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When a silo fails it can be devastating, in more ways than one.  Loss of the vessel, contamination of the material it contains, loss of material, clean-up, replacement costs, AND possible injury or loss of life (the most important). 

Have you ever seen a silo fail, or that has failed (the aftermath)?  I recently surfed around the web and searched for images and documents that might tell me more about why silos fail and what can be done to make sure this doesn't happen.  I thought I'd post what I have found.

What I Did

After searching for images of silo failures and collapses I spent some time at the website of a company I feel is an expert in bulk solids storage, silo design and bulk solids flowability.  The company is Jenike & Johanson.  I discovered a wealth of information, some about silo failures and collapses, and I will review it with you here.

I reviewed "plastic-frictional theory" at the website for the Granular Volcano Group.  This was exciting (wink-wink).  But seriously, this highlighted a cause of silo failures specific to a certain class of material in specific industries.

I also uncovered a paper that discussed the failure of a grain storage facility in France.  This occured in 1997 and caused 11 deaths and 1 injury (twelve victims overall).  Let me start my summarization with this paper.

INERIS Report On Grain Elevator Silo Collapse

INERIS is a public research body employing 525 people with an annual budget of about 47 million Euro.  INERIS was founded in 1990, is an ISO-9001:2000 certified organization and has as its mission to "assess and prevent accidental and chronic risks to people and the environment originating from industrial activities, chemical substances and underground works".  A noble and worthwhile endeavor.

The report documents the sequence of events in the failure of this grain elevator.  The facility consisted of three rows of reinforced concrete silos, 44 in total.  The facility measured approximately 100m long, 20m wide and 40m high.  The capacity of the facility was 130,000 metric tonnes of grain.  The company owning the facility had 21 people to operate the facility, almost half were killed and injured during the catastrophe.  In fact, ten of the victims were found in the administrative/technical offices "at their workplace, apparently not having had the time to react to the incident".

The picture of the failed facility, figure 5 of the INERIS report, is quite graphic and indicates the failure of the entire center section of the facility with silo structures remaining to the right and left of the center.  The cause of the silo failure/collapse was an explosion, no big surprise for a grain elevator.  However, INERIS examined a tremendous amount of information and exhaustively studied the facility.  Their description of "The Explosion Sequence", section 3.2 on page 11 of the report is fairly detailed in how the explosion likely traveled and eventually resulted in the facility destruction and collapse.  They also conclude that "it is possible that the collapse of the structures was facilitated by structural weaknesses".  However, these possible structural weaknesses were not speculated upon.

The summary conclusion as to the cause of the explosion which resulted in the facility collapse is as follows:

"Following analysis of the various possibilities, and bearing in mind that no component parts of the centralised dust removal circuit were found, it appears plausible that the explosion arose either from mechanical impacts or friction in the fan of the centralised dust removal circuit, or from an incipient fire caused by self-heating in the dust chamber".

I found this report a facinating read and it reminds me of the dangers associated with the facilities handling powders and other bulk solids, especially grain elevators.  Careful design of the structure, proper equipment and maintenance are all critical to plant safety and silo integrity.

Why Silo Failures Happen - Jenike & Johanson Case Study

As a recognized industry expert in bulk solids storage and handling (Jenike, not me) I reviewed the information at the Jenike & Johanson website.  I uncovered a wealth of information on this subject, including newsletter stories, two white papers and a great case study that exemplifies the need for proper design, construction, inspection and use of silos containing powders and bulk solids.

The case study is an easy read and interesting.  This case study documents a silo failure that occurred in the southwestern USA in 1996.  The silo was a large 80ft diameter bolted steel vessel containing 9,000 tons of fly ash.  The only employee near the facility was very lucky as he recognized a warning sound just seconds before the silo collapsed and he hurried away, avoiding being buried under the 20 feet of flyash that covered the ground where he had previously been standing.

After studying the collapse it was concluded that several factors worked together to lead to the failure of the vessel.  These included a design issue and a manufacturing issue.  In addition, the infrequent discharge of the vessel may have precipitated the failure, when taken in conjunction with the other issues.  It is possible that had even just one of these issues not been present that the silo may not have failed. 

Click here for the case study.  It's number 2 on the list.  Also, I suggest you check out the papers titled "Load Development and Structural Considerations in Silo Design" and "Silo Failures: Case Histories and Lessons Learned".

I hope this trip down the "silo failure" lane has been interesting.  I do not work for either of the organizations mentioned, nor do I know anyone who does.  But their research and documentation helped me learn more about this problem, "silo failures - why do they happen".  I hope you enjoy the review.

Joe Lewis
Vice President - Marketing & Sales
jlewis@monitortech.com
Monitor Technologies LLC
www.monitortech.com
www.flexar.info

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