Abstract:
A bucket pair in a turbomachine includes a first bucket having an airfoil and a shank; a second adjacent bucket having a second airfoil and a second shank adjacent the first shank; a first axial slot in the first shank; and an elongated, straight damper pin adapted to seat in the first axial slot, the damper pin formed with slanted forward and aft end faces.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to turbomachines and particularly, to damper pins and seal pins disposed between adjacent buckets on a rotor wheel. 
         [0002]    As is well known, turbines generally include a rotor comprised of a plurality of rotor wheels, each of which mounts a plurality of circumferentially-spaced buckets. The buckets each typically include an airfoil, a platform, a shank and a dovetail, the dovetail being received in mating dovetail slot in the turbine wheel. The airfoils project into a hot gas path downstream of the turbine combustors and convert kinetic energy into rotational, mechanical energy. During engine operation, vibrations are introduced into the turbine buckets and if not dissipated, can cause premature failure of the buckets. 
         [0003]    Many different forms of vibration dampers have been proposed to minimize or eliminate vibrations. Vibration dampers are often in the form of an elongated damper pins that fit between adjacent buckets and provide the damping function by absorbing harmonic stimuli energy produced as a result of changing aerodynamic loading. A damper pin is typically retained in a groove formed along one circumferentially-oriented “slash face” in the turbine blade shank region of one of each pair of adjacent buckets. The damping pin is centrifugally loaded during operation and, in order to prevent bucket-to-bucket binding, the groove must be machined so as to allow the pin to float relatively freely within the groove. At the same time, highly-compressed air is often extracted from the compressor of an axial turbine for the purpose of cooling turbine components, particularly those in the hot gas path downstream of the combustion. This cooling air is required to maintain the temperature of the turbine components at an acceptable level for operation, but comes at a cost to overall turbine efficiency and output. Any of the cooling flow that leaks out of the turbine components is essentially wasted. The pocket created by a damper pin groove provides a large leakage path for cooling flow to escape from the bucket shank region. The cooling efficiency can also be impaired by ingress of hot gas from the hot gas path into the bucket shank region. 
         [0004]    In one prior arrangement, the damper pin has reduced-cross-section ends supported on shoulders formed in the bucket shank, with annular seals at the interfaces between the reduced-cross-section ends and the main body portion of the pin to minimize leakage along the damper pin groove. 
         [0005]    For industrial gas turbines utilizing long bucket shank designs, a further approach to seal against cross-shank leakage is to provide radial seal pins between the shanks of adjacent buckets on the fore and aft sides of the shank, below the axially-extending damper pin. Like the damper pin, the radial seal pins are seated in seal pin grooves formed on the same slash face as the damper pin groove, and engage the substantially flat sides of the shank of the adjacent bucket. The sealing effectiveness of these cross-shank seals is an important factor in increasing the bucket life by minimizing thermal stress. Even when using both damper pins and radial seal pins, however, gaps remain between the radial seal pins and the reduced-cross-section ends of the axially-oriented damper pin, again creating readily-available leakage paths for hot combustion gases flowing past the buckets. 
         [0006]    It would therefore be desirable to provide a more reliable sealing feature in order to prevent, minimize or control the escape of cooling flow from a pressurized shank cavity, prevent or minimize flow from leaking across the turbine blade from the forward wheel space to the aft wheel space in the case of a non-pressurized shank cavity, and/or to prevent ingress of hot gas path air into the shank region. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    In one exemplary but nonlimiting embodiment, there is provided a bucket pair in a turbomachine comprising a first bucket having an airfoil and a shank; a second adjacent bucket having a second airfoil and a second shank adjacent the first shank; a first axial slot in the first shank; and an elongated, straight damper pin adapted to seat in the first axial slot, the damper pin formed with axially slanted forward and aft ends. 
         [0008]    In another exemplary embodiment, there is provided a bucket for a turbine machine rotor wheel comprising an airfoil portion, a shank portion and a dovetail mounting portion, the shank portion including opposite side faces, one of the side faces formed with an axially-oriented slot extending between forward and aft ends of the shank portion; an elongated damper pin of uniform cross-section seated in the axially oriented slot, the elongated damper pin formed with axially slanted forward and aft ends. 
         [0009]    In still another aspect, the invention provides a bucket for a turbine machine rotor wheel comprising an airfoil portion, a shank portion and a dovetail mounting portion, the shank portion including opposite side faces, one of the side faces formed with an axially-oriented slot extending between forward and aft ends of the shank portion; an elongated damper pin of uniform cross-section seated in the axially oriented slot, the elongated damper pin formed with slanted forward and aft end faces; wherein the forward and aft ends of the shank portion include material substantially covering the slanted forward and aft ends; and wherein first and second substantially radially-oriented grooves are formed at forward and aft ends of the shank portion, the first and second seal pins located in the first and second substantially radially oriented grooves. 
         [0010]    The invention will now be described in connection with the drawings identified below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a perspective view of a gas turbine bucket and damper pin assembly; 
           [0012]      FIG. 2  is a partial side elevation showing a pair of circumferentially-adjacent buckets with a damper pin located therebetween; 
           [0013]      FIG. 3  is a partial perspective view of another gas turbine bucket and damper pin assembly, wherein the damper pin is provided with discreet seal elements in one prior arrangement; 
           [0014]      FIG. 4  is a perspective view of a damper pin with attached discreet seal elements of the type shown in  FIG. 3 ; 
           [0015]      FIG. 5  is a partial end view of a pair of adjacent buckets incorporating a damper pin/seal of the type shown in  FIGS. 3 and 4 ; 
           [0016]      FIG. 6  is a partial side elevation of a damper pin and radial seal pin configuration in accordance with a first exemplary but nonlimiting embodiment of this invention; 
           [0017]      FIG. 7  is a partial end view of a pair of adjacent buckets incorporating the damper pin and radial seal pin arrangement of  FIG. 6 ; 
           [0018]      FIG. 8  is a partial perspective view of the turbine shank portion on which the damper pin rests; and 
           [0019]      FIG. 9  is a partial side elevation of a bucket incorporating a damper pin and radial seal pin configuration in accordance with another exemplary but nonlimiting embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]      FIGS. 1 and 2  illustrate a conventional bucket  10  including an airfoil  12 , a platform  14 , a shank  16  and a dovetail  18 . The dovetail  18  is utilized to secure the bucket  10  to the periphery of the rotor wheel (not shown), as is well understood in the art. A damper pin  20  is located along one axial edge (or slash face)  22  adjacent (i.e., radially inward of) the bucket platform  14  with the leading end  24  of the damper pin  20  located nearer the leading edge of the bucket, and the trailing end  26  of the damper pin located nearer the trailing edge of the bucket. 
         [0021]    It will be appreciated that a similar pin  20  is located between each adjacent pair of buckets  18 ,  118  on the turbine wheel, as apparent from  FIG. 2 . Specifically, the damper pin  20  is located in a groove  28  extending along the entire slash face  22  of the bucket  118 . The damper pin  20  includes a substantially cylindrical body portion  30  between a pair of substantially semi-cylindrical, opposite ends  24 ,  26  interfacing at shoulders  39 . This configuration creates flat support surfaces  32 ,  34  (best seen in  FIG. 1 ) that are adapted to rest on the machined bucket platform surfaces or shoulders (one shown at  36  in  FIG. 2 ) at opposite ends of the groove  28  formed in the bucket slash face, thereby providing good support for the pin while preventing undesirable excessive rotation during machine operation. 
         [0022]      FIGS. 3 and 4  illustrate a long bucket  37  where radially-oriented seal pins  38 ,  40  are used in combination with a damper pin  42 . In this instance, the damper pin  42  is formed or provided with “piston ring” seals  44 ,  46  at opposite ends where the damper pin transitions to reduced-cross-section ends  48 ,  50 . Note, however, that there is still a considerable gap between the radially outer ends  52 ,  54  of the seal pins  38 ,  40  and the ends  48 ,  50  of the damper pin  42 . In addition, as made clear from  FIG. 5  (where the reduced-cross-section end  48  is visible between adjacent buckets  37 ,  137 ) the slot in which the damper pin is located is open at both ends, allowing cooling air to escape through the clearance spaces between the pin and the groove in which it is seated, especially along the reduced-cross-section ends  48 ,  50 . 
         [0023]    It should be understood that the grooves in which the damper pin  42  and radial seal pins  38 ,  40  are seated are provided on only one side of the bucket, and that they engage flat surfaces on an adjacent bucket. In other words, each bucket in a circumferential row of buckets is formed such that the damper/seal pins seated in grooves formed on one side of a bucket engage flat surfaces of an opposite side of an adjacent bucket. 
         [0024]      FIG. 6  illustrates a bucket  56  provided damper pin/radial seal pin configuration in accordance with an exemplary but nonlimiting embodiment of the invention. Specifically, an axially-extending, substantially round damper pin  58  is formed with slanted forward and aft ends defined by, for example 45° surfaces  60 ,  62  that rest on similarly slanted surfaces  62 ,  64  formed internally of the shank, at opposite forward and aft ends thereof. More specifically, surfaces  62 ,  64  are slanted in opposite linear or axial directions. This design eliminates the need for the reduced-cross-section ends and the “piston ring” seals as shown in  FIG. 4 , and allows the length of the damper pin  58  to be shortened. The slanted ends or surfaces  60 ,  62  also allow the radial seal pins  68 ,  70  to be extended in length in a radially outward direction to close the gap between the radial seal pins and the damper pin. For example, in one exemplary embodiment, the radial pin seal slots  72 ,  74  in the shank may be extended radially outwardly (toward the airfoil) by 0.140 inch, to obtain greater sealing performance. Note also that the upper or radially outer ends of the seal pin slots or grooves  72 ,  74  are at least partially overlapped by the slanted end surfaces  60 ,  62  of the damper pin  58 . This arrangement also allows additional shank material to be retained at the ends of the damper pin groove surfaces  62 ,  64  to substantially cover the ends of the damper pin  58  as shown in Figs,  7  and  8 , further reducing the area of the leakage path. In one example, the thickness of the shank from the edge of the damper pin  58  to the forward face  72  of the shank may be on the order of 0.320 inch, thus providing sufficient material for the “cover plates”  75  ( FIG. 7 ). A comparison of the buckets  76 ,  176  in  FIG. 7  illustrates the significant reduction is leakage area resulting from the addition of the cover plates  75 . Thus, there are two aspects of the exemplary design that reduce leakage at each of the forward and aft ends of each pair of adjacent buckets. First, the gap between the upper end of the radial pin  68  and the damper pin  56  (as defined by the solid ligament or web  76 ) can be reduced, and second, an increase in shank material at the opposite ends of the damper pin groove  78  enables the ends of the damper pin  56  to be substantially covered by the cover plates  75 . 
         [0025]    In another exemplary but nonlimiting example shown in  FIG. 9 , the forward side radial pin  80  may be shortened in length and angled more sharply toward the damper pin  82 , allowing the outer end of the radial seal pin  80  to be located further up the slanted surface of the damper pin  82  to reduce the leakage area between the damper pin and seal pin. Because pressures are greater at the radially outer end of the forward side shank portion, leakage is more likely in this area than in radially inner areas of the shank. 
         [0026]    For both described embodiments, the radially outer edges of the seal pin grooves (one referenced by numeral  84  in  FIG. 8 ) are shown to be substantially parallel to the slanted surface of the damper pin groove (one referenced by numeral  86  in  FIG. 8 ). It will be understood, however that these surfaces need not be parallel. The angle of edge  84  is the contact angle for the radial pin, and that angle may be optimized depending on various parameters including the loading on the pin under various operating conditions, diameter of the pin, etc. In addition, the dimensions for the damper pin and seal pin grooves, the diameter and length dimensions of the damper pin and seal pins will also be application specific, also taking into account thermal growth characteristics; damper pin-to-bucket mass ratio to insure effective vibration damping; and other parameters understood by those skilled in the art. 
         [0027]    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.