Hazmat Containers - Part 1: Highway Bulk Containers

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Placards on tankers provide information concerning the hazard class of the material inside, but a great deal can also be learned about the physical characteristics and hazards of the material from the tank itself. For example, the MC/DOT 312/412 is used exclusively for the transport of corrosive materials. The tanker, however, may carry oxidizer, flammable, poison or corrosive placards.

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Photo by Robert Burke
An MC/DOT 312/412 acid-hauling tanker. Notice the small tank and the heavy reinforcing rings around the outside. Such tankers may carry placards warning of the presence of oxidizers, flammables, poisons or corrosives.


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Photo by Robert Burke
An MC/DOT 306/406 with multiple compartments. Notice the elliptical shape of the container.

Placards are placed on a vehicle based on the U.S. Department of Transportation (DOT) system of listing the most severe hazard. Corrosive materials, as with most hazardous materials, can have more than one hazard. If response personnel become familiar with the various types of bulk highway containers and the products they typically haul, they can better identify "hidden" hazards of materials involved in accidents.

The DOT defines a highway bulk cargo tank as "any tank attached to or forming a part of any motor vehicle or any bulk liquid or compressed gas packaging not permanently attached to any motor vehicle, which by reason of its size, construction or attachment to a motor vehicle, is loaded or unloaded without being removed from the vehicle." The term "bulk hazardous material" is defined by the DOT as "any container with over 119 gallons liquid capacity." Other vehicles are also used to transport dry bulk and liquid/dry mixtures.

Specifications for construction of highway transportation bulk tanks are in accordance with American Society of Mechanical Engineers (ASME) requirements. Additionally, the DOT stipulates procedures for manufacturing, maintaining, testing, inspecting and repairing bulk containers. Construction materials are also specified by the DOT, and include aluminum, carbon steel, high-strength/low-alloy, mild steel and stainless steel. Tanks are designed around the hazards of the materials they will be hauling.

DOT regulations and construction standards apply to both interstate and intrastate vehicles. The most common bulk containers used to transport hazardous materials are MC/DOT 306/406, 307/407, 312/412, MC 331, MC 338, dry bulk and tube trailers (MC stands for motor carrier and DOT stands for Department of Transportation). Since Aug. 31, 1993, the manufacture of vehicles with the MC 300 series specifications has not been allowed. Vehicles manufactured after the above date are required to conform to design specifications for the DOT 400 series. DOT 400 series tanks are built using stronger metals, are larger, and have a larger volume and a lower center of gravity. MC 300 series tanks will remain in service as long as they meet testing and inspection requirements.

The MC/DOT 306-406 tanker is primarily an atmospheric pressure non-insulated flammable liquid container that is hydrostatically tested to 3 psi. Capacities vary from 2,000 to 9,000 gallons. It generally has an elliptical shape, although some manufacturers make a round version, and is used to haul gasoline, diesel, aviation fuel and other flammable liquids. Materials used to construct these tanks include aluminum, steel and stainless steel. Baffles installed within the container limit product movement during transportation.

These tanks may have multiple compartments and may be carrying several different materials. Specification plates, usually located on the right frame rail of the trailer, are small and difficult to read. They may be of more use during training and pre-planning than during an incident. The plate contains information about the type of tank, manufacturer, construction material, date built, design and test pressures and number of compartments and capacities. Specification plates for other bulk tanks contain similar information and are located in the same general area.

Relief valves on 306/406 tanks are spring loaded and remain closed during transportation. Valve operation can be mechanical, pneumatic or hydraulic. Each valve is equipped with an automatic heat-activated closure system. This system is usually a fusible link, but could operate by some other means which operates at temperatures up to 250 degrees Fahrenheit.

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Photo by Robert Burke
An MC/DOT 307/407 insulated tanker used for molten liquids. Notice the “HOT” placard on the rounded tank.


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Photo by Robert Burke
An MC/DOT 307/407 insulated tanker carrying a corrosive liquid. Notice the horseshoe shape of the container from the rear.

There is also a secondary closure system, which is separate from the fill and discharge valves and mounted inside the tank. Manual controls for the secondary system are usually on the left front of the container. This type of tank is unloaded on the bottom. Bottom valves are designed to shear off in the event of a collision of such force that the valve would be damaged. There may be up to 10 gallons of fuel in the valve and piping system under the tank. Fill openings on top of the tank are protected by manhole covers that are securely closed.

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Photo by Robert Burke
An MC 331 high-pressure tank carrying propane.

Tanks are provided with rollover protection and a safety device which prevents the cover's from releasing when excessive internal tank pressure exists. Vacuum and relief vents are located on top of the barrel of the tank or internally. The vacuum vent is set to open at six ounces of vacuum and the relief vents open with as little as 1 psi. Both vents are designed to prevent the release of product during a rollover. If mounted on the outside of the tank, valves must be protected from rollover damage.

Some 306/406 tanks have vapor recovery systems in place to prevent vapors from reaching the atmosphere. Bumpers extend six inches from any vehicle part and serve to protect the vehicle and provide a method of gauging the impact of an accident on the vehicle. Response personnel can use the information during damage assessment. If the bumper is significantly damaged, the tank's baffles are also likely to have beendamaged and their integrity compromised.

The MC/DOT 307/407 is a low-pressure tank used for the transport of flammable liquids, Class 6.1 poisons (poison liquids) and light corrosives. Working pressure in the tank is 25 psi and not greater than 40 psi. Tank capacities range from 2,000 to 8,000 gallons and they are constructed of aluminum or stainless steel. This tank may be insulated or uninsulated. Insulated tanks have a horseshoe shape when viewed from the rear. Uninsulated tanks are generally round and some have reinforcing rings similar to the 312/412 but, the tank diameter is much larger. Insulated tanks that no longer meet low-pressure specifications are sometimes used to transport molten solids such as asphalt. These materials have temperatures in excess of 300F and can be a thermal hazard.

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Photo by Robert Burke
An MC 338 insulated cryogenic tanker carrying liquid argon.

Vehicles carrying molten materials carry a "HOT" placard and sometimes also have a miscellaneous hazardous materials designation. Valving location and information is the same as for the 306/406. Pressure lines may be present on the 307/407, which contains inert gases that may be injected into the tank to absorb moisture or used to assist in off loading the product. Pressure vents are installed in the 307/407 tanks to limit the internal pressure to 130% of the maximum allowable working pressure. Vents are pressure activated by a spring-loaded mechanism. Fusible and or frangible (breakable) venting may be provided with fusible vents activating at 250F. Frangible disks are designed to burst at not less than 130% or more than 150% of the maximum allowable working pressure.

MC 312/DOT 412 tankers are used exclusively to transport corrosive liquids. Corrosives are usually heavy materials, so the tank has a small diameter with reinforcing rings around the outside. These tanks are considered low-pressure containers much like the 307/407. The 312/412 tanks are constructed of black iron steel, stainless steel or an aluminum alloy, with a lining that will resist degradation and reaction with the tank's contents. Unlike the previously mentioned tanks, these tanks are loaded and unloaded from the top. No valves or piping can be found on the underside of the tank. Valves and venting systems are much the same as the previously mentioned tanks. The 307/407 tanks can be insulated or uninsulated. They may have a steam pad that can be used to heat the container's contents and speed the off loading process. It is located at the rear of the tank.

MC 331 tanks are high-pressure containers used to haul liquified gases including, propane, LPG, chlorine and anhydrous ammonia. Liquefied gases are gases which have been liquified by bringing the gas to its critical temperature and pressure. At this point, the gas turns into a liquid. Propane, for example, has a critical temperature of 206F and a critical pressure of 617 psi. At that temperature and pressure, the propane gas becomes a liquid. Butane has a critical temperature of 306F and a critical pressure of 555 psi. Once liquefied, the gas is kept in the liquid state by pressurizing the tank to keep a constant artificial atmosphere pressing down on the liquid to keep it from returning to the gas state.

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Photo by Robert Burke
A dry bulk container. The materials in these tanks are usually not hazardous; it is the form of the material that creates the hazard - in this care, flour, which when suspended in air in the presence of an ignition source can create a dust explosion.

Liquids in these tanks are at atmospheric temperature. Whatever the temperature is outside, the liquid inside will be nearly the same. Liquefied gases exist in the tanks well above their boiling points. The only thing keeping the material from boiling and turning back to a gas is the pressure in the container. These tanks are usually constructed of steel with working pressures generally between 100 and 500 psi. Propane has a working pressure of 250 psi, while anhydrous ammonia has a pressure of 265 psi.

These tanks are never filled to the top. There is usually a 20% vapor space allowed above the liquid level. Usually, these tanks are not insulated but rather painted with white or aluminum paint to reflect radiant heat. When a pressure container such as the 331, with a boiling liquid inside, is exposed to flame on the vapor space, the container will quickly fail and produce a boiling liquid expanding vapor explosion (BLEVE). During a BLEVE, when the container opens up, all of the liquid inside instantly turns into a gas because it is existing above its boiling point. If there is an ignition source present, a fire ball may also be created.

According to the National Fire Protection Association (NFPA), a BLEVE will occur within 15 to 20 minutes after flame impingement starts. If flame impingement occurs on the liquid level of the tank, the liquid in the tank will absorb the heat and protect the container. The heat from the fire will cause an increase in the pressure in the tank and it will be vented through the relief valve.

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Photo by Robert Burke
A mixer truck used to make ammonium nitrate blasting agent at a remote site.

If the relief valve cannot relieve the pressure as fast as it builds up, the container may still fail. Relief valves are designed only to relieve pressures created by increases in ambient temperature, not flame impingement. Excess flow valves are installed at product discharge openings. They operate in the event of a failure in discharge hoses or piping. Venting systems are either mechanical pressure relief or frangible disks. Valve protection during a rollover is much the same as for other MC series tanks.

MC 338 tanks are used for the transportation of cryogenic gases, sometimes referred to as refrigerated liquids. These materials are very cold with boiling points of -130F for carbon dioxide to -452F for liquid helium. Common cryogenics include oxygen, nitrogen, helium, argon, and others.

Many of the materials carried in 338 tanks are considered inert gases. That is, they do not readily react chemically to other materials, are not flammable and are not poisonous. They do, however, have significant hazards when released as liquids or gases. Liquids are extremely cold and can cause frostbite and solidification of anything they contact, including body parts. Liquids also have large expansion ratios, producing huge amounts of vapor from a small spill. In some cases, as little as one gallon of a cryogenic liquid can produce over 900 gallons of gas. While these gases are inert in many cases, they can still displace the oxygen in the air and cause simple asphyxiation.

Great care should be taken when dealing with liquid or vapor leaks from a MC 338 container, which actually is a tank within a tank. Between the inner and outer tanks there is a space for insulation to help keep the material within the tank from heating up and vaporizing. The tanks are not refrigerated and even with the insulation a certain amount will vaporize, increasing the pressure in the tank and causing the excess pressure to be exhausted through the relief valve. This is a normal process and does not indicate a leak in the tank.

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Photo by Robert Burke
A tube trailer carrying compressed hydrogen, a flammable gas.

Cryogenic tanks are constructed of steel on the outer tank and special alloys of steel on the inside to withstand extremely cold temperatures and internal pressures of 23.5 to 500 psi.

Some additional types of bulk containers warrant discussion because of dangers of the products inside. Dry bulk tanks carry dry materials of very fine particle size. These materials rarely are placarded and are not regulated by the DOT. However, if released in air in the presence of an ignition source, they could produce a dust explosion. The only dry bulk tanks I have ever seen placarded were those carrying ammonium nitrate, with an oxidizer placard, and those carrying ammonium nitrate fuel oil mixture, which is placarded as a blasting agent/Class 1.5 explosive.

Tube trailers are used to transport high-pressure gases. Unlike the MC 331, there are no liquids in these containers and therefore no protection against flame impingement anywhere on the surface of the tank. Pressures in these tanks may be in excess of 3,000 psi. These trailers are actually a series of small pressure tanks placed on a flatbed trailer, then banded and cascaded together. Accidents have been reported where the tanks have come loose during an accident and come through the cab of the truck.

Another type of truck used for hazardous materials is the blasting agent mixer. This truck looks very much like an agricultural feed truck. It has three tanks - one for dry ammonium nitrate, one for fuel oil and one for a mixture of the two. Once mixed, the blasting agent is off-loaded through a mechanical arm, much like grain from a feed truck. This vehicle usually carries three placards - oxidizer, flammable liquid and explosive 1.5, also known as blasting agent.

Knowing the design characteristics of hazmat containers can assist response personnel in safely dealing with a hazardous materials incident involving a bulk container. They can more effectively identify the dangers of the hazardous material as well as provide effective damage assessment on the container.


Robert Burke, a Firehouse® contributing editor, is the fire marshal for the University of Maryland and has served on state and county hazmat response teams. Burke is a veteran of over 17 years in career and volunteer fire departments, serving as assistant chief and deputy state fire marshal. He holds an associate's degree in fire protection technology and a bachelor's degree in fire science, and is pursuing a master's degree in public administration. Burke is an adjunct instructor at the National Fire Academy and Maryland Fire and Rescue Institute, and is the author of the textbooks Hazardous Materials Chemistry For Emergency Responders, published in 1997, and Counter-Terrorism for Emergency Responders, published in 1999. He can be reached on the Internet at robert.burke@worldnet.att.net.

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