A steam turbine with an improved cooling system for the casing

Abstract: A steam turbine includes a rotor (1) which extends along a longitudinal axis and having at least two different pressure turbine portions (3,4) disposed around and along the rotor (1). Each of the turbine portions (3,4) has multiple stages. A casing (2) encloses the rotor (1) and has at least one steam inlet (6). A dummy ring (10) is provided stationarily around the rotor (1) to separate and seal between the two different pressure turbine portions (3,4). The dummy ring (10) defines a nozzle chamber (13) for receiving the steam from the steam inlet port (6) and a plurality of nozzles for directing the steam from the nozzle chamber toward the higher pressure turbine portion (3). The dummy ring (10) and the casing (2) define a space therebetween. The space is fluidly connected to the higher pressure turbine portion (3). A steam passage (22) extends between the space and a steam passage downstream of the last stage of the higher pressure turbine portion (3). The steam passage (22) allows the steam within the space to flow to the steam passage downstream of the last stage of the higher pressure turbine portion (3) to cool the casing (2).

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to a steam turbine and, in particular, to a steam turbine with an improved cooling system for the casing. 2. Description of the Related Art With reference to Figure 3, a prior art stream turbine includes a rotor 100 extending along a longitudinal axis, and a casing 102 enclosing the rotor 100. A high pressure turbine portion 103, an intermediate pressure turbine portion 104 and a low pressure turbine portion are disposed within the single casing 102 around and along the rotor 100. Provided within the casing 102 is a dummy ring 110 which separates the high and intermediate pressure turbine portions 103 and 104 and seals therebetween. The dummy ring 110 and the casing 101 define a space 118 therebetween. The space 118 is filled with steam so that the steam within the space 118 is held there. The steam within the space 118 is heated by thermal transfer from the high pressure and temperature steam supplied to the high pressure turbine portion 103 so that the portion of the casing 102 enclosing the space 118 is also heated. This results in the thermal deformation of the casing 102. U.S. 5,149,247 discloses a steam turbine according to the preamble of claim 1. As a further feature of this prior art steam turbine, a steam flow passage for cooling the casing enters the steam passage downstream of the third stage of a multi-stage higher pressure turbine portion at a point downstream of a steam outlet port of the higher pressure turbine portion. U.S. 2,796,231 discloses a steam turbine which includes one multi-stage pressure turbine unit. An inner casing comprises an annular chamber through which steam can flow entering the chamber via a plurality of radial passages formed in an intermediate pressure stage of the multi-stage pressure turbine. The steam leaving the chamber can be circulated by a plurality of passageways formed in the inner casing and a plurality of small pipes extending through an exhaust passage between the inner casing and an outer casing and entering back into the turbine at a stage downstream of the stage from which the steam is fed to the chamber. Summary of the invention It is the technical problem underlying the invention to provide a steam turbine which at least partly overcomes the difficulties of the prior art steam turbines mentioned above, and is in particular able to provide for effective cooling of the turbine casing. The invention solves this problem by providing a steam turbine having the features of claim 1. DESCRIPTION OF THE DRAWINGS These and other objects and advantages and a further description will now be discussed in connection with the drawings in which: Figure 1 is a generally sectional view of a steam turbine according to the preferred embodiment of the invention; Figure 2 is an enlarged section illustrating in detail a portion of the steam turbine indicated by "A" in Figure 1; and Figure 3 is a generally sectional view of a steam turbine of the prior art. DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to Figures 1 and 2, the preferred embodiment of the invention will be described. Figure 1 shows a sectional view of a stream turbine according to the preferred embodiment of the invention which includes a rotor 1 extending along a longitudinal axis and a casing 2 for enclosing the rotor 1. A high pressure turbine portion 3, an intermediate pressure turbine portion 4 and a low pressure turbine portion 5 are disposed within the single casing 2 around and along the rotor 1. In this particular embodiment, the high pressure turbine portion 3 includes first, second and third stages 3a, 3b and 3c which are provided around and along the rotor 1 (Figure 2). Although it is not shown in detail in the drawings, the casing 2 includes higher and lower shell halves which are joined to each other at a horizontal plane by a plurality of bolts, as is well-known in the art. The casing 2 includes a high pressure steam inlet port 6 through which high pressure steam 30 is supplied to the high pressure turbine portion 3 and a high pressure steam outlet port 7 through which the steam used in the high pressure turbine portion 3 is exhausted. The casing 2 further includes an intermediate pressure steam inlet port 8 through which an intermediate pressure steam 32 is supplied to the intermediate pressure turbine portion 4. The casing 2 further includes a low pressure steam inlet port 9 through which a low pressure steam 33 is supplied to the low pressure turbine portion 5. Provided within the casing 2 is a dummy ring 10 which separates the high and intermediate pressure turbine portions 3 and 4 and seals therebetween (Figure 2). The high pressure steam 30 flows into the high pressure turbine portion 3 through the high pressure steam inlet port 6 to drive the high pressure turbine portion 3, after which the steam used in the high pressure turbine portion is exhausted through the high pressure steam outlet port 7. The intermediate pressure steam 32 flows into the intermediate pressure turbine portion 4 through the intermediate steam inlet port 8 to drive the intermediate pressure turbine portion 4, after which it flows into the low pressure turbine portion. The low pressure steam 33, supplied through the low pressure steam inlet port 9, flows into the low pressure turbine portion 5 together with the steam from the intermediate pressure turbine portion 4 to drive the low pressure turbine portion 5. The steam used in the low pressure turbine portion 5 is exhausted through an exhaust chamber 11. The dummy ring 10 defines a nozzle chamber 13 which is fluidly connected to the high pressure steam inlet port 6 through steam passages (not shown) provided between the casing 2 and the dummy ring 10, and to a plurality of nozzles 12 through which the high pressure steam is directed to the high pressure turbine portion 3 and, in particular, to the first stage 3a of the high pressure turbine portion. For example, the steam is supplied to the nozzle chamber 13 at approximately 560 °C and supplied to the first stage 3a at approximately 500 °C. The casing 2 and the dummy ring 10 define a space 18 therebetween. The space 18 is fluidly connected to the high pressure steam turbine portion 3 at a portion between the second and third stages 3b and 3c through a gap 16. Therefore, the space 18 is filled with steam from downstream of the second and third stage 3b through gap 16. However, in the prior art, the space 18 is not fluidly connected to another portion within the casing 2 so that the steam within the space 18 is held there. The steam within the space 18 is heated to at least 500 °C by thermal transfer from the high temperature steam within the nozzle chamber 13 and between the nozzles 12 and the first stage 3a through the dummy ring 10. The heated steam within the space 18 then heats the portion of the casing 2 enclosing the space 18 to at least 500 °C. This results in the thermal deformation of the casing 2 and the increase in the stress in the bolts connecting the upper and lower shell halves of the casing 2. The embodiment shown in Figure 2 includes external piping or a steam passage 22 extending between the space 18 and a steam passage 15 downstream of the third stage 3c of the high pressure turbine portion 3. The steam passage 15 is fluidly connected to the high pressure steam outlet port 7. The external piping 22 allows the steam within the space 18 to flow to the steam passage 15 and establishes a steam flow passage, for cooling the casing 2, from the high pressure turbine portion 3 between the second and third stages 3b and 3c, through the gap 16, the space 18, and the external piping 22 to the steam passage 15 downstream of the third stage 3c of the high pressure turbine portion 3. The expansion of the steam through the first and second stages 3a and 3b of the high pressure turbine portion 3 reduces its temperature from approximately 500 °C to approximately 450 °C. This reduces the temperature of the casing 2 whereby the amount of the thermal deformation of the casing 2 and the stress in the bolts for connecting the upper and lower shell halves of the casing 2, are reduced. The external piping 22 may includes a valve 21 for controlling the flow rate of the steam through the piping 22. Controlling the flow rate of the steam controls the overall heat influx to the casing 2 and thus controls the temperature of the casing 2.

Claims: A steam turbine comprising: a rotor (1) extending along a longitudinal axis and having at least two different pressure turbine portions (3, 4, 5) disposed around and along the rotor, each of which has multiple stages, wherein a higher pressure turbine portion (3) has a first, a second, and a third stage (3a, 3b, 3c); a casing (2) for enclosing the rotor, the casing including a steam inlet port (6) and a higher pressure steam outlet port (7); a dummy ring (10), provided stationarily around the rotor, for separating and sealing between two different pressure turbine portions, the dummy ring defining a nozzle chamber (13) for receiving the steam from the steam inlet port and a plurality of nozzles (12) for directing the steam from the nozzle chamber toward the higher pressure turbine portion (3), the dummy ring and the casing defining a space (18) therebetween, the space being fluidly connected to the higher pressure turbine portion between the second and third stage (3b, 3c) of the higher pressure turbine portion; and a steam flow passage (22), extending between the space and a steam passage (15) downstream of the last stage (3c) of the higher pressure turbine portion, for allowing the steam within the space to flow to the steam passage (15) downstream of the last stage of the higher pressure turbine portion to cool the casing; characterized in that the steam flow passage (22) enters the steam passage (15) downstream of the last stage (3c) of the higher pressure turbine portion (3) at a point (Y) upstream of the high pressure steam outlet port (7). A steam turbine according to claim 1, wherein the steam passage includes a valve (21) for controlling the flow rate of the steam through the steam flow passage to control the cooling of the casing.

Inventors: MAGOSHI RYOTARO, NAKANO TAKASHI, MAGOSHI, RYOTARO, NAKANO, TAKASHI, Magoshi, Ryotaro, c/o Takasago Machinery Works, Nakano, Takashi, c/o Takasago Machinery Works
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI HEAVY INDUSTRIES, LTD.
Application year: 1999
Main classification: F01D25
IPC codes: F01D25