Diferences Between Hoppers and Silos

As a new type of modern silo, the steel silo has the characteristics of short construction period, high standardization degree and low foundation cost. They plays an important role in dry bulk storage industries, can be used for cement storage, fly ash storage, coal storage, etc.

In the design process, when selecting the basic form of the steel silo, in addition to considering the impact of natural factors (location, hydrology, temperature, etc.) on the quality of the project, the form of the silo bottom should also be considered.

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 distinctive advantages and architectural features. The different construction methods adopted at the bottom of the steel silo have different effects on the storage material.

What is the purpose of the conical bottom?

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.

Hopper Silo Bottom Types

The bottom of the conical silo is divided into all-steel bottom,  semi-steel bottom and 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.

Which to Choose? Hopper or Silo?

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 1500t. 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.

Conclusion

From the above analysis, it can be seen that when the steel silo has a large diameter and capacity, 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.