Abstract:
The damper system includes a ceramic composite shroud in part defining the hot gas path of a turbine and a spring-biased piston and damper block which bears against the backside surface of the shroud to tune the vibratory response of the shroud relative to pressure pulses of the hot gas path in a manner to avoid near or resonant frequency response. The damper block has projections specifically located to bear against the shroud to dampen the frequency response of the shroud and provide a thermal insulating layer between the shroud and the damper block.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a damping system for damping vibration of 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. 
   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. 
   BRIEF DESCRIPTION OF THE INVENTION 
   In accordance with an aspect of the present invention, there is provided an attachment mechanism between a ceramic composite shroud and a metallic support structure which utilizes the pressure distribution applied to the shroud, coupled with a loading on the shroud to tune the shroud to minimize damaging vibratory response from pressure pulses of the hot gases as the buckets pass the shrouds. To accomplish the foregoing, and in one aspect thereof, there is provided a spring mass damping system which includes a ceramic composite shroud/damping block, a damper load transfer mechanism and a damping mechanism. The damper block includes at least three projections for engaging the backside of the shroud, thereby spacing the damper block surface from the backside of the shroud, affording a convective insulating layer, and reducing heat load on the damper block. The three projections are specifically located along the damper block to tune the dynamic response of the system. The load transfer mechanism includes a piston having a ball-and-socket coupling with the damper block along with a spring damping mechanism in the socket region of the outer shroud block. The ball-and-socket coupling uses a pin retention system enabling relative movement between the piston and damper block. Local film cooling is also provided to enhance the long-term wear capability of the coupling. The piston engages the spring through a thermally insulating washer and preferably also through a metallic washer, both being encapsulated within a cup supplied with a cooling medium. The cooling medium maintains the temperature of the spring below a temperature limit in order to maintain positive preload on the shroud. Various other aspects of the present invention will become clear from a review of the ensuing description. 
   In a preferred embodiment according to the present invention, there is provided a damper system for a stage of a turbine comprising a shroud having a first surface defining in part a hot gas path through the turbine, a shroud body for supporting the shroud, a damper block having at least three projections raised from a surface thereof and engaging a backside surface of the shroud opposite the first surface and a damping mechanism carried by the shroud body and connected to the damper block for applying a load to the damper block and the shroud through the engagement of the projections with the backside surface of the shroud thereby damping vibratory movement of the shroud. 
   In a further preferred embodiment according to the present invention, there is provided a damper system for a stage of a turbine comprising a shroud formed of a ceramic material having a first surface defining in part a hot gas path through the turbine, a shroud body for supporting the shroud, a damper block carried by the shroud body and engaging the shroud, the damper block being formed of a metallic material and a damping mechanism carried by the shroud body and connected to the damper block for applying a load to the damper block and the shroud to dampen vibratory movement of the shroud, the damping mechanism including a spring for applying the load to the damper block. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       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; 
       FIG. 2  is a cross-sectional view thereof as viewed in an axial forward direction relative to the hot gas path of the turbine; 
       FIG. 3  is a perspective view illustrating the interior surface of a damper block with projections for engaging the backside of the shroud; and 
       FIG. 4  is an enlarged cross-sectional view illustrating portions of the damper load transfer mechanism and damping mechanism. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   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. 
   The damper system of the present invention 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. 
   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. 
   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 . 
   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. 
   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 . 
   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 housing  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 housing  62 . The cap  64  has a central opening or passage  67  enabling cooling flow from compressor discharge air to flow within the housing 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 . 
   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. 
   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.