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Good day All,

I've read in at least 2 pieces of literature that as a Rule of Thumb for Lube oils "Every 10 Degree C rise in temperature over 80 degrees C decreases the life of the oil by about half".

If this is in fact a good rule to follow, can anyone advise on the following question:

What is the "exposure time" that the oil can experience that 10 degree rise above 80 degrees C (90 degrees C for example) before it experiences this 50% reduction in life? Does it start happening right away? Does it happen after 15 minutes? Does it happen after 1 month?

Let's assume that the sump is small and the unit is running continuously and there is little, if any opportunity to test the oil in an effort to monitor its Oxidative stability/degradation.

Let's also assume that the operating temperature of the bearing is 90 degrees C (not normal for this unit) and management wants an answer as to how long the bearing/lube can run at this temperature before the lube fails.

Thanks.

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quote:
What is the "exposure time" that the oil can experience that 10 degree rise above 80 degrees C (90 degrees C for example) before it experiences this 50% reduction in life? Does it start happening right away? Does it happen after 15 minutes? Does it happen after 1 month?

"Life" represents cuumulative effect of aging which may occur at different temperatures.
So I think the use of the word "life" as above is causing you problems. I think it more understandable if you think in terms of "aging rate". i.e. if we increase the oil temperature by 10C, we double the aging rate (for aging that obeys Arrhenius relation).

If we operate a system for 1 year at 80C and 1 year at 90C, it has undergone thermal aging equivalent to 3 years at 80C. IF we had reason to think the 80C life was 5 years, then the relationship predicts we have used up 60% of the oil's life.

A couple of big caveats:
Caveat #1 - We had a big IF in there (IF we had reason to think the 80C life was 5 years). Describing an oil's life in units of time can difficult if we don't have experience in similar machinery application and conditions.
Caveat #2 - Thermal aging may not be the only process. Contamination may be occuring. That can affect the oil aging. To complicate things further, aging byproducts can become contaminants.


Let's assume that the sump is small and the unit is running continuously and there is little, if any opportunity to test the oil in an effort to monitor its Oxidative stability/degradation.

quote:
Let's also assume that the operating temperature of the bearing is 90 degrees C (not normal for this unit) and management wants an answer as to how long the bearing/lube can run at this temperature before the lube fails.

Tough question. It brings up the two caveats from above:
#1 - Same as caveat 1. As a starting point, do we have a basis for estimating the life at 80C?
#2 - If we can answer #1, then we can use Arrhenius relation to provide a prediction. But we still have to add caveat #2 above to our conclusion.

And no matter what, our estimate cannot account for unexpected conditions. There is always the possibility that the equipment in question sees unexpected different conditions than the experience upon which you based your time estimate. Especially contamination (air, moisture, other). These things can be detected if you're monitoring but obviously not if you're just going by prediction without monitoring.
RM
Last edited by Registered Member
basically the assumption about the rate of oxidation is right for a uninhibited mineral oil - although also a base temperature of 100C is used in stead of 80 C.

that situation changes when a oxidation inhibitor is used. a oxidation inhibitor does not prevent oxidation completely but usually is capable of doing two things: neutralize a large part of the radicals formed as a result of primary oxidation and to a certain extend prevent chain reactions that may result from radicals not being neutralized. as long as sufficient inhibitor is available, oxidation can be kept under control, but the inhibitor is changed in the process and therefore it works once only....when running at high temperature continues, also the oxidation inhibitor may be used up more or less completely and then the rate of oxidation will rise exponentially with all kinds of secondary chain reactions as well - resulting in varnish and lacquer deposits and severe oilthickening.

most oxidation inhibitors are based on compounds of zinc and sulphur and are multifunctional, in the sense of also being a anti wear agent. that means that when there also is quite a large need for its antiwear capabilities, the amount of inhibitor available will be reduced, which may result in shorter oil life.

the situation thus is thus a bit more complex then the assumption referred to, although it is a fact that if temperature is higher reactions will occur at a substantial faster rate.
RM
I agree additive depletion is another complicating factor.

quote:
basically the assumption about the rate of oxidation is right for a uninhibited mineral oil - although also a base temperature of 100C is used in stead of 80 C.

I didn't understand your meaning about 100C here. Assuming the life were known at 80C, the life could be estimated at 90C for aging that obeys Arrhenius without any reference to 100C of course

Are there some standard/published oil lives somewhere based on 100C? Does some oil test deliver estimated calendar life at 100C?
RM
quote:
Originally posted by electricpete:
I agree additive depletion is another complicating factor.

Are there some standard/published oil lives somewhere based on 100C? Does some oil test deliver estimated calendar life at 100C?


not that i know of, but maybe some proprietary tests could be designed that correlate well with actual practice. the 100 deg C is arbitrary - most applications run on a lower temperature, whereas for example engine oils and oils for automatic transmissions may run at much higher temperatures.

as far as i know it is more or less a rule of thumb: "above 100 deg C every 10 deg C temperature increase will double the rate of oxidation". no doubt there might be a more scientific type of explanation for it, but in the past 30 years or so i never heard anything else then the rule of thumb mentioned above. i guess popularity of the "rule" more or less is based on its simplicity - no sophisticated math is needed :-)

that same rule of thumb may be applied more or less to other situations as well. i know a bearing manufacturer that states suggested regreasing intervals at 70 deg C, each 15 deg C above that temperature the regreasing interval should be halved, each 15 deg lower then the reference temperature the regreasing interval may be doubled.
RM
Gentlemen,

Thank you very much for your time and your contribution to my question.

I realize that there is some reading/researching that I need to get started on to better grasp the thoughts that you have put forward.

Once again, your contributions are appreciated.



On another note Electricpete, did you ever solve your problem with the High levels of Phosphorous in your Turbine Oil?

Thanks
RM

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