Explanation of typical topics in bulk technology

Glossary, Explanation of typical topics in bulk technology


Below is an explanation of a number of important concepts in the field of bulk technology. First of all, an introduction to the substance we call bulk. Then a short explanation of two important flow patterns: mass flow and core flow, and why flow does not always go well: bridging.

Bulk solids

Mass flow

Core flow




For a more detailed explanation of phenomena in the transport and storage of bulk goods, see the articles in the reading_room.

Bulk Solids

Powder, solids, dry matter, granular material, bulk material...


Bulk solids do not have an unambiguous name; it is not an unambiguous material. Bulk solids do neither behave as a solid, nor a liquid, nor a gas.
It can flow, but it is not a liquid: a liquid cannot absorb shear stresses at rest: you cannot build a pile out of it. Bulk material can, that is why it does not necessarily flows out of a certain outlet.

A definition of this matter could be: a number of particles that in principal can move freely, but that definitely show interaction and are also influenced by gases and/or liquids that are present between these particles.

Bulk Solids

Whatever it is called, through fundamental knowledge and years of experience BSE is specialist in the field that we call bulk technology.

Mass flow Mass flow
Mass flow: the whole contents of the silo is moving

Mass flow

FIFO: first-in, first-out


  • first in - first out.
  • little segregation.
  • steady flow and a well controllable discharge.
  • no ageing or decay.
  • no contamination.
  • fit for HACCP.
  • possibility of "following" product batches.


  • For abrasive products: wear of the wall.
  • Risk of silo quaking.

When mass flow occurs, the whole contents of the silo is moving as soon as product is withdrawn from the silo. For more information on mass flow design see article silo design.

When mass flow does not occur, the flow type in the silo is called core flow, funnel flow or ratholing, see below.

Core flow, funnel flow, ratholing

Sometimes useful, usually undesirable

The type of flow where part of the product in the silo is at rest (stagnant zones) is called funnel flow, core flow or ratholing.

Core flow Core flow, funnel flow
Core flow, Funnel flow: a part of the silo contents is at rest

Funnel flow is only applicable for coarse, free flowing products, where ageing or decay is not important. In all other cases mass flow should occur, for the following reasons:

  • The product will be liable to ageing and decay, when the silo is filled again before it is completely empty.
  • Contamination will occur if the silo is used for several products.
  • Flooding: the collapse of stagnant zones leads to uncontrollable flow of product.
  • Flow will stop completely in the case that the product keeps building up, so that the outlet is blocked.


Bulk solids do not flow through every hole

Arching or bridging

Bridging is one of the most apparent and troublesome problems with silos. It can occur because product particles jam over the opening, but most of the 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 five metres!

When a product is withdrawn from a silo, it must converge in the hopper. The product is supported by the hopper wall. Arches of product are formed.
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 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.
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 baking of sand cakes. With dry sand this is not 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.