Preventing dust explosions
January 1, 2008
This is the first in a two-part series on preventing grain dust explosions. The second article will appear in an upcoming issue of World Grain.
The ATEX Directives in the European Union (E.U.) and the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR) 2002 in the U.K. provide the legal framework for promoting the safety and health of workers potentially at risk from explosive atmospheres.
The regulations place obligations on manufacturers, suppliers, owners and operators of equipment for use in potentially explosive atmospheres. Owners and operators of equipment must:
• classify places where explosive atmosphere may occur into zones;
• carry out a risk assessment of any work activities involving dangerous substances;
• eliminate or reduce the risks as much as reasonably possible;
• provide equipment and procedures to deal with accidents and emergencies; and
• provide information and training to employees.
These regulations have been in place for existing and new plants in the E.U. since June 30, 2006. Other countries have their own codes and regulations to deal with potentially explosive dusts.
Given the millions of tonnes of cereals processed throughout the world annually, it could be argued that dust explosions are relatively rare. However, from 1995 to 2005 in the United States (U.S.) agricultural sector, there were 106 dust explosions causing 16 fatalities, 126 injuries and an estimated $162million of damage to grain storage and processing facilities.
Undoubtedly, there are many dust explosions, fires and “near misses” throughout the world that go unreported.
In addition to the immediate catastrophic consequences, there is also the potential for loss of production capability, risk of prosecution and litigation, and adverse publicity causing damage to a company’s public image and customer base
A common mistake is to believe that because a plant has been operating for years without a fire or dust explosion, it must be safe. This is never a valid basis for determining whether or not a facility is doing a good job preventing dust explosions.
As far as prevention and control of dust explosions are concerned, the risk assessment is undoubtedly the critical exercise and should be the starting point for all plant managers. Once completed, the risk assessment will provide plant managers with a detailed picture of how well the plant and its personnel are protected and highlight areas that need attention. This is an invaluable tool that can be used for safety management, capital expenditure planning and continuous improvement.
The risk assessment should be led by a person competent in the field of explosion protection (i.e. competent by experience, training or both). Many companies will bring in external specialists to act as the competent person.
In addition to the competent person, the risk assessment should involve employee and company management representatives and team members with practical knowledge and experience of the plant, such as operators and engineers.
The objective of the risk assessment is to determine the risks to employees and others by:
• identification of areas and processes in the plant where an explosive atmosphere could occur;
• identification of potential sources of ignition, whether the sources of ignition could give rise to a fire or explosion and how the sources of ignition are controlled; and
• establishing the likelihood of an explosion occurring;
• identifying what measures are in place to mitigate the effects of an explosion and whether these measures are adequate.
Particularly in complex and older plants, the risk assessment can appear to be a daunting and time-consuming exercise for the busy plant manager. To make the exercise manageable, it is recommended to go through the production process from raw material delivery to finished product out load, breaking down each step of the process into “bite-size” chunks. In a flour mill, for example, this could be:
• cereal delivery and intake;• silo storage;• cereal transfer;• cereal cleaning;• milling;• finished product storage;• finished product load out;• byproduct storage;• byproduct out load.
In addition, all potential sources of ignition should be considered and adequately controlled. These sources may include naked flames; hot surfaces; burning material; welding or cutting; mechanical friction; electric sparks and electrostatic discharges.
SYSTEMS AND PROCEDURES
Emergency and evacuation plans should be in place to deal with a fire/explosion or other emergencies occurring. All employees and others must understand how to respond in an emergency. The plan should be regularly reviewed.
Hot Work such as welding and grinding should be prohibited except under a written Hot Work Permit. This is a permit issued by a responsible person who verifies that it is safe to proceed with the work and verifies safety after completion of the work.
Contractors and visitors should be made aware of the explosion hazards and the emergency plans. This can be done by induction or training, such as a short video presentation, before work commences.
Contractors should only work under a Permit to Work system that specifies where, when, and how the work will be done. Tools and equipment must not provide a potential source of ignition, including portable electrical appliances, power tools and mobile phones.
Effective plant maintenance is vital to plant safety and control of potential ignition sources. Priority attention should be given to critical protective devices such as bearing temperature sensors, bucket elevator misalignment sensors and rotary valves to ensure that they are well maintained, functioning correctly and regularly tested.
Plant automation systems also play a vital role, through correct sequencing and interlocking, in ensuring both the risks and effects of an explosion are minimized. Emergency shutdown procedures in the event of explosion are especially important. Consider abnormal events such as a power cut.
Regular housekeeping inspections should take place, since proper housekeeping can be critical in preventing secondary explosions from occurring. Dust layers can ignite or be roused into dust clouds contributing to devastating secondary explosions. Out-of-sight areas, such as overhead beams and ledges, can harbor the driest and finest dust layers which are potentially more explosive.
Finally, training plans should include basic explosion awareness training for all employees. Operational and maintenance staff should be provided with more in-depth training.
Explosion venting is a common protection measure in grain handling facilities. But if poorly designed, it will offer limited or no protection in the event of an explosion.
The purpose of explosion venting is to allow the pressure, flame front and burning debris from an explosion to escape to a safe area.
The vent or bursting panel must open rapidly at a known pressure. The vent should be correctly sized and fitted with a burst detector switch.
Even from a small vessel, an explosion flame front can be many feet long. Therefore, vent ducts must direct the explosion to a safe area with minimum resistance. The ducts must be as short and straight as possible; bends and long ducts must be avoided. The duct outlets must never be blanked off, nor should they be weakened, damaged or corroded, as they need to be strong enough to contain the effects of an explosion.
Also, the vessels being vented must be strong enough to contain the reduced explosion pressure.
REVIEWING THE RISK ASSESSMENT
The completed risk assessment will present a clear picture of how well the plant is protected. Deficiencies will also be highlighted and must be acted upon. Some can be easily and quickly resolved, for example, by a change in procedures. Others may require major investment over a period of time and be technically challenging, and in some cases expert advice may also be required The risk assessment must be regularly and routinely reviewed, with the review forming an integral part of your management systems. A review should also be done if there is a modification to the plant or a change in working processes or practices.
Ged Begley is a freelance grain handling and milling technologist based in Scotland. He has worldwide experience in cereal milling and has specialist knowledge and extensive experience in wheat debranning, wheat gluten processing and the Scotch Whisky Industry. He can be contacted at Begleyconsulting@aol.com . Tel: +44 1292 28072.