Flow patterns might explain silo collapse

Cement Australia’s silo, containing concrete powder, collapsed about 10 days ago.

According to the ABC and Advertiser, emergency services were called to a private port on Veitch Road around 9.40am on Sunday 14th June.

There were no injuries although the sound of the collapsing steel structure was heard more than a kilometre away.

A spokesman for the Metropolitan Fire Service said the silo was “brand new” and was filled to about three-quarters of its 3,000 tonne capacity.

The collapse is being investigated by SafeWorkSA.

Emeritus Professor Peter Arnold of the Faculty of Engineering & Information Sciences at the University of Wollongong identified non-symmetrical flow patterns and erratic flow patterns as causes behind, globally, the fairly regular collapses of silos.

“The main reason that silos collapse is because non-symmetrical flow patterns develop inside them, so the loads become uneven all around and that’s what tends to bring them down,” explained Arnold, one of Australia’s pre-eminent bulk solids academics.

This scenario might arise when the silo is being emptied via an eccentric outlet.

“If you have a chute on the side, the material will all be going down to that chute, but the rest of it is dead so that gives it a very non-symmetrical loading,” said Arnold. “And generally they’ve all been designed on the basis that everything is nice and symmetrical.”

To avoid this issue, Arnold said that silos need to be loaded symmetrically, while for unloading the outlet needs to be on the axis of symmetry.

In terms of erratic flow patterns, a sudden collapse of material can cause silo damage or collapse.

“Material might have formed an arch, and the material under the arch has flown out, and then suddenly the arch gives way and you get this large mass of material dropping,” said Arnold.

An academic paper — Structures, Stability, Silos and Granular Solids: a Personal Adventure —  by J. M. Rotter of the Institute for Infrastructure and Environment at the University of Edinburgh concurred with Arnold.

In it, Rotter said of the “challenging problem of eccentric solids flow” that “Where a channel of flowing solids occurs adjacent to the silo wall, very low pressures are exerted by the flowing solid, and this unsymmetrical pressure pattern is probably the most damaging of all to both metal and concrete circular silos.

“Such eccentric flows probably cause more buckling problems in metal silos than any other phenomenon, and it can again be seen that a low pressure in the flowing zone is the root cause of the problem.”

In the aftermath the Cement Australia silo collapse, one firm was keen to offer its forensic engineering services to get to the root cause of the collapse.

Frank Soto of Soto Consulting said that the silo collapse is exactly the kind of industrial incident that needs forensic engineering.

Apart from determining the most probable cause of failure, it will deliver the information quickly and effectively so the owner/operator, constructor and various authorities can move swiftly towards resolution, said Mr Soto.

“Thankfully nobody was injured in this incident, but we know for sure the constructors and new owners of the plant would want to know why this happened,” said Mr Soto.

“It is exactly this type of situation when government authorities and investigators need a forensic specialist to identify precise causes of equipment failure, system or structure collapse and other incidents such as explosion or fire.”

Forensic Engineering can be defined as post or pre-failure analysis often for investigative resolution in order to identify system failures in design, manufacture or construction. The goal is to accurately identify the sequence of events leading to ultimate failure or catastrophic event.

“We have in-house expertise and digital engineering virtual analysis tools to do detailed computerised analysis to determine if the equipment in question was being used in a way not intended by the original designer or whether the designer had considered the full breadth of loading conditions,” said Mr Soto.

“We are brought in as technical experts to review and model potential causes of failure in the plant, equipment and systems.

“Comprehensive checks are made to analyse if equipment was being commissioned and operated as indicated or whether there was a failure in the design and construct review processes.”