Solids in powder or grain form that are neither flammable nor toxic are stored in tanks (silos) that are in contact with the atmosphere through solid-gas separation systems.The silo design pressure is usually limited and the mechanical structure is not usually able to support conditions of overpressure or underpressure.
Generally, in order to avoid dust emissions into the atmosphere that are higher than the levels permitted by law, contact of the silo with the atmosphere passes through suitable dust collector systems, such as cyslones separators or filter systems.
Material loading into a silo may be realized in one of the following ways:
- by a mechanical handling system;
- by a pneumatic positive-pressure system (in dense- or dilute-phase flow);
- a pneumatic negative-pressure system.
While mechanical product loading does not usually affect the pressure conditions inside the silo, during pneumatic loading, bearing in mind the gas involved, there may be dangerous pressure conditions inside the silo, caused by:
- malfunctions in the machine that generates the gas flow (compressor or vacuum pump);
- wrong set up of loading conditions or human errors, particularly in loading systems placed on vehicles;
- cleaning of the pneumatic loading system under high gas flow rate or pressure;
- malfunctions in the dust collector system (e.g. obstruction of filter elements).
Unloading a silo can be done by:
- a mechanical handling system;
- a pneumatic negative-pressure system.
Also in this case abnormal internal pressure conditions may be caused by:
- wrong use of fluidisation pads to assist off-loading;
- malfunction in the dust collector system (e.g. obstruction of filter elements);
- malfunction in the vacuum system, which provokes a higher depression than that designed for the equipment;
- sudden discharge from the silo of the stored material.
These abnormalities cause an increase or decrease in pressure inside the silo, which may damage the silo structure.
The international standard ISO 8456, concerning storage equipment for large quantities of bulk solid materials, makes it necessary to use suitable pressure restabilising devices for silos and hoppers which, due to their loading and unloading methods, are subject to internal pressure variation.
These devices, usually known as pressure relief valves, are basically valves that allow air into and out of the silo and therefore guarantee the safety of the silo if there is a change in internal pressure.
The factors to be considered when choosing a relief valve are:
- the silo¿s range design pressure;
- type of solids stored, in order to choose a suitable material for the construction of the valve body, casing, plug and seals.
Thanks to their very simple construction, relief valves require no particular maintenance. A periodical check should, however, be made to make sure that no dust has built up on the valve that may affect its correct working, and therefore the valve itself should be cleaned.
A silo containing explosive solids in powder or granular form can be protected in a number of ways:
- by inerting the contents of the silo inert, which means lowering the oxygen content to below the value required for combustion;
- by using devices that release fire-extinguishing substances in order to avoid the spread of fire, and therefore the creation of excessive pressure;
- by adopting reliev ventings that allow the pressure wave that develops to escape as quickly as possible in order to keep the internal silo pressure below the design value.
Storing materials under controlled atmospheres or nitrogen is a suitable system, as it prevents any risk of explosion, although it is often very expensive due both to the cost of the inert gas and the maintenance required on the inertising system. For this reason it is usually used only to protect silos that contain dangerous materials.
The use of an extinguisher system inside the silo, activated by sensors that pick up the increase in pressure, control and put out the fire before it gets out of hand, guarantees good protection for the silo. On the other hand, it requires a large amount of maintenance and a careful choice of the extinguishing substance, which must be compatible with the physical and chemical characteristics of the stored material.
The use of panels or doors which open under high pressure to allow combustible gases and vapours to escape, is, in some cases, the cheapest and simplest solution for controlling explosions.
The vapours in the silo escape only when a certain pressure is reached; the escape path for the gases can be created by a vent panel breaking or by a vent door opening.
Due to the different type of opening, the vent door has a greater inertia than a vent panel: this reduces its efficiency.
These devices have been designed to ensure that the pressure inside the silo during an explosion is in any case lower than the silo design pressure.
The gases are unloaded directly into the atmosphere or conveyed to a duct.