Klausbruckner & Associates News » Advances in Dust Explosion Risk Assessment

Advances in Dust Explosion Risk Assessment

August 15th, 2011

Recent dust explosion accidents and their aftermath highlighted a renewed need to focus on a better understanding of the methods used in preventing these incidents. A two-phase study was commissioned by the Fire Protection Research Foundation of NFPA to develop and verify a more accurate methodology to assess dust explosion hazards while resolving the controversies associated with the current standards. Part I of this study has now been completed (Updated on 8/29/2011).

Combustible dust incidents are an unfortunate but regular occurrence, recalling some of the more recent ones at AL Solutions in December 2010, New Cumberland, West Virginia, the combustible iron powder incident in January 2011 of the Hoeganaes Corporation plant in Gallatin, Tennessee, or one of the most widely publicized combustible dust explosion of the Foxconn Chengdu plant (China), which was caused by airborne aluminum dust in May of 2011.

Most of these incidents could have been prevented, if the proper risks were identified early and subsequently the necessary steps were taken to mitigate any potential hazards. The appropriate standards are widely available and are updated frequently to include the latest science and technology in dust explosion protection.

The development of combustible dust standards has come a long way, but also has shown to be challenging due to the diversity of dusts, particle size, and their properties, facilities, and processes involved.

The Need for a New Methodology

One of the foremost standards used in dust explosion assessment and control is NFPA 654. For years it has been used for determining whether an explosion hazard exists in a building compartment. Unfortunately application of the prescribed criteria in assessing the risk hazards have not always been easy, or consistent with other standards, nor without controversy. Perhaps most importantly using these as the foundation for enforcement seem to cause much concern due to its lack of sufficient technical justification [1].

In response, the Fire Protection Research foundation sponsored and initiated a two-phase project that will establish the technical basis for determining whether a compartment is considered a “dust explosion hazard,” with the overall objective to improve NFPA 654. The Phase I portion of this investigation has now been completed developing a ‘strawman’ method to assess dust hazards and verifying its predictions by comparison to historic explosion data [1]. The upcoming Phase II part of the study will perform tests to further validate the new methodology.

Basics of the New Approach

The newly developed methodology deviates from the previous approach of either only considering the thickness of dust layers or, on the other hand, the total dust amount in any given space as a means of predicting explosion hazards, by only taking into consideration the amount of dust that is suspended in air. This new approach is applicable to a variety of dust hazard scenarios that include common dust hazards encountered in agricultural and food processing, combustible metal processing, wood processing and wood-working facilities.

It is stipulated that the amount of dust that can be moved into air by a disturbance, the “entrainment fraction”, can be estimated by this new methodology and this metric is primarily dependent on only a few but key parameters, such as dust characteristics, layer thickness, geometry, type, magnitude and duration of the disturbance. If this new approach can be verified in Phase II of this study, it will prove that controlling the amount of dust that can accumulate in the air will prevent a dust explosion.

In More Detail

It is argued that most dust explosions

(1) Start with dust being raised into the air through a sudden event, “disturbance”, such as rupture of air lines, explosions nearby, or a by-product of the manufacturing process,
(2) While in the remaining cases dust is suspended in air, which can lead up to an explosion.

Dust and layer properties are argued to play a significant role in the dust entrainment rates i.e., it is of importance as to how much dust is suddenly suspended in air, which in turn determines the size of the “explosible” (dust) cloud volume. A property of the dust layer assumed by the strawman method is the presence of spherical shaped dust articles. However, this new method can also account for the effect of non-spherical shaped particles, through the application of a correction factor for the threshold velocity, U_t (more discussions further down).

Deflagration Vents

Once such explosible dust cloud is ignited, it can develop pressures up to 10 times the magnitude of the initial pressure. Sometimes deflagration (pressure release) vents are used to limit maximum pressures during an explosion. While they can reduce pressures to save buildings and structures from failing, they may not be able to reduce the pressures sufficiently to levels that would make them safe for occupants. But perhaps more concerning are secondary explosions caused by deflagration venting from the ‘explosion room’ into a neighboring room with combustible dust deposits. In all of these cases NFPA 654 and 664, Standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities, specify the maximum amount of dust allowable in these spaces (including the threshold layer thickness). However disagreements between these two standards make it difficult to apply the prescribed assessment criteria in hazard control. Further discussions on this topic can be found in Scott Stookey’s article, “Dust Deflagrations: Recognizing And Regulating The Hazard” [3].

The Proposed New Methodology

The new methodology can be reduced into two basic components:

a. Selection of the types, magnitudes and durations of the maximum disturbances,
b. Calculation of the mass of the dust entrained from the deposits.

It is the objective of the new method to focus on the second component i.e., the mass of the dust that can accumulate and will become airborne during an incident. No longer will it be required to use an empirical value for the mass entrainment factor as specified by the old method.

However, since these standards cover dusts exhibiting a wide spectrum of properties, the project results could be extrapolated to most other dusts. In fact, large-scale test data for coal dust rock dust mixtures as well as sand and soil were used in the preliminary validation of the strawman method described in this report.

The following equation is proposed to estimate the entrainment mass flux [1], and which is to be further validated in the next phase of this project:

m” = 0.002*ρ*U*(U^1/2 – U_t² / U^3/2) and U > U_t

where:

m” entrained mass flux in kg/m2-s

ρ gas density in kg/m3

U free stream velocity in m/s

U_t threshold velocity in m/s.

The threshold velocity, U_t, is the minimum air velocity at which dust removal from the layer begins, and it depends on factors such as particle size, particle shape and particle density. The research report provides algebraic correlations and charts to estimate this parameter.

A comparison of predictions of the strawman method to large scale coal dust and cornstarch explosion test data demonstrated good agreement. Further validation tests and comparisons to the predictions of this new methodology will be performed in the upcoming second phase of this study.

Examples

A detailed discussion and application of the new methodology is given by example of two common scenarios in the report:

EXAMPLE A: Catastrophic burst of an indoor equipment (a 74 ft³ dust collector sitting on the floor, in the middle of a plant bursts at 0.5 barg. The floor of the plant is covered with a thick layer of aluminum dust made up of 100 micron with spherical particles)
EXAMPLE B: Dust deflagration venting from a room into another room (100 m³ primary enclosure communicating with the rest of the building through a 6’ by 8’ opening. The floor of the building is covered with a thick layer of aluminum dust made up of 100 micron spherical particles).

Seminars and Symposiums

NFPA is sponsoring a Dust Explosion Hazards 2-Day Symposium on September 20 – 21, 2011 in Detroit, MI. Experts will present sessions covering dust explosion fundamentals, hazards assessments, hazard control, and more.

You might also want to check your calendar for a free “NFPA Dust Hazards Assessment Webinar” on August 23, 2011, at 12:30-2:00 pm (EST).