Silos and Hoppers - Bulk Handling Global
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 feedersand 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.
The angle of repose of a bulk solid or powder has influence only on the capacity of silos or hoppers. This is related to the contour of the stockpile. Contrary to common belief, the angle of repose is not a measure of the bulk solid or powder flowability.
The following graph 1 illustrates typical silo pressure distribution for a cylindrical silo with conical converging wall hopper.
Graph 1 - Typical silo pressure distribution per AS 3774-1996 Load on bulk solids containers.
Sketch 1 - Bin flow obstructions
Converging walls of a pyramid shaped hopper form a valley angle where the side walls meet. These angles can at times be considerably less than the dimensioned hopper wall angle, and often lead to silo and hopper flow problems.
Hopper wall angles can only be determined by measurement of the bulk solid or powder on the wall material, at the range of consolidated silo pressure. The addition of hopper liners to assist in rectifying silo and hopper flow problems can at times lead to higher consolidation pressures, and additional feeder loads.
Care must be taken with silos and hoppers, if introducing bin discharge aids such as air cannons or skimmer valves. These devices if fitted need to be placed at, or near to, the silo critical arching dimension to be effective. These types of devices can create surge loading if silo bridges occur, potentially leading to structural damage and safety issues.
The design of silos and hoppers need to include structural consideration to accommodate any vacuum pressures due to a silo bridge collapse, or positive silo pressures due to filling conditions.
If you are about to design, purchase a silo, require assistance in solving flow problems, wanting to minimise your silo cleaning costs, contact us and we will be happy to offer assistance in your silo requirements and solve any flow problem issues.
