|Dust Explosions - A Solvable Problem|
Issue 47: Dust Explosions - A Solvable Problem
By John M. Cholin, P.E., FSFPE
Research conducted by the
Occupational Safety and Health Administration (OSHA) uncovered an
average of five fatalities and over 28 additional injuries each year
from combustible dust deflagrations and explosions.1 And when
OSHA responded to the spate of large-loss incidents in the middle of
the last decade with its National Emphasis Program (NEP), it found that
there was wide-spread non-compliance with safety standards in the
facilities it inspected. Failure to comply with safety criteria, such as
those found in many of the NFPA "dust standards" for decades, can lead
to large-loss dust explosions.
A combustible solid of sufficiently small average particle size will
propagate a deflagration when suspended in air in a sufficient
concentration and ignited.2 When a deflagration is confined
within an enclosure, the release of heat and combustion products causes
an increase in pressure that can cause the rupture of the confining
enclosure. The deflagration can be more hazardous to people than the
resulting building collapse or missile impact.
The technology for protecting the building occupants from enclosed vessels such as bins, bunkers, silos, mixers, blenders, and dust collectors, all of which can pose an explosion hazard, is well known. The relevant NFPA standards 3,4,5,6,7 stipulate how these enclosed vessels should be protected. While different standards prescribe slightly different solutions in some cases, the concepts are universally applicable across all of the combustible classifications (agricultural, forest products, metals, sulfur and general dust including plastics and chemicals).
Where the sundry NFPA standards differ is in the prescribed method
for hazard identification, if any, and the criteria for establishing the
obligation to manage the hazard, especially as it pertains to
occupiable building compartments. In this area there are two changes
occurring in the NFPA dust standards.
The first is a transition from using the term "deflagration" to mean
the expanding fire ball that propagates through the building interior to
the term "flash fire." This later term has been embraced by the process
chemical community and is in both NFPA 654 and NFPA 655. It has not
been embraced in NFPA 664, but is under consideration by the technical
committees responsible for NFPA 61 and NFPA 484.
The second is the development of clear criteria for determining if a
deflagration or flash fire hazard exists in a building compartment. This
determination is critical as hazard management obligations, including
the use of fire resistant clothing for employees, are triggered by
whether the building compartment is a deflagration or flash fire hazard.
For the past two revision cycles, NFPA 654, the general dust document that covers chemicals, plastics and dusts not covered by other standards, has used a 1/32nd inch (0.8 mm) layer depth criterion as the threshold for deeming a building compartment an "explosion hazard". However, the language used for this purpose was not as clear as it could have been. NFPA 664, the standard dealing with forest products, used a 1/8th inch (3 mm) layer depth criterion. Again, the language used was not as clear as it could have been. However, when the layer depth criteria in NFPA 654 and 664 are corrected for net heat of combustion and bulk density, they yield similar quantities of energy per unit of accumulation area. That provided a level of internal consistency between documents. Furthermore, there is no loss history for facilities that comply with these criteria.
The hazard determination criteria for the other dust standards are
quite different. NFPA 484, the combustible metals standard, does not
allow any dust accumulation. Neither does the current edition of NFPA
655. However, the 2012 edition of NFPA 655 is adopting language similar
to that in NFPA 654 and 664. Because of the bulk density of powdered
sulfur, the layer depth criterion ends up at 1/16th inch (1.6
mm). An alternate method for hazard assessment is also included in the
new NFPA 655 that based upon accumulated mass. How NFPA 61 and NFPA 484
deal with the hazard determination issue is yet to be seen as both of
these documents are in revision.
The bottom line is that none of the tragic human injury and loss of life from dust explosions is necessary. None of the loss of facility and production capacity is necessary. The basic principles of how to prevent these incidents has been known for decades. Those basic principles are pan-industrial and are the basis for the prescriptive criteria in the NFPA occupancy design standards that pertain to dusts. While the improvements in these NFPA standards is welcome and is certainly the product of a lot of hard work by numerous dedicated committee members, with the notable exception of NFPA 61, there is no loss history in facilities that fully complied with current NFPA dust standards. NFPA 61 is unique in that it does not have an explicit fugitive dust layer accumulation criterion that is to be used for determining where there is a dust deflagration or flash fire hazard within a building. When considering an agricultural products facility covered within the scope of NFPA 61, a prudent design engineer will adopt the dust layer criteria of either NFPA 654 or NFPA 664, corrected for net heat of combustion and bulk density, for identifying those portions of the facility where a deflagration or flash fire hazard exists.
Despite having the ability to prevent dust explosions for decades, they still occur. A major contributing factor is operator awareness. Operators may be unaware of the hazard posed by combustible dusts, especially dust accumulations in building interiors. Fire protection engineers are obligated to apprise clients of this hazard. Any dry, solid material that is metallic, is edible, is eaten, was once alive, is derived from fossil life forms or is a compound that is not fully oxidized (in the chemical sense) is deflagrable if the particle size is sufficiently small.
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