Dust explosions in the food industry
01 June 2009
Over 70% of all organic materials are combustible and have the potential to result in a dust explosion. The manufacture of food products involves the storage and handling of materials such as grains and flours.
Dust explosions in the food industry
All of which can give rise to fire and dust explosion hazards.
The transport, storage and handling of organic materials using silos, bucket elevators and pneumatic powder transport, can all generate dust clouds which may be controlled by the use of dust collection systems. Processing operations such as milling, spray drying, blending, agglomeration, etc. can also produce potentially hazardous dust clouds. In addition, high temperature process operations such as baking and frying can also give rise to further fire and explosion hazards.
In the past bucket elevators have been responsible for a number of incidents involving a large loss of life, usually as a result of inadequate explosion protection of the elevator legs and its ‘head house’. Any explosion within an elevator will propagate into downstream equipment unless steps are taken to prevent this. If no protection is present in downstream equipment then there is the potential for catastrophic failure of the equipment with a commensurate risk of loss of life.
The pneumatic transfer of powders gives rise to a flammable atmosphere within the receiving vessel. The transfer of powder will result in high levels of electrostatic charge generation, which may accumulate on unearthed conductors and could produce incendive sparks. In addition the presence of a dust filter unit on the receiving vessel can also represent a source of ignition due to poorly earthed filter supports. Pneumatic transfer into vessels is therefore considered to be a relatively high-risk operation, requiring some form of explosion protection. It is also necessary to prevent explosion propagation from the vessel to both
upstream and downstream items of equipment.
Dust explosions in the food industry
Dust collection systems, such as filter units, will generate flammable atmospheres within the unit during the bag cleaning part of the filter cycle. Furthermore the pneumatic transfer of the dust laden air through the unit will result in high levels of electrostatic charge generation. Historically it has been the filter bags and their support cages that have been the cause of the greatest number of incidents within filter units. It is for this reason that filter units handling combustible dusts are normally provided with explosion relief panels. With dust collection systems there is the possibility of flame propagation back to the equipment being served by the unit. This is of particular concern for centralised units where there is the potential for explosion propagation back to all the equipment served by the unit.
Dust collection units such as cyclones can also contain a flammable atmosphere, although having a lower explosion risk than filter units due to the lower risk of generating an internal electrostatic ignition source. However, protection is still required where there is the potential for burning material to enter the cyclone or there is a risk of explosion propagation from connected equipment.
Spray dryers can be considered to represent a relatively high explosion risk. The operation of a spray dryer can result in the formation of a flammable atmosphere within the dryer. In addition, the formation of excessive material layers on the walls of the dryer or excessive heating can result in the generation of burning nests of material due to the material self-heating. Ignition of the dust cloud within the spray dryer may occur directly or as a result of explosion propagation from downstream items when the burning material comes into contact with another flammable dust cloud.
The manufacturing process can involve high temperature operations such as baking and frying, resulting in the potential for fires in the ovens or fryers due to a loss of temperature control or an accumulation of material within the equipment, which in turn may self-heat, resulting in a fire.
Dust explosions in the food industry
Although the most significant hazards are typically found within the manufacturing areas it is also necessary to consider the hazards and consequences of business interruption associated with the warehousing and transportation of the finished goods.
All these hazards must be assessed in a formal risk assessment and the appropriate safety measures must be in place to control the identified hazards. These would include a combination of hardware systems -- trips and explosion vents – and software systems such as trip/testing procedures, control of ignition sources and hazardous area classification, per unit operation.
Burgoyne Consultants has been providing advice for the food industry since 1985 and are retained consultants for some of the largest domestic and international food manufacturers. They have a team of highly qualified scientists and engineers specialising in all areas of safety management and process safety / loss prevention. Their special strength of combined talents in uncontrolled combustion, safety management and loss prevention and extensive experience in the food industry allows them to provide practical solutions for the prevention or mitigation of fire and explosion hazards, assistance in the development and implementation of effective safety management systems and the preparation of risk assessments.
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