By discharge of silos of fine powders, often occurs ratehols. To avoid problems during the storage and discharge of cohesive bulk materials, silos should always be designed for mass flow. In the case of mass flow silos, the entire quantity of bulk material is in motion during discharge. In the case of funnel flow, only a fraction of the bulk material is in motion. This may lead to:
Depending on the bulk material properties, ideal dimensions of the silo and hopper or even the wall material, mass flow or funnel flow can be occur.
LIMITING CRITERIA FOR MASS FLOW / FLOW FUNNEL
| φe | φw | ßcrit | ||||
| 35.6 deg | 10.7 deg | 45.7 deg | MASS FLOW | |||
Excerpt from the automatic silo calculation report
After the bulk solid measurements there is an available online calculation tool to determine the limiting criterion for mass flow / funnel flow. With the measured material data, the necessary hopper angle or the appropriate wall material can be easily determined.
The flow function describes the dependence of the unconfined compressive strength (σc) to the principal stress (σ1). This is necessary in order to calculate the critical outlet diameter in the hopper. In simplified terms, the load on the bulk material bridge must be greater than the unconfined compressive strength so that the material can flow.
Excerpt from the automatic silo calculation:
CALCULATION OF THE CRITICAL OUTLET DIAMETER
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| flow function σc(σ) = | FL(σ) = 5.1 deg * σ + 401 Pa | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| bmin = | 0.15 m |  |
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| hmin = | 1.61 m | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| outlet > critical bridge = | TRUE | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
The most of the discharge equipment don’t activate the whole area of the silo outlet. The screw conveyer reclaim on the back side an it is no more place to take material from places in the front area. The belt conveyor or vibratory conveyor must discharge from front side and the belt slide under the powder on the back side. The working of the bin activator is more complicated. The bin activator works discharges not over the full covered area, but only in a small unpredictable area and coarse ratholing. Depending of the eccentricity of the inside couple and the local differences of powder compaction, the powder flows at the place of the smallest flow resistance. The rest of powder compacts under vibration and get les flow-able in the time, so if a flow channel is established it will stay on the same place – the rest of material compacts.
By collapsing of rate holes dust clouds occurs which can form a danger for dust explosion. The dust concentration we can not control, it changes with the time because the dust settles on the surface. That means that the critical dust concentration exist for certain time by each ratehole collapse. The only possibility is to prevent dust development – prevent hatehole where is no dust, the dust explosion can not occur. The second danger of collapsing rateholes is that when the ratehole collapse, the collapsed powder mixes with air and fluidizes. This lied to hydrostatic pressure on silo walls. This hydrostatic pressure lead to high wall l pressure – much more than normal silo pressure. From this point of few the ratholling should be prevented to.
From this, we can learn that it is not only necessary to establish a big outlet to prevent bridges, but the discharge equipment has to activate the entire outlet area to.
An requirement for preventing rathole is that the diameter of the silo must not be much bigger than the dimension of the outlet. Is the diameter big, then an central flow area will be establish, possible safe for bridging but not safe for ratholling. In oversized diameter the powder forms an central flowing zone and a dead zone in near the silo walls. According to the cohesion of the powder a stable ring formed wall can stay stable. In the highest dept of this ring formed wall a pressure is acting correspondent to the height of the column. This pressure can not be bigger than the unconfined compressive strength of the powder. That means that if we further discharge that the ring formed wall must claps and develops dust. To prevent rathole in an oversized silo, the silo outlet must be oversized too.
By right dimesion the whole silo content is
flowing like a piston in a cylinder and no rateholing is possible.
A wheat flour silo where the collapse of the flowing channel have caused damage of the silo walls. The original discharge opening was designed large enough to provide a mass flow and prevent the bridging. However, the discharge device which has been used, a screw discharger with an agitator above them, had activate only a small part of the discharge opening and have therefore introduced an funnel flow. Ratholing with stable walls from the bottom up to the top was a daily problem. Sometimes it collapses by them self, sometimes with manual help. In all those cases the powder flowing to the funnel fluidizes. The rebars of the concert walls have been plastic deformed because the wall at location of the funnel was loaded by hydrostatic pressure.
The pressure in the rebars have been measured before and after redesign. Before redesign when a funnel collapsed, the stresses reaches a very high value. During this test, the funnel collapses three times. The first time at a high level causing very high stresses in the rebar’s and other three time by nearly empty silo. Under this circumstances, it was to dangerous to use the silo in original design.
The redesign was realized by simply changing the original discharge device because the silo outlet itself was big enough. A Siletta discharge device was placed under the original silo opening which activate the whole silo outlet, an ratholing was not more possible and uniform flow over the entire outlet area was achieved. Because of not existing ratholes, there are no collapsing of the funnels and the pressure in the rebar’s is getting in the area of tolerated stresses.
This example shows how dangerous can be a wrong choice of the discharge device and how simple can be the solution when using the right discharge device.
From the examples from industrial practice we have seen that the wrong choice of the discharge equipment can lead to dangerous situations regarding dust explosion or static overload os the silo walls. By proper design and choice of the right discharge equipment, all problems can be solved and a safe and static safe and technologic adopted installation can be archived.
