their "latest" research!). Thus, if you have not already done so, you may like to do the sludge analysis done too - just to find out at what sludge content you are compared to the original lube oil (ie new lube oil's sludge content also shall me measured!)

If you were to be in a decision making position, you may elect to change the lube oil based on ASTM4378 recommendation alone.

Please let us know more about your equipment (power, type, service, length of LO in service, any abnormal conditions due to low RPVOT such as high brg temp etc.), so that it will be useful for all. Thanks


(PS: I just joined the forum today. the forum is very useful. I have lots of fundamentals to learn on lube oils. I would apprecialte if anyone can tell me about a website/ free literature to aid my knowldege in this field)

we are also facing similar problems. In two of our turbines the rpvot values of the oils were tested as 175 and 80. Ruler test results are less than 15%. THE TAN is still good 0.16,0.17. Viscosity of the oil is also good. 46 ,47 Cst aoo C ( ISO VG 46). All other properties ( air release,demulsibility,foam etc..)are ok. RPVOT value of fresh oil is 650 min. Eventhough the rpvot and ruler test shows very low oxidation resistance, why is not reflecting in TAN and viscosity? can anybody explain me?

Few questions:
Have you done a varnish potential test?
What type of turbine are you testing?
Is the oil warm when sampled?

Just because antioxidants are depleted (ruler detecting phenols and/or amines ?), rate at which basestock oxidizes is dependent on operating temperature of system, e.g., gas runner hotter than steam so TAN, viscosity, varnish increase at faster rate after antioxidant depletion for gas than steam.

Another possibility, Wooton has shown varnish contains carboxylic acids and Livingstone has shown varnish worse for systems that cycle. So if samples were taken once oil had a chance to cool down for a period of time, the acid containing varnish would have a chance to settle out/accumulate on surfaces and oil sample would have low TAN, normal viscosity (accumulation of oxidation polymers cause of viscosity increase) even though oil oxidation occurring.

Hello
I began the work in the hydropower generation plant from 2 years, while the turbine oils in service have more then 20 years.
So, I don’t know the new oil RPVOT value.
My question is:
Is there a minimum limit of RPVOT value for in-service turbine oils (except the 25%of initial RPVOT)?