A mixture of GST32 and AW46 is suspected to have been discovered in a 1650 gallon sump tank of our 50MW turbine generator. Would it be possible to exchange the contaminated oil with fresh GST32 on the run and have effective results? The cost of shutting down is far more than the cost of 7500 gallons of GST32 that would be used for the exchange. The additive package of the hydraulic oil is also a concern that questions the negative impact on the turbine and system. Please help...Thanks in advance.

What is the proportion of hydraulic to turbine oil? This could make all the difference in the answer.

The exact proportions are only speculated. Thirty, fifty-five gallons drums were used to fill the reservior and the analyzed viscosity is now running close to forty one cSt. The thought is that, at least ten drums of AW46 would have to have been used in order to change the viscosity to that extent. As a test, a 50/50 mixture (AW46/GST32) was sent for analysis and the resultant viscosity was 39 cSt. This type of problem may be more common than I realize, and hopefully more experienced insight will emerge. Thanks again.

The difference between turbine oil and hydraulic oil is, first, the base oil.The turbine base oil have to pass more tests than hydraulic base oil BUT some companies make only one base oil (...cost production...) and it is a turbine base oil. You have to verify with your oil company if they use the same base oil.
Second, the additives, a turbine oil have more Anti-Oxydant and anti-foam, and hydraulic oil more Anti-Wear. What are you afraid of about the additives? You don't mention. Now for the viscosity, check the difference of the viscosity at the operation temperature since the viscosity you have (39cSt) is at 40°c or 100°F. Probably, the viscosity is still good.
Also, I assume that you probably have an analysis program, so, have more check-up to be sure the mixed oil is still good for the application. You probably not need to change the oil. Give us the final result.

Base oils used in formulation of both AW and RO oils should not concern you at all. They are most likely petroleum-based, and therefore, fully compatible. Additives from AW and RO oils may pose problems if they are found to be incompatible. Antiwear additive is OK as long as you keep your oil dry, because water can easily displace antiwear additive from metal surfaces. In that case, your oxidation protection might be somewhat compromised. Changed viscosity may cause you some problems (higher operational temp), even though I doubt that because the increase is not that drastic. However, it has a different impact on different equipment. Therefore, my advice to you is to monitor operational temp (oil, bearings, etc.), and check for possible increase in air entrainment and foaming. A significant increase in operational temp will tell you that oil is too thick and replacement should be considered. If you see significantly more entrained air and/or foaming, you may have some additives incompatibility problems. Also, keep you oil clean and dry. Short of changing entire oil bulk, you can try to do a “reverse dilution” while the unit is running. I mean, you can drain a “safe” portion of current oil and add your R&O ISO 32 oil. How much you can drain out at one time and not jeopardizing the system, depends on the capacity of oil in the sumps. If you are careful enough, that way, you can lower current viscosity with such “reverse dilution” to the point of almost replacing existing oil. Of course, this way you can not flush the system, and it could take more gallons of oil than the system holds.

I agree with John's recommendations. The only addition that I have is that Group I and Group II basestocks should not be mixed. Although running a compatibility test in a lab with new GST and new AW oil will not appear to have any adverse affects, it is not wise to have this mixture in an operating system since the solubility of the mixture is different. This may result in an increased potential for varnish. The biggest danger is when a new Group II fluid is mixed with a used Group I fluid. Many times this results in instant varnish formation. As long as this scenario did not occur, follow Jonn's recommendations on ramped up oil analysis testing. I would also add demulsibility to the tests that John has already recommended. Flush and change fluid at your first available planned outage. If the AW fluid was added to the GST, I would perform a bleed and feed as soon as possible.

Thanks all for your input. As planned, we flushed while the turbine was in service with thirty-five hundred gallons of GST 32. A follow-up analysis of the oil revealed a decrease in viscosity from 41 cSt to 33 cSt. The zinc levels decreased from 111 to 31 ppm. The process of exchanging the oil was performed as a bulk (single truck instead of individual drums) operation. We're still not sure how the suspected AW 46 was introduced into the system. Unfortunately, the drums that were used are no longer around and can't be found.
abeaudoin, there was concern that the zinc would affect (chemically attack) the copper in the tubes of the oil coolers. One noted change was a decrease of ten degrees in bearing temperature after the exchange. We suspect the temp decrease is purely due to lower viscosity. The oil has been dry and we will monitor the ability to keep it dry. Hinderance of water and oil separation was another concern with the additives. Thanks to all of you.

You did right by changinig it out. GST Containes Acidic additives, while normal AW hydraulics contain Alkaline additives. Here is an extract of a Chevron paper:
"Cause of Incompatibility
In rare cases, an adverse reaction may occur between two oils at high temperatures or pressures. Most often, however, the cause of incompatibility is the neutralization of an acidic additive in one oil by an alkaline additive in the other oil. The reaction occurs in the presence of water. A soap (usually a calcium soap) precipitates as a grease-like gel which can interfere with lubrication and oil flow. Laboratory investigation shows that the neutralization reaction between oil additives requires water and time.
Raising the temperature speeds the reaction. Generally, in-service oils contain small amounts of water, which are sufficient to start the neutralization process and result in possible oil incompatibility.
Unfortunately, the reaction is not reversible. Removal of the water by drying the system and the oil does not destroy the gel or eliminate the soap.
Incompatibility is enhanced when one of the oils is in use for some time and is oxidized or contaminated."

Widman,
Thanks for the info. We're watching this closely. Is it likely that the gel will be detected in our monthly samples? Would an early indication be a decrease in TAN? I'm learning more and more everyday. Thanks again, Sid Hand

I am concerned at the Zinc levels. That it was 100 plus and dropped to 30's is a good sign. The Anti wear packaged widely used is ZDDP and is dosed in most Hydraulic oils. In the presence of moisture the package breaks down and setles on the filters as whitish deposits of ZN SO4.

Last few reports indicate that things are in order but on the long run I would prefer to see Zinc below 10 pm.

Hussam Adeni