Axial seal system for a gas turbine steam-cooled rotor

Abstract: An axial seal assembly is provided at the interface between adjacent wheels (18) and spacers (32) of a gas turbine rotor and disposed about tubes (58) passing through openings in the rotor adjacent the rotor rim and carrying a thermal medium. Each seal assembly includes a support bushing (80) for supporting a land (70) of the thermal medium carrying tube (58), an axially registering seat bushing (82) disposed in the opposed opening and a frustoconical seal (84) between the seal bushing and seat. The support bushing (80) includes a radial flange (88) having an annular recess (90) for retaining the outer diameter edge (93) of the seal, while the seat bushing (82) has an axially facing annular surface (96) forming a seat for engagement by the inner diameter edge (98) of the seal.

Description: The present invention relates to a gas turbine having a rotor with buckets cooled by a thermal medium supplied and returned in generally axial directions along and adjacent the rim of the rotor and particularly relates to seal assemblies for axially sealing tubes carrying the thermal medium at junctures between axially stacked wheels and spacers to seal between cavities insulating the tubes from the wheels and spacers and cavities surrounding the joints between the wheels and spacers. In gas turbines, the rotor is typically formed of axially stacked wheels and spacers bolted together in alternating relationship with one another. The wheels mount circumferentially-spaced buckets about their periphery, while the spacers lie in radial opposition to nozzles, the buckets and nozzles forming part of and residing in the hot gas path in which the products of combustion flow, rotating the rotor about its axis. In more advanced designs of gas turbines, the buckets of the first stage and preferably the first and second stages are cooled by a thermal medium which passes axially through a bore tube assembly, outwardly in radial supply tubes in the aft disk of the rotor and forwardly through a plurality of axially extending, circumferentially-spaced tubes passing through aligned openings in the stacked wheels and spacers. The spent cooling medium is returned from the first and second stages along the rim of the rotor to the aft disk by a plurality of circumferentially-spaced return tubes in communication with radial return tubes for returning the thermal medium to the bore tube assembly. It has been determined that as the rotor structure heats up during turbine start-up, the wheel and spacer rim areas move closer together due to the temperature changes. That is, the rims of the rotor heat up faster than the interior portions of the rotor where the tie bolts are located such that the rims of the wheels and spacers move toward one another. This phenomena is known as rotor clap. As the rotor structure approaches steady-state operating temperatures, this rotor clap diminishes and eventually disappears. It is essential, however, during both start-up and steady-state operations, that the seals between the cavities surrounding the tubes within the wheel and spacer openings and cavities surrounding the seal are maintained. As a consequence of these requirements, there has developed a need for a seal that is sufficiently compliant to absorb significant compressive displacement forces caused by rotor thermal growth during start-up, yet sufficiently stiff to resist substantial transverse centrifugal forces during steady-state operation. Moreover, the seal must return to its original assembled position as the compressive displacement forces are removed and the turbine approaches steady-state operation. The seal must therefore be sufficiently robust in the radial direction to support itself under high centrifugal loadings. US-A-4,720,236 discloses a gas turbine that comprises a spring seal member with an I-shaped cross-section. One end of the spring seal member is in contact with a support, and the other end is located against a stator vane support. US-A-5,695,319 discloses a gas turbine that comprises a plurality of flow channels with brush seal members located in a number of item. The brush seal members are supported along the top of the seal housing and the sealing elements extend toward the channel bottom surface. In accordance with a preferred embodiment of the present invention, there is provided a seal assembly for location between axially opposed faces of the wheels and spacers of the gas turbine rotor assembly and about the thermal medium carrying tubes. The seal assemblies effectively seal between the cavities in the wheels and spacers openings about the tubes and cavities external to the seals between the registering faces of the wheels and spacers while accommodating high compressive stresses during turbine start-up and high centrifugal loadings during steady-state operation. The present seal between the cavities surrounding the tubes and the cavities surrounding the seals prevents the medium in the cavities surrounding the seals, the preferred medium being bore cooling air, from entering into or exiting from the cavities surrounding the tubes. Air transfer between cavities would induce a flow through the cavities surrounding the tubes, which would then greatly increase heat transfer from the tubes to the wheels and spacers. Increased heat transfer from the tubes to the wheels and spacers contributes to rotor failure. To accomplish the foregoing, the present invention,provides a seal assembly comprising a support bushing, a seal element and a seat bushing. Each support bushing is carried by a spacer adjacent an end face thereof and provides a support for the lands of the thermal medium carrying tube. The support bushing has a radially outwardly directed flange in the region of the interface between the wheels and spacers. The seat bushing is disposed in the axially registering opening through the end face of the opposed wheel and has a radial flange mounting an annular axially facing bearing surface. A frustoconical seal element extends between the support bushing and the seat bushing. Preferably, the larger diameter of the seal element bears in a recess of the seal bushing and the inner smaller diameter edge bears against the annular bearing surface of the seat bearing. A retainer flange or stop is formed along the outer margin of the flange of the support bushing to provide support for the seal element upon axial movement of the wheel and spacer toward one another whereby high compressive forces may be absorbed. As the turbine moves toward steady-state operation, the rotor clap diminishes and the seal returns to its original assembled condition. In that condition, the seal element is stiff in a radial direction to absorb the high centrifugal field caused by rotor rotation. In a preferred embodiment according to the present invention, there is provided in a gas turbine having a rotor comprised of axially stacked rotor wheels and spacers and aligned openings spaced from an axis of rotation of the rotor, the aligned wheels and spacers receiving a tube extending in an axial direction, a seal assembly about the tube at an interface of one of the wheels and one of the spacers, comprising a generally annular seal support in part received in a first opening in one of the wheels and spacers and including a generally radially extending flange having an annular recess in axial registration with an opposed face of another of the wheel and spacers, a seat bushing in part received in a second opening of another wheel and spacer in axial opposition to the first opening and having an seat and a seal between the seal support and the bushing, the seal including an annular frustoconically-shaped element having a radially outer edge portion engaging the seal support in the recess thereof and a radially inner edge portion engaging the seat for sealing between the seal support and the seat. An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:- FIGURE 1 is a cross-sectional view of a portion of a gas turbine illustrating a turbine section and the thermal medium carrying tubes for supplying cooling thermal medium to buckets; FIGURE 2 is a fragmentary perspective view of portions of the turbine stages of Figure 1 with parts in cross-section and illustrating the location of seal assemblies according to the present invention; FIGURE 3 is an enlarged cross-sectional view of a portion of a thermal medium carrying tube illustrating the seal assemblies between opposed faces of a spacer and a wheel; FIGURE 4 is an enlarged fragmentary cross-sectional view of a preferred embodiment of seal assembly according to the present invention; FIGURE 5A is a perspective view of a support bushing forming part of the seal assembly of Figure 4; FIGURE 5B is an axial end view of the support bushing; FIGURE 5C is a cross-sectional view thereof taken about on line 5C-5C in Figure 5B; FIGURE 6A is an axial end view of a sealing element employed with the seal assembly hereof; FIGURE 6B is a cross-sectional view thereof taken generally about on line 6B-6B of Figure 6A; FIGURE 6C is a perspective view of the seal element of Figure 6A; FIGURE 7A is an axial end view of a seat bushing forming part of the seal assembly hereof; FIGURE 7B is a cross-sectional view thereof taken about on line 7B-7B in Figure 7A; and FIGURE 7C is a perspective view of the seat bushing. Referring to Figure 1, there is illustrated a turbine section incorporating the present invention. The turbine section 10 includes a turbine housing 12 surrounding a turbine rotor R. Rotor R includes in the present example four successive stages represented by wheels 14, 16, 18 and 20, carrying a plurality of circumferentially spaced buckets or blades 22, 24, 26 and 28, respectively. The wheels are arranged alternately between spacers 30, 32 and 34. The outer rims of spacers 30, 32 and 34 lie in radial registration with a plurality of stator blades or nozzles 36, 38 and 40, the first set of nozzles 42 lying forwardly of the first stage buckets 22. Consequently, it will be appreciated that a four-stage turbine is illustrated wherein the first stage comprises nozzles 42 and buckets 22; the second stage, nozzles 36 and buckets 24; the third stage, nozzles 38 and 26 and, finally, the fourth stage, nozzles 40 and buckets 28. The rotor wheels and spacers are secured one to the other by a plurality of circumferentially spaced bolts 44 which pass through aligned openings in the wheels and spacers. A plurality of combustors, one being illustrated at 45, are arranged about the turbine section to provide hot gases of combustion through the hot gas path of the turbine section in which the nozzles and buckets are located for rotating the rotor. The rotor also includes an aft disk 46 formed integrally with a bore tube assembly, generally designated 48. At least one and preferably both of the buckets 22 and 24 of the first two stages are provided with a thermal medium for cooling, the thermal medium preferably being cooling steam. Cooling steam is provided and returned through the bore tube assembly 48. With reference to Figures 1 and 2, and in a preferred embodiment, the bore tube assembly includes an annular passage 50 supplied with cooling steam, from a steam plenum 52 for flow to a plurality of radially extending supply tubes 54 provided in the aft disk 46. Tubes 54 communicate with circumferentially spaced, axially extending thermal medium supply tubes in communication with cooling passages, not shown, in the first and second-stage buckets. Spent or returned cooling steam at an elevated temperature flows from the first and second-stage buckets through a plurality of circumferentially spaced, axially extending thermal medium return tubes 58. Return tubes 58 communicate at their aft ends with radially inwardly extending return tubes 60 which, in turn, flow into the central bore of the bore tube assembly 48 for returning the cooling medium to a supply or to steam turbines for use in a combined-cycle system. It will be appreciated from the foregoing description that the axially extending supply and return tubes 58, respectively, lie adjacent the rim of the rotor. Also, each supply and return tube extends through axially aligned openings 62 and 64 through the axially stacked wheels and spacers, respectively. Referring to Figures 2 and 3, a return tube 58 is illustrated. The supply and return tubes are similar, except as otherwise noted, and a description of one will suffice as a description of the other for purposes of describing the seal assembly hereof. Each tube comprises a thin-walled structure having a plurality of raised lands 70 at axially spaced locations along the length of the tube. The axial locations of the lands 70 coincide with the locations of support bushings of the seal assemblies hereof disposed in part in the openings through the wheels and spacers. Between the lands are thin-walled sections 72 of the tubes. From a review of Figure 3, it will be appreciated that the outer exterior surfaces of the lands 70 are radially outwardly of the exterior surface of the thin-walled sections 72. Transition sections 74 having outer arcuate surfaces are provided between each land 70 and an adjacent thin-walled section 72. Note that the tubes are spaced radially inwardly from the cylindrical surfaces of the openings through the wheels and spacers and thereby define insulation cavities 76. Seal assemblies, generally designated S, are provided at the interface of axially adjacent wheels and spacers to seal between cavities 76 and the respective cavities 78 between the wheels and the spacers surrounding the tubes. As illustrated in Figure 2, four seal assemblies are provided for the return tube and lie at the interfaces of the wheel 16, spacer 32; spacer 32 and wheel 18; wheel 18 and spacer 34; and spacer 34 and wheel 20. Each seal assembly S includes, in accordance with a preferred form of the present invention and referring to Figure 4, a seal support, preferably a support bushing 80, a seat bushing 82 and a seal 84. Each support bushing 80 includes a sleeve 86 received in a counterbore of its associated spacer 32 formed in the opening 64. Each support bushing 80 also lies at an axial location coincident with a land 70 of the tube and provides support for the tube at that axial location. Referring particularly to Figures 5A-5C, bushing 80 has a radially outwardly directed flange 88 adjacent an end of the sleeve 86 which lies axially beyond the counterbore of opening 64. The axial end face of flange 88 also includes an annular recess 90 for receiving an outer diameter edge of the seal 84. The outer margin 92 of flange 88 serves as a stop for the seal 84 as described below. The seat bushing 82 illustrated in Figures 7A-7C includes a sleeve 94 having an interior diameter greater than the interior diameter of the sleeve 86 of support bushing 80. As illustrated in Figure 3, the land 70 transitions to the thin-wall section 72 such that the thin-wall section 72 lies within and is spaced from the seat bushing 82. Seat bushing 82 includes an enlarged radial flange mounting an axially facing bearing surface 96 in axial opposition to the flange 88 of bushing 80. Bearing surface 96 serves as a seat for the inner diameter edge 98 of the seal 84. Referring to Figures 6A-6C, the seal 84 comprises a shallow, frustoconical section extending between its inner diameter 98 and the outer diameter 93. The angle α made along the inside face of the seal element 84 and a plane perpendicular to the rotor axis is about 13° and lies within a range of 10-15°. Thus, the angle is shallow. From a review of Figure 4, it will be appreciated that the support bushing 80 and seat bushing 82 are axially spaced one from the other upon initial assembly. Seal 84 lies within annular recess 90 with its outer diameter edge lying within the recess and bearing against the outer stop or flange 92. The inner diameter edge 98 bears against the annular seat 96 with a compressive force. During turbine start-up, the wheels and spacers have a tendency when heated to move toward one another responsive to thermal growth of the rotor. The significant compressive forces tending to displace the bushings toward one another are absorbed by the seal element. After start-up and as the turbine approaches steady-state operation, the seal assembly moves back to its original assembled condition. During steady-state operation, however, high centrifugal loads are applied to the rims of the turbine rotor and, hence, to the seals. The frustoconical annular shape of the seal assemblies, however, provide sufficient stiffness to withstand these high centrifugal loads, while simultaneously serving their function as seals. It will be appreciated that the support bushings and seat bushings are not necessarily mounted in counterbores of the spacers and wheels, respectively. The wheels and spacers could mount the support bushings and seat bushings, respectively. Also, while the seal assemblies are illustrated as lying between the second stage wheel 16 and spacer 32; spacer 32 and third stage wheel 18; wheel 18 and spacer 34; and spacer 34 and fourth stage wheel 20, it will be appreciated that the seal assemblies could be applied at other locations.

Claims: A seal assembly (S) for a gas turbine (10) wherein the gas turbine comprises a rotor (R) comprised of axially stacked rotor wheels (14, 16, 18, 20) and spacers (30,32,34) and aligned openings spaced from an axis of rotation of the rotor, the aligned wheels and spacers receiving a tube (58) extending in an axial direction, the seal assembly being located about said tube at an interface of one of said wheels and one of said spacers, the seal assembly being characterized by : a generally annular seal support (80) in part received in a first opening in one of said wheels and spacers and including a generally radially extending flange (88) having an annular recess (90) in axial registration with an opposed face of another of said wheel and spacers; a seat bushing (82) in part received in a second opening of said another wheel and spacer in axial opposition to said first opening and having a seat (96); and a seal (84) between said seal support (80) and said bushing (82), said seal including an annular frustoconically-shaped element having a radially outer edge portion (93) engaging said seal support (80) in said recess (90) thereof and a radially inner edge portion (98) engaging said seat for sealing between said seal support and said seat. A seal assembly (S) according to claim 1 wherein said support (80) has a radially outer flange (92) about said recess (90) projecting toward said another wheel and spacer forming an outer limiting stop for said seal. A seal assembly (S) according to claim 1 wherein one of said seal support (80) and said seal bushing (82) supports said tube (58). A seal assembly (S) according to claim 3 wherein another of said seal support (80) and said bushing (82) is spaced from said tube (58). A seal assembly (S) according to claim 1 wherein one of said seal support (80) and said seal bushing (82) supports said tube (58) and another of said seal support and said seal bushing is spaced from said tube, said radially inner edge portion of said seal bearing against an annular bearing surface (96) of said seat bushing in axial opposition to said seal support enabling said seal for compliance and compression in an axial direction. A seal assembly (S) according to claim 1 wherein said frustoconically-shaped seal (84) forms a shallow included angle α with a plane perpendicular to the axis of said rotor and passing through said seal. A seal assembly (S) according to claim 1 wherein said tube (58) is spaced from the opening in one of said wheel (32) and said spacer (18) to define an air gap therebetween, said seal (84) sealing between said air gap and a cavity between said wheel and spacer about said seal assembly.

Inventors: MASHEY THOMAS CHARLES, MASHEY, THOMAS CHARLES
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 2000
Main classification: F01D5
IPC codes: F01D5, F02C7, F01D11, F16J15