There have been too many dust flash fires and explosions in the process industries. The new standard, NFPA 652, on the Fundamentals of Combustible Dust is designed to stop this – but are you compliant?
Most powders handled in industry can form explosible dust clouds if particle size is small, moisture content low, and concentration in the cloud high enough. If the dust cloud finds a strong enough ignition source then a dust explosion is possible.
Every year, dust cloud flash fires and explosions cause catastrophic events involving fatalities, injuries, community impact, facility damage, and economic losses. It is clearly essential that effective precautions be taken to control the risk and protect employees, contractors, the public, and facilities, against the effects of fires and explosions.
This article discusses the requirements of the new NFPA 652: Standard on the Fundamentals of Combustible Dust and provides some practical measures for controlling dust flash fire and explosion hazards that are usually associated with the handling, processing, and storing of combustible powders.
Conditions for Dust Cloud Flash Fires and Explosions
Back to basics: Most people are familiar with the fire triangle. Simply put, three components need to be present in one location and at the same time 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:
1. 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)
2. Sufficient oxidant, which typically the oxygen in air will provide
3. An ignition source with sufficient energy
It should be no surprise that conditions 1 and 2 are usually present at least at some point during any powder/dust handling, transfer, processing, dust collection, and packaging operations. And of course, simultaneous existence of an energetic ignition source will result in a dust cloud flashfire.
If a deflagration occurs and is confined in a closed process vessel or room/building, pressure can build that is sufficient to rupture the confining enclosure; the event becomes a “dust explosion” (Ref. NFPA 654).
Standards, Relationships and Hierarchy
To help industry better understand and control dust flash fires and explosions in their operations and processes, the National Fire Protection Association (NFPA) has produced several codes and standards over the years. Pertinent NFPA publications include:
• NFPA 61, Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Products Facilities
• NFPA 68, Guide for Venting of Deflagrations
• NFPA 69, Standard on Explosion Prevention Systems
• NFPA 77, Recommended Practice on Static Electricity
• NFPA 484, Standard for Combustible Metals, Metal Powders, and Metal Dusts
• NFPA 499, Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas
• NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing and Handling of Combustible Particulate Solids
• NFPA 655, Standard for Prevention of Sulfur Fires and Explosions
• NFPA 664, Standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities
Although these publications have been enormously helpful, they have also been the source of some confusion in various industries regarding consistency and specific applicability of some of their requirements.
NFPA 652 was issued by the Standards Council on August 18, 2015 and became effective on September 7, 2015. Its declared purpose is to provide general requirements for management of combustible dust fire and explosion hazards. It also directs the users to appropriate NFPA industry or commodity-specific standards by establishing relationship and hierarchy between them. These industry or commodity-specific standards (listed above) are: NFPA 61, NFPA 484, NFPA 655, NFPA 654, and NFPA 664.
Basically, if the requirement in an industry or commodity-specific NFPA standard differs from or specifically prohibits the requirement in NFPA 652, then the requirements in the industry or commodity-specific NFPA standard shall be permitted. However, if an industry or commodity-specific NFPA standard neither prohibits nor provides a requirement then the requirement in NFPA 652 shall apply.
Requirements of NFPA 652
NFPA 652 requires that the owner/operator of a facility where potentially combustible dusts might be present shall be responsible for the following:
• 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 shall not be used as basis for deeming a particulate non-combustible or non-explosible.
• Conducting a Dust Hazard Analysis (DHA)
DHA is a systematic review to identify and evaluate potential dust fire, flash fire, and explosion hazards in a process or facility where combustible/explosible powder is handled or processed. DHA must be conducted by someone with proven expertise in hazards associated with handling and processing combustible particulate solids.
For existing processes, the owner/operator shall schedule the DHA to be completed within three years of the issue date of NFPA 652 (September 2015), although some industry or commodity-specific standards might have a longer compliance period. For new construction or modifications more than 25% of the original cost, DHA shall be completed as part of the project.
• Managing identified fire, flash fire, and explosion hazards
The owner/operator of the facility shall be 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, and isolation, and fire protection.
It should be noted that a documented risk assessment acceptable to the authority having jurisdiction (AHJ) shall be permitted to be conducted in lieu of prescriptive requirements of NFPA 652 to determine level of design and protection features for the building, protection for the equipment, ignition source control, and explosion protection.
• 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 shall apply to new and existing facilities and processes.
Towards Compliance: Safety from Dust Flash Fire and Explosion Hazards
Managing dust cloud flash fire and explosion hazards could include:
• Data collection on your powders is a good first step. Specific testing on representative samples from your facility provides the highest confidence in data compared to those obtained from literature sources. Typical tests/data to consider include:
* Explosibility Screening (go/no go)
* Minimum Ignition Energy (MIE)
* Minimum Ignition Temperature of Dust Cloud (MIT)
* Minimum Ignition Temperature of Dust Layer (LIT)
* Self-Heating (Thermal Stability)
* Minimum Explosible Concentration (MEC)
* Limiting Oxygen Concentration (LOC)
* Explosion Severity (Pmax and Kst)
* Electrostatic Chargeability
* Volume Resistivity
• Avoidance/Control of explosible dust cloud (Fuel)
Consideration should be given to appropriate equipment design for dust containment and release source reduction. Additionally, correctly designed, operated, and maintained local exhaust ventilation should be installed in those locations where any dust release is anticipated. Since dust accumulations on surfaces outside of processing equipment could result in devastating secondary dust cloud explosion hazards, an effective housekeeping program must be enacted to manage dust accumulations on all surfaces in the facility.
• Avoidance/Control of Oxidant
Any explosion is prevented if the oxidant concentration is maintained below the Limiting Oxidant Concentration (LOC), by replacing the oxidant with an inert gas. Inert gas blanketing can be considered if the vessel containing the explosible atmosphere can be totally contained. It should be noted that LOC for combustion is dependent on the nature of the dust (fuel) and the inert gas that is used. Commonly used inert gases are nitrogen, carbon dioxide, and argon. NFPA 69 provides requirements for installing systems for the prevention and control of explosions in enclosures that contain flammable mixtures, including inert gas blanketing.
• Elimination/Control of Ignition Sources
Ignition sources that could be present during normal and foreseeable abnormal conditions must be identified and controlled. Common ignition sources include heat sources, friction and impact sparks, electrical arcs and sparks and electrostatic discharges.
• Determine powder’s onset temperature for self-heating (laboratory test simulating the process) and maintain the process temperature below powder’s onset temperature for self-heating.
• Perform regular inspection and maintenance of plant to prevent overheating due to bearing failure, misalignment, rubbing, etc.
• Perform welding, cutting and other hot work operations under a hot work permit system according to NFPA 51B.
• All metal (conductive) items of the plant that could be electrostatically charged must be electrically grounded with a resistance of ground path to earth not exceeding 1 megohm (106 ohms). Where bonding/grounding system is all metal, resistance in continuous ground paths typically is less than 10 ohms.
• If electrically isolated from ground, the accumulated electrostatic energy on human body during normal activity could reach 10 to 30mJ. So, in locations where an explosible dust cloud or indeed any flammable atmosphere with a Minimum Ignition Energy (MIE) less than 30mJ might exist operators must be adequately grounded. Static dissipative footwear used together with conductive or static dissipative flooring with a total resistance between 106 ohms and 108 ohms provides a means to control and dissipate static charges from the human body.
• Arcs, sparks, and hot surfaces are expected during normal operation of many electrical devices. Additionally, failure may also create ignition sources. It is therefore important that appropriately rated electrical devices are selected and installed where flammable atmospheres are/may be present. This is achieved by conducting a Hazardous Area Classification according to the requirements of Article 500 of the National Electrical Code (NFPA 70). The intent of Article 500 is to prevent electrical equipment from providing a means of ignition for an ignitable atmosphere.
• Explosion Protection
If the formation of an explosible dust cloud atmosphere cannot be prevented and all sources of ignition cannot be reasonably eliminated or excluded, then the possibility of a dust cloud explosion exists. Under such conditions, explosion protection measures must be considered to protect people and minimize damage to facilities. Explosion protection measures should be considered in addition to taking all reasonable steps to reduce the possibility of formation/spread of dust clouds and to exclude potential ignition sources. Explosion Protection measures include:
o Explosion venting to a safe place according to NFPA 68: Guide for Venting of Deflagrations.
o Explosion suppression by injecting a suppressant according to NFPA 69: Standard on Explosion Prevention Systems.
o Containment by explosion resistant construction (Design based on ASME Boiler and Pressure Vessel Code, Section VIII, Division I).
o Explosion isolation (mechanical or chemical) to prevent deflagration propagation between connected equipment and/or work areas.
Effective management of dust flash fires and explosions in your facilities often requires:
• Having pertinent data for the understanding of the combustibility, ignition sensitivity, electrostatic, self-heating, and explosion characteristics of the dust(s)
• Having good understanding of all operations and processes
• Conducting a dust hazard analysis (DHA)
o Identifying locations where explosible atmospheres could be present
o Identifying potential ignition sources that could be present under normal and foreseeable abnormal conditions
o Evaluating all potential dust fire, flash fire, and explosion hazards in the process and facility
• Ensuring safety through properly defined fire and explosion prevention and protection measures - “Basis of Safety”
• Establishing effective written safety management systems
• Regular review and maintenance of management programs, process safety data and information, training, process-control systems, processing equipment, and the facility to ensure the continued safety of people, the community, and the business.
Vahid Ebadat Ph.D., M.Inst.P, MIET, C.Phys. is CEO of Solent Process Safety Inc. He has worked extensively as a process and operational hazards consultant for the chemical, pharmaceutical, food, oil/gas, and paper/wood industries. For more information, contact Dr. Ebadat at email [email protected] or 609-610-6570, or visit solent-safety.com.
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