Silo Technology

Silos, feed hoppers and other bulk material containers are often not the focus when planning a plant, as they generally do little to add value by refining or manufacturing a product. On the other hand, they can have a negative impact on plant throughput or product quality if they are not designed properly. Despite this, many silos are built without considering the behavior of the bulk material to be stored. This leads to the well-known effects, e.g. flow problems, which can often be recognized by the marks of hammer marks on the silo body. Similar problems also arise when handling bulk materials on a small scale (e.g. dosing). The aim is therefore to design the silo / container / chute in such a way that the bulk material flows without problems. This is achieved by carrying out the design on the basis of shear analyzes with the bulk material in question. With this, the seven silo problems,arching, funnel flow, ratholing, flooding, segregation, eccentric discharge, silo deformation and vibrations (silo music) avoided or reduced. The "Problem solver means: mass flow". The basis for a correct process engineering silo design is knowledge of the flow profile.

If you have problems with your silo, as described above, or would like to design your silo correctly before planning, please contact us. We help you to be successful.

Arching

Arching is a problem in both funnel and mass flow. A stable arch is formed over the outlet opening, so that the bulk material flow comes to a standstill. In the case of fine-grained and cohesive bulk solids, the cause of arching is the compressive strength (bulk solid strength) based on the adhesive forces between the individual particles. Here, too, arching can be avoided by having a sufficiently large outlet opening, which is calculated on the basis of shear tests.

The critical diameter that must not be fallen below is determined.

You have problems with bridging - please contact us.

Funnel flow

Funnel flow occurs, for example, when the silo discharge hopper is not steep or smooth enough. In this case, the bulk material in the filled silo cannot slide down directly on the hopper walls. Dead zones form and the bulk material only moves downwards in a flow zone that extends upwards from the outlet opening.

If the silo is operated as a buffer, the bulk material remains in the dead zones for a very long time and can change its properties (e.g. spoil). Funnel flow can be avoided by properly dimensioning the silo based on shear tests.

You have problems with funnel flow - please contact us.

Ratholding

The rathole formation is a consequence of the core flow. Bulk material can become so consolidated in the dead zones due to funnel flow that it can no longer flow out due to gravity alone. This can be recognized by an empty "funnel" reaching up from the outlet opening.

A stable rathole is created in the funnel flow silo in that only the bulk material flows out vertically via the outlet opening. The remaining bulk material - the dead zones - remain in the silo due to the compressive strength and form the walls of the shaft. If the bulk material tends to cake over time, it can solidify more and more in the dead zones, where there is no bulk material movement for a long time, so that the risk of rathole formation increases.

In extreme cases, the bulk material can only be moved again with great effort (mining). Ratholing and arching, can be avoided by a sufficiently large outlet opening, which can be calculated on the basis of shear tests.

You have problems with shaft formation - please contact us.

Flooding

Flooding is often the result of insufficient de-aeration of the bulk material and is therefore associated with too short time at rest. However, the retention time of freshly filled bulk material can be very short, especially in a funnel flow silo, if it is drawn off during filling and the filled bulk material reaches the outlet opening through the dead zone in a short time. A bulk material that can be easily fluidized with air (e.g. flour, fine lime) does not have the time to de-aerate and will therefore flood out of the outlet opening like a liquid uncontrolled.

This can result in an increased level of dust in the environment, up to and including the discharge device being flooded. In the case of mass flow, on the other hand, the retention time is longer with the same filling quantity and the same mass flows.

Flooding is also a consequence of collapsing bridges. The falling bulk material falls in the direction of the outlet, where it compresses the gas area. The gas, usually air, mixes intensively with the bulk material and fluidizes it.

Depending on the bulk material (e.g. alumina, d50: approx. 80 µm) it can be so strongly fluidized that it can pass closed gate valves.

Other reasons for flooding are, e.g.:

large extraction rates from the silo
aeration devices
air cannons

You have problems with flooding bulk material - please contact us.

Segregation

When filling a silo, you always have to expect that the product will separate over the silo cross section. With centric filling, the smaller particles are usually found in the silo axis and the larger particles on the silo wall. If funnel flow prevails in the silo, the product from the middle (fines) will flow out first, while the coarser product from the silowall will follow later. This creates a temporally segregated flow of bulk material at the outlet, which not only represents a reduction in quality when filling small containers, but can also make stationary operation of subsequent processes impossible.

Powder and bulk solids tend to segregate if the particles differ in particle size, shape or density. In many cases, one would like to prevent or at least limit segregation, since the bulk material is required in subsequent processes with a constant composition, e.g. because a product is to be filled in partial quantities of the same composition (e.g. in sales packaging), or because a subsequent process (e.g. grinding or combustion) can only take place in a stationary manner if the product composition is constant. Segregation can also lead to fluctuating bulk density, making volumetric dosing less accurate.

You have problems with segregation - please contact us.

Eccentric discharge

If the design is unfavorable, discharge devices can lead to one-sided bulk material discharge and thus to funnel flow. One-sided flow of bulk material leads to an unfavorable asymmetrical load on the silo wall.

Significantly smaller normal wall stresses occur in the area of the flow zone than in the rest of the area. In the immediate vicinity of the flow zone, on the other hand, slightly larger stresses occur. The silo wall is flattened in the area of the flow zone (larger radius of curvature) and slightly more bent next to it, which is accompanied by bending moments in the silo wall (relative to the vertical) of different signs.

Causes of eccentric emptying can be:

Funnel flow silo with asymmetric flow zone, e.g. with a non-centric outlet or asymmetric funnel
Silo with several outlet openings, not all of which are active
Discharge gates not fully open
Silo with a discharge device that pulls off one side

You have problems with silo deformation, or eccentric flow - please contact us.

Vibrations - silo music

Shaking and self-excited vibrations can occur when bulk material flows in silos. The frequency of the vibrations caused by the bulk material can be greater than 1 Hz, sometimes even in the audible range > 20 Hz, but small amplitudes usually predominate. If audible vibrations are generated, this is referred to as "silo music". However, shocks with periods ranging from seconds to hours can also occur in an irregular sequence

“silo quaking” or “silo music”. Depending on the surface, the Impact loads spread far into the surrounding area, so that silo vibrations and silo music are often associated with adverse effects on the environment, but are also unpleasant for the operating personnel and possibly unfavorable for the loading of the silo structures.

The cause of silo vibrations and silo music is the sudden flow and deceleration of the bulk material.

You have problems with vibrations - please contact us.

Problem solver: mass flow

Frequently occurring problems when storing bulk materials in silos have been described in the previous sections. Most of the problems shown there are related to funnel flow. A number of the problems shown can therefore be avoided simply by designing for mass flow:

Ratholes are solidified dead zones and can therefore only form with funnel flow.
The residence time distribution in the mass flow silo is narrow (“first in – first out”), so that unfavorably long and unknown residence times of the bulk material that occur with funnel flow can be avoided.

Flooding is often the result of insufficient de-aeration of the bulk material and is therefore associated with too short time at rest. However, the time of freshly filled bulk material can be very short, especially in a funnel flow silo, if it is drawn off during filling and the filled bulk material reaches the outlet opening through the dead zone in a short time. In the case of mass flow, on the other hand, the retention time is longer with the same filling quantity and the same mass flows.
The segregation that occurs during filling across the silo cross-section has a strong effect on the temporal composition of the discharged bulk material in the case of funnel flow. In the case of mass flow, on the other hand, sufficient back-mixing often occurs in the hopper.

In the case of fine-grained and cohesive bulk solids, the cause of arching is the compressive strength (bulk solid strength) based on the adhesive forces between the individual particles. Here, too, arching can be avoided by having a sufficiently large outlet opening. This can also be evaluated on the basis of shear tests.

You have problems with your silo - please contact us.

Flow profile: mass flow and funnel flow

If bulk material flows out of a silo, a distinction is made between mass flow and funnel flow. With mass flow, the entire silo content is in motion when bulk material is discharged. Mass flow is only possible if the hopper walls are sufficiently steep and/or smooth. On the other hand, if the hopper wall is too flat or too rough, funnel flow occurs. With funnel flow, initially only the bulk material in the area above the outlet opening is in motion.

The bulk material in the "dead zones", which form from the hopper walls in the edge area of the silo, is only discharged when the silo is completely emptied. The dead zones can extend to the surface of the bulk material, so that a flow funnel is formed there and the funnel flow can be clearly recognized as such from above. However, there is also the possibility that the dead zones are only in the lower area of the silo, so that the flow profile cannot be determined by looking at the surface of the bulk material. In addition, dead zones can also form asymmetrically, which has an unfavorable effect on the load on the silo walls.

You have problems with your silo - please contact us.