FIELD OF INVENTION
The invention relates to a method of welding of finned tubes to headers, which
serve the purpose of heat recovery steam generator for industrial plant. More
particularly, the invention relates to an improved method of flux core arc welding
for welding of finned tube top headers in manufacturing of heat recovery steam
generator (HRSG) modules.
BACKGROUND OF THE INVENTION
To produce heat recovery steam generator plants, a plurality of welded joints
have to be made between the finned tubes and header. These so-called HRSG
modules form a large part of industrial boilers. It is known that the Tungsten
Inert Gas welding (TIG) process, in particular Gas Tungsten Arc Welding (GTAW)
is used to weld such joints in the root region and followed by Shielded Metal Arc
Welding (SMAW) or, TIG welding in the subsequent passes. The TIG welding
process has the advantages that the weld quality is superior devoid of the
defects such as lack of fusion, gas hole, etc. However, it has the disadvantage
that welding can normally only be carried out by a highly skilled welder and
takes more time to complete, as the deposition rate is low. Normally, for the
header to tube welding, the HRSG module mounted on a pallet is loaded on an
'A' Frame by using crane and the welding is carried out in down hand position.
This method require lot of lead time in loading, reversing the module assembly
and unloading of the same from the 'A' frame including facilities like cranes and
necessary man power. Moreover, this method will not be suitable for higher
length modules, where the roof height or level of crane elevation will be a limiting
one. Thus, there does not exist any simple method, for welding of finned tubes to
header assembly in HRSG.
Japanese patent JP 413 5064, considered as the closest document to the present
invention, is hereby incorporated in this specification.
JP 4135064 discloses a method and apparatus for welding a plurality of rows of
heat exchanger tubes with their corresponding header stubs. These stubs are pre-
welded to the header in earlier welding operations, and are placed in rows. The
invention is directed to a method which enables welding of a plurality of heat
exchanger tubes to these stubs. The tubes are carried to the header stubs in a
carrier (22), are held in position for welding and then welded to the stubs. The
carrier (22) transports and holds the tubes in different levels according to the
levels of the header stubs. Moreover, it discloses simple butt-welding of the tubes
to the header stubs.
The present invention teaches a method of welding the tubes directly to the
header in a single row. It does not require any carrier or similar apparatus to
perform the invention. And it teaches a more complicated and inventive welding
comprising both root pass including subsequent passes in a single step. It
eliminates the additional step of pre-welding of the strips to the header and
reduces the number of weld joints and chances of joint failure.
According to publication number WO 2006/093907 having the title "Method for
repairing heat recovery steam generator tube to header damage", the method is
for repairing a heat recovery steam generator header to tube attachment damage
through an access window in the header by removing the header to tube
attachment damage and then inserting a tapered stub into a header penetration
being centered axially, immediately over the tube and effecting a profile weld
between the header and the tapered stub tube through the header inside
diameter.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved method of flux
arc welding for welding of finned tube to header in the manufacturing of HRSG
modules.
Another object of the invention is to propose an improved method of flux arc
welding for welding of finned tube to header in the manufacturing of HRSG
modules, which is highly productive.
A further object of the invention is to propose an improved method of flux arc
welding for welding of finned tube to header in the manufacturing of HRSG
modules which eliminates use of complicated work handling devices and thus less
costly.
A still further object of the invention is to propose an improved method of flux arc
welding for welding of finned tube to header in the manufacturing of HRSG
modules, which reduces the man-power requirement.
These and other objects and advantages of the invention will be apparent from the
ensuring description.
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 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, the invention provides an improved Flux Core Arc
Welding (FCAW) welding in the HRSG modules, by keeping the modules in
horizontal position. By this method, each tube will be welded in down hand
including in overhead positions. Accordingly, there is provided an apparatus and
process for welding of HRSG header, which enables the finned tube to header
welding in both down hand and overhead position. The advantages of the
invention can be seen, inter alia, in the fact that the finned tube to header can be
welded in a single operation by employing FCAW (Flux Core Arc Welding) welding
that is instead of two step operations first by TTG welding and slower SMAW.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - shows an embodiment of the apparatus according to the invention. This
welding system uses a flux cored welding process using the special wire and the
positioning of the tube to header is illustrated.
Figure 2 - shows the actual complete mockup assembly after flux core welding.
DETAILED DESCRIPTION OF A PREFERRED 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 per Figure 1, the tube to header joint is completely made by
making use of only Flux core welding process. All the passes including root pass
are carried-out by flux core wire welding. Commercially available welding power
source for FCAW welding can be used and flux wire of DW 100 has been used.
Figure 2 shows the preferred embodiment of the invention used for welding of a
tube (1) to a header (2) by a skilled welder in down hand welding position at the
top and in overhead position in the bottom side of the tube (1) to header joint (2).
According to this embodiment, the FCAW torch (3) with gas shielding is held at
suitable angle with respect to the joint (4) and welding is carried out. A suitable
supporting means (5) is provided.
During welding trials, the root weld has been checked by LPI. After completion of
welding with two filler passes, the weld was subjected to MPI. After MPI, the
openings were closed with dummy plates and end covers were also welded by
using FCAW process. Inlet and outlet nipples were welded and the Assembly had
been subjected to stress relieving and LPI was carried out prior to hydraulic test.
Hydraulic test was carried out successfully at 161 Kg/cm2 (1.5 X of design
pressure).
Flux core arc welding is a type of welding used for various purposes in an industry
and the present invention does not claim "Flux core arc welding" but suggests an
improved method of arc welding of finned tubes to header assembly during the
manufacturing of heat recovery steam generator. Referring to cited document US
2006261053, the specification clearly teaches about a flux cored, gas shielded arc
welding electrode and not any method of welding. It suggests an electrode for
depositing a weld metal with low diffusible hydrogen and having tensile strength
greater than 90 Kpsi and yield strength greater than 80 Kpsi and the electrode
includes a nickel free steel sheath with a carbon content of less than 0.05% of the
sheath and a core in the range of 13-17% of the electrode. Hence the subject
matter of the invention as per cited document US 2006261053 is entirely different
and does not make the present invention 'obvious' and 'lacks novelty'.
Location of surface flaws and sub-surface flaws is essential for many of the
industrial components since the failure to do so may lead to catastrophic failures of
the components during service. For locating gross surface flaws, visual inspection
with the aid of magnifying glasses is adequate. However, the location of minute
cracks may not be possible even with the aid of magnifying devices. In such cases,
penetrant testing and magnetic particle testing can locate the defects. For locating
only surface flaws, liquid penetrant inspection is sufficient while for locating slightly
sub-surface and surface flaws in ferromagnetic materials, magnetic particle
inspection cam be used. Penetrant testing (PT) is generally considered to be one
of the easiest methods to locate discontinuities that are open to the surface. PT
can be used on any specimen except exteremely porous materials. Penetrant
testing is also commonly called as Liquid Penetrant Inspection (LPI), Liquid
Penetrant Testing (LPT), Dye Testing (DT) or Dye Penetrant Testing (DPT).
For locating only surface flaws, Liquid Penetrant inspection is sufficient while for
locating slightly sub-surface flaws and surface flaws in ferromagnetic materials,
magnetic particle inspection can be used. The technique is basically simple and
easy to operate.
The technique can be applied for welds, raw material, billets, plates, castings,
finished components, hot rolled bars, etc. The technique enables detection of
cracks, seams, inclusions, lack of fusion, porosity, lamination and other similar
flaws.
This can be used for detecting discontinuities on surface or sub-surface detects at
a depth of about 3-6 mm depending on the material and the parameters used.
This method is also called Magnetic Particle (MP) or Magnetic Testing (MT).
In the prior art, the job has to be kept in an inclined position so that the welding
can be done in down hand condition. It has the following inherent disadvantages:
• This requires a frame to hold the module in inclined position and a fixture to
hold the job.
• As the height of the module goes beyond certain limits, say beyond the
height between the floor and the crane hoist, then, the job cannot be
handled at the shop floor.
• We have to construct a separate shop with higher roof width only to handle
such modules, which will be in need of huge investment in terms of
exclusive crane, shop, building as well as man power.
• For putting the module on the frame for welding, to till it for the other side
and to unload the same we should deploy more than at shop for safe
handling.
• To make any repair work, again the job has to be hold by the crane or put
into the frame.
In the proposed method, we can keep the module in horizontal position at a height
where the welder can stand and do the welding. It results in
• Reduced handling time
• Reduced utilization of cranes
• Avoid the need for a separate frame for welding
• Any length/height of the module can be welded in the existing shop itself.
Therefore the need for exclusive shop is avoided and the investment
required for this, could be used productively for other purposes.
A comparative study of the new and old method is given below which clearly
indicates the distinct advantages of the new invented method over the old
method:
WE CLAIM
1. An improved method of flux core arc welding (FCAW) for manufacturing of
heat recovery steam generator (HRSG), comprising:
fixing a finned tube (1) to a header (2) and positioning a module of the
HRSG in the horizontal position on a supporting means (5) and carrying out
flux core arc welding of the tube (1) to header (2) in both root pass
including subsequent passes in a single step.
2. The method as claimed in claim 1, wherein the FCAW welding of the finned
tube (1) to header (2) is carried out in both downward and overhead
positions.
3. An improved method of flux core arc welding (FCAW) for manufacturing of
heat recovery steam generator (HRSG) as substantially herein described
and illustrated with the accompanying drawings.

An improved method of flux core arc welding (FCAW) for manufacturing of heat
recovery steam generator (HRSG) consists of fixing a finned tube (1) to a header
(2) and positioning a module of the HRSG in the horizontal position on a
supporting means (5) and carrying out flux core arc welding of the tube (1) to
header (2) in both root pass including subsequent passes in a single step. The
FCAW welding of the finned tube and header is carried out in both downhand
and overhead position.