Abstract:
An inner mounting ring ( 20 ) for gas turbine flow path components such as shroud ring segments ( 24 ). The inner ring ( 20 ) may be mounted to an outer ring ( 22 ) on radially slidable mounts ( 26, 28 ) that maintain the two rings ( 20, 22 ) in coaxial relationship, but allows them to thermally expand at different rates. This allows matching of the radial expansion rate of the inner ring ( 20 ) to that of the turbine blade tips ( 32 ), thus providing reduced clearance ( 33 ) between the turbine blade tips ( 32 ) and the inner surface of the shroud ring segments ( 24 ) under all engine operating conditions. The inner ring ( 20 ) may be made of a material with a lower coefficient of thermal expansion than that of the outer ring ( 22 ).

Description:
FIELD OF THE INVENTION 
     The invention relates to mounting devices for gas turbine flow path components, and particularly those for mounting shroud ring segments to minimize clearance between the turbine blade tips and the inner surface of the shroud ring segments under steady-state operating conditions. 
     BACKGROUND OF THE INVENTION 
     A gas turbine shaft supports a series of disks. Each disk circumference supports a circular array of radially oriented aerodynamic blades. Closely surrounding these blades is a refractory shroud that encloses the flow of hot combustion gasses passing through the engine at temperatures of over 1400° C. The shroud is assembled from a series of adjacent rings supporting flow path components that are typically made of one or more refractory materials such as ceramics. Shroud rings that surround turbine blades are normally formed of a series of arcuate segments. Each segment is attached to a surrounding framework such as a metal ring called a blade ring that is, in turn, attached to the engine case. Close tolerances must be maintained in the gap between the turbine blade tips and the inner surfaces of the shroud ring segments to ensure engine efficiency. However, the shroud ring segments, blade ring, blades, disks, and their mountings are subject to differential thermal expansion during variations in engine operation, including engine restarts. This requires a larger gap and a corresponding efficiency reduction during some stages of engine operation. 
     Differences among coefficients of linear thermal expansion in flow path components and their support structures dictate the magnitude and variability of blade tip clearances. In prior designs, flow path components such as shroud ring segments are attached directly to support structures such as blade rings. Thus, when the support structures expand, the flow path components are pulled with them. This creates a large blade clearance requirement, partly because of the time delay between heating of flow path components and their more-insulated support structures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in the following description in view of the drawings listed below. Herein “axial” means oriented with respect to the axis  16  of the engine turbine shaft  15 . An “axial plane” is a plane that includes the axis  16 . 
         FIG. 1  is a conceptual sectional view taken on a plane normal to the turbine axis showing an inner ring  20  according to the invention mounted within an outer ring  22 . 
         FIG. 2  is a more detailed sectional view of a joint between upper and lower halves of the inner and outer rings of  FIG. 1 . 
         FIG. 3  is a perspective view of an upper section of an inner ring  20 A. 
         FIG. 4  is an enlargement of an end of the inner ring of  FIG. 3 . 
         FIG. 5  is an enlargement as in  FIG. 4  from a viewpoint parallel to the axis. 
         FIG. 6  is a sectional view, taken on an axial plane, of a shroud ring segment  24  mounted in an inner ring  20  which is in turn mounted in an outer blade ring  22 . 
         FIG. 7  is a view as in  FIG. 6  with the shroud ring segment  24  exploded for clarity. 
         FIG. 8  is a view of the inner ring formed from first and second halves. 
         FIG. 9  is a view of an alternate embodiment of the alignment tabs  46  and  50  and tab slots  48 . 
         FIG. 10  illustrates an assembly method for the inner and outer rings and mounts. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present inventors have recognized that isolating the thermal expansion of a shroud ring from that of its support structure could minimize differential radial expansion rates between the shroud ring and turbine blades during engine operational transients. This would allow minimizing the radial expansion rate of the shroud ring, thus allowing less clearance between the blades and the shroud ring, increasing power output and efficiency. 
       FIG. 1  is a conceptual view of a cross section of a gas turbine  14  with a turbine shaft  15 , a shaft axis  16 , a disk  17 , and blades  18  in a case  19 . An inner ring  20  according to the invention is mounted within an outer ring  22 . Shroud ring segments  24  are mounted on the inner ring  20 . The outer ring  22  may be made of a first material with a first coefficient of linear thermal expansion, and the inner ring  20  may be made of a second material with a lower coefficient of thermal expansion than that of the first material. The inner ring  20  is attached to the outer ring  22  by a plurality of radially slidable mounts  26 ,  28  that allow radial sliding movement between the inner and outer rings  20 ,  22 . A clearance  30  between the rings  20 ,  22  provides radial clearance for differential expansion of the rings. The mounts  26 ,  28  allow the inner ring  20  to expand independently of the outer ring  22  in order to match the radial expansion characteristics of the turbine blade tips  32 . A material with a relatively low coefficient of thermal expansion is suggested for the inner ring  20 . In one embodiment, a nickel-iron-cobalt alloy sold under the trade name designation INCOLOY® alloy 909 (UNS NI9909) may be used. INCOLOY alloy 909 is known to have the following chemical composition: nickel 35.0-40.0%; cobalt 12.0-16.0%; niobium 4.3-5.2%; titanium 1.3-1.8%; silicon 0.25-0.50%; aluminum 0.15 maximum; carbon 0.06 maximum; iron balance. A material for the inner ring may be further selected for improved wear and oxidation resistance at elevated temperatures. 
     As shown in  FIG. 2  the inner ring  20  may have first and second halves or sections  20 A,  20 B that are bolted together at a joint  34 . A pair of bolts  36  may pass through the abutting ends of the sections  20 A,  20 B to connect them. Recessed holes  38  for such bolts  36  are shown in  FIGS. 3 and 4 , which also show segment locking holes  55 . As shown in  FIGS. 4 ,  5  and  8  a key clamp  40  is defined in each joint  34  between the upper and lower sections  20 A,  20 B of the inner ring  20 . 
     The outer ring  22  may also have first and second halves or sections  22 A,  22 B that are similarly joined at abutting ends. The resulting joint  42  forms a key slot  44  in the outer ring  22  opposite the key clamp  40  in the inner ring  20 . A key  46  may be clamped in the key clamp  40  as shown in  FIG. 2 , and the bolts  36  may pass through it. The key  46  is radially slidable in the key slot  44 . This mounting mechanism fixes the rotational position of the inner ring  20 , but allows relative radial movement between the inner ring  20  and the outer ring  22 . Alternately (not shown) the key  46  may be fixed in the outer ring  22  and slidable in the inner ring  20 , or slidable in both rings. 
     Upper and lower tabs slots  48  and tabs  50  may be provided on the outer and inner rings  20 ,  22  as illustrated in  FIG. 1 . The tabs  50  slide radially in the tab slots  48 . The interfacing of these tab slots  48  and tabs  50  keeps the inner ring  20  centered laterally within the outer ring  22 . Alternately as in  FIG. 9  the tabs  50  may be disposed on the inner ring  20 , and the tab slots  48  may be on the outer ring. Alternately (not shown) the inner ring  20  may be made in four sections, and the tabs  50  may be formed using keys  46  at the resulting upper and lower joints  28  similarly to the other two joints  26  shown. 
     The key slots  44  and/or the tab slots  48  may be formed as enclosed chambers except for an open radially inner end that receives the key  46  or tab  50 . Such a chamber fixes the inner ring  20  in the outer ring  22  against movement parallel to the turbine axis  16 . Thus, the only freedom of movement between the inner and outer rings is a centered radial expansion. However, not all of the key slots  44  and tab slots  48  need be axially restrictive. A combination of four radially slidable mounts  26 ,  28  at four cardinal points as shown is ideal because it maintains a coaxial relationship of the rings  20 ,  22 , while allowing differential radial expansion of them, and allowing assembly of them. 
     For assembly  70  as illustrated in  FIG. 10 , the lower half of the inner ring  20 B may be inserted  72  into the lower half of the outer ring  22 B along the radial direction allowed by the tab slots  48  and tabs  50 . This forms a lower half inner/outer ring assembly, which is then rolled  74  into the engine, with or without the rotor in place. Before the upper half of the ring assembly is made, the rotor must be in place  75 . A respective key  46  is then placed  76  in each end of the lower half of the inner ring  20 B. The upper and lower sections  20 A,  20 B of the inner ring are then bolted together  77 ,  78 , clamping the respective keys  46  between them. Finally, the upper outer ring section  22 A is lowered  79  over the upper inner ring section  20 A along the radial direction allowed by the tab slots  48  and tabs  50 . The upper and lower outer ring sections  22 A,  22 B are then connected together  80 , trapping the keys  46 . This retains the keys  46  radially slidably within the key slots  44  in the abutting ends of the outer ring sections  22 A,  22 B. 
     As shown in  FIGS. 6-7  shroud ring segments  24  may be assembled onto the inner ring halves  20 A,  20 B by sliding the shroud ring segments  24  into tracks  52  in each inner ring half  20 A,  20 B before the other assembly steps above. Alternately the shroud ring segments  24  may be assembled onto the inner ring  20  by other means known in the art. A track-and-slide assembly geometry is illustrated in  FIGS. 6-7 , which also show air cooling channels  54  and gas seals  56 . Bosses  58  are provided for mounting the outer ring  22  to the engine case  19 . 
     While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.