Patent Publication Number: US-2003221315-A1

Title: Mehtod for repairing damaged rotor blades of the turbine part of a gas turbine

Description:
FIELD OF THE INVENTION  
       [0001] The present invention deals with the field of gas turbine technology. It relates to a method for repairing damaged rotor blades of the turbine part of a gas turbine in accordance with the preamble of claim  1 .  
       [0002] A method of this type is known, for example, from U.S. Pat. No. 5,644,394.  
       DISCUSSION OF BACKGROUND  
       [0003] The blades and vanes of gas turbines are exposed to different loads in operation. This is particularly true of the rows of blades and vanes in the turbine part of the gas turbine where the hot combustion gases coming out of the combustion chamber are expanded in such a manner as to perform work. It will automatically be understood that, of the rows of blades and vanes of the turbine part, in particular the first rows are subject to high levels of load, since this is where the gas temperatures and velocities are highest.  
       [0004] Since the operating temperatures of gas turbines are being raised ever higher in order to increase efficiency, it is necessary for ever higher-quality materials to be used for the blades and vanes. At the same time, the blades and vanes are coated with protective layers in various ways in order to protect the blade or vane material from attack by the aggressive hot gases and to extend the service life.  
       [0005] However, during operation, these thermal barrier coatings may become partially detached or may flake off. Since it is extremely expensive to replace damaged blades or vanes of this type, and such an operation would also lead to long shutdown times for the gas turbine, it is desirable for the blades or vanes to be repaired in the installed state by the damaged regions of the coating being replaced and, if necessary, any other damage to the blades or vanes being eliminated in situ.  
       [0006] It has already been proposed in U.S. Pat. No. 5,644,394, which was cited above, to introduce a specially adapted treatment tool with a rotating grinding head into the interior of the machine through small inspection openings in the casing of a gas turbine, in the manner of a catheter, in order to treat damaged front edges of blades or vanes fitted in the turbine, with optical control by means of a camera device carried along with the tool. This requires differently shaped tool inserts to be used for differently arranged blades or vanes.  
       SUMMARY OF THE INVENTION  
       [0007] Accordingly, one object of the invention is to provide a method which, in a particularly simple way, enables the rotor blades in the turbine part of a gas turbine which are subject to particularly high levels of load to be treated for repair purposes without the turbine part having to be dismantled.  
       [0008] This object is achieved by the combination of features described in claim  1 . The core idea of the invention consists in treating the rotor blades of the first row through the spaces between the guide vanes arranged at the inlet of the turbine part. Since these spaces are distributed over the circumference and have similar dimensions to the rotor blades belonging to the first row, it is possible for tools which are suitable for repair purposes to reach all the rotor blades belonging to the first row relatively easily and under substantially identical conditions.  
       [0009] A preferred configuration of the method according to the invention is distinguished by the fact that treatment devices which can be introduced from the outside through the spaces between the guide vanes belonging to the first row until they reach the rotor blades are used to treat the rotor blades, and that the treatment devices are inherently moveable, in particular flexible, and/or singularly or multiply jointed.  
       [0010] For the surface treatment of the rotor blades, it is particularly advantageous if the treatment devices comprise a grinding device for grinding and/or polishing the surface of the rotor blades, a brush for applying a film of liquid to the surface of the rotor blades, a device for drying a film of liquid located on the surface of the rotor blades, in particular in the form of a hot-air blower.  
       [0011] If the rotor blades are provided on their surface with a protective coating, defective regions of the coating which have formed during operation of the gas turbine are preferably removed by the treatment and replaced by a replacement coating.  
       [0012] In particular, the repair method is distinguished by the fact that, in a first step, the defective coating is removed by means of a grinding process, that, in a second process, a film is applied in the region of the coating which has been removed, forming a protective replacement coating during a subsequent heat treatment, and that, in a third step, the rotor blades which have been provided with the film are subjected to a heat treatment.  
       [0013] The film is preferably applied to the surface of the rotor blades in liquid form and is subjected to a drying process before the subsequent heat treatment.  
       [0014] The bonding of the replacement layer can be improved by the surface of the rotor blades being roughened, in particular by etching, before the film is applied.  
       [0015] A further improvement results if the surface of the rotor blades is polished immediately after the grinding operation and if the film is applied a plurality of times, and is in each case subjected to a drying process between two application operations.  
       [0016] In principle, it is conceivable for the heat treatment of the applied film to be carried out separately. However, the process becomes particularly simple if the heat treatment used is the heating of the gas turbine during the recommissioning which follows the repair operation. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0017] A more complete appreciation of the invention and many of the dependent advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:  
     [0018]FIG. 1 shows a longitudinal section through the inlet region of the turbine part of a gas turbine;  
     [0019]FIG. 2 shows a view of the inlet region of the turbine part shown in FIG. 1 in the axial direction;  
     [0020]FIG. 3 shows a view in the radial direction of the first rows of the guide vanes and rotor blades of the turbine part shown in FIG. 1, diagrammatically depicting the use of various types of grinding devices within the context of the invention;  
     [0021]FIG. 4 shows an illustration similar to that presented in FIG. 3, illustrating the use of a brush for applying a liquid film to the rotor blades belonging to the first row;  
     [0022]FIG. 5 shows an illustration similar to that presented in FIG. 3 illustrating the use of a hot-air blower for drying a liquid film which has been applied to the rotor blades belonging to the first row; and  
     [0023]FIG. 6 shows a block diagram illustrating the various steps involved in a repair method in accordance with a preferred exemplary embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0024] Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, FIG. 1 illustrates a longitudinal section through the inlet region of the turbine part  11  of a gas turbine  10 . A plurality of rows of rotor blades, of which only the first row of rotor blades  17  is shown in the drawing, for the sake of simplicity, are arranged in succession on a rotor  12  which is mounted in such a manner that it can rotate about the axis  24 . Rows of guide vanes  16 ,  18  which are secured to a hot-gas casing  13  which concentrically surrounds the rotor  12  are arranged between the rows of rotor blades. The space between the rotor shaft and the hot-gas casing  13  forms a hot-gas passage  14 , through which hot gas  15  flows from right to left, being expanded in such a manner as to perform work. The hot gas  15  comes out of a combustion chamber (not shown in FIG. 1), the outlet of which is adjoined by the turbine part  11 . A configuration of this type is illustrated, for example, in U.S. Pat. No. 5,575,617. The first row of guide vanes  16  is positioned in front of the first row of rotor blades  17 , toward the combustion chamber.  
     [0025] The rotor blades  17 , as the first row of rotor blades, are exposed to a particularly high load from the hot gas  15  flowing into the hot-gas passage  14 . Therefore, it is frequently the case that defects occur at these blades first, manifesting themselves by flaking of the thermal barrier coating which has been applied to the surface of the rotor blades. If these defects in the protective layer are not eliminated immediately, the base material of the blades is attacked by oxidation and/or corrosion, which can then lead to greater damage extending beyond the surface. It is therefore important for damage to the protective layer of this type to be eliminated as quickly as possible, with the minimum possible outlay and with a short gas turbine shutdown time.  
     [0026] According to the invention, the damaged surfaces of the rotor blades  17  belonging to the first row are now repaired with the blades in the installed position by suitable treatment devices being introduced into the hot-gas passage  14  from the inlet of the turbine part  14  through the spaces ( 23  in FIG. 3) between the guide vanes  16  and being used to treat the surface of the rotor blades  17 . Although the guide vanes  16  overlap in the axial direction (FIG. 2; for this reason, the rotor blades  17  are only shown in dashed lines in the drawings), as shown in FIGS.  3 - 5 , if the guide vanes  16  and rotor blades  17  are placed in a suitable angular position with respect to one another, it is nevertheless easy for a treatment device to reach the pressure surfaces of the rotor blades  17  which are particularly exposed to the hot gas through the spaces  23  (running obliquely) between the guide vanes  16 , if the treatment device is designed or guided in a suitable way (FIGS.  3 - 5 ).  
     [0027] The diagrammatic sequence of the repair method, including its main steps, is represented in FIG. 6: first of all, in a first step, the damaged regions of the coating are removed from the rotor blade. This is preferably implemented by means of a (mechanical) grinding process, which may be followed by a polishing process, in order to smooth the surface. For this purpose, in accordance with FIG. 3, a rotating grinding device  19  with a grinding head  20  can be guided through the space  23  to the rotor blade  17 . It is possible to use a rigid grinding device  19 , a flexible, inherently moveable grinding device  19 ′ or a grinding device  19 ″ provided with a plurality of joints  25 , depending on the spatial conditions and the extent of the damaged area. However, other abrasive processes, such as sand-blasting or the like, are also conceivable.  
     [0028] After the polishing operation, the treated surface is prepared to receive a replacement coating by being roughened. The roughening is preferably carried out chemically by etching the surface. When the blade has been prepared for good bonding of the replacement coating in this way, in accordance with FIG. 4 a brush  21  is used to apply a film of liquid to the blade, and this liquid, after a heat treatment, forms the replacement coating. A liquid or suspension of this type is commercially available, for example, under the name “SermaLoy J”, and during a subsequent heat treatment, by means of diffusion, forms a coating with an aluminum-containing alloy (aluminide diffusion coating).  
     [0029] The film of liquid, after it has been applied, is dried as shown in FIG. 5, for example by means of a hot-air blower  22  which has been introduced into the space  23 . Liquid can then be brushed on again.  
     [0030] Once the film which has been applied to the blade has the required thickness and consistency, in order to form the replacement coating it can be subjected to a subsequent heat treatment, during which diffusion into the blade takes place. In principle, it is conceivable for this subsequent heat treatment to follow the application of the film as a separate process step. However, it is particularly simple if the heating of the gas turbine during the recommissioning which follows the repair is used for the heat treatment.  
     [0031] Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.  
                               LIST OF DESIGNATIONS                                                10   Gas turbine           11   Turbine part           12   Rotor           13   Hot-gas casing           14   Hot-gas passage           15   Hot gas (from the combustion chamber)           16   Guide vane (1 st  row)           17   Rotor blade (1 st  row)           18   Guide vane (2 nd  row)           19,19′,19′′   Grinding device           20   Grinding head           21   Brush           22   Hot-air blower           23   Space           24   Axis           25   Joint