Patent Publication Number: US-11661864-B2

Title: Turbine casing, gas turbine, and aligning method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a turbine casing, a gas turbine, and an aligning method. 
     2. Description of the Related Art 
     A gas turbine, for example, includes: a compressor that generates compressed air; a combustor that generates combustion gas by mixing a fuel with the compressed air and combusting the fuel; and a turbine driven by the combustion gas. A turbine casing constituting the contour of the gas turbine is generally divided into a plurality of parts in an axial direction (JP-2013-181503-A). 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1 
     
         
         JP-2013-181503-A 
       
    
     SUMMARY OF THE INVENTION 
     When the gas turbine is assembled, piece-part assembly of only the turbine casing is performed before final assembly performed by internally incorporating a rotor. In a step of this piece-part assembly, axial alignment (centering) of each part is performed in a state in which the turbine casing is erected vertically. In work of the axial alignment, an alignment jig having a dial gage attached to an arm that swings around a pole may be used. Specifically, the parts are placed so as to cover the pole, distances between the pole and the inner circumferential surfaces of the parts are measured by the dial gage by swinging the arm around the pole, and the positions of the parts are adjusted in a horizontal direction such that all the circumferences of the inner circumferential surfaces are at equal distances from the pole. The axes of the parts are aligned with each other with the pole as a reference by performing such work for all of the stacked parts as needed. 
     However, this work takes time, and the dedicated alignment jig needs to be prepared. 
     It is an object of the present invention to provide a turbine casing, a gas turbine, and an aligning method that obviate a need for a dedicated alignment jig, and make it possible to shorten the time of piece-part assembly. 
     In order to achieve the above object, according to the present invention, there is provided a turbine casing divided in an axial direction into a first casing and a second casing coupled to each other by flanges of the first casing and the second casing, the first casing and the second casing each being divided into two parts as viewed from the axial direction, the two parts being an upper half casing and a lower half casing, the turbine casing having three or more sets of a first radial reference surface and a second radial reference surface in a circumferential direction, the first radial reference surface being disposed in a flange peripheral portion of the first casing the second radial reference surface being disposed in a flange peripheral portion of the second casing, each first radial reference surface being located at an equal distance from a turbine central axis, each second radial reference surface being located at an equal distance from the turbine central axis, positional relation between the first radial reference surface and the second radial reference surface being equal in each set. 
     According to the present invention, it is possible to obviate a need for a dedicated alignment jig, and shorten the time of piece-part assembly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic configuration diagram of an example of a gas turbine according to one embodiment of the present invention; 
         FIG.  2    is a diagram extracting and showing a first casing and a second casing constituting a turbine casing of the gas turbine according to one embodiment of the present invention; 
         FIG.  3    is a sectional view taken along a line III-III in  FIG.  2   ; 
         FIG.  4    is an enlarged view of a part indicated by an arrow IV in  FIG.  2   ; and 
         FIG.  5    is a sectional view taken along a line V-V in  FIG.  3   . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention will hereinafter be described with reference to the drawings. 
     Gas Turbine 
       FIG.  1    is a schematic configuration diagram of an example of a gas turbine according to one embodiment of the present invention. The gas turbine shown in the figure is a prime mover that drives a load apparatus (not shown). The gas turbine includes a compressor  10 , combustors  20 , a turbine  30 , and an exhaust chamber  35 . A casing of the compressor  10  (compressor casing  11 ) is supported by a leg portion  12 . A casing of the turbine  30  (turbine casing  31 ) is supported by a leg portion  32 . The exhaust chamber  35  is supported by a leg portion  38 . The load apparatus is typically a generator. However, a pump may be applied as the load apparatus. Incidentally, when the gas turbine is referred to as a “gas turbine engine,” the turbine may be referred to as a “gas turbine.” 
     The compressor  10  has an air intake  13  for taking in air and an inlet guide vane (IGV)  14  within the compressor casing  11 . The compressor  10  further includes a stage portion in which stator blades  15  and rotor blades  16  are arranged alternately in the direction of the central axis of the turbine in the rear of the inlet guide vane  14 . The combustors  20  are plurally arranged annularly on a peripheral portion of a combustor casing  21  between the compressor  10  and the turbine  30 . The turbine  30  includes, within the turbine casing  31 , stator blades  33  and rotor blades  34  arranged alternately in the direction of the central axis of the turbine. On a downstream side of the turbine casing  31 , the exhaust chamber  35  is disposed via an exhaust casing  36 . The exhaust chamber  35  has an exhaust diffuser  37  continuous with the turbine  30 . 
     In addition, a rotor  5  is located so as to pass through the centers of the compressor  10 , the combustors  20 , the turbine  30 , and the exhaust chamber  35 . An end portion of the rotor  5 , which is on the compressor  10  side, is rotatably supported by a bearing  6 . An end portion of the rotor  5 , which is on the exhaust chamber  35  side, is rotatably supported by a bearing  7 . A part of the rotor  5 , which belongs to the compressor  10 , is formed by superposing, in an axial direction, a plurality of disks having a plurality of rotor blades  16  fitted to peripheral portions thereof. A part of the rotor  5 , which belongs to the turbine  30 , is formed by superposing, in the axial direction, a plurality of disks having a plurality of rotor blades  34  fitted to peripheral portions thereof. In the example of  FIG.  1   , the end portion of the rotor  5 , which is on the exhaust chamber  35  side, is coupled as an output power shaft to a driving shaft of a load apparatus (not shown). 
     In the above constitution, air taken into the compressor  10  from the air intake  13  is compressed while passing through the inlet guide vane  14 , the cascade of the stator blades  15 , and the cascade of the rotor blades  16 , so that a high-temperature and high-pressure compressed air is generated. In the combustors  20 , a fuel supplied from a fuel system is mixed and combusted with the compressed air supplied from the compressor  10 . A high-temperature combustion gas is thereby generated, and is supplied to the turbine  30 . A liquid fuel or a gaseous fuel is used as the fuel. The high-temperature and high-pressure combustion gas as an operating fluid generated in the combustors  20  passes through the cascade of the stator blades  33  and the cascade of the rotor blades  34  in the turbine  30 , and thereby drives and rotates the rotor  5 . A part of the output power of the turbine  30  is used as power to the compressor  10 . The rest of the output power of the turbine  30  is used as power to the load apparatus  4 . The combustion gas that has driven the turbine  30  is discharged as exhaust gas via the exhaust chamber  35 . In the present embodiment, a single-shaft gas turbine is illustrated. However, application targets of the invention include a two-shaft gas turbine. The two-shaft gas turbine includes a high pressure turbine and a low pressure turbine having rotary shafts separated from each other, and has a configuration in which the high pressure turbine is coaxially coupled to the compressor, and the low pressure turbine is coaxially coupled to the turbine. 
     Turbine Casing 
     The above-described gas turbine is provided with a turbine casing that includes the rotor  5 . The turbine casing is divided in the direction of the central axis of the turbine into divided casings as a plurality of cylindrical parts, specifically the compressor casing  11 , the combustor casing  21 , the turbine casing  31 , the exhaust casing  36 , and the like. The compressor casing  11  and the combustor casing  21  have vertical annular flanges (for example, flanges  21   v  and  31   v  to be described later with reference to  FIG.  2    or the like) in mutually opposed portions thereof. The compressor casing  11  and the combustor casing  21  are coupled to each other by fastening these flanges with bolts (not shown). The same is true for the combustor casing  21  and the turbine casing  31 . The same is true for the turbine casing  31  and the exhaust casing  36 . In addition, the number of divisions in the axial direction of the turbine casing can be changed. 
     Further, the parts of the turbine casing such as the compressor casing  11 , the combustor casing  21 , the turbine casing  31 , and the exhaust casing  36  are each divided into two parts, that is, an upper half casing and a lower half casing as viewed from the axial direction. Each upper half casing and the lower half casing corresponding to the upper half casing include horizontally extending flanges (for example, flanges  21   hl ,  21   h   2 ,  31   hl , and  31   h   2  to be described later with reference to  FIG.  2   ) in mutually opposed portions thereof. The upper half casing and the lower half casing are coupled to each other by fastening these flanges with a large number of bolts (not shown). 
     Alignment Structure 
       FIG.  2    is a diagram extracting and illustrating a first casing and a second casing constituting the turbine casing.  FIG.  3    is a sectional view taken along a line III-III in  FIG.  2   .  FIG.  4    is an enlarged view of a part indicated by an arrow IV in  FIG.  2   .  FIG.  5    is a sectional view taken along a line V-V in  FIG.  3   . In the assembly of the gas turbine, piece-part assembly of the turbine casing is performed before final assembly performed by assembling the rotor  5  ( FIG.  1   ) into the turbine casing.  FIGS.  2  to  5    represent a mode at a time of alignment between the first casing and the second casing, which is performed in the piece-part assembly of the turbine casing. Specifically,  FIGS.  2  to  5    show a state in which the second casing is stacked on the first casing with the turbine central axis set vertical. 
     In the specification of the present application, parts (divided casings) of the turbine casing, which are mutually coupled by flanges so as to be adjacent to each other in the axial direction of the turbine, will be described as the first casing and the second casing as appropriate.  FIG.  2    illustrates the turbine casing  31  and the combustor casing  21  as the first casing and the second casing. In addition, the exhaust casing  36  and the turbine casing  31  as well as the combustor casing  21  and the compressor casing  11  correspond to the first casing and the second casing. In addition, the objects of the first casing and the second casing may be opposite from each other. 
     The turbine casing is provided with three or more sets (four sets in the present example) of radial reference surfaces at intervals in a circumferential direction in the opposed portions of the first casing (turbine casing  31  in this case) and the second casing (combustor casing  21 ). Each set of radial reference surfaces includes a first radial reference surface  31 R ( FIG.  3    and  FIG.  5   ) provided in a peripheral portion of a flange  31   v  of the turbine casing  31  and a second radial reference surface  21 R ( FIG.  5   ) provided in a peripheral portion of a flange  21   v  of the combustor casing  21 . 
     Each of the first radial reference surfaces  31 R is located at an equal distance from the turbine central axis (at a position at a distance R 1  from the turbine central axis). In addition, each first radial reference surface  31 R is constituted by a surface parallel with the turbine central axis and facing outward in a radial direction of the turbine casing  31  among inner wall surfaces of a flat groove-shaped slit  31   x  provided in the peripheral portion of the flange  31   v  of the turbine casing  31 . In the present embodiment, a flat surface is adopted as the first radial reference surface  31 R, and as viewed in the direction of the turbine central axis, the first radial reference surface  31 R is orthogonal to a plane including the turbine central axis at the position at the distance R 1  from the turbine central axis. That is, each of the first radial reference surfaces  31 R constitutes a tangent to a same circle having the turbine central axis as a center as viewed in the direction of the turbine central axis. However, the first radial reference surface  31 R does not need to be a flat surface as long as the first radial reference surface  31 R is in predetermined positional relation to the second radial reference surface  21 R. For example, the first radial reference surface  31 R may be a curved surface. 
     Each of the second radial reference surfaces  21 R is located at an equal distance from the turbine central axis (at a position at a distance R 2  from the turbine central axis). In addition, each second radial reference surface  21 R is constituted by a surface parallel with the turbine central axis and facing outward in a radial direction of the combustor casing  21  among inner wall surfaces of a flat groove-shaped slit  21   x  provided in the peripheral portion of the flange  21   v  of the combustor casing  21 . In the present embodiment, a flat surface is adopted as the second radial reference surface  21 R, and as viewed in the direction of the turbine central axis, the second radial reference surface  21 R is orthogonal to a plane including the turbine central axis at the position at the distance R 2  from the turbine central axis. That is, each of the second radial reference surfaces  21 R constitutes a tangent to a same circle having the turbine central axis as a center as viewed in the direction of the turbine central axis. However, the second radial reference surface  21 R does not need to be a flat surface as long as the second radial reference surface  21 R is in predetermined positional relation to the first radial reference surface  31 R. For example, the second radial reference surface  21 R may be a curved surface. 
     The positional relation between the first radial reference surface  31 R and the second radial reference surface  21 R is equal in each set of the first and second reference surfaces. While the first radial reference surface  31 R and the second radial reference surface  21 R may be flush with each other (that is, R 1 =R 2 ), it suffices for the mutual positional relation to be equal in each set, and the first radial reference surface  31 R and the second radial reference surface  21 R do not necessarily need to be flush with each other.  FIG.  5    illustrates a case where the second radial reference surface  21 R is more distant from the turbine central axis by a distance dR than the first radial reference surface  31 R, and there is a step between the radial reference surfaces  31 R and  21 R. 
     In addition, as shown in  FIG.  4   , the turbine casing is provided with at least one set of groove-shaped notches  31   y  and  21   y  (two sets at a pitch of 180° in the present example) in the opposed portions of the first casing (turbine casing  31 ) and the second casing (combustor casing  21 ). 
     The notch  31   y  is provided on one side of the upper half casing  31 U and the lower half casing  31 L (lower half casing  31 L in  FIG.  4   ) of the turbine casing  31  so as to face the other side (upper half casing  31 U). That is, the notch  31   y  is provided at a corner part at which end surfaces of the flanges  31   v  and  31   h   2  of the turbine casing  31  intersect each other. A surface of the upper half casing  31 U, which is opposed to the lower half casing  31 L (that is, an end surface of the flange  31   h   1  of the upper half casing  31 U), is partly exposed to face the notch  31   y . The end surface of the flange  31   h   1 , which faces the notch  31   y , constitutes a first roll reference surface  31 C for alignment in the circumferential direction between the turbine casing  31  and the combustor casing  21 . Hence, the first roll reference surface  31 C corresponds to a section of the flange  31   v  among end surface portions of the flange  31   hl  of the upper half casing  31 U of the turbine casing  31 . 
     The notch  21   y  is provided on one side of the upper half casing  21 U and the lower half casing  21 L (lower half casing  21 L in the present example) of the combustor casing  21  so as to face the other side (upper half casing  21 U). That is, the notch  21   y  is provided at a corner part at which end surfaces of the flanges  21   v  and  21   h   2  of the combustor casing  21  intersect each other. The circumferential positions of the notches  21   y  and  31   y  correspond to each other, and the notches  21   y  and  31   y  face each other in the direction of the turbine central axis. In addition, a surface of the upper half casing  21 U, which is opposed to the lower half casing  21 L (that is, an end surface of the flange  21   h   1  of the upper half casing  21 U), is partly exposed to face the notch  21   y . The end surface of the flange  21   hl , which faces the notch  21   y , constitutes a second roll reference surface  21 C for alignment in the circumferential direction between the turbine casing  31  and the combustor casing  21 . Hence, the second roll reference surface  21 C corresponds to a section of the flange  21   v  among end surfaces of the flange  21   hl  of the upper half casing  21 U of the combustor casing  21 . 
     The first roll reference surface  31 C and the second roll reference surface  21 C are both the end surfaces of the upper half casings  31 U and  21 U opposed to the lower half casings  31 L and  21 L. The circumferential positions of the first roll reference surface  31 C and the second roll reference surface  21 C therefore coincide with each other with high accuracy in a state in which the turbine casing is assembled. The width dimensions in the circumferential direction of the notches  31   y  and  21   y  may be made to coincide with each other, but do not necessarily need to coincide with each other. 
     In addition, as shown in  FIG.  5   , the flange  21   v  of the second casing (combustor casing  21 ) is provided with a through hole  41  parallel with the turbine central axis. In addition, an end surface of the flange  31   v  of the first casing (turbine casing  31 ) is provided with a knock hole  42  corresponding in position to the through hole  41 . The knock hole  42  is a pin hole for inserting a knock pin  43 . A dimensional tolerance between the outside diameter of the knock pin  43  and the inside diameter of the knock hole  42  is set as small as possible, and there is practically no clearance between the outer circumferential surface of the knock pin  43  and the inner circumferential surface of the knock hole  42 . The knock pin  43  is partly inserted into the knock hole  42  of the flange  31   v  of the turbine casing  31  through the through hole  41 . A remaining part of the knock pin  43 , which projects from the knock hole  42 , is located within the through hole  41 . In addition, a bush  44  is located within the through hole  41 . 
     The bush  44  is a cylindrical member, and functions as a spacer fitted so as to cover the knock pin  43  and filling a clearance between the knock pin  43  and the through hole  41 . A dimensional tolerance between the outside diameter of the knock pin  43  and the inside diameter of the bush  44  is set as small as possible, and there is practically no clearance between the outer circumferential surface of the knock pin  43  and the inner circumferential surface of the bush  44 . On the other hand, though not shown in the schematic diagram of  FIG.  5   , the inside diameter of the through hole  41  is set slightly larger than the outside diameter of the bush  44 , and thus there is a predetermined clearance between the inner circumferential surface of the through hole  41  and the outer circumferential surface of the bush  44 . The bush  44  is fixed to the flange  21   v  of the combustor casing  21  (for example, the inner wall of the through hole  41 ) by welding (welding portion  45 ) in a state in which the bush  44  is fitted to the knock pin  43 . Consequently, the knock pin  43  is constrained by the knock hole  42 , the bush  44  is constrained by the knock pin  43 , and the through hole  41  is constrained by the bush  44 , so that the flanges  31   v  and  21   v  constrain each other. 
     Incidentally, while description has been made with reference to  FIGS.  2  to  5    by taking the configuration of the opposed portions of the turbine casing  31  and the combustor casing  21  as an example, the structures of the radial reference surfaces, the roll reference surfaces, the knock pin, and the like are similarly provided also to the opposed portions of the other first and second casings. For reference,  FIG.  2    illustrates radial reference surfaces X and roll reference surfaces Y. 
     Turbine Casing Aligning Method 
     Referring to  FIGS.  2  to  5   , description will be made of a step of axial alignment and angular alignment between the first casing and the second casing adjacent to each other in the axial direction, the axial alignment and the angular alignment being performed in a process of the piece-part assembly of the turbine casing. In performing this step, the upper half casing  31 U and the lower half casing  31 L of the turbine casing  31  are coupled to each other and formed into a cylindrical shape in advance. The same is true for the upper half casing  21 U and the lower half casing  21 L of the combustor casing  21 . The turbine casing  31  and the combustor casing  21  are provided in advance with the first radial reference surfaces  31 R, the second radial reference surfaces  21 R, the first roll reference surfaces  31 C, and the second roll reference surfaces  21 C described earlier. 
     The inside diameter of the turbine casing  31  is, for example, machined (turned) by a machining center, for example, in a state in which the turbine casing  31  is in a cylindrical shape. At this time, the first radial reference surfaces  31 R (slits  31   x ) are formed in advance by, for example, milling or the like at the same time (by one time of setup work). When each first radial reference surface  31 R is formed in the same setup as the inside diameter machining, all of the first radial reference surfaces  31 R can be formed at equal distances from the turbine central axis with high accuracy. The notch  31   y  related to the first roll reference surface  31 C and the knock hole  42  may be also formed at the time of the inside diameter machining of the turbine casing  31 . However, the notch  31   y  and the knock hole  42  do not need a high position accuracy, and therefore may be machined in another step. 
     Similarly, when the inside diameter of the combustor casing  21  is machined (turned), the second radial reference surfaces  21 R (slits  21   x ) are formed in advance by, for example, milling or the like at the same time (by one time of setup work). When the second radial reference surfaces  21 R are formed in the same setup as the inside diameter machining, all of the second radial reference surfaces  21 R can be formed at equal distances from the turbine central axis with high accuracy. The notch  21   y  related to the second roll reference surface  21 C and the through hole  41  may be also formed at the time of the inside diameter machining of the combustor casing  21 . However, the notch  21   y  and the through hole  41  do not need a high position accuracy, and therefore may be machined in another step. 
     At a time of the piece-part assembly of the turbine casing, alignment (axial alignment and angular alignment) is performed between the turbine casing  31  and the combustor casing  21  thus fabricated separately. On a horizontal disk surface, for example, the combustor casing  21  is placed on the turbine casing  31  by using a crane, for example, such that the flanges  31   v  and  21   v  of the turbine casing  31  and the combustor casing  21  face each other in a posture in which the turbine central axis is oriented vertically. When the combustor casing  21  is stacked on the turbine casing  31 , the circumferential positions of the slits  31   x  and  21   x  and the notches  31   y  and  21   y  are made to roughly coincide with each other in advance. 
     Next, steps of axial alignment and angular alignment of the combustor casing  21  with respect to the turbine casing  31  are performed. These axial alignment and angular alignment steps are performed in succession or in parallel with each other, and at a time of the axial alignment, the work of the angular alignment is also performed. When the steps of the axial alignment and the angular alignment are performed in succession, either step may be performed first. When necessary, the work of the axial alignment and the angular alignment may be repeated alternately multiple times. 
     Description will first be made of the step of the angular alignment of the combustor casing  21  with respect to the turbine casing  31 . In the step of the angular alignment, while the combustor casing  21  is pulled upward by a crane or the like, the positions in the circumferential direction of the turbine casing  31  and the combustor casing  21  are made to coincide with each other by, for example, manually rotating (rolling) the combustor casing  21  about the central axis. The circumferential positions of the first roll reference surface  31 C and the second roll reference surface  21 C are made to coincide with each other by thus finely adjusting the angle of the combustor casing  21 . As one method at the time, for example, a contact fitting M ( FIG.  4   ) is made to abut against the roll reference surface  31 C or  21 C (roll reference surface  31 C in the example of  FIG.  4   ) within the notches  31   y  and  21   y . Then, a clearance G between the contact fitting M and the roll reference surface  21 C or  31 C (roll reference surface  21 C in the example of  FIG.  4   ) is measured by a scale, a clearance gage, or the like, and the combustor casing  21  is moved such that the clearance G becomes zero. The angle of the combustor casing  21  with respect to the turbine casing  31  can be thereby adjusted. 
     Next, the step of the axial alignment will be described. Also in the step of the axial alignment, as in the step of the angular alignment, while the combustor casing  21  is pulled upward by a crane, the center of the combustor casing  21  is made to coincide with the center of the turbine casing  31  by, for example, manually moving (shifting) the combustor casing  21  in a horizontal direction. The positional relations between the first radial reference surfaces  31 R and the second radial reference surfaces  21 R are made to be equal at all positions in the circumferential direction by thus finely adjusting the position in the horizontal direction of the combustor casing  21 . Specifically, step dimensions between the first radial reference surfaces  31 R and the second radial reference surfaces  21 R are measured as the positional relations between the first radial reference surfaces  31 R and the second radial reference surfaces  21 R by a measuring instrument such as a dial gage or the like, and the steps are made to be equal at all of the positions in the circumferential direction. That is, the steps between the first radial reference surfaces  31 R and the second radial reference surfaces  21 R are all made to be a value within an allowable value set in advance for the distance dR ( FIG.  5   ). 
     At this time, when the width dimension of the slit  31   x  or  21   x  is adjusted to the width of a magnet base retaining the dial gage, for example, a point to be measured by the dial gage is positioned easily by setting the magnet base in the slit  31   x  or  21   x . Efficiency of axial alignment work is improved when the dial gage is thus installed at each position in the circumferential direction, and the position of the combustor casing  21  is adjusted while the measured value of each dial gage is viewed. 
     Effects 
     (1) As described above, three or more sets of first radial reference surfaces  31 R and second radial reference surfaces  21 R are provided in the circumferential direction to align the axes of the first casing and the second casing adjacent to each other in the axial direction. All of the first radial reference surfaces  31 R are located at equal distances from the turbine central axis, and all of the second radial reference surfaces  21 R are also located at equal distances from the turbine central axis. Therefore, when adjustment is made such that the positional relations between the first radial reference surfaces  31 R and the second radial reference surfaces  21 R are equal at three positions or more, the centers of the first casing and the second casing can be made to geometrically coincide with each other. Thus, according to the present embodiment, a dedicated alignment jig is not needed for the work of the axial alignment of the first casing and the second casing. In addition, since the work is easy, a work time of the axial alignment of the first casing and the second casing can be shortened, and thus a time of the piece-part assembly of the turbine casing can be shortened. 
     (2) Selecting specific positions of unprocessed flanges as the first radial reference surfaces  31 R and the second radial reference surfaces  21 R is conceivable in theory. However, in this case, in addition to highly accurate perfect circles of the flanges, a very strict centering accuracy with respect to the outside diameter of the flanges is required when the inside diameter of the first casing and the second casing is processed. 
     Accordingly, in the present embodiment, the flange peripheral portions of the first casing and the second casing are provided with the slits  31   x  and  21   x , and inner wall surfaces of the slits  31   x  and  21   x  are set as the first radial reference surfaces  31 R and the second radial reference surfaces  21 R. The slits  31   x  and  21   x  can be machined in the same setup as inside diameter processing by using a machining center or the like at the same time as the inside diameter processing of the first casing and the second casing, for example. A distance between each first radial reference surface  31 R and the turbine central axis can be thereby made uniform with high accuracy. Similarly, a distance between each second radial reference surface  21 R and the turbine central axis can also be made uniform with high accuracy. 
     Whereas a high accuracy of distance of the first radial reference surface  31 R and the second radial reference surface  21 R from the turbine central axis is required of the slits  31   x  and  21   x , the functions of the slits  31   x  and  21   x  are not affected even when the slits  31   x  and  21   x  are slightly shifted in position along the first radial reference surface  31 R and the second radial reference surface  21 R. 
     However, as long as the above-described essential effect (1) is obtained, the slits  31   x  and  21   x  do not necessarily need to be provided to form the first radial reference surface  31 R and the second radial reference surface  21 R. For example, protruding portions are formed in advance at parts where the first radial reference surface  31 R and the second radial reference surface  21 R are intended to be formed on the peripheral surfaces of the flanges  31   v  and  21   v  of the first casing and the second casing. Then, a mode is conceivable in which the first radial reference surface  31 R and the second radial reference surface  21 R are formed by grinding down end surfaces of the protruding portions by machining. In this case, the first radial reference surface  31 R and the second radial reference surface  21 R are located more distant from the turbine central axis than the peripheral surfaces of the flanges  31   v  and  21   v.    
     (3) As described above, the first roll reference surface  31 C and the second roll reference surface  21 C are provided whose circumferential positions coincide with each other when the circumferential positions of the first casing and the second casing adjacent to each other in the axial direction are made to coincide with each other. Hence, at the time of the piece-part assembly of the turbine casing, the relative circumferential positions of the first casing and the second casing can be made to coincide with each other by making the positions of the first roll reference surface  31 C and the second roll reference surface  21 C coincide with each other. Hence, according to the present embodiment, a dedicated alignment jig is not needed when the first casing and the second casing are aligned with each other in the circumferential direction. In addition, since the work is easy, a work time of the alignment in the circumferential direction of the first casing and the second casing can be shortened, and thus the time of the piece-part assembly of the turbine casing can be shortened. 
     (4) In the present embodiment, the lower half casings  31 L and  21 L of the first casing and the second casing are provided with the notches  31   y  and  21   y  opposed to the upper half casings  31 U and  21 U. Then, the end surfaces of the flanges  31   h   1  and  21   h   1  of the upper half casings  31 U and  21 U, which face these notches  31   y  and  21   y , are set as the first roll reference surface  31 C and the second roll reference surface  21 C. An accuracy of reference positions can be secured easily by setting the end surfaces as the roll reference surfaces. 
     In addition, since the notches  31   y  and  21   y  are provided, a fitting, a scale, or the like having a flat surface, such as the contact fitting M in  FIG.  4   , can be made to abut against the roll reference surfaces, as described earlier. It consequently becomes easy to measure the step between the first roll reference surface  31 C and the second roll reference surface  21 C, that is, an amount of displacement between the circumferential positions of the first casing and the second casing. This also contributes to facilitating the work of the angular alignment of the first casing and the second casing. 
     However, boundaries between the upper half casings  31 U and  21 U and the lower half casings  31 L and  21 L can be visually recognized even without the notches  31   y  and  21   y . Therefore, the notches  31   y  and  21   y  are not necessarily needed in adopting the opposed surfaces of the upper half casings  31 U and  21 U and the lower half casings  31 L and  21 L as a reference. 
     In addition, when positional accuracy of the first roll reference surface  31 C and the second roll reference surface  21 C can be secured, the opposed end surfaces of the upper half casing and the lower half casing do not necessarily need to be set as the roll reference surfaces. For example, when processing positions in the circumferential direction of surfaces facing the circumferential direction (for example, surfaces N 1  and N 2  in  FIG.  3    and  FIG.  5   ) among the inner wall surfaces of the slits  31   x  and  21   x  can be aligned with each other with high accuracy, these surfaces can also be used as the roll reference surfaces. 
     (5) In addition, in order to fix the relative positions of the first casing and the second casing, the first casing is provided with the knock hole  42 , and the knock pin  43  is erected in the knock hole  42 , while the second casing is provided with the through hole  41  having a dimensional margin with respect to the knock pin  43 . Hence, even when the centers of the through hole  41  and the knock hole  42  are slightly displaced from each other after completion of the axial alignment and the angular alignment of the first casing and the second casing, the knock pin  43  can be easily driven into the knock hole  42  via the through hole  41 . In addition, there is a clearance between the knock pin  43  and the through hole  41 , and at a point in time that the knock pin  43  is driven in, the knock pin  43  and the second casing are not in fixed relation. However, the bush  44  is fitted to the knock pin  43 , and the through hole  41  and the bush  44  are welded to each other. Consequently, the knock pin  43  is fixed by the bush  44  and the welding portion  45 , and the first casing and the second casing can be fixed to each other in a state in which the first casing and the second casing are already aligned with each other. 
     In general, the flanges of the first casing and the second casing may be subjected to common hole machining after alignment work, and the first casing and the second casing after the alignment may be fixed to each other by driving a knock pin into the common hole. In this case, the first casing and the second casing need to be transferred to a machine tool while the first casing and the second casing are maintained in a stacked state after the alignment, and the common hole machining requires a number of man-hours. 
     On the other hand, in the present embodiment, as described above, the inside diameter of the through hole  41  is set at a dimension having a margin with respect to the outside diameter of the knock pin  43 . Therefore, even when the centers of the through hole  41  and the knock hole  42  are slightly displaced from each other, the knock pin  43  can be driven into the knock hole  42 , and the first casing and the second casing can be fixed by covering the knock pin  43  with the bush  44  and welding the bush  44 . Thus, a strict positional accuracy is not required of the through hole  41  and the knock hole  42 . The through hole  41  and the knock hole  42  can therefore be made in advance before alignment of the first casing and the second casing. Hence, after completion of the alignment work, the first casing and the second casing can be fixed to each other on the spot, and the first casing and the second casing do not need to be transferred to a machine tool for common hole machining. This also contributes to shortening the step of the piece-part assembly of the turbine casing. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           21 : Combustor casing (second casing) 
           21 C: Second roll reference surface 
           21 L: Lower half casing 
           21 R: Second radial reference surface 
           21 U: Upper half casing 
           21   v : Flange 
           21   x : Slit 
           21   y : Notch 
           31 : Turbine casing (first casing) 
           31 C: First roll reference surface 
           31 L: Lower half casing 
           31 R: First radial reference surface 
           31 U: Upper half casing 
           31   v : Flange 
           31   x : Slit 
           31   y : Notch 
           41 : Through hole 
           43 : Knock pin 
           44 : Bush 
         dR: Step (positional relation between the first radial reference surface and the second radial reference surface) 
         R 1 : distance between the first radial reference surface and a turbine central axis 
         R 2 : Distance between the second radial reference surface and the turbine central axis