When engineers
think of chemical vessels made
from thermoplastics, they often
feel that they are venturing into
uncertain territory. The long-term
material properties, construction
methods and an apparent lack of
detailed design standards via which
quality and known life can be assured,
raise questions for engineers used
to the highly defined codes for
steel structures.
In the case of welded structures
made from extruded thermoplastic
sheet, these assumptions could not
be further from the truth.
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Material Testing
In contrast to the low density
plastics utilized in roto-moulding,
extruded or pressed thermoplastics,
e.g. HDPE and polypropylene, have
been around for more than 50 years.
The results of these tests determine
precisely what stress
conditions, applied for a known
period of time will result in
material
failure for any extruded thermoplastic
so tested.
Major producers of thermoplastic
sheet have also developed a
testing regime to determine the
effect
of chemicals on the integrity
of each material over time.
This testing measures the environmental
(chemical) stress-cracking
resistance
or ESCR of thermoplastics and
is defined in the European
standard EN 12814. There exists
no similar
testing regime for the low
density plastics utilised in roto-moulding.
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Design and Construction
Guidelines
In 1973 the first
all encompassing design and construction
standards were developed by the DVS
in Germany. These new standards -
DVS2205 and EN12573 - detail how
to design and construct thermoplastic
vessels for chemical processing and
containment that ensures that the
end product will last the required
lifetime whilst holding a specific
chemical at the required temperature. |
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Design
Software
To aid in the design
of chemical vessels Dotmar and their
partner Roechling in Germany have
a specialized design program called
RITA that works directly off the
DVS guidelines. It simply requires
basic inputs of relevant data and
provides a detailed output of how
to manufacture the tank.
For more complicated,
odd-shaped vessels, Dotmar has instituted
a program of development using sophisticated
design methods, still relying on
the time-honoured material properties
that have ensured safety of the vessels
for over 30 years.
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Sample
RITA input page:
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