Managing the risk from combustible dust explosions involves a number of strategies for preventing explosions, as well as isolating, venting and suppressing explosions if they occur. Complying with applicable NFPA standards is a pathway toward minimizing risk
Combustible dust explosions present an ongoing risk to plants within the chemical process industries (CPI) that handle combustible powders or bulk solids. While dust explosions are not as common as fires, if an explosion occurs, the potential for extensive damage can be an order of magnitude higher. Managing dust explosion risks involves a number of strategies, including reducing the likelihood of an explosion through prevention activities, as well as implementing measures to isolate, vent and suppress explosions if they do occur.
Safety measures established by the U.S. Department of Labor’s Occupational Safety and Health Administration (OSHA; Washington, D.C.; www.osha.gov), through the OSHA Combustible Dust National Emphasis Program, provide a good basis for combustible dust safety. The program recommends following practices in applicable National Fire Protection Association (NFPA; Quincy, Mass.; www.nfpa.org) standards. This article presents an overview of NFPA standards that are relevant for combustible dusts, along with considerations for managing combustible dust risks.
Dust explosion science
Dust explosion hazards exist in a wide range of industries, including chemical, petrochemical, grain, food, wood, paper and pharmaceutical industries. If the material being processed is combustible and capable of generating dust, then there is a potential dust explosion hazard. Common process equipment that presents an explosion risk include dust collectors, filter receivers, pneumatic conveyors, mechanical conveyors, size-reduction equipment, dryers, bins and storage silos.
A deflagration is an expanding flame front traveling through a fuel-air mixture at a velocity lower than the speed of sound. The resulting hot expanding gases rapidly pressurize any containing vessel, causing it to explode, unless it is capable of withstanding the full explosion pressure. This pressure can exceed 100 psi. The maximum explosion pressure is referred as Pmax, and is commonly expressed in the units barg. Pmax is one of several combustion characteristics referenced for protection system design. Others include the following:
• Pmax (barg) — Maximum explosion pressure
• Kst (bar . m/s) — An explosibility index determined from measuring the rate of pressure rise with respect to time in a test vessel
• Minimum ignition energy (MIE; mJ) – The minimum ignition energy needed to ignite the fuel-air mixture
• Auto-ignition temperature (°C) — The lowest temperature at which a dust layer or dust cloud will spontaneously ignite. These are determined by two different test apparatuses
• Minimum explosible concentration (MEC; g/m3) — The minimum concentration of a combustible dust in air that will support a deflagration
These explosibility characteristics are affected by the physical characteristics of the material. For example, grinding a material and reducing the particle size can increase the rate of combustion and increase the Kst value. It is therefore important that the specific material being handled is tested. If published data are used, care should be taken that the data correspond to a material that is representative of the material being handled.
Fire and explosion prevention
Deflagrations can occur both within process equipment and from the ignition of dust external to the process. To reduce the potential for deflagrations external to the process, electrical components, such as lights and motors, should be suitably rated according to the area hazard classification, which is based on the frequency of dust being present and the characteristics of the dust. The method of determining area classifications is detailed in the National Electric Code, NFPA 70, Article 500 Hazardous (Classified) Locations. Additionally, it is important to note that effective dust control and housekeeping is key to limiting the potential for external deflagrations. From a cost standpoint, the best approach is to limit fugitive dust through adequate dust collection and by maintaining a sealed process. Regular cleaning of all horizontal surfaces, including roofing supports, is a costly alternative to keeping the dust contained in the first place. Blowing down the process with compressed air can create a deadly combustible dust cloud and must be avoided. Housekeeping and dust control are detailed in NFPA 652.
Within process equipment volumes, unless it is possible to fully contain the maximum explosion pressure, displace the oxygen or to operate below the explosible dust concentration, the primary method of reducing the potential of a deflagration is to monitor and prevent potential ignition sources. Potential ignition sources include:
• Overheated bearings
• Mechanical equipment failures, such as impact sparks from a hammer mill
• Rubbing or slipping conveyor belts
• Static electricity
• Foreign material entering the process
• Exothermic chemical reactions
• External welding or other hot work
• Process overheating
Having identified potential ignition sources, risk management solutions can be applied, such as bearing temperature monitoring. Testing the material being handled to determine its MIE and auto-ignition temperature will be helpful in determining the best prevention strategies.
While methods such as monitoring and controlling potential ignition sources are key aspects of explosion prevention, they do not completely eliminate the risk. This necessitates the application of solutions to manage the pressure and flame in the event of a deflagration and prevent propagation to adjacent process volumes. NFPA 652 recognizes the following passive and active methods of protection:
• Reduction of oxidant concentration
• Deflagration venting
• Deflagration venting via flameless vents
• Deflagration pressure containment
• Deflagration suppression
• Dilution with a non-combustible dust to render the mixture non-combustible
Of the methods of protection listed, the most common are explosion venting and explosion suppression (Figure 1). We will discuss these approaches in more detail.
Dust explosion venting
During the early stages of a dust or gas explosion, explosion vents quickly open at a predetermined burst pressure, allowing the rapidly expanding combustion gases to escape into the atmosphere and limit the pressure generated inside the process equipment to calculated safe limits. Venting is the most widely adopted protection mechanism because it provides an economical solution and is often considered “fit-and-forget” solution. However, it is important to note that vents need to be regularly inspected, according to guidance contained in NFPA 68.
For decades, explosion vents have traditionally been designed using a “composite” approach that sandwiches plastic film between more resistant stainless-steel sheets with holes or slots cut into them. These vents are designed to open at typically 1 to 1.5 psi set pressure. With this type of technology, over time, the holes and slots in the stainless-steel sheets can admit particulate matter and debris. The buildup of solids can eventually affect the functionality of the vent. A vent that becomes heavier in weight will open slowly and less efficiently.
A better solution is a single-section explosion vent, comprised of a solitary sheet of stainless steel in a domed configuration. Perforations around the perimeter aid opening at the desired low-set pressure are protected with gasket materials. The single-section domed design produces a vent that is more robust, lighter in weight and largely eliminates the potential for buildup or contamination.
Despite their popularity, explosion vents will not work for every application. With venting, the combustion process releases a large ball of flame into the atmosphere. While this might be an acceptable consequence for outdoor equipment, such as silos, for applications within a plant, it could endanger personnel or equipment, and even lead to a secondary explosion.
In cases where a flame ball must be avoided, flameless venting can be deployed. Flameless vents incorporate an enclosure covering the non-process side of the explosion panel. This enclosure provides a path to atmosphere for the expanding hot gases through a stainless-steel mesh which absorbs the heat and prevents the transmission of flame. This style of vent adds considerable weight, due to the size and construction of the enclosure. External supports may therefore be required. When evaluating the cost of this solution, explosion isolation needs to be considered. This isolation is normally incorporated into the design of explosion-suppression systems. This should be considered when comparing flameless venting solutions to explosion suppression.
For processes where an explosion would ideally be prevented altogether, suppression systems are the optimal alternative. Explosion suppression equipment detects a dust explosion in the first milliseconds of the event, signaling explosion suppressors to rapidly release a flame-quenching medium, such as sodium bicarbonate, into the process equipment (Figure 2). This effectively stops the explosion in its infancy and results in a reduced explosion pressure that is safe for the protected equipment.
For a process running 24 hours a day, 7 days a week, a suppression system can be desirable because the speed of cleanup and refit allows for a quick return to production. In contrast, with venting or flameless venting, the explosion fully develops in the process equipment, requiring cleanup, attending to fire-related damages and other consequences that take time before the process can be brought back into operation. A typical suppression system consists of sensors and several explosion suppressors that propel an extinguishing agent into the process equipment. Pressurized nitrogen is typically used to provide the motive power.
In the event of a deflagration, there is a potential for the flame front to propagate via interconnections between equipment volumes, triggering secondary explosions of increasing severity. For this reason, where a dust explosion hazard exists, NFPA 652 calls for isolation devices in accordance with NFPA 69. Isolation methods accepted by NFPA 69 include the following:
• Chemical barriers
• Flap valves
• Float valves
• Pinch valves
• Slide-gate valves
• Material chokes (rotary valves)
Broadly, explosion isolation can be categorized as passive or active, per NFPA 69. A common example of passive isolation is a flap valve, which is most commonly installed horizontally as a one-way valve on the inlet duct to a dust collector. The flap is open during normal operation, latching closed against a seat in response to the cessation of air flow and a pressure wave traveling in the opposite direction. Recent advances in flap-valve design enable some models to be installed vertically and in ducts where the direction of airflow is in the same direction as the potential fireball. This enables them to be used, for example, on the exhaust duct of a dust collector.
An example of an active isolation method is chemical isolation, which typically consists of an explosion pressure detector that triggers a chemical suppressor. Chemical isolation is not limited to duct orientation or air flow direction. Furthermore, chemical isolation can be used on rectangular ducts and casings with moving internals such as drag conveyors. Chemical isolation does not restrict the pipe in any way, eliminating concerns about pressure losses.
NFPA dust standards
Plant owner/operators are required to document the risk of dust explosions at their facilities in the form of a Dust Hazard Analysis (DHA), the scope of which is detailed in NFPA 652 (Standard on Fundamentals of Combustible Dusts). The deadline for implementing this DHA was September 7, 2020. The DHA is applicable to existing plants, processes and new projects. In NFPA 61, for the food and grain industry, the deadline has been extended to January 1, 2022 for existing plant and processes. The main components of a DHA are the following:
• Identifying where in the process or facility the potential exists for fires and explosions. This involves determining material combustion characteristics, identifying potential ignition sources, identifying external dust emissions, and noting the potential for explosion propagation between interconnected equipment volumes and building compartments
• Identifying safe operating ranges
• Identifying existing protection strategies and equipment
• Providing a plan for implementation of any additional protection equipment and strategies needed to manage the risk
A range of safety measures are available to process-plant owner/operators in meeting the OSHA Combustible Dust National Emphasis Program requirements. These are documented within the NFPA standards 61, 68, 69, 652, 654, 655 and 664. Compliance with these standards ensures that your process plant upholds a level of safety is acceptable to employees, the general public, the allied industry, state and federal fire marshals, insurance companies, and OSHA.
Understanding NFPA standards
In response to the 2008 fatal explosion of an Imperial sugar refinery, OSHA created the Combustible Dust National Emphasis directive, which outlines policies and procedures that create or handle combustible dusts. Violations to the OSHA NEP result in fines. States may provide and administer their own programs, which, if not identical to the federal requirements, must be at least as effective. No state is exempt from compliance with the intent of the OSHA NEP.
The directive states that “NFPA standards should be consulted to obtain evidence of hazard recognition and feasible abatement methods.” Therefore, NFPA standards are the foundation of OSHA’s enforcement tools for managing combustible dust hazards. OSHA hasn’t created its own combustible dust standards, which is why the NFPA standards remain the basis for managing combustible dust hazards.
The relevant NFPA standards are described below.
NFPA 652. NFPA 652 provides a framework for the minimum requirements to be met for protecting against dust explosions and includes the requirement for a DHA. NFPA 654 provides prescriptive solutions for dust explosion protection without being specific to a particular commodity or industry. All industries with combustible dust hazards are required to comply with this standard. The reasoning leading to the development NFPA 652 standard came from users trying to follow the standards for combustible dust hazards but finding discrepancies and conflicting, inadequate explanations. NFPA 652 provides the minimum requirements that must be met for managing combustible dust hazards. This does not eliminate the need to meet the requirements of industry or commodity specific standards.
NFPA 654. NFPA 654 is the Standard for the Manufacturing, Processing and Handling of Combustible Particulate Solids. This standard provides safety measures to prevent and mitigate fires and dust explosions in facilities that handle combustible particulate solids. It includes prescriptive measures to protect a range of process equipment such as dust collectors, conveyors and dryers. It is not industry- or commodity-specific, and is therefore applicable to a range of industries and commodities not covered by other specific standards.
NFPA 68. NFPA 68 is the Standard on Explosion Protection by Deflagration Venting Standard on Explosion Protection by Deflagration Venting. This document focuses on explosion venting, that is, devices and systems that vent combustion gases and pressures resulting from a deflagration within an enclosure, for the purpose of minimizing structural and mechanical damage.
NFPA 69. NFPA 69 is the Standard on Explosion Prevention Systems, which covers the following methods for prevention of deflagration explosions: control of oxidant concentration, control of combustible concentration, explosion suppression, deflagration pressure containment, and spark extinguishing systems.
NFPA 61. NFPA 61 is the Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities. This standard covers all facilities engaged in handking dry agricultural bulk materials, including grains, oilseeds, agricultural seeds, legumes, sugar, flour, spices, feeds, and other related materials, such as starch.
NFPA 484. NFPA 484 is the Standard for Combustible Metals. This standard covers all metals and alloys in a form that is capable of combustion or explosion, and it outlines procedures that shall be used to determine whether a metal is combustible or noncombustible form. It also applies to processing or finishing operations that produce combustible metal powder or dust such as machining, sawing, grinding, buffing and polishing.
NFPA 664. NFPA 664 is the Standard for the Prevention of Fire and Explosions in Wood Processing and Woodworking Facilities. This standard establishes the minimum fire and explosion prevention requirements for facilities that process wood or manufactured wood products using wood or cellulosic fibers, creating wood chips, particles or dust. Examples include wood flour plants, industrial woodworking plants, furniture plants, plywood plants, composite board plants, lumber mills, and production-type woodworking shops and carpentry shops that meet minimum requirements for plant size or dust collection flow rates.
NFPA 655. NFPA 655 is the Standard for Prevention of Sulfur Fires and Explosions. This standard covers explosion and fire hazards encountered in the handling, crushing, grinding, and pulverizing of bulk and liquid sulfur.
There is always more than one way to achieve combustible dust safety. The expertise is in reviewing each option for a particular industrial process and arriving at a combination of housekeeping and technologies that is technically effective, as well as cost effective, in meeting the owner-operator’s responsibilities under OSHA and NFPA standards.
Edited by Scott Jenkins
Clive Nixon is a sales manager for BS&B Pressure Safety Management (7455 East 46th Street, Tulsa, OK 74145; Phone: 918-622-5950; Email: firstname.lastname@example.org). Nixon supplies explosion protection solutions to the food, chemical, wood, and grain industries, and has over 20 years of experience in the field of industrial explosion protection. He holds a degree in technical communications from Northeastern University, and an higer national certificate (HNC) in electrical and electronic engineering from Slough College of Technology in Berkshire, England. He has presented seminars on the subject of industrial explosion protection for the International Powder & Bulk Solids Conference/Exhibition Conference, Rosemont, Ill.; Interphex Pharmaceutical Manufacturing Conference & Exhibition, New York City; NFPA World Safety Conference & Expo, Baltimore, Md.; The Department of Engineering Professional Development, University of Wisconsin-Madison; Powtech, Sao Paulo; and e-Week, Albany, N.Y.