WO1998016764A1

WO1998016764A1 - Brush seal for gas turbine combustor-transition interface

Info

Publication number: WO1998016764A1
Application number: PCT/US1997/013218
Authority: WO
Inventors: Graydon L. Whidden; Raymond E. Chupp
Original assignee: Siemens Westinghouse Power Corp; Westinghouse Electric Corp
Current assignee: Siemens Energy Inc; Westinghouse Electric Corp
Priority date: 1996-10-16
Filing date: 1997-07-29
Publication date: 1998-04-23
Anticipated expiration: 1999-04-16
Also published as: JPH10153129A

Abstract

A sealing mechanism for minimizing the leakage of air through the mechanical coupling of the combustor and the transition section of a gas turbine. In accordance with a preferred embodiment of the invention, the sealing mechanism comprises a brush seal that can be mounted to the transition section or to the combustor. The brush seal minimizes leakage of air into the transition section due to tendency of such air to flow past the mechanical coupling of the combustor and transition piece. As a result, air that would otherwise leak into the transition section flows into the combustor to provide for leaner mixtures of fuel and air in the combustor and gas turbines incorporating the present invention have low levels of NOx emissions and improved overall operating efficiency.

Description

BRUSH SEAL FOR GAS TURBINE COMBUSTOR-TRANSITION INTERFACE

BACKGROUND OF THE INVENTION

The present invention relates to combustors capable of burning fuel in compressed air for a power generation system, such as a gas turbine power system. More particularly, the present invention relates to a self- accommodating brush seal at the interface of the combustor and the transition area of the gas turbine. The combustor- transition seal significantly reduces the amount of leakage of air into the transition section that would otherwise flow into the combustor, thus reducing the amount of harmful NOx produced by combustion and increasing the efficiency of the turbine system.

In a gas turbine, fuel is burned in compressed air, produced by a compressor, in one or more combustors Generally, in the combustion section of the turbine a chamber is formed by an outer casing and a plurality of combustors are circumferentially arrayed within the chamber. Also within the chamber, and mechanically coupled to the outer shell, or liner, of each combustor is a transition section (one for each combustor) which distributes the hot combustion gases to the turbine for expansion.

Air from the compressor section is distributed into the combustion section chamber such that the air is expose-i to the outer surfaces of the combustor liner and the transition section before the air flows through the inlet mcc the combustors. As is known, an adequate supply of air intake into the combustor is necessary for, among other things, reducing to acceptable levels the emissions of environmentally harmful by-products of the cαmoustion process, primarily NOx. The availability of more air input to the combustor provides for burning of the fuel at a lean fuel-to-air mixture ratio. However, m known turbine designs there is a substantial amount of leakage of air through the mechanical coupling where the combustor interfaces with the transition piece that would otherwise flow across the outer surfaces of the combustor and transition section and into the combustor inlet.

Currently known spring clips, which serve to mechanically couple the aft end of a combustor to the front end of a transition piece, permit approximately 2-3% of the total air flow from the compressor section to leak into the transition section and bypass the combustor. It is also known that after extended service, approximately 1-2 years, these spring clips allow even more of the total air to bypass the combustor. In known diffusion type flame combustors, these leakage levels are acceptable. However, with the advent of lean pre-mix combustors operating at high firing temperatures, NOx emission levels cannot be maintained below acceptable levels with the known air flow losses permitted by the existing spring clips at the combustor/transition interface. Sealing devices are known to minimize leakage at the interface of the aft end of the transition section and the first stage nozzle support elements. Such known seal mechanisms are disclosed in U.S Patent Nos. 5,265,412 and 5,400586 -- both to Bagepalli et al . However, these patents disclose a brush seal device for sealing the space between the transition piece and the first stage nozzle to minimize leakage of the combustion products into the transition-nozzle interface. These patents do not disclose means for sealing the interface of the combustor and the transition area to minimize leakage of compressed air into the transition area, wherein that air would otherwise flow into the combustor for the generation of the combustion products. It is therefore desirable to provide a gas turbine combustion section having a seal at the interface of the aft end of the combustor and the front end of the transition section, so as to reduce the formation of NOx and to increase the overall efficiency of the turbine system.

SUMMARY OF THE INVENTION Accordingly, it is a general object of the curren_t invention to provide a gas turbine combustion section wherein the amount of leakage of compressed air that would otherwise be available to supply the combustors is reduced, with the attendant benefits of operation of the combustors with very lean mixtures of fuel and air and increased turbine efficiency.

Briefly, this object, as well as other objects of the current invention, is accomplished in a gas turbine comprising a compressor for compressing air, a combustor for producing a hot gas by burning a fuel in the compressed air, a transition piece disposed proximate the combustor with a gap between the transition piece and the combustor, a brush seal disposed in the gap to minimize the compressed air from leaking through the gap, and a turbine for expanding the hot gas produced by the combustor. According to one embodiment of the invention, the brush seal is a densely packed bed of directionally compliant bristles clamped between an upstream and downstream retainer . The upstream and downstream retainer provide mechanical support for the sealed pressure load. A retaining ring that provides support for the brush seal can be attached to either the transition piece or the combustor, such that the brush seal can be mounted to either the transition, with the bristles slightly deflected by the combustor, or to the combustor, with the bristles contacting the transition piece. Axial and radial movement between the combustor and the transition piece is accommodated by the bristles in the seal.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a longitudinal cross-section through a portion of a gas turbine in the vicinity of the combustion section. Figure 2A is an exploded view of Section A-A of FIG. 1, showing a brush seal in accordance with a preferred embodiment of the invention.

Figure 2B is an exploded view of Section B-B of FIG. 2A.

Figure 2C is a cross-sectional view of a brush seal taken along lines C-C of FIG. 2A.

DETAIT.-Fm rvRRCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, there is shown in Figure 1 a portion of a gas turbine 1 in the vicinity of the combustion section 6. In addition to the combustion section

6, the gas turbine comprises a compressor section 2 and a turbine section 3. The current invention concerns a combustor

4 for the gas turbine 1 -- specifically, a combustor designed to operate efficiently and generate very low levels of NOx

(e.g., less than approximately 9 ppmv when the gas turbine is operating at its base load firing temperature on gas fuel and without the use of water or steam injection) .

The combustion section 6 comprises a chamber 7 formed by an outer casing 18 of the gas turbine and in which a plurality of combustors 4 are circumferentially arrayed.

Each combustor comprises a pre-mixing zone 14 and a combustion zone 10 downstream of the pre-mixing zone. Fuel 11, 12 and compressed air 8 from the compressor 2 are mixed in the pre- mixing zone 14 and burned in the combustion zone 10. A casing

13 extends outward from the front face of the casing 18 and encloses a portion of the pre-mixing zone 14, as well as the combustor fuel supply piping. A transition section (or transition piece) 5 comprises a duct that is mechanically coupled to a vessel 16, which vessel 16 encloses the combustion zone 10 and generally forms the aft end of the combustor 4. The transition section 5 directs the hot gas 9 produced by the combustor 4 to the turbine 3 for expansion.

The forward portion of the transition section 5 can be mechanically coupled to the aft end of the combustor vessel

16 using a variety of known mechanical couplings which are known to one of ordinary skill in the art. As shown in FIG. 2A, one currently known mechanical coupling is a spring clip 20. The spring clip 20 is an annular flexible member that comprises a plurality of leaves spaced closely together in the circumferential direction. The spring clip 20 is mounted to the combustor vessel 16 and the transition section 5 rests on the upper curved surface 21 of the spring clip. Due to the presence of the small gap (not shown) between each of the leaves of the spring clip, the leaves deflect in the radial direction under the influence of the transition section such that the transition section generally remains in contact with the spring clip. Accordingly, the spring clip 20 ensures that the combustor is centered with respect to the inlet of the transition section and the contact between the transition section 5 and the upper curved surface 21 of the spring clip minimizes the amount air leaking into the transition area over the top of the spring clip. However, due to the presence of the small gap between each of the leaves that form the spring clip, air flowing from the compressor section to the combustor inlet is known to leak through the gaps and into the transition section. Accordingly, and as described above, spring clips allow for substantial leakage of air into the transition section that would otherwise be available for input into the combustors, resulting in decreased turbine efficiency and undesirable emissions of NOx. In order to reduce the amount of leakage through the mechanical interface between the combustor vessel 16 and the transition piece 5, a seal mechanism in accordance with the present invention is shown in FIGS. 2A-2C. Brush seal 22 comprises a densely packed bed of directionally compliant bristles 24 clamped between an upstream retainer 26 and a downstream retainer 28. The upstream and downstream retainers 26, 28 provide mechanical support for the sealed pressure load on the brush seal. Retaining r ng 30 provides further mechanical support for the brush seal onto the inner surfac of the transition section 5. The means for mechanically affixing the brush seal within the combustor-transitier. section interface is not intended to be limited to the coupling described herein and other suitable means for mechanically securing the brush seal into position will be known to those skilled in the art .

Referring to FIG. 2C, the ends of the bristles ₂₄ of the brush seal 22 are formed into a circumferential surface 32 such that the bristles conform to the outer circumferential surface of the combustor vessel 16. The densely packed bed of directionally compliant bristles 24 can be optimally designed to minimize leakage of air through the seal in accordance with the description set forth in "Generalizing Circular Brush Seal Leakage Through a Randomly Distributed Bristle Bed," R.E. Chupp and G.F. Holle, Journal of Turbomachinery, Volume 118, January 1996, which is herein incorporated by reference as if set forth in its entirety. In the embodiment shown in FIG. 2A, a brush seal in accordance with the present invention is shown mounted to the transition section 5 with the bristles 24 slightly deflected by the combustor vessel 16. However, the brush seal can also be mounted to the combustor vessel, using the same type of retention mechanism described herein, such that the bristles 24 contact the transition piece 5.

Referring again to Figure 1, the operation of the combustor 4 is as follows. During start-up, the compressor 2 is spun-up to ignition speed and as the compressor rotor accelerates, compressed air 8 from the compressor 2 flows into the chamber 7 of the combustion section. A portion of the compressed air 8 passes across the outer surfaces of the combustor vessel 16 and the transition piece 5. Referring tc FIG. 2A, a portion of the compressed air 8 has a tendency to flow through the spring clip 20 at the interface of the combustor and the transition section. However, in accordance with the present invention the brush seal 22 minimizes leakage of the air that tends to flow through the spring clip and intc the transition section such that this air is available fcr input into the combustor to provide a more lean mixture cf fuel and air in the combustor. In accordance with the present invention, leakage through known spring clips would be reduced to less than one percen . A brush seal in accordance with the present invention also accommodates thermal growth and other movement between the combustor and transition section in the axial and radial direction. Sealing mechanisms, such as the brush seal 22, in accordance with the present invention can be used for sealing the interface between the combustor and transition section in conjunction with any other known mechanical coupling devices for mechanically securing the combustor to the transition, such as known stand-off pins. The sealing mechanism must only be positioned downstream of the combustor-transition mechanical coupling to minimize the amount of air leaking through the coupling and into the transition section.

The current invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.

Claims

CLAIMS:

1. A gas turbine, comprising: a compressor for compressing air; a combustor for producing a hot gas by burning a fuel in said compressed air; a transition piece disposed proximate said combustor with a gap between said transition piece and said combustor; a brush seal disposed in said gap to seal said compressed air from leaking through said gap; and a turbine for expanding said hot gas produced by said combustor.

2. The gas turbine according to claim 1, wherein said brush seal comprises a densely packed bed of directionally compliant bristles.

3. The gas turbine according to claim 1, further comprising coupling means for mechanically coupling said transition section and said combustor.

4. The gas turbine according to claim 3, wherein said brush seal is positioned downstream of said coupling means.

5. The gas turbine according to claim 2, said bristles having a sealing end, wherein said brush seal is mounted onto said transition piece such that said sealing end of said bristles maintain a sealing engagement with said combustor.

6. The gas turbine according to claim 5, wherein said bristles are arranged such that said sealing end of said bristles form a circumferential surface.

7. The gas turbine according to claim 4, said bristles having a sealing end, wherein said brush seal is mounted onto said combustor such that said sealing end of said bristles maintain a sealing engagement with said transition piece.

8. The gas turbine according to claim 1, further comprising a flexible spring clip disposed in said gap upstream of said brush seal.

9. The gas turbine according to claim 8, wherein said spring clip comprises a plurality of closely spaced leaves .

10. The gas turbine according to claim 9, said spring clip having a curved upper surface, said spring clip mounted to said combustor such that said transition piece contacts said curved upper surface of said spring clip.