Abstract:
In a gas turbine including a nozzle retaining ring having a first annular axially facing sealing surface and a shroud segment having an axial registering second surface. To minimize or prevent leakage flow between the retaining ring and shroud segments, a generally U-shaped seal having reversely folded U-shaped marginal portions is received in a cavity formed in the second surface. At operating conditions, the marginal portions seal against the base of the cavity and the first surface of the retaining ring to prevent leakage flow past the retaining ring/shroud segment interface. To install the seal, the seal body is first compressed and maintained in a compressed state by applying one or more wraps about the seal body and an epoxy is used to secure the seal when compressed in the cavity. At operating temperatures, the retention means releases the seal to engage marginal portions against opposite sealing surfaces of the shroud segments and retaining ring.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to resilient seals in turbines and particularly to methods of compressing a seal and retaining the compressed seal in a restrained condition as well as to methods for installing and releasing the seal in situ to seal against adjacent turbine parts. 
     In a gas turbine, hot gases of combustion flow from combustors through nozzles and buckets of the various turbine stages. Compressor discharge air is typically used to cool certain of the turbine elements. It will be appreciated that there is a need for seals at various locations in the turbine, as well as different types of seals. In co-pending U.S. patent application Ser. No. 10/028,928, filed Dec. 28, 2001 (Attorney Docket No. 839-1127) and Ser. No. 10/029,003, filed Dec. 28, 2001 (Attorney Docket No. 839-1124), there is disclosed a similar seal for use at two different locations within the turbine. For example, one of the disclosed seals may be used for sealing between the nozzle segments and a nozzle support ring to provide a seal supplemental to the chordal hinge seals sealing against leakage flow from the high pressure compressor discharge region into the lower pressure hot gas path. Another similar seal may be utilized for sealing between the nozzle retaining ring and shroud segments. Leakage paths or gaps sometimes appear between these sealing systems during turbine operations. In these and other sealing sites in a turbine, it is therefore desirable that seals be deployed between these sealing surfaces. Seal installation between these close-fitting sealing surfaces is difficult and it has been found desirable to compress the seals prior to and during installation to avoid damage to the seals and/or ancillary structure. Accordingly, there is a need for a seal which can be restrained in a compressed condition prior to and during installation and forms an effective seal under turbine operation conditions as well as for methods of installing and relieving the restrained seal for use. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In accordance with a preferred embodiment of the present invention, there is provided a resilient seal having sealing portions laterally spaced from one another and restrained in reduced lateral dimension prior to and during installation of the seal into the turbine. Subsequent to closure of the seal within sealing surfaces of the turbine parts, an operating condition of the turbine relieves or releases the restrained (compressed) seal, enabling the sealing surfaces to resiliently engage the adjacent sealing surfaces of the turbine parts to form the seal. In a particular preferred embodiment, the seal includes an elongated seal body having a generally U-shaped body portion in cross-section and a pair of reversely extending, generally U-shaped marginal portions in cross-section along opposite sides and at distal ends of the U-shaped body portion. In a preferred form, the seal is formed of sheet metal, preferably a pair of complementary-shaped sheet metal plates secured to one another in back-to-back relation and formed into the aforementioned configuration. Prior to installation, the seal is placed in a resiliently compressed or restrained state and maintained in that compressed state during installation. That is, the lateral sealing surfaces of the seal body are displaced toward one another and resiliently restrained in that condition prior to installation of the seal into the turbine to reduce the lateral extent of the seal. In that manner, the seal can be installed into a seal cavity on one of the turbine parts without any portion of the seal projecting from the seal cavity, enabling the assembly of the turbine parts without interference between the seal and the turbine parts. 
     To compress and maintain the seal in a compressed condition in accordance with a preferred embodiment hereof, the seal body is passed longitudinally between a pair of laterally spaced side-by-side rollers. The rollers displace the seal portions, e.g., the U-shaped marginal portions, toward one another to reduce the lateral extent of the seal. The compressed seal is then passed through a rotating bobbin holder ring which mounts a bobbin on its periphery for rotation about the elongated seal. As the compressed seal passes through the rotating bobbin holder ring, the fiber from the bobbin is wrapped about the compressed seal, maintaining the seal in its compressed condition. The bobbin holder ring may be alternately rotated about the seal in opposite directions to provide alternate clockwise and counterclockwise wrapping of the fiber about the seal. By wrapping the fiber in opposite directions, torsional effects on the seal due to the compressed wrapping are nullified. 
     The fibers are preferably formed of a material which will disintegrate at a turbine operating condition. Specifically, the fibers may be formed of carbon or a Kevlar® material known as Kevlar® 29. These carbon or Kevlar® fibers will disintegrate as the turbine heats up, releasing the seal from its compressed installation condition to an operable condition with the marginal sealing surfaces expanding to engage against sealing surfaces of the turbine parts, forming an effective seal. Preferably, the wrapped seal may be adhesively secured within the seal cavity to ensure that it resides completely within the cavity and does not fall out of the cavity during installation. At or below turbine operating temperatures, the epoxy and restraining fibers burn up and release the seal without leaving significant residue. 
     In a preferred embodiment according to the present invention, there is provided in a turbine having parts including a pair of adjacent surfaces and a flexible seal in sealing engagement with the adjacent surfaces, the seal having a pair of sealing portions preloaded to sealingly engage the pair of adjacent surfaces, respectively, upon installation of the seal into the turbine, a method of installing the flexible seal in the turbine, comprising the steps of locating the seal between the adjacent surfaces, maintaining the seal between the adjacent surfaces with the sealing portions thereof in a first position poised and biased for movement into sealing engagement with the adjacent surfaces and releasing the sealing portions of the seal in situ for movement from the first position into a second position in sealing engagement with the respective adjacent surfaces in response to a turbine operating condition. 
     In a further preferred embodiment according to the present invention, there is provided for a turbine having parts including a pair of adjacent surfaces and a flexible seal for sealing between the adjacent surfaces, the seal having a generally U-shaped body portion and a pair of sealing surfaces laterally spaced from one another along opposite sides of the U-shaped body portion, a method of forming the flexible seal for installation of the seal in the turbine, comprising the steps of resiliently displacing the sealing surfaces of the seal toward one another in a generally lateral direction into a turbine installation condition to reduce lateral spacing between the sealing surfaces relative to one another and preload the sealing surfaces for movement away from one another into a sealing condition, retaining the sealing surfaces in the installation condition while installing the seal between the pair of adjacent turbine surfaces and enabling the sealing surfaces for resilient movement away from one another into the sealing condition engaging and sealing against the adjacent surfaces of the turbine subsequent to closure of the sealing surfaces about the seal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary schematic side elevational view of a portion of a gas turbine in which seals constructed in accordance with a preferred embodiment hereof may be utilized; 
     FIG. 2 is a representative example of a seal constructed in accordance with a preferred embodiment of the present invention interposed between a nozzle segment retaining ring and a shroud segment; 
     FIG. 3 is a schematic representation of the seal of FIG. 2 in a natural state having a lateral dimension larger than the depth of the seal cavity, causing interference between adjacent sealing parts upon installation of the seal; 
     FIG. 4 is a schematic representation of a seal compressed for installation in accordance with the present invention; 
     FIG. 5 is an enlarged end view of the seal prior to installation and illustrating the process of compressing the seal between a pair of rollers; 
     FIG. 6 is a plan view of the compressed seal being wrapped by a fiber; 
     FIG. 7 is a schematic representation of a bobbin ring mounting a bobbin for wrapping the seal with fiber; and 
     FIG. 8 is a plan view of a wrapped seal with the fibers extending both clockwise and counterclockwise directions about the seal. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, there is illustrated a representative example of a turbine section of a gas turbine, generally designated  10 . Turbine  10  receives hot gases of combustion from an annular array of combustors, not shown, which transmit the hot gases through a transition piece  12  for flow along an annular hot gas path  14 . Turbine stages are disposed along the hot gas path  14 . Each stage comprises a plurality of circumferentially spaced buckets mounted on and forming part of the turbine rotor and a plurality of circumferentially spaced stator vanes forming an annular array of nozzles. For example, the first stage includes a plurality of circumferentially-spaced buckets  16  mounted on a first-stage rotor wheel  18  and a plurality of circumferentially-spaced stator vanes  20 . Similarly, the second stage includes a plurality of buckets  22  mounted on a rotor wheel  24  and a plurality of circumferentially-spaced stator vanes  26 . Additional stages may be provided, for example, a third stage comprised of a plurality of circumferentially-spaced buckets  28  mounted on a third-stage rotor wheel  30  and a plurality of circumferentially-spaced stator vanes  32 . It will be appreciated that the stator vanes  20 ,  26  and  32  are mounted on and fixed to a turbine casing, while the buckets  16 ,  22  and  28  and wheels  18 ,  24  and  30  form part of the turbine rotor. Between the rotor wheels are spacers  34  and  36  which also form part of the turbine rotor. It will be appreciated that compressor discharge air is located in a region  37  disposed radially inwardly of the first stage. 
     Referring to the first stage of the turbine, the stator vanes  20  forming the first-stage nozzles are disposed between inner and outer bands  38  and  40 , respectively, supported from the turbine casing. As noted above, the nozzles of the first stage are formed of a plurality of nozzle segments  41 , each mounting one, preferably two, stator vanes extending between inner and outer band portions and arranged in an annular array of segments. A nozzle retaining ring  42  connected to the turbine casing is coupled to the outer band and secures the first-stage nozzle. Shroud segments  43  arranged in an annular array thereof surround the rotatable buckets, e.g., the buckets  16  of the first stage. The shroud segments include an axial facing surface  46  (FIG. 2) which lies in sealing engagement with a confronting axial facing surface  48  of the nozzle retaining ring  42 . A nozzle support ring  44  (FIG. 1) radially inwardly of the inner band  38  of the first-stage nozzles engages the inner band  38 , particularly the inner rail  52  thereof. 
     As noted previously, however, in turbine operation, the nozzle retaining ring  42  and the shroud segments  43  may tend to form leakage gaps between the axially confronting sealing surfaces  46  and  48  whereby leakage flow may occur across such gaps. In order to minimize or prevent such leakage flow into the hot gas path  14 , and, as described and illustrated, in co-pending U.S. patent application Ser. No. 10/028,928, filed Dec. 28, 2001 (Attorney Docket 839-1127), there is provided a seal for sealing between the nozzle retaining ring  42  and the shroud segments  43 . It will be appreciated that a similar seal may be employed to seal between various other parts of a turbine, e.g., the inner rail  52  and the nozzle supporting ring  44 , e.g., as set forth in U.S. patent application Ser. No. 10/029,003, filed Dec. 28, 2001 (Attorney Docket No. 839-1124). A representative example of such seal, generally designated  70  (FIG.  2 ), includes a seal body  71  having a first, generally U-shaped portion  72  in cross-section and a pair of reversely extending, generally U-shaped marginal sealing portions  74  in cross-section along opposite sides of the U-shaped portion  72 . Preferably, the seal body  71  is formed of sheet metal. In a particular embodiment hereof, a pair of sheet metal plates  76  and  78  are secured in back-to-back relation to one another, for example, by welding, to form the seal body  71 . 
     Still referring to FIG. 2, one of the sealing surfaces  46  and  48  of the shroud segments  43  and the nozzle retaining ring  44  is provided with a cavity  80  for housing the seal  70 . Preferably, the cavity  80  is formed in the shroud segments  43  with the cavity  80  opening generally axially through surface  46  and toward the axially opposite sealing surface  48  of the nozzle retaining ring  44 . The cavity  80  includes a base  82  and radially opposed surfaces  84  and  86 , respectively. The cavity  80  extends in an arcuate path about the axis of the turbine rotor and lies radially outwardly of the hot gas path  14 . It will also be appreciated that the seal  70  is provided preferably in arcuate lengths in excess of the arcuate length of the individual shroud segments, preferably in 90° or 180° lengths, and therefore spans the joints between the shroud segments. Consequently, the seal  70  is located to substantially preclude any leakage flow past the axially opposed surfaces  46  and  48 . 
     In a natural state of the seal body as illustrated in FIG.  3  and in its sealing state illustrated in FIG. 2, the laterally outer extremities of the marginal U-shaped portions  74  extend outwardly beyond the lateral extent of the main U-shaped portion  72 . Also, in the natural state of seal body  71  illustrated in FIG. 3, the marginal sealing portions  74  extend laterally in excess of the depth of cavity  80 . This lateral dimension inhibits or precludes assembly of the turbine paths having the sealed surfaces, e.g., surfaces  46  and  48 , due to potential interference of those parts with the protruding seal body  71 . For example, the projecting marginal portion  74  may snag on the retaining ring  42  or snap off entirely upon installation of the mating parts, e.g., surfaces  46  and  48 . This, of course, could render the seal ineffective during turbine operation. 
     Because the marginal sealing portions  74  are biased or preloaded for sealing engagement against the respective base surface  82  of cavity  80  and the sealing surface  48  in use, and also to avoid interference between the seal and sealing parts during assembly of the seal, the seal  70  must first be compressed during installation. Otherwise, and with references to FIG. 3, a marginal portion  74  will project from the cavity  80  when the seal body  71  is initially placed in the cavity. 
     To install the seal body  71 , the body is first compressed to a configuration which, when inserted into the cavity  80 , enables the seal body to lie wholly within the confines of the cavity  80  as illustrated in FIG.  4 . That is, the lateral dimension between the marginal sealing portions  74  is reduced (FIG. 4) to a dimension equal to or less than the lateral dimension between the base surface  82  and the sealing surface  48 . Means are provided to maintain the seal body in such compressed state during installation. Such means, for example, may comprise a wrap  92  provided about the entire length or portions of the length of each seal segment. The wrap restrains the marginal seal portions  74  of the seal in the compressed condition of the seal with the lateral extent of the marginal seal portions  74  reduced. Such wrap may comprise Kevlar® 29 and may comprise a continuous wrap or a segmented wrap about sections of the seal. Alternatively, a high-strength plastic such as Lexan™ or Ultem™ clips may hold the seal  70  in a compressed state during assembly. 
     Referring now to FIGS. 5-8, the seal  70  is wrapped in a compressed state with a wrap  92  as indicated previously such that, upon assembly, the seal  70  may reside completely within the cavity  80 . To compress the seal, the elongated seal  70  is advanced through a pair of compression rollers  100 , the spacing between which can be adjusted to adjust the degree of lateral compression of the seal. Thus, when the seal  70  passes through the rollers  100 , the lateral margins  74  of seal  70  are displaced laterally toward one another, reducing the overall lateral dimension of the seal. The rollers  100  thus induce a bias or preload on the seal in its compressed condition. As illustrated in FIG. 6, the rollers  100  feed the elongated seal  70  through a wrapping mechanism by which the wrap  92  is wound about the seal  70  to maintain the seal under compression and reduced in lateral dimension. 
     Referring to FIG. 7, the wrapping mechanism includes a bobbin  102  mounted on a rotatable ring  104 . The bobbin carries the wrap  92  and the ring is rotatable in opposite directions, one of the rotary directions being illustrated by the arrow  106  in FIG.  7 . The ring  104  is rotated by powered gears or friction rollers, not shown, which are equipped with a direction reversal mechanism. Consequently, as the seal  70  passes through the ring  104  and the ring is rotated, the fiber  92  is wrapped about the compressed seal  70  in one direction. By reversing the rotational direction of the ring  104 , the wrap may be disposed about the seal body  71  in a reverse direction. For example, alternate wraps in clockwise and counterclockwise directions may be provided. By providing for reversal of the wrap about the ring, torsional effects due to the compressed wrapping are completely eliminated. The seal  70  thus emerges from the wrapping mechanism in a restrained compressed condition, as illustrated in FIG.  8 . 
     With the seal  70  wrapped as illustrated in FIG.  8  and reduced in lateral dimension, the seal can be located wholly within the sealing groove, for example, groove  80 , of the shroud segments. Epoxy  83  (FIG. 4) may be applied to the margins of the seal to maintain the seal in the cavity during installation. 
     As the turbine reaches operating conditions, e.g., high operating temperatures, the retaining means, e.g., the wrap or wraps, release the seal from its compressed state, enabling the seal to expand under natural bias or preload in a lateral (axial) direction. Where epoxy is used to retain the compressed seal in the cavity, the operating conditions, e.g., high operating temperatures, similarly cause the epoxy to melt and release the seal. Such expansion of the seal body  71  locates surface portions  91  (FIG. 2) of the marginal portions  74  into engagement against the base  82  of the cavity  80  and the sealing surface  48  of the nozzle retaining ring  42 . Consequently, the marginal portions  74  of the seal remain biased or preloaded into sealing engagement with the opposed sealing surfaces notwithstanding relative movement of the surfaces  46  and  48  or the opening of one or more leakage gaps therebetween. It will be appreciated that a metal-to-metal line contact with good sealing performance is thus provided by the seal  70  to prevent any leakage flow past the confronting axial surfaces  46  and  48 . 
     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.