Sealed stores

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When a new grain store is being planned, there should be no question as to whether or not it should be possible to seal it effectively to make it air-tight for fumigation. The benefits of sealed stores are such that the small costs involved during initial construction (negligible in many cases) should not warrant consideration. Despite claims to the contrary, there are no disadvantages to building sealable stores, and when circumstances arise where ventilation is required (e.g. to aerate the grain), ventilators can be provided to allow this to be done.

Low-cost sealing is most easily achieved at the design stage. Retro-sealing of stores which have not been designed to be sealed can be expensive, and in some cases (particularly with small stores) it can be uneconomical. Sealing technology has been developed extensively in Australia where the warm climatic conditions are highly conducive to insect pests and where the need arose to develop means of effectively and economically controlling them. In Queensland, Australia, all stores constructed since 1975 have been built to strict gastightness standards, and for many years all new stores have been built to such standards throughout the country. In Western Australia, some 60% of stores are sealed, most of which have been retro-sealed in the last 15 years.

Fumigation of grain is much cheaper and more effective than the use of chemical protestants, and residue problems are avoided. Furthermore, once fumigated, grain in a sealed store can be maintained free from insects because the sealing prevents access for reinfestation. 100% mortality can only be achieved if grain is fumigated in properly sealed stores, which are routinely tested to check their gas-tightness (see below).

Diurnal temperature variations cause pressure changes in the air inside the store, and it is usually uneconomical to design stores (in particular their roofs) to withstand the pressure differentials that can occur. Some means of ventilating sealed stores is thus necessary to avoid these pressure exceeding safe values. Pressure-relief valves should be pressure actuated; in other words they should remain sealed when pressures are below the critical value. One option is to use an oil bath with a baffle extending below the liquid surface such that air-pressure inside the store will displace the oil and allow air to pass below the bafle. Non-evaporative oil should used for the purpose.

Another type of valve uses counter-weighted diaphragms which lift off gasket seals when the pressure reaches a preset value such as are used in the oil industry for protecting oil tanks from damage as a result of internal pressure changes. See Figure 7.12 (see Figure 7.12. Oil-bath and Diaphragm-type pressure relief valves.) (opposite).

Steel bins are particularly prone to diurnal pressure variations due to the thermal conductivity of the steel. Internal air temperature changes can be minimised by painting the steel surfaces white since this will significantly reduce day-time steel temperatures by reflecting much of the sun's radiation. It should be noted that galvanised surfaces do not reflect as much radiation as white surfaces, and these should be painted white also.

 

Condensation, Moisture Migration and Moisture Diffusion

The question of condensation risk often arises in any discussion about sealed stores, especially in the case of steel structures. Yet there is little evidence to support the theory that sealed stores actually cause condensation. Theoretically, there should be less risk of condensation in sealed stores than in unsealed ones provided the grain stored in them is at or below its 'safe' storage moisture content.

In a properly sealed store at constant temperature, the grain and air mixture are in moisture-equilibrium with each other. Different grains have different moisture equilibria with air, and for each grain type, the moisture equilibrium level changes slightly with temperature. Typically, however, grain at a 'safe' moisture level of 12 to 13% is in equilibrium with air with Relative Humidity of about 65%.

Condensation will occur only where the air temperature drops below its Dew Point. In the case of a grain storage, it is only the air in the 'head-space' above the grain surface that is likely to experience any rapid decline in temperature (e.g. at night time), since the grain itself is an excellent heat insulator, and temperatures within the grain mass will change only very slowly. From a psychrometric chart (Figure 5. 3) it can be seen that air at 65 % RH and 25C will need to cool to 18C before reaching its dew point. Cooling rates can be quite rapid with steel surfaces, however the air inside will lose its heat mainly through convection which will be much slower. In the process, the RH of the cooled air will be reduced by grains at the surface absorbing water vapour. It should be noted that only very small quantities of water are involved, and that the change in grain moisture content will be very small. Even if condensation on the underside of the roof was to occur, the amount of water that would be deposited would be very small. For instance, a 40 metre diameter tank silo with 1 metre of head-space above the grain has a head-space air volume of around 1500 cubic metres. If the head-space air is initially at 25C and 65% RH, and it cools to 10C without moisture absorption by the grain, then (from the psychrometric chart) the initial air moisture is 13 grammes per kilo-gramme of dry air, and the final moisture is about 8 gm/kg. From this it can be calculated that the amount of moisture condensing would be around 9 kg, or enough to raise the moisture content of the top centimetre of grain by 0.08%. Where bins are only partially filled with grain (i.e. where there is a large head-space volume) there is potential for an increased amount of condensation. However the total amounts of water remain low provided the store is sealed and not open to entry of moist air from outside.

Experience in Australia suggests that problems with condensation do not occur in sealed stores when the grain is kept at or below its safe moisture level. Problems will occur if grain moisture levels are high, or if insect infestations occur, since in both circumstances biological activity will cause a localised increase in temperature and moisture content. This creates thermal instability in the grain mass, resulting in a convection movement of air and moisture which in turn enlarges the volume of affected grain. This chain reaction can ultimately result in massive spoilage with wetting and crusting of the grain surface if not controlled. Crusting is likely to occur whether the store is sealed or not, the problem emanating from within the grain mass itself, and is a result of poor storage management. Sealable storage provides a management tool which can be used to reduce the risk of such occurrences.

Moisture diffusion is not the same as moisture migration. It will occur in a grain mass in which temperature differentials exist for extended periods of time, such as when warm grain is kept in storage during cold winter conditions. This has sometimes been reported to cause condensation problems in sealed stores. It may be noted that diffusion is a physical process of moisture and heat redistribution, which is quite separate from moisture migration resulting from insect or fungal 'hot spots' which are heat generating. Such situations are, however, better handled by aerating the grain to equalize temperatures, than by selecting ventilated (or unsealed) structures for grain storage. It may be noted that without aeration, average temperatures in a grain bulk will follow average outside temperatures with a delay of 2 or 3 months, depending on the size of the bulk.

 

Methods of Sealing

The easiest stores to seal are fully-welded steel silos, since the structure is effectively sealed by virtue of its welded construction. Concrete silos can also be sealed with ease provided joints are properly detailed, and care is taken to prevent cracking of the walls (as discussed above). In either case (with both steel and concrete silos), attention is needed to the design of openings in the structures: grain inlets and discharge valves, man access doors, etc. These must be fitted with suitable gaskets to ensure sealing when closed. Sealable discharge valves require careful detailing, since they also have to withstand grain pressures. Various methods are commonly used which overcome this problem (Figure 7.13. Two alternative arrangements (schematic) for sealing silo discharge valves. A, a valve plate which can be lifted to seal against the silo base; and B. a flexible seal attached to the silo base which can be tightened down against the valve plate.).

As discussed earlier, bolted light-gauge steel bins, and other similar structures such as steel sheds and warehouses, are less easy to seal unless special care is taken during design and construction to ensure that all bolts are fitted with suitable sealing washers, and joints between adjoining plates are carefully sealed with appropriate sealants. Silicone sealants are well suited to sealing surfaces which are to be fixed together, such as overlapping sheets of iron. Silicones should be 'neutral cure' type which will not cause corrosion of the steel surfaces. Joints between roof and wall require special detailing to minimise the gap between them to allow easy sealing.

Another method of sealing of joints, particularly when retro-sealing structures, involves the use of high-build "co-polymer" acrylic coatings. These are easier to apply than silicone sealants, and can be either brush or spray applied. High quality coatings can bridge small gaps, and will remain flexible enough to accommodate movements over a long period of time. This is particularly useful for sealing existing joints which cannot be opened easily for the application of silicone type sealants. Acrylic coatings require minimum surface preparation and can be applied over concrete to seal cracks, and also to steel surfaces to seal over joints.

When gaps are wider than about 2mm, acrylics perform better if a flexible fabric 'bandage' is used as a bridging medium to support the coating. This is especially the case if relative movements across the joint are likely.

Joints with gaps wider than about 10mm can be sealed with closed cell polyurethane foam, which can be spray-applied over adjoining surfaces to act as a bridging medium. In the case of very wide gaps, solid infill such as light gauge steel can be used in combination with other sealants. Polyurethane application involves spraying of two liquid chemical components which, when mixed together in the spray gun, expand on contact with the air to form a rigid foam.

 

Testing for Gas-Tightness

Testing of stores for gas-tightness should be carried out when they are commissioned. It should also be carried out routinely each time a fumigation is to be undertaken. The standard gas-tightness test was evolved by the CSIRO Stored Grain Research Laboratory, Australia, in the early 1970's and has proved an easy and effective means of determining the suitability of stores for effective fumigation.

The test involves applying a positive or negative pressure differential between the inside and outside of the sealed structure, for instance by means of a small fan or air compressor. Once the test pressure is reached, the air supply is cut off (and sealed) and the drop pressure recorded over a period of time. The acceptance criterion for an empty store is that the time taken for the pressure to drop to half its initial value (its half-life decay time) should be not less than 15 minutes. For a full store the time should be not less than 8 minutes. Test pressures are normally in the range of 0.75 to 1.5 kPa, depending on the strength of the structure being tested. Tests should be carried out at times of relatively constant air temperature so that pressure decay periods are not affected by temperature generated pressure variations.

It may be noted that the pressure decay standard is independent of storage volume. The standard is thus much more tolerant of air-leaks in large stores than in small ones. It can thus be deduced that small stores are more difficult to seal than large ones, since much more attention has to be given to sealing of small leakage paths. Nevertheless, farm-bins of 50 and 100 tonne capacity are often sealed in Australia, and a growing market exists there for sealable bins sold by small-bin manufacturers.


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