FAQ CATEGORY: Combustible Dusts and Explosion Safety

Explosion prevention begins when one piece of the Dust Explosion Pentagon is removed.

Remove Combustible Dust 

If you can go without storing combustible dust in your facility, you don’t have a dust explosion hazard. This approach is safer than any other prevention technique because the fuel is no longer present.

Concentration Reduction

Keeping fuel below the minimum explosive concentration, or MEC, can be done through regular cleaning of your present equipment or selecting new equipment that doesn’t allow dust to accumulate inside it or in the ductwork.

Oxidizer Reduction 

Oxidizer reduction is inerting the atmosphere to bring the oxygen level down below the limiting oxygen concentration, or LOC. This method involves the injection of an inerting gas like nitrogen into a closed system.

Spark Detection and Control 

With this active system, possible ignition sources are detected. Examples include hot screws, smoldering piles, or a hot ember that has been sucked into a dust collection system. These hazards can be detected through the presence of smoke, radiation and high temperatures, and must activate a control method such as:

• an abort gate that shunts a hot ember out of the processing line;
• a suppression system that quenches it before it can get downstream and start the incipient stages of a flash fire or explosion.

Proper Hot Work Systems

Hot work such as welding and cutting should not be done in a dusty environment or on tanks or hoppers containing combustible dust. Clean up the material or empty it from the equipment being worked on before hot work begins.

Avoid Self-Ignition

Self-ignition can happen in silos where smoldering combustion is deep inside the stored material and turns into flaming combustion when it reaches the surface. This event can ignite a dust or gas explosion in the headspace.

Self-ignition can also occur in equipment like spray dryers. Material sticks to the inside edge, heats up and becomes an ignition source for a dust explosion. It is critical to remove this material safely; striking the vessel to dislodge the buildup could ignite an explosion.

Ignition Source Control

Minimum Ignition Energy, or MIE, is the minimum amount of energy required to initiate the combustion of a cloud of dust, vapor or gas. The lower the number, the more hazardous it is. Making sure the ignition sources in your system or surface temperatures are below the MIE of the dust cloud removes the ignition piece of the Dust Explosion Pentagon and is the final way to prevent an explosion from occurring.

Explosion mitigation - protecting workers, equipment or the environment from an incident or incident sequence  –  begins after the incipient stages have already begun.

Containment

Containment refers to increasing the confinement. Explosion-proof pressure vessels on a dust collector can avoid the need for an isolation channel between them. In other cases, such as hammer mills, the equipment is built strong enough to withstand an explosion inside.

Venting

Venting is a passive approach. The explosion vent is designed to open at a set pressure. When an explosion reaches the set pressure, the vent opens and the pressure is expelled into the surrounding area.

Flameless Venting

Flameless venting usually involves a passive explosion vent panel and flame-inhibiting device located within equipment such a dust collector. As an explosion begins, the explosion vent panel opens, permitting flame and dust to enter the flame-inhibiting device, reducing the flame’s temperature below the dust’s ignition temperature and thereby keeping the flame from spreading and causing an explosion.

Suppression Systems

Suppression systems actively monitor for the beginning of an explosion. A pressure sensor detects an incident at its incipient stage and activates a control system. This system could suppress the flame or inert the atmosphere so that the flame can’t develop further.

The 5 Elements of a Dust Explosion

There are five required elements for a dust explosion. They are sometimes referred to as the Dust Explosion Pentagon:

• A fuel, which is the combustible dust
• An oxidant, which is typically the oxygen in the air
• An ignition source capable of igniting materials when they are dispersed as a cloud
• Dispersion, which is when the accumulated dust is spread out and creates a dust cloud
• Confinement, which leads to pressure rise and a potential vessel rupture, facility obstruction or structural collapse

Deflagration

A term often used in connection with dust fires and explosions is ‘deflagration’ and is defined as combustion which propagates through a gas or across the surface of an explosive at subsonic speeds, driven by the transfer of heat. The term is used for both flash fires and explosions. With a dispersed dust cloud, deflagration can cause an unconfined flash fire or, when confined, an explosion that ruptures the containment vessel.

The Two Explosions

Combustible dust explosions often involve two explosions: primary and secondary. The primary explosion is the first to occur when dust suspension in a confined space is ignited and explodes. The first explosion will dislodge other dust that has accumulated which, when airborne, also ignites. A secondary dust explosion is often more destructive than the primary one.

The Occupational Safety and Health Administration (OSHA) sets federal standards for workplace safety and has developed regulations that apply to dust explosions. These include standards addressing issues such as materials handling, as well as other safety issues, such as emergency exits. Failure to comply with OSHA regulations could carry serious fines.

Standards set by the National Fire Protection Association (NFPA), on the other hand, are guidelines published by scientists and other leading experts in the field of fire safety. The organization’s primary list, “Standard on the Fundamentals of Combustible Dust,” constitutes the most advanced recommendations to protect workers and facilities that encounter combustible dust. NFPA recommendations don’t carry any weight of law, but they are highly respected and are often looked to for guidance when making federal and state regulations.

Many metal powders are used as raw materials in 3D printing. These metal powders can be highly combustible, and the 3D printing process generates dust. NFPA 484 begins to outline proper handling and processing of this combustible dust and metal powders used in additive manufacturing.

NFPA 652, Standard addresses the fundamentals of Combustible Dust to mitigate fire, flash fire, and explosion hazards. This new standard provides the basic principles and requirements for identifying and managing the fire and explosion hazards of combustible dusts and particulate solids. This standard serves a wide variety of industries including chemical, wood processing, metals, and agricultural.

Yes, NFPA 484 permits the use of downdraft benches and environmental control booths as long as it collects less than .5 lbs of dust is collected and emptied each day, and a DHA (Dust Hazard Analysis) has been conducted.

Aluminum, magnesium, titanium, zirconium, sodium, lithium and potassium have the potential to combust when in powder or dust form.

According to NFPA 484, Legacy Metals include aluminum, magnesium, niobium, tantalum, titanium, zirconium and hafnium.

Nanometals are very fine metallic particles smaller than 500 nanometers.

To reduce the accumulation of static electrical charge, be sure all components of your dust collection system are properly bonded and grounded

DHA is a systematic review to identify and evaluate the potential for fire, flash fire and explosion hazards associated with the presence of one or more combustible particulate solids arising from your processes or present in your facilities.

There are five critical elements to a thorough Dust Hazard Analysis:

• Methodology and scope
• Material characterization
• Process characterization
• Hazard analysis and recommendations
• Administrative controls and recommendations

For more information around each component, link to DHA White Paper here.

The NFPA 484 Standard addresses the proper handling of combustible metal dusts to reduce the risk of explosion. This standard applies to all facilities that process or finish potentially combustible metals, creating metal dust or powder.

The National Fire Protection Agency (NFPA) defines combustible dust as “any finely divided solid material that is 420 microns or smaller in diameter and presents a fire or explosion hazard when dispersed and ignited in air.” Any combustible material can burn rapidly when in a finely divided form. If such a dust is suspended in air in the right concentration, under certain conditions, it can become explosible. Left uncontrolled, dusts may migrate from the point of production/release, increasing the portion of the facility subjected to combustible dust fire and explosion hazards. Even materials that do not burn in larger pieces, given the proper conditions, can be explosible in dust form. Typical metalworking materials that can form combustible dusts include titanium, aluminum, magnesium and iron. The amount of dust required to produce a dangerous dust cloud can be surprisingly small — a dust layer as little as 1/32 inch thick can be of concern in some situations.

The first step in preventing a dust explosion is understanding the nature of your dust. Having your dust professionally tested will allow you to properly address the dangers, if any exist. A professional analysis of your dust will provide you with the dust’s Pmax and Kst values, figures that will tell you how potentially explosive the dust is.

If your dust is combustible, a full Process Hazard Analysis is advised. This analysis looks at how your dust is produced and how it propagates throughout your facility. Next, a list of recommendations will address ways to mitigate the dangers. These might include:

• Altering the process so that dust particulates are coarser and less explosive
• Adding screens or walls to limit the spread of dust or the spread of an explosion
• Identifying and reducing possible sources of ignition

Actively filtering your air is another effective way to prevent a dust explosion. This practice prevents dust from reaching concentration levels necessary to produce an explosion. Many kinds of dust collectors are available, including both source capture and ambient capture units. A source capture unit collects dust close to the source or application. This is the most effective way to prevent dust from getting in the air. Ambient capture equipment filters the air throughout the entire facility, preventing an accumulation of dust in the air or on surfaces.

For more information on preventing dust explosions, check out OSHA’s manual: Combustible Dust in Industry: Preventing and Mitigating the Effects of Fire and Explosions.

The biggest air quality problem that could lead to a fire is the accumulation of dust in the air or on surfaces in a facility. When combustible dust combines with oxygen in the right proportion in an enclosed space, a dust explosion is possible. Manufacturers must be careful to keep dust concentrations at low enough levels to avoid this possibility. A vigilant approach to dust collection can prevent fires and explosions while also protecting workers’ health.

The accumulation of dust on surfaces is another air quality issue that can worsen a dust explosion. During a dust explosion, often the initial blast stirs up dust that has settled throughout a facility, creating a new cloud of combustible material that ignites and causes a massive secondary explosion. Again, using dust collectors avoids this air quality problem.

The Occupational Safety and Health Administration (OSHA) enforces standards for workplace safety, and the agency has developed a list of standards that apply to dust explosions. Some of these standards apply to workplace practices, such as materials handling, while others address typical safety issues, such as emergency exits. It is a manufacturer’s responsibility to comply with these standards and to reduce the risks of dust explosions. To meet these OSHA requirements, manufacturers must address their air quality. Most likely this will mean exhausting or filtering their air in order to reduce dust concentrations.

Another set of standards for manufacturers to look to for guidance is the National Fire Protection Association’s (NFPA) “Standard on the Fundamentals of Combustible Dust.” While these standards are not federal regulations and carry no weight of law, they represent the leading science on the risks and mitigation measures. If OSHA changes its regulations on these matters, the agency will likely look to the NFPA standards for guidance.

Manufacturers should indeed be aware of possible changes in OSHA regulations in the future. In 2009, OSHA began the process to develop a more comprehensive standard to regulate combustible dust. The regulatory process began with issuing an “advance notice of proposed rulemaking” and holding a series of stakeholder meetings. One of the next steps was to convene a panel required by the Small Business Regulatory Enforcement Fairness Act (SBREFA). The process will likely take a while, as is usually the case, but the eventual change could apply to a wide swath of manufacturers and could compel them to make serious changes to their air quality.

Dust explosions occur when combustible dusts build up in the air and combust rapidly, causing a strong pressure wave to form. They are a deadly hazard in a variety of workplaces, from grain silos to plastics factories. A dust explosion requires several factors to be present at once. These include:

• A combustible dust at the right concentration level
• Oxygen
• An enclosed space
• An ignition source

Sometimes these factors are combined into a graphic known as the “Dust Explosion Pentagon.” The component in this graphic called “dispersion” is also known as concentration. If a concentration of dust is too low, there is not enough of it present to fuel an explosion. If the concentration is too high, there is not enough oxygen to support combustion.

While some combustible dusts are easy to guess—wood and paper dust, for example—others aren’t, such as aluminum dust. Combustible dusts become more dangerous as particulates become finer. These dusts feature a high ratio of surface area to volume, adding to their combustibility. When these dusts combine with oxygen within a range of concentrations, a dust explosion is possible.

In these conditions, all that is needed for an explosion is an ignition source. This source can be anything from a cigarette to a spark to an overheated wheel bearing. Under the right conditions, some combustible dusts can self-ignite as a result of static that builds up as particulates rub against one another. The ignition causes the dust to combust quickly—a process called deflagration that creates a wave of high air pressure. Sometimes this explosion can stir dust that has settled in the space, creating a cloud of new dust—a fuel source for an enormous secondary explosion. A dust explosion can blow out walls in a facility and kill or injure workers within the space or nearby.

The combustibility of dust depends on a number of factors. For a quick check, look at this chart from the Occupational Safety and Health Administration (OSHA). If your dust is on that list, then you need to be concerned about combustible dust.

Just how combustible your dust is, however, depends on many things you won’t find on that list nor be able to see in your facility. For example, particle size has a major impact on the combustibility of a dust. In general, the smaller the particle, the more dangerous the dust.

To understand the specific dangers of your dust, it is best to have it tested by professionals. They can analyze a sample of your dust and produce a full report describing it. Their report would include two key numbers: the Pmax and Kst values. These numbers measure the explosive power of your airborne dust. The Pmax value is the maximum pressure that might be produced by an explosion of your dust. The Kst value—also called the deflagration index, measuring the relative severity of the explosion—is determined by several factors, including the size and chemical nature of your particulates, along with their moisture level.

Having at least a cursory understanding of these values is useful in assessing the risks. For example, the larger the Kst value, the stronger the possible explosion. A Kst value of 0 will result in no explosion. A value over 0 and below 200 could result in a “weak explosion.” A value between 200 and 300 could result in a “strong explosion.” These substances might include cellulose or wood flour. A value over 300 could see a “very strong explosion.” These materials might include aluminum or magnesium dust.

Testing the KST factor of your combustible dust will likely require a professional service. These experts have equipment to ignite your dust in a controlled setting that allows for precise measurements to be made. This kind of test would produce both Kst and Pmax values. The Kst value measures the “relative explosion severity compared to other dusts,” as OSHA says. The Pmax value is the maximum pressure that might be produced by an explosion of your dust.

Dusts vary widely in their Kst values, even among the same substance. Factors such as particle size, particle shape and moisture content affect the Kst value. This is why it is essential to have your dust tested. Simply looking up your dust on a list of what is combustible and what is not is not adequate. Having your dust professionally tested will ensure that you understand the dangers.