|1. Powder blockage
|4. Discharging aids
Every factory operator having a silo or a hopper in his process has known this situation : downstream operations, for example a conveying like, a mixing step or a filling step, is requested powder but it will not flow out of the silo !
Most of the time, if possible to open and inspect the hopper, once will witness that the tip of the cone, above the discharge valve, is empty, but the powder is blocked above forming a kind of arch similar or bridge.
In other cases, one will see a funnel going from top to bottom of the silo, clear of product, but with all the materials on the side of the funnel, and not falling down.
The 1st case is called arching or bridging, and the second case is called ratholing or channelling.
Arching and ratholing is related to the flowability properties of the bulk solids. To understand on what the flowability depends, it is interesting to consider the forces that apply in between particles :
F = Fv + Fc + Fe + Fes [Lawrence]
F = the total force of adhesion in between particles
Fv = Van der Waals force
Fc = capillary force (related to the moisture of the product)
Fe = electrical force
Fes = electrostatic force
Flowability is not only related to the force in between particles as, for example, the geometry of the particles can be more or less interlocking and thus flow more or less well, but the lower the adhesion forces in between particles, the easier it will flow.
To be noted that the capillary forces can be kept at reasonable level if the relative humidity in the process environment is < 65 % RH [Lawrence].
Arching happen when a sufficiently cohesive powder is stored in a hopper which has not steep enough cone walls and / or a large enough outlet. Indeed, to flow, powders need to be submitted to a constraint sufficiently important. The constraint is dependent on the outlet shape of the silo and how easily the powder can use it as a support to consolidate itself and create arches.
If the angle of the outlet is too flat, the constraint will be reduce, it will be easy for the powder to resist gravity and form an arch. Similarly, if the outlet is small, below a critical outlet diameter that can be determined by powder testing, it will be easier for the powder to support an arch in between opposite walls that are very near to each other.
Arches can be avoided by design by calculating the hopper wall angle required and the critical outlet diameter required in order to always ensure enough constraint to break arches and make the powder flow.
In case it is not possible to avoid arches by design, discharging aids must be installed.
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Ratholes will form in hoppers which have not been designed as mass flow silos and which have a too small outlet. Product will only flow through a channel at the center of the hopper and the rest of the product will stay on the sides, not moving. As for the arches, the powder can consolidate itself sufficiently so that the constraint which happens on the powder on the side of the hopper is not sufficient to make it flow.
Ratholes can be avoided by designing the hopper with a diameter of the outlet bigger than the outlet rathole critical diameter. This diameter can be found by submitted the powder to shear cell testing in order to determine its flow properties. To be noted that the rathole diameter is usually bigger than the arching diameter.
If not possible to have an outlet diameter large enough, or in other terms, if the hopper is not mass flow, some discharging aids can be used in order to make the product collapse and flow. Note that such method can be hazardous on big silos as a lot of materials suddenly falling can damage the structure of the equipment, thus the importance to have a robust design from the start.
Details on discharging aids such as bin activator, fluidizing pads or knockers can be found in the page how to solve blockages and flow problems in silos.