When we opened the bearings of our steam turbine this year we found a coating of varnish. As we opened other parts of the system we found that the coating was throughout the system including valves coolers and tanks. Thinking that the oil was oxidizing due to its age, 15 to 25 years, we sent samples to be tested. The results came back indicating that the oil had very little oxidation, a low oxidation number and the viscosity was similar to new oil. Any suggestions as to what is causing the varnish and what we should look for?

Roger,

The situation you are describing sounds like a long term moisture problem. I have seen bearing lube systems with water contamination, and the varnish builds up as a result of excessive temperatures at the friction surface causing localized oxidation, but not destroying the lubricant itself. I would monitor the system closely for water contamination and have a RPVOT done to make sure the lube is still in good shape.

Steem turbines are a little ticky, I will also recomend you keep a close eye to the humidity levels, measured on % of saturation at a certain temperature.