Abstract:
A method and apparatus for joining the ends of concentric inner and outer tube to a double containment fitting are disclosed, as are related methods of fabrication and systems. The apparatus uses a threaded coupling to compressively engage and seal the ends of both the inner and outer tubes to the double containment fitting. The double containment fitting includes inner and outer noses and a threaded annular body and annular nut. The inner nose may be at least partially inserted within a flared end of the inner tube, and the outer nose may encircle the inner tube and be inserted within the flared end of the outer tube. The inner and outer noses and the inner and outer tubes may be engaged in a relationship within bores through the annular body and the annular nut. Threadably engaging the annular nut with the annular body may establish seals between the components.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates generally to the coupling of fluid flow components. More particularly, the present invention relates to polymeric fittings for coupling polymeric fluid flow tubing. Embodiments of the present invention relate especially to polymeric fittings for coupling concentric inner and outer polymeric tubes, containment systems including the same, and associated methods. 
       BACKGROUND 
       [0002]    Numerous industries utilize fitting arrangements in many applications to compressively seal the ends of non-threaded tubing. In particular, fitting arrangements of this type are used extensively in the semiconductor processing industry, where plastic tubes are used to confine dangerous fluids including, for example, fluids that are corrosive, highly acidic, at a high temperature, and/or under significant pressure. In applications such as semiconductor processing, the fluids involved react with and/or may be contaminated by the use of metallic components and conventional fittings. Thus, in such industries, plumbing components are often made of highly inert materials such as fluoropolymers (e.g., PFA and PTFE) for wetted components. 
         [0003]    Containment integrity becomes critical in processes using caustic and dangerous fluids. Under such conditions, the use of concentric tubing is well known. In a concentric tube arrangement, outer tubing surrounds a given length of inner tubing that may be transporting fluid. This arrangement may provide containment from internal leaks and/or protection against external damage. The outer tube may additionally carry heated or cooled fluids used to heat or cool the fluids flowing through the inner tube. Thus, it is desirable under these types of settings to use fitting arrangements with the ability to seal concentric inner and outer plastic tubing against leaks. 
         [0004]    One conventional fitting for sealing concentric inner and outer plastic tubes, is disclosed in U.S. Pat. No. 5,498,036 to Kingsford. Specifically, the Kingsford fitting comprises concentric tubes and three additional members: an annular fitting body, an intermediate annular body, and an annular nut. The annular fitting body has a circular bore extending therethrough, a cylindrical nose portion on one end, and external threads. The intermediate annular body has a circular bore extending therethrough, a cylindrical nose portion on one end, a collar on the opposing end, and both internal and external threads. The annular nut has a circular bore extending therethrough, a shoulder on one end, and internal threads. The inner tube may be compressively engaged between the cylindrical nose portion of the annular fitting body and the collar of the intermediate annular body when the annular fitting body is threaded into the intermediate annular body. Similarly, the outer tube may be compressively engaged between the cylindrical nose portion of the intermediate annular body and the shoulder of the annular nut when the intermediate annular body is threaded into the annular nut. Thus, both the inner and outer tubes are sealed in one fitting. 
         [0005]    Double containment fittings like the one described above, that feature multiple threading engagements, possess some inherent problems. Fluoropolymers are relatively soft materials. The softness of the material makes it difficult to hold tolerances and shape while machining. In addition, the material&#39;s high coefficient of thermal expansion prohibits high-speed machining due to frictionally induced heating and expansion of the material. Thus, additional thread cutting passes add significantly to the time required to machine a single fitting unit. The probability that a flaw in a threaded engagement will cause a hazardous leak is also increased with each additional threaded engagement. Every threaded connection has a potential to leak, thus, the more threaded connections there are in a containment system, the greater the probability that there will be a leak. 
         [0006]    As may be appreciated, it would be advantageous to provide a fitting arrangement for jointly sealing concentric inner and outer plastic tubing with minimal use of threaded couplings. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    Embodiments of the invention relate to a double containment fitting for the ends of concentric tubing, systems including the same, methods of sealing the end of concentric tubing, and methods of making a double containment fitting. 
         [0008]    According to one embodiment of the invention, a double containment fitting for joining to the ends of concentric inner and outer tubes comprises an annular body with a first end portion, a second, opposing end portion, and a bore therethrough, wherein a portion of an outside surface of the second end portion includes threads thereon, an inner nose with a first end portion, a second, opposing end portion, and a bore therethrough, wherein the first end portion is configured to be receivable by the bore of the annular body, an outer nose with a first end portion, a second, opposing end portion, and a bore therethrough, wherein the bore of the outer nose is configured to telescopically receive the inner tube and the first end portion of the outer nose is configured to be receivable by the bore of the annular body, and an annular nut having a first end portion, a second, opposing end portion, and a bore therethrough, wherein the bore is configured to receive a portion of the concentric inner and outer tubes with the second end portion of the outer nose positioned between the concentric inner and outer tubes, and wherein at least a portion of a surface defining the bore includes threads thereon, and the threads of the annular nut are configured to engage with the threads of the annular body. 
         [0009]    Another embodiment of the present invention comprises a method for joining the ends of concentric inner and outer tubes to a fitting, the method comprising compressing a flared end of the inner tube between an inner nose and an outer nose, compressing a flared end of the outer tube between the outer nose and an annular nut, and threading the annular nut to an annular body, with the inner and outer tubes and the inner and outer noses positioned therebetween. 
         [0010]    Still another embodiment of the present invention comprises a method of forming a kit for a double containment fitting, including molding an inner nose comprising a first end portion, a second end portion, and a bore therethrough, molding an outer nose comprising a first end portion, a second end portion, and a bore therethrough, molding an annular body comprising a first end portion, a second end portion, a bore therethrough, and at least one flexible wall section positioned between the first end portion and the second end portion, wherein the bore is configured to receive the first end portion of the inner nose and the first end portion of the outer nose, and molding an annular nut configured to couple with the annular body. 
         [0011]    Yet another embodiment of the present invention comprises a double containment system including an outer tube having a flared end, an inner tube extending through the outer tube and having a flared end, and a double containment fitting closing the flared end of the outer tube and joining the flared end of the inner tube in fluid communication with a passageway therethrough, the double containment fitting comprising: an annular body with a first end portion, a second, opposing end portion, and a bore therethrough, wherein a portion of an outside surface of the second end portion includes threads thereon; an inner nose with a first end portion, a second, opposing end portion, and a bore therethrough, wherein the first end portion is configured to be receivable by the bore of the annular body; an outer nose with a first end portion, a second, opposing end portion, and a bore therethrough, wherein the bore of the outer nose is configured to telescopically receive the inner tube and the first end portion of the outer nose is configured to be receivable by the bore of the annular body; and an annular nut having a first end portion, a second, opposing end portion, and a bore therethrough, wherein the bore is configured to receive a portion of the concentric inner and outer tubes with the second end portion of the outer nose positioned between the concentric inner and outer tubes, and wherein at least a portion of a surface defining the bore includes threads thereon, and the threads of the annular nut are configured to engage with the threads of the annular body. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0012]    For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which: 
           [0013]      FIG. 1A  shows an exploded cross-sectional view of an embodiment of a double containment fitting of the present invention; 
           [0014]      FIG. 1B  depicts a cross-sectional view of an embodiment of an annular body of the present invention; 
           [0015]      FIG. 1C  depicts a side view of the annular body of  FIG. 1B ; 
           [0016]      FIG. 1D  depicts a cross-sectional view of an embodiment of an annular nut of the present invention; 
           [0017]      FIGS. 2-4  show cross-sectional views of the double containment fitting of  FIG. 1A  in partially assembled states; 
           [0018]      FIG. 5  shows a cross-sectional view of the double containment fitting of  FIG. 1A  in an assembled state; 
           [0019]      FIG. 6  shows a cross-sectional view of an embodiment of an annular body of the present invention; 
           [0020]      FIG. 7  shows a cross-sectional view of another embodiment of an annular body of the present invention; and 
           [0021]      FIG. 8  is a simplified sketch used to illustrate embodiment of methods of the present invention for making and/or assembling a double containment fitting. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    Referring in general to the accompanying drawings, various aspects of the present invention are illustrated in the context of embodiments of a fitting and methods for assembling such a fitting with concentric tubes. Common elements of the illustrated embodiments are designated with like reference numerals. It should be understood that the figures presented are not meant to illustrate actual views of any particular portion of a particular fitting, but are merely idealized schematic representations which are employed to more clearly and fully depict the invention. 
         [0023]      FIG. 1  depicts an exploded cross-sectional view of a first embodiment of a double containment fitting of the present invention. The fitting  1  may be used with concentric tubing  10 . The concentric tubing  10  may include an inner tube  12  and an outer tube  16 . The inner tube  12  may be at least partially received within the outer tube  16 . The inner tube  12  may have a flared end  13 , and the outer tube may have a flared end  17 . The concentric tubing  10  may be used for transporting fluids. The double containment fitting  1  enables the flared ends  13 ,  17  of either or both the inner tube  12  and outer tube  16  to be engaged and/or sealed within the double containment fitting  1 . The flared end  17  of the outer tube  16  may be closed from fluid communication in the double containment fitting  1 , and a passageway  14  through the inner tube  12  may be in fluid communication with an opening  2  of the double containment fitting  1 . 
         [0024]    The double containment fitting  1  includes an annular body  20 , an inner nose  30 , an outer nose  40 , and an annular nut  50 .  FIG. 5  shows the double containment fitting  1  and the concentric tubing  10  in an assembled state. As shown in  FIG. 5 , the inner nose  30  may be at least partially inserted within the flared end  13  of the inner tube  12 , and the outer nose  40  may encircle a section of the inner tube  12  and be partially inserted within the flared end  17  of the outer tube  16  in the assembled state. The inner nose  30 , the inner tube  12 , the outer nose  40 , and the outer tube  16  may be at least partially telescopically positioned within the annular nut  50  in the assembled state. The annular nut  50  and the annular body  20  may be joined with a threaded connection. In other embodiments, the annular nut  50  and the annular body  20  may be joined with a snap-fit (or other mechanical interference connection), using an adhesive, or by forming a direct bond between the annular nut  50  and the annular body  20  (e.g., using a thermal bonding process or an ultrasonic bonding process). 
         [0025]    The inner tube  12 , outer tube  16 , annular body  20 , inner nose  30 , outer nose  40 , and annular nut  50  may be formed of any material possessing good chemical and thermal resistance and capable of accommodating the types of fluids, pressures, temperatures, etc. to which the double containment fitting  1  may be exposed. Suitable materials include, but are not limited to, polymeric materials. As a nonlimiting example, these components may be formed from fluoromer materials such as, for example, tetrafluoroethylene (TFE), polytetrafluoroethylene (PTFE), fluorinated ethylene-propylene (FEP), perfluoroalkoxy fluorocarbon resin (PFA), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), ethylene-tetrafuoroethylene copolymer (ETFE), polyvinylidene fluoride (PVDF), and polyvinyl fluoride (PVF). Many other polymer materials also may be used including, for example, polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC), polypropylene, polyethelyne, high density polyethylene, acrylonitrile butadiene styrene (ABS), a thermal setting plastic, a thermal plastic, or a plastic with property enhancing additives. 
         [0026]    The inner tube  12 , outer tube  16 , annular body  20 , inner nose  30 , outer nose  40 , and annular nut  50  may be formed using, for example, a molding process (e.g., compression molding, injection molding, transfer molding, etc.). Optionally, features such as threads may be added to molded parts using machining processes (e.g., turning, milling, and drilling), which may comprise computer numerical control (CNC) processes.  FIG. 8  depicts a method of making the components of the double containment fitting  1 . The components may be molded (act  100 ) and then threads  24 ,  54  by be formed in the annular nut  50  and the annular body  50  (act  110 ). 
         [0027]      FIG. 1B  is an enlarged cross-sectional view of the annular body  20  shown in  FIG. 1A . The annular body  20  may include a first end  21 , an opposing second end  22 , and a bore  23  that extends through the annular body between the first end  21  and the second end  22 . The annular body  20  may be configured to at least partially telescopically receive the inner nose  30 , the flared end  13  of the inner tube  12 , and the outer nose  40 . 
         [0028]    The bore  23  of the annular body  20  comprises multiple portions. A first end portion  23   a  is configured to receive the outer nose  40  and may include an annular groove  26  for mating with an annular lip  44  (see  FIG. 2 ) of the outer nose  40  and forming a fluid-tight seal therebetween. A second portion  23   b  of the bore  23  has a diameter d b  smaller than a diameter d a  of the first end portion  23   a . The diameter d b  of the second portion  23   b  of the bore  23  may be smaller than the outside diameter of the outer nose  40 . Thus, the outer nose  40  may not be inserted within the bore  23  beyond the first portion  23   a.    
         [0029]    A third portion  23   c  of the bore  23  may be defined by a flexible wall  27  of the annular body  20 . The length l c  the third portion  23   c  of the bore  23  may change as forces are applied to the annular body  20  during assembly of the double containment fitting  1 , as further described hereinbelow. The diameter d c  of the third portion  23   c  of the bore  23  may change along the length l c  of the third portion  23   c . The inner surface of the annular body  20  in the third portion  23   c  may thus have a tapered profile, as shown in  FIG. 1B . The flexible wall  27  may be thinner than other walls of the annular body  20  to enable the flexible wall  27  to deform under longitudinal tension on the annular body  20 . The outside surface  29  of the annular body  20  may fold radially inwardly at the flexible wall  27 , then folds radially outwardly again to meet the threads  24  of the annular body  20 . In other words, the portion of the annular body  20  that defines the third portion  23 C of the bore  23  may comprise one or more bends, folds, or flutes  28  to facilitate deformation (e.g., flexing) thereof. 
         [0030]    A fourth portion  23   d  of the bore  23  may be configured to receive a first end portion  31  of the inner nose  30 . The fourth portion  23   d  of the bore  23  may include an annular groove  25  for mating with an annular lip  34  (see  FIG. 2 ) of the inner nose  30  and forming a fluid-tight seal therebetween. A fifth portion  23   e  of the bore  23  may have a diameter d e  smaller than a diameter d d  of the fourth portion  23   d . The diameter d e  of the fifth portion  23   e  of the bore  23  may be smaller than the outside diameter of the inner nose  30 . Thus, the inner nose  30  may not be inserted within the bore  23  beyond the fourth portion  23   d.    
         [0031]      FIG. 1C  depicts a side view of the annular body  20 . The annular body  20  may include one or more flat surfaces  29  (which, optionally, may define a hexagonal outer profile) configured to enable a tool (e.g., a wrench) to be used to provide relative rotation between the annular body  20  and the annular nut  50  ( FIG. 1 ). The first end  21  of the annular body  20  may be configured for coupling to an external component (not shown). An outside surface  3  of the second end  22  of the annular body  20  may include threads  24  thereon that are configured to engage with complementary threads  54  on the annular nut  50  ( FIG. 1 ). 
         [0032]    Returning to  FIG. 1 , the inner nose  30  comprises a first end portion  31 , an opposing second end portion  32 , and a bore  33  extending through the inner nose  30  between the first end portion  31  and the second end portion  32 . The first end portion  31  of the inner nose  30  may have an outside diameter larger than an outside diameter of the opposing second end portion  32  of the inner nose  30 . The bore  33  may have a substantially uniform diameter therethrough. The first end portion  31  of the inner nose  30  is configured to be received within the bore  23  of the annular body  20 . The opposing, second end portion  32  of the inner nose  30  may be inserted into the flared end  13  of the inner tube  12 . The first end portion  31  of the inner nose  30 , having a larger outside diameter than both the opposing, second end portion  32 , and the inside diameter of the flared end  13  of the inner tube  12  may prevent the inner nose  30  from being inserted too far longitudinally into the flared end  13  of the inner tube  12 . The diameter of the passageway  14  extending through the inner tube  12  may be substantially similar to the diameter of the bore  33  extending through the inner nose  30 . Thus, there is an at least substantially continuous fluid flow passage extending through the inner nose  30  and the inner tube  12  when the second end portion  32  of the inner nose  30  is inserted into the flared end  13  of the inner tube  12 . 
         [0033]    The outer nose  40  may comprise a first end portion  41 , an opposing, second end portion  42 , and a bore  43  extending through the outer nose  40  between the first end portion  41  and the second end portion  42 . The first end portion  41  of the outer nose  40  is configured to be received within the bore  23  of the annular body  20 . In the assembled state ( FIG. 5 ), the outer nose  40  may at least partially encircle the second end portion  32  of the inner nose  30 , and the flared end  13  of the inner tube  12  may be disposed between the first end portion  41  of the outer nose  40  and the second end portion  32  of the inner nose  30 . The bore  43  within the first end portion  41  of the outer nose  40  may have a diameter larger than the diameter of the bore  43  within the opposing, second end portion  42  of the outer nose  40 . The flared end  13  of the inner tube  12  may be received by the bore  43  within the first end portion  41  of the outer nose  40 . The flared end  13  of the inner tube  12  may have a larger outside diameter than the bore  43  within the opposing, second end portion  42  of the outer nose  40 . Thus, the outer nose  40  may be prevented from traveling longitudinally beyond the flared end  13  of the inner tube  12 . The opposing, second end portion  42  of the outer nose  40  may be configured to be at least partially received by the flared end  17  of the outer tube  16 . 
         [0034]    The annular nut  50  may at least partially telescopically receive the outer tube  16 , which in turn at least partially receives the outer nose  40  and the inner tube  12 . The second end portion  32  of the inner nose  30  may be at least partially telescopically received by the inner tube  12 , and the first end portion  31  of the inner nose  30  may be at least partially telescopically received by the annular body  20 . The annular nut  50  may engage with the annular body  20  to seal the double containment fitting. The annular nut  50  may include a first end  51 , an opposing second end  52 , and a bore  53  extending through the annular nut  50  between the first end  51  and the second end  52 . 
         [0035]      FIG. 1D  is an enlarged cross-sectional view of the annular nut  50 . The bore  53  of the annular nut  50  may be defined by an inside surface  56  of the annular nut  50 . A portion of the inside surface  56  may include threads  54  complementary to, and configured to engage with, the threads  24  of the annular body  20 . 
         [0036]    The bore  53  of the annular nut  50  may include multiple portions or sections. A first portion  53   a  of the bore  53  is configured to telescopically receive the outer tube  16 , but the diameter d 53a  of the first portion  53   a  of the bore  53  is too small to receive the flared end  17  of the outer tube  16 . The annular nut  50  may, in some embodiments, comprise an annular gripper  60 , which may be inserted into the second portion  53   b  of the bore  53 , and which is described in further detail hereinbelow. A third portion  53   c  of the bore  53  may be configured to telescopically receive the flared end  17  of the outer tube  16 . A fourth portion  53   d  of the bore  53  may be defined by or comprise the threads  54  on the inside surface  56  of the annular nut  50 . 
         [0037]    An annular gripper  60  may be positioned within the bore  53  of the annular nut  50 . The annular gripper  60  may be configured to grip an outer surface of the outer tube  16 . The annular gripper  60  may be or comprise a ring (e.g., a split ring), and may comprise a softer material than the material of the annular nut  50 . The softer material of the annular gripper  60  may more readily conform to the surface of the outer tube  16 . The annular gripper  60  may be positioned in the second portion  53   b  of the bore  53  of the annular nut  50 , between the third portion  53   c  and the first portion  53   a . The diameters of the second portion  53   b  and the third portion  53   c  may be larger than the diameter of the first portion  53   a . Thus, the gripper  60  may contact a longitudinally facing inner wall  55  of the annular nut  50 . 
         [0038]    A manner in which the double containment fitting  1  may be assembled with the inner tube  12  and the outer tube  16  is described below with reference to  FIGS. 2-4 , each of which depicts the double containment fitting  1 , the inner tube  12 , and the outer tube  16  in a partially assembled state. 
         [0039]    Referring to  FIG. 2 , the second end portion  32  of the inner nose  30  may be inserted into the flared end  13  of the inner tube  12 , and the first end portion  31  of the inner nose  30  may be inserted into the bore  23  of the annular body  20 . The second end portion  42  of the outer nose  40  may be inserted into the flared end  17  of the outer tube  16 , and the first end portion  41  of the outer nose  40  may be inserted into the bore  23  of the annular body  20  and positioned over the flared end  13  of the inner tube  12  and the second end portion  32  of the inner nose  30 . The annular nut  50  may be positioned over the outer tube  16 , and the first end  51  of the annular nut  50  may be positioned over the flared end  17  of the outer tube  16 . 
         [0040]    Turning to  FIG. 3 , the annular nut  50  may be threaded onto the annular body  20 . As the threads  54  of the annular nut  50  are threaded onto the threads  24  of the annular body  20 , the annular nut  50  will slide relative to and over the annular body  20  (i.e., in the leftward direction of  FIG. 3 ). As the annular nut  50  will slide relative to and over the annular body  20 , the annular gripper  60  will abut against and grip the outer tube  16  at the junction between the flared end  17  and the body portion  18  of the outer tube  16 , thus retaining the outer tube  16  in the annular nut  50 . 
         [0041]    Optionally, a surface of the annular gripper  60  may include a plurality of annular ridges (not shown), arranged in a stair-step manner to approximate the angle of a surface of the shoulder region of the outer tube  16 , between the flared end  17  and the body portion  18 . The plurality of annular ridges may seize the outer tube  16  shoulder surface, retaining the outer tube  16  in place within the annular nut  50 . 
         [0042]      FIG. 3  depicts the threads  54  of the annular nut  50  partially engaged with the threads  24  of the annular body  20 . The body portion  18  of the outer tube  16  is received by the first portion  53   a  of the bore  53  of the annular nut  50  with the flared portion  17  of the outer tube  16  in the third portion  53   c  of the bore  53  of the annular nut  50 . The annular gripper  60  prevents the flared end  17  of the outer tube  16  from sliding through the bore  53  of the annular nut  50 . The second end  42  of the outer nose  40  is received by the flared end  17  of the outer tube  16 , and the inner tube  12  extends through the outer tube  16  and the bore  43  of the outer nose  40 . An end of an annular passageway  19  between an outside wall of the inner tube  12  and an inside wall of the outer tube  16  is sealed at the ends of the inner tube  12  and the outer tubes  16  by the outer nose  40 . The first end  31  of the inner nose  30  is received by the bore  23  of the second end  22  of the annular body  20 . The second end  32  of the inner nose  30  is received by the flared end  13  of the inner tube  12 . The inner tube  12  is telescopically received by the bore  43  of the outer nose  40 . The first end portion  41  of the outer nose  40  is received by the bore  23  of the second end  22  of the annular body  20  and the second end  42  of the outer nose  40  is received by the flared end  17  of the outer tube  16 . The outer tube  16  is telescopically received by the bore  53  of the annular nut  50 , and retained therein by the annular gripper  60 . 
         [0043]    In  FIG. 3 , the threads  54  of the annular nut  50  are shown engaged to the threads  24  of the annular body  20 . After the annular gripper  60  abuts against the flared end  17  of the outer tube  16 , as the annular nut  50  and annular body  20  are further threaded together, the inner nose  30 , the flared end  13  of the inner tube  12 , the outer nose  40 , and the flared end  17  of the outer tube  16  are longitudinally compressed together between the annular body  20  and the annular nut  50 . 
         [0044]    Referring to  FIG. 4 , threading the annular nut  50  and the annular body  20  further together (from the configuration of  FIG. 3 ) will cause the second end portion  42  of the outer nose  40  to abut against the flared end  13  of the inner tube  12 , and, in turn, cause the flared end  13  of the inner tube  12  to abut against the second end portion  32  of the inner nose  30 . As the annular nut  50  is even further threaded onto the annular body, the first end portion  31  of the inner nose  30  is caused to be seated in the annular body  20  and to provide an interference fit between the annular lip  34  of the inner nose  30  and the complementary annular groove  25  of the fourth portion  23   d  ( FIG. 1B ) of the bore  23  of the annular body  20 .  FIG. 4  illustrates the double containment fitting  1  in this configuration, after the annular lip  34  of the inner nose  30  has been seated in the annular groove  25  of the annular body  20 . 
         [0045]      FIG. 5  depicts the annular nut  50  and the annular body  20  in the fully engaged configuration. As can be seen by comparing  FIG. 5  to  FIG. 4 , further threading of the annular nut  50  onto the annular body  20  from the configuration of  FIG. 4  will cause the flexible wall  27  of the annular body  20  to deform, which allows the second end  22  of the annular body  20  to move further toward and into the annular nut  50 , while the first end  21  of the annular body  20  is prevented from moving further toward the annular but  50  due to interference between the annular gripper  60 , the flared end  17  of the outer tube  16 , the outer nose  40 , the flared end  13  of the inner tube  12 , and the inner nose  30 . In other words, as the annular nut  50  is further threaded onto the annular body  20 , a tensile force may be applied to the annular body  20 , which may cause the flexible wall  27  to stretch in the longitudinal direction or to otherwise deform. 
         [0046]      FIG. 1B  illustrates the annular body  20  in a neutral configuration, with no external forces applied thereto.  FIG. 6  illustrates a cut-away view of the annular body  20  under tension along a longitudinal axis of the annular body  20 , as would be the case in the fully engaged configuration (shown in  FIG. 5 ) of the double containment fitting  1 . With the annular body  20  under tension, as shown in  FIG. 6 , the length l c  of the third portion  23   c  ( FIG. 1B ) of the bore  23  through the annular body  20  may increase as the flexible wall  27  stretches or deforms. Thus, the length l c  of the third portion  23   c  of the bore  23  through the annular body  20 , and the length of the annular body  20  itself are greater when the annular body  20  is under tension than when no tensile forces are applied to the annular body  20 . The double containment fitting  1  is configured such that the annular body  20  is placed under tension as the components of the double containment fitting  1  are assembled together with the inner tube  12  and the outer tube  16 , and the annular nut  50  is threaded onto the annular body  20 , and the annular body  20  is configured such that at least a portion of the annular body  20  will deform when it is placed under tension. 
         [0047]    Referring again to  FIG. 5 , as the annular nut  50  is further threaded onto the annular body  20 , the flexible wall  27  deforms (e.g., elastically deforms), and the second end  22  of the annular body  20  moves further into the annular nut  50  until the first end portion  41  of the outer nose  40  is seated in the annular body  20 , and an interference fit is provided between the annular lip  44  of the outer nose  40  and the complementary annular groove  26  of the first portion  23   a  ( FIG. 1B ) of the bore  23  of the annular body  20 . After an interference fit is provided between the annular lip  44  of the outer nose  40  and the complementary annular groove  26  of the annular body  20 , the double containment fitting  1  may be in the fully-engaged configuration with the inner tube  12  and the outer tube  16 . In other words, the annular body  20 , the inner nose  30 , the inner tube  12 , the outer nose  40 , the outer tube  16 , and the annular nut  50  may be configured such that, as the annular nut  50  is threaded onto the annular body  20 , the first end portion  41  of the outer nose  40  and the annular body  20  are the last parts to fully engage with one another. The biasing force of the flexible wall  27  of the annular body  20 , when flexed, enables the components of the double containment fitting  1  to be coupled in such a manner as to provide fluid tight seals therebetween, yet the biasing force is not strong enough to strip the complementary threads  24 ,  54  of the annular body  20  and the annular nut  50 , respectively. 
         [0048]    In the fully engaged configuration shown in  FIG. 5 , the outer nose  40  provides a seal between the inner tube  12  and the outer tube  16 , and seals the passageway  19  from the exterior of the outer tube  16  and the interior of the inner tube  12 . Similarly, the inner nose  30  provides a seal between the inner tube  12  and the annular body  20 , and seals the passageway  14  from the exterior of the inner tube  12 . The passageway  14  of the inner tube  12  is in fluid communication with the bore  33  of the inner nose  30  and the bore  23  of the annular body  20 . 
         [0049]    The annular nut  50  and the annular gripper  60  engage the flared end  17  of the outer tube  16 , and force the outer tube  16  toward the outer nose  40 . The outer nose  40  engages the flared end  13  of the inner tube  12 , and forces the inner tube  12  toward the inner nose  30 . The first end portion  41  of the outer nose  40  engages with the annular body  20 , which forces the outer nose  40  back against the outer tube  16 , such that the flared end  17  of the outer tube  16  is pinched between the annular gripper  60  and the outer nose  40  to form a fluid-tight seal. The first end portion  31  of the inner nose  30  engages with the annular body  20 , which forces the inner nose  30  back against the inner tube  12 , such that the flared end  13  of the inner tube  12  is pinched between the outer nose  40  and the inner nose  30  to form a fluid-tight seal. Furthermore, the interference fit between the annular lip  34  of the inner nose  30  and the annular groove  25  of the annular body  20 , and the interference fit between the annular lip  44  of the outer nose  40  and the annular groove  26  of the annular body  20 , provide a fluid-tight seal that prevents fluid from flowing from the passageway  14  to the exterior of the double containment fitting  1  through a pathway extending between the annular body  20  and the inner nose  30  and between the annular body  20  and the outer nose  40 . 
         [0050]      FIG. 7  depicts another embodiment of an annular body  20 ′ of the present invention. The annular body  20 ′ includes a first end portion  21 ′, an opposing, second end portion  22 ′, and a bore  23 ′ extending through the annular body  20 ′ between the first end portion  21 ′ and the second end portion  22 ′. The annular body  20 ′ includes a flexible wall  27 ′ which comprises multiple flutes  28 ′ or folds, which may allow the flexible wall  27 ′ to deform and the length of the flexible wall  27 ′ to increase under tension more than the length of the flexible wall  27  of the annular body  20  shown in  FIG. 1B , which comprises a single flute  28 . 
         [0051]    Embodiments of double containment fittings of the present invention (such as, for example, the double containment fitting  1  shown in  FIGS. 2-5 ) provide a seal for the end of concentric tubes using a single threaded coupling. 
         [0052]    Although the foregoing description contains many specific details, these should not be construed as limiting the scope of the present invention, but merely as providing illustrations of some exemplary embodiments. Similarly, other embodiments of the invention may be devised which do not depart from the spirit or scope of the present invention. Moreover, features from different embodiments of the invention may be employed in combination. The scope of the invention is, therefore, indicated and limited only by the appended claims and their legal equivalents, rather than by the foregoing description. All additions, deletions, and modifications to the exemplary embodiments of the invention, as disclosed herein, which fall within the meaning and scope of the claims, are embraced thereby.