Abstract:
The present invention, in one embodiment, provides a low leakage sealing system comprising a pipe for directing the flow of a fluid medium and a housing surrounding the pipe. The housing is spaced radially outward of the pipe defining an annular space. A sealing assembly, comprising an inner seal, outer seal and a final seal are disposed in a spaced relationship in an axial direction in the annular space so as to reduce leakage flow therethrough that is created by the flowing fluid medium pressure. In addition, a ledge is disposed downstream from the sealing assembly and the final seal.

Description:
BACKGROUND OF INVENTION  
         [0001]    The present invention relates to a sealing system for steam turbines and particularly relates to a pipe or snout for flowing steam wherein the pipe is sealed to separate casings which have different magnitudes of thermal expansion relative to one another and hence are movable relative to one another and to the pipe.  
           [0002]    Steam turbines require sealing systems that can prevent leakage between the steam inlet snout and the surrounding distinct inner and outer shells and a nozzle box (hereafter sometimes collectively referred to as a housing). In current seal designs for this purpose, the seal system consists of sets of rings that seal between the inlet snout and each of the inner and outer shells and nozzle box. For example, for sealing in the annular space between the snout and a shell, a plurality of sealing rings are axially stacked one against the other. Alternate sealing rings in the stack have large and small diameters, respectively. The smaller diameter sealing rings bear and seal against the exterior surface of the pipe or snout, while the larger diameter sealing rings bear and seal against the interior wall surface of the shell. Thus, with the rings alternately sealing radially against the snout and shell walls and sealing axially against one another at opposed axial sealing faces, relative movement between the parts is facilitated.  
           [0003]    In one such prior sealing system, the smaller diameter sealing rings have a coefficient of thermal expansion less than the coefficient of thermal expansion of the snout whereby the snout expands a greater amount than the smaller sealing rings to ensure a tight seal between the smaller diameter sealing rings and the snout wall as operating temperatures increase to steady state. In that same prior sealing system, the coefficient of thermal expansion of the larger diameter sealing rings is approximately the same as or larger than the coefficient of thermal expansion of the outer shell such that the larger diameter rings expand more than the shell expands. This ensures a tight seal between the larger diameter sealing rings and the shell wall when the system heats up to operating temperature.  
           [0004]    In these prior systems, however, there remain leakage paths due to the relative movement of the various parts of the system, e.g., misalignments and vibrations occur even at operating temperatures. Consequently, the sealing rings may lose contact with one another and/or the interfacing sealing component and yield leakage flow. With axially stacked sealing rings, the leakage flows may occur between the sealing rings and the snout or shell walls, or both, or between the axial sealing faces of the sealing rings per se. Accordingly, there is a need for a low leakage snout sealing system for steam turbines.  
         SUMMARY OF INVENTION  
         [0005]    The present invention, in one embodiment, provides a low leakage sealing system comprising a pipe for directing the flow of a fluid medium and a housing surrounding the pipe. The housing is spaced radially outward of the pipe defining an annular space. A sealing assembly, comprising an inner seal, outer seal, and a final seal are disposed in a spaced relationship in an axial direction in the annular space so as to reduce leakage flow therethrough that is created by the flowing fluid medium pressure. In addition, a ledge is disposed downstream from the sealing assembly and the final seal. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0006]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:  
         [0007]    [0007]FIG. 1 is a side representational view of a steam inlet pipe or snout surrounded by an outer shell, an inner shell and a nozzle box and a sealing system disposed therebetween in accordance with the prior art;  
         [0008]    [0008]FIG. 2 is an enlarged crossview of a prior art sealing system illustrating axially stacked sealing rings and leakage paths about the sealing rings;  
         [0009]    [0009]FIG. 3 is a view similar to FIG. 2 illustrating a low leakage sealing system according to one embodiment of the present invention;  
         [0010]    [0010]FIG. 4 is an enlarged cross-sectional view of FIG. 2 illustrating a low leakage sealing system according to one embodiment of the present invention.  
         [0011]    [0011]FIG. 5 is a view similar to Figure w illustrating a low leakage sealing system according to another embodiment of the present invention; and  
         [0012]    [0012]FIG. 6 is a view similar to FIG. 2 illustrating a low leakage sealing system according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0013]    A sealing system, generally designated  10 , between a snout or pipe  12  (hereinafter snout) and an outer shell  14 , an inner shell  16  and a nozzle box  18 , the shell and box being sometimes individually or collectively referred to as a housing  19  (see FIG. 1). These components, disposed around centerline  17 , form part of a turbine in which a flowing fluid medium, for example, steam, is passed through the snout  12 . It will be appreciated that some turbines operate at high inlet pressures (typically in the range about  3500  psia) thus necessitating the use of at least two separate shells, an inner shell  16  and outer shell  14 , nested one inside the other.  
         [0014]    A sealing system  10  is required between snout  12  and each of the shells  14  and  16  and nozzle box  18 , which are movable relative to one another as the steam turbine, for example, is brought up to operating temperature. In prior sealing systems, a series of axially stacked sealing rings  20  are disposed between the snout  12  and the interior walls of the housing  19 . For example, as illustrated in FIG. 2, large and small diameter sealing rings  20  and  22 , respectively, are axially stacked one against the other and engage the wall of shell  14 , on the one hand, and the outer wall of the snout  12  on the other hand. It will be appreciated, however, that leakage steam flows from the high pressure side toward the low pressure side, i.e., from the top to the bottom of drawing FIG. 2. The leakage paths are thus between the sealing rings  20  and  22  and the walls of the housing, e.g., shell  14  or nozzle box  18 , and snout  12  (see FIG. 1). On the rightside of FIG. 2 is illustrated a similar arrangement of sealing rings with a potential leakage path, in which two patterns of leakage (left and right of centerline  17 ) are shown, between the opposed axial sealing faces of the sealing rings. The potential leakage paths in both the left and right sides of FIG. 2 are illustrated by the heavy, solid black lines.  
         [0015]    In one embodiment of the present invention shown in FIGS. 3 and 4 (in further detail in FIG. 4), the sealing system of the present invention, generally designated  11 , comprises an annular sealing assembly  26  and an annular final seal  32 . Sealing assembly  26  and final seal  32  are juxtapositionally disposed so as to be supported axially by at least one support ledge  34 . Juxtapositionally disposed means that final seal  32  typically sits radially between inner  28  and outer  30  seal. Sealing assembly  26  comprises an inner seal  28  and outer seal  30  (both generally designated as sealing assembly  26  in the Figures) and is typically supported by a support ledge  34 . It will be appreciated that the shape of final seal  32  conforms to the shape of the inner seal  28  and outer seal  30 , or in other words, the shape of final seal  32  is adapted to the profile established by the inner seal  28  and outer seal  30 . For example, in one embodiment, the trapezoidal shape of final seal  32  conforms to the profile of the inner seal  28 , and outer seal  32 , at their respective interfaces so that the final seal  32  is disposed against those surfaces as shown in FIG. 4. It will be appreciated that the shape (in cross-sectional view) of sealing assembly  26  and final seal  32  includes, but is not limited to, square, trapezoidal, triangular, circular, rectangular or irregular shapes.  
         [0016]    The ledge  34 , which is typically disposed downstream from final seal  32  and sealing assembly  26 , prevents final seal  32  and sealing assembly  26  from being blown out by the turbine pressure. It will be appreciated that ledge  34  is typically made in segments to facilitate assembly. In operation, when the turbine is pressurized, for example, pressure forces acting on the upstream surface of final seal  32  will drive it between inner seal  28  and outer seal  30  serving to displace inner seal  28  radially inward and displace outer seal  30  radially outward so as to enhance the reduction of leakage flow therebetween. Thus, the upstream pressure against final seal  32  results in a seal being formed resulting in a reduction of leakage flow. Specifically, a reduction of leakage flow exists at the interfaces of the final seal  32  and inner  28  and outer  30  seals, at the interface between inner seal  28  and snout  12 , and at the interface between the outer seal  30  and the outer shell  14 . It will be appreciated that support ledge  34  may include, but is not limited to, an annular locking ring.  
         [0017]    Sealing assembly  26  and final seal  32  materials typically include, but are not limited to, graphite materials, carbon materials and the like. It will be appreciated that such materials may be compressible materials. The constant pressure force during operation of the turbine, for example, will result in the compressible material filling any gaps that may be created due to radial or angular misalignment, wear, or distortion of the parts. It will be appreciated that such materials do not have to be annular and can be in the form of a rope or yarn, for example. The rope can be formed annularly by cutting the rope at angle so as to have overlap between rope ends. In addition, the yarn ends can be interwoven or wrapped to form an annular seal.  
         [0018]    In another embodiment, it will be appreciated that sealing system  11  may be combined with at least one existing sealing ring  20  or  22  to create a more robust design (see FIG. 5). For example, sealing system  11  may be placed upstream of the sealing rings  20 ,  22 . Here, sealing system  11  works in conjunction with sealing rings  20 ,  22  to reduce leakage flow therebetween. It will be appreciated that in any embodiment, more than one sealing system  11  may be used, solely or in combination, with sealing rings  20 ,  22 .  
         [0019]    In another embodiment, sealing system  11  may be combined with sealing rings  20 ,  22  in any order to choke any leakage flow therebetween. It will be appreciated that the number and location of sealing rings  20 ,  22  and sealing systems  11  may be varied. Here, the combination of sealing system  11  and sealing rings  20 ,  22  constitutes a reliable design in that, should the sealing rings  20 ,  22  fail, the sealing system  11  remains in effect to choke any leakage flow therethrough.  
         [0020]    It will be apparent to those skilled in the art that, while the invention has been illustrated and described herein in accordance with the patent statutes, modification and changes may be made in the disclosed embodiments without departing from the true spirit and scope of the invention. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.