Neal Langerman, a chemist
with the San Diego-based consulting company called Advanced Chemical Safety, speculated the explosion in West happened this way:
Anhydrous ammonia tanks are never more than 85 percent filled with the liquid chemical. The other 15 percent of the space is taken up by ammonia in gas form. When the fire got to the tanks it heated the part of the tank over the gas bubble to the point where it melted.
At that point, Langerman said, the gas would have escaped and been ignited by the fire, which was well over the 1204 F ignition temperature. The liquid ammonia in the tank would then have vaporized, escaped and caught fire too--all in the fraction of a second.
“The explosive release of the ammonia--both gas and liquid--would have very quickly transitioned to an actual detonation,” said Langerman, who has also served as an official of the American Chemical Society.
In fires that threaten stores of anhydrous ammonia the strategy is to spray water on the tanks to keep them cool enough so that the metal doesn’t melt. Rising pressure inside the tanks blows out relief valves, which allows the contents to vent and burn in a controlled way. All that, however, must be done from a safe distance.
“What the standard says is: You back off, up to a mile if necessary, and you only apply water remotely. You get your people out of there
,” Langerman said.
The three key questions at this point are: Where did the fire start? Was West Fertilizer operating the depot according to standards? Did the firefighters recognize the particular risk of stored anhydrous ammonia?