FIELD OF THE INVENTION
The invention relates to an apparatus and process for the circumferential welding
of finned tubes to headers which serves the purpose of heat recovery steam
generator for industrial plant. More particularly, the invention relates to an
improved Gas Metal Arc Welding (GMAW) process for welding of finned tube to
headers in vertical position to form heat recovery steam generator modules in
industrial boilers.
BACKGROUND OF THE INVENTION
Manufacturing of a Heat Recovery Steam Generator HRSG involves a number of
welded joints between the finned tubes and header. The HRSG modules so
formed constitute a large part of industrial boilers. It is known that a Gas
Tungsten Arc Welding process (GTAW) in particular is used to weld such joints in
the root region, followed by Shielded Metal Arc Welding (SMAW) in the
subsequent passes or by TIG welding itself.
The TIG welding process has the advantages that the weld quality is superior,
and the defects such as lack of fusion, and gas hole, are avoided. However, it
has the disadvantages that the deposition rate is low and requires highly skilled
welder to achieve sound welds.

Normally, for the header to tube welding, the HRSG module mounted on a pallet
is loaded on an 'A' Frame by using a crane and the welding is carried out in down
hand position. This method requires substantial amount of lead time in loading,
reversing and unloading of the module assembly from the 'A frame, including
provisions of cranes and necessary man power.
Moreover, the prior method is not suitable for higher modules, where the roof
height or level of crane elevation delimits all of the activities.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved Gas Metal Arc
Welding (GMAW) process for welding of finned tube to headers in vertical
position to form heat recovery steam generator modules in industrial boilers,
which constitutes a circumferential welding of finned tube to header.
Another object of the invention is to propose an improved gas metal arc welding
(GMAW) process for welding of finned tube to headers in vertical position to form
heat recovery steam generator modules in industrial boilers, which adapts an
identical welding process for all the passes.

A still another object of the invention is to propose an improved gas metal arc
welding (GMAW) process for welding of finned tube to headers in vertical
position to form heat recovery steam generator modules in industrial boilers,
which improves the productivity without affecting quality.
Yet another object of the invention is to propose an improved gas metal arc
welding (GMAW) process for welding of finned tube to headers in vertical
position for form heat recovery steam generator modules in industrial boilers,
which eliminates the need of a highly skilled welder.
A further object of the invention is to propose an improved gas metal arc welding
(GMAW) process for welding of finned tube to headers in vertical position for
form heat recovery steam generator modules in industrial boilers, which reduces
the number of movement in the welding process to form the HRSG module.
A still further object of the invention is to propose an improved gas metal arc
welding (GMAW) process for welding of finned tube to headers in vertical
position for form heat recovery steam generator modules in industrial boilers,
which eliminates the requirement for costly work handling devices.

A further object of the invention is to propose an improved gas metal arc welding
(GMAW) process for welding of finned tube to headers in vertical position to form
heat recovery steam generator modules in industrial boilers which is enabled to
change the welding position from overhead to vertical.
At the outset of the description, which follows, it is to be understood that the
ensuing description only illustrates a particular form of this invention. However,
such a particular form is only an exemplary embodiment and the teachings of the
invention are not intended to be taken restrictively.
SUMMARY OF THE INVENTION
To overcome these difficulties of finned tube to header welding for industrial
boiler and to improve the productivity, it is proposed that by keeping the (HRSG)
modules in vertical position be welded in a Robotic Gas Metal Arc Welding
System. By simultaneously operating two robot in either side, each robot is
allowed to do welding in vertical position. For root pass welding, both the robot
can approach the welding positions from bottom to top i.e. vertically upward
position and subsequent pass by moving the robot from top to bottom i.e.
vertical down.

Accordingly, there is provided an apparatus and process for welding of HRSG
header, which enables the finned tube to header welding in vertical up and
vertical down position. The advantages of the invention can be seen, inter alia,
that the finned tube to header can be welded in a single welding process by
employing R-GMAW (Robotic Gas Metal Arc Welding) i.e. instead of two step
operations for example, by TIG welding and then by slower (SMAW).
Further, by introducing the Robot where the welding is totally programmed,
instead of relying on the skill of a welder an ordinary operator can be deployed
instead of skilled welder. And by repeating the same program and continuously
working both quality and productivity can be highly improved.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - shows basic arrangement for positioning of (HRSG) module.
Figure 2 - shows a welding system with two robot in either side.
DETAILED DESCRIPTION OF A PREFERED EMBODIMENT
OF THE INVENTION
Referring now to the drawings. Wherein like reference numerals designate
identical or corresponding parts throughout the several views, the welded joint

shown in Figure 1 is to be made with the least possible outlay by the process
according to the invention. As shown in figure 1, two end headers (1, 2) in which
a plurality of tubes (3) are inserted and welded, the tubes are also welded with a
fin (4). A device (5) is provided to hold the module in vertical position. A Robotic
Gas Metal Arc Welding process is implemented by positioning the module in
vertical position. The tube to header joint is completely made by making use of
only Robotic Gas Metal Arc Welding. All the passes including root pass are done
by robotic gas metal arc welding. Commercially available welding power source
for Robotic welding can be used with Gas metal arc welding. Figure 2 shows the
preferred embodiment of invention used for welding of tube to header by placing
the robot in either direction using vertical up for root pass welding and vertical
down for filler pass welding. According to this embodiment, an in-built welding
torch corresponding to accessibility and approach to the joint areas is used. In
the laboratory test of the above R-GMAW only embodiment, the tube to header
joint was successfully welded.
A header with 8 tubes (Figure 2) has been welded according to the invention
with established welding parameter. After completion of welding with root pass
followed by subsequent filler pass, the weld was subjected to Magnetic Particle
inspection. After Magnetic Particle inspection, the openings were closed with
dummy plates and end covers were also welded by using manual (GMAW)

process. Inlet and outlet nipples were welded and the Assembly was subjected to
stress relieving and Liquid Penetrant Inspection was carried out prior to hydraulic
test. Hydraulic test was carried out successfully at 161 Kg/cm2 (1.5 X of design
pressure).
As shown in figures 1 and 2, item 1 and 2 shows the two header in either side,
item 3 is groove where tube is to be welded with header in either side, item 4 is
the tubes, item 5 is the finned welded over tubes, and 7 and 8 are the robots
placed suitably in either side of HRSG.

WE CLAIM:
1. An improved gas metal arc welding (GMAW) process for welding of finned
tube to headers in vertical position to form heat recovery steam generator
modules in industrial boilers comprising the steps of:
- fixing a plurality of two end finned tubes to headers to form a HRSG
module and positioning the module vertically on a device; and
- joining of tube to header by robotic gas metal arc welding using solid wire
or flux / metal cored filler wires.

2. The process as claimed in claim 1, wherein the welding is carried out in
vertical position from either side which gives a uniform quality.
3. The process as claimed in claim 1, wherein welding is carried out in
vertical up position for root pass, and in vertical down position for filler
pass welding.
4. The process as claimed in claim 1, wherein the welding parameters are
optimized to achieve desired fusion between the tube and the header.

5. The process as claimed in claim 1, wherein the welding is completed in a
single step and eliminates the lead time required for loading reversing and
unloading of the module assembly from the 'A' frame.
6. The process as claimed in claim 1, wherein the welding is done in
horizontal position for higher length modules delimited by roof height or
level of crane elevation.