Safe storage is top priority

The April 17, 2013, lethal fire and explosion at a West, Texas, facility storing ammonium nitrate (AN) fertilizer has prompted a chemical advisory from three federal agencies that implement chemical safety regulations.  The advisory describes the risks of explosion and fire associated with AN when it comes into contact with combustible organic materials and sometimes with inorganic materials; it then focuses on proper storage, including keeping temperatures in a safe range, avoiding contact with combustible materials, and using correctly designed and built enclosures.  Also addressed are fire protection, community emergency planning, and emergency response—all general guidelines applicable to the management of large quantities of any explosive substances. 

AN is also subject to multiple laws and regulations, particularly when it is combined with a combustible material.  The advisory summarizes these requirements and also references other sources of information, including industry codes, accident reports, and technical reports about the chemical properties that make AN a high safety concern. 

AN is a low-cost fertilizer that can supply copious amounts of nitrogen, which is critical for growing crops.  As a result, it is manufactured, distributed, and sold in large quantities.  AN is also an oxidizer—a material that readily yields oxygen, a key ingredient in explosive reactions—and much smaller quantities are used in the manufacture of industrial explosives.  Pure AN is stable and will explode only under extraordinary circumstances.  However, the addition of combustible materials, such as sugar, grain dust, seed husks, or other organic contaminants, even in fairly low percentages, in the presence of heat creates a dangerous combination, and the AN mixture becomes far more susceptible to detonation.  This characteristic of AN underlies most of the recommendations in the advisory.

Even before the West, Texas, disaster, the catastrophic potential inherent in AN was well known.  As with West, Texas, two relatively recent events in France highlight the extraordinary potential impact of AN incidents.  In September 2001, 10 days after the terrorist attacks in the United States, an explosion involving 200 to 300 tons of AN in a fertilizer factory in Toulouse resulted in the deaths of 30 people, 2,500 injuries, destruction of the factory, and heavy damage to an additional 10,000 buildings. 

While the exact cause of the incident remains unknown, storage of incompatible material with AN is believed to have been a factor.  Two years later, an explosion in a farm warehouse in St. Romaine en Jarez, where 3 to 5 tons of AN were stored in bags, killed 26 people.  Improper storage methods are thought to have played a role.  Other incidents, including two in Germany in 1921 and one in Texas City, Texas, in 1947, involved AN that had been combined with other substances that served as the explosive material—ammonium sulfate in one case and wax used as a coating in another.  The advisory indicates that “corrective steps” were taken following both the 1921 and 1947 incidents.  The recent incidents seem to indicate that adherence to those steps deteriorated over time. 

The advisory highlights three lessons learned from investigations of accidents involving AN:

• AN will self-compress/self-confine under some conditions, becoming much more likely to explode.
• AN poses an explosion risk when stored near other sensitizing agentsthat can add fuel to the AN—such as grain, sugar, seeds, sawdust, and especially petroleum fuels such as diesel.  AN may also be sensitized by certain inorganic contaminants, including chlorides and some metals, such as aluminum powder, chromium, copper, cobalt, and nickel.
• AN is a powerful oxidizer and a rich source of nitrate, which provides energy to an explosion.  In general, for fertilizer blends containing AN, the more nitrogen they contain, the greater the explosion hazard they present.  Blended fertilizers containing AN and chloride compounds and blended fertilizers containing AN contaminated with combustible materials or incompatible substances pose increased explosion hazards.  The presence of fuel and/or heat (and especially both) near AN is a very high hazard situation.

Following are key hazard-prevention measures the advisory indicates should be included in plans for the correct management of large quantities of AN.

Storing AN

• Avoid heating AN in a confined space.
• Ensure that AN is not exposed to strong shock waves from explosives.  AN storage near high explosives or blasting agents must conform to the Bureau of Alcohol, Tobacco, Firearms, and Explosives’ (ATF) Table of Separation Distances at 22 CFR 555.220.
• Avoid contamination of AN with combustible materials or organic substances such as packing materials, dust, seed, oils, and waxes.  Avoid collocating AN, especially bulk AN in bins, with dust-producing organics such as grains or seeds.
• Avoid contamination of AN with inorganic materials that may contribute to its sensitivity to detonation, including chlorides and some metals, such as aluminum powder, chromium, copper, cobalt, and nickel.  Pay attention to the materials used to build storage areas and cribs.  Wood and metals that will be in contact with AN must be specially treated to prevent impregnation.  Metals can be treated with epoxy tar or chlorinated rubbers to prevent corrosion of the metal and contamination of AN.
• Maintain the pH of AN solutions within the safe operating range of the process.  In particular, avoid low pH (acidic) conditions.  If possible, do not collocate acids in an AN storage area.  Keep molten or solid AN out of confined spaces, especially sewers or drains where it can react with organic materials.
• OSHA's standard for explosives and blasting agents at 29 CFR 1910.109(i) contains requirements for AN stored in the form of crystals, flakes, grains, or prills, including fertilizer grade, dynamite grade, nitrous oxide grade, technical grade, and other mixtures containing 60 percent or more of AN by weight.  AN should also be handled in accordance with safe practices found in National Fire Protection Association’s(NFPA) 400 Hazardous Materials Code, Chapter 11.

Building design

• Store AN only in one-story buildings with no basements unless the basement is open on one side.
• Use fire-resistant walls within 50 feet of combustible building materials.
• For flooring in storage and handling areas, use material that is noncombustible or protected from impregnation by AN.
• Avoid installing or remove or close-off open drains, traps, tunnels, pits, or pockets into which molten AN can flow and be confined.
• Keep buildings dry; prevent water seepage through roofs, walls, and floors.
• Have adequate ventilation or ensure self-ventilation in the event of a fire to avoid pressurization.
• Do not place AN into storage when the temperature of the product exceeds 130°F (54.4°C).

Containers

• Piles of bags, drums, and other containers should be no closer than 36 inches below the roof or supporting beams.
• Bags should be stored no less than 30 inches from walls or partitions.
• Piles of bags, drums, and other containers should not exceed a height of 20 feet, width of 20 feet, and length of 50 feet, unless the building is of noncombustible construction or protected by automatic sprinklers.
• Maintain aisles of at least 3 feet width between piles.

Bulk storage

• Bins for storing bulk AN should be kept clean and free of materials that could contaminate the AN.  Bins should not be constructed of galvanized iron, copper, lead, or zinc unless suitably protected.  Aluminum or wooden bins should be protected against impregnation by AN.
• Piles of bins must be adequately sized, arranged, and moved periodically to minimize caking.  Height or depth of the piles should be limited by the pressure-setting tendency of the product, but in no case, should the pile be higher than 36 inches below the roof or supporting beams.
• Do NOT use dynamite, explosives, or blasting agents to break up or loosen caked AN.
• Protect piles of AN from absorbing moisture from humid air by covering them with water-impermeable sheeting or using air conditioning.
• Do not store AN with organic chemicals, acids, or other corrosive materials, materials that may require blasting during processing or handling, compressed flammable gases, flammable and combustible materials, or other contaminating substances.  Stored AN should be separated from incompatible substances by using separate buildings, 1-hour fire-resistant walls, or a minimum separation distance of 30 feet.

Fire protection

• AN storage areas should be equipped with automatic sprinkler systems or have automatic fire detection and alarm systems if the areas are not continuously occupied. This is especially important when the facility in question is close to the public.
• Facilities should not store more than 2,500 tons of bagged AN without an automatic sprinkler system.
• An automatic sprinkler system, if installed, should be provided in accordance with NFPA 13, Standard for the Installation of Sprinkler Systems.
• Suitable fire control devices such as hoses and appropriate portable fire extinguishers (AN is an oxidizer and not all fire extinguishers are appropriate) must be provided throughout the warehouse and loading areas.  Water supplies and fire hydrants should be available.
• Store AN fertilizer in separate buildings or separate AN with approved fire walls from organic, combustible, or reactive materials such as grains, wood or other organic materials, urea and urea compounds, flammable liquids or gases, corrosive acids, chlorates, chromates nitrites, permanganates, or finely divided metals or sulfur.
• AN fertilizer should not be stored in the same building with explosives or blasting agents unless conditions in ATF’s Table of Separation Distances of Ammonium Nitrate and Blasting Agents from Explosives and Blasting Agents (27 CFR 555.220) are met.
• Prohibit smoking in AN storage areas.

Community emergency planning

AN is a hazardous chemical covered under the OSHA Hazard Communication Standard (HCS).  Therefore, facilities that handle and store AN are required by law to submit information regarding the hazards of AN to their state or tribal emergency response commission, local emergency planning committee, and local fire department.

The owner/operator of an HCS-covered storage facility should develop a site emergency response plan that includes:

• Coordination with local first responders;
• Joint training with first responders, if possible;
• Employee training;
• Community outreach;
• Analysis of what may be at risk in a serious accident and appropriate planning;
• Signs that clearly mark high hazard areas, safe areas, emergency contact numbers, fire-fighting equipment, and other essential areas during an emergency response; and
• A site and area evacuation plan.

Regulations, codes, and information

In addition to OSHA’s HCS, the manufacture of or processes involving AN are subject to the following regulations:

• Clean Air Act general duty clause.
• Clean Air Act risk management program (RMP).
• Emergency Planning and Community Right-to-Know Act (EPCRA) Sections 303 (emergency response plans), 311 (material safety data sheets), and 312 (emergency and hazardous chemical inventory forms).  Section 311 does not apply to any substance used in routing agricultural operations or to a fertilizer held for sale by a retailer to the ultimate customer.  However, this exemption does not apply to AN mixed or formulated with other chemicals to produce a fertilizer mix.
• OSHA Section 29 CFR 1910.109, which applies to explosives and blasting agents.
• Department of Homeland Security’s chemical facility anti-terrorism standards (CFATS).
• Department of Transportation’s hazardous materials regulations (HMR).
• ATF’s explosives regulations at 29 CFR 555.

The advisory also lists codes and sources of information on AN published by standards organizations and industry associations, including the NFPA, Compressed Gas Association, European Fertilizers Manufacturers Association, The Fertilizer Institute, National Safety Council, Agricultural Retailers Association, and Institute of Makers of Explosives.

The AN advisory

William C. Schillaci