Abstract:
A shell structure for a turbine engine that includes a plurality of diaphragms, wherein each diaphragm comprises: an annularly formed support structure that positions and secures a row of circumferentially spaced stator blades, and wherein the plurality of diaphragms are configured and axially stacked such that each diaphragm abuts the diaphragm that is positioned directly upstream from it and the diaphragm that is positioned directly downstream from it such that a first shell structure is formed; a second shell structure disposed outboard of the first shell structure; and an intermediate chamber defined between an outboard face of the first shell structure and an inboard face of the second shell structure; wherein: a downstream end and an upstream end of the first shell structure define the axial length of the first shell structure; one or more downstream radial ports are defined at the approximate axial position of the downstream end of the first shell structure, each downstream radial port fluidly connecting the intermediate chamber to the main flow path of the turbine engine; and between the axial position of the downstream axial ports and the axial position of the upstream end of the first shell structure, the first shell structure is configured such that the intermediate chamber is substantially sealed from the main flow path.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present application relates generally to systems, and apparatus for improving the efficiency and operation of turbine engines, which, as used here and unless specifically stated otherwise, is meant to include all types of turbine or rotary engines, including steam turbine engines, combustion turbine engines, aircraft engines, power generation engines, and others. More specifically, but not by way of limitation, the present application relates to systems, and apparatus pertaining to turbine engine diaphragms and shells. 
         [0002]    Turbine engines, such as steam turbines or gas turbines, typically include multiple turbine stages with each stage including a pair of interspersed stator rings and rotors. The stator rings (diaphragms) include nozzles or stator airfoils, and the rotors include buckets or rotating airfoils. To rotate the rotor, the nozzles direct high pressure and high temperature fluid onto the buckets in a direction that causes the buckets to rotate with a speed proportional to the fluid pressure. 
         [0003]    To support the diaphragms and for maximum fluid utilization, the typical turbine engine includes one or more outer shells or casings. The outer shell isolates the working fluid from the ambient conditions outside the turbine engine, and also supports and provides alignment to the diaphragms. As the shell structures are constantly exposed to high pressures and temperatures, these structures are typically made from good quality and high-grade alloy metals that can withstand high pressures and temperatures; the higher the fluid&#39;s working pressure and temperature, the thicker the shell structure, and the better the metal quality. So, for very high pressure and temperature applications, turbine engines include three or more concentric shell structures. Each shell structure provides a layer of isolation from the temperature and pressure inboard of that shell, thereby splitting up the pressure and temperature change. As the inner most shell structure is subjected to the highest temperatures and pressures, this shell structure is the thickest and made from very expensive high-alloy steel; the outer shells, however, are exposed to intermediate pressures and temperatures, and therefore these structures are relatively thinner and made from lighter metals. 
         [0004]    To reduce costs, attempts have been made to remove one or more shell structures. One such attempt removes the inner shell, leaving only the outer shell both to support the diaphragm and to contain the pressure and temperature within the turbine engine. That structure, however, provides only limited utility in high temperature and pressure applications, as the outer shell cannot, by itself, contain the high fluid pressure and temperature. Further, as only one shell structure is utilized the shell is thicker as compared to the outer shells utilized in three shell structures. The shell is cast from very expensive high-alloy steel, which again results in very expensive designs. 
         [0005]    As a result, there remains a need for innovative approaches to more efficient and cost effective shell structures and diaphragms for turbine engines. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    The present application thus describes a shell structure for a turbine engine that includes a plurality of diaphragms, wherein each diaphragm comprises an annularly formed support structure that positions and secures a row of circumferentially spaced stator blades in a manner such that the stator blades deliver a working fluid flowing through a main flow path to a row of circumferentially spaced rotor blades in a manner consistent with efficient turbine engine operation, and wherein the plurality of diaphragms are configured and axially stacked such that each diaphragm abuts the diaphragm that is positioned directly upstream, if present, from and the diaphragm that is positioned directly downstream, if present, from it such that a first shell structure is formed; a second shell structure disposed outboard of the first shell structure, the second shell structure comprises a cylindrically formed rigid structure that is in proximity to and surrounds the first shell structure; and an intermediate chamber defined between an outboard face of the first shell structure and an inboard face of the second shell structure; wherein: a downstream end and an upstream end of the first shell structure define the axial length of the first shell structure; one or more downstream radial ports are defined at the approximate axial position of the downstream end of the first shell structure, each downstream radial port fluidly connecting the intermediate chamber to the main flow path of the turbine engine; and between the axial position of the downstream axial ports and the axial position of the upstream end of the first shell structure, the first shell structure is configured such that the intermediate chamber is substantially sealed from the main flow path. 
         [0007]    The application further describes a shell structure for a steam turbine, the shell structure comprising: a plurality of diaphragms, wherein each diaphragm comprises an annularly formed support structure that positions and secures a row of circumferentially spaced stator blades in a manner such that the stator blades deliver a working fluid flowing through a main flow path to a row of circumferentially spaced rotor blades in a manner consistent with efficient turbine engine operation, and wherein the plurality of diaphragms are configured and axially stacked such that each diaphragm abuts the diaphragm that is positioned directly upstream, if present, from and the diaphragm that is positioned directly downstream, if present, from it such that a first shell structure is formed; a second shell structure disposed outboard of the first shell structure, the second shell structure comprises a cylindrically formed rigid structure that is in proximity to and surrounds the first shell structure; and an intermediate chamber defined between an outboard face of the first shell structure and an inboard face of the second shell structure; wherein: a downstream end and an upstream end of the first shell structure define the axial length of the first shell structure; the intermediate chamber and the second shell structure are substantially sealed at the upstream end of the shell structure to isolate the shell structure from temperature and pressure conditions further upstream of the shell structure; and between the downstream end and the upstream end of the first shell structure, the first shell structure is configured such that the intermediate chamber is substantially sealed from the main flow path. 
         [0008]    These and other features of the present application will become apparent upon review of the following detailed description of the preferred embodiments when taken in conjunction with the drawings and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    These and other features of this application will be more completely understood and appreciated by careful study of the following more detailed description of exemplary embodiments of the invention taken in conjunction with the accompanying drawings, in which: 
           [0010]      FIG. 1  is an isometric view of a conventional diaphragm; 
           [0011]      FIG. 2  is a partial elevation cut-away view of an exemplary turbine assembly accordingly to some embodiments of the present invention; 
           [0012]      FIG. 3  is a partial elevation cut-away view of the exemplary turbine of  FIG. 2 , depicting annulus ventilation; 
           [0013]      FIG. 4  is a partial elevation cut-away view of the exemplary turbine of  FIG. 2 , depicting steam extraction according to embodiments of the present invention; and 
           [0014]      FIG. 5  is a flowchart illustrating an exemplary method for selecting a radial port for steam extraction. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    Embodiments of the present invention describe a turbine shell structure composed of a diaphragm shell and an outer shell. The diaphragm shell, which includes multiple axially abutting diaphragms, may be substantially sealed to prevent steam or fluid leakage from within the diaphragm shell structure, thereby eliminating the inner shell requirement. To this end, the diaphragm shell may include novel sealing devices that ensure sufficient isolation of the working fluid from the outer shell. Sealing reduces the outer shell thickness and facilitates usage of less expensive metals to fabricate the outer shells. Absence of an inner shell implies that the outer shell can be located closer to the diaphragms, which reduces the outer shell radius, and thereby decreases the amount of material employed to fabricate the outer shell. In this manner, low-cost and efficient turbine engines can be manufactured. 
         [0016]    In some embodiments, an annulus formed between the diaphragm shell and the outer shell may not be sealed at the downstream end; in such embodiments, fluid from downstream stages can cool this annulus. Cooling the annulus further decreases the outer shell&#39;s thickness requirement, as the outer shell is exposed to lower temperatures. Alternatively, for regenerative-Rankine cycle turbines, the annulus can be sealed and the fluid, such as steam or gas can be extracted from any turbine stage through the sealed annulus. These and other embodiments will be described in detail with reference to the Figures. 
         [0017]    To describe clearly the invention of the current application, it may be necessary to select terminology that refers to and describes certain machine components or parts of a turbine engine. Whenever possible, common industry terminology will be used and employed in a manner consistent with its accepted meaning. However, it is intended that any such terminology be given a broad meaning and not narrowly construed such that the meaning intended herein and the scope of the appended claims is unreasonably restricted. Those of ordinary skill in the art will appreciate that often certain components may be referred to with several different names. In addition, what may be described herein as a single part may include and be referenced in another context as consisting of several component parts, or, what may be described herein as including multiple component parts may be fashioned into and, in some cases, referred to as a single part. As such, in understanding the scope of the invention described herein, attention should not only be paid to the terminology and description provided, but also to the structure, configuration, function, and/or usage of the component as described herein. 
         [0018]    In addition, several descriptive terms may be used herein. The meaning for these terms shall include the following definitions. The term “rotor blade,” without further specificity, is a reference to the rotating blades of either the compressor or the turbine, which include both compressor rotor blades and turbine rotor blades. The term “stator blade,” without further specificity, is a reference the stationary blades of either the compressor or the turbine, which include both compressor stator blades and turbine stator blades. The term “blades” will be used herein to refer to either type of blade. Thus, without further specificity, the term “blades” is inclusive to all type of turbine blades, including compressor rotor blades, compressor stator blades, turbine rotor blades, and turbine stator blades. 
         [0019]    Further, as used herein, “downstream” and “upstream” are terms that indicate a direction relative to the flow of working fluid through the turbine. As such, the term “downstream” means the direction of the flow, and the term “upstream” means in the opposite direction of the flow through the turbine. Related to these terms, the terms “aft” and/or “trailing edge” refer to the downstream direction, the downstream end and/or in the direction of the downstream end of the component being described. In addition, the terms “forward” or “leading edge” refer to the upstream direction, the upstream end and/or in the direction of the upstream end of the component being described. The term “radial” refers to movement or position perpendicular to an axis. It is often required to describe parts that are at differing radial positions with regard to an axis. In this case, if a first component resides closer to the axis than a second component, it may be stated herein that the first component is “inboard” or “radially inward” of the second component. If, on the other hand, the first component resides further from the axis than the second component, it may be stated herein that the first component is “outboard” or “radially outward” of the second component. The term “axial” refers to movement or position parallel to an axis. Moreover, the term “circumferential” refers to movement or position around an axis. 
         [0020]    Referring now to the figures, it will be recognized by those in the art that the exemplary embodiments depicted and described herein all deal with steam turbines. It will also be recognized, however, that the exemplary embodiments are selected only to illustrate one application of the principles set out in the present disclosure. Those of skill in the art will be able to employ those principles across the range of various forms of turbine engines, as described above. As pointed out below, the invention set out in the present disclosure is set out solely in the claims, and none of the following discussion should be seen as limiting that scope. 
         [0021]      FIG. 1  is a perspective view of a conventional turbine engine diaphragm  100 , composed of a pair of semi-annular diaphragm ring segments  102 -A and  102 -B (hereafter referred to as diaphragm segments  102 ) joined at a horizontal split or joint surfaces (line AB). Each diaphragm segment  102  supports a semi-annular row of radially spaced airfoils  104  between a diaphragm ring  106  and diaphragm web  108 . The airfoils  104  increase the fluid velocity and alter the flow direction so that the fluid impinges on buckets, causing them to rotate. The diaphragm segments  102 -A and  102 -B are joined by suitable fasteners, such as with horizontal bolts  110 . Typically, two horizontal bolts  110  are used—one for each diaphragm segment joint surface. Crush pins  112 -A,  112 -B, and  112 -C (hereafter referred to as crush pins  112 ) along the diaphragm circumference form a close tolerance fit to prevent the diaphragm  100  from rattling. Typically, three crush pins  112  are employed, one at each diaphragm segment joint surface, and one at the diaphragm bottom. Other fastening devices known to those in the art may be employed to meet the requirement of particular implementations of the disclosure. It will be understood that the number of bolts and pins, and the type of bolts and pins utilized to join the diaphragm segments  102  may vary depending on the type of turbine engine. 
         [0022]    During assembly, diaphragms, such as the diaphragm  100 , are fixed into shells or casings (not illustrated). Centering pins  114  and support bars  116  facilitate this mounting. Centering pins  114 , typically located at the diaphragm bottom, transversely align the diaphragm  100  in the shell, and support bars  116 , typically present on the outboard face of the diaphragm  100 , hold the diaphragm  100  within the shell/casing. These support bars  116  engage similar extensions present on the inboard face of the shell, locking the diaphragm  100  in place and providing vertical support and alignment. Further, a lifting provision  118  is provided on the diaphragm  100  to lift the diaphragm  100  in and out of the outer shell. 
         [0023]      FIG. 2  is a partial elevation cut-away view of a turbine assembly, schematically illustrating an exemplary turbine shell structure  200 .  FIGS. 2-4  depict a turbine assembly of a multiple stage high-pressure steam turbine, with the arrow illustrating the steam flow (downstream) direction. It will be appreciated, however, that the turbine shell structure  200  can be employed in various applications, such as gas turbines, nuclear turbines, fossil fuel turbines, high-pressure turbines, intermediate pressure turbine stages, or low-pressure turbine stages, without departing from the scope of the invention. A high-pressure steam turbine stage is illustrated here merely as an example to describe the various parts of the turbine shell structure  200  and not to dictate the scope of the present invention. 
         [0024]    The turbine shell structure  200  includes a diaphragm shell  202 , an outer shell  204  disposed outboard of the diaphragm shell  202 , and an intermediate chamber  206  (interchangeably referred to as an annulus  206 ) formed between the outboard side of the diaphragm shell  202  and the inboard side of the outer shell  204 . The outer shell  204  and the annulus  206  may extend approximately the full axial length of the diaphragm shell  202 . 
         [0025]    The outer shell  204  may be a rigid structure in proximity to and surrounding the diaphragm shell  202 . Moreover, the outer shell  204  may make contact with the diaphragm shell  202  along at least a portion of the outboard face of the diaphragm shell  202 . The outer shell  204  contains the high pressure and high temperature steam within the turbine. Therefore, the outer shell&#39;s diameter and thickness vary depending on the application; for example, in low-pressure conditions, the outer shell&#39;s thickness is relatively less than the thickness required for higher-pressure situations. Further, the outer shell  204  may be integrally formed of a metal. The material used varies depending on the turbine application, the pressure and temperature conditions in the turbine, and so on. Typical materials include carbon steel, chromium-molybdenum alloy steel, chromium-molybdenum-vanadium alloy steel and high chromium content steel. 
         [0026]    The annulus  206  includes a relatively narrow hollow space that may vary in radial width along the axial length of the diaphragm shell  202 . In addition, outboard of the downstream diaphragm shell end, the annulus  206  may be in fluid communication with the annulus  206  outboard of the upstream diaphragm shell end. 
         [0027]    The diaphragm shell  202  can include a number of multiple axially-abutting diaphragms, such as diaphragms  208 -A,  208 -B,  208 -C, and  208 -D (hereafter collectively referred to as diaphragms  208 ). For simplicity,  FIG. 2  depicts four axially stacked diaphragms  208 , it will be understood, however, that the number of diaphragms  208  may vary. Some implements may provide a structure incorporating three diaphragms  208 ; alternatively, the diaphragm shell  202  can include more than four diaphragms  208 , and that number can be increase six or more diaphragms  208  in appropriate installations. Based on the disclosure set out herein, those in the art will be able to adapt the number of diaphragms to the needs of particular implementations. Depending on the application, the stages required, or the turbine type, the number of axially stacked diaphragms  208  in the diaphragm shell  202  can vary. Further, one or more diaphragm shells  202  can be axially disposed within the outer shell  204 , depending on the number of stages required. 
         [0028]    Each diaphragm  208  may include a diaphragm ring  210 , a diaphragm web  212 , and multiple circumferentially spaced nozzles or stator blades  214  supported between the diaphragm ring  210  and the diaphragm web  212 . The abutting diaphragms  208  substantially seal the main flow path by forming a shell structure that may perform the function of a conventional inner shell. 
         [0029]    Here, the diaphragms  208  are mechanically coupled. Other designs can achieve the functional equivalent of that arrangement by suitable coupling mechanisms. In one embodiment, diaphragms  208  may be coupled in pairs, for example, so that the first upstream diaphragm  208 -A can be mechanically coupled to the adjacent downstream diaphragm  208 -B. This diaphragm  208 -B, in turn, can be mechanically coupled to the next downstream diaphragm  208 -C, with this process repeating until all the diaphragms  208  are coupled. In an alternate embodiment, axial holes may be drilled through the diaphragms  208  and one or more rods may bolt all the diaphragms  208 . For coupling pairs of diaphragm rings  210 , any mechanical fitting can be employed. These fittings are widely known in the art and it will be understood that the fitting utilized do not dictate the scope of the present invention. 
         [0030]    One exemplary coupling method is shown in  FIG. 2 . Here, rabbet joints  216  and stacking bolts  218  combine to join the diaphragm rings  210 . Abutting diaphragm edges are structured to form the rabbet joint  216 , with one diaphragm  208  including a groove and the other including an extension, which fit together. For example, diaphragm  208 -A can include a groove while diaphragm  208 -B can include an extension. A hole may be drilled through this rabbet joint  216 , and the stacking bolt  218  may extend through the hole, sealing the rabbet joint  216 . Alternatively, a hole can be drilled through the entire stack of diaphragms  208 , and a single stacking bolt, such as the stacking bolt  218 , can secure all the diaphragms  208 . Each diaphragm pair further includes radial dowels  220 ; depending on the application, the number of radial dowels  220  can vary. In one embodiment, each diaphragm pair can include six radial dowels  220 , three on the diaphragm  208  top half, and three on the bottom half. The radial dowels  220  provide circumferential diaphragm alignment and transfer the reactionary torque from the diaphragms  208  to the outer shell  204 . Other embodiments can include more or fewer radial dowels  220 , as known in the art. These radial dowels  220  are typically pins or metal blocks that fit into holes in the abutting portions of two diaphragms  208 , partly in one diaphragm  208  and partly in the other, to align them with respect to each other. 
         [0031]    The mechanically coupled diaphragms  208  may be substantially sealed to eliminate or reduce steam leakage from the main flow path into the annulus  206 . Various sealing mechanisms may be employed to achieve this result; examples can include a barrier fit  222 , one or more sealing key(s)  224 , one or more horizontal bolt(s)  226 , one or more axial support(s)  228 , or an axial seal  230 . The upstream end of the turbine shell structure  200  may be axially disposed adjacent the trailing edge of the turbine&#39;s first stage (not illustrated), which operates at the highest pressure and highest temperature in the steam turbine. The barrier fit  222  may circumferentially fit around the annulus  206  and the outer shell  204  at this upstream end to isolate the high pressure and high temperature steam upstream of the barrier fit  222 , so that ambient conditions downstream of the barrier fit  222  can be controlled as desired. Controlling these conditions ensures that the outer shell  204  is not exposed to the relatively high first stage pressures and temperatures. The barrier fit  222  can be formed using a flexible, heat and pressure tolerant material to accommodate the differential axial expansions between the first stage upstream of the barrier fit  222  and the outer shell  204 . Among the flexible materials employed to fabricate the barrier fit  222  are, carbon steel, chromium-molybdenum alloy steel, chromium-molybdenum-vanadium alloy steel, high chromium alloy steel, and nickel alloys. 
         [0032]    The sealing key  224  can extend along each diaphragm&#39;s horizontal joint, thereby eliminating or minimizing leakage paths along mating faces of diaphragm segments. In one embodiment, the diaphragms  208  may be closely stacked so that the sealing keys  224  from each diaphragm  208  can be located as close to each other as possible. This arrangement provides effective sealing along the horizontal joints. Alternatively, the sealing key  224  can form a gapless seal along the horizontal joint of the diaphragms  208 . To this end, retainer keys (not illustrated) may cover the gaps between the diaphragms  208 , and the sealing key  224  can extend all along the horizontal joints. Sealing keys are adequately known in the art, and therefore, they will not be described in detail here. 
         [0033]    The horizontal bolts  226 , which may be provided along the diaphragm&#39;s horizontal joint, may also minimize leakage. Conventionally, one horizontal bolt is employed per diaphragm  208 . The embodiments of the present invention, however, may utilize multiple horizontal bolts  226  per diaphragm  208 . In one embodiment, the horizontal joints may be sealed using as many horizontal bolts  226  as can axially fit along the diaphragm&#39;s horizontal joint. Alternatively, a specified number of horizontal bolts  226  can be employed per diaphragm  208 . Sealing keys  224  and horizontal bolts  226 , collectively, eliminate or substantially reduce leakage into the annulus  206  through the horizontal joint. 
         [0034]    In addition to horizontal seals (such as the sealing keys  224  and the horizontal bolts  226 ), the axial support  228  or the axial seal  230  may be employed to seal the annulus  206 . Axial supports are conventionally employed in steam turbines and can be employed as well in other turbine applications. These supports provide fixtures to axially support the diaphragms  208 . The axial support  228  provides a locational fit around the last downstream diaphragm, diaphragm  208 -D, and does not extend circumferentially around the diaphragm  208 -D; as a result, this support may be useful in certain single-flow engines in which steam is allowed to flow in the annulus  206 . Alternatively, the axial seal  230  may be employed to seal the annulus  206  between the diaphragm shell&#39;s downstream end and the outer shell  204 . The axial seal  230  is a circumferential seal that substantially seals the annulus  206 . It will be understood that various other known seals can be employed to seal the annulus  206  without departing from the scope of the present invention. 
         [0035]    The sealed diaphragm shell  202  restricts the high pressure and high temperature working steam within the steam flow path, thereby eliminating the need for an inner shell. Further, the axial seal  230  offers the advantage of sealing the annulus  206 , thus controlling the temperature and pressure within the annulus  206 . This measure allows fabrication of thinner and less costly outer shells, which in turn can reduce the turbine&#39;s cost considerably. 
         [0036]    The diaphragm shell  202  may further include conventional bolts and supports to retain the diaphragms  208  in position within grooves in the outer shell  204 . For example, in one embodiment, vertical supports  232  (support bars  116  of  FIG. 1 ) may be provided along the circumference of the diaphragm rings  210 . Each diaphragm ring  210  can include two vertical supports  232  (only one vertical support  232  is illustrated) to support the diaphragm shell  202  in the outer shell  204 . In another embodiment, alternate diaphragm rings  210  may include vertical supports  232 . Crush pins  234  may be employed, as well, to fix the diaphragm shell  202  and to preclude the diaphragms  208  from rattling. In one embodiment, three crush pins  234  are employed per diaphragm  208 ; these crush pins  234  are utilized in the diaphragm&#39;s lower half. In addition, one or more centering pins  236  may be circumferentially disposed at the bottom end of the diaphragms  208 . As depicted in the figure, two centering pins  236 —one at the bottom end of diaphragm  208 -D and the other at the bottom end of diaphragm  208 -B can be used. It will be understood, however, that the centering pins  236  may be disposed at the top end of the diaphragms instead, or that the centering pins  236  may be disposed on the diaphragms  208 -A and  208 -C or any other combination of diaphragms  208  without departing from the scope of the present invention. These centering pins  236  are relatively larger than conventional centering pins  114  and provide transverse alignment. It will be understood that other conventional bolts, fits, supports, or nuts may be employed to fix, support, align, or seal the diaphragm shell  202  and the outer shell  204 . These tools are widely known in the art and require no further explanation here. 
         [0037]    In certain extremely high-pressure situations, the turbine shell structure  200  may include a second outer shell (not illustrated) outboard of the outer shell  204  to split the pressure and temperature change. The second outer shell can have a cylindrical rigid structure in proximity to and surrounding the outer shell  204 . It will be appreciated that the second outer shell&#39;s thickness and composition may vary depending on the pressure and temperature of the desired application. 
         [0038]    The turbine shell structure  200  can be employed in high pressure and temperature applications ranging from about 1800 Psi to about 4500 Psi and 900° F. to about 1150° F., and preferably in applications ranging from about 2400 Psi to about 3500 Psi and 1000° F. to about 1100° F. In very high pressure and temperature applications, 3-wall structures are typically implemented, which include concentric rings of an outer shell, an inner shell, and a nozzle box. By implementing at least some of the teachings of the present invention, the inner shell can be eliminated, and the outer shell  204  can be disposed circumferentially closer to the diaphragm shell  202  as compared to the conventional 3-wall structures. This assembly produces a low-cost turbine structure, because the inner shell is eliminated and the outer shell  204  diameter is decreased (which reduces the material requirement). Further, as the outer shell  204  is subjected to controlled temperature and pressure conditions, less expensive materials can be employed for fabricating that component. 
         [0039]    In relatively lower pressure and temperature applications, one-wall structures are typically implemented, which include only an outer shell. In these applications, a conventional outer shell is subjected to the stage pressure and temperature. Therefore, the outer shell thickness is governed by stage conditions, which implies that the outer shell is thicker in the upstream stages than in the downstream stages. The sealed diaphragm shell  202  according to some embodiments of the present invention, however, ensures either that the outer shell  204  is substantially shielded from any stage conditions (when the axial seal  230  is used), or exposed to only the relatively low final stage conditions (when the axial support  228  is used). This property of the diaphragm shell  202  facilitates reduction in outer shell thickness and utilization of less expensive material to manufacture the outer shell  204 , thereby reducing turbine costs. 
         [0040]      FIG. 3  illustrates another embodiment of the present invention, in a partial elevation cut-away view of the turbine shell structure  200  illustrating annulus pressurization &amp; ventilation. Pressurization relates to tailoring the pressure in the annulus to minimize pressure drop across the membranes, while ventilation relates to selecting the “optimum” annulus temperature depending on the membrane material utilized. In this embodiment, the axial support  228  is utilized instead of the axial seal  230 . As a result, the annulus  206  is not sealed, but rather is exposed to the final stage&#39;s pressure. Lower temperature and pressure steam from the main flow path may leak into the annulus  206  from the final downstream stage (diaphragm  208 -D), and this steam cools the annulus  206  from the downstream end to the upstream end, adjacent to the barrier fit  222 . From the upstream end, the steam may exit the outer shell  204  through external pipes  302 , and these pipes can reinsert the steam at any stage of the diaphragm shell  202 . Alternatively, instead of reinserting the steam, the external pipes  302  may direct steam from the upstream end to a condenser, a feed water heater, or any similar external device known in the art. 
         [0041]    In other embodiments, steam from any intermediate stage may be utilized, instead, to cool the annulus  206 . Each turbine stage operates at different ambient temperatures and pressures, and generally, the steam pressure and temperature both drop as the steam travels downstream. Depending on the required pressure drop, an appropriate stage can be selected to cool or pressurize the annulus  206 . To this end, the axial seal  230  can be utilized instead of the axial support  228 , and the entire annulus  206  can be sealed. Then a radial port  304  may be drilled in the appropriate stage, and steam from that stage can be utilized to cool or pressurize the annulus  206 . Again, external pipes  302  are utilized to circulate the steam from the annulus  206  back into the diaphragm shell  202 . The external pipe  302  may include a valve  306  that re-inserts steam at a required pressure in the diaphragm shell  202 . Depending on the pressure and the temperature of the circulated steam, radial ports  304  are drilled for re-inserting the steam at the appropriate stage. For example, but not by way of limitation, if the working steam from stage  3  cools the annulus  206 , then after circulation, the steam can re-enter the main flowpath either through stages  4  or  5 , depending on the temperature and pressure of the re-inserted steam. 
         [0042]    Because steam is extracted from a relatively cool stage, the ventilation cools the outer shell  204  to the temperature of that stage. That cooling action permits reduction of the outer shell  204  wall thickness, bolting, and shell requirements, reducing the outer shell&#39;s cost. 
         [0043]      FIG. 4  illustrates another embodiment of the turbine shell structure  200 . The figure is a partial elevation cut-away view schematically illustrating an extraction arrangement to extract steam from the diaphragm shell  202 . In certain applications, such as extraction turbines and regenerative-Rankine turbines, steam is withdrawn from one or more stages, at one or more pressures, for various applications, such as heating, plant process, or feed-water heater needs. 
         [0044]    This disclosure describes two steam extraction embodiments. In one embodiment, steam may be extracted from any stage marked  2 ,  3 , or  4 ; and in the second embodiment, steam can be extracted from the stage marked  5 . In the first embodiment, the axial seal  230  seals the annulus  206 . Depending on the required temperature and pressure, radial ports  304  can be drilled in the direction of the arrows, which permit steam to enter the annulus  206  from that stage. An extraction connection  402  draws the steam from the annulus  206  and provides the steam to feed-water heaters (not shown). The extraction connection  402  can include typical extraction ports, pipes, or tubes, which extend from the annulus  206  to the outboard face of the outer shell  204  and from the outboard face of the outer shell  204  to an extraction assembly. A typical extraction assembly may be employed beyond the extraction connection  402 . 
         [0045]    For the second embodiment, the axial support  228  is utilized. Here, steam from the final downstream stage enters the annulus  206 . The extraction connection  402  extracts this steam from the annulus  206  through the outer shell  204 . Using the turbine shell structure  200  described in this disclosure, steam can be extracted from any stage by easily varying the annulus seal from the axial seal  230  to the axial support  228 . 
         [0046]      FIG. 5  is a flowchart illustrating an exemplary method for selecting an appropriate radial port for steam extraction. The method may include determining the required steam pressure and temperature for extraction at step  502 . Here, based on the application, the extraction pressure and temperature required may vary. For example, for feed water heating applications, higher temperature and pressure steam may be used, while for cooling applications, it may be more desirable to use lower temperature and pressure steam. 
         [0047]    At step  504 , the last downstream diaphragm  208 -D may be selected. The next step, step  506 , may compare the stage pressure with the extraction pressure to determine the most appropriate diaphragm stage. It will be understood that to extract steam from the turbine shell structure  200  the stage pressure should be greater than the extraction pressure. Further, it will be understood that the turbine shell structure  200  can also facilitate steam admission if the pressure of the steam is less than the admission pressure. The comparison starts from the last downstream diaphragm  208 -D. If the pressure at this diaphragm  208 -D is lesser than the extraction pressure required, the method  500  can move to the next upstream diaphragm (diaphragm  208 -C) and compare that stage&#39;s pressure with the extraction pressure. This process continues until the method  500  determines upon comparison that a certain stage&#39;s pressure is greater than the extraction pressure required. 
         [0048]    Step  508  may compare extraction temperatures with the temperatures at each stage to determine the most appropriate stage. For feed water heating application, the stage is selected such that the stage temperature is greater than the extraction temperature; while, for cooling applications, the stage is selected such that the stage temperature is less than the extraction temperature. In one embodiment, (in a feed water heating application) the method  500  starts comparing from the diaphragm  208  selected by step  504 . If that diaphragm stage&#39;s temperature is greater than the extraction temperature required, the method  500  proceeds to step  510 . Alternatively, the method  500  returns to step  506  and determines the pressure and temperature of the next upstream diaphragm  208 . This process continues, until the method determines the most appropriate stage. 
         [0049]    At step  510 , a determination may be made whether the most appropriate diaphragm  208  is the last downstream diaphragm  208 -D. If the most appropriate stage is the last downstream diaphragm  208 -D, the process may move to step  512  and the axial support  228  can be utilized, which allows steam from the final stage to enter the annulus  206 . The extraction connection  402  delivers the final stage steam to an external device, such as the feed-water heater (not shown). If, on the other hand, any other stage is selected as the most appropriate stage, the method  500  may proceed to step  514 , and the axial seal  230  can be utilized. At this step, the annulus  206  is substantially sealed and a radial port, such as radial port  304  may be drilled in the most appropriate diaphragm stage. Steam from this stage is allowed to escape into the annulus  206  and exit the steam turbine through the extraction connection  402 . 
         [0050]    As one of ordinary skill in the art will appreciate, the many varying features and configurations described above in relation to the several exemplary embodiments may be further selectively applied to form the other possible embodiments of the present invention. For the sake of brevity and taking into account the abilities of one of ordinary skill in the art, all of the possible iterations are not provided or discussed in detail, though all combinations and possible embodiments embraced by the several claims below or otherwise are intended to be part of the instant application. In addition, as indicated by the description above, setting out several exemplary embodiments of the invention, those skilled in the art will perceive improvements, changes, and modifications. Such improvements, changes, and modifications within the skill of the art are also intended to be covered by the appended claims. Further, it should be apparent that the foregoing relates only to the described embodiments of the present application and that numerous changes and modifications may be made herein without departing from the spirit and scope of the application as defined by the following claims and the equivalents thereof.