Abstract:
A leaktight seal includes plaquettes that are housed between platforms of “hammer legged” vanes and a periphery of a rotor disc. The plaquettes lie above a throat that receives legs between a collar. The plaquettes are completed with heels that also lie between the legs. Centrifugal forces distort the seal and produce excellent leaktightness between the disc and the platforms.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a leaktight seal for a circular vane stage and in particular a stage for “hammer legged” vanes, i.e. vanes being fastened to a disc using a leg with a bulbous cross-section that is inserted into a circular throat provided in a peripheral surface of the disc. 
     2. Description of the Related Art 
     If we refer to FIG. 1, vanes  1  of this kind can be seen on the circumference of discs  2  of a gas turbine stator  3  where they alternate with immobile vanes  4  used to rectify a gas flow and that are fastened to a stator  5 . The vanes  1  comprise a blade  6  that constitutes the active section of the vanes and that lies in a gas flow chamber  7 , a platform  8  used to define chamber  7  and that is adjacent to blade  6 , a leg  9  that is inserted into a circular throat  10  of the relevant disc  2  and a fastener  11  that connects leg  9  to platform  8  and that passes through a collar  12  of throat  10  to reach the outside. The leg  9  has a bulbous cross-section, in other words, it widens from the zone at fastener  11  and given that collar  12  is narrower than the leg, said leg  9  is maintained in throat  10  despite the centrifugal forces. Disc  2  is, however, provided with a hole at a specific point on its circumference that widens collar  12  to enable legs  9  of vanes  1  to be inserted successively in throat  10  through said hole before vanes  1  are slid around disc  2  to reach their definitive position. 
     Re-circulated air may be noticed under platforms  8 , in other words, air that is returned upstream towards the compressors in the machine and the low pressures, in the opposite direction to the flow of gas in chamber  7 . This re-circulated air travels between vane  1  and disc  2  through throat  10  despite the parts being adjustable in this zone. This re-circulation leads to performance loss of the machine. Various kinds of leaktight seals have been put forward in order to reduce the performance loss such as circular Inconel wires  14  that are housed in throats  15  with a small cross-section cut into a peripheral surface  16  of disc  2 , said wires  14  being under platforms  8  next to legs  9 . The drawback with this solution is that the centrifugal forces cause wires  14  to leave throats  15  and move upwards to platforms  8  where they become worn or even cut. A leaktight seal that does not cause this damage to the vanes  1  is therefore preferred, particularly given that vanes  1 , that are generally titanium, are expensive and also that the loss of a piece of a platform  8  can lead to serious damage to the compressor. 
     SUMMARY OF THE INVENTION 
     The present invention provides a suitable solution to the above-mentioned problem. The solution consists in a leaktight seal for a circular vane stage with legs that are inserted into a circular throat of a disc, blades, or platforms that are adjacent to the blades and which lie on a peripheral surface of the disc onto which the throat opens in the form of a circular collar, and fasteners that connect the legs to the platforms, said legs being inserted into the collar. The seal comprises a plate that is housed between the peripheral surface and the platforms, said plate being provided with apertures into which the fasteners are inserted. The plate is divided into plaquettes that are abutted and that are defined by a cutting line through the apertures, said apertures having a surface area that is smaller than a cross section of the vane legs. The seal is characterized in that it comprises heels that are joined to the plaquettes and that together constitute the components of the seal. The heels lie under the upper oblique flanks of the throat. 
     European Patent No. 210,940 describes a leaktight seal the upper section of which lies near the platforms of the vanes and that corresponds to the first part of this definition. This known seal does not, however, include the lower heels which provide additional leaktightness, which is improved like the others by the centrifugal forces present during operation. 
     Leaktightness is improved if the plate has edges that fold towards the inside of the disc and if the edges are inserted into circular grooves that are provided in the peripheral surface of the disc. 
     Excellent construction of the seal, that greatly facilitates assembly, is provided if the cutting lines comprise horizontal and circular lines and if the plaquettes form an obtuse angle with the heels. The horizontal and circular lines join at the heels using joints that are extended with slots between the heels and the plaquettes. The components of the seal are interconnected in pairs with the joint of each of the components of a pair being housed in the slot of the other component of the same pair, thus constituting a hinge. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in greater detail using the following figures that illustrate certain embodiments: 
     FIG. 1 (described above) shows a zone of a gas turbine compressor where the invention may be used, 
     FIGS. 2 and 3 show a first embodiment of the invention, and 
     FIGS. 4 and 5 show a second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention has been added to the apparatus in FIG. 1 except that the wires  14  that it replaces have been eliminated as well as throat  15  that houses said wires. Referring to FIGS. 2 and 3, a seal  17  comprises a band or plate  18  that is more or less annular and that is inserted between the peripheral surface  16  of disc  2  and the platforms  8  when the rotor is assembled. Plate  18 , however, has downward-sloping edges  19 , in other words, edges that are folded towards the inside of the disc  2  and that enter grooves  20  which are provided in the peripheral surface  16 . Finally, the plate  18  is angularly divided along longitudinal cutting lines  21  and also by a circular cutting line  35  that is equidistant between downward-sloping edges  19 . The plate  18  is divided such that it is actually constituted by abutted plaquettes  22  that lie on angular semi-sections. Plaquettes  22  are provided with apertures  23  intended to receive the fasteners  11  of vanes  1 , the surface area of which is smaller than the cross-section of legs  9 . Some plaquettes  22  are provided with further perforations  24  thus extending other perforations  25  that are pierced through corresponding platforms  8 . Perforations  24  and  25  are intended to receive screws (not shown) the heads of which are inserted into the threads of disc  2  in order to immobilize seal  17  and vanes  1 , said seal  17  and vanes  1  being joined by platforms  8  to resist circular movements. An arrangement of this kind is standard with vanes inserted into a common circular throat  10  in the disc  2 . A single pair of perforations  24  and  25  may be sufficient but two pairs may also be envisaged, as in the present embodiment. Characteristically, seal  17  also comprises heels  26  that are respectively connected to plaquettes  22  using joints  27  that combine to form an obtuse angle. In this embodiment, and in order to reduce play, the width of fasteners  11  and legs  9  of vanes  1  (in the angular direction of disc  2 ) is almost equal to that of apertures  23  and the width of heels  26  lies over most of the distance between apertures  23 . Therefore, almost all of throat  10  is occupied. Moreover, joints  27  extend over almost half of the width of heels  26  and leave a slot  28  between each pair of plaquettes  22  and heels  26  into which the joint  27  of another pair of plaquettes  22  and heels  26  is inserted. Plaquettes  22  of the two pairs are joined and extend in the longitudinal direction of the machine. In other words, the pairs of plaquettes  22  and heels  26  are grouped together in pairs that overlap, each heel  26  lying under plaquette  22  of the other component of the pair and the joints  27  lying in an extension along the circular cutting line with sufficient play to constitute a hinge  29 . Once the joints  27  have been inserted into slots  28 , it is therefore possible to fold the assembly thus obtained in order to draw heels  26  closer together and to insert them into throat  10  using an inward movement passing through the collar  12 . Heels  26  are then unfolded to separate from one another, causing the plaquettes  22  to be parallel. The arrangement shown in the figures where heels  26  are in contact with upper oblique flanks  30  of throat  10  is thus achieved. It is understood that the centrifugal forces caused by rotor  3  rotating will press heels  26  against flanks  30 , plaquettes  22  against platform  8 , and the downward-sloping edges  19  against outer surfaces  31  of the grooves  20  due to the deflection of plaquettes  22 . A double leaktight barrier against re-circulated air is thus obtained with plaquettes  22  and their downward-sloping edges  19  producing a first barrier between platforms  8  and disc  2  and heels  26  producing a second barrier between plaquettes  22  that are pressed against platforms  8  and disc  2 . 
     A slightly different embodiment of seal  17 ′ is shown in FIGS. 4 and 5 where the plates  32  are twice as narrow and can therefore lie between cutting lines  21  in twice the number, from the center of one of the apertures  23  to the center of the adjacent aperture  23 . In other respects, this second embodiment is similar to the previous embodiment. FIG. 5 is a perspective view that shows one of the pairs of components that are folded in order for the heels  26  to pass through collar  12 . FIG. 5 also shows a hole  33  used to enlarge a specific area of collar  12  to enable legs  9  of vanes  1  to be inserted. However, the first embodiment, in which a pair of plaquettes  22  was associated with vanes  1 , required an even number of vanes. It may also be noted that, where hinges  29  were required in the first embodiment to fold heels  26  to enable them to be inserted in the throat  10 , this step is no longer necessary in the second embodiment as the heels  26  are capable of passing through the hole  33  before they are slid into position in the throat  10  in the same way as legs  9  of vanes  1  and to alternate with said legs  9 . The components of seal  17 ′, comprising two plaquettes  22  that are interconnected with two heels  26 , are then replaced by immobile vanes  4 . Hinges  29  are, however, preserved in order for the flexibility of seal  17 ′ to be maintained and for perfect leaktightness to be achieved.