Coal, Gas

Oil Proves Durable in Cogen Application

Issue 11 and Volume 110.

In 1999, the Eastman Co.’s Texas operations celebrated its 50th anniversary and announced an agreement with CSW Energy Inc. to construct and operate a 440-MW cogeneration facility at its Longview, Texas, site.

Eastman Cogen, formerly known as AEP Eastex Cogen, came online in 2001 providing power for the Eastman Chemical Co. as well as operating as a merchant power plant. Since powering up, its steam turbine and two GE Frame 7 gas turbines have been running around the clock.

Signs of Durability

In April 2005, Eastman Cogen contacted Mark Mamaghani, manager of strategic alliances for ConocoPhillips Co. and its Conoco brand lubricants, to consult on results from oil analysis testing. Samples of the Conoco Diamond Class Turbine Oil that had been in the system since startup were pulled from two gas turbines. The samples were sent to ConocoPhillips Technical Services Labs in Ponca City, Okla., for confirmation of rotating pressure vessel oxidation test (RPVOT) and total acid number (TAN) analysis. RPVOT measures oxidation stability of a turbine oil and TAN measures the oil’s concentration of acidic components, including organic acids formed by oil oxidation. The better an oil can stand up to extreme temperatures without losing its oxidation fighting capabilities, the better its varnish and deposit control on the internal turbine components will be.

New Diamond Class turbine oil’s oxidation stability is D-943, which is predicted to last 22,000 hours. The oil analysis results from Ponca City, shown in Table 1, showed that the used oil was still in good condition, even though the turbines at Eastman Cogen ran continually for more than three years, meaning that the oil had accumulated more than 30,000 hours of use, including at least 23,000 fired hours. Fired hours are those hours during which the turbine is generating electricity and is under extreme heat. When a turbine is not generating electricity, the lube oil system is still running and the oil is under stress, but less than when it is generating.

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Table 1 illustrates that the new Diamond Class Turbine Oil 32 is typically in the 2000 minute range. After 30,000 hours of use, the oil in both gas turbines dropped to the 1500-plus-minute range.

“We were pleased by these results,” said Mamaghani. “The D-943 oxidation stability test is a test that the turbine oil formulators use to gauge how well a turbine oil will stand up to the elements in a gas turbine application. A D-943 oxidation stability of 22,000 hours, roughly translates to 22,000 fired hours before the oil has to be changed out. The oxidation levels of the used Conoco Diamond Class Oil were better than some new premium turbine oils and far surpassed our own performance expectations.”

Oxidation Resistance

John Paul Jones has served as plant manager for Eastman Cogen from its beginning. Prior to his tenure with Eastman, Jones had been involved in the start up of several different plants around the country. This experience prompted his decision to use Conoco products from the start. Jones says he used Conoco lubricants at other facilities and chose the brand because of its high oxidation resistance levels.

Hydroclear Diamond Class Turbine Oil was developed particularly to meet the demands of severe-service gas turbines. The oil is formulated with premium-quality, hydrocracked paraffinic base oils and fortified with additives that provide high oxidation resistance.

The hydrocracking process subjects the oil feed to high temperatures and high pressure in the presence of hydrogen and a proprietary catalyst, changing the shape of the molecules to make them more thermally stable. The process also removes impurities from the oil, extending its service life.

Because gas turbines operate at high temperatures, they cause conditions that accelerate the rate of thermal degradation and oil oxidation. Hydrocracked oils’ stability at high temperatures plays a major part in preventing the accumulation of harmful deposits and varnish, protecting components for a long time. This allows control systems to run cleaner with fewer mishaps. The ability of hydrocracked oils to handle severe service situations can extend drain intervals on gas turbines two- to three-fold.

Jones and his crew at Eastman Cogen carry out regular monitoring of their oil stock, conducting monthly analysis and performing oxidation tests every six months.