Claims:We claim
1. An improved method for distortion control in welding of plates onto pre-existing attachments of flat fin welded tubular panel, the said method comprising:
placing a plurality of restraint mechanism of locking a wedge inside an L shaped solid bar whose one end is fixed to a rigid surface, the said restraint mechanism is positioned in a plurality of locations all along a length of the panel (7);
locating a centreline of the length of the panel (7);
following a weld sequence, wherein the weld that is present in a plate that is located closest to the left of the centreline (7) of the panel is carried out first and continuing in the increasing order of distance of separation of the plate welds from the centre line of the panel (7) by choosing the welds alternately on the left and right sides of the centreline of the panel.

2. The method as claimed in claim 1, wherein an order of carrying out the various welds and weld passes comprising: are as provided below
performing track weld with the sequence as claimed in claim 1;
introducing slits in fins which are located at an interval spacing of every two metres on both left and right sides of the centreline of the panel;
following the weld sequence for:
a first pass of the first longitudinal weld of each of the plates;
a first pass of the second longitudinal weld of each of the plates;
a transversely placed welds of each of the plates;
a second pass of the first longitudinal weld of each of the plates;
a second pass of the second longitudinal weld of each of the plates;
a third pass of the first longitudinal weld of each of the plates;
a third pass the second longitudinal weld of each of the plates;
welding and closing the slits; and
removing the wedges after completion of all the plate welds of the panel.

3. The method a claimed in claim 2, wherein performing the tack welds comprising placing a plurality of tack welds of length greater than 25 mm on all longitudinal and transverse weld lines connecting the plate with a underlying tube lug (3).

4. The method as claimed in claim 2, wherein the slits having a minimum length equal to 25 percent of the length of the fin.

, Description:FIELD OF THE INVENTION
[001] The present disclosure, in general, related to a method and a technique for control of distortion that is caused due to one-side welding of several discretely placed heavy plates which are welded onto preexisting attachments of tubular panels, used in power boiler application.
BACKGROUND OF THE INVENTION
[002] The tubular panel is an arrangement that consists of alternately placed tubes and fins welded with one another, used in the boiler industry for absorption of the heat energy of the fuel burnt in the boiler by the water that is allowed to flow through the inside of the tubes in these tubular panels. The size of the tubes and fins are decided based on the rating of the boiler. These are typically fabricated for large lengths and widths to forms the walls of the boiler and hence referred to as the water walls of the boilers.
[003] These water walls need to be erected at sites and there are many other elements that are supported from the water walls. For instance, attachments like tube lugs are welded on the tubular panel to act as a resting supports on which the buck stays will be mounted. Over these tube lugs, there are certain plates that need to be welded which are in turn required for providing supports for other assembly elements are placed in position when the tubular panels are erected in thermal power plant sites.
[004] Since the plates are getting welded onto the tube lugs, the panel gets distorted heavily, the reason being the localized and intense heat input on the panel in the discrete locations where the plates are getting welded. This distortion is perceived as a major bottleneck in achieving enhanced levels of productivity in such panels. Owing to the localized application of heat during welding, the zones of material which melted and fused by the heat of welding tries to shrink, but the adjoining zones partially prevent this shrinkage and partially undergo plastic deformation by way of yielding. The contraction of the weld during the cooling phase leads to the formation of a shrinkage force in the weld. This shrinkage force and the plastically deformed zones together constitute a shape change of the component alongside the shrinkage in the length of the weld. This change of shape manifests as the distortion in the tubular panels after welding of plates onto tube lugs.
[005] The presence of distortion leads to many unwelcome effects like loss of aesthetics, problem of matching in the later stage of assembly at site, reduced load carrying capacity etc. Any amount of welding whether high or low will always lead to some residual distortion which cannot be eliminated completely but can be brought down significantly and can be kept within permissible limits.
[006] Prevention is better than cure and hence control is better than correction of distortion as the latter process results in an inordinate delay in completion of fabrication leading to increase of ‘takt’ time. Accordingly, the present invention proposes to control the distortion that is experienced in tubular panels when plates are welded onto tube lugs which are preexisting (already welded onto) on such tubular panels.
[007] Control of distortion can be effected by following certain techniques like weld sequencing, disposing components back-to-back and simultaneous welding, introduction of high restraints and clamps etc. However, it is to be noted that there is no one universal solution for control of distortion that is applicable for all welded components. The most suitable control method for a given case has to be tailor-made and has got to be ad-hoc in nature so that the distortion is kept within the stipulated limits. Since, every case is unique and has no precedent seen in the prior art, the evolution of a procedure with an ingenious welding procedures qualifies for an invention. Although, the present invention is proposed exclusively for the purpose of control of welding induced distortion that is seen when plates are welded on to tube lugs which are preexisting on the tubular panels is novel in nature, a case which is not seen in any of the existing inventions, a scan of prior art in the field of welding and distortion control is presented below to buttress the claim of novelty of the invention.
[008] US Patent Document US 7028882 describes a method for depositing an overlay weld on a boiler tube panel comprising a plurality of tubes with adjacent tubes joined together and then straightening the bow that occurs due to overlaying. However, there are no techniques revealed for welding the tubular panel with control of distortion. But, the method disclosed herein, in this invention relates to a method of controlling the distortion of the tubular panel that is likely to arise when plates are welded onto tube lugs which are preexisting attachments in such tubular panels.

[009] US patent 7703660 B2 refers to a method of determination by numerical modeling and application of an optimum welding sequence that reduces the welding induced distortion and residual stress formation while depositing hard-facing layers on a boiler water wall panel. Our invention is different in the sense that it is concerned with a method of application of weld sequencing aimed at controlling bowing distortion which is a consequence of welding of plates onto tube lugs which are preexisting attachments of tubular panels. There is no hard-facing operation talked about in the proposed invention.
[0010] US patent US 6023044 discusses a control method in a multi-layer welding where the weld line and a gap width of work pieces to be welded are detected by a laser sensor mounted on a robot, during a welding for a first layer and welding conditions are adjusted in accordance with the detected gap width for a second and subsequent layer and performed by using the stored data in such a manner that the welding torch is made to follow the weld line, and the welding conditions are adjusted in accordance with the gap width. But, in our invention, a method of employing a technique with specialized weld sequence scheme, without any need of sensors or robot, for control of welding induced bowing distortion encountered while welding plates over preexisting attachments in finned tubular panels has been described.
[0011] US patent 5591363 describes a process of depositing layers of weld metal onto a ferrous NiMoV low alloy steel turbine component, where during the deposition of a first layer of weld metal, low levels of amperage are used to prevent a dramatic increase in a level of hardness of the HAZ and during the deposition of a second layer of weld metal, higher levels of amperage are used to temper the heat affected zone. But, our invention relates to a method of welding plates onto preexisting tube lug type of attachments present in finned tubular panels wherein the disclosure includes a technique consisting of an ad-hoc weld sequencing scheme aimed to controlling the welding induced bowing distortion happening in such panels during welding of plates.
[0012] Indian patent application number 818/Kol/2015 describes a method of carrying out attachment welds in thin panel type of structures but specifies a welding method comprising a prebending technique which cannot be employed in the present case of proposed invention. Further, the former invention discloses a weld sequence method for attachments which are connected only by longitudinal welds and not by transverse welds, whereas in the present case, the weld sequence is revealed for a case of plate welded onto panels both by longitudinal and transverse welds.

[0013] Indian patent application number 201631017624 reveals a method of controlling distortion while welding of burner panels of boilers. This is pertaining to carrying out welding in sequential manner and in multi-planes by simultaneously deploying a plurality of welders. But, this neither covers the method of attachments nor welding of attachments with both longitudinal and transverse welds. The instant invention relates to controlling of distortion while welding of plates onto tube lugs in tubular panels. Neither multi-plane welding nor simultaneous deployment of a multiple welders are being talked about in our invention.
OBJECTS OF THE INVENTION
[0014] It is therefore an object of the invention to provide a technique involving a weld sequencing scheme for the control of distortion that is caused by the welding of thick plates onto tubes lugs, which are preexisting attachments in a finned tubular panel.
SUMMARY OF THE INVENTION
[0015] According to the invention, a distortion control method comprising a weld sequencing technique for controlling the longitudinal bow type of distortion caused by welding of plates on to preexisting attachments of the fin welded tubular panel is disclosed. The tubular panel consists of alternately placed tubes and fins for a designed length. Fillet welds connect each tube and an adjoining fin on both the top side and hind side of the panel. These panels are typically fabricated as long welded structures. In certain type of tubular panels, certain group of attachments are required to be welded onto the tubular panel. The first type of attachment is the tube lugs that are placed in the space between two adjacent tubes. These are discretely placed and welded all through the length of the panel based on design and field erection requirements. After these tube lugs are welded, the second type of attachment namely thick plates are positioned over the already existing tube lugs and are welded with those tube lugs. These plates are connected to the tube lugs both by longitudinal welds and transverse welds. This invention discloses the technique of controlling the welding distortion that is caused during welding of such plates onto preexisting tube lugs in such panels.
[0016] The panel is positioned in a work table and the plates that are required to be welded onto tube lugs are positioned at the respective places. After such placement of all the plates over the tube lugs, a restraining arrangement consisting of a wedge inserted into a rigid L shaped solid bar is placed on the edges of the panel at multiple locations. Then, the tack welds are made first in all the plates. The plate that is placed to the left of the centerline of the panel is chosen first. A plurality of tack welds of minimum length 25 mm is first placed in the transverse direction and then in the longitudinal direction. Then, the plate that is placed to the right of the centerline of the panel is chosen next. The tack welds are placed following the same procedure stated above. Likewise, the tack welds are placed in each of the plates alternately on the both the left and right sides of the center line of the panel and in the increasing order of separation of the plates from the center line of the panel. Then, slits of minimum length equal to 25 percent of the length of the fin are made in fins that are lying at an interval of every two-meter span taken from the center line of the panel, both in the right and left sides of the panel.
[0017] Then, the full welding is begun and completed in the following manner. As followed during tack welding, the plate that is placed to the left of the centerline of the panel is chosen first. The first pass of the fillet weld (along the length of the plate) shall be done on one of the two longitudinal weld lines associated with the plate. Then, the plate that is placed to the right of the centerline of the panel is chosen next. The first pass of one of the longitudinal fillet weld is made. Then, this procedure is repeated alternately on plates on either side of the center line of the panel in the increasing distance of separation from the center line of the panel. In the same manner, the first pass of the other longitudinal weld of each of the plates are completed. Then, the top and bottom transverse welds of each of the plates are completed in the same manner described above. This is followed by the second pass alone being laid in the first longitudinal weld in each of the plates in the same sequence as followed earlier. Then, the second pass of the other longitudinal weld of each of the plates are laid in the same manner as followed earlier. This is further followed by the third pass in the first longitudinal weld of each of the plates in the same sequence and third pass of the other longitudinal weld of each of the plates in the same sequence. This completes the overall welding. While welding is being carried out, heavy dead weights (of magnitude at least 3 to 5 tons) shall be placed over as many number of plates as possible to further restrain the distortion behavior of the panel. This produces the panel within a very least distortion and within the allowable limits.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
[0018] These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
Figure 1 illustrates a schematic sketch of tubular panel with alternately placed tubes and fins;
Figure 2 illustrates an arrangement of plates placed on attachments (tube lugs) of tubular panels;
Figure 3 illustrates a cross sectional view of tubular panel across section AA shown in figure 2 (Cross sectional view of tube lug (3) shown separately);
Figure 3(a) illustrates a restraining arrangement (consisting of wedge forced into solid L shaped bar) placed on the edges of the panel when viewed from left extremity of the panel;
Figure 4 illustrates a scheme of welding of plates over tube lugs of tubular panel shown in figure 2;
Figure 5 (a) illustrates sequential welding of first pass of one longitudinal weld of all the plates alternately with respect to the centerline based on the increasing distance of separation of weld plates (Weld sequence: 1 – 2 – 3 – 4 – 5 – 6 etc.);
Figure 5 (b) illustrates sequential welding of first pass of the second longitudinal weld of all the plates, alternately with respect to the centerline based on the increasing distance of separation of weld plates (Weld sequence: 1 – 2 – 3 – 4 – 5 – 6 etc.);
Figure 5 (c) illustrates welding of intermittent transverse welds of all the plates alternately with respect to the centerline based on the increasing distance of separation of weld plates (Weld sequence: 1 – 2 – 3 – 4 – 5 – 6 etc.);
Figure 5(d) illustrates sequential welding of second pass of one longitudinal weld of all the plates alternately with respect to the centerline based on the increasing distance of separation of weld plates (Weld sequence: 1 – 2 – 3 – 4 – 5 – 6 etc.);
Figure 5(e) illustrates sequential welding of second pass of the second longitudinal weld of all the plates, alternately with respect to the centerline based on the increasing distance of separation of weld plates (Weld sequence: 1 – 2 – 3 – 4 – 5 – 6 etc.);
Figure 5 (f) illustrates sequential welding of third pass of one longitudinal weld of all the plates alternately with respect to the centerline based on the increasing distance of separation of weld plates (Weld sequence: 1 – 2 – 3 – 4 – 5 – 6 etc.); and
Figure 5(g) illustrates sequential welding of third pass of the second longitudinal weld of all the plates, alternately with respect to the centerline based on the increasing distance of separation of weld plates (Weld sequence: 1 – 2 – 3 – 4 – 5 – 6 etc.).
[0019] Further, skilled artisans will appreciate that elements in the drawings are illustrated for simplicity and may not have been necessarily been drawn to scale. For example, the flow charts illustrate the method in terms of the most prominent steps involved to help to improve understanding of aspects of the present invention. Furthermore, in terms of the construction of the device, one or more components of the device may have been represented in the drawings by conventional symbols, and the drawings may show only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the drawings with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
[0020] For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated system, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates.
[0021] It will be understood by those skilled in the art that the foregoing general description and the following detailed description are explanatory of the invention and are not intended to be restrictive thereof.
[0022] Reference throughout this specification to “an aspect”, “another aspect” or similar language 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 phrase “in an embodiment”, “in another embodiment” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.
[0023] The terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a process or method that comprises a list of steps does not include only those steps but may include other steps not expressly listed or inherent to such process or method. Similarly, one or more devices or sub-systems or elements or structures or components proceeded by "comprises... a" does not, without more constraints, preclude the existence of other devices or other sub-systems or other elements or other structures or other components or additional devices or additional sub-systems or additional elements or additional structures or additional components.
[0024] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skilled in the art to which this invention belongs. The system, methods, and examples provided herein are illustrative only and not intended to be limiting.
[0025] In an example, the tubular panels are made of alternately placed tubes (1) and fins (2) which are welded to each other by means of fillet welds as shown in figure 1. The length and breadth of the panel are decided based on design requirements. These panels constitute the water walls of a typical boiler used for steam generation application. During field erection of such tubular panels, at certain elevations, these panels are required to be supported using certain attachments that are welded onto these panels. In some such types of panels, attachments called as tube lugs (3) (figure 2) are placed at discrete locations between two adjacent tubes and on one side of the panel only. The outline of the cross section of typical tube lug is shown in figure 3. The tube lugs are welded with both the adjacent tubes within which they are placed, above the fin connecting these adjacently placed tubes (figure 3). A set of tube lugs consisting of fifteen numbers are placed side by side in one location as shown in figure 2. Similarly, a plurality of sets of such tube lugs are placed all through the entire length of the such panel which are typically more than 10 metres long. Above the tube lugs, there is another attachment which is a rectangular plate (4) (figure 2) that is positioned in such a manner that it sits on three contiguously placed tube lugs (3). The cross section view along section A-A marked in figure 2 is shown in figure 3. It is evident from figure 3 and figure 4 that the plates that are placed over the tube lugs are getting welded onto them both using two longitudinal welds (5) one each on the first and the third tube lug in a given location and two transverse welds (6) which are present in the top and bottom of the plate (4). It can be noted from figure 4 that the transverse welds (6) are intermittently placed along the width of the underlying tube lugs (3).
[0026] The longitudinal and the transverse welds namely (5) and (6) connecting the plates with the underlying tube lugs (3) are fillet welds of leg sizes ranging in between 10 mm to 25 mm. In a typical panel, the number of locations where the plates are welded onto tube lugs could be as high as fifteen and when such huge quantum of weld connecting the plates to the tube lugs are made, it is very likely to cause bowing type of distortion in the panels. The present invention is being proposed to provide a solution to this distortion issue by means of providing an ad-hoc welding procedure incorporating an appropriate weld sequence scheme. It is assumed that the panel welded with tube lugs are already available and the method of distortion control when welding the plates onto the tube lugs alone is considered as the scope of invention.
[0027] Firstly, the panel already welded with tube lugs is taken and positioned in a work table. Then, the restraining mechanism consisting of a wedge (4x) locked into an L shaped solid bar (4y) is placed on multiple locations all along the panel. All these restraints are placed along the edges of the panel (figure 2) as a means to counteract the shrinkage force of the weld, the prime factor for production of distortion in the panel. The wedge (4x) is locked tightly into the L shaped solid bar (4y) whose one end is fixed onto a rigid surface as shown in figure 3(a).

[0028] The steps of the welding procedure are listed below:
a) The centreline (7) of the panel is located as shown in figure 5(a) and the first plate that is located closest to the centreline and present to the left of the centreline is selected first. A plurality of tack welds of length not less than 25 mm is placed on all longitudinal and transverse weld lines connecting this plate with the underlying tube lug (3).
b). Then, slits of minimum length equal to 25 percent of the length of the fin are made in fins that are lying at an interval of every two-meter span taken from the center line of the panel, both in the right and left sides of the panel.
c). Next, the plate which is located closes to the right of the centreline is chosen. A plurality of tack welds of length not less than 25 mm is placed on all longitudinal and transverse weld lines connecting this plate with the underlying tube lug (3).
d). Similar to the procedure specified in steps (a) and (b), the tack welds in all the remaining plates shall be completed based on the increasing order of distance of separation of each of the plates from the panel centreline, alternately with respect to left and right sides of the centreline of the panel. The sequence numbering is shown in figures 5(a) to 5(g).

After completion of tack welding, the full welding of the longitudinal and transverse welds is made as per the steps described hereinafter.
e). Similar to step (a), the first plate that is located closest to the centreline and present to the left of the centreline is selected first. The first pass of one of the longitudinal weld is made in this plate.
f). Then, the next plate that is that is located closest to the centreline and present to the right of the centreline is selected next. The first pass of one of the longitudinal weld is made in this plate.
g). Similar to the steps (e) and (f), the first pass of one of the longitudinal weld is made in all the remaining plates in the increasing order of the separation of the respective plates from the centreline of the panel by choosing the welds alternately on the left and right sides of the centreline of the panel as per the sequence 1-2-3-4-5-6- etc. sequence indicated in Figure 5(a).
h). Then, the first pass of the second longitudinal weld is made in all the plates starting from the closest plate that is located to the left of the centreline (7) of panel and continuing in the order of increasing distance of separation of the plates from the centre line of the panel (7) by choosing the welds alternately on the left and right sides of the centreline of the panel as per the sequence 1-2-3-4-5-6- etc. sequence indicated in Figure 5(a).
i). After completion of steps (a) to (h), the panel will look as depicted schematically in figure 5(b).
j). Then, the transverse welds (6) (seen as intermittent welds in figure 5(c)) of each of the plates are completed following the same sequence of welding as described in point (h) above i.e. the transverse welds are made in all the plates starting from the closest plate that is located to the left of the centreline (7) of panel and continuing in the order of increasing distance of separation of the plates from the centre line of the panel (7) by choosing the welds alternately on the left and right sides of the centreline of the panel as per the sequence 1-2-3-4-5-6- etc. sequence indicated in Figure 5(c).
k). After the completion of transverse welds as specified in point (j) above, then the second pass of the first longitudinal weld is made in all the plates starting from the closest plate that is located to the left of the centreline (7) of panel and continuing in the order of increasing distance of separation of the plates from the centre line of the panel (7) by choosing the welds alternately on the left and right sides of the centreline of the panel as per the sequence 1-2-3-4-5-6- etc. sequence scheme indicated in Figure 5(d).
l.) Then, the second pass of the second longitudinal weld is made in all the plates starting from the closest plate that is located to the left of the centreline (7) of panel and continuing in the order of increasing distance of separation of the plates from the centre line of the panel (7) by choosing the welds alternately on the left and right sides of the centreline of the panel as per the sequence scheme 1-2-3-4-5-6- etc. indicated in Figure 5(e).
[0029] Similar to the points (k) and (l) mentioned above, the third pass of the first longitudinal weld in each of the plates is completed next followed by the completion of the third pass of the second longitudinal weld in each of the plates, both these group of welds being made with the sequence scheme 1-2-3-4-5-6- etc. indicated in Figure 5(f) and Figure 5(g) respectively.
m). Finally, the slits that were made in fins all along the length of the panel earlier (as described in in step (b)) are welded and closed.
n). The wedges (4x) are finally removed and the panel is taken for further processing as necessary.
[0030] The present invention as narrated herein above should not be read and construed in a restrictive manner, as some modifications, adaptations and alterations are possible within the scope and limit of the invention as defined in the encompassed appended claims.