Patent Publication Number: US-9903210-B2

Title: Turbine blade tip shroud

Description:
This application is the US National Stage of International Application No. PCT/US2014/038700 filed May 20, 2014, and claims the benefit thereof. The International Application claims benefit of the 21 May 2013 filing date of U.S. provisional patent application No. 61/825,601. All applications are incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to gas turbine blade tip shrouds and particularly to a seal rail and cutter tooth configuration for a tip shroud. 
     BACKGROUND OF THE INVENTION 
     A gas turbine blade has a tip that closely brushes a surrounding shroud. The shroud channels the working gas flow through circular arrays of blades. Each circular array is called a turbine stage, the first stage being just after the combustion section. The inner lining of the shroud is made abradable so that the blade tips can cut a path in it to minimize the blade tip-to-shroud clearance. This minimizes leakage of the working gas from the pressure side to the suction side of each blade. Some blade designs include a tip shroud as shown in  FIG. 1 , which is a plate on the blade tip. A seal rail may extend radially outward from the plate. The rail is aligned circumferentially along the rotation direction. It cuts a narrow groove in the shroud lining for working gas sealing. The rail may include wider portions called teeth that cut the groove wider than the rail to allow for proper blade to shroud clearances for tolerances and rotor axial movement. 
     A disadvantage of adding a tip shroud and seal rail to a blade design is added weight. Cantilevered portions of the tip shroud must be rigid to resist flexing from centrifugal force. This limits possible weight reductions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in the following description in view of the drawings that show: 
         FIG. 1  is a perspective view of a prior art turbine blade with a tip shroud. 
         FIG. 2  is a top view of a prior art tip shroud and seal rail. 
         FIG. 3  is a sectional view taken on line  3 - 3  of  FIG. 2 . 
         FIG. 4  is a top view of two adjacent tip shrouds showing aspects of an embodiment of the invention. 
         FIG. 5  is a sectional view taken on line  5 - 5  of  FIG. 4 . 
         FIG. 6  is a top view of a tip shroud showing aspects of a second embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a prior art turbine blade  20 A with a tip shroud  22 A. The blade has a root  23 , a platform  24 , and an airfoil  25  with a leading edge  26  and a trailing edge  27 . A transverse profile  30 M of the airfoil midsection is shown with a pressure side P and a suction side S. An axial direction  28  of the working gas flow and a circumferential direction  29  of blade rotation are shown. “Axial” means parallel to the turbine rotation axis. The circumferentially oriented seal rail  32 A has wider portions or teeth  34 ,  35  for cutting a groove in the shroud liner. 
       FIG. 2  is a top view of a prior art turbine blade  20 B showing a tip shroud  22 B, a platform  24 , and an airfoil  25  with a leading edge  26  and a trailing edge  27 . A transverse profile  30 T of the airfoil tip is shown with a dashed line. An axial direction  28  of the working gas flow and a circumferential direction  29  of blade rotation are shown. A circumferentially oriented seal rail  32 B has first and second teeth  38 ,  39  for cutting a groove in the shroud liner. Cooling air outlets  40  pass through the tip shroud from cooling chambers in the airfoil  25 . The rail and teeth have fillets  42 . 
       FIG. 3  is a sectional view taken on line  3 - 3  of  FIG. 2 , showing an abradable shroud liner  44  with a groove  46  therein that is cut by the teeth  38 ,  39 . Abradable shroud liners are made of ceramic that may be porous and/or may have a honeycomb structure to increase abradability. Gas leakage over the blade tip is impeded by the top of the seal rail  32 B closely clearing the top of the groove  46 . 
       FIG. 4  is a top view of two adjacent tip shrouds  22 C,  23 C showing aspects of an embodiment of the invention. An axial direction  28  of the working gas flow and a circumferential direction  29  of blade rotation are shown. A circumferentially oriented seal rail  32 C has first and second teeth  48 ,  50  for cutting a groove in the shroud liner. The first tooth  48  or both teeth may be proximate or over a stacking axis  52  of the blade. The stacking axis is a radial line from the turbine axis through the center of mass of the blade. Proximity of the teeth to the stacking axis minimizes bending moment on the blade about the stacking axis. Cooling air outlets  40  may pass through the tip shroud from cooling chambers in the blade via the blade tip  30 T. The rail and teeth may have fillets  42 . 
     The teeth  48 ,  50  may be smoothly rounded or bumps extending upstream and downstream from the seal rail  32 C. The top leading edge of each tooth (the edge touched by lead lines  48 ,  50 ) may be sharp, with an included angle B ( FIG. 5 ) such as 90 to 100 degrees when viewed in section, while the side surfaces  49 ,  51  may be smoothly rounded. This combination produces clean cutting by the sharp edges plus smoothing of the sides of the groove  46  ( FIG. 5 ) by the rounded sides of the teeth. The sides  49 ,  51  can fly on the boundary layer of gas on the sides of the groove in some conditions, minimizing resistance. Such an air bearing effect is maximized by the tooth sides being smooth and rounded. Only one tooth  48 ,  50  is needed on each side of the rail  32 C. 
     The rail  32 C may have front and back portions  54 ,  56  with respect to the rotation direction  29 . The front portion  54  of the rail is ahead of the teeth  48 ,  50 . It may be aligned with the rotation direction  29  as shown by centerline  58 . The back portion  56  of the rail is behind the teeth. It may be angled back to the extended front centerline  58  as shown so that the back end  62  of the rail aligns with the front end  64  of the rail on the following tip shroud  23 C. The back portion  56  of the rail may span linearly from the peak  66  or maximum lateral extent of the back tooth  50  to the back end of the rail  62  centered on the extended centerline  58 . This configuration minimizes mass in the back portion  56  of the rail for a given width thereof, since the shortest distance between two points is a straight line. The angle A between centerlines  58 ,  60  of the front and back portions of the rail may be in a range such as 2 to 3 degrees. 
     The tip shroud and seal rail as shown in  FIG. 4  may be used in original turbine manufacture or on replacement blades, which are also called buckets. This provides reduced mass and friction over the prior art of  FIG. 2 . 
       FIG. 5  is a sectional view taken on line  5 - 5  of  FIG. 4 , showing an abradable shroud liner  44  with a groove  46  cut therein by the teeth  48 ,  50  of  FIG. 4 . 
       FIG. 6  is a top view of a tip shroud  22 D with aspects of a second embodiment of the invention. An axial direction  28  of the working gas flow and a circumferential direction  29  of blade rotation are shown. A circumferentially oriented seal rail  32 D has first and second teeth  68 ,  70  for cutting a groove in the shroud liner. The first tooth  68  or both teeth may be proximate or over the stacking axis  52  of the blade. The rail and teeth may have fillets  42 . 
     The teeth  68 ,  70  may be formed by smoothly rounded lateral departures or bumps on the seal rail  32 D as shown. The top leading edge of each tooth (the edge touched by lead lines  68 ,  70 ) may be sharp, while the sides  69 ,  71  may be smoothly rounded. Only one tooth  68 ,  70  is needed on each side of the rail  32 D. 
     The rail  32 D may have front and back portions  74 ,  76  with respect to the rotation direction  29 . The front portion  74  is ahead of the teeth  68 ,  70 , and may be aligned with the rotation direction  29 . The back portion  76  is behind the teeth. It may span linearly from the peak  78  or maximum lateral extent of the back tooth  70  to a back end  62  of the seal rail that is centered on an extended centerline  60  of the front portion  72  of the rail. This configuration minimizes mass in the back portion  76  of the rail for a given width thereof, since the shortest distance between two points is a straight line. The front and back portions  74 ,  76  of the rail may have a common uniform thickness, although this is not a requirement. 
     The tip shroud and seal rail as shown in  FIG. 6  may be used in original turbine manufacture or on replacement blades, which are also called buckets. 
     The teeth pairs  48 / 50 ,  68 / 70  may be formed in a comma shape as viewed from above ( FIGS. 4 and 6 ). This forms a smooth transition and allows a more constant rail thickness than in prior art ( FIG. 2 ), resulting in more uniform cooling and thermal expansion of the rail. 
     While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.