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Chapter 4
Steam Turbine Technology

Excerpt

Today, large steam turbine-generators with ratings in excess of 1000 MW can be built as tandem-compound single-reheat steam turbines with water-cooled stator windings of the generator. Supercritical main steam pressure and elevated main and reheat steam temperatures of about 1100°F (600°C) are recommended. Figure 4-1 shows the perspective view of a steam turbine-generator of Siemens Westinghouse design. This about 800-MW reference unit features:

• Two stop and control valve combinations for main steam

• A single-flow barrel-type HP turbine

• Two stop and control valve combinations for reheat steam

• A double-flow 2-shell IP turbine

• Two double-flow LP turbines

• A hydrogen-cooled generator with water-cooled stator windings

• A spring-supported turbine-generator foundation

• Two condensers solidly attached to the LP turbine exhaust hoods

• Two LP bypass stations with stop and control valve combinations

The HP turbine of such a unit must be designed to reliably and efficiently handle the high main steam pressure and temperature operating conditions. Figure 4-2 shows the single-flow barrel-type HP turbine, which can best perform this task. The perspective view shows how the 2 combined main steam stop and control valve casings become an integral part of the HP turbine outer casing front section. Since the barrel-type front section of the HP turbine outer casing carries most of the pressure difference, the inner casing can be built with only a light vertically arranged joint without mass concentration and with only light bolting. The relatively large outer casing exhaust section is radial mounted to the smaller front section. Since the exhaust section is exposed to only low steam temperatures, it can be cast from 1% chromium steel, whereas the front section of the outer casing, the inner casing, and the rotor are made from advanced 10% chromium steel as indicated in Figure 4-3. The rotational symmetric shapes of the inner casing and the front section of the outer casing minimize thermal stressing. The inner casing is mounted into the front section of the outer casing by a threaded ring.

The inlet steam is admitted from the 2 stop and control valve combinations into a full-arc admission chamber of the inner casing to reduce the rotor metal temperature and consequently to minimize creep and lowcycle fatigue. The spiral admission chamber is designed for optimal radial steam flow distribution for the first stage blading. The full-arc admission with a tilted first-stage stationary blades is illustrated in Figure 4-4. This design concept, with a low reaction-type blading, reduces the temperature to which the rotor surface is exposed. This full-arc admission design is well suited for variable pressure operation, eliminating thermal stressing due to load swings. By using advanced martensitic 10% chromium steel components, this HP turbine design can easily handle the main steam conditions of 4350 psig (300 bar) and 1112°F (600°C) without the introduction of austenitic steel alloys.

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