Lube Oil Varnishing - Reasons for Concern
Q: I am a power plant maintenance manager responsible for several gas turbines. We received a letter from our OEM about lube oil varnishing. The OEM suggests testing is optional. We have never experienced a trip due to varnish but have recently changed lubricants. Should I be concerned?
A: Yes, this may be a cause for concern. As you may be aware, varnish is the product of oil degradation due to contamination. This contamination may take several forms such as: hot spots in bearings, static discharge from filters (arcing), and the implosion of microscopic air bubbles (microdieselling). Other causes of varnish may include additive depletion, formulation changes in turbine oils, cyclic operation of the turbine, and the design of the lube oil system itself. If the varnish contamination level in the lubricant is too high, severe performance problems may occur.
There are several methods developed by ASTM to determine oil degradation, but there are few tests to determine the amount of varnish contamination in a lube oil system. Some tests are as follows:
- FTIR scan of the native spectra
- Colorimetric patch
- Ultracentrifuge
- Gravimetric analysis
Depending on the root cause of varnish formation, routine oil testing may not indicate a problem. The few tests that are available have some repeatability and reproducibility concerns associated with them. These tests should be done routinely, but the cost of some of these tests may be prohibitive to conduct monthly. BTS is researching and developing a cost effective method that will quantitatively and qualitatively measure the amount of varnish contamination in the lube oil system.
The recommended course of action if varnish is detected would be to invest in an oil purification system to remove the varnish build up. Monitor the temperature of the lube oil and bearings at several points. Routinely top off the lubricant to boost the anti oxidant package of the lube oil. The last point is to keep the lubricant clean, cool, and dry.-DPW
