Bulk-Solids Storage and Flow: Silos and Stockpiles

This article provides guidance on the selection and sizing of silos and stockpiles in bulk-solids-handling operations, and clarifies misconceptions surrounding gravity-reclaim storage systems for bulk-solids materials

Gravity-reclaim systems for bulk-solids handling rely solely on gravity to withdraw material from storage systems, such as stockpiles and silos. These systems consist of storage, reclaim and transfer components. In the case of stockpiles and silos, the material is usually reclaimed by, but not limited to, apron, belt, vibrating pan and screw feeders. The reclaimed material is then transfered to downstream processing units.

These types of bulk-solids systems are common in several industry sectors, including mining and minerals, agriculture, cement and aggregates and power generation. Gravity-reclaim systems are simple and economical storage choices, but they may experience flow problems, such as arching, ratholing, erratic flow, limited storage capacity, limited discharge flowrate, caking, segregation and flooding. Designs that account for the flow properties of the material to be stored can help prevent these problems.

When comparing gravity-reclaim stockpiles and silos from a bulk-solids handling perspective, what are the key differences between these storage systems? What flow issues could affect operations? What key aspects of a storage system might be impacted by selecting a stockpile or silo? And more importantly, how should engineers design a stockpile or silo for reliable flow? These are some of the questions for which this article will provide guidance.

Material properties

A common question that arises during the design of a plant in which bulk-solid materials are to be handled and stored is whether a silo or a stockpile should be used to achieve the required surge or storage capacity. Answering this question requires posing a series of follow-on questions about the process requirements, material properties and their effect on flow behavior. For example:

• What is the required storage capacity?
• Is it cohesive?
• Is it free flowing?
• Is it suitable for gravity flow?
• Does it degrade over time or when exposed to the elements?
• Is it prone to segregation?
• Is it dusty?
• Is it air retentive?
• Is it easy to fluidize?
• What is the moisture content range?
• Is the material exposed to freezing conditions?

To answer these questions, the properties of each material need to be determined by laboratory testing. Generally, the specific properties of individual bulk-solid materials cannot be found in reference books or material libraries. Flow properties will depend on chemical nature, particle-size distribution, particle shape, moisture content, time at rest and environmental conditions, such as relative humidity and temperature.

A material is considered to have good flowability if it flows easily when handled in the selected storage system. However, flowability is not inherent to the material by itself, but a quality of the handling system, which results from the combination of material-flow properties, equipment design and operating conditions.

Once the material properties and process requirements are established, engineers can determine if a gravity-flow storage system is practical, whether mass flow or funnel flow is appropriate, how many outlets are needed, outlet dimensions and shape, outlet(s) location, feeder dimensions, live capacity and so on. Knowing your materials properties in early design stages will allow for more accurate forecasting for cost and operational performance.

Stockpile versus silo selection

Is a stockpile better than a silo, or vice versa? It is sometimes erroneously thought that stockpiles are easier to manage than silos. However, designers must consider what it effectively means for one system to be “easier to manage” than another, since flow issues can arise in both systems.From a bulk-solids-flow perspective, the phrase “easy to manage” refers to a minimal risk of flow issues occurring, and a storage system that performs as expected in terms of capacity, throughput and material quality. An easy-to-manage storage system would also have a relatively simple and effective solution for flow issues in the unlikely chance they did occur, and the impact to the system performance would be minor.

One line of thinking for why a stockpile might be “easier” has been as follows: When gravity flow stops and no more material can be reclaimed by feeders, it is still possible to push the material into the feeders from above (if there is no arching). But the risks associated with such an operation should be taken into account. Does the possibility of pushing material into the feeder qualify as “easier to manage”? Or is it perhaps the thought that because there can be a higher number of outlets and larger outlet dimensions at the base of a stockpile, flow issues will not occur?

Without knowing the particulars of the material’s flow properties, it is difficult to be sure that an outlet is large enough to prevent arching or ratholing in the material. Unfortunately, many stockpiles and silos have outlet shapes and sizes that are not consistent with good material flow.

The simple truth is that neither silos nor stockpiles are “better” than the other. The same material can be as easy to handle in a silo as in a stockpile if they are properly designed for the application and process conditions. Of course, selecting one over the other will have implications on cost, total storage capacity, live capacity, overall size, footprint, dust generation and so on.

In some ways, silos and stockpiles are not that different. As shown in Figure 1, a stockpile, specifically the section above the slab and reclaim hopper(s), can be considered as a flat-bottom silo with no walls.

Flow patterns

The main difference between silos and stockpiles is the flow pattern that can develop when material is reclaimed from the storage system. There are three possible flow patterns: mass flow, funnel flow and expanded flow.

When withdrawing material from a hopper, a mass-flow pattern will develop if the outlet is large enough to prevent arching and the hopper walls are steep enough to promote material flow against the walls. In the case of a silo, a mass-flow hopper will mobilize the full silo content toward the outlet, and the material slides down against the silo walls.

However, if the hopper walls are not steep enough to promote flow, yet the outlet is large enough to prevent arching, an active, steep-sided flow channel will develop directly above the outlet, which will be surrounded by stagnant material. As the level of material drops in the flow channel, layers of stagnant material may destabilize and fall into the active flow channel. The remaining material left inside will form a drawdown angle. A rathole, or stable empty flow channel, will form if the material has sufficient cohesive strength to support itself and will collapse when the flow-channel diameter is sufficiently large such that the induced stress at the flow-channel walls overcomes the cohesive strength. The location of this unstable condition, which leads to the rathole failure, depends on the cohesiveness of the material.

Expanded flow occurs when a funnel-flow pattern develops above a mass-flow section. The mass-flow section is sized to activate the full cross-sectional area of the funnel-flow section outlet, which is typically sized to prevent the formation of a stable rathole. Figure 2 depicts these three flow patterns.

A typical silo consists of a cylindrical section, which usually has a circular or rectangular cross section, followed by a hopper section, which usually has a conical, wedge, pyramidal, transition or chisel-like shape. They can also be flat-bottomed. The hopper section can have one or multiple outlets and can be designed to be mass flow, funnel flow or expanded flow. Stockpiles, on the other hand, are limited to funnel flow or expanded-flow patterns. Expanded flow can only be achieved when the reclaim system, which consists of one or more reclaim hoppers and feeders, operates in mass flow.The correct design of an expanded-flow reclaim system will provide the desired flow channel size and, prevent the formation of a rathole or limit its proportions to ensure that the operational requirements are met.

Design considerations

Two flow issues that have the most impact on performance when operating stockpiles or silos are pluggages and limited live capacity. Depending on the process configuration and requirements, segregation, variable discharge rates and irregular residence time may also affect performance. If handling fine and dry powders, issues like erratic flow, flowrate limitations and flooding must be considered and addressed. For the above-mentioned flow issues, the risk of occurrence is highly influenced by the flow pattern developed when material is withdrawn.

Risk of pluggages (that is, arching by interlocking or cohesion) is managed by sizing the outlet larger than the critical outlet size. The critical outlet size varies depending on the outlet and hopper geometry, the bulk-solids characteristics and the flow properties, such as particle-size distribution and cohesive strength, the sensitivity to consolidation pressure, time at rest and the flow pattern that develops inside the hopper when material is withdrawn. Interlocking arching occurs when the outlet dimensions are not sufficiently large to prevent blockage of large particles, originating either from handling of excessively coarse material or lumps breaking off stagnant zones and falling into the flow channel in funnel-flow operation. An example of this is frozen lumps from the pile surface in freezing conditions.

Live capacity refers to the amount of material that can be withdrawn from a stockpile or silo by gravity alone. Limited live capacity is a consequence of funnel or expanded flow, with the worst-case scenario being formation of stable ratholes (empty flow channels), as shown in Figure 3. For certain materials, cohesive strength increases with time, resulting in progressively steeper and narrower flow channel walls [1]. Rathole formation is a critical aspect to consider when designing funnel-flow storage silos and stockpiles.

Note that ratholes are often unstable and subject to sudden collapse from internal or external excitation, such as that caused by vibrators, air cannons, operation of gates or feeders and so on. When this occurs, a large quantity of material can suddenly fall into the rathole, resulting in significant impact loads to the hopper and reclaim system. This can result in structural failure, flooding from aeration or liquefaction, leading to uncontrolled discharge and silo vibration or quaking. Dynamic loads generated by ratholes becoming unstable and collapsing is one of the many structural concerns to keep in mind when designing silos or stockpiles.

Material segregation will occur by three main mechanisms, sifting, fluidization and dusting segregation. When forming a pile of material, the fines will naturally sift through the coarse material, with a higher concentration of fine particles in the pile center and a higher concentration of coarse particles in the pile periphery. Sifting segregation cannot be avoided, but the impact on the particle size distribution of the discharged product can be minimized in mass flow or exacerbated in funnel flow. Considering that in funnel flow the discharge pattern is “first-in-last-out,” as shown in Figure 4, the particle-size distribution (PSD) of the discharge product will be significantly affected by the storage system filling and reclaim operation. On the other hand, in mass flow, the discharge pattern is “first-in, first-out,” which promotes remixing of the segregated material upon discharge, resulting in a more consistent and stable PSD.

Discharge rate variations are exacerbated in funnel flow because of the following reasons:

• Variable consolidation pressure at the outlet with material level in the silo, leading to unsteady bulk density
• Erratic-flow or no-flow behavior
• Segregation, leading to variations in PSD

For most applications, unless high accuracy rates are required, mass-flow volumetric feeders will provide steady discharge rates.

Flooding occurs when material becomes sufficiently aerated, resulting in a loss of solids particle-to-particle contact and causing the bulk solid material to behave more like a fluid.

In funnel flow, silos handling pneumatically filled material, the risk of uncontrollable discharge due to flooding is exacerbated because the aerated material flows through a narrow and steep flow channel toward the outlet without sufficient residence time for de-aeration. The residence time will be further reduced if a stable rathole forms.

Unstable ratholes also present the risk of large masses of material falling suddenly into the discharge zone, causing it to aerate and flood downstream equipment. The resulting large impact loads induce great hydrodynamic pressures and, freeboard vacuum or depressurization, potentially leading to silo implosion or buckling. The silo and supporting structure will fail if not designed to withstand these types of extreme loadings.

Mass-flow silos are recommended for materials that are prone to flooding to promote adequate de-aeration and to achieve a safe and reliable discharge. Mass-flow silos are, however, typically taller because of steeper hopper requirements. They are also prone to abrasive wear by sliding friction.

From a bulk-solids-handling perspective, funnel flow should be considered when the opening is large enough to overcome ratholing, a “first-in-last-out” flow pattern is acceptable, the material does not degrade with time, particle segregation is not a concern and de-aeration of fine material is not required. For materials that are cohesive, degradable, prone to caking, prone to flooding (fines), gain strength over time or when particle segregation is a concern, consider mass flow.

Storage capacity

Having established the fact that stockpiles cannot operate via mass flow, and how the discharge pattern can either exacerbate or minimize flow issues seen in silos and stockpiles, we can now consider how selecting a silo or a stockpile as a storage system might impact key aspects, such as total storage capacity, live capacity, minimum level of operation and product variation.

Total storage capacity is the total amount of material stored in a silo or stockpile, and can be expressed as volume or mass. The volumetric capacity is determined by the material repose and drawdown angles, pile height and the shape of the pile for a stockpile, or the silo geometry.

A large storage-capacity requirement is often the deciding factor selecting a stockpile. In general terms, one stockpile can store considerably more material than one silo. However, silos require a smaller footprint for the same capacity.

Live capacity corresponds to the amount of material that can be reclaimed by gravity flow only. In mass flow storage, the total capacity always coincides with the live capacity because all the material moves towards the outlet, whereas in funnel flow storage, the stagnant material may be unrecoverable, resulting in reduced live capacity. The live capacity in funnel and expanded flow is determined by the drawdown angle or rathole(s) geometry.

A stockpile live capacity will never be equal to its total storage capacity. In fact, it is generally much smaller. For example, a conical stockpile with two outlets handling a free-flowing material (no ratholes) can provide a maximum live capacity of 33% of the total storage capacity [2]. It is the live capacity that determines the maximum surge capacity provided by a silo or stockpile.

Silos and stockpiles with multiple outlets have a minimum height to ensure that no discharge outlet is starved of material, ensuring simultaneous feed through all outlets. The minimum height depends on the height of the stagnant zones, which in turn, is determined by the drawdown angle or rathole geometry, as depicted in Figure 5.

As shown in Figure 6, the live capacity of a stockpile can be maximized by the correct sizing and placement of multiple outlets and reduce the minimum level of operation. To achieve this, outlets need to be sized and located close enough to allow the flow channels above each outlet to intersect and form a combined flow channel with a combined flow channel diameter large enough to overcome the critical rathole diameter.

As discussed previously, sifting segregation is the main driver for particle-size variation when forming a pile, because stockpiles cannot be mass flow, and it is not possible to “remix” the material as it discharges. Therefore, depending on the downstream system configuration, this can significantly impact the process performance. One way to deal with this is to collect discharged material from all outlets in one common conveyor.

Even though multiple outlet silos can be designed to operate with mass flow, it is easy for the flow pattern to change to funnel flow during operation. If one outlet is not active, stagnant zones will form inside the silo or, if material is simultaneously discharged at different rates through different outlets, a preferential flow channel could form. When designing a multiple-outlet silo for a cohesive material, consider if smaller, one-outlet silos are also a possibility.

As previously established, the best-case scenario for a stockpile is expanded flow. This requires mass-flow reclaim systems, which consist of mass-flow reclaim hoppers equipped with mass-flow feeders to ensure a fully active outlet-cross-sectional area. An incorrect feeder design could result in funnel-flow reclaim, which will result in partially active outlets and narrower flow channels with higher risk of arching and rathole formation. Improper feeder design is one of the commonly observed issues in silos and stockpiles.

Costs and retrofits

As mentioned at the beginning of this article, silos are not better than stockpiles and stockpiles are not better than silos. The same material can be as easy to handle in a silo as in a stockpile if they are properly designed for the application. The opposite is also true: the same material can be as hard to handle in a stockpile as in a silo if they are not designed for reliable material flow.

A specific storage-system configuration may have a smaller upfront cost, but unanticipated flow issues will increase the cost unexpectedly.

This rise in cost may be driven by, for example, unmet throughput quotas, plugged equipment (leading to unplanned downtime), limited live capacity, additional maintenance costs to re-establish operations, clean-up of spilled material or structural failures. Add to that the fact that once a storage system has been built, the positive impact of retrofitting the system will be limited by space, budget and technical constraints.

In several cases, retrofit options are limited to installing a mass-flow feeder interface to activate the entire outlet. The improvement will be significant, but it will not necessarily achieve optimal performance. This could have been achieved if the material-flow properties were known before making decisions on outlet dimensions and position, feeder technology and dimensions, headroom requirements, conveying technology and so on. It is much harder (nearly impossible) to increase the dimensions of the opening of a stockpile slab or increase the width of a tunnel to fit bigger feeders. Moving the outlets closer together to ensure the flow channels intersect is also not generally feasible.

Final remarks

Regardless of which storage system you select, silos and stockpiles need to be designed according to your material and process conditions in order to ensure reliable handling. Key to a reliable and robust design is knowing the material-flow properties at the operating conditions. And for that, flow-property testing is always going to be the first step in the design process. 

Edited by Scott Jenkins

Acknowledgement

All diagrams provided by the author.

References

1. Gálvez, A. and Holmes, T.M., Storage Time at rest: A Forgotten Source of Flow Problems, Powder Bulk Solids, Vol.43, No.11 Pages 8-12, November 2025.

2. F.J. Cabrejos, F.J. and Goodwill, D.J., Tunnel Reclaim from Ore Stockpiles, Bulk Solids Handling, Vol. 16, No. 3, July/September 1996.

Author

Alida Gálvez is a project coordinator at Jenike & Johanson Ltd. (3397 American Drive, Unit #10, Mississauga, Ont., Canada; Phone: 905-694-9769; Email: agalvez@jenike.com) based in Toronto. She has B. Sc. in mechanical engineering and has been working with Jenike for over four years. Her expertise is in bulk-solids handling and has worked mainly in mining-related projects for national and international clients.