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withdrawn from a silo, it must converge in the hopper.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.
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.