Page 22 - Failure Analysis Case Studies II

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levels HL for the first leak and different ratios of sloshing (d* = 6/D) are assumed, it can be seen
from Fig. 9 that the silo must have been filled to the top, and the liquid must have sloshed relatively
far after hitting the ground to have produced the observed pattern on the site.

4. CONCLUSIONS

Thc visual inspection at the site of the accident showed the typical picture of failure by
overpressure. Indications of an explosion or a chemical reaction, which could have produced
the overpressure, were not found. A pressure above atmospheric pressure can also be excluded
because the necessary safety devices were installed and operative.
According to the manufacturer of the silo, it was permissible to fill the silo with liquid up to
the seventh ring (counted from the bottom), i.e. ca 10 m high.
The amount of 407 t, admitted by the user, corresponds to a filling height of ca 14m.
The assessment of the filling height from the observed spurting distance also points to a filling
level practically at the top of the silo.
The design, manufacture and assembly of the silo can be judged as proper, suitable and
according to normal engineering practice.
Tests on the material also indicate a higher level than the seventh ring (counted from the
bottom). This is supported by the observed deformations in the failed bolted joint of the silo.
An additional argument against the statement of the user related to the filling level is the fact
that the silo was still filled to the middle of the tenth ring (counted from the top), although
the whole neighbourhood was covered with slurry from the silo.
Based on these findings it can be said that failure of this silo was caused by filling it to too
high a level with liquid instead of forage.
It cannot be completely excluded that a mix-up in the way of counting the rings has contributed
to the failure. Whereas one would normally count the rings starting from bottom, as for
buildings, the manufacturer of the silo counts the rings starting from top, because the silo is
erected that way, assembling first the top, then putting rings under the top ring until the
intended height of the silo is reached.

Acknowledgemenf-The calculations were performed by R. Primas, Section Materials and Structural MechdnicsiJoining
Technology of EMPA.