Oil performance

Dear Joy
As you comment, the varsnish problems are related with high temperatures of operation. It is a comun problem in Gas Turbines. In your case, maybe the high temperatures in bearings (95 to 103) could be causing the varnish problem. You are not going to be able to predict this problem with typical oil analisys like AN, FTIR, or RBOT. You should run a special test for detection of varnish, maybe the Varnish Potential Rating.

As second step, I don't think that you should change to a synthetic lubricant. Maybe a Group II or III Oil could be better thant the current oil. You should look for a +10,000 hours in ASTM D-943 for this application.

Of course, I strongly recommend a Flushing Procedure for changing the oil in the next outage.

I hope my comments will be useful for you.
Regards,
Cristián.

Thankyou for your valuable Comments and Advice.
what I learned from Mobil is that MOBIL DTE Light-32 belongs to Group III, however the TOST value is 4200 min as per datasheet.

Hence, in this current context, if we plan to switch to a synthetic substitute, what property and spec. should be looked for/considered for selecting the appropriate lubricant.I presume that D-943 value for synthetics will be above +10000 hrs. Your comment on this is highly appreciated.

Dear Joy

Mobil DTE Light 32, doesn't belong to Group III. It belongs to Group I (Purity <90%, VI<120, etc).

You could try with a Hydrocraked Oil. They are from Group II and III, and their cost is not so high (1.5-2.5 USD/liter). If you wants to try with a synthetic oil, you should look for a good thermal stability and resitance to oxidation. But, you should have in mind that the change from a mineral oil to a sinthetic could represent some risk for your system. In the other hand Hydrocracked Oils are compatibles with mineral oils.

Please, any other doubt you have, write to schmidmend@hotmail.com.

Regards,

Cristián,

I don’t know on what basis you made your claim that this oil DTE Light is Group 1 oil. MSDS data sheet for DTE Light oils under CHRONIC TOXICOLOGY (SUMMARY)--- clearly states that ……(mineral) base oils in this product are severely solvent refined and/or severely hydrotreated, which was confirmed to Joy by Mobil reps. Can you share with us why you think this oil is Group-1?

I would be careful about trying a switch to a hydrocracked oil to solve a varnish problem. To solve an oxidation problem, yes, but not a varnish problem. There is a solvency issue with the HC oil, the varnish will fall out faster in an HC oil.

I suggest you find the root cause of the varnish problem, it will almost always be air/foam related. And beleive it or not, reservoir design is probably assisting at the fault.

You probably have too much entrained air in the oil from the return line, going straight back into the oil pump(remember you can not see up to 10% entrained air in the oil, so don't look for it, test for it!) . The pump pressures micro-diesel the bubbles, varnish forms and migrates to cold areas since it is polar.

Pulp and paper industry studies show that the rectangle shoe-box reservoir is not very effective, and many times allows the oil in and right out again, without really co-mingling it with stagnant oil. Thus, up to 70% of your oil may be sitting around doing nothing while the other 25% is doing all the work. And it gets even harder to trace when your sample points are all hitting at or near the 70% zones, which makes everything look look fine in the oil tests.

I agree, use a VPI test(Focus Lab Thailand) or VPR test(Analyst USA)to monitor, perhaps look into Electrostatic filtration systems for a short-term fix, but the best recommendation is to find the source of the air or foam problem, then your varnish problem will go away.

Regards,
Andy Sitton

Thanks Andy Sitton for valuable Guidance.
Do you mean to say that the foam/air can cause varnish on Synthetic oil too.
Do you think that the indicated bearing temperatures are not exceeive, as the Actual Oil temperature at the pad surface will be (say 10-15 Deg) above the above mentioned temperature.your comments are appreciated.

yes, there has to be air for varnish formation(usually in the from of foam), however the synthetic will be more resistant.

I'm not sure about the pad temps. The problems I have investigated gave only the reservoir temps. We have seen reservoir temps of 70-80 deg C, with foaming in the sightglasses, varnish deposits all over. The varnish builds up to the point of plugging the Moog servo valves. Electrostatic filtration and reservoir modifications, have usually solved the problem.

Andy Sitton