Abstract:
A turbomachine has a number of interspersed stages of rotor blades and stator vanes. A split case has first and second case sections meeting along a junction extending essentially along a longitudinal first plane. The metallic spring seal extends along the junction. The spring seal may be installed in a remanufacturing of an existing turbomachine.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     Benefit is claimed of U.S. patent application Ser. No. 60/647,080, filed Jan. 25, 2005, and entitled “Split Case Seals and Methods”, the disclosure of which is incorporated by reference herein as if set forth at length. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The invention relates to turbomachinery. More particularly, the invention relates to the sealing of split cases of steam turbines and industrial gas turbines.  
         [0003]     Stationary turbine installations include steam turbines and industrial gas turbines. A principal use is for power generation. Common configurations for such turbines include horizontally-split cases with upper and lower case sections joined along a pair of diametrically opposed mating flanges. The flanges may be sealed such as with a gasket material.  
         [0004]     Some stationary turbines are subject to particularly stressful use patterns. So-called peaker turbines are used intermittently and often relatively briefly. For example, a peaker turbine in a power generation facility may be used to address peak loads whereas the other turbines are more constantly used. Use may also be temporary to replace the capacity of another turbine which has been temporarily dropped from service. The peaker turbine may be of like kind or dissimilar to other units in a facility. For example, a power plant with a number of steam turbines may use an industrial gas turbine to both address peak loads and address steam system failures. Peaker use often involves rapid starting from a shutdown condition to a high power condition. The stressful use pattern of peaker turbines may cause case deformation and sealing failure.  
       SUMMARY OF THE INVENTION  
       [0005]     Accordingly, one aspect of the invention involves a method for modifying a turbomachine split case. First and second case sections are separated along their junction. A channel is machined at least in the first case section along the junction. A metallic spring seal segment is inserted in the channel. The first and second case sections are reassembled to compress the spring seal segment. The method may be performed during regular maintenance or more comprehensive remanufacturing of the turbomachine.  
         [0006]     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is a schematic side view of a stationary turbomachine.  
         [0008]      FIG. 2  is a schematic transverse sectional view of the turbomachine of  FIG. 1 .  
         [0009]      FIG. 3  is a view of a sealing face of a case section flange of the engine of  FIG. 1 .  
         [0010]      FIG. 4  is a view of a seal.  
         [0011]      FIG. 5  is a transverse sectional view of the seal of  FIG. 4 , taken along line  5 - 5 .  
         [0012]      FIG. 6  is a transverse sectional view of the flange of  FIG. 3 , taken along line  6 - 6 . 
     
    
       [0013]     Dimensions shown in the drawings are merely exemplary for one particular implementation.  
         [0014]     Like reference numbers and designations in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0015]      FIG. 1  shows a turbomachine  20  having a central longitudinal axis  500  which forms an axis of rotation of the turbomachine rotors. The axis  500  falls along a transverse horizontal centerplane  502 . The turbomachine has a case assembly including an exemplary front (upstream) case  22 , intermediate case  24 , and rear case  26 . The intermediate case  24  is shown having ends fore and aft  28  and  30  at junctions with the front and rear cases.  
         [0016]      FIG. 2  shows the intermediate case  24  as including upper and lower halves  32  and  34 . In the exemplary turobomachine, these halves are symmetric across a vertical centerplane  504 . The halves  32  and  34  are joined along respective pairs of flanges  40 A,  42 A and  40 B,  42 B having faces  50  at junctions along the horizontal centerplane  502 . The flanges of each pair may be secured together such as via bolting.  FIG. 3  shows the face  50  of the flange  42 A, the flange  42 B being a mirror image. Along the intermediate case  24 , its interior surface  52  varies in diameter relative to the centerline  500  to accommodate the diameters of the various rotating components. The flange generally follows such change in radius.  
         [0017]     The surface  50  may initially be essentially flat and uninterrupted. A channel  60  may then be machined through the surface  50  to accommodate a seal.  FIG. 3  shows the channel  60  closely spaced apart from the interior surface  52  along a majority of and essentially the entirety of the intermediate case length (e.g., in excess of 90%). An exemplary channel  60  is essentially a right channel with a pair of sidewalls and a base.  
         [0018]      FIGS. 4 and 5  show further details of an exemplary seal  62 . The exemplary seal is a spring compression seal having a C-sectioned outer jacket  64  and a coiled energizing spring  66  concentrically within the jacket (e.g., along a seal centerline  510 ). The exemplary spring  66  has an outer diameter labeled as D s . The exemplary jacket  64  has a diameter D j  which forms a relaxed thickness of the seal and jacket normal to a plane  512  discussed below. The jacket extends more than 180° around the axis  510  to form two sealing faces. The exemplary jacket extends approximately 270° around the axis  510  in a relaxed condition.  
         [0019]     The seal  62  and its centerline  502  extend from a first end  68  to a second end  70  which, when installed, fall close to the intermediate case ends  28  and  30 . The exemplary seal and its centerline include several straight portions  72 ,  74 ,  76 , and  78  interspersed with curved portions or bends  80 ,  82 , and  84 . The exemplary channel  60  also extends between first and second ends  98  and  100 . In the exemplary embodiment, these ends are recessed from the intermediate case ends  28  and  30  by lengths L 1  and L 2  (measured parallel to the axis  500 ). The channel has straight portions  102 ,  104 ,  106  and  108  and bends  110 ,  112 , and  114   6 corresponding to the straight portions and bends of the seal.  FIG. 4  shows sections corresponding to the seal straight portions as having lengths L 3 , L 4 , L 5 , and L 6  (measured along the seal and to the middle of each adjacent bend). The corresponding channel sections have lengths L′ 3 , L′ 4 , L′ 5 , and L′ 6 . In the exemplary embodiment, L 4′  and L 5′  may respectively very closely correspond to L 4  and L 5  whereas L 3′  and L 6′  may be slightly greater than L 3  and L 6  to provide room for longitudinal expansion (e.g., thermal expansion) of the seal.  
         [0020]     The exemplary seal bends are parallel to each other so that the seal centerline falls along a plane  512 . When the seal is installed, the plane  512  may be parallel to and spaced apart from the plane  502  (e.g., by half the depth of the channel  60  as shown in  FIG. 6 ). In the installed condition, the seal is compressed (e.g., to the channel height H) below the relaxed thickness D j . The compression may close an initial gap (if any) between the jacket and spring and further compress the jacket and spring to an out-of-round condition. The compression may thus also narrow the gap between longitudinal edges of the jacket. In the exemplary installed condition of  FIG. 6 , the flanges are fully mated to each other (with contact of their faces  50 ) compressing the seal within the channel  60 . However, stress may cause local or general separation of the flanges either transiently or more permanently. When these separations occur, advantageously, the seal compliance maintains the seal in engagement with both flanges, preventing escape of hot gases. The present seals may be supplemented or complemented by additional sealing means. For example, a ceramic fiber-filled RTV silicone may be used to seal at the ends of the seals  62 .  
         [0021]     Exemplary jacket material is UNS N07718 (SAE AMS 5596) sheet, a (gamma) precipitation-hardenable nickel-chromium alloy containing significant amounts of iron, niobium, and molybdenum along with lesser amounts of aluminum and titanium. Its microstructure consists principally of a Ni—Cr—Fe—Mo solid-solution matrix. Exemplary spring material is UNS N07090 (SAE AMS 5829) wire, a nickel-chromium-cobalt alloy being precipitation hardenable, having high stress-rupture strength and creep resistance at high temperatures (up to about 950° C.).  
         [0022]     An exemplary turbomachine  20  is a stationary industrial gas turbine (IGT) for electrical power generation. Implementations of the invention may involve: (1) remanufacture/retrofit of an existing IGT; and/or (2) a reengineering of an existing IGT configuration prior to manufacture of further units. An exemplary remanufacture/retrofit implementation is performed on-site with the IGT shut down. The upper and lower case halves are unbolted and separated along their junction. The channels  60  are then machined (e.g., via conventional milling) in the associated flanges. The seals  62  may then be inserted in the respective channels. The case halves may be reassembled and the bolts tightened to compress the seals. In one example, the channels are machined in the flanges of the upper case half. This permits machining to be performed away from the rest of the IGT so that there is better access to the flanges and less chance of introducing debris to the IGT. Although machining may be performed with the upper case half inverted, reassembly involves facing the channels downward. Accordingly, the seals may be retained by wax, adhesive, or the like during replacement of the upper case half. This material may be sacrificed (e.g., melted/vaporized) upon IGT operation.  
         [0023]     One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, when implemented as a remanufacturing of an existing engine or a reengineering of an existing engine configuration, details of the existing configuration may influence or dictate details of any particular implementation. Accordingly, other embodiments are within the scope of the following claims.