Dr Robert Berry
Senior Consulting Engineer
The Wolfson Centre for Bulk Solids Handling Technology - University of Greenwich
1. No-flow and erratic flow are results of poor silo, bin, tank or hopper design. Can they be avoided?
When purchasing a new silo, bin, tank or hopper, the user should always be ensured that it is designed with consideration of the bulk material or materials that it is intended to store over its life cycle.This design process requires a consideration of the bulk solid flow properties for both the calculation of the structural loadings and the optimum geometry for the converging section: the wall angles, outlet size and the design of the feeding device under the outlet.Correct specification of this converging geometry and the feeder is particularly critical for cohesive materials – which would otherwise hang up or discharge erratically from storage. For existing silos, bins, tanks or hoppers that exhibit discharge problems, there are a number of options for geometric modifications or bolt-on discharge aids that could be used to solve these problems.
2. The result in obstruction and so-called arch or rat hole. What are the best practices for handling them if they occur?
If the flow challenges are just to re-initiate flow after periods of shutdown, then there are a number of bolt-on discharge aids (vibrators, air injectors) that could be used. The potential implications for these approaches will be discussed.However, if the problems are severe and continual intervention is required to maintain flow from storage, then this suggests a more fundamental redesign of the silo, bin, tank or hopper.Options include: a composite hopper, the use of flow pattern correcting inserts (e.g., inverted cone over outlet), lining the inner walls with low-friction material or fitting aeration panels, all of which are intended to modify the flow pattern in the silo.However,the suitability of the different methods is dependent on the shape/configuration of the silo, bin, tank or hopper and the properties of the bulk solid stored. Examples will be discussed at the upcoming training.
3. What are the results of using a hammer to handle them?
The issues with using a hammer to solve poor flow problems include:
- Resulting damage of the equipment over time,creating an increasingly rough and irregular surface on the inside of the hopper that will further restrict the flow of the contained bulk solid over time,
- Health and safety of the workers hammering the equipment,
- Potential for structural damage and loss of containment of the silo, bin, tank or hopper.
4. Testing of friction, compressibility, segregation and degradation can solve these costly flow problems. How is it done?
Strength, friction and compressibility tests enable fundamental measurements of the bulk solid flow properties that are used to assess pressures acting on the material during discharge and static storage. These are used to evaluate how the material will flow through the plant. Other useful characterisation tests discussed include particle size segregation, which occurs at the free surface of the heap that is formed when material is poured into a silo or stockpile on the ground, particle degradation (breakage) occurring when material experiences an impact against a solid surface, e.g., at transfer point or silo loading.How these measurements are obtained and used to inform plant design will be discussed at this training.
5. Attendees of this training can look forward to numerous case studies. Can you give us a sneak peek into what they will hear about?
- An opportunity to measure the cohesive strength of a bulk solid with the “sand castle” test
- Undertake a tutorial to calculate the optimum hopper geometry for reliable flow of a cohesive material, following the Jenike silo design method
- Calculation of the pressures in the silo using the Janssen method (as used in the Eurocode)
- How to calculate the contribution of the outlet pressure of a silo to the power calculation for the feeding device, e.g., belt, screw, chain and many more
I look forward to meeting all the delegates and hearing their questions.