Abstract:
A brush seal ( 9 ) comprising a plurality of bristles packed together in a bristle layer ( 10 ). The bristles ( 10 ) extend from a first component ( 4 ) towards a facing surface of a second component ( 2 ). A movable plate ( 14 ) is disposed substantially parallel to and adjacent to the bristle layer ( 10 ). The plate ( 14 ) is movable relative to the bristle layer ( 10 ) in a direction parallel to the bristle layer ( 10 ). An edge ( 22 ) of the movable plate ( 14 ) adjacent to and facing the facing surface of the second component ( 2 ) is arranged to air ride on the facing surface of the second component ( 2 ). To promote the air riding the edge ( 22 ) of the movable plate ( 14 ) may be accordingly profiled. Alternatively the edge ( 14 ) may comprise an enlarged foot member ( 24 ) which defines an enlarged surface ( 26 ). Furthermore a recess ( 30 ) may be defined in the movable plate ( 14 ).

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to fluid seal arrangements for sealing high pressure areas from low pressure areas and in particular to improvements to brush seals. 
     BACKGROUND OF THE INVENTION 
     Brush seals have been developed for sealing high pressure regions from low pressure regions for, in particular, gas turbine engine applications where a seal is required between relatively movable parts, typically between rotor shafts and a stationary housing. The seal prevents or restricts leakage flow along the shaft. Such brush seals offer improved sealing as compared to conventional labyrinth type seals and can better accommodate radial movements of the shaft. 
     Brush seals comprise a layer of bristles which are sandwiched between annular front and backing plates which are mounted on and extend radially from the stationary housing surrounding the shaft. The backing plate typically extends radially further inwards towards the shaft than the front plate with the bristles extending yet further still with the bristle tips typically wiping against the surface of the shaft. Although the bristles provide a significant degree of sealing it has been found that the clearance between backing plate and the shaft has a significant effect on seal performance with smaller clearances reducing the leakage flow. However since, unlike the bristles, the backing plate is solid and fixed a minimum clearance between the backing plate and the shaft is required in order to accommodate anticipated radial movement of the shaft. Such movement is caused by thermal growth, centrifugal growth, eccentric mounting of the shaft, shaft vibration and transient movement due to loads on the shaft. 
     One proposal to address this problem and provide an improved seal is described in U.S. Pat. No. 5,351,971. In this proposal a radially movable backing plate is suggested which can move radially when contacted by the shaft in order to accommodate the anticipated radial movement of the shaft. Since the backing plate can now move radially a smaller clearance between the backing plate and shaft can be used as compared to a seal with a fixed backing ring and the seal performance can be improved. 
     A problem however with this proposal is that the shaft must contact the backing plate in order to move the backing plate. Such contact will generate friction between the backing plate and the shaft even when non abrasive low coefficient coating are used, which will generate heat and will cause wear of the backing plate and/or shaft. Such wear increases the clearance between the backing plate and shaft which reduces the seal performance. Furthermore wear of the shaft will reduce its strength and may give rise to a stress concentration requiring the shaft to be prematurely replaced or in extreme cases could lead to shaft failure. 
     It is therefore desirable to provide an improved seal arrangement which addresses the above problems of contact between the backing plate and shaft whilst minimising the clearance therebetween to provide improved seal performance and/or which offers improvements generally. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a brush seal comprising a plurality of bristles packed together in a bristle layer with the bristles mounted on and extending from a first component towards a facing surface of a second component, and a movable plate disposed substantially parallel to the bristles and adjacent to the bristle layer, the plate being movable relative to the bristle layer in a direction parallel to the bristle layer; characterised in that the movable plate and an edge of the movable plate adjacent to and facing the facing surface of the second component are arranged to, in use, air ride on the facing surface of the second component. 
     Preferably the edge of the movable plate adjacent to the second component is profiled so as to promote air riding of the movable plate adjacent to the facing surface of the second component. 
     Alternatively the edge of the movable plate adjacent to the second component comprises an enlarged foot member which extends from the remainder of the movable plate so as to define an enlarged surface substantially parallel to and facing the facing surface of the second component. 
     Furthermore the plate recess walls may define a recess in the edge of the movable plate adjacent to the second component, with the recess defined in the movable plate having an open side facing the facing surface of the second component. A duct within the movable plate may interconnect the recess with a source of pressurised fluid and in operation supplier pressurised fluid to the recess. The depth of the recess may vary along the length of the edge of the movable plate adjacent to the second component. 
     Preferably the facing surface of the second component in the region facing the movable plate is sufficiently smooth so as to promote air riding of the movable plate. 
     A support plate may extend from the first component towards the second component, at least a portion of the support plate abutting the movable plate and supporting the movable plate in a position adjacent to the bristles. Preferably a first portion of the support plate is spaced from the movable plate with a second portion of the support plate extending towards and abutting the movable plate such that a chamber is defined between the support plate and the movable plate. A duct may interconnect the chamber with a source of pressurised fluid. 
     The movable plate is preferably segmented. 
     At least a portion of the movable plate adjacent to the bristle layer may be spaced from the bristle layer. A chamber may thereby be defined by between the movable plate and the bristle layer. Preferably, in use, pressurised fluid is arranged to be supplied to between a region between the movable plate and the bristle layer. 
     Preferably the first component is a stationary housing, the second component is a rotatable shaft and the movable plate is annular. 
     The movable plate may be disposed either downstream or upstream of the bristle layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described by way of example only with reference to the following figures in which: 
     FIG. 1 shows a schematic sectional view of a seal arrangement according to the present invention; 
     FIG. 2 shows a schematic sectional view in the sealing direction shown by arrow A of the seal arrangement shown in FIG. 1; 
     FIG. 3 shows a schematic sectional view similar to that shown in FIG. 1 of a second embodiment of a seal arrangement according to the present invention; 
     FIG. 4 shows a schematic sectional view similar to that shown in FIG. 1 of a further embodiment of a seal arrangement according to the present invention; 
     FIG. 5 is a section along line X—X, in a plane perpendicular to the rotational axis, through the part of the radially inner portion of the backing plate shown in FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1 there is shown a seal  9  according to the present invention for sealing a gap D between a shaft  2  and a housing  4 . The shaft  2  rotates, as shown by arrow B about an axis  1  within the stationary housing  4 . The seal  9  segregates a first upstream region  6  at a pressure P 1  from a second downstream region  8  at a lower pressure P 2  and prevents or restricts a leakage flow of fluid along the shaft  2 , in the sealing direction A from the higher pressure region  6  to the lower pressure region  8 . It will be appreciated that the terms upstream and downstream are used throughout this description in relation to the direction A of leakage flow and sealing direction indicated by arrow A. 
     The seal  9  is of a brush seal type and comprises a plurality of metallic bristles  10  which extend from a seal body  21  located and attached to the stationary housing  4 . The radially outward ends of the bristles are welded to the seal body  21  whilst the distil radially inward ends are arranged to just wipe against the outer surface  3  of the shaft  2 . Alternatively the distil ends of the bristles  10  are in very close proximity to the shaft outer surface  3 . The bristles  10  thereby seal the upstream region  6  from the downstream region  8  along the shaft  2 . As shown in FIG. 2 the bristles  10  are disposed around the shaft  2  and inner bore of the stationary housing  4  and as known in the art, the bristles  10  are circumferentially angled in the direction of rotation B of the shaft  2 . Typically the bristles  10  are angled at an angle of 45° to the radial direction. 
     Upstream of, and adjacent to, the bristles  10  is an annular front plate  12  which is attached to the seal body  21  and housing  4 . The front plate  12  extends radially from the seal body  21  and housing  4  to a point radially part way along the radial length of the bristles  10 . 
     Extending radially from the seal body  21  axially downstream of the bristles  10  is an annular support plate  16 . A lip  16   a  extends axially from the radially inner distal end other support plate  16  toward the bristles  10 . An annular backing plate or ring  14 , which is just those substantially parallel to the bristles  10 , is sandwiched between the bristles tended and the lip  16  AE with an upstream face of the backing plate  14  adjacent to the bristles  10 . The backing plate is movable in a radio director and relative to the lip  26 , bristles  10  and in a radio direction relative to the lip  26 , bristles  10  and seal body  21  whilst it is actually located and supported by the lip  16  a and support plate  16 . The lip  16  a also has a curved profile to reduce the friction between the lip  16  AE and backing plate  16  to the two assists and permit radial movement of the backing plate or ring  14 . In addition to reduce friction a chamber or cavity  18  is defined by the lip  16  AE, support plate  16 , and seal body  21 . Preferably a duct  20 , connected to a source love pressurized air (not shown), is provided in the seal body  21  to supply pressurized air to the cavity  18  and generate a cavity pressure therein. The cavity pressure is approximately equal to the upstream pressure P 1  such that there is a small net force in the upstream direction on the backing plate to reduce the friction between the  16  a lip and backing ring  14 . The exact value of the cavity pressure is experimentally determined in the cavity pressure is fine tuned, i.e. raised or lowered celestial permit the required radial movement of the backing ring  14 . 
     It will be appreciated however that a lip  16   a  need not be provided and the backing ring  14  could be simply sandwiched between an upstream face of the support plate  16  and the bristles  10 . This however will increase the friction between the backing plate  14  and support plate  16  which may restrict, to some degree, the radial movement of the backing ring  14 . 
     To facilitate radial movement of the backing plate  14  a clearance  28  is provided between the outer peripheral edge of the backing plate  14  and the seal body  21 . The dimensions of the clearance  28  are such that the backing plate  14  can move sufficiently radially, in response to radial movement of the shaft  2 , so that the inner periphery of the backing plate  22  will not contact the shaft outer surface  3 . Radial movement of the shaft  2  is to be expected in operation due to runout and thermal bowing of the shaft  2 , movement of the shaft  2  due to turning acceleration, shaft vibration, and eccentricity of the shaft  2  or seal  9  mounting with respect to the axis of rotation  1 . 
     To inhibit rotational movement of the backing plate  14 , whilst still permitting radial movement of the backing plate  14 , anti rotation lugs engaging in radial slots may be provided between the backing plate and seal body or support plate. In the interests of clarity these have not been shown. It will also be appreciated that other means can be used to prevent rotation of the backing plate  14 . 
     The diameter of the radially inner bore of the backing plate  14  is only slightly greater than the diameter of the outer surface  3  of the shaft  2  such that there is only a very small clearance c between the radially inner peripheral edge  22  of the backing plate  14  and the shaft outer surface  3 . Conventionally the clearance between the backing plate needs to be sufficient to accommodate radial movement of the shaft  2  without the backing plate contacting the backing ring  14 . However since the backing ring  14  is radially movable the clearance c now only needs to be sufficient to accommodate centrifugal and thermal growth of the shaft and consequently can be much smaller. Typically in gas turbine brush seal applications the clearance between a fixed backing ring and the shaft  2  is between 1 to 2 mm cold reducing by 0.2 mm at operating temperature, whereas with the movable backing ring  14  clearances below 1 mm down to close to the shaft outer diameter at operating conditions can be used. By reducing the clearance c the leakage flow through the seal  9  is reduced and the seal  9  performance is improved giving a significant reduction in leakage flow over a conventional fixed backing ring brush seal with a 1 to 2 mm clearance. 
     In the embodiment shown in FIG. 1, at the radially inner extent  22  of the backing ring  14  there is a circumferential flange  24  which extends in an axially downstream direction generally parallel to the shaft outer surface  3 . The flange  24  defines an enlarged foot on the backing ring  14  and provides an enlarged radially inner peripheral surface  26  to the backing plate  14  of a increased area. This inner peripheral surface  26  is separated from the shaft outer surface  3  by the very small clearance c. 
     In operation fluid will leak and flow from the high pressure region  6  to the lower pressure region  8  along the shaft  2  within the clearance c between inner peripheral surface  26  of the backing plate  14  and the shaft outer surface  3 . The enlarged surface area of the inner peripheral surface  26  of the backing plate and the small clearance c cause a cushion of air to be generated between the inner peripheral surface  26  and the shaft outer surface  3 . The backing plate  14  will float radially on this cushion of air; an aerodynamic phenomenon know as air riding in which the inner surface  26  is not only lifted away from the outer shaft surface  3  but is also sucked radially inwardly towards to shaft outer surface  3  to maintain an equilibrium minimum clearance c therebetween. The air riding maintains the small clearance c even during radial movement of the shaft  2 . Contact between the backing plate  14  and the shaft  2  is thereby avoided, or reduced, and is not required in order to radially move the backing plate  14  as is the case with conventional movable backing plate brush seals. Wear of the backing plate  14  and shaft  2  is also avoided or reduced. Consequently seal performance does not deteriorate as rapidly as with conventional brush seals in which wear increases the clearance c between the backing plate  14  and shaft  2 . Damage to the shaft  2  through such wear, which in extreme cases with conventional seals can lead to shaft  2  failure, is also reduced or avoided. 
     An alternative embodiment of the present invention is shown in FIG.  3 . The seal  9   a  is generally similar to the seal  9  shown in FIG.  1  and like reference numerals have been used for like features. In this embodiment however no flange  24  is provided on the backing plate  14   a.  Instead a recess  30  is defined within the radially inner periphery  22   a  of the backing plate  14   a  by upstream and downstream backing plate recess walls  31 , 32 . The recess  30  circumscribes the inner periphery  22   a  of the backing plate  14   a  with an open side of the recess  30  facing radially inwardly towards the outer surface  3  of the shaft  2 . In operation the recess  30  traps some of air leaking between the shaft  2  and backing plate  14   a.  This creates an air cushion between the inner the backing plate  14   a  and the shaft outer surface  3  with the backing plate  14   a  air riding adjacent to the shaft outer surface  3 . In a similar way to the embodiment shown in FIG.  1  air riding of the backing plate  14   a  maintains the clearance c between the backing plate  14   a  and the shaft surface  3 . To further promote air riding of the backing plate  14   a  a duct  21  may be provided within the backing plate  14   a  to supply pressurised air from the chamber  18  defined by the support plate  16  to the recess  30 . Alternatively a duct may directly interconnect the recess  30  with the upstream region  6 , albeit through the bristles  10 . 
     The recess  30  defined in the radially inner periphery  22   a  of the backing plate  14   a  may be divided circumferentially to form a number of pockets rather than a single circumferential recess  30 . To further enhance air riding the radial depth d of the recess  30  or pockets may vary circumferentially as shown in FIG.  5 . Over the circumferential length of sectors of the circumference of the plate  14   a,  or over the circumferential length of the individual pockets, the radial depth d of the recess  30  may increase gradually, in the direction of rotation B of the shaft  2 , to a maximum. 
     It will be appreciated that in other embodiments the inner periphery  22   a  of the plate  14  may be profiled or shaped in different ways in order to promote air riding of the plate. 
     In operation the bristles  10  are forced against the backing plates  14  by the pressure difference between the upstream  6  and downstream  8  regions and the backing plate  14  provides axial support for the bristles  10 . The load between the bristles  10  and the backing plate  14  may however restrict and impede radial movement of the backing plate  14 . To overcome this problem a radially outer portion  15  of the face of the backing plate  14   c  facing the bristles  10  may be recessed and spaced axially from the bristles as shown in FIG. 4. A cavity  19  is then defined between the face of the backing plate  14   c  and the bristles  10  with the bristles  10  contacting and being supported by only a small portion of the backing plate  14   c.  A duct  17  may also be provided and defined within the backing plate  14   c.  The duct  17  interconnects the cavities  18 , 19  on either side of the backing plate  14   c  and supplies pressurised air to the backing plate cavity  19 . The pressure in both cavities  18 , 19  will be similar and is substantially the same as the upstream pressure P 1 . This has the affect of reducing the pressure differential across the bristles  10  which reduces the load between the bristles  10  and the backing plate  14   c.  Recessing of the backing plate and supplying pressurised fluid to the cavity  19  is described in EP 0,778,431 albeit in relation to a fixed backing plate. As described in that patent a further advantage, which is also present in the seal  9   c  shown in FIG. 4, is that radial movement and flexibility of the bristles  10  is enhanced so reducing bristle  10  wear. 
     In all of these embodiments the inner peripheral surface of the backing ring and the outer surface  3  of shaft  2  are preferably made as smooth as possible in order to promote air riding of the backing plate. 
     It will be appreciated that in all of these embodiments the backing plate  14  may be segmented with a number of individual abutting segments making up a complete annular backing plate assembly. The individual segments would be able to move radially independently of each other but would be held arch bound to prevent the assembly collapsing in below a minimum diameter, for example the shaft  2  diameter. Air riding of the individual segments will maintain a minimum clearance between the radially inner edge of the segments and the outer shaft surface  3  with the air riding to a certain degree preventing the segments from moving too far radially outwards. By segmenting the backing plate  14  inadvertent build up of hoop stresses due to thermal effects which may buckle or otherwise damage the backing plate is avoided. In addition the individual segments will move radially more easily than movement of the solid complete backing plate. A segmented backing plate will therefore be more responsive. 
     Although in all of the embodiments described above an air riding backing plate  14  is described it will be appreciated that the arrangement could be reversed with the backing plate  14  becoming a air riding front plate. This would have the advantage that the bristles  10  would be forced away from the radially movable plate and so the would be free to move radially more easily.