Industrial silos: construction, types and capacity selection

Main silo components

A typical design includes a shell, lower discharge section, roof, supports and process connections. Depending on the task, it may include ladders, platforms, level sensors, filters, aeration components and devices that promote stable product flow.

The shell carries its own weight, product weight, bulk material pressure and external climatic loads. Outdoor installations must also account for wind, snow and temperature effects.

Why product properties matter

The material name alone is insufficient for calculation. Bulk density, moisture content, particle size distribution, caking tendency, flowability and particle surface characteristics are important.

Two products with similar density may behave differently during discharge. Outlet geometry is therefore selected according to actual material behaviour.

Loads required by SP 359.1325800.2017

A silo cannot be calculated as an ordinary vertical tank. Bulk material pressure depends on wall friction, internal friction, geometry, filling method and flow pattern. During discharge, pressure distribution may differ significantly from storage conditions.

SP 359.1325800.2017 requires consideration of product weight, horizontal wall pressure, vertical friction, bottom and hopper pressure, process equipment, temperature effects and air pressure during ventilation or pneumatic discharge. For dusty products, accidental pressure from a possible dust-air explosion is considered separately.

Design situations are checked during both filling and discharge. Eccentric filling or discharge produces asymmetric loads, so connection locations are part of the structural model, not merely an operational convenience.

• maximum filling level and product mass;
• horizontal product pressure on walls;
• product friction against vertical surfaces;
• loads on a flat or conical bottom;
• wind, snow, temperature and equipment weight;
• dynamics of vibrating equipment and pneumatic conveying;
• special actions, including a dust explosion where possible.

Main configurations

• outdoor silos for open-site installation;
• indoor silos integrated into a process building;
• loading silos designed for rapid dispatch;
• multi-compartment designs for storing several fractions;
• custom designs for restricted dimensions and unusual layouts.

Determining required capacity

First, calculate the required material stock by mass using line throughput, delivery intervals and reserve stock. Then convert mass to volume using the design bulk density.

Nominal geometric capacity differs from useful working capacity. Process headroom is required at the top, and the filling pattern depends on the product feed method.

Why the discharge section is critical

The main objective is predictable product movement towards the outlet. An unsuitable hopper shape can cause bridging, stable flow channels and uneven discharge.

For materials with difficult flow properties, auxiliary devices and geometry are selected to suit those properties, rather than as a universal set of options.

Sespel’s industrial experience

Sespel manufactures bulk storage silos in various configurations. Its aluminium silo specification lists diameters of 2.4–4 m and storage capacities of 30–240 m³, with a shell made from extruded aluminium profiles joined by friction stir welding.

Loading silos are also available, with discharge sections selected according to material properties and the plant’s process arrangement. This illustrates why capacity is only one silo selection parameter.

Why one bulk density value is insufficient

SP 359 requires consideration of changes in bulk material properties due to composition, particle size distribution, moisture, temperature, age and other factors. Calculation using one average bulk density may therefore be insufficient.

Useful design input includes the bulk density range, internal friction angle, friction coefficient against the wall material, caking and bridging tendencies, moisture, temperature, abrasiveness and dust explosion characteristics. For non-standard products, testing is advisable to determine these properties.

What to include in the specification

For a steel silo, the designer also determines the consequence class, design load combinations, and material and weld requirements. Final capacity is just one design outcome, not sufficient input on its own for ordering.

• material name and physical properties;
• required stock in tonnes and useful capacity;
• filling method and rate;
• required discharge rate;
• installation location and available space;
• shell material and maintenance requirements.