Patent Publication Number: US-2022221089-A1

Title: Ferrule coupling for joining ducts together

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. patent application Ser. No. 16/020,402 filed Jun. 27, 2018, and entitled “FERRULE COUPLING FOR JOINING DUCTS TOGETHER,” which is hereby incorporated by reference in its entirety. 
    
    
     FIELD 
     The present disclosure relates generally to a ferrule coupling for joining together lines, such as ducts, and, more particularly, to an apparatus and method for joining an outer duct and an inner duct using a ferrule coupling assembled using two ferrules or three ferrules. 
     BACKGROUND 
     Certain applications may require the use of lines, pipes, or ducts with multiple walls. As one example, an aircraft may have fuel lines formed by dual wall duct systems. For example, a fuel line may include an inner duct and an outer duct that are joined to each other with the outer duct spaced apart from the inner duct. In some cases, the inner duct may be used for a fluid of a first type, while the outer duct may be used for a fluid of a second type. In other cases, the inner duct may be used for a main flow of fluid, while the outer duct may be used to contain leakage or vapors released from the inner duct. A coupling may be used to both join the inner duct and the outer duct and control a distance between the inner duct and the outer duct. For example, a coupling may be used to ensure that the outer duct is spaced apart from the inner duct to allow a flow of fluid or provide a containment volume between the inner duct and the outer duct. 
     Some currently available couplings for joining an inner duct and an outer duct use two ferrules that are welded together at one or more weld locations. Over time, the performance of the coupling at these one or more weld locations may decline due to temperature changes, stress, or other factors. For example, the coupling may become susceptible to cracking or separation at the one or more weld locations. Further, welding the two ferrules together makes welding or other permanent methods of joining the ferrules to prevent for disassembly of the components once joined. Thus, one or more apparatuses and methods for ferrule couplings unaffected by heat-based stresses may be desired. 
     SUMMARY 
     In one example embodiment, an apparatus comprises an inner ferrule and an outer ferrule. The inner ferrule has a plurality of engagement sections that define a plurality of gaps between the plurality of engagement sections. The outer ferrule has an engagement area disposed around at least a portion of the inner ferrule and mechanically joined to the plurality of engagement sections to join the inner ferrule to the outer ferrule. 
     In another example embodiment, another apparatus is provided. The apparatus comprises an inner ferrule, an intermediate ferrule, and an outer ferrule. The intermediate ferrule includes a plurality of engagement sections. The intermediate ferrule is disposed around at least a portion of the inner ferrule and is mechanically joined to the inner ferrule. The outer ferrule includes an engagement area disposed around at least a portion of the intermediate ferrule. The engagement area is mechanically joined to the plurality of engagement sections. 
     In yet another example embodiment, a method is provided. The method may include coupling an outer duct to an outer ferrule. A plurality of engagement sections of an inner ferrule is joined to an engagement area of the outer ferrule. Further, an inner duct is joined to the inner ferrule. The outer ferrule may be disposed around the inner ferrule. 
     In still yet another example embodiment, a method is provided for moving fuel through a duct system in an aircraft. The fuel is moved through an inner duct that is joined with an outer duct through a joining of an inner ferrule attached to the inner duct and an outer ferrule attached to the outer duct. A leakage from the fuel flowing through the inner duct is captured within a region that is formed between the inner duct and the outer duct by a plurality of engagement sections of the inner ferrule that protrude radially outward from the inner ferrule and that define a plurality of gaps between the plurality of engagement sections. 
     The features and functions may be achieved independently in various embodiments of the present disclosure or may be combined in yet other embodiments in which further details may be seen with reference to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features believed characteristic of the illustrative embodiments are set forth in the appended claims. The illustrative embodiments, however, as well as a preferred mode of use, and further objectives and features thereof, will best be understood by reference to the following detailed description of an illustrative embodiment of the present disclosure when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is an illustration of a perspective view of an aircraft in accordance with an example embodiment. 
         FIG. 2  is an illustration of an isometric view of a duct system formed using a ferrule coupling in accordance with an example embodiment. 
         FIG. 3  is an illustration of an isometric exploded view of the duct system from  FIG. 2  in accordance with an example embodiment. 
         FIG. 4  is an illustration of a cross-sectional view of the duct system from  FIG. 2  in accordance with an example embodiment. 
         FIG. 5  is an illustration of a side exploded view of the duct system from  FIG. 4  in accordance with an example embodiment. 
         FIG. 6  is an illustration of a front view of the duct system from  FIG. 2  in accordance with an example embodiment. 
         FIG. 7  is an illustration of a rear view of the duct system from  FIG. 2  in accordance with an example embodiment. 
         FIG. 8  is an illustration of a cross-sectional view of an inner ferrule in accordance with an example embodiment. 
         FIG. 9  is an illustration of a cross-sectional view of an outer ferrule in accordance with an example embodiment. 
         FIG. 10  is an illustration of an isometric view of a duct system formed using a ferrule coupling having a different configuration in accordance with an example embodiment. 
         FIG. 11  is an illustration of an isometric exploded view of the duct system from  FIG. 10  in accordance with an example embodiment. 
         FIG. 12  is an illustration of a cross-sectional view of the duct system from  FIG. 10  in accordance with an example embodiment. 
         FIG. 13  is an illustration of a side exploded view of the duct system from  FIG. 12  in accordance with an example embodiment. 
         FIG. 14  is an illustration of a front view of the duct system from  FIG. 10  in accordance with an illustrative embodiment. 
         FIG. 15  is an illustration of a cross-sectional view of an inner ferrule in accordance with an example embodiment. 
         FIG. 16  is an illustration of a cross-sectional view of an intermediate ferrule in accordance with an example embodiment. 
         FIG. 17  is an illustration of a cross-sectional view of an outer ferrule in accordance with an example embodiment. 
         FIG. 18  is a flowchart illustration of a process for assembling a duct system in accordance with an example embodiment. 
         FIG. 19  is a flowchart illustration of a process for assembling a duct system in accordance with an example embodiment. 
         FIG. 20  is a flowchart illustration of a process for moving fuel through a duct system in an aircraft in accordance with an example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The illustrative examples provide a duct system that uses a ferrule coupling formed using multiple ferrules. This multi-piece ferrule coupling may be used to connect an inner duct and an outer duct to form a duct system in which the outer duct is spaced apart from the inner duct. The outer duct may form an encapsulated shroud around the inner duct. The ferrule coupling described in the illustrative examples may be formed using at least two ferrule components that are assembled to form the ferrule coupling without welding. The ferrule coupling forms an electrostatic bond path between the inner duct and the outer duct. This type of ferrule coupling may be robust and may reduce or eliminate the need for rework of each ferrule component, thereby allowing the electrostatic bond path to be maintained. Further, this multi-piece ferrule coupling may have a longer service life than a ferrule coupling formed by welding two ferrules together. 
     With reference now to the figures,  FIG. 1  is an illustration of an aircraft, depicted in accordance with an illustrative embodiment. Aircraft  100  includes wing  102  and wing  104  attached to fuselage  106 . Aircraft  100  includes engine  108  attached to wing  102  and engine  110  attached to wing  104 . Aircraft  100  also includes tail section  112 . Horizontal stabilizer  114 , horizontal stabilizer  116 , and vertical stabilizer  118  are attached to tail section  112 . 
     Aircraft  100  is an example of one type of platform that includes duct systems that include multi-piece ferrule couplings in accordance with the illustrative embodiments described below. For example, without limitation, aircraft  100  may include a fuel system that includes fuel lines that at least partially include duct systems joined together with multi-piece ferrule couplings in a manner similar to duct system  200  of  FIG. 2  below or duct system  1000  of  FIG. 10  below. 
     In these illustrative examples, aircraft  100  takes the form of a refueling aircraft, such as a tanker, having refueling boom  120 . Fuel may be transferred from aircraft  100  to another aircraft or other type of vehicle or platform through refueling boom  120 . A duct system, such as duct system  200  in  FIG. 2  or duct system  1000  in  FIG. 10  below, may be used with fuel lines that are located within the refueling aircraft or outside the refueling aircraft. For example, these types of duct systems may be used with fuel lines that are connected to or located within refueling boom  120 . 
     In other illustrative examples, aircraft  100  may be some other type of aircraft and one or more duct systems implemented similar to duct system  200  of  FIG. 2  may be used to transport fuel or other fluids or gases within an inner duct. The outer duct may then serve to lock in any fluids or gases that leak from the inner duct. As such, an inner duct may be disposed within an outer duct, ferrules may be used to join together the inner duct and the outer duct and dispose the outer duct in a fixed spatial relationship to the outer duct. 
     Such ferrules may include a multi-piece ferrule coupling with two, three, or four or more ferrule components. As the components of such a multi-piece ferrule coupling may be mechanically joined together, electrostatic bonding is not needed to join together the components. In this way, areas of the ferrules weakened from heat-based stresses of electrostatic bonding may be prevented. For the purposes of this disclosure, “joining” may refer to one or both of direct coupling (e.g., two components that are directly connected and, thus, contacting one another) or indirect coupling (e.g., two components that are held in one or more fixed spatial relations to each other through one or more other components). Thus, “join” or “joining” may also be referred to as “couple” or “coupling,” herein. 
     A technical effect of the illustrative embodiments described herein includes reduced maintenance of fuel lines. For example, the multi-piece ferrule couplings may reduce or eliminate the need for rework of each ferrule component. Further, the duct systems using these types of ferrule couplings may be stronger and more reliable over time because the ferrule couplings include at least one interface between the multiple ferrules of the multi-piece ferrule coupling that is stronger and more resistant to temperature-based stresses, cracking, and/or other inconsistencies as compared to ferrule couplings in which all ferrule interfaces are formed by welding. 
     Thus, a technical effect of the illustrative embodiments described herein includes duct systems that are more reliable compared to traditionally manufactured ducts, such as ducts with welded ferrules. Further, even after rework, the ferrule couplings described by the illustrative embodiments herein do not weaken or eliminate an electrostatic bond path between the inner and outer ducts of the duct system. Accordingly, these ferrule couplings, and thereby the duct systems using these types of ferrule couplings, may have longer service lives. 
       FIGS. 2-9  are illustrations of different views of duct system  200  and components of duct system  200 . Duct system  200  is a multi-walled duct system that uses a dual ferrule coupling. 
     With reference now to  FIG. 2  is an illustration of an isometric view of a duct system formed using a ferrule coupling in accordance with an example embodiment. Duct system  200  may include at least two ducts and may include any number of ferrules, ferrule couplings, or combination thereof. In one illustrative embodiment, duct system  200  includes inner duct  202 , outer duct  204 , and ferrule coupling  206 . Ferrule coupling  206  is used to mechanically join inner duct  202  and outer duct  204 . 
     Ferrule coupling  206  includes inner ferrule  208  and outer ferrule  210 . Inner ferrule  208  is joined to inner duct  202 . When inner ferrule  208  is joined to inner duct  202 , inner ferrule  208  may be disposed around, or overlap, at least a portion of an exterior (not shown) of inner duct  202 . Outer ferrule  210  is joined to outer duct  204 . When outer ferrule  210  is joined to outer duct  204 , outer ferrule  210  may be adjacent to, surround, or overlap at least a portion of exterior  212  of outer duct  204 . 
     The joining of inner ferrule  208  to inner duct  202  and outer ferrule  210  to outer duct  204  may be performed using any number of techniques including, but not limited to, at least one of welding, electrostatic bonding, adhesive bonding, swaging, or some other type of process. For example, inner duct  202  may be swaged within inner ferrule  208 . In some illustrative examples, outer ferrule  210  is welded to outer duct  204  at weld interface  214 . Weld interface  214  may be formed using, for example, a butt welding technique. For example, weld interface  214  may be formed where an edge of outer ferrule  210  abuts an edge of outer duct  204 . 
     Inner ferrule  208  may be joined with outer ferrule  210  to thereby join inner duct  202  with outer duct  204 . As depicted, when inner ferrule  208  and outer ferrule  210  are joined, outer ferrule  210  may at least partially overlap inner ferrule  208 . 
     In these illustrative examples, inner ferrule  208  may be mechanically joined with outer ferrule  210 . A first component, such as inner ferrule  208 , may be “mechanically joined” to a second component, such as outer ferrule  210 , either directly or indirectly. Further, mechanically coupling two components may mean coupling the two components, without welding. In some cases, mechanically joining two components creates an electrostatic bond between the two components. Inner ferrule  208  may be mechanically joined to outer ferrule  210  via threading, friction fit, swaging, bolting, insertion of at least one dowel in a corresponding opening, adhesive, some other mechanical coupling technique, or a combination thereof. 
       FIG. 3  is an illustration of an isometric exploded view of duct system  200  from  FIG. 2  in accordance with an example embodiment. As depicted, inner duct  202 , outer duct  204 , inner ferrule  208 , and outer ferrule  210  may be substantially cylindrical. Further, when assembled to form duct system  200 , inner duct  202 , outer duct  204 , inner ferrule  208 , and outer ferrule  210  may be substantially coaxial with respect to axis  300 . 
     Inner ferrule  208  extends between end  301  and end  302 . As depicted, inner ferrule  208  has inner surface  303  and outer surface  304 . Inner ferrule  208  includes body  305  and plurality of engagement sections  306  disposed along outer surface  304 . In these illustrative examples, plurality of engagement sections  306  may be a plurality of discrete, discontinuous engagement sections disposed substantially circumferentially around body  305  of inner ferrule  208  at or near end  302  of inner ferrule  208 . In particular, each of plurality of engagement sections  306  may be located at a different position along outer surface  304  of body  305  of inner ferrule  208 . 
     In these illustrative examples, plurality of engagement sections  306  may protrude or extend radially outwards from body  305  relative to axis  300 . Engagement section  307  is an example of one of plurality of engagement sections  306 . Engagement section  307  has a selected height such that engagement section  307  protrudes outward. 
     Plurality of engagement sections  306  defines plurality of gaps  308  between plurality of engagement sections  306 . When inner ferrule  208  is joined to outer ferrule  210 , plurality of gaps  308  helps define openings through which fluid may flow. Further, each of plurality of engagement sections  306  has threads. For example, engagement section  307  has threads  309 . 
     In one illustrative example, inner ferrule  208  includes grooves  310  disposed on inner surface  303  of body  305  of inner ferrule  208 . Grooves  310  are disposed along inner surface  303  between end  301  and end  302 . In this illustrative example, grooves  310  are disposed along a middle portion of inner surface  303  located between end  301  and end  302  of inner ferrule  208 . As depicted, grooves  310  may extend fully around inner surface  303  of body  305  of inner ferrule  208 . In some illustrative examples, grooves  310  may extend only partially around inner surface  303 . In other illustrative examples, without limitation, grooves  310  may extend in a discontinuous manner around inner surface  303  to form multiple grooved areas along inner surface  303 . 
     Inner duct  202  may be fit at least partially within inner ferrule  208  using grooves  310 . For example, without limitation, grooves  310  may take the form of swage grooves. Inner duct  202  may be swaged (e.g., roller swaged) within inner ferrule  208  using grooves  310  (e.g., swage grooves) to form a swaged joint between inner ferrule  208  and inner duct  202 . 
     Outer ferrule  210  extends between end  312  and end  314 . As depicted, outer ferrule  210  has inner surface  316  and outer surface  318 . Outer ferrule  210  includes body  320  and engagement area  322 . Engagement area  322  of outer ferrule  210  may be a substantially continuous area, or feature, disposed along inner surface  316  of body  320  of outer ferrule  210 . In this illustrative example, engagement area  322  is located along a middle portion of inner surface  316  between end  312  and end  314 . In these illustrative examples, engagement area  322  may extend fully around inner surface  316  of body  320  in an annular manner. In other illustrative examples, engagement area  322  may be disposed only partially around inner surface  316  or may be formed in a discontinuous manner around inner surface  316 . 
     Plurality of engagement sections  306  of inner ferrule  208  may be engaged with engagement area  322  to join inner ferrule  208  to outer ferrule  210 . In other words, engagement area  322  may be mechanically joined to plurality of engagement sections  306  to join inner ferrule  208  to outer ferrule  210 . 
     Engagement area  322  may include threads  324  disposed along inner surface  316  of outer ferrule  210 . In these illustrative examples, the threads on each of plurality of engagement sections  306  may be sized and shaped for engagement with threads  324  of engagement area  322  of outer ferrule  210 . In other words, threads  324  may correspond with the threads of plurality of engagement sections  306 . For example, threads  309  of engagement section  307  may be sized and shaped for engagement with threads  324 . In other words, threads  309  may correspond to threads  324 . 
     In these illustrative examples, inner duct  202 , outer duct  204 , inner ferrule  208 , and outer ferrule  210  may each be comprised of one or more materials including, but not limited to, metal and metal alloys. Body  305  of inner ferrule  208  may be comprised of a material that is harder than the material forming inner duct  202  to allow for swaging. In some cases, body  305  of inner ferrule  208  may also be comprised of a material that is harder than the material forming body  320  of outer ferrule  210 . For example, without limitation, body  305  of inner ferrule  208  may be comprised of a first aluminum alloy, such as 2024-T851 aluminum, while body  320  of outer ferrule  210  may be comprised of a second aluminum alloy, such as 6061-T6 aluminum. 
       FIG. 4  is an illustration of a cross-sectional view of duct system  200  taken with respect to lines  4 - 4  in  FIG. 2  in accordance with an example embodiment. As depicted, inner ferrule  208  engages outer ferrule  210  to thereby join inner duct  202  with outer duct  204 . A fluid (e.g., a liquid(s), a gas(es), or combination thereof) may flow through channel  400  through inner duct  202 . In these illustrative examples, at least one of inner ferrule  208  or inner duct  202  defines an outer circumference of channel  400 , which may be a first fluid volume. 
     In this illustrative example, inner duct  202  has been swaged within inner ferrule  208 . Further, threads  309  of engagement section  307  of inner ferrule  208  are engaged with threads  324  of engagement area  322  of outer ferrule  210 . Further, weld interface  214  between outer ferrule  210  and outer duct  204  may be more clearly seen. 
     Ferrule coupling  206  formed by inner ferrule  208  and outer ferrule  210  provides separation between inner duct  202  and outer duct  204 . In particular, ferrule coupling  206  spaces apart outer duct  204  from inner duct  202  to define region  402  between inner duct  202  and outer duct  204 . Specifically, at least one of outer ferrule  210  or outer duct  204  defines an outer circumference of region  402 , which may be a second fluid volume. 
     Outer duct  204  may be separated from inner duct  202  in a radial direction relative to axis  300 . Region  402  may be used to collect any leakage of fluid flowing through channel  400  within inner duct  202 . When the fluid flowing through inner duct  202  is fuel, region  402  may be used to capture a leakage of the fuel or a leakage of fuel vapors. For example, outer duct  204  may function as a “shroud” that prevents any leakage of fluid out of inner duct  202  from escaping duct system  200 . In other illustrative examples, region  402  may provide for the flow of a fluid different from the fluid flowing through channel  400  in a same or different direction. 
     In still other illustrative examples, channel  400  may be used for transferring fluid and filling, while region  402  may be used for venting. As one illustrative example, when duct system  200  is used within, for example, refueling boom  120  of  FIG. 1 , region  402  may be used to provide venting. When fuel is transferred through refueling boom  120  into, for example, a fuel tank of another aircraft, vapors in the fuel tank may be pushed out as fuel fills the fuel tank. These vapors may be vented out of the fuel tank through region  402  between inner duct  202  and outer duct  204 . 
     The engagement of threads on plurality of engagement sections  306  from  FIG. 3  with threads  324  of engagement area  322  establishes ferrule interface  404  between inner ferrule  208  and outer ferrule  210 . Ferrule interface  404  establishes an electrostatic bond between inner ferrule  208  and outer ferrule  210 , and thereby, between inner duct  202  and outer duct  204 . Ferrule interface  404  may be stronger and more resistant to temperature-based stresses, cracking, and/or other inconsistencies as compared to an interface formed by welding. Further, ferrule interface  404 , even if reworked later in the life of duct system  200 , may be capable of maintaining the electrostatic bond between inner ferrule  208  and outer ferrule  210  and thereby, between inner duct  202  and outer duct  204 . 
       FIG. 5  is an illustration of a side exploded view of duct system  200  from  FIG. 4  in accordance with an example embodiment. As depicted, each engagement section of plurality of engagement sections  306  is individually threaded. Thus, coupling inner ferrule  208  to outer ferrule  210  may include threading together inner ferrule  208  and outer ferrule  210  such that the threads on each of plurality of engagement sections  306  engage the corresponding threads  324  of engagement area  322  of outer ferrule  210 . 
       FIG. 6  is an illustration of a front view of duct system  200  taken with respect to lines  6 - 6  in  FIG. 2  in accordance with an example embodiment. As depicted, plurality of gaps  308  between plurality of engagement sections  306  help define plurality of openings  600  through which fluid may flow. Plurality of openings  600  allow fluid to flow into or out of region  402 . For example, plurality of openings  600  may allow fluid to flow in a substantially axial direction relative to axis  300 . 
       FIG. 7  is an illustration of a rear view of duct system  200  from  FIGS. 2-6  taken with respect to lines  7 - 7  in  FIG. 2  in accordance with an example embodiment. Plurality of openings  600  may also be visible in this view. 
       FIG. 8  is an illustration of a cross-sectional view, similar to that of  FIG. 4 , of only inner ferrule  208  in accordance with an example embodiment. Threads  309  of engagement section  307  may be more clearly depicted in this view. Further, grooves  310  may be more clearly depicted. Inner ferrule  208  has stop feature  800  located adjacent to grooves  310 . Stop feature  800  helps provide a guide for the positioning of inner duct  202  of  FIGS. 2-3  within inner ferrule  208 . When inner duct  202  is swaged within inner ferrule  208  in the direction of arrow  801 , stop feature  800  provides a stop for this swaging action to help prevent further swaging in the direction of arrow  801 . 
     Inner ferrule  208  also includes outer groove  802 . Outer groove  802  may be used during the manufacturing of inner ferrule  208  (e.g., as a data point reference), may receive a sealing component, such as an O-ring, may be interfaced with one or more other components (e.g., may be used to join inner ferrule  208  to a different ferrule), or a combination thereof. 
       FIG. 9  is an illustration of a cross-sectional view, similar to that of  FIG. 4 , of only outer ferrule  210  in accordance with an example embodiment. Threads  324  of engagement area  322  may be more clearly depicted in this view. As depicted, outer ferrule  210  may include portion  900  having a smaller diameter than the rest of outer ferrule  210 . Portion  900  defines stop feature  902 . Stop feature  902  helps provide a guide for the positioning of inner ferrule  208  of  FIGS. 2-3  within outer ferrule  210 . When inner ferrule  208  is threaded into outer ferrule  210  in the direction of arrow  904 , stop feature  902  provides a stop for this threading action to prevent further threading in the direction of arrow  904 . 
     Further, outer ferrule  210  includes outer groove  906 . Outer groove  906  may be used during the manufacturing of outer ferrule  210  (e.g., as a data point reference), may receive a sealing component, such as an O-ring, may be interfaced with one or more other components (e.g., may be used to join outer ferrule  210  to a different ferrule), or a combination thereof. 
       FIG. 10  is an illustration of an isometric view of a duct system formed using a ferrule coupling having a different configuration in accordance with an example embodiment. Duct system  1000  may include at least two ducts and may include any number of ferrules, ferrule couplings, or combination thereof. In one illustrative embodiment, duct system  1000  includes inner duct  1002 , outer duct  1004 , and ferrule coupling  1006 . Ferrule coupling  1006  is used to mechanically join inner duct  1002  and outer duct  1004 . In these illustrative examples, ferrule coupling  1006  includes inner ferrule  1008 , outer ferrule  1010 , and intermediate ferrule  1012 . 
     In some cases, the intermediate ferrule  1012  may also be referred to as an inner ferrule. For example, intermediate ferrule  1012  may be a first inner ferrule and inner ferrule  1008  may be a second inner ferrule. The first and second inner ferrules may be joined together to form a sub-coupling that is joined to outer ferrule  1010 . 
     Inner ferrule  1008  is joined to inner duct  1002 . Outer ferrule  1010  is joined to outer duct  1004 . Inner ferrule  1008  and outer ferrule  1010  are joined through intermediate ferrule  1012 . Accordingly, joining inner ferrule  1008  with outer ferrule  1010  through intermediate ferrule  1012  thereby joins inner duct  1002  with outer duct  1004 . As depicted, when duct system  1000  is assembled, outer ferrule  1010  may at least partially overlap inner ferrule  1008 . The joining of inner ferrule  1008  to inner duct  1002  and outer ferrule  1010  to outer duct  1004  may be performed using a mechanical technique, electrostatic bonding, adhesive bonding, some other technique, or a combination thereof. 
     In some illustrative examples, outer ferrule  1010  may be positioned adjacent to and in contact with an edge of outer duct  1004 . In other illustrative examples, outer ferrule  1010  may be positioned around at least a portion of exterior  1013  of outer duct  1004 . In one illustrative example, outer ferrule  1010  is welded to outer duct  1004  at weld interface  1014 . Weld interface  1014  may be formed using, for example, a butt welding technique. In other illustrative examples, outer ferrule  1010  may be joined to outer duct  1004  in some other manner. In some cases, inner ferrule  1008  may be welded to inner duct  1002  in a similar manner. 
     In other illustrative examples, inner ferrule  1008  and outer ferrule  1010  may be joined to inner duct  1002  and outer duct  1004 , respectively, through swaging (e.g., roller swaging, rotary swaging, axial swaging, or some other type of swaging) or some other joining technique. In some cases, adhesive may be used in joining inner ferrule  1008  and outer ferrule  1010  to inner duct  1002  and outer duct  1004 , respectively. 
       FIG. 11  is an illustration of an isometric exploded view of duct system  1000  from  FIG. 10  in accordance with an example embodiment. As depicted, inner duct  1002 , outer duct  1004 , inner ferrule  1008 , outer ferrule  1010 , and intermediate ferrule  1012  may be substantially cylindrical. Further, when duct system  1000  is assembled, inner duct  1002 , outer duct  1004 , inner ferrule  1008 , outer ferrule  1010 , and intermediate ferrule  1012  may be substantially coaxial with respect to axis  1100 . 
     Outer ferrule  1010  extends between end  1101  and end  1102 . As depicted, outer ferrule  1010  has inner surface  1103  and outer surface  1104 . Outer ferrule  1010  includes body  1105  having engagement area  1106 . Engagement area  1106  is formed along inner surface  1103  of body  1105  of outer ferrule  1010 . Engagement area  1106  may be referred to as, in some cases, a retaining feature. In one illustrative example, engagement area  1106  may be shaped and sized to form a pocket or notch for positioning and retaining at least a portion of intermediate ferrule  1012 . In one illustrative example, the engagement area  1106  includes a groove. 
     In this illustrative example, inner ferrule  1008  has end  1107  and end  1108 . Further, inner ferrule  1008  has inner surface  1109  and outer surface  1110 . Inner ferule  1008  includes body  1112  having engagement area  1114 . Depending on the implementation, engagement area  1114  may be a single continuous engagement area or may be comprised of multiple engagement areas. 
     Inner ferrule  1008  also includes stop feature  1116  and stop feature  1117 . Stop feature  1116  may be located at or near end  1108 . Stop feature  1117  may be located at or near end  1107 . In one illustrative example, stop feature  1117  is located near but spaced away from end  1107 . Stop feature  1116  may be, for example, a lip or other type of protrusion at the edge of inner ferrule  1008  at end  1108 . 
     In this illustrative example, engagement area  1114  is located between stop feature  1116  and stop feature  1117 . Engagement area  1114  may be referred to, in some cases, as a retaining feature. Engagement area  1114  may be shaped and sized for receiving, positioning, and retaining at least a portion of intermediate ferrule  1012 . 
     As depicted, inner ferrule  1008  may include plurality of protrusions  1118  that protrude outwards from body  1112  of inner ferrule  1008 . Plurality of protrusions  1118  may be discrete, discontinuous protrusions disposed substantially circumferentially around inner ferrule  1008 . In this illustrative example, plurality of protrusions  1118  is located adjacent to and extends axially away from stop feature  1117 . Plurality of protrusions  1118  does not extend all the way to stop feature  1116 . 
     Engagement area  1114  includes plurality of retaining areas  1122  (e.g., gaps) defined between plurality of protrusions  1118 . In other illustrative examples, plurality of retaining areas  1122  may be considered separate from engagement area  1114 . 
     When duct system  1000  is assembled, intermediate ferrule  1012  is positioned between inner ferrule  1008  and outer ferrule  1010 . Intermediate ferrule  1012  helps join inner ferrule  1008  with outer ferrule  1010 , while also providing a spacing or separation, in a radial direction relative to axis  1100 , between inner ferrule  1008  and outer ferrule  1010 . 
     Intermediate ferrule  1012  includes body  1123 , plurality of engagement sections  1124 , and plurality of tab elements  1126 . In one illustrative example, body  1123  takes the form of a band or nearly annular band. For example, body  1123  includes gap  1125 . Plurality of engagement sections  1124  are disposed along and protrude outwards from body  1123 . Plurality of engagement sections  1124  may include discontinuous engagement sections that are separated by plurality of gaps  1127 . Plurality of engagement sections  1124  may engage engagement area  1106  of outer ferrule  1010 . For example, when engagement area  1106  includes a groove, plurality of engagement sections  1124  may be disposed within the groove when duct system  1000  is fully assembled. Further, when duct system  1000  is fully assembled, plurality of gaps  1127  between plurality of engagement sections  1124  form openings (or channels) through which fluid may flow. 
     Plurality of tab elements  1126  are positioned adjacent to plurality of engagement sections  1124  and extend axially relative to axis  1100  in one direction away from body  1123 . In this illustrative example, plurality of tab elements  1126  includes a corresponding tab element for each of plurality of engagement sections  1124 . In other illustrative examples, a tab element in plurality of tab elements  1126  may be sized such that the tab element is positioned adjacent to multiple engagement sections of plurality of engagement sections  1124 . 
     When intermediate ferrule  1012  is joined to inner ferrule  1008 , plurality of engagement sections  1124  and plurality of tab elements  1126  are positioned and retained within engagement area  1114 . Specifically, plurality of tab elements  1126  are positioned and retained within plurality of retaining areas  1122 . Accordingly, plurality of protrusions  1118  on inner ferrule  1008  may be shaped and sized to define plurality of retaining areas  1122  for receiving plurality of tab elements  1126 . In other words, plurality of retaining areas  1122  may be complementary to plurality of tab elements  1126 . 
     Once intermediate ferrule  1012  is joined with inner ferrule  1008 , stop feature  1116  and stop feature  1117  may help prevent axial movement of intermediate ferrule  1012  relative to inner ferrule  1008  relative to axis  1100 . Plurality of protrusions  1118  may help prevent rotation of intermediate ferrule  1012  relative to inner ferrule  1008  about axis  1100 . 
     In these illustrative examples, inner ferrule  1008  and intermediate ferrule  1012  may be mechanically joined via, for example, an interference fit (or friction fit). Gap  1125  in body  1123  of intermediate ferrule  1012  provides a degree of flexibility to body  1123  that allows intermediate ferrule  1012  to be wrapped around and fit over inner ferrule  1008  with an interference fit. Gap  1125 , which may also be referred to as a discontinuity, may allow intermediate ferrule  1012  to be deformed in a manner that allows intermediate ferrule  1012  to be fitted around inner ferrule  1008 . Thus, gap  1125  allows intermediate ferrule  1012  to be more easily joined to inner ferrule  1008 . 
     In one illustrative example, intermediate ferrule  1012 , when no forces are exerted on or applied to intermediate ferrule  1012 , may have an inner diameter that is smaller than the outer diameter of inner ferrule  1008 . This difference in diameters may allow intermediate ferrule  1012  to be fitted around inner ferrule  1008  with an interference fit. 
     In these illustrative examples, each of plurality of tab elements  1126  may have a height selected such that plurality of tab elements  1126  are substantially flush with plurality of protrusions  1118  of inner ferrule  1008  when intermediate ferrule  1012  is joined to inner ferrule  1008 . In other illustrative examples, one or more of plurality of tab elements  1126  may have a height greater than or lower than plurality of protrusions  1118 . 
     When inner ferrule  1008  is joined with intermediate ferrule  1012 , the two ferrules form a sub-coupling that may then be joined to outer ferrule  1010 . In particular, when inner ferrule  1008  and intermediate ferrule  1012  are joined to form the sub-coupling, plurality of engagement sections  1124  may fit within and be retained within engagement area  1106  of outer ferrule  1010 . 
     In these illustrative examples, inner duct  1002 , outer duct  1004 , inner ferrule  1008 , intermediate ferrule  1012 , and outer ferrule  1010  may each be comprised of one or more materials including, but not limited to, metal and metal alloys. As one illustrative example, each of inner duct  1002 , outer duct  1004 , inner ferrule  1008 , intermediate ferrule  1012 , and outer ferrule  1010  may be comprised of a same or different aluminum alloy. 
     In these illustrative examples, plurality of engagement sections  1124  of intermediate ferrule  1012  may be comprised of a material that provides both sufficient compliancy and strength. In particular, plurality of engagement sections  1124  may need to be sufficiently compliant such that the sub-coupling of inner ferrule  1008  and intermediate ferrule  1012  may be inserted through outer ferrule  1010  and plurality of engagement sections  1124  may be slid into and fit within engagement area  1106  of outer ferrule  1010 . Plurality of engagement sections  1124 , however, may also need to be sufficiently strong and hard to allow each of plurality of engagement sections  1124  to retain its shape to maintain adequate separation between inner ferrule  1008  and outer ferrule  1010  over time. 
     Intermediate ferrule  1012  is used as a structural and mechanical separator between inner ferrule  1008  and outer ferrule  1010 . Intermediate ferrule  1012  may be joined to inner ferrule  1008  after the welding operation has been performed to weld inner ferrule  1008  with inner duct  1002 . Further, the sub-coupling formed by intermediate ferrule  1012  and inner ferrule  1008  may be joined with outer ferrule  1010  after the welding operation has been performed to weld outer ferrule  1010  to outer duct  1004 . In this manner, intermediate ferrule  1012  may be used after a buildup of heat from the welding operations has dissipated and the weld interfaces between inner ferrule  1008  with inner duct  1002  and outer ferrule  1010  to outer duct  1004  has cooled. Engagement area  1114  of inner ferrule  1008  and engagement area  1106  of outer ferrule  1010  work together to retain intermediate ferrule  1012  as part of ferrule coupling  1006  and within duct system  1000 . 
       FIG. 12  is an illustration of a cross-sectional view of duct system  1000  taken with respect to lines  12 - 12  in  FIG. 10  in accordance with an example embodiment. As depicted, inner ferrule  1008  is joined with intermediate ferrule  1012  and outer ferrule  1010 . A fluid (e.g., a liquid(s), a gas(es), or combination thereof) may flow through channel  1200  through inner duct  1002 . 
     In this illustrative example, inner duct  1002  has been joined within inner ferrule  1008  through an interference fit. Intermediate ferrule  1012  has been joined with inner ferrule  1008  through an interference fit. As depicted, each of plurality of engagement sections  1124  fits within and is retained within engagement area  1106  of outer ferrule  1010 . 
     Ferrule coupling  1006  formed by inner ferrule  1008 , outer ferrule  1010 , and intermediate ferrule  1012  provides separation between inner duct  1002  and outer duct  1004 . In particular, ferrule coupling  1006  spaces apart outer duct  1004  from inner duct  1002  to define region  1202  between inner duct  1002  and outer duct  1004 . Outer duct  1004  may be separated from inner duct  1002  in a radial direction relative to axis  300 . Region  1202  may be used to collect any leakage of fluid flowing through channel  1200  within inner duct  1002 . For example, outer duct  1004  may function as a “shroud” that prevents any leakage of fluid out of inner duct  1002  from escaping duct system  1000 . In other illustrative examples, region  1202  may provide for the flow of a fluid different from the fluid flowing through channel  1200  in a same or different direction. 
     In still other illustrative examples, channel  1200  may be used for transferring fluid and filling, while region  1202  may be used for venting. As one illustrative example, when duct system  1000  is used within, for example, refueling boom  120  of  FIG. 1 , region  1202  may be used to provide venting. When fuel is transferred through refueling boom  120  into, for example, a fuel tank of another aircraft, vapors in the fuel tank may be pushed out as fuel fills the fuel tank. These vapors may be vented out of the fuel tank through region  1202  between inner duct  1002  and outer duct  1004 . 
     The engagement of threads on plurality of engagement sections  1124  with engagement area  1106  establishes ferrule interface  1204  between inner ferrule  1008 , intermediate ferrule  1012 , and outer ferrule  1010 . Ferrule interface  1204  establishes an electrostatic bond between the three ferrules and thereby, between inner duct  1002  and outer duct  1004 . Ferrule interface  1204  may be stronger and more resistant to temperature-based stresses, cracking, and/or other inconsistencies as compared to an interface formed by welding. Further, ferrule interface  1204 , even if reworked later in the life of duct system  1000 , may be capable of maintaining the electrostatic bond between inner ferrule  1008 , intermediate ferrule  1012 , and outer ferrule  1010  and thereby, between inner duct  1002  and outer duct  1004 . 
       FIG. 13  is an illustration of a side exploded view of duct system  1000  from  FIG. 12  in accordance with an example embodiment. As depicted, outer ferrule  1010  includes stop feature  1300  that helps define engagement area  1106 . Stop feature  1300  also helps provide a guide for the positioning of the sub-coupling formed by inner ferrule  1008  and intermediate ferrule  1012  of  FIGS. 10-12  within outer ferrule  1010 . When this sub-coupling is fit within outer ferrule  1010 , stop feature  1300  prevents the sub-coupling from moving further in the direction of arrow  1302 . 
       FIG. 14  is an illustration of a front view of duct system  1000  taken with respect to lines  14 - 14  in  FIG. 10  in accordance with an illustrative embodiment. As depicted, plurality of gaps  1127  between plurality of engagement sections  1124  of intermediate ferrule  1012  help define plurality of openings  1400  through which fluid may flow. Plurality of openings  1400  allow fluid to flow into or out of region  1202 . For example, plurality of openings  1400  may allow fluid to flow in a substantially axial direction relative to axis  1100 . 
       FIG. 15  is an illustration of a cross-sectional view, similar to that of  FIG. 13 , of only inner ferrule  1008  in accordance with an example embodiment. As depicted, inner ferrule  1008  also includes outer groove  1500 . Outer groove  1500  may be used during the manufacturing of inner ferrule  1008  (e.g., as a data point reference), may receive a sealing component, such as an O-ring, may be interfaced with one or more other components (e.g., may be used to join inner ferrule  1008  to a different ferrule), or a combination thereof. 
       FIG. 16  is an illustration of a cross-sectional view, similar to that of  FIG. 13 , of only intermediate ferrule  1012  in accordance with an example embodiment. 
       FIG. 17  is an illustration of a cross-sectional view, similar to that of  FIG. 13 , of only outer ferrule  1010  in accordance with an example embodiment. As depicted, outer ferrule  1010  includes outer groove  1700 . Outer groove  1700  may be used during the manufacturing of outer ferrule  1010  (e.g., as a data point reference), may receive a sealing component, such as an O-ring, may be interfaced with one or more other components (e.g., may be used to join outer ferrule  1010  to a different ferrule), or a combination thereof. 
     While the illustrative examples described herein include continuous engagement areas and discontinuous engagement sections disposed on certain portions of the ferrules, other example embodiments include engagement areas or engagement sections disposed on other portions of the ferrules. In other illustrative examples, an engagement area described herein may be substituted with engagement sections, and engagement sections may be substituted with an engagement area. Corresponding engagement areas and engagement sections may be mechanically joined to each other. For example, corresponding engagement areas and engagement sections may be mechanically joined through interference fit, swaging, bolting, adhesive bonding, some other technique, or a combination thereof. 
       FIG. 18  is a flowchart illustration of a process for assembling a duct system in accordance with an example embodiment. Process  1800  illustrated in  FIG. 18  may be implemented using duct system  200  from  FIGS. 2-9 . 
     Process  1800  begins by joining outer duct  204  to outer ferrule  210  (step  1802 ). In one illustrative example, outer duct  204  may be welded to outer ferrule  210 . Next, plurality of engagement sections  306  of inner ferrule  208  may be mechanically joined to engagement area  322  of outer ferrule  210  (step  1804 ). 
     At step  1804 , mechanically joining plurality of engagement sections  306  with engagement area  322  joins inner ferrule  208  to outer ferrule  210 , such that outer ferrule  210  is at least partially disposed around inner ferrule  208 . In some illustrative examples, thread locker or some type of sealant or adhesive may be applied to one or more of plurality of engagement sections  306  or engagement area  322  to aid in securing inner ferrule  208  and outer ferrule  210  together. In these illustrative examples, each of plurality of engagement sections  306  may then be threaded together with engagement area  322 . The threads on plurality of engagement sections  306  may be torqued to a predetermined torque value to provide adequate electrostatic bonding contact between inner ferrule  208  and outer ferrule  210 , and thereby inner duct  202  and outer duct  204 . 
     Inner duct  202  is joined to inner ferrule  208  (step  1806 ), with the process terminating thereafter. In one illustrative example, inner duct  202  may be swaged (e.g., roller swaged) to grooves  310  disposed on inner surface  303  of inner ferrule  208 . In one or more examples, this swaging may be performed according to the Aerospace Standard set by the Society of Automotive Engineers (SAE) (e.g., SAE AS4060). 
       FIG. 19  is a flowchart illustration of a process for assembling a duct system in accordance with an example embodiment. Process  1900  illustrated in  FIG. 19  may be implemented using duct system  1000  from  FIGS. 10-17 . 
     Process  1900  may begin by joining outer duct  1004  to outer ferrule  1010  and inner duct  1002  to inner ferrule  1008  (step  1902 ). In one illustrative example, outer duct  1004  may be welded to outer ferrule  1010  and inner duct  1002  may be welded to inner ferrule  1008 . In other illustrative examples, some other type of process may be used to perform the joining of these components. For example, in some cases, inner duct  1002  may be swaged within inner ferrule  1008 . 
     Thereafter, intermediate ferrule  1012  is joined to inner ferrule  1008  to form a sub-coupling (step  1904 ). This sub-coupling may also be referred to as a ferrule sub-coupling. The sub-coupling is then joined to outer ferrule  1010  (step  1906 ), with the process terminating thereafter. The joining of the sub-coupling to outer ferrule  1010  joins inner ferrule  1008  to outer ferrule  1010 , and thereby, inner duct  1002  to outer duct  1004 . In some illustrative examples, the joining performed at step  1604  and  1606  may be performed through interference fits. During these joining processes, the various stop features of inner ferrule  1008  and outer ferrule  1010  may be used to help guide and align the different ferrules relative to each other. In some illustrative examples, sealant or adhesive may be used to provide additional support and restraint. 
       FIG. 20  is a flowchart illustration of a process for moving fuel through a duct system in an aircraft in accordance with an example embodiment. Process  2000  illustrated in  FIG. 20  may be implemented using, for example, duct system  200  from  FIGS. 2-9 . 
     Process  2000  may begin by moving the fuel through inner duct  202  that is joined with outer duct  204  through a joining of inner ferrule  208  attached to inner duct  202  and outer ferrule  210  attached to outer duct  204  (step  2002 ). At step  2002 , inner duct  202  and outer duct  204  may at least partially form a fuel line or portion of a fuel line system for an aircraft, such as aircraft  100  in  FIG. 1 . In some embodiments, the aircraft may be a refueling aircraft, such as a tanker aircraft. 
     A leakage from the fuel flowing through inner duct  202  is captured within region  402  that is formed between inner duct  202  and outer duct  204  by plurality of engagement sections  306  of inner ferrule  208  that protrude radially outward from inner ferrule  208  and that define a plurality of gaps  308  between plurality of engagement sections  306  (step  2004 ), with the process terminating thereafter. At step  2004 , the leakage may be a leakage of the fuel itself or a leakage of vapors. The leakage of fuel or vapors that is captured, or collected, within region  402  may be allowed to flow through plurality of gaps  308  between plurality of engagement sections  306 . 
     Thus, the different example embodiments provide two different configurations of ferrule couplings that may be used to join two lines, such as ducts, together. In one example embodiment, a ferrule coupling comprises an inner ferrule and an outer ferrule. The inner ferrule has a plurality of engagement sections. The outer ferrule has an engagement area that allows for the outer ferrule to be disposed around at least a portion of the inner ferrule and mechanically join the engagement area to the plurality of engagement sections to join the inner ferrule to the outer ferrule. 
     In another example, another apparatus is provided. The apparatus comprises an inner ferrule, an intermediate ferrule, and an outer ferrule. The intermediate ferrule includes a plurality of engagement sections and is disposed around at least a portion of the inner ferrule and mechanically joined to the inner ferrule. The outer ferrule includes an engagement area and is disposed around at least a portion of the intermediate ferrule. The engagement area and the plurality of engagement sections are mechanically joined to the plurality of engagement sections. 
     The flowcharts and block diagrams in the different depicted embodiments illustrate the architecture, functionality, and operation of some possible implementations of apparatuses and methods in an illustrative embodiment. In this regard, each block in the flowcharts or block diagrams may represent a module, a segment, a function, and/or a portion of an operation or step. In some alternative implementations of an illustrative embodiment, the function or functions noted in the blocks may occur out of the order noted in the figures. For example, in some cases, two blocks shown in succession may be executed substantially concurrently, or the blocks may sometimes be performed in the reverse order, depending upon the functionality involved. Also, other blocks may be added in addition to the illustrated blocks in a flowchart or block diagram. For example, while the processes described herein detail a manufacturing process with a certain sequence of assembly, other processes may include different sequences for the steps of the process, as needed. 
     As used herein, the phrase “at least one of,” when used with a list of items, means different combinations of one or more of the listed items may be used and only one of the items in the list may be needed. The item may be a particular object, thing, step, operation, process, or category. In other words, “at least one of” means any combination of items or number of items may be used from the list, but not all of the items in the list may be required. For example, without limitation, “at least one of item A, item B, or item C” or “at least one of item A, item B, and item C” may mean item A; item A and item B; item B; item A, item B, and item C; item B and item C; or item A and C. In some cases, “at least one of item A, item B, or item C” or “at least one of item A, item B, and item C” may mean, but is not limited to, two of item A, one of item B, and five of item C; three of item B and six of item C; or some other suitable combination. 
     The description of the different illustrative examples has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the embodiments in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. Further, different illustrative examples may provide different features as compared to other desirable embodiments. The embodiment or embodiments selected are chosen and described in order to best explain the principles of the embodiments, the practical application, and to enable others of ordinary skill in the art to understand the disclosure for various embodiments with various modifications as are suited to the particular use contemplated.