BEYOND THE
FENCE LINE Mike Schmidt, Bluefield Process Safety, USA, argues that the industry is responsible for process safety even after chemical products leave the plant, and shares some ideas on how to meet that responsibility.
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here is not much talk about process safety once hazardous chemicals are loaded and shipped. But process safety – the concern with fires, explosions and toxic releases – does not end at the fence line. Wikipedia lists 35 incidents in its article on ammonium nitrate (AN) disasters. Of those, over a third involved the transport of AN; four were explosions of cargo ships loaded with AN; three were explosions of freight trains pulling cars loaded with AN; and six were explosions of trucks hauling loads of AN. They follow a common pattern: an incident – sometimes a wreck, sometimes an onboard failure – ignites a fire. The fire burns and attempts are made to bring the fire under control. Then, without warning, the AN explodes, with dramatic consequences. By way of example, an AN truck explosion on a highway in Arkansas, US, in 2019 was heard as far away as Little Rock, almost 100 miles (160 km) to the north (Figure 1).
Stability of AN AN is not combustible. It is an oxidiser, however, and will accelerate the combustion of flammable and combustible materials. It is stable at ambient temperatures and pressures, and is reported as having very low shock sensitivity as a pure compound. Prior to the 1921 explosion at Oppau in Germany, BASF reportedly used small charges of dynamite to disaggregate solid masses of AN/ammonium sulfate mixtures as many as 20 000 times without incident. However, AN is sensitised when contaminated with organic materials. In fact, ANFO (the bulk industrial blasting agent) typically consists of a 94% AN/6% fuel oil mixture. Spilled diesel fuel at truck accidents has been suspected as a sensitiser in some cases of AN
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truck explosions. Occasionally, reporters or the public will seize on terms such as ‘fertilizer grade’ versus ‘explosive grade’ to explain different stabilities or energies of explosion between various inventories of AN. The difference is in the porosity of the prill. Agricultural grade is not very porous, so it ships a little denser. Explosive grade is porous, so it can absorb fuel oil more readily to make ANFO. Without fuel oil, though, explosive grade AN is just AN, and no more or less likely to explode. The Institute of Makers of Explosives (IME, based in Washington, D.C., US) recommends that trailers be dedicated to hauling AN, and if not, that they be washed out and dried before being used to haul AN. Contaminants from previous loads that the IME particularly warns about include: Animal fats. Baled rags, burlap or cotton bags. Charcoal. Foam rubber. Hay. Metal powders. Sawdust. Straw. Vegetable oil. Wood, wood chips or wood shavings. Other impurities are known as sensitisers for AN, including chlorates, mineral acids and metal sulfides. That does not mean that uncontaminated AN will not explode. It is the sudden and catastrophic decomposition of AN that results in explosions. The melting point of AN is 169.6˚C (337.3˚F), and uncontaminated AN starts decomposing at 210˚C (410˚F).
