HAZARDS AT THE COOPER COUNTY DERAILMENT
In addition to the visible hazard of the burning pool of phosphorus, responders were faced with the unknown condition of the car that had been subjected to impact damage and localized heating for almost five hours.
Phosphorus, which has a melting point of 110°F, is loaded as a liquid in a closed (piped) system that excludes air. If air were present, the product would spontaneously ignite, as it is hotter than its 86°F autoignition temperature. Air is excluded from an empty car before loading bypurging with nitrogen. Finally, a water layer is added in the headspace to isolate the phosphorus in case a relief device or seal opens.
Heat is not maintained on the cars and the product slowly cools, usually solidifying en route. Heat is carefully applied at the off-loading site. The melting process is like the heating of candle or canning wax in a pan and the liquefied phosphorus is viscous (like melted wax). If heat is applied too rapidly, the melted product next to the metal container can boil, while the poor heat conduction of the solid part resists a quick melt and could trap the hot vapors of locally heated material. In a closed container such as a tank car, this is a serious situation.
The circumstances for a critical situation were present at the Cooper County incident. The relief devices were on the top of the car, the localized heating was on the bottom, and the product was partially solidified. It is now’ apparent that the breech made by the rail was probably relieving much of the internal pressure (and thus rapidly expanding the pool). The possibility exists that this space would not have been adequate to maintain low-risk pres-
sures if the fire had continued to grow. Two results were possible: The pressure could have forced the contents to relieve out of a topmounted relief device or the body of the car could have failed where the heat of the flame front and impact damage caused the greatest stress. While operation of a pressure-relief device was the greater possibility, even that would have contributed to increased flame spread and threatened the entry team. Controlling the fire and cooling the car stopped the localized heating and removed that risk.