Abstract:
This disclosure describes an improved method of electron beam (“EB”) welding utilizing a collection pocket. The method includes providing a first surface and a second surface, forming a collection pocket in at least one of the first surface and the second surface, coupling the first surface to the second surface at a joining location, and EB welding the first surface and the second surface to each other at the joining location. The collection pocket captures and contains excess weld material to prevent the excess material from escaping the joining location, and also reduces an amount of wall thickness required for EB welding. A method of reconditioning gas turbine components is also disclosed.

Description:
TECHNICAL FIELD 
       [0001]    The field of the invention relates to electron beam (“EB”) welding. 
       BACKGROUND OF THE INVENTION 
       [0002]    Gas turbines include numerous components. These components may include a combustor for mixing air and fuel for ignition, a turbine blade and rotor assembly for producing power, and a fuel nozzle assembly for providing fuel to the combustor for operation of the gas turbine. Fuel nozzle assemblies in gas turbines often include a fuel nozzle end cover with at least one fuel nozzle insert that is brazed into the fuel nozzle end cover. 
         [0003]    Gas turbine components, including fuel nozzle assemblies, are frequently located near the combustor and typically must withstand high temperatures for extended periods of time. As a result, durability limits of these components are often reached or exceeded, requiring replacement, repair, and/or reconditioning/refurbishing of the components for continued operation of the gas turbine. 
         [0004]    Replacing, repairing, and/or reconditioning gas turbine components, including fuel nozzle inserts, is often challenging, due to the limitations of traditional brazing and EB welding. EB welding is useful in gas turbine assemblies because EB welded joints have increased ability to handle tension from thermal strain, compared to brazed joints. EB welded joints also have the ability to yield and distribute loads. However, traditional EB welding may require a geometric structure, such as a backing shelf or other geometric configuration, to work effectively. Additionally, there is a requisite loss of wall thickness and weakening of wall integrity required to form a backing shelf, a limited containment of excess weld material escaping the weld joints, and also a limited number of repairs that can be performed, due to loss of wall thickness. As a result, a new and improved method of EB welding is needed that addresses these problems, among others. 
       SUMMARY 
       [0005]    This summary presents a high-level overview of various aspects of the invention and a selection of concepts that are further described below in the detailed description section. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in isolation to determine the scope of the claimed subject matter. The scope of the invention is defined by the claims. 
         [0006]    In brief, and at a high level, this disclosure describes, among other things, an improved method of EB welding that reduces leakage of excess weld material, improves integrity of welded surfaces, and allows for greater versatility of EB welding, due to reduced geometric requirements. The method may include forming a collection pocket in at least one of a first and a second surface that are to be EB welded together, coupling the first and second surfaces together at an EB welding location, and EB welding the first and second surfaces together at the EB welding location. The collection pocket may be located at least partially between the first and second surfaces at the EB welding location to collect and retain excess weld material. The method may be used in tight-tolerance or thin-walled applications where EB welding with a backing shelf, which provides alignment and a barrier, may be difficult or impossible due to geometric constraints. The method, in one exemplary application, allows for improved replacement and reconditioning of a fuel nozzle insert in a fuel nozzle assembly of a gas turbine. 
         [0007]    In a first embodiment, an electron beam (EB) welded turbine component is provided. The component comprises an insert, or a component thereof, and a receiving component comprising a base material that forms a cavity corresponding to a shape of at least a portion of the insert or the component thereof. The outer surface of the insert or the component thereof is EB welded to an inner surface of the cavity at a first location, and, at the first location, at least one of the outer surface of the insert or the component thereof and the inner surface of the cavity includes a collection pocket. 
         [0008]    In a second embodiment, a method of reconditioning a turbine component with electron beam (EB) welding is provided. The method comprises providing a receiving component comprising a base material that forms a cavity having an inner surface, providing an insert or a component thereof having an outer surface, forming a collection pocket on at least one of the inner surface and the outer surface, coupling the inner surface to the outer surface at a first location, and EB welding the inner surface and the outer surface together at the first location. 
         [0009]    In a third embodiment, a method of EB welding gas turbine components is provided. The method comprises providing a first component having a first surface, providing a second component having a second surface, forming a collection pocket in at least one of the first surface and the second surface, and EB welding the first surface to the second surface. 
         [0010]    Although the EB welding methods, devices, and systems described in this disclosure are described in the context of gas turbine components, assemblies, and systems, the methods described herein may be used for joining any two surfaces where effective EB welding is desired, and should not be limited merely to components, assemblies, and systems of gas turbines. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0011]    Illustrative embodiments of the present invention are described in detail below with reference to the attached drawing figures, wherein: 
           [0012]      FIG. 1  depicts a partial, cross-sectional, perspective view of a fuel nozzle assembly with an end cover and multiple fuel nozzle inserts, in accordance with an embodiment of the present invention; 
           [0013]      FIG. 2  depicts a partial, side elevation, fragmentary view of a fuel nozzle insert of  FIG. 1  connected to a fuel passageway, in accordance with an embodiment of the present invention; 
           [0014]      FIGS. 3A-3D  depict perspective views of a set of components that together form an assembled fuel nozzle insert, in accordance with an embodiment of the present invention; 
           [0015]      FIGS. 4A-4F  depict an angled, perspective, cross-sectional view of a fuel nozzle end cover having a cavity in which a fuel nozzle insert is pre-installed, removed, and refurbished/reconditioned, respectively, in accordance with an embodiment of the present invention; 
           [0016]      FIG. 5  depicts a traditional EB welding configuration utilizing a backing shelf, in accordance with an embodiment of the present invention; 
           [0017]      FIGS. 6A-6B  depict first and second exemplary EB welding configurations with an undercut geometry that forms a collection pocket, in accordance with an embodiment of the present invention; 
           [0018]      FIG. 7  depicts an exemplary EB weld utilizing a collection pocket, in accordance with an embodiment of the present invention; 
           [0019]      FIG. 8  depicts a block diagram of a method of reconditioning a turbine component with EB welding, in accordance with an embodiment of the present invention; and 
           [0020]      FIG. 9  depicts a block diagram of a method of EB welding gas turbine components, in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    The subject matter of the various embodiments of the present invention is described with specificity in this disclosure to meet statutory requirements. However, the description is not intended to limit the scope of invention. Rather, the claimed subject matter may be embodied in various other ways to include different features, components, elements, combinations, and steps, similar to the ones described in this document, and in conjunction with other present and future technologies. Terms should not be interpreted as implying any particular order among or between various steps unless the stated order of steps is explicitly required. Many different arrangements of the various components depicted, as well as use of components not shown, are possible without departing from the scope of the claims. 
         [0022]    At a high level, the present invention generally relates to an improved method of EB welding that incorporates a collection pocket. The collection pocket may be formed in one or more surfaces that are to be EB welded together, to reduce the deposit of excess weld material around the EB weld, and also to reduce an amount of wall thickness required to form a secure EB weld connection, due to the reduced requirement for specific wall geometry (e.g., a backing shelf). For example, the method may be applied to replacement of an insert, such as a fuel nozzle insert in an assembly of a gas turbine. In such an example, a pre-installed insert, such as a fuel nozzle insert, may be removed from a receiving component, such as a fuel nozzle end cover, leaving a cavity, an outer surface of a fuel nozzle insert component may be coupled to an inner surface of the cavity at an EB welding location, and the outer surface may be EB welded to the inner surface at the EB welding location. Additionally, prior to EB welding the surfaces, a collection pocket may be formed on at least one of the inner surface and the outer surface, such that the collection pocket is at least partially between the inner and outer surfaces at the EB welding location. In this regard, in any application where two surfaces are being EB welded together, the collection pocket may be formed on one or both of the surfaces that are EB welded, such that in either scenario, the collection pocket is at least partially between the surfaces that are EB welded. 
         [0023]    Having described some general aspects of the invention, reference is now made to  FIG. 1 , which depicts a partial, cross-sectional, perspective view of a fuel nozzle assembly  100  with an end cover  102  and multiple fuel nozzle inserts  108  located in the end cover  102 , in accordance with an embodiment of the present invention. In  FIG. 1 , the end cover  102  includes a plurality of cavities  106  that are each configured to receive at least a portion of a corresponding fuel nozzle insert  108 . The fuel nozzle insert  108  may be brazed, EB welded, or otherwise secured to an inner surface  110  of the cavity  106 , and may, prior to installation, be provided as one or multiple components. The configuration of the end cover  102  shown in  FIG. 1  may limit the ability to provide backing shelves for traditional EB welding of replacement fuel nozzle inserts  108  without causing thin wall issues between the fuel nozzle inserts  108  in the end cover  102 . As a result, fewer repairs for replacing the fuel nozzle inserts  108  may be possible, due to the ever increasing proximity of the cavities  106  for the fuel nozzle inserts  108  in the end cover  102 . 
         [0024]    Referring now to  FIG. 2 , a partial, side elevation, fragmentary view of one of the fuel nozzle inserts  108  of  FIG. 1  connected to a fuel passageway  112  is provided, in accordance with an embodiment of the present invention. In  FIG. 2 , the end cover  102  is shown with the cavity  106  formed such that it can receive at least a portion of the fuel nozzle insert  108 . Further, the fuel nozzle insert  108  includes a plurality of components  114  which, when installed and assembled, form the fuel nozzle insert  108 . The end cover  102  includes a base material  116  that forms a shape of the cavity  106 . The fuel nozzle insert  108  is brazed, EB welded, or otherwise secured to the base material  116  at the cavity  106  to join the end cover  102  and the components  114  of the fuel nozzle insert  108 . 
         [0025]    Referring now to  FIGS. 3A-3D , a series of fuel nozzle insert components  120 ,  122 ,  124 ,  128  that may be EB welded to an end cover  102  to form an installed, assembled fuel nozzle insert  108  is provided, in accordance with an embodiment of the present invention. A traditional braze repair and reconditioning of a fuel nozzle insert  108  may be performed with a single insert. However, the EB welding method described herein may utilize multiple insert components  120 ,  122 ,  124 ,  128  that are distinct, as shown in  FIGS. 3A-3D , allowing for a sequenced installation.  FIG. 3A  depicts a first component  120  which may be coupled and EB welded in a cavity  106  of the end cover  102 .  FIG. 3B  depicts a second component  122  which may be coupled and EB welded in the cavity  106  of the end cover  102  at a location distinct from the first component  120 .  FIG. 3C  depicts a third component  124  which may be coupled and EB welded in the cavity  106  of the end cover  102  at a location distinct from the first and the second components  120 ,  122 . Additionally,  FIG. 3D  depicts a spacer  128  which may be installed at an opening  154  of the cavity  106  after assembly of the fuel nozzle insert  108  in the end cover  102  by EB welding the components  120 ,  122 ,  124  in place in the cavity  106 . 
         [0026]    Referring now to  FIGS. 4A-4F , an angled, perspective, cross-sectional view of a fuel nozzle end cover  102  having a cavity  106  in which a fuel nozzle insert  108  is pre-installed, removed, and refurbished/reconditioned, respectively, is provided, in accordance with an embodiment of the present invention. In  FIG. 4A , the end cover  102  is shown with the fuel nozzle insert  108  brazed to the inner surface  110  of the cavity  106 .  FIG. 4B  shows the cavity  106  after machining to remove the fuel nozzle insert  108 , leaving the inner surface  110  of the cavity  106  exposed, and an opening  154  in the cavity  106 . Further, by using the EB welding process described herein, only a minimal amount of wall thickness (i.e., base material  116 ) must be removed when extracting the fuel nozzle insert  108 , due to the reduced welding surface geometry required for EB welding with a collection pocket  118  (as shown in  FIGS. 4C-4F ) instead of a backing shelf or other geometric feature. 
         [0027]      FIG. 4C  depicts an installation of a first component  120  of a fuel nozzle insert  108  in the cavity  106  of the end cover  102 . The first component  120  includes an outer surface  132  on which a plurality of collection pockets  118 , which may be curved indentations or depressions in the outer surface  132 , are formed. The inner surface  110  of the cavity  106  and the outer surface  132  of the first component  120  are coupled at a first location  134 , and the collection pocket  118  is disposed, or located, between the inner surface  110  of the cavity  106  and the outer surface  132  of the first component  120  at the first location  134 . 
         [0028]    The first location  134  may be described as a portion of the inner surface  110  of the cavity  106  and a portion of the outer surface  132  of the first component  120  that are in contact with each other, and between or in which the collection pocket  118  is disposed, or located. The first component  120 , once coupled against the inner surface  110  of the cavity  106 , may be EB welded from first and/or second ends  136 ,  138  of the first location  134 , joining the material of the inner surface  110  and the outer surface  132  at the first location  134 . As the EB welding is performed, even without a backing shelf or other geometric feature built into the surfaces  110 ,  132 , the collection pocket  118  may help to receive, retain, collect, and store excess weld material (e.g., weld blow, weld spatter, weld leakage, etc.) escaping the first location  134 . In gas turbine assemblies, excess weld material outside of EB welded joints may interfere with operation of the gas turbine, or cause detrimental effects to the gas turbine, and as a result, it is desirable to avoid such excess material buildup. Reducing excess weld material and maintaining maximum wall thickness by EB welding with a collection pocket may allow for repeated reconditioning processes, as well as protection of internal components, which may extend the life of the end cover  102  or another gas turbine component which is EB welded. 
         [0029]      FIG. 4D  depicts an installation of a second component  122  of the fuel nozzle insert  108  in the cavity  106  of the end cover  102 . The second component  122  includes an outer surface  140  on which a collection pocket  118 , which may be a curved indentation or depression in the outer surface  140  of the second component  122 , is formed. The inner surface  110  of the cavity  106  and the outer surface  140  of the second component  122  are coupled at a second location  142  that is separate from the first location  134 . The collection pocket  118  is disposed, or located, between the inner surface  110  of the cavity  106  and the outer surface  140  of the second component  122  at the second location  142 . The second location  142  may be described as a portion of the inner surface  110  and a portion of the outer surface  140  that are in contact with each other, so that EB welding of the surfaces  110 ,  140  may occur, and between or against which the collection pocket  118  is positioned or formed. EB welding may be performed from either end  136 ,  138  of the second location  142 , to join the outer surface  140  of the second component  122  and the inner surface  110  of cavity  106  at the second location  142 . 
         [0030]      FIG. 4E  depicts an installation of a third component  124  of the fuel nozzle insert  108  in the cavity  106  of the end cover  102 . The third component  124  includes an outer surface  144  on which a collection pocket  118 , which may be a curved indentation or depression in the outer surface  144  of the third component  124 , is formed. The third component  124  further includes an inner surface  146  that is coupled to the outer surface  140  of the second component  122  at a third location  148 , the coupling including a traditional backing shelf  150  where excess material from EB welding at the third location  148  may be collected and contained. Additional EB welding may be performed between the inner surface  146  of the third component  124  and the outer surface  140  of the second component  122  at the third location  148 . 
         [0031]    Furthermore, the inner surface  110  of the cavity  106  and the outer surface  144  of the third component  124  are coupled at a fourth location  152  that is separate from the first, second, and third locations  134 ,  142 ,  148 . A collection pocket  118  is disposed, or located, between the inner surface  110  of the cavity  106  and the outer surface  144  of the third component  124  at the fourth location  152 . The fourth location  152  may be described as a portion of the inner surface  110  of the cavity  106  and a portion of the outer surface  144  of the third component  124  that are in contact with each other, so that EB welding of the surfaces  110 ,  144  can occur, and between or against which the collection pocket  118  is located. EB welding may occur from first or second ends  136 ,  138  of the fourth location  152 , to join the outer surface  144  of the third component  124  and the inner surface  110  of the cavity  106 .  FIG. 4F  depicts an installation of the spacer  128 , which may be welded or otherwise coupled around an opening  154  of the cavity  106  in the end cover  102 . 
         [0032]    In the assembly process illustrated in  FIGS. 4C-4F , it should be noted that the collection pocket  118  may be at least partially positioned, formed, or located on either or both surfaces at each EB welding location, including the first, second, and fourth locations  134 ,  142 ,  152 . For example, at the first location  134  where the outer surface  132  of the first component  120  and the inner surface  110  of the cavity  106  are joined, the collection pocket  118 , although depicted as formed in the outer surface  132  of the first component  120 , may alternatively or additionally be formed in the inner surface  110  of the cavity  106 . Additionally, one or multiple collection pockets  118  may be used in each weld location. The collection pocket  118  may also include a curved contour (shown in  FIGS. 6A and 6B ), which may help to collect and channel excess weld material into the collection pocket  118 . Furthermore, the collection pocket  118  may be formed at each EB welding location such that it is in fluid communication with the EB weld so that it can receive, collect, and retain at least a portion of any excess weld material generated from the EB welding of the corresponding surfaces. 
         [0033]    Referring now to  FIG. 5 , an exemplary traditional EB welding configuration  500 , as used in the third location  148  shown in  FIG. 4E , utilizing a backing shelf  502 , is provided, in accordance with an embodiment of the present invention. In  FIG. 5 , a first surface  504  is coupled to a second surface  506 . The first surface  504  and the second surface  506  each include a multi-directional geometry that forms a backing shelf  502  which may be used to control an amount of excess weld material escaping from at least one of the ends  136 ,  138  of the weld, and help secure the first surface  504  to the second surface  506 . 
         [0034]    Referring now to  FIGS. 6A and 6B , an exemplary EB weld configuration  600  with an undercut geometry that forms a collection pocket  118  is provided, in accordance with an embodiment of the present invention. As described herein, the collection pocket  118  is an integrated feature of first and second surfaces  602 ,  604  of the weld configuration  600 . As shown in  FIG. 6A , the first surface  602  is coupled to the second surface  604 , and the collection pocket  118  is formed or shaped into the first surface  602 , such that the first and second surfaces  602 ,  604  can be EB welded together at an EB welding location  606  with the collection pocket  118  between the first and the second surfaces  602 ,  604 . In this respect, the collection pocket  118  may collect excess weld material generated from EB welding the first and second surfaces  602 ,  604  at least partially together at the EB welding location  606 . 
         [0035]    The EB welding location  606  may be described as the length between the first end  136  and the second end  138  along which the first and second surfaces  602 ,  604  are coupled and EB welded. The EB welding may be performed from either end  136 ,  138  of the EB welding location  606 , including both ends, depending on the geometric arrangement of components and structures to which the first and second surfaces  602 ,  604  are joined (i.e., the accessibility of each end  136 ,  138  for performing EB welding).  FIG. 6B  shows an alternative configuration  608  where the collection pocket  118  is formed or shaped into the second surface  604 , instead of the first surface  602 . 
         [0036]    The collection pocket  118  may be formed or constructed to include different shapes, sizes, and/or orientations. For example, the collection pocket  118  may have straight portions, curved portions, or be defined by shapes formed in adjacent surfaces joined together for EB welding. Additionally, the collection pocket  118  may be circular, ovular, elliptical, square, rectangular, and/or symmetrical or asymmetrical. Additionally, the collection pocket  118  may be positioned on the inner surface  110  of the cavity  106  and may be oriented towards an interior  111  of the cavity  106 , or the collection pocket  118  may be positioned on an outer surface (e.g., outer surface  132 ) of an insert component (e.g., component  120 ) and may be oriented away from the interior  111  of the cavity  106 , as exemplified in  FIGS. 4C-4F . 
         [0037]    Referring now to  FIG. 7 , an exemplary EB weld  700  utilizing a collection pocket  118  is provided, in accordance with an embodiment of the present invention. In  FIG. 7 , an EB welded portion  704  is formed from the first end  136  of the EB welding location  606 . Further,  FIG. 7  depicts the EB welded portion  704  joining a portion of the first and second sides  602 ,  604  during the associated EB welding process. As the first and second sides  602 ,  604  are EB welded together, excess weld material  702  produced from EB welding the first and second sides  602 ,  604  is deposited into the collection pocket  118 , rather than out the second end  138  of the EB welding location  606 . This helps to reduce buildup of the excess weld material  702  outside of the EB welding location  606 . As shown in  FIG. 7 , the EB welded portion  704  forms a tapered, or nail-like, shape in the first and second surfaces  602 ,  604 . 
         [0038]    Referring now to  FIG. 8 , a block diagram of a method  800  of reconditioning a turbine component with EB welding is provided, in accordance with an embodiment of the present invention. At a block  810 , a receiving component, such as the fuel nozzle end cover  102  shown in  FIG. 1 , is provided, the receiving component comprising a base material, such as the base material  116  shown in  FIG. 2 , that forms a cavity, such as the cavity  106  shown in  FIG. 2 , having an inner surface, such as the inner surface  110  shown in  FIG. 2 . At a block  812 , an insert, such as the fuel nozzle insert  108  shown in  FIG. 1 , or a component thereof, such as the first, second, or third components  120 ,  122 ,  124  shown in  FIG. 4F , having an outer surface, such as one of the outer surfaces  132 ,  140 ,  144  shown in  FIG. 4F , is provided. At a block  814 , a collection pocket, such as the collection pocket  118  shown in  FIG. 4F , is formed on at least one of the inner surface and the outer surface. At a block  816 , the inner surface is coupled to the outer surface at a first location, such as the first location  134  shown in  FIG. 4F . At a block  818 , the inner surface and the outer surface are EB welded together at the first location. 
         [0039]    Referring now to  FIG. 9 , a block diagram of a method  900  of EB welding gas turbine components is provided, in accordance with an embodiment of the present invention. At a block  910 , a first component, such as the first component  120  shown in  FIG. 4F , having a first surface, such as the outer surface  132  shown in  FIG. 4F , is provided. At a block  912 , a second component, such as the end cover  102  shown in  FIG. 4F , having a second surface, such as the outer surface  110  in the cavity  106  shown in  FIG. 4F , is provided. At a block  914 , a collection pocket, such as the collection pocket  118  shown in  FIG. 4F , is formed in at least one of the first surface and the second surface. At a block  916 , the first surface is EB welded to the second surface. 
         [0040]    A system for reconditioning a turbine component with EB welding is also provided, in accordance with an embodiment of the present invention. The system may comprise a fuel nozzle end cover comprising a base material that forms a cavity having an inner surface, and a fuel nozzle insert or a plurality of components thereof, wherein the fuel nozzle insert or the plurality of components thereof include a respective outer surface that is EB welded to the inner surface of the cavity at a separate location. Additionally, at each separate location, one of the inner surface of the cavity and the outer surface of the fuel nozzle or respective component thereof includes a collection pocket. 
         [0041]    Removal of pre-installed fuel nozzle inserts in the end cover, and installation of a replacement fuel nozzle insert, may be performed in multiple steps. For example, for an existing brazed insert, or otherwise installed insert, a horizontal boring mill, or other device, may be used to remove the pre-installed insert and leave a semi-finished cavity. Next, a vertical boring machine, or other device, may be used to provide a machined finish to the cavity. Then, the inner surface of the resulting cavity may be further prepared as needed for proper EB welding (e.g., polishing, finishing, stress relief, heat treating, etc.), and installing of the components may be commenced. After completing the EB welding process, additional pressure testing, heat treating, or polishing may occur to provide a fully finished, reconditioned fuel nozzle insert. 
         [0042]    A further exemplary process of replacing or reconditioning a fuel nozzle insert may include rough machining a pre-installed fuel nozzle insert to remove at least a portion of the material forming the pre-installed insert, final machining the cavity in which the pre-installed insert was located, cleaning the cavity, EB welding new components into the cavity to form the replacement fuel nozzle insert in the cavity, heat treating the new fuel nozzle insert and end cover, and final machining the fuel nozzle insert and end cover. Additionally, pressure testing, visual inspection, and other testing may be performed. After completion of the replacement, final assembly and final inspection of the fuel nozzle assembly may be performed, as well as flow testing and flow adjustments. 
         [0043]    In addition to combustion end covers and fuel nozzle inserts, the methods described herein may be utilized for EB welding other turbine components and assemblies, in addition to other non-gas turbine related surfaces and components. Such additional components and assemblies of gas turbines may include fuel nozzles, transition duct picture frames, blade squealer tips, or any other turbine component assembly or component that may be welded or brazed. 
         [0044]    The collection pocket described herein may be incorporated into a variety of welding applications. One such welding application is the construction of a fuel nozzle assembly in a gas turbine, as described above. Additionally, EB welding utilizing a collection pocket may be used to improve traditional EB welding with a backing shelf. Non-limiting examples of EB welding with a collection pocket include joining airfoils and shrouds, fuel manifold construction, fuel nozzle tip attachment, connection of tubing, and/or any other scenario in which one turbine component is inserted in, and/or coupled to, another turbine component in order to EB weld the turbine components together. 
         [0045]    Embodiments of the technology have been described herein to be illustrative rather than restrictive. Alternative embodiments will become apparent to readers of this disclosure. Further, alternative means of implementing the aforementioned elements and steps can be used without departing from the scope of the claims, as would be understood by one having ordinary skill in the art. Certain features and sub-combinations are of utility and may be employed without reference to other features and sub-combinations, and are contemplated as within the scope of the claims.