A selection of common problems
An arch (left) is a stable bulk solid structure above the outlet opening of a hopper. With poorly flowing bulk solids, the cause is the bulk strength (compressive strength of the consolidated bulk solid). If an outlet opening is not large enough in relation to the particle size, arching can occur due to wedging of the particles.
The formation of a stable rathole (right) occurs when stagnant zones form during discharge, in which the bulk solid remains at rest even during discharge. If this bulk solid is sufficiently cohesive or if its strength increases during the rest period (so-called time consolidation), it remains stable due to its strength when the silo is emptied and, thus, forms the walls of a "rathole".
Bulk solids consisting of different particles with respect to size, shape or density tend to segregation when being moved. A very common mechanism is segregation when filling containers or forming piles.
The video demonstrates how segregation during flow on the inclined bulk solid surface ("sieve effect") causes the larger yellow particles to accumulate preferentially on the outside, while the finer red particles are found preferentially in the center. If this bulk solid were subsequently discharged from a central outlet opening and funnel flow were present, the bulk solid from the center would flow out first, and only later the bulk material stored in the dead zones. The discharged bulk solid flow would thus first contain an increased concentration of the red particles, and later of the yellow ones.
Essential for assessing and avoiding problems: The flow profile
The flow profile is the velocity distribution in the silo during discharge. Two basic flow profiles can take place: Either funnel flow (only part of the bulk solid in the silo is moving) or mass flow (the entire bulk solid mass in the silo is moving).
The animation shows in simplified form* the bulk solid movement during funnel flow. Inside the silo, the "flow zone" has formed in which the bulk material flows to the outlet opening. Starting from the inclined hopper walls, there is a "stagnant zone" in which the bulk material rests even during discharge. A stagnant zone is created, for example, by hopper walls that are not steep enough with respect to the wall friction angle, but also by inward protrusions or a discharge device that does not withdraw material from the entire outlet opening.
*) simplified, because in fact the bulk solid would flow faster in the silo axis than close to the stagnant zone or the silo wall
If the hopper wall is steep enough with respect to the wall friction angle (larger friction angles require steeper walls), the bulk solid can also flow across the hopper wall. Thus, if the bulk solid is discharged over the entire discharge opening, the entire silo content is in motion, which is shown in simplified form* in the animation.
Unlike funnel flow, a mass flow silo shows approximate
first-in-first-out behavior, which is advantageous for bulk
How can the problems be avoided?
The arching problem shown above can be solved by a sufficiently large outlet opening. For cohesive bulk solids, the required size can be determined from the flow properties measured with shear testers (especially important are the compressive strength, the time consolidation, and the bulk density). If arching is due to wedging of particles, the outlet size must be a multiple of the particle size.
The formation of stable ratholes can in principle also be avoided by a sufficiently large outlet opening, but the dimensions required for this are often very large. In addition, bulk solids often tend to consolidate over time. As a result, the strength of the bulk solid stored at rest in the stagnant zone of a funnel flow silo for a long time increases so that flow of this material due to gravity becomes even more unlikely. The better solution is therefore "mass flow": In a mass flow silo, no dead zones are formed that can lead to ratholing!
The segregation occurring when filling a silo or bin can hardly be avoided. If funnel flow prevails when a bin is emptied, the bulk material flows first from the center of the container, followed by the material from the periphery. Thus, the segregation that occurs above the cross-section during filling leads to a temporally segregated product at the outlet opening. If individual packs were to be filled, customers would have to accept different compositions of the individual packs. To avoid segregation, in most cases the best solution is to accept it when filling the bin, but design the hopper in such a way that mass flow occurs when material is discharged. As a result, bulk solid from the center and from the periphery of the bin is remixed when approaching the outlet opening.
Other problems can also be avoided with mass flow: The residence time distribution in the silo is narrow - unlike funnel flow, where it may not be known at all if a silo is used as a buffer. Thus, mass flow helps in batch traceability and in avoiding over-ageing of products. Also, mass flow will avoid a (fine-grained) bulk solid just fed in appearing too quickly at the outlet opening and not having enough time to deaerate. The latter could lead to dust formation and even flooding. Flooding is liquid-like, hardly controllable flow of the fluidized bulk solid.
Hopper wall inclination and outlet dimensions to avoid the problems described above are determined within the scope of silo design for flow This requires the knowledge of the flow properties of the bulk solid, which you can measure with our Ring Shear Testers.
More about silo design for flow:
For advice on bulk solids and silo technology, such as silo
design for flow, to design new silos or to redesign existing
equipment, please contact Schwedes + Schulze Schüttguttechnik
Schwedes + Schulze Schüttguttechnik GmbH offers, among other things, the measurement of flow properties (also for structural silo design) and silo design for flow.