Abstract:
A system for rapidly assembling solar receiving tubes and solar energy systems comprises a welding station is described. The welding station provides for rapidly assembling solar receiver tubes by welding together two or more solar receiving tubes and comprises means for receiving and restraining solar receiver tubes and a welding station comprising an orbital or a rotational weld head.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Application claims the benefit of priority under 35 U.S.C. 119(e) to U.S. Provisional Application No. 61/220,120, filed Jun. 24, 2009, of U.S. Provisional Application No. 61/238,195, filed Aug. 30, 2009, both of which are incorporated by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to apparatus and methods for assembling pipes, and more particularly to methods and systems for joining tubes for solar receivers. 
     2. Description of Related Art 
     Solar thermal power plants may be used to obtain electric power from the sun. In such plants, the solar flux impinges on tubes through which a heat exchange medium flows. In some solar thermal power plants, tubes are situated in a solar collector, such as along the axis of a parabolic trough. The heated heat exchange medium from the tubes may be used in a thermodynamic cycle to generate electric power. 
       FIGS. 1-3  show a typical prior art tube  100  for use in solar thermal plants. Tubes  100  are sometimes referred to as “solar receiver tubes” or “heat collection elements (HCE).” Tube  100  may be, for example and without limitation, a SCHOTT solar receiver tube model PTR 70 (SCHOTT Solar, Inc., Albuquerque N. Mex.). Tube  100  having a length L includes an outer tube  110  having a diameter D capped at each end by a flange  115 , an inner tube  111  coaxial with outer tube  110  and having a diameter d, and a bellows  113  that connects the flange and inner tube. Tubes  110  and  111 , bellows  113  and flange  115  are sealed to form a volume  112 , which is evacuated to provide a high thermal insulation between tubes  110  and  111 . 
     In general, the length L is from 5 feet (1.5 m) to 20 feet (6 m), the diameter D is from 2 inches (50 mm) to 7 inches (0.18 m), and the diameter d is from 1 inch (25 mm) to 4 inches (0.1 m). 
     Typically, tube  110  is a glass tube, and tube  111 , flange  115 , and bellows  113  are metal. Tube  110  is generally transparent to sunlight to facilitate the solar heating of a heat exchange medium that may flow through tube  111 , as indicated by arrows in  FIG. 1 . 
     In certain embodiments, tube  111  protrudes longitudinally beyond the end of each flange  115  by a distance S, which it typically from 0.375 inches (10 mm) to 4 inches (0.1 m). The portion of tube  111  that so protrudes is referred to as a collar  114 . In certain other embodiments, solar energy systems are formed from multiple solar receiver tubes  100  by joining collars  114  of adjacent tubes. Collar  114  may includes an index, which may be the center line of the tube weld joint, which may be used to rotationally align adjacent tubes for welding. 
     Due their length, L, and glass components, solar receiver tubes tend to be fragile, and difficult to join, typically by welding, since the collars  114  protrude beyond the ends of the glass outer tube  110  by a relatively small distance from each end. Further, collars  114  are adjacent to bellows  113 , on whose integrity the vacuum of volume  112  depends. Solar receiver tube are thus difficult to join without damaging the more fragile glass outer tube  110  or the bellows  113  joining tubes  110  and  111 . There is a need in the art for methods and apparatus that permit the easy and rapid joining of such tubes to facilitate more efficient assembly of solar energy systems. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention overcomes the limitations and problems of the prior art by providing an apparatus and method for rapidly tubes, which maybe used for solar energy systems. A welding station provides for rapidly assembling tubes by welding together two or more such tubes. 
     In one embodiment, an apparatus for joining two or more tubes is provided. Although not part of the invention, the solar receiver tubes have a longitudinal axis extending from a first end and a second end, and include an outer tube and a coaxial inner tube. The apparatus includes a weld head, a first means for receiving a first solar receiver tube, and a second means for receiving a second solar receiver tube. The first means allows the first end of the inner tube of the first solar receiver tube to be positioned near the weld head. The second means for receiving the second solar receiver tube, where the second means allows the first end of the outer tube of the second solar receiver tube to be positioned with the first end of inner tube of the second solar receiver tube near the weld head. The apparatus further includes translations stages to position the first ends of the received inner tubes in the weld head. 
     In another embodiment, the apparatus accepts a third solar receiver tube and joins three tubes. In yet another embodiment, the weld head is an orbital weld head. 
     In one embodiment, a method for joining tubes in a welding apparatus having a first weld head and a second weld head is provided. The method includes accepting a first solar receiver tube into the welding apparatus; accepting a second solar receiver tube into the welding apparatus; adjusting a welding apparatus translation stage to abut ends of the first and second solar receiver tube; orbital welding the first and second solar receiver tubes using the first weld head; accepting a third solar receiver tube into the welding apparatus; adjusting a welding apparatus translation stage to abut ends of the first and second solar receiver tube; and orbital welding the second and third solar receiver tubes using the second weld head. 
     These features together with the various ancillary provisions and features which will become apparent to those skilled in the art from the following detailed description, are attained by the joining apparatus and method of the present invention, preferred embodiments thereof being shown with reference to the accompanying drawings, by way of example only, wherein: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a prior art solar receiver tube; 
         FIG. 2  is an end view  2 - 2  of the tube of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view  3 - 3  of the tube of  FIG. 1 ; 
         FIG. 4  is a side view of one embodiment of a welding station; 
         FIG. 5  is a top view of the welding station of  FIG. 4 ; 
         FIG. 6  is an end view of the welding station of  FIG. 5 ; 
         FIG. 7  is a side view of a second embodiment of a welding station; 
         FIG. 8  is a sectional top view of an alternative embodiment welding station; 
         FIG. 9  is an end view of the welding station of  FIG. 8 ; 
         FIG. 10  is one embodiment of a purge trap disc assembly; 
         FIG. 11A-11C  illustrates a process for welding together solar receiving tubes; 
         FIG. 12  illustrates two tubes welded together; 
         FIG. 13  an embodiment of a double joining welding station; 
         FIG. 14  illustrates three tubes welded together; 
         FIG. 15  is a side view of a second embodiment of double joining welding station for joining three tubes; 
         FIG. 16  is a top view of the welding station of  FIG. 15 ; 
         FIG. 17  is an end view of the welding station of  FIG. 15 ; 
         FIG. 18A  is a top view  18 - 18  of  FIG. 16  illustrating a first position of an end cap assembly; 
         FIG. 18B  is top view  18 - 18  of  FIG. 16  illustrating a second position of an end cap assembly; 
         FIG. 19A  is a side view  19 - 19  of  FIG. 15  illustrating a first position of an end cap assembly; 
         FIG. 19B  is side view  19 - 19  of  FIG. 15  illustrating a second position of an end cap assembly; 
         FIG. 20A  is a top view  20 - 20  of  FIG. 16  illustrating a retracted position of a weld head and process chamber; 
         FIG. 20B  is a top view  20 - 20  of  FIG. 16  illustrating an extended position of the weld head and process chamber with a sealed process chamber; 
         FIG. 21A  is a side view  21 - 21  of  FIG. 15  illustrating a retracted position of the weld head and process chamber; 
         FIG. 21B  is a side view  21 - 21  of  FIG. 15  illustrating an extended position of a weld head and process chamber; 
         FIG. 21C  is a side view  21 - 21  of  FIG. 15  illustrating the extended position of a weld head and process chamber with a sealed process chamber; 
         FIG. 21D  is a side view  21 D- 21 D of  FIG. 21C ; 
         FIG. 21E  is a side view  21 E- 21 E of  FIG. 21C ; and 
         FIG. 22  is a view of one embodiment of a welding station control panel. 
     
    
    
     Reference symbols are used in the Figures to indicate certain components, aspects or features shown therein, with reference symbols common to more than one Figure indicating like components, aspects or features shown therein. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 4-6  are schematics of one embodiment of a welding station  400 , where  FIG. 4  is a side view,  FIG. 5  is a top view, and  FIG. 6  is an end view. Welding station  400  includes a stand  416  for positioning the welding station on the ground G and to support tube stations  410 , specifically a first tube station  410 - 1  and a second tube station  410 - 2 , at a height H. The height H may be, for example and without limitation, from 3 feet (0.3 m) to 5 feet (1.5 m) above the ground. 
     Tube stations  410 - 1  and  410 - 2  are each adapted to receive a solar receiver tube, such as two tubes  100 , and both stations include two or more assemblies  422  for supporting the tube ends. Thus, for example and without limitation, four assemblies  422 - 1 ,  422 - 2 ,  422 - 3 , and  422 - 4  are shown in  FIG. 4 , with station  410 - 1  associated with first assembly  422 - 1  and second assembly  422 - 2 , and station  410 - 2  associated with third assembly  422 - 3 , and fourth assembly  422 - 4 . In general, assemblies  422  are positioned near the ends of tubes provided to stations  410 - 1  and  410 - 2 . 
     Assemblies  422  may each be the same or may be different, and may include, for example and without limitation, various combinations of tube guides, supports, translation stages, and locking mechanisms to aid in the guiding, positioning and restraining of each tube within welding station  400 . 
     Welding station  400  also includes a weld head  425  and an associated weld head process chamber  424  located between stations  410 - 1  and  410 - 2 , and ancillary equipment for welding including, for example, a process gas supply  426 , supply line  421  to provide gas to an accepted tube  100 , a process gas controller  427 , a power supply  428 , a welding remote control unit  429 , and a welding process controller and cooling unit  430 . 
     Weld head  425  is adapted to fit with space  2 S and a distance (D−d)/2 to weld adjacent tubes within tube stations  410 - 1  and  410 - 2 . Weld head  425  may be any weld head suitable for welding the ends of pipes, and may be, for example and without limitation, a standard orbital welder or a rotational welder, with unit  430  selected as an appropriate unit. Thus, for example and without limitation, weld head  425  may be used with an Arc Machine 9-7500 welder (Arc Machines, Inc., Pacoima, Calif.) and unit  430  may include an Arc Machines model  207  power supply controller with its mating 207-CW cooling package. 
     Weld head process chamber  424  may include two halves, which may or may not be hinged, to permit the chamber to open and receive tubes  100 , and may also include connections to receive a gas from process gas controller  427 . Process gas may thus be provided to the outside of tubes  100  during welding. 
     In one embodiment, as illustrated in  FIG. 6 , assemblies  422  include lowering guides  631 , also referred to as a “V supports,” to guide accepted solar receiver tubes  100  (shown in phantom lines) into welding station  400 . Lowering guides  631  may also include several rubber protected, radius-saddle type supports that form to the curvature and support the outer diameter of an accepted tube  100 . 
     As further illustrated in  FIG. 6 , one or more components of assembly  422  may include one or more translation stages  632  to permit adjusting of an accepted solar receiver tube  100  along one or more of: the length of the tube (the “X” axis); in the plane of stand  416  and transverse to the length of an accepted tube (the “Y” axis); and in the plane of stand  416  and traverse to X and Y (the height along the “Z” axis). In addition and without limitation, weld head  425 , and or lowering guides  631  may also be mounted on individual translation stages  632 . 
       FIG. 7  is a side view of a second embodiment of a welding station  700 . Welding station  700  is generally similar to welding station  400 , except were explicitly noted. 
     Welding station  700  includes: assembly  422 - 2  including perch  723 - 1  and a support  631 - 1  mounted on an X-Y translation stage  734 - 1  and Z-axis translation stage  740 ; and assembly  422 - 3  which includes perch  723 - 2  and a support  631 - 2  mounted on an X-Y translation stage  734 - 2 . 
     For an orbital weld head  425 , the weld head includes an electrode  742  and a rotor  745  that may be moved into place for welding via a retraction mechanism  744 , and adjusted using fine tuning adjustments of X-Y translation stage  733 , X-axis translation stage  739 , Z-axis translation stage  735  and  741 . Alternatively, weld head  425  may translate relative to stand  416  and not include retraction mechanism  744 . 
     Translation stages  734 - 1  and  734 - 2 ,  733 ,  735 ,  739 ,  740 , and  741  may be, for example and without limitation, screw slide mechanisms. 
     For illustrative purposes, welding station  700  is shown as having accepted first tube  100 - 1 , having a glass outer tube  110 - 1  and a flange  115 - 1 , and second tube  100 - 2 , having a glass outer tube  110 - 2  and a flange  115 - 2 . In addition, each glass tube  110 - 1  and  110 - 2  is shown as having a corresponding nipple  736 - 1  and  736 - 2  that remains from the tube manufacturing process. In one embodiment, nipples  736 - 1  and  736 - 2  are aligned and oriented in the same plane. 
     Perches  723 - 1 ,  723 - 2  are adapted to accept and support tubes, such as tubes  100 - 1  and  100 - 2 . Thus, for example, perches  723 - 1 ,  723 - 2  each have a radius that is adapted to accept metal flange portion  115  and tube  110 , respectively. In addition, one or more of perches  723 - 1 ,  723 - 2  may be grounded to form a ground for welding adjacent tubes. Other means for restraining the tube  100  for welding include, but are not limited to, clamps, saddles, perches, straps, and combinations thereof. 
     Process chamber  424  may be positioned to cover weld head  425  and provides a protected or sealed environment around the ends of accepted solar receiver tubes  100 - 1  and  100 - 2 . In one embodiment, the interior of process chamber  424  is configured to be purged with a gas provided, for example, by process gas supply  426 . 
     Translation stages  739 ,  740 ,  741  provide adjustments of accepted tubes  100 - 1  and  100 - 2  along the X, Y, and Z axes. Specifically, X-axis translation stage  739  permits adjustment of the electrode  742  for alignment of the weld joint with the tube ends, translation stage  740  permits adjustment of tube-to-tube alignment in the plane of the stand, and the Z axis translation stage  741  permits vertical adjustments to fine tune electrode concentricity for welding. In one embodiment, glass supports  631  are mounted to the stages  740  in the welding station stand and restrain outer glass tubes of the solar receiver tubes. 
     Retraction mechanism  744  allows weld head  425 , including the electrode  742  and rotor  745 , into position for welding. Retraction mechanism  744  may, for example and without limitation, be slide mounted and air or electrically actuated. Preferably, the weld head  425  is retractable to a position that is below the bottom edge of the received solar receiver tubes  100 - 1 ,  100 - 2 . The retractable weld head  425  facilitates placement of the solar receiver tubes  100 - 1 ,  100 - 2  during loading and unloading. 
     In one embodiment, translation stages  733 ,  734 - 1  and  734 - 2  permitting fine adjustment in the X-Y plane for tube-to-tube alignment of accepted tubes  100 - 1  and  100 - 2 . In an alternative embodiment, the translation stages  734 - 1  and  734 - 2  include the V-support  631 , and the Y-axis adjustment may be achieved by sliding weld head  425  in a machined slot mounted to stand  416 . Thus, for example, tubes are manually presented to the rotational weld fixture and indexed on receiver tube metal collar/bellows not requiring a Z-axis adjustment on receiver tube supports. Z-axis adjustment using translation stage  735  may provide for adjusting the position of electrode  742 , enabling centerline seem adjustment of the electrode to the joint. 
     In certain embodiments, it is advantageous to provide a process gas to a joining location on the inside of accepted tubes  100 , especially when the tubes are being welded.  FIG. 10  shows a purge trap disc assembly  1000 . Assembly  1000  is used to provide control of gases on the interior of tubes  111 - 1  and  111 - 2  near ends that are being joined. Assembly  1000  includes a first disc  1001  and a second disc  1003 . Discs  1001  and  1003  have outer diameters that match the inner diameters of tubes  111 , and has a pair of circular seals  1005  that seat against the metal tube  111  of each tube, isolating a region about the abutted collars such that process gas can be provided to the inside metal surface near the location at which welding will occur. Disc  1103  has a line  1104  that may be connected to supply line  421 , and disc  1101  has an opening  1102 . Assembly  1000  may thus receive gas from supply  427 , which purges the space within tubes  111  between discs  1001  and  1003 . A sparging process gas is vented through the sealed portion of tube during welding, eliminating metal oxidation on the interior of weld seam. 
       FIG. 8  is a sectional top view of an alternative embodiment welding station  800 , and  FIG. 9  is an end view of the embodiment of welding station  800 . Welding station  800  is generally similar to welding stations  400  and  700 , except where explicitly discussed. 
     Welding station  800  includes a supporting channel  846 , optionally equipped with a protective cap  847 , attached to the welding station stand  416 , an assembly  422 - 1  including a saddle support  837 - 1 ; and an assembly  422 - 4  including a saddle support  837 - 2 . Saddle supports  837 - 1 ,  837 - 2  are adapted to accept and support tubes, such as tubes  100 - 1  and  100 - 2 . 
     In welding station  800 , supporting channels  846  has a width sufficient to support the outer diameter of solar receiver tubes  100 - 1 ,  100 - 2  on saddles  837 - 1 ,  837 - 2 , respectively, each of which may be coupled to a translation stage  734 . One or more channels may be connected together, or be individually fixed to a welding station stand. In one embodiment, the outer glass portions  110  of the solar receiver tubes longitudinally contact cushions on the saddles  837 . In this embodiment, the lowering guides  631 , and stages  734  are both optional. 
       FIGS. 11A-11C  are illustrative of, but not meant to limit, one method of using a welding station, such as welding station  400 ,  700 , or  800 . Initially, any clamps on assemblies  422 , weld head  425 , and process chamber  424  are open, permitting acceptance of solar receiver tubes  100 . 
     For embodiments where the weld head  425  is mounted on a retraction mechanism  744 , the weld head  425  is positioned below the tubes until needed for welding, at which time it is raised into welding position, and the tubes are translated relative to the electrode  742 . 
     First, as shown in  FIG. 11A , a first receiver tube  100 - 1  is lowered onto lowering guides  631  of assemblies  422 - 1  and  422 - 2 . 
     One illustrative example, which is not meant to limit the scope of the present invention, is described with reference to the embodiment of  FIG. 7 . The lowering guides facilitate placement of the solar receiver tube on the perch  723 . Thus, for example the electrode  742  is required to rotate about the adjoining solar receiver tube ends with a nearly constant arc gap. The lowering guides direct the metal-flanged ends  115  of the solar receiver tubes onto the perches  723  and coarsely index mating the tube ends end-to-end. The X axis position may then be adjusted so that the electrode  742  points to the centerline (dashed line) of the eventual weld joint. 
     In certain embodiments, lowering guides  631  include a portion that accepts the collar  114  index to maintain the weld stub of the tube in proper rotational relationship to a rotational weld fixture electrode of the welding station. 
     Next, with reference to  FIG. 11B , a second tube  100 - 2  is lowered onto lowering guides  631  of second tube station  410 - 2 . In one embodiment, the weight of tubes  100 - 1  and  100 - 2  is sufficient to secure the tubes for welding. In another embodiment, positive restraint mechanisms, such as clamps, are provide to restrain one or more of tubes  100 - 1 ,  100 - 2 . X-Y translation stages  632 ,  733 ,  734 - 1  and  734 - 2  are then adjusted for tube-to-tube alignment of accepted tubes  100 - 1  and  100 - 2  such that the ends of their respective tubes  111  are in contact. 
     Next, with reference to  FIG. 11C , purge trap disc assembly  1000  is attached to supply line  421  and is inserted into an open end of the metal tube  111  of tube  100 - 1 , and process chamber  424  is placed about the ends of tubes  100 - 1  and  100 - 2 . Process gas is then supplied to assembly  1000  and process chamber  424  to provide a controlled gas mixture for the welding of tubes  100 - 1 ,  100 - 2  to eliminate or reduce metal oxidation at the weld site. 
     At this point tubes  100 - 1  and  100 - 2  are ready for welding. An operator engages the sequence start button of the welding process controller and cooling unit  430 , and a preprogrammed weld sequence fuses the ends of their respective tubes  111 . 
     After welding, assembly  1000  is removed from the welded tubes  1200 , process chamber  424  is opened, any straps or clamps are opened, and the welded tubes  1200  are removed from welding station  700 .  FIG. 12  shows the welded tubes  1200  as including tubes  100 - 1  and  100 - 2 . 
       FIG. 13  is an embodiment of a double joining welding station  1300  for joining three tubes  100 . Welding station  1300  is generally similar to welding stations  400 ,  700 , and  800  except where explicitly discussed. 
     Welding station  1300  includes first tube station  410 - 1 , second tube station  410 - 2 , and a third tube station  410 - 3 . Tube stations  410 - 1  and  410 - 2  are adapted to receive tubes  100 - 1  and  100 - 1 , as described in other embodiments of the welding station, and tube station  410 - 3  includes assemblies  422 - 5  and  422 - 6  to accept a third tube  100 - 3 . Assemblies  422 - 5  and  422 - 6  are generally similar to previously discussed assembly  422 . 
     Welding station  1300  includes a first weld head  425 - 1  and a second weld head  425 - 2 . Weld heads  425 - 1  and  425 - 2  are generally similar to weld head  425 , including mounting, positioning and translation capabilities. Welding station  1300  also includes a first weld head process chamber  424 - 1  and a second weld head  425 - 2  disposed inside a second weld head process chamber  424 - 2 . Weld heads  425 - 1  and  425 - 2  are generally similar to weld head  425 , including mounting, positioning and translation capabilities, and process chambers  424 - 1  and  424 - 2  are generally similar to process chamber  424 . 
     Welding station  1300  also includes a first supply line  421 - 1  and a second supply line  422 - 2 . First supply line  421 - 1  provides gas to a first disc assembly  1000  that may be inserted to the junction of tubes  100 - 1  and  100 - 2 . Second supply line  421 - 2  provides gas to a second disc assembly  1000  that may be inserted to the junction of tubes  100 - 2  and  100 - 3 . 
     Welding station  1300  is used in a manner similar to the method illustrated in  FIG. 7 . A first tube  100 - 1  is placed in station  410 - 1  and a second tube  100 - 2  is placed in station  410 - 2 . Tubes  100 - 1  and  100 - 2  are welded, and then a third tube  100 - 3  is placed in station  410 - 3 . Third tube  100 - 3  is then welded to tube  100 - 3 . 
     After welding, assemblies  1000  are removed from the welded tubes  1400 , process chambers  424 - 1  and  424 - 2  are opened, any straps or clamps are opened, and the welded tubes  1400  are removed from welding station  1300 .  FIG. 14  shows the welded tubes  1400  as including tubes  100 - 1 ,  100 - 2 , and  100 - 3 . 
       FIGS. 15-22  are illustrative of a second embodiment of double joining welding station  1500  for joining three tubes  100 . Welding station  1500  is generally similar to welding stations  400 ,  700 ,  800 , and  1300 , except where explicitly discussed. 
       FIG. 15  is a side view,  FIG. 16  is a top view, and  FIG. 17  is an end view of welding station  1500 . Welding station  1500  includes two weld head and process chambers  1540 : a first weld head and process chamber  1540 - 1  and a second weld head and process chamber  1540 - 2 . Weld head and process chamber  1540  may be generally similar to weld head  425  and weld head process chamber  424 . 
     When adapted to weld three tubes, welding station  1500  includes three tube stations  1510  for accepting tubes  100 . Tube stations  1510  may be generally similar to tube stations  410 . Tube stations  1510  include a tube station  1510 - 1  is adapted to accept tube  100 - 1 , a tube station  1510 - 2  to accept tube  100 - 2 , and a tube station  1510 - 3  to accept tube  100 - 3 . Tube stations  1510 - 1  and  1510 - 2  may be used to position end of tubes  100 - 1  and  100 - 2  near or within weld head and process chamber  1540 - 1 . Tube stations  1510 - 2  and  1510 - 3  may be used to position end of tube  100 - 2  and  100 - 3  is near or within weld head and process chamber  1540 - 2 . 
     Tube stations  1501 - 1 ,  1501 - 2 , and  1501 - 3  include assemblies  1520  for supporting tubes  100 , which may be generally similar to assemblies  422 . Thus, for example and without limitation, welding station  1500  includes six assemblies  1520 - 11 ,  1520 - 12 ,  1520 - 21 ,  1520 - 22 ,  1520 - 31 , and  1520 - 31 , with tube station  1510 - 1  associated with first assembly  1520 - 11  and second assembly  1520 - 12 , tube station  1510 - 2  associated with first assembly  1520 - 21  and second assembly  1520 - 22 , and tube station  1510 - 3  associated with first assembly  1520 - 31  and second assembly  1520 - 32 . Assemblies  1520  may also include guides  631 . 
     In one embodiment, welding station  1500  includes an adjustable stand  1530  for positioning the welding station on the ground G and to support tube stations  1510 . Stand  1530 , which may be generally similar to stand  416 , includes legs  1531 ,  1533 ,  1535 , and  1537  which support platforms  1538 - 1 ,  1538 - 2 ,  1538 - 3 , and  1538 - 4 . Platform  1538 - 1  includes assembly  1520 - 11 , platform  1538 - 2  includes assemblies  1520 - 12  and  1520 - 21 , platform  1538 - 3  includes assemblies  1520 - 22  and  1520 - 31 , and platform  1538 - 4  includes assembly  1520 - 32 . 
     Pairs of rails  1539 - 1 ,  1539 - 2 , and  1539 - 3  are connected at assemblies  1520  to form a rigid stand  1530 . In one embodiment, rails  1539 - 1 ,  1539 - 2 , and  1539 - 3  are segments of rails spanning the length of welding station  1500 , and support platforms  1538 - 1 ,  1538 - 2 ,  1538 - 3 , and  1538 - 4  may be placed along the rail to adapt the length of tubes  100 - 1 ,  100 - 2 ,  100 - 3 . In another embodiment, rails  1539 - 1 ,  1539 - 2 , and  1539 - 3  are separate rails that telescope in support platforms  1538 - 1 ,  1538 - 2 ,  1538 - 3 , and  1538 - 4  to adapt the length of tubes  100 - 1 ,  100 - 2 , and  100 - 3 . 
       FIG. 17  shows one embodiment of platform, such as  1538 - 1 , as including a pair of tubular portions  1703  and  1705  which are connected to legs  1531 , and spanning portion  1701 . Tubular portions  1703  and  1705  are sized to accept tubular portions  1539  as an insert. Spanning portion  1701  may include components to support tubes, weld heads, and/or process chambers. 
     Weld head and process chamber  1540  is provided on stand  1530  to allow each weld head  425  to retract (move down) to allow tubes  100  to be placed and extended (moved up) to weld tubes. Thus, for example, weld heads  425 - 1 ,  425 - 2  are attached to platforms  1538 - 2 ,  1538 - 3  via retraction mechanisms  744 - 1 ,  744 - 2 , and the platforms each have a hole  1601 - 1 ,  1601 - 2  to permit movement of weld heads  425 - 1 ,  425 - 2  for tube placement and welding (as in  FIGS. 21A ,  21 B, and  21 D) Weld heads and process chambers  1540  also permit outside covering of tubes  100  by process chambers  424 , as described subsequently. 
       FIGS. 18A and 18B  are top views  18 - 18  of  FIG. 16  and  FIGS. 19A and 19B  are side views  19 - 19  of  FIG. 15  illustrating the end cap in the first and second position, respectively. 
     Welding station  1500  includes a pair of end cap assemblies  1800  at the each end of the tubes which are to be welded. In the embodiment illustrated, welding station  1500  includes a pair of end cap assemblies  1800 , one at an end of tube  100 - 1  and at one at an end of tube  100 - 3 . End cap assemblies  1800  may be used to push the tubes  100  together for welding and/or provide access to a process gas, such the gas from process gas supply  426 . End cap assembly  1800  may, for example, form part of an end assembly  1520 - 11  and  1520 - 32 . 
     End cap assembly  1800  includes a perch  1801  having a curved surface to accept a tube  100 , a motor  1803  which is attached to platform  1538 , a rod  1805 , and an end cap  1807  having a line  1810  that passes through the end cap within the location of an accepted tube  111 . One or more of end cap assembly  1800  may also provide an electrical connection to an accepted tube  111  for grounding the tube. 
     Motor  1803  pushes or pulls on rod  1805 , moving end cap  1807  in a longitudinal direction of accepted tube  100 . In the first position of  FIGS. 18A ,  19 A, motor  1803  pushes end cap  1807  to an extreme position, permitting the easy acceptance and placement of tube  100 . In the second position of  FIGS. 18B ,  20 B, motor  1803  pulls end cap  1807  onto the end of tube  111 . Motor  1803  may provide sufficient force to move tubes  100  together. Motor  1803  may also provide sufficient force to provide a seal of end cap  1807  on tube  111 . Line  1801 , which passes through end cap  1807 , may then be used to provide and/or remove purge cases from tubes  111 - 1 ,  111 - 2 , and  111 - 3 . 
     Motor  1803  may be, for example and without limitation, a pneumatic device such as a model CDY2S25H-100 pneumatic air linear table slide (SMC Corporation of America, Noblesville, Ind.). 
       FIGS. 20 and 21  illustrate various configurations of the weld head and process chamber, where  FIG. 20A  is a top view  20 - 20  of  FIG. 16  illustrating a retracted position,  FIG. 20B  is a top view  20 - 20  of  FIG. 16  illustrating an extended position with a sealed process chamber,  FIG. 21A  is a side view  21 - 21  of  FIG. 15  illustrating a retracted position,  FIG. 21B  is a side view  21 - 21  of  FIG. 15  illustrating an extended position,  FIG. 21C  is a side view  21 - 21  of  FIG. 15  illustrating the extended position with a sealed process chamber,  FIG. 21D  is sectional view  21 D- 21 D of  FIG. 21C , and  FIG. 21E  is a sectional view  21 E- 21 E of  FIG. 21C . 
     Weld head and process chambers  1540 - 1 ,  1540 - 2  each includes a weld head retraction mechanism  744  which includes a motor  2101  attached to a platform, such as platform  1538 - 3 , through translation stage  632 . Retraction mechanism  744  further includes a rod  2103  that extends from motor  2101  to platform  2104 , on which weld head  425  is mounted. As shown in  FIGS. 20A and 21A , a process chamber bottom portion  2105  is attached to platform  1538 - 3  surrounding hole  1601 - 2 , permitting welding head  425  to move to tubes  100 , as shown, for example, in  FIGS. 21A and 21B . 
     In one embodiment, platform  2104  seats against portion  1701 , and a removable lid  2007  may be placed on bottom portion  2105  after tubes for welding have been received to form process chamber  424 , as shown, for example in  FIGS. 21B and 21D . 
     Welding station  1500  may alternatively include clamps to restrain on or more tubes  100 . As an example, which is not meant to limit the scope of the present invention, center tube  100 - 2  is held in place at each end with clamp  2000 - 1 ,  2000 - 2  (or, in general, clamp  2000 ), as shown in  FIGS. 15 ,  16 ,  17 ,  20 A,  20 B,  21 A,  21 B, and  21 C. Clamp  2000  may, for example and without limitation be a swivel clamp (manufactured, for example, by DE-STA-CO Industries, Auburn Hills, Mich.) having a motor that rotates an arm  2101  to accept a tube (as in  FIG. 20A ) or over an accepted tube, and then pulls the tube onto the stand  1530 . 
     Thus, for example, with tubes  100  placed in tube stations  1501 - 1 ,  1501 - 2 , and  1501 - 3 , the center tube is clamped by clamps  2000  ( FIGS. 20A and 21A ). Tubes  100  are aligned using the various translation stages. In one embodiment, tubes  100  are aligned in the y-z plane. Thus, for example,  FIG. 21E  illustrates a saddle  2109 , which may be part of assembly  422 , which includes a curved portion  2111 , and is attached to translation stages to affect y and z axis adjustments. In another embodiment, a laser alignment system is used to illuminate the tube ends to facilitate alignment. End cap assemblies  1800  are then moved to force end caps  1807  onto the tube ends. Lines  1801  may then be attached to lines  421 . Retraction mechanism  744  raises weld head  415  to a pair of adjacent tubes  100  ( FIGS. 20B and 21B ). Lid  2107  is then be placed on bottom portion  2105  to form process chambers  724 , as is shown, for example, in  FIG. 21D . Process gas is then provided to process chambers  724  and the interior of tubes  111 , and the tubes are welded. 
     As shown in  FIG. 22 , which is one of a pair of control panel  2200  for each weld head of welding station  1400  which may include, but is not limited to: providing high and low flow rates to tubes  111  for purging and exhaust  2201 , process gas flow rate and pressure sensors and meters  2203 , an oxygen analyzer  2205  to measure and monitor the quality of the purge gases; switch  2207  to control end cap assemblies  1800 ; switch  2209  to control clamp  2000 ; switch  2211  to control retraction mechanism  744 ; and safety interlocks  2113  for purge, pneumatics, welding. 
     Reference throughout this specification to “one embodiment,” “an embodiment,” or “certain embodiments” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment,” “in an embodiment” or “in certain embodiments” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments. 
     Similarly, it should be appreciated that in the above description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment of this invention.