Abstract:
A shroud support apparatus for a ceramic component of a gas turbine having: an outer shroud block having a coupling to a casing of the gas turbine; a spring mass damper attached to the outer shroud block and including a spring biased piston extending through said outer shroud block, wherein the spring mass damper applies a load to the ceramic component; and the ceramic component has a forward flange and an aft flange each attachable to the outer shroud block.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates to ceramic matrix components for gas turbines and, specifically, to testing of ceramic matrix turbine bucket shrouds.  
         [0002]     The present invention relates to a support and damping system for ceramic shrouds surrounding rotating components in a hot gas path of a turbine and particularly relates to a spring mass damping system for interfacing with a ceramic shroud and tuning the shroud to minimize vibratory response from pressure pulses in the hot gas path as each turbine blade passes the individual shroud.  
         [0003]     Ceramic matrix composites offer advantages as a material of choice for shrouds in a turbine for interfacing with the hot gas path. The ceramic composites offer high material temperature capability. It will be appreciated that the shrouds are subject to vibration due to the pressure pulses of the hot gases as each blade or bucket passes the shroud. Moreover, because of this proximity to high-speed rotation of the buckets, the vibration may be at or near resonant frequencies and thus require damping to maintain life expectancy during long-term commercial operation of the turbine. Ceramic composites, however, are difficult to attach and have failure mechanisms such as wear, oxidation due to ionic transfer with metal, stress concentration and damage to the ceramic composite when configuring the composite for attachment to the metallic components. Accordingly, there is a need for responding to dynamics-related issues relating to the attachment of ceramic composite shrouds to metallic components of the turbine to minimize adverse modal response.  
         [0004]     Ceramic matrix composites can withstand high material temperatures and are suitable for use in the hot gas path of gas turbines. Recently, melt-infiltrated (MI) silicon-carbon/silicon-carbon (SiC/SiC) ceramic matrix composites have been formed into high temperature, static components for gas turbines. Because of their heat capability, ceramic matrix composite turbine components, e.g., MI-SiC/SiC components, generally do not require or reduce cooling flows, as compared to metallic components.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0005]     The invention may be embodied as a shroud support apparatus for a ceramic component of a gas turbine having: an outer shroud block having a coupling to a casing of the gas turbine; a spring mass damper attached to the outer shroud block and including a spring biased piston extending through said outer shroud block, wherein the spring mass damper applies a load to the ceramic component; and the ceramic component has a forward flange and an aft flange each attachable to the outer shroud block.  
         [0006]     The invention may also be embodied as a shroud support for a melt-infiltrated ceramic matrix composite inner shroud for a row of turbine buckets of a gas turbine, said rig comprising: a metallic outer shroud block having a coupling to a casing of the gas turbine; a spring mass damper attached to said outer shroud block and further comprising a spring biased piston extending through said outer shroud block, wherein said piston is pivotably coupled to a pad; said ceramic matrix inner should having a forward flange and an aft flange each attachable to said outer shroud block, and wherein said pad applies a load to said ceramic component and pre-loads the forward and aft flanges.  
         [0007]     The invention may be further embodied as a method for testing a ceramic stationary component of a gas turbine comprising: securing an outer shroud block to a casing of the gas turbine; attaching a forward flange and an aft flange of the component to the outer shroud; loading the component between the forward flange and the aft flange by applying a bias force to the component with a spring mass damper, and exposing the component to a hot gas stream in the gas turbine, wherein the bias force and the attachments of the forward flange and aft flange secure the component.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a cross-sectional view through an outer shroud block as viewed in a circumferential direction about an axis of the turbine and illustrating a preferred damper system according to the present invention.  
         [0009]      FIG. 2  is a cross-sectional view thereof as viewed in an axial forward direction relative to the hot gas path of the turbine.  
         [0010]      FIG. 3  is a perspective view illustrating the interior surface of a damper block with projections for engaging the backside of the shroud.  
         [0011]      FIG. 4  is an enlarged cross-sectional view illustrating portions of the damper load transfer mechanism and damping mechanism.  
         [0012]      FIG. 5  is a close-up, cross-sectional view of a forward attachment for the shroud.  
         [0013]      FIG. 6  is a close-up, cross-sectional view of an aft attachment for the shroud.  
         [0014]      FIG. 7  is a close-up, cross-sectional view of a pin hole in forward flange of the shroud.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     Referring now to  FIGS. 1 and 2 , there is illustrated an outer shroud block or body  10  mounting a plurality of shrouds  12 .  FIG. 1  is a view in a circumferential direction and  FIG. 2  is a view in an axial forward direction opposite to the direction of flow of the hot gas stream through the turbine. As seen from a review of  FIG. 2 , the shroud block  10  carries preferably three individual shrouds  12 . It will be appreciated that a plurality of shroud blocks  10  are disposed in a circumferential array about the turbine axis and mount a plurality of shrouds  12  surrounding and forming a part of the hot gas path flowing through the turbine. The shrouds  12  are formed of a ceramic composite, are secured by bolts, not shown, to the shroud blocks  10 , and have a first inner surface  11  ( FIG. 2 ) in contact with the hot gases of the hot gas path.  
         [0016]     The outer shroud block fits into the casing  104  of the gas turbine. The rig is mounted in the casing  104  on for example a casing  104  that extends inwardly from an inner wall  106  of the casing. The T-hook  107  may be arranged as an annular row of teeth that engages opposite sides of the a groove  110  extending the length of the outer shroud block  10 . The blocks  10  fit within a plenum cavity  108  within the casing and near the rotating portion of the gas turbine.  
         [0017]     The outer shroud blocks  10  may be formed of a metal alloy that is sufficiently temperature tolerant to withstand moderate high temperature levels. A small portion of the metal outer shroud block, e.g., near the inner shroud  12 , may be exposed to hot gases from the turbine flow path. The outer shroud block  10  connects to the gas turbine engine casing  104  by latching onto the T-hooks of the casing. The outer shroud block  10  may be a unitary block that slides over the T-hook or may be a pair of left and right block halves that are clamped over the T-hook. A slot  110  in an outer surface of the outer shroud block is configured to slide or clamp over the T-hook  107 .  
         [0018]     The damper system includes a damper block/shroud interface, a damper load transfer mechanism and a damping mechanism. The damper block/shroud interface includes a damper block  16  formed of a metallic material, e.g., PM2000, which is a superalloy material having high temperature use limits of up to 2200° F. As illustrated in  FIGS. 1 and 3 , the radially inwardly facing surface  18  ( FIG. 3 ) of the damper block  16  includes at least three projections  20  which engage a backside surface  22  ( FIG. 1 ) of the shroud  12 . Projections  20  are sized to distribute sufficient load to the shroud  12 , while minimizing susceptibility to wear and binding between the shroud  12  and damper block  16 . The location of the projections  20  are dependent upon the desired system dynamic response which is determined by system natural frequency vibratory response testing and modal analysis. Consequently, the locations of the projections  20  are predetermined.  
         [0019]     Two of the projections  20   a  and  20   b  are located along the forward edge of the damper block  16  and adjacent the opposite sides thereof. Consequently, the projections  20   a  and  20   b  are symmetrically located along the forward edge of the damper block  16  relative to the sides. The remaining projection  20   c  is located adjacent the rear edge of the damper block  16  and toward one side thereof. Thus, the rear projection  20   c  is located along the rear edge of block  16  and asymmetrically relative to the sides of the damper block  16 . It will be appreciated also that with this configuration, the projections  20  provide a substantial insulating space, i.e., a convective insulating layer, between the damper block  16  and the backside of the shroud  12 , which reduces the heat load on the damper block. The projections  20  also compensate for the surface roughness variation commonly associated with ceramic composite shroud surfaces.  
         [0020]     The damper load transfer mechanism, generally designated  30 , includes a piston assembly having a piston  32  which passes through an aperture  34  formed in the shroud block  10 . The radially inner or distal end of the piston  32  terminates in a ball  36  received within a complementary socket  38  formed in the damper block  16  thereby forming a ball-and-socket coupling  39 . As best illustrated in  FIG. 2 , the sides of the piston spaced back from the ball  36  are of lesser diameter than the ball and pins  40  are secured, for example, by welding, to the damper block  16  along opposite sides of the piston to retain the coupling between the damper block  16  and the piston  32 . The coupling enables relative movement between the piston  32  and block  16 .  
         [0021]     A central cooling passage  42  is formed axially along the piston, terminating in a pair of film-cooling holes  44  for providing a cooling medium, e.g., compressor discharge air, into the ball-and-socket coupling. The cooling medium, e.g., compressor discharge air, is supplied from a source radially outwardly of the damper block  10  through the damping mechanism described below. As best illustrated in  FIG. 4 , the sides of the piston are provided with at least a pair of radially outwardly projecting, axially spaced lands  48 . The lands  48  reduce the potential for the shaft to bind with the aperture of the damper block  10  due to oxidation and/or wear during long-term continuous operation.  
         [0022]     The damper load transfer mechanism also includes superposed metallic and thermally insulated washers  50  and  52 , respectively. The washers are disposed in a cup  54  carried by the piston  32 . The metallic washer  50  provides a support for the thermally insulating washer  52 , which preferably is formed of a monolithic ceramic silicone nitride. The thermally insulative washer  52  blocks the conductive heat path of the piston via contact with the damper block  12 .  
         [0023]     The damping mechanism includes a spring  60 . The spring is pre-conditioned at temperature and load prior to assembly as a means to ensure consistency in structural compliance. The spring  60  is mounted within a cup-shaped block  62  formed along the backside of the shroud block  10 . The spring is preloaded to engage at one end the insulative washer  52  to bias the piston  32  radially inwardly. The opposite end of spring  60  engages a cap  64  secured, for example, by threads to the block  62 . The cap  64  has a central opening or passage  67  enabling cooling flow from compressor discharge air to flow within the block to maintain the temperature of the spring below a predetermined temperature. Thus, the spring is made from low-temperature metal alloys to maintain a positive preload on the piston and therefore is kept below a predetermined specific temperature limit. The cooling medium is also supplied to the cooling passage  42  and the film-cooling holes  44  to cool the ball-and-socket coupling. A passageway  65  is provided to exhaust the spent cooling medium. It will be appreciated that the metallic washer  50  retained by the cup  54  ensures spring retention and preload in the event of a fracture of the insulative washer  52 .  
         [0024]     It will be appreciated that in operation, the spring  60  of the damping mechanism maintains a radial inwardly directed force on the piston  32  and hence on the damper block  16 . The damper block  16 , in turn, bears against the backside surface  22  of the shroud  12  to dampen vibration and particularly to avoid vibratory response at or near resonant frequencies.  
         [0025]      FIG. 5  is an enlarged view of a forward flange section  68  and the flange connector pin  70 . The flange connector pin(s)  70  is inserted through an aperture(s)  72  of the forward flange  68  of the shroud  12 . The pin  70  holds the shroud in place in the support block  10  and against the damper block  16 . The pin  70  fits into a pin aperture  74  in the block, which includes a recess for the pin head. The pin aperture  74  extends across a gap  76  in the outer shroud block  10  to receive the forward flange  68 .  
         [0026]     The forward flange connector pin  70  includes a cooling passage  78  for cooling air. Cooling air flows through a cooling conduit  80  in the shroud block  10  to the pin. The pin  70  includes an axial cooling passage  78  that provides cooling air to the pin. Radial cooling passages  82  in the pin head allow cooling air from the conduit  80  to flow through the pin. Cooling gas passing through the pin and recess  62  is exhausted into the cavity  84  formed between the shroud block  10  and damper block  16 .  
         [0027]      FIG. 6  is an enlarged view of a cross-section of the aft flange  86  and attachment bolt  88 . The bolt screws into a threaded hole  90  in a side surface of the outer shroud block  10 . A retention pin  92  locks the bolt in the outer shroud block. The aft attachment bolt securely fixes the aft flange  86  of the shroud  12  to the outer surface block.  
         [0028]     The metal aft attachment bolt  88  is cooled by cooling air passing through the bolt and out passage  96  in the block  10 . An axial passage  98  in the bolt allows cooling air to enter and cool the bolt.  
         [0029]      FIG. 7  is an enlarged view of the pin hole  72  in the forward shroud flange  68 . The pin hole includes a cylindrical center section  100  and conical sections  102  on opposite sides of the center section. The conical sections may have a tapered slope of about 10 degrees with respect to the cylindrical surface of the center section. The outer surface of the shroud, including the flange and conical sections may be coated with an environmental barrier coating (EBC) conventionally used for silicon-carbide fiber-reinforced silicon carbide ceramic matrix composites (SiC/SiC CMCs)—which may be used to form the shroud. The cylindrical surface of the pin hole may be masked during EBC deposition.  
         [0030]     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.