Tips
A silo design checklist Tim Ti m J. J.Le Leas asee
WL Por Port-L t-Land andSys System tems, s, Inc Inc..
Designing a silo Designing silo that that will handl handlee yourr dry bul you bulk k mate materia rial’ l’ss char characacteristics, your operational requirements, and your geographical location is a complex task ta sk.. Th Thee ch chec eckl klis istt in th this is ar arti ticl clee outlines some of the key silo designfact sign factors ors yo you u need needto to cons consider ider..
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o de desi sign gn a si silo lo th that at wi will ll re reli liab ably ly store st ore an and d di disch schar arge ge yo your ur ma mate te-riall ov ria over er th thee lo long ngte term rm,, yo you u ne need ed to ga gath ther er a ra rang ngee of in info form rmat atio ion n ab abou outt yourr dry bu you bulk lk mat materi erial, al, app applic licati ation on requireme qui rements nts,, and inst install allati ation on sit site. e. This includes identifying your material’s unique uni quepro propert perties ies,, ho how w the thesil silo o wil willl operat er ate, e, wh what at ro roof of an and d wa wall ll op open enin ings gs it willl req wil requir uire, e, wha whatt int intern ernal al and ex exter ternal nal load lo adss wi will ll be ex exert erted ed on th thee ve vesse ssel, l, an and d how the silo’s geographical location willl af wil affec fectt th thee str struct ucture ure.. Wh Wheth ether er yo your ur company contracts an independent consul con sultin ting g eng engine ineer er or a sil silo o sup suppli plier er to desig des ign n th thee sil silo, o, th thee fol follo lowi wing ng ch chec eckli klist st of cri criti tica call fa fact ctor orss to co cons nsid ider er ca can n he help lp yougetstarte you getstarted. d.
J Coefficient of friction between the mat the materi erial al and sil silo o wa wall ll,, hop hoppe perr wall, wal l, and othe otherr surf surfaces aces
Moisturecontent content J Moisture J Temperature Partic ticle le siz sizee (di (distri stribu butio tion n and andmax max-J Par
imum imu m and andmin minimu imum m part particl iclee size sizes) s)
J Abrasiveness J Corrosiveness J Friability J Explosibility or deflagration index inde x (KSt value)
For example, your material’s bulk density will be used to calculate the
silo’ss vo silo’ volu lume me and the str struct uctura urall lo loads ads on th thee ve vesse ssel. l. Th Thee an angl glee of rep reposewill osewill help the design engineer determine the silo’s hoop (or circumferential) tension, useful volume, and hopper geometry. The coefficient of friction will be used to calculate the vertical load lo adss on th thee si silo lo wa wall ll an and d ho hopp pper er du durring discharge and to design the hopper. Particle size information can be usedto used to det determi ermine ne whe whethe therr you yourr mat mateerial is likely to segregate during silo fill and discharge and thus whether the silo’s hopper and other elements mustt be de mus desi sign gned ed to pr prev even entt th this is pro probblem. The other material characteristics listed here will also affect which structur stru ctural al elem elements ents,, con constru structio ction n materials, and components the engineer selec sel ects ts for you yourr sil silo. o. If yo you u do don’ n’tt ha have ve previous experience in storing and handling the material your silo will hold, it’s wise to have it tested by an independen indep endentt materia materiall charac characteriza terization tion lab to identify and quantify as many of the these se char characte acterist ristics ics as poss possible ible.. The silo’ silo’ss oper operatio ational nal requirements To de desig sign n a pro prope perly rly per perfor formin ming g sil silo, o, you yo u nee need d to toun under dersta stand ndho how w th thee sil silo o wi will ll
Your material’ material’ss characteristics In a silo, the cylinder and hopper geometries and other structural elements men ts are aredes design igned ed to ha handl ndlee a par partic tic-ular dry bulk material with specific characteristics. characterist ics. Here are the major material mate rial cha charact racterist eristics ics to iden identify tify:: Bulk lk de densi nsity ty (in (inpo poun unds ds pe perr cu cubic bic J Bu foot or kilograms per cubic meter)
Angl glee of rep repose ose J An
Success Succe ssfu fully lly des design igning ing a sil siloo re requ quire iress an inin-dep depth th un under derst stand anding ing of you your r material’s characteristics, your operational requirements, and the silo’s geographical geograp hical location.
function in your application. Consider thesecriticaloperatingrequirements:
Internaland external loads on the silo
J What storage capacity the silo
You mustanticipate whatinternaland external loads, including peripheral and imposed loads, will be exerted on the silo by the structures and components included in its design. Typical examples of silo components that exertsuch loads are:
requires
J Whether the silo will provide
long-term storage or be filled and discharged frequently
J Whether the silo must minimize material degradation
J Whether the silo must minimize material segregation
J Whether the silo must provide concentric or eccentric discharge
J How many discharge outlets the silomust have
J Whether the silo must provide completecleanout at discharge
J What fill and discharge flowrate capacities the silomust handle
J Whetherthe silo must beequipped with aeration or fluidization devicestoaidflow
J What service life the silo must provide
J Piping along the silo wall or
table’s elevation, theboringrefusal elevation (at which an obstruction,such as a rock layer, prevents the boring drill fromprogressingfarther), the expected total and differential soil settlement based on the silo structure’s anticipated dead and live loads, recommended ground improvements (such as overexcavating and backfilling) if your site requires such improvements, and the recommended foundation type (such as shallow, piling, or other).
J Equipment such as baghouses,
Putting it all together
J Ladders and stairs acrossthe roof
conveyors, and jib cranes
J Floors inside the silo to provide support for or access to conveyors, discharge equipment, or process equipment below the hopper J Bridge and tower supports for conveyors leading to the siloroof
J Supportsfor material-level-probe and temperature-probe cables
The silo’s geographical location The silo’s required openings You also need to determine where the silo will be penetrated by equipment or have other openings so the silo can be designed to accommodate them. Silo wall, roof, or hopper openings are typically requiredfor:
J Manways, doors, and other accessopenings for workers
J Material fill inlets outlets J Materialdischarge outlets J Truck or railcar drive-through doors
J Conveyors J Material-level and temperature probes
J Vents for r eleasing dust, gas fumes, heat,or moisture
J Explosion relief or suppression devices
J Aerationor fluidization devices
As with any design project, your silo design’s success depends on accurately defining and understanding your design assumptions. While the checklist here covers the major factors to consider in designing a silo, the list isn’t exhaustive. Expect to work closely with the design engineer and share many additional details about your storage and handling needs as the design project progresses. The result will be a silo that meets your needs and performs reliably for many years. PBE
Some or all of the following design parameters will depend on your silo’s location:
J Buildingcode requirements J Zoning requirements (including zoning limitations that may require a variance)
J Wind loadrequirements J Snow loadrequirements J Seismic requirements J Frost requirements J Silo footprint and height restric-
For further reading Find more information on designing silos and other storage vessels in articles listed under “Storage” and information on defining material characteristics in articles listed under “Particle analysis” in Powder and Bulk Engineering’s comprehensive article index at www.powderbulk .com and in the December 2008 issue.
tions
J Soil conditions The last parameter — soil conditions — will affect your silo’s foundation design.To determine these conditions at your site, your company(or thesilo supplier) must contract an independent geotechnical firm to conduct a soil analysis. This analysis includes soil borings and a geotechnical report thatwilltypically cover the following site information: the allowable soilbearing capacity, the groundwater
Tim J. Lease, PE, is president of WL Port-Land Systems, Inc., 305 Mount Lebanon Boulevard, Ste. 400, Pittsburgh, PA 15234; 412-344-1408, fax 412-344-1412 (
[email protected], www.wlport-land.com). He has 25 years of experience in bulk solids storage andhandling.
