Abstract:
An apparatus and process for depositing an overlay weld on a boiler tube panel comprising a plurality of tubes with adjacent tubes joined together with membranes therebetween, and then straightening the panel in response to bowing that occurs as a result of depositing the overlay weld. The apparatus generally comprises an overlay welding apparatus that deposits an overlay weld on a surface of a boiler tube panel, and a straightening apparatus for straightening the panel following deposition of the overlay weld. The straightening process entails placing the boiler tube panel between two members, and the pressing a portion of the panel therebetween.

Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention generally relates to welding equipment and processes. More particularly, this invention relates to a welding apparatus and process for depositing an overlay weld on a boiler tube panel and then straightening the panel for installation in a boiler. 
   2. Description of the Related Art 
   Overlay welding generally involves depositing weld material over a surface region in a series of beads with some lateral overlapping, forming a continuous layer of weld material that increases the thickness and strength of the region. Overlay welds are often deposited by shielded metal-arc welding (including gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW)) in the restoration of tubes and large vessels used in industries such as utilities, co-generation refining, petrochemical, pulp and paper, and waste-to-energy. Particular uses of overlay welds include reinforcing and repairing individual boiler tubes, nozzles, and pipe sections of a boiler and the reinforcement and repair of boiler tube panels comprising a number of boiler tubes joined by web or membrane bars. Boiler tubes and tube panels reinforced or repaired with a corrosion-resistant overlay weld offer much longer tube life, reducing costly unscheduled maintenance outages and significantly improving boiler availability. 
   The reinforcement and repair of boiler tube panels have been performed manually or with the use of an automatic or semiautomatic welding machine. Accuracy and consistency of all parameters are critical during an overlay welding process, especially when the panel must be both supported and manipulated during welding, as is the case with overlay welding operations performed on boiler tube panels prior to installation in a boiler. Further complicating the overlay process is the need to comply with a wide range of specifications, such as minimal weld penetration and deposit thickness, low dilution, complete fusion, homogeneous deposits, and very low heat input. 
   Distortion has been a particular problem encountered when overlay welding boiler tube panels prior to installation as a result of the large size of tube panels (for example, about fifty feet (about fifteen meters) in length and about six feet (about two meters) in width) and the relatively large heat input of GMAW and GTAW welding process. Distortion in the form of bowing or bending of a panel is particularly likely when welding is performed on only one side of the panel, e.g., the side that will face the interior of the boiler and therefore most likely to be deteriorated from corrosion. As a solution, complex fixtures have been proposed as well as welding techniques that reduce the amount of heating. It would be desirable if an overlay welding apparatus and process were available that were sufficiently uncomplicated for use by operators with limited experience, yet capable of consistently producing undistorted boiler tube panels. 
   SUMMARY OF INVENTION 
   The present invention provides an apparatus and process for depositing an overlay weld on a boiler tube panel comprising a plurality of tubes with adjacent tubes joined together with membranes therebetween, and then straightening the panel in response to bowing that occurs as a result of depositing the overlay weld. The apparatus generally comprises means for supporting the boiler tube panel during overlay welding, and means for straightening the boiler tube panel following deposition of the overlay weld. The supporting means comprises a frame adapted to support the boiler tube panel by contacting a first surface thereof, at least one welding carriage mounted adjacent the frame and adapted for travel along the frame, and an overlay welding head mounted to the welding carriage and adapted for depositing an overlay weld on a second surface of the boiler tube panel oppositely disposed from the first surface of the boiler tube panel. The straightening means comprises a first member having a contact surface with a profile that is complementary to the first surface of the boiler tube panel, a second member having a contact surface with a profile that is complementary to the second surface of the boiler tube panel, means for causing the first and second members to move toward each other to press a portion of the boiler tube panel therebetween and for causing the first and second members to move away from each other to release the boiler tube panel, and means for positioning the boiler tube panel between the first and second members. 
   The process carried out with the above-described apparatus generally entails supporting the boiler tube panel on the frame so that the frame contacts the first surface of the boiler tube panel, overlay welding the second surface of the boiler tube plate with at least one overlay welding head supported by a welding carriage that travels along the frame, positioning the welded boiler tube panel between the first and second members, and then straightening the boiler tube panel by moving the first and second members toward each other so that the first and second surfaces of the boiler tube panel are contacted with contact surfaces of the first and second members, respectively, so that a portion of the boiler tube panel is pressed therebetween. The first and second members are then moved away from each other to release the boiler tube panel. 
   According to optional but preferred aspects of the invention, the apparatus includes electronic circuitry that provides better control of the overlay welding head and various parameters of the overlay welding operation. For example, the apparatus preferably includes a feedback capability that can address errors and failures of various components of the welding apparatus, including shielding gas, carriage travel, etc., thereby reducing the incidence of cold laps, burn-throughs, and overlay deposited with improper shielding gas pressure. The invention utilizes the above improvements and preferred aspects to provide an apparatus and process that are capable of consistently producing an undistorted boiler tube panel, yet the apparatus and process are sufficiently uncomplicated to be used by operators with limited experience. 
   Other objects and advantages of this invention will be better appreciated from the following detailed description. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIGS. 1 and 2  are perspective views of opposite ends of an overlay welding apparatus in accordance with a preferred embodiment of the present invention. 
       FIG. 3  depicts a boiler tube panel on which an overlay weld was deposited with the apparatus of  FIG. 1 . 
       FIG. 4  is a perspective view of a straightening apparatus in accordance with the preferred embodiment of the present invention. 
       FIG. 5  is a detailed view of a ram and scallop bar adapted for engaging opposite surfaces of the panel of  FIG. 3 . 
       FIG. 6  is a block diagram of a control system for the overlay welding apparatus of  FIGS. 1 and 2  in accordance with the preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
     FIGS. 1 and 2  depict an overlay welding apparatus  10  and  FIGS. 4 and 5  depict a straightening apparatus  12  in accordance with a preferred embodiment of the invention. As seen in  FIGS. 1 and 2 , which depict opposite ends of the welding apparatus  10 , a boiler tube panel  14  is shown as being horizontally supported on the apparatus  10 , with the lower surface  34  of the panel  14  being contacted by an elongate frame  16  and the upper surface  36  of the panel  14  facing upward. The frame  16  is shown as being constructed of individual frame members  22  oriented in a transverse direction to the panel  14  and frame  16 , and spaced apart to define slots  24  between adjacent frame members  22 . Retaining members  26  are shown as secured to the frame  16  with bolts  28  anchored in the slots  24 , enabling the retaining members  26  to be adjusted transversely inward and outward relative to the panel  14  for gripping the lateral edges of the panel  14 . The frame  16  further includes a header bar  30  for securing one longitudinal end of the boiler tube panel  14  to the frame  16  and a tail bar  32  for securing the opposite longitudinal end of the panel  14  to the frame  16 . 
   The apparatus  10  is shown as further comprising a track  18  and a pair of welding assemblies  20  mounted to the track  18 . Each welding assembly  20  is adapted for depositing an overlay weld  38  on the upper surface  36  of the panel  14 . The particulars of the overlay weld  38  will depend in part on the application for the panel  14 , including the type of material being welded, and therefore will not be discussed in any detail here. Each overlay weld assembly  20  comprises a welding carriage assembly  40 , an index arm  42  extending therefrom, and a welding head or torch  44 . The carriage assemblies  40  are mounted to the track  18  and travel along the longitudinal lengths of the panel  14  and frame  16  by a carriage motor  41 . The index arms  42  position their respective torch  44  in the lateral direction of the tube panel  14 . The torches  44  may be of any suitable metal-arc type, though a gas metal arc welding (GMAW; or metal inert gas (MIG)) torch is preferred that makes use of a wire filler material and shielding gas. A wire feed unit  46  and shielding gas line  48  are shown in  FIGS. 1 and 2  for this purpose. The torches  44  are each equipped with a torch height slide  50  for adjusting the distance of the torch  44  from the surface  36  of the panel  14 , and an oscillator  52  for causing the torch  44  to oscillate in a direction parallel to the weld bead, i.e., parallel to the travel of the torch  44  relative to the panel  14 . 
     FIG. 6  is a block diagram of a control system  54  for one of the overlay welding assembly  20 . According to a preferred embodiment, the control system  54  corresponds to a control system disclosed in commonly-assigned U.S. Pat. No. 6,548,783 to Kislovsky, whose content pertaining to the control system is incorporated herein by reference. As such, the control system  54  will be summarized below, with more specific details being set forth in Kislovsky. 
   In accordance with Kislovsky, the control system  54  includes a welding power source  56  that provides the welding current for one of the torches  44 . The power source  56  is a pulsed power source preferably capable of delivering direct or alternating current, depending on the welding process, the wire (filler) material, the type of shielding used, and the material of the tube panel. The power source  56  is indicated in  FIG. 6  as being controlled by a weld voltage ON/OFF control  58 , which enables the welding current to be quickly terminated under certain conditions, as discussed below. As noted above, the welding torch  44  is equipped with an oscillator  52 , which in  FIG. 6  is shown to be operated by an oscillator motor  60  that is controlled by an oscillator driver  62 . The driver  62  preferably utilizes digital potentiometers to regulate the speed of the motor  60 , which may be calibrated in strokes per minute. The output of the driver  62  is preferably limited to 0 to 30 VDC at 1.5 amperes. 
     FIG. 6  further depicts the control system  54  as including a programmable automatic torch height control  64 . Through positional feedback from a Hall-effect current sensor  66 , the torch height control  64  is able to precisely control the distance between the torch  44  and the adjacent surface  36  of the panel  14  by sensing weld current and controlling the movement of the index arm  42 . Alternatively, the torch height controller  64  could make use of a variety of other sensors, such as a temperature sensor or a laser or ultrasound distance sensor. In any event, through feedback the torch height control  64  controls a slide motor  82  mounted to the torch height slide  50  to maintain a constant arc length for its torch  44  as it passes over the contour of the panel  14 , thereby maintaining a substantially constant voltage drop. 
   A wire feed motor  68  feeds wire filler to the overlay welding head  44  at an appropriate speed, which will depend in part on the material, diameter, etc., of the weld wire used. A controller  70  for the wire feed motor  68  preferably utilizes feedback to regulate the wire feed rate to a programmed value, and communicates with the weld voltage ON/OFF control  58 . If wire feed speed is below a preset lower limit to the desired speed, an OFF signal is sent to the ON/OFF control  58 , which in turn shuts down the welding power source  56 . 
   The control system  54  is also shown as including a closed-loop (feedback) controller  72  for the carriage motor, a monitor  74  for shielding gas pressure, and an indexer motor controller  76 . The controller  72  and monitor  74  are preferably programmed to have preset ranges for their respective targets, and communicate with the ON/OFF control  58  so that the control  58  is able to interrupt welding current to the power source  56  for the torch  44  if the carriage motor  41  or shielding gas pressure is outside their permitted ranges. For example, if carriage travel speed falls below a preset lower limit, the carriage motor controller  72  sends an OFF signal to the ON/OFF control  58 , which in turn shuts down the welding power source  56  to prevent burn-through. As another example, the shielding gas pressure monitor  74  is preferably operated to continuously monitor shielding gas pressure, which typically will have two preset limits, e.g., a lower 25 psi (about 0.21 MPa) limit and an upper 70 psi (about 0.64 MPa) limit. If the shielding gas pressure is within this range, the weld voltage ON/OFF control  58  is enabled. Audible and visual warnings preferably occur if a gas pressure at the lower or upper limit of the acceptable range is detected. At gas pressures sufficiently outside the permitted range (e.g., 20 psi (about 0.18 MPa)), the ON/OFF control  58  preferably shuts down the power source  56 , thereby avoiding a circumstance in which an overlay weld must be removed and a weld repeated because of improper gas pressure. 
   The indexer motor controller  76  controls an indexer motor to cause the index arm  42 , and therefore the welding torch  44  mounted thereto, to move in the transverse direction of the boiler tube panel  14  and frame  16 . Potentiometers are provided for adjusting the timing of the index cycle, and a switch for manually changing the direction of indexer travel. The extreme limits for the travel of the welding torch  44  along the weld path are set by limit switches. In a typical welding cycle, the carriage motor  41  causes the welding assembly  20  to travel the length of the track  18  until one of the limit switches is tripped, upon which the indexer motor controller  76  causes the indexer motor to move the index arm  44 , and therefore the torch  44 , a predetermined distance toward or away from the track  18 . After the index sequence, the welding carriage assembly  40  reverses its travel direction on the track  18 , such that a bead is formed with some lateral overlap with the immediately preceding bead. 
   A weld mode controller  78  controls the sequence of events during the welding cycle. An automatic welding cycle can preferably be initiated through a switch on a pendant control  80  that communicates with the weld mode controller  78 . The pendant control  80  is preferably suspended to be accessible to an operator standing near the welding apparatus  10 , and preferably houses input controls for travel speed of the welding assembly  20  along the frame  16 , the electric power source  56 , the wirefeed motor controller  70 , and the shielding gas monitor  74 . Though the welding apparatus  10  is intended for operation in a machine welding cycle, it is foreseeable that the controller  78  could allow for both automatic and manual welding modes. 
   The control system  54  is also shown as having various input devices, including a selector switch  84 , a digital meter  86 , and various potentiometers to allow an operator to visually check all parameter settings and adjust them as necessary during the weld overlay process. Finally, the torch height controller  64  is shown as communicating with a data logger  88 , which can be controlled, for example, to operate after the welding current from the power source  56  reaches a predetermined level.  FIG. 6  also shows the controls  64 ,  70 , and  72 , oscillator driver  62 , and welding power source  56  as communicating with the data logger  88  for the purpose of monitoring the weld process parameters, enabling evaluation of the weld quality in real time. With such information, an accurate determination can be made of the run time for each operation. 
   The straightening apparatus  12  depicted in  FIGS. 4 and 5  is adapted to straighten the boiler tube panel  14  after it has undergone overlay welding with the welding apparatus  10 . A typical post-weld condition of the panel  14  is represented in  FIG. 3 , evidencing that the lower and upper surfaces  34  and  36  of the panel  14  are convex and concave, respectively. The panel  14  is shown as being transported to and through the straightening apparatus  12  with bands  90  that can be adjustably positioned along the length of the panel  14 . The straightening apparatus  12  comprises a frame  92  and a column  94  that is mounted to the frame  92  so as to define a passage  96  through which the panel  14  is fed during the straightening operation. A ram  98  ( FIG. 5 ) is mounted to the column  94  and actuated with a hydraulic cylinder  100  for applying a large force, e.g., nineteen tons (about 9000 N), to the panel  14  when positioned in the passage  96 . Mounted to the frame  92  and opposing the ram  98  is a scallop bar  102  for contacting the concave overlaid surface  36  of the panel  14 . As seen in  FIGS. 4 and 5 , the scallop bar  102  is configured complementary to the transverse contour of the panel  14  so that generally uniform pressure is applied to the tubes and membranes of the panel  14 . The scallop bar  102  is shown as being longer than the transverse width of the panel  14  so that the entire width of the panel  14  is contacted in a single operation. In contrast, the ram  98  is depicted as carrying an adapter  104  ( FIG. 5 ) that is sized to apply the force generated by the ram  98  to only a limited transverse portion of the convex surface  34  on the panel  14 . Similar to the scallop bar  102 , the adapter  104  is contoured to be complementary to the contour of the panel  14  as defined by the tubes and membranes of the panel  14 . However, the adapter  104  as shown in  FIG. 5  is sized to engage only two tubes and their three adjoining membranes. Furthermore, the adapter  104  is narrower in width than the scallop bar  102 , so that the adapter  104  will engage the approximate apex of the curvature for that portion of the panel  14  supported by the scallop bar  102  during the straightening process. 
   To enable straightening of the entire transverse width of the panel  14 , the ram  98  is slidably mounted between a pair of tracks  106  on the column  94  and suspended by a cable  108  coupled to a motor  110  that can be operated to raise and lower the ram  98  on the column  94 . With this arrangement, straightening can be performed across the width of the panel  14  by sequentially pressing the panel  14  with the adapter  104 , releasing the panel  14 , repositioning the ram  98  to be aligned with an adjacent transverse portion of the panel  14 , and then actuating the ram  98  to apply pressure to the adjacent transverse portion of the panel  14 . This operation can be repeated until the entire transverse width of the panel  14  has been straightened, after which the panel  14  is repositioned within the passage  96  to align a new transverse width portion of the panel  14  between the scallop bar  102  and adapter  104 . 
   While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims.