Patent Publication Number: US-2012025472-A1

Title: Labyrinth seal

Description:
This application claims priority under 35 U.S.C. §119 to Great Britain Application No. 1008968.8, filed 28 May 2010, the entirety of which is incorporated by reference herein. 
     BACKGROUND 
     1. Field of Endeavor 
     The present disclosure relates to a labyrinth seal, and in particular to a labyrinth seal for sealing between static and rotating parts in a gas turbine or steam turbine. Embodiments relate to a combined labyrinth and brush seal. 
     2. Brief Description of the Related Art 
     Labyrinth seals are commonly used to provide a seal between static and rotating parts in an axial flow turbine such as a gas turbine or steam turbine. Typically, a labyrinth seal is formed by a plurality of arcuate labyrinth seal segments. The arcuate labyrinth seal segments may also carry a plurality of arcuate brush seal segments, thereby forming a combined labyrinth and brush seal. 
     Conventional labyrinth seals can suffer from a number of problems including leakage at the confronting ends of adjacent arcuate labyrinth seal segments and vibration within the seal as a result of relative movement between adjacent arcuate labyrinth seal segments. 
     There is, therefore, a need for an improved labyrinth seal. 
     SUMMARY 
     According to a first aspect, a labyrinth seal includes a plurality of arcuate labyrinth seal segments, wherein confronting ends of adjacent arcuate labyrinth seal segments have planar faces which are oriented radially and axially but which are canted away from a strictly axial orientation by a cant angle. 
     By canting the radially and axially oriented planar faces of confronting ends of adjacent arcuate labyrinth seal segments away from a strictly axial orientation, additional frictional forces arise between adjacent arcuate labyrinth seal segments, thereby damping the relative movement between adjacent arcuate labyrinth seal segments and reducing vibration within the labyrinth seal. 
     The cant angle may be between about one degree and six degrees. 
     In preferred embodiments, the cant angle is between two degrees, plus or minus one degree, and five degrees, plus or minus one degree. A minimum cant angle of about two degrees may be appropriate because the manufacturing tolerances are typically plus or minus one degree and because a cant angle of less than one degree may have little or no effect on reducing vibration. A maximum cant angle of about five degrees may be appropriate because, taking into account the aforesaid manufacturing tolerances, a cant angle of greater than six degrees may result in unacceptably large frictional forces between the confronting ends of adjacent arcuate labyrinth seal segments, thereby resulting in an unacceptably stiff labyrinth seal. 
     The labyrinth seal may include a plurality of arcuate brush seal segments retained in the plurality of arcuate labyrinth seal segments, thus forming a combined labyrinth and brush seal. 
     Each arcuate brush seal segment typically includes an arcuate bristle retainer and a plurality of bristles which are retained by the arcuate bristle retainer and typically inclined away from the radial direction of the labyrinth seal. Confronting ends of the arcuate bristle retainers of adjacent arcuate brush seal segments are typically inclined away from the radial direction, at substantially the same angle of inclination as the bristles. This ensures that there are no triangular gaps between the bristles at the confronting ends of adjacent arcuate brush seal segments. 
     The confronting ends of the arcuate bristle retainers of adjacent arcuate brush seal segments are preferably axially oriented, usually in a strictly axial orientation. 
     Each arcuate brush seal segment typically has the same angular extent as the arcuate labyrinth seal segment which retains it. Each arcuate labyrinth seal segment typically comprises a plurality of axially spaced arcuate fins which normally have the same angular extent as the arcuate labyrinth seal segment. The labyrinth seal normally forms part of the static structure of a gas turbine or steam turbine and cooperates with a radially adjacent rotating surface. Typically, the arcuate brush seal segments and fins extend from an inner surface of each arcuate labyrinth seal segment towards a radially adjacent rotating surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention of the present application will now be described in more detail with reference to exemplary embodiments of the apparatus and method, given only by way of example, and with reference to the accompanying drawings, in which: 
         FIG. 1  is a diagrammatic perspective view of part of an embodiment of a combined labyrinth and brush seal in a disassembled state showing confronting ends of adjacent seal ring segments; 
         FIG. 2  is a diagrammatic radially inward looking view on the radially outer surface of an arcuate labyrinth seal segment of  FIG. 1 ; and 
         FIGS. 3 and 4  are simplified radially inward looking and axial views of the seal segments shown in  FIG. 1  after assembly, showing the regions of contact between confronting ends of adjacent arcuate seal segments. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Embodiments will now be described by way of example only and with reference to the accompanying drawings. 
       FIG. 1  is a perspective view of the confronting ends of two adjacent seal ring segments  10 ,  12  of a labyrinth seal. In order to form a complete labyrinth seal, it will be understood that a plurality of the seal ring segments  10 ,  12  are arranged end-to-end to form a continuous seal ring. The labyrinth seal forms part of the static structure of a gas turbine or a steam turbine and cooperates with a radially adjacent rotating surface so that a seal is formed between the static and rotating parts. 
     Each seal ring segment  10 ,  12  includes an arcuate labyrinth seal segment  14  having fins  15  which extend from a radially inner surface of the arcuate labyrinth seal segment  14  towards the surface of a radially adjacent rotating part (not shown). It is common practice to utilize labyrinth seals whose sealing elements include only fins  15  or the like, as described above. However, in the present case, a combined labyrinth and brush seal is formed by retaining arcuate brush seal segments  16  in the arcuate labyrinth seal segments  14 . It is preferred that each arcuate brush seal segment  16  has the same angular extent as a corresponding arcuate labyrinth seal segment  14 . The arcuate brush seal segments  16  reduce leakage and thereby increase the sealing efficiency of the labyrinth seal. 
     The arcuate brush seal segment  16  has an arcuate bristle retainer  18  which has a generally T-shaped cross-section, and the head  20  or cross-bar of the T-shaped bristle retainer  18  and most of its stem  22  are located in a correspondingly shaped slot  23  in the arcuate labyrinth seal segment  14 . The end of the arcuate bristle retainer  18  is oriented axially (i.e., is not inclined away from the strictly axial direction), as best shown by the dashed line  30  in  FIG. 3 . A plurality of bristles  24  are retained by the arcuate bristle retainer  18  and the bristles  24  are inclined away from the radial direction of the labyrinth seal at a common angle, for example 45 degrees, so that if notionally extended beyond their length, they would be nominally tangential to a circle of smaller diameter than the labyrinth seal. 
     In order to avoid a triangular gap between the bristles  24  at confronting ends of the adjacent arcuate brush seal segments  16 , which would leave the bristles  24  adjacent the triangular gap unsupported by neighboring bristles and therefore liable to damage, the confronting ends of the arcuate bristle retainers  18  of adjacent arcuate brush seal segments  16  are also inclined away from the radial direction of the labyrinth seal at the same angle of inclination as the bristles  24 . As a result, the end of one of the arcuate brush seal segments  16  projects from the end face of the seal ring segment  10  whilst the confronting end of the adjacent arcuate brush seal segment  16  is recessed inwardly from the end face of the seal ring segment  12 . 
     The confronting end of each adjacent arcuate labyrinth seal segment  14  has a generally planar face  26  which is oriented in the radial and axial directions of the labyrinth seal. The generally planar face  26  of the confronting end of each adjacent arcuate labyrinth seal segment  14  is oriented in a strictly radial direction, as best seen in  FIG. 4 , but is canted away from a strictly axial orientation, denoted by the line  28  in  FIGS. 2 and 3 , by a cant angle x. As discussed above, this increases the frictional forces between the confronting ends of adjacent arcuate labyrinth seal segments  14 , thereby increasing damping and reducing unwanted vibration which can otherwise arise in the labyrinth seal during operation of a gas turbine or steam turbine. 
     In typical embodiments, the cant angle x is between about 2 and 5 degrees, these minimum and maximum cant angles being subject to a typical manufacturing tolerance of about ±1 degree. As discussed above, a cant angle x of less than about 1 degree is believed to have little or no effect on reducing vibration whilst a cant angle x of greater than about six degrees is believed to result in an unacceptably stiff seal as a result of unacceptably large frictional forces. 
     Although various embodiments have been described in the preceding paragraphs, it should be understood that various modifications may be made to those embodiments without departing from the scope of the following claims. For example, the arcuate brush seal segments  16  (when present) could have any suitable configuration, possibly having confronting ends that are not inclined away from the radial direction. 
     While the invention has been described in detail with reference to exemplary embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents. The entirety of each of the aforementioned documents is incorporated by reference herein.