Silo Design for Reliable Flow and Strength

The design of bins and silos for storage of bulk solid material requires the flow properties of the bulk solids which must be stored. The flow behaviour of a bulk material is defined by the bulk density, the effective angle of internal friction, the unconfined yield strength, the wall friction angle. Spearpoint Engineering is able to assist clients with the measurement of these parameters through a Jenike Shear Tester or a ring Shear Tester.

Just after filling an empty silo, the filling stress state prevails where the vertical pressure on the vertical segment increases in a degressive way. With the height to diameter ratio of the silo sufficiently large, a constant vertical pressure is attained. The leading reason for this course is the shear stresses acting between the bulk solid and the silo walls even if the bulk solid is at rest. As the result of the shear stresses, the silo walls carry a part of the weight of the bulk solid. The stresses acting in a hopper are different from those acting in the vertical segment. The vertical stress in the hopper decreases less in the upper part of the hopper and then more near the imaginary hopper apex. As soon as some bulk solid is discharged for the first time after filling, the stresses in the hopper termed emptying stress state prevails. When the flowing downwards in the hopper, the bulk solid is compressed in the horizontal direction so that the walls of the hopper carry larger part of the weight of the bulk solid and, hence, the vertical stress in the lower part of the hopper become smaller than the filling. In principle, in the vertical section of the silo the stresses remain unchanged at discharge. Spearpoint Engineering is able to assist clients with the calculations of these loads either analytically using the relevant standards or computationally using Discrete Element Methods (DEM) in computer software that caters for the Multiphysics of the bulk materials.

Central to the load calculations, is the reliable flow of the bulk material in the silos. Two different modes of flow can be observed if a bulk solid is discharged from a silo, that is, mass flow and funnel flow. In the case of mass flow, the whole content of the silo is in motion at discharge. The mass flow is only possible, if the hopper walls are sufficiently steep and/or smooth, and the bulk solid is discharged across the whole outlet opening. In the case of the flat or too rough hopper wall, funnel flow appears. In the funnel flow the bulk solids placed in motion are in the area above the outlet, that is, concentric with the outlet. The bulk solids adjacent to the hopper walls are at rest and they are referred to as dead or stagnant zone. These bulk solids can be discharged only when the silo is emptied completely. Spearpoint Engineering is able to assist clients with the calculations of the hopper slope required for mass flow and the minimum outlet size to prevent arching using Jenike’s Theory.

In addition, the design checks for global stability and static equilibrium, strength of the structure and joints, fatigue, serviceability limit state (deflection) should be conducted. Spearpoint Engineering is able to assist clients with these checks including damage tolerant design philosophy where the internal walls of the vertical segments are allowed to either wear-off or corrode during the service life of the silo.

Figures below illustrates the CAE Design and Physical Silo Design upgrade for the storage of wet spent grain at SAB Brewery designed by Spearpoint Engineering for the replacement of the existing silo which has reached its 20-year life cycle, respectively.