Thursday, May 1, 2008

Bizarre Anhydrous Ammonia Release

Earlier this week a leak of anhydrous ammonia from two 5-gallon propane tanks shut down I-10 for about three hours. The two tanks were being transported in the back seat of a car, a car also transporting two apparent methamphetamine manufacturers. The tanks probably overheated causing an increase in pressure in the tanks. The newspaper article says that the tanks ruptured releasing the gas; it is possible, however, that a safety device on the tanks, a pressure relief valve (PRV), was responsible for the released gas.

The only reported injuries were chemical burns on the exposed skin of the two personnel transporting the tanks. When the toxic gas was released they stopped the car and tried to get away on foot. They were apprehended shortly after response personnel showed up at the scene.

Traffic was stopped and local businesses were told to shelter-in-place while the authorities dealt with the situation. Water was used to knock down the cloud and cool the cylinders (this is why I suspect a PRV release not a tank rupture). Acetic acid (vinegar) was brought in to neutralize the remaining ammonia in the tanks.

Improvised Chemical Device

This is actually a pretty good example of an improvised chemical device, though that was probably not the intention. Liquid anhydrous ammonia is put into an empty propane tank. Propane tanks are pressure tanks, but they are not designed to withstand the pressure produced when anhydrous ammonia goes from the liquid phase to the gas phase upon heating.

If the PRV is operating properly when the pressure reaches a pre-set, safe (to prevent a violent rupture of the tank) pressure level the PRV will allow a steady stream of the toxic fumes to be released to the atmosphere. If the PRV is damaged or malfunctions, and the heat is high enough, the tank will ‘explode’ (the chemical industry prefers to call it something non-violent like a ‘pressure relief event’) and release all of the gas at one time along with flying pieces of metal.

From a terrorist’s point of view either type of release can be used to advantage. The explosive release can provide physical casualties to go along with the chemical effects if they could be ‘detonated’ near people. The prolonged PRV release ties up emergency personnel for a longer period of time and the objectionable cloud (not toxic as the concentration decreases through dispersion) will bother people for a longer period of time.

Drug War Weapons?

It is surprising to me that a device such as this has not already been used. I would have expected to see this used in low-level drug wars between competing manufacturers of methamphetamines. They have the experience in handling this material and know where to get the ammonia. While I have not seen any news reports of this kind of internecine attack, it could be that it was not recognized as a weapon by investigating police. It would have probably been written off as a drug-addled act of incompetence instead being identified as a weapon in a drug war.

Anhydrous Ammonia Legislation Required?

Anhydrous ammonia is almost a perfect chemical for this type of attack; toxic, volatile, and a liquid at reasonable temperatures. This is, of course, why it is also so widely used in cooling and refrigeration systems. This combined with its use as a fertilizer on many farms makes it a relatively easy chemical for Methamphetamine manufacturers to obtain. Many of the legitimate users have relatively small amounts of material on hand and have inadequate or non-existent security procedures protecting their supplies.

Once DHS completes their release of the proposed rule on the control of ammonium nitrate, they might want to start work on similar rules for anhydrous ammonia. The same type registration system would probably be useful for anhydrous ammonia. The rules should also probably include the requirement to have inexpensive locks and alarm systems on all valves on storage and piping systems.

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