Abstract:
The present invention provides, in one embodiment, an annular turbine seal for disposition in a turbine between a rotatable component having an axis of rotation and a turbine housing about the same axis of rotation. The turbine seal has a plurality of arcuate seal carrier segments that have an abradable portion secured to the seal carrier segments. In addition, at least one spring is disposed on the seal carrier segment to exert a force and maintain the seal carrier segment adjacent to the rotatable component.

Description:
BACKGROUND OF INVENTION  
         [0001]    The present invention relates generally to rotary machines, and more particularly to a seal assembly for a rotary machine such as steam and gas turbines.  
           [0002]    Rotary machines include, without limitation, turbines for steam turbines and compressors and turbines for gas turbines. A steam turbine has a steam path that typically includes, in serial-flow relationship, a steam inlet, a turbine, and a steam outlet. A gas turbine has a gas path which typically includes, in serial-flow relationship, an air intake (or inlet), a compressor, a combustor, a turbine, and a gas outlet (or exhaust nozzle). Gas or steam leakage, either out of the gas or steam path or into the gas or steam path, from an area of higher pressure to an area of lower pressure, is generally undesirable. For example, a gas path leakage in the turbine or compressor area of a gas turbine, between the rotor of the turbine or compressor and the circumferentially surrounding turbine or compressor casing, will lower the efficiency of the gas turbine leading to increased fuel costs. Also, steam-path leakage in the turbine area of a steam turbine, between the rotor of the turbine and the circumferentially surrounding casing, will lower the efficiency of the steam turbine leading to increased fuel costs.  
           [0003]    It is known in the art of steam turbines to position, singly or a combination, variable clearance labyrinth-seal segments and brush seals in a circumferential array between the rotor of the turbine and the circumferentially surrounding casing to minimize steam-path leakage. Springs hold the segments radially inward against surfaces on the casing that establish radial clearance between seal and rotor but allow segments to move radially outward in the event of rotor contact. While labyrinth seals, singly or in combination with brush seals, have proved to be quite reliable, their performance degrades over time as a result of transient events in which the stationary and rotating components interfere, rubbing the labyrinth teeth into a “mushroom” profile and opening the seal clearance.  
           [0004]    Accordingly, there is a need in the art for a rotary machine having good leakage control between stationary and rotating components.  
         SUMMARY OF INVENTION  
         [0005]    The present invention provides, in one embodiment, an annular turbine seal for disposition in a turbine between a rotatable component having an axis of rotation and a turbine housing about the same axis of rotation. The turbine seal has a plurality of arcuate seal carrier segments that have an abradable portion secured to the seal carrier segments. In addition, at least one spring is disposed on the seal carrier segment to exert a force and maintain the seal carrier segment adjacent to the rotatable component. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0006]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:  
         [0007]    [0007]FIG. 1 is a schematic, cross-sectional exploded view of one embodiment of the instant invention.  
         [0008]    [0008]FIG. 2 is a schematic, cross-sectional exploded view of another embodiment of the instant invention.  
         [0009]    [0009]FIG. 3 is a schematic, cross-sectional exploded view of another embodiment of the instant invention.  
         [0010]    [0010]FIG. 4 is a schematic, cross-sectional exploded view of another embodiment of the instant invention.  
         [0011]    [0011]FIG. 5 is a schematic, cross-sectional exploded view of another embodiment of the instant invention. 
     
    
     DETAILED DESCRIPTION  
       [0012]    A rotary machine  100 , for example, a steam turbine, typically comprises a rotating turbine bucket  110  disposed in a stationary turbine housing  120  and which turbine bucket  110  is supported by conventional means, not shown, within turbine housing  120  (as shown in FIG. 1). An abradable seal  130 , generally designated  130 , disposed between rotating turbine bucket  110  and stationary turbine housing  120 , comprises an arcuate seal carrier segment  140  disposed adjacent to turbine bucket  110  separating pressure regions on axially opposite sides of arcuate seal carrier segment  140 . Arcuate seal carrier segment  140  includes an abradable portion  150  radially disposed on seal carrier segment first surface  190 . As used herein, “on”, “over”, “above”, “under” and the like are used to refer to the relative location of elements of rotary machine  100  as illustrated in the Figures and is not meant to be a limitation in any manner with respect to the orientation or operation of rotary machine  100 . It will be appreciated that while only one arcuate seal carrier segment  140  and one abradable portion  150  are illustrated, typically a plurality of abradable seals  130  having at least one abradable portion  150  and at least one arcuate seal carrier segment  140  are provided about turbine bucket  110 . Abradable portion  150  is of a design for obtaining close clearances with the radial projections or ribs  160  and the grooves  170  of the bucket cover  180 . For example, during operation, ribs  160  and grooves  170  wear away part of abradable portion  150  leaving a profile matching that of ribs  160  and grooves  170  on abradable portion  150  resulting in a close clearance between the components. The clearance is typically in the range between about 0.02 mm and about 0.7 mm. It will also be appreciated by one of ordinary skill in the art that the location, number and height of ribs  160  and grooves  170  located on bucket cover  180  may be varied. In addition, turbine bucket  110  components (e.g. bucket cover  180 ) facing abradable portion  150  may be varied as well, for example, there may not be a bucket cover  180  and therefore the turbine bucket  110  surface may be flat.  
         [0013]    Abradable seal  130  segments are typically spring-backed and are thus free to move radially when subjected to movement during normal conditions of startup. For example, abradable seal  130  segments are free to move radially when there is a variance from the normal rotational profile between abradable seal  130  and turbine bucket  110 . In one embodiment, springs  185  exert a force to keep abradable seal  130  disposed adjacent to bucket cover  180  and allow some radially outward movement of arcuate seal carrier segment  140  during transient events, for example, during startup and shutdown. Springs  185  typically comprise, but are not limited to, leaf springs or coil springs. Springs  185  apply a radial force, when assembled in the rotary machine, that is typically in the range of about 2 to about 5 times the weight of the arcuate seal carrier segment  140  that it is supporting. In operation, springs  185  only need to provide enough force to seat arcuate seal carrier segment  140  radially toward turbine housing  120  and keep arcuate seal carrier segment  140  disposed adjacent to turbine bucket  110 , bucket cover  180  or blades (see FIG. 2). As a result of “seating” arcuate seal carrier segment  140  radially toward turbine housing  120 , the gap “G” (see FIG. 1) between seal carrier segment  140  and turbine housing  120  is minimized thus reducing gas or steam leakage in the turbine area of a gas or steam turbine (see FIG. 2). For example, steam turbine applications, the weight of an individual arcuate seal carrier segment  140  is typically in the range of about 10 pounds to about 25 pounds. Thus, springs  185  must provide at least this level of force in order to provide enough force to seat arcuate seal carrier segments  140  radially toward turbine housing  120 . In another embodiment, spring  185  is disposed on a plurality of arcuate seal carrier segments  140 . In another embodiment, a single spring is disposed on the entire annular array of arcuate seal carrier segments  140 .  
         [0014]    In another embodiment, the spring system of the present invention is adapted to be used in conjunction with other means to apply pressure to arcuate seal carrier segments  140 . For example, springs work in conjunction with gas pressures (illustrated in phantom in FIG. 2) for providing a force to keep abradable seal  130  disposed adjacent to bucket cover  180  or turbine buckets  110 . In this embodiment, arcuate seal carrier segment  140  is initially pushed axially toward turbine housing  120  by the upstream pressure which is caused by the expansion of the gas through the turbine and dictated by the design of the gas or steam path geometry and flow (see FIG. 1). This upstream pressure eventually fills the cavity between turbine housing  120  and arcuate seal carrier segment  140  and further forces arcuate seal carrier segment  140  radially inward to reduce the clearance with turbine buckets  110 , for example, after the turbine has been brought up to speed. In one embodiment, at least one spring  185  is disposed on each of the arcuate seal carrier segments  140 .  
         [0015]    In one embodiment, abradable portion  150  composition typically comprises a first component comprising cobalt, nickel, chromium, aluminum, yttrium (hereinafter referred to as CoNiCrAlY) and a second component selected from the group consisting of hexagonal boron nitride (hexagonal BN) and a polymer. Typical polymers used are thermosets, such as polyesters and polyimides. In another embodiment, abradable portion  150  composition typically comprises a component comprising nickel, chromium and aluminum, and another component comprising clay (e.g. bentonite) (hereinafter referred to as “NiCrAl+clay”). Another embodiment is a composition typically comprising a first component consisting nickel and graphite (hereinafter referred to as “Ni+Graphite”) or a second component comprising of stainless steel. Another embodiment is a composition typically comprising nickel, chromium, iron, aluminum, boron and nitrogen (hereinafter referred to as “NiCrFeAlBN”). Another embodiment comprises a first component comprising chromium, aluminum and yttrium (hereinafter referred to as “CrAlY”) and a second component selected from the group consisting of iron, nickel and cobalt. Furthermore, abradable portion  150  may consist of a composition typically comprising a first component comprising chromium and aluminum (hereinafter referred to as “CrAl”) and a second component selected the group consisting of iron, nickel and cobalt. Other embodiments of abradable portion  150  composition may include a material composed of metal fibers that are pressed or sintered together or infiltrated with resin or other material, for example, Feltmetal™ (offered for sale by Technectics Corp., DeLand, Fla.) and a nickel based alloy with high resistance to oxidation, for example, Hastelloy™ (offered for sale by Technectics Corp., DeLand, Fla.). It will be appreciated that abradable portion  150  is disposed on seal carrier segment first surface  190  by brazing or thermal spraying, for example. In addition, it will be appreciated by one of ordinary skill in the art that the thermal spray may be adjusted to introduce porosity into the abradable portion. Operating conditions for abradable portion  150  composition is typically in the range between about 20° C. and about 700° C.  
         [0016]    Referring to FIG. 1, abradable portion  150  nominally projects from arcuate seal carrier segment  140  a distance “t” which corresponds to the maximum expected radial incursion of the turbine buckets  110  or blades into the abradable portion  150  of abradable seal carrier  130  in a radial direction. Consequently, the distance “t” corresponds to the radial deflection of the turbine buckets  110  and its calculation is dependent on the predicted deflection of rotary machine  100  and the radial deflection of arcuate seal carrier segments  140  during transient or steady-state operation. Abradable portion  150  radial distance “t” is typically in the range between about 0.5 mm and about 5 mm. In one embodiment, abradable portion  150  arcuate length “l” and width “w” is equal to the arcuate length and width of the arcuate seal carrier segment  140  (see FIG. 5). It will be appreciated that arcuate length and width of abradable portion  150  may vary depending upon the application.  
         [0017]    In accordance with another embodiment of the instant invention (see FIG. 2), there is provided a springbacked abradable seal  130  formed by the combination of an abradable portion  150  and at least one labyrinth tooth  200 . It will be appreciated that the location and number of labyrinth teeth  200  on arcuate seal carrier segment  140  may be varied. In one embodiment, labyrinth teeth  200  are typically located at the periphery of each arcuate seal carrier segment  140  as shown in FIG. 2. Here, at least one labyrinth tooth  200  profile extends  360 ° about the edge annular array of seal carrier segments (not shown).  
         [0018]    In accordance with another embodiment of the instant invention (see FIG. 3), there is provided a springbacked abradable seal  130  formed by the combination of an abradable portion  150  and at least one brush seal  210 . It will be appreciated that the location and number of at least one brush seal  210  may be varied depending upon desired application. In operation, it will be appreciated that the combined abradable portion  150  and at least one brush seal  210  may move radially inwardly and outwardly with the tips of the bristles  220  engaging the turbine bucket covers  180  substantially throughout the full 360° circumference of the rotor.  
         [0019]    In accordance with another embodiment of the instant invention (see FIG. 4), there is provided a springbacked abradable seal  130  formed by the combination of an abradable portion  150 , at least one brush seal  210  and at least one labyrinth tooth  200 . It will be appreciated that the location and number of at least one brush seal  210  and at least one labyrinth tooth  200  may be varied depending upon desired application. For example, in steam or gas turbines, solid particles are typically centrifuged outward at the blade tips. The labyrinth tooth  200  and brush seal  210  serve as auxiliary seals in case of excessive erosion of the abradable portion. Depending upon at least one brush seal  210  bristle angle, there may be a lack of bristles  220  at the ends of arcuate seal carrier segment  140 . The lack of bristles  220  at the ends of arcuate seal carrier segment  140  does seriously compromise or degrade the sealing capability because of the structural combination with abradable portion  150 , at least one labyrinth tooth  200  or both.  
         [0020]    It will be apparent to those skilled in the art that, while the invention has been illustrated and described herein in accordance with the patent statutes, modification and changes may be made in the disclosed embodiments without departing from the true spirit and scope of the invention. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.  
         [0021]    [0021]