Engineering

Improving storage silo reliability

All too often storage silos and hoppers are relegated to the backburner of design. Corin Holmes explains why determining the bulk solids’ flow and mechanical properties must be the first step in any design.

Bins and silos are a common sight at most industrial plants around the world. The smallest of these, may contain only a few kilograms of material while the largest may have a capacity of tens of thousands of tons of material. A common function of bins and silos is to provide storage or surge capacity for the material being handled. The material stored in a bin or silo is generically referred to as a bulk solid. This term encompasses an extremely wide range of materials varying in size, from submicron powders to run of mine ores and as chemically different as sugar is to limestone. [1]

Sometimes designers simply just “copy-paste” from one application and process to a new application and process with little thought to the potential effects. Dr. Andrew Jenike developed the science of modern-day bulk material handling technology in the 1960s [2] but it is still very common to see design basis documents that have little to no information other than specifying the generic name of the bulk solid to be handled and maybe a single bulk density value. Variations in particle size, chemical composition, moisture content, and others can dramatically alter the flow properties of a bulk solid. Whether you are looking to increase storage capacity, design a storage new system, or handle a new type of material in your system determining the bulk solids’ flow and mechanical properties must be the first step in any design.

Reliability Issues

Reliability issues can range from the storage silo’s inability to discharge a bulk solid to difficulty controlling discharge rate. Some of the most common issues are outlined below.

1. No flow condition

A no flow condition can be one of the most serious problems as downstream operations may be starved of material. Interruption in discharge can be due to arching or ratholing. Arching, a form of stable obstruction, can develop within a storage vessel at, or just above, the outlet as it is the region with the narrowest cross section. [1] Arching can be caused by mechanical interlocking (i.e. large particles relative to the outlet size) or by cohesion (i.e. a bonding of particles together). Ratholing occurs when a stable flow channel empties while the remaining material remains stagnant.

2. Flow rate issues

For some fine powders the discharge rate from a storage silo can be extremely high if the material becomes fluidized. The flooding material could overwhelm the downstream process and could pose serious health and safety issues such as excessive dust generation. In some cases, flooding or collapsing of ratholes, may cause the silo to experience unacceptably high structural loads and collapse of the structure. 

Alternatively, the material discharge rate may be much lower than that required. Increasing the speed of the feeder may not correct the problem if it is due to a flow rate limitation through the hopper outlet.

3. Segregation

Many bulk materials handled in silos consist of a blend of particles of different size, density, or chemical composition. If the material discharging from your vessel has wide variations in these properties over time, it can result in quality control problems or affect downstream operations.  

4. Limited live capacity

If ratholing occurs in your bin or silo, the live or usable capacity of the vessel is limited to the volume of the rathole. This volume can be as little as 10% of the silo volume, thus a silo which ratholes is an uneconomical storage vessel that needs to be refilled frequently.


SOLVING RELIABILITY ISSUES

In a project, the ability to influence the final cost, quality, and schedule is the highest during testing and concept development, while the cost of any change is at its lowest (Figure 1). The ability to influence the cost, quality, and schedule decreases rapidly as the project progresses, while the cost of any change increases significantly. Clearly, the earlier the project leverages the science developed by Dr Jenike the maximum the benefit can be.

Figure 1: Project timeline
Figure 1: Project timeline

As can be seen in Figure 2, cohesive strength can be affected by moisture content. Where moisture is low the cohesive strength of the material is also low, however, the material may be dusty and more frictional. In this case, it may be critical to measure the dust extinction moisture content to determine the minimum moisture for which no dusting occurs. Alternatively, if the material is fine, then there is potential for flow rate limitation, a result of low permeability of the material, or in extreme situations, there is a potential for flooding, caused by aeration of the material.

Figure 2: Cohesive strength and moisture content relationship
Figure 2: Cohesive strength and moisture content relationship

As the moisture content of the material continues to increase, so too does the strength of the material. This leads to greater arching and ratholing potential as the material sits at rest in a storage silo or hopper. Eventually, the moisture increase may make it impossible to discharge material out of a bin through gravity alone. 

Sometimes, material may gain enough moisture that it becomes saturated and, as the water acts as a lubricant, the material begins to lose strength. There may be a concern however that, at this moisture level, the material may become liable to liquefaction. 

Bulk solids’ flow properties can be measured using standard material flow properties testing methods, according to ASTM D6128. Measurements such as compressibility, cohesive strength, wall friction, permeability, and abrasive/wear characteristics, are among the many properties that can be measured to characterise the material and inform design. [3] Often these characteristics will vary as a function of parameters such as moisture content, temperature, chemical composition, and particle size, shape, and distribution. These variations must be considered in development of a robust flow properties testing program to accurately inform the subsequent design of any storage silo and feeder. 

DESIGN FOR FLOW

Once you have determined the flow properties of your bulk solid, you can use proven procedures to design silos that will result in reliable discharge. The first step is to determine the appropriate flow pattern, mass flow or funnel flow. Funnel flow bins have stagnant material along hopper walls that extends into some portion of the cylinder or vertical sided portion of the bin. Mass flow occurs when all the material is in motion whenever any is withdrawn.  

Many bins discharge bulk solids in a funnel-flow pattern of discharge – i.e. where some of the material moves during discharge while the rest remains stationary (Figure 3). This first-in last-out flow sequence is acceptable if the material is relatively coarse, free-flowing, nondegradable, and if segregation during discharge isn’t important. If the material meets all four of these characteristics, a funnel-flow bin 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. Fluidized powders often cannot fully deaerate in funnel flow so material remains fluidized in the flow channel and floods while discharging. The first-in last-out flow sequence can cause some materials to cake, segregate, or spoil. In extreme cases, unexpected structural loading, such as when ratholes fail, may result in downstream equipment failure.

Figure 3: Major material flow patterns in a storage silo and hopper

Flow reliability issues such as those described can be prevented with storage vessels specifically designed to move materials in a mass flow pattern. With mass flow, the material flow and bulk density are uniform and reliable; there are no stagnant regions, so bin level indicators work reliably, and material doesn’t cake or spoil. The first-in first-out flow sequence minimizes segregation and material residence time is uniform, so fine powders can deaerate. Mass flow bins 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 mitigating segregation is important.  

The first thing to consider in designing for mass flow is the hopper slope. The smoothness of the wall liner surface affects wall friction [2,4] and generally, the smoother the surface the less frictional resistance. Lower friction allows design of less steep hopper walls. 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 [2] to select the appropriate hopper slope. (Note that the hopper angle differs for conical and slotted outlet hoppers.) 

The second thing to consider is outlet size. As noted above, there are two types of flow obstructions that can occur with bulk materials. For the first type, particle interlocking, the minimum outlet size required to prevent an interlocking arch is directly related to the size of the particles. As a rule of thumb, a circular outlet must be sized about six to eight times that of the largest particle size while for wedge shaped hoppers the opening width should be at least three to four times the largest particle size. 

For the second type, cohesive obstructions, the materials’ flow function must be determined to understand the materials’ cohesive strength. The strength is directly related to the ability of the bulk material to form arches and ratholes in bins and silos. Typically to overcome arching, conical outlets are twice as large as planar outlets.  

DON’T FORGET THE FEEDER

The last consideration is the outlet area, it must be fully live. Even the most carefully designed mass flow hopper can discharge in funnel flow if the feeder does not provide uniform withdrawal of material from the entire hopper outlet (Figure 4). 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 used at a hopper outlet to regulate flow and is detrimental to reliable flow as it prevents discharge from a portion of the hopper outlet.

Figure 4: Properly designed mass flow belt feeder

Understanding the flow properties for your bulk material and how equipment design affects flow patterns and possible development of flow obstructions in storage silos and feeders will ensure that you are protected against design flaws. Conditions at and below the hopper outlet are just as important as the overall hopper geometry. 

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. 

References

[1] Carson, J. W., “How to Improve the Reliability of Bins and Silos”, Plant Services, March 2006, pp 50-52

[2] Jenike A.W., Storage and Flow of Solids, University of Utah Engineering Experiment Station, Bulletin No. 123, Nov. 1964.

[3] Carson J. W., and Marinelli J., 1994. Characterize Bulk Solids to Ensure Smooth Flow, Chemical Engineering, Vol. 101, no. 4, April, pp 78-90.

[4] Royal, T. A. and Carson, J. W.:  “Fine Powder Flow Phenomena in Bins, Hoppers and Processing Vessels.”  Bulk 2000 (1991).  

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