This article discusses the flow problems that occur when handling chemicals and other bulk solids in silos and tanks. It also delves into how to avoid some of these flow problems and discusses topics such typical flow patterns and solids flow properties.
The impact of improper design
A poorly designed silo is the natural outcome of an inadequate understanding of the solid being stored. Five possible conditions will occur as a result: no flow, erratic flow, flooding, counter-current air flow and segregation.
Clearly a no flow condition negatively affects your process. This condition can occur when you try to initiate flow by opening a gate or starting a feeder. Two problems can occur: an arch or a rathole can form  (Figure 1).
Figure 1. Two types of conditions cause solid flow to stop completely.
An arch (bridge, dome) can form over your outlet. This arch is capable of supporting the entire contents of the silo above. Extreme methods may be required to initiate flow. Gravity flow is desired as gravity requires no maintenance; however, flow aid devices, such as sledgehammers, vibrators, and air blasters, may be required to assist gravity.
The second no flow condition occurs when a stable rathole (pipe, core) forms. Some material typically discharges through a preferential flow channel; however, because of a materials cohesive strength, the flow channel empties out resulting in a stable rathole and no flow.
Erratic flow develops when a silo experiences both ratholing and arching. What usually happens is that flow is initiated and a stable rathole develops. To maintain flow, you are required to collapse the rathole by use of some flow aid device. The collapsing material arches as it impacts the outlet and flow has to be re-initiated until a rathole forms again and the scenario repeats. Erratic flow can affect the structural integrity of the silo or tank.
Some fine powders can easily flood from silos. If a stable rathole forms and additional material is added or falls into the channel from above, the falling material becomes fluidized or aerated in the channel. The feeding device at the outlet, which is designed to discharge a bulk solid under control, cannot control a fluid and the product floods from the silo.
Fine powders oftentimes exhibit limiting discharge rates, typically because of counter-current airflow. This flow rate limitation is a function of the materials permeability. Flow rate limitations typically develop because of a vacuum that is created as material flows in a hopper. Air or gas from the outlet flows counter to the material to satisfy this vacuum, causing a flow rate limitation. The usual approach to solve this type of flow problem is to increase the speed of the feeder. However, the powder cannot flow any faster through the outlet, so increased feeder speed wont help.
Segregation problems occur in many industries. This happens when a solid composed of a range particle sizes or densities separates. The major cause is sifting, where fine particles sift between coarse particles. As an example, upon forming a pile of material with differing particle sizes, typically the fine particles would concentrate under the fill point while the coarse particles would roll or slide to the outside. There are several other mechanisms of segregation that can be troublesome if uniform density or mixed material is required for a process.
Its worth considering the production troubles spawned by inconsistent flow from a silo. This is especially important when expensive improvements like a variable pitch conveyor are considered for the bottom of a silo to improve flow. These troubles can include: reduced live storage capacity, spoiling or caking of a product, and vibration, which in turn can damage weigh cells and other delicate instrumentation and even lead to structural failure of the bin itself.
Spoilage or caking can occur due to stagnated material that resides in the silo for days, weeks, months, and even years. Because this product remains stagnant, agglomerates can form, bacteria can grow and chemicals can react. If a product gains cohesive strength after storage at rest, it can cake, not only creating flow difficulties but also an undesirable product for your customers.
As bridges and ratholes collapse, they expose the silo to excessive vibrations. Imagine that a product has ratholed. Suddenly, this rathole collapses either on its own or due to some external force. The volume of material that impacts within the silo can cause significant vibrations, which may eventually affect its structural integrity.
Silos have unfortunately failed. Failure due to vibrations is just one area of concern. A preferential flow channel can expose the silo to asymmetric loads. These loads can easily be great enough to cause dents in the silo sidewalls or even collapse the vessel.
To avoid the flow problems mentioned earlier and their effects, you must measure your materials flow properties. Second, you must be aware of the type of flow pattern that develops and, last, you must ensure reliable feeder design.
There is a battery of tests that will help you identify materials flow properties. Property tests will identify, prior to equipment fabrication, just how a solid will flow in a particular geometry . The validity of a proposed geometry can be confirmed by determining a materials cohesive properties, wall friction properties, and compressibility and permeability values.
Measuring a materials cohesive strength allows you to determine opening sizes to prevent arching and ratholing. Cohesive strength is a physical, chemical, or electrical bond that causes a flow obstruction to occur. Many bulk solids when poured from a container will flow like a liquid. Under these conditions, the material has no cohesive strength. However, when the solid is squeezed in your hand, it may gain enough strength due to compaction to retain the shape of your hand.