What is bridging in a silo?
If a product is "free flowing" problems with bridging will not occur. An example is sand, provided that the particles are reasonably round and approximately the same size, and that the sand is not moist.
From this it becomes clear that the name of a product is not enough to know what the flow properties will be. Also the conditions play an important role.
Most products are cohesive; particles stick together. This makes that the product is not free flowing, so that bridging can occur in a silo.
Flow in a silo is in fact the continuous yielding of bridges. Bridging is only then a problem if a bridge does not collapse when flow is required. Here two factors are of concern:
- the strength of a bridge;
- the force acting on that bridge.
Strength of the bridge
The product in a silo will experience pressure from the above lying product. Through this silo pressure the product gets a certain strength (the unconfined yield strength).
Compare this to forming a snow ball and a "sand ball" with dry sand the latter is not possible.
The strength of a product depends on:
- the composition;
- the particle size (distribution);
- the pressure that it has undergone;
- the moisture content;
- the temperature;
- the storage duration.
Especially the last factor is important in practice, as appears from starting-up problems after a weekend.
To calculate the strength of a bridge, the strength of the product must be measured as a function of stress. From the above may be clear that it is important to measure the properties under the applicable conditions.
Force on the bridge
To let the product flow out of a silo, the binding strength of the product must be broken. Stable bridging will not occur as long as bridges that are formed in a closed silo, yield when the opening is free.
Bridges will collapse if the gravitational force is greater than the strength of the product. Products with a low bulk density are therefore more likely to give bridging problems.
Whether stable bridges occur is dependant on the bulk density, the shape of the silo, the wall friction, the internal friction and of course the cohesion of the product. If these parameters are known (measured), then the critical diameter can be calculated. The opening to apply must be greater than this critical diameter to prevent bridging.
How to design for bridging?
At this part of the silo design the minimum diameter of the opening is determined. The procedure is as follows:
- When different products or conditions are concerned, first the most critical product or condition is determined. This is done with a qualitative tester, with which bridging behaviour can be compared.
- The wall friction and internal friction are measured, and the occurring silo stresses are calculated.
- The bridging properties, i.e. the strength of the product, is measured with the Jenike shear cell, under the applicable conditions, and for different stress levels.
- If applicable the time consolidation is measured, for the period that the product can be at rest in the silo.
- The critical diameter, the diameter where stable bridges can be formed, is calculated.
- If a greater critical diameter is found than an opening that can be used in practice, a solution must be found in the form of bridge breakers, vibrating bottoms, grate bottoms, aeration, etc.
Application of solutions that can avoid or break bridges, can be selected based on data available from the design procedure and if necessary from additional tests. For example:
- When applying a bridge breaker it is important to place it at the position where bridging will occur. From the design this diameter is known.
- Recirculation of the product will cancel time consolidation. For a given opening the maximum period of storage can be determined.
- If an aerated bottom is considered, the influence of aeration on the flow properties can be investigated.
With the design method and supporting tests, one can find an optimum solution for each situation.