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  FlowPropertiesofPowdersandBulkSolidsandSiloDesignforFlow DietmarSchulzeThe phrase good flow behavior usually means that apowder or bulk solid flows easily. Products are poorlyflowing iftheyexperienceflowobstructions(archesor pipes, figure 1), or consolidate during storage or transport. The reason for the latter is the compressivestrength(unconfinedyieldstrength)ofabulksolid.Fig.1:Arching(ontheleft),piping(ontheright)Fig.2: Measurementofunconfinedyieldstrengthff = / (1)Fig.3: Unconfined yield strength in dependence onconsolidation stress; lines of constantflowabilityff  Additionally,infigure3theboundariesoftherangesof the classifications listed above are shown as straightlines. The ratio ff and thus the flowability of a specificbulk solid change with consolidation stress .Therefore, for flowability measurements testers arerequired which make possible the adjustment of defined consolidation stresses. This is fulfilled byappropriatesheartesters.Fig.4: RingShearTester(shearcell,inprinciple) FlowabilityRingsheartester  Figure2showsahollowcylinderwithfrictionlesswalls,filled with a fine-grained, cohesive bulk solid. First thebulk solid is consolidated by the consolidation stress. Subsequently the hollow cylinder is removed andthe cylindrical bulk solid specimen is loaded with anincreasing vertical compressive stress until thespecimen (f ls). The stress failure iscalled unconfined yieldstrength .The unconfined yield strength typically increaseswithconsolidationstress .CurveAshowsthetypicalincrease of unconfined yield strength in depend-enceon consolidationstress (figure3). theratio ff of consolidation stress to unconfined yieldstrength is used to characterize flowability numeri-cally:The larger ff , the better a bulk solid flows. Often thefollowingclassificationisapplied:ff <1 notflowing1<ff <2 verycohesive(tonon-flowing)2<ff <4 cohesive4<ff <10 easy-flowing10<ff free-flowingTypical shear testers are the Schulze Ring Shear Testers(figures4and5)[1,3]whicharewidelyusedinindustryandresearch.  1cc1c1c 1cc 1 cccccccc1 breaks ai causingcompressive strength or Usually   cnot flowing very cohesivecohesivefree-flowingeasy-flowingff = 1 c ff = 2 c ff = 4 c ff = 10 c  1              c   A area A  Schwedes + Schulze Schüttguttechnik GmbHConsultants (silo design and powder testing) Am Walde 3, D-38302 WolfenbüttelPhone +49 5331 8573-312, Fax -313E-Mail: mail@schwedes-and-schulze.comInternet: www.schwedes-and-schulze.comDr. Dietmar Schulze SchüttgutmesstechnikRing shear testers and software Am Forst 20, D-38302 WolfenbüttelPhone +49 5331 9-35490, Fax -78001E-Mail: mail@dietmar-schulze.comInternet: The bulk solid specimen (figure 4) is contained in anannularshearcell( ring )andloadedfromthetopwitha vertically acting force N through the lid in order toadjust the stress level. During testing the shear cellrotates slowly in direction of arrow , while the lid isprevented from rotation by two tie rods. Thereby thebulksolidspecimenissheared.Forces(F +F )actingin the tie rods are measured. From the results of aprescribed test procedure the flow properties of thebulksolidarecalculated[1,2,3].  1 2 With the automatic Ring Shear Tester RST-01.pc(figure5,right)thetestprocedure,includingloadingof the specimen and shearing, and also the evaluationare performed computer-controlled. For very smallsamples of fine powders as occurring, e.g., inpharmaceutical industry, the smaller automatic RingShear Tester RST-XS.s is available, which is alsooperatedcomputer-controlled(figure5,left).Fig.5: Computer-controlledRingShearTestersRST-01.pcandRST-XS.s Advantages of Ring Shear Testers RST-01.pc andRST-XS.sare:Measurement of well defined flow properties, e.g.,flowability, caking, bulk density .....Different stress levels.Easy operation. Automatic testing & evaluation.Thus a Ring Shear Tester is an advantageousalternative to so-called simple flowability testers,since it is easy to operate, whereby neverthelessclearlydefinedphysicalquantitiesaremeasured[2,3].When a silo is discharged, two different flow patternscan prevail: mass flow and funnel flow (figure 6) [3,4].Incaseofmassflowthewholesilocontents,i.e.,everyparticle, move during discharge. Mass flow is onlypossible if the hopper walls are sufficiently steepand/or smooth. If the latter is not the case, funnel flowprevails. In case of funnel flow, only a portion of thebulk solid in the silo moves downwards duringdischarge while the rest of the bulk solid remainsstationarythusformingstagnantzones.Themostfrequentproblemsemergingwhenpowdersorbulksolidsarestoredordischargedare[3,4]: Arching (figure 1)Piping (figure 1)Irregular flow and floodingBroad residence time distribution: The bulk solid inthe stagnant zones of a funnel flow silo remains inthesiloforextremelylongtimeperiodsSegregation due to particle size, density or shape,which leads to an unsteady product composition attheoutletinfunnelflowsilosFig.6: Massflow(ontheleft),funnelflow(ontheright)In a funnel flow silo all problems listed above canappear, while in the case of mass flow only theproblemofarchingmustbeconsidered.Alsopracticalexperience shows that funnel flow is a primary causeofflowproblems.For the design of a mass flow silo the slope of thehopper walls to ensure mass flow and the minimumoutletdimensionrequiredtoavoidstablearcheshaveto be determined. For this the flow properties of thebulk solid have to be measured with an appropriateshear tester [3]. With sufficient experience in theapplication of these quantities one can design newsilos as well as redesign existing silos or other equipmentexhibitingflowproblems.[1] Schulze, D.: Powder Handling & Processing8 (1996) 3, pp. 221-226[2] Schulze, D.: Powder & Bulk Engineering 10(1996) 4, pp. 45-61 and 10 (1996) 6, pp. 17-28[3] Schulze, D.: Powders and Bulk Solids -Behavior, Characterization, Storage and Flow,Springer, Berlin Heidelberg New York (2008)[4] Jenike, A.W.: Storage and Flow of Solids, Bull.123, Engng. Exp. Station, Univ. Utah, Salt LakeCity (1964)  FlowofbulksolidsinsilosLiterature (c) D. Schulze 2014. All rights reserved.Misprints and errors excepted. RST-XS.s RST-01.pc
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