Abstract:
Embodiments of the invention relate generally to steam turbines, steam turbine systems, and methods for their operation and, more particularly, to high-pressure-intermediate-pressure (HPIP) steam turbine configurations having pressure drop-limited seals and exhibiting reduced mid-packing and/or end-packing leakage. One embodiment of the invention provides a combined high-pressure-intermediate-pressure (HPIP) steam turbine comprising: a high-pressure section; an intermediate-pressure section sharing a common rotor with the high-pressure section; at least one packing ring disposed along a length of the rotor and between the high-pressure section and the intermediate-pressure section; and at least one conduit having a first end disposed adjacent a packing ring and a second end disposed adjacent a steampath stage of the high-pressure section.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Embodiments of the invention relate generally to steam turbines, steam turbine systems, and methods for their operation and, more particularly, to high-pressure-intermediate-pressure (HPIP) steam turbine configurations having pressure drop-limited seals and exhibiting reduced mid-packing and/or end-packing leakage. 
         [0002]    All combined HPIP steam turbine configurations are susceptible to mid-packing leakage between the high-pressure (HP) section and the intermediate-pressure (IP) section. This mid-packing leakage is driven by a significant pressure ratio between the HP and IP sections (represented by arrow A in the drawings), as well as high operating temperatures in these areas. Typically, such leakage represents between 70% and 90% of the total leakage loss in the steam turbine. In separate HPIP sections, the highest leakages correspond to the high pressure drop and high temperature areas, which are the HP and IP inlet areas. 
         [0003]    A schematic cross-sectional view typical of a known combined HPIP steam turbine is shown in  FIGS. 1A-B . With reference to  FIG. 1 , the steam turbine  100  comprises an HP section  110 , an IP section  150 , and a mid-packing section  130  herebetween. The HP section  110  includes a plurality of steampath stages  112 A-E. The mid-packing section  130  includes a plurality of packing rings  140 A-E positioned along a length of a rotor  134 . Leakage is reduced by the proximity of packing teeth  136  of the rotor  134  to corresponding teeth of packing rings  140 A-E. Such proximity is shown more clearly in the detailed view of  FIG. 1B . Steam flow is generally along path A, from the HP section  110  to the IP section  150 . 
         [0004]    One approach to reducing mid-packing leakage and improving overall turbine efficiency has been to increase the number of packing teeth  136  along rotor  134 . This restricts flow, which would otherwise be expected to reduce leakage to some degree. However, the additional packing teeth require a greater bearing span, which in turn results in a more flexible rotor and necessarily larger radial clearances. These larger radial clearances increase leakage flow. As a result, little improvement is seen with respect to reduced mid-packing leakage or turbine efficiency. 
         [0005]    Other sealing devices, such as those incorporating brush seals and leaf seals, provide small radial clearances and have been employed in other contexts. However, brush seals, leaf seals, and similar devices (referred to herein as pressure drop-limited seals) cannot be used where a large pressure difference is found across the seal. Typically, such pressure differences cannot be greater than about 21.1 kg/cm 2  (about 300 p.s.i.) Pressures at the HP section of a typical HPIP steam turbine may be as high as 316 kg/cm 2  (4500 p.s.i.), while pressures at the IP section are generally between 28.1 kg/cm 2  (400 p.s.i.) and 35.2 kg/cm 2  (500 p.s.i.). As a result, the pressure difference across each packing ring is too great for the use of pressure drop-limited seals such as brush seals or leaf seals. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    One embodiment of the invention provides combined high-pressure-intermediate-pressure (HPIP) steam turbine comprising: a high-pressure section; an intermediate-pressure section sharing a common rotor with the high-pressure section; at least one packing ring disposed along a length of the rotor and between the high-pressure section and the intermediate-pressure section; and at least one conduit having a first end disposed adjacent a packing ring and a second end disposed adjacent a steampath stage of the high-pressure section. 
         [0007]    Another embodiment of the invention provides a separate high-pressure-intermediate-pressure (HPIP) steam turbine comprising: a high-pressure section; an intermediate-pressure section; a low-pressure section; at least one rotor engaging at least one of the high-pressure section, the intermediate-pressure section, or the low-pressure section; an end-packing region having at least one packing ring disposed along a length of the at least one rotor; and at least one conduit having a first end disposed adjacent the end-packing region and a second end disposed adjacent a steampath stage of the high-pressure section. 
         [0008]    Yet another embodiment of the invention provides a steam turbine system comprising: a high-pressure-intermediate-pressure (HPIP) steam turbine comprising: a high-pressure section; an intermediate-pressure section; at least one rotor engaging at least one of the high-pressure section and the intermediate-pressure section; at least one packing ring disposed along a length of the at least one rotor; and at least one conduit having a first end disposed adjacent a packing ring and a second end disposed adjacent a steampath stage of the high-pressure section; and a steam-generating apparatus for providing a quantity of steam to the steam turbine. 
         [0009]    Still another embodiment of the invention provides a method of reducing mid-packing leakage in a high-pressure-intermediate-pressure (HPIP) steam turbine, the method comprising: directing a quantity of steam between a steampath stage of a high-pressure section of the HPIP steam turbine and a mid-packing section of the HPIP steam turbine containing at least one packing ring. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    These and other features of this invention will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings that depict various embodiments of the invention, in which: 
           [0011]      FIG. 1A  shows a schematic cross-sectional view of a know combined high-pressure-intermediate-pressure (HPIP) steam turbine; 
           [0012]      FIG. 1B  shows a detailed view of a portion of the HPIP steam turbine of  FIG. 1A ; 
           [0013]      FIG. 2  shows a schematic cross-sectional view of a combined HPIP steam turbine according to one embodiment of the invention; and 
           [0014]      FIG. 3  shows a schematic cross-sectional view of a packing ring containing a pressure drop-limited seal suitable for use in various embodiments of the invention; and 
           [0015]      FIG. 4  shows a schematic cross-sectional view of a separate HPIP steam turbine according to one embodiment of the invention. 
       
    
    
       [0016]    It is noted that the drawings of the invention are not to scale. The drawings are intended to depict only typical aspects of the invention, and therefore should not be considered as limiting the scope of the invention. In the drawings, like numbering represents like elements between the drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0017]      FIG. 2  shows a schematic cross-sectional view of a combined high-pressure-intermediate-pressure (HPIP) steam turbine according to an embodiment of the invention. As in known devices, such as that shown in  FIG. 1A , the HPIP steam turbine  200  of  FIG. 2  includes a high-pressure (HP) section  210 , an intermediate-pressure (IP) section  250 , and a mid-packing section  230  therebetween. The mid-packing section  230  includes a rotor  234  disposed between the HP section  210  and the IP section  250  and a plurality of packing rings  240 A-C along its length. 
         [0018]    The embodiment of the invention shown in  FIG. 2 , however, includes packing rings  240 A-C having pressure drop-limited seals (shown in  FIG. 3  and described in greater detail below). The use of pressure drop-limited seals is made possible by the inclusion of conduits or “leak-off lines”  260 ,  262 ,  264  between the mid-packing section  230  and steampath stages  212 A,  212 C,  212 E of the HP section  210 , through which a quantity of high-pressure steam may travel (e.g., along path B, through conduit  264 ). Steam flow through the conduits determines the pressure difference across the packing rings  240 A,  240 B,  240 C. 
         [0019]    The conduit or leak-off line may be of any material capable of containing and permitting the movement of steam having a temperature and pressure found within the HP section. Each conduit or leak-off line  264  includes a first end  264 A adjacent a packing ring  240 C and a second end  264 B adjacent a steampath stage  212 A of the HP section  210 . (For the sake of simplicity, the first and second ends of the conduit or leak-off line are shown only with respect to conduit or leak-off line  264 . It should be understood, however, that other conduits or leak-off lines  260 ,  262  would also include first and second ends.) 
         [0020]    The resulting pressure difference across the pressure drop-limited seal(s) may vary, depending, for example, on the number of conduits or leak-off lines employed. The pressure difference across the seal should, however, be no greater than the maximum design pressure difference of the seal. The maximum design pressure difference of typical brush seals, for example, is between about 14.1 kg/cm 2  (about 200 p.s.i.) and about 21.1 kg/cm 2  (about 300 p.s.i.). In the case where a brush seal is used, therefore, the pressure difference across the brush seal may be no greater than, and preferably less than, about 21.1 kg/cm 2  (about 300 p.s.i.), more preferably less than about 17.6 kg/cm 2  (about 250 p.s.i.), and still more preferably less than about 14.1 kg/cm 2  (about 200 p.s.i.). 
         [0021]    Some embodiments of the invention may include as few as one conduit or leak-off line, while other embodiments of the invention, such as that shown in  FIG. 2 , may contain a plurality of such conduits or leak-off lines. In addition, some embodiments of the invention may include one packing ring having a pressure drop-limited seal while other embodiments of the invention may contain a plurality of such packing rings. For example, it may be possible, in some embodiments, to substantially reduce mid-packing leakage and/or improve turbine efficiency by the inclusion of a single packing ring having a pressure drop-limited seal, such as a brush seal, and a single conduit or leak-off line. Other conventional packing rings, such as those shown in  FIGS. 1A-B  ( 140 A-E) may also be employed in such embodiments. 
         [0022]    In other embodiments of the invention, the inclusion of a plurality of packing rings containing pressure drop-limited seals, while further reducing mid-packing leakage and/or improving turbine efficiency, may confer an additional benefit in that it affords the ability to reduce the overall length of the rotor  234  through the use of fewer, more efficient packing rings (i.e., those containing pressure drop-limited seals). As noted above, increasing the number of packing rings results in an increased bearing span (longer rotor), greater rotor flexibility, and larger radial clearances. Contrarily, the use of fewer packing rings, as embodiments of the invention make possible, allows for a reduced bearing span (shorter rotor), less rotor flexibility, and smaller radial clearances. Thus, while embodiments of the invention reduce mid-packing leakage and/or improve turbine efficiency through the use of packing rings having pressure drop-limited seals, the reduced bearing span made possible by the use of such packing rings may yield an additional reduction in mid-packing leakage and/or improvement in turbine efficiency. 
         [0023]      FIG. 3  shows a schematic cross-sectional view of a packing ring  340  including a brush seal  342 , suitable for use in an embodiment of the invention, and as shown in  FIG. 2  ( 240 A-C). The brush seal  342  comprises a plurality of brushes  342 A radially oriented substantially perpendicular to a longitudinal axis of the rotor to be sealed against (not shown) and disposed between an HP block  342 B and an IP block  342 C. 
         [0024]    As noted above, the invention is not limited to the use of brush seals. Any pressure drop-limited seal may be employed, leaf seals being one additional example. The pressure drop-limited seals useful in practicing the various embodiments of the invention include an element (e.g., brush, leaf, etc.) capable of movement in response to a pressure difference across the seal. For example, a brush of a brush seal bends or otherwise moves toward a side of the seal experiencing a lower pressure and away from a side of the seal experiencing a higher pressure. Leaves of a leaf seal move similarly in response to a pressure difference across the seal. 
         [0025]    In some embodiments of the invention, the packing ring  340  may further include teeth  344 ,  346 , as are often found in known packing rings. In other embodiments of the invention, the reduction in mid-packing leakage effected by the brush seal or leaf seal may obviate the need or desirability for such teeth. 
         [0026]      FIG. 4  shows a schematic cross-sectional view of a separate HPIP steam turbine  400  according to an embodiment of the invention. Here, rather than a single mid-packing section between the HP section  410  and the IP section  450 , the steam turbine  400  includes four end-packing sections  470 ,  430 A,  430 B,  490 , one on either side of the HP section  410 , one adjacent the IP section  450 , and one adjacent a low-pressure (LP) section  452 . 
         [0027]    In the embodiment shown in  FIG. 4 , the conduits  460 - 466  function similarly to those shown in  FIG. 2 , permitting the flow of steam between the HP section  410  and the end-packing sections  470 ,  430 A,  430 B,  490 . Thus, packing rings  420 A-B,  440 A-B,  440 C-D,  480 A-B having a pressure drop-limited seal may be employed in each end-packing section  470 ,  430 A,  430 B,  490 . In other embodiments, such packing rings may be employed in fewer than each end-packing section. 
         [0028]    Steam turbine systems according to the invention may include a combined HPIP steam turbine or a separate HPIP steam turbine such as those described above and a steam-generating apparatus, such as a boiler, for providing a quantity of steam to such an HPIP steam turbine. Other components of such a steam turbine system will be known to one skilled in the art. 
         [0029]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any related or incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.