Pattern-abradable/abrasive coatings for steam turbine stationary component surfaces   

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.

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.

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.

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.

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.

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.

SUMMARY

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.

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.

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.

The invention will now be described in detail in connection with the drawings identified below.

BRIEF DESCRIPTION OF THE DRAWINGS

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;

FIGS. 2-5 illustrate various surface patterns that may be employed on the surface of the seal elements shown in FIG. 1;

FIGS. 6-8 illustrate an exemplary but nonlimiting embodiment of a pattern-abradable seal and an opposed seal tooth;

FIGS. 9-11 illustrate an exemplary but nonlimiting embodiment of an anti-swirl feature incorporated into the patterned-abradable seal;

FIGS. 12-14 illustrate a second exemplary but nonlimiting embodiment of an anti-swirl elements added to a patterned-abradable seal;

FIGS. 15-17 illustrate a different surface patterns that may be employed on the surface of the seal added to a pattern-abradable seal;

FIGS. 18-20 illustrate a third exemplary but nonlimiting embodiment of anti-swirl elements added to a pattern-abradable seal;

FIGS. 21-23 illustrate a fourth exemplary but nonlimiting embodiment of anti-swirl elements added to a pattern-abradable seal;

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;

FIG. 25 illustrates a nozzle root seal arrangement incorporating pattern-abradable seals in accordance with another exemplary but nonlimiting embodiment of the invention;

FIG. 26 discloses pattern-abradable seals incorporated in a packing ring segment in accordance with another exemplary but nonlimiting embodiment of the invention; and

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

FIG. 24 also illustrates a pattern-abradable/abrasive coating or insert 194 on a downstream, stationary component, adjacent the bucket tip shroud 13B.

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.

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.