Cement Import Terminals in bolted Steel Silo Design
Cement import terminals with bolted steel silos are being used more and more. The advantages are the high flexibility, a fast assembly, and the possibility to dismantle them if necessary and to rebuild them at another location. Innovative solutions in construction and assembly make this silo variant attractive for terminal operators. There is a clear trend towards larger silo volumes for bolted steel silos with conical outlet. If the limit was approx. 3,000 t storage capacity about 10 years ago, capacities of 7,000 t are already reached today. However, this tendency also holds dangers and reaches constructive limits, which in turn limit the flexibility of such plants. How are bolted silo plants constructed, what must be considered in the design and where are the dangers of such a silo plant?
This blog will help you to find answers to these questions.
Design notes and constructional structure
For a bolted steel silo of e.g. 5000 t storage capacity, the silo diameters vary between 12 and 15 m, depending on the manufacturer. The behaviour of cement and cement-like products during storage and especially during discharge must be considered in the design of steel silo plants. For the handling of cement an aeration floor is preferably screwed to the silo cone. This prevents bridging of the material and reduces adhesions which can lead to a core flow of the material. The diameter of the aeration bottom is about half the diameter of the cylindrical part of the silo. This causes eccentric loads during the discharge of the material, which must be considered in the silo construction.
picture from shutterstock
The individual construction solutions differ, among other things, in the type of bolting, sealing and assembly. An efficient silo system uses gravity for the material transport. Therefore, the silo system is often required to be able to be driven underneath. This requirement has a considerable influence on the design of the substructure and on the force dissipation of the construction either by a ring support of the cylindrical part or by the insertion of vertical stiffeners. In case of a ring support of the cylindrical part, the substructure can also be made of concrete. The screw connection variants for the individual silo sheets are panel construction, overlapping screw connection and "butt on butt" screw connection. With the panel construction method, the panels are designed with additional stiffeners. Thus, such panels can be up to 9 m long. All screw connections are outside and parallel to the silo wall. EPDM seals are located between the flange that are bolted together. The sketch shows the detail of the bolting of the panels. Force flow is conducted over the plates and not over the bolts (no shear forces).
For all other screw connection variants, overlapping or butt to butt (see sketch), work must be carried out from inside and outside. For this purpose, scaffolds must also be erected on the inside, which makes the assembly and maintenance of the silo plants considerably more complex. A further variant of the substructure is the so-called skirted design. Here the cylindrical part is simply pulled further down. Internal vertical stiffeners are necessary for this construction method. Innovative assembly concepts, i.e. building the silo system from top to bottom, reduce the assembly time and increase the safety at work, as suspended loads can be largely dispensed with.
The causes of silo damage
The collapse of a steel silo not only causes economic losses for the company, but also poses a danger to the life and limb of workers.
Bolted steel silos are similar in appearance. However, in fact they are very different due to the different construction and depending on the materials stored in the silo and the force generated. Each silo must be designed strictly according to the materials to be stored, otherwise safety cannot be guaranteed. This starts with the consideration of the correct bulk density and ends with eccentric loads when the material is discharged from the silo. A silo system should be designed according to Eurocode. Most silo damages are due to insufficient structural strength. Many of these steel silos showed significant deficiencies in both circumferential tensile strength and vertical compressive strength. The quality of the steel used can also lead to structural damage. Improper operation is another cause of silo damage. Some users have arbitrarily increased the load on the silo body after the construction of the steel silo by adding equipment on the silo ceiling or by adding transitions and stairs. Even more serious, however, are additional lateral silo discharge points which change the flow behaviour in the silo and lead to incalculable forces. Incorrectly designed dedusting equipment or non-functioning over/under pressure flaps and false air flaps can also cause considerable silo damage.
In summary, it can be said that cement import terminals in bolted steel silo design have long been established as an alternative to cylindrical concrete silos, flat storage halls or dome silos. However, the tendency to increase the storage capacity of the individual silo systems requires a close coordination between the actual silo supplier and the suppliers of the cement handling system because there is a risk that the forces from the material, e.g. during the discharge, acting on the silo walls are underestimated. Operators who are considering the construction of a terminal in bolted steel silo design should seek advice from independent experts on which design is the best solution for which requirements.