Abstract:
A technique for refurbishing nozzle diaphragm sections of a gas turbine replaces an eroded section of the nozzle diaphragm with a replacement part designed to engage a slot machined in the nozzle diaphragm. The replacement part is formed of a material with capable of sustained exposure to higher temperature than the original eroded section, and with a similar coefficient of expansion as the material used for manufacture the original nozzle diaphragm. The combination of the nozzle diaphragm and the replacement part conform to the original manufacturer&#39;s dimensional specifications for the nozzle diaphragm.

Description:
TECHNICAL FIELD 
     The present invention relates to the field of gas turbines, and in particular to a technique for refurbishing gas turbines. 
     BACKGROUND ART 
     In a gas turbine, gas is typically produced by the combustion of fuel. The gas is then passed over a collection of stationary nozzles, which discharge jets of gas against the blades of a turbine rotor, forcing the rotor to rotate. The rotation of the rotor drives the external load of the turbine, such as an electrical generator. 
     One problem with gas turbines is that the high temperatures in the turbine eventually cause degradation, such as burning, of packings or diaphragms, where the diaphragms are connected to the nozzles. 
     SUMMARY OF INVENTION 
     A technique for refurbishing nozzle diaphragm sections of a gas turbine replaces an eroded section of the nozzle diaphragm with a replacement part designed to engage a slot machined in the nozzle diaphragm. The replacement part is formed of a material that is capable of sustained exposure to higher temperature than the original eroded section, and with a similar coefficient of expansion as the material used for manufacture the original nozzle diaphragm. The combination of the nozzle diaphragm and the replacement part conform to the original manufacturer&#39;s dimensional specifications for the nozzle diaphragm. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an implementation of apparatus and methods consistent with the present invention and, together with the detailed description, serve to explain advantages and principles consistent with the invention. In the drawings, 
         FIG. 1  is a lateral view of a conventional gas turbine nozzle segment and attached diaphragm segment; 
         FIG. 2  is a lateral view of the gas turbine diaphragm segment of  FIG. 1 , illustrating a section to be machined off and replaced during refurbishment; 
         FIG. 3  is a lateral closeup view of the gas turbine diaphragm segment of  FIG. 2 , after removal of the rail section; 
         FIG. 4  is a lateral view of a replacement rail section according to one embodiment; 
         FIG. 5  is lateral view of the replacement rail section of  FIG. 4  fitted into the gas turbine diaphragm segment of  FIG. 3 ; 
         FIG. 6  is a bottom view of the replacement rail section of  FIG. 4 ; and 
         FIG. 7  is a top view of the replacement rail section of  FIG. 4 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     As stated above, a problem with some gas turbines, such as the General Electric Frame  7 FA gas turbine, is that they operate at a temperature that can exceeds the ability of the materials used by the original equipment manufacturer to make the diaphragm. A conventional gas turbine nozzle is typically comprised of a plurality of nozzle segments  110 , each of which has an associated diaphragm or packing member  120  attached to the nozzle segment, such as shown in  FIG. 1 . 
     The nozzle segment  110  is typically manufactured from a high-temperature nickel-based superalloy or other similar material selected for its ability to withstand the high temperatures of hot gas in the turbine, which can reach approximately 2000° F. (1090° C.). The diaphragm  120  is attached radially interior to the vanes of the nozzle segment and forms an air seal around the rotor. 
     Because the diaphragm segments  120  are not directly exposed to the hot turbine gas, they are typically manufactured from a lower temperature material, such as a cast nickel-iron known in the art as a Ni-Resist (ASTM 439), which is an austenitic cast iron that is often used for heat and corrosion resistant applications. Although the diaphragm  120  is not directly exposed to the high heat, due to heat conduction through the nozzle segment  110  where the diaphragm  120  attaches to the nozzle segment  110 , the diaphragm segments  120  often exceed the temperature limits of the Ni-Resist material, typically approximately 1500° F. (815° C.). The excessive temperature causes oxidation and erosion of the diaphragm segments  120 , most commonly in the aft hook area  130 , sometimes referred to as a rail section, where the diaphragm segment  120  attaches to the nozzle segment  110 . This oxidation and erosion, sometimes referred to as burning of the rail section is typically discovered when the turbine is taken out of service for repair and refurbishment. Although the disclosure below and in the drawings is set forth using a replacement for the aft hook area  130 , the techniques disclosed herein can be employed to replace other heat-damaged areas of the diaphragm  120 , as needed. 
     Some refurbishers have repaired the eroded rail surface by machining the eroded surface to remove the eroded and corroded portions, welding on additional Ni-Resist material, and remachining the diaphragm  120  to the original equipment manufacturer&#39;s dimensional specifications. Such a technique is difficult to perform, because the casting porosity and heavy oxidation of the Ni-Resist material makes it difficult to weld. Furthermore, the conventional welding refurbishment technique does not permanently solve the problem, and the diaphragm is subject to the same erosion and oxidation, because the repair does not solve the problem of the excessive temperature. Thus, the repaired diaphragm may develop the same erosion and oxidation as before, requiring redoing the repair procedure. 
     Others have replaced the entire diaphragm section  120  with a replacement diaphragm section manufactured from a higher temperature material, such as a stainless steel. While the replacement diaphragm made of stainless steel is capable of withstanding higher temperatures than the Ni-Resist material, the cost of replacing the diaphragm with a new diaphragm made of stainless steel is undesirably high, because of the higher materials and manufacturing costs for the stainless steel diaphragm, as well as the waste of the original diaphragm, which in most part is not subject to the higher temperatures, and does not suffer the erosion and oxidation as a result, and does not need the higher-temperature material. 
     As disclosed herein, embodiments of the present invention avoid the high costs of a complete replacement and the lack of durability and difficulty of an original material Ni-Resist material repair. A portion  210  of the aft hook section  130  of the diaphragm  120  (indicated in  FIG. 2  with a dashed line, is machined off, removing the area of erosion and corrosion. The removed portion  210  is typically larger than the actual area of erosion and oxidation. Then, as illustrated in  FIG. 3 , a slot  310  is milled longitudinally into the diaphragm segment  120 .  FIG. 3  illustrates one embodiment of the slot  310 , an approximately T-shaped slot. The slot configuration of  FIG. 3  is illustrative and by way of example only, and other slot shapes can be used. 
     A replacement rail insert section  400  is machined, typically from a solid block of a stainless steel material. Most of the rail insert section  400  is machined or milled to match the configuration of the section  210  of the diaphragm  120  that was eroded and removed as described above. But a tab section  410  is machined to match the T-slot  310  in the diaphragm  120  illustrated in  FIG. 3 . Turning to  FIG. 5 , the rail insert  400  is illustrated inserted into the diaphragm  120 , with the T-slot  310  filled by the tab section  410 . The use of a matching T-slot  310  and tab  410  helps provide structural stability for the refurbished diaphragm  120 . Although as described above the replacement section  400  is machined to the desired configuration, other techniques for forming the replacement section  400  can be used, such as casting or forging. 
     In some embodiments, the rail insert  400  is machined roughly the same width as but slightly wider than the diaphragm section  120 , then further machined in place after assembly to better match the diaphragm section  120 , providing a smooth interface where the tab  410  engages the slot  310 . A retainer screw  510  can be used at either end or both ends of the tab-slot interface to hold the rail insert section  400  in place, preventing motion relative to the diaphragm  120 . 
     As illustrated in top view in  FIGS. 6 and 7 , the rail insert is curved to match the curvature of the diaphragm section  120 . In addition, as is also illustrated in  FIGS. 6 and 7 , other tooling holes for sensors, screws, or other fasteners can be drilled or otherwise formed in the rail insert section  400  as desired. 
     If necessary or desired, other openings can be formed or machined in the rail insert  400  for allowing placement of sensors or other conventional elements as used in the original diaphragm section  120  before refurbishment. Where seals, such as the seal  230  of  FIG. 2 , or slots such as the inter-section seal slot  220  also shown in  FIG. 2  were present in the original diaphragm section  120 , the rail section insert  400  is milled, drilled, or otherwise configured to provide for such seals or slots, to conform to the original configuration of the diaphragm  120 . The resulting assembly thus meets the original equipment manufacturer&#39;s specifications for the diaphragm section  120 , even though the rail insert section  400  is of a different material than originally specified, and is capable of withstanding the higher temperatures. 
     The rail insert  400  is preferably manufactured from a block of material. The material used is selected for its ability to sustain high temperatures and with a similar coefficient of expansion as the Ni-Resist material used in the diaphragm  120 . In one embodiment, the rail insert  400  is manufactured from a stainless steel, such as a type 310 stainless steel. The resulting refurbished nozzle segment and diaphragm is then reassembled with other such segments into the turbine nozzle. Depending on the damage observed or discovered when the turbine nozzle was disassembled for refurbishment, any number of the diaphragm segments  120  can be refurbished as described above, providing a refurbished gas turbine nozzle at a lower cost and with less wasted materials than a complete replacement with higher temperature material, but providing improved durability over a conventional repair procedure that replaces eroded material with the original material. 
     While certain exemplary embodiments have been described in details and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not devised without departing from the basic scope thereof, which is determined by the claims that follow.