WO2004053365A1

WO2004053365A1 - Seal assembly

Info

Publication number: WO2004053365A1
Application number: PCT/GB2003/005276
Authority: WO
Inventors: Peter Francis Crudgington
Original assignee: Cross Manufacturing Co 1938 Ltd
Current assignee: Cross Manufacturing Co 1938 Ltd
Priority date: 2002-12-07
Filing date: 2003-12-04
Publication date: 2004-06-24
Anticipated expiration: 2005-06-07
Also published as: AU2003285585A1; GB0228586D0

Abstract

A seal assembly (10) for sealing between a stator and an element (11) rotating about an axis (12) stator to separate a higher pressure region from a lower pressure region. The assembly includes a housing (13) for disposal circumjacent the rotating element and defining a radially opening channel (17) facing the axis. The assembly further includes a seal body (14) having a first portion (16) slidably received in the channel for inward and outward movement therein and a second portion having a surface (18) for forming a seal at an interface with the rotating element. The assembly also has at least one resilient element (15) for urging the seal body in an outward sense to form the seal. The resilient element restricts or prevents circumferential movement of the seal body relative to the axis.

Description

Seal Assembly

This invention relates to seal assemblies for sealing between a stator and an element rotating about an axis.

In the field of gas turbines, steam turbines, compressors, gas pumps, turbo-chargers and vacuum pumps there is often the need to provide a seal between a stator and a rotating shaft, or the like, lying within the stator to divide a higher pressure region on one side of the seal from an axially spaced lower pressure region on the other side of the seal.

As will be known to any person skilled in the art, there are a huge number of proposals for seals of this type. In general they consist either of brush seals or solid sealing shoes, which are mounted from movement in and out of a radially facing channel formed in a housing on a stator. The sealing shoe is urged towards the rotor by a spring. Each of these approaches have their problems. The brush seals can be deflected by the pressure drop, unless particular design features are built in, and they are subject to wear and deterioration.

Shoe seals, examples of which are shown in European Patent Applications EP-A-0803859 and EP-A-0995933, can be ineffective if the seal ring shoes or elements making up the seal move radially and so there is a tendency to pin them against this movement. This results in friction, which can inhibit proper operation of the seals. As can be seen in the above-mentioned European patent applications the ring elements making up the seal are typically urged towards the rotating member by an encompassing garter spring, so the movement of only one ring element will influence the movement of the others. US-A-5026252 shows a hybrid arrangement, which attempts to combine the features of bristle seals and shoe seals, but the result is to introduce a potential leakage path between the bristle ends and the shoe and it is difficult to ensure uniform bristle distortion as a result of radial movements of the shoe. Further the location of the shoe relies entirely on the weld to the very thin bristles and circumferential movement is also not well addressed.

The present invention consists in a seal assembly for sealing between a stator and an element rotating about an axis within the stator to separate a higher pressure region from a lower pressure region, the assembly including a housing for disposal circumjacent the rotating element and defining a radially opening channel facing the axis, a seal body having a first portion slidably received in the channel for inward and outward movement therein and a second portion having a surface for forming a seal at an interface with the rotating element and at least one resilient element for urging the seal body towards the axis to form the seal wherein the resilient element restricts or prevents circumferential movement of the seal body relative to the axis.

Conveniently there is a plurality of seal bodies or shoes arranged circumferentially side-by-side in the channel to abut end to end. The abutting ends of the respective pairs of seal bodies may form a labyrinth seal and additionally or alternatively a member or members may be provided for extending across the joint between the seal body abutting ends.

In a preferred embodiment the resilient element is a leaf spring and, for example, the leaf spring may extend generally circumferentially between the housing and the seal body. The leaf spring may be cranked and the relative lengths of the portions of the leaf spring either side of the crank may be selected so that only radial movement relative to the axis can occur. Preferably there are at least two leaf springs, which would usually be circumferentially in line.

Alternatively there may be at least a pair of compression springs located in respective pairs of aligned recesses in the housing and seal body.

The second portion of the seal body may be axially wider than the first portion to provide an extension for lying in the lower pressure region.

The seal surface may be profiled for forming a hydrodynamic film or films between the seal surface and the rotating element or, alternatively it may have one or more cavities for connection to a high pressure source to form a hydrostatic bearing. In this latter case the seal body may include one or more conduits extending between the cavities and a face of the seal body, which is exposed to the higher pressure region in use.

The seal body may be made from a plurality of parts and the seal surface may be constituted by different material from the bulk of the seal body. Thus it may be formed by a trim attached to the main part of the seal body, for example by rivets, or it may be formed by a coating.

The invention further includes a rotary machine including a stator, a rotatable element and seal assembly as set out above. In that case the seal body may be dimensioned such that the integral of the pressure drop across the seal assembly on the housing or stator size is at least approximately equal to the integral of equivalent pressure drop across the seal side.

Although the invention has been defined above it is to be understood it includes any inventive combination of the features set out above or in the following description.

The invention may be performed in various ways and specific embodiments will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic vertical section through a seal disposed in relation to a rotating shaft or disc;

Figure 2 is a vertical section through Figure 1 on the line ll-ll;

Figure 3a illustrates the seal surface of the seal illustrated in Figure 1 in an embodiment which forms hydrodynamic films, whilst Figure b illustrates an embodiment which forms a hydrostatic bearing;

Figures 4a to c are schematic illustrations of abutting joints formed between seal bodies;

Figures 5a and b respectively show an integral seal body and a multi- component seal body;

Figure 6 illustrates the pressure drops across either side of the seal body;

Figures 7a to c illustrate different spring constructions for use in the invention;

Figure 8 illustrates a particular embodiment of the Figure 7a proposal; Figure 9 illustrates an alternative construction of the Figure 7c proposal;

Figure 10 illustrates a further embodiment of the seal assembly;

Figure 11 illustrates a modification of the seal assembly;

Figure 12 illustrates a further modification of the seal assembly; and

Figure 13 illustrates the pressure drops across either side of the seal body of Figure 11.

Turning to Figures 1 and 2 a seal assembly, generally indicated at 10, is shown in relation to a rotor or disc 11 , which is rotating about an axis 12. The seal assembly comprises a housing generally indicated at 13 and a seal body or shoe 14. The housing 13 and seal body 14 are interconnected by springs 15.

The shoe 14 has a first portion 16, which is slidingly received in a channel 17 defined by the housing 13 for radial movement relative to the rotor or disc 11. The springs 15 act in a sense to urge the seal body 14 against the peripheral surface of the rotor or disc 11. The shoe 14 includes a second portion, which defines a sealing surface 18 that forms the seal at the interface with the rotor or disc 11. As can be seen the second portion can include an extension or plough 19 for increasing the size of the seal surface 18.

As is best illustrated in Figure 2, the springs 15 comprise leaf springs extending between the housing 13 and a circumferentially spaced point on the shoe or seal body 14. In the particular embodiment these springs are in line. It will be understood that the springs are relatively resilient in the radial direction allowing for self adjustment in the shoe position so that the seal can be formed as will be discussed in more detail below, but are relatively stiff in the circumferential direction so that little or no circumferential movement of the shoe can take place. This means that the designed gaps 21 between adjacent shoes

14 can be maintained, without introducing locking pins and the like, which, as has been indicated before, result in high friction.

In general with seals of this type, the intention is to float the seal surface 18 just above the rotor 11 so that any leakage gap is extremely small, but equally wear of the seal surface can be reduced or avoided.

Figure 3a illustrates the use of cavities 22 to form hydrodynamic pads, in a known manner. Alternatively the seal surface 18 may simply be slightly frustoconical in shape so as to form a hydrodynamic film. In Figure 3b cavities 23 are connected to the high pressure side of the seal assembly by conduits 24 allowing a hydrostatic bearing to be formed.

Figures 4a to c illustrate various possible abutments between the ends

21. It will be appreciated that in all of the drawings the gaps are shown as being much greater than they are, simply to allow for clear illustration. As the gaps 22 are carefully designed, the maintenance of their design characteristics by the circumferential stiffness of the springs 15 is particularly beneficial.

In Figures 5a and b, a cross-section of the shoe 14 is illustrated. In

Figure 5a the seal body or shoe 14 is illustrated. In Figure 5a the seal body or shoe 14 is an integral element, whereas in Figure 5b the sealing surface 18 is carried on another element 25, which may be a layer of material riveted to the main body of 14 or may simply be a coating.

Figure 6 illustrates the pressure profile on the inner and outer sides of the seal body 14. It will be seen that by suitable choice of the dimensions L_a and U so that the integrals of the pressure curves on either side are equal. Figures 7a to c illustrate different spring arrangements. In Figure 7a compression springs 26 are used instead of the leaf springs 15 and these sit in aligned pairs of recesses in the housing 13 and the shoe or seal body 14 respectively. This combination of springs again restricts circumferential movement, without introducing significant friction in the radial direction. Figure 7c is essentially the Figure 1 embodiment, whilst Figure 7c introduces a development of that concept, where the springs 15 are cranked at 27. By carefully selecting the point of cranking, the spring can be constructed, in a known manner, so that it will only allow radial movement. This idea is developed in Figure 8, where the crank springs 15 are shown as being cut out as an integral part of the shoe and then they may be welded, riveted or otherwise attached, for example as illustrated in Figure 9.

In Figure 10 shims 28 are mounted to extend across the gaps 22, enhancing the effectiveness of the seal. These shims or thin shoes 28 are mounted by their leaf springs onto the main seal mounting.

Figure 11 shows a modification of the design in which the shoe 14 is made axially shorter. To make this function the pressure above the shoe 14 has been reduced to an intermediate pressure Pint. This intermediate pressure has been achieved by placing holes 30 and 31 in the front and back plate, respectively, of the shoe 14. In order to reduce the pressure above the shoe 14, the holes 31 are sized larger than the holes 30. The relationship between the hole sizes is governed by the theory of compressible flow and is highly dependant on the overall pressure ratio across the seal. The various pressures are further illustrated in Figure 13. Figure 12 shows a seal working on a similar principle to the arrangement of Figure 11 , but in this case several radial slots are formed in the shoe 14 to control the pressure above the shoe. Slot 32 in the front plate of the shoe 14 is narrower than slot 33 in the back plate of the shoe so as to create a reduced pressure Pint as in the seal of Figure 11. Again, the relationship of the slot widths/areas is governed by compressible flow theory.

Claims

1. A seal assembly (10) for sealing between a stator and an element (11 ) rotating about an axis (12) stator to separate a higher pressure region from a lower pressure region, the assembly including a housing (13) for disposal circumjacent the rotating element and defining a radially opening channel (17) facing the axis, a seal body (14) having a first portion (16) slidably received in the channel for inward and outward movement therein and a second portion having a surface (18) for forming a seal at an interface with the rotating element and at least one resilient element (15) for urging the seal body in an outward sense to form the seal wherein the resilient element restricts or prevents circumferential movement of the seal body relative to the axis.

2. An assembly (10) as claimed in any one of the preceding claims wherein the seal surface (18) is profiled for forming a hydrodynamic film or films between the seal surface and the rotating element (11 ).

3. An assembly (10) as claimed in Claim 1 or 2 comprising a plurality of seal bodies (14) arranged circumferentially side-by-side in the channel (17) to abut end-to-end.

4. An assembly (10) as claimed in Claim 3 wherein the abutting ends (21) of the respective pairs of seal bodies (14) form a labyrinth seal.

5. An assembly (10) as claimed in Claim 3 or Claim 4 further including a member or members (28) for extending across the joint between the seal body (14) abutting ends (21).

6. An assembly (10) as claimed in any one of the preceding claims wherein the resilient element is a leaf spring (15). 7. An assembly (10) as claimed in Claim 6 wherein the leaf spring (15) extends generally circumferentially between the housing (13) and the seal body (14). 8. An assembly (10) as claimed in Claim 7 wherein the leaf spring (15) is cranked so that only radial movement relative to the axis (12) can occur.

9. An assembly (10) as claimed in any one of Claims 6 to 8 wherein there are at least two leaf springs (15) for the seal body (14).

10. An assembly (10) as claimed in Claim 9 wherein the leaf springs (15) are circumferentially in line.

11. An assembly (10) as claimed in any one of Claims 2 to 4 wherein there are at least two resilient elements and the resilient elements are compression springs (26) located in respective pairs of aligned recesses in the housing (13) and seal body (14). 12. An assembly (10) as claimed in any one of the preceding claims wherein the second portion is axially wider than the first portion (16) to provide an extension for lying in the lower pressure region.

13. An assembly (10) as claimed in any one of Claims 3 to 12 wherein the seal surface (18) has one or more cavities (23) for connection to a high pressure source to provide a hydrostatic bearing.

14. An assembly (10) as claimed in Claim 13 wherein the seal body (14) includes one or more conduits (24) extending between the cavities (23) and a face of the seal body (14), which is exposed to the higher pressure region in use.

15. A seal assembly (10) as claimed in any one of the preceding claims wherein the seal body (14) is made from a plurality of parts.

16. A seal assembly (10) as claimed in any one of the preceding claims wherein the seal surface (18) is constituted by a different material from the bulk of the seal body (14).

18. A rotary machine including a stator, a rotatable element (11 ) and a seal assembly (10) as claimed in any one of the preceding claims.

19. A machine as claimed in Claim 18 wherein the seal body (14) is dimensioned such that the integral of the pressure drop across the seal assembly (10) on the housing (13) side is at least approximately equal to the integral of the equivalent pressure drop across the seal side.