Patent Publication Number: US-6341937-B1

Title: Steam turbine with an improved cooling system for the casing

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 FIG. 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 lower pressure turbine portion  105  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 . 
     SUMMARY OF THE INVENTION 
     The invention is directed to solve the above mentioned prior art problems, and the objective of the invention is to provide a steam turbine with an improved cooling system for the casing. 
     The invention provides a steam turbine which includes a rotor which extends along a longitudinal axis and has at least two different pressure turbine portions disposed around and along the rotor. Each of the turbine portions has multiple stages. A casing encloses the rotor and has at least one steam inlet. A dummy ring is provided stationarily around the rotor to separate and seal between the two different pressure turbine portions. The dummy ring defines a nozzle chamber for receiving the steam from the steam inlet port and a plurality of nozzles for directing the steam from the nozzle chamber toward the higher pressure turbine portion. The dummy ring and the casing define a space therebetween. The space is fluidly connected to the higher pressure turbine portion. Piping extends between the space and a steam passage downstream of the last stage of the higher pressure turbine portion. The steam passage allows the steam within the space to flow to the steam passage downstream of the last stage of the higher pressure turbine portion to cool the casing. 
    
    
     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: 
     FIG. 1 is a generally sectional view of a steam turbine according to the preferred embodiment of the invention; 
     FIG. 2 is an enlarged section illustrating in detail a portion of the steam turbine indicated by “A” in FIG. 1; and 
     FIG. 3 is a generally sectional view of a steam turbine of the prior art. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIGS. 1 and 2, the preferred embodiment of the invention will be described. 
     FIG. 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  3   a ,  3   b  and  3   c  which are provided around and along the rotor  1  (FIG.  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  3  is exhausted from the high pressure turbine portion  3 . 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 (FIG.  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 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  3   b  and  3   c  through a gap. Therefore, the space  18  is filled with steam from downstream of the second and third stage  3   b  through gap. 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 casing  1  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  3   b  and  3   c  through a gap  16 . Therefore, the space  18  is filled with steam from downstream of the second and third stage  3   b  through gap  16 . However, in the prior art, the space  18  is not fluidly connected 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  3   a  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 FIG. 2 includes external piping or a steam passage  22  extending between the space  18  and a steam passage  15  downstream of the third stage  3   c  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 Z from the high pressure turbine portion  3  between the second and third stages  3   b  and  3 , through the gap, the space  18 , and the external piping  22  to the steam passage  15  downstream of the third stage  3   c  of the high pressure turbine portion  3 . The expansion of the steam through the first and second stages  3   a  and  3   b  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 case  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 . 
     It will also be understood, by those skilled in the art, that the forgoing description is a preferred embodiment of the disclosed invention and that various changes and modifications may be made without departing from the spirit and scope of the invention.