Defuse Dust Dangers

Carefully consider and then counter risks of fire and explosion

By Dirk Willard, Contributing Editor

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When the West Fertilizer Company in West, Texas, blew up on April 17, 2013, killing 14 people, it must have taken Donald Adair, the owner of the plant, by surprise. In his 2011 emergency plan, Adair described the worst-case scenario for his plant as a 10-minute release of gas! Perhaps we chemical engineers don't appreciate the risks posed by dust as well as we do those of flammable fluids.

Unfortunately, it's not as easy to assess the danger from heat.


One way to bolster your understanding of dust's risks is to check out Chemical Processing's free on-demand webinar "Dust Control: How to Identify & Manage Explosion Hazards." (Scroll down to the "Available To View Now!" section.)

You must estimate the severity of the risk and then its probability. NFPA 499, "Recommended Practice for the Classification of Combustible Dusts and of Hazardous Locations for Electrical Installations in Chemical Process Areas," lists many compounds that produce combustible dusts. If yours isn't on the list, test according to ASTM E1226, "Standard Test Method for Explosibility of Dust Clouds."  

In simplified terms, ASTM E1226 involves disturbing a small volume of dust with a pulse of air, followed, after a prescribed time delay, by ignition with a small electrical charge. The dust must contain < 5% moisture by weight and have particles smaller than 420 microns in diameter (i.e., ones that pass through a U.S. No. 40 standard sieve). The test takes place in a bomb of at least 20 liters at ranges of dust concentrations, fuel/air ratios, and electric charges. The goal of this test is to estimate the maximum pressure, the rate of pressure rise with time, and the dust deflagration index, Kst, a measure of relative explosive severity; these parameters also are useful in designing deflagration vents. OSHA defines a dust as a hazard if its Kst exceeds zero; this definition won't protect you if your process produces fines, especially those smaller than 15 microns, which easily are converted to an aerosol. A Kst between 0 and 200 (when measured in bar-meters/sec) indicates a weak explosive risk typical of sugar.

ASTM E1226 can pose several problems: measuring the dust density accurately; accounting for the pressure spike from the igniters; maintaining a dry sample; mixing issues affecting dust and air combustion; and, perhaps, comparisons between bombs of different volumes. So, get as much data as you can on dust properties, do more bomb runs, evaluate the equipment and procedure for systemic faults, and compare your test data against a known standard.

With the severity estimated, it's time to consider the probability that a spark or heat could initiate a fire or explosion. Probability tests involve measurement of the minimum ignition energy (MIE), the minimum explosible concentration (MEC), the auto-ignition temperature (AIT), and the limiting O2 content (LOC). Except for the AIT, tests are for dust clouds. ASTM E2019 covers measuring the MIE of a dust cloud; ASTM E1515 the MEC of a cloud; ASTM E1491 the AIT of a cloud; and ASTM E 2021 the AIT of layered dust. No LOC test is approved in the US; ASTM has WK41004 in the works but Europe has DIN EN 14034-4:2004.

The spark risk is measured in milliJoules (mJ). OSHA states that "materials that ignite above 0.50 joules (500 mJ) are not considered sensitive to ignition by electrostatic discharge." Between 500 and 100 mJ, equipment and people must be grounded to reduce the risk of ignition. An MIE less than 25 mJ is extremely hazardous, posing a risk during bulk operations, e.g., pneumatic conveying, silo storage, etc.

German data from 1965–1985 show that electrical discharge represents only 10% of the ignition sources in 426 accidents. Unfortunately, it's not as easy to assess the danger from heat. Fire caused by grinding or another physical action, drying or even self-heating represents the greatest potential, and is poorly understood. I couldn't find any correlation directly connecting MIE and fire risk; it's more of an article of faith that a low MIE is a fire risk.

So, let's move on to mitigation. Here're some ideas: 1) keep surface temperatures 170°F below  the AIT; 2) avoid rubbing of rotating parts; 3) reduce rotating speed; 4) maintain strict grounding policy (see: "Move Against Static Electricity"); 5) measure and decrease available oxygen; and 6) cut the quantity of dust by good housekeeping. Also, read the article "Dust Explosion Standard Gets Significant Revisions," which highlights important revisions to that standard for prevention of fire and dust explosions.

Hopefully, by focusing on temperature as well as electricity we can avoid more surprises for plant managers.


dirk.jpgDIRK WILLARD is a Chemical Processing contributing editor. He recently won recognition for his Field Notes column from the ASBPE. Chemical Processing is proud to have him on board. You can e-mail Dirk at dwillard@putman.net
   

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  • Editor's note: This response came in via email and the author, Dennis Hendershot, gave us permission to post it here on his behalf:

    In the September 2013 issue of Chemical Processing, in the "Defuse Dust Dangers" article Mr. Willard strongly implies that the April 17, 2013 ammonium nitrate (AN) explosion in West, Texas was a dust explosion. While the investigation is still in progress, I would be surprised if this turns out to be the case. It is much more likely that this explosion was an explosive decomposition of the ammonium nitrate stored in bulk at the facility, when exposed to heat from a fire. In August 2013, the EPA issued a bulletin "Chemical Advisory: Safe Storage, Handling, and Management of Ammonium Nitrate" (http://www.epa.gov/emergencies/content/rmp/index.htm). On page 2 of this bulletin it states that "Although pure AN is stable at ambient temperature and pressure under many conditions....AN may explode when exposed to strong shock or when subjected to high temperatures in confinement." There have been several major incidents involving the decomposition of large quantities of ammonium nitrate. For example: A September 2001 explosion in Toulouse, France involving 200-300 tons of ammonium nitrate caused 30 fatalities, 2,500 injuries, and damaged 10,000 buildings.

    An April 1947 explosion of the SS Grandcamp docked in Texas City, Texas, which contained approximately 2,300 tons of ammonium nitrate. There were at least 581 fatalities and thousands of injuries. This incident is generally considered to be the worst industrial accident in United States history.

    A September 1921 explosion of an estimated 450 tons of AN fertilizer mixture at a BASF plant in Oppau, Germany resulted in 561 fatalities and more than 2000 injuries.

    Mr. Willard's main message in the column message is to highlight the dangers of dust explosions, and the relevant standards for managing these hazards. This is a very important message, but it would be more appropriate to cite other incidents to illustrate the hazard. There are a number of examples on the US Chemical Safety Board web site (www.csb.gov) - for example:

    The February 2008 explosion at the Imperial Sugar refinery in Port Wentworth GA The January 2003, an explosion at West Pharmaceutical Services in Kinston, NC The February 2003 explosion and fire at CTA Acoustics in Corbin, KY

    In 2006 the CSB released a report titled "Combustible Dust Hazard Investigation" which describes the results of their nationwide study of dust explosion hazards (http://www.csb.gov/combustible-dust-hazard-investigation/).

    Here is the Wikipedia page on ammonium nitrate explosions (http://en.wikipedia.org/wiki/Ammonium_nitrate_disasters). They seem to be happening about once every 4 years of so somewhere in the world.

    Dennis Hendershot

    Reply

  • Editor's Note: Dirk Willard offered this response. He gave us permission to add the comment:

    While I understand Mr. Hendershot's attention to details specific to Ammonium Nitrate (AN) I think he misses the point: Donald Adair, the plant manager underestimated the risk of handling and storing AN. True, we don't know the final cause. My point in reference to West Fertilizer is that we, as chemical engineers underestimate the risk of dust and powder. I have three years of experience with Ammonium Perchlorate, Nitroglycerine-based plasticizers and other components in solid rocket propellants. I authored three paper and contributed to two others while doing research at the US Air Force Rocket Propulsion Laboratory at Edwards, AFB in California. The military uses AN in their engine starter cartridges, and I've worked with AN before, including investigation of a fire involving AN drying in our mixer ingredient ovens. From my hands-on experience with these powders I can't really see much difference between a dust and a powder. I'm afraid I must leave these fine details to safety experts.

    Dirk Willard

    Reply

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