Powder and Bulk Engineering, November 2000 35

Taking the guesswork out of silo design

Joe Marinelli A.O. Smith Engineered Storage Products Co.

The material flow problems created by a poorly de- signed silo can bring your production to a grinding halt. If you want to prevent flow problems, you need to work closely with the silo manufacturer to ensure that your new silo is properly designed to handle your material. After describing problems that can result from poor silo design, this article explains how to work with a silo manufacturer in designing a new silo. Sections cover understanding flow patterns, identifying your material’s flow properties, and using flow properties to design your silo. A related sidebar gives tips on choosing a feeder for your silo. (This article concentrates on steel and other metal silos; much of the information can also be applied to concrete silos.)

he way bulk solids processors and handlers order storage silos has been evolving in recent years. In T the past, when a company was ready to buy a silo,

tank, or other storage vessel, its engineering department or an outside consultant or engineering fm typically would recommend the silo’s design. The silo manufacturer usu- ally fulfilled the order based on this design recommenda- tion and concentrated on manufacturing responsibilities - typically, the silo’s fabrication materials and quality control during vessel fabrication.

But with belt-tightening trends across the industry - companies downsizing, engineering firms disappearing, and consulting services becoming more costly - silo

manufacturers are now providing more design services to companies like yours. Today, you can expect a silo manu- facturer to design your silo based on your material’s flow properties. This means that the manufacturer not only pro- vides quality materials and fabrication, but tests your ma- terial’s flow properties and translates the results into a silo design that promotes reliable flow.

The silo manufacturer’s design process is based on the sci- entific approach to bulk solids flow and storage developed by Andrew W. Jenike in the 1950s. This industry pioneer’s shear testing method is now the ASTM standard for estab- lishing a solid’s cohesive and wall friction properties.’ By basing your silo’s design on established solids flow sci- ence, the silo manufacturer can take the guesswork out of the design process.

Problems resulting from poor silo design A silo designed without following a scientific approach to bulk solids flow can have any of several discharge prob- lems. Common problems include flow stoppage, erratic dis- charge, flooding, a limited discharge rate, and segregation.

Flow stoppage and erratic discharge. Row stoppage and erratic discharge are common signs that a silo has been poorly designed for its application. These problems can lead to inconsistent product output, production intermp- tions, and - potentially - the silo’s structural failure.

Flow from the silo can stop (called a no-flow condition) when a gate is opened or feeder is started at the silo outlet. Commonly, this indicates that either an arch (also called a bridge or dome) or rathole (sometimes called a core or

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36 Powder and Bulk Engineering, November 2000

pipe) has formed in the silo. The arch, as shown in Figure 1 a, forms over the outlet and supports the entire silo con- tents. A flow aid that applies a force greater than gravity (such as a sledgehammer, vibrator, or air blaster) typically must be used to overcome the arch’s strength and force the material to flow.

When a rathole causes the no-flow condition, as shown in Figure lb, some material still discharges, but the mater- ial’s cohesive strength eventually causes the flow channel to empty out, resulting in what’s called astable ruthole. At this point, material flow stops and a flow aid must be used to collapse the rathole and restart flow.

Erratic discharge occurs when both an arch and a stable rathole form in the silo. The rathole commonly develops when the material begins flowing in the silo. Then when a flow aid collapses the rathole, the collapsing material forms an arch as it impacts the silo outlet. The arch now supports the entire silo contents, and the only way to get material flowing again is to use a flow aid. Once material is flowing, the rathole redevelops and the whole cycle re- peats, creating erratic flow. This problem results in erratic bulk densities, and the excessive loads can even damage the silo’s structural integrity.

Flooding. Flooding occurs when a stable rathole forms and additional material (typically a fine powder) is added to or falls into the flow channel from above. As the mater- ial falls into the channel, it becomes entrained in the chan- nel’s air and becomes fluidized (aerated). Because the feeder at the silo outlet is designed to discharge deaerated material, it can’t control the aerated material’s fluid-like flow. As a result, the material flows uncontrollably from the silo outlet.

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2. Limited discharge rate. A limited discharge rate is typi-

occurs when material flows into the silo’s hopper section; as the material converges in the narrow hopper section, the air between the particles is squeezed out. This creates a vacuum, which draws air from outside the silo upward through the outlet and into the hopper section, counter to the discharging material, thus limiting the discharge rate. Increasing the speed of the feeder at the outlet can often solve this problem. However, because there’s a limit to how fast material can flow through a given opening, faster feeding can only help the limited discharge rate up to a cer-

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Segregation. Segregation occurs when a material com- posed of different particle sizes or densities separates dur- ing silo discharge. The major cause is sifting - that is, when fine particles flow downward between coarse parti- cles in the material. The same phenomenon can be seen

sizes: The fine particles will typically concentrate under the

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when a pile is formed from materials with different particle

fill point and the coarse particles will roll or slide to the out- side. Maintaining a material’s uniform particle size or den- sity or its blend ratio during silo discharge can be difficult because other segregation mechanisms -including air en- trainment and frictional drag (which separates particles sliding on a surface) -can also affect the material.

2.

Your material‘s cohesive strength is a physical, chemical, or electrical bond that can obstruct flow in a silo.

The keys to designing a silo that will smoothly discharge your material are understanding what types of flow patterns can develop in a silo and identifying your material’s flow properties. Your knowledge of both topics will help you work with the silo manufacturer to develop the silo design.

Understanding flow patterns Silos can be designed for one of three material flow pat- terns: funnel flow, mass flow, or expanded flow.

Funnelflow. A funnel-flow silo is limited primarily to handling coarse, free-flowing materials that don’t degrade over time and materials such as plastic pellets that are used in processes where segregation is acceptable. Why? The silo’s shape produces a flow pattern that prevents smooth discharge.

The funnel-flow silo typically has a shallow hopper sec- tion to meet low headroom requirements and reduce fab- rication costs. Examples of shallow hopper sections,

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which are typically made of carbon steel, include conical (typically 60 degrees or less) and pyramid types. These are shown in the funnel-flow silo designs in Figure 2. A silo with a flat bottom also produces a funnel-flow pattern.

Powder and Bulk Engineering, November 2000

The walls in this silo’s shallow hopper section aren’t steep or smooth enough to force material to flow along them. In- stead, friction develops between the wall surface and the material, preventing the material from sliding along the walls and causing some material to remain stationary while other material flows. A narrow flow channel forms in the silo, usually directly over the outlet, and the first ma- terial to flow into the silo is usually the last to flow out (calledfirst- in lust-outflow) .

The funnel-flow silo is also subject to other problems:

*If the narrow channel empties out, a stable rathole can form. The stable rathole can decrease the silo’s live (us- able) capacity, produce areas of stagnant material that can cake or spoil, increase segregation problems, or cre- ate stresses that can cause the silo’s structural failure.

If the stored material has enough cohesive strength, it can form an arch over the outlet, stopping flow.

These problems explain why the silo is chiefly suited to coarse, free-flowing materials that aren’t likely to degrade over time and plastic pellets and similar materials that can segregate without affecting later processing.

Mussflow. A mass-flow silo’s shape enables it to handle cohesive materials, materials that degrade or spoil over time, easily segregated materials, and fine powders. The silo typically has a taller hopper section than a funnel-flow

silo does. Mass-flow silos with steep conical and wedge- shaped transition hopper sections are shown in Figure 3. The hopper sections are typically made of stainless steel or a coated steel to provide smoother walls. The conical sec- tion has a circular outlet (Figure 3a); the wedge-shaped transition hopper has a slot-shaped outlet (Figure 3b).

Because the mass-flow silo’s hopper section walls are steep and smooth, they overcome the friction that develops be- tween the wall surface and material. All material in the silo is in motion whenever any material is discharged, and ma- terial flows to the outlet in a first-in first-out sequence.

The mass-flow silo outlet is sized to prevent arching. Al- though mass flow itself can’t prevent arching, knowing your material’s flow properties enables you to design a large enough outlet to prevent arching. This makes the silo suitable for cohesive materials and materials that degrade or spoil over time. The silo also minimizes segregation by reuniting fine and coarse particles at the silo outlet, mak- ing the silo suitable for easily segregated materials. And as long as they are in the silo long enough to deaerate, fine powders can’t flood from the mass-flow silo.

Expandedflow. An expanded-flow silo, as shown in Fig- ure 4, has a combination hopper section that consists of an upper funnel-flow section on a lower mass-flow section. The mass-flow pattern in the mass-flow section expands into the upper funnel-flow section, creating a flow pattern that combines the best characteristics of funnel-flow and mass-flow silos: the funnel-flow silo’s headroom savings with the mass-flow silo’s first-in first-out flow.

If the silo’s upper funnel-flow section is properly de- signed, the material will move in a funnel-flow pattern but won’t form a stable rathole because the section’s outlet is

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large enough to destabilize a rathole. The mass-flow sec- tion must have a steep enough wall angle and large enough outlet to provide mass flow. Because of its high fabrication costs, the expanded-flow design is cost-effective only for silos 25 feet in diameter or larger, but the design is suitable for virtually any material.

Identifying your material’s flow properties The silo manufacturer will run a series of flow property tests on your material to evaluate how it will flow. These tests measure the material’s cohesive strength, wall fric- tion properties, compressibility, and permeability.*

Cohesive strength. Your material’s cohesive strength is a physical, chemical, or electrical bond that can obstruct flow in a silo. Many materials flow like a liquid when poured from a container. Under such conditions, the mate- rial has no cohesive strength. But when you squeeze and compact such a material in your fist, it may gain enough cohesive strength to retain your hand’s shape after you open your fist.

In the cohesive strength test, the silo manufacturer uses a bench-scale lab testing device, such as a direct shear tester, to measure your material’s cohesive strength as a function of applied consolidation pressure. A material sample is placed in a shear cell on the shear tester and then both com- pressive and shear loads are applied to the sample to simu- late flow conditions in a silo. After the compressive load consolidates the sample, shear pressure is applied to it until the consolidated sample fails (collapses). The point at which this occurs provides a measure of the material’s cohesive strength.

Powder and Bulk Engineering, November 2000 39

This procedure is then repeated under different conditions, such as different temperatures and storage times at rest, providing a value of cohesive strength versus consolidating pressure (the material’sflow&nction). These results help the manufacturer determine how large the silo outlet must be to prevent your material from arching or ratholing.

Wall&ctionproperties. A material’s wall friction proper- ties are expressed as a wall friction angle (or coeficient of slidingfriction). The lower your material’s wall friction angle, the less steep the walls in your silo’s hopper section must be to provide mass flow.

The silo manufacturer can measure the wall friction angle with the direct shear tester or similar device. The test deter- mines how much force it takes to slide a material sample across a stationary wall surface; the friction that develops between the material and wall surface resists this force. For a given wall surface and material, the wall friction angle isn’t necessarily constant but often varies with normal pressure (the pressure perpendicular to the wall surface), usually decreasing as normal pressure increases. The man- ufacturer can use your material’s wall friction angle to de- termine the wall angle your silo’s hopper section requires to achieve mass flow.

Compressibility. Bulk density is an essential value in ana- lyzing a material’s flow properties and is often character- ized as either loose or packed bulk density. But a material has more than two bulk densities; in fact, a material’s bulk density inside a silo has a range of values depending on the consolidation pressure applied to it.

The compressibility test measures this range of bulk densi- ties for your material. In the test, the silo manufacturer compresses a material sample in a small cup and measures the material’s deflection as a result of the compression. The test is repeated at various compression loads.

The manufacturer will use your material’s resulting val- ues, called compressibility values, to calculate the hopper section angle and outlet size required to achieve mass flow in your silo. The values also determine the silo loads, which the manufacturer will use to specify the silo’s wall thickness and fabrication method.

Permeability. The silo manufacturer determines your ma- terial’s permeability by passing air or another gas through a representative column of the material. The gas flowrate is measured while the pressure across the material column is regulated. This allows the manufacturer to determine the material’s permeability as a function of its bulk density.

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The manufacturer uses this permeability value to calculate the limiting discharge rate of your material in a mass-flow

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silo at various outlet sizes. This allows the manufacturer to choose a silo outlet size that will provide the discharge rate you need.

Using flow properties to design your silo As you can see, the flow properties identified by the cohe- sive strength, wall friction properties, compressibility, and permeability tests all play apart in your silo’s design.

The first three measurements, along with flow factor val- ues from silo design charts: are used to calculate the min- imum outlet size that your silo must have to prevent your material from arching or ratholing.

The wall friction angle also determines the wall angle the hopper section must have to achieve mass flow with your wall surface and material. Used in conjunction with data from the silo design charts, this data yields the required wall angle for either conical or wedge-shaped hopper sections.

*The results of all four tests - along with information about silo loads, dynamic effects, and flow patterns - help the manufacturer design the silo’s structure to prevent failures ranging from a dent in a steel silo’s shell to the silo’s catastrophic collapse. Basing your silo’s design on the silo loads and dynamic effects your material applies is critical because another material can produce completely different loads and effects in the same silo. Load problems can also result when your silo outlet design is incompati- ble with the rest of the silo design - for instance, an outlet at the silo’s side can impose asymmetric loads in a silo that’s designed to discharge from the center. Load prob- lems can also develop when a mass-flow pattern develops

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in a funnel-flow silo, because the loads applied by mass flow are greater than those applied by funnel flow and can jeopardize the silo’s structural integrity.

By using your material’s flow properties and other appli- cation requirements to design your silo, the silo manufac- turer can provide a silo that not only will work well at startup but provide years of reliable service. PBE

References 1. American Society for Testing and Materials standard D 6128-97,

available from ASTM, 100 Barr Harbor Dr., West Conshohocken, PA 19428; 610-832-9500, fax 610-832-9555 (www.astm.org).

2. J.A. Marinelli and J.W. Carson, “Characterize bulk solids to ensure smooth flow,” Chemical Engineering, April 1994.

3. A.W. Jenike, Storage and Flow of Solids, Bulletin 123, University of Utah Engineering Experimentation Station, Salt Lake City, November 1964.

For further reading Look for information on selecting steel and concrete silos in the article, “Storage silos-steel or concrete?,’’ else- where in this issue. Find more information on silo design and other storage topics in articles listed under “Solids flow” and “Storage” in Powder and Bulk Engineering’s comprehensive “Index to articles” (in the December 1999 issue and at www.powderbulk.com).

Joe Marinelli, who leads the consulting$rm Solids Han- dling Technologies, FortMill, S. C. prepared this article for A. 0. Smith Engineered Storage Products Co., PO Box 996, Parsons, KS67357; 316-421-0200, fax316-421-9122.