Patent Publication Number: US-6984106-B2

Title: Resilent seal on leading edge of turbine inner shroud

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to gas turbines, and, in particular, to a resilient seal for reducing air leakage and improving turbine engine efficiency. 
   In industrial gas turbines, shroud segments are fixed to turbine shell hooks in an annular array about the turbine rotor axis to form an annular shroud radially outwardly and adjacent to the tips of buckets forming part of the turbine rotor. The inner wall of the shroud defines part of the gas path. Conventionally, the shroud segments are comprised of inner and outer shrouds provided with complimentary hooks and grooves adjacent to their leading and trailing edges for joining the inner and outer shrouds to one another. The outer shroud is, in turn, secured to the turbine shell or casing hooks. Typically, each shroud segment has one outer shroud and two or three inner shrouds. 
   Two common designs have been used for configuring inner shrouds, i.e., an opposite hook design and a C-clip design. The opposite hook design is the more traditional approach and incorporates oppositely projecting hooks on the leading and trailing edges that are retained by the outer shroud. 
   The C-clip design is schematically illustrated in  FIG. 1 . As can be seen, like the traditional opposite hook design, the C-clip design also includes leading and trailing edge hooks  10 ,  12  projecting in opposite directions. However, in the C-clip design, the trailing edge hook  12  is retained with a separate C-clip  14 , rather than being retained by the outer shroud  16 , as in the opposite hook design. 
   Traditional inner shroud designs use a sealing scheme around the leading edge hook of the inner shroud. This scheme typically consists of an axial chording gap and a cloth seal segment gap for leakage control around the leading edge hooks. In the chording gap, there is a surface-to-surface gap between parts of the inner shroud and the outer shroud of the turbine. The chording gap is related to thermal chording which forms a gap between mating parts at an elevated temperature. The resulting equivalent gap is generally on the order of five to ten mils. Thus, the chording gap allows a significant amount of air to leak out from between the inner and outer shrouds into the hot gas path of the turbine, which reduces the operating efficiency of the turbine. 
   The cloth seal segment gap depends on the thermal growth or expansion of the inner shroud due to heating and manufacturing process capabilities. Here again, however, the cloth seal segment gap also allows air to leak out into the gas path of the turbine, again reducing the operating efficiency of the turbine. 
   A third inner shroud design, which is disclosed in U.S. patent application Ser. No. 10/348,010, filed Jan. 22, 2003, the contents of which are incorporated herein by reference, modifies the traditional stage one inner shroud to reverse the leading edge hooks, as compared to the traditional opposite hook design and the C-clip design. This reverse hook design also allows the use of a resilient seal on the leading edge hook of the inner shroud to improve turbine engine efficiency by reducing air leakage from between the inner and outer shrouds. 
   BRIEF DESCRIPTION OF THE INVENTION 
   In an exemplary embodiment of the invention, a sealing arrangement for a stator shroud of a multi-stage gas turbine comprises at least one shroud segment having a leading edge and a trailing edge, each shroud segment comprising an outer shroud and at least one inner shroud connected thereto, the outer shroud having grooves defined adjacent to and along the leading and trailing edges, the at least one inner shroud having a leading edge axially projecting tab portion and a trailing edge axially projecting tab portion for respectively engaging the grooves of the outer shroud, the engagement connecting the inner shroud to the outer shroud, and a resilient seal located on the leading edge axially projecting tab portion of the at least one inner shroud so as to be between the leading edge axially projecting tab portion and a retaining ring that contributes to holding the inner shroud in place. The resilient seal is preferably W-shaped and made from a nickel-based alloy. 
   In another exemplary embodiment of the invention, a sealing arrangement for a stator shroud segment comprises an outer shroud having a leading edge and a trailing edge, the outer shroud comprising a leading edge hook and a trailing edge hook, both the hooks of the outer shroud projecting in a first axial direction, a plurality of inner shrouds each having a leading edge and a trailing edge, each of the inner shrouds comprising a leading edge hook and a trailing edge hook, both the hooks of the inner shroud projecting in a second, axial direction, diametrically opposite the first axial direction, the leading and trailing hooks of each the inner shroud being respectively engaged with the leading and trailing hooks of the outer shroud, the engagement connecting the inner shroud to the outer shroud, and a resilient seal located on a leading edge of the leading hook of the inner shroud so as to be between the leading hook of the inner shroud and a retaining ring that contributes to holding the inner shroud in place. The resilient seal is preferably W-shaped and made from a nickel-based alloy, such as a product named “Waspaloy”. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of this invention will be more completely understood and appreciated by careful study of the following more detailed description of the presently preferred exemplary embodiments of the invention taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a schematic shroud segment circumferential end view of the inner shroud and a circumferential section view of the outer shroud, the schematic showing a conventional C-clip inner shroud retention design; and 
       FIG. 2  is a schematic circumferential end view of a shroud segment including an inner shroud with a reverse leading edge hook and the resilient seal of the present invention on the reverse leading edge hook. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   As mentioned above,  FIG. 1  schematically illustrates a conventional C-clip design for an inner shroud  18 . As shown in  FIG. 1 , the inner shroud  18  includes an inner shroud leading edge hook  10  and an inner shroud trailing edge hook  12  for engagement with corresponding leading and trailing edge hooks  20 ,  22  of an outer shroud  16 . The inner shroud trailing edge hook  12  is secured to the trailing edge hook  22  of the outer shroud  16  with a separate C-clip  14 , rather than being maintained in place by outer shroud  16 , as in the traditional opposite hook design. However, like the traditional opposite hook design, the C-clip design includes an axial chording gap  19  and a cloth seal segment gap  21 , both at the inner shroud leading edge hook  10 . 
   Referring to  FIG. 2 , there is illustrated a shroud segment, generally designated  100 , comprised of an outer shroud  116  and a plurality of inner shrouds  118 . Although the illustrated shroud segment  100  would typically include two or three inner shrouds  118 , only one inner shroud  118  is shown in  FIG. 2  for purposes of clarity. As described in greater detail below, the inner shrouds  118  have hooks  110  and  112  adjacent to their leading and trailing edges, respectively, for circumferentially and axially slidable engagement, in final assembly, in grooves  126  and  128  defined by hooks  120 , 122  of the outer shroud  116 . In the illustrated embodiment, an impingement cooling plate  124  is mounted between the shrouds for impingement cooling of the inner wall surfaces of the inner shroud segment  118 , in a conventional manner. 
   In the illustrated embodiment, the outer shroud  116  has a radially outer dovetail  130  for engagement in a dovetail groove  132  defined by leading and trailing hooks  134 , 136  forming part of the fixed turbine shell or casing for securing the shroud segment to the casing. It will be appreciated that an annular array of shroud segments  100  are formed about the rotor of the gas turbine and about the tips of the buckets on the rotor, thereby defining an outer wall or boundary for the hot gas flowing through the hot gas path of the turbine. In  FIG. 2 , the inner shroud seal slots  170 , the stage one nozzle structure  172 , stage one bucket  174  and stage two nozzle structure  176  are shown for completeness and reference. 
   With reference to  FIG. 2 , which is a detailed circumferential end view of a shroud segment  100  showing mating parts, it can be seen that a reverse hook shroud configuration is provided to engage and hold the inner shrouds  118  to the outer shroud  116 . The outer shroud  116  is engaged by leading and trailing casing hooks  134 , 136 , as described above, and an outer shroud anti-rotation pin  138  is provided to extend into a corresponding slot  140  to circumferentially lock the outer shroud  116  with respect to the casing  142 . In the illustrated embodiment, outer shroud seal slots  144  are shown as are air metering holes  146  and impingement plate  124 . At the leading edge of the outer shroud, inner shroud anti-rotation pin bores  148  are further provided to align with corresponding holes  150  and to receive inner shroud anti-rotation pins  152 . 
   As further illustrated in  FIG. 2 , the leading edge hook  120  of the outer shroud  116  is reversed so as to include a tab portion  154  projecting axially upstream, away from the trailing edge. The trailing edge hook  122  of the outer shroud  116  also includes a tab portion  156  that projects axially upstream, toward the leading edge, in the same direction as the tab portion  154  of the leading edge hook  120 . Thus, the grooves  126  and  128  of the outer shroud  116  both open axially in the upstream direction. 
   The hooks  110  and  112  of the inner shroud  118  are engaged with the leading and trailing edge hooks  120 ,  122 , and in particular with the grooves  126 ,  128  of the outer shroud  116 . More particularly, in the illustrated embodiment, the leading edge hook  110  of the inner shroud comprises a tab portion  158  that projects axially downstream, towards the trailing edge, so as to axially and radially engage the hook  120  of the outer shroud  116 , to axially and radially lock the outer and inner shrouds. A receptacle or hole  150  is defined in the leading edge hook of the inner shroud for receiving the inner shroud anti-rotation pin  152  inserted through the corresponding bore  148  defined in the outer shroud leading edge portion. 
   The trailing edge hook  112  of the inner shroud similarly includes a tab portion  160  extending axially downstream, towards the trailing edge, in the same direction as the leading edge tab portion  158  to axially and radially lock with the trailing edge hook  122  of the outer shroud. 
   According to the present invention, the air leaking out through the chordal gap between the outer shroud  116  and the inner shroud  118  is substantially reduced by the addition of a resilient seal  181  that is positioned between the leading edge hook  110  of inner shroud  118  and a retaining ring  178  that contributes to holding inner shroud  118  in place. Preferably, seal  181  is shaped like a “W” or “E”, the bellows of an accordion, the Greek letter “Ω”, or any other shape that allows seal  181  to be “springy” or compressible. Seals of this type are made by a number of companies that include the Fluid Sciences business unit of PerkinElmer, Inc. and Advanced Products Company. The use of resilient seal  181  results in a gap on the order of 1 mil (plus segment gaps), which significantly reduces the amount of air flow that leaks from between the leading edge hook  110  and the leading edge groove  126  of shrouds  118 ,  116 , respectively, into the hot gas path of the turbine. Thus, the resilient seal of the present invention is effectively the limiting element of the leakage flow path, providing up to an 80% reduction in this component of the leakage flow over the traditional chording gap arrangement. Resilient seal  181  reduces the amount of air leakage so that more air will pass through the turbine and be available for useful work and cooling, rather than being just wasted energy. This results in a higher operating efficiency for the turbine. The use of resilient seal  181  causes most of the air leakage past seal  180  to be routed into a cavity below plate  124  and reduces leakage out of such cavity below plate  124 . 
   The reversed hook inner shroud design shown in  FIG. 2  includes an axial chording gap  182  between the leading edge hook  110  and the leading edge groove  126  of shrouds  118  and  116 , respectively, and a cloth seal segment gap  183 , also shown in  FIG. 2 . However, because resilient seal  181  is located at the leading edge hook  110  of inner shroud  118  so as to be between leading edge hook  110  and retaining ring  178  that contributes to holding inner shroud  118  in place, seal  181  substantially blocks the air that leaks through chording gap  182 . It should also be noted that seal  181  can be made from a single piece of material or a plurality of pieces of material for all of the inner shrouds  118  positioned in the annular array of shroud segments about the turbine rotor axis. Preferably, seal  181  is made from two pieces of material that each extend half way around the array. 
   The material from which seal  181  is made is preferably a metal alloy that can withstand the temperatures that are seen at the location of seal  181 . When such temperatures range between 1200 to 1300° F., preferably, this metal alloy is a product named “Waspaloy”, a nickel-based alloy. For lower temperatures, preferably seal  181  is made from “Inconel 718”, another nickel-based alloy. It should be noted that “Waspaloy” and “Inconel 718” are made by many companies, such as, for example, Principal Metals and Diversified Metals, Inc. Seal  181  is resilient, even though it is made from a metal-based material, because it is made in a springy or compressible shape, and it is made using a very thin material. 
   While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.