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Combustible Dust

Published: 10th Sep 2011 in OSA Magazine

Manufacturing industries don’t appear high risk like the oil, gas, petrochemical, and mining sectors. Appearances are deceiving. After all, how dangerous is shopping at the grocery store while navigating the aisles, or comparing department store prices on electronic products, gadgets, and wizardry?

Pull the curtain back and we’ll discover how earlier in the product lifecycle, bulk raw products of wood, food, metal, plastic or paper are magically transformed through chemical and mechanical processes into appealing finished products.

Combustible dust is generated from combustible particulate solids (CPS) during this process, providing a potentially explosive atmosphere in the workplace.

Primary dust explosions occur when a minimum explosive concentration of suspended dust combines with an ignition source inside process equipment. This explosion pentagon has five elements: heat, fuel, oxygen, suspension and confinement.

Vapour cloud explosions and flash fires in many high risk industries have the same elements, with a pressure wave and subsonic self-sustaining flame/reaction front propagating outwards.

Catastrophic secondary dust explosions occur when the pressure wave from a primary explosion lifts dust accumulated on horizontal surfaces into suspension. Next, the slower moving subsonic flame front of the primary deflagration ignites suspended dust with devastating results.

Global combustible dust catastrophes

High risk added another member to the family as combustible dust hazards received global attention on May 20, 2010.Three production workers were fatally injured during metal polishing operations at an electronics factory in Chengdu, China, due to an aluminum dust explosion.

A week later and 14 time zones to the east, a powder metallurgy plant in Gallatin,TN USA, experienced a catastrophic hydrogen explosion, resulting in two fatalities from severe burn injuries. According to preliminary investigations, metal dust contributed to the severity of the incident.

On a disturbing note, within four months of the recent Tennessee incident, two workers were fatally burned and one injured in two earlier metal dust flash fires at the facility.

As the global combustible dust fuse continues to burn, hopping from continent to continent, no manufacturing sub-sector is spared.Who is next on what appears to be borrowed time?

The majority of incidents do not occur without warning. Several weeks prior to the China dust explosion, workers complained about unhealthy aluminum dust concentrations due to poor industrial ventilation at another electronics facility. Dust incidents are neither isolated nor unusual; they occur regularly on an international scale.

Definition of combustible dust

So what is a combustible dust? In the past, fire protection professionals agreed it was a particle 420 microns or smaller and passes through a US #40 Standard Sieve. This mindset changed following catastrophic events involving CPS fibres
or agglomerates too large to pass through a sieve.

If dust can be suspended in air or other oxidising mediums – regardless of size or shape – and ignite, then it is defined as combustible dust. When evaluating deflagration hazards, particle size of dust should not be the overall determining factor. Before a flash fire or dust explosion can ensue, suspensions require a minimum explosive concentration (lower explosive limit) and ignition source.

All dusts should be considered combustible until laboratory testing indicates otherwise. Many stakeholders do not understand how dust is combustible, especially when hazards are not immediately perceived, like gasoline vapours we smell when fueling our car.
Explosion severity (pressure wave and flame front) is just as intense; the only difference is higher ignition sensitivity (more difficult to ignite).

As surface area increases during the manufacturing process, ignition sensitivity is lowered and explosion severity increases. This concept of increased surface area is the key to developing a healthy respect for the hazards associated with combustible dust.

Surface area increases when CPS are transformed from raw bulk product during manufacturing processes generating dust. When starting a fire with logs gathered from the woods, a log by itself will not burn. It is necessary to chop up the log into kindling and tinder to increase surface area. Now, with smaller pieces, the campfire starts easily.

Identifying combustible dust hazards

Stakeholders must take action now in addressing dust hazards posing fire and explosion risks in their workplace, disrupting operations, or even worse, resulting in numerous fatalities and injuries. This requires identifying process conditions (equipment), situations (ignition sources) and materials (combustible dust) contributing to an explosive atmosphere.

While identifying process materials posing a dust hazard, be aware flammable vapours in the vicinity of combustible dust lower ignition sensitivity and increase explosion severity of the dust. This powerful combination is defined as a ‘hybrid mixture’ and kicks like a mule, also a hybrid.

A comprehensive process hazard analysis provides information necessary for developing administrative, engineering and personal protective equipment (PPE) control measures. Hazard identification provides valuable insight into initiating events such as process upsets, external events and inadequate administrative controls – housekeeping, training, maintenance and inspection.

Explosion prevention and protection standards have been developed by The International Electrotechnical Commission (IECEx). Several Asian countries adopted IECEx standards. Additional information is available at www.iecex.com

Process conditions – equipment

Process conditions include particle abrasion and material break down during sieving, pouring, grinding, micronising, and pneumatic conveying of CPS. These actions continually generate dusts; the possibility of combining with an ignition source is omnipresent.

Equipment involved in ignition (EII) in process areas includes:
• Blenders/mixers dryers
• Dust collectors
• Pneumatic conveyors
• Silos, hoppers, bins, tanks
• Hoses, loading spouts
• Bucket elevators

Size reduction equipment including mills, grinders and pulverisers generate dust clouds that are ignitable in the presence of an ignition source. Hot surfaces and friction arise from grinding, presenting fire and explosion hazards. Other types of equipment have similar ignition characteristics. IEC 60079-10-2 Ed. 1.0 Explosive atmospheres – Part 10-2: Classification of areas – Combustible dust atmospheres, provides EH&S professionals with information on levels of explosion protection. Work areas are classified according to duration and frequency of dust layer and dust cloud accumulation.

Resources identifying frequency and duration of dust accumulations:
• Process flow diagrams
• Facilities and equipment layout
• Equipment specifications/designs
• Process and instrumentation diagrams (PID)
• Material/energy flow rates
• Utility locations
• Condition fire/explosion suppression protection equipment

Each facility has unique process equipment, yet a commonality exists across manufacturing sectors. Dust collectors, pneumatic ductwork, dryers, mixers, silos, hoppers and bins are utilised globally. Identifying equipment generating dust assists in developing explosion protection barriers, minimising the possibility of combining with ignition sources.

Process situations – ignition sources

A variety of ignition sources are inherently present in the manufacturing process. Identifying potential sources and implementing control measures is essential in reducing the probability of dust fires and explosions.

Primary ignition sources in process areas include:
• Electrical equipment (arcing and sparking)
• Static electricity (ESD)
• Hot surfaces
• Flames and hot gases (hot particles)
• Mechanically generated sparks
• Powered industrial trucks (forklifts)
• Spontaneous combustion, exothermic reactions

EN 1127 Explosive atmospheres – Explosion prevention and protection – Part 1: Basic concepts and methodology, provides an excellent overview of risk assessment and reduction of the ignition sources noted above. This basic concept and methodology standard is one of several explosion protection standards accepted by the United Nations Economic Commission for Europe (UNECE).

UNECE recently published a Common Regulatory Framework for Equipment Used in Environments with an Explosive Atmosphere, promoting a globally harmonised adoption of explosive atmosphere standards. Most importantly, UNECE endorses the IECEx (the IEC System for Certification to Standards relating to Equipment for use in Explosive Atmospheres) as the recommended global best practice model for verifying conformity to international standards.

Process materials

Identifying fire and explosion hazards associated with dust begins with reviewing Safety Data Sheets (SDS).A problem arises in utilising SDSs lacking ignition sensitivity data. Minimum ignition temperature (flash point), minimum ignition energy (static electricity hazard), or minimum explosive concentration (lower explosion limit) data are often missing.

If process equipment has hot surfaces where dust accumulates, one needs to know the temperature at which the dust ignites, either in suspension or in layers. Control measures cannot be safely implemented until hazards are identified and evaluated.

Evaluating combustible dust hazards

Risk assessments should not rely on laboratory testing alone to evaluate fire and explosion hazards. Other factors such as process situations, conditions and process changes during a facility’s lifecycle must be considered.

A comprehensive process hazard analysis is necessary, including one of the following methods: Preliminary Hazard Analysis, Fault Tree Analysis, Event Tree Analysis, or Failure Modes and Effects Analysis.

Combustibility Testing (Burning Behavior/BZ Class) in accordance with VDI 2263 determines whether a dust fire will spread through process equipment. High BZ class dusts require higher levels of preventative actions.

Go/no-go testing evaluates ability of a dust to ignite while suspended in air. Information is available on the GESTIS-DUST-EX website
http://www.dguv.de/ifa/en/gestis/

Further characterisation of dust samples includes ignition sensitivity, flammability limits and explosion severity.

Ignition sensitivity

• Minimum Ignition Energy (MIE) – electrostatic behavior
• Minimum Ignition Temperature (MIT) – cloud and layer
• Exothermic Decomposition

Experimental data for probability of occurrence (ignition sensitivity) and severity of consequence (explosion severity) helps identify specific accidents likely to occur, their causes and adverse effects.

Flammability limits

• Minimum Explosive Concentration (MEC) – lower flammability limit
• Limiting Oxygen Concentration (LOC)

MEC is difficult to conceptualise. Values can only be determined in the laboratory; monitoring equipment doesn’t exist as it does for flammable vapours. LOC is monitored when dusts with low ignition sensitivity must be inerted with Nitrogen to prevent effective ignition.

Explosion severity

• Kst (normalised rate of pressure rise as a function of time)
• Pmax (maximum pressure)

Engineering designs for venting and suppression protection utilise K st and P max values. An extreme example in understanding the violence of aluminum dust ignition is Solid Rocket Boosters containing aluminum dust, which has a higher K st than wood dust. Therefore, mitigative controls for equipment handling wood dust have different design parameters than aluminum dust.

Engineering controls – built environment

Initially, hierarchal controls either eliminate combustible dust fire and explosion hazards or substitute with less hazardous materials (Inherent Design). In most instances this is not possible.

The next approach in prevention and mitigation of fires and explosions is engineering controls, which reduce the severity of consequence when a primary explosion occurs in process equipment. Since these controls are fixed, they are considered the built environment:
• Deflagration venting
• Deflagration suppression
• Pressure containment
• Oxidant concentration reduction (inerting)

Isolating interconnected process equipment must be considered in the event of a deflagration. For instance, dust collectors experience a high occurrence of fires and primary explosions.

Problems arise when a pressure wave and subsonic fireball travels upstream through the ductwork into the plant. Engineering controls isolating deflagrations from interconnected process equipment include:
• Rotary valves
• Chokes
• Mechanical fast-acting valves
• Chemical isolation
• Flame front diverters

Spark detection and flame suppression

Most international media attention focuses on secondary catastrophic explosions or flash fires, which are rare events. Non-consequential combustible dust related fires, many of which occur in ductwork or dust collectors, are more common but largely ignored in news accounts.

Process equipment may produce sparks. Ventilation hoods pick up the sparks and convey them through the ductwork to the dust collector. Spark detection and flame suppression systems extinguish sparks or embers before they reach the dust collector, thus preventing a fire or explosion.

Primary explosions cannot be totally eliminated in the manufacturing process. Engineering controls substantially reduce the severity of inevitable deflagrations. Industrial ventilation systems convey dust throughout the process. Additional ignition sources exist due to process upsets, human error and external conditions.

On the other side of the equation, alongside severity of consequence, is probability of occurrence or the likelihood of an event disrupting the production process.

Administrative controls – operational environment

Safety management programmes comprising administrative controls are the next layer of protection.

Administrative controls include training personnel, good housekeeping, maintenance and inspection programmes, management of change, hot work permits and hazard communication.

Training the workforce on fire and explosion hazards of combustible dust provides a solid foundation, minimising probability of future incidents. Contractors grinding, chipping, cutting and welding adjacent to process equipment must receive training on the fire and explosion hazards of combustible dust.

The majority of combustible dust incidents are non-consequential fires with no fatalities, injuries, or property damage. Yet in many instances these fires are precursors to catastrophic secondary explosions. Good housekeeping is critical. Minimising dust accumulations on horizontal surfaces through scheduled housekeeping prevents catastrophic secondary explosions.

One essential maintenance procedure includes lubrication of bearings to prevent excessive heat build-up. Inspecting hot surfaces such as piping, ductwork and process equipment daily is crucial in preventing ignition of dust. Hot surfaces must be 80% below the minimum ignition temperature of combustible dust.

Globally, there is a high occurrence of dust related fires and explosions during hot work. Hot work permits reduce the likelihood of igniting dust on horizontal surfaces and inside process equipment.

Typically, SDSs lack ignition sensitivity data and should be supplemented with hazard communication.

PPE – flame resistant clothing (FRC)

Engineering and administrative controls prevent and mitigate dust fires and explosions, yet cannot entirely prevent incidents from occurring; they only minimise probability and reduce severity. The last layer of protection, donning flame resistant clothing (FRC), minimises severity from intense thermal effects of ComDust flash fires and explosions.

During high pressure and heat deflagrations, ensuing fireballs engulf personnel, causing life threatening third degree burns. In many cases, workers succumb to injuries shortly after incidents.

Without FRC, street clothing will catch on fire like a flaming torch. Ironically, injuries are more severe than if workers did not have clothing on. Flame resistant clothing will not prevent burns; however, FRC minimises severity of injuries and increases chances of survival. Post-injury quality of life is also improved.

FRC is required in high risk industries where potentially explosive atmospheres are present. Stakeholders must acknowledge life threatening burns from ComDust incidents have the same level of severity as other
industrial sectors.

Conclusion

Hopefully your awareness and understanding of the fire and explosion hazards of combustible particulate solids and combustible dust has been raised. The subject is, at times, very complex and spread across many disciplines involving diverse manufacturing processes, sharing similar process conditions and situations.

Administrative and engineering controls reduce probability of occurrence and severity of consequence of these fires and explosions. Due to the inherent nature of manufacturing processes generating a fuel source in conjunction with potential ignition sources, incidents will continue to happen.

The donning of flame resistant clothing – personal protective equipment – by facility personnel provides an added layer of protection for when the inevitable happens.

Disclaimer
Although every effort has made in providing this information the authors accept no responsibility or liability for any consequences arising from the use of such information.

Combustible dust fire and explosion hazards are a multi-disciplined topic where the diverse hazards have their own unique circumstances specific to the worksite. 

Authors’ details:

John C Astad, Combustible Dust Policy Institute

John Astad, Director, Combustible Dust Policy Institute, Santa Fe,Texas, is dedicated to educating stakeholders on the prevention and mitigation of combustible dust fires and explosions in the workplace. John also conducts site evaluations and hazard awareness training. He is currently researching the prevalence of non-consequential combustible dust fires, which are precursors to secondary catastrophic dust explosions.

John attended University of Houston-Clear Lake with a BS Business and Public Administration, majoring in Environmental Management.

You can reach him at
E: john@combustibledust.com 
Twitter: @comdust
T: 409-539-7396
W: www.combustibledust.com

Teresa A Long, Combustible Dust Policy Institute

Teresa A Long, Research Associate at the Combustible Dust Policy Institute, served as the global focal point for reactor/train qualifications and FDA compliance audit programmes within the Polyolefins R&D characterisation group at Dow Chemical. Prior to that,Teresa performed analytical testing and method development while working at American Electric Power’s Dolan
Engineering Laboratory.

Teresa earned her AAS in Chemical Technology from Texas State Technical College along with a certificate in Environmental Health & Safety Technology. She is pursuing an Environmental Science degree at University of Houston-Clear Lake, specialising in Industrial Hygiene.

E: teresa.long60@gmail.com 

www.osedirectory.com/health-and-safety.php

Published: 10th Sep 2011 in OSA Magazine

Author


John C Astad and Teresa A Long


John C Astad, Combustible Dust Policy Institute

John Astad, Director, Combustible Dust Policy Institute, Santa Fe,Texas, is dedicated to educating stakeholders on the prevention and mitigation of combustible dust fires and explosions in the workplace. John also conducts site evaluations and hazard awareness training. He is currently researching the prevalence of non-consequential combustible dust fires, which are precursors to secondary catastrophic dust explosions.

John attended University of Houston-Clear Lake with a BS Business and Public Administration, majoring in Environmental Management.

You can reach him at
E: john@combustibledust.com or
Twitter: @comdust
T: 409-539-7396
W: www.combustibledust.com

Teresa A Long, Combustible Dust Policy Institute

Teresa A Long, Research Associate at the Combustible Dust Policy Institute, served as the global focal point for reactor/train qualifications and FDA compliance audit programmes within the Polyolefins R&D characterisation group at Dow Chemical. Prior to that,Teresa performed analytical testing and method development while working at American Electric Power’s Dolan
Engineering Laboratory.

Teresa earned her AAS in Chemical Technology from Texas State Technical College along with a certificate in Environmental Health & Safety Technology. She is pursuing an Environmental Science degree at University of Houston-Clear Lake, specialising in Industrial Hygiene.

E: teresa.long60@gmail.com 


John C Astad and Teresa A Long

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