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ROBERT BURKE
Published: March 1995

Tank car of chlorine, Class 2.3 Poison Gas, Inhalation Hazard

Hazard Class 2 is composed of materials that are gases under pressure. The gases in this category may also be liquefied in order to ship larger quantities more economically. The liquefaction of gases presents a hazard in itself. Liquefied gases have large liquid to gas expansion ratios. That is to say, a very small amount of a liquid leaking from a container can form a very large gas cloud. This increases the danger of flammability if an ignition source is present, and asphyxiation or toxicity when vapor clouds form. Some liquefied gases such as propane and butane are ambient temperature liquids. Cryogenic liquefied gases on the other hand are very cold liquids. This difference in temperatures can cause problems for responders unless handled properly.

The compressed gas class is divided into three subclasses. 2.1 materials are flammable compressed gases, 2.2 materials are non-flammable compressed gases, and 2.3 materials are poison compressed gases. Each of the compressed gas categories present their own special hazards in addition to the hazard of being under pressure in a specially designed and regulated pressure container. Pressures range from atmospheric, 5 psi to 6000 psi. The higher the pressure the more substantial the container must be to bold the pressure. The higher the pressure the greater the danger when the pressure is released or the container fails.

Class 2.1 materials are flammable. The DOT defines a flammable gas as "any material which is a gas at 20 C (68 F) or less and 14.7 psi of pressure or above, which is ignitable when in a mixture of 13 percent or less by volume with air or has a flammable range with air of at least 12 percent regardless of the lower limit."

Chlorine Ton Cylinders

Certain physical conditions must be present however in order for the materials to burn. These physical characteristics are sometimes referred to as "parameters of combustion". From fire science we know that materials must be in the vapor state (gas state) in order to burn. Many of these materials are already gases, some of them may be found liquefied, or in the liquid state. These liquefied gases are already above their boiling point. The only thing keeping them a liquid is the fact they are under pressure. If that pressure is released such as when a Boiling Liquid Expanding Vapor Explosion (BLEVE) occurs, all of the liquid turns into a gas instantly. Being a flammable gas it may ignite if an ignition source is present. When that ignition occurs it must occur within the flammable range of that particular material.

Flammable range has two components, a lower explosive limit, and an upper explosive limit. Ignition can only occur between the two. Flammable range is the point in which there is enough oxygen and enough fuel present for combustion to occur. There are two common terms applied to flammable range to express where the mixture is located. The first is the upper explosive limit (UEL). When a material is above the upper explosive limit, it is considered too rich to burn. That means there is plenty of fuel for combustion but not enough oxygen. The second term is the lower explosive limit (LEL). When the material is below its lower explosive limit there is plenty of oxygen for combustion to occur, but not enough fuel. So for combustion to take place a material must be within its flammable range. Just the right mixture of oxygen and fuel must be present for combustion to occur. Different materials have different flammable ranges. The only way to determine if a material is present in a mixture with air that is in its flammable range is to use a meter to monitor for the lower explosive limit of the material. Monitoring instruments check for a percentage of the lower explosive limit. The rule of thumb according to the EPA is when you reach 25% of the lower explosive limit; it becomes too dangerous for personnel to proceed any further.

The DOT has established criteria for a material to be classified as a flammable gas. First of all its lower explosive limit must be below 13%. If it is then that it is classified as a flammable gas. Some materials have wide flammable ranges which make them much more dangerous than materials with narrow ranges. So the DOT says if a material has a flammable range greater than 12 percentage points regardless of what the lower limit is than it is classified as a flammable gas.

Propane and Butane are two very common flammable liquefied compressed gases. Propane and Butane have boiling points of -42.5 C and -0.5 C respectively. Both materials are above their boiling points under ambient temperature conditions in many parts of the country year round. This makes the materials very dangerous when a leak or fire occurs, especially if there is flame impingement on the container. Because the materials are already above their boiling points, flame impingement, radiant heat transfer, or increases in ambient temperature can cause the materials to boil faster. Faster boiling causes an increase in vapor pressure within the container. Even though the containers are specially designed to withstand pressure, and have relief valves to release excess pressure, there are limits to the pressure they can tolerate. If the pressure build-up in the container exceeds the ability of the tank to hold the pressure or the relief valve to relieve the pressure, the container will fail.

Cryogenic Duwar Container

There have been numerous incidents over the years involving pressure containers and flammable gases. One of the most recent incidents happened in August 1994 in White Plains New York. A propane tanker crashed into a house and exploded killing two people. Other incidents include Kingman Arizona where a railroad tank car containing propane caught fire with flame impingement on the vapor space. The car BLEVED killing twelve firefighters and one civilian. In Waverly Tennessee a Butane car in a train derailment was damaged. Two days after the accident the car BLEVEd when ambient temperatures caused pressure increase in the tank and the tank BLEVEd because of a weakened area in the tank caused by the accident. Sixteen people were killed including police and firefighters with over eighty injured many having experienced serious burns.

When dealing with emergencies with pressure containers and flammable gases great caution should be taken. Flame impingement on the vapor space of a container is a no win situation. A BLEVE is going to occur, it's just a matter of time. To try to fight a fire under those conditions is to play Russian roulette. If the impingement is on the liquid space the liquid will absorb the heat for a period of time. There will also be an increase in vapor pressure within the tank because of the flame impingement. This can still be a dangerous situation if not handled properly. Copious amounts of water are needed to cool the tank down. Conditions involving the tank must be monitored constantly for changes, including liquid level, pressure coming out of the relief valve, and signs of tank failure.

Hazard subclass 2.2 includes materials that are non-flammable and non-poisonous compressed gases. The materials in this class can be compressed gases, liquefied gases, pressurized cryogenic gases and gases in solution. These materials are non-flammable, but that does not mean the containers cannot BLEVE under flame impingement conditions on the container from a fire involving other materials, or from a damaged or weakened container. In Verdigris Oklahoma a tank car was being filled with anhydrous ammonia. It was unknown that there was a weakened place on the tank car that gave way from the pressure of the ammonia and resulted in a BLEVE. There was no fire, just a vapor cloud which traveled down wind defoliating trees and turning other vegetation brown. One worker who was filling the tank car was killed in the incident. Pressure containers can be very dangerous under accident conditions, ambient temperature changes, flame impingement and damage to containers can cause BLEVEs.

"The D.O.T definition of a non-flammable gas is a material that exerts in the packaging an absolute pressure of 41 psi or greater at 20 C, and does not meet the definition of Division 2.1 or 2.3." If the pressure is less than 41 psi they do not belong in this category. Many cryogenic materials are shipped at atmospheric pressure so are not considered a compressed gas. Cryogenics are not a D.O.T. hazard class as a group, but if they are shipped above 41 psi they are considered a compressed gas. Cryogenics may carry other placards such as flammable gas, poison gas, and oxidizer. If cryogenics do not have any other placardable hazard, they are not required to be placarded under DOT regulation. Materials under this class that are shipped as liquefied gases, such as cryogenics, exhibit other hazards not indicated by the placard. Liquefied Refrigerated Gases such as cryogenics are very cold materials. Cryogenics or refrigerated liquids as they are sometimes called have boiling points of -130 F below zero or greater. Liquid helium has a boiling point of -452 F below zero, it is the coldest material known to man. It is also the only material on earth that doesn't ever exist as a solid.

These gases are liquefied into cryogenic liquids by a process of pressurization, cooling, and ultimate release of pressure. Therefore they do not require pressure to keep them in the liquid state unless they will be in the container for a long period of time, then they are pressurized above 41 psi. These materials have very large expansion ratios some as much as 900 or more to 1. Because of this expansion ratio if the material is flammable, or toxic these hazards are intensified because of the potential of large gas cloud production from a very small amount of liquid. This means that 1 gallon of a material can produce over 100 gallons of a gas. Those materials that are not under pressure above 41 psi, or are not flammable, or poisonous, are not required to display placards. These materials can still pose serious danger to responders. These materials have large expansion ratios and can displace oxygen in the air, which can harm responders by asphyxiation. Asphyxiation is not poisoning. Asphyxiation is just simply not enough oxygen to breath. Being very cold these materials can cause frostbite and solidification of body parts. When the parts thaw out the tissue is irreparably damaged. Many of the inert gases from the periodic table of chemical elements are extracted from the air and turned into cryogenic liquids. These include Helium, Neon, and Argon. Other air components such as Oxygen and Nitrogen are also turned into cryogenic liquids for shipment and storage. Nitrogen as a cryogenic is also considered inert. Oxygen however is very reactive with hydrocarbon-based materials. Liquid Oxygen in contact with an asphalt surface such as a parking lot or highway can create a contact explosive. Dropping an object, driving over the area or even walking on the area can cause an explosion to occur.

Flammable Gas 2.1 Propane Bulk Tank

There are also some "foolers" in this category. Anhydrous Ammonia is regulated by the DOT as a Non-Flammable Compressed Gas. This is the only country in the world that placards anhydrous ammonia this way. Everywhere else it is a poison gas. Not to mention that it is also flammable under the right conditions. If it is inside a building or in a confined space it may very well be within its flammable range and burn if an ignition source is present. In Shreveport Louisiana one firefighter was killed and one badly burned in a fire involving anhydrous ammonia. A leak developed inside a cold storage plant. The firefighters donned Level A chemical protective clothing and went inside to try to stop the leak. Something caused a spark and the anhydrous ammonia caught fire.

If anhydrous ammonia met the DOT definition of a flammable gas it would be placarded as such. It does not however meet the DOT definition of flammable gas, which is a LEL of less than 13% and a flammable range of greater than 12 percentage points. It has a LEL of 16% and a range of 16% to 25%, which is 10 percentage points.

Other cryogenic materials such as such as hydrogen are liquefied and placarded as flammable gases. Liquid oxygen is placarded as an oxidizer or non-flammable compressed gas. Cryogenic liquid fluorine is placarded as a poison. There may be nothing more than the container that tells you that it is a cryogenic in addition to being a flammable, oxidizer, or poison.

Subclass 2.3 materials are Poison Gases. They are an inhalation hazard, and some may also be absorbed through the skin. "The D.O.T. definition of poison gas is a material that is a gas at 20 C or less at 14.7 psi and is so toxic to humans as to pose a hazard to health during transportation, or in the absence of adequate data on human toxicity, is presumed to be toxic to humans because when tested on laboratory animals it has an LC50 value of not more than 5000ml/m3." These materials are considered so toxic that when transported the vehicle must be placarded regardless of the quantity. The potential exists for 2.3 materials to affect large populations by creating gas clouds that are toxic. Examples of some 2.3 poison gases are fluorine, chlorine, carbon monoxide, hydrogen sulfide, and chloro picrin-methyl bromide mixture. Poison gases may be encountered as gases, liquefied gases or cryogenics. The placard will indicate poison gas, it will not tell you the material has been liquefied or turned into a cryogenic liquid. The container type will help determine the physical state of the materials.

Non-Flammable Gas 2.2 Cryogenic Liquid Tanker

In Atlanta a small-pressurized cylinder fell from a truck in the garage of the Hilton Hotel. The resulting leak of chlorine, a 2.3 poison gas, sent thirty-three people to the hospital including six firefighters and four police officers. Chlorine is one of the most common poison gases transported and stored. In addition to being a poison chlorine is also a very strong oxidizer. Chlorine will behave much the same way as oxygen in accelerating combustion during a fire. Chlorine is also corrosive. It may be encountered in 150 lb. cylinders, 1-ton containers, and in tank truck and rail tank car quantities. Because chlorine is so common its hazards are sometimes taken for granted. There was a time when firefighters handled chlorine leaks with turnouts and SCBAs. That is no longer an acceptable practice. Poison gases pose a large threat to not only the public but also to emergency responders and the proper protective clothing and SCBA must be worn for protection. Chlorine is used as a swimming pool chlorinator, a water treatment chemical and has many other industrial uses.

Particular attention must be paid to compressed gases in emergency response situations. Compressed gases present responders with multiple hazards including poisons, flammables, oxidizers, cryogenics and the hazard of the pressure in the container. If the container fails or opens up the container can become a projectile or through pieces of the container over a mile from the incident scene. Learn to recognize pressure containers and be very cautious when there is flame impingement on a pressure container.

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