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Designing a silo that will handleyourdrybulkmaterial’s charac-teristics, your operational re-quirements, and your geo-graphical location is a complextask. The checklist in this articleoutlines some of the key silo de-sign factorsyouneed toconsider.

Todesign a silo that will reliablystore and discharge your mate-rial over the long term,youneed

to gather a range of information aboutyour dry bulkmaterial, application re-quirements, and installation site. Thisincludes identifying your material’suniqueproperties, how the silowill op-erate, what roof and wall openings itwill require, what internal and externalloadswill be exerted on the vessel, andhow the silo’s geographical locationwill affect the structure.Whether yourcompany contracts an independentconsultingengineeror a silo supplier todesign the silo, the following checklistof critical factors to consider can helpyougetstarted.

Yourmaterial’s characteristics

In a silo, the cylinder and hoppergeometries and other structural ele-ments are designed to handle a partic-ular dry bulk material with specificcharacteristics. Here are the majormaterial characteristics to identify:

� Bulk density (in pounds per cubicfoot or kilograms per cubicmeter)

� Angle of repose

� Coefficient of friction betweenthematerial and silo wall, hopperwall, andother surfaces

� Moisture content

� Temperature

� Particlesize (distributionandmax-imumandminimumparticlesizes)

� Abrasiveness

� Corrosiveness

� Friability

� Explosibility or deflagrationindex (K

Stvalue)

For example, your material’s bulkdensity will be used to calculate the

silo’s volume and the structural loadson thevessel. The angle of reposewillhelp the design engineer determinethe silo’s hoop (or circumferential)tension, useful volume, and hoppergeometry. The coefficient of frictionwill be used to calculate the verticalloads on the silowall and hopper dur-ing discharge and to design the hop-per. Particle size information can beused todeterminewhether yourmate-rial is likely to segregate during silofill and discharge and thus whetherthe silo’s hopper and other elementsmust bedesigned toprevent this prob-lem. The other material characteris-tics listed here will also affect whichstructural elements, construction ma-terials, and components the engineerselects for your silo. If you don’t haveprevious experience in storing andhandling the material your silo willhold, it’s wise to have it tested by anindependentmaterial characterizationlab to identify and quantify as manyof these characteristics as possible.

The silo’s operationalrequirements

To design a properly performing silo,youneedtounderstandhowthesilowill

A silo design checklistTimJ.Lease WLPort-LandSystems, Inc.

Tips

Successfully designing a silo requires an in-depth understanding of yourmaterial’s characteristics, your operational requirements, and the silo’sgeographical location.

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function in your application. Considerthesecriticaloperatingrequirements:

� What storage capacity the silorequires

� Whether the silo will providelong-term storage or be filled anddischarged frequently

� Whether the silo must minimizematerial degradation

� Whether the silo must minimizematerial segregation

� Whether the silo must provideconcentric or eccentric discharge

� How many discharge outlets thesilomust have

� Whether the silo must providecomplete cleanout at discharge

� What fill and discharge flowratecapacities the silomust handle

� Whether thesilomustbeequippedwith aeration or fluidization de-vices toaid flow

� What service life the silo mustprovide

The silo’s required openings

You also need to determinewhere thesilo will be penetrated by equipmentor have other openings so the silo canbe designed to accommodate them.Silo wall, roof, or hopper openingsare typically required for:

� Manways, doors, and other ac-cess openings forworkers

� Material fill inlets outlets

� Material discharge outlets

� Truck or railcar drive-throughdoors

� Conveyors

� Material-level and temperatureprobes

� Vents for releasing dust, gasfumes, heat, ormoisture

� Explosion relief or suppressiondevices

� Aerationor fluidizationdevices

Internal and external loadson the silo

Youmust anticipatewhat internal andexternal loads, including peripheraland imposed loads,will be exerted onthe silo by the structures and compo-nents included in its design. Typicalexamples of silo components thatexert such loads are:

� Ladders and stairs

� Piping along the silo wall oracross the roof

� Equipment such as baghouses,conveyors, and jib cranes

� Floors inside the silo to providesupport for or access to convey-ors, discharge equipment, orprocess equipment below thehopper

� Bridge and tower supports forconveyors leading to the silo roof

� Supports formaterial-level-probeand temperature-probe cables

The silo’s geographical location

Some or all of the following designparameterswill depend on your silo’slocation:

� Building code requirements

� Zoning requirements (includingzoning limitations that may re-quire a variance)

� Wind load requirements

� Snow load requirements

� Seismic requirements

� Frost requirements

� Silo footprint and height restric-tions

� Soil conditions

The last parameter— soil conditions— will affect your silo’s foundationdesign.Todetermine these conditionsat your site, your company (or the silosupplier) must contract an indepen-dent geotechnical firm to conduct asoil analysis. This analysis includessoil borings and a geotechnical reportthatwill typically cover the followingsite information: the allowable soil-bearing capacity, the groundwater

table’s elevation, theboring refusal el-evation (atwhich anobstruction, suchas a rock layer, prevents the boringdrill fromprogressing farther), the ex-pected total and differential soil set-tlement based on the silo structure’santicipated dead and live loads, rec-ommended ground improvements(such as overexcavating and backfill-ing) if your site requires such im-provements, and the recommendedfoundation type (such as shallow, pil-ing, or other).

Putting it all together

As with any design project, your silodesign’s success depends on accu-rately defining and understandingyour design assumptions. While thechecklist here covers the major fac-tors to consider indesigning a silo, thelist isn’t exhaustive. Expect to workclosely with the design engineer andshare many additional details aboutyour storage and handling needs asthe design project progresses. The re-sult will be a silo that meets yourneeds and performs reliably formanyyears. PBE

For further reading

Find more information on designingsilos and other storage vessels in arti-cles listed under “Storage” and infor-ma t ion on defin ing mate r i a lcharacteristics in articles listed under“Particle analysis” in Powder andBulk Engineering’s comprehensivearticle index at www.powderbulk.comand in theDecember2008 issue.

Tim J. Lease, PE, is president of WLPort-Land Systems, Inc., 305 MountLebanon Boulevard, Ste. 400, Pitts-burgh, PA 15234; 412-344-1408, fax412-344-1412 (tlease@wlport-land.com, www.wlport-land.com).He has 25 years of experience in bulksolids storageandhandling.