At the Calpine-Oneta power plant located in Broken Arrow, Okla., in 2007, the turbines were not operating efficiently because of severe problems with varnish and carbon buildup. These byproducts of turbine oil degradation, which result from high heat and friction, were fouling servo valves, pencil filters, reservoir interiors and piping. The recurrence of varnish and carbon buildup was hindering proper operation of critical hydraulic components. The resulting sludge not only played havoc in obvious places but collected in areas not so obvious such as turbine piping and heat exchangers.

To overcome these problems, Broken Arrow decided to try Dow TF-25, which is sold exclusively in North America by American Chemical Technologies Inc. (ACT) as EcoSafe TF-25. It could have been a risky move, as the recommended operating parameters for fluids on these turbines were written originally for mineral-oil-based lubricants. By contrast, Dow TF-25 is composed of a unique polyalkalene glycol (PAG) chemistry, and PAG chemistry is fundamentally different from mineral oil.

Fortunately, the Dow PAG fluid has had decades of proven performance in other turbo-machinery applications, says the manufacturer, which gave Broken Arrow confidence to try it.

In the documentation for the GE-made turbines, not all of the recommended lubricant properties specified in GE K 32568F, Lubricating Oil Recommendations for Gas Turbines with Bearing Ambience Above 500°F, provide a relevant correlation to the performance of the PAG based fluid. According to the G.E. specification, "The [recommended lubricant properties] are only recommended values. An oil that has been shown to perform successfully in the field may still be used even if all the [recommended lubricant properties] have not been satisfied."

"The recommended properties for lubricating oil in the GE documentation do not have any specifications for PAG based fluids," says Chuck Carn, strategic marketing manager for Dow Fluids and Lubricants. "However, the G.E. specification allows for the use of fluids that have been shown to perform successfully in the field. Our experience verifies that we have met the performance criteria."

Why the Change?

When mineral-oil-based turbine oils are subjected to the rigors of normal gas powered turbines, servos stick and filters get plugged. This means more time for maintenance personnel to work on the machinery. Adding to those costs are the lost production of turbines that are offline. So, the price Calpine was paying for less than ideal lubrication was quite high.

After unsuccessfully testing several methods and systems that claimed to remove or reduce the varnish, Oneta Energy Center (OEC) decided to move towards replacing the lubricant with a product that would better resist or eliminate the varnish or the mechanism causing it to form.

Lubrication is important for installations like the one in Broken Arrow, as gas turbines are built to run at high operating temperatures and require fine-tuned hydraulic controls. Selecting the right lubricant and keeping the oil clean are two of the keys to fewer break downs, less maintenance or downtime. Localized hot spots in turbine oil can initiate oxidation, resulting in the formation of sludge, varnish and hard carbon deposits.

According to its manufacturer, EcoSafe TF-25 can shrink the potential for varnish formation in gas turbines used in either base load service or high stress peaking operations.

To solve the problems of contamination Calpine was experiencing, a complete turnkey operation was performed to remove the old lubricant and byproducts. A fluid system flush and cleaning was performed on the turbine at Broken Arrow. All the old varnish was removed, as was the sludge that had accumulated during the normal operation.

After thorough cleaning of the turbine, the PAG fluidwas used to replace the old lubricant. The results were immediately promising, according to Dow. OEC noticed a slight drop in bearing metal temperatures by approximately 2 to 3°F under similar operating conditions. This reduction in heat is assumed as a result of less friction/energy loss in the bearings.