Handbook of Turbomachinery Second Edition Revised - Ventech!

Even though designers make the exhaust hood as large as possible,
because of the large volume of steam flow, sonic velocities can be reached at
the steam exit. As a result, losses accumulate similar to flow through a pipe,
and the exhaust hood is potentially subject to flow-accelerated corrosion.
Exhaust hood redesign to optimize flow paths is an area that is showing
considerable economic payback [23]. These redesigns are being performed
using tools such as computational fluid dynamics (CFD), a technique that
was not available to the early designers.
Fossil and Nuclear Turbine Designs Compared
Fossil Turbine Designs
An overall schematic of atypical fossil turbine is shown in Fig. 28. Steam
enters from the main steam lines through stop and control valves into the
HP section or cylinder. As noted earlier, the first or control stage is spaced
somewhat apart from subsequent stages to allow for stabilization of the
flow.
After passing through the HP turbine, cold reheat piping carries the
steam to the reheater and returns in the hot reheat piping to the integrated
HP and IP cylinder to pass through the IP turbine. The return to the IP
turbine typically passes through a stop and intercept valve. The flow exits
the IP through the IP exhaust hood and typically passes back along the
spacing between the inner and outer casing of the combined HP and IP
turbine. The flow then passes through crossover piping to the LP.
During its expansion through the LP turbine(s), the steam crosses the
steam saturation line, also termed the phase-transition zone (PTZ). Exit is
through the LP exhaust hood. The typical modern steam turbine has a
number of extraction points throughout all its sections, the extracted steam
being supplied to the feedwater heaters.
Nuclear Turbine Designs
Nuclear steam supply is generally at lower pressures and temperatures than
in fossil fuel units; as a result, considerably higher flow rates are required to
generate the same power output. There is generally an HP section coupled
with several LP sections.
Nuclear HP turbines are usually of a double-flow design. Because the
supplied steam has little or no superheat, expansion through the nuclear HP
turbine leads to high moisture levels (typically 10–15%). If it were exhausted
directly to the LP turbine, exit moisture from the LP would be on the order
Copyright © 2003 Marcel Dekker, Inc.