Factors such as environmental and geopolitical instability have contributed to increasing the demand for safe storage of grains in increasing volumes in recent years. The correct design of a silo plant is the most important moment for a successful investment. I will focus on the the main parameters to consider when developing the concept of the industrial silo storage facility in this article.
Dr. Martino Celeghini
CESCO EPC CEO
INTRODUCTION
CESCO EPC GmbH, formerly known as PETKUS Engineering GmbH, is an internationally operating company based in Konstanz – Germany, which designs, manufactures, and supplies industrial plants for grain logistics and processing with handling, storage and grinding systems including all structural steel structures required for the storage or processing plant.
The products and services it offers are mainly divided into four business areas: Inland Grain Terminals, Port Grain Terminals, Turnkey Plants and Dry Milling Plants for Deep Processing.
CESCO designs down to the detail and produce in the factories in Germany, Spain and Italy the various types of storage silos, the steel structures intended for its plants, and all the handling equipment.
CESCO offers a complete service starting from concept and detail engineering, procurement and manufacturing, erection and commissioning for new plants, or refurbishing and upgrading of existing plants.
CESCO deals with the grain from harvest to transformation and/or distribution and does not deal directly with the processes, since it is mainly a contractor, that is, it builds complete grain storage and handling plants and integrates, if necessary, individual process machines from specialized suppliers, as preferred. by customers, buying their equipment or collaborating in parallel with them. CESCO boasts a wide range of qualified references in Europe, the Soviet Bloc, the Middle and Far East.
BACKGROUND: CURRENT TRENDS AND CONSEQUENCES
Factors such as environmental and geopolitical instability have contributed to increasing the demand for safe storage of grains in increasing volumes in recent years.
On the one hand, high temperatures, drought, and water shortages, had reduced the grain production in Europe/MENA, leading to increased import demand. Also, snowmelt in northern areas, heavy rain, cold snowy weather, and strong wind forces require safe silo design with higher wind loads in design and construction.
On the other hand, geopolitical/economic instability has generated a greater need for self-sufficiency in grain-importing countries. Demand shocks and volatility in grain prices put pressure on government investors to set up new grain storage and milling plants as strategic investments to free the country from short-term import dependency and ensure security for food supply and market prices.
Today there is an increased need for customer consulting and expertise in the entire scope of supply, design, machine supply, engineering, controls, supervision, and training. In addition, there has been an increase in the demand for financing of large projects by German or European banks with export credit insurance.
DESIGN OF THE STORAGE PLANT
The correct design of a silo plant is the most important moment for a successful investment. The following criteria are the main parameters to consider when developing the concept of the industrial silo storage facility:
Structural, architectural and management design:
This first part includes the evaluation of the soil to optimize the costs of excavations and foundations, the assessment of local regulations such us: rules on external local loads (like for wind, snow, and particularly seismic ones), the architectural rules concerning the size of new buildings (e.g. distance from existing buildings, roads, fences etc.) or the safety and hygiene at work (e.g. dust and noise emissions or fire safety). Other factors to consider are the time necessary for the investment and the constructive typology of the structures, in reinforced concrete or steel design.
Functional and operational design:
The correct evaluation of the type of silos depends also on the functional properties of the plant and the purpose of the investment, which may lead to use of flat bottom silos, hopper bottom silos or of mixed solutions.
The flow diagram gives the functional representation of a plant and serves to evaluate the interfaces with existing infrastructures and facilities and the interlocks to them, if any. It also shows the required simultaneous operations mainly based on: intake, silos loading, recirculation, silos unloading and transfer to process, allowing to develop and optimize the lay-out in order to reduce the number of handling machine and the conveyors length, without reducing the efficiency of the plant.
It is essential to know the nature of the products to be treated, and to take in consideration how many different varieties of grain need to be stored. And for each variety is mandatory to consider the density, humidity, impurity content and the source of the product (farmer, domestic market, imported) in order to identify the most appropriate system for the conservation of the product. Likewise, also is important to take into account the number and tonnage of products to be treated simultaneously, to determine the number/size of the silo and conveying equipment needed, as well as the daily and annual operating hours of the plant, or the total daily and annual tonnage to be conveyed/processed, without forgetting the methods of delivery and shipment of the products.
Finally, another factor is the level of automation, for the installation of a modern control system with PLC and computer visualization, with interlocking program and interface with existing installations.
With the figures identified by the questions listed above, the concept of the plant can be developed allowing to pass to the design of the silos as follows.
SILO DESIGN
Silos can be divided into two main categories:
— Raw material storage silo (before processing)
— Process silo (bins inside the industrial process)
— Raw material storage silos
— The following parameters need to be analyzed to determine the correct design of storage silos:
— Number of filling and emptying cycles
— Concrete or corrugated steel sheet design
— With flat or hopper bottom. The following table gives the main guidelines for the correct choice of silo bottom.
— Permissible load on the ground
— Minimum and maximum single silo capacity
— Minimum number of silos
— Range available for corrugated sheet metal silos with flat and hopper bottoms
— Service structures such as ladders, walkways, and platforms for loading lines
— Tunnels or housing spaces for outloading lines
Cereal and rice processing plants such as flour mills, feed mills, seed plants, rice mills, malting plants, breweries, etc. always include the following relevant components:
— Silos for finished products
The following parameters need to be analyzed to determine the correct design of process silos:
Process silos must be fitted with hoppers anyway due to the high number of filling and emptying cycles
Two main categories of the process silos can be used: cylindrical or prismatic silos. For process silos prismatic silos are often used due to their advantages, such us:
Higher storage capacity for the same volumes occupied
No empty space between the silos with higher “sanitation” and no residual in empty state
Completely realized with bolted design. This facilitate the erection procedure that can be performed by a local team following the instruction of the CESCO supervisor
Transport by container or truck. The bolted solution with modular elements allows to pack all the components and to minimize the required transport volume and, consequentially, the shipping costs
— Loading / Unloading systems
— Silos upstream of the process
— Silos downstream of the process
Loads resulting from stored product: Difference between solid, liquid, grains
The following picture shows the difference in shape of solid, bulk, and liquid products when stored on the floor and stored inside a container as well as the loads transferred to the wall of the container by solid, bulk, and liquid products when stored. This results in different horizontal and vertical loads on the wall.
Friction load on the walls
Only horizontal loads affect the walls of a silo containing liquid, while both horizontal and vertical loads affect silos containing bulk product. The silo wall must be designed accordingly: either with a thin sheet with vertical stiffener, or with thicker sheet preventing buckling. The pressure acting on the bottom of a bag complies with the weight of the content, while the pressure acting on the bottom of silos is only part of the weight of the content. The balance is transferred to the bottom by the wall.
Euro Code vs. ANSI/ASAE loads
ANSI/ASAE are the US-American standards, which are usually applied on the international market, while the EURO Code is the European standard, mainly required in the European market. EU standards are more severe since are based on bulk density of stored product of 0,8 - 0,9 kN/m³, while the US-American standards specify a bulk density of 8,3 kN/m³. Still the ANSI/ASAE standards allow for max 6% compacting of material, which, if considered, leads to a similar maximum bulk density of 8,8 kN/m³ the two standards are very similar if correctly considered.
Funnel flow / Mass Flow
There are two general flow patterns that can occur when a bulk solid is discharged from a hopper: funnel flow and mass flow. In funnel flow, an active flow channel forms above the outlet, with stagnant material remaining at the periphery. In mass flow, the entire solids bed is in motion when the material is discharged from the outlet. The loads induced on the silo bottoms with a mass flow are higher than with a funnel flow and leads to a totally different design of the silo. This information must be known in advance in order to correctly design the silo
INTEGRATED DESIGN OF SILOS AND PROCESS STRUCTURES
The design of the process plant as such and the related storage is usually performed by companies which are not involved in the design of the dedicated steel structure and silos, and therefore which do not know the peculiar specification or preconditions of the same, nor can develop an optimized and integrated design.
In addition, process plants, and particularly extension of existing process plants, must be often installed in limited areas thus involve the optimization of required volume and areas.
CESCO has matured long experience, upgraded professionality, and very significant references in the optimization of the design of integrated silos and structure for process building, by:
Improving the static cooperation between the process silos and the process building
Selecting structural typology allowing the optimization of the dedicated room
Simplifying the foundation design
Combining the best structural specification of reinforced concrete and steel
Preventing useless and unused rooms
Including the potential future extension in the first design
Improving the safety of the personal during the operation and the maintenance
Preventing dead areas, where dust or impurities might rest and allow for contamination by insect or parasites
Installing absolute smooth wall silos
Installing absolute smooth cladding and roofing without purlins
The CESCO steel design solutions for silos and building in process plants allow for:
Design up to the minimum detail and complete pre-fabrication
Short delivery time compared to alternative concrete design
Transport by container or truck
Erection by local team with CESCO supervision
Total time of the investment up to commissioning very much shorter than any alternative solution
The use of prismatic silos in steel buildings allows an optimization of the static, of the structural design and the integration between the silos and the building itself with significant benefits in the realization of steel process building. The positive impact on the reduction of the investment costs is therefore very significant.