A “Toolbox” for Forensic Engineers

252 Forensic Materials Engineering: Case Studies
Weld
Weld
Figure 7.37 Tank with internal cone for holding raw paint. It collapsed because
of the poor support for the inner cone. The base of the tank was supported by
the main weld and four vertical panels, two of which are shown. A steel support
frame should have been used instead.
to the tank building was over a small bridge incapable of carrying a full
tanker. Needless to say, this and many other tanks had to be replaced by tanks
of the correct design.
7.6.8 Other Tank Failures
Such tanks continue to fail, however; one recent failure in 1998 involved a
paint tank. It had been designed with an inner sloping floor, as shown in
Figure 7.37. This was essentially to allow all the contents to be mixed within
the tank and then drained away for further processing. The tank failed the
first time it was used, collapsing in a misshapen heap on the factory floor,
the contents draining away over the factory floor. The weakness of the design
shown is that all the mass of the contents is taken by a single weld, with
support for the inner cone only provided by four plastic panels. They failed
quickly and the weld had peeled away, failing progressively, so that almost
all the weld had disappeared. The design was again at fault, with the absence
of reliable support recommended by DVS 2205, such as a steel frame resting
on the ground, and bearing the weight of the cone and contents. Fortunately,
the design was a one-off, made by a small company for a specific contract,
and had not been repeated elsewhere.
Thermoplastic storage tanks have displaced not just metal or alloy equivalents
but other polymer types, especially glass fiber–reinforced vessels.
Although much more expensive, the reinforcement from the glass fiber makes
for a much stiffer wall, a factor that allows tall silos to be constructed.
Nevertheless, failures have been reported by Meyer Ezrin, 9 some of which
have actually been caused by the glass used. In another case reported by Derek