Patent Application
20130058756
This invention relates to gas turbines in which cooling air is introduced into the interstage disc cavities containing the stator to rotor shaft seals. More particularly, it relates to an arrangement which substantially confines the ingress of hot main gas flow into the interstage disc cavities to regions capable of withstanding high temperatures, thereby reducing the cooling air requirements to provide increased turbine efficiency.
Gas turbines such as those used to drive electric power generators have a number of rotor discs axially spaced along a rotor shaft to form interstage disc cavities. Designs for these components are varied. See, for example, U.S. Pat. Nos. 7,052,240 and 6,668,114 each incorporated herein by reference. Generally, the stages of the stator extend radially inward from the turbine casing into the interstage disc cavities. Each stator stage includes a number of stator vanes secured to the turbine casing and a seal assembly which seals against the rotor discs to prevent main gas flow from bypassing the vanes.
The combination of each stator section with the upstream and downstream rotor discs forms annular disc cavities. Cooling air bled from the compressor is introduced into the interstage disc cavities to cool and purge the seal assemblies. Typically, the cooling air flows axially and radially outward through the disc cavities and passes outward through a rim seal into the main gas flow.
Despite the provision of the rim seal and an adjoining rim seal cavity about the exit of the disc cavity, it is common for some of the main gas flow to at times ingress into the disc cavities. For example, pressure variations induced by the rotating parts may cause recirculation of gases within the cavities, and this can draw the very hot main gas flow toward the stator, rendering components vulnerable to thermal damage. Sufficient cooling gas must be provided in order to protect the rotor seals from the hot main gas ingress. This reduces the overall efficiency of the gas turbine.
There is a need, therefore, for an improved interstage disc cavity design in a gas turbine which provides greater protection from thermal damage and which results in improved operating efficiency. More particularly, there is a need for a reduction in the volume of cooling air needed to cool components in the interstage disc cavities of a gas turbine. It is desirable that such a design will reduce the amount of heating which may occur within the interstage disc cavities of a gas turbine due to ingress of main gas flow.
A full understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which:
Referring to
The gas turbine 1 also includes a stator 23 having a number of stator stages or sections 25, each extending radially inward from the turbine casing 7 into the interstage disc cavities 13. Each of the stator sections 25 includes a plurality of stator vanes 27 secured to the turbine casing 7 in axial alignment with the main gas flow 17 and the rotor blades 15. As best viewed in
Each interstage seal housing 29, being of a U shape, comprises upstream and downstream arms, 30u and 30d. Each arm extends radially outward from an innermost position along the rotor 5. See
A rotor inner flange 411 extends in a downstream direction from the first stage rotor disc 9-1 at a mid position along the rotor disc. A relatively smaller rotor outer flange, functioning as a rim seal 41o, extends in a downstream direction from near an outermost portion of the first stage rotor disc 9-1. Each of the flanges 411 and 410 is along a surface 9-1s of the disk 9-1 which faces the upstream arm 30u of the seal housing 29. A relatively small rotor outer flange, also functioning as a rim seal 42o, extends in an upstream direction from near an outermost portion of the second stage rotor disc 9-2. The rim seals 41o and 42o are circumferentially continuous flanges which each restrict a portion of the main gas flow 17 from entering the cavity 13, i.e., the region between the blade rotor discs 9-1, 9-2 and the U-ring interstage seal housing 29. The flange 41i and rim seals 41o, 42o may be integrally formed, e.g., via a casting process, along the rotor disc surfaces.
A first seal housing flange, operating as a first flow discourager flange 43, is located in a mid position along the seal housing upstream arm 30u. The flange 43 extends outward from the arm 30u in an upstream direction in close proximity to the rotor inner flange 411. The flange 43 thereby further limits hot gas of the main flow 17 from traveling through the labyrinth seal 37. A second seal housing flange, operating as a second flow discourager flange 44, is located near an outermost radial position of the upstream arm 30u. The flange 44 also extends outward from the arm 30u in an upstream direction. The discourager flanges 43, 44 are circumferentially continuous flanges which extend about the rotor 11.
In accord with an embodiment of the invention, cooling air bled from the compressor (not shown) is introduced through the stator vanes (not shown) into interstage disc cavity regions (the disc cavities 13) through cooling air inlets such as shown in
The seal assembly 28 is a combination of components, including (i) the interstage seal housing 29, positioned in the disc cavity 13 and having a seal housing surface 30s spaced away from the surface 9-1s of the first stage rotor disc 9-1, (ii) a portion of the first stage rotor disc 9-1 having a surface 9-1s which faces the upstream arm 30u of the housing 29 and extends along the subregions 52, 54 and 56 of the disc cavity 13 from the labyrinth seal 37 at least to the rim seal 41o, and (iii) a portion of the second stage rotor disc 9-2 having a surface 9-2s which faces the downstream arm 30d of the seal housing 29 and extends along a portion of the disc cavity 13 from the labyrinth seal 37 at least to the rim seal 42o. Along the surface 9-1s, between the labyrinth seal 37 and the rim seal 41o, the combination of the rotor inner flange 41i and discourager flange 43 are in close proximity to one another to thereby restrict flow 17 from movement toward the labyrinth seal 37. Further, with the discourager flange 44 positioned radially outward with respect to the flange 43, the air inlet extends through the upstream arm 30u of the seal housing 29 to inject cooling air 50 in the subregion 52 of the cavity 13 which is between the discourager flange 44 and the rim seal 41o.
With the arrangement of discouragers 43 and 44 and the air inlet 47 positioned to inject cooling air into the first subregion 52, ingress of hot gas from the main flow 17 into the cavity 13 is limited and hot gas which enters the cavity is diluted by the injected cooling air, this resulting in a lower temperature as the air and hot gas mix in the circular flow path of the subregion 52. With the purge flow pressure, i.e., the relative pressure of the cooling air 50, higher than the pressure of the hot gas flow, the purge air mixes directly with the ingested hot gas to provide effective cooling to the rotor disc. The hot gas ingested into the cavity 13 is largely contained in the first subregion 52 which is a radially outermost recirculation zone. With the foregoing features, the purge flow requirement can be reduced while maintaining a sufficiently cool thermal environment to sustain the longevity of components, thereby providing for improved efficiency of turbine power generation.
In one embodiment of the invention a gas turbine has been disclosed having a rotor mounted for rotation within a turbine casing. The rotor includes a shaft and at least first stage and second stage rotor discs axially displaced on the rotor shaft to form an interstage disc cavity therebetween. The rotor includes a plurality of rotor blades extending radially outward from each of the rotor discs into a main gas flow. The turbine includes a stator having a plurality of stages, a first of the stator stages extending radially inward to the interstage disc cavity from the turbine casing toward the rotor shaft. Each of the stator stages includes multiple stator vanes axially aligned with the rotor blades in the main gas flow and terminating radially inwardly with a seal housing which provides a seal about the rotor shaft. The first of the stator stages includes an attachment portion connecting the seal housing to at least one stator vane. A combination, comprising the seal housing, the first stage rotor disc, a surface of the first stage rotor disc which faces the seal housing, the second stage rotor disc and a surface of the second stage rotor disc facing the seal housing, form a seal assembly about the interstage disc cavity. The seal housing includes a first portion facing the surface of the first stage rotor disc and a second portion facing the surface of the second stage rotor disc. First and second seal housing flanges each extend outward from the first portion of the seal housing, each extending toward the first surface of the first stage rotor disc. The seal housing flanges may be integrally formed with the seal housing, e.g., via a casting process. Inner and outer rotor flanges each extend outward from the first stage rotor disc along the surface of the first stage rotor disc toward the seal housing of the first of the stator stages. The inner rotor flange and first seal housing flange are positioned radially inward relative to the second seal housing flange and the outer rotor flange is positioned radially outward relative to the second seal housing flange. The outer rotor flange functions as a rim seal. The inner rotor flange and first seal housing flange extend toward one another in close proximity to limit movement of main gas flow along the rotor shaft. The first portion of the seal housing includes a cooling air inlet positioned to inject air in an outer region of the disc cavity between the outer rotor flange and the second seal housing flange.
In a related method, applied to such a gas turbine having at least first stage and second stage rotor discs axially displaced on the rotor shaft to form the interstage disc cavity, at least first and second interconnected flow regions are formed in the disc cavity 13 where the first flow region 52 is positioned radially outward with respect to the second flow region 56 to provide a flow path (52, 54, 56) wherein the first flow region 52 initially receives a portion of the main gas flow before that portion is received by the second flow region 56. A seal, e.g., the labyrinth seal 37, is positioned as the first seal in the flow path. A flow of air, different from the main gas flow, is injected into the flow path (52, 54, 56) so that the portion of the main gas flow which is received by the second flow region 56 is mixed with the flow of air before reaching the first seal 37 in the flow path. Although the first flow region is described by example as the region 52, it may be the region 54 or another flow region of the disc cavity. Similarly the second flow region may be the flow region 54 or another flow region of the disc cavity 13. Further, the first seal, being the first seal in the flow path, may be a seal positioned before the labyrinth seal 37. In the illustrated embodiment, first, second and third interconnected flow regions 52, 54, 56 are formed in the disc cavity 13 to provide the flow path where the first and second flow regions are positioned radially outward with respect to the third flow region so that the second flow region initially receives a portion of the main gas flow before that portion is received by the third flow region, and the first seal is positioned in the third flow region.
While various embodiments of the present invention have been shown and described herein, it will be apparent that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.
