Prevention and mitigation of dust explosions
By Pulp & Paper Canada
The pulp and paper industry has experienced few reported explosions in the last 25 years when compared to other industrial plants that handle, process or otherwise generate combustible dusts. The Chemical Safety Board reported in its 2006...
By Pulp & Paper Canada
The pulp and paper industry has experienced few reported explosions in the last 25 years when compared to other industrial plants that handle, process or otherwise generate combustible dusts. The Chemical Safety Board reported in its 2006 Combustible Dust Hazard study that, in a 25-year period in the United States, from 1980 to 2005, there were a total of 281 explosion incidents, 24% of which were attributed to wood and paper dust1. These explosions resulted in 11 fatalities and 88 injuries and occurred in dust collectors, particle size reduction equipment, silos, storage bins and inside of plant buildings where housekeeping issues existed. Out of the 66 incidents two were associated with paper dust; pulp dust was never mentioned in the database details.
The following may be some reasons why pulp and paper plants experience a lower frequency of dust explosions when compared to some other industries. Generally speaking, manufacturing facilities that produce pulp and paper products generate particles that do not lend themselves to the production of fine uniformly distributed dust clouds, when placed into suspension. In many cases the particles produced from paper and pulp dryers and cutting machines, and dust that escapes from equipment where the pulp is processed, tend to be non-uniform in size and are strand-like in shape. These particles, when mixed with the smaller spherical shaped particles, result in the formation of agglomerations when settling out. The agglomerations, when disturbed by air movement including blow-down with compressed air, tend to result in suspensions that are isolated in nature, of small volume and not dense enough to create a cloud that will propagate flame, in the presence of a credible ignition source.
The relatively low bulk density of pulp and paper dusts (typically less than 160 kg/m3 or 10 lb/ft3) also offers a benefit in terms of explosion risk. A relatively thick accumulation of paper or pulp dust must be present to present a dust explosion risk in a room or building when compared to other combustible dusts such as coal, agricultural and food dust, metal dust and even wood2. For example, an accumulation of up to 1/3 in. for pulp and paper dusts compares to accumulations of 1/32 – 1/8 in. for most other combustible dusts.
Another way of looking at this is that more paper and pulp dust particles must be present in a cloud of dust in a given volume of air to reach the Minimum Explosible Concentration (MEC) when compared to more dense dusts having the same MEC value.
Finally, paper and pulp dust is extremely hygroscopic and wetting of the particles above 5% can offer a benefit in terms of increasing the sensitivity of the dust cloud to ignition and lowering the severity of the resulting explosion.
Risk of fire is significant
The above factors are considered to be important reasons why the frequency of reported explosions in the paper and pulp industries seems to be somewhat lower than other industries. The operator should not however, be lulled into a false sense of security that their plant is not at risk for explosion based on the favorable properties of pulp and paper dust just described. Prudent administrative and engineering controls must be put into place and followed if the plant is to be operated at an acceptable level of risk.
The risk of dust fire in these plants is also significant. The relatively low bulk density of the dust and agglomerations that are produced, in conjunction with dust generation, actually increase the risk of fire due to increased surface area and porosity that allows air to easily mix with the dust.
Some pulp and paper plants have a green wood section where trees are received, debarked and chipped in preparation for digestion. These areas may generate dry wood dust that easily forms dense combustible clouds, when suspended, and can lead to fire and explosions if proper controls are not in place. The incident history reveals that many fires and explosions have occurred where wood dust is handled. There is a significant risk of fire and explosion, where wood dust is involved, if the hazards are not managed effectively.
Conditions required for dust explosions to occur
Three elements are required for a fire: (1) a fuel; (2) an oxidant, typically the oxygen in air; and (3) a sufficiently energetic ignition source. These three elements are commonly referred to as the “fire triangle”. If any one of these three elements can be removed, fire cannot be initiated. The first two of these elements – the fuel and oxidant – when in an appropriate ratio, are referred to as the flammable atmosphere.
For an explosion involving a combustible dust to occur, two additional requirements are necessary: (4) suspension or mixing of the combustible dust in air, in small enough particle sizes, above the lower explosive limit, and (5) confinement. These two additional requirements combine with the fire triangle to form the “Explosion Pentagon”.
Generally speaking, dust particles must be of small diameter, usually below 0.45-0.5 mm (450 to 500 microns); however, in the real world, dust particles are not often spherical in nature and can be needle shaped or flake like. (See the 2012 edition of the NFPA 6643 standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities for a definition of deflagrable (having the ability or readiness to burst into flame) wood dust.)3 Laboratory testing is always advised to determine if a specific dust is combustible.
The term used to identify the minimum density of a dust cloud necessary to cause combustion is called Minimum Explosible Concentration (MEC) and is usually reported in units of grams per cubic meter (in air) or ounces per cubic foot. Typical wood dusts will exhibit MECs between 40 and 120 g/m3 or 0.04 – 0.120 oz/ft3. The oxidizer is the 21% oxygen that is present in the air. In wood, paper and pulp manufacturing facilities there is always enough oxygen to support combustion, unless the process is inerted.
Credible ignition sources that have been found to be the cause of the majority of explosions in dust handling/processing plants include welding and cutting, heating from dryers, sparks
generated by mechanical equipment failure, sparks generated by mechanical impacts, hot surfaces, open flames and burning materials, self-heating, electrostatic discharges, and electrical sparks.
The basis of safety for dust cloud explosion hazards
Any measures put into place to remove one or more elements of the fire triangle would be considered to be a “Basis of Safety.” Typically, control of fuel or removal of oxygen, if possible, is the preferred basis of safety. In some cases reliance on control of ignition sources may be the only practical measure for ensuring safety. It should be noted however, that there are situations where, although the operator may completely identify all sources of ignition that may be present in the plant, control of these sources may not always be possible or practical. Hence, under such conditions control of ignition sources is used as a secondary basis of safety, in conjunction with other measures such as control of fuel, oxidant, or explosion protection.
Both NFPA 654 and 664 standards require that the design of the fire and explosion safety provisions be based on a process hazard analysis (PHA) of the facility, processes and associated fire or explosion hazards. This analysis considers explosibility characteristics of the dust, conditions that can produce the dust cloud, identification of potential ignition sources and the likelihood of the event. Alternately there are prescriptive requirements in the standards but these requirements do not consider the risk (likelihood x severity) of the event. Expert help is generally required to assess the hazards and to provide recommendations on the management of the risk.
Management of dust explosion hazards in your facility
A systematic approach
to identifying dust cloud explosion hazards and taking measures to ensure safety in any facility involves:
• Determining the dust cloud’s ignition sensitivity and explosion severity characteristics through appropriate laboratory tests on representative dust samples;
• Identifying areas of the facility where combustible dust cloud atmospheres could exist under normal and/or abnormal conditions;
• Identifying potential ignition sources that could exist under normal and/or abnormal conditions;
• Preventing the formation of explosible dust clouds in the plant and reducing the extent and duration of any clouds that may be formed;
• Taking measures to eliminate/control ignition sources; and
• Taking measures to protect against the consequences of dust cloud explosions.
With regard to control of dust and dust generation, a three-pronged approach to management of the dust explosion risk is suggested. This approach includes:
• Reducing or eliminating dust generation;
• Limiting or restricting dust migration;
• Removing/cleaning dust accumulations.
In addition, engineering controls should be installed on equipment that is particularly at risk for dust explosion including dust collectors, mills, storage bins and silos.
Examination of the dust explosion history in pulp and paper manufacturing facilities in the U.S. indicates that the frequency of explosions in this sector is less than other industries where combustible dusts are processed, handled or otherwise generated. The reason for this statistic may be, in part, due to the low bulk density of pulp and paper dust and the agglomerative nature of the dust that is generated. These characteristics offer some benefit with regard to controls to manage the risk. The risk should not be downplayed however, and an understanding of the factors that determine if a dust explosion hazard exists is essential. A systematic approach to identifying dust cloud explosion hazards and taking measures to ensure safety is required if the combustible dust explosion risk is to be effectively managed.
1. The Chemical Safety Board, Investigation Report – Combustible Dust Hazard Study, Washington, D.C., November 2006.
2. The National Fire Protection Association, NFPA 654 “Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids”, (2013), Section 6.1.6 – Layer depth criterion to assess the dust explosion hazard based on bulk density. Quincy, MA. 02169-7471.
3. The National Fire Protection Association, NFPA 664 “Prevention of Fires and Explosions in Wood Processing and Wood Working facilities” (2012), Quincy, MA. 02169-7471.
Steven J. Luzik, PE, CFEI, is a senior process safety specialist at Chilworth Technology, Inc. with more than 30 yearsí experience in the area of fire and explosion hazards. He can be contacted at 609-799-4449 or firstname.lastname@example.org. For more information about Chilworth, visit www.chilworth.com.