Patent Publication Number: US-2003223872-A1

Title: Covers for turbine buckets and methods of assembly

Description:
BACKGROUND OF THE INVENTION  
       [0001] The present invention relates to a multiple-layered bucket cover for turbines and particularly relates to a multi-layered bucket cover having an intermediate perforated layer and methods of assembly.  
       [0002] A turbine rotor typically mounts a plurality of circumferentially spaced, generally radially extending airfoils or buckets. Covers are conventionally provided on the tips of the buckets, the covers forming a 360° annulus about the buckets with a small clearance between the outer surface of the cover and the surrounding shroud.  
       [0003] There are a number of different structures and methods for securing covers to the tips of the buckets. One such structure is known as a button tenon/cover configuration. In that construction, one or more tenons projecting generally radially from each bucket pass through corresponding openings in the cover and are peened, preferably automatically, along the outer surface of the cover. This button tenon/cover configuration provides substantial pull strength, i.e., sufficient structural integrity between the cover and bucket, to preclude removal of the cover from the end of the bucket under centrifugal forces. The button tenon/cover configuration, however, does not provide adequate cover sealing. That is, the buttons form a series of radially outward projections along the outer surface of the cover, necessitating increased clearance between the rotating component, i.e., the covers, and the surrounding stationary component, i.e., a shroud, thereby increasing tip leakage losses. The button tenon/cover configuration, however, has the advantage of enabling the peening operation to be performed automatically.  
       [0004] In another configuration, known as a “foxhole” tenon/cover configuration, the tenon on the bucket is recessed from the outer opening of the cover. Because of the absence of buttons projecting beyond the outer surface of the cover, the “foxhole” configuration enables tighter tip clearance with the surrounding stationary component, affording improved cover sealing and reduced tip leakage losses. However, foxhole tenon/cover configurations require a manual peening process to secure the covers to the buckets. This process is physically laborious and costly. Accordingly, there is a need to provide a bucket cover which affords sufficient pull strength and which can be both automatically peened and provide adequate cover sealing to minimize tip leakage losses.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0005] In accordance with a preferred embodiment of the present invention, there is provided a bucket cover formed of multiple layers or elements in an arcuate configuration spanning the outer tip of the buckets. The bucket cover is provided in multiple arcuate segments forming a complete annulus about the periphery of the rotor, each segment comprised of multiple elements. Preferably, each bucket cover segment includes an inner element, an outer element and intermediate element for disposition between the inner and outer elements. The segments, and hence the elements, may span three or more buckets and are fitted on tenons of the buckets.  
       [0006] Particularly, the elements are provided with circumferentially spaced openings in registration with one another for receiving the tenons on the ends of the buckets. It will be appreciated that the tenons have a reduced profile as compared with the airfoil profile of the buckets. A radius or chamfer is provided between the reduced profile of the tenon and the airfoil profile at the tip of the bucket. Each opening through the inner element has a radial inwardly increasing chamfer to overlie the radiussed portion at the juncture between the tenon and the bucket airfoil. The intermediate element has openings corresponding to the profile of the tenon. The outer element has openings having a radially outwardly directed chamfer. The inner, intermediate and outer elements are disposed sequentially on the buckets with the openings receiving the tenons. Projecting ends of the tenons may then be peened, preferably automatically, to secure the elements to the buckets. Any excess material of the tenons is removed, for example, by machining to provide a smooth, continuous surface along the outer circumference of the cover. In this manner, tight clearances can be maintained between the cover and the surrounding stationary component.  
       [0007] To provide the necessary pull strength, cavities are provided in the intermediate elements between adjacent tenons. The cavities are void of material but are closed by end walls of the intermediate element facing upstream and downstream directions of hot gas flow through the turbine and bounded by the outer and inner surfaces of the inner and outer elements, respectively. In this manner, a considerable weight reduction in the cover is effected and hence any tendency of the covers for displacement radially outwardly due to centrifugal force is minimized and significantly reduces the pull strength. While the cover elements can be radially aligned with one another at their circumferentially adjacent joints, the elements may also be staggered relative to one another such that the joints between circumferential adjacent elements are misaligned or staggered in a circumferential direction relative to one another.  
       [0008] By utilizing the foregoing described configuration, a flush tenon/cover configuration having adequate pull strength is provided which advantageously can be formed using automatic peening machinery. Simultaneously, cover sealing is improved by tightening clearances between the cover annulus and the surrounding shroud.  
       [0009] In a preferred embodiment according to the present invention, there is provided a cover for a turbine bucket having a tenon adjacent a tip of the bucket comprising discrete inner and outer arcuate cover elements and an intermediate arcuate cover element therebetween, the elements having generally radially aligned openings for receiving the bucket tenon, the outer element having a radially outward chamfer for receiving peened bucket tenon material to retain the elements on the bucket.  
       [0010] In a further preferred embodiment according to the present invention, there is provided a rotatable component for a turbine comprising a plurality of circumferentially spaced buckets rotatable about an axis and terminating in radially outwardly extending tenons, a cover for the buckets including a plurality of discrete arcuate cover segments, each segment having inner, outer and intermediate arcuately extending elements, the elements having generally radially aligned openings at circumferentially spaced locations along the segments for receiving tenons, the tenons being peened to secure the elements on the buckets and forming a generally smooth continuous outer surface with an outer surface of the outer element.  
       [0011] In a further preferred embodiment according to the present invention, there is provided a method of assembling covers on buckets of a rotary component of a turbine, including the steps of providing inner, outer and intermediate arcuate cover elements having openings therethrough for receiving tenons formed on the ends of the buckets, locating the inner, intermediate and outer cover elements in succession on the tenons of the buckets with ends of the tenons projecting from the outer element, peening projecting ends of the tenons to secure the elements to the buckets and providing a smooth continuous arcuate surface along the outer surface of the cover including along the peened ends of the tenons. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0012]FIG. 1 is a fragmentary cross-sectional view of a portion of a turbine illustrating a turbine rotor with buckets and a cover and associated turbine stator stages;  
     [0013]FIG. 2 is a schematic fragmentary axial cross-sectional view illustrating application of the multiple-layer bucket cover of the present invention on the tenons of the buckets of the rotor;  
     [0014]FIG. 3 is a view similar to FIG. 2 illustrating the completed bucket cover assembly; and  
     [0015]FIG. 4 is a cross-sectional view taken generally about on line  4 - 4  in FIG. 3. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0016] Referring now to the drawings, particularly to FIG. 1, there is illustrated a rotor  10  mounting a plurality of circumferentially spaced buckets, one of which is illustrated at  12 , having a bucket cover  14 . Axially adjacent stator vanes  16  and  18  of the fixed component of the turbine are also disclosed in the turbine flowpath  20 . Labyrinth-type seals  22 , as well as a brush seal  24 , are illustrated for sealing about the covers  14  during operation of the turbine.  
     [0017] Referring now to FIG. 2, the cover  14  includes a plurality of arcuate cover segments  23 , each comprised of cover layers or elements, collectively identified at  25 . As illustrated, the collective cover elements  25  of each segment  23  include an arcuately extending outer element  26 , an inner cover element  28  and an intermediate cover element  30 . The cover elements  26 ,  28  and  30  extend arcuately and span a plurality of buckets  12 , for example, four to twenty buckets, depending upon the stage. The arcuate cover segments are secured on tenons  32  formed on the tip of each of the buckets  12 . The cover segments  23  butt adjoining cover elements at opposite ends in a circumferential direction at locations between the buckets to form a complete annulus about the buckets  12 . Likewise, each of the outer, inner and intermediate elements  26 ,  28  and  30 , respectively, butt one another in a circumferential direction. Thus, the joints  31  between the segments  23  have their inner, intermediate and outer elements aligned in a radial direction with one another. Alternatively, the elements may be staggered in a circumferential direction relative to one another whereby the joints between the corresponding elements  26 ,  28  and  30  are staggered circumferentially relative to one another.  
     [0018] As illustrated in FIG. 2, the buckets  12  terminate at their outer tips in tenons  32 . Each of the cover elements  26 ,  28  and  30  has openings at circumferentially spaced positions to receive the tenons  32 . As illustrated in FIG. 4, the tenons  32  are reduced in cross-sectional configuration from that of the bucket airfoil shape illustrated at  34  in FIG. 4 by the dashed lines. Each tenon  32  also terminates short of both the leading and trailing edges of the corresponding bucket. Their side surfaces also lie inwardly of the suction and pressure sides of the airfoil surfaces of buckets  12 .  
     [0019] As illustrated in FIG. 2, the juncture of the tenons  32  and the radial outer end of the airfoils of the buckets  12  are radiussed or chamfered as indicated at  36 . The openings  38  through the inner element  28  are also radiussed for engagement with the radii  36  at the juncture of the tenons and bucket airfoils. As illustrated in FIG. 4, the intermediate element  30  includes circumferentially spaced openings  40  in the general shape of, and for receiving the tenons  32 . Referring back to FIG. 2, the openings  42  through the outer elements  26  are also radiussed to provide generally concave surfaces  44  about the tenons  32 . It will be appreciated that each of the cover elements  26 ,  28  and  30  comprises discrete elements which are separately and sequentially applied to the tenons  32  of the buckets  12  during their assembly onto the rotor  10 .  
     [0020] As best illustrated in FIGS. 2, 3 and  4 , the intermediate element  30  also includes cavities  46  at spaced circumferential locations about the cover  12 . Preferably, each cavity  46  opens radially outwardly and inwardly of the intermediate element  30 . It will be appreciated, however, that the intermediate element may have a cavity formed therein rather than a cavity formed through the element. The cavity  46  is bounded by end walls  48  and  50  and also by margins of adjacent webs  52 . Each web  52  spans between the end walls  48  and  50 , the webs  52  surrounding the openings  40  receiving tenons  32 . The end walls  48  and  50  face upstream or downstream flow directions along the turbine flowpath. In assembly, it will also be appreciated that the cavity  46  is bounded by the inner and outer surfaces of the outer and inner elements  26  and  28 , respectively. In final assembly, it will therefore be appreciated that each of the cavities  46  along the cover and between the buckets is entirely closed.  
     [0021] To assemble the cover onto the rotor  10  and particularly on the tips of the buckets, the inner, intermediate and outer elements of each cover segment  23  are disposed on the tenons  32  in sequence. Once located on the tenons, as illustrated in FIG. 2, the tenons may be peened. Because the tenons project outwardly of the outer surface of each outer cover element  26 , the tenons may be peened by automatic machinery. During peening, the material of the tenons is deformed to expand laterally to fill the space between the tenons and the margins of the openings through the elements  26 ,  28  and  30 . Particularly, the deformed material fills the radius or concave openings  44  about the openings through the outer cover element  26 . Because the material is deformed by the peening operation into the enlarged area of the openings through the outer cover element  26 , the inner, intermediate and outer elements, which are stacked one against the other, are secured to the buckets  12  about the tenons  32 .  
     [0022] As illustrated in FIG. 3, the peening operation will result in a small button, indicated  54  in FIG. 3, along the outer surface of the outer element. These buttons, which are formed by excess material of the tenons, are then machined off to provide a smooth, continuous outer arcuate surface about the annular cover. That is, the outer surface of the tenon conforms to the arcuate circumferential shape of the covers.  
     [0023] With the foregoing construction, it will be appreciated that cover sealing is provided with reduced tip leakage losses because tight clearances can be maintained between the outer surface of the cover and the surrounding shroud or seal surfaces. While generally flush tenon/cover configurations do not provide adequate pull strength, i.e., there is insufficient strength to maintain the cover on the buckets during high centrifugal loads, the flush tenon/cover configuration of the present invention has substantial pull strength because of the reduction in weight of the cover afforded by the formation of the cavities  46  in the intermediate elements  30 . With this configuration, it will therefore be appreciated that the pull strength requirements are met. Importantly, the tenons can be automatically peened and provide tight cover sealing due to the flush tenon/cover design. The perforated cover design meets these requirements.  
     [0024] 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.