“You know you’re making fuel, right?”

This is a question I often found myself asking as new wood pellet and biomass plants started sprouting up in North America, especially in the Southeast U.S. I didn’t say this to be rude, but to point out potential combustible dust and ignition issues.

Every engineering firm and large equipment manufacturer seemed to think they could build a wood pellet plant better than the last, so naturally, many of the designs are different. Some are efficient, and some not so much. Some were built with the proper engineering controls designed in, and for others, fire and explosion protection seemed to be an afterthought.

The Problem, Complacency
In my job, I get the opportunity and privilege of making site visits to many of these wood pellet operations, as well as many other types of combustible dust processes. Often, there is a disconnect from safety theory to actual practice. I go through many variations of safety orientation at these plants, and then walking through the facility, will see posters that say “safety first,” all the while walking around on layers of combustible dust!

What if I told you combustible dust could be just as flammable or explosive as gasoline? Or that wood dust bin explosions are just as powerful as grain elevator explosions? Would that change how you view combustible wood or biomass dust?

Some established and reputable companies dismiss the idea of building a conveyor that is dust-tight. But if a plant had a combustible gas leak, it would not continue to operate. Instead, it would be shut down, and the leak fixed prior to starting back up. If they had a fuel, gasoline or other flammable liquids spill, their first responsibility would be to evacuate the area, fix the problem and clean up the mess.

Yet, when it comes to combustible dust, we seem to be blind to the potential hazard. Because we work around it every day, over time, some level of complacency tends to develop. Training and housekeeping become critical to preventing fires, explosions and catastrophic secondary explosions.

Another thing that makes me cringe is hearing someone say, “That’s the way we have always done it,” or “We have been running this way for years,” or “Fires are just part of the process.” This is complacency, lack of education to the hazard, and really, a lack of respect for the danger of combustible dust. Plants change. Machinery wears over time, products change, specifications change. Change management is critical to stay on top of safety. So is constant improvement and training.

Combustible dust layers on equipment and machinery, walls, floors, rafters, cable trays, conduit, and piping are an indication of this disconnect, complacency and understanding of a potential combustible dust incident. Many times after touring plants with layers of combustible dust around it, I find they have indeed had previous fires or explosions. Understand, if you are having fires and explosions, no matter how minor, they are potential precursors to a bigger event. You may be witnessing leading indicators of future problems.

Any time you move or manipulate a combustible product, you are creating friction and heat as well as combustible dust, and therefore have potential for fires and explosions.

Fire Triangle/Explosion Pentagon
The three main areas of concern for creating fires in the biomass process are dryers, hammer mills and pelletizers. Conveyors and other moving machinery are a secondary concern for creating fires. These processes create friction and heat, which is one leg of the Fire Triangle, along with oxygen and fuel. Inherently, you have all the ingredients for a fire.

If your dust is in an enclosure that contains a dust cloud such as a bin or dust collector, you not only have all the ingredients for a fire, but also all of the requirements for an explosion. Having combustible dust in suspension in a confined area or vessel are the last sides of the dust explosion pentagon. If you are storing wood dust or pellets in enclosures, you have dust collectors, you have all the ingredients necessary for an explosion.

We see heat, friction and mechanical sparks as the cause of many of these fires and explosions.
The controlling document for protecting wood biomass and wood pellet plants and processes is the NFPA 664 Standard for the Prevention of Fires and Explosions in Wood Processing and Wood Working Facilities. For agricultural-based biomass, see NFPA 61 Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities. NFPA 664 requires a dust hazard analysis  (DHA) to identify and mitigate all potential combustible dust and ignition hazards.

The primary areas of concern for explosions are enclosed vessels such as bins and hoppers, dust collectors and storage silos, and secondarily, enclosed conveyors such as bucket elevators. To protect these processes and equipment, facilities use a hierarchy of controls.

Hierarchy of Controls
The hierarchy of controls includes elimination, substitution, personal protective equipment (PPE), administrative controls and engineering controls, the last three of which this article will discuss.
Personal protective equipment (PPE) and gear are the easiest to apply to keep personnel safe. Most of us know that when operating in areas where there are potential fire and explosion hazards, proper PPE must be worn. For example, safety glasses, hard hat, hearing protection, gloves and steel-toed boots are the most common. When working around processes where there is potential for combustible dust fires and explosions, you should also add fire-resistant clothing. While this article focuses on combustible dust, this also applies to processes with flammable gasses, other flammable products and hybrid mixtures.

Administrative controls such as housekeeping, hazard communication and management of change are a primary level of prevention. For example, changing from softwoods to hardwoods, or adding a dryer to the process, necessitates hazard analysis, hazard communication and management of change.

Engineering controls come in a variety of shapes and sizes. Several specific areas relevant to combustible dust are area classification for electrical controls; separation, isolation and segregation of dangerous processes from each other; and layered protection systems including fire prevention, and fire and explosion protection.

• Area classification: Two documents are used for the classification of combustible dust and hazardous locations—NFPA 499 Recommended Practices for the Classification of Combustible Dusts and Hazardous Locations for Electrical Installations in Chemical Process Areas, and NFPA 70, article 500 of the National Electrical Code. Also, refer to NFPA 77 Recommended Practice on Static Electricity, NFPA 79 Electrical Standard for Industrial Machinery, and again, NFPA 499 Recommended Practice for the Classification of Combustible Dusts and of Hazardous Locations for Electrical Installations in Chemical Process Areas.

• Class/Division Hazardous Location: Hazardous locations are described as locations where electrical equipment might present an ignition hazard. Class II Hazards are locations where combustible or conductive dusts are present (or may be present) in quantities sufficient to produce explosive or ignitable mixtures.

Division refers to the probability of hazardous materials and mixtures. Division 1 has a high probability of producing an explosive or ignitable mixture due to it being present continuously, intermittently, or periodically from the equipment itself under normal operating conditions. Electrical equipment in these areas must meet the criteria for explosion proof rating.

Division 2 has a low probability of producing an explosive or ignitable mixture, and is present only during abnormal conditions for a short period of time, such as a container failure or system breakdown. See NFPA 652 Standard on the Fundamentals of Combustible Dust requirement for a DHA, which requires all such facilities to perform a DHA and risk assessment for each process that handles or creates combustible dust.

Engineering Principles
An engineering principle outlined in NFPA 664 is to isolate, segregate and separate the various hazardous parts of the process. For example, there is a design of hammer mill process that uses a plenum in between the mill and the dust collector, thus creating a bomb. It is preferable to have a choke in between, such as a screw conveyor or an airlock, and to remove the dust collector to a remote location, outside the building, thus isolating a potential explosion or deflagration.

Another principle of design found in NFPA 664 is layered protection systems for fire prevention, fire protection and explosion protection. NFPA 664 Chapter 8 applies to processes and systems such as mechanical conveyors, pneumatic conveyors, classifying and dust collection systems. Conveyors and ducts with a fire hazard are required to have fire prevention and/or fire protection.

Fire prevention is typically spark detection. Spark detection and extinguishing systems are a primary tool to prevent sparks from propagating into fires by detecting and suppressing sparks or embers in the incipient stage. Spark detection systems are typically applied to mechanical conveyors, pneumatic conveyor and dust collection systems.

Fire protection is typically deluge and sprinkler systems. We may also utilize other types of hazard monitoring equipment such as bearing temperature, heat detection, spark, ember, flame, smoke, CO detection and emissions monitoring, as well as other types of suppression, control and isolation devices. Interlocking machinery, conveyors, fire dumps and proper sequencing of shutdowns are also critical engineering controls.

Explosion protection includes explosion venting or suppression, as well as chemical or mechanical isolation, to control and prevent a deflagration from transferring down or upstream, thus preventing catastrophic secondary explosions.

Vessels, Venting
Vessels and dust collectors with a deflagration hazard are required to have explosion protection and isolation. Mechanical or chemical isolation of these vessels and dust collectors is also required. See NFPA 68 Standard on Explosion Protection by Deflagration Venting, and NFPA 69 Standard on Explosion Prevention Systems.

Explosion vents have to be engineered based on the explosive characteristics of the dust, thus dust testing is required. Venting needs to vent to a remote area away from buildings, machinery and people. Blast radius areas should be defined. Bins inside the building can be vented outside, provided the distance to the exterior wall is short enough. Alternately, indoor explosion vents, also called flameless venting, can be utilized. These flameless vents use a mechanical or chemical flame barrier to suppress the flame front, but still emit a pressure wave.

Where explosion venting cannot be used, chemical explosion protection and isolation must be. Explosion protection systems consist of an optical and pressure sensor, a control panel and chemical canisters strategically located on the vessel and connected ducting.

After implementing the above, you should consult your insurance company, as well as local codes and authorities having jurisdiction

With proper analysis, design, engineering and administrative controls, training and housekeeping, you can reduce the probability of risk and the severity of consequences, while maintaining safety and business continuity, a safe environment for employees and stakeholders, and safeguarding your reputation in the industry.
 

Author: Jeff Nichols
CEO, Industrial Fire Protection
jnichols@industrialfireprotection.com
(770) 266-7223