Abstract:
A method for assembling a turbine assembly is provided. The method includes providing at least two buckets that each include an axial entry dovetail, a tip and an airfoil extending therebetween. The method also includes coupling the at least two buckets to a rotor wheel by inserting the axial entry dovetail into at least one complementary-configured mating dovetail slot defined in the rotor wheel, and coupling a bucket cover to the tip of the at least two buckets such that the bucket cover substantially circumscribes the rotor wheel in a continuous band.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to the use of turbine assemblies, and more particularly, to rotating airfoils used with turbine assemblies. 
         [0002]    Airfoils, or buckets, used with steam turbines are subjected to harmonic stimulus from numerous sources during operation of the steam turbines. Harmonic stimulus frequencies coinciding with a bucket&#39;s natural frequency may cause significant resonance in the bucket. Over time, this resonance may cause high cycle fatigue in the bucket and may contribute to reducing the useful service life of the bucket. 
         [0003]    At least some known bucket and associated bucket cover designs include tangential entry dovetail buckets and segmented tip shrouds. Tangential entry dovetail buckets are assembled to the wheel through an assembly gate and then packed circumferentially about the wheel circumference. Known segmented tip shrouds include four or more discrete bands connecting the tips of buckets coupled together about the periphery of the rotor assembly. During operation however, such designs may engender numerous vibratory modes, i.e., natural frequencies, within the per revolution operating frequency range. Moreover, relatively low natural frequencies may enhance the susceptibility of such bucket and bucket cover designs to significant per rev resonance, which may compromise the mechanical integrity of the bucket and/or bucket cover. Increasingly, such designs may be unable to withstand increased turbine output. 
         [0004]    Other known bucket and cover designs include axial entry dovetail buckets having integral shrouds. In such designs, circumferentially adjacent shrouds lock-up with speed forming a continuously coupled structure. Such designs may also engender vibratory modes within the per revolution operating frequency range and may be unable to withstand increased turbine output. Additionally, these designs may not be well suited for variable speed applications where the amount of lock-up, or coupling, varies as a function of speed. Fabricating the axial entry buckets with integral shroud can be expensive and fixturing limitations have primarily limited their use to the last stage of a turbine. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0005]    In one embodiment, a method for assembling a turbine assembly is provided. The method includes providing at least two buckets that each include an axial entry dovetail, a tip and an airfoil extending therebetween. The method also includes coupling the at least two buckets to a rotor wheel by inserting the axial entry dovetail into at least one complementary-configured mating dovetail slot defined in the rotor wheel, and coupling a bucket cover to the tip of the at least two buckets such that the bucket cover substantially circumscribes the rotor wheel in a continuous band. 
         [0006]    In another exemplary embodiment, a turbine assembly is provided. The assembly includes a rotor wheel including at least one dovetail slot defined therein and at least two buckets that each includes an axial-entry dovetail, a tip and an airfoil extending therebetween. Each of the at least two buckets is coupled to the rotor wheel via the dovetail slot. The assembly also includes a bucket cover coupled to the tip of the at least two buckets such that the bucket cover substantially circumscribes the rotor wheel. 
         [0007]    In yet another exemplary embodiment, a steam turbine assembly is provided. The assembly includes a rotor wheel including at least one dovetail slot defined therein, and at least two buckets each including an axial entry dovetail, a tip and an airfoil extending therebetween. Each of the at least two buckets is coupled to the rotor wheel and the dovetail is at least one of a straight entry dovetail, a slanted entry dovetail and a slanted entry dovetail. The assembly also includes a bucket cover coupled to the tip of the at least two buckets such that the bucket cover substantially circumscribes the rotor wheel and increases the natural frequency of the at least two buckets. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a cross-sectional schematic view of a portion of an exemplary steam turbine; 
           [0009]      FIG. 2  is a perspective view of an axially-mounted bucket that may be used with the steam turbine shown in  FIG. 1 ; and 
           [0010]      FIG. 3  is an enlarged perspective view of an exemplary bucket cover. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]      FIG. 1  is a partial cross-sectional schematic view of a portion of an exemplary steam turbine  10  including an impulse rotor assembly  12  and a plurality of axially spaced rotor wheels  14  used to couple axial buckets  16  to rotor assembly  12 . A series of nozzles  18  extend in rows between adjacent rows of buckets  16 . Nozzles  18  cooperate with buckets  16  to form a stage and to define a portion of a steam flow path, or a hot gas flow path, indicated by the arrow  15  that extends through turbine  10 . 
         [0012]    In operation, high pressure fluid enters an inlet end (not shown) of turbine  10  and moves through turbine  10  generally parallel to an axis  19  of rotor assembly  12 . The steam strikes a row of nozzles  18  and is directed downstream against buckets  16 . The hot gas then passes through the remaining stages, thus forcing buckets  16  and rotor assembly  12  to rotate. The term “axial” as used herein is intended to be in a direction generally parallel to axis  19 . 
         [0013]      FIG. 2  is a perspective view of an exemplary bucket  16  that may be used with steam turbine  10  (shown in  FIG. 1 ). Bucket  16  includes a straight entry axial-entry dovetail  22 , a base  28 , an airfoil  30  and a tip  34 . Dovetail  22  includes a radially inner end  24  and a radially outer end  26 . It should be appreciated that although axial dovetail  22  is described as being a straight entry type dovetail, dovetail  22  may be any type of axial dovetail, such as, but not limited to, a slanted or curved entry dovetail, that enables buckets  16  to function as described herein. Further, it should be appreciated that the dovetail cross-sectional area may be any cross-sectional area, such as, but not limited to, square-shaped, rectangular, and/or triangular, that enables buckets  16  to function as described herein. Base  28  extends between outer end  26  and airfoil  30 . Airfoil  30  extends from a root  32  adjacent to base  28  to bucket tip  34 . In the exemplary embodiment, bucket tip  34  includes a tip platform  36  and tenons  38  and  40 . Tip platform  36  is oriented generally parallel to base  28 , and tenons  38  and  40  extend substantially perpendicularly away from tip platform  36 . It should be appreciated that bucket airfoil  30  may extend at any angle away from base  28  and tenons  38  and  40  may extend at any angle away from tip platform  36  that enables buckets  16  to function as described herein. 
         [0014]    A maximum load for each bucket  16  is partially determined by its natural frequency. Thus, raising the natural frequency of each bucket  16  generally increases the maximum tolerable load for that bucket  16 . Continuously coupling bucket tips  34  together facilitates increasing the natural frequency of each bucket  16 . Consequently, in the exemplary embodiment, and as described in more detail below, a continuous bucket cover (not shown in  FIG. 2 ) circumscribes rotor assembly  12  and is coupled to each bucket tip  34  to facilitate increasing the natural frequency of each bucket  16 . Moreover, a continuous bucket cover decreases the modes of vibration of each bucket  16 . 
         [0015]      FIG. 3  is an enlarged perspective view of an exemplary chain link bucket cover  42  used with buckets  16  and rotor assembly  12 . More specifically, bucket cover  42  includes a plurality of links  44  coupled together to form a chain link type bucket cover  42 . In the exemplary embodiment, each link  44  is a plate that includes a radially outer top surface  46 , a radially inner bottom surface  48 , a first side  50 , an opposite second side  52 , a first end  54  and an opposite second end  56 . Each end  54  and  56  includes a pair of side edges  55  that form an apex  57  and a pair of openings  58  and  60  that extend through cover  42  from top surface  46  to bottom surface  48 . Openings  58  and  60  are each sized and shaped to mate with one of complementary-shaped and oriented tenons  38  or  40 . Likewise, second end  56  includes a pair of openings  62  and  64  that are shaped and oriented to mate with one of the complementary-configured tenons  38  or  40 . In the exemplary embodiment, openings  58  and  60  are identical to openings  62  and  64 . 
         [0016]    Links  44  are arranged in an alternating overlapping pattern that forms a continuous overlapping chain link bucket cover  42 . Generally, links  44  are oriented such that first end  54  of an “over” link  66 ,  72  overlaps second end  56  of an “under” link  68 ,  70 , and such that the second end  56  of each over link  66 ,  72  overlaps with the first end  54  of an under link  68 ,  70 . More specifically, in the exemplary embodiment, when fully assembled, the bottom surface  48  of the first end  54  of an over link  66  is positioned substantially flush against the top surface  46  of the second end  56  of an under link  68 . Moreover, over link  66  and under link  68  are aligned with respect to each other such that opening  58  of over link  66  aligns with opening  62  of under link  68 , and such that opening  60  is aligned with opening  64 . Similarly, the bottom surface  48  of second end  56  of over link  66  is positioned substantially flush against the top surface  46  of first end  54  of under link  70 . In addition, over link  66  and under link  70  are aligned so that opening  62  of over link  66 , aligns with opening  58  of under link  70 , and such that opening  64  is aligned with opening  60  of under link  70 . When links  42  are aligned, buckets  16  are coupled to bucket cover  42  by inserting tenons  38  and  40  into complementary-configured openings defined by openings  60  and  64 , and/or openings  58  and  62 . Tenons  38  and  40  are riveted over the “over” links  66  and  72  creating a rigid connection. The “under” links  68  and  70 , by comparison, are assembled with a slip fit to allow for expansion. It should be appreciated that although the bucket cover  42  in the exemplary embodiment is an over and under overlapping chain link cover, other various exemplary embodiments may use any other type of continuous bucket cover that enables bucket cover  42  to function as herein described. 
         [0017]    Because bucket cover  42  is coupled to each bucket  16 , as opposed to extending only over groups of buckets  16 , bucket cover  42  continuously circumscribes the periphery of rotor wheel  14  and rotor assembly  12 . Continuously coupling the bucket cover  42  to each bucket tip  34  results in a stiffer assembly having higher natural frequency and greater load capability, thus facilitating preventing harmful vibrations and related stresses from developing in buckets  16  during periods of increased turbine output. Moreover, because the cover  42  is not integral with bucket tip  34 , bucket  16  is more cost effective. That is, because a shroud isn&#39;t required, each bucket  16  is less expensive to manufacture and repair. Furthermore, the exemplary embodiment described herein is easier to assemble, is better suited for variable speed applications than integrally covered axial entry designs, and may be used at all stages of a turbine, not only the last stage. 
         [0018]    In contrast, due to the gap inherent in tangential entry dovetail designs, a bucket cover, such as chain link bucket cover  42 , cannot be continuously coupled to a complete row tangential-entry of buckets  16  and at the same time continuously circumscribe rotor wheel  14 . The axial entry dovetail  22  of the exemplary embodiment is more efficient than tangential entry dovetail designs because a complete row of tangential entry buckets  16  is installed about rotor wheel  14  without a gap. This geometry enables bucket cover  42  to be coupled to each bucket tip  34  in a complete row of buckets  16  while continuously circumscribing rotor wheel  14 . As a result, the complete row of axial entry dovetails  22  yields less unbalance. 
         [0019]    In each embodiment, the above-described combination of axial entry dovetail bucket  16  and chain link bucket cover  42  facilitates increasing the tolerable loads of buckets  16 . More specifically, in each embodiment, the combination of axial entry dovetail bucket  16  and chain link bucket cover  42  results in a significant increase in bucket  16  natural frequencies, fewer modes of bucket  16  vibration and increased overall mechanical reliability. Furthermore, each embodiment eliminates cover lock-up at off design speeds. As a result, turbine operation with higher output is facilitated. Accordingly, steam turbine performance and component useful life are each facilitated to be enhanced in a cost effective and reliable means. 
         [0020]    Exemplary embodiments of axial entry dovetail buckets and bucket covers are described above in detail. The axial entry dovetail buckets and bucket covers are not limited to use with the specific steam turbine embodiments described herein, but rather, the axial entry dovetail and buckets can be utilized independently and separately from other components described herein. For example, the axial entry dovetail with continuous chain link bucket cover may be used with any utility, industrial or mechanical drive steam turbine. Moreover, the invention is not limited to the embodiments of the axial entry dovetail and bucket cover described above in detail. Rather, other variations of axial entry dovetail and bucket cover embodiments may be utilized within the spirit and scope of the claims. 
         [0021]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.