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Bulk solids   Mass flow   Core flow   Bridging   Lab equipment  

Arching of bridging: Flow depends on the opening diameter


Bulk solids do not flow through every hole

Bridging is one of the most apparent and troublesome problems with silos. It can occur because product particles jam over the opening. The opening must be approx. 7 times the size of the biggest particle to avoid this kind of bridging.
But a lot of bridging problems occur with powders. Here it is the cohesion, the sticking of particles, that makes the product hang up. Dependent on the circumstances (for example storage time) a stable bridge of a powder can reach span of 3 metres!

When a product is withdrawn from a silo, it must converge in the hopper. This causes stress arches the product, supported by the hopper wall.
Flow in a silo is the continuous yield (collapse) of these arches (temporary bridges). Bridging occurs when an arch does not yield.

Bridging occurs when the outlet opening is too small.

For every situation a so-called critical outlet diameter can be determined. When the outlet of the silo is bigger than this critical diameter, the product will flow from the silo. If the existing diameter is smaller, bridging will occur.
The critical diameter depends on the shape of the hopper and on the silo pressures, but mainly on the unconfined yield stress of the product. This unconfined yield stress is the cohesion at a certain pressure, and is highly dependent on the conditions.
Compare this to the forming of a snow ball, and the baking of sand cakes. With dry sand the latter is impossible.

The unconfined yield stress of a product depends on:
* the composition
* the particle size distribution
* the exerted silo pressure
* the moisture content
* the temperature
* the storage time
Especially storage time is important in many cases, as appears from start-up problems after a week-end. To calculate the critical diameter, measurements are therefore performed under the applicable conditions.

For silo design concerning bridging see the article silo design.



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