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Tribology and Lubrication Technology Lubrication Fundamentals December 2013 : Page 28

Figure 5 | General Electric LM2500 Gas Turbine Steam turbines can take on many different configura-tions, an analysis of which goes beyond the scope of this article. However, all multistage steam turbines require cool, clean oil supplied to their journal bearings. This oil is supplied from several types of systems, but the key is that the oil must be delivered at the right flow rate, pressure and temperature. Often the driven equipment, such as the compressor, is also lubricated with the same oil, again at the right level of cleanliness, flow, pressure and tempera-ture. Furthermore, additional equipment also might be lu-bricated by this same system such as various control valves. solublize varnish rather than letting it form deposits. More modern formulations along with effective filtration generally manage excessive varnish formation, or at least they should. Such oils typically have a viscosity index (VI) in the range of 95-100 so that the viscosity doesn’t vary too much from startup to peak temperatures. GAS TURBINES Industrial gas turbines, which are basically land-based jet en-gines (windmills notwithstanding), have some similarities to steam turbines except that steam is replaced with hot com-bustion gases. Thus, water contamination is not so much a problem with gas turbines. However, sump temperatures can range between 50 C-100 C with hot spot peaks up to 280 C. Speeds also are higher on the order of 3-7,000 rpm. Therefore, the performance requirements of the lubricating oil are necessarily higher, while lubricating oils based on hydro-treated basestocks are used for the less stringent ap-plications. Clearly, gas turbines are a good application for synthetic lubricants such as polyalphaolefins and ester-based oils. Many of the larger, higher-performing industrial gas turbines (like the one shown in Figure 5) often use an aircraft engine core and require similar lubricants. For more advanced air-craft jet engines, which really are just higher-performing gas turbines, we have to kick it up a notch. These turbines use primarily ester-based lubricants for their wide-temperature range capabilities such as those described by military specifications MIL-L-7808 and MIL-L-23699. For even higher-temperature resistance with oper-ating temperatures as high as 300 C, polyphenylether lubri-cants, as described in MIL-L-87100, could be used. But these are not for the financially faint of heart, and the polyphenyl-ethers have poor low-temperature performance, which is not so critical for an industrial gas turbine but can be a show-stopper for an airplane. Clearly, turbomachinery, like the applications described above, affect our everyday lives in many ways. Keeping them going around allows us to go around. WWW .S TLE. OR G In compressor applications, steam turbines and the compressor itself are similar in that they are both turbomachines. In general terms, steam turbines can operate up to speeds of 3,000 rpm, oil sumps around 40 C-70 C with peak or hot spot temperatures as high as 150 C. Clearly, aside from par-ticulate contamination, which is always a problem, steam turbines are subject to the effects of water and steam. Typi-cally turbine oils for steam turbines must be primarily re-sistant to rust and oxidation, the so-called R&O oils. Addi-tionally, turbine oils may contain antiwear and EP additives. Further, they should exhibit good demulsibility of water and be low foaming. Interestingly, when one thinks about the highly formulat-ed automotive oils (10-20 percent additives), turbine oils by contrast are typically formulated with not much more than 1 percent additives. Therefore, the base oils are important. They can vary from Groups II, III, III+ or IV. The higher re-fined oils tend to have more inherent oxidation stability but, especially in the old days, they also tended to exhibit more varnish formation, likely because less refined oils tend to 28 TRIBOL OG Y & L UBRIC A TION TE CHNOL OG Y

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