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Quantities of hazardous materials in portable containers are sometimes so small that the DOT does not consider them a serious hazard and does not require placarding or labeling. These materials are placed in a special class known as Other Regulated Materials (ORM-D). For example, charcoal lighter would be classified as a flammable liquid in bulk quantities; however, in quart cans for consumer use it is an ORM-D. Large numbers of small containers in a shipment or in fixed storage collectively can create a serious hazard, even though the individual container quantity is small.
Photo by Robert Burke
Containers of very flammable acetylene gas
Cryogenic hazardous materials (those with boiling points below -130 degrees Fahrenheit) are shipped, stored and used in two types of portable containers: Dewar and cylinder. Dewar containers are non-pressurized vacuum-jacketed flasks with a five- to 20-liter capacity, very much like a Thermos bottle. Cylinder containers are insulated with separate vacuum-jackets and 100- to 200-liter capacities.
Cryogenic containers are usually found at gas-bottling plants, college and university research facilities, private-sector research facilities and are replacing the oxygen gas cylinder in welding operations in many areas. Common cryogenic hazardous materials include oxygen, nitrogen, argon, hydrogen, natural gas, helium and others. Cryogenic liquids have very low boiling points and high liquid-to-vapor expansion ratios. Even a small Dewar or cylinder could produce a significant vapor cloud during a release, causing the displacement of oxygen in confined areas.
While many of the cryogenic liquids are "inert," or non-hazardous, the vapor can still be an asphyxiation hazard when the oxygen in the air is displaced. In addition, the contents of cryogenic containers are extremely cold with temperatures between -130F and -456F. Contact with these liquids could cause frostbite or solidification of body parts, or both.
Cryogenic liquids and liquefied gases such as propane and butane do not have the same physical characteristics. While cryogenics are very cold, liquefied gases exist at whatever ambient temperature is surrounding the container - if the ambient temperature is 100F, then the liquefied gas is also around 100F.
Portable tanks of liquefied compressed gases can be found in many sizes and shapes. They can contain many types of gases, but commonly contain propane or other liquefied petroleum gas (LPG) used for home barbecue grills, heating living spaces, as a fuel for motorized equipment, soldering and others.
These containers shouldn't be overfilled. There should be a vapor space when the container is filled, usually around 20% of the volume of the container, to allow for expansion of gas within the container during increases in ambient temperature. While not as extensive as cryogenic liquids, liquefied petroleum gases also have large liquid-to-gas expansion ratios in the area of 270 to 1, so even a small container can produce a large amount of flammable gas when released to the atmosphere.
Pressures in LPG containers can range from 15 to 230 psi, depending on the ambient temperature. The higher the temperature outside the container, the higher the pressure inside the container will be. Container bursting pressures are generally about four times greater than the working pressures of LPG. Rapid heat buildup can still cause container rupture if the pressure cannot be relieved fast enough.
Liquefied petroleum gases are heavier than air and are found in basements or other low-lying areas during a release. They have no natural odor and are colorless. An odorizer is added to allow leaks to be detected.
Compressed gas cylinders are also common and often used for oxygen, nitrogen, carbon dioxide, fire extinguishing agents, hydrogen and many others. These containers are made of heavy steel and have operating pressures of 3,000 to 6,000 psi. Just the pressure alone in the container can present a significant physical hazard if the valves are knocked off or the containers are exposed to high heat or direct flame contact. These containers can rocket a great distance and present an impact hazard to building occupants and response personnel.