Chemical Profiling II: What You See Does Make a Difference

IN PART 1, “CHEMICAL PROFILING: A SAFE APPROACH for First Responders” (May 2007), I discussed the first responder reference materials, the Department of Transportation (DOT) Emergency Response Guidebook and the National Institute for Occupational Safety and Health (NIOSH) Pocket Guide to Chemical Hazards. These are only two tools used for profiling chemicals. There are other clues used to recognize and identify hazardous materials such as dispatch information; occupancy and location; container color and shape; placards, labels, and markings; and shipping papers and facility documents.

If you look at a response based on closest to farthest, dispatch information puts you at the farthest point from the incident and in the safest position. The closer you get to the incident, the greater the danger. On scene, using shipping papers and facility documents puts you even closer. Observing placards and container shapes puts you as close as you need to be. Anything in transit falls under the DOT, as was discussed in Part 1. That having been said, all hazardous materials in transit must have placards on all four sides of bulk containers (those with a maximum liquid capacity of greater than 119 gallons, a maximum net capacity greater than 882 pounds for solids, and a maximum water capacity greater than 1,001 pounds for gases) or labels on two sides for nonbulk containers (those with a maximum liquid capacity of less than 119 gallons, a maximum net capacity of less than 882 pounds, and a maximum water capacity of less than 1,001 pounds for gases).


Certain characteristics of container shapes are strong indicators of the contents. A look at some of the most common characteristics of over-the-road cargo tanks will help you identify the family of the hazardous material within the container and, consequently, the harm associated with that substance.

According to the DOT Hazardous Materials Information System (HMIS), 20,310 hazardous materials incidents occurred in transit in 2006. Of those incidents, 17,128 (84 percent) occurred on the highway. When breaking down hazardous materials incidents proportionally by type of carrier, the statistics are quite enlightening.1

Let’s look at the following statistics:

  • Nonpressure: 57 percent
  • Low Pressure: 21 percent
  • Corrosive: 12 percent
  • Pressure: 10 percent
  • Cryogenic: < 1 percent
  • Tube: < 1 percent

What these statistics mean is that 78 percent of the highway incidents we encounter involve materials in a nonpressure or low-pressure cargo tank transporting a flammable/combustible liquid or poison/corrosive. The lack of any significant pressure is a plus for responders; however, beware! Even a little pressure in one of these vessels should be respected; injury can occur.



MC 306/DOT 406 or Atmospheric Pressure Cargo Tank

This workhorse of over-the-road trailers is most commonly identified with hauling petroleum products, but it can carry other liquid products. These cargo tanks can be easily identified by their elliptical shape and flat ends (photo 1). They have separate compartments for different products. Note the valves to determine the number of compartments (photo 2).

Photos 1-2, 3-6, and 12-14 courtesy of Engineer (Ret.) Bill Hand, Houston (TX) Fire Department; others by author.
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The largest compartments usually are the front and the rear compartments. Trailer-length vapor recovery system pipes (photo 3) run along one or both sides of the manway on the top of the 306/406. An emergency shutoff control (photo 4) is on the driver’s side of the trailer just above the fifth wheel.

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In metropolitan areas, the majority of atmospheric pressure cargo tanks today have electronic integrated dome covers (photo 5).

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  • Oval cross-section, nonpressure/low pressure (3 psi);
  • Single-shell aluminum construction;
  • 9,000-gallon capacity;
  • Emergency shutoff located on driver’s side of trailer above the fifth wheel; and
  • Transports flammable/combustible liquids and nonhazardous materials.

MC 307/DOT 407 or Low-Pressure Cargo Tank

The workhorse of the chemical industry, the low-pressure trailer can be easily identified by its stiffening rings and top-center loading domes with rollover protection. It may be insulated or noninsulated. In noninsulated trailers, stiffening rings are visible. Insulated carriers usually have a horseshoe shape, and there are no visible stiffening rings (photo 6).

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  • Circular or horseshoe-shaped cross section;
  • Pressures up to 40 psi;
  • Double shell with insulation most common;
  • One or two compartments;
  • 6,000- to 7,000-gallon capacity;
  • Emergency shutoff located on the driver’s side above the fifth wheel; and
  • Transports flammable/combustible liquids, mild corrosives, poisons.

MC 312/DOT 412

This “heavy” corrosive carrier is characterized by its smaller-diameter circular cross-section in relation to the 307/407 (photo 7). It, too, has stiffening rings; however, loading and unloading are done from the rear, and there normally is a strip of corrosion-resistant paint around this area (photo 8). Obvious rollover protection protects all piping at the rear (photos 9, 10).

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  • Circular cross-section, smaller diameter;
  • Overturn protection at dome and valves;
  • Capacity 5,000 to 6,000 gallons;
  • Transports high-density corrosive liquids;
  • Emergency shutoff on the driver’s side above the fifth wheel; and
  • Transports high-density corrosive liquids such as caustic soda, nitric acid, and hazardous waste.

MC 330/331 High-Pressure Cargo Tanks

In many parts of the nation, people rely on butane or propane for use in everyday living. Without liquefied petroleum gas (LPG) high-pressure cargo tanks (photos 11, 12), their lives would be significantly more difficult.

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These cargo tanks are easily recognized by their rounded, circular ends and white color. Photo 12 shows a “bobtail” pressure cargo tank, so named because of its smaller size in comparison to the tank in photo 10. Loading and unloading are accomplished by an inlet marked “vapor spray” and two outlets marked “vapor” and “liquid” (photo 13). Shutoffs are just above the inlet/outlets (photo 14).

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  • Pressure not <100 or >500 psi;
  • Single-shell, noninsulated tank;
  • Capacity 2,500 to 11,500 gallons; and
  • Shutoffs are above the product inlet/outlets; and
  • Transports LPG, anhydrous ammonia.

MC 338 Cryogenic Liquid Cargo Tank

This rolling “thermos bottle” can be identified not only by its somewhat circular rounded ends but also by a “closet” in the rear of the trailer, where the controls are located. This trailer has a double shell to keep the material inside at the recommended temperature (photos 15, 16). Keep in mind that a cryogenic is defined as any substance with a temperature of -130°F or colder, which can cause extreme tissue damage on contact.

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A design feature of this trailer is its road-relief valve. On occasion, you may receive a call for a trailer that is releasing product. Most times, the release is the result of this valve’s doing its job of alleviating built-up pressure from warming that occurs in transit caused by external ambient temperatures (photo 17). It is also noteworthy that there is ice buildup in the control box caused by product loading/unloading (photo 18).

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  • Insulated “thermos bottle” design;
  • Double shell with relief protection;
  • 8,000-gallon capacity;
  • Vapor discharge from the road-relief valve;
  • Transports liquid oxygen, liquid nitrogen, liquid argon, liquid hydrogen.

Compressed Gas Trailers (Tube Trailers)

This trailer could be called a hybrid. The DOT does not classify tube trailers as cargo tanks. They are actually modified semitrailers with cylinders or “tubes” that have independent pipes and valves permanently mounted to the trailer (photos 19, 20). All of the cylinders contain the same compressed gas product. A very important attribute of these vessels that pose a definite hazard to first responders is their operating pressures, which range from 3,000 to 5,000 psig.

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  • Minimum water capacity of 1,000 pounds;
  • Cylinders range from nine to 24 inches in diameter;
  • All cylinders cascade to a single discharge manifold; and
  • Transports gases under pressure such as oxygen, ethylene, and methane-not liquefied gases.


The next three cargo trailers are referred to as nonspecification cargo tanks, meaning that they are not DOT 49 CFR (Code of Federal Regulations) tanks. However, they must be placarded when transporting hazardous materials.

Dry-Bulk Cargo Tank/Pneumatically Offloaded Hopper Trailer

This trailer (photo 21) has working pressures of 20 to 80 psig. It is unloaded through W-shaped compartments on the underside. Some of these trailers may have a compressor mounted on the rear to assist with unloading. This cargo tank can carry solids as well as slurries. Some of the materials they may transport are fertilizers, oxidizers, and some water-reactive materials.

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Molten Sulfur Tank

This tank (photos 22, 23) looks very similar to an insulated low-pressure cargo tank (MC 307/DOT 407). The tank on this trailer is heavily insulated to maintain the heat of the contents. Steam coils inside the shell keep the sulfur molten. Although it is a nonspecification trailer, it will be placarded and stenciled “Molten Sulfur UN 2448.” Some of these trailers may have a breathing air cylinder on the underside near the ladder. The driver can hook up to this cylinder while working with the product instead of being hampered by an SCBA.

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Asphalt Trailer

This trailer (photos 24, 25) also looks like a nonpressure or insulated low-pressure cargo tank. Like the molten sulfur trailer, it is heavily insulated. However, the asphalt trailer may have burner tubes where the molten sulfur trailer has steam coils for heating the product inside and may carry propane bottles to fuel the burners. This trailer will be placarded “Hot UN 3257.”

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. . .

The process of chemical profiling should not be rushed. With practice, however, you will be able to accomplish it in a relatively short time. As a first responder, it is up to you to become familiar with the tools at your disposal to accomplish chemical profiling. Using the recognition and identification clues for these bulk carriers is an excellent place to start to safely mitigate or control a hazardous material incident before the hazmat team arrives. Remember: What you see does make a difference!


1. DOT statistics from Office of Hazardous Materials Safety, Research and Special Programs Administration, now absorbed into the Pipeline and Hazardous Materials Safety Administration for the year 1998.

ROBERT SHELTON, a 15-year veteran of the fire service, is a member of the Cincinnati (OH) Fire Department, where he is a hazmat specialist/rescue tech on Squad 52. He is also a member of the Hamilton County (OH) US&R Task Force, a fire science and hazmat instructor for Cincinnati State Technical and Community College, a classroom presenter for FDIC, an instructor for the International Association of Fire Fighters, and a contributor to the Fire Engineering Roundtable.

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