Fire as a Weapon

Yesterday on my personal blog I discussed some of the process safety issues associated with the recent food processing facility fire in St. Charles, MN. That fire resulted in the evacuation of the town of almost 4,000 because of the fear of the possible imminent explosion of five anhydrous ammonia storage tanks in the facility. Not only was there concern about the potential release of 30,000 lbs of anhydrous ammonia, a toxic inhalation hazards (TIH) chemical, but the catastrophic failure of pressure tanks would have resulted in concussive blast effects and flying bits of metal. Aggressive action by local fire departments prevented the failure of the storage tanks. The pressure relief releases of anhydrous ammonia to the atmosphere were controlled to maintain safe off-site concentrations of the toxic gas. All in all it was a successful emergency response, the facility was essentially destroyed by the fire, but there were no serious injuries or deaths and no significant chemical exposures. While process safety people are interested in examining incidents like this for lessons to be learned, the chemical security community should also look at such incidents. Accidents are much more common than terrorist attacks (thank goodness) so we should take a look at serious accidents as surrogates for terrorist attacks. Fire as a Release Initiator Because of the physical characteristics of toxic inhalation hazard (TIH) chemicals, they are stored in pressure tanks. The construction of these tanks is such that they are much more resistant to explosions and fire arms than are most chemical storage tanks. This makes for a slightly easier to defend potential target. This is especially true when one considers that for maximum effect there must be a catastrophic breach of the storage tank to get the highest concentration toxic plume. Small holes make for slower releases and faster dispersion to sub-toxic levels. The ‘easiest’ way to get a catastrophic breach is to over-pressure the storage tank, quickly increase the pressure in the tank to more than its rated pressure capacity. The word ‘easiest’ is in quotes because chemical engineers are well aware of these potential hazards and spend a great deal of time designing the tank and its associated hardware and software systems to prevent over-pressure situations. Redundant temperature and pressure sensors, cooling systems, valve interlocks, high-level and high-pressure alarms and pressure relief systems are just some of the tools that are used to prevent over-pressure situations. There is one over-pressure scenario that makes chemical engineers cringe; the fire case. This is where there is ‘direct impingement’ of flames from a vigorous fire on the walls of the tank. The heat from the fire increases the head space pressure by increasing the temperature. With low boiling point chemicals (like TIH chemicals) the liquid in the tank quickly reaches the boiling point and it expands exponentially during conversion from liquid to gas. At the same time the heat from the flames on the metal walls of the tank will reduce the strength of the metal decreasing the pressure rating. In short order a very hot fire will result in the explosive failure of the pressure tank, effectively releasing the entire contents instantaneously. Given the rapid action involved, the astronomical increases in pressure, and the weakening of the vessel walls, there is no practical safety system that can prevent the catastrophic failure of the pressure tank in a fire case. The only solution is to keep combustible materials away from the storage tanks so that there can be no fire that will directly impinge on the walls of the tank. This is why TIH chemicals are most often stored in tanks remote from structures and other storage tanks. New Attack Scenario Security planners need to keep the above facts in mind when they look to provide effective security for large PIH storage tanks. They need to re-evaluate the normal chemical engineering effort to protect the tank from an over-pressure situation. They need to insure that there are no combustible materials stored in the vicinity of the tank, even on a temporary basis. Then they need to look at potential attack scenarios that would allow a terrorist to introduce combustible materials into the area around the tank. The simplest attack scenario would be for the terrorist to place a flammable liquid on the ground beneath the TIH storage tank and ignite it. This can be done by routing hoses or puncturing transfer lines (typically an insider attack). Another successful technique would be to cause the catastrophic failure of a nearby flammable (or even combustible) liquid storage tank and igniting that spill. A small explosive device that would be of little use against a pressure tank would be very effective against the typical sheet metal construction of most storage tanks. A similar attack on a properly placed portable storage tank or delivery truck would accomplish the same ends. For facilities that do not have the luxury of being able to keep their TIH storage tanks separate from other chemical storage, or must keep the tank in or near buildings that can burn, special precautions must be taken to avoid the affects of the fire case. Typical fire suppression systems are probably not adequate. A deluge system that keeps a high volume water flow across the entire surface of the tank for the duration of the fire is necessary. This does come with the attendant problem of stopping the potentially contaminated runoff from leaving the site. Again, learning from the mistakes or successes of others is the least expensive way to learn a lesson. A truly intelligent person looks at unrelated situations and extrapolates the lessons learned to their own situation. Protecting TIH storage tanks from fire case situations is not just a safety requirement. It should also be an objective of facility security officers.