Patent Publication Number: US-6981847-B2

Title: System for connecting and locking rotor blades of an axial compressor

Description:
The present invention relates to a system for connecting and locking rotor blades of an axial compressor. 
   More precisely, the invention relates to a system for connecting and locking rotor blades which are fixed circumferentially and which are positioned in an array on the rotor disc of an axial compressor of a gas turbine. 
   The term “gas turbine” denotes the whole of a rotary beat engine which converts the enthalpy of a gas into useful work, using gases obtained directly from a combustion process and supplying mechanical power on a rotating shaft. 
   The turbine therefore usually comprises one or more compressors or turbocompressors, which compress air drawn in from the outside. 
   Various injectors supply the fuel, which is mixed with the air to form a fuel-air mixture for ignition. 
   The axial compressor is driven by a turbine, properly so called, or turboexpander, which supplies mechanical energy to a user by converting the enthalpy of the gases burnt in the combustion chamber. 
   The turboexpander, the turbocompressor, the combustion chamber (or heater), the output shaft for the mechanical energy, the control system and the starting system form the essential components of a gas turbine machine. 
   As regards the operation of a gas turbine, it is known that the fluid enters the compressor through a set of inlet ducts. 
   In these channels, the gas is characterized by low pressure and low temperature, but as it passes through the compressor the gas is compressed and its temperature rises. 
   It then enters the combustion (or heating) chamber, where it undergoes a further significant temperature rise. 
   The heat required to increase the gas temperature is supplied by the burning of liquid fuel introduced by injectors into the heating chamber. 
   The combustion is initiated by sparking plugs when the machine is started. 
   At the outlet of the combustion chamber, the gas, at high pressure and high temperature, passes through suitable ducts, reaches the turbine, where it gives up some of the energy accumulated in the compressor and in the heating (combustion) chamber, and then flows to the outside through the exhaust ducts. 
   Since the work transmitted by the gas to the turbine is greater than the work absorbed by the gas in the compressor, a certain quantity of energy remains in the shaft of the machine, and this work, after deduction of the work absorbed by the accessories and by the passive resistance of moving mechanical parts, constitutes the useful work of the machine. 
   Where the compressor is concerned, the maximum compression pressure is limited by the strength of the materials used. 
   Given the conditions of pressure, temperature and velocity of the rotating members in which the compressor is made to operate, it will be understood that the various components, and in particular the blading, are particularly stressed and therefore subject to rapid deterioration. 
   To enable maintenance and replacement to be carried out, the blades of the rotor disc are not made in one piece with it, but are fixed by their base projections which are inserted into suitable seats formed on the rim of the rotor disc. 
   In the connections of the rotor blades, the fixings are subjected, during the operation of the machine, to high perpendicular, bending, and possibly torsional stresses. 
   It will be appreciated, therefore, that the blade connection procedure is a crucial aspect of the design of any rotor. 
   In axial turbines, the most common type of blade fixing makes use of seats formed in the rotor disc, having sides with a grooved profile, in which the terminal portions, or roots, of the blades are engaged. 
   These seats can be made in the form of peripheral grooves extending essentially parallel to the axis of rotation of the rotor disc, so that the blades are inserted in an essentially axial direction. 
   A different type of blade fixing is provided by using what is known as circumferential fixing, in which a circumferential groove is formed on the outer circumference of the rotor disc to enable the blades to be inserted in the radial direction. 
   A particularly significant problem in the field of the design of rotor blades for axial compressors is the problem of providing connections which reduce to a minimum the down time for maintenance and replacement operations. 
   A first object of the present invention is therefore that of permitting the speedy assembly, dismantling and replacement of blades of the type fixed circumferentially to the rotor, by providing a blade connecting and locking system, with a reduced number of parts, which simplifies the removal of the locking devices and the replacement of the blades without any need to dismantle the rotor. 
   One disadvantage encountered in the connections of blades to rotor discs in the prior art is represented by the assembly tolerances: this is because excessive clearance in the assembly of the blades can cause dangerous vibrations, while the absence of such clearance can give rise to shrinkage due to the prevention of thermal expansion, causing additional stresses. 
   A second object of the present invention is therefore to provide a blade connecting and locking system which ensures correct assembly tolerances. 
   Another object of the present invention is to provide a system for connecting and locking rotor blades of an axial compressor which provides high reliability during the operation of the machine. 
   The system for connecting and locking blades which are fixed circumferentially to a rotor disc of an axial compressor according to the invention comprises the fixing of a plurality of blades positioned in an array along the circumference of a rotor disc, by the introduction of a shaped root of each blade, by the use of a means for positioning and locking the blades, into a circumferential seat formed along the circumference of the rotor disc, this seat being capable of housing slidably in a radial arrangement the roots of the blades and the positioning and locking means. At least one insertion slot, intersecting the said circumferential seat, is provided for the insertion of the roots of the blades and the positioning and locking means. 

   
     The characteristics of the system for connecting and locking rotor blades of an axial compressor according to the present invention will be made clearer by the following description and by the attached drawings, which relate to one embodiment, described by way of example and without restrictive intent, and in which: 
       FIG. 1  is a perspective view of the connecting and locking system according to the invention; 
       FIG. 2  is a partial plan view of a rotor disc designed for the connecting and locking system according to the invention; 
       FIG. 3  is a section taken through the line III—III of  FIG. 2 ; 
       FIG. 4  is an exploded view in partial section of details of the system according to the present invention; 
       FIG. 5  is a schematic section illustrating the connecting and locking system according to the invention; 
       FIG. 6  is a lateral view of a detail of the system according to the invention. 
   

   With reference to the figures, a multi-stage axial compressor comprises a rotor disc  1  having a plurality of stages  2 , each comprising, along its circumference, an array of circumferentially fixed blades  10 . 
   The blades  10  of each array are essentially identical, since their aerodynamic and structural behaviour must be identical. 
   The structure of a blade  10  essentially comprises three main portions: a quadrangular platform  11 , preferably trapezoidal; a portion with an aerodynamic profile  12  designed to compress the air and extending from the upper face of the platform  11 , and a root  13  which acts as the fixing in the rotor disc  1  and extends from the lower face of the platform  11 . 
   The root  13  is the portion by which the blade  10  is connected to the rotor disc  1 , preventing the expulsion of the blade by centrifugal force. 
   The root  13  is shaped in such a way as to form a partial fixing in a correspondingly shaped circumferential seat  3 , formed along the circumference of the rotor disc  1 . 
   In this context, it should be pointed out that, although reference is made to a rotor disc  1  carrying the blades  10 , in some compressors a plurality of blading stages are connected directly to the rotor shaft which is designed for the purpose, by the provision of a number of circumferential seats equal to the number of bladed stages to be fitted. 
   The fixing of the root  13  in the circumferential seat  3  is considered to be a partial fixing, since it allows the blade  10  to slide along the circumference of the rotor disc  1  but prevents its movement in the axial direction. 
   In order to form the partial fixing between the blade and disc, the root  13  of the blade  10  and the circumferential seat  3  have profiles which match each other, and which can be made in various forms to meet different requirements of design and construction. 
   The root  13 , when seen from the front with respect to the direction of sliding in the circumferential seat  3 , appears shaped in the form of a dovetail with rounded corners. As illustrated in  FIG. 1 , each root has opposite flat sides in the circumferential directions of the rotor disc. 
   In its upper part, in the portion near the platform  11 , the root  13  has a pair of recesses  13 ′ which can engage with corresponding counterparts  3 ′ formed along the walls of the circumferential seat  3 . 
   The root  13  also has at its base a pair of projections  13 ″ retained in corresponding bends  3 ″ formed in the walls of the circumferential seat  3  near the base. 
   Preferably, the recesses  13 ′, the counterparts  3 ′, the projections  13 ″ and the bends  3 ″ are made in pairs in the corresponding elements, but different forms of fixing which are equally effective can have only one shaped side. 
   The root  13  has a thickness s measured in the direction of sliding of the blade  10  within the circumferential seat  3 , and extends centrally with respect to the platform  11  which has in the same direction a side whose measurement L is essentially equal to twice the thickness s. 
   The blades  10  are locked in the seat  3  by positioning and locking means, comprising at least one block  20 , also shaped in the form of a dovetail with rounded corners, and having a thickness s essentially equal to the thickness of the root  13 , subject to the various tolerances specified for assembly, and having a profile essentially reproducing that of the root  13 , so that it can be inserted into, and slide within, the circumferential seat  3 . As illustrated, block  20  has opposite flat sides in circumferential directions of the rotor disc. 
   In particular, the block  20  has in its upper part recesses  20 ′ which reproduce the profiles of the counterparts  3 ′ formed along the walls of the circumferential seat  3 , and at its base a pair of projections  20 ″ identical to the projections  13 ″ of the roots  13  and capable of being retained in the bends  3 ″ of the walls of the circumferential seat  3 . 
   The block  20  also has a thickness s, measured in the direction of sliding of the blade  10  and the block  20  within the circumferential seat  3 , which is essentially equal to the thickness s of the roots  13 , subject to the necessary assembly tolerances. 
   To achieve effective locking, at least two blocks  20  are provided, these being positioned a certain distance apart, according to the procedures which will be made clear in the rest of the description. 
   Each block  20  has a central through hole  21 , which passes vertically through it, for the insertion of a dowel  22 . 
   The dowel  22  of each block  20  comprises a body  23  and a head  24  designed for engagement in part in a corresponding blind hole  5  formed in the base of the circumferential seat  3  for fixing each block  20  to the rotor disc  1 . 
   For fixing the block  20  to the rotor disc  1 , the central hole  21  is threaded in the area which houses the body  23  of the dowel, which is also correspondingly threaded. Accordingly, at least a portion of the dowel is threaded to enable threading the dowel in the hole  21 . 
   Therefore, when the dowel  22  is screwed in, the head  24  is made to bear on the base of the blind hole  5 , thus locking the corresponding block and consequently the whole array of blades  10 . 
   To enable the roots  13  and the blocks  20  to be inserted radially into the circumferential slot  3 , at least one insertion slot  4  is provided, intersecting the said circumferential seat  3 . 
   Preferably, a single insertion slot  4  is provided, in order to increase the reliability of the system, but the provision of two insertion slots  4  in diametrically opposite locations with respect to the rotor disc provides better balancing during rotation. As illustrated in  FIG. 2 , the blind holes  5  are spaced apart symmetrically with respect to the insertion slot  4 . 
   In this case, the components of the whole connecting and locking system are doubled. 
   The insertion slot  4  is, in practice, an aperture of essentially quadrangular shape, and its dimensions are slightly greater than the dimensions of the roots  13  and of the blocks  20 , because sufficient assembly clearance is provided to enable the roots  13  and the blocks  20  to be inserted radially into the circumferential seat  3 . 
   Pairs of securing blades  10 ′, located next to each block  20 , are also provided for the assembly of the system according to the invention. 
   These securing blades  10 ′ are essentially identical to the blades  10 , but each of them has an aperture  14 , which is generally semicircular, or quadrangular if particular constructional requirements have to be met. 
   This aperture  14  is formed on the edge of the platform  11 , adjacent to the corresponding edge of the other securing blade making up the pair. 
   These apertures are made in central positions, to allow access to the dowel  22 . 
   In a corresponding way, a small block or bush  20   a  extends from the upper face of the block, this bush also being formed in a central position and having the central hole  21  passing through it. 
   The bush  20   a  is designed to be inserted into the said semicircular or quadrangular apertures  14  formed in the platforms  11  of the securing blades  10 ′. 
   If the apertures  14  are made quadrangular in order to meet constructional requirements, the bush is also made quadrangular. 
   In order to understand more clearly the advantages of the connecting and locking system according to the invention, reference should be made to its assembly on the rotor disc  1 . 
   The blade  10  are first inserted through the insertion slot  4  and are slid circumferentially along the circumferential seat  3 , after which a securing blade  10 ′ is inserted, followed by a block  20  and then another securing blade  10 ′, in such a way that the two semicircular apertures  14  are jointed to form an aperture which can receive the bush  20   a.    
   Two other blades  10  are then inserted, and finally two more blades  10 ′, with the second block  20  between them, are inserted in the same way as before. 
   Finally, the whole array is slid within the circumferential seat  3  until the two blocks  20 , or more precisely their central holes  21 , are brought in line with the blind holes  5 , so that the dowels  22  can be screwed in until their heads  24  enter in part the blind holes  5 . 
   When the assembly is complete, the blades  10  and the securing blades  10 ′ are in contact with each other along the edges of their platforms  11  perpendicular to the direction of sliding of the blades, and a space is provided between the roots  13  of the two pairs of contiguous securing blades  10 ′ for housing the blocks  20 . 
   The decision to position and fix the blocks at a spacing enabling four blades, namely two blades  10  and two securing blades  10 ′, to be placed between them, was made in order to provide an optimal solution to the problem of the tolerances and clearances required for carrying out the assembly. 
   However, it should be emphasized that this decision was also dependent on the dimensions of the blades of one stage, and that it could, therefore, be modified, with the insertion of a different number of blades  10  between the blocks. 
   In particular, this decision makes it possible to keep the blades which are close to the insertion slot  4  in their predetermined positions, and avoids a situation in which the insertion of a greater number of blades between the two blocks might, as a result of an unforeseen sum of tolerances, cause one of the blades to be too closely aligned with the insertion slot, thus risking the expulsion of this blade. 
   Advantageously, the provision of a single insertion slot for the whole array of blades of each stage of the rotor disc further reduces the possibility of occurrence of such problems. 
   In this context, it should be noted that, in the arrangement according to the preferred embodiment of the invention, on completion of assembly, two contiguous blades are positioned symmetrically with their platforms  11  covering the insertion slot  4 , these platforms having the function of re-creating the flow duct in the areas above the root housing slot. 
   Therefore, given the values of the thickness s of the root  13 , the width L of the platform  11  which is equal to twice the thickness s, and the width of the insertion slot  4  which is slightly greater than the thickness s, the roots of the two blades are essentially aligned in the insertion slot  4 , and it is therefore easy to imagine how a minimal displacement of the blade could bring its root into a position of excessive projection into the insertion slot, thus making the locking unstable or even causing the blade to be expelled from the circumferential seat during the rotation of the rotor disc. 
   Finally, the arrangement according to the invention makes it possible to avoid an excessive closeness of the blocks which, by creating irregularities in the circular symmetry of the array of blades, perturb the rotation of the rotor disc. 
   The above description clearly indicates the characteristics of the connecting and locking of blades on a rotor disc of an axial compressor of a gas turbine which is the object of the present invention, and also makes clear the additional advantages, which include, in addition to those mentioned previously:
         an increased average life of the components;   a higher rotation speed of the machine or an increase in the temperature of the fluid, or an appropriate combination of the two factors.       

   Finally, it is evident that the system designed in this way can be modified and varied in numerous ways, all included within the scope of the invention; moreover, all the components can be replaced with technically equivalent elements. 
   In practice, the materials used, as well as the shapes and dimensions, can be varied at will according to technical requirements.