Abstract:
A method of reforming an annular attachment flange of an outer annular portion of a gas generator case, including inserting a first fixture element within the annular portion, the first fixture element conforming to an inner surface of the flange and forcing the flange radially outwardly passed a desired orientation, pressing the attachment flange against the first fixture element with a second annular fixture element surrounding the flange and conforming to an outer surface of the flange, heating the outer portion pressed by the fixture elements to a temperature and time period corresponding to an annealing of the material of the outer portion, forcing cooling of the outer portion, and disengaging the cooled outer portion from the fixture elements so that the released flange extends at the desired orientation. A gas generator case including a reformed outer portion and an unused inner portion is also discussed.

Description:
TECHNICAL FIELD 
       [0001]    The application relates generally to gas turbine engines and, more particularly, to the repair of a gas generator case in such engines. 
       BACKGROUND OF THE ART 
       [0002]    Gas turbine engines generally include a gas generator case surrounding the combustor. Some gas generator cases include an internal portion defining a diffuser through which fluid flow communication is provided between the compressor section and the combustor and/or a bearing housing for receiving a bearing supporting a main shaft. When damage occurs to the internal portion, the case is usually replaced in order to keep the relative position of the elements of the case within relatively tight tolerances. 
       SUMMARY 
       [0003]    In one aspect, there is provided a method of repairing a gas generator case of a gas turbine engine, the case having an outer annular portion surrounding a damaged inner portion and having an annular flange at one end thereof defining a sole connection between the outer and inner portions, the annular flange extending at a first acute angle from an axial direction of the case, the method comprising: cutting between the annular flange and the inner portion along a circular path concentric with the flange; removing the damaged inner portion; clamping the outer portion in a fixture having an outer element conforming to an outer surface of the annular flange and an inner element conforming to an inner surface of the annular flange and pressing against the outer element, the fixture forcing the annular flange to a second acute angle from the axial direction smaller than the first angle, the fixture being made of a same material as that of the outer portion; heating the clamped outer portion to a temperature and time period corresponding to an annealing of the material of the outer portion; forcing cooling of the outer portion; removing the cooled outer portion from the fixture; and inserting a new inner portion within the outer portion and attaching the new inner portion to the flange. 
         [0004]    In another aspect, there is provided a method of reforming an annular attachment flange at an extremity of an outer annular portion of a gas generator case of a gas turbine engine to a desired orientation for attachment to a remainder of the gas generator case, the method comprising: inserting a first fixture element within the annular portion made of a same material as that of the flange, the first fixture element conforming to an inner surface of the flange and forcing the flange radially outwardly passed the desired orientation; pressing the attachment flange against the first fixture element with a second annular fixture element made of a same material as that of the flange, surrounding the flange and conforming to an outer surface of the flange; heating the outer portion pressed by the fixture elements to a temperature and time period corresponding to an annealing of the material of the outer portion; forcing cooling of the outer portion; and disengaging the cooled outer portion from the fixture elements so that the released flange extends at the desired orientation. 
         [0005]    In a further aspect, there is provided a gas generator case comprising an outer portion reformed following the above method, the outer portion surrounding and being concentric to an inner portion defining a diffuser at an upstream end thereof, a bearing housing connected to the diffuser and located radially inwardly thereof and concentrically thereto, and an annular attachment flange extending downstream from the upstream end and located radially outwardly of and concentrically to the diffuser, the inner portion being in a unused state, the attachment flanges of the inner and outer portions being interconnected. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0006]    Reference is now made to the accompanying figures in which: 
           [0007]      FIG. 1  is a schematic cross-sectional view of a gas turbine engine; 
           [0008]      FIG. 2  is a schematic rear view of a gas generator case which can be used in a gas turbine engine such as shown in  FIG. 1 ; 
           [0009]      FIG. 3  is a schematic cross-sectional view of one side of the gas generator case of  FIG. 2 ; 
           [0010]      FIG. 4  is a schematic cross-sectional view of a connection flange of an outer portion of the gas generator case of  FIG. 2 , illustrating a deformation created during assembly of the gas generator case; 
           [0011]      FIG. 5  is a schematic cross-sectional view of the outer portion of the gas generator case clamped in a fixture for reforming the connection flange; and 
           [0012]      FIG. 6  is a top view of the fixture of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0013]      FIG. 1  illustrates a gas turbine engine  10  of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a compressor section  14  for pressurizing the air, a combustor  16  in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section  18  for extracting energy from the combustion gases and for rotating a drive shaft  12  which rotates a propeller through an appropriate reduction gearbox (not shown). Although the engine  10  is illustrated here as a turboprop engine, alternately the engine may be of another type, for example a turboshaft or a turbofan. 
         [0014]    Referring particularly to  FIGS. 2-3 , the engine  10  includes a gas generator case  20  which surrounds and contains the combustor  16 . The gas generator case  20  generally includes annular and concentric inner and outer portions  22 ,  24 . 
         [0015]    The outer portion  24  defines the outer wall of the combustor cavity  26  containing the combustor  16 , and includes a port  28  for bleeding pressurized air from that cavity, for example for circulation into the cabin of the aircraft. The upstream end of the outer portion  24  is defined by an annular flange  30  ( FIG. 2 ) extending at an angle from the axial direction A of the case. The inner portion  22  also includes an annular flange  32  ( FIG. 2 ) in alignment with the annular flange  30  of the outer portion  24 , both flanges  30 ,  32  being interconnected and defining the sole connection between the inner and outer portions  22 ,  24 . 
         [0016]    The inner portion  22  defines a diffuser  34  providing the fluid flow communication between the compressor section  14  and the combustor  16 . The diffuser  34  includes angled tubes  36  which are circumferentially regularly spaced apart, and which are each connected to a respective curved diffuser pipe  38  (see  FIG. 2 ), with the tubes  36  and pipes  38  being shaped to pressurize the accelerated flow exiting the compressor impellor. The diffuser  34  is approximately axially aligned with the connection flange  30  of the outer portion  24 , and the inner portion  22  has an upstream end  40  protruding from the outer portion  24 . 
         [0017]    The inner portion  22  also defines a bearing housing  42  concentric to the diffuser  34 . The bearing housing  42  is located radially inwardly and downstream of the diffuser  34 , and receives a bearing supporting the high pressure shaft. The bearing housing  42  is contained within the outer portion  24 . The inner portion  22  further includes oil passages  44 ,  45  in fluid communication with the bearing housing  42  and with an oil circulation system (not shown) of the engine  10 , to provide an oil flow to and out of the bearing housing  42 . 
         [0018]    During initial manufacturing of the gas generator case  20 , the inner and outer portions  22 ,  24  are moulded with excess material and are attached through welding of the connection flanges  30 ,  32 . The connection flanges  30 ,  32  deform during the welding of the two portions  22 ,  24 . This is illustrated by  FIG. 4 , where α is the blueprint acute angle between the connection flange  30  of the outer portion  24  and the axial direction A, or angle before initial assembly, and α′ is the acute angle of the connection flange  30  once the gas generator case  20  is assembled, i.e. after welding during production. It can be seen that the value of the acute angle of the flange  30  with the axial direction A becomes larger after production welding, i.e. the end  46  of the flange  30  moves radially inwardly. After the inner and outer portions  22 ,  24  are connected, machining of various attachment flanges, holes and other features is performed starting from the center of the gas generator case  20  with a progression in which newly formed surfaces are used as datums to machine the next surfaces, resulting in a stacking of tolerances at the connection flanges  30 ,  32  which necessitates the relative position of the connection flanges  30 ,  32  to be within relatively tight tolerances. However, the deformation of the connection flanges  30 ,  32  during the initial assembly makes it difficult to replace only the inner portion  22  upon damage while remaining within required tolerances. For example, in a particular embodiment, a maximum offset of 0.02″ is allowed between the connection flanges  30 ,  32  before attachment. 
         [0019]    Accordingly, in the embodiment shown, when the inner portion  22  of the gas generator case  20  is damaged, the diffuser pipes  38  are removed, for example by cutting a ring (not shown) interconnecting each diffuser pipe  38  and its respective tube  36 , and the outer and inner portions  22 ,  24  are separated by cutting the connected flanges  30 ,  32 , preferably along or adjacent to the weld connection that was performed during initial manufacturing. The outer portion  24  is left with its connection flange  30  attached thereto, which is deformed such as shown in  FIG. 4 . 
         [0020]    Referring to  FIGS. 5-6 , the outer portion  24  is clamped in a fixture  50  to reform the connection flange  30 . More particularly, the fixture  50  includes an annular outer element  52  conforming to the outer surface  54  of the connection flange  30  and an annular or circular inner element  56  conforming to the inner surface  58  of the connection flange  30 . The outer and inner elements  52 ,  56  clamp the connection flange  30  between them. In the embodiment shown, this is done by having the outer and inner elements  52 ,  56  both connected to a same bottom plate  60  through respective fasteners  62 ; other configurations are also possible, for example a configuration where the outer and inner elements  52 ,  56  are directly interconnected. The outer and inner elements  52 ,  56  are shaped to force the connection flange  30  radially outwardly. In a particular embodiment, the outer and inner elements  52 ,  56  force the connection flange  30  to an angle which is smaller than the blueprint angle α; the connection flange  30  is thus placed in a position where the end  46  is moved further radially outwardly than the desired blueprint position. 
         [0021]    In the embodiment shown, the connection flange  30  has a curved profile and as such the contacting surfaces of the outer and inner elements  52 ,  56  of the fixture  50  are correspondingly curved. 
         [0022]    The clamped outer portion  24  is then heated to a temperature and for a time period corresponding to annealing of its material. The clamped outer portion  24  is then forced cooled at least up to a given temperature, after which it can be passively cooled. In a particular embodiment, the clamped outer portion  24  is heated under vacuum or partial pressure of argon. Once the outer portion  24  has reached a temperature at which it can be handled, it is removed from the fixture  50 . 
         [0023]    In a particular embodiment, the outer portion  24  is made of an austenitic nickel-chromium-based superalloy such as for example Inconel® 718, and the clamped outer portion  24  is heated to a temperature of 1750° F.±25° F. for about one hour, and then forced cooled at least until the temperature of the clamped portion  24  gets below 800° F. 
         [0024]    The elements of the fixture  50  are made of a same material as that of the outer portion  24 , in order to minimize the differences of heat expansion between the fixture  50  and outer portion  24 . 
         [0025]    In a particular embodiment, upon release from the fixture  50 , the connection flange  30  relaxes and moves slightly radially inwardly, thus increasing its angle with the axial direction A. The angle of the connection flange  30  within the fixture  50  is selected such that after the reforming and release from the fixture  50 , the connection flange  30  moves back to extend at or approximately at the blueprint angle α from the axial direction A. 
         [0026]    The reformed outer portion  24  is then attached to a new inner portion  22  by attaching the reformed connection flange  30  to the connection flange  32  of the new inner portion  22 . In a particular embodiment, the flange  32  of the new inner portion  22  and reformed connection flange  30  are attached using an Electron Beam (EB) welding process, which may provide weld shrinkage of approximately 0.015″ with a variability of 0.003″. The diffuser pipes  38 , which in a particular embodiment are not replaced, are reattached to the respective diffuser tube  36  of the new inner portion using a new ring (not shown), for example by welding. 
         [0027]    In a particular embodiment, the replacement of the inner portion  22  is performed when cracks are detected in the oil passages  44 ,  45  communicating with the bearing housing  42 , which may lead to undesirable smoke in the cabin. The geometry of the oil passages  44 ,  45  may not allow the cracks to be successfully plugged or otherwise repaired, for example because of lack of accessibility and/or stresses in the repaired zone during use of the engine  10  which may lead to re-opening of the repaired cracks. Replacement of the inner portion  22  may provide for a less costly repair than the replacement of the complete gas generator case  20 . 
         [0028]    The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. For example, the inner portion may define only one of a diffuser and a bearing housing. Other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.