Factors influencing the choice of bulk store
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Compared to most other foodstuffs, such as meats and vegetables, grains are relatively easy to store. If grain is kept insect-free and below its safe moisture content, it will keep for many years with minimal loss of quality or nutritional value. Low temperature is an important factor in minimising insect activity and in maintenance of nutritional quality in general. Storage at or below the safe moisture content is essential for prevention of deterioration caused by microorganisms and insects (see Chapters 2 and 8 respectively).
Where insects are present, temperatures are high, and most especially where moisture content is above safe levels, then storage of grain becomes both risky and difficult, and losses will be difficult to avoid. It is in these circumstances that the type of store and its design become critical to the safety of the stored grain. It is worth remembering that most often, the value of the grain (in dollars-per-tonne) is usually greater than the cost of the structure in which it is stored. Minor expenditure in improving the quality of the store can thus be quickly recovered if commodity losses are commensurably reduced.
Whilst the choice of storage design is wide, the essential requirements needed to store grain safely remain the same. These are that the storage structure must keep the grain free from water ingress, insects, rodents and birds. The store should also permit easy and economical disinfestation of grain in the event of insect infestation and, if grain is to be stored at moisture content above 'safe' levels, provision should be made for cooling the grain. These matters are discussed in more detail in the following sections.
Sue of Grain Storage
Mathematical, graphical or computer modelling can be helpful in determining the volume of storage that is required. In some cases, it is a relatively simple exercise to determine the requirements - for instance where harvested grain is to be received over a short period, dried over a longer period and dispatched over 12 months, it is a simple matter to calculate the buffer storage requirements for wet and dry grain. This type of situation can be illustrated by a simple graphical model as shown in Figure 7.1 (see Figure 7.1. Model for determining the Volume of storage required.). Here, a quantity "Q" tonnes of grain is assumed to be received into storage over a period "tr" days, and dried over a period "td" days. It is then dispatched over period "to". If we assume a uniform rate of drying and dispatch, then the maximum wet storage requirement is:
fw*Q where fw (the wet storage factor) = (1 - tr/td);
and the maximum dry storage requirement is:
fd*Q where fd (the dry storage factor) = (1 - td/to).
More complex models are required where the rates of receival and out-turn are less clearly defined; for instance at a shipping import terminal where the rate of receival will depend on ship arrival rates, berth availability etc. and where out-turn rates may depend on the inland transport system. In such situations computer models are very helpful in optimising various design parameters including the storage volume, number of berths, ship unloader capacity and out-turn capacity. An optimum-cost solution can thus be determined through sensitivity analysis, by varying the values of the input parameters.
Specialised software is available for such modelling exercises, however in simpler cases models can be developed on spreadsheets to produce satisfactory results.
Specialised programming languages are available which are especially useful in simulation modelling 'systems', for instance, where it is desired to look at complete storage and handling systems in a region. Such languages are available for PC applications include SLAM II (produced by Pritsker Corporation of the USA), SIMAN (produced by Systems Modelling Corporation of the USA) and GPSS/H (produced by Wolverine Software Corporation). These modelling languages are not inexpensive, and require training to use them effectively.
Selection of Storage Type
Once a storage need is identified, the choice arises as to the type of store that is most suitable for a particular application. The following storage options may need to be considered: round or rectangular, tall or short, steel or concrete, flat floored or hoppered, permanent or temporary.
Some basic guide-lines, or principles, are offered:
(i) Round or Rectangular
In terms of structural cost per tonne of storage, round stores are generally more economical than rectangular ones. The reasons are simple:
Firstly: grain exerts a horizontal pressure on the structure which contains it. A round store will resist this pressure through the development of hoop tension forces which are very efficiently resisted (eg by steel reinforcement). A rectangular structure must resist grain loads through the development of bending stresses which are less efficiently resisted than tensile loads since both tensile and compressive forces have to be resisted. In addition, in the case of a rectangular 'horizontal' store, the walls act as retaining walls and their foundations have to resist overturning moments caused by the grain loads. Foundations for cylindrical structures have mostly to resist the vertical loads imposed on them from the walls.
Secondly: the roof structure of a rectangular structure has to carry its loads in bending, compared to the roof of a cylindrical structure which can be designed as a shell (for instance a cone), which carries its loads in direct compression and tension.
Another important advantage of cylindrical structures is that they have less joints. In the case of silos where the bins are independent (i.e. not connected to each another), there is a joint between the wall and the floor, and a joint between the wall and the roof. It is thus usually a relatively simple matter to seal these joints to make the structure air-tight and suitable for fumigation. Ideally the roof should be rigidly attached to the wall, since this not only makes sealing easier, but also greatly increases the stiffness of the wall in resisting bending stresses. In such cases silicone type sealants are useful for sealing the roof-wall joint, however where a sliding joint is required bituminous based mastic sealants sandwiched between the joining surfaces have been successfully used.
Rectangular structures, by comparison, have more joints (for instance at the comers) and by the nature of their construction, sealing for fumigation is generally more difficult to achieve.
Horizontal stores have another inherent disadvantage, in that they require more complicated and longer conveying systems to place grain in them. Usually an internal conveyor is required with a tripper (or similar device) to spread the grain over the floor surface. A cylindrical store, on the other hand, requires only a central point for filling.
(ii) Tall or Short
In the case of flat bottom stores, structural efficiency is also increased by minimising the height of the structure. For a given volume of storage, the lower the height of the walls, the more grain pressure is applied directly to the floor surface and the less load there is on the walls. Furthermore, the minimum structural surface area (and hence cost) for a given volume of grain, is achieved if the wall height is relatively low. For instance in the case of a cylindrical 'tank' with a conical roof, the minimum surface area of walls and roof is achieved when the wall height is around half the radius of the bin. It is thus no coincidence that the lowest cost stores are generally in the shape of squat cylindrical 'tanks' where the walls are relatively low compared to the diameter.
To take an example: a rectangular warehouse or shed structure may typically have a wall height of say 8 metres, and a length about 21/2 times the width. Thus to store 12,000 tonnes of corn (or say 16000 cubic metres) with a repose angle of 30°, it may be calculated that the length of the shed will be about 60 metres and the width 24 metres. By comparison, a tank store with the same volume and same wall height will have a diameter of about 41 metres. A comparison of the structural surface areas of the two alternatives gives the following results:
Table 7.1. Comparison of structural areas for 12,000 tonne stores.
|Comparison of Structural Areas for 12,000 tonne Stores|
|Wall Surface||1510 mē*||1000 mē|
|Roof Surface||1660 mē||1524 mē|
|Floor Surface||1440 mē||1320 mē|
|Total Area:||4610 mē||3844 mē|
* Including 'gable' end walls.
Since the structural components are represented by the surface areas, then the comparison can be used as a preliminary gauge of relative cost. Additionally as mentioned earlier, the walls and roof of the tank store will usually be lighter and less costly than those of the warehouse.
Tank storage is not, however, suited to all situations; for instance where high throughputs are required it is usually desirable to have a self-emptying bin using a sloping floor. In such cases, it is more economical to design higher walls of smaller diameter: to minimise the cost of hopper bottoms (which are usually suspended above ground level) and the risks of ground water infiltration, and to facilitate conveyor design and installation. In instances where land values are high, or where space is limited, it may be expedient to opt for tall bins, even with flat floors to maximise space utilisation.
The chief disadvantage of flat bottomed stores is associated with the difficulty of emptying them and in removing the 'dead' grain that is not discharged by gravity. There are various means of achieving this; for instance portable conveying equipment may be used, or pneumatic systems, or front-end loaders. Another commonly used alternative is the sweep conveyor (usually a screw or auger) which automatically rotates about the bin centre and draws the grain towards the discharge point. Sweep conveyors are usually buried under the grain when the bin is full, and operate only after gravity discharge is completed. Sometimes they are suspended above the grain surface and lowered with winches to perform their function. Either way, it should be born in mind that the investment in permanently installed sweep conveyors can be high, and is seldom warranted if they are to be operated only once or twice per year.
Nevertheless, where low throughputs are required, such as when a store is to be filled and emptied only two or three times a year, the lower capital investment in a large diameter flat store outweighs the additional capital and operational costs associated with emptying it. An analysis of capital and operational costs is usually necessary to evaluate minimum cost solutions where a store may be emptied more than (say) five times per year.
(iii) Construction Materials
The choice of construction material is usually between steel and concrete, though in some countries timber or masonry are still used as alternatives.
The choice between steel and concrete is dependent on a number of considerations, all of which ultimately come down to capital and operational costs. The fact that in most countries steel and concrete are both so widely used indicates that these costs are generally not dissimilar.
Where price considerations do not dictate the type of construction, the following observations may be helpful:
Some suppliers give warranties that their bins can achieve and retain their air-tightness. Where this is proven to be the case, this type of bin can be quite satisfactory for longterm storage. The extra cost of sealing such bins is in the order of US$ 5.00 per tonne.
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