Dear Mr. Rusinek,

Over here in the US we lose silos quite often- the typical farmer

has upright silos to store animal feed year round. What happens quite often is that the material/animal feed gets stuck on the side of the silo and builds up and keeps on building up as the silo is filled- then what happens is gravity takes over and the entire contents of the silo above the bridging drop and the force of the material and gravity can and will rupture a silo and knock it down from it own weight.

The force of the material hitting the bottom of the silo- If the stored material is already at a high angle of repose due to the uneven loading as a result of the bridged material above,

and it drops on the material at the base which is already offset in angle will project the force of the dropping material outward and downward and possibly knocking a silo off its foundation causing it to fall over.

You do not state in your posting what type of silo you use be it pressurized with low pressure air or an unpressurized storage so I would also say that fastener failure due to stresses on the silo skin is also a possibility.

The other problem relates to operations as we have had many failures/explosions over the years due to the simple fact that the vent doors were not opened when cement was delivered to a bulk hopper site. ■

Another possible cause of bursting is inner wall being too smooth or polished by a high bulk density material in a tall silo.

The lack of much adhesion at the walls means the total vertical load acts on the junction between the walls & the bottom cone or base - causes structural failure.

Never line silos with teflon or similar low friction material. ■

I had posted a query on hopper lining (capacity of 40 tons) and when I read on here I saw this. I am particularly keen on low friction liner material. Is teflon any good at high temperature then? ■

I think that it is important that the comment " do not line silos with teflon or similar low friction material" is understood to apply to the vertical section by readers. The use of appropriate wall materials on the convergence is a different issue altogether - in which case not only should the wall friction properties of the wall material (relative to the bulk solid) be taken into account, but also the longevity of the material/coating. Low friction can be useful if mass flow (or self draining) is sought - however the structural implications of achieving mass flow in a large silo should be fully investigated before changes are made. Equally, the wear resistance to sliding abrasion (as the bulk solid flows) should also be taken into account in the selection process. Be aware that some surface coatings puport to give low friction and wear resistance (substantiated by the pristine appearance of the coating after the silo has been emptied). However, it is often the case that the wall friction is insufficiently low to achieve mass flow - hence flow does not occur at the wall (except at draining)and the coating remains in perfect condition - giving the appearance of being wear resistant.

Regards

Richard Farnish

The Wolfson Centre for Bulk Solids Handling Technology, Univ.Greenwich, London, UK

r.j.farnish@gre.ac.uk

www.bulksolids.com ■

There are many reasons for silo failures. Eccentric flow is a common cause by raising unbalanced stresses around the circumference. This is often brought about by extracting product from a non-central location, it being superficially attractive to make an entry at the side of a silo wall to create an extra discharge point in an existing storage unit. Collapsing arches also impose high impact loads on the bottom section of a container, invariably a result of inadequate design for reliable flow. Dust explosions can cause catastrophic damage unless suitable provision is made for containment, suppression or venting. ‘Thermal ratcheting’ is another culprit for failures. This effect is the consequence of accumulative cycles of differential expansion and contraction of the silo shall and its contents. The stored material settles further each time the silo shell expands and is then subjected to increasing compaction loads as the shell cools on the progressively consolidated contents. External effects cannot be ignored. Vehicles occasionally run into supports and earthquakes occur. It would be helpful to know the reason for the enquiry to enable more constructive suggestions or comment to be put forward. ■