There was a staggering amount of damage and multiple deaths caused by Wednesday’s explosion at the West fertiliser Company in West, Texas.
Here’s why fertiliser plants are so prone to explosions.
The likely culprit behind this explosion is their anhydrous ammonia.
According to construction permits from 2006, the fertiliser company kept the anhydrous ammonia in two 12,000 gallon permanent storage tanks. The fertiliser is used on farms directly, but can also be combined with nitric acid to make ammonium nitrate, another volatile chemical.
We don’t know how much ammonium nitrate the plant had on hand when the explosion happened, but that could have added to the explosion.
A nasty gas
Anhydrous ammonia is a gas at room temperature. It becomes a liquid when held at higher pressure, which is how it was stored at this fertiliser plant.
Heat causes the liquid to expand as it tries to become a gas and pushes against the tank. So, the warmer things are around the storage tank, the more pressure the chemical needs to be under to keep it a liquid.
(Anhydrous ammonia doesn’t pose the same explosion problem as a gas because the molecules are too far apart so the heat doesn’t build up as quickly).
An explosive reaction
The tanks should be able to handle pressures up to 250 pounds per square inch. If the temperature of the tank reaches about 120 degrees Fahrenheit, though, the pressure inside the tank will be 271 psi.
The heat from the fire, the cause of which is unknown right now, could have heated the tanks until they couldn’t handle the pressure and burst.
Then, the anhydrous ammonia would have combined explosively with the fire fighter’s water as they were battling the blaze.
The problem? The reaction creates heat. When a lot of anhydrous ammonia and water combine, it can create a lot of heat, and even an explosion.
The tanks are supposed to be inspected every day to look for leaks, and have other safety precautions in place. Because of these precautions, the company claimed that there was “no” risk of fire or explosion in a emergency planning report submitted to the Environmental Protection Agency (EPA).
In the report, the company said that the worst case scenario would be a 10-minute leak of ammonia gas which would pose no risk. According to reporter Lee Fang of The Nation, the plant hasn’t been inspected in the last five years.
They’ve been wrong before: The plant was fined in 2006 for their inappropriate risk management planning. Their emergency planning reports had failed to meet federal standards, WFFA.com reports.
These emergency plans are created by companies that deal with toxic or reactive chemicals so when an incident happens they are prepared to deal with it appropriately.
It is widely known that emergency responders shouldn’t spray liquid anhydrous ammonia with water. We can’t say for sure if this was in the emergency response plan, though.
The compound’s material safety data sheet, which should be on hand where ever the compound is held, states that firefighting procedures in the vicinity of anhydrous ammonia should include:
Must wear protective clothing and a positive pressure SCBA. Stop source if possible. If a portable container (such as a cylinder or trailer) can be moved from the fire area without risk to the individual, do so to prevent the pressure relief valve of the trailer from discharging or the cylinder from rupturing. Fight fires using dry chemical, carbon dioxide, water spray or alcohol-resistant foam. Cool fire-exposed containers with water spray. Stay upwind when containers are threatened. Use water spray to knock down vapor and dilute.
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