TURBINE FLUIDS: How does PAG stack up against mineral oils after a decade of experience?

An increasing number of turbine owner/operators suffering varnish issues associated with the use of mineral-oil lubricants has been migrating to non-varnishing polyalkylene glycol (PAG) over the last decade (see sidebar for a chemistry refresher).

EcoSafe® TF-25, based on a PAG formulation developed by Dow Chemical Co and marketed exclusively by American Chemical Technologies Inc (ACT), continues to attract converts—most notably among the industry’s shrinking pool of experienced engineers responsible for the care of gas and steam turbines.

Chemistry backgrounder

The stress experienced by a turbine lubricant contributes significantly to the ageing of petroleum oil, causing the non-polar fluid to oxidize. However, the resulting byproducts of decomposition are polar and insoluble in the base oil; they come out of solution as varnish. Polyalkylene glycol, by contrast, is a polar fluid and, while it too oxidizes, the byproducts of decomposition are polar and infinitely soluble in the base stock. No varnish is produced.

New-product introductions in the electric-power industry are notoriously difficult given the unwritten code that warns against being the first to try a new product. The typical response: “No Number Ones!” Generally there is little, if any, upside for users taking such risk.

EcoSafe TF-25 gained a toehold 10 years ago at two combined cycles powered by 7FA gas turbines experiencing expensive random trips attributed to varnish. In the seven years following the product’s powerplant debut in November 2007, 50 turbines, in round numbers, had converted to PAG from mineral oil. It took only three more years to convert another 50 units. Momentum continues to build.

Does this trend suggest mineral oils might be a superannuated lubricant for turbine service? No. Or, that there are no effective methods for removing varnish from mineral oils? No. What you will learn from the two 10-year case histories summarized below is that EcoSafe TF-25 was an effective solution at these locations given their specific conditions, and something you might consider under similar circumstances.

Tests conducted at both generating plants after a decade of service suggest the expected useful life of EcoSafe TF-25 is in the neighborhood of 35 years. Might a judiciously selected and properly maintained mineral oil last as long? Some experts say it’s possible.

Consider attending the upcoming meeting of the Steam Turbine Users Group (STUG, www.stusers.org), Aug 27-30, 2018, in Louisville, to become more knowledgeable about the properties of turbine fluids and their care. You’ll have the opportunity to learn about such things as (1) sampling best practices for accurately tracking the condition of your fluid to assure its proper maintenance, (2) the forgiving nature of PAG with respect to water intrusion—often overlooked—which is a primary cause of mineral-oil degradation in steam turbines, etc.

Northeastern Station, Oologah, Okla, was the first powerplant to convert from mineral oil to EcoSafe TF-25. The 7FA known as Unit 1 in its 2 × 1 baseload, must-run combined cycle made the switch in November 2007, Unit 2 the following March. Reason: In 2005, about four years after commissioning, the gas turbines began experiencing seemingly random trips due to “loss of flame,” a somewhat generic condition with several possible root causes.

Investigation of operating data indicated the fuel-gas control valves were not functioning properly. A physical inspection of the hydraulic control circuits found that a tenacious gummy material, later identified as varnish, had been depositing throughout the lube and hydraulic systems. When it collected in servo valves, the varnish caused fuel valves controlled by the servos to misoperate, restricting fuel flow and causing the random turbine trips. In 2007, the plant tripped about 50 times (three times in one weekend alone) and suffered 18 servo failures, according to plant personnel.

Plant management sought a non-varnishing lubricant after speaking with colleagues dealing with varnish issues who advised that seemingly practical “solutions”—such as heat tracing, oil conditioners, some varnish mitigation equipment, etc—would likely be ineffective. Personnel at a sister plant recommended contacting ACT having had success with PAG chemistry in steam-turbine hydraulic systems. Good suggestion.

In relatively little time, system-wide varnish deposits were returned into solution because of the inherent detergency and solubilizing characteristics of EcoSafe TF-25. Subsequent inspections of servos, now serviced every three to five years, and the lube and hydraulic systems have shown them varnish-free. The bottom line: Since conversion to PAG, the gas turbines have not tripped because of a varnish-related problem.

A goal of the battery of tests conducted on the 10-year-old fluid was to determine its remaining life. According to the OEM’s gas turbine lubricant recommendations document, GEK 32568h, a turbine oil/fluid should operate trouble-free until its antioxidant (AO) level drops to 25% of the new value. High-performance liquid chromatography was used to determine the AO content in Northeastern’s turbine fluid. It calculated the plant’s AO loss at 1.99% annually meaning an expected life of 37.7 years.

Tests also revealed the decade-old fluid had the same lubricity and load-carrying capability as new fluid. A mini-traction machine was used to measure both sliding and rolling frictional forces.

Unit 4 at Oneta Energy Center, Broken Arrow, Okla, one of four 7FAs that power the facility’s two 2 × 1 combined cycles, also was converted to TF-25 in November 2007 when what was later identified as varnish inhibited operation of the gas turbine’s inlet guide vanes. Result: The unit would not start.

In the 10 years since converting to PAG (about 41,000 fired hours and 1500 starts later), plant staff reports EcoSafe TF-25 has performed flawlessly. Tests suggest the fluid should last an additional 20 years. CCJ