A Combustible Dust Hazard Analysis or Assessment (DHA) is an investigation to identify potential dust hazards in the processes at a facility and document how such hazards are managed - specifically, fire, deflagration, and explosion hazards due to the presence of combustible dusts.
What is a Dust Deflagration Hazard?
Deflagration is simply defined as, “to burn violently, or to make something burn violently”. A dust deflagration hazard is a condition that presents the potential for harm or damage to people, property, or the environment due the combustion of a sufficient quantity of combustible dust suspended in air (or another oxidizing medium). A dust deflagration hazard in an enclosure that is capable of bursting or rupturing the enclosure due to the development of excessive internal pressure from the deflagration constitutes a defined explosion hazard.
Dust Hazard Analysis (DHA) Process
A Dust Hazard Analysis is a systematic analytical review of a facility and its processes to identify combustible dust hazards to employees, property, and the public at large. Aspects of a facility’s operation, such as individual pieces of process equipment, ducts, and dust collection system, are examined individually and aggregately to determine administrative or engineering safeguards that should be implemented to reduce the risk of a combustible dust event. Potential ignition sources are assessed and methods to eliminate or reduce possible ignition of a dust cloud are examined.
The DHA is typically conducted in a manner that meets the National Fire Protection Association (NFPA) guidelines described in NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids. NFPA 652, Standard on the Fundamentals of Combustible Dust was utilized for assessing the general requirements necessary to manage fire, flash fire, and explosion hazards posed by combustible dusts identified in the DHA.
Threat of Secondary Explosions
Even in instances when the observed accumulation of dust external to equipment is minimal, it is important to note that the greatest hazard associated with combustible dust comes from the threat of secondary explosions.
What causes secondary explosions?
Secondary explosions occur when a primary explosion, often inside process equipment or in an isolated area, sends pressure waves through a facility that dislodges fine dust accumulated on floors, walls, and overhead surfaces. This fine dust then forms a cloud that spreads into a large area. If this dust cloud is ignited, a large, potentially devastating flash fire or explosion could occur.
NFPA and OSHA guidelines state that a dust layer 1/32 inch (0.79 mm) thick spread over just 5% of the floor area of a facility is sufficient to pose a combustible dust hazard. Dust that settles on rafters and piping above the floor can account for covering as much as 10% of the floor area of a building.
How can you reduce the risk of a combustible dust explosion?
Cleaning overhead and relatively inaccessible areas can be difficult and even dangerous. Instead of removing accumulations of combustible dust, preventing the escape of fugitive dust from process equipment is the safest, most effective method of reducing the risk of a combustible dust explosion.
Prevention of Dust Explosions
There are two basic types of preventive measures: Administrative Measures and Engineering Controls.
Administrative procedures include detailed housekeeping, hot work, and lockout/tagout procedures that address the hazards of performing maintenance work in areas where combustible dusts are present.
Proactive housekeeping practices, coupled with routine inspections of equipment and dust accumulation levels, can greatly reduce the hazard level in a facility.
Vigilant maintenance of equipment with an emphasis on fugitive dust emissions can dramatically reduce dust accumulation levels, and thus the hazard level, in a facility.
Employers must inform workers of the hazards associated with combustible dusts present, as outlined in OSHA’s Hazard Communication requirements.
Engineering controls include the mechanical equipment necessary to prevent a deflagration from spreading to occupied areas and/or safely extinguish or vent the deflagration before structural failure of equipment or buildings.
Spark detection and suppression systems are used in ductwork to extinguish sparks or burning embers before they reach cyclones or dust collectors which may contain sufficient airborne dust concentrations to cause a deflagration or explosion.
If a deflagration does form inside a piece of equipment, deflagration suppression and venting systems are designed to either extinguish or vent the pressure of the deflagration before the equipment fails and an explosion is allowed to propagate through the building.
Isolation devices (active and passive) are used to prevent a deflagration inside a dust collector from spreading through the ductwork to other areas of the facility.
Combustible Dust Flash Fire Hazard
Proper PPE for Flash Fires are Industry Requirements
NFPA 654 states that operating and maintenance procedures address personal protective equipment (PPE) including flame-resistant garments in accordance with the workplace hazard assessment required by NFPA 2113. This is due to the specific hazards of a flash fire.
Flash fires burn extremely fast, usually lasting less than three (3) seconds.
Non-flame resistant garments can be ignited by the flash fire and continue to burn after the initial fire has burned out.
The body area beneath non-flame resistant clothing is often burned more severely than exposed skin.
The survival rate of personnel exposed to a flash fire drops significantly as the percentage of the total body area affected increases.
Flame resistant clothing can drastically reduce the effect of a flash fire.
Personnel that operate in dusty environments may also need to wear conductive footwear and grounding straps to prevent the buildup of static electricity.
Such clothing may not be warranted for operation of the wire drawing equipment during routine operations.
Use of flame resistant clothing during maintenance should be considered.
KERAMIDA’s team of industrial hygienists and engineers are ready to assist you with conducting a Combustible Dust Hazard Analysis of your facility. Contact us today to schedule a site visit, or call us at (800) 508-8034 to speak with one of our safety experts.