Abstract:
The invention concerns a gas turbine having a picture frame, a first vane, and a sealing arrangement to seal a gap between the picture frame and the first vane, the sealing arrangement including two seals arranged in series between the picture frame and the first vane. In exemplary embodiments, one of the seals is a honeycomb seal, a dogbone seal, a hula seal or a piston seal and the other seal is a honeycomb seal, a dogbone seal, a hula seal or a piston seal. A method of supplying cooling fluid to the gap between the picture frame and the first vane is also disclosed.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to sealing arrangements in gas turbines, and particularly to sealing arrangements comprising two seals to seal the gap between the first vane and the picture frame. 
       BACKGROUND OF THE INVENTION 
       [0002]    In a gas turbine, there is a gap between the picture frames in the combustor outlet and the first vane of the turbine. Movement of the two parts relative to one another can be considerable, and a gap must be left between the parts to avoid contact. The gap is generally purged with cooling air. It has been appreciated that it would be advantageous to improve the design around this gap to, for example, reduce the cooling air requirements. 
       SUMMARY OF THE INVENTION 
       [0003]    The invention is defined in the appended independent claims to which reference should now be made. Advantageous features of the invention are set forth in the dependent claims. 
         [0004]    According to a first aspect of the invention, there is provided a gas turbine comprising a picture frame, a first vane, and a sealing arrangement to seal a gap between the picture frame and the first vane, the sealing arrangement comprising two seals arranged in series between the picture frame and the first vane. This can help to reduce leakage and hot gas ingestion in the gap between the picture frame and the first vane, particularly on the inner platform (inner diameter) between each picture frame and the first vane (rocking vane). In particular, this can seal the gap during all operating conditions despite first vane movements in both the axial and radial directions (compared to the gas turbine longitudinal axis). The noble parts (first vane and picture frame) of the gas turbine are not affected, and the solution can be retrofitted to existing turbines. Use of two seals can also provide redundancy, so that the gap can still be sealed if one of the seals fails or the performance of one or both the seals deteriorates. The seal closest to the hot gas flow can reduce hot gas ingestion in particular, and the seal furthest from the hot gas flow can reduce and/or control the flow of cooling fluid in particular. 
         [0005]    In one embodiment, one of the seals is a honeycomb seal, a dogbone seal, a hula seal or a piston seal and the other seal is a honeycomb seal, a dogbone seal, a hula seal or a piston seal. A piston seal can compensate for both axial and radial displacement, as the piston can be pushed up against the first vane by pressure behind the piston seal in the piston volume. In one embodiment, one of the seals is a honeycomb seal, the first vane comprises a sealing portion arranged to seal the gap in combination with the honeycomb seal. 
         [0006]    In one embodiment, at least one of the seals is a hula seal, and at least one of the at least one hula seals is a conical hula seal. A conical hula seal can be particularly suited to cope with vane movements in both the axial and radial directions (compared to the gas turbine longitudinal axis). 
         [0007]    In one embodiment, the conical hula seal comprises an inner part, a plurality of fingers and an outer part, wherein the inner part is attached to one end of each of the plurality of fingers and the outer part is attached to the other end of each of the plurality of fingers. 
         [0008]    In one embodiment, at least one of the seals is a dogbone seal, and the picture frame comprises a socket for holding the dogbone seal. In one embodiment, at least one of the seals is a piston seal, the piston seal comprises a piston seal front end for contacting a first vane, the majority of the piston seal front end is at least half the width of the widest part of the piston seal in a radial direction relative to a gas turbine longitudinal axis, and the piston seal front end has a tapered portion for contacting the first vane. 
         [0009]    In one embodiment, the first vane comprises a first vane contact surface to contact at least one of the seals. In one embodiment, the first vane contact surface is parallel or substantially parallel to a surface of the picture frame on the opposite side of the gap. 
         [0010]    In one embodiment, the first vane contact surface is conical or spherical. This can help better seal the gap. In one embodiment, the first vane contact surface is angled such that an elastic range of movement of the first vane relative to the picture frame is minimised. This can minimise the gap width and make sealing the gap easier. 
         [0011]    According to a second aspect of the invention, there is provided a method of cooling a gas turbine comprising a picture frame, a first vane, and a sealing arrangement to seal the gap between the picture frame and the first vane, the sealing arrangement comprising two seals arranged in series between the picture frame and the first vane, comprising the step of supplying cooling fluid to the gap between the picture frame and the first vane. This can purge the gap to reduce hot gas ingestion. 
         [0012]    In one embodiment, the method comprises the step of maintaining a higher pressure at the end of the gap furthest from a hot gas flow than the pressure at a hot gas flow end of the gap. 
         [0013]    In one embodiment, at least one of the seals is a piston seal and the method comprises the step of supplying cooling fluid to a volume between the piston seal and the picture frame such that the piston seal remains in contact with the first vane. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings in which: 
           [0015]      FIG. 1  shows a cross-section of an embodiment of the invention comprising a honeycomb seal and a conical hula seal; 
           [0016]      FIG. 2  shows a cross-section of an embodiment of the invention comprising a honeycomb seal and a dogbone seal; 
           [0017]      FIG. 3  shows a cross-section of an embodiment of the invention comprising two honeycomb seals; 
           [0018]      FIG. 4  shows a perspective view of a conical hula seal on a bulkhead; 
           [0019]      FIG. 5  shows a top view of the conical hula seal of  FIG. 4 ; 
           [0020]      FIG. 6  shows graphically the movement of a point on the first vane surface during gas turbine operation; 
           [0021]      FIG. 7  shows a cross-section view of a piston seal; and 
           [0022]      FIG. 8  shows a cross-section view of an alternative piston seal. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]      FIG. 1  shows a first vane  1  (rocking vane) on one side of a gap  24 ,  26 ,  44 ,  46 , to be sealed, with a first sealing portion  2  and a second sealing portion  7  (vane tooth). On the other side of the gap  24 ,  26 ,  44 ,  46  to be sealed is a picture frame  4  and a bulkhead  5  attached to the picture frame  4 . The bulkhead  5  is shown as a separate component to the picture frame  4  in the embodiments in this application, but the bulkhead  5  may also be an integral part of the picture frame  4 . In the gap  24 ,  26 ,  44 ,  46  between the first vane  1  and the picture frame  4  and bulkhead  5 , a first seal (honeycomb seal  3 ) and a second seal (conical hula seal  6 ) are arranged. 
         [0024]    The honeycomb seal  3  is arranged between the picture frame  4  and the bulkhead  5  on one side of the gap  44 ,  46 . The first sealing portion  2 , which is a protrusion extending from the first vane  1 , extends across the gap  44 ,  46  to interact with the honeycomb seal  3  and seal the gap  44 ,  46 . 
         [0025]    The conical hula seal  6  extends between the second sealing portion  7  and the bulkhead  5 . Preferably, the surface  12  of the bulkhead adjacent to the conical hula seal  6  and the surface  14  of the second sealing portion  7  adjacent to the conical hula seal  6  are parallel or substantially parallel (in use they will not always be parallel), both extending in a hula seal direction  16 , the hula seal direction  16  being at an angle from the longitudinal axis direction  18 . The longitudinal axis direction  18  is the direction of the longitudinal axis of the gas turbine (not shown), which is generally also parallel to the axis of the first vane  1  and the picture frame  4 . 
         [0026]    In  FIG. 2 , an alternative embodiment is shown in which a dogbone seal  8  is provided as the second seal rather than a conical hula seal. The dogbone seal  8  comprises a bulkhead portion  54  adjacent to the bulkhead, a first vane portion  56  adjacent to the second sealing portion  7 , and a central portion  58  joining the bulkhead portion and the first vane portion. The bulkhead portion  54  and the first vane portion  56  are typically cylindrical. The bulkhead  5  also differs from the bulkhead in  FIG. 1  in that a socket  20  is provided to hold the dogbone seal  8 . In  FIG. 2 , the last part of the gap  24  is effectively part of the compressor plenum. 
         [0027]    In  FIG. 3 , an alternative embodiment is shown in which a second honeycomb seal  9  is provided as the second seal. The bulkhead  5  comprises a recess  30  in which the second honeycomb seal  9  is placed. In addition, the second sealing portion  7  comprises a second sealing portion nose  10 . This second sealing portion nose  10  has the same function as first sealing portion  2 . The first seal (honeycomb seal  3 ), as in the embodiments in  FIGS. 1 and 2 , is provided to seal a first gap  44 ,  46  which extends in a direction  34  perpendicular to the longitudinal axis direction  18 . The second seal (second honeycomb seal  9 ) is provided to seal a second gap  24 ,  26  which extends in a direction  38 , the direction  38  being at an angle A from the longitudinal direction  18 . 
         [0028]    Hula seals such as that shown in  FIG. 1  will now be described in more detail. In annular hula seals, the hula seal describes a ring in which the inner part and the outer part are at the same distance from the central axis of the hula seal (i.e. the axis through the centre of the hula seal, which is the hula seal longitudinal axis in the longitudinal axis direction  18 ). In contrast, a conical hula seal has an inner part that is closer to the central axis of the hula seal than the outer part. In other words, the hula seal direction in an annular hula seal is parallel to the longitudinal axis direction  18  (hula seal longitudinal axis), whereas the hula seal direction in a conical hula seal is at an angle from the longitudinal axis direction  18 . 
         [0029]    A cone is a three-dimensional geometric shape that tapers smoothly from a flat base; the base may be circular or may be another shape. Mathematically, a conical hula seal is conical frustum, being in the shape of the surface of the frustum of a cone (i.e. a section of the cone not including the apex, also known as a truncated cone), in contrast to an annular hula seal in which the hula seal follows the shape of the surface of a cylinder. It should be noted for completeness that hula seals do not strictly describe a precisely annular or conical shape, as can be seen in  FIG. 3  for example, where a conical hula seal  6  follows a hula seal direction  16  but the curvature of the fingers deviates slightly from a perfect cone. 
         [0030]      FIGS. 4 and 5  show a conical hula seal  6  attached to a bulkhead  5 , which is in turn attached to or part of a picture frame  4  (see  FIG. 1 ). The conical hula seal  6  comprises an inner part  64  extending in a circumferential direction  69  relative to a longitudinal axis direction  18  (hula seal axis direction, see  FIG. 1 ). The inner part  64  is configured to be attached to the surface  12  (see  FIG. 1 ) of the bulkhead  5 . A plurality of fingers  66 , each attached to the inner part  64 , extend away from the inner part  64  in a hula seal direction  16  (see  FIG. 1 ). An outer part  68  is provided at the distal end of the fingers  66  from the inner part  64 . The conical hula seal extends in a hula seal direction  16  (see  FIG. 1 ) at an angle A from the longitudinal axis direction  18 . A cross section of the conical hula seal  5  along the line I-I in  FIG. 5  would correspond to the view of the conical hula seal  5  in  FIG. 1 . It is noted that the bulkhead of  FIG. 4  is a different design to the bulkhead of  FIG. 1 . 
         [0031]    The fingers  66  do not extend in a straight line, but curve such that the direction of each finger describes an angle greater than angle A (see  FIG. 1 ) nearer the inner part  64  and an angle smaller than angle A near the outer part  68 . The outer part  68  is arranged to slidably contact the surface  12  (see  FIG. 1 ) of the bulkhead  56 . 
         [0032]    The hula seal may be an entire ring or may be made up of multiple individual hula seals (hula seal segments). The hula seal of  FIG. 5  is a hula seal segment. In one example, 20 hula seals are placed around the 360° annular joint, each extending 15° around the circumference in the circumferential direction  40 . The other types of seals described herein may also be segmented in this way. 
         [0033]    In a gas turbine, a plurality of picture frames (sequential liner outlets) are arranged around the longitudinal axis of the gas turbine in a ring, with the picture frames typically being attached to sequential liners and with the sequential liners surrounding can combustors. In other words, the picture frames all intersect a plane perpendicular to the longitudinal axis. The sealing solution discussed in this application can be applied on the inner diameter of the picture frames (the edge of the picture frames closest to the longitudinal axis). Preferably, each picture frame has a separate seal segment or segments; that is, seal segments do not extend across multiple picture frames in the circumferential direction relative to the longitudinal axis. This can allow for single vane/blade assembly and/or disassembly during manufacture, maintenance and repair. 
         [0034]    The sealing solution discussed in this application is preferably for sealing a gap between two static parts such as the gap between the picture frame and the first vane. 
         [0035]    When in use, the second sealing portion surface  14  (more specifically, a particular spot on the second sealing portion surface  14 ) will move along a path similar to that shown in  FIG. 6  during its operation cycle. The range of movement fits within an angled rectangular box  148 , angled at an angle α from the longitudinal axis direction  18 . Preferably, angle A (see  FIGS. 1 and 3 ) is the same or substantially the same as angle α. There are two directions of movement, the sliding range  158  and the elastic range  159 . Movement in the direction of the sliding range  158  can be due to rocking (rotation) of the first vane, for example, and movement in the direction of the elastic range  159  can be due to thermal expansion/contraction of components, for example. 
         [0036]    Initially, at the assembly or cold position  150  in  FIG. 6 , the first vane  1  is in a certain position. During startup, the first vane starts moving in a direction as shown by movement line  156 , passing through point  151  which may be a first maximum extent in the sliding range. At full load, the first vane position may be at point  152 , and during steady state operation the first vane position may be at point  153 . During operation, the loading may vary, and the first vane position may vary accordingly, moving on or near the movement line  156  in the area between and around points  152  and  153 . On shut down, the first vane then cools and continues further around the movement line  156 . During shut down, the first vane reaches a second maximum extent on the sliding range at point  154 . Once fully cooled, the first vane will have completed a full circle of movement line  156 , arriving back at its cold position at point  150 . The movement line  156  of  FIG. 6  and the description of the points on the curve are an approximation, and deviations may occur from this approximation. 
         [0037]    During assembly of the gas turbine, the sealing portion  2  is crushed into the honeycomb seal  3  and generally stays in contact during operation. Similarly, the second sealing portion nose  10  would be crushed into the honeycomb seal  9 . 
         [0038]    In use, cooling fluid may be supplied to the gap between the picture frame and the first vane. This can help reduce hot gas ingestion and can purge the gap. 
         [0039]    A further alternative seal is shown in  FIG. 7 , namely a piston seal  15 . As with the other seals described herein, the piston seal  15  is designed to seal the gap  24 ,  26  between the second sealing portion  7  of the first vane  1  and the bulkhead  5 , which is attached to the picture frame  4 . At a front end of the piston seal, a piston seal front end  107  is arranged to contact the second sealing portion surface  14 ; the front end of the piston seal is shaped like one half of a dogbone seal. A back end  105  of the piston seal  15  is arranged in a piston volume  102  of the bulkhead  5 . The piston seal  15  can move relative to the bulkhead  5 . At a back end  104  of the piston volume  102 , a source of pressure (not shown) such as a source of cooling fluid is provided. Optionally, a notch  108  for an additional seal (not shown; for example a rope seal) is provided in one or both of the side walls of the piston. This notch could alternatively be in one of both of the side walls of the piston volume. This additional seal can help reduce leakage past the back end  105  of the piston seal  15 . 
         [0040]      FIG. 8  shows an alternative embodiment of the piston seal of  FIG. 7 , with an alternative shape of piston seal front end  107 . In this design, a majority of the extent (preferably at least 75%) of the piston seal front end (i.e. the part of the piston seal extending across the gap to contact the first vane) is more than half the width of the piston volume (the width of the piston volume in the direction perpendicular to the longitudinal axis direction  18 ; this is normally the same or very similar to the width of the main part of the piston seal). This design may be more rigid than the embodiment in  FIG. 7 . The part of the front end  107  adjacent to the first vane is preferably tapered. The taper is preferably a convex shape; this can allow better contact with the first vane over a range of operating conditions. 
         [0041]    The piston seal  15  may be various shapes besides the examples in  FIGS. 7 and 8 . 
         [0042]    As with many of the other embodiments described herein, the second sealing portion surface  14  in  FIG. 6  is preferably substantially parallel to the bulkhead surface, on the opposite side of the gap. 
         [0043]    The piston seal embodiment in  FIGS. 7 and 8  is shown with a piston volume  102  in the bulkhead  5  parallel to longitudinal axis direction  18 , but the piston volume  102  may be angled in other directions, for example perpendicular to the second sealing portion surface  14 . The piston seal would also be re-orientated accordingly in line with the piston volume. 
         [0044]    The depth of the piston volume  102  from the bulkhead surface  12  to the back end  104  of the piston volume  102  may be determined during a design phase based on the length of the piston seal  15  and the expected size of the gap between the bulkhead surface  12  and the second sealing portion surface  14 . 
         [0045]    When in use, the pressure in the cavity in between the back end  104  of the cavity  102  and the back end  105  of the piston seal  15  is preferably higher than the pressure in the gap  24 ,  26  on either side of the piston seal  15 . At a minimum, the pressure at the back end  105  should be high enough to keep the piston seal front end  107  adjacent to the second sealing portion surface  14 . The honeycomb seal  3  and/or the second honeycomb seal  9  may also have a similar pressure profile, with a source of pressure such as a source of cooling air provided to push the honeycomb seal  3 /second honeycomb seal  9  against the first sealing portion and second sealing portion nose respectively. 
         [0046]    A limited number of specific embodiments are described above. More generally, various sealing solutions can be used to seal the gap between the picture frame  4  and the first vane  1 . For the first seal, a honeycomb seal is shown in  FIGS. 1 to 3 , but any of the other seals described in this application could also be used as the first seal. Similarly, any of the seals in this application could be used as the second seal. Any combination of seals is also possible. For example, both the first and second seals could be conical hula seals, with the picture frame and first vane designed so that the picture frame and first vane have the structure shown surrounding the conical hula seal in  FIG. 3  for both the first seal and the second seal. Three or more seals in series could also be provided. 
         [0047]    The first and second seals would be in series; that is, the second seal would be in a different part of the gap to the first seal in the axial direction perpendicular to the longitudinal axis direction  30 . In other words, any escaping hot gas from the hot gas flow would have to pass through both the conical hula seal and the second seal. 
         [0048]    The seals described herein may be an entire ring or may be made up of multiple individual seals (seal segments). For example, the conical hula seal  6  of  FIG. 4  is a hula seal segment. In one example, 20 seals are placed around the 360° annular joint, each extending 15° around the circumference in the circumferential direction  69 . 
         [0049]    Each seal can describe a partial or full ring as described above. The longitudinal axis direction  18  will typically be perpendicular to the plane of this partial or full ring. For seal segments, the longitudinal axis of each seal segment is the longitudinal axis of a full ring of seal segments. The seal longitudinal axis will generally also be the longitudinal axis of the picture frame at the end of the sequential liner (the combustor outlet) and/or the gas turbine longitudinal axis. 
         [0050]    The first sealing portion  2  and the additional seal portion  10  may be various shapes; these parts are required to extend across the gap from the first vane to interact with a honeycomb seal. Similarly, the socket  20  could vary in shape depending on the shape of the dogbone seal and the recess  30  could vary depending on the shape of the second honeycomb seal and the bulkhead. 
         [0051]    The bulkhead  5  is shown varying in shape in the different embodiments shown in the Figures to accommodate the different types of seal. These bulkhead shapes are examples, and other bulkhead shapes are also possible. For example, a bulkhead of the shape shown in  FIG. 1  could be amended by the addition of a socket  20  for the dogbone seal but otherwise retain its shape. The bulkhead surface, second sealing portion surface and first vane shape can also vary depending on the seals used. 
         [0052]    The vane tooth  7  is a part of the first vane  1  in the embodiments above, but may also be a separate component attached to the first vane. The first sealing portion  2  and the additional seal portion  10  may also each be either an integral part of the first vane  1  or separate components attached to the first vane. 
         [0053]    The honeycomb seal  3  as a first seal is shown between the picture frame and the bulkhead in the embodiments described above. Alternatively, the honeycomb seal  3  could be attached only adjacent to the picture frame or only adjacent to the bulkhead. 
         [0054]    In  FIG. 3 , the honeycomb seal  9  as a second seal is angled at an angle A from the longitudinal axis direction  18 . Alternatively, a second honeycomb seal at a different angle could be used, for example a honeycomb seal similar to the honeycomb seal as a first seal in  FIGS. 1 to 3  or a honeycomb seal at an angle parallel to the longitudinal axis direction. The design of the bulkhead  5  and the second sealing portion  7  would need adjusting accordingly. Similarly, the honeycomb seal  3  as a first seal of any of the embodiments could be angled in a different direction to that shown in  FIGS. 1 to 3 . 
         [0055]    The honeycomb is generally orientated so that it can elastically deform under the pressure of the first sealing portion  2  or the second sealing portion surface  14 . The lines shown in the honeycomb seal  3  and second honeycomb seal  9  show the walls of the honeycomb; that is, the hexagonal nature of the honeycomb structure would be seen if the seal was viewed in the direction of the lines shown in the honeycomb. The honeycomb seal  3  shown in the Figures is aligned such that the hexagonal honeycomb structure extends in a circumferential direction relative to the gas turbine longitudinal axis. The second honeycomb seal  9  shown in  FIG. 9  has the hexagonal honeycomb structure at a different angle, extending perpendicular to the second sealing portion surface  14 . 
         [0056]    Although only a conical hula seal is shown in the Figures, an annular hula seal may also be used. A radial hula seal could also be used, which is the extreme case where the hula seal direction is perpendicular to the longitudinal axis direction  18 . In the case of a conical hula seal, the hula seal direction  16  can be at an angle A other than that shown in the examples, and is preferably set such that angle A is within 15° of angle α, more preferably within 5°, and most preferably at an angle A=α. This is the angle that minimises the relative movement of the first vane and the picture frame/bulkhead during gas turbine operation. This minimises the elastic range  159  and thus enables the gap between the first vane and the picture frame/bulkhead, specifically between the second sealing portion surface  14  and the bulkhead surface  12 , to be minimised. This minimises the required range of movement of the conical hula seal. Similarly, the bulkhead surface  12  associated with the conical hula seal is preferably at an angle within 15° of angle α, more preferably within 5°, and most preferably at an angle A=α, and the second sealing portion surface is preferably within 15° of angle α, more preferably within 5°, and most preferably at an angle A=α. 
         [0057]    The dogbone seal  8  is shown as extending parallel to the longitudinal axis direction  18  in  FIG. 2 , but may also extend at an angle to the longitudinal axis direction  18 . Other shapes of dogbone seal could be used besides the example described above; the same is true for the hula seals, dogbone seals, piston seals and honeycomb seals described herein. 
         [0058]    In the embodiment of  FIG. 2 , a further socket (not shown) may be provided in the second sealing portion  7  to hold the dogbone seal  8  in place. When the dogbone seal is held at both ends in this way, it will generally need to be made of an extendable material, as the distance between the two sockets will vary when in use. When the dogbone seal is held at only one end, it is generally necessary for the gap to be narrower on the side of the dogbone seal that is at a lower pressure. The further socket (not shown) is optional because the pressure difference can hold the seal in place—the volume  24  (of the gap between the picture frame  4 /bulkhead  5  and the first vane  1 ) that is closer to the longitudinal axis than the dogbone seal  8  is at a higher pressure than the volume  26  (of the gap between the picture frame  4 /bulkhead  5  and the first vane  1 ) further from the longitudinal axis than the dogbone seal  8 . The pressure difference, along with the shape of the gap, which is narrower on the side of the dogbone seal that is at a lower pressure, keeps the dogbone seal in a position that seals the gap. As a result, it is not essential to provide support on the second sealing portion  7 . Alternatively, a socket could be provided in only the first vane  1  (for example in the vane tooth  7 ) and not in the bulkhead  5 . 
         [0059]    Considerable variation is possible in the shape, direction, width and length of the gap between the picture frame and the first vane. The first gap  44 ,  46  largely extends in a direction perpendicular to the longitudinal direction  18 , with the first sealing portion  2  extending into the gap to help seal the gap along with honeycomb seal  3 . As implied above when discussing the angles of the various seals, this gap may extend in a different direction in alternative embodiments. Similarly, second gap  24 ,  26  may extend in a direction other than that shown in  FIGS. 1 to 3 . The shape of the various parts of the first vane (first sealing portion, second sealing portion, second sealing portion nose, second sealing portion surface) may also vary accordingly. 
         [0060]    As with the gap sealed by a dogbone seal  8  in the embodiment in  FIG. 2 , the gaps may also vary in width along their extent. 
         [0061]    For ease of reference, different parts of the gap between the first vane and the picture frame/bulkhead have been denoted with reference numerals. The gap comprises three parts, a first gap  44 ,  46 , an intermediate region  48  and a second gap  24 ,  26 . Dotted lines are shown to delineate where these three regions of the gap could start and finish. The first gap  44 ,  46  between the first vane and the picture frame/bulkhead corresponds to the part of the gap with the first seal, and is divided into a volume  44  adjacent to the hot gas flow (not shown) and a volume  46  on the other side of the first sealing portion  2  to the hot gas flow. 
         [0062]    The pressure is typically higher in the volume  46  on the other side of the first sealing portion  2  to the hot gas flow than in the volume  44  adjacent to the hot gas flow; this can allow for a purging flow to leak through the seal, avoiding hot gas ingestion past the first sealing portion and the first seal. 
         [0063]    There is then an intermediate region  48  of the gap between the first gap  44 ,  46  and the second gap  24 ,  26 , although this intermediate region  48  is optional and the first gap  44 ,  46  and second gap  24 ,  26  may lead directly to one another. 
         [0064]    The second gap  24 ,  26  has a similar structure to the first gap  44 ,  46 , with a volume  24  in the gap on the far side of the second sealing portion nose  10  relative to the hot gas flow and a volume  26  in the gap on the near side of the second sealing portion nose  10  relative to the hot gas flow (adjacent to the intermediate region  48  or the first gap  44 ,  46 ). 
         [0065]    In the case of a piston, dogbone or hula seal, the seal itself would split the first or second gap region rather than the first sealing portion or the second sealing portion nose as shown in the honeycomb seals embodiment of  FIG. 3 . In the case of a hula seal, an additional portion of the volume of the gap is between the hula seal itself and the bulkhead. 
         [0066]    Various modifications to the embodiments described are possible and will occur to those skilled in the art without departing from the invention which is defined by the following claims. 
         [0000]    
       
         
               
             
               
               
             
           
               
                   
               
               
                 REFERENCE SIGNS 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 first vane 
               
               
                 2 
                 first sealing portion 
               
               
                 3 
                 honeycomb seal 
               
               
                 4 
                 picture frame 
               
               
                 5 
                 bulkhead 
               
               
                 6 
                 conical hula seal 
               
               
                 7 
                 second sealing portion 
               
               
                 8 
                 dogbone seal 
               
               
                 9 
                 second honeycomb seal 
               
               
                 10 
                 second sealing portion nose 
               
               
                 12 
                 bulkhead surface 
               
               
                 14 
                 second sealing portion surface (first vane contact surface) 
               
               
                 15 
                 piston seal 
               
               
                 16 
                 hula seal direction 
               
               
                 18 
                 longitudinal axis direction 
               
               
                 20 
                 socket 
               
               
                 24 
                 (partial) volume of the second gap (end of 
               
               
                   
                 the gap furthest from the hot gas flow) 
               
               
                 26 
                 (partial) volume of the second gap 
               
               
                 30 
                 recess 
               
               
                 34 
                 first gap direction 
               
               
                 38 
                 second gap direction 
               
               
                 44 
                 (partial) volume of the first gap (hot gas flow end of the gap) 
               
               
                 46 
                 (partial) volume of the first gap 
               
               
                 48 
                 intermediate region of the gap 
               
               
                 54 
                 dogbone seal bulkhead portion 
               
               
                 56 
                 dogbone seal first vane portion 
               
               
                 58 
                 dogbone seal central portion 
               
               
                 64 
                 inner part 
               
               
                 66 
                 finger 
               
               
                 68 
                 outer part 
               
               
                 69 
                 circumferential direction 
               
               
                 102 
                 piston volume 
               
               
                 104 
                 back end of the piston volume 
               
               
                 105 
                 back end of the piston seal 
               
               
                 107 
                 piston seal front end 
               
               
                 108 
                 notch 
               
               
                 148 
                 angled rectangular box 
               
               
                 150 
                 assembly/cold position 
               
               
                 151 
                 startup 
               
               
                 152 
                 full load 
               
               
                 153 
                 steady state operation 
               
               
                 154 
                 extreme point in shut down 
               
               
                 156 
                 movement line 
               
               
                 158 
                 sliding range 
               
               
                 159 
                 elastic range 
               
               
                 A 
                 angle 
               
               
                 α 
                 angle