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processing and processors0 pages
PROCESSING & PROCESSORS
The use of water soluble film
for pipe purging*
When stainless steel pipes
and tubes, hereafter refer-
red to only as pipes, (and
some made of other materi-
als such as chrome moly
carbon steel, copper nickel,
titanium, etc.) are welded
together or to fittings, it is
desirable to purge the
oxygen out of the inside of
the material near the weld
zone to prevent it from re-
acting with the hot metal
and causing coking, porosi-
ty, uneven penetration and
so on.
Mostly, argon is used for
purging because it is an
inert gas and because it is
readily available for
welding. In some countries
it is customary to use nitro-
gen for some of this work,
although it cannot be uni-
versally applied for purging
as for example it can
combine with hot titanium
to cause undesirable nitri-
des of the parent metal.
To obtain a quality purge
is not easy and it has been
customary to fill the pipe as-
sembly with gas and keep
the gas running at what is
thought to be a suitable flow
rate for an estimated period
of time.
In today's climate of ever
improving quality control
and increasing demands on
procedures with traceability,
it is inappropriate to use
such hit and miss methods
to guarantee a satisfactory
purge.
Clearly it is not desirable
to simply pour gas into a
pipe assembly in the hope
that a good purge will be
achieved. The principle dis-
advantage of this is that it
will cost the operator more
money than is necessary,
particularly for larger dia-
meter and longer length
assemblies.
In addition, the time
taken to remove all of the
oxygen causes lengthy
waiting periods by costly
specialists. Furthermore
there is no guarantee that
such voluminous quantities
of gas and protracted
purging periods will actually
prevent a bad weld.
When confronted with
Solid Disc Purge Dam Assemblies
Diagram 2
large volumes to purge, the
specialist will attempt to
confine the volume by
damming the pipe in some
way.
Many specialists still use
poor quality materials to
dam pipes. Cloth, wooden
discs, foam bungs are all
common. Such materials ge-
nerally have a high vapour
pressure and can emit con-
taminating vapours into the
space being purged. Clearly
it is better to use specialist
products which are made
for the purpose of pipe
purging.
Some of the specialist
products available include
purge bladders (see
diagram 1 above), solid
rubber disc purge dam as-
semblies (see diagram 2
above) and water soluble
materials for users to manu-
facture their own dams (see
diagram 3 opposite).
Bladders and rubber disc
assemblies require an open
end to enable the assembly
to be retrieved after the
STAINLESS STEEL FOCUS
"
