1. Field of the Invention
The present invention relates generally to a turbine exhaust system for a steam turbine that discharges from an exhaust duct the steam having passed through a turbine blade. In particular, the invention relates to an exhaust system for a high pressure or an intermediate pressure turbine.
2. Description of the Related Art
Electric generating plants generate electric power by rotating a turbine with steam produced by a steam generator such as a boiler. An electric generating plant generally includes a plurality of turbines adapted for different steam pressures; for example, a high pressure turbine, an intermediate turbine, and a low pressure turbine. After being passed through from the high pressure turbine to the low pressure turbine to finish rotating work, the steam is finally led into a condenser. The steam then condenses into condensed water and returns to the steam generator. The exit of each high, intermediate, and low pressure turbines is linked with a turbine exhaust system that guides steam to the subsequent stage equipment such as a lower pressure turbine, a condenser, etc. The turbine exhaust system includes an exhaust hood defined between an inner casing covering a turbine rotor and an outer casing further covering the inner casing. The steam that has passed through the turbine blade is delivered to the subsequent stages via the exhaust hood.
A common exhaust hood changes the direction of a steam flow delivered from a turbine from an axial-flow direction to a direction perpendicular thereto in a very short distance. Therefore, exhaust hoods tend to disturb the steam flow and cause pressure loss. In particular, exhaust hoods of high and intermediate pressure turbines have a shorter flow passage than those of low pressure turbines. Further, parts of high and intermediate pressure turbines are made thicker than those of low pressure turbines in order to withstand pressure. Exhaust hoods of high and intermediate pressure turbines are thus more likely to be affected by their inner components such as flanges compared to low pressure turbines.
An example of conventional technologies made in consideration of the above matters is disclosed in JP-2007-40228-A. According to the publication, an annular flow guide is provided at the leading end side of the exit portion of last stage rotor blades. The flow guide rectifies the flow and in turn reduces flow turbulence. The flow guide disclosed in JP-2007-40228-A is an annular flow guide constructed by combining a convexly curved flange with a disk-like steam guide. In contrast, flared annular flow guides are often used in real machines.
Incidentally, flow guides of a low pressure turbine serve as a diffuser for converting kinetic energy to pressure energy. In addition, exhaust hoods of low pressure turbines have less spatial restriction than those of high and intermediate pressure turbines. In regard of this, a flow guide having a vertically asymmetric shape (whose lower side is long) is proposed in the aim of improving diffuser effect (JP 3776580).