Abstract:
A brush seal includes first and second clamp plates, each clamp plate having an opposed clamping surface, and a bristle pack clamped between the clamping surfaces. The clamping surface of at least one of the clamp plates includes a surface feature extending across the clamping surface, the feature having a protrusion.

Description:
[0001]    This is a Divisional Application of application Ser. No. 14/816,557 filed Aug. 3, 2015, which is a Divisional Application of application Ser. No. 13/591,947 filed Aug. 22., 2012, The disclosure of the prior applications is hereby incorporated by reference herein in its entirety. 
         [0002]    The present invention relates to brush seals, which may be in strip form for linear sealing applications, or in annular form for circular and non circular sealing applications. 
     
    
     BACKGROUND 
       [0003]    Brush seals are used in many industries and commonly take the form of a pair of clamping plates between which is clamped a bristle pack. The clamping plates may be of different heights, providing different levels of support to the bristles depending on the direction of application of force. A section through a conventional annular brush seal  2  is illustrated in  FIG. 1 . The seal  2  comprises a first annular clamp plate  4 , a second annular clamp plate  6  and a bristle pack  8  sandwiched between the plates  4 ,  6 . A circumferential weld  10  is applied around the outer circumference of the seal  2  to attach the bristle pack  8  firmly to the first and second plates  4 ,  6 . The clamping plates  4 ,  6  each comprise an opposed clamping surface  12 ,  14 , that contacts the bristle pack  8  and over which a clamping force is applied to hold the bristle pack  8  in place. The clamping surfaces  12 ,  14  are planar and parallel, resulting in a substantially constant separation between the surfaces across the entire clamped region of the bristle pack  8 . As illustrated in  FIG. 1 , one of the first and second plates  4 ,  6 , may be deeper in a bristle direction than the other of the plates, such that the bristle pack  8  is supported along a greater extent of its height on one side. In the seal  2  illustrated in  FIG. 1 , the second plate  6  is deeper than the first plate  4 , providing additional support to the bristle pack  8  on the right side of the bristle pack, as seen in the Figure. 
         [0004]    It will be appreciated that the sectional view of  FIG. 1  is also representative of a linear blush seal having rectangular clamping plates  4 ,  6  and a linear bristle pack and outer edge weld. 
         [0005]    During manufacture of a seal, the component parts of the seal are assembled and a clamping load is applied to the clamping plates  4 ,  6  while the weld operation takes place. The bristle pack  8  may contain a great number of individual bristles, and the packing arrangement of the bristles may vary across the clamped region of the bristle pack. Such variations in packing arrangement can impact upon the clamping load experienced across the bristle pack  8 . The result is a bristle loading pattern that varies in an uncontrolled manner across the clamping region, with regions of maximum clamping load, where the bristles of the bristle pack are strongly constrained and unable to move, and regions of lesser loading, where some movement of the bristles is possible. These regions of greater and lesser clamping load cannot be accurately identified, meaning the precise location at which bristles are rigidly fixed, and hence the effective bristle length, is unknown. Even very small variations in clamping load applied to the plates, or in plate separation, during the weld operation can impact upon the effective length of the bristle pack. These variations also affect the weld depth around the circumferential or outer weld. 
         [0006]    The uncontrolled nature of the variation in clamping means that the effective bristle length and the weld depth are not constant around any one seal or between seals, resulting in considerable variation in bristle pack behaviour. Such in-part and part-to-part variation in bristle pack behaviour can result in seal components being badly matched to the mechanical environment in which they are employed, and hence to substantial variation in component life. 
         [0007]    Issues of bristle packing and clamping load variation are particularly acute in annular brush seals. Owing to the radially extending nature of the bristle pack, the same number of bristles must be packed into a cylindrical packing area that reduces with reducing radius. The packing density of bristles increases towards the radially inner edge of the clamping region, owing to the reduction in circumference with decreasing radius. Consequently, the clamping force experienced by the bristle back increases with decreasing radius. The deeper the clamping plates in the radial direction, the greater the variation in packing density, and hence in clamping load over the plates. 
         [0008]    The present invention seeks to address some or all of the above mentioned disadvantages. 
       SUMMARY OF INVENTION 
       [0009]    According to the present invention, there is provided an annular brush seal comprising first and second annular clamp plates, each clamp plate comprising an opposed annular clamping surface, and a pack of radially extending bristles clamped between the clamping surfaces of the clamp plates, wherein the damping surface or at least one of the clamp plates is profiled, such that a separation between the clamping surfaces increases with decreasing radius. 
         [0010]    The clamping surface may be profiled such that separation between the clamping surfaces is inversely proportional to radius. In this manner, it may be ensured that the cylindrical packing area available to the bristles remains substantially constant with changing radial location. 
         [0011]    Both clamping surfaces may be profiled, the two profiles cooperating to provide the desired relationship between separation and radial location. 
         [0012]    The profiled clamping surface may further comprise an annular surface feature causing a step change in the profile of the surface. A step change in the surface profile causes a corresponding step change in the load experienced by the bristle pack, thus providing a defined radial location of maximum loading. 
         [0013]    The annular surface feature may comprise an annular shoulder. The annular shoulder may be arranged such that the step change in surface profile causes a step increase in clamping surface separation with a reduction in radius. 
         [0014]    The annular surface feature may comprise an annular protrusion, such as an annular rib formed on the clamping surface. 
         [0015]    The annular surface feature may comprise an annular recess. The annular recess may extend past a radially inner edge of a clamped region of the bristle pack. 
         [0016]    The annular surface feature may comprise a cooperating protrusion and recess. The recess may be located on a radially inner side of the protrusion. 
         [0017]    According to another aspect of the present invention, there is provided a brush seal comprising first and second clamp plates, each clamp plate comprising an opposed clamping surface, and a bristle pack clamped between the clamping surfaces of the clamp plates, wherein the clamping surface of at least one of the clamp plates comprises a linear surface feature extending across the clamping surface and causing a step change in the profile of the damping surface. 
         [0018]    A step change in the clamping surface causes a corresponding step change in the load experienced by the bristle pack, thus providing as defined radial location of maximum loading. 
         [0019]    The linear surface feature may extend across the clamping surface substantially to the full extent of the bristle pack clamped between the clamping surfaces. 
         [0020]    The surface feature may comprise an integral feature of the clamp plate. Alternatively, the surface feature may comprise a separate component cooperating with the clamp plate. 
         [0021]    The surface feature may comprise a shoulder. The shoulder may be arranged such that the step change in surface profile causes a step increase in clamping surface separation in a direction towards the free ends of the bristles. 
         [0022]    The surface feature may comprise a protrusion, such as a rib formed on the clamping surface. 
         [0023]    The surface feature may comprise a recess. The recess may extend past an inner edge of a clamped region of the bristle pack. 
         [0024]    The surface feature may comprise a cooperating protrusion and recess. The recess may be located on an inner side of the protrusion. 
         [0025]    The brush seal may comprise an annular seal. The clamp plates may comprise annular clamping surfaces and the surface feature may comprise an annular surface feature. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0026]    For a better understanding of the present invention, and to show how it may be carried into effect, reference will now be made, by way of example, to the following drawings, in which:— 
           [0027]      FIG. 1  is a sectional view of a conventional annular brush seal; 
           [0028]      FIGS. 2 and 3  are sectional views of a seals having a recessed clamping plate; 
           [0029]      FIG. 4  is a sectional view of a seal having a protrusion on a clamping surface; 
           [0030]      FIG. 5  is a sectional view of a seal having a cooperating protrusion and recess on a clamping surface; 
           [0031]      FIG. 6  is a sectional view of a seal having a profiled clamping plate; and 
           [0032]      FIG. 7  is a sectional view of a seal having a protrusion formed on a profiled clamping plate. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0033]    The present invention addresses the disadvantages of the prior art in two ways. The increased packing density experienced at radially inner locations of annular seals is addressed by profiling the clamping surface or surfaces, such that separation of the clamping surfaces varies inversely with radius, providing a substantially constant cylindrical packing area. This profiling may be combined with, or as an alternative to, a step change in the clamping surface, caused for example by a shoulder, recess or protrusion. Such a step change in clamping surface causes a corresponding step change in clamping force experienced by the bristle pack, hence defining a precise radial location for maximum axial loading. This location is then the location at which the bristles become fixed, and defines the effective length of the bristles. In this manner, variation in bristle free lengths is minimised and a more repeatable bristle clamping position is established. Aspects of the present invention thus allow for consistent and repeatable bristle pack behaviour around a single seal and between different seals. 
         [0034]    With reference to  FIG. 2 , a first embodiment of the present invention comprises an annular brush seal  102  having a first annular clamp plate  104 , a second annular clamp plate  106  and a bristle pack  108  sandwiched between the plates  104 ,  106 . A circumferential weld  110  is applied around the outer circumference of the seal  102  to attach the bristle pack  108  firmly to the first and second plates  104 ,  106 . The clamping plates  104 ,  106  each comprise an opposed clamping surface  112 ,  114 , that contacts the bristle pack  108  and over which a clamping force is applied to hold the bristle pack  108  in place. The clamping surfaces  112 ,  114  are planar and parallel. A first one of the clamping surfaces  112  comprises an annular recess  116  that commences at a shoulder  118  and extends from the shoulder  118  to the radially inner edge  120  of the clamping surface  112 . The separation between the clamping surfaces  112 ,  114  is thus increased by the depth of the recess over the annular area of the recess. Consequently, the clamping force experienced by the bristle pack is reduced over this annular area. The maximum clamping force experienced by the bristle pack  108  is at the radial location of the shoulder  118  at which the recess commences. This is illustrated on the Figure as R 1 . At this radial location, the bristles in the bristle pack become fixed, and the effective length of the bristle pack may thus be measured from this radial location. 
         [0035]    Another embodiment of the present invention is illustrated in  FIG. 3 . This embodiment largely corresponds to that of  FIG. 2 , the seal  202  comprising first and second annular clamp plates  204 ,  206  and a bristle pack  208  sandwiched between the plates  204 ,  206 . A circumferential weld  210  is applied around the outer circumference of the seal  202 . As in  FIG. 2 , the clamping plates  204 ,  206  each comprise an opposed clamping surface  212 ,  214 , that contacts the bristle pack  208  and over which a clamping force is applied to hold the bristle pack  208  in place. In the embodiment of  FIG. 3 , an annular recess  218  is formed on a second one of the clamping surfaces  216 . The recess  216  commences at an annular shoulder  218  and extends radially inwardly past the radially inner extent  220  of the first clamping surface  214 . The recess  216  performs a similar function to the recess  116  of the embodiment of  FIG. 2 , effecting a step change in the clamping force experienced by the bristle back  208  and defining a radial location for maximum loading at the radially outer shoulder  218  where the recess commences. This radial location, illustrated as R 2  on the Figure, is the location at which the bristles in the bristle pack become fixed, and the effective length of the bristle pack may thus be measured from this radial location. The recess  216  may continue to the radially inner extent of the second clamping surface  214 , or may terminate at a second radially inner shoulder  222 . As illustrated in the Figure, the radially inner shoulder  222  is positioned radially inward of the radially inner extent of the first clamping surface  212 , such that the separation between the clamping surfaces remains constant radially inward of the first shoulder  218 . 
         [0036]      FIG. 4  illustrates another embodiment of the present invention, in which a defined radial location for maximum loading is provided by an annular rib or protrusion  324  formed on the first clamping surface  312 . The seal  302  of  FIG. 4  comprises first and second clamping plates  304 ,  306  having opposed clamping surfaces  312 ,  314  and sandwiching a bristle pack  308  therebetween. An annular weld  310  extends around the outer circumference of the seal  302 . An annular rib  324  protrudes from a first one of the clamping surfaces  312 , providing a step change in the separation between the clamping surfaces  312 ,  314  at the radial location of the rib  324 . This step change corresponds to a step increase in the clamping load experienced by the bristle pack  308 , and hence provides as defined radial location for maximum axial loading experienced by the bristle pack  308 . This radial location, illustrated as R 3  on the Figure, is the location at which the bristles in the bristle pack become fixed, and the effective length of the bristle pack may thus be measured from this radial location. 
         [0037]    With reference to  FIG. 5 , the annular rib of the embodiment of  FIG. 4  may be formed on either the first or the second clamp plate, and may optionally be combined with the recess of  FIG. 3 . The embodiment of seal  402  shown in  FIG. 5  comprises first and second clamping plates  404 ,  406  having opposed clamping surfaces  412 ,  414  and sandwiching a bristle pack  408  therebetween. An annular weld  410  extends around the outer circumference of the seal  402 . An annular rib  424  protrudes from a second one of the clamping surfaces  414 , providing a step change in the separation between the clamping surfaces  412 ,  414  at the radial location of the rib  424 . The rib  424  is combined with a recess  416 , formed radially inwards of the rib  424  and extending radially inwardly past the radially inner extent  420  of the first damping surface  414 . The step change in separation between the clamping surfaces caused by the cooperating rib  424  and recess  416  corresponds to a step change in the clamping load experienced by the bristle pack  408 . This change thus provides a defined radial location for maximum axial loading experienced by the bristle pack  408 . This radial location, illustrated as R 4  on the Figure, is the location at which the bristles in the bristle pack become fixed, and the effective length of the bristle pack may thus be measured from this radial location. 
         [0038]    As discussed above, the present invention also provides for the establishment of a uniform clamping load in an annular brush seal by profiling one or both of the clamping surfaces. A seal embodying this concept is illustrated in  FIG. 6 . The seal  502  comprises a first annular clamp plate  504 , a second annular clamp plate  506  and to bristle pack  508  sandwiched between the plates  504 ,  506 . A circumferential weld  510  is applied around the outer circumference of the seal  502  to attach the bristle pack  508  firmly to the first and second plates  504 ,  506 . The clamping plates  504 ,  506  each comprise an opposed clamping surface  512 ,  514 , that contacts the bristle pack  508  and over which a clamping force is applied to hold the bristle pack  508  in place. A first one of the clamping surfaces  512  is profiled, such that the separation between the clamping surfaces increases with decreasing radius. The profile of the first clamping surface  412  is idealised to provide an inversely proportional relationship between separation of the clamping surfaces and radius. Such a relationship allows for the cylindrical packing area available to the bristle pack  508  to remain substantially constant, even as the radial location under consideration changes. 
         [0039]    The cylindrical packing area available to the bristle pack at a radius r is calculated by the formula: 
         [0000]      Packing area=2 πrs    
         [0000]    where s is the separation distance between the clamping surfaces at the radius r. 
         [0040]    Thus, with the parallel damping surfaces of the prior art, a bristle pack must be forced into a cylindrical packing area that decreases towards the radially inner edge of the seal. In contrast, according to the present invention, the cylindrical packing area may remain constant with changing radius, owing to the clamping plate profiling that results in a relationship between separation s and radius r of: 
         [0000]    
       
         
           
             s 
             ∝ 
             
               1 
               r 
             
           
         
       
     
         [0041]    It will be appreciated that this relationship represents an idealised form of the invention, and that the profile of the clamping surface  512  need not exactly match the ideal in order to deliver the advantages of the invention. The substantially constant packing area available to the bristle pack ensures that clamping force remains largely constant with changing radius, allowing greater control of bristle packing and more even distribution of packing load. 
         [0042]    Features of the various embodiments of the invention described above may be combined in an advantageous manner, as illustrated for example in  FIG. 7 .  FIG. 7  illustrates a seal  602  in which the profiled clamping surface of the embodiment of  FIG. 6  is combined with the recess of  FIG. 2 . The seal  602  of  FIG. 7  comprises a first annular clamp plate  604 , a second annular clamp plate  606  and a bristle pack  608  sandwiched between the plates  604 ,  606 . A circumferential weld  610  is applied around the outer circumference of the seal  602  to attach the bristle pack  608  firmly to the first and second plates  604 ,  606 . The clamping plates  604 ,  606  each comprise an opposed clamping surface  612 ,  614 , that contacts the bristle pack  608  and over which a clamping force is applied to hold the bristle pack  608  in place. A first one of the clamping surfaces  612  is profiled, such that the separation between the clamping surfaces increases with decreasing radius. The profile preferably allows for an approximately inversely proportional relationship between the separation of the clamping surfaces and the radial location at which the separation is measured. The first one of the clamping surfaces  612  further comprises an annular recess  616  that commences at a shoulder  618  and extends from the shoulder  618  to the radially inner edge  620  of the clamping surface  612 . The shoulder  618  at which the recess commences causes a step change in the clamping surface separation, and hence a step reduction in the clamping force experienced by the bristle pack  608  over the annular area of the recess. The maximum clamping force experienced by the bristle pack  608  is at the radial location of the shoulder  618  at which the recess commences. This is illustrated on the Figure as R 6 . At this radial location, the bristles in the bristle pack become fixed, and the effective length of the bristle pack may thus be measured from this radial location. 
         [0043]    The present invention thus addresses the key areas of bristle free length variability and weld process control, allowing for the formation of repeatable bristle packs, with little variation around a single part and also from part-to-part. 
         [0044]    It will be appreciated that the invention may be applied to circular or non circular seals. The invention may also be applied to brush seal strips for linear sealing problems where a defined bristle or fibre length is necessary to reduce or remove variability in seal performance. The invention may be employed in any application where free fibre or bristle length is important, and particularly in applications where the bristle pack has a radially extending element; the reduction in cylindrical clamping area with radius causing ambiguity in the exact location of bristle or fibre clamping. The invention may be applied to metallic or non metallic seals used in gas or steam turbines, pumps or compressors.