Patent Publication Number: US-7905709-B2

Title: Advanced firtree and broach slot forms for turbine stage 1 and 2 buckets and rotor wheels

Description:
FIELD OF THE INVENTION 
     The invention is directed to turbines and, more particularly, to an improved configuration for the root portion, known as a firtree, of a turbine bucket and the corresponding turbine wheel broach slot into which the bucket fits. More specifically, the present invention provides improved firtree/broach slot configurations that reduce the number of buckets required and the stresses acting on the buckets and wheel at the point of their attachment. 
     BACKGROUND OF THE INVENTION 
     The stages of a typical gas turbine can have as many as 92 buckets that radially extend from a rotor or wheel. Each bucket has a root portion that is configured to mate with a corresponding broach slot in the wheel. The firtree/broach slot configurations are designed to reduce stresses that occur transiently and at normal operating speeds. 
     Prior known firtree/broach slot configurations are disclosed in Goodwin, U.S. Pat. No. 4,260,331 issued on Apr. 7, 1981, Pisz et al., U.S. Pat. No. 4,824,328 issued on Apr. 25, 1989, Dierksmeier et al., U.S. Pat. No. 5,688,108 issued on Nov. 18, 1997, Heppenstall, U.S. Pat. No. 5,741,119 issued on Apr. 21, 1998, Dierksmeier et al., U.S. Pat. No. 5,836,742 issued on Nov. 17, 1998, and Dierksmeier et al., U.S. Pat. No. 5,863,183 issued on Jan. 26, 1999. Each one of these prior art patents describes the particular details of the geometric assimilation of lines, arcs, and angles of its disclosed firtree/broach slot configuration for the purposes of reducing centrifugal forces, bending moments, and vibrations and the consequential peak stresses that result at the attachment points. 
     It is desirable to reduce the number of buckets to be attached to the wheel for a number of reasons, including fewer parts (less cost), less required cooling air, higher natural frequencies, less profile losses (skin friction), and reduced overtip leakage. However, a reduction in the number of buckets also results in each individual bucket being heavier as it covers a longer circumferential length. Simply scaling the size of the buckets and slots on existing firtree and broach slot configurations, while maintaining the same size wheel, to reduce the number of buckets will not minimize the stresses acting at the attachment points. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved firtree/broach slot configuration or form that enhances the transfer of load from the bucket (buckets, also known as blades, include the airfoil, shank, and firtree attachment) to the wheel (also known as disk) for a high temperature turbine stage having only 60 buckets. 
     Another object of the present invention reduces the magnitude of the pull force on the rotor wheel by the bucket firtree and wheelpost known as the dead rim annulus. 
     Further objects of the present invention are to reduce the magnitudes of the concentrated stresses in the form for improved low cycle fatigue (LCF) and high cycle fatigue (HCF) capability of both the bucket and the wheel, and improve the necessary capacity for delivering cooling air to the buckets (air passage area). 
     Still further objects of the present invention are to reduce the capacity for leaks across the stage through the firtree, and equalize the load transfer from the bucket to the wheelpost among the tangs. 
     The present invention is designed with the intent and goal of improved fuel efficiency over previous designs. Several measures have been taken in the hot gas path to contribute to this goal, among them being a reduced bucket count. Stages  1  and  2  in the turbine have 60 buckets rather than the typical 92 bucket count. The benefits of reduced bucket count include: fewer parts (cost), less required cooling air, higher natural frequencies, less profile losses (skin friction), reduced overtip leakage, etc. 
     However, a reduced count also results in each individual bucket being heavier as it covers a longer circumferential length. This increased weight and circumferential length have been accounted for in the new firtree form since the prior art forms were typically designed for as many as 92 buckets. 
     The new firtree form has unique dimensions and relationships between the bucket and wheel necessary for enhancing transfer of the bucket load into the wheelpost, while reducing concentrated stresses and rotor pull. The new firtree form was arrived at by iteration of form parameters and thermo-mechanical loading. This form has certain key features that have improved this load transfer successfully. 
     This form may be scaled to larger or smaller sizes provided, however, that the rotor wheel or disk diameters are correspondingly scaled to larger or smaller sizes or that the two sides of the bucket and wheel are offset similarly, i.e., wider or narrower. In addition, although a preferred range of tolerances for the dimensions of the bucket and wheel are provided herein, those skilled in the art will recognize that a broader range of tolerances could also be employed in practicing the invention. 
     Although the intended use for this form is the GE 6C IGT model gas turbine, it, or any scale of it, may be applied to other applications where blades or buckets are attached to a rotating wheel or disk in a high temperature environment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a portion of a turbine wheel with attached buckets; 
         FIG. 2A  represents a cross-sectional schematic drawing of a portion of a bucket at the attachment and depicts the firtree profile; 
         FIG. 2B  represents a cross-sectional schematic drawing of a portion of a turbine wheel at the attachment and depicts the broach slot profile; 
         FIG. 3A  shows a forward view a bucket interlocked between corresponding wheelposts; 
         FIG. 3B  shows an aft view of a bucket interlocked between corresponding wheelposts; 
         FIG. 4  represents an interior cross-sectional schematic drawing of the attachment portion of a bucket; 
         FIG. 5  shows the slot opening area beneath the bucket for cooling air delivery; 
         FIG. 6  shows gaps between an installed bucket and an adjacent wheelpost in the operating (loaded outward) condition; 
         FIG. 7  shows a perspective view of the upper edge of a wheelpost; 
         FIG. 8  shows a perspective view of the upper edge of a wheelpost with an installed bucket; 
         FIGS. 9 and 10  show dimensional aspects of a bucket; 
         FIGS. 11 and 12  show dimensional aspects of the corresponding broach slot in which the bucket of  FIGS. 9 and 10  installs; and 
         FIG. 13  schematically shows zones for slight dimensional changes from those of the preferred embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Key and fundamental elements of the invention are defined by two series of lines, arcs, and ellipses of which the adjacent components are tangent. One series depicts the profile or form of the firtree shape of the bucket root while the other series depicts the profile or form of the corresponding broach slot of the rotor wheel into which the firtree shape is fitted. 
       FIG. 1  shows a portion of an assembled rotor wheel  10  to include buckets  11  fitted into corresponding broach slots  12 . Thus, the profile of the wheel broach slot  12  (best seen in the unfilled broach slot in  FIG. 1 ) is substantially filled by the portion of the bucket  11  termed the bucket root (best seen by the filled wheel broach slots in  FIG. 1 ). 
       FIG. 2A  shows in cross-sectional schematic form the profile of bucket root  21  of bucket  11 . Bucket root  21  comprises three sets of curved tangs  22 ,  23 ,  24  and three sets of fillets  25 ,  26 ,  27 . One tang and fillet, from each set of tangs and fillets, is disposed on either side of centerline C. On either side of center line C and above tangs  22  are disposed fillets  25 . Tangs  22  are disposed on either side of centerline C between fillets  25  and  26 . Tangs  23  are disposed on either side of centerline C between fillets  26  and  27 . Tangs  24  are joined to each other at centerline C and are disposed below fillets  27 . 
     Each one of fillets  25 ,  26 ,  27  comprises an inwardly curved radial surface at its center together with two substantially straight surfaces on either side of the curved radial surface. In the case of fillet  25 , the central curved surface is joined to the lower straight surface by way of a transitioning arc. For each fillet  25 , curved surface  200  is connected to straight surface  201  at its upper end that also forms an upper portion of bucket root  21 , and transitioning arc  226  at its lower end. The other end of arc  226  connects to straight surface  202  that also forms a part of tang  22 . For each fillet  26 , curved surface  203  is sandwiched by upper straight surface  204  that also forms a part of tang  22  and lower straight surface  205  that also forms a part of tang  23 . For each fillet  27 , curved surface  206  is sandwiched by upper straight surface  207  that also forms a part of tang  23  and lower straight surface  208  that also forms a part of tang  24 . 
     Each one of tangs  22 ,  23  comprises an outwardly curved radial surface sandwiched by straight surfaces on either side. For each tang  22 , curved surface  209  is sandwiched by upper straight surface  202  that also forms a part of fillet  25 , and lower straight surface  204  that also forms a part of fillet  26 . For each tang  23 , curved surface  210  is sandwiched by upper straight surface  205  that also forms a part of fillet  26  and lower straight surface  207  that also forms a part of fillet  27 . 
     Each one of tangs  24  comprises an outwardly curved surface sandwiched by curved and straight surfaces on either side. For each tang  24 , outwardly curved surface  211  connects at its upper end to elliptical surface  227  that transitions into straight surface  208  that also forms a part of fillet  27 . At its lower end, surface  211  connects to another outwardly curved surface  212  with the curved surfaces  212  of each tang  24  being joined at the centerline C. 
       FIG. 2B  shows in cross-sectional schematic form the profile of broach slot  12  of rotor wheel  10 . Broach slot  12  comprises the physical space between two adjacent wheelposts  13  and is thus defined by the same set of curves. Broach slot  12  comprises three sets of tangs  28 ,  29 ,  30  and three sets of fillets  31 ,  32 , and  33 . The fillets and tangs of broach slot  12  are complimentary to the tangs and fillets of bucket root  21  so that bucket root  21  can be fitted within broach slot  12 . 
     Each one of tangs  29 ,  30  comprises an outwardly curved radial surface sandwiched between straight surfaces. For each tang  29 , curved surface  216  is sandwiched by upper straight surface  217  that also forms a part of internal fillet  31 , and lower straight surface  218  that also forms a part of fillet  32 . For each tang  30 , curved surface  219  is sandwiched by the upper straight surface  220  that also forms a part of fillet  32  and lower straight surface  221  that also forms a part of fillet  33 . 
     Each one of tangs  28  comprises an outwardly curved surface connected to a straight surface at its upper end and transitioning to a straight surface at its lower end by way of an elliptical curve. For each tang  28 , curved surface  213  connects at its upper end to straight surface  214  that forms a top surface adjacent to another broach slot  12 . At its lower end, surface  213  connects to elliptical surface  229  that transitions into straight surface  215  that also forms part of fillet  31 . 
     Each one of fillets  31 ,  32  comprises an inwardly curved radial surface sandwiched by substantially straight surfaces on either side. For each fillet  31 , curved surface  222  is sandwiched by upper straight surface  215  that also forms a part of tang  28 , and lower straight surface  217  that also forms a part of tang  29 . For each fillet  32 , curved surface  223  is sandwiched by upper straight surface  218  that also forms a part of tang  29  and lower straight surface  220  that also forms a part of tang  30 . 
     Each one of fillets  33  comprises an inwardly curved surface  224  connected on each end to another inwardly curved surface. At its upper end, surface  224  connects to curved surface  228  that transitions it into straight surface  221  that also forms a part of tang  30 . At its lower end, surface  224  connects to curved surface  225  with these surfaces  225  of each fillet  33  being joined at the centerline C. 
       FIGS. 3A and 3B  show forward and aft views of bucket root  21  interlocked within wheelposts  13  (or installed in broach slot  12 ). In  FIGS. 3A and 3B , empty broach slot  12  is adjacent to the slot with the bucket root  21  installed and shows in perspective upper tang  28  of wheelpost  13 . A horizontal (axial) air channel  31  is formed between surfaces  224  and  225  of the broach slot and the bottom flat surface of the bucket root and communicates with vertical (radial) air passages  41 , shown in  FIGS. 4 and 5 . Air channel  31  allows an adequate amount of cooling air to the bucket, while maintaining an adequate live rim radius to reduce the amount of dead weight in the firtree and wheelpost. More particularly, as shown in  FIG. 4 , the neck above the bottom tang on the firtree (between fillets  27 ) has been sized to permit passage of sufficient airfoil cooling air while maintaining an adequate thickness to carry the necessary loading at reasonable stress levels. 
     As shown in  FIG. 6 , a small gap  60  exists between a bucket root  21  and wheelpost  13  in wheel  10 , when the bucket root is inserted into the broach slot  12 . This gap or clearance is provided to facilitate the insertion of the buckets into the broach slots and to accommodate manufacturing tolerances. 
     As shown in  FIGS. 7 and 8 , center region  70  of upper tangs  28  of wheelpost  13 , looking at a tangential cross-section, has been scalloped away to reduce weight, which reduces rotor pull and stresses in wheelpost  13 . The lobes  71  on the end remain to seal against the bucket to reduce leakage across the firtree/shank region. 
     The bucket root  21 , as described above, incorporates a uniquely sized and interleaved triple fillet and tang arrangement so as to distribute concentrated stresses evenly over a larger region, thus lowering peak stresses and improving LCF capability. The arrangement allows for a significant reduction from 92 buckets and wheelposts to 60 buckets and wheelposts for the first two stages of a turbine. 
     The radial thickness of bottom tang  24  as set by surface  14  in  FIG. 4  has been uniquely sized such that an equalized distribution of loading exists among the tangs. This stiffness adjustment results in even stress distributions throughout the firtree and wheelpost thus improving the LCF capability of the parts as well as reducing peak crush stresses on the bearing faces. 
     The fillets, between the tangs on the bucket firtree, and on the wheelpost have been sized to reduce occurrence of peak stresses thus improving LCF capability. 
     The fillet above the top tang on the bucket firtree incorporates a compound fillet so as to distribute the concentrated stresses over a larger region, thus lowering peak stresses and improving LCF capability. The top of the wheelpost, as the form transitions away from the contact face and into the top sealing lobe, incorporates an elliptical curve to make this transition. Likewise, the bottom of the bucket firtree, as the form transitions away from the contact face and into the bottom-sealing lobe, incorporates an elliptical curve to make this transition. 
     The divergence angles D of the contact faces (angle to centerline of dovetail), shown in  FIGS. 10 and 12 , are set at 21.000° so that the appropriate balance between the crush stresses on the contact faces and the peak stresses in the adjacent fillets is achieved. The divergence angles E also shown in  FIGS. 10 and 12 , of the array of tangs on each side of the form, have been set at 20.782° so that the appropriate balance among various limits (p/a stress, crush stress, peak stresses, etc.) has been maintained. 
       FIGS. 9 and 10  provide exemplary and preferred dimensions of the bucket and  FIGS. 11 and 12  provide exemplary and preferred dimensions for the broach slot into which the bucket of  FIGS. 9 and 10  is inserted. In all cases, the preferred relative dimensions with respect to the buckets and wheelposts shown in  FIGS. 9-12  are such that the line and curve segments fall within offsets of the defined profile at ±0.001 inches. Of course, those skilled in the art will recognize that minor changes beyond those tolerance ranges will not impact, to any substantial effect, the practice of the invention, and therefore should be considered to be within the scope of the invention. For example, a set of joined lines and curves falling within a tolerance zone defined by profile offsets of ±0.01 inches may still meet the intent of the invention. Further, the sides of the bucket dovetail or broach, mirrored by the centerline, may be separated differently and still fall within this scope. For example, dimensions L 1 , L 2 , L 3 , L 4 , L 9  and L 10  in  FIG. 9  could be increased or decreased by a constant amount to change the overall width of the bucket dovetail. 
     As shown in  FIG. 9 , the angle A that depicts the angular orientation of tang pressure faces  202 ,  205  and  208  relative to horizontal equals 50.000°. The angles B of the first tang  22  and the second fillet  26  equal 56.087°. The angles F of the second tang  23  and lowermost fillet  27 , shown in  FIG. 10 , equal 56.964°. In all of the angular measurements described in this application, the angle to be measured is defined by tangent lines along the outer boundaries of the portions of the bucket or wheelpost to be measured or between the center line of the bucket or wheelpost and a line defined by the intersection points resulting from at least two sets of the aforementioned intersecting tangent lines. 
       FIG. 9  also shows that the termination of upper fillet  25  forms a 90.000° angle with the center line C through the bucket as denoted by angle C′. In  FIG. 10  angles D and E are measured from center line C to lines defined by points at which tangent lines along the first and second fillets and tangs, respectively, intersect. Angles D and E are respectively 21.000° and 20.782° . As shown in  FIG. 10 , intersecting tangent lines T 1  and T 2  along the pressure faces of the bottommost tang do not lie on either line that forms the angle E of 20.782° with the center line. 
       FIG. 9  shows a number of dimensional relationships L 1  through L 13 , L 29  and L 31  which define the relative position of the tangs and fillets that form the geometric configuration of the bucket. 
     L 1  measures 1.6300 inches and L 2  measures 0.7846 inches, with L 1  representing the outermost distance or width of the bucket from center line C and L 2  representing the distance from the center line C to the intersection point of the tangent lines formed along either side of tang  22 . L 29  measures 0.6268 inches and defines the distance from center line C to the intersection point of tangent lines drawn along either side of tang  23 . L 11  measures 0.4654 inches and depicts the distance from the center line C to the intersection point of a line drawn through intersection points defined above with respect to tangs  22  and  23  and a tangent line along upper straight surface  208  of tang  24 . 
     L 5  to L 8  define the distances from the bottom surface of tang  24  to, respectively, the uppermost straight portion of fillet  25 , the intersection point of tangent lines drawn along tang  22 , the intersection point of tangent lines drawn along tang  23 , and the intersection point of a line drawn through the intersection points defined above with respect to tangs  22  and  23  and a tangent line along upper straight surface  208  of tang  24 . These distances L 5  through L 8  are, respectively, 1.9836 inches, 1.2588 inches, 0.8429 inches, and 0.4177 inches. 
     Distance measures L 11 , L 31  depict the distance from the bottom of tang  24  to the points from which the radii of curvatures for the curved portions of tang  24  are defined. L 12  and L 13  depict the distance from the bottom of tang  24  to, respectively, the intersection point of tangent lines drawn along fillet  27 , and the intersection point of tangent lines drawn along fillet  26 . L 11 , L 31 , L 12 , and L 13  measure, respectively, 0.3792 inches, 0.5556 inches, 0.7855 inches and 1.2092 inches. 
     Dimensions L 3  and L 4 , respectively, give the distance from center line C to the intersection point of tangent lines along fillet  27  and the intersection point of tangent lines drawn along fillet  26 . L 3  and L 4  measure, respectively, 0.1568 inches and 0.3194 inches. 
     As noted above, tang  24  is formed in part by two radial curves having center points offset from either side of center line C. A third radial curve forming tang  24  has its center point on center line C the distance L 31  from the bottom of tang  24 . Distance L 9  shows the offsets to the right and left of center line C (offset is only shown to the right of center line C in  FIG. 9 ) and measures 0.0327 inches. The offset radii are shown in  FIG. 10  as R 1  and measure 0.3762 inches. The radius for the curve having its center point on the center line is shown in  FIG. 10  as R 13  and measures 0.5556 inches. 
     L 27  denotes the width of the uppermost tangs  22  which measures 1.3850 inches, and L 28  denotes the width of the intermediate tangs  23  which measures 1.0543 inches. 
     In addition to radii R 1  and R 13 ,  FIG. 10  also shows radii R 2  through R 6  which respectively represent the radius of the lowermost fillet  27 , the radius of the intermediate tang  23 , the radius of fillet  26 , the radius of the uppermost tang  22  and the radii of the uppermost fillet  25 . These radii R 2  through R 6  are respectively, 0.0897 inches, 0.1037 inches, 0.0741 inches, 0.0959 inches, 0.0983 inches (R 6′ ) and 0.3342 inches (R 6 ). 
     Curve  227  joins tang  24  with fillet  27  and is an elliptical radius with semi-major axis 0.0356 inches and semi-minor axis 0.0036 inches. 
     As noted above,  FIGS. 11 and 12  show the dimensions related to the corresponding broach slots. In  FIGS. 11 and 12  the angles A, B, C′ and D through F are identical in measurement to the complementary angles A, B, C′ and D through F in  FIGS. 9 and 10 . 
       FIG. 11  shows a number of dimensional relationships L 14  through L 26 , L 30  and L 32  that define the relative position of the tangs and fillets that form the geometric configuration of the broach slot. 
     L 14  measures 1.4000 inches and L 15  measures 0.7893 inches, with L 14  representing the outermost distance or width of the wheelpost from center line C and L 15  representing the distance from the center line C to the intersection point of the tangent lines formed along either side of fillet  31 . L 30  measures 0.6315 inches and defines the distance from center line C to the intersection point of tangent lines drawn along either side of tang fillet  32 . L 23  measures 0.4701 inches and depicts the distance from the center line C to the intersection point of a line drawn through the intersection points defined above with respect to fillets  31  and  32  and a tangent line along upper straight surface  221  of fillet  33 . 
     L 18  to L 21  define the distances from the bottom of fillet  33  to, respectively, the uppermost straight portion of tang  28 , the intersection point of tangent lines drawn along fillet  31 , the intersection point of tangent lines drawn along fillet  32 , and the intersection point of a line drawn through the intersection points defined above with respect to fillets  31  and  32  and a tangent line along the upper straight surface  221  of fillet  33 . These distances L 18  through L 21  are, respectively, 1.9836 inches, 1.2592 inches, 0.8433 inches, and 0.4181 inches. 
     Distance measures L 24 , L 32  depict the distance from the bottom of fillet  33  to the points from which the radii of curvature for the curved portions of fillet  33  are defined. L 25  and L 26  depict the distance from the bottom of fillet  33  to, respectively, the intersection point of tangent lines drawn along tang  30 , and the intersection point of tangent lines drawn along tang  29 . L 24 , L 32 , L 25 , and L 26  measure, respectively, 0.3852 inches, 0.5616 inches, 0.7859 inches and 1.2096 inches. 
     Dimensions L 16  and L 17 , respectively, give the distance from center line C to the intersection point of tangent lines along tang  30  and the intersection point of tangent lines drawn along tang  29 . L 16  and L 17  measure, respectively, 0.1615 inches and 0.3241 inches. 
     Fillet  33  is formed by two radial curves having center points offset from either side of center line C and a third radial curve with its center point on center line C the distance L 32  from the bottom of fillet  33 . The offset radii are shown in  FIG. 12  as R 7′  measuring 0.3822 inches and R 7″  measuring 0.1248 inches. Distance L 22  shows the offsets to the right and left of center line C for the offset radial curves R 7′ (the offset is only shown to the right of center line C in  FIG. 11 ) and measures 0.0327 inches. The radius for the curve having its center point on the center line is shown in  FIG. 12  as R 7  and measures 0.5616 inches. 
     In addition to radii R 7  through R 7″ ,  FIG. 12  also shows radii R 8  through R 12  which respectively represent the radius of tang  30 , the radius of fillet  32 , the radius of tang  29 , the radius of the uppermost fillet  31  and the radius of the uppermost tang  28 . These radii R 8  through R 12  are respectively, 0.0897 inches, 0.1037 inches, 0.0741 inches, 0.0959 inches, and 0.3282 inches. 
     Curve  215  joins tang  28  with filet  31  and is an elliptical radius with semi-major axis 0.0356 inches and semi-minor axis 0.0028 inches. 
       FIG. 13  schematically depicts that the bucket dovetail and wheel broach profiles can be formed within a range of tolerances as shown by the heavy and dotted lines. For example, with respect to the bucket, its outer dimensions could be altered from the solid line to a shape within the dotted lines. 
     In  FIG. 13 , ‘A’ represents the combination of lines and curves making up the bucket dovetail or wheel broach profile as defined exactly. ‘B’ represents the zone bound by offsets of ‘A’ by ±0.001 inches and contains profile variations that meet the preferred embodiment. ‘C’ represents the zone bound by offsets of the individual mirrored sides of ‘A’ by ±0.01 inches and contains profile variations that fall within the scope of the invention. 
     In particular, all of the dimensions for the bucket and wheel could be scaled larger or smaller than those given for the preferred embodiment. Furthermore, the two sides of the bucket (and corresponding broach slot) could be spaced differently by increasing or decreasing dimensions L 1 , L 2 , L 3 , L 4 , L 9 , L 10  which would result in different bottom fillet radii  227 ,  211  and  212  for the bucket. Similarly, increasing or decreasing the corresponding dimensions of the broach slot would result in different bottom fillet radii  228 ,  224  and  225 . 
     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.