Coal

Turbine Lubricants: Biodegradable, Preventative Alternatives

Issue 2 and Volume 116.

By Sharryn Dotson, online editor

AEP’s Northeastern Station was one of the first plants to use Dow’s EcoSafe TF-25 lubricant.

Turbine operators at gas and coal-fired power plants have lots of choices they can make when it comes to choosing a lubricant to keep their machines working. Many companies tout that their product is the best at combating varnish, a by-product of turbine oil degradation, and carbon build up. Other companies say their oils are the best for lubricating parts.

One company says they have a fluid that actually helps get rid of and prevent varnish build up, while another company says their oil is not only good for the turbine, but good for the environment.

How it works

Dow Chemical’s product, called Dow Turbine TF-25, or EcoSafe TF-25, is a polyalkaline glycol (PAG) based lubricant and is exclusive to North America.

“The product is detergent in nature – it can clean up the varnish from older systems,” said Brian Goldstein, product marketing manager with Dow. “The by-products of degradation are such that when the product degrades, the by-products are dissolved back into the fluid.”

That is because it has abundance of oxygen in the molecular backbone.

“It is a well-known fact that when non-polar mineral oil and synthetic hydrocarbon molecules decompose by high temperature or oxidation, they will make by-products that are polar in nature,” said Govind Khemchandani, senior technical specialist with Dow. “The PAG chemistry is polar in nature.”

The chemical has a faster air release than that of a mineral oil, Goldstein said.

“Air release means you have the potential to reduce micro air bubbles and reduce component wear and oxidation of the fluid itself,” Goldstein said. “Better air release for OEMs (original equipment manufacturers) can lead to small sump design, improve the way OEMs like GE and Siemens can design their turbines and use less oil in their designs.”

Operators perform a turnkey operation to remove the old lubricant and by-products. A fluid system flush is completed, including draining and cleaning the fluid reservoir of used oil and debris. Then a fresh charge of PAG-based turbine oil is added to the reservoir. No extra flushing is necessary to remove existing varnish. The higher thermoconductivity helps to sweep heat away from bearings, hydraulics and areas where you may have high heat in a combustion turbine.

Case Studies

One instance where the fluid has been proven is in testing Dow GE 7FA gas turbines. Jeff Freeman, senior technical specialist with Dow, said that sticking servo valves caused by oil varnish was a major cause of turbine trips.

“When a turbine trips, you not only lose electrical production, but we’re using exhaust to generate steam in an HR unit,” Freeman said. “We lose the electricity and the steam.”

The costs of those trips can quickly add up.

“Each maintenance incident that causes you to trip leads to opportunity costs between $75,000 and $200,000 per incident,” said Chuck Carn, strategic marketing manager with Dow.

Freeman said trips in Dow’s unit were attributed to sticking in the servo valves, a device used to convert an electrical signal to a mechanical force through hydraulics to accurately position the five fuel valves and the compressor inlet guide vanes on a GE 7FA gas turbine. If any of the servos “stick” and do not respond to the outputs of the control system, the combustion process becomes too “lean” and will result in a flameout with resulting turbine trip.

Initially, Dow changed out the servos every 18 months to manage the varnish issue. This was a short-term solution as they investigated the different varnish removal technologies and searched for the best long-term solution. It was during the investigation that they came up with the EcoSafe TF-25 product.

“The conversion process was simple as we just drained the old oil, cleaned the reservoir, changed all the filters and installed the TF-25,” Freeman said. “We didn’t have to change any of the control settings. We’ve been operating for two years since the first conversion.”

Freeman said he also looked at the components after 18 months and saw no evidence of varnish.

“We have had no issues with varnish and we have not had to change any filters any more frequently,” Freeman said.

There were immediate differences noticed when the fluid was initially changed. “The bearing temperatures did decrease by 5 F to 10 F,” Freeman said.

Jim Kovanda, vice president of American Chemical Technologies, the North America supplier of EcoSafe TF-25, said the results from initial testing of the fluid looked promising.

The turbines at AEP’s Northeastern Station experienced frequent varnish-caused trips before the TF-25 was used.

“After four years of operation, the fluid’s acid value has remained stable and well within the fluid monitoring limits and no varnish formation has been reported by utility companies using TF-25,” Kovanda said.

One of the first utilities to use TF-25 in their gas turbine operations, AEP’s David Wilkes said the company has run two General Electric 7FA gas turbines at base load for four years without incident in Unit 1 at the Northeastern Station in Oologah, Okla. The first conversion was completed in November 2007 and the second in March 2008.

Calpine Corp. said its 980 MW Oneta combined-cycle power plant in Broken Arrow, Okla. ran for 18,000 straight hours of operation as of September 2010. Previously, the four GE 7FA turbines had been tripping frequently because of varnish and carbon build-up.

Both sets of turbines experienced frequent trips before the fluid was replaced in 2007 with the PAG. Since the change in 2007 at both plants, the utilities said they have not experienced any varnish-related trips.

“Green” oil

Kluber Lubrication North America L.P. said it has a line of biodegradable and bio-based formulations for the power generation industry.

“Market-driven influences include changes in environmental regulations, or the need to meet specific lifetime usage criteria, as well as banned substances,” said Dieter A. Becker, president and CEO of Kluber Lubrication North America L.P. “However, we have also chosen to change the chemical makeup of products in order to develop the next generation of products, which include a focus on green, bio-based, as well as biodegradable formulations.”

Becker said while the biodegradable oils would be a good fit to help plant owners lower their power plant emissions, not many power generators have used it.

“Kluber has not seen the demand from the industry to go to biodegradable lubricants in either wind power generation or coal-fired power plants, where they sell traditional and high performance synthetic gear oils,” Becker said. “If they did use the bio oils, it would certainly help with their carbon footprint.”

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