Saturday 17th Aug, 2019

Ask an engineer: How do I protect my storage silo and feed hopper from baked in design flaws?

In this regular column, experts from specialist bulk materials engineering firm Jenike & Johanson answer readers' queries around problems at their sites. In this edition, Corin Holmes explains how good design can keep silos performing reliably. 

In this regular column, experts from specialist bulk materials engineering firm Jenike & Johanson answer readers’ queries around problems at their sites. In this edition, Corin Holmes explains how good design can keep silos performing reliably. 

All I have to do is determine the storage capacity required and select a feeder that will achieve a certain discharge rate. So, how hard can it be?

All too often storage silos and feed hoppers are relegated to the back-burner of design, to the point where some are simply a copy of another application and process, both undervalued and unconsidered.

Look at the solids flow problems outlined below and ask yourself what they have in common.

• A lithium producer handles a spodumene concentrate having a variety of particle sizes and densities in a surge bin. The contents of this bin are then continuously discharged to the grinding circuit. However, every so often operators observe that the grinding circuit loses efficiency.

• A coal-fired power plant collects fly ash in a storage silo and meters it via a screw feeder to a conditioner (mixing auger and tank) prior to loading it to a truck for transport to the tailings pit. Periodically, the fly ash floods uncontrollably through the system, overloading the screw feeder, mixing unit and the receiving truck. The result, apart from serious health, safety and environmental issues from dust generation, is lost production and significant cleanup costs.

• An iron ore producer ships to its port via train loaded by a train-load-out (TLO) bin. Operators sometimes experience a situation where either the material floods from the TLO and engulfs the rail car or where limited discharge rate occurs. This causes significant delays in getting the material to port.

• A grain supply company has a series of storage silos, and although the design intent was to have fully active storage capacity where the entire contents of the silos would flow, the actual useable capacity of these silos is less than half. In this case, there are large stagnant regions in the silos, which prevent them from being emptied completely and periodically operators have to dig out the contents.

All of these case histories involve a storage container (bin, silo, bunker), which exhibits a funnel flow pattern; where some of the material moves during discharge while the rest remains stationary (shown in Figure 1).

Much of the bulk material in a storage container becomes stagnant along the bottom due to either shallow hopper angles or wall roughness.
Figure1. Much of the bulk material in a storage container becomes stagnant along the bottom due to either shallow hopper angles or wall roughness.

This first-in last-out flow sequence is acceptable if the material is relatively coarse, free-flowing, and nondegradable, and if segregation during discharge isn’t important. If the material meets all four of these characteristics, a funnel-flow container can be the most economical storage choice.

With many materials, however, funnel flow can create serious problems with product quality or process reliability. Arches and ratholes may form, and flow may be erratic.

Fluidised powders often cannot fully deaerate in funnel flow, such that the material remains fluidised in the flow channel and floods while discharging. The first-in last-out flow sequence can even cause some materials to cake, segregate, or spoil. In extreme cases, unexpected structural loading, such as when ratholes collapse, results in downstream equipment failure.

Problems such as those described in the case histories can be prevented with storage containers specifically designed to move materials in a mass flow pattern, in which all the material moves whenever any is discharged (shown in Figure 2).

All of the bulk material is flowing at once, which can prevent operations and maintenance problems.
Figure 2. All of the bulk material is flowing at once, which can prevent operations and maintenance problems.

With mass flow, the material flow and bulk density are uniform and reliable. In addition, there are no stagnant regions so level indicators work reliably and material doesn’t cake or spoil. The first-in first-out flow sequence minimises segregation and material residence time is uniform, so fine powders deaerate.

Mass flow containers are suitable for fine powders, cohesive (non-free flowing) bulk materials, materials that tend to degrade when stored for extended periods of time without movement, and when segregation is important.

To predict and therefore control how a material will flow in a given container, you must determine the material’s flow properties. Flow properties can be measured in a bulk solids testing lab under conditions that accurately simulate the handling process and environment.

We are often called to inspect storage containers at client sites, and one of the first things we look for is called “hammer rash”. This is always a result of operators “encouraging” or reinitiating flow with some sort of mechanical aid and is a direct result of a mismatch between hopper geometry and material flow properties.

The four problems described in the case histories above could have been avoided if the flow pattern selected was mass flow, but only if the minimum requirements for hopper geometry are met. So, how do we ensure that we achieve this goal?

The first thing to consider is the hopper slope (sometimes referred to as the hopper half-angle), whether the feature of a stand-alone feed hopper or the converging portion of a larger container. The smoothness of the interior surface affects wall friction, and generally the smoother the surface (e.g., new, smooth vs. rusted carbon steel) the less frictional resistance there is for the bulk solid to slide during discharge. Lower friction allows a design of less steep hopper walls to achieve mass flow. The required steepness and smoothness of the hopper is determined by conducting tests to measure wall friction, and then use of a set of design charts to select the appropriate hopper slope, which differ for conical and slotted outlet hoppers.

The second thing to consider is outlet size. There are two types of flow obstructions that can occur with bulk materials. The first is particle interlocking where particles lock together mechanically. The minimum outlet size required to prevent an interlocking arch is directly related to the size of the particles, provided that the particles are at least six millimetres or larger. As a rule of thumb, a circular outlet must be sized about six to eight times that of the largest particle size. Wedge hoppers must have an opening width that is at least three to four times the largest particle size.

If most of the particles are less than around six millimetres in size, flow obstructions can occur by cohesive arching. Particles can bond together physically, chemically, or electrically. In order to characterise this bonding tendency (called cohesiveness of a bulk material), its flow function must be determined. This information can be generated in a testing laboratory by measuring the cohesive strength of the bulk material as a function of the consolidation pressure applied to it. The strength is directly related to the ability of the bulk material to form arches and ratholes in storage containers.

A third consideration when designing a mass flow container is the required discharge rate. All bulk materials have some maximum rate at which they will flow through a hopper opening of a given size. Usually this rate is far in excess of the required rate, especially if the bulk material consists primarily of coarse particles.

Fine powders, on the other hand, have considerably lower maximum discharge rates when exiting from a container. This is due to the interaction between air (or gas) and solid particles as reflected in the permeability of the material. Solid/gas interactions are complex and, in many cases, counter-intuitive. While trial-and-error methods can be used, the results are often disappointing.

Proprietary two-phase flow computer programs have been developed that can reliably predict how solids and gases will interact. Problems such as settlement and limiting flow rate can be evaluated, as well as ways to overcome flow-rate restrictions by the introduction of small, controlled amounts of air.

The last consideration is the outlet area, which must be fully live. Even the most carefully designed storage container can discharge in funnel flow if the feeder does not provide uniform withdrawal of material from the entire hopper outlet (such as shown in Figure 3).

Properly designed mass flow belt feeder interface.
Figure3. Properly designed mass flow belt feeder interface.

This is the case even if the outlet is large enough to prevent arching and the walls are steep enough and smooth enough to allow flow along them. This problem frequently occurs when a gate or valve is left partially closed in an attempt to regulate flow and is detrimental to reliable flow as it prevents discharge from a portion of the hopper outlet. Conditions at and below the hopper outlet are just as important as the overall container geometry.

Understanding the flow properties for your bulk material and how equipment design affects flow patterns and possible development of flow obstructions in storage containers and feeders will ensure that you are protected against baked-in design flaws.

Do you have a bulk solids handling question? Jenike & Johanson has developed the science of bulk solids flow and specialises in applying it to solving the most challenging bulk solids handling problems. So why not put them to the test with your question? The harder, the better.