Patent Publication Number: US-2009223053-A1

Title: Method for repairing heat recovery steam generator tube-to-header damage

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. Ser. No. 11/364,604, filed on Feb. 28, 2006, which claims the benefit of the filing date of U.S. Provisional Application for Patent Ser. No. 60/656,959 filed Feb. 28, 2005, both of which are incorporated herein by reference. 
    
    
     FIELD OF INVENTION 
     The present invention is directed to a method for repairing heat exchanger tube and tube attachment damage, such as for heat transfer applications. More specifically, the disclosed method is related to a method for repairing a heat recovery steam generator (HSRG) tube damage and tube to header attachment damage, for applications such as electricity producing power plants, such as fossil fuel plants. 
     BACKGROUND 
     One of the life limiting factors for many heat recovery steam generators (HRSGs) is associated with damage that occurs at header-to-tube attachments. This attachment is particularly troublesome due to thermal differences experienced between the header and the tubes during cyclic operation. Tubes attached to the header tend to cool very rapidly to the temperature of the incoming water and the bulk wall temperature of the header tends to respond much more slowly. Thermal shock results and often leads to thermal fatigue failures at the toe of the tube weld. Corrosion fatigue in the vicinity of the root of the toe welds have also been noted for units that experience extensive cycling. 
     Restricted axial expansion of tubes induces forces and bending moments on both tubes and the tube attachments. Tubes are restricted from moving as they are attached to inlet and outlet headers at each end. This results in restricted thermal expansion. Tube temperatures quickly follow the temperature of the fluid flowing through them. Anomalies in temperature and flow rate of the fluid between tubes in the same row can cause significant differences in the average temperatures of the set of tubes attached to the same headers. During start-ups and shutdowns, these transient conditions place abnormal stresses on the tubes and tube attachments, eventually resulting in damage to the attachments. 
     At present, the industry addresses tube attachment damage from the outside of the header. This requires cutting your way past a number of tubes in a tube bundle, performing the weld repair from the outside of the tube (often requiring partial tube replacement), and then re-welding all of the tubes that were cut to gain access to the header-to-tube damage. This approach (commonly referred to as “cutting your way in and welding your way out”) is time consuming, costly, and more often than not, results in poor weld quality due to limited accessibility. 
     Commonly assigned U.S. Pat. No. 6,596,957 is directed to a method and apparatus and prefabricated replacement tube for localized waterwall repair, and is incorporated by reference as if fully written out below. Also, commonly assigned U.S. patent application entitled “Method for Inspection and Repair”, filed on even date, is incorporated by reference as if fully written out below. 
     SUMMARY 
     Repairing HRSG header-to-tube damage from inside the header (as opposed to the outside) significantly reduces the opportunity for subsequent failures to repaired tubes and provides a more fatigue-resistant attachment design to those currently found in the industry. The method provided reduces the number of welds required as well as the time required to complete the repair. 
     A method is provided for repairing header to tube attachment damage comprising: providing an access window in the header opposite the attachment damage location: removing the tube damage; inserting a tapered stub tube into a header penetration centered axially immediately over the tube; effecting a profile weld between the header and the tapered stub tube at the header penetration from the inside surface of the header; welding the tapered stub tube to the existing tube through the header inside diameter; welding shut the header access window; and completing the post weld heat treatment of all the welds. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-section of a plug inserted into a header to tube attachment. 
         FIG. 2A  is a schematic illustration of a partial penetration weld of a header to tube attachment. 
         FIG. 2B  is a schematic illustration of a full penetration weld of a header to tube attachment. 
         FIG. 2C  is a schematic illustration of a forged integral nipple and full penetration weld of a header to tube attachment. 
         FIG. 2D  is a schematic illustration of separate nipple and full penetration welds between the nipple and header to tube attachment. 
         FIG. 2E  is a schematic illustration of a base plate mounted to a header with an alignment ring and an alignment plug. 
         FIG. 2F  is a schematic illustration of a base plate mounted to a header with an EDM assembly mounted to the base plate. 
         FIG. 3  is a schematic illustration of another embodiment of a repair tooling on a header to tube assembly. 
         FIG. 4A  is a schematic illustration of a repair device on a header, removing a tube. 
         FIG. 4B  is a schematic illustration of a primary mandrel and a mandrel extension. 
         FIG. 4C  is a schematic illustration of a mandrel gripper and a retaining ring. 
         FIG. 4D  is a schematic illustration of a cutting shaft and cutting tooling. 
         FIG. 4E  is a schematic illustration of a cutting assembly. 
         FIG. 5A  is an elevational view of a tapered stub tube. 
         FIG. 5B  is a cross-sectional view of a tapered stub tube. 
         FIG. 6A  is a schematic view of a stub tube insertion tool and the stub tube. 
         FIG. 6B  is a schematic illustration of a welding assembly with a GTA profile welding head. 
         FIG. 6C  is a schematic illustration of a GTA profile weld head. 
         FIG. 6D  is a schematic illustration of a GTA rotary axis weld head. 
         FIG. 7A  is a schematic illustration of the tapered stub tube attachment to a header in cross-section. 
         FIG. 7B  is a schematic illustration of another embodiment of the tapered stub tube attachment to a header. 
         FIG. 8  is a flow chart of the procedure for carrying out an embodiment of the method. 
     
    
    
     DETAILED DESCRIPTION  
     The present method provides a more pragmatic approach to address HRSG header-to-tube attachment damage than is currently used by industry today. This form of repair is often more difficult than conventional tube repairs due to limited access to the damage location. As a result, tube and header to tube attachment failures are often left in place and simply plugged. FIG. I illustrates a plug weld configuration. Plugging requires removal of an access window within the header  11  one hundred eighty (180) degrees away from the stub tube  12  attachment location, machining and insertion of a plug  13  into the damaged tube bore, welding of the plug into place, and then re-installation of the window. 
     Repair methodology is complicated by the number of different attachment configurations used by industry. Header to tube attachment weld designs used by original equipment manufacturers (OEMs) include: 
     a) partial penetration welds  14  (shown in  FIG. 2A ); 
     b) fall penetration welds  15  (shown in  FIG. 2B ); 
     c) forged nipples  21  with full penetration welds  15  (shown in  FIG. 2C ); 
     d) separate nipples  22  and full penetration welds  15  between nipple  22  and header  11  and tube  12  (shown in  FIG. 2D ). 
     The present method is applicable to all four attachment configurations. 
     For the purposes of the present method, the phrase header to tube attachment damage also defines damage to the tube away from the attachment location. Additionally, the term tube should be interpreted as defining both tubes and pipes. 
     The present method addresses the attachment damage from the inside of the header as opposed to “cutting your way in and welding your way out.” In certain embodiments, the header diameter can range from about 3 inches to about 12 inches. The attachment damage repair can be performed from either the upper or lower header. For illustrative purposes only, the following description is based on a repair from the upper header. In this approach, the first step is to create an access window in the header, about 180 degrees across from the damaged tube. The access window may be created either manually or automatically. When performed automatically, a base plate  50  is mounted on the header  11  and secured, for instance, with clamping straps  51 .  FIG. 2E  is a schematic illustration of the base plate  50  mounted on the header  11 . The base plate may serve as the platform from which all major steps can be launched. The base plate is mounted approximately 180 degrees away, and centered axially along the header from the attachment damage location. 
     In one embodiment, an electrical discharge machining (EDM) assembly  52  is mounted on the base plate  50 . The EDM electrode, fabricated to match the contour and diameter of the header, is inserted into the EDM assembly.  FIG. 2F  is a schematic illustration of an EDM assembly  52  for providing access windows  31  into the header  11 . The EDM assembly is activated to remove a plug from the header, approximately 180 degrees from the attachment damage location. The resulting hole in the header serves as the access window  31  to the inside of the header and tube. The diameter of the access window should be greater than the stub tube diameter to allow for insertion of the stub tube.  FIG. 3  is a schematic illustration of tooling  33  for repair of header  11  to tube attachment damage. 
     Following preparation of the access window  31 , the base plate may be centered exactly over the damaged tube. This can be accomplished by using an alignment ring  53  and alignment plug  54 .  FIG. 2E  is a schematic illustration of a base plate  50  with an alignment ring  53  and alignment plug  54 . First, the center alignment plug and alignment ring are mounted to the base plate  50 . Next, the clamping straps  51  are loosened and the base plate adjusted until the alignment plug slips through the header  11  and into the tube  12  with the attachment damage. The base plate is then properly aligned for the remaining steps, and the clamping straps can be tightened. The alignment ring and plug are then removed. 
     After aligning the base plate, the attachment damage may be removed by either severing and withdrawing the damaged portion, or machining (or grinding) through the damaged area. 
     To sever and withdraw the attachment damage, a tube-cutting assembly is mounted on the base plate and a cutting device is extended therefrom through the header and into the tube beyond the attachment damage. In certain embodiments, the repair may accommodate off-set tubing (often referred to as dog-leg tubing) as this is where many failures have been noted.  FIG. 4A  illustrates a schematic representation of a cutting assembly  35  shown on an 8-inch header  11 , removing a tube  12  at a 30-degree angle. 
     Next, the cutting device is activated and the tube is cut proximate to the header. The cutting location is typically about 1″ to 4″ from the outer diameter surface of the header. However, the distance may vary depending on the location of the damage. As a result, the damaged portion including the attachment damage and a small part of the tube only remain connected to the header. Next, the damaged portion is severed from the header, such as by boring or EDM. The diameter of the resulting header penetration should be at least equivalent to the outer diameter of the attached damaged tube portion, and sized appropriately for securing the top of the stub tube  40 , shown in  FIG. 5A , in the header penetration, such as by press fitting or roll expanding. The severed piece is then withdrawn from the header through the access window  31 . 
     Alternatively, the attachment damage may be removed by machining. In this embodiment, an EDM assembly  52 , shown in  FIG. 2F , with a flat bottom electrode is mounted on the base plate  50 . Next, the electrode is extended therefrom into the header  11  and the assembly is activated. The electrode generates a flat bottom, cylindrical depression inside the lower header wall centered axially immediately over the attachment damage location. The depression provides a suitable surface for subsequent machining steps. The diameter of the depression may be at least equivalent to the outer diameter of the tube, and sized appropriately for securing the top of the stub tube in the header penetration, such as by press fitting or roll expanding. 
     Next, the EDM assembly is removed from the base plate and a two piece mandrel is inserted into the tube with the attachment damage.  FIG. 4B  illustrates the primary mandrel  56  and mandrel extension  57 . The mandrel captures and secures the damaged tube to prevent movement, and creates a centering device for subsequent cutting operations. First, the primary mandrel  56  is inserted through the header and into the tube. The end of the primary mandrel inserted into the damaged tube includes grippers  58  and a retainer ring  59 .  FIG. 4C  illustrates the mandrel grippers  58  and retainer ring  59 . A tighten-up draw bar expands to secure the grippers within the tube section to prevent movement of the tube. A mandrel extension  57  is then attached to the primary mandrel  56  and extends out of the header through the access window. 
     After securing the tube with the mandrel, a hollow cutter shaft  60  with a cutter tool holder  61  and tooling is slid over the mandrel.  FIG. 4D  illustrates the cutter shaft  60  and cutter tool holder  61 . A cutting assembly  35  is then mounted to the base plate  11 .  FIG. 4E  illustrates the cutting assembly  35 . Next, the cutter shaft  60  is attached to the cutting assembly drive  63 . The shaft is then manually lowered into place, to within approximately ⅛″ of the bottom of the flat bottomed depression. 
     The cutting assembly  35 , shown in  FIG. 4E , is activated and the cutting tool is lowered until it contacts the flat bottom depression. At this point, the plunge machine process begins and continues until the header is penetrated and the tube is machined to a sufficient depth below the header, thereby removing the attachment damage. Depending on the location of the damage, the tube may be machined, as in one embodiment about 1 inch to about 4 inches, below the header outer diameter to facilitate installation of the new stub tube. The mandrel, in place, may capture the severed tube. 
     Next, the cutting tool is replaced with an end-prep tool and the tube is then prepped for welding. After prepping the tube, the cutter assembly and the mandrel extension are removed to allow for the insertion of the new stub tube. 
     The stub tube  40  is tapered so that the top of the stub tube  41  fits snuggly within the header penetration above the tube.  FIG. 5A  and  FIG. 5B  illustrate the joint stub tube  40 . This allows the stub tube to be secured in position in the header penetration, such as by press fitting or roll expanding. The top of the stub tube may be fabricated to match the contour of the inside diameter of the header  11 , shown in  FIG. 7A . The bottom of the stub tube has a diameter slightly greater than the existing tube  12 , and provides backing  42  for the stub tube to tube welding operation. Additionally, it may optionally include a transition, or sleeve  43  on its end that allows the stub tube section to slip over the existing tube, thereby guaranteeing alignment for the welding step. 
     The first step to insert the stub tube is slipping the stub tube  40  and an insertion tool  45  over the primary mandrel  56 .  FIG. 6A  is a schematic view of a stub tube  40  and insertion tool  45 . The insertion tool  45  rigidly secures the stub tube  40  as it is slipped over the mandrel  56 . The insertion tool  45  also automatically rotates the contoured stub tube to meet the contour of the inside diameter of the header  11 , shown in  FIG. 7A , thereby assuring proper alignment. In one embodiment, a press fixture is mounted to the base plate  50  and the insertion tool is attached to a lever on the fixture. The stub tube  40  is then press fit into place in the header  11  penetration. As the stub tube  40  is press fit into the header  11 , the opposite end secures the existing tube in place. As a result, the bottom of the stub tube  42  is also positioned for stub tube-to-tube welding. Alternatively, the stub tube may be roll expanded to secure it in the header penetration. 
     Next, the header penetration and the top of the stub tube  41  may be prepared for welding. The EDM assembly  52  with a profiled electrode is mounted on the base plate  50 . The EDM chamfers the header penetration and upper stub tube surface  41  in preparation for welding. The EDM assembly is then removed from the base plate. 
     Following chamfering, either weld may be performed. However, it is preferred to perform the header to stub tube (HST) weld first. Depending on the material of construction, preheating may be required before welding. For example, P/T91 material commonly used for HSRG configurations requires preheating to 400° F. Preheating is accomplished by methods known in the industry. 
     The HST weld may be performed either manually or automatically. When automatic welding is used, a welding assembly  70  is mounted on the base plate  50  and a GTA profile weld head  71  is inserted in the welding assembly.  FIG. 6B  illustrates a welding assembly  70  with a GTA profile weld head  71  extended into the header  11 . The GTA profile weld head  71  shown in  FIG. 6C , allows for X, Y and Z axes of motion. This is necessary due to the contour of the stub tube and header. The welding device is then extended therefrom into the header to perform the profile weld of the stub tube to header. 
     Next, the welding device is withdrawn from the header and the weld held is replaced with a different welding device to perform the stub tube-to-tube weld.  FIGS. 7A and 7B  illustrate the stub tube  40  to header  11  and stub tube-to-tube  12  attachments. This is accomplished by inserting an internal welding device, such as a device for gas tungsten arc welding (GTAW), gas metal arc welding (GMAW), laser welding, or plasma transferred arc welding (PTAW), and welding the tube from the inside. In one embodiment, a rotary axis GTA weld head  72 , shown in  FIG. 6D , is used to perform the stub tube-to-tube weld. After attaching, the welding device is extended therefrom through the header  11  and inside the stub tube  40  to the abutment of the stub tube and existing tube  15 . A height indicator enables the user to determine vertical position of the weld head to properly position it for welding. The height indicator also allows for incremental changes for multi-pass welding. Finally, the stub tube  40  to existing tube  12  weld is performed  15 . 
     After completing the stub tube to tube weld, the access window  31  must be welded shut. It is preferable to use the original plug. However, another suitable piece may be substituted. If the original plug is used, it is necessary to build up the plug due to any losses attributable to the EDM process. It may be welded using a B9-type filler and the GTAW process. The buildup should be sufficient so that a 30 degree weld prep can be machined around its diameter to match the 30 degree weld prep used for the access hole. The latter weld prep could be performed before removal of the weld base plate by simply reinserting the tooling device. Once the two weld preps have been completed, the plug can be reinserted into the header by tack welding the plug into place, preheating the header, and completing the final weld to secure the plug back in place. 
     The present method is adapted for heat recovery steam generator (HRSG) attachment repairs. However, the method may be employed to address other situations such as shell and tube heat exchangers, nozzle penetrations in steam drums, and the like, such as those used in the paper and pulp, chemical and petroleum industries. 
     The individual actions with respect to one embodiment of the method for repairing header to tube attachment damage are outlined in the flow chart of  FIG. 8 , and are set out below.
           80  Position tooling.     81  Remove access window from header opposite the damage.     82  Generate a flat bottom depression inside the header, centered axially, immediately over the attachment damage location.     83  Insert cutting tool to penetrate the outer surface of the header.     84  Machine the tube proximate to the header.     85  Insert prep tool to prepare remaining tube.     86  Insert new tapered stub tube and secure in header.     87  Perform profile weld between header and tapered stub tube.     88  Perform weld between existing tube and tapered stub tube.     89  Weld shut the access window.     90  Complete a post weld heat treatment of all welds.       

     It will be understood that the embodiments described herein are merely exemplary and that a person skilled in the art may make many variations and modifications without departing from the spirit and scope of the invention. All such modifications and variations are intended to be included within the scope of the invention as described herein. It should be understood that the embodiments described above are not only in the alternative, but can be combined.