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2.1.2010

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3.12.2009

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3.12.2009

17.11.2009

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1.11.2009

30.10.2009

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27.9.2009

2.9.2009

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31.8.2009

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31.8.2009

27.8.2009

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27.8.2009

25.5.2009

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15.5.2009

31.3.2009

24.3.2009

9.3.2009

18.2.2009

17.12.2008

5.12.2008

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31.10.2008

17.6.2007

21.3.2007

28.6.2006

12.4.2006

30.1.2006

3.12.2005

5.3.2005

27.1.2000

New glossary of terms on storage and flow of particulate solids

17.11.1999

Prof. Michael Rotter, Ed U, Scotland, UK

The following are taken from the forthcoming Guide to the Economic Design of Metal Silos, to be published by Elsevier under the auspices of the British Materials Handling Board early in 2000. Aeration: Where air is entrained in a fine particulate solid (powder), the solid is said to be aerated. This can arise either from deliberate air injection, from pneumatic conveying or from the filling or discharge processes. Apex of hopper A conical hopper is terminated by the outlet. However, the geometry of the cone is defined in terms of the geometric apex, which is a point in space below the outlet. Arching Formation of an arch, bridge or dome in the stored solids over the silo outlet, usually in the hopper. This term refers only to stable static arches, which arrest the flow. Axisymmetric The term should not be applied, as in the present Glossary, to silos with polygonal cross-sections. It has a strict meaning in mechanics for bodies which can be described in a cylindrical coordinate system and have phenomena which are invariant with respect to the circumferential coordinate. Barrel section The upper part of a silo with vertical parallel walls. Bin This term should simply be synonymous with the term silo. To give it an alternative meaning for only part of the silo makes it ambiguous and unclear. Bending moment The bending moment per unit width of wall is a couple within the shell wall (no net force resultant). Bending moments can be in the vertical or circumferential direction, according to the direction of the stresses induced by the bending moment (note contrast with beam theory). Cohesion The component of the shear strength of a particulate solid which is independent of the applied normal stress. According to Eurocode 1 Part 4 Silos and Tanks, a solid is termed one of low cohesion if the unconfined yield stress is less than 14 kPa when the solid is pre-compressed to a stress of 100 kPa. Consolidation The process by which the particles of a stored solid are squeezed together into a tighter packing, principally by the application of stress (especially shear stress). In soil mechanics, this refers to the movement of interstitial water, but for dry granular solids in silos, whilst it may involve interstitial air movement, it is not concerned with air pressures. Effective transition The position on vertical wall of a mixed flow silo where the flow channel boundary strikes the barrel section vertical wall. Equivalent surface The level to which the silo would be filled if the same volume of material were arranged with a level surface Expanded flow A flow pattern combining mass flow within a lower hopper with funnel flow in the upper part Feeder A device conveying material away from the outlet of the silo. Fluidised solid A stored fine particulate solid with a high proportion of interstitial air, in which the interstitial air pressure supports some part of any external stress. The air may be introduced either by aeration or by the filling method. The interstitial air pressure significantly changes the properties of the solid. Flow pattern The geometry of the moving solids when flow is well established. The silo is still close to its maximum filling condition. Free flowing material A material which does not develop significant cohesion when subjected to stress levels occurring in silos. Funnel flow A flow profile in a silo which is nearly full in which solid adjacent to some part of the wall remains stationary whilst other parts move. The stationary parts may be only near the outlet, or may extend adjacent to the walls as far as the surface. This category may be sub-divided into Mixed Flow and Pipe Flow. Homogenising silo A silo in which the stored solid is, at least at times, completely fluidised. Hopper half angle Angle between the vertical axis of the conical hopper and the hopper wall Internal flow When pipe flow occurs in a silo with a concentric or only slightly eccentric outlet, the flow may be termed "internal". The material in a vertical or nearly vertical channel above the outlet is in motion, but is completely surrounded by stationary solid. For the flow to be termed internal, the flow channel must not intersect the silo walls before it reaches the top surface of the solid . Mass flow The condition under which all particles of the solid are in motion when the silo is discharging Membrane stresses A stress component in the shell wall which is invariant through the wall thickness. Membrane stresses are mostly governed by equilibrium considerations, and control the buckling response. Mixed flow A flow pattern in which the lower part, near the outlet, has stationary material against the wall, but the upper part has solids moving across the entire cross-section. The upper limit of the stationary solids is termed the "effective transition". The silo bottom may be a hopper or a flat bottom. Particulate solid A material formed from many discrete particles which interact with each other through contacts but may move independently. The assembly is assumed to contain enough particles for the statistical mean of any property to be independent of the number of particles. This term includes the terms bulk solid, particulate material and powder (the latter has a special meaning within this definition). Pipe flow A flow pattern in which the material in a vertical or nearly vertical channel above the outlet is in motion, but is surrounded by stationary solid. The flow may intersect the silo wall if the outlet is eccentric. This term is used to distinguish clearly between two types of funnel flow. If an internal flow channel does not reach the silo wall within the height of the stored solid, the flow is a here termed internal pipe flow. Pipe flows are divided into two categories: internal pipe flow and wall contact pipe flow. Powder A granular bulk solid in which more than 50% of the mass is made up of particles smaller than 50 microns (0.05mm) in diameter. This limit is given for guidance only: the change in behaviour from coarse solids to powders is gradual as the particle size distribution changes. Pressure The force per unit area exerted on the silo wall by the particulate solid. The term "pressure" is reserved for the interface between the solid and the structure to match with standard structural engineering terminology. This limited usage ensures that the term "stress" can refer to structural stresses within the silo wall, needed for the purposes of structural design, or to internal stresses developing within the particulate solid. Rathole A stable empty vertical or near-vertical channel above the outlet, reaching to the solid surface. Flow is arrested. Segregation The partial separation of different sizes or types of particle into different spatial locations. A consequence of segregation is that the solid contained within the silo is no longer homogeneous. Segregation can occur with respect to particle size, particle density, chemical or surface properties. Self-cleaning silo A silo is termed self-cleaning if the entire stored solid contents discharge by gravity through the outlet. Self-cleaning may be achieved with the assistance of discharge aids. Shear cell An apparatus used to determine the strength properties of a particulate solid by translating one part of it relative to another under stressed conditions. Silo A container used for the storage of particulate bulk solids. Slender silo A silo in which the overall aspect ratio hc/dc of the stored solid exceeds 1.50. This definition comes from Eurocode 1 Part 4 Silos and Tanks Squat silo A silo in which the overall aspect ratio hc/dc of the stored solid is less than 1.50. This definition comes from Eurocode 1 Part 4 Silos and Tanks Stress The force per unit area existing within the particulate solid (statistical mean of inter-particle contact forces). Surcharge The total vertical force acting on any horizontal cross-sectional area of the silo (integral of the vertical stresses within the solid at a given level). Transition The position on the silo wall where the barrel section and the hopper meet (see also "effective transition"). Unit weight The weight of a quantity of particulate solid divided by its total volume. Wall contact pipe flow An eccentric flow involving a pipe flow in contact with the wall of the silo. The flow channel cross-section involves a part in contact with the wall and part which involves a solid-solid rupture surface. I have omitted definitions where that which is in the existing Glossary is so close to that in the new Guide that no comment is necessary. I hope the above will be useful.

4.8.1999