Abstract:
The manufacture of clad tubes ( 30 ) is described. Tube ( 10 ) intended to line the walls of a combustion chamber is made of a high strength material to contain the high-pressure steam created. However, these tubes ( 10 ) are typically not corrosion/erosion resistant. Manufacture of tubes ( 10 ) with both high strength and high resistance to corrosion/erosion would be prohibitively expensive. Therefore, tubes ( 10 ) are covered with a non-corrosive material to protect them. This is done by surface welding a strip ( 20 ) of high alloy material to the outer surface ( 12 ) of the tubes ( 10 ). It is preferable to use electric high frequency resistance welding to surface weld the strip ( 20 ) onto tube ( 10 ). The strips ( 20 ) are preferably attached with little melting and metal dilution allowing the strip  20  to keep its corrosion/erosion resistance properties.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of U.S. Provisional Patent Application 61/352,448 filed Jun. 8, 2010 and therefore incorporates this Provisional Patent Application and claims priority from this application and the benefit of its earlier filing date. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates generally to a method for cladding tubes, and more particularly, to a method of wrapping strips of material to outer surface of tubes to clad them. 
       BACKGROUND OF THE INVENTION 
       [0003]    Steam generation pipes within a boiler are exposed to corrosive and erosive environments that cause the premature failure of pipes and tubes due to wall thinning leading to rupture. 
         [0004]    The steam generated typically used in running a turbine for production of electricity and in chemical processes for providing energy to initiate a chemical reaction. Some boilers include one or more walls, each formed of a plurality of tubes, the walls being secured to one another thereby surrounding a combustion chamber within the boiler. Additional groups of tubes can be disposed within the combustion chamber. 
         [0005]    Each of the tubes also has an inside surface defining a passage extending therethrough. One end of each of the plurality of tubes can be in fluid communication with a water supply header while an opposing end of each of the plurality of tubes can be in fluid communication with a steam header. During operation of the boiler, combustion generally occurs in the combustion chamber and heats water flowing through the passages, creating steam that is fed to the steam header. The outer surfaces of the tubes in the combustion chamber and throughout the boiler are exposed to fuel, combustion, heat and combustion byproducts that corrode the tubes. As a result, the useful life of the tubes is reduced. 
         [0006]    There have been a number of methods employed to add protective coverings to standard pipes and tubes to improve their resistance to increase strength, or to prevent corrosion and erosion. Virtually all of the methods that weld the protective coverings require the covering to be completely melted to adequately attach the covering to the tube. 
         [0007]    In conventional welding, a welding rod is melted at its tip. The structure being welded has a trough of material that is also melted. The molten welding rod and the molten surface mix together to create a ‘bead’. The ‘bead’ has a composition that is a mixture of both the molten welding rod and the molten surface. Since a significant amount of welding rod and a significant amount of surface are mixed, there is significant mixing of the metals. Therefore, if the welding rod is made of a high concentration of a high-grade metal and the surface being welded has a lower concentration of the high-grade metal, the resulting mixture (‘bead’) has a lower concentration of the high-grade metal as compared with the original welding rod. This results in the dilution of the concentration of the high-grade metal in the mixed metal bead. 
         [0008]    Therefore, as more of the welding rod and more of the surface are melted, more dilution occurs. The diluted metal has less corrosion resistance, erosion resistance and/or less strength. 
         [0009]    Therefore, by welding the entire surface of an object, such as tubing, requires a large amount of heat. The large amount of heat may distort the tubing and it is often difficult to control the amount of covering material deposited to optimum thicknesses. This method of cover tubing is difficult to implement. 
         [0010]    Typically tubes operating in corrosive or erosive environments are coated, using techniques such as thermal spray or vapor deposition to provide a more protective surface layer. In the most aggressive environments clad tubing produced by co-extrusion has been used. However limitations in the integrity of the bond formed in this way can lead to debonding particularly during long exposures in thermal cycling conditions as a result of the stresses associated with the mismatch in thermal expansion coefficients between the austenitic and ferritic steels. 
         [0011]    Currently, there is a need for a method of protecting boiler tubes from erosion and corrosion that may be easily applied without the need for large amounts of energy. 
       SUMMARY OF THE INVENTION 
       [0012]    In the present invention, a strip of non-corrosive material is applied to the outer surface of the tube to protect tube from corrosion. 
         [0013]    The present invention may be embodied as a method for producing clad tubes ( 30 ) by: 
         [0014]    providing a first tube ( 10 ); 
         [0015]    providing an elongated strip ( 20 ); 
         [0016]    surface welding an inner surface ( 22 ) of the strip ( 20 ) and the outer surface ( 12 ) of the tube ( 10 ) while helically wrapping the strip ( 20 ) around the outer surface ( 12 ) of tube ( 10 ); and pressing the strip ( 2 ) to the tube ( 10 ) as it is being surface welded. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    Referring now to the Figures, which are exemplary embodiments, and wherein the like elements are numbered alike: 
           [0018]      FIG. 1  is a perspective view of a strip of material being applied to the outer surface of a tube according to one embodiment of the present invention. 
           [0019]      FIG. 2  is a top plan view of the strip of material being applied to the outer surface of a tube of  FIG. 1 . 
           [0020]      FIG. 3  is an elevational view of the strip of material being applied to the outer surface of a tube of  FIGS. 1 and 2 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0021]      FIG. 1  illustrates a tube  10  less expensive material, such as a low-alloy steel, that is lack properties such as corrosion resistance, erosion resistance or high strength of a that is intended to be used in a boiler. Without protection, corrosion and erosion of the tube  10  reduce the tube wall thickness to a thickness that does not have the strength to retain the pressure of steam within the tubes. When this occurs, they burst. This low-alloy steel tube  10  should be protected to reduce corrosion and erosion, and the thinning of the tube walls. 
         [0022]    A strip  20  that is made of a material that exhibits corrosion resistance, erosion resistance, or additional strength is shown here partially wrapped around the outside surface  12  of tube  10 . It is preferably wrapped or wound around the tube in a helical fashion while being welded using surface welding techniques creating clad tubing  30 . 
         [0023]    The strip  20  is manufactured from a suitable corrosion/erosion resistant material that can withstand high temperatures and corrosive environments, such as austenitic steel. While the strip  20  is described as being manufactured from austenitic steel, it is contemplated that the cladding tube can be manufactured from other corrosive-resistant, erosion-resistant, high strength or other cladding materials, depending upon its intended use. 
         [0024]    As shown in  FIG. 1 , it is preferable that the strip  20  be surface welded on its inner surface  22  to the outer surface  12  of tube  10  where they meet ay an interface  14 . 
         [0025]    One type of electrical resistance weld is a high frequency weld. In this type of weld, a high-frequency alternating current is passed through the strip  20  and the tube  10  setting up a current path. The current flows through the surface of the strip  20  and tube  10  and creates resistive heating in the metal, similar to a toaster heating wire. 
         [0026]      FIG. 2  is a top plan view of the strip  20  of material being applied to the outer surface  12  of a tube  10  of  FIG. 1 .  FIG. 3  is an elevational view of the strip of material being applied to the outer surface of a tube of  FIGS. 1 and 2 . 
         [0027]    With reference to  FIGS. 2 and 3 , a frame  50  is shown having rollers  51  are used to support the tube  10  as it is being processed. Rollers  51  allow the tube to be rotated. A motor  61  causes rotation of the tube  10 . A second motor  71  causes longitudinal motion of the tube  10 . Preferably, the motors as well as other aspects of the system activated, coordinated and controlled by the controller  100 . 
         [0028]    Strip  20  is stored on and provided from a roll  24 . A guide  26  is angled with respect to a longitudinal axis of the tube  10 . As the tube is rotated by the controller  100  and motors  61 ,  71 , the strip  20  is provided from a supply roll  24  guided by guide  26 , pressed against the tube  10  by pressing roller  28  and spirally wound around tube  10 . 
         [0029]    A contact  41  is coupled to a lead of a welding unit  90  and is positioned to make contact with the strip  20  at a location marked “A” near the location “B” where the strip  20  contacts the pipe  10 . 
         [0030]    A second contact  43 , coupled to a second lead of the high frequency welder unit  90  is positioned to contact the tube  10  at a location marked “C”. 
         [0031]    The welding unit  90  is activated and controlled by controller  100 . When activated, it causes a surface current to flow between the first contact  41  and the second contact  43 . Since there is a large current, even a small inductance in the strip  20  and/or the tube  10  causes significant heat to be created. 
         [0032]    The current passes between the surface of strip  20  at location “A”, through the meeting to the tube  10  and strip  20  at location “B” and to the second contact  43  at location “C”. 
         [0033]    The current route between A-B-C creates a “V” shape. Due to the nature of surface currents, they converge and concentrate their energy at location “B” where the welding occurs. 
         [0034]    Since the heat is provided by a surface current, it is applied evenly at over the inside surface of the strip  20  and the outside surface  12  of the tube  10 . The amount of metal melted at both the strip  20  and the tube  10  is very small compared to conventional welding. There is significantly less mixing of the metals and significantly less dilution. 
         [0035]    During the surface welding of the present invention, there is substantially less mixing, dilution and the weld is not just in a bead, but also along the inner surface  12  of the strip  10 . Therefore, if a high nickel steel is used as strip  10 , it will be diluted less by using high frequency welding as compared with conventional welding, and therefore keep more of its corrosion resistant properties. This results in significant cost savings. 
         [0036]    This type of weld applies heat only to the region being welded and does not melt the tube and strip material overall. Therefore, there is less warping and distortion of the tube  10  and strip  20  as compared with prior art methods that require melting of the outer protective material, and the corrosion-resistance of the strip alloy is not diluted by mixing with the lower grade alloy of the tube material. 
         [0037]    Once the strip  20  and tube  10  are heated, they melt slightly at the surfaces  12 ,  22 . Using high-frequency resistance welding, the surface currents melt only 5-15% of the thickness of the strip  20 . It may be about 0.040 inches thick. This is considerably less than the 0.1-0.3 inches that are common to conventional welding of similar geometry and use. A pressing roller  28  presses the strip to the tube  10  thereby causing the molten inner surface  22  of strip  22  to forge to the molten outer surface  12  of pipe  10 . 
         [0038]    The rotation and longitudinal movement of tube  10  are chosen by controller  100  so that the strip  20  is spirally wrapped onto tube  10 . Since the current flows also through the edges  31 ,  33  of strip  20 , the edges also heat up. If the rotation and longitudinal motion of the tube  10  are correctly chosen, the tape will fit flush against the tube  10  and the previous wrapping of the strip  20 . Since there is also a concentration of current flow as a first edge  31  of strip  20  meets the second edge  33  near interface  14 . This concentration of current also causes the adjacent edges  31 ,  33  of the spiraled strip  20  to melt and fuse together. Therefore, the strip edges may also be forged together causing one wrap of the strip  20  to bond to the previous wrap of the strip  20 . 
         [0039]    Preferably, the welding is done an inert atmosphere. Therefore, a source of an inert or non-reactive gas  97 , such as neon, argon or xenon passes through an input line  99  into an inert enclosure  95 . The inert enclosure encompasses the welding area and seals it to the degree that it can maintain a generally inert atmosphere. This reduces or eliminates the oxidation and other reactions that occur during the welding. 
         [0040]    In this embodiment of the present invention, the tube  10  is rotated as the strip  20  is would around its outer surface. It may also be that a device would rotate around the tube  10 . 
         [0041]    The resulting clad tubing  30  exhibits strength due to the tube  10  being made of a high strength material. The clad tubing  30  also exhibits corrosion resistance due to strip  20  covering tube  10 . Tube  30  is significantly lower cost than a tube made entirely of a high-strength, corrosion resistive material. 
         [0042]    In an alternative embodiment, the tube  10  may be preheated prior to wrapping the strip  20  onto pipe  10 . Many different preheaters may be used, however, an inductively coupled coil  80  is provided in  FIG. 2 . The coil  80  induces a rapidly changing current in tube  10  that results in resistive heating. The use of the preheating coil  80  increases the effectiveness of the device. 
         [0043]    To implement the present invention, it was found that existing tube fin applying machinery might be reconfigured to attach the metal strip  20  to the surface of a tube  10 . This results in low start up costs and dual use of existing machinery. 
         [0044]    While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.