Facts At Your Fingertips: Hopper Outlet Geometry and Arching

When transferring stored bulk-solids materials from hoppers, bins and silos, flow stoppage can occur because of bridging or arching at the vessel outlet. Hopper outlets must be large enough to prevent cohesive arches or stable ratholes from developing. Determining the size and shape of the hopper outlet is critical to ensuring that the bulk material flows. This column provides information on the interplay between bulk material properties and vessel geometry and how those relate to outlet size and shape. Arch and rathole formation Bulk solids can experience a range of conditions within a bin, silo or storage hopper. Consolidation pressures range from zero at the surface, to relatively large values at increasing depth within the container. If a solid gains cohesive strength because of the pressures applied to it, an arch or rathole could form. An arch (also called a bridge or dome) is a stable obstruction that forms over the point of narrowest cross-section of the storage vessel (usually the discharge outlet). The arch supports the rest of the bin contents, preventing discharge (Figure 1, left). A rathole is a stable pipe or vertical cavity that empties out over the outlet. Material is left stranded in stagnant zones that usually remain…