FIELD OF THE INVENTION
The present invention relates to an improved welding method to minimize welding induced distortion during fabrication of boiler headers. More particularly, the invention relates to a welding method with an improved weld sequencing technique for fabrication of assembly boiler headers involving minimum welding induced distortion.
BACKGROUND OF THE INVENTION
Boilers are widely used for generation of power in thermal power plants. The boiler consists of various pressure vessels for its efficient operation. Headers are used for collecting stream from various components in the boiler. This Header is fabricated using welding and it consists of thick walled pipe / plate formed pipe welded itself and also welded with plurality of tubes. The holes are made on the pipe and tubes are inserted into the set on type edge and fillet welding is carried out. During the fillet welding due to huge plastic deformation, the longitudinal bending distortion is induced in the header pipe. Similarly the welded tubes are undergoing angular distortion due to the fillet welding of tube with header. The bending of pipe and angular distortion of stubs leads to mechanical correction of pipe after welding. The correction is very complicated and is not possible in some of the headers. This is because of the rapid heating and rapid cooling effects associated with welding which leads to change of shape and dimensions.
Once the component is distorted, some correcting methods may have to be resorted to. But this will lead to undue increase in both cycle time of manufacture and fabrication cost of the component. Thus, during welding, extreme care and caution is to be exercised in order to control welding induced distortion. This welding distortion can only be controlled by the application of some procedures and this cannot be eliminated completely. One effective way of controlling distortion is by using a suitable welding technique which will, to a large extent, control the distortion happening as a result of welding. Such welding techniques are case-specific and have to be developed keeping into consideration the material nature and the

component geometry. Further, the welding techniques cannot be generalized and have to be developed by the welding engineer for different configurations of the component with different design of weld joints and weld locations.
US patent 20040094514A1 describes an automatic orbital Welding machine, as well as a welding process for joining pipes of a bundle to cylindrical bodies of larger diameter, preferably header cylindrical bodies. The invention is related to the welding process of header fabrication and the distortion control, weld sequencing, etc. is not disclosed.
US patent 5221818A teaches a method and an apparatus for welding tubes to a tube sheet uses a gas tungsten arc welding (GTAW) torch assembly that is small enough to be inserted through small access holes in a box header. This invention is closely related to the apparatus arrangement for the header fabrication.
US 1420241 A discloses a manufacturing process for headers for tubular boilers. In this case, the headers are composed of two or more pieces, which is generally the case when they are made of sheet metal. The method of this invention, two similar trough like vessels, made of cast or wrought sheet metal, are placed together with the edges of their adjacent sides are securely connected together by welding; when the ends are made integral with the trough their edges are also welded together.
All these disclosed prior art are related to the fabrication and welding methods pertaining to header fabrication. The welding sequencing and its method of distortion control is not taught.
US 3291962 teaches a welding method in which the ends of tubes arranged in a row are received in aligned counter-bores in the header, and the weld metal is continuously deposited in a sinuous or scalloped path first along one side of the tube row and then along the other side to the point of beginning. The resultant weldments thus includes substantially semi-circular deposits of weld metal at each such tube junction with the header, on opposite sides of the centreline of the counter bores, but with the ends of the semi-circular weld deposits overlapping

between adjacent tubes and along that centreline. This complete weldment can be accomplished in much less time and by a single welder resulting in greater manufacturing efficiency and economy since it is in effect a continuous welding process. Here also, the weldment laying procedure is disclosed, but the distortion control technique is not revealed in the invention.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a welding method with an improved weld sequencing technique for fabrication of assembly boiler headers involving minimum welding induced distortion.
Another object of the invention is to propose a welding method with an improved weld sequencing technique for fabrication of assembly boiler headers involving minimum welding induced distortion, which allows laying of multiple welds in header stub welding.
A further objects of the invention is to propose a welding method with an improved weld sequencing technique for fabrication of assembly boiler headers involving minimum welding induced distortion, in which welds in various locations of a boiler header are sequentially laid to control and minimize the welding induced distortion.
SUMMARY OF THE INVENTION
According to the invention, there is provided a welding method with an improved weld sequencing technique for fabrication of assembly boiler headers involving minimum welding induced distortion. The disclosed invention teaches a sequencing technique in laying the welds in a boiler header assembly that includes a fillet welding of stubs to header. The header assembly can be made of carbon steel or alloy steel of any grade and involves laying multiple welds at various locations all through the length of the component and therefore is prone to severe welding induced distortion, if not welded with an innovative sequence of depositing weld

beads. The method of laying the weld beads in a sequential manner in header assembly is explained in this invention.
The weld layers during the welding of header assembly which is a pipe of welded structure, consists of a plurality of stubs and other members attached to it. The base pipe is taken first and the required holes are drilled into it and stubs inserted in the holes and welded to it using any known welding process. After drilling the number of holes along the length of header, the tubes called stubs are inserted into the header and tack welded. The tack welding of tube to header is positioned parallel to the pipe’s longitudinal axis. After the necessary pre heating, the stubs are tack welded. The weld sequence to be employed for the individual stub to header welding shall follow the following weld sequence. The stub to header welding generally consists of minimum three weld passes and more than three passes can also be arrived with similar consideration. After tack welding, a root pass welding is carried out. The start, stop and welding direction of the circumferential welding shall be parallel to the pipe’s parallel axis. The second pass shall be started opposite to the root pass welding in both start, stop and welding direction. The third weld pass is carried out in such a way that, the start and stop points are opposite to root pass and welding direction is opposite to the second pass. All the welders shall follow the similar welding sequence in a line if more number of welders are employed.
Similarly all the stubs are welded along the length of the pipe. The welding sequence of the stubs along the header pipe is given as follows. A welder starts individual stubs from the center of pipe and fillet welding is proceeded towards the end of the pipe. If more number of welders to be employed, then even number of welders could be used starting the welding from the center of the pipe and proceed towards the end of the pipe.
The header may have multiple numbers of stub rows along the length of header pipe. The individual stub row is selected for welding sequentially as explained below. The bending distortion produced by the individual rows on pipe is calculated using a modified Blodgett formulae. Based on the calculated distortion values, the row which

produces the lower distortion is selected for welding first, and then the row which gives slightly larger distortion and lies opposite to the first selected row is welded. Similarly the selection is made for different rows of stubs in a header.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 – A full view of the Header stub assembly of a boiler.
Figure 2 – schematic illustration of Tack welding employed between the stub and header.
Figure 3 – welding sequence between fillet welds (three welds) for single stub to header welding.
Figure 4 – schematic view of welding sequence of stubs along the length of header pipe.
Figure 5- Illustration of welding sequence of different rows of stubs around the circumference of pipe.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
According to the invention, there is disclosed a method for depositing weld layers in a sequential manner, in fabrication of header assembly of a boiler, in order to minimize the welding induced distortion.
It is known that a header stub assembly constitutes a pipe of welded structure that contains a plurality of tubes called stubs positioned all through the length of the pipe called header, the stubs being welding on the header in a multiple numbers of rows along the circumference of the pipe.
The various rows of stubs along the length of the header assembly of a boiler are welded in a sequential manner in order to reduce distortion. According to the invention, the weld layers are deposited in a sequential manner that would minimize the welding induced distortion during welding of the header assembly.

The constructional details pertaining to the header assembly of a boiler is explained with reference to figure 1.
The following are the different components that comprise the boiler header :-
1) Header Pipe
2) Stubs
3) Tack welding
4) Centre axis
5) Fillet welding
6) Welding direction
7) Ends of pipe
8) Stub row 1
9) Stub row 2
A typical header stub arrangement is shown in Figure.1. The header pipe (1) with all the holes drilled at multiple locations is positioned on a rigid base with a support. After positioning, of the header pipe (1), the stubs (2) are positioned on the header in order. After positioning the stubs (2) the stubs (2) are tack welded (3) with the pipe (1). The tack welds (3) are made in a longitudinal direction of the pipe as shown in Figure 2. During tack welding a pre heating if necessary needs to be followed as per procedure depending on the material to be welded.
After tack welding of the tubes with the header pipe along the length of it, the stub (2) to header (1) welding generally consists of three fillet weld passes (5). The start, stop and welding direction of the circumferential welding shall be parallel to the pipe’s parallel axis (4) (Figure 3). After tack welding, root pass welding is carried out (Figure 3a). The second pass shall be started opposite to the root pass welding in both start, stop and welding direction (Figure 3b). The third weld pass is carried

out in such a way that, the start and stop points are opposite to root pass and welding direction is opposite to the second pass (Figure 3c). All the welders shall follow the similar welding sequence in a line if more number of welders is employed. Similarly all the stubs are welded along the length of the pipe.
The welding sequence of stubs along the header pipe is given as follows (Figure 4). The welder will start individual stubs from the center axis (4) of pipe (1) and fillet welding (5) is preceded towards the either end (7) of the pipe. The direction (6) of welder movement is also shown in the figure. If more number of welders to be employed, then even number of welders could be used starting the welding from the center (4) of the pipe and proceed towards either ends (7) of the pipe.
The header may have multiple numbers of stub rows (Figure 5) around the circumference of header pipe. The bending distortion produced by the individual rows (8,9) on pipe (1) is calculated using modified Blodgett’s design formulae. Based on the calculated distortion values, the row (8) which produces the lower distortion is selected for welding first, and then the row which gives slightly larger distortion and lies opposite to the first selected row (9) is welded. Similarly the selection is made for different rows of stubs in a header based on the calculation.
The disclosed weld sequence is applicable for all welded headers consisting of a plurality of stubs welded along the length of header pipe. It is applicable for all sizes of headers used in the boiler. The length of the assembly is not a limiting factor for use of the technique disclosed in the invention.
This weld sequencing technique disclosed in the invention is applicable to all type of welded header consisting of any amount of stubs with any number of stub rows attached to the assembly in different orientations.
The method of welding is not necessarily manual. This weld sequence technique can be applied even in semi-automatic or fully automated welding methods using programmed robots as well.

The material of the header stub assembly is not limited to grade of carbon steel alone and the material is not a limiting factor for the use of this technique. The weld sequence technique disclosed in this invention can also be used for other grades of steel as well.
The variables of welding process used like speed of deposition of weld layer, welding current, arc voltage etc. are not a limiting factor for the application of the said weld sequencing technique to header assembly of boilers.
The pipe position of welding as per ASME like 1G, 2G, 5G, 6G (Groove and fillet welding) etc. is not a limiting factor for the application of the said technique and is covered within the scope of the invention.
The use of additional accessories like fixtures or jigs is not excluded from the scope of the invention. The said weld layer sequencing technique can be employed with or without the use of such accessories.
The number of welders employed for applying the said weld layer sequencing technique is not a limiting factor for its use.

WE CLAIM :
1. A welding method with an improved weld sequencing technique for
fabrication of assembly boiler headers involving minimum welding induced
distortion, a header stub assembly constitutes a pipe of welded structure that
contains a plurality of tubes called stubs positioned all through the length of
the pipe called header, the stubs being welding on the header in a multiple
numbers of rows along the circumference of the pipe, the method comprising
the steps of :-
- positioning the header pipe on a rigid base with a support, the header pipe
provided with a plurality of drilled holes at multiple locations; of drilled holes at multiple locations;
- disposing a plurality of stubs over the header pipe in a spaced apart fashion and tack welding the stubs along a longitudinal direction of the pipe;
- welding of the stubs to header using three fillet weld passes in which the start, stop, and welding direction of the circumferential welding shall be parallel to the pipe axis, the three weld passes include a root pass, a second pass starting opposite to the root pass welding in both start, stop and welding direction, the third pass carried out such that the start and stop points remain opposite to the root pass welding and that the welding direction maintained opposite to the second pass, wherein the welding sequence of the individual stubs along the header pipe is commenced by the welder from the center axis of the pipe and wherein the fillet welding is preceded towards either ends of the header pipe.
2. The method as claimed in claim 1, wherein the bending distortion produced
by individual stub rows on the pipe is calculated, and wherein the stub row
which is likely to produce a calculated lower distortion value is selected for
welding first and followed by welding to the stub-rows in an increasing order
of calculated distortion values.

3. The method as claimed in claim 1, comprising a pre-heating of the components during the tack-welding.
4. The method as claimed in any of the preceding claims, wherein the configuration of the weld joints between stub and header can be of any known type of groove like the 'J' V, single bevel, socket joint etc. and the stub row orientation can be ranging from 0 degrees to 360 degrees etc. with respect to header axes.
5. The method as claimed in claim 1, wherein the welding procedure adaptable can be any of Gas metal arc welding (GMAW), Shielded metal arc welding (SMAW), Gas tungsten arc welding (GTAW) or any other variations of these methods or a combination of these types.