By Richard Farnish and Suzi Fraser Dominy
Keep silos discharging consistently and reliably or run the risk of quality problems, downtime and increased labor costs.
The storage and discharge arrangements of feed grains have significant influence over the quality of the product from downstream processes. Despite this fact, it is often found that storage vessel design is not given the amount of attention that is due such a fundamental and yet vitally important piece of equipment. Often, the materials handling equipment used is specified and sourced without taking the flow properties of the particular material being handled fully into account, resulting in erratic flow or even no flow at all.
The most commonly encountered symptom that indicates that all is not well with a silo is the presence of "silo rash," which is caused by operatives beating the silo with blunt objects in an attempt to initiate flow (usually after the silo has been in an undisturbed condition for a period of time).
There are several causes for material not flowing from a silo, including mechanical arching, cohesive arching and "rat holing."
In the case of mechanical arching, the root cause of the problem is that the outlet is too small. It is usual to dimension the outlet to a size that is at least 10 times that of the largest particle expected.
The formation of cohesive arches and "rat holes" can be eliminated if the strength of the bulk solid is prevented from reaching a level such that it can support itself. There are several approaches to achieving this.
One approach is to design the silo such that outlet size is sufficient that the stresses induced within the material are such that they overcome any potential strength that is present and thus the material around the central flow channel will collapse. Alternatively, the hopper can be designed so its ability to discharge precludes the build-up of strength that can develop within the material, which could contribute to the formation of a "rat hole."
An increase in fines content in a bulk solid can have a detrimental effect on flow properties and increase the cohesiveness of the material. In effect, the fines act as a "filler" between the larger particles and reduce their propensity to move independently of adjacent large particles.
If a particular material has become "troublesome" (in terms of the frequency with which cohesive arches are being encountered), compared with previous handling experiences with the same material, then perhaps a change has occurred in the process or a variation in the raw material from which the bulk solid is created. Either factor could be responsible for an increased level of degradation within the bulk solid and hence an increased level of fines.
In the case of degradation caused by the process, it may be that the installation of a new item of process equipment has resulted in the material being handled more "roughly" than was the case previously. Alternatively, the increase in fines level may indicate that the efficiency of dust handling apparatus has reduced.
A change in supplier may also result in an increase in fines content, even though the raw material is nominally "identical" in specification. This can be caused by variations between suppliers in the way in which the raw material is handled or processed.
TAKE STEPS TO AVOID PROBLEMS.
It is not unknown for two visually "identical" materials that both meet a given specification to exhibit dramatically different flow and conveying characteristics, as well as exhibit different levels of resistance to degradation, when put through the same process plant.
If a silo is being modified (or designed from scratch) to handle a material that is known to be cohesive, there are steps that can be taken to eliminate cohesive arching. One countermeasure is to use conditioning air. Such techniques can involve installing a plenum around the discharge section at a height above the outlet commensurate with the position of the maximum arching condition for the material to be stored.
In this way, a low volume of air is introduced into the material, which then permeates to the top surface. The passage of air disturbs the stored material and prevents the build-up of strength, which would otherwise contribute to the formation of a cohesive arch.
However, it should be noted that fine powdery material such as flour does not respond well to introduction of air, resulting in "channels" of air traveling up through the material instead of the desired dispersion. (Note: This approach will not help to eliminate mechanical arches that are formed by larger particles, as the air that would be introduced will "channel" its way through the bulk solid with little effect.)
If aeration methods are adopted, it is most important that the correct amount of air is dispersed through the product. If an excessive amount is introduced, it is possible to aerate the material to the extent that it becomes fluidized and will flood uncontrollably upon the discharge valve being opened.
The use of individual air injection points for aeration purposes within a discharge section is not usually recommended. Any intrusion within the discharge section presents an obstruction to flow and should be avoided at all costs if mass flow is to be maintained. It is often the case that very fine material will flow back into the nozzles and distribution pipes when the air is turned off or reduced.
Although the cost of a plenum chamber is much higher than that associated with individual injection points, it offers much improved air distribution and no obstruction to the flow path of the material and eliminates the risk of material flowing back.
Other devices that are marketed to "break" arches do so by introducing energy into the material. These devices take the form of vibrators, inflating pads or other mechanical methods. For some materials this will work, but for other materials such devices can add to the strength of the bulk solid by encouraging the fines to pack more tightly between the larger particles.
If vibrators are installed, they invariably are found near to the outlet. In this position, the vibrations imparted into the stored material may be concentrated below the point at which arching may occur, and in the case of fibrous material (such as wheat germ), the effects are often damped out.
Compaction of the material can also occur if the vibrators are activated prior to the outlet being opened. Equipment such as vibrators, air pads, etc., have their applications, but should be used judiciously.
In summary, if your silos or hoppers do not discharge material in a consistent and reliable manner, you may run the risk of not only reducing the quality of your end product, but of downtime and increased labor costs due to stoppages. The knock on effects of these two symptoms can be substantial if wastage and lost productivity are considered.
Correctly tailoring equipment to handle specific materials may appear to be an expensive undertaking, but the financial returns in terms of consistent product quality, reduced wastage and elimination of downtime because of flow problems will often more than justify the initial outlay.
The best way to ensure reliable flow from silos is to design or modify the discharge section to have geometry suitable for the material to be stored.