In many facilities, dry bulk material storage and related handling systems are critical for reliable and efficient plant operations, and in many instances existing systems have been in service for many years. Over the years it is likely that raw material and product requirements have changed, throughputs have increased, and cycle times decreased. Many facilities are looking to de-bottleneck or upgrade existing systems while some are considering new installations. There are a myriad of considerations when planning such a project from system capacity, throughput, bin/silo flow regime/geometry to material specific concerns such as combustible dust issues, first in-first out requirements, to moisture control, and many more.
Depending on the complexity of the system as well as the various constraints that it may be subject to and general facility related considerations, developing a path forward can be a daunting task. For the project to succeed, it’s critically important to get the design right. There are a number of key points to consider as you begin this process. For this article, we have chosen to focus on 12 key considerations that impact the design of the system.
The order of importance will depend on your plant and process requirements, but each deserves careful consideration.
Consideration 1: Stored Material Properties and Characteristics
The design of a bulk material storage and handling system always begins with understanding the bulk material properties as they relate to flow within the storage vessel and the forces that the material exerts on the vessel. A vessel suitable for whole kernel corn, for example, probably would not perform well for flour or cornstarch. The following are a few of the key bulk material properties/parameters that need to be investigated and understood:
- Loose Bulk Density
- Compacted Bulk Density
- Angle of Repose
- Friction Angle
- Particle Size, Shape, and Size Distribution
- Cohesive Strength
Consideration 2: Storage Bin Type, Flow Pattern, and Geometry
Once material parameters are understood, the type of bin, silo, or hopper can be determined based on the predicted flow pattern of the material. For example, some materials if placed in the incorrect geometry bin may “funnel flow” or “rat-hole”, where material flows down the center of the vessel, but there is stagnant, unmoving material hung in the hopper due to improper geometry. A properly designed bin, silo, or hopper would exhibit “mass flow”, where all material is moving downward during withdrawal, with no stagnant zones. Ideally the system can be designed without the need for special discharge mechanisms, but in some situations, a mechanical device is required to influence mass flow, again depending on material properties and bin geometry. These mechanical devices can take many forms from vibratory bottoms, to large screw type mechanisms. Generally, bins that do not require special discharge equipment can be considered Gravity Systems, and those that do are referred to as Activated Bottom Systems.
Consideration 3: Combustible Dust and Explosion Venting
Not all bulk materials and their dusts are combustible, but when they are, it’s a significant issue that needs special consideration. If there is any question as to whether the material or the dust that it generates is combustible, it should be tested by a reputable testing organization. At a minimum the dust should have an Initial Dust Characterization performed and a Go-No Go test (ASTM E1226). Based on these tests you can determine if the material is combustible, or not, and if it is, you would need to follow up with additional testing such as Explosion Severity (ASTM E1226), and Minimum Explosion Concentration (ASTM E1515). Given the potential consequences, this might be your bulk dry storage system’s most important design concern.
In the recent past, the National Fire Protection Agency (NFPA) Standard for Explosion Venting was strictly a recommendation. Now it’s the law. That means every storage silo must be compliant by 2020 – even existing bins. Many dusts from dry storage are combustible, making the proper venting extremely important. Make sure you review and understand the new standards and meet or exceed them.
Consideration 4: Construction Materials
A big part of the bin design process is deciding what it should be made of. That starts with the material you’ll be storing. Is it a food product? Is it corrosive? These and other questions should inform your decisions about construction materials, particularly if you need to use stainless steel or have the silo coated on the inside.
Those decisions impact cost as well, so they’re important for multiple reasons. But the integrity of the product or the ingredient should be first and foremost.
Consideration 5: Moisture Control
As we mentioned above, clumping can be a concern for certain products, particularly if the time in storage is more than a day or two. Ambient moisture makes that problem even more likely, so does air from pneumatic conveying systems. Be sure to design your system with adequate venting, and possibly with conditioned air, and in colder climates pay particular attention to the material’s moisture content, temperature, and the need to insulation on the bid to minimize condensation so that moisture doesn’t impact productivity or product quality.
Consideration 6: Sizing and Logistics
Obviously, you want to size your storage silos right so that you have enough inbound material to keep your process moving, and enough outbound storage to satisfy your packaging or bulk transit requirements. Keep in mind weather delays, holidays, and weekends when evaluating delivery and pick-up of your materials. As a rule of thumb, make sure your bins are sized to keep at least five days worth of material on hand. And make sure you design a clear path within the plant to get supplies to your silos quickly when those delays inevitably occur.
Consideration 7: Seasonal Supply
Speaking of sizing, that’s a particularly big design consideration when you’re dealing with seasonal ingredients. If the price of the material you’re storing fluctuates from time to time, you’re probably buying when the price is low and storing for a longer term. That means you need more storage volume. Size your storage system accordingly.
Consideration 8: Intake/Withdrawal
You don’t just need to store your ingredients, of course. You need to get them into storage and into your process practically and reliably as well. Many types of conveying systems are common both in receiving or transporting bulk materials into storage and reclaiming them from storage to feed a process or to loadout to a packaging system or bulk transport such as truck or rail. Depending on the material properties, the convey rate and distance, mechanical conveyors such as screw, belt, chain drag, and bucket elevators are common as well as various types of pneumatic conveying. Be sure your conveying systems are appropriate for the material you’re moving and that it’s properly engineered to handle the requirements of your bulk material system.
Consideration 9: Access Points
Maintenance should always be a primary design consideration, but it’s particularly important for these systems, where key equipment components are often situated at the bottom of the storage vessel, in a skirt or tunnel, and space is limited, and potentially where confined space situations exist. Be sure to fully understand the maintenance requirements of the equipment associated with your storage and reclaim systems and to make proper provisions such as draw-out space (pulling a screw conveyor out of its trough for example), monorails, and any confined space requirements such as retrieval system space, ladders, stairs, and manways.
Just remember that maintenance on these systems tends to be urgent, so the easier you can make it for the technicians and mechanics to get in and get to work, the sooner you’ll be back up and running.
Consideration 10: Isolation
When problems arise in your storage system, it’s important to be able to contain them so they don’t impact other parts of your process. Some designs don’t include an isolation device at the bottom of the bin. Don’t let yours be one of them. Include an isolation gate – a small investment that can save you big. If there’s a potential that you may need to enter the bin for maintenance operations, be sure you incorporate isolation valves/gates on all bin inlets to comply with your company’s Lock-Out / Tag-Out (LOTO) policy.
Consideration 11: Location
Where you place your dry bulk storage silo is practically as important as the system you choose. The location needs to easily enable material delivery by truck or rail and ensure that vendors making those deliveries can get in, unload and move out as efficiently as possible. That’s why a location away from the plant’s main building is often ideal. It will likely improve access for your partners and workflow for your associates.
Consideration 12: Dust Control
Dust isn’t just a respiratory issue for your associates; it’s an environmental concern for you and your neighbors. That’s why managing it isn’t an EPA request; it’s a requirement. So be certain your bin is equipped with proper Dust Collectors and sufficiently sized Vent Filters to keep your team safe and your organization on the right side of the regulators.
Experience and Knowledge Make All the Difference
As we mentioned at the outset, dry bulk storage systems can be tricky, and the strict regulations don’t make it any easier. That’s why it’s essential to have an experienced engineering partner get the design right.
If you’re someone who’s new to systems like these, you should also gather as much intelligence as you can early on. Start by looking through our online design and installation guide. Use every resource available to give yourself the knowledge and perspective you need. That knowledge will prove invaluable to your project’s bottom line success.