Steam turbine and power generating equipment

Details Steam turbine and power generating equipment Steam turbine and power generating equipment

2002-12-23

2004-09-14

Look, Edward K. (Department: 3745)

Rotary kinetic fluid motors or pumps

Working fluid passage or distributing means associated with...

Casing with axial flow runner

C415S108000

Reexamination Certificate

active

06790002

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a casing structure for a steam turbine included in a thermal power generation system installed in, for example, a combined power plant, and a power generating system using the steam turbine provided with the casing structure.
BACKGROUND OF THE INVENTION
Recently, many combined-cycle power plants provided with a gas turbine and a steam turbine in combination have been constructed. Generally, the improvement of steam conditions is directly related with the improvement of the efficiency of a power plant provided with a steam turbine. Therefore, the increase of the pressure and temperature of steam for driving a steam turbine included in a combined-cycle power generating system has been required to improve the efficiency of the power generating system and to enhance the output of the power generating system.
As shown in
FIG. 8
, a casing
110
of a high-pressure stage
5
of a conventional steam turbine for combined-cycle power generation is a single-wall casing. Usually, the thickness of the wall of the single-wall casing must be increased to improve the pressure withstand strength when inlet steam pressure is raised. In the event that the pressure and temperature of the steam are raised to improve the efficiency of the steam turbine provided with the conventional single-wall casing and to enhance the output of the same, an increased pressure stress and an increased thermal stress are induced in the casing owing to increase in the thickness of the wall of the casing. The casing is thus damaged by thermal fatigue or high temperature low-cycle fatigue during the operation, and the operation of the turbine affected.
The risk of steam leakage from the horizontal flange of the casing is increased by increase in thermal deformation of the casing, resulting in the marked degradation of the reliability of the steam turbine. Steam leakage involves the direct discharge of high-temperature, high-pressure steam into the atmosphere, which is fatal to the operation of the steam turbine, and increases the risk of fire and injury.
Since an excessively high thermal stress is induced in the casing having a thick wall at the start of the steam turbine, it must take a long time for starting up time of the turbine to reduce the level of the thermal stress. However, in a case, such as a combined-cycle power plant which is required quick start-up, the extension of the starting up time delays the start up of the combined-cycle power plant and increases the operating cost of the power generating system.
When the output of the steam turbine provided with a conventional single-wall casing structure is increased by raising the pressure and temperature of the main steam, the casing must be made of 12-Cr steel or 9-Cr steel, which has strength at high temperatures but expensive, instead of a conventional low alloy steel. The high material cost of the casing is a principal factor that increases the cost of the steam turbine.
The linear thermal expansion coefficients of the 12-Cr steel and the 9-Cr steel are smaller than those of conventional low alloy steels, typically CrMoV steels. Therefore, the thermal expansion of a casing made of 12-Cr steel or 9-Cr steel is smaller than that of the conventional casing. Thus, the expansion difference (the difference between the respective axial thermal expansions of the casing and the rotor with respect to a reference position corresponding to a thrust bearing of the turbine) is greater than that in the conventional turbine. This results in reduction of axial clearances between the rotor, i.e., a rotating body, and the components of the casing, i.e., stationary members. Due to this, the rotor and the components of the casing contact with each other, resulting in so-called axial-rubbing, causing the intense vibration of the shaft that hinders the continuation of the operation of the turbine.
Recently, a conventional combined-cycle steam turbine employs a double-wall casing structure including an inner casing
111
and an outer casing
112
entirely covering turbine stages from the high-pressure first stage
7
to the high-pressure exhaust stage
8
of the high-pressure section
5
, as shown in
FIG. 9
, with a view to solving the foregoing problems. This known double-wall casing structure will be referred to as “complete double-wall casing structure”, for simplicity.
Basically, thermal stress induced in a casing is proportional to the temperature difference between the outer and inner surfaces of the casing. Supposing that a casing is a thin-wall cylindrical structure for simplicity, steady circumferential thermal stress due to the temperature difference between the outer and inner surfaces in the thin-wall cylindrical structure is expressed by: &sgr;&thgr;t=0.714&agr;×E×T, where &sgr;&thgr;t is steady thermal stress, &agr; is the linear thermal expansion coefficient of the material of the thin-wall cylindrical structure, and T is the temperature difference between the outer and inner surfaces in the thin-wall cylindrical structure.
The temperature difference T
1
between the outer and inner surfaces of the casing in the single-wall casing structure can be divided into 0.7×T
1
in the outer casing of the double-wall casing structure, and 0.3×T
1
in the inner casing of the same. Therefore, a steady thermal stress that will be induced in the inner casing of the double-wall casing structure is on the order of 0.7 times a thermal stress that will be induced in a single-wall casing structure. A steady thermal stress that will be induced in the outer casing of the double-wall casing structure is on the order of 0.3 times the thermal stress that will be induced in the single-wall casing structure. Thus, the steady thermal stress induced in the casing of the high-pressure section can be effectively reduced by using a double-wall casing structure.
Supposing that a casing is a thin-wall cylindrical structure for simplicity, circumferential stress induced in the thin-wall cylindrical structure due to the internal pressure therein is expressed by: &sgr;&thgr;p=a×p/t, where &sgr;&thgr;p is circumferential stress, and t is the thickness of the thin-wall cylindrical structure. Thus, the pressure difference P
1
between the internal and the external pressure of the casing of the single-wall casing structure can be divided into 0.7×P
1
for the outer casing of a double-wall casing structure, and 0.3×T
1
for the inner casing of the double-wall casing structure.
Supposing that a casing is a thin-wall cylindrical structure, the radius of an inner casing of a double-wall casing structure is about 0.9×a and that of an outer casing of the double-wall casing structure is about 1.5×a, where a is the radius of a single-wall casing. Therefore, the wall thickness of the single-wall casing is a×P
1
/&sgr;1, the wall thickness of the outer casing of the double-wall casing structure is about 0.45×a×P
1
/&sgr;2 and the wall thickness of the inner casing of the double-wall casing structure is about 0.63×a×P
1
/&sgr;3, where &sgr;1 is a circumferential pressure stresses induced in the single-wall casing, and &sgr;2 and &sgr;3 are circumferential pressure stresses induced in the inner and outer casings of the double-wall casing structure, respectively.
If those circumferential stresses may be equal, i.e., &sgr;1=&sgr;2=&sgr;3, the respective wall thicknesses of the inner and outer casings of the double-wall casing structure may be about 0.63 times and about 0.45 times the wall thickness of the single-wall casing, respectively.
Conversely, the respective wall thicknesses of the inner and outer casings of the double-wall casing structure may be about 0.9 times and about 0.65 times the wall thickness of the single-wall casing, respectively, if it is desired to limit the pressure stress induced in the double-wall casing structure to a value 0.7 times the pressure stress induced in the single-wall casing. That is, the double-wall casing structure ach

Affiliated with

Also associated with

Rating

Steam turbine and power generating equipment does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Steam turbine and power generating equipment, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Steam turbine and power generating equipment will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3271429