A Thin Film of Lubricant Can Often be the Difference Between Selling into a Summer-time Wholesale market at $50 to 100/MWh and answering to the irate company president about a failed bearing that caused a forced outage. Maximizing lubricants`functional properties and extending their operating lifetimes, therefore, is a most worthy endeavor. Conventional solvent-refined lubricants have, on the whole, done their job effectively over the years, while synthetic lubricants have been developed to address particularly demanding applications. These products, however, sit at opposite ends of the price-value spectrum. To fill the gap, hydrocracked lubricants are now being offered with functionality approaching that of synthetics, but at lower prices.
Lubricants must satisfy a range of service demands in today`s power plant, from high-speed gas turbines to general purpose electric motor lubrication. In all cases, however, the goal is the same: reliable, long-term performance and protection. Differences in lubricants show up downstream in terms of varying lubricant lifetimes, maintenance requirements, inspection frequency, equipment damage, etc. Hydrocracked lubricants, first introduced by Petro-Canada and now being offered by Conoco, Chevron and others, enable users to cost-effectively benefit from synthetic-like properties. Hydrocracked lubricants are formulated via hydrocracking, in which crude oil molecules are chemically altered and rearranged into building blocks with favorable characteristics. The resulting base oil is far more pure (99+%) than solvent-refined base stocks, making it more resistant to oxidation, more responsive to additives, more pumpable at low temperatures and lighter in color.
The Heat Is On
As more and more cogeneration and combined-cycle plants are brought into operation, adding more heat to the cycle, more stress is placed on the gas turbines, heightening the importance of reliable lubrication. For 10-weight turbine oil products, that`s a lot to ask. Temperature plays a critical role in lubrication. According to Joel Reber, industrial lubricants product manager for Conoco, a lubricant should be selected not only to provide a long life at normal temperatures, but also to handle “hot spots” in the system that can trigger thermal decomposition.
In a GE Frame 7EA gas turbine, for example, inlet lubricating oil temperatures are only about 130 F, but after making a 30-second pass through the turbine bearing, lubricant temperatures can rise to 155-200 F. Along the way, the lubricant encounters hot spots in excess of 300 F that can initiate oxidation. As the oil oxidizes, the resulting oxidized molecules become less thermally stable, leading to varnish, sludge and hard-carbon deposits that ultimately hamper turbine performance. Hydrocracked lubricants offer improved oxidation resistance because their purity enhances the effectiveness of anti-oxidant additives. Unstable aromatics, typically the first lubricant components to form sludge and varnish compounds, are essentially absent in hydrocracked products, but make up 3 to 5 percent of solvent-refined lubricants.
As power plant operators strive to extend the time between outages to maximize revenue and reduce maintenance expenses, they are counting on lubricant manufacturers to pitch in as well. Extending turbine oil service life by improving oxidation resistance translates directly into reduced maintenance costs. Most OEMs require turbine oils to achieve 2,000 or more hours on the ASTM turbine oil oxidation test (D-943), in which oxygen is bubbled through a turbine oil sample in a test tube with steel and copper catalysts until oxidation reaches a predetermined limit. Conoco has tested its Diamond Class Turbine Oil and observed oxidation test values greater than 20,000 hours, said Reber. Turbine OEMs would like to increase the oxidation test specification above 2,000 hours, but are reluctant to do so because of concerns over the worldwide availability of high-quality lubricants and the impact on equipment warranty guarantees.
DuPont`s Sabine River Works cogeneration plant in south Texas generates about 100 MW electric power and supplies up to 450,000 lb/hr of high-pressure (550 psi) steam for the nylon and ethylene co-polymer facilities. The plant had been using conventional solvent-refined oils for turbine lubrication for many years, occasionally experiencing oxidation and foaming problems. In an effort to extend the lubricant`s functional lifetime, plant management decided to switch to Conoco Diamond Class Turbine Oil. An unplanned outage at Sabine means significant additional costs for replacement power, emphasizing the need for high availability.
Figure 1 shows rotating bomb oxidation test (RBOT) results for the GE 7EA gas turbine at Sabine. The RBOT test (ASTM D-2272) provides a quicker indication of lubricant service life than the turbine oil oxidation test, by subjecting samples to higher temperatures to simulate hot spot conditions. Figure 1 compares RBOT results for three lubricants used at Sabine over the years: a conventional solvent-refined turbine oil, a premium solvent-refined oil and a hydrocracked oil. Whereas the conventional lubricant lost most of its oxidation resistance after only six months in service, and the premium solvent-refined product lost more than 50 percent by 15 months, the hydrocracked product still retained greater than 70 percent of its initial oxidation resistance after 15 months in service.
According to Ed Gormley, technical associate with DuPont, the turbine oil is typically replaced at every 18-month shutdown. At previous outages since switching to the hydrocracked product, DuPont still replaced the oil for insurance reasons, despite minimal signs of lubricant breakdown. In September 1998, at the facility`s last 18-month shutdown, DuPont was sufficiently satisfied with the long-term stability characteristics of the oil that it decided to forego oil replacement.
Reliant Energy (formerly Houston Lighting & Power) has been using hydrocracked products on Unit 1 of its four-unit, supercritical gas-fired 2,200 MW PH Robinson plant for about one year. The bulk of the experience has been gained with a Voith fluid drive coupling on a shaft-driven boiler feed pump. Voith specifies a hydraulic oil to protect the gear set. Reliant had been using a top-tier solvent-refined turbine oil (longer life than hydraulic oil) to extend drain intervals and match scheduled downtime. The 300+ F temperatures in the coupling, however, led to early oil breakdown, varnish deposits, and repeated blow-out of the coupling`s fusible plugs, ultimately reducing pump capacity.
Since switching to a Conoco hydrocracked hydraulic oil, the system has been running 9 F cooler and D-943 oxidation times have increased from 2,500 hours to 4,500 hours. Reliant also has realized significant maintenance cost savings. “With the previous oil, every time the unit tripped, we`d have to clean out both filters, usually multiple times, at $1,000 a filter,” said Richard Laws, Reliant optimization operations supervisor. “That`s no longer necessary. In two start-ups, we paid for the price of the new oil.”
Reliant is also using a hydrocracked turbine oil on a gas recirculation fan bearing on Unit 1. Previously, the solvent-refined oils had lasted for only a few months in the 230 F bearing cavity environment before turning to sludge, leaving varnish deposits and precipitating carbon seal leakage. With the hydrocracked oils, sludge buildup has been slowed substantially, varnish deposits have disappeared and oil life has increased to 6-8 months or more. Reliant has plans to extend the use of hydrocracked lubricants to Robinson`s Unit 2 and to the 2 x 750 MW supercritical Cedar Bayou plant.
Steam turbines also derive benefits from hydrocracked lubricants, although the full benefits may be a little longer in coming. Unlike a gas turbine, where the 1,500-2,000 gallon oil reservoir can be drained and re-filled relatively easily and inexpensively, the 20,000 gallon oil reservoirs on some steam turbines often dictate a gradual lubricant replacement program. Such a “top-off” program, however, can still yield almost immediate results. In one evaluation of three GE steam turbines with 3,300 gallon reservoirs, for example, oxidation times (RBOT) increased by 69, 89 and 97 percent after a 5 percent top-off with hydrocracked turbine oil (Figure 2).
Another key lubricant property for steam turbine applications is demulsibility, the ability of oil to separate from water. Water infiltration into a steam turbine bearing can cause a breakdown of the oil film and potentially lead to turbine failure. The ASTM demulsibility test (D-1401) measures the time it takes in minutes for 40 ml oil to separate from 40 ml water after being mixed together in a Waring blender. Typically, this is reported as the time until 3 ml of emulsion are left, but hydrocracked lubricants are commonly reported at 0 ml of emulsion. The improved demulsibility enables even marginal oil-water separators to be used in a given plant, potentially avoiding costly investment.
Changes in the power generation industry are resulting in the modification of duty cycles for certain units. Plants that once were purely baseload are now commonly cycled to follow load. This places additional stress on power plant components, particularly when called on to perform a rapid start-up under cold weather conditions. Low-temperature start-ups on unpressurized bearing lubrication systems, for example, force motor bearings to operate for a significant period of time with only a residual oil film left over from the previous run period. Hydrocracked lubricating oils ease concerns about this limited safety margin because the absence of waxy compounds affords excellent low-temperature mobility.
The purity of hydrocracked lubricants also eases maintenance. There is a noticeable difference in clarity between new and used oil samples taken from a gas turbine oil reservoir containing solvent-refined oil and from a reservoir containing a hydrocracked turbine oil (see photo). While the solvent-refined turbine oil is opaque after use, the hydrocracked lubricant remains transparent after use, enabling maintenance personnel to readily identify contaminant build-up during sample inspections. Furthermore, although their clear color may make initial level checks more difficult, hydrocracked oils quickly take on enough color to ease distinction.