I have never read a book comprised entirely of punctuation. Such a text would be dull and nonsensical at best. Stringing together a series of commas, periods, semicolons, and perhaps for added excitement a few interrobangs, would not be a sure recipe for the next bestseller.
Furthermore, if you were to read this paragraph aloud you would not call out and draw attention to each piece of punctuation. Conversely, I have no desire to read a book where the punctuation is either absent or grossly misused. With any of these extremes, writing would go awry rather quickly.
The solids storage vessel seems to share a similar identity in the life of a plant or process. Although silos and hoppers are required, they remain mostly unnoticed. They hide in plain sight, blending into the scenery, frequently becoming a part of the backdrop.
For most chemical, manufacturing, or other industrial facilities this is because the storage of bulk materials is not a primary function. Bins and hoppers are not built as independent entities, and do not serve as an end to themself. Rather they hold a material so it can then be used for a specific purpose. Silos serve as appendages to the key equipment that accomplishes a plant’s ultimate purpose.
Most equipment or processes involve a material undergoing some type of transformation. Material is being altered in some way, often to create a final product or during material consumption. This is seen in mixers, dryers, reactors, crushers, mills, and an assortment of other plant equipment.
In these pieces of equipment material exits having undergone some type of change. Meanwhile, should a bulk solid exit a silo in an altered state, something has gone quite wrong. This makes silos rather boring, as maintaining the status quo is their key attribute.
If material alteration is not the goal of silage, then what purpose should hoppers serve? The first is storage. Prior to use, consumption, delivery, or transportation, material typically must wait in a hopper. The amount of storage at a minimum should correspond to a time or volume based on the process or batch requirements.
Silos can also serve as a means of dealing with fluctuations in a material supply chain. If a shipment of material is delayed to a plant, a silo full of material can allow for continued plant operation. The silo here provides the option to enter the domain of risk management.
For instance, in the event of an equipment breakdown, a downstream surge hopper would allow for a window of time before a further downstream process would be interrupted. This feature of capacity and risk management can backfire, though, if the hoppers or silos are not designed properly.
Just as the surge hopper in the previous example allowed for continued operation, should material stop feeding from that hopper this would also interrupt the downstream process. Increases in bin sizes are typically accompanied with increasing pressures.
A material’s flowability is a function of pressure, and therefore an increase in pressure often leads to worsening flow problems. As a result, by trying to manage one risk and increasing the storage capacity, another risk may be introduced since larger hoppers can experience an increased frequency and severity of flow problems [1].
Unfortunately, materials in silos do exhibit a host of flow problems and other material handling challenges. The implications of this reality have been thoroughly discussed in other literature, so the subject will be observed in brief here [2]. Silos seem prone to misbehave in a myriad of ways when handling bulk solids.
Figure 1 summarises a few of the personality traits more commonly seen in silos behaving inappropriately. These problems are specific to a combination of a unique silo with a particular material under certain conditions.
This means that altering any of these three variables has the potential to add or amplify problems in the silo’s operation; for instance, fluctuations in temperature can lead to problems that may have been otherwise absent [3].
Now that silos have been brought from the background to the foreground of our attention, and we are aware of their inclination to be a source of problems, we can look at an appropriate design path. Proper silo design requires that we:
1. Size the silo capacity appropriately.
a. Ensure enough storage based on process or batch requirements.
b. If possible, consider some level of risk management in sizing a silo, leaving room for equipment or process interruptions upstream of the silo.
c. In selecting capacity review the supply chain of material arriving at and leaving the facility.
2. Design the silo to handle the specific materials that will be stored.
a. Account for material flowability such that the materials enter and exit the silo in a controlled and desired manner [4].
b. Ensure that the state materials leave the silos is unchanged or is only altered in a desirable manner. For example, some materials may enter a silo at an elevated temperature and then leave after being allowed to cool.
There are other factors that are of equal importance but are outside of this current discussion. This includes structural integrity, available space, environmental impact, and cost to name a few.
Finally, just as the silo cannot be overlooked, so too the silo’s various ancillary equipment must be taken into careful consideration. This often includes, the feeding device, gates, dust collection, explosion panels, instrumentation, and supporting structural members all of which can impact the silo operation.
Silos serve a specific role within a much larger ecosystem of plant operation. They support the key processes that occur as material is consumed or altered. Without proper design and usage, silos become incapable of this function.
The procurement and erection of a storage silo should not be approached lightly. The rigours of accounting for specific material properties and environmental conditions are neither optional nor insignificant. Just as we wish for our punctuation to have a secondary nature in writing, so too our silos should share a similar role. Poor use of punctuation is a distraction to writing. Poor hopper design is a costly and unnecessary bane to plant operation.
REFERENCES
1. Roberts, A.W. (1991, 1998): Characterisation for hopper and stockpile design; Characterisation of Bulk Solids. USA, Canada. Blackwell Publishing Ltd. pp. 85-131.
2. Myers, Christopher W., et al. Understanding Process Plant Schedule Slippage and Startup Costs. Rand, 1986.
3. Schulze D, Schwedes J, Leonhardt C, Kossert J,: Design of a silo for the storage of 10,000 t Sulpher Granules; bulk solids handling Vol. 18 (1998) No. 2, pp. 211-217
4. Carson J, Marinelli J,: Solve Solids Flow Problems in Bins, Hoppers, and Feeders (1992) Chemical Engineering Progress, May 1992, pp. 22-28.
