Abstract:
A gas turbine nozzle can be refurbished to reduce downstream deflection. The outer shroud of the gas turbine nozzle is held in a fixture, and then the nozzle is heated. The heated nozzle is then reshaped by a force exerted upon the inner shroud of the gas turbine, reducing the downstream deflection of the nozzle. After the deformation of the nozzle, an aft hook of the nozzle that has been adjusted by previous refurbishment efforts can be rebuilt to remove the previous adjustments.

Description:
TECHNICAL FIELD 
     The present invention relates to the field of gas turbines, and in particular to a technique for refurbishing gas turbine nozzles. 
     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 gas loading on the nozzles and the high temperatures in the turbine, eventually cause the stationary turbine nozzles to deform. This is a particular problem with turbines where the nozzles are made of cobalt-based superalloys and use a cantilevered design. 
     SUMMARY OF INVENTION 
     In one embodiment, a method of refurbishing a gas turbine nozzle comprises mounting the gas turbine nozzle in a fixture, heating the gas turbine nozzle to a predetermined temperature range, and applying force to the heated gas turbine nozzle distal from the fixture sufficient to reshape the gas turbine nozzle by a calculated amount. 
     In another embodiment, an apparatus for refurbishing a gas turbine nozzle comprises a mounting fixture, configured to hold an outer shroud of the nozzle, a hydraulic jack, positioned below an inner shroud of the nozzle, adapted to exert an upward force on the inner shroud, a heat source, disposed with the nozzle, and a plurality of thermocouples, positioned with the nozzle and adapted for monitoring the temperature of the nozzle. 
     In another embodiment, an apparatus for refurbishing a gas turbine nozzle, comprises a means for holding a first portion of the nozzle, a means for heating the nozzle, a means for exerting an upward force on a second portion of the nozzle, distal from the first portion of the nozzle, and a means for supporting the second portion of the nozzle. 
     Other systems, methods, features, and advantages consistent with the present invention will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that such additional systems, methods, features, and advantages be included within this description and be within the scope of the invention. 
    
    
     
       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 drawing of an exemplary collection of stationary nozzles for a gas turbine, removed from the turbine for repair and refurbishment; 
         FIG. 2  is a radial view of a gas turbine nozzle illustrating the deformation caused by downstream deflection; 
         FIG. 3  is a side view of a hook of a gas turbine nozzle, illustrating a typical conventional adjustment to the hook; 
         FIG. 4  is a radial view of a gas turbine nozzle illustrating the incomplete refurbishment of a conventional DSD refurbishment. 
         FIG. 5  is a side view illustrating a technique for reshaping a gas turbine nozzle according to various embodiments; 
         FIG. 6  is a side view illustrating an apparatus for reshaping a gas turbine nozzle according to one embodiment; 
         FIG. 7  is a side view illustrating an apparatus for reshaping a gas turbine nozzle according to another embodiment; 
         FIG. 8  is a side view illustrating the use of insulating blankets on the gas turbine nozzle according to one embodiment; 
         FIG. 9  is a side view illustrating one embodiment of induction heating coils for heating a gas turbine nozzle; 
         FIG. 10  is a top view illustrating a plurality of thermocouples used to monitor the heating of the gas turbine nozzle according to one embodiment; and 
         FIG. 11  is a side view illustrating a reshaped gas turbine nozzle mounted with a saddle fixture for inspection after reshaping according to one embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In a cantilevered gas turbine design, a ring  100 , as illustrated in a radial view in  FIG. 1 , is composed of a plurality of circumferentially spaced apart stationary nozzles  110 , each of which includes vanes supported between radially inner and outer bands or shrouds. Each nozzle  110  is typically an arcuate segment with two or more vanes joined between the inner and outer shrouds, as shown in more detail in  FIG. 2 . Each vane is an airfoil, and the vanes are typically cast with the inner and outer shrouds to form the nozzle. 
     Each nozzle  110  is cantilevered from the outer shroud, using hook to hold the nozzle in place. As the stationary nozzles  110  deform in a downstream direction, commonly referred to as downstream deflection (DSD), the nozzle  110  provides reduced axial clearances and radial seal clearances are compromised. As a consequence of the compromised clearances caused by the DSD, sealing effectiveness is reduced, which can result in high wheel space temperatures. 
       FIG. 2  is an illustration of a radial view of a typical nozzle  110  in both its original and an exemplary deformed condition caused by DSD. The nozzle  110  is exemplary and illustrative only and other nozzle configurations are known in the art. Similarly, the deformation is exemplary and illustrative only, and each nozzle  110  may have a deformation that is different from any other nozzle  110 . As shown in  FIG. 2 , the original configuration of the nozzle  110  as manufactured is shown in solid lines, and a deformed configuration is shown in dashed lines, with the deformation exaggerated for clarity of the drawing. The nozzle  110  is fixed in place by the hooks  240  of the outer shroud  210  when mounted in the turbine, and the vanes  230  and inner shroud  220  are deflected downstream (to the right in  FIG. 2 ). A box  250  engages with the inner shroud  220 , and contains a plurality of packing teeth  260 . 
     Conventional refurbishment techniques attempt to rotate the nozzle  110  into the original position by adjusting one of the hooks  240 . As illustrated in  FIG. 3 , a portion  310  of the hook  240  is machined away, and a pad  320  is built up by welding or brazing onto a radially inward surface  330  of the hook, causing the outer shroud  210  to rotate upwardly from its original position when installed back in the turbine, which brings the deformed nozzle  110  back to the position illustrated by dashed lines in  FIG. 4 , which shows the nozzle  110  (in solid lines) in its original state and the refurbished nozzle  110  (in dashed lines). As repeated DSD refurbishments are performed using this conventional technique, the repeated machining of the hooks  240  can also open segment seal slots  340 , as shown in  FIG. 3 . 
     But as can be easily seen in  FIG. 4 , the conventional adjustment of the hook  240  does not actually change the geometry of the nozzle  110 , but merely rotates the nozzle  110  to attempt to reduce the DSD. Furthermore, repeated conventional DSD refurbishment can change the outer sidewall flow path, and does not solve deformation problems such as the angled packing teeth  260 , which can contribute to high wheel space temperatures. 
       FIG. 5  is a line drawing in side view that illustrates one embodiment of a technique for refurbishing the gas turbine nozzle  110 , even one that has been refurbished multiple times with conventional techniques. A work surface  510 , typically a workbench or table, provides a place to mount fixture  500  to hold the nozzle  110  above the work surface  510  a sufficient working distance. The fixture  500  is composed of a plate  520 , to which is attached brackets  530  and  540 , configured to engage hooks  240 . The box  250  (not shown) is typically removed from the nozzle  110  during the refurbishment. The fixture  500  is typically made of steel, although other suitably strong and heat-resistant materials can be used. The construction of the fixture  500  in  FIG. 5  is exemplary and illustrative only and other configurations can be used. In particular, the fixture  500  can be of an integral construction or composed of additional elements than the elements shown in  FIG. 5 . The fixture  500  can be welded or otherwise suitably attached to the work surface  510  as desired. 
     Once the nozzle  110  is mounted on the fixture  500 , the nozzle  110  is heated, then deformed in an upstream direction to counter the effect of downstream deflection, by force exerted from beneath the nozzle  110  upwardly, shown by arrow  560 . In some embodiments, an additional force, shown by arrow  550 , is exerted onto the inner shroud  220  toward the fixture  510 . 
     By pushing upward on the heated nozzle  110 , the deformation caused by DSD is actually reversed, bringing the nozzle  110  closer to its configuration when newly manufactured. Instead of merely rotating the deformed nozzle  110 , the nozzle  110  is reshaped to reduce or eliminate the deformation, rotating the vanes  230  and inner shroud  220  relative to the hooks  240  and outer shroud  210 . After the nozzle  110  is reshaped, if the nozzle  110  had previously been refurbished by the conventional hook adjustment technique, the modified hook  240  is rebuilt by removing the pad  320  that was added to the undersurface  330  of the hook  240 , and welding back a pad onto an upper surface of the hook  240  where the previous refurbishment had machined off a portion  310  of the original hook  240 . This rebuilding of the hook  240  can close segment seals  340  that may have been opened by the earlier refurbishments. 
     The order of steps of the above technique of first heating the nozzle  110 , then reshaping it, and finally rebuilding the hook  240 , can be rearranged, by first rebuilding the hook  240 , then reshaping the nozzle  110  sufficiently on fixture  500  to rotate the vanes  230  and inner shroud  220  back into their original position relative to the hook  240 . But the reordered technique is not as good as the preferred technique, because the hook  240  cannot be positioned as precisely. When the hook is rebuilt last, the desired position of the hook  240  can be calculated by an operator of the reshaping apparatus, then the nozzle  110  reshaped to approximately the right shape. After the heated nozzle  110  is reshaped, the hook  240  can be rebuilt to precisely the desired configuration, ensuring the nozzle  110 , when put back into the gas turbine, is within or close to the manufacturer&#39;s specifications. 
     Superalloys such as the cobalt-based superalloys frequently used in the construction of the nozzles  110  are not generally considered pliable under heating, and are metallurgically created to attempt to avoid deformation at high temperatures. So one of skill in the art would have expected that heating the nozzle  110  would not allow for the controlled force reshaping necessary for refurbishment of the nozzle  110 , but would have caused fractures or other metallurgical damage to the nozzle  110 . Applicants have tested the nozzle  110  and found no such damage to the nozzle  110  after the reshaping treatment. 
     In an embodiment where both the force  560  and force  550  are used, the inner shroud  220  can be caused to rotate in an additional dimension. But in experimental testing, it was determined that use of the force  560  is typically sufficient, and that the rotation caused by the force  550  tends to occur without the force  550  as the nozzle  110  is pushed closer into its original configuration. In such an embodiment, illustrated in  FIG. 7 , a jack shaft  700  moves through a fixture  710 , mounted to the work surface  510 , under pressure from another hydraulic jack (not shown). Other techniques can be used to exert the force  550  on the inner shroud  220 . 
     Force  560  is applied by pressure from a hydraulic jack, typically raising one or more jack shafts  600  through the work surface  510 . Preferably, at least two jack shafts  600  are used, exerting force equally or differentially as desired on the inner shroud  220 . With a differential jacking, a desired radial rotation of the inner shroud  220  and vanes  230  can be performed if needed. Once the nozzle  110  has been jacked up sufficiently, jack stands can be inserted to allow the inner shroud  210  to rest on the jack stands and withdrawal of the jack shafts  600  while allowing the reshaped nozzle  110  to cool, before completing the refurbishment by adjustment of the hook  240 , as described above. Any convenient kind of jack stand can be used, for example, a screw-type jack stand, such as the jack stands  910  in  FIG. 9  or a fixed height stand, such as a cylinder machined to a predetermined height appropriate for the nozzle  110 . The jack stands can be affixed to the work surface  510 , or movable as desired. 
     To heat the nozzle  110  prior to reshaping, the nozzle  110  is first insulated using insulating blankets  800 , as shown in  FIG. 8 . This is in part for safety of the operator. Because the heating is done by induction heating, the insulating blankets  800  can be applied before wrapping the nozzle  110  with the induction heating wires, as shown in  FIG. 8 . The type and position of the insulating blankets  800  illustrated in  FIG. 8  is exemplary and illustrative only, and any convenient insulating blankets and positioning thereof can be used. 
     In one embodiment, the heating is achieved by using induction heating coils  810 , which are typically composed of copper tubing, with a high temperature insulation mesh surrounding the tubing. The tubing has high frequency electricity provided to it, and cooling water on the inside, creating an electromagnetic effect that induces electrical currents within the part surrounded by the high frequency magnetic field. Because an even temperature is desirable for the reshaping of the nozzle  110 , the induction heating coils are wrapped around the nozzle  110 . Various configurations of the coils can be used, such as shown in  FIGS. 8 and 9 . In one embodiment, a configuration that is a pinched ovoid shape (roughly that of a peanut) is used, although other configurations can be used as convenient or desired. In some embodiments, such as shown in  FIG. 9 , the induction heating coils are wrapped around the nozzle  110  as a whole. In other embodiments, the induction heating coils can be wrapped through gaps between the vanes  230 . 
     The use of induction heating is exemplary and illustrative only. Other heating techniques can be used, such as quartz lamps, resistance heating, flame heating, etc. 
     Typically, a plurality of thermocouples  1010 , as illustrated in  FIG. 10 , allow an operator to monitor the temperatures at various locations on the nozzle  110 , to ensure no hot spots, and also to allow the operator to control the power in the induction coils  810 , bringing up the power to the coils and the ramp rate to achieve the desired temperature of the nozzle  110 . Any convenient thermocouple control and monitoring mechanism can be used. 
     The temperature used for this technique is dependent upon the materials used to construct the nozzle  110 . For some nozzles  110 , the superalloy metal is heated to approximately 2000° F., and generally between 1800° F. and 2100° F. The specific temperatures are exemplary and illustrative only, and different superalloy metals would require heating to a different range. For any nozzle  110 , however, the nozzle  110  should be heated to a temperature above a hardening temperature, but below a melting point of the metal. 
     After refurbishment, the nozzle  110  can be checked for compliance with the manufacturer&#39;s specifications by placing the nozzle  110  into a testing saddle such as the exemplary and illustrative saddle fixture  1102  of  FIG. 11 , which mimics the surrounding components in the gas turbine. Then the positioning of the nozzle  110  can be checked at predetermined points, such as points  1100 - 1195 . For example, dimension  1100  is one dimension that is directly affected by DSD. After reshaping, the nozzle  110  should be within manufacturer&#39;s tolerances. 
     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.