Chemical Engineering September 2010 - 43

8.0
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FIGURE 4. Shown here is view of a
BCF weld of PVDF (polyvinylidene luoride).
The energy for melting the polymer
is introduced from the outside; the
formation of a crevice-free and smooth
inner surface in the joining region is
achieved by temporarily inserting a balloon
illed with compressed air
A good investment over decades
Although polymer-based piping systems
have not reached the status
of metal-based piping in CPI usage,
there are a variety of quite demanding
segments in CPI where the specific
advantages of polymer pipes have
been understood, systematically and
successfully exploited for many years
now [6]. The global chlorine industry,
basic-chemical synthesis, logistic partners
for chemical media, surface technologies
(for example, galvanizing and
pickling industry), pulp and paper,
as well as power plants are the most
important fields of application. The
combination of the practical experience
gained from these sectors and the
knowledge derived from other fields of
application leads to a considerable pool
of data supporting the assessment and
further understanding of the longterm
performance as well as to assist in the
decision making for material selection.
The continuous evolution of that
knowledge-pool is efficiently supported
by both an active international network
of polymer-corrosion specialists
as well as international and national
joint efforts in polymer-corrosion research
of institutes, universities and
industry-partners. In this concern the
immense contribution (30 years of active
research) of the Swerea KIMAB
(former Swedish Corrosion Institute,
SE) as well as the recent activities of
the NACE (U.S.), Eurocorr (EU), FGK/
GfKorr and KRV (Germany) should be
recognized. The result of this work is
primarily a steadily increasing pool
of longterm-field references, thoroughly
evaluated case studies of key
applications as well as the buildup of
systematic databases to extrapolate
or forecast the corrosion process of a
FIGURE 5. After 17 years of continuous
contact with 35 wt.% hydrogen peroxide
(25°C), only 60 µm of corrosion
attack is measured with PVC-U piping
components. Such minor traces of corrosion
do not inluence the function or
safety of the system at all
given polymer in contact with chemical
media under varying service conditions.
Figures 5 and 6 provide an impressive
example of the status of such
efforts in this field.
Physical and natural barriers
A comparatively high thermal expansion,
longterm creeping under mechanical
load as well as a significant
reduction of mechanical properties
under increasing temperature are the
well-known, inherent shortcomings of
polymers that have to be considered
in any polymer-product application.
Those processes are, of course, wellunderstood,
and corresponding material
data are always considered in
the material selection, design and
dimensioning. Polymer-specific safety
factors and very reliable data to
quantify the longterm creep behavior
support this selection process. When
using smaller pipe-dimensions (O.D.
typically < 110 mm) in the CPI, many
experienced end-users specify the application
of thick-walled piping systems
for safety reasons (for instance,
pipe series 9 for PVC-U). As already
mentioned, the use of dual laminates
for handling of chemical media at elevated
temperatures or higher pressure
(or both) is also a widely applied
tool to fully exploit the potential of
chemical resistance properties of a
given polymer and to surmount some
of the polymer-inherent shortcomings.
Keeping a proper bonding of the
inliner to the FRP sleeve maintained
as well as not exceeding the critical
wall-thickness of the inliner are necessary
preconditions to guarantee a
safe operation.
The energy-transporting properties
of polymers can strongly be influenced
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10 11 12 13 14
0.0823
0.0763
15 16 17 18 19 20
HNO3
FIGURE 6. This graph show the isothermal
diffusion coefficients of mixed acid (HNO3/
HF; T= 65°C) with variable concentration
(wt.%) in PVC-U determined from the combination
of laboratory-scale immersion testing
and simulations (source: Swerea KIMAB [7])
0.0883
by compounding with suitable functionalizing
additives; by doing so, even
an increase of the thermal conductivity
of a few orders of magnitude under
retention of the electrical insulationproperties
is possible.
The non-destructive monitoring of
corrosion processes in polymer piping
is still an unfulfilled demand of
many end-users and the realization
of such a concept is currently not in
sight. However, efficient management
of corrosion knowledge, practical experience,
a rational understanding of the
chemical processes and materials involved
can already give a good and reliable
basis for a sustainable and safe
operation. Competent manufacturers
are always good partners for the endcustomer
in case of need.
Quo vadis industrial piping ?
Despite being a small branch of the
industry, industrial piping and piping
for the CPI have always been
very innovative. During the last few
years, the impulse for innovation in
the chemical industry shows a focus
at the site of compounder and processors
rather than at the site of the
raw material suppliers. In the field of
materials and piping concepts, a trend
towards more pressure-resistant and
stiffer pipes has become evident. The
various approaches are quite versatile
and cover innovations such as layered
structures, continuous welding of oriented
polyolefin tapes on polyolefin
pipes made from the same polymer;
and the use of innovative copolymers
or fillers, to name just a few. The topics
" functional layers " and " nano-technology "
have just emerged in practically
relevant concepts for this type of industry
and are expected to strongly
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