Abstract:
A sealing system for reducing a gap between a tip of a turbine blade and a shroud of a turbine engine. As a turbine engine reaches steady state operating conditions, components of the sealing system reach their maximum expansion and reduce the size of the gap located between the blade tips and the engine shroud, thereby reducing the leakage of air past the turbine blades and increasing the efficiency of the turbine engine. The sealing system includes a ring segment having a sealing surface positioned proximate to a tip of a turbine blade. The ring segment may be coupled to a blade ring using a spindle having a coefficient of thermal expansion greater than the coefficient of thermal expansion for the blade ring.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention is directed generally to turbine engines, and more particularly to systems for sealing gaps between blade tips and shrouds in turbine engines.  
       BACKGROUND  
       [0002]     Typically, gas turbine engines are formed from a combustor positioned upstream from a turbine blade assembly. The turbine blade assembly is formed from a plurality of turbine blade stages coupled to discs that are capable of rotating about a longitudinal axis. Each turbine blade stage is formed from a plurality of blades extending radially about the circumference of the disc. Each stage is spaced apart from each other a sufficient distance to allow turbine vanes to be positioned between each stage. The turbine vanes are typically coupled to the shroud and remain stationary during operation of the turbine engine.  
         [0003]     The tips of the turbine blades are located in close proximity to an inner surface of the shroud of the turbine engine. There typically exists a gap between the blade tips and the shroud of the turbine engine so that the blades may rotate without striking the shroud. During operation, high temperature and high pressure gases pass the turbine blades and cause the blades and disc to rotate. These gases also heat the shroud and blades and discs to which they are attached causing each to expand due to thermal expansion. After the turbine engine has been operating at full load conditions for a period of time, the components reach a maximum operating condition at which maximum thermal expansion occurs. In this state, it is desirable that the gap between the blade tips and the shroud of the turbine engine be as small as possible to limit leakage past the blade tips.  
         [0004]     However, reducing the gap cannot be accomplished by simply positioning the components so that the gap is minimal under full load conditions because the configuration of the components forming the gap must account for emergency shutdown conditions in which the shroud, having less mass than the turbine blade and disc assembly, cools faster than the turbine blade assembly. In emergency shutdown conditions, the diameter of the shroud reduces at a faster rate than the length of the turbine blades. Therefore, unless the components have been positioned so that a sufficient gap has been established between the turbine blades and the turbine shroud under operating conditions, the turbine blades strike the shroud because the diameter of the shroud is reduced at a faster rate than the turbine blades. Collision of the turbine blades and the shroud often causes catastrophic results. Thus, a need exists for a system for reducing gaps between turbine blade tips and a surrounding shroud under full load operating conditions while accounting for necessary clearance under emergency shutdown conditions.  
       SUMMARY OF THE INVENTION  
       [0005]     This invention relates to a sealing system for reducing a gap between a tip of a turbine blade and a shroud of a turbine engine. As a turbine engine reaches steady state operation, components of the sealing system reach their maximum expansion and reduce the size of the gap located between the blade tips and the engine shroud, thereby reducing the leakage of air past the turbine blades and increasing the efficiency of the turbine engine. In at least one embodiment, the sealing system includes a turbine blade assembly having at least one stage formed from a plurality of turbine blades. The sealing system also includes a blade ring radially surrounding the turbine blade assembly such that the blade ring may radially expand and contract during operation as a result of thermal expansion or contraction. A ring segment having at least one surface positioned in close proximity to at least one tip of the turbine blade assembly may be positioned such that the ring segment forms a gap between the at least one surface of the ring segment and the plurality of blades. A spindle may be fixed to the blade ring at a first end of the spindle and coupled to the ring segment at a second end of the spindle for supporting and positioning the ring segment in close proximity with at least one tip of the plurality of blades. The spindle may be formed from a material having a coefficient of thermal expansion that is greater than a coefficient of thermal expansion for a material forming the ring segment.  
         [0006]     While the turbine engine is at rest, there exists a gap between the blade tips and the ring segments. During operation, the ring segments reach maximum operating temperature before the turbine blade assembly. As the ring segments are heated, the spindle lengthens a greater amount than the blade ring. In other words, the length of the spindle increases a greater distance than the diameter of the blade ring increases. As a result, the ring segment attached to the end of the spindle undergoes a net radial displacement towards the tips of the blades. As the turbine blade assembly reaches its maximum operating temperature, the blades lengthen to their steady state operating positions. Operating a turbine engine using this sealing system reduces the gap between the tips of the turbine blades and the ring segments by about 0.04 inches to about 0.05 inches, depending on the difference in thermal expansion coefficients between the spindle and the blade ring. The larger the difference in coefficients of the spindle and the blade ring, the larger the reduction in gap spacing. Upon shutdown, even in emergency conditions, the ring segment undergoes a net radial displacement away from the blade tips, thereby preventing the blade tips from contacting the ring segments.  
         [0007]     An advantage of this invention is that the size of the gap between blade tips and shrouds of turbine engines may be reduced without introducing the possibility that the blade tips may contact the shroud, thereby damaging the turbine engine.  
         [0008]     These and other embodiments are described in more detail below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The accompanying drawings, which are incorporated in and form a part of the specification, illustrate embodiments of the presently disclosed invention and, together with the description, disclose the principles of the invention.  
         [0010]      FIG. 1  is a perspective view of an embodiment of this invention.  
         [0011]      FIG. 2  is a side view of the embodiment of this invention taken at  2 - 2  in  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]     As shown in  FIGS. 1-2 , this invention is directed to a sealing system  10  for a turbine engine. In particular, the sealing system  10  is operable to reduce a gap  12  between one or more tips  14  of a turbine blade  16  in a turbine engine  18  and a surrounding shroud  20  while the turbine engine  18  is operating. The gap  12  exists in the turbine engine  18  so that the tips  14  do not contact the shroud  20 . In at least one embodiment, the turbine engine  18  includes a turbine blade assembly  22  formed at least in part from a plurality of turbine blades  16  coupled to a disc  24 . The blades  16  may be coupled to the disc  24  at various points along the disc  24  and may be assembled into rows, which are commonly referred to as stages  23 , having adequate spacing to accommodate stationary vanes between adjacent stages of the blades  16 . The stationary vanes are typically mounted to a casing of the turbine engine  18 . The disc  24  may be rotatably coupled to the turbine engine  18 .  
         [0013]     The turbine engine  18  may also include a plurality of blade rings  26 . The blade rings  26  may be positioned radially surrounding the turbine blade assembly  22  such that the blade ring  26  may radially expand and contract during operation as a result of thermal expansion or contraction. The size and configuration of the blade rings  26  depend on the size and configuration of the turbine engine  18 .  
         [0014]     A ring segment  28  may be coupled to a blade ring  26  using a spindle  30 . The ring segment  28  may have at least one sealing surface  32  positioned in close proximity to at least one tip  14  of the plurality of turbine blades  16  of the turbine blade assembly  22 . The ring segment  28  may be positioned so that a gap  12  is formed between the tips  14  of the turbine blades  16  and the ring segment  28 .  
         [0015]     In at least one embodiment, the ring segment  28  may be supported by a single spindle  30 . The spindle  30  may be attached to the ring segment  28  substantially at a center point  34  of the ring segment  28 . The spindle  30  may be fixed to the blade ring  26  at a first end  36  and coupled to the ring segment  28  at a second end  38  for supporting and positioning the ring segment  28  in close proximity with at least one tip  14  of the plurality of turbine blades  16 . The spindle  30  may be fixed to the blade ring  26  at the first end  36  using one or more bolts, welds, interference fits, or other appropriate mechanical connectors. The spindle  30  may be fixed so that as the temperature of the spindle  30  increases, and the length of the spindle  30  thereby increases. As a result, the second end  38  of the spindle  30  extends from the blade ring  26 . In at least one embodiment, the turbine blades  16  are substantially of equal lengths and the ring segment  28  is positioned in close proximity to all of the tips  14  of the turbine blades  16 . In at least one embodiment, the spindle  30  may be positioned substantially parallel to a radial axis  39  extending from an axis of rotation  40  of the turbine blade assembly  22 . Spindle  30  may be formed from a material having a coefficient of thermal expansion greater than a coefficient of thermal expansion for the material forming the blade ring  26 . For instance and not by way of limitation, the spindle  30  may be formed from A286 disc alloy having a coefficient of thermal expansion of about 9.7 inch per inch per degree Fahrenheit, and the blade ring  26  may be formed from IN909 having a coefficient of thermal expansion of about 4.5 inch per inch per degree Fahrenheit.  
         [0016]     In at least one embodiment, as shown in  FIG. 2 , a web  44  may be coupled to the ring segment  28  and extend away from the sealing surface  32 . As shown in  FIG. 1 , the web  44  may extend circumferentially around the axis of rotation  40  of the turbine blade assembly  22 . As shown in  FIG. 2 , the web  44  may extend from the ring segment  28  such that the web  44  may be substantially parallel to the spindle  30 . The web  44  may also include a sealing portion  46  that may be generally parallel to the sealing surface  32  of the ring segment  28  and a hook  47  at a first end  48  that is opposite to the second end  50  coupled to the ring segment  28 . The spindle  30  may be coupled to the ring segment  28  using one or more bolts  61 , or other suitable releasable mechanical connections. In particular, a mechanical connector (not shown) may be passed through an orifice  51  in the hook  47  and an orifice  53  in a flange  49  of the spindle  30  and coupled to the ring segment  28  to attach the ring segment  28  to the spindle  30 . In alternative embodiments, the hook  47  may be discontinuous and may be present at intermittent locations along the length of the web  44 .  
         [0017]     Under steady state operating conditions, the web  44  may thermally expand toward an isolation ring  42  and seal the ring segment  28  to the isolation ring  42  using a seal  45 . The seal  45  may be, but is not limited to, a spring seal, or other seal capable of withstanding the high temperatures present in the turbine engine  18 . The isolation ring  42  may extend circumferentially around the axis of rotation  40  of the turbine blade assembly  22 . The isolation ring  42  may be used to seal the ring segment  28  to the supporting turbine components. The isolation ring  42  may include one or more channels  43  for positioning the seal  45  between the ring segment  28  and the isolation ring  42 .  
         [0018]     During operation, the temperature of the turbine engine  18  increases, which causes the blade ring  26 , the ring segment  28 , and the turbine blades  16  forming the turbine blade assembly  22  to heat up. Each of the blade ring  26 , the ring segment  28 , and the turbine blades  16  expand as the temperature of each component increases. In particular, as the temperature of the turbine engine  18  increases, the length of each turbine blade  16 , the diameter of the blade ring  26 , and the length of the spindle  30  increase. Because the coefficient of thermal expansion of the spindle  30  is greater than the coefficient of thermal expansion of the blade ring  26 , the ring segment  28  coupled to the spindle  30  undergoes a net positive radial displacement towards the tips  14  of the turbine blades  16  even though the diameter of the blade ring  26  is increasing. In other words, as the tip of the blades  16  lengthen towards the ring segment  28 , the sealing surface  32  of the ring segment  28  extends towards the tip of the turbine blades  16 . This configuration results in a steady state, hot running blade tip clearance reduction of between about 0.04 inches and about 0.05 inches, depending on the difference in coefficients of thermal expansion between the spindle  30  and the blade ring  26 .  
         [0019]     In the event the turbine engine  18  is shutdown quickly, such as during emergency shutdown, the spindle  30  cools more quickly than the turbine blade assembly  22  because the spindle  30  has less mass than the turbine blade assembly  22 . As the spindle  30  cools, the ring segments  28  may be withdrawn toward the blade ring  26  so that the sealing surface  32  of the ring segment  28  does not contact the tips  14  of the turbine blades  16 . Because the coefficient of thermal expansion of the spindle  30  is greater than the coefficient of thermal expansion of the blade ring  26 , the spindle  30  is retracted a greater distance than the distance that the blade ring  26  is reduced as the blade ring  26  cools. Thus, the gap  12  between the tips  14  of the turbine blades  16  and the sealing surface  32  of the ring segment  28  is increased as the temperature of the turbine engine  18  is reduced.  
         [0020]     The foregoing is provided for purposes of illustrating, explaining, and describing embodiments of this invention. Modifications and adaptations to these embodiments will be apparent to those skilled in the art and may be made without departing from the scope or spirit of this invention.