Bending
of Plates
Direction
of Bending
The
shell plate will have more strength in the direction of rolling compared to its
transverse direction; hence as far as possible the direction of bending of the
plate shall be same as that of the original direction of rolling which is in
the direction of the length of the plate,
Pre pinging
Once the shell plate is fed into the bending
machine, first both the ends are pressed to the required shape, after pre
pinging the profile at the ends is checked using template and if satisfactory,
full bending is carried out. Usually bending is carried out in a few stages so
that the elongation of the shell will be negligible and no further trimming of
the plate will be needed.
Setup
for Welding of Long Seams and Circular Seams
Upon completion of bending, the profile all around
the shell shall be checked and deviation in profile from the template beyond
acceptable levels shall be repaired.
In addition, to that the joint configuration shall
be checked for its compliance to the design and its uniformity throughout the
length of the seam. (E.g. root gap, root face, throat, and v – angle etc.)
Precaution
to control Distortion
Distortion of weld joints is more problematic in the
case of longitudinal seams when compared to the circumferential seams,
We have three proposals for the reduction of
distortion.
When the thickness of the shell plate is less than
16mm, single V double welded joints are recommended. In such cases the groove
shall be provided as far as possible at the inside.
When the thickness of the shell plate is greater
than 16mm, double V joint are recommended, in this case as far as possible the
joint configuration shall be unequal V with the major V on the inside. While
the welding from inside is being carried out, there shall be at least 2 spiders
in position at both end of the shell. While providing the spiders care shall be
taken to avoid the weld area to facilitate easy access to the welder.
Upon completion of welding from inside and prior to
the back gouging, two more stiffeners shall be provided
Tack
Welding of Joints
Tack
welding on the seam or the use of clamps and wedges may be resorted to for
holding the joint in position. Provided a qualified welder does the tack weld,
using qualified welding procedure. Its starting and stopping points shall be
properly prepared by grinding prior to full welding. Tacks shall be provided at
a spacing of 200 to 250mm with the length of tack ranging from 25 to 40mm.
Run
in and Run out Plates
2 pieces of plate of 50 to 100mm length shall be
provided at the beginning and end of each seam, preferably with the V
configuration. This will help the welder to start the weld from the run in
plate and by the time the welder reaches the shell, welding will be stabilized
and weld on the shell will be free from defects. This is only possible for
longitudinal seams.
Stiffeners
to maintain Circularity
All
shell section shall be provided with spiders for maintaining the circularity
while rotating the shells on floor as well as n rotator.
At least 4 spiders shall be given at both the ends of the shell avoiding the
longitudinal seam area.
Pre
cleaning of Welds
A weld seam shall be thoroughly cleaned and shall be
free from rust, grease, mill scale, or any other foreign material not only on
the weld but also on either side of the groove to a width of at least 25mm.
Inter
pass cleaning
After every pass, the weld has to be cleaned using
proper tools to remove all slag and spatters from the groove.
Back
gouging/ Grinding
In the case of long seams/ Cir. Seams it is
recommended to complete the welding from inside first and then perform the back
gouging from outside. This may be carried out either by carbon electrode
gouging or grinding alone. In the case of carbon electrode gouging it shall be
ensured that all block carbon gouge marks on the gouged area are removed by
grinding into sound metal by at least 1mm.
This is to prevent the carbon particles from going into the molten
metal, which may cause detrimental defects.
While back gouging care shall be taken to follow,
It is not going to a depth beyond half the thickness
of the plate in order to reach the sound metal.
The width of the V is not exceeding the designed
Width.
NDT
after Back gouging
The LPT of the back gouged joint is always useful in
detecting gross defects like lack of fusion, cluster porosity, slag inclusions
etc. in the root run and hence is recommended for all full penetration welds.
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