Flow issues in silos and bins

Bins and hoppers can clog frequently if they are not designed, installed or operated properly. When this happens, operators are left to their own devices and typically resort to manual intervention to restore flow to the downstream process. “Hammer rash” is often a sign of such intervention (see Figure 1).

Given this context, several key concepts need to be understood in the design stage to prevent such problems.

Typical flow problems

Common flow issues associated with poorly designed silos are as follows:

Stable rathole

When discharging, a material flows only through a narrow channel (rathole) and the rest remains stagnant in the bin. In those situations, material typically sluffs down the top surface into the flow channel. Under certain stress conditions of stress in the material, a the flow channel empties and material flow ceases, forming a stable rathole. At times, a stable rathole can collapses, and the falling material imposes dynamic loads on the silo, which may result in structural failure, obstruction of the opening or irregular flow (see Figure 2A).

Stable arch

If the material is strong enough, another problem that can develop is the formation of a stable arch across the hopper outlet (see Figure 2B). If manual intervention breaks the arch, this too can dynamically load the hopper and may result in damage. More often than not, a stable arch interrupts flow and can play havoc on the process downstream.

Segregation

When filling a bin that contains coarse and fine particles, segregation will occur naturally as the material level in the bin rises, forming a conical pile at the top. Fine particles accumulate under the loading point (typically in the centre of the bin) while coarser particles roll down the slope of the conical pile towards the silo wall (see Figure 2C). This is known as segregation—an undesirable effect in some processes—and in such cases, can be mitigated with proper design. In funnel flow, fines in the central core discharge first through a central flow channel followed by sluffing of the coarser material in the outer section. The particle size distribution of the discharged material is highly variable.

In a mass flow bin, however, the material segregated on filling is remixed at the outlet on discharge. The flow pattern is such that material moves down the bin uniformly across the cross-section, in horizontal slices. When the moving slice reaches the outlet, coarse and fine particles are “re-mixed”, thus reducing the effects of segregation in the process.

Flow-rate limitation and flooding: problems specific to fine powders

In addition to potential ratholing, powders, like cement or flour, can become aerated when filling a bin and, if the silo is not designed with sufficient retention time for the material to become deaerated by the time it reaches the outlet, it will flow out of the silo uncontrollably, like water.

Other times, when the powder is deaerated, the desired discharge rate cannot be achieved because the material’s low permeability prevents the required flow of air from entering the pore space in the powder, creating a vacuum in the pore space that impedes flow. Properly designed bins that consider the material’s flow properties can prevent such problems.

Requirements to consider when designing a bin

Consult an experienced bin design expert

Before designing a silo, it is important to seek advice from a bin design specialist and avoid some of the pitfalls described above. An expert, experienced in designing bins for similar materials, applies years of relevant knowledge to each application and maximizes success.

Establish material flow properties

Just as the mineralogy of ores is not the same everywhere, the flowability of materials also varies. Therefore, for a given application, the only way to ensure that a bin will provide reliable feed to the process is to design it for the measured flow properties of the material to be handled and under intended operating conditions. Establishing material flow properties at a reputable laboratory is the first step in determining the appropriate design parameters for reliable flow for the application, such as hopper geometry and liner requirements.

Physical operating conditions, like material time at rest, moisture content and temperature, must be properly established in advance so testing best represents the intended operating conditions in the bin.

Consider the potential for variability in the material’s flow characteristics over time, depending on its origin. It is important to bracket the material samples and test representative composites to ensure appropriate design.

Select the flow pattern

Material in a bin can flow in three patterns:

• Mass flow
• Funnel flow
• Expanded flow

The choice depends mainly on operating requirements and material flow properties.

In the case of mass flow, all particles start to flow as soon as the discharge is opened. A mass flow pattern is recommended when a controlled feed rate is required, a first-in, first-out (FIFO) operating mode is needed, or segregation is to be avoided. Generally, mass flow can be achieved if the hopper walls are smooth enough and steep enough to cause material to slide along the walls, and if the bin outlet is large enough to prevent the formation of a stable material arch, i.e., the stress in a potential arch is larger than the material’s strength.

In a funnel flow pattern, the hopper wall is too shallow or too rough for material to slide along it, and a channel forms, through which the material flows. A funnel flow pattern is recommended when the material is free flowing and does not exhibit degradation or sifting concerns. Funnel flow patterns provide uncontrolled flow and are not preferred in many applications.

In the case of expanded flow, the discharge lower hopper is designed to operate in mass flow while the upper portion operates in funnel flow. An expanded flow configuration is recommended for large diameter silos and stockpiles where a mass flow pattern throughout is impractical due to headroom constraints or capital cost.

Discharge feeder

A well-designed bin does not guarantee reliable flow unless a properly designed feeder is located at the outlet. In fact, a mass flow bin will operate in funnel flow if the feeder is not correctly designed.

Benefits of mindful bin design

A properly designed bin can provide the following benefits:

• Reliable and predictive feed control
• Preservation of perishable material
• Mitigation of the risk of spontaneous combustion when handling self-heating materials, such as certain types of coal and food products
• Structural bin safety
• Reduced wear
• Reduced corrosion
• Prevention of unplanned shutdowns and production losses
• Facilitation of smooth start-ups
• Prevention of costly retrofits after construction

Conclusion

Bins and hoppers are critical components in any material handling operation. Flow patterns and material flow pressures are complex, and reliable flow is not achieved unless the design is performed by experts experienced in bin design. BBA’s bin design experts have designed numerous silos for many types of difficult bulk solids all over the world. Reach out to our bin design experts for your challenging application. They can help you provide a reliable and predictable feed system that is always available when you need it.