Abstract:
An exhaust hood for an axial steam turbine that includes a radial channel, downstream from the normal flow pattern. The radial channel guides the exhaust steam flow in upper half of the hood in the flow momentum direction. Due to this pattern of flow direction, vortex generation in upper exhaust hood is reduced and increased flow diffusion results. The geometric arrangement can eliminate the outer casing of the exhaust hood over the axial length of the turbine inner casing, allowing the turbine inner casing to be supported directly by a foundation for the steam turbine.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates generally to steam turbines and more specifically to exhaust hoods for efficiently diffusing steam to a condenser. 
         [0002]    In the discharge of exhaust steam from an axial flow turbine, for example discharge of this exhaust steam to a condenser, it is desirable to provide as smooth a flow of steam as possible and to minimize energy losses from accumulation of vortices, turbulences and non-uniformity in such flow. Usually the exhaust from the turbine is directed into an exhaust hood and from there through a discharge opening in the hood in a direction essentially normal to the axis of the turbine into a condenser. It is desirable to achieve a smooth transition from axial flow at the exhaust of the turbine to radial flow in the exhaust hood and thence a smooth flow at the discharge opening of this hood into the condenser. 
         [0003]    In the constructing of an effective exhaust hood for use with such an axial flow turbine it is desirable to avoid acceleration losses within any guide means employed therein and to achieve a relatively uniform flow distribution at the discharge opening of the exhaust hood for the most efficient conversion of energy in the turbine and effective supplying of exhaust steam to the condenser to which it is connected. 
         [0004]    It is also desirable to achieve optimum efficiency at the last stage buckets of the turbine prior to exhaust from the turbine by achieving a relatively uniform circumferential and radial pressure distribution in the exit plane of the last stage buckets. Usually, attempts have been made to accomplish these results while employing a hood having as short an axial length as possible, so as to limit the axial size of the turbine train. 
         [0005]    The prior art has employed, in the exhaust duct connected to the turbine, vanes, which have smoothly curved surfaces for effectively changing the axial flow of the steam from the turbine to the generally radial flow. For example of such an arrangement for converting the axial flow of the exhaust from the turbine to radial flow is shown in U.S. Pat. No. 3,552,877 by Christ et al. Further developments in prior art exhaust hoods for axial flow turbines, such as U.S. Pat. No. 4,013,378 by Herzog, have incorporated multiple sets of vanes for further smoothing flow. The exhaust hood includes a first set of guide vanes arranged in an exhaust duct connected to the turbine adjacent the last stage buckets thereof. These vanes are curved to provide a relatively smooth transition of steam flow from an axial direction to a generally radial direction. A guide ring circumferentially surrounds the first set of guide vanes and a plurality of secondary vanes are circumferentially spaced around this guide ring. Steam, which is discharged radially from the first set of vanes to the secondary vanes, is directed by the secondary vanes to the discharge opening of the exhaust hood. The secondary vanes are substantially equally spaced around the guide ring and are curved, at different angles to effect different angles of discharge of steam from these vanes. The angles of discharge are chosen so as to direct the steam toward the discharge opening of the exhaust hood in a manner achieving substantially uniform flow distribution across the exit plane of the last stage buckets and across the plane of the discharge opening. However, while such vanes may be optimized for one set of flow conditions, they may operate with significantly less effectiveness at other flows. 
         [0006]    Diffusers, for example, are commonly employed in steam turbines. Effective diffusers can improve turbine efficiency and output. Unfortunately, the complicated flow patterns existing in such turbines as well as the design problems caused by space limitations make fully effective diffusers almost impossible to design. A frequent result is flow separation that fully or partially destroys the ability of the diffuser to raise the static pressure as the steam velocity is reduced by increasing the flow area. For downward exhaust hoods used with axial steam turbines, the loss from the diffuser discharge to the exhaust hood discharge varies from top to bottom. At the top, much of the flow must be turned 180 degrees to place it over the diffuser and inner casing, then turned downward. Pressure at the top is thus higher than at the sides, which are in turn higher than at the bottom. 
         [0007]      FIG. 1  illustrates a perspective partial cutaway of a double flow steam turbine a portion of a steam turbine. The steam turbine, generally designated  10 , includes a rotor  12  mounting a plurality of turbine buckets  14 . An inner turbine casing  16  is also illustrated mounting a plurality of diaphragms  18 . A centrally disposed generally radial steam inlet  20  applies steam to each of the turbine buckets and stator blades on opposite axial sides of the turbine to drive the rotor. The stator vanes of the diaphragms  18  and the axially adjacent buckets  14  form the various stages of the turbine forming a flow path and it will be appreciated that the steam is exhausted from the final stage of the turbine for flow into a condenser beneath (not shown). 
         [0008]    Also illustrated is an outer exhaust hood  21 , which surrounds and supports the inner casing of the turbine as well as other parts such as the bearings. The turbine includes steam guides (not shown) for guiding the steam exhausting from the turbine into an outlet  26  for flow to one or more condensers. With the use of an exhaust hood supporting the turbine, bearings and ancillary parts, the exhaust steam path is tortuous and subject to pressure losses with consequent reduction in performance and efficiency. A plurality of support structures may be provided within the exhaust hood.  21  to brace the exhaust hood and to assist in guiding the steam exhaust flow. An exemplary support structure  30  is situated to receive and direct the steam exhaust flow  35  from the steam turbine  10 . The diffusion of the steam is restricted to the volume in the exhaust hood  21 . 
         [0009]    The exhaust hood  21  includes an upper hood  22  and a lower hood  23 . The upper and lower hoods are joined along a horizontal seating surface  33 . An upper part of the lower hood  23  is reinforced with support members  34  providing a support frame  36 . The weight borne by the support frame  36  is transferred at support ledge  27  to a foundation  40 . 
         [0010]      FIG. 2  illustrates a schematic elevation view of a prior an exhaust hood for the double flow steam turbine  10  including an exhaust flow path  35 . The steam turbine LP section consists of an inlet domain  20 , turbine stages (nozzles  18  and buckets  14 ) and an exhaust hood  22  with diffuser  25 . One of the main functions of the exhaust hood is to recover the static pressure and guide the exhaust steam flow  35  from last stage buckets  15  to the condenser steam outlet  26  to the condenser (not shown) underneath. The exhaust hood  21  includes the upper exhaust hood  22  and the lower exhaust hood  23 . Flow from the last stage buckets  15 , which could have very high swirl and high flow gradient in radial direction, enters the condenser through exhaust hood  21 . Part of the flow  28  directly flows down to condenser through the lower exhaust hood  23  and the remaining flow  29  travels through upper exhaust hood  22 . The flow in the upper exhaust hood  22  is directed by flow guide  32  and begins to turn 180 degrees from a vertically upward direction to downward direction over the inner casing  16  to reach the condenser. This results in strong vortex formation  38  behind the steam guide  24  in upper exhaust hood and minimizes the effective flow area between the steam guide and outer wall of the hood, thereby increasing losses in the steam path as well. This phenomena decreases the flow diffusion in upper half of exhaust hood, results in degradation of exhaust hood performance, which has direct impact on the last stage bucket performance. 
         [0011]    Accordingly, it would be desirable to eliminate vortex flow in the upper exhaust hood and provide improved flow patterns and diffusion performance, particularly in the upper exhaust hood. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0012]    The present invention relates to an exhaust arrangement for an axial flow steam turbine in which a radial channel to the turbine condenser partially eliminates vortices in the upper exhaust hood and improves hood performance. 
         [0013]    Briefly in accordance with one aspect of the present invention, an exhaust arrangement is provided for an axial flow steam turbine. The exhaust arrangement includes an inner turbine casing with a plurality of turbine stages providing an axial steam flow path through the inner turbine casing and an exhaust outlet from a plurality of buckets of a last turbine stage. A turbine condenser is mounted below the steam turbine. An exhaust hood is provided at a downstream end of the steam turbine where the exhaust outlet flows through a diffuser into a dual path to the turbine condenser. A bearing cone and a plurality of annular steam guides define a diffuser flow path for the exhaust outlet flow. A first exhaust path of the dual path extends through a lower section of the diffuser to a lower section of the exhaust hood and then essentially downward to the condenser. An upper section of the exhaust hood is in fluid communication with an upper section of the diffuser. A downstream radial channel of the exhaust hood is in fluid communication with the upper section of the exhaust hood and is further in fluid communication with the turbine condenser below. A second exhaust path of the dual path flows through the upper section of the diffuser into the upper section of the exhaust hood, downstream axially to the radial channel and then downward through the radial channel to the turbine condenser. 
         [0014]    According to another aspect of the present invention, an axial flow steam turbine is provided. The steam turbine includes an inner casing with a plurality of turbine stages providing an axial steam flow path through the inner casing and an exhaust outlet from a plurality of buckets of the last turbine stage. A turbine condenser is mounted below the steam turbine. A foundation is provided for the steam turbine. An exhaust hood at a downstream end of the steam turbine includes at least one exhaust path through a radial channel of a dual exhaust path from the exhaust outlet of the inner turbine casing to the turbine condenser. The exhaust hood is mounted to the inner turbine casing at an axial end of the inner casing. Support means are provided for the steam turbine such that the inner casing is supported directly from the foundation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0015]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0016]      FIG. 1  illustrates a perspective partial cutaway of a double flow steam turbine including a prior art exhaust hood; 
           [0017]      FIG. 2  illustrates a schematic elevation view of a prior art exhaust hood for the double flow steam turbine including an exhaust flow path; 
           [0018]      FIG. 3  illustrates a schematic elevation view of a first embodiment of the inventive exhaust arrangement for an axial flow steam turbine; 
           [0019]      FIG. 4  illustrates a top view of an embodiment of the steam turbine and exhaust arrangement with the upper exhaust hood removed; 
           [0020]      FIG. 5  illustrates a three-dimensional side view of the exhaust arrangement structure with a radial channel; 
           [0021]      FIG. 6  illustrates a three-dimensional end view of the exhaust arrangement structure with a radial channel; 
           [0022]      FIG. 7  illustrates an isometric view of one lateral side of the exhaust arrangement viewed from the turbine inner casing end; 
           [0023]      FIG. 8  provides a cutaway side view of the second exhaust path in the second embodiment of the exhaust arrangement; and 
           [0024]      FIG. 9  provides an isometric view of one lateral side of the exhaust arrangement. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    The following embodiments of the present invention have many advantages, including improving static pressure recovery in a low pressure (LP) exhaust hood and thereby improving the heat rate or output of the steam turbine. Further, a very simple geometry construction results from the invention, thereby helping, to reduce weight by eliminating a portion of the outer casing of the exhaust hood that covers the inner casing, thereby saving cost. 
         [0026]    A further advantage of the geometrical construction for the hood provides an opportunity to rest the inner turbine casing on the foundation for the turbine, which lead to enhanced machine reliability. 
         [0027]    The present invention incorporates a concept of a radial channel, which guides the flow in upper half of the hood in the flow momentum direction. Due to this pattern of flow direction, the vortex generation in upper exhaust hood may be reduced and hence an increase in flow diffusion would result. The radial channel may be disposed behind the end wall of the exhaust diffuser to direct the flow from upper half of exhaust hood towards a turbine condenser as shown in  FIG. 3 . This radial channel configuration will help to minimize the vortex generation in upper half of the hood. Since there is no inner casing in radial channel there will be smooth transition of flow over 180 degrees to the turbine condenser, which will improve the flow diffusion, and hence provide low pressure section efficiency improvement. Also, better diffusion of flow in upper section of the exhaust hood helps to achieve uniform pressure gradient between the last stage bucket (LSB) exits and the exhaust inlet, which has a favorable impact on LSB performance. 
         [0028]    A first embodiment of the present invention provides an exhaust arrangement  121  for an axial flow steam turbine as illustrated in  FIG. 3 . An inner turbine casing  116  includes one or more turbine stages of nozzles  114  and buckets  118  providing an axial steam flow path through the inner turbine casing  116 . An exhaust outlet flows from multiple last stage buckets  115 . An exhaust hood  125  is coupled to a downstream axial end  127  of the inner turbine casing  116 . A turbine condenser  140  is mounted below the exhaust hood  125  for condensing and subcooling the exhausted steam. For a dual axial steam turbine, an exhaust hood  125  is coupled at each downstream axial end  127  of the inner casing  116  with one or more turbine condensers  140  accepting the exhausted steam. 
         [0029]    The exhaust hood  125  provides a dual exhaust path from the last stage buckets  118  to the turbine condenser  140 . The exhaust hood  125  may include an upper exhaust hood  122  and a lower exhaust hood  123  separated conventionally along a horizontal joint  135  ( FIG. 4 ). The exhaust hood  125  includes a diffuser  150 , a lower section  155 , an upper section  160 , and a downstream radial channel  170 . A first exhaust path  180  for steam discharging into the exhaust hood  125  from the last stage buckets  118  includes a lower section  151  of the diffuser  150 , the lower section  155  of the exhaust hood  125  and a downward discharge into the condenser  140 . The second exhaust path  190  flowing from the last stage buckets  118  of the inner casing  116  includes an upper section  152  of the diffuser  150 , the upper section  160  of the exhaust hood  125 , and a downstream radial channel  170  of the exhaust hood  125  discharging downward to the turbine condenser  140  below. 
         [0030]    The diffuser  150  is formed between an inner wall  154  of a hearing cone  155  and steam guides  156 ,  157 . The axial downstream ends of the bearing cones engage with a divider wall separating the upper section of the exhaust hood from the downstream section. 
         [0031]    The lower half  151  of the diffuser  150  opens into the lower section  155  of the exhaust hood  125 . The lower section  155  of the exhaust hood opens downwardly into the turbine condenser  140 . The upper half  152  of the diffuser  150  opens into the upper section  160  of the exhaust hood  125 . An opening  161  for steam flow from the axial downstream end  161  of the upper section  160  of the exhaust hood  125  to the downstream radial channel  170  is provided between the upper exhaust hood easing wall  125  and the outer end  166  of the circumferential divider wall  165 . The radial channel  170  connects the upper section  160  of the exhaust hood with the turbine condenser  140  below. The radial channel  170  includes an upper space  171  between a plane of the divider wall  165  and an endwall  172 . The upper space  171  may be formed as a semi-annulus above the rotor shaft  112 . 
         [0032]    The radial channel  170  may also include two descending exhaust spaces  173  to the turbine condenser  140 . The descending exhaust spaces  173  may be positioned axially downstream from the divider wall  165  and be open radially to the upper section  171  of the radial channel above and to the turbine condenser  140  below. The two descending exhaust spaces  173  together may be formed around the rotor shaft  112 , which extends axially through the exhaust arrangement  121  and divider wall  165 . The exhaust spaces  173  may lie axially between the divider wall  165  and end wall  174 . The two descending exhaust spaces  173  may be generally aligned in parallel for the vertical descent to the turbine condenser  140 . The two descending exhaust spaces  173  may be an integral part of the exhaust arrangement  121  or may be enclosed in external ductwork. Each of the descending exhaust spaces  173  may include an inner sidewall  175  ( FIG. 6 ), wherein an opening space  176  is provided there-between. The opening space  176  between the descending exhaust spaces  173  of the radial channel  170  may be sufficiently large to allow personnel access to the bearing cone  145  areas. 
         [0033]    Because the exhaust hood  125  mates with an axial end  127  of the turbine inner casing  116 , the spaces  177 ,  178  above and below and around the turbine inner easing are not utilized for the exhaust hood.  FIG. 4  provides a top view of the steam turbine  100  with the upper exhaust hood removed. Spaces  177  and  178  are available to mount the turbine inner casing  116  to the foundation directly. At least one support arm  185  from each lateral side  186  of the turbine inner casing  116  may extend to the pads  187  on foundation wall  80 . The exhaust hood  125  may include a reinforced section  135  which also seated on the foundation wall  80  to provide support for the exhaust hood. 
         [0034]    With the upper exhaust hood  1 . 22  removed, the tap of steam guide  157  and the top surface of the inner wall  144  of the bearing cone  145  are exposed. A general flow pattern  200  of exhaust along the second exhaust path is illustrated between the upper steam guide  157  and the inner wall  144  of the bearing cone  145 , continuing over the inner wall  144 , and around and over the divider wall  165 . 
         [0035]    The radial channel may be formed with different shape and contouring of outer casing as shown in  FIGS. 5-6 . In a second embodiment of the present invention, the configuration of the radial channel is modified. The two descending exhaust spaces of the radial channel in fluid communication with the upper section of the radial channel and with the turbine condenser may include an exhaust space on each lateral side of the exhaust hood. The descending exhaust space on each respective lateral side may extend radially outboard relative to the exhaust hood in a path to the turbine condenser below. The descending exhaust space may further curve upstream axially such that it descends vertically alongside the outer radial casing of the exhaust hood in a vertical path to the turbine condenser below. Alternatively the vertically descending exhaust space may be enclosed in a separately enclosed volume that exhausts downward to the turbine condenser in a parallel path relative to the condenser flow from the lower section of the exhaust hood. 
         [0036]      FIG. 5  illustrates a three-dimensional side view of the exhaust arrangement structure  121  with the external casing of the exhaust hood removed. Steam exhausted from turbine inner casing  116  flows in the second exhaust path  190  between upper steam guide  157  and bearing cone  145  into upper exhaust section of exhaust hood  125 . Flow continues over divider wall  165  into the upper section  171  of radial channel  170  between divider wall  165  and end wail  172 . Flow continues downward through exhaust section  173  of radial channel  170  on way to condenser (not shown) below. 
         [0037]      FIG. 6  illustrates a three-dimensional end view of the exhaust arrangement structure  121  with a radial channel. The radial channel  170  includes an upper section  171  into which exhaust steam flow passing over divider wall  165  ( FIGS. 3 ,  4 ,  5 ) enters. Due to endwall  172 , the exhaust steam flow is forced downward into two descending exhaust spaces  173  on the way to the condenser below ( FIG. 3 ). The two descending exhaust spaces include an inner radial surface (wall)  175 . The two descending exhaust spaces  173  fold around rotor shaft  112  ( FIGS. 3 ,  4 ) and may allow a space  176  below the rotor shaft for personnel access to the bearing cone area. 
         [0038]      FIG. 7  illustrates an isometric three-dimensional sectional view of the exhaust arrangement structure  121  viewed from the turbine inner casing end. Exhaust flow paths are shown as dashed lines within the individual volumes. The first exhaust flow path  180  flows from the diffuser volume between the lower steam guide (not shown) and the bearing cone (not shown) to the lower exhaust volume. The second, exhaust path  190  flows from the diffuser volume between the upper steam guide (not shown) and the bearing cone (not shown) into the upper hood section  160 , then into the upper section  171  of the radial channel  170  and then into the descending exhaust section  173  (one shown) on the path to the turbine condenser below (not shown). 
         [0039]      FIG. 8  provides a cutaway side view of the second exhaust path in the second embodiment of the exhaust arrangement  205 . The second exhaust path from the upper half of inner casing outlet  216  flows between the steam guides  257  and an inner wall of the bearing cone  245  into the upper section of the second embodiment of exhaust hood  205 . The divider wall  265  extends in a radial direction from the bearing cone  245 . The second exhaust flow path  290  passes axially from the upper section  260  of the exhaust hood  205  to the radial channel  270  in the space between the divider wall  265  and the outer casing  225  of the exhaust hood. The second exhaust flow path  210  is forced to turn downward in the upper section  271  of the radial channel  270  by the endwall. A curved descending exhaust space  273  further directs the flow downward, upstream axially, and outboard relative to the exhaust hood outer casing. The second exhaust flow path  290  continues downward to the condenser in a flow parallel to the first exhaust path  280  from the lower section  255  of the exhaust hood. 
         [0040]      FIG. 9  illustrates an isometric view of one lateral side of the exhaust arrangement viewed from the turbine inner casing end. A first exhaust flowpath  280  from the lower half space of inner casing outlet flows between the steam guide  256  and an inner wall of the bearing cone (not shown) into the lower section of the exhaust hood and then downward to the turbine condenser. The second exhaust path  290  from the upper half of inner casing outlet  250  flows between the steam guide  257  and an inner wall of the bearing cone (not shown) into the upper section  260  of the exhaust hood. The second exhaust path  290  from the upper section  160  of the exhaust hood passes over the divider wall  265  into the radial channel  270  of the exhaust hood. The rear wall  272  of the downstream section forces the flow in a downward direction, passing into the curved descending exhaust space  273  which directs the flow outboard radially and upstream axially to a space  295  outboard of and parallel to the exhaust path from the lower section of the exhaust hood. The downward path may be in a same space or as space walled-off. 
         [0041]    While various embodiments are described herein, it will be appreciated from the specification that various combinations of elements, variations or improvements therein may be made, and are within the scope of the invention.