400 Business Park Drive 3485 Empresa DriveTyngsboro, MA 01879-1077 San Luis Obispo, CA 93401-7328tel: (978) 649-3300 fax: (978) 649-3399 tel: (805) 541-0901 fax: (805) 541-4680

Also: Toronto, Canada and Viña del Mar, Chilewww.jenike.com

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June 18, 2009

Mr. Dennis A. Blauser, CEOMarietta Silos, LLC2417 Waterford RdMarietta OH 45750

RE: Advantages of reinforced concrete silos

Dear Mr. Blauser:

You have asked Jenike & Johanson, Inc. to comment on the advantages of reinforced concrete overmetal (typically steel or aluminum) when used to construct large (30 ft diameter and larger) silos to storebulk solids such as fly ash, cement, lime, etc. In this letter report we focus on design and operationalissues, not cost. Both slip form and jump form construction are considered.

First, some caveats:

1. We assume proper design of the silo. This includes [1]1:a. Operating requirements and conditions are thoroughly defined before design is started.

This includes such factors as silo capacity, discharge rate and frequency, mixture andmaterial uniformity, material friability, pressure and temperature differences, safety andenvironmental concerns, etc.

b. The range of relevant properties of the bulk solid (or solids) to be stored is known. Thisusually requires flow property tests to be performed by a specialized testing laboratory.

c. The silo’s functional design has been generated by engineers experienced in solids flow.d. Detailed design has been performed by competent silo engineers, with consideration

given to all reasonably expected loading conditions.2. We assume proper construction. The silo must be constructed in close agreement with the

detailed design, and deviations, if any, must be approved by the silo design engineer.3. We assume proper silo operation. Routine inspections must be carried out. [2] Any changes in

the bulk solid being stored or in operating conditions must be properly considered before

____________________1 Numbers in brackets refer to References at end of text. A copy of each paper is appended.

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implementation, and any signs of distress must be promptly investigated with appropriate actiontaken.

Before commenting on the benefits of reinforced concrete construction, it is perhaps helpful to providesome background concerning horizontal and vertical loads that must be considered in designing a silo.[3] Horizontal loads are due to hoop tension, out-of-round flexure in a circular wall, and out-of-planeflexure in a flat wall. One of the leading causes of the latter two phenomena is non-uniform pressuresthat result from eccentric fill or withdrawal from a silo. Vertical loads are due to compression from theaccumulated frictional drag of the silo contents, and also compression (and tension) due to overturningcaused by external loading such as wind and seismic.

In a reinforced concrete silo, the main criterion governing wall thickness is horizontal loads. Verticalloading is a major consideration only at wall openings, columns, pilasters, etc. In metal silo design, onthe other hand, the first consideration is always vertical loading, as explained below.

What then are the advantages of reinforced concrete over metal (bolted or welded) for silo construction?

From our experience, a reinforced concrete silo’s ability to withstand the effects of eccentric loads is oneof its main advantages over metal, especially when considering large silos.

A silo wall constructed of concrete is reinforced by steel reinforcing bars (rebar), and the amount ofrebar is proportioned to carry the hoop tension. It is easy to vary the area of rebar in a unit height of thewall as the hoop tension requirement varies. This is achieved by varying the bar size, spacing, or both.The amount of rebar can be increased to account for the effects of loads other than hoop tension, such asflexure due to non-uniform pressures, connectivity with other structures, thermal loading, seismic orwind loading, etc.

The situation is very different with metal silos. The first consideration when designing such a silo isvertical loading. The critical longitudinal buckling stress for a cylindrical shell (i.e., the strength of acylindrical silo wall in vertical compression) is inversely proportional to its radius of curvature. Out-of-round bending causes the radius to be both increased and decreased, at different places. It is imperativethat a proper analysis is made to predict possible changes in radius, both anticipated and accidental. If asuitable plate thickness to provide the required buckling resistance is uneconomical, vertical stiffeners(internal or external) are required. A flat wall should always be reinforced with vertical stiffeners.

Vertical buckling of inadequately stiffened metal silos causes more silo failures than any othermechanism. [4] Once a silo wall has buckled, it cannot be restored but must be replaced.Buckling often leads to fracture of joints between plates, followed by rapid collapse of the silo.

Once the plate thickness for a cylindrical metal silo wall has been selected according to vertical bucklingstrength requirements, the next check is the thickness and connections -- welded or bolted -- towithstand applied hoop tension. Where non-uniform pressures are present, it is unlikely (except in smallsilos) that an increased plate thickness can be found that provides a large enough section modulus toresist out-of-round flexure. Therefore, external ring stiffeners are usually required to provide flexuralstiffness, and these are often found to be quite heavy members. A flat wall always requires externalstiffeners.

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External horizontal stiffeners have additional problems. One must consider that the effects of suchreinforcement are localized so if they are too widely spaced, deformation may occur between them.Additional welding or bolting is required, which drives up cost. Finally, the presence of external ledgesprovides significant area for fugitive material to build up, which can result in corrosion that is hiddenfrom view until a dangerous situation has developed.

Other advantages of reinforced concrete for silo construction include:

1. Theses silos have good resistance to corrosion. This includes both corrosion of internal walls dueto the stored bulk solid and also external corrosion caused by moisture. Metal corrosion is a wellknown problem.

2. There is no concern about electrolytic effects at welds or liner connections.3. Careless detailing of metal walls may leave inward facing ledges or welds, which can obstruct

flow and increase wall pressures. This is avoided with concrete.4. Concrete is better able to resist abrasive wear than most metals.5. Concrete is more robust and thus better able to withstand impact loads.6. Concrete has higher wall friction angles with most bulk solids than most metals. This results in

higher frictional drag down the cylinder walls and hence lower pressures acting normal (i.e.perpendicular) to cylinder and hopper walls.

7. It is possible to construct interconnected structures using reinforced concrete. This allows forinterstices that can be used for storage and/or process equipment.

8. There is no concern about weld quality or stress risers, such as bolted connections.9. There is no concern about leakage to the environment, which can be a problem when storing fine

powders in bolted silos.

In summary, reinforced concrete has many advantages over metal when designing and building largesilos for the storage of bulk solids. As with any structure or piece of equipment, such silos must beproperly designed, built and operated in order for these advantages to be realized.

Please contact us if you have any questions or if we can be of further assistance.

Sincerely,

John W. Carson

John W. Carson, Ph.D.PresidentTyngsboro, Massachusetts

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References

1. Purutyan, H., B.H. Pittenger, and J.W. Carson: Six Steps to Designing a Storage Vessel thatReally Works, Powder and Bulk Engineering, November 1999, Vol. 13, No. 11, pp. 56-68.

2. Carson, J. W. and Jenkyn, R. T.: How to Prevent Silo Failure with Routine Inspections andProper Repair. Powder and Bulk Engineering, Vol. 4 No. 1, January 1990.

3. Carson, J. W. and R. T. Jenkyn: Load Development and Structural Considerations in SiloDesign. Presented at Reliable Flow of Particulate Solids II, Oslo, Norway, August 1993.

4. Carson, J.W.: Silo Failures: Case Histories and Lessons Learned. Proceedings of The 3rdIsraeli Conference for Conveying and Handling of Particulate Solids, Vol. 1, p. 4.1-4.11, 2000.

Published in Powder and Bulk Engineering, November 1999 www.powderbulk.com

Designing a storage vessel for your plant requires amethodical approach. This article outlines six stepsto follow in designing, installing, and starting up astorage vessel that successfully handles your bulkmaterial under your operating conditions.

Have you experienced delays in starting up a new pro-cessing system? Or is your existing or new process-ing system performing poorly? Often the main

culprit is a poorly designed storage vessel — such as a bin,silo, or hopper — somewhere in the system. An improperlydesigned storage vessel is also more likely to fail structurallythan other plant equipment and is more prone to dust explo-sions or fires and to releasing hazardous emissions. Theseproblems produce unsafe conditions for your workers andthe community surrounding your plant.

Factors behind a poor design

What leads to improper vessel design? One cause is con-sidering your vessel design after other system equipmenthas been selected. Another common mistake is designingthe vessel without fully investigating your material’s flowproperties. Knowing your material’s name, bulk density,particle size distribution, and angle of repose just isn’tenough.

Relying on your past experience in selecting storage ves-sels can also lead to a poor vessel design. Why? You typi-cally need a new vessel because your material’scharacteristics or your operating conditions, or both, havechanged.

Designing a vessel based on an inadequate budget that’sset before the design process really starts can also result in

a poorly functioning vessel. In a project’s early stages, theengineer in charge is often expected to quote a cost for thevessel based on as little information as “the vessel will be a14-foot-diameter silo with a cone hopper.” Such a simplis-tic approach can make it hard to go back and increase thebudget. So the vessel matching those vague initial specifi-cations can be the vessel you’re stuck with in the end.

Steps in properly designing, installing, and startingup a vessel

Avoid vessel performance problems by following a de-tailed, systematic approach to designing, installing, andstarting up your storage vessel. The steps are:

1. Define your operating requirements and conditions.

2. Test your material’s flow properties.

3. Develop the vessel’s functional design.

4. Develop the vessel’s detail design.

5. Fabricate and install the vessel.

6. Start up and maintain the vessel.

Depending on your company’s size and whether you’redesigning a vessel for an existing or new process or for anentirely new plant, the testing, engineering, fabrication,and installation services you need to contract as you fol-low these steps can vary widely. For instance, if you’readding a vessel to an existing plant, you can hire an inde-pendent firm to test your material’s flow properties and dothe functional and detail designs, and then hire a fabricatorto build and perhaps install the vessel at your site. If the

Six steps to designing a storage vessel that really worksHerman Purutyan, Brian H. Pittenger, and Dr. John W. Carson Jenike & Johanson

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vessel will be part of a new plant or major plant addition,the engineering consulting firm managing your projectmay handle the flow property tests and the functional anddetail designs and work with a fabricator or vessel supplierto fabricate and install the vessel.

The following sections explain how you can follow eachstep and avoid pitfalls along the way.

1Define your operating requirements andconditions.

Identify your operating requirements and conditions beforeyou design the storage vessel. Among the most importantfactors to consider are capacity, discharge rate and fre-quency, mixture and material uniformity, material friability,pressure and temperature differences, safety and environ-mental concerns, and construction materials. Your applica-tion may require you to consider other factors as well.

Capacity. First consider your storage vessel’s required ca-pacity. For help in setting this capacity, look at your plant’sbusiness or operating strategies. For instance, a growingtrend in many plants is to reduce raw material inventoriesto free up working capital. If this is the case in your plant,your storage vessel may require a relatively small capacity.

If the storage vessel will be at your process’s front end, thevessel capacity may be dictated by the raw material’s de-livery schedule, shipping container type and size, andusage rate. For instance, if your plant receives one truck-load of material per day, one relatively small silo may beenough. But if a larger quantity is delivered by train or shiponce a month, you may need a much larger vessel (or mul-tiple vessels) to store it.

If your vessel will be located at an intermediate processstep, base the vessel’s capacity on your process require-ments. For instance, the vessel may need to hold enoughmaterial to prevent shutting down a furnace or reactorwhen an upstream problem temporarily halts materialflow. Or you may need to base the vessel capacity on thequantity of material a batch step requires or the quantityneeded to even out differences in the rates of two processsteps.

If your vessel is located at the process’s back end, base thevessel capacity on your plant’s shipping schedule, productorders or sales cycles, shipping container type and size,and your plant’s business strategies (such as a just-in-timeshipping policy).

Discharge rate. Regardless of where your storage vessel islocated, it must deliver material to a downstream process ata required rate. You need to specify the required dischargerate early in the design process and communicate it clearlyto the project engineer. For instance, are you stating the dis-charge rate as an average rate? How did you determine it?Is it based on volume (such as cubic feet per hour) or mass(such as pounds per hour)? Be specific: If your downstreamprocess requires 10 t/h of material but the vessel will dis-charge material only four times per hour for 5 minutes at atime, the instantaneous discharge rate the vessel must pro-vide is 30 t/h. A vessel that can discharge material at only10 t/h won’t be able to deliver enough material to yourprocess in those four 5-minute periods.

Also consider the minimum and maximum discharge ratesyour vessel must provide in both normal and upset condi-tions. Some processes are much more sensitive to dis-charge rate variations than others. For instance, suchvariations may not be important if your process transfers acertain-size batch after a given time. But if your processcombines multiple material streams, each from a differentvessel, into one mixture, each vessel must have a uniformdischarge rate to maintain the proper proportion of ingre-dients in the mixture.

An improperly designed storage vessel is more likelyto fail structurally than other plant equipment. Whenthe hopper section in this corn silo failed, falling corncreated a vacuum that sucked the silo’s top inward.

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Discharge frequency. Specify the vessel’s discharge fre-quency early in the design process. When a material isstored over time, some of its flow properties can change.Ensure that your vessel is designed to handle thesechanges by considering how long your material will bestored in the vessel between discharges. Will your vesselbe used in a one-shift-per-day operation that leaves mater-ial at rest in the vessel overnight? Will your process shutdown for weekends, leaving some material in the vessel?During planned shutdowns, will you empty the vessel orwill you leave material in it? How long will material re-main in the vessel during a shutdown — 1 week? Longer?

Mixture and material uniformity. If your vessel will holda mixture consisting of several ingredients, your processmay require that the mixture remain uniformly mixed dur-ing storage and discharge. If your vessel stores ingredientsfor a dry salad dressing mix, for instance, it should dis-charge all ingredients together in the right proportionsrather than herbs first, seasonings next, and croutons last.If your vessel stores dry ingredients for cement, it shoulddischarge the limestone and clay together rather than oneafter the other to your mill.

If the storage vessel will be at your process’s front end,the vessel capacity may be dictated by the rawmaterial’s delivery schedule, shipping container typeand size, and usage rate.

If your vessel will store only one material, you still mayneed to be concerned about maintaining the material’s par-ticle size uniformity during storage and discharge. Coarseand fine particles with the same chemical content can per-form quite differently. If your downstream process is de-signed to handle a wide particle size range, design thevessel to prevent the discharge of only all fines or all coarse.

Off-spec material or dust that’s returned from the processto your vessel can also affect the material’s uniformity.Early in the design process, consider whether off-spec ma-terial or dust must be returned to your vessel and how it canbe returned to prevent affecting the material’s uniformity.

Material friability. If your material is friable, a poorly de-signed vessel will degrade it. For instance, detergent ag-glomerates can break up during vessel loading anddischarge, compromising the final product’s quality. Attri-tion of pasta or cereal flakes during loading and dischargecan result in scrapped product.

Pressure and temperature differences. Your material canbehave differently depending on the gas pressure and tem-perature it’s exposed to. Identify the gas pressure in equip-ment upstream and downstream from your vessel. If thesediffer from the pressure inside the storage vessel, they canaffect your material’s flow properties. The same is true ifyour process operating temperature is different than thetemperature inside the storage vessel. Determine the tem-perature conditions, including minimum and maximumambient temperatures (especially for outdoor storage) andminimum and maximum incoming material temperatures,that can affect your material’s behavior so the vessel youdesign can handle these conditions.

Safety and environmental concerns. Determine whetheryour material is likely to explode or burn. For instance,materials such as coal and grain generate flammable or ig-nitable dust. Others such as polyethylene and polypropy-lene can contain volatiles. Use this information to designthe vessel with adequate explosion- and fire-protectionfeatures (such as an explosion vent or explosion-suppres-sion system) or to decide whether to use an inert gas (suchas nitrogen) in the vessel.

Consider whether material spilled from the vessel or fugi-tive dust or fumes released from it can injure your workersor pollute the environment. Also determine whether conta-minants, atmospheric gases, humidity, and temperature canadversely affect the material stored in your vessel. In eithercase, design the vessel to safely handle these conditions.

Construction materials. Your material’s chemical compo-sition and other properties can limit the choice of construc-tion materials for your vessel. An abrasive material canwear some wall materials. A material containing a corrosivesubstance such as a salt or acid may require a vessel withepoxy-coated walls. Residue from previous materials in up-stream equipment can also affect the walls. For instance, anacid not completely removed during equipment washdownmay linger in upstream process equipment, travel with yourmaterial into the vessel, and corrode the walls.

2 Test your material’s flow properties.

Testing to identify your material’s flow properties is criti-cal to successfully designing your storage vessel. Run testson a representative sample of your material under condi-tions that match the worst-case conditions you expect thematerial to be handled in. For instance, if you expect yourmaterial to degrade during pneumatic conveying into thevessel, test a sample of material that’s been pneumaticallyconveyed under the same conditions.1

You also need to test the material’s flow properties underthe conditions that will be present in your vessel. Whenpossible, obtain samples from your vessel supplier of the

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wall materials you’re considering so you can test the bulkmaterial’s behavior when flowing along these surfaces.

Also get samples of your bulk material from the materialsupplier you’ll use rather than another supplier. Materialswith the same chemical composition from different sup-pliers can have quite different flow properties. If the parti-cle size, shape, moisture content, or other properties of thesample differ from those of your actual material, the testresults won’t be of much help in designing your vessel.

If you can’t obtain a sample of your material because ithasn’t been produced yet, you may be able to test a samplefrom a pilot plant. Although your production-grade mater-ial may be different than this sample, testing the pilot-plantsample can help you at least establish a baseline for de-signing the vessel.

Vessel diameter can be limited by the space availablein your plant or the vessel’s construction method.

If you can’t get a material sample because you haven’t yetidentified a supplier for it, you can obtain a range of flowproperty data for the material from others who have con-ducted flow property tests. For instance, to design a silothat will receive coal from many locations around theworld, you can survey flow properties of coal from diverselocations to at least identify some vessel design basics.While clearly less accurate than testing the actual coalsamples, using this method is better than making your de-cision without any coal data.

If your vessel will hold materials from several sources orhold several grades of one material, run a series of tests onsamples of each material or grade. The tests can identifywhich samples have extreme flow properties that will af-fect your vessel design.

Before running tests, consider which flow properties youneed to identify for designing your vessel. Distinguish be-tween tests that provide qualitative, relative data and teststhat provide quantitative, absolute data. For instance, testsfor angle of repose, flow time through a funnel, and com-paction ratios will generate qualitative, relative data that,at best, may help you find differences between samplesbut won’t help you design the vessel.

Instead, use quantitative, absolute tests that identify flowproperties important to vessel design. These include testsof the material’s cohesive (shear) strength, compressibil-

ity and permeability, segregation tendencies, and abrasive-ness. Wall friction (friction between the material and ves-sel wall) is another important test. For most of these tests,you can use the Jenike shear tester, adopted as the onlystandard flow property test device by the American Soci-ety for Testing and Materials (ASTM), International Stan-dards Organization (ISO), and European Federation ofChemical Engineering.

Conduct the tests under the conditions in your process thatare most likely to adversely affect your material’s flow.For instance, a material generally becomes harder to han-dle as temperature increases (although freezing can alsomake a material harder to handle). Increasing the mater-ial’s moisture content, increasing its storage time at rest,and decreasing its particle size also can cause flow prob-lems. So run your tests with materials at the maximumtemperature and moisture and after the maximum storagetime at rest that you expect in your process. If your mater-ial has a wide particle size range with a significant portion(15 to 20 percent or more) of particles less than 1⁄4 inch,conduct the flow tests on these fine particles only.

3 Develop the vessel’s functional design.

Consider your operating requirements and conditions andthe material’s flow properties to develop your vessel’sfunctional design. This functional design specifies the fea-tures the vessel needs to function effectively in your appli-cation. The vessel features that will be designed duringthis step include:

• Cylinder height, diameter, and construction material.

• Hopper shape, slope, and construction material.

• Outlet size.

• Feeder type and size (including details for activating theentire outlet, if necessary).

Optional features that you also can determine at this stepinclude:

• Discharge valve type (slide gate, butterfly, and so on)and size.

• Hopper insert or flow aid type, location, size, and con-struction material.

To determine the vessel’s maximum diameter and height,consider your site conditions and construction factors.Vessel diameter can be limited by the space available in

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your plant or the vessel’s construction method. For in-stance, a vessel that will be fabricated in a supplier’s shopprobably can’t exceed 14 feet in diameter to be transportedin one piece to your site. Vessel height can be limited byyour surrounding structure’s height, your area’s seismic orwind-loading conditions, the amount of associatedprocess equipment that must be located above or near thevessel, on-site construction factors (such as crane size), orthe vessel foundation design (which can be limited by thearea’s soil conditions). These limitations can also deter-mine whether you need to install one or more vessels.

The key factor to consider in selecting the vessel’s otherfunctional design features is the appropriate flow patterninside the vessel. Several vessel features — including theoutlet size, cylinder and hopper shapes, hopper wall slope,hopper surface material, feeder (located at the outlet), andany required valves, hopper inserts, or flow aids — affectthe flow pattern.

The two primary flow patterns in a vessel are funnel flowand mass flow, as shown in Figure 1. In funnel flow (Fig-ure 1a), an active flow channel forms above the outlet withnonflowing material around the vessel periphery. The re-sult is a first-in last-out flow sequence, with potential cak-ing of stagnant material and sifting segregation in whichfines typically exit first. This produces uneven dischargewith inconsistent bulk density and uncontrolled flow. Asthe material level in the vessel drops, layers of the non-flowing material may or may not slide into the flowingchannel. This can produce a stable rathole in the vessel,where material outside the channel remains stagnant.

In mass flow (Figure 1b), all the material is in motionwhenever any is withdrawn from the vessel. Material fromboth the vessel center and periphery moves toward the out-let. This provides a first-in first-out flow sequence, elimi-nates stagnant material, reduces sifting segregation, andprovides a steady discharge with consistent bulk densityand uniform, well-controlled flow.

Despite the advantages of mass flow, it isn’t always possi-ble, practical, or cost-effective to install a vessel that pro-vides it. In fact, a vessel that provides funnel flow can beacceptable for some applications. For instance, you canuse a funnel-flow vessel if your material is free-flowingand coarse enough to prevent aeration and if your processwon’t be affected by mixture segregation, particle sizesegregation, or first-in last-out flow.

But when these conditions aren’t acceptable, your vesselmust provide a mass-flow pattern. Achieving mass flowrequires a vessel outlet that’s large enough to prevent ma-terial from arching (also called bridging) over the outlet,hopper walls that are smooth and steep enough to promotematerial flow at the walls, and an outlet that’s entirely ac-tive (which requires choosing a feeder that allows this).

Figure 1

Flow patterns

a. Funnel flow

Flowingmaterial

Stagnantmaterial

Flowingmaterial

b. Mass flow

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The minimum outlet size that can overcome arching is di-rectly related to your material’s cohesive strength. Use thecohesive strength test results you obtained in step 2 to cal-culate a minimum outlet size. Also consider whether a cir-cular or elongated outlet is better for your application.

Your required discharge rate also affects the outlet size. Ifthe material — particularly a fine powder — deaerates inthe vessel, the discharge rate can slow greatly. Use the per-meability test results from step 2 to calculate your mater-ial’s discharge rate for various outlet sizes.

The shape, slope, and construction materials of surfaces incontact with your material also affect the flow pattern.How cohesive and frictional your material is determineswhat hopper geometry — cone, wedge, transition, or other— your vessel will have. Because a cone hopper must be10 to 12 degrees steeper than a wedge or transition hopperof the same construction material, the wedge or transitionhopper may be more appropriate if you have limited head-room. The outlet width in the wedge or transition hoppercan also be equal to one-half the cone hopper’s outlet di-ameter while having the same effect in preventing arching,so if your material’s cohesive strength would require alarge circular outlet, using a wedge or transition outlet maywork fine and also save headroom.

Selecting the proper feeder for the vessel outlet is also im-portant because mass flow can’t occur if the feeder can’twithdraw material from the entire outlet. A lip, ledge, par-tially open gate, or mismatched flange at the outlet canalso be fatal to mass flow.

To choose a feeder, consider whether your material is fineor fluidizable. Most dry solids feeders can’t hold back flu-ids. For instance, a screw feeder can’t contain a fluidizedmaterial, so if the material in the vessel never settles duringfilling or becomes fluidized, it will flood out of the feeder.

If your vessel will have a unique design and you don’tknow all your operating conditions or material flow prop-erties, you can test the vessel and feeder design in a scalemodel. However, you need to understand which of yourmaterial’s flow properties can be scaled down, because notall of them can. For instance, wall friction results can bescaled down, but the results for the minimum outlet size toovercome arching can’t.

4 Develop the vessel’s detail design.

Developing your vessel’s detail design requires selectingits construction materials, fabrication and installationmethods, and structural design. Once the detail design iscompleted, your design engineer can generate engineeringdrawings of the vessel. The drawings will be used to fabri-

cate the vessel and in many cases are given to firms submit-ting bids for the vessel fabrication. To ensure that all rele-vant functional design details from step 3 are properlyincorporated into the detail design, you must communicateall functional design points from step 3 to the engineer.

For instance, your functional design may specify a Type304 stainless steel hopper surface with a number 2B finish,but if you don’t communicate all the details clearly, the en-gineer may interpret this specification as simply a Type304 stainless steel hopper surface. This means you mayend up with a surface fabricated from mill finish or num-ber 1 finish Type 304 stainless steel, which is much morefrictional than a number 2B finish surface and will causeflow problems.

Determining how to fabricate your vessel is one of the firstthings to consider in the detail design. The three most com-mon fabrication options are reinforced concrete, weldedsteel or aluminum, and bolted steel or aluminum.

A reinforced concrete vessel is typically used for large-ca-pacity applications. It typically has a 35-foot or larger di-ameter and a total volume greater than 30,000 cubic feet.The vessel is often fabricated in a bank or group of multi-ple vessels. A welded steel or aluminum vessel can be ofvarious sizes and fabricated in the shop or at your site.Shop fabrication, which provides close control of thewelding process, is an option only for a vessel with a diam-eter of 14 feet or less. This fabrication method also allowsa special vessel coating such as epoxy to be applied (andcured, if necessary) before the vessel is shipped. A boltedsteel vessel can have a 100-foot or greater diameter andconsists of shop-fabricated (and, sometimes, shop-coated)pieces assembled in the field.

To select a fabrication method, consider the constructionmaterial and surface coating or lining you need, the costand installation (or shipment) method for each type of fab-rication, and the service life of vessels fabricated by eachmethod.

Also consider your vessel’s installation requirements inthe detail design. If your vessel will be in a hard-to-accessarea and you have a limited amount of downtime to installthe vessel, it may have to be designed and built in sections.For instance, designing a vessel with a hopper in multiplesections may eliminate having to move walls, other equip-ment, pipes, and electrical components in your plant andreduce the vessel’s installation time. While the hopper’scost will be more than that of a one-piece unit, the overallproject’s cost can be reduced because installing the vesselin sections will require less downtime.

You need to determine the vessel’s structural design, too.This requires calculating the pressures that your material

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Sometimes a shop-fabricated vessel is shipped with a pro-tective coating. Before you load material into such a ves-sel, remove the coating and restore the surface to itsoriginal condition. It’s a common misconception that ma-terial flow will remove the coating and expose the under-lying surface. In fact, if the coating is more frictional thanthe wall surface, no flow will occur at the walls and thecoating will remain. Once you’ve loaded material intoyour vessel, the only way to remedy this problem is to un-load the vessel (now a real problem) and clean the walls.

You can have similar problems if you load your vessel witha material other than the one the vessel was designed for.Sometimes this is done in early flow trials to “work out thevessel’s bugs” — but the bugs that result can be entirely dif-ferent from those produced by your design material. If youuse a different material for flow trials, use one with flowproperties similar to those of your design material.

Loading your vessel with a material produced during aprocess startup can also produce problems. The startupmaterial often has substantially different moisture content,particle size, chemical composition, and even surfacestructure, and it won’t give accurate results in your flowtrials.

6 Start up and maintain the vessel.

After installation, those who installed the vessel should bepresent at startup. You may also want the engineer whohandled the vessel’s detail design to be present. Startup in-volves loading the vessel with your material and checkingthat the material discharges as required from the outlet.

After startup, routinely inspect your vessel to preventsmall problems from growing into large ones. Inspect thehopper’s sloping surfaces for any changes in condition,such as liner or coating wear, that can produce flow prob-lems. Inspect the vessel walls for thin spots caused by wearand corrosion, especially around the cylinder-hopper in-terface and near the outlet. This can help you circumventserious structural problems. Inspect the welds and supportstructure to identify any deterioration that needs repair.Routinely inspect the vessel’s relief valves to preventoverpressurizing the vessel, and also inspect related dustcollection filters, feeder gaskets, and seals.2

If you decide to store a different material in your vessel, besure to consider more than just the material’s bulk density.If the material’s flow properties are different from those ofyour original material, the flow pattern through the vesselcan be different and impose different stresses on the ves-sel. This can cause a vessel wall section not designed to

will apply to the vessel walls. The pressures depend on thematerial’s flow properties, the vessel flow pattern, and thevessel geometry. Various codes, including those of theAmerican Concrete Institute (ACI) and American Societyof Agricultural Engineers (ASAE), address aspects ofstructural design and vessel construction, but none coversall the conditions you must consider.

Carefully consider how to add such items as access doors,poke holes, flow aids, and level sensors to the vessel. Ifyou place them poorly, they can cause flow problems. Forinstance, an aeration pad installed on the hopper surface toprevent potential flow problems may actually create them.To avoid this situation, have the engineering drawings re-viewed by the individuals who specified the vessel’s oper-ating requirements, determined your material’s flowproperties, and prepared the functional design.

5 Fabricate and install the vessel.

Before and during fabrication, you need to ensure that thevessel fabricator correctly interprets the detail design aswell as understands the basis for the design. Otherwise, thefabricator can make a mistake, such as trying to improveyour vessel’s number 2B finish by polishing it, unawarethat polishing this surface often increases wall friction andcan stop the material flow in your vessel. Other details,such as a slightly oversized flange, can also appear to betrivial to the fabricator, who may make the flange smallerso it fits the vessel outlet better. The result can be a flowstoppage.

Before installation, inspect the vessel against the detail de-sign to ensure that the vessel meets your design specs andintent. It’s easier to fix problems now than after the vesselis installed.

In mass flow, all the material is in motion whenever anyis withdrawn from the vessel.

At installation, which can be handled by a vessel supplier,an engineering contractor, or your plant staff, ensure thatthe hopper’s sloping walls — where wall friction is criti-cal — are protected. If the vessel requires field welding,these surfaces should be draped with fireproof cloth to pre-vent weld spatter from marring them. Minimize any weld-ing on the sloping surface, and vertically orient welds sothey follow the direction of material flow.

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handle high stress to experience loads exceeding the sec-tion’s design limits, resulting in vessel failure. Beforeplacing a different material in your vessel, measure thematerial’s flow properties and review your vessel designin light of these properties.

Some final advice

While it’s impossible to cover all the details of designing astorage vessel for reliable operation in one article, the sixsteps described here provide a good road map for startingthis journey. Work with experts — whether they’re fromyour firm or are independent engineers or consultants —to carefully consider the details of your application. Mak-ing the design project a top priority and directing the ap-propriate resources toward it can help you design a vesselthat performs reliably and safely. PBE

References1. Find more information on how to conduct flow property tests in

articles listed under “Solids flow” in Powder and Bulk Engineering’scomprehensive “Index to articles” (in the December 1998 issue andon PBE’s Web site, www.powderbulk.com).

2. See “How to prevent silo failure with routine inspections and properrepairs,” by Dr. John W. Carson and Richard T. Jenkyn in Powderand Bulk Engineering, January 1990, pages 18-25.

For further reading

Find more information on selecting storage equipment inarticles listed under “Storage” and “Solids flow” in Pow-der and Bulk Engineering’s comprehensive “Index to ar-ticles” (in the December 1998 issue and on PBE’s Website, www.powderbulk.com).

Herman Purutyan is a senior project engineer at Jenike& Johanson, One Technology Park Drive, Westford, MA01886; 978/392-0300, fax 978/392-9980 (e-mail: hpurutyan@jenike.com). Brian H. Pittenger is a senior pro-ject engineer and Dr. John W. Carson is president of thecompany.

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Published in Powder and Bulk Engineering, January 1990 www.powderbulk.com

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One Technology Park Drive • Westford, MA 01886-3189 • Tel: (978) 392-0300 • FAX: (978) 392-9980Also: San Luis Obispo, CA • Toronto, Canada • Viña del Mar, Chile

www.jenike.com

Load Development and StructuralConsiderations in Silo Design1

ByJ.W. Carson, Ph.D.

andR.T. Jenkyn, P.Eng.

1 Source: Carson, J. W. and R. T. Jenkyn: Load Development and Structural Considerations in Silo Design. Presented atReliable Flow of Particulate Solids II, Oslo, Norway, August 1993. Used with the permission of the publisher.

SYNOPSIS

Each year an alarming number of silos, bins,and hoppers fail due to bad design, poorconstruction or improper use. Jenike &Johanson engineers have been called in toinvestigate more than 50 structural failures inthe last five years alone.

Many failures are the result of loadingconditions not anticipated by the designer. Inthis paper we describe design procedures thatwe have found to be successful. In particular wecover bin load calculations for various fillingconditions and flow patterns, force resultants,and design requirements.

INTRODUCTION

Although statistics are not available, hundredsof industrial and farm silos, bins, and hoppersfail in one way or another each year. Sometimesthe failure is a complete dramatic structuralcollapse. Other times cracks are found in a

concrete wall, or dents in a steel shell, either ofwhich might appear harmless to the casualobserver. Nevertheless, these are danger signalswhich indicate that corrective measures areprobably required.

The economic cost of repairs to this essential –though frequently neglected – component of abulk material handling system is never small.The owner faces the immediate costs of lostproduction and repairs, personnel in the vicinityare exposed to danger, and the designer andbuilder face possible litigation because of theirliability exposure.

What can be done to avoid these problems? Inthis paper we show some of the problems thatcan occur, why they occur, and the straight-forward steps that can be taken to avoid, or atleast minimize, such problems.

2

SILO DESIGN

The design of bins and silos to store bulk solidsinvolves bulk material, geometric, and structuralconsiderations.

Bulk material considerations are importantbecause the frictional and cohesive properties ofbulk solids vary from one solid to another, andthese properties affect material behaviorconsiderably. In addition, a given bulk solid’sflow properties can vary dramatically withchanges in numerous parameters, includingparticle size, moisture, temperature, andconsolidating pressure. This variability ofproperties makes testing at actual conditionsmore important for proper bin and silo designthan may at first appear.

When considering the geometric design of asilo, potential problems include arching acrossan outlet, ratholing through the material, and theflow pattern during discharge. A bulk material’spropensity to arch or rathole is primarily relatedto it’s cohesiveness, while its flow patternduring discharge depends upon internal frictionas well as the friction that develops between thematerial and the silo’s hopper walls. The goal ofgeometric design is to maximize the useablecapacity of a silo while minimizing its capitalcost, overall height, etc.

Established design procedures [1] includeselection of the optimum hopper angles andminimum outlet dimensions. The idealdischarge mode is one where, at steady state, allmaterial flows without obstruction. This isreferred to as mass flow. The discharge modewhere only some of the material flows is calledfunnel flow. In mass flow, the material does notnecessarily move at a uniform rate throughout:velocity variations across any horizontal cross-section are possible.

The structural design of a silo requires, amongother things, knowledge of the distribution ofpressures and shear stresses on its walls (causedby the stored material) and how that distributionvaries during charging, storage at rest,discharging, and recharging.

Of the three major aspects of silo design (bulkmaterial, geometric, and structural), the binloads aspect of structural design is the leastunderstood. But unless the structural design isdone properly, the integrity of the silo may becompromised. Silo collapse is far too common,yet agreement amongst designers on proceduresfor determining silo loads has not beenforthcoming. This is very apparent when oneconsiders existing codes of practice. There isvery little detailed guidance concerning thevarious loading conditions – some static, somedynamic – which can co-exist.

Even if existing codes were “better,” it isunreasonable to expect that any code of practicewould contain a definitive set of instructionscovering all cases that might have to beconsidered. Usually none but the simplest casescan be described. Over-enthusiastic compliancewith the letter, to the exclusion of the spirit andintent of a code of practice, can be misleading,and even dangerous.

In some countries, codes are recommendationsonly, so compliance with them is notmandatory. However, for practical purposes inthe event of a failure, a code (assuming thatone exists) is a minimum mandatory standard.In other words, an engineer may have the rightto exercise independent engineering judgmentwhen creating a design, and may even go backto first principles. But if a problem occurs andthe engineer must justify his design, he willhave difficulty doing so unless it is as good asthe minimum provided by the applicable code(or codes), or the inapplicability of the code hasbeen documented [2].

3

Codes are particularly weak in the area ofeccentric flow channel formation. In fact evenflow experts often cannot agree on where a flowchannel will form in a funnel flow bin or silo, itssize, shape, etc. Because of this uncertainty inthe ability to predict the occurrence of flowchannels, some designers feel that it is prudentto assume the occurrence of worst case flowchannels if there is any doubt at all. Part of theirrationale is that they consider it to be dangerousto fine tune a design on the basis that somedefinite predicted flow regime will occur, thatoperators will operate the silos according to adefinite set plan, or that the material’s flowproperties will not vary [3]. While such anapproach should be conservative, it may be toocostly to implement.

Several committees in various countries arecurrently working to revise silo design codes.Many are having great difficulty in enactingnew procedures for the design of silos toaccommodate flow channels even though theyknow that they occur and they know that manysilo failures have been caused by such flowchannels. Every day there are new engineerswho are charged with the design of new silos.Most of these new engineers look first to thecodes for information on the design of thesestructures, hoping and expecting that the codeswill point them in the right direction. To do this,a code need not be perfect, but it must reflectthe latest in technology and be rational.Hopefully, papers like this one will fill some ofthe gaps while codes are being revised.

CAUSES OF SILO FAILURES

There are many different causes of silo failures[4]: shortcomings in the design procedure,construction, usage, maintenance, or somecombination thereof. This, in turn, means thatmore than one individual or group often bearssome responsibility when a failure occurs.

Potentially responsible parties include thedesigner, builder, building material supplier,owner, user, and others.

Failures Due to Design Errors

Silo design requires specialized knowledge. Thedesigner must first establish the material’s flowproperties, then consider such items as flowchannel geometry, flow and static pressuredevelopment, and dynamic effects. Problemslike ratholing and vibration have to beprevented, while assuring reliable discharge atthe required rate. Non-uniform loads, thermalloads, and the effects of non-standardfabrication details must be considered. Aboveall, the designer must know when to be cautiousin the face of incomplete or misleadinginformation, or recommendations that comefrom handbooks, or from people with the “it’salways been done this way” syndrome.

Having established the design criteria, acompetent design has to follow. Here thedesigner must have a full appreciation of loadcombinations, load paths, primary andsecondary effects on structural elements, and therelative flexibility of the elements. Specialattention must be given to how the most criticaldetails in the structure will be constructed sothat the full requirements and intent of thedesign will be realized.

Flow-related loading conditions which,unfortunately, many designers fail to anticipateinclude:

• Bending of circular walls caused byeccentric withdrawal. If the withdrawalpoint from the hopper is not located on thevertical centerline of the silo, and if theresulting flow channel intersects the silowall, non-uniform pressures will developaround the circumference of the silo leadingto horizontal and vertical bending moments.

4

Many silo designers incorrectly account forthese non-uniform pressures by onlyincreasing hoop pressures. The problem ofbending moments is particularly commonwhen using silos with multiple hoppers inwhich only one or two of the hopper outletsare used at a time.

• Non-symmetric pressures caused by inserts.Support beams and other types of internalscan impose non-symmetric pressures on thesilo wall leading to unacceptable bendingstresses.

• Self-induced vibrations. Bins and silossometimes vibrate. This can be either a highfrequency, low amplitude type of cyclicvibration, or a low frequency, highamplitude erratic vibration leading toshocks. The latter have been known to causestructural failures [5].

• Local peak pressure at a point where afunnel flow channel intersects a silo wall.

• Mass flow occurring when funnel flow wasexpected.

• Migration of moisture from wet to dryparticles within the stored solids, whichcauses the dry particles to expand andimpose large radial loads on a silo. (This isan uncommon problem.)

Failures Due to Construction Errors

In the construction phase there are two ways inwhich problems can be created. The morecommon of these is poor workmanship. Unevenfoundation settlement and faulty construction(such as using the wrong materials or not usingadequate reinforcement, such as insufficientquantity of rebars) are but two examples of sucha problem. This can usually be avoided byhiring only qualified builders, by close

inspection during construction, and by enforcinga tightly written specification [6].

The other cause of construction problems is theintroduction of badly chosen, or evenunauthorized, changes during construction inorder to expedite the work. Any changes indetails, material specifications, or erectionprocedure, must be given careful considerationby both the builder and silo designer.

Failures Resulting from Silo Usage

If a bulk material other than the one for whichthe silo was designed is placed in it, the flowpattern and loads may be completely different.The load distribution can be radically changed ifalterations to the outlet geometry are made, if aside outlet is put in a center discharge silo, or ifa flow controlling insert or constriction is added.The designer should be consulted regarding theeffects of such changes before they areimplemented. Some of the problems which canoccur include:

• Collapse of large voids. A collapsing arch orrathole induces tremendous dynamic loadson the structure, which can cause thestructure to fail. Vibrating bin dischargershave also been known to fall off bins andsilos because of this mechanism.

• Development of mass flow in silos designedstructurally for funnel flow. Mass flow candevelop if the walls become smoother withtime or if the properties of the bulk solidbeing stored change. This generally resultsin much higher loads at the top of the hoppersection, which can result in structuralfailure.

• Drastic means of flow promotion. Highpressure air cannons and even dynamite aresometimes used to restore flow. The result

5

may be more dramatic than the user anddesigner anticipated!

• Buckling of an unsupported wall below anarch of stored bulk material.

• Metal fatigue caused by externally-mountedbin vibrators.

• Dust explosions.

Failures Due to Improper Maintenance

Maintenance of a silo comes in the owner’s oruser’s domain, and must not be neglected. Thereare two types of maintenance work which arerequired [7]. The first is the regular preventativework, such as the periodic inspection and repairof the liner used to promote flow, protect thestructure, or both. Loss of a liner may beunavoidable with an abrasive or corrosiveproduct, yet maintaining a liner in properworking condition is a must if the silo is tooperate as designed.

The second area of maintenance involveslooking for signs of distress, (e.g., cracks, walldistortion, tilting of the structure) and reactingto them. If evidence of a problem appears,expert help should be immediately summoned.An inappropriate response to a sign thatsomething is going wrong can precipitate afailure even faster than leaving it alone,including the common instinct to lower the silofill level.

Wear due to corrosion and/or erosion can beparticularly dangerous. For example, as carbonsteel corrodes, the reduced wall thickness caneventually lead to a structural failure. Thisproblem can be compounded through erosivewear of the silo wall. Erosive wear can also be aproblem in reinforced concrete silos handlingabrasive bulk materials such as coarse ores.

SILO LOADS

The loads which bulk materials exert on silostructures can generally be divided into twocategories: those due to initial fill and thosewhich are as a result of flow. Initial fill loadsdevelop, as the name implies, when a silo isfilled from an empty condition without anywithdrawal taking place. The term flow-inducedloads, on the other hand, is somewhat of amisnomer since it implies that the material mustbe in motion for these loads to develop. In fact,the only requirement is that there be somewithdrawal of material which allows the flowinduced loads to develop. Once this occurs, flowcan be stopped and then restarted withouthaving any appreciable effect on the silo loads.In addition, the rate of discharge is usually not asignificant variable in affecting the magnitudeof the silo loads. The primary reason for this isthat most bulk materials are not viscous orvisco-elastic, so their rate of movement has littleeffect on their frictional properties.

Initial Fill

As with all of the loading conditions describedherein, it is convenient to consider first thevertical-sided portion of the silo (generallycalled the cylinder section), and then the hopper(i.e., sloped section of the silo in which thecross-sectional area is changing with height).

If a silo is filled at a point which coincidesclosely with the silo’s centerline, the loadswhich develop on the cylinder walls aregenerally less than those which are flow-induced and are therefore of little interest as faras structural design is concerned. If there issome reason to consider these loads, werecommend the use of the Janssen equation witha K j value (ratio of horizontal to verticalpressures) of 0.4 and with wall friction angle f¢equal to a value determined from tests (seesection MATERIAL FLOW PROPERTIES

6

below). For a circular cylinder of diameter D,the Janssen equation is:

p =g D4 m

1 - e-4 m K j z / D[ ] (1)

t = m p 2)

m = tan ¢ f 3)

See NOMENCLATURE section at end of paperfor a description of each term.

Other types of fill conditions can result in loadson the cylinder walls which are larger than thosewhich are flow-induced. In particular, considerthe conditions which occur when a silo is filledoff-centered, or if it is filled along a ridge (suchas would occur if a continuous belt tripper fillsystem were used). Pressures around the siloperimeter at any elevation caused by theseconditions, can be calculated using thefollowing procedure:

• At any point on the cylinder’s perimeter,measure vertically up the wall to theelevation where the material surfacecontacts the wall, z1.

• Cut the surface profile with a horizontalslice at the elevation just determined (i.e.,where the material surface contacts thewall). Calculate the volume of the surchargeabove that slice, then divide that volume bythe area of the slice, to give an effectiveadditional head above the slice, z2.

• Apply Janssen’s equation, usingz = z1 + z2.

• Repeat this for sufficient points around thesilo perimeter to define the distribution.

While this condition is usually rather localizedto a region immediately below the materialsurface, it can occur at any elevation as the silois being filled.

As far as the hopper section is concerned, webelieve that the following equation adequatelypredicts the initial fill pressures which actnormal (i.e., perpendicular) to the walls of aconverging conical hopper no matter what typeof flow pattern occurs during discharge.

p = gh - z

ni

+qg

-hni

Ê

Ë Á Á

ˆ

¯ ˜ ˜ 1 -

zh

Ê Ë Á

ˆ ¯ ˜

ni +1Í

Î Í Í

˙

˚ ˙ ˙

(4)

ni = 2 1+tanf'tanq c

Ê

Ë Á Á

ˆ

¯ ˜ ˜ - 3 (5)

Note that “z” in equation (4) starts with a zerovalue at the top of the hopper, not at the top ofthe cylinder as in equation (1). The value of qcan be calculated by taking the Janssenhorizontal pressure p at the bottom of thecylinder and dividing by Kj (recommendedvalue = 0.4)

For hopper geometries other than conical,numerical integration of the equations ofequilibrium is required.

As will be shown below, in the case of a massflow hopper the initial fill loads govern thestructural design of the hopper in roughly itsbottom two-thirds, whereas flow-induced loadsgovern in the upper third. See Fig.1. In mostfunnel flow hoppers, their structural design canbe based upon initial fill loads.

7

Mass Flow – Single Outlet

Mass flow is a condition in which all of thematerial is in motion whenever any iswithdrawn. As indicated in the SILO DESIGNsection above, particles can be flowing atdifferent velocities and still satisfy therequirements for mass flow as long as they aremoving.

A mass flow bin or silo can still exhibit a no-flow condition of arching if the outlet is toosmall relative to the particle size (arching due tointerlocking) or if the outlet is too small relativeto the material’s cohesive strength. Mass flowsilos can also develop self-induced vibrations asmaterial discharges [5].

If we assume that the outlet size is large enoughto prevent the formation of a stable arch, andfurthermore that self-induced vibrations do notoccur upon discharge, the loads that develop onthe silo walls are fairly well defined. In thecylinder section, a good starting point is to usethe Janssen equation but with a range of Kj andwall friction values as follows:

0.25 £ Kj £ 0.6 (6)

f' calc. = f' meas. ± 5o (7)

The “plus” sign should only be used in thisequation when calculating maximum shear

stresses for cylinder buckling calculations.Otherwise the “minus” sign should be used.

If an applicable silo code predicts higherpressures, it should be used for the reasonsstated in the SILO DESIGN section above.

In the hopper section, we recommend the use ofthe following equation [8] to predict flow-induced loads in conical hoppers:

p = g Kfh - znf

+qg

-h

nf

Ê

Ë Á Á

ˆ

¯ ˜ ˜ 1 -

zh

Ê Ë Á

ˆ ¯ ˜

n f +1Í

Î Í Í

˙

˚ ˙ ˙ (8)

Kf =1

23

1 +tanf'tanqc

Ê

Ë Á Á

ˆ

¯ ˜ ˜ -

16(s' / gB)tanqc

È

Î Í Í

˘

˚ ˙ ˙

(9)

nf = 2Kf 1 +tanf'tanqc

Ê

Ë Á Á

ˆ

¯ ˜ ˜ - 3 (10)

The value of “z” in equation (8) starts at zero atthe top of the hopper, as in equation (4). Thevalue of q can be calculated by taking theJanssen horizontal pressure p at the bottom ofthe cylinder and dividing by Kj. To beconservative, a minimum value of Kj should beused for the calculation of p.

These equations result in higher pressures inroughly the upper third of the mass flow hopperthan occur during initial fill, but lower pressuresin the bottom two-thirds of the hopper section.See Fig. 1.

Because of the rapid switch in the state of stressthat occurs at the top of a mass flow hoppersection, some increase in wall pressure is oftenexperienced in the section of the cylinder justabove the top of the hopper. To account for thiscondition, we recommend that the peak pressurebe spread along the vertical wall as shown inFig. 2. First, draw a circular arc centered on thetheoretical apex of the conical hopper, and

Fig. 1: Mass flow hopper

t

Flow

Initial fill

P

t = m p

p

8

passing through the top of the cone. Theelevation of the highest point on the arc isapproximately the maximum elevation at whichthe increased peak pressure is experienced. Thewall pressure distribution below this elevation(down to the top of the cone) can be assumedlinear.

A silo in which the fill and withdrawal pointsare located along the vertical centerline, andwhich behaves in mass flow, will probablyexperience some non-uniformity of pressuresaround its circumference. This could be causedby the wall being out-of-round or out-of-plumb,the intrusion of construction joints, orsegregation of the contained bulk material. It iscommon practice, although by no means alwayscorrect, to compensate for these effects bymultiplying the calculated wall pressure p bysome “over pressure factor” for the purpose ofdesign. We recommend that this should be aminimum requirement, and that a designershould make a rational attempt to estimatepressure non-uniformities and their effects.

Funnel Flow – Single Outlet

As noted above, since there is no flow along thehopper walls in a funnel flow pattern (exceptperhaps when the hopper is being emptied at the

end of the discharge sequence), it is reasonablein most cases to consider that the designpressures acting normal to the hopper walls arethe same as those which occur during initial fill.Therefore no additional calculations are neededfor the hopper section. This presumes, of course,that the outlet size and feeder arrangements aresuch that no arching or ratholing can occur asmaterial is discharged. It is also important thatthere be no self-induced silo vibrations acting tomagnify pressures [5].

As far as the cylinder section is concerned, thereare two main conditions to consider. First, if theflow channel does not intersect the cylinderwall, it is safe and reasonable to assume that thepressures acting against the walls will be thesame as during initial fill. If, on the other hand,the flow channel does intersect the cylinderwall, one must consider whether or not the flowchannel is centered (i.e., intersects the cylinderwall at the same elevation around itscircumference). If the flow channel is centered,one can assume a Janssen stress field above theeffective transition (i.e., the elevation at whichthe flow channel intersects the cylinder walls).As with mass flow cylinder pressures, werecommend using a range of Kj and wall frictionvalues as described above.

At the effective transition where the flowchannel strikes the wall, there is a rapid increasein wall pressure due to the convergence whichthe material is undergoing. Within the flowchannel itself, it is reasonable to assume that thepressures will vary as if this were a mass flowhopper but with the hopper angle replaced bythe flow channel angle, and the wall frictionvalue replaced by the internal friction ofparticles sliding on each other. How thispressure distribution is transmitted to thevertical walls of the cylinder is not well-defined.It is safe, but probably somewhat conservative,to assume that the pressure which acts normal to

Fig. 2: Spreading of mass flow pressure peak into cylindersection

t

t

P

p

p

t = m pR

9

the cylinder walls is the same pressure whichacts normal to the flow channel.

As with the conditions which occur at thebottom of a cylinder just above a mass flowhopper, there is some progression of thispressure peak, which occurs just above theeffective transition in a funnel flow silo. For thiswe recommend that the total radial outwardforce given by the peak pressure, multiplied bythe effective area over which it acts, beconverted to a smaller uniform pressure spreadover a wall height equal to one vertical bendinghalf wave length. This should be centered at theelevation of the effective transition. See Fig. 3.

Since the side slope of the flow channel – andthus the elevation at which it intersects thecylinder wall – is variable, the above procedureshould be used to develop an envelope of peakpressures to be used in design of the cylinderwall.

If the flow channel is not symmetric but stillintersects some or all of the cylinder wall, theloading conditions become much more complex.The resulting eccentric flow channel can causenon-uniform pressures to act on the silo walls.In cylindrical reinforced concrete silos thiscauses horizontal and vertical bending momentswhich act in addition to the membrane forcesand can lead to serious cracking if the walls arenot designed to withstand such loading, as isoften the case with concrete silos constructedwith a single layer of reinforcing steel. Inaddition, there are many documented cases ofdented or collapsed steel bins and silos as aresult of eccentric flow channels. The shape ofthe flow channel, the locations at which the flowchannel intersects the silo walls, and thepressure within the flowing and non-flowingregions must all be estimated to permit thesebending moment calculations.

Several studies have been conducted in anattempt to predict the shape of flow channels infunnel flow bins. One of the older and betterknown of these studies is that which wasperformed by Giunta [9]. He postulated that fora silo having a circular outlet with a diameterlarge enough to prevent arching and ratholing,the flow channel shape would consist of a coneemanating from the outlet and flaring out tosome diameter. In the upper portion of the bin orsilo, he postulated that the flow channel shapewould be cylindrical with a diameter set by themaximum size of the conical flow channel.Giunta tested his hypothesis on an 18 in.diameter flat-bottom bin having a single, centraloutlet. Test materials included industrial starch,pulverized coal, and iron ore concentrate. Hefound reasonably good agreement between theactual flow channel shape and his theory.

There are a number of limitations in applyingGiunta’s work as pointed out by Carson et al[11]. Unfortunately, as the work of these authorsillustrates, there is no straightforward anduniversal method by which the shape of a funnelflow channel can be predicted.

With non-free flowing bulk solids, relativelysteep flow channels form which tend to becomemore or less circular in cross-section some

Fig. 3: Funnel flow hopper – flow channel intersectingcylinder wall

t

P

p

t p

Pressurepeak

Distributedpressure peak

t = m p

Effectivetransition

Vertical bending halfwave length of cylinder

10

distance above the outlet. If the outlet is circularand its diameter is less than the bulk solid’scritical rathole diameter, a stable rathole willform whose diameter is approximately the sameas that of the outlet. With elongated outlets, thediameter of the flow channel will beapproximately equal to the length of thediagonal of the outlet. Again, if this diameter isless than the bulk solid’s critical ratholediameter, the flow channel will empty out whenthe silo level is lowered. The diameter of theresulting rathole will be approximately equal tothe diameter of the flow channel.

In both of the above cases, the wall pressure willbe essentially constant at any elevation unlessthe outlet is near the wall. Only then will thesteep flow channel intersect the wall. However,if this occurs, the resulting horizontal bendingmoments can be very large because of thehighly non-uniform wall pressures.

The other extreme is with free flowingmaterials. As shown by Carson et al, the steadystate flow channel angle with such materials isconsiderably less steep than the anglespostulated by Giunta. Furthermore, the authorsfound that with eccentric outlets, the resultingflow channel expanded at roughly the sameangle as in a bin with a centered outlet, and theeccentric flow channel’s axis of symmetry wasapproximately vertical. See Fig. 4.Unfortunately, this study failed to identify anycorrelation between steady state flow channelangle and material flow properties such aseffective angle of internal friction or angle ofrepose. Clearly, much more work needs to bedone with larger models, more bulk solids, andfull scale silos before any definitive conclusionscan be reached. In the meantime, the authors ofsilo design codes should write silo designrequirements to reflect a high degree ofuncertainty, not only about actual pressures, butalso about the angle of convergence of flowchannels and their boundaries.

Bulk solids that fall in between the extremes ofthose that are free flowing and those whichrathole, produce flow channels which fallbetween the extremes described above. Eachcase needs to be studied closely so as to avoidproblems with the design.

Expanded Flow – Single Outlet

An expanded flow silo is defined as one inwhich the lower hopper section has walls whichare steep enough and smooth enough for flow tooccur along them, whereas in the upper sectionof the hopper the walls are either too shallow ortoo rough for this to occur. Provided that theflow channel in the lower hopper sectionexpands sufficiently to prevent ratholing at thetop of this section (i.e., the diameter of the flowchannel exceeds the critical rathole diameter ofthe material), ratholing will not occur within thesilo. Furthermore if one assumes that the outletis sufficiently large such that arching does notoccur, and that no self-induced vibration occursduring discharge, then the followingcombination of loads can be considered. (SeeFig. 5) In the cylinder section and in the upperportion of the hopper where flow does not occuralong the hopper walls, the bin loads will be thesame as those which would occur in a funnelflow silo of the corresponding dimensions. The

Fig. 4: Flow channels with centered and eccentric outletsin a funnel flow bin or silo

a) Centered outlet b) Eccentric outlet

Flowchannelangles

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lower hopper section where flow does occuralong the hopper walls, can be designed as ifthis were a mass flow hopper. However, sincesome convergence of the flow channel willoccur above this section, there will be no peakpressure at the top of this hopper section asoccurs at the top of a mass flow hopper where itintersects the cylinder. Therefore, the governingloading condition is usually that of initial fillpressures.

Multiple Outlets

If more than one outlet is present in a silo, it isessential to design the silo structurally towithstand the worst possible loading condition[12]. This usually occurs when one or more ofthe outlets is active while the rest are inactive.Even if all of the outlets are active but aredischarging at different rates, preferential flowchannels can develop even though functionallythe silo is designed for mass flow.

To account for these various design conditions,the silo should be designed for funnel flowloading conditions with an off-centered flowchannel occurring above one or more of theactive outlets. The most severe combination offlow channels must be considered whencalculating the eccentric loads.

MATERIAL FLOW PROPERTIES

Most silo design codes include, either in thecode itself or in the commentary section, atabulation of “typical” properties of a number ofbulk materials. One should approach the data insuch tables very cautiously. Interpolatingproperties or guessing properties on the basis ofsuperficial similarities in the description ofmaterials should be vigorously avoided. It isimportant to remember that it is not possible toknow, or to look up, the required flow propertiesof a granular material from its generic namealone. This is true not only of the bulk materialby itself, but also of the surface on which it issliding. For example, providing values, or arange of values, for wall friction of “coal onsteel” sounds simple but can be very misleading.Before using such data, one should consider thefollowing questions:

• What type of coal (e.g., bituminous, lignite,anthracite) was used in developing the datain this table?

• What was the particle size, moisture content,ash content, etc. of the coal which is beingdescribed?

• What type of steel and what surface finishwere used for the tests? If carbon steel wasused, was the variation from a smooth,polished surface to a rough surface (e.g., dueto corrosion) considered? If stainless steelwas used, was the surface rough (mill finishplate) or smooth (2B finish sheet or polishedplate)? If the steel was mechanicallypolished, was the direction of polish linestaken into account?

In our opinion, most such tabulations provide adisservice to design engineers in that they temptthe engineer to use them in spite of the warningswhich are given either within the table or inaccompanying text. An engineer can be lulled

Fig. 5: Expanded flow silo

pt

t p

t

Pp

Mass flowinitial pressuredistribution

Funnel flowdistribution(see Fig. 3 ifflow channelintersectscylinder wall)

t = m p

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into a sense that he or she has some quantitativedata that is useful for design, whereas in fact, nosuch assumption is valid.

Material flow tests should be run wheneverpossible to accurately quantify the flowproperties (and range of flow properties) of thebulk material to be handled. This is particularlyimportant when the bulk material being handledis not free flowing, or when its flow propertiesare unknown, uncertain, or variable. Definingwhether or not a material is “free flowing” issomewhat subjective and a matter of debate. Inour opinion, the best way to define this is tobase it on the flow properties of the bulkmaterial and how those flow properties dictatethe type of flow which will occur in a given binor silo. For example, if it is known (eitherthrough experience or through flow propertiestests) that a given bulk material will not form astable arch or rathole in a given bin or silo, onemight reasonably conclude that this material inthis silo is “free flowing.” This same material inanother silo having a different flow pattern orsilo dimensions might no longer be considered“free flowing.”

If tests are to be done, we recommend thefollowing [13]:

• Flow function and effective angle of internalfriction. Measurements of a material’scohesive strength and internal friction anglesshould generally be run on the fine fractionof the bulk material, since it is the fineswhich exhibit most strength. Furthermore,concentrations of fines are usuallyunavoidable because of particle segregation[14]. Once these parameters have beenmeasured, it is possible to follow designprocedures to calculate minimum outletdimensions to prevent arching as well ascritical rathole diameters.

• Bulk density. Generally this is measured byconsolidating the bulk material to variouspressures and then measuring the resultingbulk density at those pressures. Such testsshould be run both on the fine fraction (inorder to use the resulting values to calculatearching and ratholing dimensions) as well ason the full particle size range. The largervalue should be used when calculating binloads.

• Wall friction. Generally it is easier to runthis test on the fine fraction of the material,and the resulting values typically don’t varysignificantly with particle size. It isimportant to run this test on both thematerial of construction of the cylindersection as well as that of the hopper.Consideration should be given to variationsin the initial condition of the silo walls aswell as conditions that can occur after usagedue to abrasive wear, corrosion, etc. Ingeneral, the smoother the wall surface, thehigher the wall pressure acting against it.

• Abrasive wear. A tester is available [15]which can quantitatively predict the actuallife of a bin or silo wall material due to abulk material sliding across it. This testercan also be used to determine the change inwall friction due to wear.

Each of the above parameters can vary with thesame bulk solid if any one or more of thefollowing conditions change:

• Moisture content• Time of storage at rest• Particle size distribution• Temperature• Chemical changes

Note that we have not included in the abovelisting the measurement of the value of Kj. Inour opinion, this parameter is more silo-

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dependent than material-dependent. Therefore,attempts to measure its value for a given bulksolid are inappropriate.

FORCE RESULTANTS

Tension

In a circular bin or hopper wall with uniformpressure on the circumference, the onlyhorizontal force resultant is ring tension. This iseasy to calculate and accommodate in design.

If the hopper bottom is supported at its top edge(i.e., the junction with the vertical wall), it willbe loaded in tension along the line of slope, aswell as ring tension. This too is easy to calculateand design for, but it is important to check formeridional bending.

Vertical Force, Upper Section

There is a vertical compression force in thewalls of the upper silo section due to theaccumulation of wall friction effects from thetop surface down to the level of the support.This is the sum of the horizontal outwardpressures at each increment of depth, multipliedby the depth increment and the wall frictioncoefficient. Add to this any loads from the roofclosure and self weight.

The critical buckling stress in the wall is thecriterion governing the thickness required tocarry this vertical compression. This conditionseldom dictates the thickness of reinforcedconcrete walls, but is a major consideration indesigning thin-walled steel or aluminum silos.

Bending in Flat Walls

Flat walls appear in rectangular bins or hoppers,or in a chisel-shaped hopper between a circularupper section and a slotted outlet. This bendingis always combined with tension in the plane of

the wall. In the upper section of a bin, verticalcompression may also be present. A flatreinforced concrete wall in bending must havetwo layers of reinforcing steel, adequatelyanchored at the ends by lap splices running intothe adjoining walls. In a steel design it is usuallyassumed that the tension or compression iscarried by the wall plate, and the bending iscarried by the external stiffeners.

The flat walls of a rectangular or chisel-shapedhopper, operating in mass flow, must remain asnearly flat as possible, or the mass flow patternmay be lost.

Horizontal Bending of a Circular Wall

This is the major resultant of a funnel flow,single eccentric flow channel reaching the upperbin wall. The horizontal radial outward pressureof the material on the wall is not uniform on thecircumference, so out-of-round bending isinduced. Non-uniform pressures insymmetrically filled and emptied silos can alsoresult in bending which needs to be evaluated.

Combined bending and tension effects can bestbe calculated using a finite element model of thebin wall loaded by the internal pressurescalculated over the whole circumference andheight. Alternatively, a hand calculation ofbending and tension in a ring can be performed.

The most important effect on a steel plate shellis the reduction in vertical buckling strengthresulting from an increase in the radius ofcurvature when the shell deflects out-of-round.If the construction is of reinforced concrete, thereinforcing steel must be provided in two layers,with adequate capacity for the bending and ringtension at any point.

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Vertical Bending of Upper Wall

In mass flow, as well as in a case of funnel flowat the point that the flow channel strikes thewall, a peak pressure develops at the effectivetransition. This may be on the full perimeter oran isolated patch, and is also transient. In funnelflow this peak pressure may be several timesgreater than the pressures above and below, andoccurs on a very shallow band. The forceresultant is bending in the vertical direction. In aconcrete wall the result may be the developmentof horizontal cracks.

Vertical Force on a Flat Bottom

This is calculated using a value of Kj which willmaximize the vertical pressure. One mustremember that a large portion of the grossweight of contained material is carried by thebottom when the height-to-diameter ratio issmall. This portion decreases rapidly as theheight-to-diameter ratio increases.

Forces at Ring Beam

Perhaps the most common, even typical, designof a steel storage silo is circular, with a verticalupper section and a conical bottom hopper,supported at discrete points around thecircumference of a ring beam at the junctionbetween the two parts. A concrete silo willcommonly have a steel bottom hopper supportedfrom a ring beam which is either separate fromthe vertical wall, or built into the wall. This ringbeam accumulates the meridional tension fromthe hopper shell, and possibly the gross weightof the bin by vertical friction load from theupper wall. The tension from the hoppercontributes a horizontal and vertical component.The horizontal component from the hoppercreates compression in the ring beam.

The sum of the vertical forces creates bending,shear, and torsion in the ring beam. The bending

moments are negative (tension top) over thesupport points, and positive at mid-span. Shearoccurs at the supports. Torsion develops due tothe curvature of the beam, and is at a maximumat the points of contraflexure of the spans.

An additional force resultant is the rollingmoment. The line of action of the vector sum ofthe forces applied to the ring beam is unlikely topass through the shear center of the beam crosssection. The beam therefore tends to be rolledinside out. The net effect of rolling is anadditional vertical moment, applied at all pointson the circumference.

The ring beam must be designed toaccommodate all these forces in combination.

OTHER CONSIDERATIONS

Feeder Design

In addition to the geometry and materials ofconstruction of the silo, equally important is thetype of feeder which is used, as well as detailsof the interface between the hopper and thefeeder. This is particularly important if a massflow design is to be used in which case thefeeder must ensure that the outlet area is fully“live” [16, 17]. Feeder design is also importantwith funnel flow or expanded flow silos since,depending upon the details of the interface, theflow channel may either be centered oreccentric. Also important is the operation of agate at the outlet. If such a gate is used inanything but a full open or full closed position,it may upset the development of mass flow orthe type of flow channel which develops infunnel flow or expanded flow. A partially closedgate – even if only just projecting into flowingmaterial – can prevent flow along significantportions of the hopper wall.

15

Thermal Loading

Many bulk solids are fed into silos at atemperature significantly different from that ofthe surroundings. In such cases, calculationshave to be made to estimate values for rate ofheat flow out of, or into, the silo, temperaturegradients through the wall, and change oftemperatures in the silo contents. From this,design can proceed to such things as heatinginput, selection of insulation, (e.g., to maintainthe contents at a carefully controlledtemperature, to prevent freezing) orstrengthening the walls to safely resist thermalstresses.

There are two distinct and different conditionsto be analyzed [18]. The worst thermal effectsare usually found in the walls of a silo above ahot material surface. Here the temperature ismaintained at a high level while fresh materialcontinues to be fed into the silo. As hot materialcontinues to be fed into the silo, the surfacerises. Material already in place, and successivelevels of wall, are buried. Material at a hightemperature comes in contact with the wall at alower temperature. This causes a brieftemperature excursion affecting a narrow bandof the wall, following which all the temperatureswill start to fall as heat flows through the wall tothe outside, and a zone of cooled materialdevelops against the wall.

The other condition to be considered in designexists below the material surface, wheretemperatures fall as heat flows to the outside. Atemperature gradient develops through somethickness of the granular material, from the hotinterior to the cooler wall. Gravity loads willtherefore co-exist only with reduced thermalloads. It is of interest to know the time taken forthis temperature gradient to develop to somecritical point, such as temperature falling belowfreezing at the inside face of the wall.

NOMENCLATURE

D = cylinder diameterh = hopper height

Kf = defined by equation (9)Kj = Janssen ratio of horizontal to vertical

pressureni = defined by equation (5)nf = defined by equation (10)p =pressure acting normal (i . e . ,

perpendicular) to a silo or hopper wallq = vertical pressure acting at top of hopperz = vertical coordinatez1 = vertical distance along cylinder wall

starting at point of intersection of toppile

z2 = additional vertical height added to z1 toaccount for pile height

g = bulk densityqc = conical hopper angle (measured from

vertical)m = coefficient of sliding friction between

bulk solid and wall surfaces¢/gB = see Fig. 58 to 62 of ref. [1]

t = shear stress acting along wall surface indirection of flow

f¢= wall friction angle between bulk solidand wall surface

REFERENCES

[1] Jenike, A.W.: Storage and Flow Solids,University of Utah EngineeringExperiment Station, Bulletin No. 123,Nov. 1964.

[2] Samuels, B.: Silo Design: Setting theStandard. Presented at a meeting ofAmerican Concrete Institute Committee313, Vancouver, March 31, 1993.

[3] Boaz, I. B., Private communication.

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[4] Jenkyn, R. T. and Goodwill, D. J.: SiloFailures: Lessons to be Learned.Engineering Digest, September 1987.

[5] Purutyan, H., Bengtson, K. E., and Carson,J. W.: Flow-Induced Silo Vibrations.Proceedings, Powder & Bulk SolidsConference/Exhibition, Chicago, May1993.

[6] Carson, J. W., Jenkyn, R.T., and Sowizal,J. C.: Reliable and Economical Handlingof Bulk Solids at Coal-Fired Power Plants.Bulk Solids Handling, Vol. 12, No. 1, pp.11-16, Feb. 1992.

[7] Carson, J. W. and Jenkyn, R. T.: How toPrevent Silo Failure with RoutineInspections and Proper Repair. Powderand Bulk Engineering, Vol. 4, No. 1,pp. 18-23, January 1990.

[8] Jenike, A. W.: Effect of Solids FlowProperties and Hopper Configuration onSilo Loads. Unit and Bulk MaterialsHandling (Loeffler, F.J., and C.R. Proctor,eds.), 1980, ASME, pp. 97-106.

[9] Giunta, J.S.: Flow Patterns of GranularMaterials in Flat-bottom Bins.Transactions of the ASME, Journal ofEngineering for Industry, 91, Ser. B,No.!2, pp. 406-413.

[10] Carson, J.W. and Johanson, J.R.:Vibrations Caused by Solids Flow inStorage Bins. Proceedings, InternationalPowder and Bulk Solids Handling &Processing Conference, Rosemont, IL,May 1977.

[11] Carson, J.W., Goodwill, D.J., andBengtson, K.E.: Predicting the Shape ofFlow Channels in Funnel Flow Bins andSilos. Presented at the American ConcreteInstitute 1991 Spring Convention, Boston,March 17-21, 1991.

[12] Carson, J.W. and Goodwill, D.J.: TheDesign of Large Coal Silos for Safety,Reliability and Economy. Bulk SolidsHandling Vol. 4, No. 1, pp. 173-177,1984.

[13] Marinelli, J. and Carson, J.W.: SolveSolids Flow Problems in Bins, Hoppers,and Feeders. Chemical EngineeringProgess, pp. 22-28, May 1992.

[14] Carson, J.W., Royal, T.A., and Goodwill,D.J.: Understanding and EliminatingParticle Segregation Problems. BulkSolids Handling, Vol. 6, No. 1, pp. 139-144, February 1986.

[15] Johanson, J.R. and Royal, T.A.: Measuringand Use of Wear Properties for PredictingLife of Bulk Materials HandlingEquipment. Bulk Solids Handling, Vol. 2,No. 3, pp. 517-523, 1982.

[16] Bridge, D.T. and Carson, J.W.: How toDesign Efficient Screw and Belt Feedersfor Bulk Solids. Proceedings. Powder andBulk Solids 12trh Annual Conference,Rosemont, IL, May 1987.

[17] Marinelli, J. and Carson, J.W.: Use ScrewFeeders Effec t ive ly . ChemicalEngineering Progress, pp. 47-51,December 1992.

[18] Jenkyn, R.T.: How to Calculate ThermalLoadings in Silos, Bulk Solids Handling,Vol. 14, No. 2, pp. 345-349, April/June1994.

One Technology Park Drive • Westford, MA 01886-3189 • Tel: (978) 392-0300 • FAX: (978) 392-9980Also: San Luis Obispo, CA • Toronto, Canada • Viña del Mar, Chile

www.jenike.com

Silo Failures: Case Histories and Lessons Learned 1

byDr. John W. Carson

1 Source: Carson, J. W.: Silo Failures: Case Histories and Lessons Learned, presented at the Third Israeli Conference forConveying and Handling of Particulate Solids, Dead Sea Israel, May 2000

Silos and bins fail with a frequency which ismuch higher than almost any other industrialequipment. Sometimes the failure only involvesdistortion or deformation which, whileunsightly, does not pose a safety or operationalhazard. In other cases, failure involvescomplete collapse of the structure withaccompanying loss of use and even loss of life.

Presented are numerous case histories involvingstructural failure which illustrate commonmistakes as well as limits of design.

1. INTRODUCTION

Although statistics are not available, hundredsof industrial and farm silos, bins, and hoppersexperience some degree of failure each year. [1-3] Sometimes the failure is a complete anddramatic structural collapse. Other times thefailure is not as dramatic or as obvious. Forexample, cracks may form in a concrete wall, ordents in a steel shell, either of which mightappear harmless to the casual observer.Nevertheless, these are danger signals whichindicate that corrective measures are probablyrequired.

The economic cost of a silo failure is neversmall. The owner faces the immediate costs oflost production and repairs, personnel in thevicinity are exposed to significant danger, andthe designer and builder face possible litigationbecause of their liability exposure.

The major causes of silo failures are due toshortcomings in one or more of four categories:design, construction, usage, and maintenance.Each of these is explored below, with examplesand lessons learned.

2. FAILURES DUE TO DESIGN ERRORS

Silo design requires specialized knowledge.The designer must first establish the material'sflow properties [4], then consider such items asflow channel geometry, flow and static pressuredevelopment, and dynamic effects. Problemssuch as ratholing and self-induced silo vibrationhave to be prevented, while assuring reliabledischarge at the required rate. Non-uniformloads, thermal loads, and the effects of non-standard fabrication details must be considered.Above all, the designer must know when to be

2

cautious in the face of incomplete or misleadinginformation, or recommendations that comefrom handbooks, or from people with the “it'salways been done this way” syndrome.

Having established the design criteria, acompetent design has to follow. Here thedesigner must have a full appreciation of loadcombinations, load paths, primary andsecondary effects on structural elements, and therelative flexibility of the elements. [5,6] Specialattention must be given to how the most criticaldetails in the structure will be constructed sothat the full requirements and intent of thedesign will be realized.

Five of the most common problems whichdesigners often ignore are described below,along with a few examples of each.

2.1 Bending of circular walls caused byeccentric withdrawal

This is one of the most common causes of silostructural problems, since it is so oftenoverlooked. It results when the withdrawalpoint from the hopper is not located on thevertical centerline of a circular silo [7,8], and isparticularly common when using silos withmultiple hoppers in which only one or two ofthe hopper outlets are used at a time. If theresulting flow channel intersects the silo wall,non-uniform pressures will develop around thecircumference of the silo leading to horizontaland vertical bending moments. See Figure 1.Many silo designers incorrectly account forthese non-uniform pressures by only increasinghoop tension. [9,10]

Some examples:

• A silo storing sodium sulfate consisted of a4.3 m diameter by 15 m tall cylinder section,below which was a short conical hopper, atransition hopper, and 460 mm diameter

screw feeder. A significant inward dentdeveloped about mid-height in the cylindersection. It extended about one-quarter of theway around the circumference and wascentered slightly offset from the long axis ofthe screw at its back end. The problem wascaused by eccentric withdrawal due to animproperly designed screw feeder. SeeFigure 2.

• A silo consisting of a 3.5!m diametercylinder, 20° (from vertical) cone section,3!m diameter vibrating discharger, andpantleg discharge chute was used to storereground PVC flake. Flow was meteredthrough each chute leg using a rotary valve.The vibrating discharger was used

Fig. 1, Non-uniform pressures caused by eccentricwithdrawal

Fig. 2, Constant pitch screw feeder caused eccentricwithdrawal

3

infrequently (30 sec. on, 5 minutes off), andonly one leg of the pantleg was used most ofthe time. A dent formed in the cylindersection centered over the active pantleg.

• A blending silo utilized 24 external tubes towithdraw plastic pellets at various elevationsfrom the cylinder and cone sections.Significant wrinkles developed in thecylinder section above several of the tubes.

The lessons to be learned here are:

• Whenever possible, design your silo forcenter fill and center withdrawal.

• If eccentric fill or withdrawal iscontemplated, perform a structural checkfirst to make sure that the silo can withstandthe non-uniform loading conditions andresulting bending moments.

• Be particularly careful with silos that havean elongated hopper outlet. An improperlydesigned screw feeder or belt feederinterface, or a partially opened slide gate,will often result in an eccentric flow patternwith accompanying non-uniform loads.

• If a sweep arm unloader is used, be awarethat operating it like a windshield wiper(back-and-forth in one area) will create apreferential flow channel on one side of asilo.

• If multiple outlets are required, considersplitting the discharge stream outside of thesilo below the main central withdrawalpoint.

• If a vibrating discharger is used but notcycled on and off on a regular basis, aneccentric flow channel may form,particularly if a pantleg chute is below theoutlet.

• Consider non-uniform pressures whendesigning silos with blend tubes.

2.2 Large and/or non-symmetric pressurescaused by inserts

Support beams, inverted cones, blend tubes, andother types of internals can impose largeconcentrated loads and/or non-symmetricpressures on a silo wall leading to unacceptablebending stresses.

Two examples:

• A tear developed in the cone section of a4!m diameter silo storing reground polyesterpellets. This tear was located where asupport strut for an inverted conical insertwas welded to the cone wall. Uponemptying the silo, it was found that theinsert support plates were severely deformedand detached from the cone wall.

• Tests showed that a certain agglomeratecould experience particle attrition under theloads generated in a large silo. To reducethe potential of this happening, an insert wasdesigned to be located in the cylindersection of an 8 m diameter silo. This 15 mtall inverted cone extended from just belowthe transition to within 2 m of the top of thesilo. The designers were provided with theloads, which would act on this insert;however, they believed the values to be tooconservative, so they designed the supportstructure for smaller loads. Shortly afterbeing put into operation, the insert supportsfailed, causing the insert to fall and impact aBINSERT“ inner cone below, the supportsof which also failed as a result of the impact.

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Lessons learned:

• Don't ignore loads on inserts, since they canbe extremely large. [11] In addition, non-uniform pressures may develop if the flowpattern around the insert is even slightlyasymmetric.

• Open inserts (such as a BINSERT“ or blendtube) can also have large loads acting onthem. Consideration must be given to theconsequences of the insert becomingplugged, thereby preventing material fromflowing through it. In this case, the verticalload greatly exceeds the dead weight of thematerial inside the insert and the cone ofmaterial above it.

2.3 Ignoring flow patterns and materialproperties

Sometimes mass flow develops in silos, whichwere structurally designed for funnel flow. [4]Even if this doesn't occur, the local pressurepeak, which develops where a funnel flowchannel intersects a silo wall, can bedevastating. [6]

In some circumstances, ignoring the propertiesof the bulk solid to be stored can be worse thanassuming an incorrect flow pattern. Consider,for example, designing a steel silo to store coal.Lacking a sample of coal which could be testedto form the design basis, the designer may resortto an often quoted design code [12] which liststhe wall friction angle for “coal on steel,” withno consideration as to the type of coal, itsmoisture, particle size, ash content, or the typeof steel, its surface finish, etc. Flow andstructural problems are common when thisapproach to design is taken.

Two examples:

• Several bolted silos storing lubricated plasticpellets split apart along a radial seam nearthe top of the hopper section. Although thesilos were designed structurally for funnelflow, no flow tests were performed to see ifthis flow pattern would occur. Lab testsperformed after the failure showed that massflow developed along the 45° cone walls.See Figure 3.

• Two similar bolted silos also storing plasticpellets failed in a similar manner. Lab testsshowed that the wall friction was not lowenough for mass flow. However, the wallfriction angle was much lower than the silodesigner assumed. Thus, less of the pelletmass was supported by shear along thevertical cylinder walls, resulting in muchhigher wall pressures in the hopper than wasassumed by the designer. See Figure 4.

Lessons learned:

• Know your material's flow properties, andthe type of flow pattern that is likely todevelop in your silo. [13]

Fig. 3, Comparison of wall normal pressures due toassumed funnel flow and actual mass flow

5

• If the flow properties are likely to vary (due,for example, to changes in moisture, particlesize, temperature, different suppliers), makesure that the silo is designed to handle thisvariation.

• If your design is close to the massflow/funnel flow limit, consider the possibleeffects of slight changes in materialproperties or the interior surface of the silo(particularly its hopper section). The latteris particularly important if the hopper wallsare likely to be polished with use.

• Buyers beware! If you don't know whichflow pattern is going to develop in your silo,or the possible consequences of designingfor the wrong one, retain the services of asilo expert who can advise you.

• Using tables of values of material propertiesis risky at best and should only be used as alast resort if no samples of the actualmaterial to be stored are available. A betterapproach would be to check with a siloexpert who may have past experiencehandling the material. Inclusion ofadditional safety factors in the design, to

account for unknown variations, is alsooften warranted.

2.4 Special considerations with bolted tanksand reinforced concrete construction

Many silos are constructed of bolted metalpanels (usually steel or aluminum), while othersare constructed of reinforced concrete. Bothtypes of construction have specific designrequirements.

Bolted connections transfer loads throughvarious load paths, and can fail in at least fourdifferent modes: bolt shear, net section tension,hole tear-out, and piling around bolt holes.Which mode results in the lowest failure loaddepends on specifics of the metal (e.g., its yieldand ultimate strengths, thickness), the bolts(e.g., size, strength, whether or not fullythreaded, how highly torqued), spacing betweenbolt holes, number of rows of bolts, etc. [14-16]

Compressive buckling must also be considered,particularly if the bolted silo has corrugatedwalls or is constructed from aluminum.

Reinforced concrete construction presentsdifferent problems [17,18]. Concrete is strongin compression but very weak in tension. Thus,reinforcing steel is used to provide resistance totensile stresses. A silo that has only a singlelayer of horizontal reinforcing steel is capable ofresisting hoop tension, but has very littlebending resistance; therefore if non-uniformpressures occur (e.g., due to an eccentric flowchannel), the silo is likely to crack.Unfortunately, the inside face of the silo wall,where cracks are difficult to detect, is where themaximum tensile stresses due to bending aremost likely to occur. Undetected cracks cancontinue to grow until the silo is in danger ofimminent collapse.

Fig. 4, Comparison of wall normal pressures due toassumed high wall friction and actual low friction

6

An example:

• Vertical cracking of concrete was observedin a 21 m diameter raw coal silo shortly afterit was put into operation. The cracks werelocated in the portion of the silo thatcontained a single layer of reinforcing steel.In an attempt to stop the cracks fromgrowing further, they were injected with anepoxy, but this proved ineffective. Later,post-tensioning strands were added to theoutside of the silo. Five years later, enoughdelamination had occurred on the inside ofthe wall to expose significant lengths ofrebar and allow them to be pulled out anddrop down the wall. Extensive repairs andreinforcing were required in order for thesilo to be used safely.

Lessons learned:

• Consider all the various modes by which abolted joint can fail, and follow recognizeddesign procedures.

• Check to ensure that the design canwithstand compressive buckling.

• Determine the likelihood of eccentric fill ordischarge and design accordingly. Inparticular, do not use a single layer ofreinforcement if eccentric loading ispossible.

2.5 Special considerations concerningtemperature and moisture

The walls of outdoor metal silos can expandduring the day and contract at night as thetemperature drops. If there is no dischargetaking place and the material inside the silo isfree flowing, it will settle as the silo expands.However, it cannot be pushed back up when thesilo walls contract, so it resists the contraction,

which in turn causes increased tensile stresses inthe wall. This phenomenon, which is repeatedeach day the material sits at rest, is calledthermal ratcheting. [19-23]

Another unusual loading condition can occurwhen moisture migrates between stagnantparticles, or masses of stagnant particles, whichexpand when moisture is added to them. If thisoccurs while material is not being withdrawn,upward expansion is greatly restrained.Therefore, most of the expansion must occur inthe horizontal plane, which will result insignificantly increased lateral pressures on, andhoop stresses in, the silo walls.

Two examples:

• A 24 m diameter bolted steel silo storing flyash split apart about two weeks after it wasfirst filled to capacity. Nearly 10,000 tonsof fly ash discharged in the accident, whichoccurred at night when no fly ash was beingfilled into or discharged from this silo.Calculations revealed that the silo wasunderdesigned, and the probable cause offailure was thermal ratcheting.

• A 7.3 m diameter silo stored a mixture ofwet, spent brewer's grains, corn, and otheringredients. No problems occurred as longas the material was not stored for anysignificant time. However, after sittingseveral days without discharge during aholiday period, the silo walls split apartdropping 700 tons of material onto theground. Strain gauge tests in a lab test rigshowed that when moisture migrationcaused the corn particles to swell, pressureson the silo wall increased by more than afactor of five.

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Lessons learned:

• Include factors of safety in the design ofoutdoor metal silos to account for the effectsof thermal ratcheting. [24]

• Assess the likelihood of significant moisturemigration occurring while the bulk solid isstationary, and design accordingly.

3. FAILURES DUE TO CONSTRUCTIONERRORS

In the construction phase, there are two ways inwhich problems can be created. The morecommon of these is poor workmanship. Faultyconstruction, such as using the wrong materialsor not using adequate reinforcement, anduneven foundation settlement are but twoexamples of such a problem.

The other cause of construction problems is theintroduction of badly chosen, or evenunauthorized, changes during construction inorder to expedite the work or reduce costs.

3.1 Incorrect material

Close inspection of contractors’ work isimportant in order to ensure that designspecifications are being followed. This includeschecking for use of correct bolts (size, strength,etc.), correct size and spacing of rebar, specifiedtype and thickness of silo walls, etc.

An example:

• During investigation of the fly ash silofailure described above (2.5), it wasdiscovered that less than 1% of the boltsrecovered had the specified marking on theirhead, and none of these were used in thecritical vertical seams. Strength tests onthese incorrect bolts revealed that some had

tensile strengths less than the minimumrequired for the specified bolts.

Lessons learned:

• Use only qualified suppliers and contractors.

• Closely inspect the installation.

• Make sure that specifications are clear andtightly written [25].

3.2 Uneven foundation settlement

Foundation design for silos is not appreciablydifferent than for other structures. As a result,uneven settlement is rare. However, when itdoes occur, the consequences can becatastrophic since usually the center of gravityof the mass is well above the ground.

Example:

• A 49 m diameter by 14.5 m tall grain siloexperienced a catastrophic failure one coldwinter night. Investigation revealed thatbecause of inadequate design of the concretefooting and changes to it duringconstruction, the foundation wassignificantly weakened. Failure occurredwhen the contents of the silo exertedoutward forces on the steel shell, whichoverloaded the foundation causing it tocrack. The failing foundation in turn pulledout on the steel shell. Low temperaturescreated additional thermal stresses at thebottom of the shell.

Lessons learned:

• Use experienced soils engineers andfoundation designers.

• Use reputable contractors.

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• Closely inspect the work. (See commentsabove in Section 3.1.)

3.3 Design changes during construction

Unauthorized changes during construction canput a silo structure at risk. Seemingly minordetails are often important in ensuring aparticular type of flow pattern (especially massflow), or in allowing the structure to resist theapplied loads.

Example:

• A buckle was observed in the side wall of aspiral aluminum silo storing plastic pellets.Once the silo was emptied it was discoveredthat many of the internal stiffeners had alsobuckled in the region of the shell buckling.Analysis revealed that the most probablecause of buckling was lack of sufficientwelds between the stiffeners and the shell.

Lessons learned:

• Make sure that both the silo builder anddesigner carefully consider and approve anychanges in details, material specifications, orerection procedure.

• Closely inspect all construction.

4. FAILURES DUE TO USAGE

A properly designed and properly constructedsilo should have a long life. Unfortunately, thisis not always the case. Problems can arise whenthe flow properties of the material change, thestructure changes because of wear, or anexplosive condition arises.

If a different bulk material is placed in a silothan the one for which the silo was designed,obstructions such as arches and ratholes mayform, and the flow pattern and loads may be

completely different than expected. The loaddistribution can also be radically changed ifalterations to the outlet geometry are made, if aside outlet is put in a center discharge silo, or ifa flow-controlling insert or constriction isadded. The designer or a silo expert should beconsulted regarding the effects of such changesbefore they are implemented.

4.1 Dynamic loads due to collapsing archesor ratholes, self-induced vibrations, orexplosions

When a poorly flowing material is placed in asilo which was not designed to store and handleit, flow stoppages due to arching or ratholing arelikely. Sometimes these obstructions will clearby themselves, but, more often, operators willhave to resort to various (sometimes drastic)means to clear them. No matter which methodis used, the resulting dynamic loads when anarch or rathole fails can collapse the silo. [26]

Self-induced silo vibrations can also result insignificant dynamic loads for which most silosare not designed to withstand. [27,28] Inaddition, few if any silos can withstand theloads imposed by an explosion -- either internalor external.

Two examples:

• A 13 m diameter by 23 m tall reinforcedconcrete silo stored waste coal. Below thecylinder was a 30° conical hopperterminating at a 4.6 m diameter vibratingdischarger. Flow from the silo wascontrolled by a vibrating pan feeder. Arathole formed above the discharger, thenpartially collapsed. The resulting impactseparated the vibrating discharger from thecone section and drove the vibrating panfeeder into the floor.

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• Three large bolted steel silos were used tostore distiller's dry grain with solubles.Each silo's cylinder section was 7.9 m indiameter by 15 m tall, below which was a30° conical hopper and 3 m diametervibrating discharger. Flow was controlledwith a 300 mm diameter screw feeder.Severe structural damage occurred in allthree silos, including 300 to 900 mmindentations in portions of the cylinderwalls, two completely split radial seams inone of the static hopper sections, and one ofthe vibrating dischargers dropping off fromits supports. The structural problems weredirectly related to the poor flowcharacteristics of the material. In fact, itsflow properties were so poor that plantpersonnel occasionally resorted to usingdynamite to break it up!

Lessons learned:

• Know the flow properties of your materialand the flow properties assumed in thedesign of your silo. If the source of yourmaterial changes, or if you plan to store adifferent material in your silo, have the newmaterial tested for flow properties. Getadvice from experts before putting the newor changed material into your silo.

• Use extreme caution in attempting to restoreflow if an arch or rathole forms. Underthese circumstances, personnel should not beallowed to be in close proximity to the silo.Consideration should be given to top reclaimusing experts trained in this technique.

• Avoid accumulations of dust or ignitablegases, which could cause an explosion.

4.2 Changes in flow patterns

Changing material properties or polishing of theinside surface of the silo may cause mass flow

to develop in a silo which was structurallydesigned for funnel flow. (The opposite canalso occur – funnel flow in a silo designedstructurally for mass flow – but this generally isnot as serious a problem.) Mass flow will resultin a dramatically different wall pressure loadingthan with funnel flow, particularly at the top ofthe hopper section.

Two examples:

• Six 7.9 m diameter by 22 m. tall silos wereused to store high-density polyethylene fluffand pellets. Below each cylinder sectionwas a 30° cone terminating at a rotary valvefeeder. A radial hopper seam split open onone silo, spilling one million pounds ofmaterial onto the ground. The cause of thisfailure was determined to be mass flowloads. The silo was structurally designedonly for funnel flow. See Figure 5.

Fig. 5, End-result of mass flow developing in a silodesigned structurally for funnel flow

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• Four outdoor bolted silos were used to storebarley and corn. As with the previousexample, failure occurred by splitting of aradial seam near the top of the hopper,which was the result of unexpected massflow loads. In this case, the cone walls wereapparently polished by the barley, and thewall friction decreased further when theoutside air temperature dropped belowfreezing.

Lessons learned:

• Know your material's flow properties andthe flow properties used in the design.Avoid materials and/or conditions that couldresult in a flow pattern for which the silowas not designed.

• Routinely inspect the interior of your silo,checking for abrasion marks, which mayindicate mass flow. [29]

• Inspect the exterior of a bolted silo on aregular basis. Pay particular attention to thebolted joints near the top of the hopper,noting any waviness along the edges of thesheets, elongation of bolt holes, or cracksbetween bolt holes, all of which are signs ofover-stress.

4.3 Buckling of unsupported wall

A pressurized cylinder is more resistant tocompressive buckling than an unpressurizedone. [9] In addition, if this pressure is caused bya bulk solid (as opposed to a liquid or gas), it iseven more resistant. The reason is as follows:Gas or liquid pressure is constant around a silo'scircumference and remains unchanged as thesilo starts to deform. On the other hand, thepressure exerted by a bulk solid against a silo'swall increases in areas where the walls aredeforming inward, and decreases where the

walls are expanding. This provides a significantrestraining effect once buckling begins.

Now consider what happens if an arch formsacross a silo's cylinder section, and materialbelow it is withdrawn. Not only is therestraining effect of the bulk solid lost, but thefull weight of the silo contents above the archare transferred to the now unsupported region ofthe silo walls. Buckling failure is likely whenthis occurs.

Example:

• A 7.6 m diameter by 27 m tall bolted flat-bottom silo was used to store soybean meal.Discharge occurred by a sweep arm screwunloader. The material's flow propertiesvaried considerably, from free flowing toextremely cohesive. An arch formed abovethe unloader, and spanned the full diameterof the silo. Material below this wasremoved by the unloader, so the full onemillion pounds was transferred to theunsupported thin silo wall causing it to failby vertical buckling. See Figure 6.

Lessons learned:

• Know your material's flow properties.

• If flow stops, investigate the cause beforeattempting to restart discharge.

Fig. 6, Buckling of unsupported wall above a sweep armunloader

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5. FAILURES DUE TO IMPROPERMAINTENANCE

Maintenance of a silo comes in the owner's oruser's domain, and must not be neglected. Twotypes of maintenance work are required. Thefirst is the regular preventative work, such as theperiodic inspection and repair of the wallsand/or liner used to promote flow, protect thestructure, or both. Loss of a liner may beunavoidable with an abrasive or corrosiveproduct, yet maintaining a liner in properworking condition is necessary if the silo is tooperate as designed. Other examples ofpreventative maintenance items include roofvents, level probes, feeders, dischargers, andgates.

The second area of maintenance involveslooking for signs of distress (e.g., cracks, walldistortion, tilting of the structure) and reactingto them. [29] If evidence of a problem appears,expert help should be immediately summoned.An inappropriate response to a sign thatsomething is going wrong, including thecommon instinct to lower the silo fill level, cancause a failure to occur with greater speed andperhaps greater severity.

5.1 Corrosion and erosion

Silo walls thinned by corrosion or erosion areless able to resist applied loads than when theywere new. This is a particular problem whenhandling abrasive materials or when usingcarbon steel construction in moist or otherwisecorrosive environments. Combining the effectsof abrasion with corrosion significantlyaccelerates the problem. This can occur, forexample, with special aging steels. Abrasivewear causes the surface layer to be removed,thereby exposing new material and speeding upthe aging process which significantly weakensthe structure.

Three examples:

• A coal silo was fabricated from aging steel.After about five years of use, the hopperdetached from the cylinder section while thesilo was full. The cause was determined tobe thinning of the silo wall due to abrasionfrom coal and corrosion.

• A tile silo storing coal failed after manyyears in use. This progressive failureoccurred because of weathering effects onthe exterior and corrosive conditions due towet coal on the interior. These combined tocorrode the steel reinforcing bars, whichthen failed.

• Six coal silos at a chemical plant lasted forabout 30 years, after which time two of thesix experienced a structural failure, whichprompted a close inspection of all six silos.The carbon steel walls were found to havethinned significantly, to the point that actualholes were visible in places. Corrosion,both exterior and interior, was to blame.

Lessons learned:

• Carefully inspect your silos on a regularbasis. Determine the minimum wallthickness required for structural integrityand compare to the actual wall thickness.

• Do not use aging steels for silo constructionif the surface will be exposed to abrasivewear.

• Prevent buildup of material, which couldtrap moisture on the exterior of outdoorsilos.

5.2 Lack of routine inspection

Silo failures often cause significant damage andsometimes result in death. Often these failures

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could have been prevented or the damage couldhave been minimized with information thatcould have been gained through routineinspection.

Example:

• The hopper section of a stone bin at amining operation fell off when the bin wasfull, killing a person working below. Theproblem was particularly attributed tomaterial buildup on horizontal externalstructural members which, combined withmoisture from the air, created a corrosiveenvironment, resulting in excessive thinningof the silo wall.

Lessons learned:

• Inspect silos routinely, both internally andexternally. [29] This is particularlyimportant with bolted and reinforcedconcrete silos, and silos which are exposedto a corrosive environment. For example,look for any signs of corrosion, exposedrebar, unusual cracking, or spalling ofconcrete.

• If conditions change (e.g., a differentmaterial is to be stored) or unusual eventsoccur (e.g., very high winds, an earthquake),inspect the silo before putting it back inoperation.

• Perform a detailed structural inspectionbefore designing modifications to a silo.

5.3 Improper reaction to signs of distress

A common reaction to signs of silo distress is toignore them, often because personnel areunaware of both the meaning and consequencesof doing so. Another common reaction iscuriosity. People have lost lives because, due totheir curiosity, they were in the wrong place at

the wrong time. Even if danger signs areunderstood, it is common for inappropriateaction to be taken in an attempt to “reduce” thechance of failure. In some extreme cases,catastrophic failure has been induced where,with appropriate action, the damage could havebeen relatively minor.

Two examples:

• A bolted steel silo with a sweep armunloader was used to store soybean meal.The meal hardened, so the sweep arm wasoperated back and forth to try to dischargethe meal. This process continued for sometime, even though wrinkles were observed inthe silo wall above the area where the sweeparm was operating. Eventually theindentations became so great that the silocollapsed.

• Another bolted silo storing grain stood upsome 14 years before failure. Shortly afterstartup in the spring after the grain had beensitting essentially stationary all winter, thesilo started tilting at approximately mid-height. Not realizing the consequences ofcontinued withdrawal, the owner operatedthe discharge system. Two days later, thesilo collapsed completely.

Lessons learned:

• Since a weakened silo is a very dangerousstructure, limit access to the areasurrounding it to only those personnel whoneed to be there, and make sure that theyhave the education and experience to dealwith the situation. Extreme caution shouldalways be exercised.

• At the first sign of silo distress, ceasedischarging immediately and assess theintegrity of the structure.

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• Investigate the cause of the distress. Retainexperts with knowledge of silo structures toassist in the investigation.

6. CONCLUSIONS

Silos that are designed, built, operated, andmaintained properly, will provide long life.Each of the case histories given above illustratesthe effects of one or more of the shortcomingspossible in design, construction, usage, andmaintenance. In each example, the cost ofrepairs or rebuilding, the cost of litigation, andthe cost of insurance added up to several timesthe cost of doing the job properly in the firstplace.

The best approach to the design of a silo, bin, orhopper for bulk materials is one that is reasoned,thorough, conservative, and based on measuredparameters. Design engineers are not legallyprotected by sticking to a code of practice.Compliance with the locally applicable code is,of course, necessary, but it should never beregarded, by itself, as a sufficient condition tothe performance of a satisfactory design.

It is the responsibility of the designer to ensurethat the design is based on sound, completeknowledge of the materials being handled, thatthe design is competent, and that it covers allforeseeable loading combinations. It is the jointresponsibility of the designer, builder, andowner that construction is of an acceptablestandard, and fulfills the intent of the design. Itis then the responsibility of the owner toproperly maintain the structural and mechanicalcomponents. It is also the responsibility of theowner to ensure that any intended alteration inusage, discharge geometry or hardware, linermaterial, or any other specified parameter, ispreceded by a design review with strengtheningapplied as required.

REFERENCES

1. R. T. Jenkyn and D. J. Goodwill, SiloFailures: Lessons to be Learned, EngineeringDigest, Sept. 1987.

2. J. W. Carson and D. J. Goodwill, The Designof Large Coal Silos for Safety, Reliability andEconomy, Bulk Solids Handling 4, pp. 173-177,1984.

3. J. Ravenet, Silos: Deformaciones – Fallas –Explosiones, Prevencion De Accidentes,Editores Técnicos Asociados, s.a.

4. A. W. Jenike, Storage and Flow of SolidsUniversity of Utah Engineering ExperimentStation, Bulletin No. 123, Nov. 1964.

5. A. W. Jenike, Effect of Solids FlowProperties and Hopper Configuration on SiloLoads, In Unit and Bulk Materials Handling(Loeffler, F.J., and C.R. Proctor, eds.), ASME,1980, pp 97-106.

6. J. W. Carson and R. T. Jenkyn, LoadDevelopment and Structural Considerations inSilo Design, Presented at Reliable Flow ofParticulate Solids II, Oslo, Norway, August1993.

7. A. W. Jenike, Denting of Circular Bins withEccentric Drawpoints, Journal of the StructuralDivision, Proceedings of the American Societyof Civil Engineers 93, pp. 27-35, 1967.

8. T. Johnston, Analysis of Silo Failures fromAsymmetric Flow, Presented at the 1991 SpringConvention, American Concrete Institute,Boston, MA, March 17-21, 1991.

9. E. H. Gaylord and C.N. Gaylord, Design ofsteel bins for storage of bulk solids, Prentice-Hall, 1984.

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10. G. E. Blight, Defects in accepted methods ofestimating design loadings for silos, Proc. InstnCir. Engrs. 88 Part 1, pp. 1015-1036, Dec.1990.

11. J. R. Johanson and W.K. Kleysteuber,Structural Support of Flow-Corrective Inserts inBins, Proceedings AIChE, Atlantic City,September 1966.

12. ACI Standard 313-91, Standard practice fordesign and construction of concrete silos andstacking tubes for storing granular materials1991.

13. J. W. Carson and J. Marinelli, CharacterizeBulk Solids to Ensure Smooth Flow, ChemicalEngineering 101, no. 4, pp. 78-90, April 1994.

14. AWWA Standard D103-87, Factory-coatedbolted steel tanks for water storage 1987.

15. Specification for the design of cold-formedsteel structural members American Iron andSteel Institute, 1986.

16. G. L. Kulak, J.W. Fischer and J.H.A. Struik,Guide to design criteria for bolted and rivetedjoints, 2nd ed. John Wiley, 1987.

17. D. F. Themer, Failures of ReinforcedConcrete Grain Silos, Transactions of the ASMEJournal of Engineering for Industry, pp. 460 to477, May 1969

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19. G. E. Blight, Temperature changes affectpressures in steel bins, International JournalBulk Solids Storage in Silos 1 No. 3 1985, pp. 1to 7.

20. G. E. Blight, Temperature surchargepressures in reinforced concrete silos, PowderHandling and Processing 2 No. 4, November1990, pp. 303 to 305.

21. G. E. Blight, Measurements on full sizesilos part 1: temperatures and strains, BulkSolids Handling 7, No. 6 December 1987 pp.781 to 786.

22. G. E. Blight, Temperature-induced loadingson silo walls, Structural Engineering Review 4No. 1, 1992, pp. 61-71.

23. H. B. Manbeck, Predicting thermallyinduced pressures in grain bins, Trans. ASAE 27No. 2 1984, pp. 482-486.

24. ASAE Engineering Practice, EP433 Loadsexerted by free-flowing grains on bins ASAEStandards 1991.

25. J. W. Carson, R. T. Jenkyn, and J. C.Sowizal, Reliable and Economical Handling ofBulk Solids at Coal-Fired Power Plants, BulkSolids Handling 12 No. 1, Feb. 1992, pp 11-18.

26. G. Gurfinkel, Tall Steel Tanks: Failure,Design, and Repair, ASCE Journal ofPerformance of Constructed Facilities, Vol. 2,No. 2, May 1988, pp. 99 to 110.

27. H. Purutyan, K. E. Bengtson, and J. W.Carson, Identifying and Controlling SiloVibration Mechanisms: Part I, Powder andBulk Engineering 8 No. 11, Nov. 1994, pp 58-65.

28. H. Purutyan, K. E. Bengtson, and J. W.Carson, Identifying and Controlling SiloVibration Mechanisms: Part II, Powder andBulk Engineering 8 No. 12, Dec. 1994, pp 19-27.

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29. J. W. Carson and R. T Jenkyn, How toPrevent Silo Failure with Routine Inspectionsand Proper Repair, Powder and BulkEngineering 4 No. 1, January 1990.