Silos design considerations

The other cause of constmction problems is the introduction of badly chosen, or even unauthorized, changes during constmction in order to expedite the work. Any changes in details, material specifications, or erection procedure must be given careful consideration by both the builder and silo designer. 

J. W. Carson and R. T. Jenkyn, "Load Development and Stmctural Considerations in Silo Design," presented at Eeliable Flow of Particulate Solids II, Oslo, Norway, Aug. 1993. 

This chapter provides a brief outline of the development of understanding of pressures that develop in silos and their consequences for the safety of the silo structure. More structural failures occur in silos than in any other engineered structural form, considering the numbers of each, and these failures occur in all countries and all industries. Structural design considerations for silos are therefore a key aspect of bulk solids handling systems. 

Metal and concrete silos carry their loads in very different ways, so the kinds of damage that can occur in each type are very different and the critical design considerations are different. For this reason, the later part of the chapter examines these two cases in separate sections. 

This compressive force rapidly approaches a linear increase with depth (term z/zo) (Figure 3.11c). Thus very high forces develop in the wall towards the bottom of the silo. This force is important in thin metal silos, as it becomes the critical effect because the controlling design consideration is buckling under axial compression (see Section 3.5.2). This is the reason why metal silos must have a much greater wall thickness towards the bottom than near the top. 

The simplest stress analysis of a cylindrical silo structure under symmetrical loads was presented above in Section 3.3.5. Unfortunately, this is often the only analysis that is applied, sometimes with unfortunate consequences for the structure. Metal and concrete silos carry their loads differently because metals are strong in tension but thin metal sections tend to buckle under compression. By contrast, concrete is very weak in tension, but can resist compression well. These aspects lead to different key design considerations. 

Taking into consideration all the different properties of cohesive to very cohesive powders tested (particle size distribution, moisture content, material properties etc.), the model fit can be characterised as satisfactory to good. Thus, the model has proved its effectiveness and can be accordingly applied in reliable silo design for flow and pressure calculation [26]. 

Raw materials and recycle (undersized, somewhat predensified material) are made available in day bins. It is essential that at all times enough material is available (controlled by level indicators) for continuous discharge from the silos. Activators and/or mass flow bin designs may be necessary to ensure constant flow. In most cases, the individual components of a mixture will be metered for correct analysis. It is important to take into consideration the recirculating material as it may influence the final analysis and, on the other hand, may be necessary to ensure acceptable densification. 

Pretreatment and Posttreatment - Some materials may require pretreatment processing to prepare the material for extraction or post-treatment of the residue after extraction is completed. Material handling systems such as conveyors and product silos must be designed with these considerations in mind. 

Basic considerations which govern the design of solids-flow devices (chutes, hoppers, silos, etc.) within a process plant are the following. 

An important consideration when specifying a bulk storage silo is the seismic zone in which it will be installed. These zones vary from zone 1, where there is no earthquake activity, to zone 4 in southern California where there is a serious threat. In zones 3 and 4, tank construction, foundation, and anchor bolt requirements are designed to resist the higher loads. 

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