Bulk Engineering, Hoppers, Technical articles

A guide to designing train load-out bins for maximum efficiency

Corin Holmes and Aleef Rahman from Jenike and Johanson explain that it is critical to understand the relationship between material flow characteristics and the train load-out system.

How a train load-out (TLO) bin will operate is fundamentally tied to the flow properties of the materials to be handled.

An understanding of how they relate to the TLO operation is crucial to ensuring that they will meet their operational requirements. It is not uncommon, however, to see these critical factors overlooked during design. 

Why wouldn’t it work? TLO bins are very complex, moving from a flooding phase (unrestricted flow) to a choking stage (restricted flow) as the railcar beneath is filled. 

TLO bins are typically expanded flow bins, ie they have both a mass flow and a funnel flow sections. Getting the geometry of the TLO wrong can potentially induce arching and/or ratholing, thus restricting live capacity. 

Additionally, fine materials often exhibit significant two-phase flow effects due to the movement, however slight, of interstitial gas as the materials compress and expand. Boundary conditions, such as leakage to or from systems at the top and bottom of the TLO will also influence this behaviour. Not understanding these effects may result in a severely limited TLO.

Typical flow issues observed in a TLO bin:

  1. Inadequate live capacity.
  2. Ratholing and/or cohesive arching (bridging) due to insufficient outlet size matched to the flow properties of the material handled.
  3. Shallow hopper angles preventing flow along the walls in the mass flow section.
  4. Greater dynamic loads than were anticipated, which can cause structural failure if not properly considered.
  5. Inaccurate loading of the railcars caused by erratic or restricted flow.

Depending on the design of the TLO bin, solutions to the problems noted may be complex in nature. To put things in perspective, as mentioned, a common design of a typical TLO bin (see Figure A) operates in an expanded flow which includes a funnel flow (maximising storage capacity) section followed by a mass flow section and a gate to control flow. 

Figure A: Typical TLO.
Figure A: Typical TLO.

Flow patterns

In funnel flow, material moves to the outlet through a funnel-shaped flow channel surrounded by stagnant material. 

The flow channel has a diameter approximately equal to the diagonal dimension of the active portion of the outlet. If this dimension is larger than the rathole diameter, layers of material from the top surface of the stagnant region will slide off into the flow channel as the level of material in the flow channel drops. If this occurs, the live capacity will be defined by the draw-down angle. 

If the outlet is smaller than the critical rathole diameter, the flow channel will empty without material sliding off the top surface, resulting in a rathole. The associated live capacity will then be restricted to the volume of the flow channel. 

In mass flow, all material is in motion whenever any is withdrawn from the hopper. Material from the centre and the periphery moves toward the outlet. Mass flow hoppers provide a first-in, first-out flow sequence, eliminate stagnant material, reduce sifting segregation, and provide a steady discharge with a consistent bulk density and a uniform and well controlled flow. 

Figure B: Mass flow patterns.

Requirements for achieving mass flow include sizing the outlet large enough to prevent arching and ensuring the hopper walls have sufficiently low wall (material/surface boundary) friction and are steep enough to achieve flow at the walls.  

A third type of flow pattern, expanded flow, can develop when a mass flow hopper is placed beneath a funnel flow hopper. The mass flow hopper is designed to activate a flow channel in the funnel flow hopper, which is sized to prevent the formation of a stable rathole. 

The major advantage of an expanded flow discharge pattern, particularly for large-diameter silos, is the savings in headroom compared to an all mass flow design. This approach not only reduces the capital cost but also facilitates retrofitting silos by minimising the additional headroom requirement. 

The mass flow hopper beneath the funnel flow hopper still has the benefit of discharging material reliably with a consistent bulk density. Note that segregation and material degradation problems are not necessarily minimised with an expanded flow pattern.

Diagnosing flow issues and rate restrictions to maximise efficiency

Every TLO bin is unique in its design and rate requirements. Not all TLO bins will experience the same flow issues; however’ the probability of them occurring is extremely likely if material handling fundamentals are not considered during the design stage. Understanding the system components and designing them in alignment with material flow characteristics is the first step and flow property test work (FPT) should be performed to establish the materials’ flow characteristics. 

Flow property testing provides vital information such as:

  • Bc and Bp – the minimum outlet dimensions to prevent cohesive arching in mass flow
  • Bf – the minimum outlet dimensions to prevent cohesive arching in funnel flow
  • Df – the critical outlet diameter to overcome ratholing
  • θc, θp – hopper wall angles for liner materials.

Permeability tests (at varying pressures) allow for a more accurate calculation of flow rates on fine materials compared to coarse ore flow rates. Table 1 provides a general understanding of the test work requirements to ensure the system operates as required.

Test work should also be undertaken when the material being loaded in the TLO bin changes. 

It is easy to assume that if the problems are not observed with material A that it should work for material B; however, this is a common misunderstanding.

Having established the material characteristics, they can be used to develop a TLO design or to conduct a review of a proposed design to prevent flow issues. By way of a guide,Figure D below provides a typical path forward.  

Figure D: TLO design flow path.

Rail car loading

Rail cars are typically loaded in two phases, flooding and choking (trimming). 

In the flooding phase, the gate is fully opened and material is allowed to flow unrestricted into the rail car. At some time during the railcar filling sequence, the gate is then moved to a partially open state, called the trimming phase (Figure E).

Figure E: Rail car fill rate schematic.

The position of the rail car when the gate is opened is critically important to the amount of product loaded into the car. This is especially true when loading occurs at high train speeds. As shown in Figure F, the gate is typically opened before the front of the car. An example of a rail car loading profile is shown in Figure G.

Figure F. Rail car position when gate is opened.
Figure F. Rail car position when gate is opened.

Part of the design methodology should be to conduct flow rate and train loading analyses to ensure the required tonnage rates can be achieved and loading in the rail cars is even. This is very important when considering two-phase flow effects.

Figure G

Two-phase flow and achieving peak flow rates

Boundary conditions, such as leakage at the top and bottom of the TLO, will also influence flowrate. There are several flowrate-dependent modes that can occur in a mass flow bin, depending on the solids flow rate, which are also a function of rail car speed. 

Steady flow mode is characterised by the steady gravity flow of partially deaerated material controlled by a gate. The limiting steady-state condition occurs when compaction forces too much gas out through the material top surface. 

This causes a slight vacuum to form as the material expands while flowing through the hopper sections. The result is a gas counter-flow through the hopper outlet, which forces the solids contact pressure to drop to zero and limits the steady solids flow rate.

Unsteady flow occurs at rates somewhat greater than the limiting rate. It is characterised by an erratic, partially fluidised discharge which may be controlled by some gate or feeder arrangements. At these flow rates, a steady discharge rate can be achieved only with the use of a gas permeation or injection system at an intermediate point to replace the lost gas.

Flooding mode occurs when the flow rate is too high to allow much, if any, gas to escape from the material voids. 

In this extreme, the material may be completely fluidised and flood or flush through the outlet unless the gate or feeder can control fluidized solids.

Which of these three flow modes occurs at a given discharge rate from a TLO depends not only on the flow properties of the material but also on the TLO geometry itself. Changes to the outlet size, fill and discharge rates, and level of material in the TLO can alter the mode of flow. Also, gas pressure conditions at the top and bottom (ie at the outlet) can have significant effects. Discharge rates will be significantly reduced if there is counter-current gas flow at the outlet.

Dynamic loads and the TLO

Experience with TLO design projects has shown that a high ore flow rate during the initial flooding phase, when the gate is first open, will likely result in unsteady flow of material in the TLO bin. This unsteady flow will cause dilation waves (low bulk density regions) to propagate upward in the TLO bin at a periodic frequency (Figure H). The unsteady flow and dilation wave propagation results in dynamic loads applied to the bin shell and inserts. These dynamic loads are reacted out through the bin in a similar way to a load applied to a spring. The dynamic loads are reduced when the ore discharge rate is choked (limited) by the level in the rail car, such that the peak-to-peak dynamic load is less in the choked phase compared to the flooding phase. 

Figure H: Dilation wave during discharge.


The success of designing and operating a robust TLO bin is a dynamic process which includes a variety of parameters from mechanical design, material properties, loading sequences, carriage timings, and more. More importantly, the consequences of mismanaging these parameters are costly due to the unexpected shutdowns and underachieving key performance indicators (KPIs). 

From a material handling standpoint, understanding the relationship between the material flow characteristics and the TLO system is critical to preventing flow issues, as well as ensuring discharge rate and rail loading requirements. 

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