| Silos and Hoppers - Bulk Handling Global article.
Silos are intended for the bulk storage of bulk solids and powders.
The lower portion of silo construction where the silo is transitioned to a smaller size, to enable the mounting of suitable silo discharge equipment, is called the hopper.
Depending on process requirements, silos can be used to store products various amounts of time. The terminology generally used - “instantaneous silo discharge” for products that are stored for short periods of time, and “time consolidation” for longer periods of bulk solids or powders.
Hoppers can be supplied, without the silo, and are generally used for smaller amounts of bulk solids storage. Product can also be stored in hoppers either for instantaneous discharge requirements, or over longer time consolidation periods. Hoppers can be used to receive bulk solids or powders in batch form - for example a front end loader, rail/truck dump, or receive batch ingredients as part of a process plant.
Intermediate bulk containers (IBC’s) is also a term used for “small” silos. Intermediate bulk containers are generally used for the transfer of powders within a process plant, as part of process requirements, or be used to transport bulk solids or powders from one plant to another. Intermediate bulk containers are normally air and water tight sealed.
Standard “off the shelf” silos are generally available for the storage and handling of grains. The reason for this is that grains generally have consistent product flowability, and the grain industry over time has attained reliable hopper wall friction angles for the various materials of construction.
Silos are often used to condition bulk solids or powders - heating or cooling, as well as aerating and fumigation. With proper silo design, silos can be used mix and blend products
Typically, there are two modes of product flow patterns to consider for the design of silos and hoppers – mass flow or funnel flow. In mass flow silos, during discharge, the product is in motion over the entire area of the silo or hopper if the side walls are steep enough. Funnel flow is where the product discharges through a flow channel which forms within the material in the bin. The funnel flow channel may not necessarily be central to the silo, and flow can be erratic, leading to segregation, degradation as well as increasing risk to dust explosions.
The design of silos can also include Ideal flow and Eccentric flow regimes for all bulk solids and powders. These silo flow modes are used to avoid segregation, agglomeration, caking and minimizing of dust explosions in silos.
It is important to remember that silos are designed for the effects of gravity. The maximum mass flow rate discharge from silos is governed by the bulk solids and powder flow properties characteristics, hopper wall friction angles and the size and configuration of hopper opening size.
To control the mass flow rate from a silo or hopper at a discharge rate below the maximum possible flow due to gravity, a discharge device or feeder needs to be used. Side or end discharge feeders and any live bin bottoms need to be properly interfaced.
Side or end discharge feeders have an effect on the silo draw down, and if not designed correctly, can dramatically change the silo draw down flow pattern.
The opening size of a silo hopper needs to be based on the evaluation of the critical arching dimension. Silo flow problems including silo bridging which results in loss of production, lead to high maintenance costs and safety issues, can be avoided by designing the silo and hopper to suit the bulk solids and powder flow properties characteristics. When a stable cohesive arch across the outlet of the hopper, the bulk solid or powder, has sufficient strength to support consolidated silo opening pressure.
The majority of silos or hoppers that experience flow problems, are due to either or a combination no evaluation of the product pressure relations and potential rat hole height, fines segregation, incorrect hopper opening size and ignoring the effect of side or end discharge as created be the feeder or discharger.
The effective area of the silo or hopper opening needs to considered in relation to the physical silo or hopper opening size, and the powder or bulk solid flow pattern.
If the bulk solids contain a wide range of particles sizes, segregation will occur. The requirement of properly designed silo filling spouts is required to avoid this problem. |
