Chemical Engineering January 2017 - 50

with no flat surfaces, an internal material
surface finish specified to reduce
interfacial tensions between a
liquid and the process contact surface,
and piping systems installed for
drainability with slope, no pockets,
and an internal material surface finish
also specified to reduce interfacial
tensions between a liquid and the
process contact surface.
Welding. Aside from establishing an
appropriate surface finish for process
contact surfaces of equipment
and piping, the joining of piping
becomes a critical design element
in the cleanability aspect of a system.
While mechanical-type joints
are necessary, they are inherently
more prone to capture and release
impurities in a system than that of
a properly made welded joint. And
for that reason, whether for metallic
or nonmetallic material, the welded
joint should be the default type joint,
and mechanical-type joints used
very selectively where breakout sections
or equipment connections demand
otherwise.
Welding metallic materials. In selecting
a metallic material for piping
and equipment, the welding method
most widely used and selected for
high-purity systems is the autogenous
automatic GTAW (TIG) method
performed with an orbital welder.
Both the ASME BPE standard and
SEMI standards F78-0304 - " Practice
for Gas Tungsten Arc (GTA)
Welding of Fluid Distribution Systems
in Semiconductor Manufacturing Applications "
and F81-1103 - " Specification
for Visual Inspection and Acceptance
of Gas Tungsten Arc (GTA)
Welds in Fluid Distribution Systems
in Semiconductor Manufacturing Applications "
provide criteria for weld
acceptance as well as guidance for
examinations of the welds.
An autogenous weld is one in which
no filler material is introduced to the
weld. The weld, as shown in Figure
2, is made by fusing together only the
base material of two components.
This is done by programming
the welder with essential elements
of the tubing that is to be welded.
Once the essential parameters are
programmed into the welding machine,
a weld coupon is made and
examined. Upon approval of the
50
weld coupon, production welding
can begin. As long as the same size,
wall thickness, tubing material and
purge gas does not change, welding
production can continue until
any of the following occurs:
* Shift change
* Change in purge gas source
* Change in welding machine assembly
(that is, weld head, weld head
extension, tungsten and so on)
* Change in the power supply
source
* Change in diameter or wall
thickness, or both diameter and
thickness
Figure 2 is a magnified photo of
the cross section of two 3-in. O.D.
 0.065-in. thick wall 316L stainless-steel
sections of tubing autogenously
welded together by means
of an automatic orbital welder. Notice
the smooth transition of both the
O.D. and, more particularly, the I.D.
of the weld bead as it transitions between
the base material of the two
sections of tubing and the weld bead
between them.
An acceptable weld for high-purity
fluid service is one that, upon examination,
can meet the requirements
found in the ASME BPE standard
or the requirements found in SEMI
Standard F81-1103, paragraph 7.
Welding nonmetallic materials. In
selecting a nonmetallic material for
piping and equipment, the welding
method best suited for nonmetallic
piping is BCF (bead and crevice
free) fusion welding, also referred to
as beadless, infrared (IR) welding, or
non-contact welding. This is a nonmetallic
version of automatic autogenous
welding of metallic tubing. It is a multistep
process in which the ends of the
components to be welded together
are cleaned prior to being clamped
into place in the welding machine.
After being clamped into the welding
machine, the ends of the two
sections of tubing are prepared by
squaring the ends with a facing tool
so that the end face of the two tubes
are square to one another and the
O.D. and I.D. of the tubes line up
in accordance with specified tolerances.
Butting the two ends together
will then allow the machine to check
fit-up alignment of the tube ends.
Prior to beginning the fusion process,
a bladder is placed inside the
tubing at the joint to be fused. The
expandable bladder maintains sufficient
force on the I.D. of the joint,
keeping the fused material, while in
its liquid state, from extruding into the
I.D. of the tubing. The result being an
I.D. joint surface that is flush with the
I.D. of the tube itself. And finally, fusion
of the two sections is performed,
all while the tubing sections remain
blocked and clamped in the welder.
The finished joint, as seen in Figures
3, 4 and 5, is difficult, if not impossible
to discern from the actual
tubing. Figure 3 is a cutaway at the
branch of a tee. The fusion bead
width is pointed out because it is extremely
difficult to discern from the
base material. Due only to disruption
of the base material in fusing the
two components together, causing a
slight variance in light reflection between
the base material and the weld
seam, can the seam be detected.
The same holds true in Figure 4. In
looking down at reflected light coming
from the invert of a sectioned
piece of tubing, the seam is actually
undetectable once out of reflective
range of the overhead lighting. If the
graphic did not indicate which way
the weld seam was running it would
be impossible to tell, either from the
photo or viewing the piece directly.
Figure 5 is a close-up of a BCF
fusion welded seam. The magenta
or fuchsia color comes not from the
tubing material itself, but from the
reflective attitude of errant overhead
lighting. If not for the disturbance of
melting and re-solidifying the base
material, which appears as though
a wet brush had just glided over the
surface of the tubing interior, the weld
seam would be barely noticeable.
Standards to consult
In order to properly specify, design,
procure, fabricate and handle highpurity
piping, tubing, fittings, other
wetted components and equipment,
it will be necessary to comply with or
adopt a set of standards that meet
your specific needs. To do that, it
is recommended that the following
standards be consulted:
ASME Bioprocessing Equipment
(BPE) Standard. BPE is a standard
that is focused on the high-purity reCHEMICAL
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Chemical Engineering January 2017

Table of Contents for the Digital Edition of Chemical Engineering January 2017
