1. Field of the Invention
The present invention relates to a steam turbine provided with a double-structured casing of an outer casing and an inner casing and to a steam turbine plant system provided with the steam turbine.
2. Description of the Related Art
A steam turbine having a high pressure occasionally has a casing structure having a double structure of an outer casing and an inner casing as described in, for example, JP-A 2006-307280 (KOKAI). In such a structure, exhaust steam at first stage moving blades flows between the inner casing and the outer casing via a gland portion and meets the turbine exhaust steam. Therefore, the outer casing has a design pressure which is a differential pressure between a pressure between the inner and outer casings and an external pressure of the outer casing. And, the casing structure is also influenced by the temperature of steam flowing between the inner and outer casings.
In the above-described steam turbine having the conventional double-structured casing, if the conditions of steam as a working fluid include a supercritical pressure or an ultra supercritical pressure, it is necessary to use a material having high strength for the outer casing or to increase the thickness of the outer casing. Therefore, the steam turbine had a problem that its production cost became high.