Abstract:
A turbine bucket shroud arrangement for a turbine system includes a contact region of a tip shroud, wherein the contact region is in close proximity to an adjacent tip shroud. Also included is a negative thermal expansion material disposed proximate the contact region, the contact region comprising a first volume during a startup condition and a shutdown condition of the turbine system and a second volume during a steady state condition of the turbine system, wherein the second volume is less than the first volume.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The subject matter disclosed herein relates to turbine systems, and more particularly to turbine bucket shroud arrangements, as well as a method of controlling turbine bucket interaction with an adjacent turbine bucket. 
         [0002]    Turbine systems employ a number of rotating components or assemblies, such as compressor stages and turbine stages that rotate at high speed when the turbine is in operation, for example. In general, a stage includes a plurality of free-floating blades that extend radially outward from a central hub. Some blades include a shroud that limits vibration within a stage and provides sealing to increase efficiency of the overall system. The shroud is typically positioned at a tip portion of the blade, a mid-portion of the blade or at both the mid portion and the tip portion of the blade. The shrouds are designed such that the free-floating blades interlock to form an integral rotating member during operation. 
         [0003]    Prior to rotation of the free-floating blades, a gap between contact surfaces of the shrouds is present. The distance of the gap determines how early an interlock of the shrouds occurs upon startup of the turbine system. Too large of a gap inefficiently delays the locking speed, which may result in resonance, for example. Too small of a gap results in undesirable effects at high speed operation of the turbine system. Such effects include lower damping effectiveness and flutter margin, as well as high stresses imposed on the turbine bucket due to increased transfer of forces between the contacting shrouds, for example. Therefore, current efforts to beneficially reduce the gap to provide an early interlock to address potential low speed aeromechanics issues are mitigated by the detrimental effects on tip shroud life that occur at steady state operating conditions. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0004]    According to one aspect of the invention, a turbine bucket shroud arrangement for a turbine system includes a contact region of a tip shroud, wherein the contact region is in close proximity to an adjacent tip shroud. Also included is a negative thermal expansion material disposed proximate the contact region, the contact region comprising a first volume during a startup condition and a shutdown condition of the turbine system and a second volume during a steady state condition of the turbine system, wherein the second volume is less than the first volume. 
         [0005]    According to another aspect of the invention, a method of controlling turbine bucket interaction with an adjacent turbine bucket is provided. The method includes reducing a gap disposed between a contact region of a tip shroud and an adjacent tip shroud by depositing a negative thermal expansion material proximate the contact region. Also included is engaging the contact region of the tip shroud with the adjacent tip shroud during a startup operating condition and a shutdown operating condition. Further included is decreasing a volume of the contact region during increased temperature operating conditions upon contraction of the negative thermal expansion material, wherein decreasing the volume reduces tip shroud contact forces and stresses during a steady state operating condition. 
         [0006]    These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0008]      FIG. 1  is a schematic view of a turbine system; 
           [0009]      FIG. 2  is a partial perspective view of a turbine stage of the turbine system; 
           [0010]      FIG. 3  is a top plan view of a turbine bucket shroud arrangement having a contact region; 
           [0011]      FIG. 4  is an enlarged top plan view of the contact region of  FIG. 3 ; 
           [0012]      FIG. 5  is a schematic view of the contact region comprising a composition; 
           [0013]      FIG. 6  is a schematic view of a plurality of layers of the composition; and 
           [0014]      FIG. 7  is a flow diagram illustrating a method of controlling turbine bucket interaction with an adjacent turbine bucket. 
       
    
    
       [0015]    The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    Referring to  FIG. 1 , a turbine system, shown in the form of a gas turbine engine, constructed in accordance with an exemplary embodiment of the present invention, is indicated generally at  10 . The turbine system  10  includes a compressor  12  and a plurality of combustor assemblies arranged in a can annular array, one of which is indicated at  14 . As shown, the combustor assembly  14  includes an endcover assembly  16  that seals, and at least partially defines, a combustion chamber  18 . A plurality of nozzles  20 - 22  are supported by the endcover assembly  16  and extend into the combustion chamber  18 . The nozzles  20 - 22  receive fuel through a common fuel inlet (not shown) and compressed air from the compressor  12 . The fuel and compressed air are passed into the combustion chamber  18  and ignited to form a high temperature, high pressure combustion product or air stream that is used to drive a turbine  24 . The turbine  24  includes a plurality of stages  26 - 28  that are operationally connected to the compressor  12  through a compressor/turbine shaft  30  (also referred to as a rotor). 
         [0017]    In operation, air flows into the compressor  12  and is compressed into a high pressure gas. The high pressure gas is supplied to the combustor assembly  14  and mixed with fuel, for example process gas and/or synthetic gas (syngas), in the combustion chamber  18 . The fuel/air or combustible mixture ignites to form a high pressure, high temperature combustion gas stream. Alternatively, the combustor assembly  14  can combust fuels that include, but are not limited to, natural gas and/or fuel oil. In any event, the combustor assembly  14  channels the combustion gas stream to the turbine  24  which converts thermal energy to mechanical, rotational energy. 
         [0018]    At this point, it should be understood that each of the plurality of stages  26 - 28  is similarly formed, thus reference will be made to  FIG. 2  in describing stage  26  constructed in accordance with an exemplary embodiment of the present invention with an understanding that the remaining stages, i.e., stages  27  and  28 , have corresponding structure. Also, it should be understood that the present invention could be employed in stages in the compressor  12  or other rotating assemblies that require wear and/or impact resistant surfaces. In any event, the stage  26  is shown to include a plurality of rotating members, such as an airfoil  32 , which each extend radially outward from a central hub  34  having an axial centerline  35 . The airfoil  32  is rotatable about the axial centerline  35  of the central hub  34  and includes a base portion  36  and a tip portion  38 . 
         [0019]    A tip shroud  50  covers the tip portion  38  of the airfoil  32 . The tip shroud  50  is designed to receive, or nest with, tip shrouds on adjacent rotating members in order to form a continuous ring that extends circumferentially about the stage  26 . The continuous ring creates an outer flow path boundary that reduces gas path air leakage over top portions (not separately labeled) of the stage  26 , so as to increase stage efficiency and overall turbine performance. In the exemplary embodiment shown, during high or operational speeds, adjacent airfoils interlock through the tip shroud  50  of each respective airfoil by virtue of centrifugal forces and thermal loads created by the operation of the turbine  24 . 
         [0020]    Referring now to  FIGS. 3 and 4 , the tip shroud  50  is illustrated in greater detail and is in close proximity with an adjacent tip shroud  52 . The tip shroud  50  includes a contact region  54  configured to engage the adjacent tip shroud  52  during operation of the turbine system  10 . Specifically, the contact region  54  engages an adjacent contact region  56  of the adjacent tip shroud  52 . A gap  58  is present between the tip shroud  50  and the adjacent tip shroud  52 , and more particularly between the contact region  54  and the adjacent contact region  56 . The gap  58  is present prior to startup of the turbine system  10 . The gap  58  is dimensionally selected based on a desirable early interlock of the tip shroud  50  and the adjacent tip shroud  52  upon operation of the turbine system  10  and rotation of the airfoil  32 . Subsequent to interlock of the tip shroud  50  and the adjacent tip shroud  52 , the operating environment increases in temperature, thereby resulting in thermal expansion of most components within the turbine  24 . 
         [0021]    To alleviate the stresses imposed by potential expansion of already contacted components, at least one of the contact region  54  and the adjacent contact region  56 , but typically both the contact region  54  and the adjacent contact region  56 , include a negative thermal expansion material  60 . The negative thermal expansion material  60  is defined by having a negative coefficient of thermal expansion, such that the material contracts in response to increased temperature exposure, rather than expanding. It is to be appreciated that any material having a negative coefficient of thermal expansion may be suitable for inclusion with the contact region  54  and the adjacent contact region  56 . Examples of such materials include zircon, zirconium tungstate and A 2 (MO 4 ) 3  compounds. Forming at least a portion of the contact region  54  and the adjacent contact region  56  with the negative thermal expansion material  60  advantageously allows for the gap  58  to be dimensionally reduced to facilitate an early interlock between the tip shroud  50  and the adjacent tip shroud  52 , while also reducing the contact forces associated with interaction between the tip shroud  50  and the adjacent tip shroud  52 , thereby reducing stresses imposed on various portions of the tip shroud  50 , the adjacent tip shroud  52  and the airfoil  32  attached thereto. The stress reduction is achieved by maintaining an interlock, but contracting the negative thermal expansion material  60 . In other words, the contact region  54  comprises a first volume during a startup condition of the turbine system  10  and a smaller, second volume during a steady state operating condition of the turbine system  10 . 
         [0022]    Referring now to  FIGS. 5 and 6 , the contact region  54  is schematically illustrated in greater detail. The tip shroud  50  includes a base metal region  62  that is coated or integrally formed with the contact region  54 . The contact region  54  is formed of one or more composition layers that typically include a fraction of the negative thermal expansion material  60  and a fraction of a wear resistant material. As noted above, the contact region  54  may include a single composition layer ( FIG. 5 ) or a plurality of composition layers ( FIG. 6 ). In an embodiment having a plurality of composition layers  72 , it is to be appreciated that distinct volume and/or weight fractions of the negative thermal expansion material  60  may be present in the plurality of composition layers  72 , such as a first layer  64 , a second layer  68  and a third layer  70 , as shown. In one embodiment, the fraction of the negative thermal expansion material  60  progressively increases in each layer, relative to moving away from the base metal region  62 . Specifically, the first layer  64  may include a lower fraction of the negative thermal expansion material  60  than the second layer  68 , with the second layer  68  having a lower fraction than the third layer  70 . Gradually transitioning the inclusion of the negative thermal expansion material  60  from the base metal region  62  reduces thermal fight at the interface between the contact region  54  and the base metal region  62  of the tip shroud  50 . It is to be appreciated that each of the plurality of composition layers  72  may vary in thickness from one another and may comprise the negative thermal expansion material  60  in a fraction ranging from about 0% to about 100%. 
         [0023]    The contact region  54 , whether a single layer or the plurality of composition layers  72 , may be deposited or integrated with the tip shroud  50  in a number of application processes. Examples of such processes include brazing, welding, laser cladding, cold spraying and a plasma transferred arc (PTA) process. The preceding list is merely illustrative and is not intended to be limiting of numerous other suitable application procedures. 
         [0024]    As illustrated in the flow diagram of  FIG. 7 , and with reference to  FIGS. 1-6 , a method of controlling turbine bucket interaction with an adjacent turbine bucket  100  is also provided. The turbine system  10 , as well as the tip shroud  50  and the contact region  54 , have been previously described and specific structural components need not be described in further detail. The method of controlling turbine bucket interaction with an adjacent turbine bucket  100  includes reducing a gap between a contact region of a tip shroud and an adjacent tip shroud by depositing a negative thermal expansion material proximate the contact region  102 . The contact region is engaged with the adjacent tip shroud during a startup operating condition  104 . A volume of the contact region is decreased during increased temperature operating conditions upon contraction of the negative thermal expansion material  106 . 
         [0025]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.