As the name implies the MPC test is a potential test that gives a numerical value to the patch color that is supposed to give an indication of the probability that the conditions for varnish formation are present in the lubricant. Since you have physical confirmation of a varnish problem I would not put much stock in the number. In my situation where we have a number of GE gas turbines that are famous (?) for varnish formation I always recommend on inspection of the filters and servos. I have seen cases where we have consistent high MPC values with no varnish formation and other instances where oil with a relatively low MPC number was found with a problem. As a rule I always run the ultracentrifuge test along with the MPC to see what is in the oil.  In your case is there adequate flow to the compressor? Is your lubricant suitable for the service?

thanks sir

flow is ok. we use this lubricant for another gas same gas turbine  and never have any varnish formation until now. we just have particle count high sometimes that reported from laboratory.but we cant find source of contamination 

best regards

I would see if you could capture the material causing the high counts. Are you using pore blockage or optical counts? If optical it will count the varnish precursors as contaminates but the pore blockage patch usually don't capture them because they are somewhat soluble.  I always run both in a varnish potential situation and track the difference-if I see more than 2 ISO codes between the 2 I know I have something in the oil that shouldn't be there.

dear sir

laboratory use iso 4406 for measuring particle count . does it difference to optical count or pore blockage ?what is difference between two?

Pore blockage is when they force a portion of fluid thru a paper wafer that is of a known mesh. It captures mainly foreign material in the oil such as dirt or wear particles. Any soluble contaminants will pass thru the filter (depending on size) and won't be counted. The optical method counts anything that blocks the signal, either solid material of "soft contaminates" such as water or the byproducts of varnish. Check with your lab and they can tell you which method they use. If they have an old sample of the subject machine have them run the ultracentrifuge and the optical count. Some labs retain samples for up to a month in case of a retest is required.

Good responses rgf… sounds like he has fought varnish a lot (more than me).

We have a lot of variability in particle count (optical method). I hadn't thought about the different counting techniques and what they might tell us.

Testoil has some good info on varnish testing here:

http://testoil.com/knowledge-center/reference-guides/

 What they say about the two methods, similar to rgf's comments:

•  "Particulate contamination is tested using two methods optical and pore blockage. Optical particle count passes the oil through a beam of light. Anything in the oil that interrupts the beam is counted as a particle. This method will count soft (varnish) particles. Pore blockage particle count passes the oil through a calibrated mesh screen that captures only hard particulates. A significant difference in the two results may be due to the presence of water, soft contaminants, or insoluble contaminants."

The suite of tests they (TestOil) would do for monitoring varnish potential:

• Membrane Patch Colorimetry
• Particle Count
• Ultra Centrifuge
• Acid Number
• Karl Fischer Method
• RULER
• IR Spectroscopy

example report here http://www.testoil.com/pdf/vpaw1.pdf

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You say varnish is only occuring directly on the bearing pads and only the compressor not the turbine (even though same oil system)?  That seems to suggests the local conditions at the pad have some relevance to the situation (temperature, pressure, entrained air bubbles).

Any indication of unusual bearing temperature that may have preceded the varnish?

Got a photo of the varnished pads?  What part of the pad is varnish occurring most on?  It may give some clues whether it appears at the hottest or coolest or heaviest loaded parts.  I have seen a case study of apparent varnishing of thrust pad from air entrainment / micro-dieseling and the varnishing seemed to be concentrated at hottest or highest-loaded part of thrust pad where the thinnest film was (not sure why it appears at that location)

What is the age and type of oil? 

We use a range of test methods to assess varnish potential/oxidation in our steam turbine oils to include

MPC, RULER, RPVOT, Total Acid, FTIR. 

In line with what Pete has said I think you need to establish the state of your bulk oil. This will help to ascertain whether it is local (to the bearing) conditions that are leading to the varnish or whether is is down to degradation of your entire oil charge. 

I would assess RULER values against a new sample to gain an understanding of your levels of anti oxidants. RPVOT has traditionally been used to assess similar (oxidation stability although I have found it to be a less reliable method than RULER confirmed by back to back testing (RPVOT is more used by oil companies to sell you oil!)

Acid Number in my opinion only has any relevance in the latter stages of oil degradation when anti oxidants have been completely depleted and organic acids from oxidation have been generated. Apparently however you may see a reduction in AN when the anti oxidants deplete.

Microscopic assessment of a patch can be used to establish whether micro dieseling is occurring and of course air release is a factor in this. 

I have seen incidences where MPC values have been very high (35-40) but the RULER test has come back good i.e. good levels of anti oxidants. This has been attributed to legacy insoluble's build up around the lub system from old oil that has subsequently been picked up by the circulating oil. 

In conclusion you do need to use a variety of tests to assess your oils condition and pinpoint the cause of the varnish

Gary