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Steam turbines have historically been the prime source of power for electric power generation. Turbines come in a variety of types with regard to inlet and exhaust steam conditions, casing and shaft arrangements and flow directions. This chapter will focus on steam turbines currently being applied to power generation. The steam conditions will include those currently applied to fossil fired power plants, combined cycle and nuclear power units.
Currently, fossil fired plants, whether coal fired or gas fired, typically supply steam at 1800 to 3500 psig steam pressures with 950 to 1050°F main steam and reheat temperatures. Double reheat units are few and will not be discussed in this chapter in much detail. Combined cycle plants (CC) typically have multiple drum heat recovery steam generator (HRSG) with inlet temperatures of 1050°F for main steam and reheat steam (see Chapter 27). While no new nuclear units have been installed in recent years, plans are underway to apply the AP1000 nuclear systems (see Chapters 23 and 24). Past steam conditions were typically saturated steam (0.2% moisture) and reheat temperature of 500°F. All current applications are condensing designs with regenerative extraction, except for CC units. Current application can be used up to 8 to 10 inHgA exhaust pressure. The application of air cooled condensers is more prevalent in CC applications.
Current designs are axial flow turbines. However, in some applications, radial flow stages are used in the inlet stages. Typical arrangements are tandem compound. This means more than one turbine casing's rotors are coupled together on the same shaft. For example a turbine train consisting of a high-pressure turbine (HP), an intermediate-pressure turbine (IP) and two low-pressure turbines (LP) would have all of these in line and coupled to an electric generator. Cross compound units (two or more shafts) are currently in use. Some arrangements have the HP and IP on one shaft along with a generator and the LP turbines on a separate shaft along with another generator. In many instances the low-pressure turbine shaft is running at half speed and has larger annulus areas to reduce LP turbine exhaust velocity and associated leaving losses. However, due to increased blade height and associated exit annulus area for full speed units and the higher costs, cross compound units are not currently in favor. Casing configurations for fossil units are typically high-pressure unit, intermediate-pressure unit, and one or more low-pressure units exhausting to the condenser. The high-pressure and intermediate-pressure units are frequently combined into one casing (HP-IP).