Abstract:
Memory metal plugs adapted to seal tubes are formed with a central post and laterally extended disks or rings. The plugs are formed from memory metal that is machined and heat treated to establish favorable temperature profile for the memory metal. The plugs are deformed in an apparatus that holds the plugs and cools the plugs to transform them into the martensitic state. A ram is used to force the cooled plugs through a die to decrease their diameter. These plugs can then be inserted into a tube and heated, causing them to return to their original state and thereby plug the tube.

Description:
RELATED APPLICATION  
       [0001]     This application is a regular utility application of U.S. Provisional Patent Application Ser. No. 60/741,621, filed on Dec. 2, 2005, the entire disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     Frequently, it is necessary to plug or seal a tube or circular opening. Plugs can be used to seal bores formed in diesel engines, such as the plug disclosed in U.S. Pat. No. 6,053,992. Plugs are also used to seal tubes in heat exchangers. There are a wide variety of different methods used to seal such devices, none of which are totally satisfactory. In certain applications, explosive devices are used to seal off a tube. But, this is very expensive. Mechanical devices can also be used; but, in high pressure applications, these may fail.  
         [0003]     A potentially useful plug to seal tubes is disclosed in Hall U.S. Pat. No. 5,189,789. This discloses the use of a memory metal or Nitinol plug. Memory metals are alloys that undergo a reversible transformation from an austenitic state to a martensitic state with changes in temperatures. At colder temperatures, the alloy enters the martensitic state and reverts to the austenitic state at higher temperatures. A plug in the martensitic state can be bent or shaped. When the metal reverts to the austenitic state it reverts to its original shape.  
         [0004]     The plug disclosed in Hall U.S. Pat. No. 5,189,789 is formed from such a memory metal and includes a central post with a plurality of disks that extend perpendicular to the post. The disclosed plug is placed in a bath of methanol and dry ice to cause it to enter the martensitic state. It is then forced through a die which bends or swages the disks, decreasing the exterior diameter of the plug. The plug can then be manually place into a tube and heated, causing it to revert to the austenitic state at which point in time it will bend back to its original shape, increasing its diameter and, thus, plugging the tube. These plugs are preferably formed from Nitinol, which is an alloy of nickel and titanium.  
         [0005]     Unfortunately, the plug disclosed in the Hall reference tends to break when swaged. The design of the plug as well as the disclosed method of swaging the plug produced very unreliable results.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention is based on the realization that an apparatus can be utilized that supports the unswaged memory metal plug in a holder and chills it while it is in the holder to transform it to the martensitic state. Preferably, the plug is chilled using liquid carbon dioxide or other cryogenic fluids at temperatures below the lower martensitic transition temperature of the memory metal alloy. Preferably, the plug is placed in a die and coolant is forced through the die walls onto the plug. The plug, while in the holder, can then be forced by a ram through a die to swage it, allowing it to be used.  
         [0007]     This ensures that the plug is cooled adequately. A thermocouple can also be used to measure the plug temperature and prevent the ram from activating before the plug reaches the martensitic state.  
         [0008]     The plug is designed with radiused regions between the disks and central rod which further prevents breakage.  
         [0009]     The deformed plug connected to a heat conducting holder is inserted into a tube. The holder and plug are heated, causing the plug to revert to the austenitic state and into its original configuration, thus expanding and sealing the tube.  
         [0010]     The objects and advantages of the present invention will be further appreciated in light of the following detailed description and drawings, in which: 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a perspective view of a plug for use in the present invention;  
         [0012]      FIG. 2  is a plan view of a plug and holder assembly for use in the present invention in its austenitic state;  
         [0013]      FIG. 3  is a plan view of a plug and holder assembly for use in the present invention in its deformed martensitic state;  
         [0014]      FIG. 4  is a diagrammatic depiction of the insertion of the plug of the present invention into a tube.  
         [0015]      FIG. 5A  is a cross sectional view of the apparatus used in the present invention showing the plug above the die.  
         [0016]      FIG. 5B  is a cross sectional view of the apparatus shown in  FIG. 5A  with the plug in the upper portion of the die.  
         [0017]      FIG. 6  is a diagrammatic depiction of the operation of the present invention.  
         [0018]      FIG. 7A  is cross sectional view of a Nitinol plug inserted into a tube in the swaged condition, as shown in  FIG. 3 .  
         [0019]      FIG. 7B  is a cross sectional view of a Nitinol plug inserted into a tube in its austenitic unswaged configuration. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     As shown in  FIG. 1 , the present invention utilizes a plug  10  that includes a central axial post  12  and first, second and third circular disk-shaped flanges  14 , 16 , and  18 , generally referred to as disks. The plug  10  is shown with three disks but can be made with as few as one and as many as desired. Two or more disks are preferred because multiple disks help align the plug in use. Each of the first, second and third disks have first and second surfaces  13   a  and  13   b.  At the junctures between the surfaces and post  12  are radiused portions  15 . The radiused portions can be located at any of the junctures between the posts and the disks. Preferably, they are at the juncture of the first surface  13   a  of the first disk  14  and at the junctures between the post  12  and the first and second surfaces  13   a,    13   b  of the second  16  and third  18  disks.  
         [0021]     Each of these radiuses should be greater than 0.003 inch, more preferably greater than 0.015 inch, and, in a preferred embodiment, is greater than about 0.03 inch. As shown, they are a complete radius of about 0.093. These radiused portions provide stress relief in both the deformation of the plug  10 , and during use of the plug  10 . This allows the plug to be deformed more, thus further reducing the diameter of the deformed plug.  
         [0022]     Generally, for a plug having a diameter of 1 inch, the post  12  will have a diameter of 0.10 to 0.3 and the disks will be 0.05 to 0.25 inch thick. For a 0.5 to 1 inch diameter plug, a thickness of 0.187 functions well.  
         [0023]     Plug  10  is formed from a memory metal alloy. As discussed below, it is important to select a memory metal alloy that has an appropriate temperature profile so that the conversions between the martensitic state and austenitic state are accomplished at temperatures that make the plug  10  commercially useful. Preferably, the memory metal is Nitinol. Such memory metals can be purchased. One supplier of such materials is Special Metals, Shape Memory Alloy Division, located in New Hartford, N.Y. A preferred material is one with  50  mole percent nickel and  50  mole percent titanium.  
         [0024]     Preferably, plug  10  is machined from Nitinol which converts to the martensitic state at about 0° F., and remains in the martensitic state until heated to a temperature of about 95° F., or higher. Such material is generally purchased as bar stock or rod stock, and must be further machined in its austenitic state to provide a plug  10 , as shown in  FIG. 1 . In order to form such a plug, a rod of the material having the desired cross sectional dimension is machined using, for example, a CNC lathe screw machine or grinder to provide the plug  10  with center post  12  and a plurality of disks  14 ,  16  and  18  (as shown). The peripheral edges of these disks are radiused to facilitate swaging. The trailing peripheral edges are not radiused. This provides a better seal in use.  
         [0025]     During the machining of these plugs  10  the temperature profile may be modified. Accordingly, after machining, the plugs  10  are subjected to a heat treatment to restore the shape memory response of the alloy. Preferably, subsequent to machining, the plug  10  is heated to a temperature of about 900° F. for a period of 30 minutes.  
         [0026]     The plug  10 , as shown in  FIG. 1 , is swaged or deformed utilizing an apparatus  30 , shown in  FIGS. 5A and 5B . The apparatus  30  includes a central housing  32  that has an upper opening covered with cover  34 . A die  38  is located within housing  32 . Cylindrical metal die  38  is supported in a cylindrical support  46  which in turn is supported on a ledge  48  in housing  32 . The support  46  includes an annular outer passage  41  which communicates between cryogenic inlets  42  and  44 . This outer annular passage  41  communicates with a series of holes  45  which are directed to an inner annular passage  47  which surrounds die  38 . Die  38  is in turn supported on a ledge  48  of support  46  and includes a series of holes  49  that align with the inner annular passageway  41  of the support  46 . A thermocouple  50  is located on ledge  48  in contact with die  38  to measure the temperature of the die.  
         [0027]     The die  38  includes an upper portion  52  and a lower portion  54 . The upper portion  52  includes a cylindrical passage that is not tapered and adapted to receive the unswaged plug  10  as shown in  FIG. 5A . The lower portion  56  of die  38  is tapered so that the bottom opening has a diameter equal to the desired diameter of the swaged plug. Cover  34  located above container  32  includes a central opening  56  aligned with the ram  58  of press  60  as well as the central axis of die  38 .  
         [0028]     The bottom  62  of housing  32  includes a channel  64 . A cup  66  with a rim  68  attaches to the lower portion  62  of housing  32  with the rim  68  located in channel  64 . Cup  66  is aligned directly beneath the die  38 .  
         [0029]     Exterior of housing  32  is a frame  65  with two side frame members  72  and  74  and a horizontal upper frame member  80 . Press  60  is supported on upper surface  80 .  
         [0030]     As shown in  FIG. 5A , an unbent Nitinol plug  10  (which is in the austenitic state) is positioned above die  38 . The central opening of the upper portion of die  38  is slightly smaller than the outer diameter of plug  10 . A holding rod  84  having a central bore at a first end  86  is placed on the post  12  of plug  10 , centered with the opening through cover  34 . The second end  88  of the holding rod  84  has a conical shape. An extension rod  90  is placed through the opening in cover  34 . This has a first end  92  adapted to be engaged with the ram  58  from the press  60  and a second end  94  having a conical recess adapted to engage the second end  88  of holding rod  84 .  
         [0031]      FIGS. 5A, 5B , and  6  combine to show a diagrammatic depiction of the operation of the present invention. With the plug  10  in position as shown in  FIG. 5A  and a safety shield (not shown) in place, switch  100  is activated which causes the first portion  102  of press  60  to force ram  58  down a predetermined distance which causes the extension  90  to engage the holding rod  84  and force the plug  10  into the upper untapered portion  52  of die  38 . This is shown in  FIG. 5B . Switch  100  also causes cryogenic fluid to be forced through inlets  42  and  44  through the annular passage  41  through passages  45  in the holder to the inner annular passage  47  and, subsequently, through the passages  49  in the die  38  chilling the plug  10 . Thermocouple  50  measures the temperature of the die  38  and, in turn, the plug  10 . When this temperature reaches a predetermined set temperature, generally about −50° F., the second portion  106  of press  60  is automatically activated, which forces the ram  58  further, as indicated by arrow  108 , forcing the plug  10  with the holder  84  through the die  38  into cup  66 , as indicated by arrow  110 . The tapered lower portion of die  38  bends the discs  14 ,  16  and  18 , leaving the plug  10  in the configuration shown in  FIG. 3 . Thus, the swaged plug  10 , shown in phantom in  FIG. 5B , will be collected in cup  56 .  
         [0032]     The die  38  is configured to bend the disks  14 ,  16 ,  18  between 10 and 25 degrees from their originally perpendicular relation to post  12  so that the effective plug diameter is decreased 2-5% for plugs with a nominal diameter of 0.5 to 1 inch. In the embodiment shown in  FIGS. 5A and 5B , the entry diameter of die  38  is 0.703 inch and the exit diameter is 0.688 inch corresponding to a disk bending of 15° for a plug having a nominal 0.187-inch diameter central post.  
         [0033]     To use the plug to seal a tube, the plug  10 , in the deformed state, i.e., martensitic state, as shown in  FIG. 3 , is inserted into a tube  111  as shown in  FIG. 7A  in the direction of arrow  112 . Holding rod  84 is attached to post  12  to facilitate this. As shown in  FIG. 4 , the plug  10  can be inserted anywhere in tube  111 , including at the tube plate  113 , as shown by plug  10 ( a ). The deformed plug should be of a size wherein the outer diameter of the deformed plug is about 0.03 inches less than the inner diameter of the tube.  
         [0034]     Once inserted into the tube with the holder  84  still in position, the plug is heated to a temperature effective to cause the plug to convert to the austenitic state. This should not exceed 550° F. The heating can be the result of residual heat in the tube or an external heat source such as a blow torch. Holder  84  is a thermally conducting metal such as steel. Therefore, it facilitates heating the plug  10 . When the temperature of the plug reaches the transition temperature to the austenitic state, the plug  10  reverts to its original condition, increasing its diameter and, in turn, pressing against the side walls of tube  111  as shown in  FIG. 7B .  
         [0035]     Once in position with the plug  10  back in the austenitic state and sealing the tube  111 , the holder  84  can be pulled from the plug. The friction fit between the holding rod  84  and post  12  allows one to remove the holder  84  using a pair of pliers.  
         [0036]     The compression fit between the expanded plug  10  and the inner wall of tube  111  as shown in  FIG. 7B  is sufficient with at least 0.004″ interference to withstand a pressure of about 6,000 psi. Thus, this will withstand repeated heating and cooling cycles remaining securely in place, providing a reliable seal. This, in turn, allows a heat exchange tube which has a leak to be sealed off quickly and reliably, allowing the heat exchanger to be put back into operation quickly and inexpensively.  
         [0037]     This has been a description of the present invention along with the preferred method of practicing the present invention. However, the invention itself should only be defined by the appended claims, WHEREIN WE CLAIM: