Co-Author: Jesús Chávez Sagarnaga
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My definition of 'hopper' is a sloped wall converging structure suspended along its top edge from the vertical wall portion of the bin or silo. Hoppers can be conical, wedge, pyramidal or a combination of these shapes. These converging parts are necessary and provide a practical means of directing and concentrating the discharge of the bin/silo contents to a specific point where it can be efficiently handled. Flow problems aside, it is hard to think of bins and silos without hoppers. I am primarily talking about circular silos constructed of welded steel with conical hoppers for the purpose of this discussion, although it applies to the other types of hoppers as well.
Depending on the size of the silo, the converging structure must safely hold up its contents along with the contents of the cylinder section that it is attached to. The material in the cylinder above the hopper can press down with loads as high as 50%-70% of the bin/silo contents. Most conical hoppers are generally thin plate type structures that are welded to the cylinder shells, which can also be constructed from a thin plate or something heavier like a built-up ring girder. The thin hopper plate behaves as a tensile membrane structure that moves (strains) relative to the cylinder at the attachment point as the silo is filled and emptied.
The key issue here is the welded joint that is used to attach the hopper section (see figure above). We at Jenike & Johanson have inspected many of these type weld joints and found some of them to be improperly installed when the bin/silo was initially fabricated. The weld joint is typically specified during the design phase to have a complete penetration fillet weld, but because of the limited access to the joint during the welding process, the weld metal does not penetrate the thickness of the hopper wall and can be as little as half the thickness (see photo). This reduced cross-section at the weld creates high tensile stresses in the joint. Furthermore, the movements of the hopper shell as the bin/silo is filled and emptied can cold-work (fatigue) this joint to the point of failure. We have investigated past catastrophic failures and identified this mechanism as the root cause. This unnecessary scenario ends in equipment damage, facility shutdown and sometimes the loss of life. Often during the inspection, we observe cracks in the attachment joint before the complete (catastrophic) separation of the conical hopper occurs.
The hopper attachment joints on all bins and silos should be inspected during fabrication or construction, and then at some interval established after the first inspection. Jenike & Johanson recommends at minimum that a qualified structural engineer specialized in bin/silo structures visually inspects this important attachment joint. So the question is: Is the attachment of your hopper safe?
Sam Walton
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May 11,
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Steel silos usually have two kinds of bottoms, one is the flat bottom and the other is the conical bottom(silos with this type of bottom are also called 'hopper'). They each have their own distinct advantages and architectural features. The different construction methods adopted at the bottom of the steel silo have different effects on the storage material.
The bottom of a hopper silo is designed to conical shape for several purposes.
First, the storage material can be discharged from the silo by its own gravity, without relying on other auxiliary equipment. And the discharge efficiency is high and the effect is good.
Second, adopting this design can eliminate the need for clearance, reducing the amount of work required to maintain the silo, and also reducing the damage to the materials and silo body caused by the accumulated material.
Third, in the construction process, if the full-metal structure is adopted for the conical bottom silo, it can be completed in the factory in advance to reduce the construction volume and shorten the construction period. And it is relatively easy to relocate the silo after completion.
Fourth, this design makes it possible to transport materials that are out of the silo more conveniently through vehicles such as automobiles.
The bottom of the conical silo is divided into the all-steel bottom, the semi-steel bottom and the cement bottom. There is not much difference in the use of these three bottoms.
All-steel hopper bottoms are lighter in weight due to their all-steel construction, and their basic requirements are not high. They must be equipped with a vibration discharge device to ensure sufficient effective storage capacity. The basic platform on which the hopper bottom is installed is generally about 4 meters high.
Semi-steel hopper bottoms are suitable for silos with a diameter of 6'12m. A 4'5m taper hole should be designed at the center of the base platform. At the time of construction, the lower half of the semi-steel hopper bottom is welded to the steel plate embedded in the base platform. After the completion of the tank body, the upper part is constructed with light material to ensure the material self-flow. This form of hopper bottom can effectively reduce the height and the cost of the base platform.
There is also a backfill method where a hopper silo bottom is installed in the silo body. Although this method reduces the difficulties in the construction process, it wastes the storage space and increases the engineering workload.
When selecting the steel silo, it is advisable to choose the conical bottom silo from a practical point of view. Because it has the advantages of no stock and there is no need to configure clearance equipment. However, from an economic point of view, there are certain limitations in the application of the conical bottom silo.
Facts have proved that the conical bottom design is only applicable to silos with a diameter of no more than 10m and a capacity of no more than t. The size of a steel silo needs to be determined according to the natural flow angle of the storing material. In general, the smaller the natural flow angle of the material, the greater the height of the conical bottom. The natural flow angle of many materials, such as grain, is generally around 40 degrees, so when the steel silo has a large diameter, the upper beam of the foundation support structure will have to be greatly increased.
Taking the conical bottom steel silo with a diameter of 15m as an example, in order to meet the requirements of the equipment under the silo, the height of the upper ring beam of the foundation support structure should be at least 8m. Therefore, the basic cost of the conical bottom silo is more than 40% higher than that of the flat bottom silo with the same capacity.
From the above analysis, it can be seen that when the steel silo has a large diameter and capacity, the different selection of the silo type could result in obvious construction cost difference. Therefore, when the diameter of the steel silo is greater than 10 meters, the flat bottom design should be preferred. This will greatly reduce the project cost.
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