Bulk solids are commonly stored in tanks or silos that can be configured in multiple shapes and sizes for the particular storage needs. The storage system that is installed needs to be properly designed, operated, and maintained to reliably perform its intended function.
The material to be stored is the most important consideration for designing the storage system. Every material has inherent characteristics that impact proper design. Some of these characteristics are:
- Bulk density to use for volume and structural loading
- Angle of repose
- Moisture content and hygroscopic properties
- Particle size
An understanding of the operating requirements is also needed for proper design. Considerations include:
- Storage capacity
- Long-term storage or frequent cycling
- Concern with material degradation or segregation
- Fill and discharge flow rates required
- Is aeration required?
Another important consideration is the type of discharge flow pattern that is required. The three types are funnel flow, mass flow, and expanded flow with pros and cons for each. Commonly, however, mass flow or expanded flow is the desired flow type for powders or materials that do not flow easily.
There are no “rules of thumb” when it comes to designing for mass flow or expanded flow discharge. Lab testing is required to fully understand and quantify the material’s flow behaviors that are needed for a proper design. The effort and cost required to determine these properties and produce an effective design are low compared to the cost of poor quality and operational issues arising from an inadequate design.
Proper operation and maintenance of the storage system will help ensure long-term functionality with minimal issues. Proper operation primarily means that the owner should use the storage system only in the way in which it was originally designed and constructed.
The operational needs of any facility will often change over time. The types of materials that need to be stored can be one of the changes that are considered. The structural loads imposed by the material being stored onto the structure is a function of the material properties and the type of discharge. Never use a storage system to store a material that is different from what was considered in the original design without consulting with the engineer of record or another qualified engineer. The loads imposed on the structure by different materials can vary significantly even with materials of similar bulk density and which may have a similar visual appearance.
Changes should never be made to the type of discharge (such as modifying the hopper, adding side draw discharges, changing feeder type or capacities, etc.) without first consulting with a qualified engineer. These types of changes can also significantly modify the loads imposed on the structure and inadvertently change the discharge flow type from mass flow to funnel flow.
To accommodate subsequent changes, the engineer may place restrictions on the use of the storage system such as limiting the maximum height of storage or forbidding eccentric fill or discharge. The engineer may also require design changes such as additional hopper reinforcement/support or the installation of roof intake vents. Or it may be determined that no changes are required and the storage system can be used safely under the new operating conditions. Either way, an engineer needs to determine the impact of any proposed changes to prevent compromises to the structural integrity.
Finally, regular maintenance needs to be performed on the storage system to keep it operating reliably and maximize its useful life. This includes performing periodic cleaning and inspections. All storage systems should be completely discharged on occasion for removal of any stagnant material and to allow for inspections of the interior surfaces. Full interior and exterior inspections should be periodically performed to look for cracks, weld failures, or other signs of structural distress, material build-up, moisture intrusion, corrosion or wear, etc.
The information presented here only scratches the surface regarding design, operation, and maintenance of storage systems. It is important that you consult with a qualified engineer, contractor, and/or fabricator with storage system experience to determine a solution that best meets your needs.
Jonathan Cowles is a licensed professional engineer and director of sales for WL Port-Land Systems, which has extensive experience in silo design and construction. For more information, visit www.wlport-land.com.