Patent Document

TECHNICAL FIELD 
     The present invention relates to steam turbine rotors, and more particularly to dovetail connections between steam turbine rotor wheels and steam turbine buckets. 
     BACKGROUND OF THE INVENTION 
     Dovetail connections between turbine rotor wheels and turbine buckets include wheel hooks and bucket hooks that cooperate with each other to counter the centrifugal forces that are imposed on the connections. These hooks also prevent the buckets from separating from their wheel during the operation of the steam turbine. However, a major failure potential for conventional dovetail connections is their margin for creep. This is especially true in high pressure (HP) sections of the turbine where high temperatures are produced. For a bucket and wheel dovetail connection in a turbine operating at operational temperature of between about 850 to 1000 degrees Fahrenheit and at given stress levels, the creep strength of the bucket and rotor materials is not equal. Instead, the strength of the wheel is less than that of the bucket. As a consequence of the material strength differences and the load distribution that occurs during operation, the lower strength wheel limits the magnitude of the stresses that the connection can withstand. If the stresses exceed the material strength of the wheel, creeping will occur and the connection may fail. The limitations created by the configuration and dimensions of these conventional connections and the dependence of these connections on the lower material strength of the wheel prevent conventional turbines from reaching optimal levels of performance. 
     Commonly assigned U.S. Pat. No. 5,474,423 to Seeley et al. discloses a bucket and wheel dovetail connection for steam turbine rotors. In the Seeley et al. patent, the dovetail connection includes four hooks on the rotor wheel that have crush surfaces. The crush surfaces each form an angle with a respective neck surface that is greater than ninety degrees. The radially innermost hook includes a crush surface that is at an angle of ninety degrees to its respective neck surface. The slanted crush surfaces result in lower stress concentrations in the wheel hooks. Nevertheless, the need for optimization of a dovetail connection between a wheel and a bucket still existed. 
     These conventional dovetail connections cannot support a larger vane unless the size of the connection is also increased. Applying a larger vane would cause a turbine using conventional dovetail connections to fail. Alternatively, an increase in the overall size of the dovetail connections would result in an undesirable increase in the overall size of the turbine stages and an increase in the spacing of the turbine stages. These increases would require that the overall size of the steam turbine also be increased. 
     BRIEF SUMMARY OF THE INVENTION 
     It is desirable to overcome the drawbacks in the prior art by providing a dovetail connection that optimizes the joint between a rotor wheel and a bucket and permits the use of wider vanes at the ends of the buckets without changing the size of the wheel and the other existing components of the turbine. In an embodiment of the present invention, a dovetail connection for connecting a rotor wheel and bucket for a turbine comprises a male dovetail component and a female dovetail component. The male dovetail component includes a plurality of hooks. Each of the hooks includes a crush surface, a neck and an angle formed between the crush surface and the neck. These hooks are dimensioned in accordance with at least one of the below listed tables. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of a conventional turbine rotor wheel and bucket dovetail joint; 
     FIG. 2 is a partial cross-sectional view of a turbine wheel dovetail connection in accordance with the present invention; 
     FIG. 3 is an enlarged fragmentary cross-sectional view of the hooks of the dovetail connection of FIG. 2; 
     FIG. 4 is a perspective view of the dovetail connection; 
     FIG. 5 is a graph showing the improvement in resisting creep experienced by the dovetail connection described herein; and 
     FIG. 6 is a chart showing the reduction in shear stresses and equivalents in wheel and bucket hooks described herein. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The figures illustrate portions of a steam turbine rotor that include a bucket and wheel dovetail connection. Compared to conventional dovetail connections, the dovetail connection according to the present invention reduces the average and concentrated stresses in the bucket and wheel that result from centrifugal forces created during the operation of the turbine. For example, the geometry and dimensions of the hooks prevent excessive creeping at high temperatures. Also, the dovetail connection discussed below has a higher load carrying capacity when compared to existing dovetail connections with the same external dimensions. The dovetail connection permits at least 30% more load to be distributed to the bucket than to the wheel during the operation of the turbine. This distribution takes advantage of the greater material strength of the bucket material. This is especially advantageous in the portions of the turbine where the operating temperatures reach between about 850 and 1000 degrees Fahrenheit. Additionally, the distribution of the additional stresses to the bucket permits the use of larger blades within the size constraints of a conventional steam turbine. As a result, the performance of the turbine can be increased to an optimum level without increasing its overall size. 
     FIG. 1 illustrates a portion of a steam turbine including a shaft  10  and a rotor wheel  12  secured to the shaft  10  in any well-known manner. Though not illustrated, the shaft  10  also includes additional rotor wheels that are spaced from each other along the length of shaft  10 . Each rotor wheel  12  mates with a plurality of steam turbine buckets  16  that carry a blade as is known in the art. The material used to form the wheel  12  can include, but is not limited to, Chrome-Moly-Vanadium alloy steel. The materials used to form the bucket  16  can include, but are not limited to, stainless steels including a 12% chrome stainless steel. 
     For clarity, only wheel  12  and bucket  16  have been illustrated and described herein. However, the discussions relating to the wheel  12  and the bucket  16  are equally applicable to the other wheels and buckets positioned along the length of the shaft  10 . 
     An embodiment of the wheel  12  and bucket  16  is illustrated in FIG.  2 . As discussed below, the embodiment shown in FIG. 2 includes three hooks. The geometry and dimensions of the hooks that add to the optimum performance of the turbine are discussed in the tables below. 
     As shown in FIG. 2, the rotor wheel  12  terminates along its outer radius in a male dovetail component  14 . The male dovetail component  14  includes a plurality of hooks  41 ,  42 ,  43  and a tang  45 . A turbine bucket  16  including a female dovetail component  18  is shown positioned on the wheel  12 . The female dovetail component  18  includes a plurality of hooks  51 ,  52 ,  53  that extend along the innermost portion of the bucket  16  for mating with the hooks  41 - 43 , respectively, of the male dovetail component  14 . The bucket  16  also includes a blade  20  that extends away from the female dovetail component  18 . In one embodiment, the dovetail connection includes a tangential entry-type dovetail arrangement. 
     As illustrated in FIG. 2, the first hook  41  is furthest away from the centerline of the shaft  10 . The second hook  42  is spaced between the first hook  41  and the third hook  43 . As clearly shown, the third hook  43  is closest to the centerline of the shaft  10 . 
     In the ensuing description, it will be appreciated that the dovetail hooks  41 - 43  of the male component  14  and the hooks  51 - 53  of the female component  18  are symmetric with respect to a radial plane  100  that extend normal to the axis of rotation of the shaft  10 . Also, it is accepted practice to refer only to half the dovetail hooks  41 - 43  and  51 - 53  of the components  14 ,  18 , i.e., the dovetail hooks along one side of the radial plane  100 . Thus, the description of the embodiment illustrated in FIG. 2 refers to the three hooks  41 - 43  and three hooks  51 - 53  along one side of the radial plane  100  that is parallel to and coextensive with the mid-plane, which includes the axis of symmetry, of the wheel  12  that extends in the direction of the bucket  16 . It is understood by one of skill in the art that the hooks  41 - 43  and  51 - 53  form only a portion of the dovetail joint and that each component  14 ,  18  of the dovetail joint includes six hooks as shown in FIG.  4 . 
     Above the first hook  41 , the wheel  12  includes an end surface  44  having a width of about 0.2975 inch. End surface  44  can also be referred to as the wheel rim surface. The width according to the present invention extends in an axial direction. The width extends from the radial plane  100  to an upper surface  60  of the first hook  41 . The upper surface  60  extends between the end surface  44  and an upper hook face  61 . The upper surface  60  is slanted relative to plane  100  so that it forms an angle A′ with the upper hook face  61 . FIG. 3 illustrates that angle A′ is equal to 180 degrees minus angle A. Angle A is the angle formed between the upper surface  60  and a plane that is parallel to the upper hook face  61 . As shown in FIG. 2, the upper hook face  61  extends substantially parallel to the plane  100 . 
     A contact surface  62  extends between the face  61  and a neck  63  of the first hook  41 . The contact surface  62  is slanted at an obtuse angle relative to the face  61 . The neck  63  extends substantially parallel to plane  100  and at an angle B′ to the contact surface  62 . Angle B′ is equal to 180 degrees minus angle B. As shown in FIG. 3, angle B is defined as the angle between the contact surface  62  and a line that extends parallel to the plane  100  and the neck  63  in the direction away from the neck  63  and toward end surface  44 . 
     The second hook  42  includes an upper surface  66  that extends between the neck  63  of the first hook  41  and a hook face  67 . The upper surface  66  is slanted relative to the plane  100  so that it forms an angle C′ with the second hook face  67 . FIG. 3 illustrates that angle C′ is equal to 180 degrees minus angle C. Angle C extends between upper surface  66  and the second hook face  67 . As is shown in FIG. 2, the second hook face extends substantially parallel to the plane  100 . 
     A contact surface  68  of the second hook  42  extends between the second hook face  67  and a neck  69  of the second hook  42 . As seen in FIG. 3, the contact surface  68  extends at an obtuse angle to the hook face  67 . Like the neck  63 , the neck  69  extends substantially parallel to the plane  100  and at an angle D′ to the contact surface  68 . Angle D′ is equal to 180 degrees minus angle D. As shown in FIG. 3, angle D is defined as the angle between the contact surface  68  and a line that extends parallel to plane  100  and the neck  69  in the direction away from the neck  69  and toward end surface  44 . 
     The third hook  43  is similar to the first two  41 ,  42 . The hook  43  includes an upper surface  71  that extends between and is slanted relative to the neck  69  and a hook face  72 . Like the other hook faces, the hook face  72  extends substantially parallel to the plane  100 . As a result, the upper surface  71  forms an angle E′ with the hook face  72  as shown in FIG.  3 . Angle E′ equals 180 degrees minus angle E. As illustrated, angle E is defined in a manner similar to angle A and angle C. 
     The third hook  43  also includes a contact surface  73  that extends between the hook face  72  and a neck  74 . The contact surface  73  intersects with the hook face  72  at an obtuse angle. Like the other necks, the neck  74  extends substantially parallel to plane  100  and at an angle F′ to the contact surface  73 . Angle F′ is equal to 180 degrees minus angle F. As shown in FIG. 3, angle F is defined as the angle between the contact surface  73  and a line that extends parallel to plane  100  and the neck  74  in the direction away from the neck  74  and toward end surface  44 . 
     The third hook further includes a surface  76  that extends between the neck  74  and an upper surface  77  of a shoulder  78  of the wheel  12 . The surface  76  extends at an angle relative to the neck  74  of 180 degrees minus angle G. Angle G extends between surface  76  and a plane that is coextensive with neck  74  and that extends away from neck  74  in the direction of the shaft  10 . Although it is shown as being at a slight angle to the neck  74 , the surface  76  is extended substantially parallel to the neck  74 . 
     The shoulder  78  includes the tang  45  at its outer edge. The shoulder  78  and tang  45  can provide support for the bucket when the turbine is not operating. Additional support is also provided by the upper surfaces  60 ,  66 ,  71  of each hook. As understood from the figures, when the turbine is at rest, the upper surfaces  60 ,  66 ,  71  each contact a cooperating surface on the bucket  16  to support the bucket  16  on the wheel  12 . 
     During the operation of the turbine, the centrifugal forces generated by the rotation of the wheel  12  causes the contact surfaces  62 ,  68  and  73  to engage with a contact surface  21  on the cooperating hooks  51 - 53  of the bucket  16  in order to prevent the separation of the wheel  12  and the bucket  16 . These contact surfaces  62 ,  68 ,  73  and  21  are commonly referred to as crush surfaces. Concentrated stresses result when load paths are forced to change directions abruptly. Accordingly, the slanted crush surfaces  62 ,  68 ,  73  and  21 , having the configuration described herein, cause a less severe change in direction and hence afford lower stress concentrations in the wheel  12 . Additionally, these slanted crush surfaces  62 ,  68 ,  73  and  21  transfer the stresses to the bucket  16  that, as discussed above, has a stronger material strength than the wheel  12  when the turbine is operating at temperatures of 850 to 1000 degrees Fahrenheit. By transferring the stresses to the bucket  16 , the elastic shear stresses in the hooks  41 - 43 , creep deformation (due to high temperature environment of high pressure stages) and stress concentrations within the wheel  12  are reduced relative to the prior art as shown by the creep improvement graph of FIG.  5 . Moreover, a percentage of reduction in the shear and equivalent stresses in the: wheel and bucket hooks as compared to conventional dovetail connections is shown in FIG.  6 . 
     The connection of the present invention permits the load on the bucket  16  to be increased by about 30% or greater. An increased load can include the application of a larger blade  20  on the end of the bucket  16 . Larger blades  20  can be used with the present invention without the size of the wheel  12 , the bucket  16  or the dovetail connection being increased. The use of a larger blade will improve the performance of the turbine. 
     In order to reduce the stress concentrations at the intersection of adjacent surfaces of the dovetail connection, each crush surface  62 ,  68  and  73  is spaced from its respective neck  63 ,  69  and  74  and face  61 ,  67  and  72  by fillets  91 . The radii of these fillets  91  are listed below in table VI. The radii of these fillets  91  result in a further lowering of the concentrated stresses in the wheel  12  and the bucket  16  at the dovetail connection when combined with the other dimensions and features of the present invention. The radii of fillets R 1 -R 12  are substantially equal to those used in conventional connections of this type. These fillets have radii of either 0.125 inch or 0.075 inch. As noted previously, slanted crush surfaces cause a component of force in the axial direction which gives rise to the bending of the bucket leg and an axial load on the tang  45  of the wheel dovetail. To minimize this effect, the hook thickness (height) H of all of the hooks is substantially the same as shown in the tables below. 
     As shown in FIG. 2, the height of each hook H is the distance from the beginning of its upper surface to the juncture between the crush surface for the hook and its neck. As also shown, the height L of each wheel neck is the distance between the juncture of its crush surface and the neck and the juncture of the neck and the beginning of an upper surface for an adjacent hook. 
     The magnitudes of the above-discussed angles are listed below in Table I. These angles and the dimensions discussed herein for the parameters of the dovetail connection were found to contribute to the optimum performance of the turbine by, at least, taking advantage of the increased material strength of the bucket  16  and reducing the stresses along the crush surfaces as discussed above. 
     Other dimensions relating to the disclosed exemplary embodiments are also disclosed in the tables below. These dimensions have a tolerance of +/− one ten-thousandths of an inch. These dimensions include the wheel neck width W for each surface of the hook. As seen in FIG. 2, the wheel neck width W for a given surface is the distance that the surface is spaced from the plane  100  on one side of the wheel  12 . 
     The below listed dimensions also include the clearance C between the surfaces of the wheel hooks and the surfaces of the bucket hooks during the operation of the turbine. The clearances C 11 , C 13 , C 21 , C 23 , C 31 , C 33  and C 34  between certain surfaces that do not contact each other during the operation of the turbine are substantially the same. These surfaces include the necks of the wheel hooks and the faces of the bucket hooks. These similar clearances have been referenced collectively in the tables below as Ca. Similarly, the clearances C 12 , C 22 , C 32  and C 35  for the upper surfaces of the wheel hooks that do not contact the lower surfaces of the bucket hooks are substantially the same. As a result, they have been referenced collectively in the tables below as Cb. 
     The heights from the bottom of the bucket  16  to the illustrated intersections of different sections of the bucket  16  are also included in the tables below and shown in FIG.  2 . Similarly, the angles formed by the surfaces of these sections with a plane extending parallel to the plane  100  are included in Table I. 
     Other dimensions include the distance DW/2 from the plane  100  to the outer surface of the bucket  16 . This distance is about 1.4 inches for an embodiment according to the present invention. The illustrated height S of the bucket  16  above surface  44  to the blade securing surface  78  is about 0.9 inch. The radial distance RW/2 from the plane  100  to the outer edge of the bucket is about 1.4 inches. Other dimensions such as the wheel rim diameter (WRD) can be the same as found with a conventional dovetail connection. As understood in the art, the wheel rim diameter is twice the distance from a point to the axis of rotation of the shaft. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 Angle 
                 Degrees 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 A 
                 80 
               
               
                   
                 A′ 
                 100 
               
               
                   
                 B 
                 65 
               
               
                   
                 B′ 
                 115 
               
               
                   
                 C 
                 80 
               
               
                   
                 C′ 
                 100 
               
               
                   
                 D 
                 65 
               
               
                   
                 D′ 
                 115 
               
               
                   
                 E 
                 80 
               
               
                   
                 E′ 
                 100 
               
               
                   
                 F 
                 90 
               
               
                   
                 F′ 
                 90 
               
               
                   
                 G 
                 0 
               
               
                   
                 AB1 
                 20 
               
               
                   
                 AB2 
                 20 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE II 
               
               
                   
                   
               
               
                   
                 Bucket Dimensions 
                 Inches 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 B1 
                 1 
               
               
                   
                 B2 
                 1.01 
               
               
                   
                 B3 
                 0.25 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE III 
               
               
                   
                   
               
               
                   
                 Clearance Between the Wheel and the Bucket 
                 Inches 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 C36 
                 0.02 
               
               
                   
                 Ca 
                 0.017 
               
               
                   
                 Cb 
                 0.027 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
           
               
                   
                 TABLE IV 
               
               
                   
                   
               
               
                   
                 Hook Height 
                 Inches 
               
               
                   
                   
               
             
             
               
                   
                 H1 
                 0.48 
               
               
                   
                 H2 
                 0.68 
               
               
                   
                 H3 
                 0.57 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE V 
               
               
                   
                   
               
               
                   
                 Wheel Neck Height 
                 Inches 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 L1 
                 0.2 
               
               
                   
                 L2 
                 0.15 
               
               
                   
                 L3 
                 0.15 
               
               
                   
                 L4 
                 2.113 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE VI 
               
               
                   
                   
               
               
                   
                 Fillets 91 
                 Radii (inches) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 R13 
                 0.219 
               
               
                   
                 R14 
                 0.075 
               
               
                   
                 R15 
                 0.075 
               
               
                   
                 R16 
                 0.075 
               
               
                   
                 R17 
                 0.25 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE VII 
               
               
                   
                   
               
               
                   
                 Distances From Plane 100 
                 Inches 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 W11 
                 0.2975 
               
               
                   
                 W12 
                 0.5035 
               
               
                   
                 W13 
                 0.3085 
               
               
                   
                 W21 
                 0.7725 
               
               
                   
                 W22 
                 0.5 
               
               
                   
                 W31 
                 1.0475 
               
               
                   
                 W32 
                 0.8525 
               
               
                   
                   
               
             
          
         
       
     
     With the foregoing dimensions, it will be appreciated that the dovetail shape minimizes concentrated stresses, while maintaining an overall size compatible with existing steam paths. 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. For example, the male hooks could be positioned on the bucket and the female hooks on the wheel.

Technology Category: f