Managing Explosion Hazards in Dust Collection Systems
The dust explosion risk in the ceramic industry is real, and it is important to know where explosible dust clouds can potentially form in your facility.
Most powders handled in industry can form explosible dust clouds if the particle size is small, moisture content is low, and the concentration in the cloud is high enough. If the dust cloud finds a strong enough ignition source, a flash fire (deflagration) or explosion will occur.
In the ceramic industry, many of the materials used (e.g., silica, limestone, cement) are inert, meaning they will not burn and cannot form explosible dust clouds. However, many other powders are also used that are explosible (e.g., organic powders, metals); indeed, powder mixtures involving both inert and flammable powders can form explosible dust clouds.
The dust explosion risk in the ceramic industry is real, and it is important to know where explosible dust clouds can potentially form in your facility. Dust cloud flash fires and explosions can cause catastrophic events involving fatalities, injuries, community impact, facility damage, and economic losses. This is why NFPA 652 (Standard on the Fundamentals of Combustible Dust) mandates that a Dust Hazard Analysis (DHA) is performed to evaluate fire, deflagration and explosion hazards. NFPA 652 also establishes how such hazards are to be managed.
Understanding Dust Collection
One plant feature that is readily susceptible to dust cloud explosions is dust collection systems, including filter receivers and baghouses. Reasons for the relatively high frequency of occurrence of explosions in dust collectors lie in the inherent nature of these systems. They can contain a readily ignitable dust cloud, with the smallest particle size likely to be often present. For example, the use of reverse jet or pulse jet units that dislodge powder entrained in the filters will increase the frequency of the formation of clouds of dust.
The powder within dust collectors is generally the finest found in the process, which is potentially the most sensitive to ignition. In addition, if ignited, it will give rise to the most severe explosion for that dust.
Dust collection systems also contain the possible presence of several sources of ignition. These can include impact and friction sparks due to foreign objects being sucked into the dust collection ductwork, static discharges from ungrounded metal components and insulating (plastic) ductwork, decomposition of self-heating substances, transfer of smoldering material from upstream processes, and sparks or hot surfaces from electrical and mechanical equipment in the dust collection system.
Three components need to be concurrently present in one location for a fire (or flash fire) to occur. In the case of dust cloud flash fires (deflagrations), this translates to having the following conditions simultaneously present:
- A combustible powder, with enough dust content (small particle size) to support flame spread, forming a cloud with a concentration above its minimum explosible concentration (MEC)
- Sufficient oxidant (typically provided by the oxygen in the air)
- An ignition source with sufficient energy
It should be no surprise that the first two conditions are usually present at least at some point during any powder/dust handling, transfer, processing, packaging, and dust collection operation. Of course, simultaneous existence of an energetic ignition source will result in a dust cloud flashfire. If a deflagration occurs in a dust collector or any other closed-process vessel or room/building, pressure can build up that would be sufficient to rupture the confining enclosure. The event then becomes a dust explosion.
NFPA 652 was issued by the Standards Council on August 18, 2015, and became effective September 7, 2015, with a declared purpose of providing general requirements for management of combustible dust fire and explosion hazards. It also directs users to appropriate NFPA industry or commodity-specific standards by establishing relationship and hierarchy between them. These industry or commodity-specific standards include: NFPA 61, NFPA 484, NFPA 655, NFPA 654 and NFPA 664. Table 1 can help with the identification of the applicable commodity- or industry-specific NFPA standard for powder mixtures.
Requirements of NFPA 652
NFPA 652 requires that the owner/operator of a facility where potentially combustible dusts might be present is responsible for the following issues.
Determining Combustibility and Explosibility of Materials
NFPA 652 permits the determination of combustibility or explosibility (go/no go) to be based on laboratory analysis of representative samples from the process or historical facility data or published data, so long as the data is representative of current materials and process conditions. One is also permitted to assume that a material is explosible, forgoing the laboratory analysis. However, absence of previous incidents should not be used as basis for deeming a particulate non-combustible or non-explosible.
Conducting a Dust Hazard Analysis
Conducting a dust hazard analysis (DHA) is a systematic evaluation of potential dust fire, deflagration and explosion hazards, and the recommendation of measures for their management. The DHA must be conducted by someone with proven expertise in hazards associated with handling and processing combustible particulate solids. Afterward, the DHA must be reviewed and updated at least every five years.
For existing processes, the owner/operator should schedule the DHA to be completed by September 7, 2020. For new construction or modifications representing more than 25% of the original cost, the DHA should be completed as part of the project.
Managing Identified Fire, Flash Fire and Explosion Hazards
The owner/operator of the facility is responsible for managing the identified fire, flash fire, and explosion hazards by considering the building and equipment design, housekeeping, ignition source control, personal protective equipment, dust control, explosion prevention, protection, isolation, and fire protection. This may be achieved by the prescriptive approach in accordance with Chapters 5, 7, 8, and 9 of NFPA 652, 2019 edition, in conjunction with any prescriptive provisions of applicable commodity-specific NFPA standards. Alternatively, a documented performance-based approach is permitted in lieu of the prescriptive requirements, in accordance with Chapter 6 of NFPA 652, 2019 edition.
Establishing Written Safety Management Systems
NFPA 652 requires the owner/operator to establish written management systems for operating their facility and equipment to prevent or mitigate fires, deflagrations, and explosions from combustible particulate solids. These written management systems include operating procedures and practices, training, incident investigation and employee participation. Written management system requirements apply to new and existing facilities and processes.
Dust Collection Systems
NFPA 652 requires that at each collection point, the dust collection system must be designed to achieve the minimum velocity required for capture, control and containment of the dust source. The addition of branch lines must not be made to an existing system without first confirming that the entire system will maintain the required and balanced airflow.
Dust collection systems that remove material from operations that generate flames, sparks, or hot material under normal operating conditions must not be interconnected with dust collection systems that transport combustible powders or hybrid mixtures. Air-material separators (AMS), such as filter socks, that are selected for these systems must be designed to allow for the characteristics of the combustible dust being separated from the air or gas flow. In addition, air moving devices (AMDs), such as fans, blowers or other devices that establish an airflow, must be of appropriate type and sufficient capacity to maintain the required rate of air flow in all parts of the system.
Centralized Vacuum Cleaning Systems
NFPA 652 requires that centralized vacuum cleaning systems be designed to assure minimum conveying velocities at all times, whether the system is used with single or multiple simultaneous operations. Where ignition-sensitive materials are collected, vacuum tools, as well as the hoses and pipes, must all be constructed of metal or static dissipative materials, electrically bonded together, and grounded.
Air Material Separators
Where a dust explosion hazard exists within the dirty side volume of the AMS with a volume greater than 8 ft3 (0.2 m3), measures must be taken to protect personnel from the consequences of a deflagration in that enclosure. NFPA 652 requires that a documented risk assessment acceptable to the authority having jurisdiction (AHJ) must be conducted to determine the level of protection to be provided.
NFPA 652 requires that the exhaust air from the final AMS must be discharged outside of buildings to a restricted area separated from the building’s clean air intakes. However, recycling of AMS exhaust to buildings would be permitted when all the following conditions are met:
- Combustible or flammable gases or vapors are not present in either the intake or the recycled air in concentrations above applicable industrial hygiene exposure limits or 1% of the lower flammable limit (LFL), whichever is lower.
- Combustible powder particles are not present in the recycled air in concentrations above applicable industrial hygiene exposure limits or 1% of the minimum explosible concentration (MEC), whichever is lower.
- Provisions are incorporated to prevent transmission of flame and pressure effects from a deflagration in an AMS back to the facility, unless a DHA indicates that those effects do not pose a threat to the facility or the occupants.
- Provisions are incorporated to prevent transmission of smoke and flame from a fire in an AMS back to the facility, unless a DHA indicates that those effects do not pose a threat to the facility or the occupants.
- The system includes a method for detecting AMS malfunctions that would reduce collection efficiency and allow increases in the amount of combustible powder particles returned to the building.
- The building to which the recycled air is returned meets the requirements of the housekeeping requirements of NFPA 652.
- Recycled air ducts are inspected and cleaned at least annually.
To reduce the explosion risk to a tolerable level, every dust collection system—indeed, any operation involving combustible powders—requires a basis of safety. A basis of safety from dust cloud deflagrations could include taking measures to avoid an explosion (explosion prevention) or designing facilities and equipment to protect people and equipment in the event of an explosion (explosion protection). Selection of explosion prevention and/or protection measures is usually based on:
- How much information is available on the powder’s “ignition sensitivity” and “explosion severity”
- The nature of the processes and operations
- Personnel’s knowledge and appreciation regarding the consequences of dust explosions
- Personnel’s management of preventive measures
- Effect of an explosion on personnel, environment, and the business
Table 2 summarizes the common basis of safety options for dust collection systems.