NFPA Journal

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As a result of the task group’s work, the Hazardous Materials Committee has made a number of important changes to the ammonium nitrate requirements in the 2016 edition of NFPA 400, contained in Chapter 11, Ammonium Nitrate Solids and Liquids. These requirements further protect facilities, workers, emergency responders, and the public from the hazards presented by AN in the event of a fire. The goal was to establish requirements that would limit AN’s explosive potential by preventing it from becoming molten and confined, contaminated with combustible materials, or exposed to fire conditions. The changes to the document affect both new construction and existing facilities, and seek to balance concerns over the potential for AN explosions with the recognition that ammonium nitrate is a stable material if stored and handled properly. The Committee believes the revisions made to the document protect workers, the public, and emergency responders, while allowing existing facilities to maintain operations in a manner that addresses both risk and cost.

Ammonium nitrate + NFPA 400

Ammonium nitrate is a chemical compound produced in both solid and liquid forms that is commonly used in fertilizers. Pure AN is stable, and when stored properly it poses a minimal safety hazard. When exposed to fire in certain conditions, however, it can present a significant explosive threat. The requirements for ammonium nitrate were originally part of NFPA 490, Storage of Ammonium Nitrate, officially adopted in 1965. NFPA 490 was withdrawn when the requirements of that code were incorporated into the first edition of NFPA 400, published in 2010. AN was unique in its incorporation into NFPA 400; whereas other materials in the code are covered based on their properties, such as oxidizers or flammable solids, AN was the only material to have a chapter of NFPA 400 devoted solely to it.

The first two editions of NFPA 400, published in 2010 and 2013, provided guidance on how to classify AN based on information found in safety data sheets, and noted that ammonium nitrate may be an oxidizer (a material that readily acts to promote or initiate combustion of combustible materials and that can sometimes undergo a vigorous self-sustained decomposition due to contamination or heat exposure), or an unstable reactive (a material that can undergo a violent chemical change under conditions of shock, pressure, or temperature). AN could also possess additional physical or health hazards depending on the mixture.

For the 2016 edition of the code, a key question was how to classify ammonium nitrate, since it behaves differently in the presence of combustible materials than in its pure state. The committee held extensive discussions over the classification of AN—whether it is an oxidizer, an unstable reactive, or both—and ultimately decided that the actual classification was less important than the requirement that anyone handling AN comply with the requirements in Chapter 11. The 2016 edition of NFPA 400 has been changed to point all AN storage, use, and handling to Chapter 11, regardless of classification, simplifying the use of the document.

As in other chapters in NFPA 400, the applicability of the requirements in Chapter 11 are dependent on the maximum allowable quantity, or MAQ, which is the threshold above which the requirements in Chapter 11 would apply. The MAQ in Chapter 11 was the subject of much debate, where the amounts proposed ranged from five pounds, which correlated with the MAQ for an unstable reactive, to 50 tons, which correlated with the amount of AN expected in one or two typical delivery weights for full trailer-loads. The committee reached consensus at a 1,000-pound threshold—well below the estimated 30 tons of AN that was involved in the West Fertilizer Company incident. Solid forms of AN, such as fertilizer-grade mixtures with 60 percent or more ammonium nitrate by weight, and liquid forms containing more than 70 percent by weight in quantities above 1,000 pounds are subject to the requirements in Chapter 11. Amounts below 1,000 pounds are not covered by the MAQ in Chapter 11; however, depending on the amount and classification of the materials, these smaller quantities may still be covered by the requirements of Chapters 15 or 19, which cover oxidizers and unstable reactives, respectively. The MAQ of 1,000 pounds for most of the common forms of AN is consistent with the amount of AN that requires a permit in NFPA 1, Fire Code.

Construction, sprinklers, alarms, and responder tactics

Most of the ammonium nitrate explosions on record involve the contamination of AN with combustible materials such as fuel oil, or the proximity of AN to combustible building materials. In the case of West Fertilizer, storage bins were constructed of wood and the building contained a significant quantity of combustible material in the form of seeds.

The 2016 edition of NFPA 400 was modified to address the serious hazard created by the presence of combustible materials in contact with ammonium nitrate. Chapter 11 now requires all new construction to be non-combustible, without exception; the use of wooden or other combustible bins for storage of AN—previously allowed if protected against impregnation by AN—has been prohibited. Additionally, new construction for AN storage, regardless of type, will be required to be sprinklered.

While establishing the criteria for new construction was straightforward, it was a challenge for the committee to determine reasonable safety requirements for the many existing buildings used for AN storage that are of combustible construction. To deal with the hazard, a requirement for sprinklers was added retroactively to those facilities that are of combustible construction. (The West Fertilizer facility was constructed of combustible materials and was not sprinklered. The fire was intense and out of control by the time firefighters reached the facility.) The revamped Chapter 11 clarifies that water-based suppression systems are the only type allowed for AN, since the purpose of the water is to both suppress an exposure fire and to cool the ammonium nitrate. The water reduces both decomposition and the possibility of forming molten AN, which when confined can create an explosion hazard. Floor sloping and drainage requirements were clarified in Chapter 11 to provide further protection against confinement of molten or spilled AN.

A key addition to Chapter 11 is the requirement for a fire alarm system with fire detection in AN storage areas for both new and existing facilities. Oxidizers such as AN can cause the growth phase of a fire to be greatly accelerated compared to fires involving ordinary combustibles. For this reason, there is increased urgency associated with providing notification to building occupants of a need to evacuate the building faster than with ordinary combustible fires.

The West Fertilizer incident also showed that fires at AN facilities can pose a significant hazard to emergency responders and the surrounding community. In response, requirements for both pre-incident and emergency action planning were added to Chapter 11, as was a requirement for a public notification/alert system, one capable of notifying individuals within one mile of the facility of the need to evacuate (see “Out of Harm’s Way,” facing page).

To help emergency responders determine when to fight fires involving AN and when to evacuate, additional guidance has been added to Annex E of NFPA 400, along with information on the types of AN and the conditions that can cause explosions. The committee added a table that shows typical AN products and their composition and provided additional resources for information on the safe handling and storage of ammonium nitrate.

AN: What lies ahead

While the changes to NFPA 400 are intended to have a positive impact on the safe storage, use, and handling of ammonium nitrate, questions remain. In most cases, it appears that freely vented ammonium nitrate fires do not present an explosion hazard—but data on what constitutes “adequate ventilation” is currently not available. It is unclear whether or not natural ventilation could prevent the buildup of decomposition gases and the release of heat sufficient to prevent an explosion, or if mechanical or emergency “self-ventilation” would be required.

In addition, one of the key outstanding debates is how to classify different forms of AN. Classification of hazardous materials is based on the inherent properties of the material; whether or not some forms of ammonium nitrate, in the absence of combustible materials, are inherently unstable reactives is a question that remains unanswered. Should ammonium nitrate be classified based on its inherent properties, like other chemicals? Or should it be considered an unstable reactive based on the fact that combustible materials are often available as contaminants and can lead to incidents such as the one that occurred at West Fertilizer?