Abstract:
A method, a apparatus ( 150 ) and a prefabricated replacement tube ( 108 ) are provided for repairing a defect ( 112 ) in a tube ( 102 ), such as in a waterwall ( 100 ). In the method, a replacement tube ( 108 ) is prefabricated having a wall ( 116 ), a central opening ( 114 ) extending through a portion thereof and a predetermined length. A length of the tube ( 102 ), including the defect ( 112 ), equal to the predetermined length is removed. The replacement tube ( 108 ) is positioned between stubs ( 126, 128 ) of the tube ( 102 ), and a rotatable torch ( 152 ) of an automated welding apparatus ( 150 ) inserted through the opening ( 114 ) to align with a joint between an end ( 118, 112 ) of the replacement tube and a stub ( 126, 128 ). The torch ( 152 ) is rotated to join it to the stub ( 126, 128 ) along the entire joint. The torch ( 152 ) is then removed, turned 180 degrees, and reinserted to weld the remaining joint. Optionally, the replacement tube ( 108 ) is held in position before welding by slip rings ( 140, 142 ) which are slid down to cover the joints.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to a method and apparatus for repairing a defective tube, and more particularly to a method, apparatus, and prefabricated replacement tube for partial tube replacement in a heat exchanger to which access is restricted.  
         BACKGROUND OF THE INVENTION  
         [0002]    Various types of heat exchangers, such as boilers and waterwalls, are commonly used in hoods and stacks for cooling and/or treating industrial exhaust, for example from steelmaking furnaces, and by utilities (electric companies) to produce electrical energy and/or to cool power generating equipment.  
           [0003]    One problem with the operation of heat exchangers is that the heat, gases, dust and substances or fluids to which they are exposed frequently leads to increased rates of corrosion resulting in damage or defects in the tubes of the heat exchanger and subsequent leakage.  
           [0004]    A conventional approach to repairing the heat exchanger is to remove a section of the damaged tube, including the defect, over some length, and to install a new replacement tube in its place. The ends of the replacement tube and the stubs of the existing tube are prepared for welding by fitting and beveling surfaces at which they will be joined. Typically, the joining is done manually using shielded metal arc welding (SMAW).  
           [0005]    One problem with this approach is that because the weld is performed completely from the outer diameter (OD) of the tube, access is required to all sides of the tube, which is not possible for all heat exchanger designs, particularly boilers such as waterwalls. For example, tubes in a waterwall are frequently connected together along the length of their sides by a metal-webbing or membrane to give added rigidity and strength to the waterwall. Moreover, the waterwall is usually positioned abutting or in close proximity to an outer wall of the boiler (the cold-side). Thus, access is limited to more than half of the outer surface of the tube. This limited accessibility makes it difficult for the welder to achieve good weld tie-in/penetration and often results in less than desirable weld quality and may create problems in the future.  
           [0006]    Yet another problem with the above tube replacement method is manual welds performed from the OD are prone to weld defects such as reinforcement or excessive build-up of material on the inner diameter (ID) that lead to restricted fluid flow and accelerated corrosion or erosion at the weld locations.  
           [0007]    Another generally known approach for partial replacement of a tube in a heat exchanger, which avoids some of the problems of the above approach, is described, for example, in U.S. Pat. No. 4,047,659, to Drago (DRAGO). DRAGO discloses accessing the ID of the tube by cutting windows at each end of the replacement tube, the windows intersecting the ends of the replacement tube, and manually welding a portion of the attachment weld from the ID through the window. Covers for the windows are fabricated and welded over the windows from the OD completing the repair.  
           [0008]    While a significant improvement over the above approach, this approach is also not wholly satisfactory. A major shortcoming of the approach disclosed in DRAGO is the time required for measuring the section cut from the tube to be repaired, cutting a replacement tube to the correct length, cutting out the windows, preparing the ends of the replacement tube for welding, fabricating covers for the windows and welding the covers over the windows. Another problem with the approach in DRAGO is that all welds are performed manually using a SMAW process. Since the SMAW welds are full penetration, i.e., through the entire thickness of the tube, build-up on the ID for that portion of the welding done from the OD, such as the window covers, can still be a problem. Moreover, because the welds are performed manually weld quality is inconsistent, not-reproducible and can vary from weld to weld.  
           [0009]    Accordingly, there is a need for a method and apparatus for repairing a defective tube that provides a weld quality similar to that of a new installation or original fabrication. It is desirable that the method and apparatus eliminate excessive buildup of material from the ID of the repaired tube that can disrupt or reduce fluid flow through the repaired tube and lead to increased erosion/corrosion at the joints. It is also desirable that the method and apparatus enable repairs to be completed quickly with a minimum amount of down time for the heat exchanger. It is further desirable that the method and apparatus be automatic to reduce the level of skilled labor needed.  
           [0010]    The present invention provides a solution to these and other problems, and offers other advantages over the prior art.  
         SUMMARY OF THE INVENTION  
         [0011]    It is an object of the present invention to provide a method, apparatus and prefabricated replacement tube for partial tube replacement in a heat exchanger to which access is restricted.  
           [0012]    According to one aspect, the present invention provides a method for repairing a defect in a tube using an apparatus including a rotatable torch assembly, a wire feeder for supplying wire to the rotatable torch assembly, and a rotational drive assembly for supporting and rotating the rotatable torch assembly. Generally, the method involves steps of: (i) removing a section of the tube having the defect therein; (ii) fabricating a replacement tube having a wall with an opening extending through a portion thereof, and first and second ends prepared for joining to stubs of the tube formed by removal of the section of the tube having the defect therein; (iii) positioning the replacement tube between the stubs of the tube; (iv) inserting the rotatable torch assembly into the replacement tube through the opening, the rotatable torch assembly configured to align with a joint between the first end of the replacement tube and a stub; and (v) joining the first end of the replacement tube to the stub entirely along a joint therewith by rotating the rotatable torch assembly within the replacement tube. Optionally, slip rings in sliding engagement with the outer surface of the replacement tube hold it in position before it is joined to the stubs. In this embodiment, following the positioning of the replacement tube between the stubs, the slip rings slide up or down to cover the joints. The slip rings have the further advantage of serving as backing rings strengthening the joint when the replacement tube is joined to the stubs, typically by welding.  
           [0013]    In one embodiment, the replacement tube is prefabricated having a first end and a second end separated by a predetermined length, and the step of removing a section of the tube having the defect therein involves removing a length of the tube substantially equal to the predetermined length of the prefabricated replacement tube.  
           [0014]    In another embodiment, the opening is centrally located between the first and second ends, and the method further includes the steps of: (i) removing the rotatable torch assembly from the opening; (ii) reorienting the rotatable torch assembly; (iii) re-inserting the rotatable torch assembly into the replacement tube through the opening, the rotatable torch assembly configured to align with a joint between the second end of the replacement tube and another one of the stubs; and (iv) joining the second end of the replacement tube to one of the stubs substantially entirely along or around a joint formed there between by rotating the rotatable torch assembly within the replacement tube. The repair of the tube is completed by removing the rotatable torch assembly from the opening, and sealing the opening with a prefabricated cover. Typically, the cover is cover-welded or welded to the opening.  
           [0015]    As noted above, the replacement tube is joined to the stubs by welding. Preferably, the replacement tube is joined to the stubs by butt-welding or welding the first and second ends of the replacement tube to the stubs using the rotatable torch assembly. In one version of this embodiment, the apparatus is a Gas Metal Arc Welding (GMAW) apparatus, or a Gas Tungsten Arc Welding (GTAW) apparatus, and the step of welding includes providing a shield gas to the rotatable torch assembly through a rotatable coupling in the rotational drive assembly.  
           [0016]    Whichever method for joining is used, preferably, the apparatus further includes a controller for controlling power supplied to the rotatable torch assembly, and operating the rotational drive assembly, and the step of joining the replacement tube to the stubs is performed automatically once the torch is aligned with a joint. The controller controls or adjusts power supplied to the rotatable torch assembly and operates the rotational drive assembly to rotate the torch at a speed that substantially eliminates an excessive build up of material on the inner diameter (ID) of the tube at the joint. Thus, restriction in fluid flow through the repaired tube and corrosion of the joint is reduced.  
           [0017]    In another aspect, the present invention is directed to an apparatus for joining a replacement tube to stubs of a tube from which a section of the tube having a defect therein has been removed. The apparatus includes a rotatable torch assembly capable of being inserted into the replacement tube through a window in the replacement tube, to join first and second ends of the replacement tube to the stubs. Wire for joining the replacement tube to the stubs is fed to the rotatable torch assembly by a wire feeder having a wire drive assembly and a wire supply. A rotational drive assembly supports the rotatable torch assembly and rotates it within the replacement tube to join the first and second ends of the replacement tube to the stubs substantially entirely along joints therewith. Generally, the apparatus according further includes a mount to which the rotational drive assembly and the wire feeder are attached, and a latching mechanism for securing the mount to position the rotatable torch assembly within the replacement tube. In one embodiment, the latching mechanism is adapted to be partially inserted into the window to secure the mount to the replacement tube.  
           [0018]    In one embodiment, the window in the replacement tube is a centralized window centrally located between the first and the second ends. The rotatable torch assembly is adapted to be inserted into the replacement tube through the centralized window to align with the joint between the first end of the replacement tube and one of the stubs to join the first end to the stub, and to be removed from the replacement tube, reoriented, and reinserted through the centralized window to align with and join the second end to the other stub.  
           [0019]    Alternatively, the replacement tube has a number of windows including: (i) a first window located a predetermined distance from the first end of the replacement tube and through which the rotatable torch assembly can be inserted into the replacement tube to join the first end of the replacement tube to one of the stubs; and (ii) a second window located the same predetermined distance from the second end of the replacement tube and through which the rotatable torch assembly can be inserted into the replacement tube to join the second end of the replacement tube to one of the stubs.  
           [0020]    In another embodiment, the rotatable torch assembly is adapted to weld the first and second ends of the replacement tube to the stubs. In one version of this embodiment, the apparatus is a gas metal arc welding apparatus (GMAW), and the rotational drive assembly further includes a rotatable coupling through which shield gas is supplied to the rotatable torch assembly. The rotational drive assembly is adapted to rotate the rotatable torch assembly at predetermined rate based on power supplied to the rotatable torch assembly and materials of the wire, the replacement tube and the tube.  
           [0021]    In still another embodiment, the apparatus further includes a controller for automatically supplying power to the rotatable torch assembly, and operating the rotational drive assembly. The controller controls or adjusts power supplied to the rotatable torch assembly and operates the rotational drive assembly to rotate the torch at a speed that substantially eliminates a build up of material on the inner diameter (ID) of the tube at the joint. Thus, restriction in fluid flow through the repaired tube and corrosion of the joint is reduced.  
           [0022]    In yet another aspect, the present invention is directed to a prefabricated replacement tube for use in repairing a tube having a defect therein. Generally, the replacement tube has a wall with a central opening extending through a portion thereof, and first and second ends separated by a predetermined length. The ends are prepared for joining to stubs of the tube from which a section of the tube having the defect therein has been removed. The central opening is centrally located between the ends, and adapted to enable a rotatable torch to be inserted into the replacement tube through the central opening to join the ends of the replacement tube to the stubs.  
           [0023]    Preferably, the central opening is adapted to accommodate the rotatable torch of an automated gas metal arc welding apparatus, which is inserted into the replacement tube to weld the ends thereof to the stubs of the tube substantially without a build up of material at joints. More preferably, the central opening is also adapted to enable an automated welding apparatus to weld a prefabricated cover over the central opening, thereby completing repair of the tube.  
           [0024]    Optionally, the prefabricated replacement tube further includes at least one slip ring in sliding engagement with an outer surface of the wall to maintain the replacement tube in alignment with the stub while it is joined to the stubs. The slip ring is adapted to be positioned over the joint between one of the ends of replacement tube and one of the stubs.  
           [0025]    Advantages of the method, apparatus and prefabricated replacement tube of the present invention include any one or all of the following:  
           [0026]    (i) a weld quality similar to that of a new installation or original fabrication;  
           [0027]    (ii) welding is performed from the inner diameter (ID) eliminating excessive buildup or reinforcement typical of outer diameter (OD) welding processes, which can disrupt or reduce fluid flow through the repaired tube and lead to increased erosion/corrosion at the joints;  
           [0028]    (iii) weld quality is reproducible and consistent compared to manual applications;  
           [0029]    (iv) weld defects associated with manual repair methods are eliminated;  
           [0030]    (v) repair is completed entirely from one side of the tubing, for example, fire-side of a waterwall, eliminating access problems associated with welding from both sides; and  
           [0031]    (vi) prefabricated replacement tubes having standardized lengths and window configurations can be made available on demand or stocked on site eliminating delays for fabricating replacement tubes. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]    These and various other features and advantages of the present invention will be apparent upon reading of the following detailed description in conjunction with the accompanying drawings, where:  
         [0033]    [0033]FIG. 1 (prior art) is a perspective view of a portion of a waterwall for which a method and apparatus according to an embodiment of the present invention is particularly useful;  
         [0034]    [0034]FIG. 2 is a perspective view of a prefabricated replacement tube having a central opening or window according to an embodiment of the present invention;  
         [0035]    [0035]FIG. 3 is a perspective view of a prefabricated cover for the central opening in the prefabricated replacement tube of FIG. 2 according to an embodiment of the present invention;  
         [0036]    [0036]FIG. 4 is a perspective view of an alternative embodiment of a prefabricated replacement tube having multiple openings or windows according to an embodiment of the present invention;  
         [0037]    [0037]FIG. 5 is a sectional side view of an apparatus for welding a replacement tube to a tube from which a section of damaged tube has been removed according to an embodiment of the present invention;  
         [0038]    [0038]FIG. 6 is a top view of the apparatus of FIG. 5;  
         [0039]    [0039]FIG. 7 is a bottom view of the apparatus of FIG. 5;  
         [0040]    [0040]FIG. 8 is a partial sectional view of a rotational drive assembly of the apparatus of FIG. 5; and  
         [0041]    [0041]FIG. 9 is a flow chart showing steps of a method for repairing a defect in a tube according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0042]    The present invention is directed to a method, apparatus, and prefabricated replacement tube for partial tube replacement in a heat exchanger to which access is restricted.  
         [0043]    In general, the method involves the steps of: (i) providing a prefabricated repair or replacement tube having wall with an opening extending through a portion thereof, and first and second ends separated by a predetermined length and prepared for joining to stubs of a tube formed by removal of a section of the tube having a defect therein; (ii) removing the section of tube with the defect, the section having a length substantially equal to the predetermined length of the replacement tube; (iii) inserting a rotatable torch assembly of the apparatus into the replacement tube through the opening to align with a joint between the first end of the replacement tube and a stub; (iv) joining one end of the replacement tube to the stub entirely along a joint therewith by rotating the rotatable torch assembly within the replacement tube; and (v) removing, reorienting and reinserting the rotatable torch assembly to join the remaining end of the replacement tube to the remaining stub. Preferably, the opening is centrally located between the first and second ends. More preferably, the opening and the rotatable torch assembly are configured to enable the rotatable torch assembly to align correctly with either end of the replacement tube, thereby enabling the joining process to be automated. The repair of the tube is completed by removing the rotatable torch assembly from the opening, and sealing the opening with a prefabricated cover.  
         [0044]    Heat exchangers generally include a number of tubes through which a heat transfer fluid is passed. Usually, the tubes are cylindrically shaped with a diameter much less than their length, and have open ends that are sealed or coupled to a larger tube or plenum (not shown) through which the heat transfer fluid is passed. The heat transfer fluid can include a gas, such as air, nitrogen, helium, argon and carbon dioxide, a liquid, such as water, deionized water, distilled water, oil, alcohol, ethylene glycol, or a liquid metal, such as sodium. To increase the efficiency of thermal transfer the heat transfer fluid may be contained within a closed pressurized system of which the heat exchanger is but one component.  
         [0045]    [0045]FIG. 1 is a perspective view of a portion of a particular type of heat exchanger, a waterwall  100 , for which a method and apparatus (not shown in this figure) according to an embodiment of the present invention is particularly useful. Waterwalls  100  are commonly used in, for example, gas, oil and coal-fired boilers (not shown) of electric power or utility plants. For purposes of clarity, many of the details of waterwalls  100  that are widely known and are not relevant to the present invention have been omitted. Referring to FIG. 1, a waterwall  100  typically includes a number of parallel tubes  102  through which a heat transfer fluid is passed. As fuel is burned in a boiler&#39;s interior, heat is transferred into the waterwall  100  to heat water contained therein and produce steam to turn a turbine and generate electricity. Optionally, the heat transfer fluid in the waterwall  100  is pressurized to raise the boiling point reducing or eliminating boiling, and the heated heat transfer fluid, water, is coupled to second heat exchanger, a steam generator (not shown), in which water heated by the heat transfer fluid is boiled to produce steam. This embodiment has the advantage of enabling use of heat transfer fluids having greater heat transfer capacities or corrosion inhibiting properties. It should be noted that in this embodiment the method and apparatus of the present invention can be used to repair defective tubes  102  in both the waterwall  100  and the steam generator.  
         [0046]    Because the tubes  102  of the waterwall  100  have thin walls  104  relative to their cross-section, they are often joined to one another along the sides thereof by a mesh or webbing  106  to improve the strength of the waterwall. To improve heat transfer, strength and tolerance to elevated temperatures the tubes  102  and the webbing  106  are made of a metal, such as steel, copper, zinc, nickel and/or alloys thereof. As noted above, the tubes  102  are cylindrically shaped having a diameter much less than their length, and open ends that are sealed or coupled to a larger tube or plenum (not shown) through which the heat transfer fluid is passed. For example, a typical waterwall used a utility plant can include over 1,000 tubes, each having a length of up to 200 meters, and a diameter of from 2 to 10 centimeters.  
         [0047]    An embodiment according to the present invention of a replacement tube  108  for repairing a tube  102  in a waterwall  100  from which a section  110  having a damage or defect  112  therein will now be described with reference to FIGS. 1, 2 and  3 . FIG. 2 is a perspective view of a prefabricated replacement tube  108  having a window or opening  114  according to an embodiment of the present invention. Generally, the replacement tube  108  has a cylindrical wall  116  with a diameter or radius substantially the same as that of a tube  102  in the waterwall  100 , an opening  114  extending through a portion of the wall, and first and second ends  118 ,  120 , separated by a predetermined length. Preferably, for reasons described below, the opening  114  is a central opening centrally located between the first and second ends  118 ,  120 . The opening  114  is sized and shaped to accommodate a rotatable torch assembly (not shown in this figure) of the apparatus of the present invention. In the embodiment shown, the opening  114  has an oval shape with a long axis parallel with that of the replacement tube  108 . By predetermined length it is meant any one of several lengths ranging from a length little longer than that of the opening to a length nearly equal to that of a tube in a waterwall for which it is prefabricated as a replacement tube. In a preferred embodiment of the method a number of prefabricated replacement tubes  108  having diameters equal to those of tubes in a waterwall and various predetermined lengths are maintained in inventory near where they are to be used to expedite the repair process and minimize downtime of the waterwall.  
         [0048]    Edges of the first and second ends  118 ,  120 , are prepared for joining to stubs  126 ,  128 , formed by removal of the damaged section  110  of the tube  102  in the waterwall  100  having the defect  112  therein. For example, the edges of the first and second ends  118 ,  120 , can be beveled to self-align and fit flush with edges  130 , 132 , of the stubs  126 ,  128 , which are beveled or slanted in a complementary direction when the replacement tube  108  is positioned between the stubs. Similarly, an edge  134 , or edges, surrounding and defining the opening  114  can be prepared or beveled for joining to an edge, or edges, of a prefabricated cover  138 , described in detail below.  
         [0049]    The replacement tube  108  can be joined to the stubs  126 ,  128 , by soldering, brazing or welding. Preferably, the replacement tube  108  is joined to the stubs  126 ,  128 , using a Gas Metal Arc Welding (GMAW) process. Alternatively, the replacement tube  108  is joined to the stubs  126 ,  128 , by a butt-weld using a Gas Tungsten Arc Welding (GTAW) process that penetrates the full thickness of the wall  116  of the replacement tube and tube  102  undergoing repair.  
         [0050]    It will be appreciated, that the replacement tube  108  need not be made from the same metal or material as the tubes  102  of the waterwall  100  it is used to repair. Rather, the replacement tube  108  can be made from a metal or material selected to facilitate joining with the stubs  126 ,  128 , of the tube  102 , or of a material that is resistant to corrosion, and electrochemical corrosion, and is suitable for use in a wide range of environments or applications. For example, high-temperature nickel alloys, stainless-steel, or other steel alloys. Preferably, the metal or material of the replacement tube  108  has a coefficient of thermal expansion similar to that of the metal or material of tubes  102  in the waterwall  100 . More preferably, the material of the replacement tube  108  also has corrosion properties similar to that of the metal or material of tubes  102  in the waterwall  100 .  
         [0051]    Optionally, the replacement tube  108  further includes one or more slip rings  140 ,  142 , in sliding engagement with an outer surface  144  of the wall  116  of the replacement tube. The slip rings  140 ,  142 , are configured to be positioned over joints formed between the first and second ends  118 ,  120 , of the replacement tube  108  and the stubs  126 ,  128 , thereby maintaining the replacement tube in alignment or position between the stubs prior to the replacement tube being joined to the stubs. In addition, in a preferred embodiment wherein the replacement tube  108  is joined to the stubs  126 ,  128 , by a weld fully penetrating thicknesses of the replacement tube and the stubs, the slip rings  140 ,  142 .  
         [0052]    [0052]FIG. 3 is a perspective view of a prefabricated cover  138  for the opening  114  in the prefabricated replacement tube  108  of FIG. 2. Generally, the cover  138  has a size and shape substantially the same as those of the opening  114  and an edge (not shown) prepared for joining to the edge  134  of the opening  114 . For example, the edge  146  of the cover  138  can be beveled to fit flush with the edge  134  of the opening  114 . In one embodiment, the cover  138  is a piece of the wall  116  of the replacement tube  108  cut from the replacement tube. Alternatively, the cover  138  can be prefabricated from a separate tube or piece of metal having an inner and/or an outer surface with a radius similar to that of the replacement tube. The prefabricated cover  138  need not be made from the same metal or material as the replacement tube  108  or the tube  102  being repaired. Rather, the cover  138  can be made from a metal or material selected to facilitate sealing the opening  114  therewith. Preferably, the material of the cover  138 , if made from a separate tube or piece of metal, has a coefficient of thermal expansion similar to that of the metal or material of the replacement tube  108 . More preferably, the material of the cover  138  also has corrosion properties similar to those of the metal or material of the replacement tube and the tube being repaired. The cover  138  can be cover-welded or welded over or into the opening  114 .  
         [0053]    An alternative embodiment of a prefabricated replacement tube  108  having multiple openings or windows through which the rotatable torch assembly (not shown in this figure) can be inserted into the replacement tube. The windows, include a first window  114 A located a first predetermined distance from the first end  118  of the replacement tube  108 , and a second window  114 B located a second predetermined distance from the second end of the replacement tube. Generally, the first window  114 A is sized, shaped and located so that the rotatable torch assembly can be inserted into the replacement tube  108  to join the first end  118  of the replacement tube to one of the stubs  126 , and the second window  114 B is sized, shaped and located so that the rotatable torch assembly can be inserted into the replacement tube to join the second end  120  of the replacement tube to the other stub  128 . Preferably, the first predetermined distance from the first window  114 A and the first end  118  of the replacement tube  108 , and the second predetermined distance from the second window  114 B and the second end  120  of the replacement tube are substantially the same. This embodiment particularly useful for relatively long replacement tubes  108  for which a centralized window  114  would result in an impractically long rotatable torch assembly. This embodiment is also useful for situations in which the rotatable torch assembly must be kept relatively short. For example, in repairing boilers or waterwalls  100  to which access is severely limited.  
         [0054]    An embodiment of an apparatus according to the present invention for repairing a tube  102  in a waterwall  100  will now be described with reference to FIGS. 5 through 8.  
         [0055]    [0055]FIG. 5 is a sectional side view of an apparatus  150  according to an embodiment of the present invention for welding a replacement tube  108  to stubs  126 ,  128 , of a tube  102  from which a damaged or defective section  110  has been removed. Generally, the apparatus  150  includes: (i) a rotatable torch assembly  152  configured to be inserted into the replacement tube  108  through the opening  114  and to weld the first and second ends  118 ,  120 , of the replacement tube  108  to the stubs  126 ,  128 ; (ii) a wirefeed mechanism or wire feeder  156  having a wire drive assembly  158  and a wire supply  160  configured to feed filler wire or wire  162 , such as welding wire, to the rotatable torch assembly  152  to join first and second ends  118 ,  120 , of the replacement tube  108  to the stubs  126 , 128 ; (iii) a rotational drive assembly  164  supports the rotatable torch assembly  152  and rotates it within the replacement tube  108  to join the first and second ends  118 ,  120 , of the replacement tube to the stubs  126 , 128 , substantially entirely along joints formed therewith; (iv) a mount  166  to which the rotational drive assembly  164  and the wire feeder  156  are attached; and (v) a latching mechanism  168  for securing the mount  166  to position the rotatable torch assembly  152  within the replacement tube  108 .  
         [0056]    Optionally, the apparatus  150  further includes a controller  170  that can be operated or programmed to automatically provide power and filler wire  162  to the rotatable torch assembly  152 , and to control the rotational drive assembly  164 , thereby providing a uniform and defect-free joint between the first and second ends  118 ,  120 , and the stubs  126 ,  128 . In addition, performing the joining process from inside the replacement tube  108  and automating the process, substantially eliminates build up of material at joints between the replacement tube and tube  102 , thereby reducing restriction in fluid flow through the tube and corrosion/erosion of the joints after the tube has been repaired.  
         [0057]    The latching mechanism  168  can include any suitable means for temporarily attaching the mount  166  to the replacement tube  108  and/or the tube  102  undergoing repair. In a preferred embodiment, the latching mechanism  168  is adapted to be partially inserted into the opening  114  to secure the mount  166  to the replacement tube  108 . In a preferred embodiment, the latching mechanism  168  is integrally formed with the mount  166 . That is, a portion of the mount  166  is also part of the latching mechanism  168  that is inserted into the opening  114 . In one version of this embodiment, as shown, the latching mechanism  168  further includes a latch-and-release trigger  172  and a spring  174  that forces the trigger against one end or edge of the opening  114 , and, through the mount  166  and a housing frame  176 , forces the rotational drive assembly  164  against the opposing or facing end or edge.  
         [0058]    Typically, the wire feeder  156  includes an electric motor  178  driving one or more gears or drive rollers  180  that engage the filler wire  162  forcing it through a wire conduit  182  to the rotatable torch assembly  152 . The wire feeder  156  can be operated manually by an operator controlling power to the electric motor  178 , or automatically by the controller  170  to synchronize feeding of the filler wire  162  to the rotation of the rotatable torch assembly  152 .  
         [0059]    In a preferred embodiment, the apparatus  150  is a welding apparatus and the rotatable torch assembly  152  is adapted to weld or butt-weld the first and second ends  118 ,  120 , of the replacement tube  108  to the stubs  126 ,  128 . For purposes of clarity, many of the details of welding systems that are widely known and are not relevant to the present invention have been omitted. Generally, in this embodiment the apparatus  150  includes a power input terminal  184  to which power from a welding power supply  186  is applied, and a lead  188  electrically coupling the terminal to the tip  154  of the rotatable torch assembly  152 . The electrical coupling is accomplished using a power brush and a rotating contact ring in the rotational drive assembly  164 , described in greater detail below. Welding is accomplished using the concentrated heat from an electric arc formed between the welding wire  162  and the replacement tube  108  and/or the tube  102  being repaired, which is electrically connected by a ground cable (not shown) to the welding power supply  186 .  
         [0060]    In a preferred embodiment, the apparatus  150  is a Gas Metal Arc Welding (GMAW) apparatus or a Gas Tungsten Arc Welding (GTAW) apparatus, and the rotational drive assembly  164  further includes a gas inlet  190  and a rotatable coupling  192  through which shield gas is supplied to the rotatable torch assembly  152 .  
         [0061]    [0061]FIG. 6 is a top view of the apparatus  150  of FIG. 5 showing the rotational drive assembly  164  and the latching mechanism  168  inserted in the replacement tube  108 .  
         [0062]    [0062]FIG. 7 is a bottom view of the apparatus  150  of FIG. 5 showing the gas inlet  190  and a rotatable coupling  192  for supplying shield gas to the rotatable torch assembly  152 .  
         [0063]    [0063]FIG. 8 is a partial sectional view of a rotational drive assembly  164  of the apparatus  150  of FIG. 5 showing the power input terminal  184 , lead  188 , power brush  194  and a rotating contact ring  196  through which power from welding power supply  186  is electrically coupled to the tip  154  of the rotatable torch assembly  152 . Power brush  194  and contact ring  196  are electrically insulated from the housing frame  176  by insulator  198 . Spring  200  forces power brush  194  against contact ring  196 , thereby ensuring good electrical coupling therebetween.  
         [0064]    An embodiment of a method for operating the apparatus  150  according to the present invention will now be described with reference to FIG. 9. FIG. 9 is a flow chart showing steps of a method for repairing a defect in a tube  102  according to an embodiment of the present invention. Generally, the method involves:removing a section  110  of the tube  102  having the defect  112  therein (step  202 ); fabricating a replacement tube  108  having a wall  116  with an opening  114  extending through a portion thereof, and first and second ends  118 ,  120 , prepared for joining to stubs  126 ,  128 , of the tube  102  formed by removal of the section  110  of the tube  102  having the defect  112  therein (step  204 ); positioning the replacement tube  108  between the stubs  126 ,  128 , of the tube  102  (step  206 ); (iv) inserting the rotatable torch assembly  152  into the replacement tube  108  through the opening  114 , the rotatable torch assembly configured to align with a joint between the first end  118  of the replacement tube  108  and a stub  126  (step  208 ); and joining the first end  118  of the replacement tube  108  to the stub  126  entirely along a joint therewith by rotating the rotatable torch assembly  152  within the replacement tube  108  (step  210 ). Preferably, the opening  114  is centrally located between the first and second ends  118 ,  120 , and the method involves the further steps of: removing the rotatable torch assembly  152  from the opening  114  (step  212 ); reorienting the rotatable torch assembly  152  (step  214 ); re-inserting the rotatable torch assembly  152  into the replacement tube  108  through the opening  114 , the rotatable torch assembly  152  configured to align with a joint between the second end  120  of the replacement tube  108  and another one of the stubs  128  (step  216 ); and joining the second end  120  of the replacement tube  108  to one of the stubs  128  substantially entirely along or around a joint formed there between by rotating the rotatable torch assembly  152  within the replacement tube (step  217 ). The repair of the tube  102  is completed by removing the rotatable torch assembly  152  from the opening  114  (step  218 ), and sealing the opening  114  with a prefabricated cover  138  (step  220 ). Typically, the cover  138  is welded to the opening.  
         [0065]    Optionally, the step of positioning the replacement tube  108  between the stubs  126 ,  128 , of the tube  102 , step  204 , includes the step of positioning slip rings  140 ,  142 , in sliding engagement with the outer surface  144  of the replacement tube  108  over joints between the ends  118 ,  120 , of the replacement tube  108  and the stubs  126 ,  128 , thereby maintaining alignment of the replacement tube with the stubs while they are joined.  
         [0066]    As noted above, the apparatus  150  can be configured to join the replacement tube  108  with the stub, steps  210 ,  217 , by welding. Whichever method is used for joining, preferably, the apparatus  150  further includes a controller  170  for controlling power supplied to the rotatable torch assembly  152 , and operating the rotational drive assembly  164 , and the steps of joining the replacement tube to the stubs, steps  210 ,  217 , are performed automatically once the tip  154  is aligned with a joint.  
         [0067]    It is to be understood that even though numerous characteristics and advantages of certain embodiments of the present invention have been set forth in the foregoing description, together with details of the structure and function of various embodiments of the invention, this disclosure is illustrative only, and changes may be made in detail, especially in matters of structure and arrangement of parts within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.