An Approach to Silo Design

A gravity flow approach to silo design is best, since gravity discharge is efficient, cost effective and reliable. A successful gravity flow approach depends on your product’s flowability. Laboratory tests provide measurement of your product's cohesive strength, yielding a “flow function” that represents the material’s pressure/strength relationship. The material’s wall friction values and compressibility are used with the flow function to calculate outlet sizes required to prevent arches and ratholes in silos.
    
The flow pattern created by your product and the silo is critical. Funnel flow occurs when some material in a silo moves, while the rest remains stationary. The walls of the hopper section are not sufficiently steep or smooth enough to force the material to flow along them. Mass flow occurs when all the material in a silo is in motion during withdrawal. Material slides along the hopper walls because they are steep and smooth enough to overcome the friction that develops between the wall surface and bulk solid.
    
Outlet size determination yields the minimum opening size required to prevent a cohesive arch from forming. Jenike’s Bulletin 123 has design charts that yield flow factor values to use in outlet size analysis. The data obtained from the intersection of the flow function with a flow factor allows you to calculate a minimum outlet dimension for both circular and slotted openings.
    
Hopper angles for mass flow for a particular bulk solid and wall surface can be calculated from the results of a wall friction test. This test generates a wall friction angle and by using Bulletin 123 the hopper angles required for mass flow conical and/or wedge-shaped hoppers are determined. Rather than accepting 70° as a “magic angle” for mass flow, the hopper angle for mass flow is dependent on both the smoothness and steepness of the hopper wall and the properties of the bulk solid.
    
Structural considerations include knowledge of the material's flow properties, flow patterns, silo loads, and dynamic effects. Silo failures can range from catastrophic structural collapse to denting of a steel shell. If a bulk solid other than the one for which the silo was designed is deposited in the silo, the flow pattern and loads may be completely different. Side discharge outlets put in a center discharge silo can impose asymmetric loading on the silo, which can cause failure. Another common problem is the development of mass flow in silos designed structurally for funnel flow. Mass flow loads are greater than those applied by material in funnel flow. The structural integrity of the silo will be in jeopardy.
    
Sometimes gravity alone will not be able to ensure reliable flow or may not be a practical solution to a flow problem. Perhaps your material requires large circular openings to prevent arching or your particles degrade, creating fines that are extremely cohesive. At this point, it is necessary to consider some type of flow aid device. Some flow aids are mechanical devices that may not always work well. Devices such as vibrating bin dischargers, vibrators, etc., are considered active flow aid devices. Aeration devices such as fluidizing nozzles and air blasters have been successful.
    
A group of passive flow devices have also been proven successful in assisting gravity to ensure reliable flow. Devices such as chisel, transition, or wedge-type hoppers, cone-in-cone designs, spiral letdown chutes, and splitters are often more reliable than active flow aid devices. Wedge-type hoppers have been used for years. Spiral letdown chutes provide a means to gently lower product into your silo to minimize particle degradation. Splitters divide the discharge stream from a single symmetric outlet into several streams to supply product to several processes.
    
It is imperative that your material’s flow properties be measured and your silo and feeder designed to reliably handle the product. Gravity flow is important to reliably handling your product. However, flow aid approaches exist that enhance gravity. Experience is important in order to recognize existing flow problems or predict potential problems so that practical, cost-effective solutions to those problems can be developed.
    
Joseph Marinelli is a consulting engineer and president of Solids Handling Technologies. He has been providing testing and consulting services since 1972. He can be reached at [email protected].

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