TPT November 2013

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120 November 2013
Article Polysoude SAS
Several decisive factors come into play when analysing this
question and the final choice must take account of the notion
of relativity which may exist between the various situations.
It can therefore be seen from comparative tests that, for
thicknesses ranging from 12 to 15mm, the increase in
productivity between a conventional groove (angle of
approximately 20°) and a Narrow Gap groove optimised in
relation to the materials is not significant. Profitability will then
increase, reaching a factor of 3 on sections around 55 to
60mm thick.
At thicknesses of 30mm and above, apart from the specific
case of welding one-off parts, a Narrow Gap procedure
must be used. The difficulties relating to materials or other
operating restrictions are to be considered in order to choose
the Narrow Gap technique that is appropriate and compatible
with the application in question.
Today, Polysoude has a comprehensive line of Narrow Gap
welding torches for all thicknesses from 30 to 300mm.
The ranges and thicknesses to be welded are to be considered
on the basis of the line of torches created and constructed
according to various technological constraints. Each torch
has a scope which enables it to be used from maximum
thickness (maximum torch insertion depth into the groove) up
to completion of surface capping runs.
With conventional torches from 0 to 45mm, electrode stick-out
has to be adjusted over a 5 to 10mm range.
So-called conventional torches remain fully versatile torches
that are suited to all types of work. A category of intermediate
tools called “V2 and V3 nozzles” exists for 0 to 100mm thick
sections.
These nozzles have the particular feature of being insulated
and channelling the gas to the root better than a conventional
torch but without guiding the tungsten via a true electrode
lance. From an insertion thickness of approximately 45mm,
these nozzles are combined with removable trailing shield
assemblies to complete the weld, oscillated capping run
included. For thicknesses of 0 to 150mm, 0 to 250mm and
finally 0 to 300mm, a line of torches is also available with
trailing shield assemblies (removable if necessary) for all runs
from root to cap. All Narrow Gap solutions (ie 0 to 100mm
V2 and V3 nozzles, and 0 to 150 mm, 0 to 250mm and 0 to
300mm NG torches) exist as fixed electrode or oscillating
electrode versions.
The deliberate reduction in the volume of metal to be deposited,
which is the main objective of Narrow Gap applications,
highlights several operational difficulties including that relating
to the lack of visibility for operators to track the welding.
Figure 3: Increase in productivity between a conventional groove
and a Narrow Gap groove
Figure 5: Narrow Gap NG torch module for thicknesses of up to
300mm
Figure 4: Narrow Gap V2/V3 torch module for thicknesses of
0–100mm