GB2439089A

GB2439089A - A brush seal for sealing a gap in a turbomachine

Info

Publication number: GB2439089A
Application number: GB0611834A
Authority: GB
Inventors: Philip David Hemsley
Original assignee: Alstom Technology AG
Current assignee: General Electric Technology GmbH
Priority date: 2006-06-15
Filing date: 2006-06-15
Publication date: 2007-12-19
Also published as: GB0611834D0

Abstract

A brush seal 16 seals a gap G between two parts 1, 12 of a turbomachine, the brush seal 16 comprising a bristle pack 20 in which bristles 18 extend towards both parts to span the gap G. The brush seal 16 controls leakage of fluid through an annular gap G between a rotary component 1 and a static structure 12 in a turbomachine. The brush seal 16 comprises an annular bristle pack 20 secured between carrier plates 24, 25, the bristle pack 20 being held by the carrier plates 24, 25 such that the free ends of its bristles 20a, 20b form both the outer and inner circumferences of the bristle pack 20. The brush seal 16 is flexibly supported on spindles or tie rods 26 for radial movement relative to the static structure 12.

Description

BRUSH SEAL

Field of Invention

The present invention relates to turbomachinery, and in particular to improvements in brush seals, such as can be used to control flow of fluids through clearances between stationary and rotating components.

Background of Invention

In turbomachines, such as steam turbines, there is a need to control leakage of the working fluid through annular gaps (clearances) between co-axial rotating and stationary components.

One known means of controlling leakage of working fluid between rotating and stationary components of turbomachines is the so-called "brush seal". This usually comprises a dense annular array of wire bristles, which bridge the annular gap between the rotating and stationary components. On one side of the annular gap, the bristles are held fixed with respect to each other by welding, brazing or clamping their ends in an annular carrier shoe on the stationary component, with the free ends of the bristles brushing against the surface of the rotating component, or held a very short distance away from it.

In many cases, the rotating component is located radially inwardly of the stationary component. To enable the bristles to have some collective flexibility when the free ends of the bristles are brushing against the surface of the rotating component and the radial distance across the annular gap varies, the bristles do not extend exactly radially across the gap, but are oriented to extend tangentially to a circle of substantially smaller radius than the rotating component. Hence, if the carrier is fixed to the stationary structure, the collective stiffness of the bristles is such as to allow a limited amount of variation of the gap between the components. Additionally, to allow for greater variation of the gap and/or greater stiffness of the bristles, provision may be made for the seal to "float" radially against a spring arrangement that acts between the carrier and the stationary structure. In the case of such a "floating" annular carrier, the carrier will be segmented and the carrier segments mounted in such a way as to allow them to move slightly relative to each other in a radial sense.

An object of the present invention is to provide a floating segmented brush seal that is effective to minimise leakage through varying annular clearances between static and rotating components in turbomachinery, but which does not need a spring arrangement between the carrier and the stationary structure.

Summary of Invention

In its broadest aspect, the present invention comprises a brush seal for sealing a gap between two parts of a turbomachine, such as a steam turbine, the brush seal comprising a bristle pack in which bristles extend towards both parts to substantially span the gap.

in most circumstances, the bristles will extend in mutually opposing directions.

The bristle pack may have two discrete sets of bristles, each set extending towards a corresponding part of the turbomachine. Alternatively, the bristle pack may comprise a single set of bristles that extends between the parts of the turbomachine. As well known in the art per Se, the bristle pack is conveniently held in a carrier, which may comprise a carrier plate on each side of the bristle pack, effective to secure the bristles therebetween.

The brush seal of the present invention is particularly applicable to minimising leakage of fluid through an annular gap between a rotary part of the turbomachine and a static part thereof, comprising an annular bristle pack held in an annular carrier, the bristle pack being held in the carrier such that free ends of its bristles form both inner and outer circumferences of the annular bristle pack, the radial extent of the bristle pack being sufficient to substantially span the annular gap. The carrier will preferably comprise a generally annular carrier plate on each side of the bristle pack.

In this way, the innate flexibility of the bristles over their extent between the carrier and the inner and outer circumferences of the annular bristle pack is used to effectively double the range of relative radial movement between the rotary component and the static component, as compared with known single-edged brush seals.

To limit or prevent contact between free ends of the bristles and the rotary part, the brush seal may be provided with snubbing means acting between the brush seal and the rotary part, preferably between the carrier and the rotary part.

Preferably, the bristles in the bristle pack are oriented to extend tangentially to a circle of substantially smaller radius than the rotary part of the turbomachine.

In a significant aspect of the invention, the brush seal is flexibly supported for movement between the parts of the turbomachine. In particular, the brush seal may be flexibly supported on rods or the like attached to the static part. To facilitate limited radial movement between the rotary and the static parts, the rods may run as a loose fit through corresponding bores in the static part.

Mountings are provided for securing the rods to the static part and resisting forces acting in the direction of the rods, in particular forces due to the pressure of a turbomachine working fluid, such as steam, on the brush seal. With regard to the relationship between the rods and the mountings to give the required flexible support to the brush seal, the rods may be inherently flexible, and the mountings rigid. Alternatively, the rods may be rigid, and the mountings inherently flexible, or the rods and the mountings may both be flexible. Such flexible mountings may comprise stainless steel wire pads of the type used for anti-vibration mountings.

The invention also includes a turbomachine including a brush seal as described above.

The turbomachine may be a steam turbine.

Further aspects of the invention will become apparent from a study of the following

description and the appended claims.

Brief Description of the Drawings

Exemplary embodiments of the invention will now be described with reference to the accompanying drawings, which are not drawn to scale: Figure 1 diagrammatically shows a partial axial section of a steam turbine stage including a brush seal in accordance with the present invention; Figure 2 is an enlarged view of part of Figure 1 showing the brush seal in greater detail; and Figure 3 is a view similar to Figure 2, but showing a further embodiment of the invention.

Detailed Description of the Preferred Embodiments

Figure 1 shows a radially inner portion of a multi-stage steam turbine. A drum-type rotor 1 rotates coaxially within a static diaphragm structure 2 comprising an annular row of static blades 4. The row of static blades 4 is paired with an adjacent downstream annular row of moving blades 6 to make a stage of the turbine. Also shown is a row of moving blades 6' belonging to a preceding stage of the turbine. The moving blades 6, 6' have root portions 8, 8' held within circumferentially extending slots 10, 10' in the periphery of the drum rotor 1.

The diaphragm 2 comprises the static blades 4 extending between a radially outer static ring (not shown) and a radially inner static ring 12, the radially inner side of which confronts the periphery of the rotor 1. To enable assembly and maintenance of the turbine, the outer and inner static rings are segmented, with several static blades per segment.

That part of the periphery of the rotor 1 confronting the diaphragm inner static ring 12 has a shallow slant-sided recess 14, the opposed axial ends of which are spaced away from adjacent moving blade rows 6, 6' as shown. Thus, the annular recess 14 provides a reduced-diameter drum portion over part of the axial distance between adjacent rows of moving blades. Part of the radially inner side of the static ring 12 projects into the annular recess 14, thereby providing a constricted and radially stepped or sinuous path between the static ring 12 and the rotor 1. Such a stepped or sinuous path increases resistance to the leakage of turbine working fluid from the low-pressure side of the static blades to their high-pressure sides through the gap G between the inner static ring 12 and the rotor 1, so reducing such leakage.

To further minimise leakage of the turbine working fluid through the gap G, a brush seal assembly 16 is provided between the inner static ring 12 and the rotor 1. As best seen in Figure 2, the body of the brush seal 16 is housed in an annular recess 18 provided in the radially inner side of the inner static ring 12. A notable feature of the brush seal 16 is that its bristle pack 20 is double-ended, having both radially outer and radially inner annular arrays of bristles 20a and 20b respectively. The outer bristles 20a extend towards and may contact the underside of the static ring 12 within the recess 18 and the inner bristles 20b extend towards and may wipe against the periphery of the rotor I within the recess 14. To provide support for the outer and inner bristles 20a, 20b near their free ends and to keep them closely packed together under operational conditions, they are clamped between front and back annular carrier plates 24, 25, respectively. The free ends of bristles 20a and 20b extend respectively outwardly and inwardly from the carrier plates 24, 25. The whole double-ended bristle pack is securely attached, for example by welding or brazing, to central annular bosses 24a, 25a of the carrier plates.

Although the bristle pack 20 may comprise two discrete sets of bristles, an outer set 20a and an inner set 20b, it will be convenient if the bristle pack comprises outer and inner ends 20a and 20b of a single set of bristles that extend all the way from the inner diameter of the brush seal 16 to its outer diameter.

To accommodate changes in the dimension of the gap G caused, e.g., by operational transients, such as differential thermal expansion in the rotor I and the diaphragm 2, the brush seal assembly 16 is flexibly supported on spindles or tie rods 26 attached to the inner part of static ring 12. Each rod 26 runs as a loose fit through a corresponding axial bore 28 in static ring 12, the rods and bores being equi-angularly spaced around it. They may also run as an interference or threaded fit through axial bores in the annular bosses 24a, 25a of the brush carrier plates 24 and 25, or they may be welded or brazed into position within the bosses. The axial position of the rods 26 is fixed by mountings 30, which are welded or brazed into position within a larger diameter portion 28a of each bore 28.

The invention provides a means of allowing limited radial movement of the brush seal 16 relative to the static structure with little restraint, while resisting the axial force exerted on the seal by the steam pressure. The axial pressure forces, acting in the direction of the spindles or tie rods, are resisted by securing the mounting 30 to the stems of rods 26 and to circular section end bosses 26a provided at the end of each rod 26, opposite the end that supports the brush seal 16. Limited radial movement of the brush seal 16 in synchronism with relative radial movements of the static ring 12 and rotor 1 is achieved either by making the rods 26 inherently flexible and the mountings 30 rigid, or by making the mountings 30 inherently flexible and the rods 26 rigid, or by making both the rods 26 and the mountings 30 inherently flexible. A flexible mounting 30 may comprise, e.g., a stainless steel wire pad of the type used for anti-vibration mountings. A flexible rod 26 may simply be a rod that is of sufficiently small diameter to be flexible under the radial loads exerted on the brush seal 16. Because rods 26 are a loose fit in bores 28, they allow the brush seal 16 to move radially in and out by a limited amount to compensate for radial growth or shrinkage of the rotor 1 and diaphragm 2.

The number and dimensions of the rods 26 and the bores 28 are determined in accordance with the requirements of the design of the turbine to which they are fitted, with particular attention being paid to defining the clearances which are desired between the ends of the bristles 20a and the static ring 12, and between the ends of the bristles 20b and the rotor 1. During normal steady-state operation of the turbine, it should be arranged that the turbine will run with small clearances at these points to avoid heat input to the rotor 1.

it should be understood that although the brush seal 16 is annular in form, as a matter of practical engineering, be of segmented construction, i.e. the annulus is angularly divided into a number of sectors, known as segments. Such an expedient is well-known in the art and will not be further described, except to note that a segmented construction is arranged to enable the component segments of the brush seal to move radially independently of each other. There will be at least two segments in the brush seal 16 and there may be six segments or even more. For example, the brush seal 16 may comprise the same number of segments as constitute the diaphragm 2.

If the bristles in the bristle pack 20 were oriented exactly radially so as to contact the inner static ring 12 or the rotor 1 at angles normal to their circular surfaces, the bristle pack as a whole would be very stiff, leading to damage to the bristles 20b and the rotor 1 if the radial interference between the bristles and the rotor becomes too great during turbine transients. Hence, in addition to the above- described flexible support for the brush seal 16 as a whole, the bristle pack 20 is advisedly given some internal radial flexibility by orienting the bristles to extend tangentially to a circle of substantially smaller radius than the rotor, as known in the art. In this way, the collective stiffiiess of the bristle pack is such as to allow a limited reduction of its radial dimension as the gap G becomes smaller.

The skilled person will recognise that the ends of the bristles 20b generate a certain amount of heat if they rub against the rotor 1. If excessive heat is generated in this way, the localised increase in temperature of the rotor at and near the area of contact between the bristles and the rotor may burn the bristles, and/or cause excessive wear of the bristles or rotor, and/or distortion of the rotor. This may be ameliorated or prevented in the present invention by a snubber arrangement, as shown in Figure 3. Here, excessive contact -or indeed any contact -between the bristles 20b and the rotor I can be prevented by providing the bristle pack front canier plate 24 with forwardly projecting circular section bosses 31 that co-operate with a radially projecting annular flange 32 secured to the rotor I by means of a base or foot 32a that encircles the rotor.

There will be at least two bosses 31, which will be diametrically opposed to each other across the rotor, but the number of bosses 31 will be determined in accordance with the requirements of the design of the turbine, e.g., there will be at least one boss 31 per segment of the brush seal, the bosses being equi-angularly spaced around the rotor.

The radial dimensions of the flange 32 and the gap 34 (if present) between the bosses 31 and the flange 32, controls how much contact there will be between the ends of the bristles 20b and the rotor 1 at a particular turbine condition. For example, when the radial height of the flange 32 equals the radial distance between the bosses 31 and the -s-ends of the bristles 20b, the bristles will just skim the surface of the rotor I even when the gap G is at a minimum, because the snubber arrangement will transfer all flexing of the bristles to the outer bristles 20a as they bear against the static ring 12. On the other hand, the radial height of the flange 32 can be set such that if the radial interference between the ends of bristles 20b and the rotor 1 becomes greater than a predetermined amount, any resulting additional flexing of the bristles is transferred from the inner bristles 20b to the outer bristles 20a. Of course, heat generation at the ends of the outer bristles 20a is not a problem because there is no relative movement between inner ring 12 and the outer bristles 20a.

It is envisaged that contact between the bosses 31 and the flange 32 will be intermittent, occurring only during operating transients of the turbine. At all other times, there will be a small gap 34 between the bosses 31 and the flange 32. However, if necessary, friction between the bosses 31 and the flange 32 may be reduced by providing the flange and the bosses with anti-friction coatings. Alternatively, or additionally, the bosses 31 could be journaled on extensions of the rods 26 so that they rotate on contact with the flange 32.

Such a journal bearing could comprise integral solid lubricant bearing pads, as known per Se.

Although the focus of the above description has been on use of the invention in connection with steam turbines, the skilled person will appreciate that the invention could be applicable to other types of turbomachinery, such as gas turbines.

The present invention has been described above purely by way of example, and modifications can be made within the scope of the invention as claimed. The invention also consists in any individual features described or implicit herein or shown or implicit in the drawings or any combination of any such features or any generalisation of any such features or combination, which extends to equivalents thereof. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments. Each feature disclosed in the specification, including the claims and drawings, may be replaced by alternative features serving the same, equivalent or similar purposes, unless expressly stated otherwise.

Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise", "comprising", and the like, are to be construed in an inclusive as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".

List of reference numbers.

Drum-type rotor I Static diaphragm 2 Static blades 4 Moving blades 6, 6' Moving blade root portions 8, 8' Slots 10, 101 Static ring 12 Recess 14 GapG Brush seal assembly 16 Annular recess 18 Radially outer bristles 20a Radially inner bristles 20b Front annular carrier 24 Back annular carrier plate 25 Annular bosses 24a, 25a Rods 26 Bores 28 Larger diameter bore portion 28a Mountings 30 Circular section bosses 31 Annular flange 32 Base 32a Radial Gap 34

Claims

CLAIMS: 1. A brush seal for sealing a gap between two parts of a

turbomachine, the brush seal comprising a bristle pack in which bristles extend towards both parts.

2. A brush seal according to claim I, in which the bristles extend in mutually opposed directions.

3. A brush seal according to claim 1 or claim 2, in which the bristle pack has two discrete sets of bristles, each set extending towards a corresponding part of the turbomachine.

4. A brush seal according to claim 1 or claim 2, in which the bristle pack comprises a single set of bristles that extends between the parts of the turbomachine.

5. A brush seal according to any preceding claim, in which the bristle pack is held in a carrier.

6. A brush seal according to claim 5, in which the carrier comprises a carrier plate on each side of the bristle pack.

7. A brush seal according to any preceding claim for minimising leakage of fluid through an annular gap between a rotary part of the turbomachine and a static part thereof, comprising an annular bristle pack held in an annular carrier, the bristle pack being held in the carrier such that free ends of its bristles form both inner and outer circumferences of the annular bristle pack, the radial extent of the bristle pack being sufficient to substantially span the annular gap.

8. A brush seal according to claim 7, in which the carrier comprises a generally annular carrier plate on each side of the bristle pack. -12-

9. A brush seal according to claim 7 or claim 8, in which contact between the bristles and the rotary part is limited or prevented by snubbing means acting between the brush seal and the rotary part.

10. A brush seal according to claim 9, in which the snubbing means acts between the carrier and the rotary component.

11. A brush seal according to any one of claims 7 to 10, in which the bristles in the bristle pack are oriented to extend tangentially to a circle of substantially smaller radius than the rotary part.

12. A brush seal according to any preceding claim, in which the brush seal is flexibly supported for movement between the two parts of the turbomachine.

13. A brush seal according to claim 12 as dependent from any one of claims 7 to 11, in which the brush seal is flexibly supported on rods or the like attached to the static part.

14. A brush seal according to claim 13, in which the rods run as a loose fit through corresponding bores in the static part.

15. A brush seal according to claim 13 or claim 14, in which the rods are secured to the static part through mountings capable of resisting forces acting in the direction of the spindles or tie rods.

16. A brush seal according to claim 15, in which the rods are inherently flexible and the mountings are rigid.

17. A brush seal according to claim 15, in which the rods are rigid and the mountings are inherently flexible.

18. A brush seal according to claim 15, in which the mountings and the rods are inherently flexible.

-13 - 19. A brush seal according to claim 17 or claim 18, in which the mountings comprise stainless steel wire pads of the type used for anti-vibration mountings.

16. A brush seal substantially as described herein with reference to Figures 1 and 2 or Figure 3 of the accompanying drawings.

17. A turbomachine having a brush seal according to any preceding claim.

18. A steam turbine having a brush seal according to any one of claims I to 16.

19. A turbomachine substantially as described herein with reference to Figure 1 of the accompanying drawings.

20. A steam turbine substantially as described herein with reference to Figure 1 of the accompanying drawings.