Patent Abstract:
A burner assembly for a firing system for firing fluidic fuels is provided. The burner assembly has a burner hub, an air inlet channel and a fuel inlet channel, wherein the fuel inlet channel is at least partially designed in the burner hub. A screening wall is arranged in the fuel inlet channel. The screening wall is spaced apart from a wall of the fuel inlet channel so that an intermediate space is formed between the wall of the fuel inlet channel and the screening wall. A sleeve is equipped with at least one radial positioning means that ensures a clearance of the sleeve from the wall of the fuel inlet channel. The at least one radial positioning means of the sleeve is designed as a positioning projection arranged in a circular manner, protruding out radially.

Full Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/EP2010/053060 filed Mar. 11, 2010, and claims the benefit thereof. The International Application claims the benefits of European Patent Application No. 09155441.0 EP filed Mar. 18, 2009. All of the applications are incorporated by reference herein in their entirety. 
     FIELD OF INVENTION 
     The invention relates to a burner arrangement for firing fluidic fuels and in particular a burner arrangement for a gas turbine installation. 
     BACKGROUND OF INVENTION 
     Burner arrangements for firing fluidic fuels are used inter alia to operate gas turbines in power plants and other large machine applications. What are known as dual fuel burners are used in particular here, being provided optionally or combined to fire liquid and gaseous fuels, for example natural gas and fuel oil. 
     The burner arrangements have correspondingly large dimensions and feature a complex structure with a number of fuel supply ducts. Thus for example a centrally disposed smaller dimensioned pilot burner with its own fuel supply and air supply is frequently used to stabilize the flame of a large main burner, which is disposed around the pilot burner. The large main burner is mainly operated in lean mixture mode with excess oxygen to achieve more favorable emission values. However lean mixture mode means that the flame of the main burner is subject, at least in certain operating states, to fluctuations which are compensated for by a constantly igniting action of the pilot burner. Such a burner arrangement is set out for example in EP 0 580 683 B1. 
     One challenge with such burners is the mechanical stress resulting due to irregular thermal distribution in the walls of the metal housing, known as the hub, in which the annular supply ducts of the gas and oil energy carriers are disposed relatively close to one another. An annular gas chamber feeds the main burner on the input side in relation to the flow direction of the incoming air upstream of what are known as the swirl blades which swirl and mix the air flow with the combustion gas or through the swirl blades. An oil supply is also present, being generally disposed closer to the burner output than the gas supply. It comprises an annular oil chamber and an oil supply duct leading to the annular chamber, said duct being disposed in the hub wall between the annular gas chamber and the pilot burner. 
     Since gas is less dense than oil, it takes up a larger cross section, with the result that the dimensions of the gas supply are much larger than those of the oil supply. The part of the burner hub with the gas supply therefore has a larger outer surface facing the air duct than the oil supply. The air supply is effected with precompressed air, which has passed through a compressor, with the result that due to compression said supplied air has a temperature that is already above 400° C. The region of the burner hub with the gas supply is therefore quickly heated to a temperature in the region of above 400° C. and remains at this operating temperature. The oil supply duct leading to the annular oil chamber in contrast is further away from the hot air supply duct so that the oil in the oil supply duct is barely heated and therefore only has a temperature of around 50° C. 
     Since on the one hand the burner hub is significantly heated in the region of the annular gas chamber and on the other hand the adjacent oil supply duct is much cooler, the wall between the annular gas chamber and the oil supply duct is subject to a large temperature gradient. The temperature gradient causes thermal stress which shortens the service life of such burner hubs and makes it necessary to use a high-quality material with the costs this entails. Such stresses also occur in other regions where a cold fuel is carried through a hot hub region. 
     SUMMARY OF INVENTION 
     An object of the present invention is to reduce thermally induced stresses in the burner hub of the burner arrangement. 
     This object is achieved by a burner arrangement as claimed in the independent claim. The dependent claims contain advantageous embodiments of the invention. 
     A burner arrangement for a firing installation for firing fluidic fuels comprises a burner hub, at least one air supply duct and at least one fuel supply duct for each type of fuel, the at least one fuel supply duct being configured at least partially in the burner hub. Disposed in at least one fuel supply duct is a shielding wall, which is at a distance from the wall of the fuel supply duct, so that an intermediate space that is not part of the flow path of the fuel flowing through the fuel supply duct is formed between the wall of the fuel supply duct and the shielding wall. The shielding wall is formed by a sleeve introduced into the fuel supply duct. To ensure the correct radial position of the sleeve in the fuel supply duct, it is equipped with at least one radial positioning means, which ensures a gap between the sleeve and the wall of the fuel supply duct, it being possible to select the gap in particular in respect of the maximum permitted heat transfer rate. Restrictions result here however from the space available in the hub. The at least one radial positioning means of the sleeve is embodied as a positioning projection that is disposed to run in a circle and projects radially outward. 
     In the inventive burner arrangement the intermediate space forms a poor heat-conducting region compared with the surrounding metal of the burner hub, thermally insulating the metal of the hub from the flowing fuel and thereby limiting the exchange of heat between the fuel and the burner hub. In particular the sleeve can feature at least one positioning projection respectively running in a circle in the region of its two ends. This makes the alignment of the sleeve apparatus more reliable and the natural vibrations that may occur due to the clearance gaps in the fuel flow are excluded. 
     The at least one positioning projection of the sleeve can further feature an annular groove, which is in particular advantageous if the positioning projection is located in the region of a connection point between the fuel supply duct and a fuel supply pipe. The annular groove then makes it possible when welding or soldering the fuel supply pipe to the fuel supply duct to avoid permanently welding or permanently soldering the positioning projection to the fuel supply duct and/or the fuel supply pipe. 
     The sleeve can also be equipped with at least one axial positioning means, which interacts with at least one axial positioning means present in the fuel supply duct to position the sleeve axially. This allows axial positioning of the sleeve without a material-fit connection. There may in particular be an axial clearance here between the axial positioning means of the sleeve and the axial positioning means in the fuel supply duct, allowing thermal expansion of the sleeve in an axial direction without generating stresses. 
     In one structurally simple embodiment the axial positioning means of the sleeve can be configured as at least one guide edge on an end surface of the positioning projection. The axial positioning means in the fuel supply duct is then embodied as a counter guide edge. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features, properties and advantages of the invention will emerge from the description which follows of an exemplary embodiment with reference to the accompanying figures, in which: 
         FIG. 1  shows a known burner arrangement, 
         FIG. 2  shows a known embodiment of the burner hub of a burner arrangement, 
         FIG. 3  shows a schematically exaggerated consequence of the thermally induced stress in the burner hub according to the prior art from  FIG. 2 , 
         FIG. 4  shows a cross-sectional view of a preferred embodiment of the inventive burner arrangement, and 
         FIG. 5  shows an enlarged partial cross-sectional view from  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
       FIG. 1  shows a burner arrangement according to the prior art, which can optionally be used in conjunction with a number of arrangements of the same type, for example in the combustion chamber of a gas turbine installation. 
     It consists of an inner part, the pilot burner system, and an outer part lying concentric thereto, the main burner system. Both systems are suitable for operation with gaseous and/or liquid fuels in any combination. The pilot burner system comprises a central oil supply  1  (medium G) with an oil nozzle  5  disposed at its end and an inner gas supply duct  2  (medium F) disposed concentrically around the central oil supply  1 . This in turn is surrounded by an inner air supply duct  3  (medium E) disposed concentrically around the axis of the burner. A suitable ignition system, for which many possible embodiments are known, can be disposed in or on the inner air supply duct  3 . This is therefore not illustrated here. The inner air supply duct  3  features a swirl blade system  6  in its end region. The pilot burner system can be operated in a manner known per se, in other words predominantly as a diffusion burner. Its task is to maintain the main burner in stable burn mode since it is generally operated with a lean mixture to reduce harmful emissions, thus requiring stabilization of its flame by means of a diffusion flame or a flame based on a less lean mixture. 
     The main burner system features an outer annular air supply duct system  4  disposed concentrically to the pilot burner system and running obliquely to this. This annular air supply duct system  4  is also provided with a swirl blade system  7 . The swirl blade system  7  consists of hollow blades with outlet nozzles  11  in the flow cross section of the annular air supply duct system  4  (medium A). These are fed from a gas supply line  19  and an annular gas duct  9  through openings  10 . The burner also features an oil supply line  23 , which opens into an annular oil duct  13 , which for its part features outlet nozzles  14  in the region or downstream of the swirl blade system  7 . 
       FIG. 2  shows an embodiment of the burner hub  18  of a burner arrangement according to the prior art in cross section. 
     The burner hub  18  features welded cast plugs not shown) in the manner of a cast part configured as a single piece, used to seal the auxiliary openings that served for the removal of the molded cores. 
     Disposed in the burner hub  18  are an annular gas chamber  9  and an annular oil chamber  13 . On the outward facing and tapering side surface of the burner hub  18  the annular chambers  9  and  13  each have a plurality of outlet openings  10  and  14 , through which the respective fuel (medium B or as the case may be medium C in  FIG. 1 ) are sprayed out into the combustion chamber  24  (see  FIG. 1 ). 
       FIG. 3  shows a schematically exaggerated consequence of the thermally induced stresses in the burner hub according to the prior art from  FIG. 2 . The stresses cause the wall  21  between the annular gas chamber  9  and the oil supply line  23  to become deformed. This deformation of the metal cast and/or welded burner hub  18  results from the temperature gradient in the wall between the oil supply duct  23 , through which the oil flows at a temperature of approx. 50° C., and the annular gas chamber  9 , which because it is heated by the compressor air in the air supply duct  4  (medium A in  FIG. 1 ) is heated to around 420° C. 
       FIG. 4  shows a segment of a cross section through an embodiment of the inventive burner arrangement. The burner arrangement comprises a burner hub  18 , in which are disposed an annular gas chamber  9  with a gas supply duct  19  (not shown in  FIG. 4 ) and an annular oil chamber  13  with an oil supply duct  23 . The basic structure of the burner arrangement corresponds to the structure described with reference to  FIGS. 1 and 2 . Therefore only the differences in respect of the burner structure described in  FIGS. 1 and 2  are described. 
     In the inventive burner arrangement a shielding wall  30  is disposed in the oil supply duct  23  such that an intermediate space  38  is formed between the wall between the annular gas chamber  9  and the oil supply line  23  on the one hand and the shielding wall  30  on the other hand. This intermediate space  38  insulates the flow path of the oil formed by the inner surface of the shielding wall  30  thermally from the wall  21  between the annular gas chamber  9  and the oil supply line  23 , since the medium present in the intermediate space, for example air or non-flowing or barely flowing oil, has a very much lower heat conductivity than the metal of the burner hub  18 . The heat conductivity of air is for example 0.023 W/mK and that of oil around 0.15 W/mK (at room temperature). The heat conductivity of metals is two to three orders of magnitude higher in contrast. The intermediate space  38  can therefore be seen as an adiabatically active thermal shield. The dimension of the gap s between the wall  21  and the shielding wall  30  can be used structurally to set a desired heat transfer rate. 
     The shielding wall is realized in the form of a sleeve  30  inserted into the oil supply duct  23 , which prevents direct contact between the cold oil flowing along the flow path in the oil supply duct  23  and the wall  21  between the annular gas chamber  9  and the oil supply line  23 . The outer diameter of the sleeve  30  is dimensioned smaller by a predefined amount than the inner diameter of the oil supply duct  23 , so that an intermediate space  38  is formed between the inserted sleeve  30  and the wall  21 , in which a medium is present with a much lower heat conductivity than the metal of the burner hub  18 . The oil flowing through the sleeve  30  disposed at a distance from the wall  21  therefore barely causes the wall  21  to be cooled, with the result that the temperature gradient between the surface on the side of the annular gas chamber and the surface of the wall  21  on the side of the oil duct becomes smaller. Therefore much fewer mechanical stresses occur than in the prior art. 
     Oil itself can be used in the simplest instance as a suitable medium in the intermediate space  38 , as long as there is no risk of ignition, as it is then not necessary to seal the intermediate space  38  off from the flow path of the oil. 
     In order to be able to mount the sleeve  30  simply in the oil supply duct  23  of the burner hub  18 , it is configured as a sleeve  30  that can be inserted into an opening in a tubular segment  37  of the oil supply duct  23 . To this end the sleeve  30  has at its upstream end an annular positioning projection  33 , preferably running in a circle, which serves as a spacer to center the sleeve body radially in the hollow space  23  and at the same time has the function of a guide edge  53 , which abuts against a complementary counter guide edge  52  present in the region of the opening of the tubular projection  37  and thus defines the position of the sleeve  30  in an axial direction. For clarification  FIG. 5  shows an enlarged partial cross sectional view of the tubular segment  37  of the oil supply duct  23  and the sleeve  30  introduced therein. 
     At its upstream end the sleeve  30  has a positioning projection  33  with an annular groove  36 . The annular groove  36  is located, when the sleeve  30  is inserted into the oil supply duct  23 , at the level of the plane in which the opening of the tubular segment  37  is located. Thus when the tubular segment  37  is welded to an oil supply pipe  32 , the weld seam  31  is located in the region of the annular groove  36 , so that when the two pipe ends  30  are connected, the positioning projection  33 , and therefore the sleeve  30 , is not permanently welded or burned into place. 
     The positioning projection  33  is disposed in a widened milled groove in the tubular segment  37  and a corresponding milled groove in the oil supply line pipe  32 . Like the milled groove in the tubular segment  37  the milled groove in the oil supply line pipe  32  also has a counter guide edge  50 , which interacts with a guide edge  51  of the positioning projection  33 . This means that the sleeve  30  is not only centered by the positioning projection  33  in the oil supply duct  23  but it is also secured in the direction of the longitudinal axis Y. 
     The described manner of positioning may already be adequate in the context of the invention but the present embodiment features a further positioning projection  35  ( FIG. 4 ), which is disposed in proximity to the downstream end of the sleeve  30 . It can effectively counter for example any natural vibrations that may occur in the sleeve  30 . The positioning projection  35  disposed at the downstream end of the sleeve  30  is also preferably embodied as an annular projection running in a circle and its preferably cylindrically embodied outer diameter extends to the wall of the hollow space  38 , so that it also helps to center the sleeve  30 . 
     All the positioning projections  33 ,  35  preferably feature a diameter that is dimensioned so that there is a sufficient gap between the walls of the hollow space  30  and the cylindrical outer surfaces of the positioning projections to compensate for different thermal expansions. This means that on the one hand the sleeve  30  is positioned accurately enough in a radial direction and on the other hand that it is never trapped during operation. The stresses that also occur in the burner hub  18  as a result of trapping are thus effectively avoided. 
     According to the invention the thermal expansion of the sleeve  30  in an axial direction Y is also embodied to be free from such trapping as it would produce stress. To this end the positioning projection  33  in the milled grooves of the tubular segment  37  and the oil supply line pipe  32  is dimensioned so that a predefined clearance d is present between the counter guide edge  50  in the milled groove of the oil supply line pipe  32  and the corresponding guide edge  51  of the positioning projection  33 , allowing thermal expansion of the sleeve in an axial direction without stresses building up in an axial direction Y as a result. 
     The sleeve  30  can be mounted in the inventive burner arrangement by introducing it into the fuel supply duct  23  through the opening of the tubular segment  37  of the fuel supply duct  23  to be connected to a fuel supply pipe  32  until the guide edge  53  of the positioning projection  33  comes up against the counter guide edge  52  in the milled groove of the tubular segment  37 . The fuel supply pipe  32  is then positioned on the upstream end of the tubular segment  37  and connected with the aid of a welding procedure to the tubular segment  37 , the annular groove  36  preventing permanent welding of the sleeve to the fuel supply pipe  32  and/or to the tubular segment  37 . 
     With the described embodiment of the sleeve  30  and of the milled grooves of the tubular segment  37  and the oil supply line pipe  32  it is possible to prevent both axial and radial stresses due to trapping of the sleeve  30 . 
     Although in the context of the exemplary embodiment the invention has been described with reference to a specific oil supply duct, it can also be applied in other fuel supply ducts. Also the sleeve does not have to have a round cross section but can also have an angular cross section.

Technology Classification (CPC): 5