Abstract:
A fuel nozzle assembly for a gas turbine includes a plurality of circumferentially spaced vanes with holes for flowing fuel from plenums within the vanes through holes in the vane walls for premixing with air. To tune the nozzle assembly, the holes are resized by reforming the existing holes to a predetermined hole size, securing plugs into the holes, and forming holes through at least certain of the plugs to diameters less than the diameter of the existing holes.

Description:
BACKGROUND AND BRIEF DESCRIPTION OF THE INVENTION 
   The present invention relates to methods for tuning gas turbine fuel nozzle assemblies and particularly relates to methods for resizing premix fuel inlet holes for supplying gaseous fuel for premixing with air within the nozzle assemblies. 
   In land based gas turbines, a fuel nozzle typically comprises a subassembly of generally concentric tubes defining a central passage for supplying diffusion fuel gas and a pair of concentric passages for supplying premix fuel gas. Spaced from and surrounding the subassembly is an inlet flow conditioner for directing and confining a flow of inlet air past a plurality of circumferentially spaced vanes carried by the subassembly. The vanes are in communication with the concentric fuel gas supply passages. Particularly, the vanes include outer premix holes and inner premix holes for supplying gas from the respective passages for mixing with the inlet air. The gas fuel mixture is swirled by the vanes downstream of the fuel inlet holes for subsequent combustion. 
   The gas fuel composition and Wobbe Index at site locations determine the fuel gas nozzle exit velocity requirement which in turn is dependent upon the fuel gas supply hole size. Where the supply holes are too large, for a given gas composition and Wobbe Index, nozzle dynamics become a concern. For example, if the gas composition changes, these concerns become real and the nozzle assembly must be retuned to preclude those dynamic concerns. 
   In accordance with an example of the present invention and in a fuel nozzle assembly for a gas turbine having a plurality of circumferentially spaced vanes with holes for flowing fuel for premixing with air within the nozzle assembly, there is provided a method of tuning the fuel nozzle assembly by changing the diameter of the premix fuel holes in the vanes. To accomplish this, the existing holes are reformed to a predetermined diameter. Plugs are inserted into the reformed holes and secured to the vanes. Holes are formed through at least three of the plugs to diameters less than the diameter of the existing holes. Thus, the original holes are resized to provide smaller holes with consequent desired tuning effects. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross sectional view of a typical fuel nozzle assembly for a gas turbine; 
       FIG. 2  is a cross sectional view thereof taken generally about on line  2 - 2  in  FIG. 1  illustrating existing premix fuel gas supply holes in the walls of the vanes; 
       FIG. 3  is a view similar to  FIG. 2  illustrating premix resized fuel gas supply holes in accordance with an aspect of the present invention; 
       FIG. 4  is an enlarged cross sectional view of enlarged outer premix holes for a vane and forming part of a method of tuning the fuel injection assemblies according to an aspect of the present invention; 
       FIG. 5  is a view similar to  FIG. 4  illustrating plugs disposed in the reformed holes; and 
       FIG. 6  is a view similar to  FIG. 5  illustrating the resized fuel supply holes. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to  FIG. 1 , there is illustrated a conventional fuel nozzle assembly generally designated  10  for a gas turbine. Generally, the fuel nozzle assembly includes a subassembly  11  and a surrounding air inlet conditioner  13 . Subassembly  11  includes a central tube  12  and a pair of concentric tubes  14  and  16  defining discrete annular fuel passages  18  and  20  respectively between tubes  12  and  14  and tubes  14  and  16 . The central tube  12  supplies diffusion gas to the combustion zone downstream, not shown, of the fuel nozzle assembly  10 . Arranged about the outer tube  16  and forming part of subassembly  11 , there are provided a plurality of vanes  22  circumferentially spaced one from the other. The vanes  22  include outer premix holes  24  supplied with gaseous fuel from the passage  20  and a plurality of inner premix gas supply holes  26  supplied with gaseous fuel from passage  18 . As best seen in  FIGS. 2 and 3 , each vane  22  has a pair of outer and inner plenums  28  and  29 , respectively, confined between opposite side walls  30  and  31  of the vane. It will be appreciated that the holes  24  and  26  lie in communication with the outer and inner plenums  28 ,  29 , respectively. 
   As illustrated in  FIG. 2 , the conventional outer premix gas supply holes  24  include a pair of radially spaced holes  32  through one wall  30  of the vane  22  and a single hole  34  through the opposite side wall  31  of the vane. Downstream portions  36  of the vanes are twisted to impart a swirl to the flow of premixed air and gaseous fuel flowing between the subassembly  11  and the inlet flow conditioner  13 , the gaseous fuel being supplied to the air stream via the outer and inner premix fuel holes  24  and  26 , respectively. As noted previously, it is sometimes necessary to retune the nozzle injector assemblies because of dynamic concerns. 
   To accomplish the foregoing, and particularly to provide resized fuel supply holes in the vanes, for example to provide smaller diameter holes in lieu of the existing gas supply holes  32  and  34  in the side walls  30  and  31 , respectively, of the vanes, the inlet flow conditioner  13  which surrounds the vanes and other portions of the nozzle subassembly is removed. The inlet flow conditioner is preferably cut into two semi-circular pieces and discarded. By removing the inlet flow conditioner  13 , the outer premix holes  24  in the vanes  22  are exposed. 
   The exposed outer premix holes are initially enlarged by an electro-discharge machining process to form a pair of holes through each of side walls  30  and  31 . For example a pair of holes  38  and  40  are formed through side walls  30  of each vane and a pair of holes  42  and  44  are formed through side walls  31  of each vane. Using electro-machining processes enables the aligned holes  38 ,  42  to be formed in one pass. Similarly, the aligned holes  40 ,  44  may form in one pass. Consequently, the existing pair of holes  32  on one vane wall  30  are enlarged by electro-discharge machining and the existing single hole  34  in the opposite vane wall  31  is likewise enlarged. The second hole  42  in the opposite wall  31  of the vane  22  is formed by passing the electro-discharge machining tool through the hole  38  in the first wall in the aforementioned single pass. In this manner, a pair of holes in each wall is formed in alignment with a pair of holes in the opposite wall, and the holes  38 ,  40 ,  42  and  44  are larger than the existing holes  32  and  34 . The holes  38 ,  40 ,  42  and  44  thus formed are then reamed preferably by hand using a carbide reamer and reaming guide to meet the required diameter for installation of plugs. Thus, the four enlarged holes in each vane, there being 10 vanes in the illustrated preferred embodiment, are each hand reamed to provide a slightly larger diameter hole. The hole diameters are preferably identical. After reaming the holes to remove burrs and cleaning the holes, for example, with acetone, the holes are degreased, e.g., in a solution of Metal Medic 7705 or equivalent, for approximately 30 minutes at 160° F. The vanes are rinsed, for example, by submergence in a warm water bath for about 10 minutes, air-dried, preferably using compressed air to remove the water from the holes an then oven-dried, for example, at temperatures between 1850° F.-1875° F. for approximately 30 to 60 minutes. After cleaning the holes with acetone, the holes are ready to receive plugs. 
   The plugs  50 ,  52 ,  54 ,  56  are secured preferably by brazing, to the walls of the vanes. Thus, after cleaning the plugs with acetone, each plug is installed into a reamed hole to lie flush with the vane surface. A small bead of brazed alloy paste is applied around the braze plugs. To complete the brazing process, the nozzle assembly is placed in a furnace which is then evacuated, e.g., to a vacuum of 5×10 −4  Torr or better. To braze the plugs to the vane walls, the furnace is ramped up to about 1675° F.-1725° F. at a rate of approximately 30° F. per minute and held for 25 to 35 minutes. The temperature is then increased to a range of 1825° F.-1875° F. and held for 10 to 15 minutes. Preferably, when the temperature exceeds 1700° F., 100-300 microns of argon are added. The assemblies are then fast-cooled with the argon within the furnace to 175° F. or below and removed from the furnace. The nozzle assemblies may then be tested for leaks. For example, a pressure test fixture, not shown, may be applied to the nozzle assembly to apply approximately 50 pounds per square inch of pressure which is held for five minutes. Water is then applied to the braze joints, or the assembly is immersed in a water tank, to check for bubbles which would indicate leaks. Assuming the absence of leaks, the nozzle assemblies are dried and the plugs are rebrazed. For example, the assemblies are again disposed in a furnace which is then evacuated to a vacuum of about 5×10 −4  Torr or better. To complete the furnace brazing, the furnace is ramped up to a temperature of between 1675° F.-1725° F. at a rate of 30° F. per minute and held for 25 to 35 minutes. The temperature is then increased to a range between 1825° F.-1875° F. and held for 10 to 15 minutes. As the temperature exceeds 1700° F., 100-300 microns of argon are added and the nozzle assemblies are fast-cooled with the argon to about 175° F. or below. Upon removal of the assemblies from the furnace, the assemblies are leak tested are once again similarly as above noted. 
   The assemblies are then tempered. For example, the assemblies are again placed in a furnace, and the furnace is evacuated to a vacuum of 5×10 −4  Torr or better. The assemblies are heated to approximately 1050° F.-1125° F. for about four hours. The assemblies are then cooled in the furnace to below 200° F. before removing from the furnace. 
   Finally, holes are now formed in the walls of the vanes, particularly through the brazed plugs. It will be appreciated that the new holes formed through the plugs may be larger in area e.g. diameter relative to the existing holes  32  and  34 . Typically, however, the new holes are provided with a smaller area e.g. a smaller diameter, relative to the existing holes  32  and  34 . Preferably, using electro-discharge machining methods are used to form holes through plugs  52 ,  54 ,  56  and  58  of a smaller size, e.g., a smaller diameter than the original existing size, e.g., diameters, of the holes. Thus, holes  60 ,  62  and  64  are formed through respective plugs  52 ,  54  and  56 . Note particularly that a smaller sized diameter hole is not formed through plug  58 . Accordingly, holes  60 ,  62  are formed through plugs  52 ,  54 , respectively in side wall  30  while hole  64  is formed through plug  56  in side wall  31 . The brazed plug  58  seals the previously formed opening  44  formed by the EDM process in side wall  31 . Also note that the openings through the one side wall  30  are angled preferably about 5° relative to a tangent through the openings. The opening  64  through the opposite side wall  31  lies on the tangent and is not angled. 
   Following the formation of the smaller diameter holes by the EDM process, the assemblies are degreased, rinsed, air-dried and dried in an oven similarly as previously described. The old but preferably a new inlet flow conditioner  13  is then cleaned and weld prepped for attachment to the returned fuel nozzle assembly. For example, the two halves of the new inlet flow conditioner are welded along a horizontal line of symmetry as well as circumferentially. Typical welding procedures are followed including inspection and fluorescent penetration inspection. 
   While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.