GB2313635A - A seal arrangement - Google Patents

Abstract

A seal arrangement is for sealing a clearance defined between two components (2 and 4). The seal comprises at least one brush seal element (18) and at least one fin element (20) disposed successively in a sealing direction. The elements (18 and 20) project from the two components (2 and 4) into the clearance (6) between the two components (2 and 4) such that when viewed in the sealing direction the elements (18) and elements (20) overlap. A leakage path through or around either element (18) or element (20) is thereby restricted by the following element producing overall a tortuous leakage path (36) through the seal. The brush seal element may contact the opposed component. The flexability of the brush seal elements eases assembly of the components.

Description

A SEAL ARRANGEMENT The present invention relates to sealing arrangements and in particular, but not exclusively, to gas sealing arrangements for components capable of relative rotational movement.

Compliant brush seals for preventing gas leakage between relatively rotating components are known. Such seals include a plurality of individual bristles, consisting of metallic wire or other fibres, extending from an annular support member towards a rotating shaft, for example. The free ends of the fibres contact, or are in close proximity with, the circumferential surface of the shaft and are manufactured to a very close tolerance in order that an effective seal may be obtained.

Labyrinth seals for a similar duty are also known. These seals comprise at least one fin like circumferential protrusion around the entire circumference of the component and extending in a radial direction from one of the relative rotating components. The tip of the fin is arranged so that it is in very close proximity to the other component such that there is only a very small path for gas leakage to occur through and an effective seal is produced. The bulk of the rotating component, excluding the seal fin, is separated from the other component by a larger clearance such that rotation is not impeded.

Radial growth of the shaft, due for example to temperature changes or centrifugal forces, has to be accommodated in both types of seal. The bristles of brush seals under normal operation will bend, and within labyrinth seals sufficient clearance between the fin tips and the other component is provided, such that growth can be managed.

However, when excesslve radial growth occurs interference between the rotating components is experienced and the effectiveness of the seal is reduced. In bristle seals the bristles tend to spread out axially along the shaft reducing the seal effectiveness and wearing the bristles away. Such wear increases the clearance between the bristle tips and shaft, further reducing the seal effectiveness. In labyrinth seals either the clearance between the fin tips and the other component is sufficiently large to accommodate the large radial growth or wear occurs to produce the required clearance. In either case the increase in the clearance reduces the seal effectiveness.

In a both conventional labyrinth and brush seal arrangements the clearance within the seals tends to produce a straight through leakage path running parallel to the components whereby the gas can more easily overcome the seal.

The present invention seeks to provide an improved seal arrangement whereby any leakage path produced by radial growth or wear is minimised and leakage through such a path is restricted. The result being a more effective seal arrangement.

According to the present invention there is provided a seal arrangement for sealing a clearance defined between two components comprising at least one brush seal element and at least one fin element disposed successively in a sealing direction, the elements projecting from the two components into the clearance between the two components such that each element projects sufficiently into the clearance so that, viewed in the sealing direction, the elements overlap and a leakage path through or around one element is restricted by the following element producing a tortuous leakage path through the seal.

::-eferably the brush seal elements and fin elements are arranged alternately in the sealing direction.

Furthermore it is preferred that the successive seal elements project alternately from the two components.

Preferably the successive seal elements are separated in the sealing direction and during normal operation do not come into contact.

The two components may be capable of relative rotational movement.

Preferably the fin element comprises a fin protrusion extending from one of the components into the clearance between the two components.

Preferably the brush element comprises a pair of spaced apart plates attached to one component, bristles being sandwiched between the plates and having free ends projecting therefrom towards the other component.

Furthermore there may be in a component separate individual local recesses from within which brush elements project.

In a sealing direction there may be at least one step in the clearance between the components with at least one of the sealing elements being attached to a different level of the step.

The present invention will now be described with reference to the accompanying drawings in which; Figure 1 is a cross sectional view of a conventional labyrinth seal, Figure 2 is a cross sectional view of one embodiment of the present seal invention, Figure 2A is a cross sectional view of one method of assembly of the embodiment shown in figure 2, Figure 3 is a cross sectional view of a second embodiment of the present seal invention, Figure 4 is a cross sectional view of a third embodiment of the present seal invention.

Referring to figure 1 which shows a conventional basic labyrinth seal for sealing two relatively rotating components 2,4. The seal is designed to prevent the axial flow of a gas through the clearance 5 between the components 2,4. The seal comprises a sealing strip 8 and a series of fin like circumferential protrusions 10 around the entire circumference of the component 2 and extending in a radial direction from the inner relatively rotating component 2. The tip of the fin 10 is arranged such that it is in close proximity to the sealing strip 8 resulting, at this point, in a defined tolerance controlled clearance 12. This small clearance 12 restricts the gas flow producing the required seal. In areas between adjacent fins 10 the size of the clearance between the two rotating components 2,4 is significantly greater than that at the fin tip 12. By careful design of the dimensions (fin height, fin pitch, fin tip thickness) of this larger clearance, the carry over and coefficient of discharge can be minimised. Additionally this larger clearance 6 forms axially thin fins 10 which reduces the likelihood of serious overheating under situations of fin 10 to sealing strip 8.

Relative radial movement of the components 2,4 caused by temperature changes, axial misalignment, or centrifugal forces for example, will cause the fins 10 to contact the sealing strip 8. This contact will cause localised wear of the sealing strip 8 in the region of the fin 10. Additionally any relative longitudinal movement of the components 2,4 will longitudinally extend the area worn away, to a possible position shown by the dotted line 14. The result of this wear is to increase the clearance 12 through which the gas can flow with reduced hindrance.

The provision of a series of fins 10 will improve the seal effectiveness by providing a series of restrictions.

However the small clearance 12, which is enlarged by wear, will be at a similar radius for each fin section. This means that the path of reduced hindrance for the gas flow will be a straight line 16 parallel to the components bypassing all of the fins 10 in a similar fashion to as if there was only one fin.

Similar problems exist in brush seal arrangements where leakage tends to occur at the free ends of the bristles and any clearance between the bristles and the other component will reduce the seals effectiveness. Wear will produce or increase such a clearance. In both cases leakage will tend to be at a similar radius for each element resulting in a straight through leakage path.

Referring to figure 2 which shows an embodiment of the present seal arrangement invention providing a seal between two relative rotating components 2,4. The seal arrangement comprises brush seal elements 18 and fin elements 20. The brush elements 18 are characterised by a plurality of closely packed bristles 26 which resemble a conventional brush seal and comprise part of component 4. Interposed between these brush seal elements 18 is a fin element 20. This fin element 20 is in the form of a radial projection from and around the circumference of component 2 forming a ridge or fin 28 on that component 2. The fin 28, and other regions where contact between parts of the seal may occur may have a hard coating in order to reduce wear. Furthermore the shape of the projection and its profile may be altered in further embodiments.

The interposition of a brush seal element 18 and fin element 20 is repeated as required to produce a suitable seal. In alternative embodiments the seal arrangement can range from comprising only one brush element 18 and one fin element 20 to comprising a plurality of each of the elements 18,20. The more elements 18,20 added the more effective the seal produced.

In this embodiment each brush seal element 18 comprises, in addition to a plurality of tightly packed bristles 26, a pair of axially spaced apart annular plates 22, 24 between which the bristles 26 are sandwiched. The bristles 26 conventionally consist of metallic wire or other fibres. They extend radially from between the plates 22,26 towards the other component, which in this embodiment is radially inward.

The bristles 26 may even extend such that their free ends act upon the other component 2.

The annular plates, comprising a front plate 22 and a back plate 24, are coaxial and have an equal outer diameter.

However the inner diameter of the back plate 24 is less than that of the front plate 22. The back plate 24 thus extends radially inward further than the front plate 22. This provides support for the bristles 26 along most of their length whilst allowing the bristles to flex in one direction especially during assembly, as shown in figure 2A. Both of the plates 22,24, in this embodiment, are attached to the outer component 4 so that the brush elements 18 comprise part of the outer component 4.

The brush element 18 and fin element 20 are configured such that the bristles 25 and the fin protrusion 28 radially extend so that they radially overlap 30, although the elements are longitudinally separated. Thus the seal assembly has an effective interference, in the seal direction, between the two components 2,4. This means that any leakage through the clearance 34 between the brush element and the other component, and through the bristles 26 is restricted by the fin 28. Also leakage through the clearance 32 at the fin 28 tip is blocked by a subsequent brush seal element 18. There is thus no direct straight through leakage path 16 through the seal, parallel to and along the components.

Leakage will occur through the clearances 32 and 34 inherent within each element 18,20. However any leakage flow through the seal will be turned through at least two sharp bends in, order to overcome the seal as shown by arrow 36.

This tortuous leakage flow path 36 restricts the leakage flow, improving the effectiveness of the seal.

In addition since the tortuous leakage path provides some restriction upon the leakage flow, the seal is less dependent upon the sealing provided by the clearances 32,34 found in each element 18,20 of the seal. This means that size of the clearances 32,34 and any changes in their size have less affect upon the seal effectiveness than would be the case in conventional arrangements.

Radial growth of the seal can be more easily be accommodated. Since the tortuous leakage path provides a further sealing means the clearances 32,34, particularly at the labyrinth fin tip 32, can be larger than would be found in a conventional seal of the similar sealing efficiency. The seal elements 18,20 can then expand and contract into these clearances 32,34 without wearing material away. This results in a more consistent and predictable seal which is less effected by radial growth.

It will be appreciated that in alternative embodiments the bristles 26 of the brush elements 18 may be in contact with the other component. In these embodiments the clearance 34 between the bristles 26 and the component will not exist.

Despite this some leakage will still occur past the element 18 since the bristles 26 do not provide a perfect barrier to a gas. This is particularly the case at the free ends of the bristles 26. Support plates 22,24 associated with the brush elements 18 and usually located at the attached end of the bristles will, however, tend to block any leakage through the bristles 26. Therefore the main leakage past the brush element 18 will tend to still occur at a similar location to the clearance 34 found in the embodiment shown in figure 2.

The seal can be assembled in a similar manner to conventional seal arrangements with the inner component 2 being inserted within the outer 4 from one end. Adopting this assembly method means that the inner diameter of the annular plates 22,24 of the brush seal elements 18 must be greater than the diameter of the fins 28, as shown in figure 2. If this is not the case the solid sections 22,24,28 of both seal elements 18,20 will interfere during assembly.

During assembly the bristles 26 will be deflected by the fins 28 as the inner component 2 is inserted 40, as shown in figure 2A. The bristles 26 are therefore resilient and suitably flexible at that point. To assist the deflection of the bristles 26 a wedge shaped insert, ramping up to the fin top, may be placed before the fins to guide the bristles 26 over the fins. The insert can then be removed after assembly.

Alternatively the fins themselves can be so shaped.

The longitudinal spacing > 2 between successive fins 28 is minimised to ensure that there is a tortuous leakage flow path 36. On the other hand the spacing is sufficient so that during assembly the brush elements 18 can be moved far enough past the fin 28 to allow enough space for the bristles 26 to drop vertically down into position 41 once assembled, as shown by arrow 38. Once the bristles 26 have dropped down the brush elements 18 can be moved to any position between successive fins 28. However the spacing L1 and L3 between a brush element 18 and the adjacent fins 28 is large enough to accommodate anticipated relative longitudinal movement of the components 2,4, and bristle 26 defection due to the gas flow, without any part of the brush elements 18 contacting the fins 28. Any contact would produce wear of the bristles 26 and fins 28 thereby reducing the effectiveness of the seal.

If other methods of assembly are used then the restrictions on the axial plates 22,24 and spacing L2 of the elements may not exist. In particular if the outer component 4 is segmented and can be assembled around the inner component 2 then these assembly considerations are eliminated.

Figures 3, and 4 show other alternative embodiments of the present invention. In figure 3 the brush elements 18 are mounted within recesses 42 in the outer component 4. This allows longer length bristles 26 to be used whilst maintaining a small clearance 32 at the fin tip and also overall 6 between the two relative rotating components 2,4.

Longer length bristles 26 may be required in order that they have the required flexibility to enable them to deflect sufficiently to allow the fins 28 to pass during assembly.

Stepping of seal elements within a seal is known. The provision of steps between seal elements means that the clearances and leakage paths of successive elements are at different radii. This produces an overall non straight through leakage path. However the leakage path in such stepped arrangements, although not straight, is not particularly tortuous. Figure 4 shows a stepped embodiment of the invention. Here steps 44 are provided between the seal elements 18,20. This arrangement increases the tortuous nature of the leakage path thereby producing an improved stepped seal. It should also be noted that the stepped arrangement allows a longer length of bristle 26 to be used in a similar manner to the recessing shown in figure 3 and described above.

The stepped seal arrangement, shown in figure 4, and the recessing of the brush elements 18, shown in figure 3, does however increase the radial space of the seal. This may be undesirable in certain applications.

It will be appreciated by one skilled in the art that other embodiments and arrangements are possible that still follow the invention of combining a fin element 20, which projects into a clearance, with a brush seal element 18 to produce seal with a tortuous leakage pathway 36. Such arrangements could include the fin element 20 comprising part of the outer component 4 and the brush seal element 18 comprising part of the inner component 2 for example. Also the disposition and number of brush seal elements and fin elements can be altered. Furthermore the detailed fin and brush element designs may be different. For example inclined bristles 26, shielding plates and other refinements can all be used within the seal.

It will further be appreciated that such a seal can be used on stationary, rotating and relative rotating components.

Claims (10)

Claims:

1. A seal arrangement for sealing a clearance defined between two components comprising at least one brush seal element and at least one fin element disposed successively in a sealing direction, the elements projecting from the two components into the clearance between the two components such that each element projects sufficiently into the clearance so that, viewed in the sealing direction, the elements overlap and a leakage path through or around one element is restricted by the following element producing a tortuous leakage path through the seal.

2. A seal arrangement as claimed in claim 1 in which the brush seal elements and fin elements are arranged alternately in the sealing direction.

3. A seal arrangement as claimed in any preceding claim in which the successive seal elements project alternately from the two components.

4. A seal arrangement as claimed in any preceding claim in which successive seal elements are separated in the sealing direction and during normal operation do not come into contact.

5. A seal arrangement as claimed in any preceding claim in which the two components are capable of relative rotational movement.

6. A seal arrangement as claimed in any preceding claim in which the fin element comprises a fin protrusion extending from one of the components into the clearance between the two components.

7. A seal arrangement as claimed in any preceding claim in which the brush elements comprises a pair of spaced apart plates attached to one component, bristles being sandwiched between the plates and having free ends projecting therefrom towards the other component.

8. A seal arrangement as claimed in any preceding claim in which in a component there are separate individual local recesses from within which brush elements project.

9. A seal arrangement as claimed in any preceding claim in which, in a sealing direction, there is at least one step in the clearance between the components with at least one of the sealing elements being attached to a different level of the step.

10. A seal arrangement as hereinbefore described with reference to and as shown in figures 2-4.