Railroad cars that are used to transport hazardous materials are, in many cases, similar to their highway counterparts, but they have much larger capacities. The primary types of railcars are box, hopper, bulk, tank, flat and tube. Types of hazardous materials carried by rail are much like those...
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Non-pressure tank cars are hydrostatically tested from 35 to 100 psi. They commonly are used to transport flammable and combustible liquids, flammable solids, oxidizers, organic peroxides, poison liquids, and corrosives. (There are exceptions; for example, DOT 111 tank cars may carry specific flammable and non-flammable gases.)
Tank cars are constructed from a variety of materials, including carbon steel, aluminum, stainless steel, nickel, chromium and iron. Single thicknesses of tank materials range from one-eighth to three-quarters of an inch. Tank car design standards are found in 49 CFR, Part 179 of the DOT Hazardous Materials Regulations. Modifications may occur to tanks that accommodate products transported because of product temperature, flammability or chemical reactivity. Tanks may be insulated externally to protect against the effects of ambient temperatures. Insulation materials can include fiberglass, polyurethane or pearlite; cork is used in some older tank cars.
Tanks may also be provided with thermal protection designed to keep tank temperatures below 800 degrees Fahrenheit during a 100-minute pool-fire or 30-minute torch-impingement test. This protection is provided by a layer of wool or ceramic fiber covered by a one-eighth-inch steel jacket. Thermal protection can also be provided through a textured coating sprayed onto a tank’s outer surface. Heat from a flame exposure is absorbed by the coating material and is not transferred to the tank metal. Metal does not hold up well to direct flame contact and will fail in 15 to 20 minutes, according to information from the National Fire Protection Association (NFPA).
Tank cars are protected from physical damage during an accident by shelf couplers and head shields. Shelf couplers are designed to stay together so they do not puncture another car during a derailment or other accident. Head shields provide an extra layer of metal to help prevent dents and punctures when a pressure tank is hit by another car or object.
Cryogenic railroad tank cars are usually constructed of nickel or stainless steel as a tank within a tank. Cryogenic tanks may also be found within boxcars. Because cryogenic liquids are very cold, insulation is placed between the two tanks and a vacuum pulled on the space to maintain the temperature. This process will allow the tank car a 30-day holding time.
Cryogenic cars transport various gases, including flammable hydrogen, liquid oxygen and poisons. Some cryogenic gases, such as nitrogen and argon, are considered inert. Temperatures of these liquefied gases can range from the warmest, carbon dioxide at –130F, to the coldest, helium at –452F. Thermal hazards of these materials are significant.
In addition to thermal hazards, cryogenic liquids have a large liquid-to-vapor expansion ratio. A small leak from a valve or container can create a large vapor cloud. Some ratios are as great as 900 to 1, meaning one gallon of cryogenic liquid can produce over 900 gallons of gas.
Multi-unit rail cars are used to transport individual tanks of gases in uninsulated “ton containers.” They are removed from the car for use, refilled and re-transported. Products carried include chlorine, phosgene, anhydrous ammonia and refrigerants. “Ton containers” have a 180- to 320-gallon water capacity and are pressure tested from 500 to 1,000 psi. They may be transported by rail or truck and can be found on special flat cars, boxcars or gondola cars, as well as on “trailer on flat car” (TOFC), or “container on flat car” (COFC) units.
High-pressure tank cars (similar to highway tube trailers) are approximately 40 feet long and contain a series of 25 to 30 steel cylinders or individual tanks that are tested to 4,000 psi. High-pressure tanks are not insulated and are equipped with pressure relief valves, and are usually used to transport helium or hydrogen.
Another type of pressure tank car is only pressurized during unloading. This car is a pneumatically unloaded, covered hopper car. Pressure is applied during the unloading process and the tank is tested to between 20 and 80 psi. This type of tank is used for dry caustic soda.