Vertical storage of bulk materials in
silos has numerous advantages over flat storage options including:
unload and transfer rates
personnel required to operate facility
equipment maintenance costs
of cross-contamination of stored materials
The most common method of dust control on truck loading is by the usage of a retractable loading spout. For materials that produce significant dust, loading spouts are also available with extraction fans and filters. Depending on the material being handled, other methods such as dust suppression fog or dust suppression hoppers may be applied.
Dust control when unloading trucks is dependent on the method used to unload the truck. In the case of pneumatic bulk trucks it is important to have a correctly sized dust filter on the silo or bin into which the truck is being unloaded.
In most cases a silo requires a bin vent filter to ensure dust does not escape from the silo during filling. The size and type of filter used is dependent on the rate and method of filling as well as the material being stored. The filter must be sized to allow all of the displaced air (and conveying air if loading pneumatically) to be vented from the silo without over-pressurizing the silo. Over-pressurization of the silo will cause the relief valve to open releasing dust into the atmosphere and in extreme conditions, could result in damage to the silo.
There are two key factors affecting how quickly a truck can be loaded:
- The rate at which material can be fed into the truck.
- The efficiency in which operators and drivers can get the truck into position.
The Feed Rate can be maximized by utilizing drive-thru silos, which allow material to free flow by gravity out of the silo directly into the truck. The use of cameras and signaling lights allow the operator to provide accurate instructions to the truck driver to position the truck for filling quickly. To further speed up the process, loading spout positioners may be employed, allowing the operator to adjust the spout position to align with the filling port on the truck rather than the driver attempting to re-position the trailer.
Depending on the characteristics of the material being loaded, and the extent of technologies employed as discussed above, a truck can be loaded in as few as 3 to 6 minutes. It is important to understand the demand on the facility (i.e. trucks per hour) in order to select the most value-added approach to filling.
We supply bolted silos in diameters of up to 47 feet (14 meters) and heights in excess of 130 feet (40 metres). Storage capacities of up to 5,000 tonnes and beyond are generally achievable depending on the density of the bulk material as well as site conditions (e.g. seismic loading).
A truck drive through allows material to flow from the silo via gravity directly into the truck, maximizing the rate at which the truck can be filled. The truck drive-thru eliminates the requirement for the conveyor and other rotating equipment for filling of the trucks. Fewer pieces of rotating equipment equates to reduced maintenance costs.
Conveyors underneath the silo on the other hand can reduce the overall height of the silo required. A conveyor set up that is fed from multiple silos can reduce the overall project cost by reducing the number of scales required . A single truck scale could be fed from any one of the silos.
The key advantage to loading a truck directly on a scale is to maximize
the utilization of the truck’s capacity. If trucks are not loaded on scale the
operators must err on the side of caution and underfill the truck to avoid an
overweight condition when the truck is later weighed. This means that less
material is being shipped out per truck. If a truck is overfilled (or grossly
underfilled) then material has to be removed or added to the truck as a
separate process, slowing down production and increasing operational costs.
When loading silos using mechanical conveying (such as a bucket elevator), chutes are often employed to direct the material from the discharge of the bucket elevator to the centre inlet of the silos. When loading a cluster of several silos, the distance from the bucket elevator to some of the silos can get quite large. Gravity flow chutes require a minimum slope to operate properly so as the horizontal distance to be traveled increases, so too does the required height of the bucket elevator. Often the costs of using a taller bucket elevator exceed the costs of installing horizontal conveyors on the silo roof to transport the material to the silos. In keeping overall heights reduced through the use of conveyors on the silo roofs, access for maintenance is improved.
By selecting the appropriate size, slope, materials and the effective use of wear liners and impact zones, the life of chutes can be maximized.
It is important to fill and empty the silo from the centre to keep structural loads balanced in the silo and to achieve effective material flow within the silo. Filling or emptying a silo off-centre creates asymmetrical loading on the silo which impacts the structural design and can significantly increase the cost of the silo.
Installing partitions in a silo creates unbalanced loading which greatly impacts the structural stability of a silo to the point of making it impractical from a structural design and fabrication perspective. It is far more cost effective to install multiple silos than to try to design and construct a larger, partitioned silo.
Some bulk materials are more fragile than others. For materials that are susceptible to degradation is important to select handling methods that minimize abrasion and wear on the bulk material particles. In pneumatic conveying this means avoiding high pressure dilute phase conveying, proper configuration of convey piping and elbow selection. In mechanical conveying it may mean using a belt conveyor rather than screw conveyor.
The choice of flow promotion equipment depends on the exact properties of the material being handled. In general we would recommend using aeration for products that are “powdered” and vibration for products that are pebbled or granular. If the product is a mix of powder and pebble/granules, a combination of aeration and vibration can also be used.
It is also important to understand that flow promotion equipment may need to be cycled on & off during the feeding period to achieve consistent feed rates - the timing of the on / off periods will depend on your material.
For most bulk materials, a guided-wave radar level transmitter is an effective method of measuring level within the silo. Alternative methods such as open-air radar and ultrasonic can work well too provided that the characteristics of the bulk material are considered such as density, dielectric constant, dustiness and so on.
Yes. With proper insulation and a simple heating
system (if necessary), the above ground tank and its content can be operational
during cold weather, even at temperatures as low as -50 deg. C.
Bolted tanks could experience failures similar to any other type of tank. But like all tanks, the risk of failure is dependent on the application and usage.
Leakage is the first and foremost concern when it comes to storage of liquids and bolted tanks are no exception. Even though our experience shows that leakage after a proper Hydrotest is rare, if leakage were to occur, it would be through the joints or a punctured hole and the repair is relatively simple. Read “How to repair a leak”.
Other modes of failure could be
pressure or vacuum built up inside the tank, which would result in damage to
the roof or tank wall. Such failures would happen when the vent on the tank is
incorrectly sized or obstructed due to lack of maintenance.
When it is possible to build a tank in the ship and delivery it to site, a bolted tank would not be price competitive. However, for applications where quality of coating, and consequently, the longevity of the tank is important, a bolted tank should be considered.
Typically, bolted tanks become price competitive with shop welded tanks at around 380 m3 (100,000 USG).
Nozzles can be placed anywhere except the bolt seams. Similar to a welded tank where weld seams are to be avoided.
Generally nozzle locations are determined during the design phase and are based on the needs of the plant.
For the vast majority of storage applications, applying a corrosion allowance is not necessary for factory coated bolted steel tanks. Typically corrosion allowance is required in field welded and field coated tanks where the underside of the steel tank floor is uncoated and can allow corrosion to occur.
Bolted tank components are coated in a factory using automated paint lines coated on all sides. Thus, there is no need to apply additional corrosion allowance when selecting the plate thicknesses.