Abstract:
A turbomachine outer case includes a pair of outer case sections, each provided with attachment flanges for securing said pair of outer case sections about an internal rotor structure, the pair of outer case sections constructed of a cement composite material. The outer case sections may be lined with a relatively thin metal alloy.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates to ground-based turbomachines in general to an external structural case that supports the axial and torsional loads transferred across a turbine during engine operation. 
         [0002]    Conventional turbine outer cases are formed from large metal-based sand castings that mount inner case structures for supporting, in the case of a gas turbine, the components of the hot gas path. Metal outer cases have thermal structural issues, however, that impact rotor alignment and blade tip clearances within the hot gas path. There have been some attempts to remedy the issues associated with metal outer cases by using multiple cases to isolate the thermal and axial structural loading. 
         [0003]    It would be desirable to develop a less complex case structure for removing or isolating thermal properties from the outer case of a turbomachine such as a gas turbine, for example, and thereby reduce the impact of the outer case on the internal hot gas path components. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    In accordance with an exemplary but non-limiting embodiment, the present invention provides a turbomachine outer case comprising a pair of outer case sections, each provided with attachment flanges for securing the pair of outer case sections about an internal rotor structure, the pair of outer case sections constructed of a cement composite material. 
         [0005]    In another aspect the invention provides a turbomachine outer case comprising a pair of substantially semi-cylindrical outer case sections, each provided with attachment flanges along opposite free ends thereof for securing the pair of substantially semi-cylindrical outer case sections about an internal rotor structure, the pair of substantially semi-cylindrical outer case sections constructed of a relatively thick cement composite material lined with a relatively thin metal material. 
         [0006]    In still another aspect, the present invention provides a method of forming an outer case section for a turbine comprising preparing a split mold with reinforcing bars and attachment hardware elements incorporated therein; pouring uncured concrete into said split mold to form a pair of reinforced concrete case sections with attachment flanges adapted to facilitate attachment of said pair of case sections together about an internal rotor structure; curing the concrete; and removing the split mold. 
         [0007]    The invention will now be described in connection with the drawings identified below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a partial schematic illustration of a turbine with a monolithic metal outer case and its relationship to the interior rotor system; 
           [0009]      FIG. 2  is a schematic end view or cross-section of a turbine split metal case of the type shown in  FIG. 1 ; 
           [0010]      FIG. 3  is schematic illustration similar to  FIG. 1  but showing a composite concrete outer case wall with reinforcement bars and local airfoil connectivity points in accordance with a first exemplary but non-limiting embodiment of the invention; 
           [0011]      FIG. 4  is a schematic and/or end or cross-section similar to  FIG. 2  but illustrating the turbine split composite case as shown in  FIG. 3 ; and 
           [0012]      FIG. 5  is a flow diagram of a process according to an exemplary embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    With reference initially to  FIGS. 1 and 2 , a ground-based turbine section  10  includes an outer case  12  that encloses an internal rotor system  14 . In the exemplary embodiment, the turbomachine is a gas turbine but the invention is not limited to gas turbines. The internal rotor system includes a plurality of stages, each of which, as understood by those skilled in the art, includes a turbine wheel mounting an annular row of blades or buckets. Three stages are represented schematically at  16 ,  18  and  20  in  FIG. 1 . 
         [0014]    The annular outer case or housing  12  is comprised of an upper substantially semi-cylindrical case half  22  and a lower substantially semi-cylindrical case half  24 , best seen in  FIG. 2 . The upper and lower case halves are secured at aligned, horizontal flange pairs  24 ,  26  and  28 , that extend length-wise along the outer case  12 , by means of suitable bolts or other equivalent fasteners (not shown) extending through axially-spaced holes in the respective flange pairs. 
         [0015]    The interior of the outer case  12  is provided with attachment points or hardware elements  32 ,  34  and  36  which support respective stationary stator or nozzle diaphragms  32 ,  40  and  42  which extend radially between the respective stages  16 ,  18  and  20 . Combustion gases exiting the turbine combustion chamber(s) flow along the rotor  14  through the plural stages  16 ,  18 ,  20  as indicated by flow arrow A in a path generally referred to as the “hot gas path”. 
         [0016]    As indicated above, the outer case or housing  12  is typically made of cast metal such as a steel alloy, giving rise to thermal gradient issues affecting rotor alignment and blade tip clearances. 
         [0017]    Turning now to  FIGS. 3 and 4 , a ground-based turbine  110  in accordance with an exemplary but non-limiting embodiment of the present invention is schematically illustrated in a manner similar to the outer case shown in  FIGS. 1 and 2 . The turbine  110  includes an outer case  112  that encloses an internal rotor system  114 . The internal rotor system  114  also includes a plurality of stages represented at  116 ,  118  and  120 , each of which includes a turbine wheel mounting an annular row of blades or buckets. 
         [0018]    As in the prior known case construction, the outer case or annular housing  112  is comprised of an upper, substantially semi-cylindrical case half or section  122  and a lower substantially semi-cylindrical case half or section  124  ( FIG. 4 ) secured at horizontal flange pairs  124 ,  126  and  128 ,  130  that extend axially along the length of the outer case  112 , by means of suitable bolts or other equivalent fasteners extending through the respective flange pairs. 
         [0019]    The interior of the outer case  112  is provided with attachment points or hardware elements  132 ,  134  and  136  which support respective stationary stator or nozzle diaphragms  138 ,  140  and  142  which extend radially between the respective stages  116 ,  118  and  120  along the hot gas path. 
         [0020]    In the exemplary but nonlimiting embodiment, the outer case  112  is comprised of a ceramic cement composite, e.g., concrete, with an internal grid of metal or composite reinforcement bars  144 . The placement or arrangement of the reinforcing bars (or “rebars”)  144  will be in accordance with usual reinforced concrete practice generally within the skill of the art. In addition, the exact chemical composition of the concrete may vary with individual applications, considering the particular thermal gradients of the system. 
         [0021]    The local metal nozzle connectivity points or hardware elements  132 ,  134  and  136  may be embedded within the concrete structure and may be of conventional design with respect to the manner in which the stator or nozzle diaphragms  138 ,  140  and  142  are secured to those hardware elements. 
         [0022]    In a preferred construction, a thin metal liner, in the form of split liner halves  146 ,  148 , is provided on the interior side of the upper and lower case halves  122 ,  124 . The split metal liner substantially conforms to the interior surfaces of the upper and lower case halves and acts as a pressure vessel which prevents combustion gas leakage from the case in the event that hairline (or larger) cracks form in the concrete outer case. The metal liner, preferably a chromium steel alloy, would also serve to enhance the connectivity points for the stator components. Other suitable liner materials include glass-based composites and high-temperature plastics or other high temperature metals. 
         [0023]    In the example embodiment, the inner metal liner may be from about ¼ to 1 inch thick, while the concrete outer case may be from about 6 to 8 inches thick, but it will be understood that these dimensions may vary. 
         [0024]    In accordance with the exemplary embodiment, the reinforced concrete case is produced in an as-cast state, about the inner metal liner, and only the local connectivity points  132 ,  134  and  136  require follow-up machining. The reinforced concrete structure is resistant to thermal stress during gas turbine operation, and will carry the axial and torsional loading of the gas turbine during operation. 
         [0025]    Manufacture of the outer case is somewhat similar to a concrete reinforced pipe section in that the reinforcement bars, metal mounting elements as shown in  FIGS. 3 and 4  can be located and fixed within a split mold. The liquid-based cement is then poured and cured, after which the split mold halves are removed (see  FIG. 5 ). 
         [0026]    The split mold with split liner halves enable the concrete reinforced case itself to be constructed in a split configuration so that the case can be fitted around an existing gas turbine rotor assembly and joined at the split flange arrangement described above. Metal flange components  150  may be secured to the mold prior to concrete pouring and curing so that the flange components  150  are at least partially embedded in the concrete, extending along exterior surfaces of the flange pairs  124 ,  126  and  128 ,  130  and providing more robust attachment points for the bolts or other fasteners used to secure the case halves. 
         [0027]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.