Patent Publication Number: US-9840928-B2

Title: Turbine diaphragm construction

Description:
TECHNICAL FIELD 
     This disclosure relates to the construction of diaphragms for turbines, and in particular, to a novel structure and assembly process for diaphragms in axial flow steam turbines. 
     TECHNICAL BACKGROUND 
     A known way of constructing a steam turbine diaphragm is to mount an annulus of static guide blades between an inner ring and an outer ring. Each such blade comprises a blade unit in which an aerofoil portion extends between an inner platform and an outer platform, the blade unit being machined as a single component. This is known as the “platform” type of construction. Each platform is in the form of a segment of a cylinder so that when the annulus of blade units is assembled, the inner platforms combine to create an inner port wall and the outer platforms combine to create an outer port wall. The inner platforms are welded to an inner ring that retains the turbine blades and provides a mount for a sealing arrangement, such as a labyrinth seal, that acts between the inner ring and a rotor shaft of the turbine. The outer platforms are welded to an outer ring that provides support and rigidity to the diaphragm. Each of the inner and outer rings comprises two semi-circular halves which are joined along a plane that contains the major axis of the diaphragm and passes between blade units so that the entire diaphragm can be separated into two parts for assembly around the rotor of the turbo-machine. 
     Existing platform constructions for HP or IP steam turbine diaphragms generally comprise solid inner and outer rings cut from thick metal plate, or forged, or formed from bar stock. Since such rings in large turbines have substantial dimensions in the axial and radial directions of the turbine, e.g., 100 mm to 200 mm, the cost of welding together the components of the diaphragm is a significant factor in the ex-works price of a large steam turbine, not least because the necessary deep penetration welds require advanced specialist welding equipment for their production. Furthermore, welds are a possible source of metallurgical defects in the diaphragm and it is also necessary to heat treat the diaphragm in order to relieve stresses in the diaphragm caused by the welding processes. 
     SUMMARY OF THE DISCLOSURE 
     In its broadest aspect, the present disclosure provides an axial flow turbine diaphragm comprising: a radially inner diaphragm ring;
         (a) a radially outer diaphragm ring;   (b) a plurality of static blade units arranged between the inner and outer rings, each blade unit comprising;
           an aerofoil portion having a stagger angle;   a radially inner platform portion that engages the radially inner ring; and   a radially outer platform portion that engages the radially outer ring;
 
wherein:
   
           (i) the radially inner ring is provided with blade unit retaining means operative to retain the inner platform portions to the inner ring;   (ii) the outer platform portion is elongate in a direction compatible with the stagger angle of the aerofoil; and   (iii) an inner circumference of the radially outer ring is provided with a plurality of blade unit retaining features, each such feature being of complementary shape and orientation to a corresponding outer platform portion of a static blade unit, and operative to retain the outer platform portion to the radially outer ring.       

     Note that the radially outer diaphragm ring includes the radially outer platform portions of the blade units as part of its structure. Thus, the present concept produces a diaphragm structure in which a radially outer port wall of the diaphragm comprises the radially outer platform portions, alternating in the circumferential direction with exposed portions of the inner circumference of the outer diaphragm ring. 
     The above construction enables the components of the diaphragm to be assembled and held together solely by mechanical means, i.e., the diaphragm can be constructed without welding or other metal melting techniques. 
     In a preferred embodiment, the radially inner platform portions of the blade units are elongate in the circumferential direction of the turbine diaphragm and an outer circumference of the radially inner ring is provided with a blade unit retaining feature of complementary shape and orientation to the inner platform portions of the static blade units, whereby the inner platform portions are retained to the radially inner ring. Hence, in this embodiment, the radially inner diaphragm ring includes the radially inner platform portions of the blade units as part of its structure. Thus, the radially inner port wall of the diaphragm will comprise the radially inner platform portions, flanked on their axially opposed (inlet and outlet) sides by exposed portions of the outer circumference of the inner diaphragm ring. 
     The preferred construction enables the components of the diaphragm to be assembled and held together solely by mechanical interlocking of its components. 
     To maintain aerodynamic smoothness, confronting ends of the radially inner elongate platform portions should preferably butt up to each other when inserted into the blade unit retaining feature of the inner ring, such that the platform portions extend continuously around the inner port wall of the diaphragm in the circumferential direction. 
     Clearly, with regard to their dimensions and surface finishes, the platform portions of the blade units and the blade retaining features of the inner and outer rings should be accurately manufactured and closely matched to each other, so that the inner and outer port walls of the diaphragm are sufficiently smooth to avoid excessive aerodynamic drag penalties. 
     To properly secure the blade units to the inner and outer diaphragm rings, the radially inner platform portions and the radially outer platform portions of the blade units have radial cross-sections shaped to fit blade unit retaining features in the form of slots or grooves having radial cross-sections with undercut or re-entrant shapes, such as dovetails. In a preferred embodiment, the radially inner and outer platform portions of the blade units are T-shaped in cross-section; for the inner platform portions the cross-bar of the T-shape is positioned radially inwards of the stem of the T-shape, whereas for the outer platform portions, the cross-bar of the T-shape is positioned radially outwards of the stem of the T-shape. 
     The radially inner and outer diaphragm rings may each comprise at least two segments. Preferably, the inner diaphragm ring has an even number of segments comprising at least four segments and the outer diaphragm ring is preferably constructed as two segments that upon assembly are united with each other on joint planes at diametrically opposed sides of the outer diaphragm ring. To avoid the joint planes cutting across the blade unit retaining features in the outer diaphragm ring, the joint planes are pitched at a scarf angle that is the same or closely similar to the stagger angle of the aerofoils. 
     The segments of the outer diaphragm ring may be united with each other by bolted joints. 
     Preferably, either the radially outer platform portions of the blade units, or the blade unit retaining features of the outer ring, or both, are provided with stop features operative against movement of the platform portions relative to the retaining features under the influence of a pressure difference across the diaphragm. 
     Further aspects of the present concept will become apparent from a study of the following description and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the concept disclosed herein will now be described, with reference to the accompanying drawings, in which: 
         FIG. 1A  is a three-dimensional perspective view of an embodiment of the present concept, showing the lower half of the outer ring of an HP or IP steam turbine diaphragm in an initial stage of assembly; 
         FIG. 1B  is an enlarged view of part of  FIG. 1A ; 
         FIG. 2A  is a three-dimensional perspective view on the pressure side of a blade unit ready for incorporation into the steam turbine diaphragm of  FIG. 1 ; 
         FIG. 2B  is a view of the suction side of the blade unit of  FIG. 2A ; 
         FIGS. 3A, 4 and 5  are views showing further stages in the assembly of the HP or IP steam turbine diaphragm, 
         FIG. 3B  is an enlarged view of part of  FIG. 3A . and 
         FIG. 6  is a diagrammatic representation of a further embodiment of the present concept. 
     
    
    
     The drawings are not to scale. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Steam turbine diaphragms are normally constructed by welding their components together, but in accordance with the present concept,  FIGS. 1A, 3A, 4 and 5  show a high pressure steam turbine diaphragm  10  and which is constructed without welding or other fusion or adhesive metal joining techniques. Referring first to  FIG. 5 , which shows the diaphragm after it has been assembled having a major axis X-X, the diaphragm  10  comprises a radially inner diaphragm ring  12 , a radially outer diaphragm ring  14  and an annular array of static blade units  16  arranged between the inner and outer rings. The illustrated embodiment is a diaphragm with a radially compact type of construction, which has a much reduced radial thickness of its inner diaphragm ring  12  compared with the more robust type of construction traditionally used for large steam turbines. In fact, as will be apparent from the description below, the inner diaphragm ring  12  of the illustrated embodiment is effectively part of all the inner platform port wall surfaces of the static blade units  16 . However, the concept discussed herein is also applicable to diaphragms having inner rings which are radially thicker than the one illustrated and/or which do not form part of the inner platform surfaces. 
     To enable assembly of the diaphragm into a turbine, the outer ring  14  is constructed in two segments, an upper half  141  and a lower half  142 , the two segments being united with each other at joint planes J. Of course, the number of segments in the outer ring  14  is at the option of the designer, consistent with requirements for cost-effective manufacture and assembly of the diaphragm  10 . In the illustrated embodiment, the joint planes J exhibit a scarf angle θ, i.e., they are inclined away from alignment with the axial direction of the assembly (the axial direction being defined by reference to the major axis X-X of the diaphragm), as explained later, and the joint is bolted at  18  on diametrically opposite sides of the outer ring  14 . 
     Returning to  FIG. 1A , bolt guide spacers  181 ,  182  are shown poised for insertion into bores  183  in planar joint faces  143 ,  144  of the lower half  142  of the outer diaphragm ring  14 . Each bolt guide spacer  181 ,  182  essentially comprises a dowel having a bore which will allow the bolts of the bolted joint  18  to pass through it, and an external diameter which allows a push-fit into the bore  183 . The bolt guide spacers  181 ,  182  are necessary because of the scarf angle of the joint faces. 
     As shown in  FIG. 4 , each planar joint face  143 ,  144  mates with a complementarily inclined planar joint face  145 ,  146  on the upper half  141  of the outer ring  14 , and a projecting part of each bolt guide spacer  181 ,  182  fits inside a complementarily dimensioned bore  184  in the upper half  141  of the outer ring  14 . Note that the outer circumference of the upper half  141  of the outer ring  14  is specially recessed at  150  on opposite sides of the ring  14  to allow insertion of bolts  185  into the bores, which run tangentially of the ring. Only a distal end portion  186  of each bolt  185  is provided with a screw thread, which screws into a complementarily threaded portion of each bore  183  in the lower half  142  of outer ring  14 . 
     When installed in the turbine, the bottom half of the outer ring (and hence the entire diaphragm) is supported within a surrounding turbine casing (not shown) by means of cross-key location features  140 , as known in the industry. 
     Referring again to  FIG. 1A , a blade unit  16  is shown poised for insertion into the lower half  142  of the outer ring  14 . Each blade unit comprises an aerofoil portion  161 , an inner platform portion  162  that engages the radially inner ring  12 , and an outer platform portion  163  that engages the radially outer ring  14 . To enable locking together of the components of the diaphragm without the use of welding or other fusion or adhesive metal-joining techniques, the inner platform portion  162  is elongate in the circumferential direction of the inner ring  12 , whereas the outer platform portion  163  is elongate in a direction which is generally transverse of the inner platform portion and compatible with the stagger angle of the blade aerofoils. Hence, when the diaphragm is fully assembled, the inner and outer platform portions  162 ,  163  are effectively cross-keyed relative to each other in the inner and outer rings  12  and  14  respectively, thus stabilising the blade units  16  within the diaphragm structure. 
     To retain the outer platform portions  163  of the static blade units  16  in engagement with the outer ring  14 , an inner circumference of the radially outer ring is provided with blade unit retaining features  147  in the form of an array of angularly spaced-apart slots, each slot  147  being of complementary shape to the corresponding outer platform portion  163  of a static blade unit  16 . In the illustrated embodiment, the outer platform portions  163  are T-shaped in cross-section, as shown more clearly in  FIG. 2A . As will be more clearly seen from  FIG. 1B , the slots  147  are also T-shaped, so that each T-shaped platform portion  163  fits inside an equivalent T-shaped slot  147  in the inner circumference of the radially outer ring  14 . 
     It should be understood that the slots  147  and the outer platform portions  163  of the static blade units  16  could be other than T-shaped in cross-section, e.g., dove-tail shaped or some other undercut or re-entrant shape that securely retains the blade units in an interlocking manner. It should also be appreciated that the slots  147  and the outer platform portions  163  of the static blade units  16  are oriented to match, or closely approximate, the stagger angle of the aerofoils  161 . Hence, the planar joint faces  143  to  146  must be pitched at the same or a closely similar angle to the stagger angle in order to avoid the joint planes J ( FIG. 5 ) cutting across any of the slots  147  in the outer ring  14 . 
     Referring to  FIGS. 1 and 5 , when the fully constructed diaphragm is part of a functioning turbine, the edges  164  of the aerofoils  161  will be their leading edges at the steam inlet side of the diaphragm and the edges  165  will be their trailing edges at the steam outlet side of the diaphragm. Hence, there will be a pressure drop across the diaphragm in the axial direction from the leading edges to the trailing edges of the aerofoils  161 . To secure the outer platform portions  163  of the static blade units  16  against movement in the slots  147  under the influence of the pressure difference between the inlet and outlet sides of the diaphragm, a stop feature  166  is provided at the inlet end of each outer platform portion  163 . In the illustrated embodiment, the stop feature  166  is in the form of a step that projects radially outwards of the rest of the platform portion  163  and fits into a matching complementary step  148  ( FIG. 1B ) cut into the inlet end of the slots  147 . Alternative stop features could be used; e.g., a step at the outlet end of the slot  147 , the step projecting radially inwards of the radially outer part of the slot and fitting into a matching complementary step cut into the outlet end of the outer platform portion  163 . 
     Returning to a consideration of  FIG. 5 , whereas the outer ring  14  comprises two segments in the form of upper and lower half rings  141 ,  142 , the inner ring  12  comprises four segments, each of ninety degrees of arc, i.e., two segments  121  in the upper half  122  of the inner ring and two segments  121  in the lower half  123  of the inner ring. Although in the illustrated embodiment, the inner ring  14  is made up of four segments to make assembly easier, it would also be possible for the inner ring to comprise only two segments, namely an upper half  122  and a lower half  123 . The number of segments in the inner ring  12  is at the option of the designer, consistent with requirements for cost-effective manufacture and assembly of the diaphragm  10 . 
     Turning now to  FIG. 3A , a segment  121  of the inner ring  12  is shown poised for attachment to the inner platform portions  162  of an assembled half ring of the static blade units  16 . Each segment  121  has a blade unit retaining feature in the form of a circumferentially extending slot  124  in the outer circumference of the segment. Attachment of segment  121  to the inner platform portions  162  is achieved by sliding the slot  124  in segment  121  onto the inner platform portions  162  of the static blade units  16 , the slot  124  being complementary in shape to the inner platform portions  162 . In the illustrated embodiment, the inner platform portions  162  are T-shaped in cross-section, as shown more clearly in  FIGS. 2A and 2B , so that each T-shaped platform portion  162  fits inside the T-shaped slot  124  in the outer circumference of the inner ring  12 , the slot  124  being shown more clearly in  FIG. 3B . 
     It should be understood that the slot  124  and the inner platform portions  162  of the static blade units  16  could be other than T-shaped in cross-section, e.g., dove-tail shaped or some other undercut or re-entrant shape that securely retains the blade units in an interlocking manner. 
     In the radially compact embodiment of  FIG. 3B , the radially inner side of each segment  121  of the radially inner ring  12  is configured as a labyrinth seal  127  for sealing directly against a rotor when the diaphragm has been assembled into a turbine, the seal being necessary to restrict leakage between relatively high and low pressure sides of the diaphragm. However, in less radially compact constructions, it is conventional for the radially inner side of a radially inner diaphragm ring to comprise a circumferentially extending recess configured to retain a separate seal therein, as shown diagrammatically in  FIG. 6 , which represents a fragmentary radial section through an inner ring  20  that is radially thicker than the inner ring  12 , and therefore can carry segments of a separately formed labyrinth seal  22  in a dovetail-shaped slot  201  or other undercut or re-entrant shape machined in its radially inner side, the radially outer side of the inner ring  20  being engaged by the inner platform portions  162  of the static blade units  16 , as previously described. As well known in the industry, other types of seal, such as brush or leaf seals, may be substituted for the labyrinth seal, and/or provision may be made for the seal to be spring-mounted in the slot  201 , so that it can automatically adjust to variations in the clearance between the inner ring  20  and the rotor surface (not shown) against which the seal acts. 
     In the traditional type of platform construction for steam turbine diaphragms, the blade units are machined as single components complete with aerofoils and inner and outer platforms, so that when the platforms are welded onto their respective inner and outer rings, the inner platforms combine to create an inner port wall and the outer platforms combine to create an outer port wall. It will be appreciated from the drawings and the above description that the present concept for platform construction is distinct from the traditional type, in that the inner and outer blade platforms are reduced to elongate attachment features  162 ,  163  that are retained in complementary-shaped blade-retaining features  124 ,  147  provided in the inner and outer diaphragm rings  12 ,  14 . In the assembled diaphragm  10 , the radially outer platform portions  163  of the blade units  16  are elongate in directions compatible with the stagger angle of the blade aerofoils  161 , whereas the radially inner platform portions  162  of the blade units  16  are elongate in the circumferential direction of the inner ring  12 . In the embodiment of the present concept illustrated in  FIG. 5 , the radially outer port wall of the diaphragm comprises the radially outer elongate platform portions 163  of the blade units  16 , alternating in the circumferential direction with exposed portions  149  of the inner circumference of the outer diaphragm ring  14 . In contrast, the radially inner port wall of the diaphragm  10  comprises the radially inner elongate platform portions  162  of the blade units  16 , flanked on their axially opposed (inlet and outlet) sides by exposed portions  126  (see also  FIG. 3B ) of the outer circumference of the inner diaphragm ring  12 . The ends of the elongate platform portions  162  butt up to each other when inserted into the inner ring  12 , so that the platform portion  162  extend continuously around the inner port wall in the circumferential direction, as do the exposed portions  126  of the inner diaphragm ring  12 . 
     It is important that the inner and outer port walls of the diaphragm are sufficiently smooth to avoid excessive aerodynamic drag penalties, and to this end the platform portions of the blade units and the blade retaining features of the inner and outer rings should be accurately manufactured and closely matched to each other with regard to their dimensions and surface finishes. 
     A sequence of assembly of the diaphragm  10  will now be described with reference to the Figures. 
     (a) The individual components of the diaphragm  10  are produced to final shape before assembly. 
     (b) As shown in  FIG. 1A  for the lower half  142  of the outer diaphragm ring  14 , the static blade units  16  are attached to the upper and lower halves of the outer ring  14  by sliding the outer platform portions  163  of the blade units fully into the slots  147  in the inner circumference of the outer ring. 
     (c) Either before or after insertion of the blade units  16  into the outer ring  14 , the bolt guide spacers  181 ,  182  may be inserted into the bores  183  in the lower half  142  (or upper half  141 ) of the outer diaphragm ring  14 . 
     (d) As illustrated in  FIG. 3A  for the lower half  142  of the outer ring  14 , when all the static blade units  16  are attached to one of the half rings, their inner platform portions  162  form a continuous circumferentially extending track, ready to receive the segments  121  of the inner ring  12 . Hence, the next stage of assembly is to attach the four segments  121  of the inner diaphragm ring  12  to the blade units  16  by sliding the T-shaped slot  124  in the outer circumference of the segments onto the T-shaped inner platform portions  162  of the blade units  16 . When the lower and upper halves of the inner ring have been attached to the inner platform portions  162  of the blade units  16 , sliding of the segments  121  relative to the inner platform portions  162  is prevented by inserting anti-rotation stop features (not shown) at the ends of the segments. Such stop features could for example comprise a step at the end of the slot  124 , which—when the segment  121  in  FIG. 3A  has been fully pushed onto the inner platform portions  162 —butts up against an end face  167  of the platform portion nearest the joint between the top and bottom halves of the diaphragm. 
     (e) After building up both the top and the bottom halves of the diaphragm  10  independently of each other, they can be joined together as indicated in  FIG. 4  by sliding the bores  184  in the top half  141  of the outer ring  141  onto the bolt guide spacers  181 ,  182 , then inserting the bolts  185  into the bores  184 , passing them through the hollow bolt guide spacers and into the bores  183  in the bottom half  142  of the outer ring  141 , and finally screwing the bolts fully home into the bottom threaded portions (not shown) of bores  183 . 
     (f)  FIG. 5  shows the fully assembled diaphragm  10 , which can easily be split into two halves for assembly into the turbine by removing the bolts  185 . 
     Adoption of the concept proposed herein confers the following advantages.
         The need for welding or other metal melting techniques in the construction of the diaphragm is completely eliminated, with consequent saving of costs and reduced manufacturing time.   Elimination of welding eliminates a possible source of defects in the structure of the diaphragm.   The type of welding normally used in the construction of diaphragms normally comprises deep penetration welds requiring advanced and expensive laser or electron beam welding equipment. Elimination of welding therefore allows more choice in the selection of production facilities for construction of turbine diaphragms.       

     The above embodiments have been described above purely by way of example, and modifications can be made within the scope of the appended claims. Thus, the breadth and scope of the claims should not be limited to the above-described exemplary embodiments. Each feature disclosed in the specification, including the claims and drawings, may be replaced by alternative features serving the same, equivalent or similar purposes, unless expressly stated otherwise. 
     For example, it is possible to envisage a diaphragm construction in which radially inner platform portions of the static blade units are retained to an inner diaphragm ring by means of bolts, or the like, instead of by an interlocking arrangement as described above. 
     Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, and the like, are to be construed in an inclusive as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.