PAO or PAG or Diester ?

What is your reason for changing to synthetic when mineral oil works well with hydrocarbon gases and is less expensive? That information will help define the best choice in lubricant.

Thanks for the reply. The compressor 2nd stg discharge temperature ranges from 135 to 155 degree C. We foudn hitish deposits choking the tips of lube oil ports, which in turn causes premature wear out of rider, piston rings & rod packing, due to dry run. These deposits were analysed to be Zn additives from the mineral oil. Moreover viscosity thins down at this temperature adding to the problem. I can still live with the probem, if I can stop the compressor once in 4 months. But that proves to be costly, due to flaring of gases during PM checks. I am also looking for some blended oil, i.e in between Synth & Mineral Oil. I think Diester should be selected, but Compressor OEM & Oil companies talk of PAG. We had a bad experience with PAG earlier in a similar application. Made lot of sludge, within a short span of time.

There was a spelling error in my last reply. Please read "hitish" as "whitish".

Dear Sushil,
I will suggest you to go for Group II/GroupIII mineral based oil having higher VI and less prone to oxidation. It will be a better alternate instead of going for costly synthetics.
What is the viscosity of oil u are using right now.
My email id dctex04@indiatimes.com

Regards
Dipendra Jha

Sushil,

First you need to justify - why synthetics? Then the choice, of whether PAO or PAG or Diester.

In India synthetics may not be manufactured by major oil companies, hence if imported, you will need to build/keep stocks thereby double/triple your inventory. The popular synthetic lubes used in India are Gear oils, Special turbine oils, special hydraulic oils, FR Fluids, Heat Transfer Fluids, Metal working oils, greases and grades used in high temperature applications like in Steel plant, Cement Plant etc.

The single largest advantage of synthetics is that its life is more as (after synthesis) it resists oxidation better than minerial oils. Hence for high temperature applications synthetics are still the best choice. Or for that matter, fill and forget type of application like the gear oil in wind turbines.

If you have finally selected the synthetic grade you should seek info from the OE on its approval. It is possible that some of the seals, gaskets may or may not be compatible.

A final word of caution, use of synthetics is recommended in a TOTALLY DRY ENVIRONMENT ( except some Glycols/Poly Glycols which have water built in). I have seen Phosphate Esters (PE) creating havoc in Servo valves of a Steel plant due to water leakage from heat exchangers.

Coming back to the "whitish deposits" indicated in your message it is most probably precipitation of ZDDP (the popular anti wear package) ON CONTACT WITH MOISTURE/WATER. Check for water ingress. What is the grade of oil in use?? Please seek low or no zinc packaged lubricant from your supplier.

Hussam Adeni

I have been working with some recip. compressors as a technical sales rep. Check out the following web page. It should answer many of your questions. http://amsoil.com/products/pc.html
These oils have much more stable zinc additives.

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Originally posted by Sushil:
Am looking for a changeover from Mineral Oil to Synthetic Oil for a Recip Compressor handling Hydrocarbon gases. Sorry, for posting a basic question, but I need to know where to use PAO, PAG or Diester based oils ? What are their specific advantages & disadantages ? If I can be directed to the right source, I will be thankful.
Regards

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quote:

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Originally posted by Sushil:
Thanks for the reply. The compressor 2nd stg discharge temperature ranges from 135 to 155 degree C. We foudn hitish deposits choking the tips of lube oil ports, which in turn causes premature wear out of rider, piston rings & rod packing, due to dry run. These deposits were analysed to be Zn additives from the mineral oil. Moreover viscosity thins down at this temperature adding to the problem. I can still live with the probem, if I can stop the compressor once in 4 months. But that proves to be costly, due to flaring of gases during PM checks. I am also looking for some blended oil, i.e in between Synth & Mineral Oil. I think Diester should be selected, but Compressor OEM & Oil companies talk of PAG. We had a bad experience with PAG earlier in a similar application. Made lot of sludge, within a short span of time.

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If you are looking for very high VI and good thermal stability then a water insoluble double end capped PAG meets these needs. It may not need an anti-wear package to lubricate the compressor, but if needed you could investigate phosphate anti-wear additives. The PAG should resist acidic impurities in the hydrocarbon stream much better than diester.

You don't describe the type of PAG that failed before, but it not have been built for thermal and hydrolytic stability.

PAO and alkylbenzenes are also possibilities, and may be able to be formulated with more thermally stable antiwear additives than ZDDP. The phosphates are commonly used in jet tubine engine lubricants with POE basestocks for very high thermal stability.

It seems your simplest solution may be to try a mineral oil with a more stable additive package, since most of your problems seem to be related to the ZDDP deposits.