Patent Publication Number: US-2007107218-A1

Title: Formed tenons for gas turbine stator vanes

Description:
BACKGROUND OF THE INVENTION  
      This invention relates generally to gas engine technology and, more specifically, to the installation of fixed shrouds on stator vanes.  
      Installing shrouds on stator vanes is a well known method for tuning the airfoil modal response due to aerodynamic flow excitation. More specifically, rotationally fixed stator vanes are often shrouded at the aft end of a gas turbine compressor where flow can be multidirectional and cause unshrouded vanes to suffer from aerodynamic stimuli. A shroud or cover ties sets of vanes together and generally results in a more robust design to address this aerodynamic stimulus issue.  
      Current industrial gas turbine compressor stator shrouds are welded or bolted to the stator vanes. To date, the manufacturing method of choice for fixed end boundary conditions is welding or casting, and for torsional free end boundary conditions, bolting and swaging.  
      Welding typical gas turbine compressor materials requires preheat treatment, postheat treatment, and final machining of the assembly to remove heat induced distortion. Accordingly, the skill level required to fabricate a typical welded shroud assembly is high.  
      The bolting methodology requires a minimum vane thickness to allow a tapped hole to be installed in a tenon by, for example, drilling, in the top of the vane. Positioning the vane during the drilling operation is critical to avoid breakout of the tenon wall. The bolt size is limited by the thickness of the vane and, on smaller gas turbines, the vane may not be thick enough to support a tenon with a tapped hole without a change to the airfoil design. With this technique, extra parts are introduced into the assembly such as bolts, washers, and bushings. Moreover, a bolted assembly can be over-torqued and the bolt can be subject to high preloading, potentially leading to premature failure in service.  
      Casting has also been used over the years but can be expensive, and long time intervals are required to develop the casting molds. In addition, some typical compressor stator vane materials cannot be readily cast, resulting in a high rejection rate.  
      Accordingly, there remains a need for a simplified manufacturing process for attaching tip shrouds to stationary gas turbine stator vanes resulting in lower cost and a shorter delivery cycle as compared to current methods of attachment.  
     BRIEF DESCRIPTION OF THE INVENTION  
      In an exemplary embodiment, this invention replaces the conventional welded or bolted attachment between shroud covers and stationary gas turbine stator vanes with a formed attachment. More specifically, in the exemplary embodiment, the radially inner tip of the stator vane is formed with a pair of radially inwardly extending tenons. The shroud cover is formed with a pair of corresponding holes adapted to receive a respective pair of tenons on each stator vane engaged by the shroud cover. The tenons are then formed by otherwise conventional tooling to secure the shroud to the set of vanes. By taking an otherwise common manufacturing process and applying it in an area where it has not previously utilized, several advantages are gained. For example, tenon forming can be accomplished with little or no change to the existing air foil shape; preheat treatment is eliminated; formed tenons can be tailored to provide frequency tuning for new designs; and the cost to form the tenons is an order of magnitude less expensive than welding. In addition, cycle time is greatly reduced over welding, and no machining is required after assembly due to process distortion. Tenon size in a formed configuration is also generally smaller than tenon size in a typical bolted configuration.  
      Accordingly, in one aspect, the present invention relates to a method of attaching a shroud cover segment to a plurality of gas turbine compressor stator vanes comprising: (a) forming each of the plurality of stator vanes with at least one tenon extending from a radially inner tip of each of the stator vanes; (b) forming the shroud cover segment with at least one aperture for each of the at least one tenon; (c) locating the plurality of stator vanes and the shroud cover segment such that the at least one tenon of each of the stator vanes projects through a respective aperture in the shroud cover; and (d) forming the at least one tenon of each of the plurality of stator vanes to create an enlarged tenon head on each of the tenons, thereby fixing said shroud cover segment to the plurality of stator vanes.  
      In another aspect, the invention relates to a method of attaching a shroud cover segment to a plurality of gas turbine compressor stator vanes comprising: (a) forming each of the plurality of stator vanes with a pair of tenons extending from a radially inner tip of each of the stator vanes; (b) forming the shroud cover segment with a pair of apertures for receiving said pair of tenons; (c) locating the plurality of stator vanes and the shroud cover segment such that the at least one tenon of each of the stator vanes projects through a respective aperture in the shroud cover; (d) forming the at least one tenon of each of the plurality of stator vanes to create an enlarged tenon head on each of the tenons, thereby fixing the shroud cover segment to the plurality of stator vanes; and (e) wherein step (d) is performed without preheat.  
      In still another aspect, the invention relates to a method of attaching a shroud cover segment to a plurality of gas turbine compressor stator vanes comprising: (a) forming each of the plurality of stator vanes with at least one tenon extending from a radially inner tip of each of the stator vanes; (b) forming the shroud cover segment with at least one aperture for each of the at least one tenon; (c) locating the plurality of stator vanes and the shroud cover segment such that the at least one tenon of each of the stator vanes projects through a respective aperture in the shroud cover; (d) forming, without preheating, the at least one tenon of each of the plurality of stator vanes to create an enlarged tenon head on each of the tenons, thereby fixing the shroud cover segment to the plurality of stator vanes; and (e) wherein the shroud cover segment extends no more than 180°.  
      The invention will now be described in connection with the drawings identified below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       FIG. 1  is a perspective view of a typical welded gas turbine stator vane shroud connection;  
       FIG. 2  is a perspective view of a typical bolted gas turbine stator vane connection;  
       FIG. 3  is a perspective view of a formed gas turbine stator vane connection in accordance with an exemplary embodiment of the invention;  
       FIG. 4 a  partial, sectional perspective view of a tenon projecting view through a hole in a tip shroud prior to forming;  
       FIG. 5  is a partial, sectional perspective view of the assembly in  FIG. 4  but with the free end of the tenon formed in accordance with an exemplary embodiment of the invention; and  
       FIG. 6  is a partial perspective view of a formed vane and shroud assembly similar to  FIG. 5  but showing a pair of formed tenons.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      With reference to  FIG. 1 , a typical welded gas turbine stator vane shroud connection is illustrated. More specifically, a stator vane and shroud assembly  10  is made up of an annular stator ring  12  to which is secured a plurality of stator vanes  14 . Each stator vane includes a dovetail mounting portion  16  that is slidably received within a complimentary dovetail shaped slot  18  in the stator ring. It will be appreciated, however, that other mounting techniques for securing the vanes to the stator ring may be employed. The stator vanes also include an airfoil portion  20 , the radially inner edge of which is formed with a tenon  21  received within a slot or opening  22  in an annular stator vane shroud  24 . In this arrangement, the shroud  24  is welded to the tenons of the respective vanes  14  in a set of vanes that are to be attached to the shroud. In this regard, the shroud cover  24  typically comprises plural arcuate segments, each of which spans a predetermined number of stator vanes within an arcuate extent of, for example, no more than 180°. The arcuate extent of the cover segments, and thus the number of stator vanes attached to each cover segment, may be varied in accordance with specific applications.  
      The shroud cover, or shroud cover segment,  24  is machined to exacting tolerances to hold tight clearances between the shroud cover and the tenons of the vanes to ensure full penetration welds. After welding, the shroud-to-stator vane assembly must be machined to remove any warpage due to the welding process. The assembly must also be preheated treated and postheat treated to relieve any residual stress due to heat input.  
       FIG. 2  illustrates another known gas turbine stator vane-to-shroud connection that utilizes bolts. More specifically, the assembly  26  again includes an annular stator ring  28  mounting a plurality of stator vanes  30 . Each of the stator vanes includes an airfoil portion  32 , the radially inner ends of which are secured to an annular shroud cover (or shroud cover segment)  34 . Tenons (not shown) on the inner tips of the stator vanes are seated in slots (also not shown) in the shroud and an assembly  36  including a threaded bolt  38  and washer  3  (as well as a bushing not visible in  FIG. 2 ) are utilized to secure the shroud to the stator vanes. As will be appreciated, this arrangement increases the number of required parts as well as the time of assembly, and raises the potential for over-torqued bolts.  
       FIGS. 3-6  illustrate a formed gas turbine stator vane-to-shroud connection in accordance with an exemplary embodiment of the invention. The assembly  42  includes an annular stator ring  44  that mounts a plurality of stator vanes  46 . The stator vanes  46  include dovetail mounting portions  48  that are received in complimentary dovetail slots  50  in the stator ring. At their opposite ends (i.e., radially inner ends), the stator vanes  52  each include a pair of projecting tenons (having substantially uniform cross-sections in the vertical direction)  56 ,  58  that are received in complimentary-shaped chamfered openings  60  in the vane shroud or shroud segment  54 , as best seen in  FIG. 4 .  
      The projecting tenons are subsequently formed utilizing otherwise conventional tools, with the resulting deformation of, for example, tenon  58 , shown in  FIG. 5 . Note that the tenon  58  has been deformed to include an enlarged head  62  that securely locks the vane shroud or shroud segment  54  to the individual stator vanes  46  within a defined arcuate segment of the stator.  FIG. 6  illustrates the manner in which both tenons  56  and  58  are reformed into enlarged heads  62 ,  64  for each of the stator vanes  46  in the set of vanes engaged by the shroud cover or should cover segment  54 .  
      It will be appreciated that the tenon heads  62  can be cold or hot formed or peened using a manual pneumatic gun impacting the tenons, or by an automated pneumatic head impacting the tenons. The tenon heads can also be formed by non-impacting processes such as orbital head forming or radial head forming.  
      The use of a pair of tenons  56 ,  58  for each stator vane  46  produces a fixed end condition that provides the same fixity as a welded connection. Moreover, the process control with peening is much simpler than welding insofar as the tenon holes cut in the shroud are used to position the shroud in relation to the stator vanes. Another advantage to utilization of the peening process is that no preheating is required and therefore, distortion due to heat is not a factor. Nor is any machining required in order to meet assembly tolerances.  
      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.