Abstract:
A pattern-abradable seal assembly is provided for a stationary steam turbine component. The seal assembly, in use, is oriented in opposition to at least one seal tooth on a rotatable turbine component so as to inhibit leakage flow across the seal assembly in one direction, the seal assembly may include an annular seal carrier having at least one axially-oriented surface; a pattern-abradable/abrasive seal coating or insert at least partially covering the at least one axially-oriented surface, the pattern-abradable/abrasive seal coating having a pattern formed thereon adapted to face and be at least partially penetrated by the at least one seal tooth. A plurality of anti-swirl elements project radially beyond the pattern and are arranged to provide at least an axial component of flow across the abradable seal assembly. The coating or insert may also be used on other stationary turbine component surfaces to direct flow in a predetermined direction.

Description:
BACKGROUND 
       [0001]    The invention relates to pattern-abradable/abrasive seals in steam turbines and especially to abradable/abrasive coatings with patterns in the shape of sealing features, anti-swirl and/or guide seal features disposed radially outwardly of shrouded nozzle/buckets or on surfaces on stationary components axially adjacent to the nozzles/buckets to reduce leakage flow, reduce swirl and/or to aerodynamically guide the leakage flow to improve turbine efficiency. 
         [0002]    It is well known to use abradable/abrasive materials which readily form seals between fixed and rotating parts of a turbine, whereby the rotating part erodes a portion of the fixed abradable material to form a seal having a very close tolerance. An important application of abradable seals in steam turbines where a rotor supporting a plurality of wheels, each of which mounts a plurality of blades or buckets rotating within a surrounding shroud. Utilizing abradable seals to minimize the clearance between the blade tips/nozzle root location and inner wall of the opposed shroud, makes it possible to reduce leakage of the working fluid which could be steam, across the blade tips and thereby enhance turbine efficiency. 
         [0003]    Similar abradable/abrasive seals are also employed in turbines along the turbine rotor section to minimize leakage flow along the rotor shaft between higher and lower pressure regions. For example, conventional labyrinth seals provide a torturous path along the rotor shaft minimizing leakage flow, and generally, comprise a plurality of radial teeth extending from the rotor, with a small cold clearance between the teeth and the opposed, stationary abradable seal. 
         [0004]    Typically, metal or ceramic abradable seals are spray-coated onto the stationary seal surface, and are effective to establish a radial clearance of about 15 mils. 
         [0005]    There is a continuing need to improve efficiency by further reducing clearances, guiding the leakage flow at a favorable angle to adjacent nozzle/buckets and by minimizing the effect of swirl or tangential flow at the seal caused by the rotating component which decreases reliability, turbine efficiency and thus turbine performance. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    Accordingly, in one exemplary but nonlimiting embodiment, there is provided a pattern-abradable/abrasive seal assembly for a stationary steam turbine component, the seal assembly, in use, oriented in opposition to at least one seal tooth on a rotatable steam turbine component so as to inhibit leakage flow across the seal assembly and/or guide the leakage flow in a first direction, the seal assembly comprising an annular seal carrier having at least one axially-oriented, annular seal surface; a pattern-abradable/abrasive seal coating at least partially covering the at least one axially-oriented, annular seal surface, the pattern-abradable/abrasive seal coating having a pattern formed therein, adapted, in use, to face and be at least partially penetrated by the at least one opposed seal tooth; and a plurality of anti-swirl elements projecting radially beyond the pattern and arranged circumferentially about the at least one axially-oriented, annular seal surface. 
         [0007]    In another exemplary but nonlimiting embodiment, there is provided a coating or insert for use on a surface of a stationary steam turbine component located along the steam path comprising: a first surface facing an adjacent rotating steam turbine component; and a first pattern-abradable/abrasive coating or insert having a pattern formed therein applied to the surface wherein the pattern is designed to direct leakage flow in a predetermined direction relative to the stationary steam turbine component. 
         [0008]    In still another exemplary but nonlimiting embodiment, there is provided a turbine bucket and abradable seal assembly comprising a bucket having a tip shroud formed with plural radially-directed seal teeth; a stationary stator component surrounding the bucket and having plural abradable seals opposing respective ones of the plural radially-directed seals teeth; wherein each of the plural abradable seals includes an abradable seal coating having a pattern formed on a surface thereof facing a respective one of the plural, radially-directed seal teeth, and at least one an anti-swirl element projecting radially beyond the pattern and arranged to provide at least an axial component of flow across the seal assembly, the at least one anti-swirl element opposed to one of the plural radially-directed seal teeth, and adapted to be at least partially penetrated thereby. 
         [0009]    The invention will now be described in detail in connection with the drawings identified below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a partial side elevation of a shrouded steam turbine bucket interacting with pattern-abradable seal elements on a radially opposed stationary stator component and on adjacent upstream and downstream nozzles; 
           [0011]      FIGS. 2-5  illustrate various surface patterns that may be employed on the surface of the seal elements shown in  FIG. 1 ; 
           [0012]      FIGS. 6-8  illustrate an exemplary but nonlimiting embodiment of a pattern-abradable seal and an opposed seal tooth; 
           [0013]      FIGS. 9-11  illustrate an exemplary but nonlimiting embodiment of an anti-swirl feature incorporated into the patterned-abradable seal; 
           [0014]      FIGS. 12-14  illustrate a second exemplary but nonlimiting embodiment of an anti-swirl elements added to a patterned-abradable seal; 
           [0015]      FIGS. 15-17  illustrate a different surface patterns that may be employed on the surface of the seal added to a pattern-abradable seal; 
           [0016]      FIGS. 18-20  illustrate a third exemplary but nonlimiting embodiment of anti-swirl elements added to a pattern-abradable seal; 
           [0017]      FIGS. 21-23  illustrate a fourth exemplary but nonlimiting embodiment of anti-swirl elements added to a pattern-abradable seal; 
           [0018]      FIG. 24  is a partial side elevation of a shrouded bucket tip and pattern-abradable/abrasive seals on a radially outward stationary stator component and on a stationary downstream nozzle in accordance with another exemplary but nonlimiting embodiment of the invention; 
           [0019]      FIG. 25  illustrates a nozzle root seal arrangement incorporating pattern-abradable seals in accordance with another exemplary but nonlimiting embodiment of the invention; 
           [0020]      FIG. 26  discloses pattern-abradable seals incorporated in a packing ring segment in accordance with another exemplary but nonlimiting embodiment of the invention; and 
           [0021]      FIG. 27  is a plot showing a reduction in tangential velocity as a function of length of an anti-swirl element on a pattern-abradable seal in accordance the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    With reference initially to  FIG. 1 , a shrouded bucket  10  is shown mounted to a rotor wheel (not shown) axially between a pair of upstream and downstream nozzle vanes  12 ,  14 . The shrouded bucket  10  is provided with a tip shroud  16  formed with a plurality of radially-projecting, axially-spaced teeth  18 ,  20  and  22 , each of which is arranged to interact with a respective seal element  24 ,  26  and  28  on the surrounding stator or stator shroud  30  (sometimes referred to herein as a “seal carrier”). The seal elements are fixed to stator seal surfaces  32 ,  34  and  36 , respectively. The seal elements are identical and, therefore, only one need be described in detail. Thus, for example, the seal  26  is a pattern-abradable seal that may be spray coated on the adjacent stator seal surface  34 , and may comprise an abradable metallic or ceramic material typically used for such purposes. 
         [0023]    More specifically, an abradable coating can be applied by thermal spraying, e.g., by plasma spraying the coating composition through a mask onto the stator shroud surface  34 . Exemplary methods of producing an abradable coating on a substrate, utilizing, for example, an abradable ceramic coating composition, is described in commonly-owned U.S. Pat. No. 6,887,528. 
         [0024]    Exemplary but non-limiting abradable patterns for the coating that forms the seal  26  are illustrated in  FIGS. 2-5 . It will be appreciated that these patterns are not drawn to scale but are enlarged for clarity. More specifically,  FIG. 2  illustrates a pattern comprised of angled, spaced and staggered “bricks”  38 . The “bricks” are arranged in circumferential rows, with one row staggered circumferentially relative to the other. Note that the angled orientation of the pattern will also serve to guide the leakage flow in a desired path relative to a downstream component.  FIG. 3  illustrates a dense, circumferentially staggered-brick pattern, with the bricks  40  arranged substantially perpendicular to the flow direction.  FIG. 4  illustrates a diamond-mesh pattern  42 , and  FIG. 5  illustrates a staggered chevron pattern  44 . In all cases, adjacent annular rows of similar pattern elements are circumferentially offset or staggered. It will be appreciated that other patterns are also within the scope of the invention. 
         [0025]    Typically, the rotating bucket teeth will penetrate from about 50 to about 100 percent of the seal thickness. For example, with a tight cold radial clearance between the bucket teeth  18 ,  20 ,  22  and the stator shroud  24 ,  26 ,  28  of about 15 mils, the abradable coating with a thickness of about 30-100 mils, may be penetrated by the teeth to a depth of about 10 to 25 mils during operation. 
         [0026]      FIGS. 6-8  illustrate the pattern abradable seal  26  schematically, with  FIG. 6  also illustrating the relative position of the seal  26  vis-à-vis a radially opposed seal tooth  20 . The seal  26  is shown as comprised of the base coating  46  and the patterned surface  48 , the latter similar to pattern  42  in  FIG. 4 . In the exemplary embodiment, the base abradable coating may have a thickness of between about 15-100 mils, and the patterned surface may have a thickness of between about and 15-100 mils (would prefer if this statement can be generalized). With a total coating thickness of between about and 30-200 mils. With this arrangement, and in an exemplary embodiment where the seal is employed for use with a shrouded bucket in the high-pressure section of a steam turbine, the cold clearance can be reduced to about 10 mils. Note that the “abradable coating” and the “base coating” may be the same material, and the depth or thickness of the “abradable coating” merely indicates the depth of the pattern itself relative to the overall coating thickness. 
         [0027]    In the exemplary embodiments, the stationary; pattern-abradable/abrasive seal is used with shrouded buckets but it is not limited to that application, and in fact, may be used wherever seal teeth are employed on rotating turbine components. 
         [0028]    It is also a feature of the invention to add anti-swirl features to the pattern-abradable/abrasive seal. These features help reduce swirl/tangential flow components and thus provide better rotor damping and improve overall turbine efficiency. For example, as illustrated in  FIGS. 9-11 , the anti-swirl feature takes the form of angled or slanted three-dimensional rectangular blocks  50  that are aligned along the downstream edge  52  of the stator shroud  54  (or stationary turbine component), at an acute angle relative to an axial centerline of the stator, overlying the pattern-abradable/abrasive seal coating, i.e., projecting radially beyond the patterned surface. With this arrangement, leakage flow entering the pattern-abradable/abrasive seal component will first impinge on the anti-swirl blocks  50  which will break up the swirling flow caused by the rotating buckets and create an axial leakage flow component by means of the angled gaps between the anti-swirl blocks  50 , before flowing around seal tooth  58  ( FIG. 9 ). The blocks  50  may be spray-coated onto the surface  56  and built up to the desired thickness, using conventional masking techniques. Alternatively, the coatings and/or anti-swirl features can be manufactured as removable inserts. The material may be the same as the patterned surface  56  and/or the base coating  60 . As already mentioned, the base coating and patterned surface may likewise be of the same material. It is noted that the anti-swirl features are located at a position (or positions) axially offset from the opposed seal tooth is that there is not contact between the seal tooth and the anti-swirl features. 
         [0029]    Another exemplary but nonlimiting embodiment is illustrated in  FIGS. 12-14  where a similar row of angled, rectangular blocks  62  are also applied along the upstream edge  53  of the shroud, bracketing the seal tooth. For  FIGS. 12-14 , reference numerals used in.  FIGS. 9-11  are also used here to designate corresponding components. Here again, the swirling or tangential flow is broken up and the leakage flow is caused to have an axial flow component as it passes through the gaps between the angled blocks  62 , over the seal tooth  58  and through the gaps between similarly angled blocks  50 . 
         [0030]    A still further exemplary but nonlimiting embodiment is shown in  FIGS. 15-17  where solid annular ribs or rings (or ring segments)  64 ,  66  are provided along the upstream and downstream edges  68 ,  70  of the patterned surface  72  which overlies the base coating  74 . 
         [0031]      FIGS. 18-20  illustrate yet another exemplary but nonlimiting embodiment of anti-swirl elements added to a pattern-abradable seal. Here, rows of angled, rectangular blocks are applied not only along the upstream and downstream edges  76 ,  78  of the stator shroud  80 , but also between the marginal rows. More specifically, marginal rows  82 ,  84  of blocks  86 ,  88 , respectively, and two intermediate rows  90  and  92  of blocks  94  and  96 , respectively, are applied to the patterned surface  98  overlying the base coating  100 . The height of the blocks in each row is dictated by the seal tooth height. With particular reference to  FIG. 18 , it may be seen that blocks  86  and  82  are the same height as blocks  96  and  92  and both row interact with relatively long seal teeth  102 ,  104  of substantially the same height. Similarly, blocks  94  and  88  in rows  90  and  84 , respectively, have substantially similar heights dictated by the relatively shorter seal teeth  106 ,  108 . The swirling tangential flow is broken up by the angled blocks and given an axial flow component but, in this embodiment, the different heights of the seal teeth cause the leakage flow to follow an even more tortuous path in the axial direction, leading to even greater sealing efficiency. 
         [0032]    In this embodiment, the seal teeth engage the anti-swirl features, i.e., the anti-swirl features also serve as seal elements, and therefore, the base surface need not be patterned. 
         [0033]      FIGS. 21-23  illustrate another exemplary but nonlimiting embodiment, utilizing multiple seal teeth  102 ,  104 ,  106  and  108  as in the previously-described embodiment, but wherein the anti-swirl features comprise plural rows  110 ,  112 ,  114  and  116  of circumferentially staggered, rectangular blocks  118 ,  120 ,  122  and  124 , respectively, arranged on the pattern-abradable seal  126  (overlying the base coating  128 ), substantially parallel to the direction of flow. The differential height of the blocks and the seal teeth remain as described in connection with the embodiment illustrated in  FIGS. 18-20  but here, there are no axial gaps between the rows  110 ,  112 ,  114  and  116  (compare  FIGS. 18 and 21 ), but there are circumferential gaps between the adjacent staggered rows as plainly evident from  FIGS. 22 and 23 , thus providing unobstructed axial passageways for leakage flow. 
         [0034]    It will be appreciated that the combination of pattern-abradable/abrasive seals and anti-swirl features is applicable to other steam turbine bucket configurations, nozzle root seals and labyrinth packing seals. In this regard, attention is drawn to  FIG. 24  which is similar to  FIG. 1  but wherein four seal teeth  130 ,  132 ,  134  and  136  are located in axially-spaced relationship along the bucket shroud tip  138 , arranged to engage opposed abradable-pattern seals  140 ,  142 ,  144  and  146  on a packaging ring segment  148  as described above. 
         [0035]    In  FIG. 25 , a nozzle root seal arrangement is disclosed wherein the seal teeth  150 ,  152 ,  154  and  156  are arranged to penetrate the pattern-abradable seal elements  158 ,  160 ,  162  and  164 . 
         [0036]      FIG. 26  discloses yet another embodiment where seal teeth  166 ,  168  and  170  on the rotating component  172  are interleaved with seal teeth  174 ,  176  and  178  on a stationary packing ring segment  180 . Between the packing ring teeth, the pattern-abradable seal elements  182 ,  184  and  186  are applied to the surfaces of the packing ring segment  180 . 
         [0037]    It will be appreciated that any of the anti-swirl elements described in connection with  FIGS. 9-23  may be employed with the seal elements shown in  FIGS. 24-26 . 
         [0038]    To demonstrate the significant reduction in tangential flow velocity achieved with the anti-swirl features described herein,  FIG. 27  plots tangential velocity against the axial length of the anti-swirl block shown, for example, in  FIG. 22 . It can be seen that there is a dramatic reduction in the high swirl component velocity from the inlet (or upstream) end to almost zero at the exit end of the anti-swirl feature. 
         [0039]    It is still another feature of the invention to utilize pattern-abradable/abrasive seal coatings or inserts on surfaces axially upstream or downstream of the rotatable components such as the blades/buckets described above,  FIG. 1 , for example, illustrates coatings or inserts  190 ,  192  on upstream and downstream vanes or nozzles  12 ,  14 , respectively. 
         [0040]      FIG. 24  also illustrates a pattern-abradable/abrasive coating or insert  194  on a downstream, stationary component, adjacent the bucket tip shroud  13 B. 
         [0041]    By designing the pattern on the coating/insert to provide defined flow paths, it is possible to direct the leakage flow at a favorable angle to the adjacent rotating or stationary component. 
         [0042]    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.