Detecting and solving lube oil varnish problems

Greg J. Livingstone and Jon Prescott, EPT Inc. and Dave Wooton, Wooton Consulting

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Autodegradation is a phenomenon that plagues gas turbines, particularly large frame units. Once the additives in turbine oil deplete, the oil becomes saturated with soluble soft contaminants. As the oil cools, the contaminants come out of solution and form varnish deposits. The contaminants' solubility at turbine operating temperatures renders electrostatic and agglomeration technologies useless for eliminating varnish problems in these applications (Figure 1).

1. Go with the flow chart. These are the key mechanisms of varnish deposition from gas turbine lube oil. Source: EPT Inc.

Although several commercial products claim to solve the lubricant varnish problem, good operating practices remain the first line of defense (see box). The vendors' claims are valid for many applications, but not for modern large frame gas turbines. The problem only gets worse when gas turbines are cycled or used for peaking service, as so many are today.

Recommendations for controlling sludge and varnish in turbine oils

A proactive program of selecting the correct lubricating oil for the right application and then closely monitoring its performance will extend lubricant life and lower maintenance expenses. Following is a short list of the key features of a good monitoring program.

Turbine oil formulation:

Select Group II or III base oils whenever possible.
Use a combination of phenolics and aminics, with more of the latter than the former for maximum aminic protection.
Exercise caution when using volatile phenolics such as BHT and DTBP. The less-volatile HPE and MBDTBP are preferred.
Avoid or minimize PANA to suppress deposit formation.

Thermal degradation:

Use Fourier transform infrared spectroscopy (FTIR) analyses for evidence of oxidation and thermal degradation.
Maintain uniform temperatures throughout the lube and hydraulic systems. Heat tracing and insulation have been effective as have modifications to increase flow in hydraulic circuits.
Take proactive measures to minimize spark damage by maintaining elevated oil temperatures, decreasing filter flux, and investigating different mechanical filter technologies. Several gas turbine plants report that taking such steps has lowered varnish levels.

Antioxidant depletion:

Closely monitor depletion of phenolics and amine antioxidants.
When the phenolic antioxidants approach the depletion level (20% to 30% of the original level), expect amine levels to begin falling and varnish potential ratings to rise.

Fluid maintenance:

Use a condition-monitoring program that incorporates RULER, MPC, and FTIR.
Employ an electrostatic oil cleaning system in combination with technology to remove soluble contaminants.

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