The Other Guys:

This issue will conclude our series pertaining to highway transportation tank trailers — in which we have previously covered the ’06 series non-pressure tank trailers, the ’07 series low-pressure tank trailers, the ’12 series of corrosive liquid tank trailers, and the MC 331 series of high-pressure cargo tank trailers. With a discussion of the “other guys” — the highway hazmat transportation trailers that we are less likely to encounter on our roadways — but of which our detailed knowledge is just as important as that pertaining to the more oft-encountered varieties of trailers. This is due to the variety of products carried and the very design features of the trailers themselves. 


As the product within MC 338 series trailers normally exists at a pressure between 25 psi and 500 psi, a pressure relief device is fitted.

The first trailer we will discuss is the MC 338 series of cryogenic liquid tank trailers.  Cryogenics are gases that are liquefied by cooling and are defined by the DOT as having a boiling point of less than -130 degrees Fahrenheit. Examples of cryogenics include liquid oxygen, liquid nitrogen and liquid argon. Due to the nature of cryogenic products and the necessity of keeping them at a low temperature, MC 338 trailers are basically a large thermos bottle on wheels consisting of a double walled tank with a vacuum space between the two tank shells filled with insulation. Such trailers normally hold 8,000 gallons to 10,000 gallons of product and have a “doghouse” cabinet at the rear of or on the underside of the trailer. The doghouse contains intake and discharge piping and valving; and possibly a pump. Oftentimes a series of coils will be seen underneath the trailer that serves to assist with the offloading of product. The product is allowed to flow into the coils, and the ambient temperature heats the product and hence increases the pressure, which allows the product to discharge. 

As the product within MC 338 series trailers normally exists at a pressure between 25 psi and 500 psi, a pressure relief device is fitted. This design leads to a source of possible hazmat calls, as cryogenics normally “boil off” in transit due to an increase in internal tank pressure and subsequent activation of the relief device. This release of product is normal and is often misinterpreted by the public as a dangerous condition. For this reason many MC 338s are stenciled with wording such as “Product Normally Vents.” MC 338 trailers also are required to have a pressure gage mounted in a location such that it is visible from the cab. In terms of response actions taken in incidents involving such trailers, hazmat personnel do not normally perform offensive actions due to the characteristics of the products transported and the tank trailers themselves. Transport company personnel or their contractors will normally handle the control of any releasing product or at a minimum will provide input to hazmat personnel in regard to handling such situations.

Another type of hazmat transportation trailer that is less commonly seen than others is the compressed gas tube trailer. Such trailers are not actually classified as a tank trailer by the DOT, but rather consist of a flatbed type trailer — usually with supports at either end — with two to 20 tube cylinders rigidly mounted. A cabinet at the rear of the trailer houses the piping and valving for the cylinders, which are cascaded together. The working pressure of the cylinders can be up to 5,000 psi. Tube trailers normally carry pressurized — not liquefied — gasses such as helium, hydrogen, and nitrogen. The piping and valving of such trailers are fairly straightforward and there has been resurgence in the last few years in their use.

Dry bulk cargo trailers are also not seen as frequently as some of the other hazmat transportation trailers. Dry bulk trailers are characterized by deep “V” shapes on their underside and are usually bottom-discharged through the use of air pressure. Although dry bulk trailers sometimes transport non-hazardous cargoes such as plastic pellets or grain, they may also transport hazardous materials such as explosives or oxidizers. 

One type of tank trailer that has not been seen to a great extent in the southeastern United States is the “frac tank,” so named for the transport of hydraulic fracturing slurries. Such chemical slurries are pumped under pressure into drill holes to fracture rock and stimulate oil or gas drilling production. Frac tanks are rectangular in appearance and have a rear-mounted axle that allows the front end of the trailer to be raised and transported over the road. When the trailer arrives at the drilling sire, the front end is lowered and the trailer becomes a semi-permanent tank. We may see an increase in these trailers on our roadways as hydraulic fracturing is likely to become more commonplace in central North Carolina. 

Hazmat personnel may also encounter a variation of the DOT 407 series of low-pressure tank trailers and the DOT 412 series of corrosive liquid tank trailers in the form of DOT 407/DOT 412 vacuum loaded tanks — either in tractor-trailer or straight truck form.  Such tank vessels are equipped with a vacuum pump for loading and exhibit the characteristics and safety features of their ’07 and ’12 series brethren. If such a tank or trailer meets both the ’07 and ’12 series specifications, it will exhibit two specification plates with a hinged cover that is latched over the specification plate not indicative of the use. Vacuum loaded tanks are often used by hazmat cleanup companies for the capture of spilled product and can also be used as septic tank pump-out vessels.

We may also find non-specification tank trailers on our roadways that are not built to a certain hazmat specification. One example of such a “non-spec” trailer appears similar to an ’07 series tank trailer in size and also similar to a ’12 series tank trailer due to the existence of external stiffening rings, however the bottom of the tank is constructed in a shallow “V” configuration.  You may ask what product these tank trailers normally transport, and the answer is one that leads to a messy situation in the event of a product release — sludge and by-products from wastewater treatment plants. Another iteration of the non-spec tank trailer is used to transport hot tar and asphalt, appearing similar to an ’07 series tank trailer that is jacketed and insulated; and sometimes is seen with propane burners and associated tanks to assure the viscosity of the product remains manageable.

The final trailer we will discuss as a component of the “other guys” is not actually a tank trailer and is actually the most common trailer on the highway, but is often overlooked as a possible hazardous materials carrier — the mixed cargo or box trailer. One normally views mixed cargo trailers as transporting standard non-hazardous cargoes, however box trailers are just as likely to carry smaller inner containers of hazardous materials. As hazmat responders, we should maintain our situational awareness and product identification skills when encountering such trailers to avoid complacency. We should also remember to approach the rear doors of a mixed cargo trailer with caution due to the possibility of any reaction occurring within and should practice opening the rear doors of box trailers with pike poles to keep us out of the range of harm when opening them in hazmat situations.

As we are concluding our study of highway transportation trailers, we should modify the traditional statement of “knowledge is power” to “knowledge is safety.” The more familiar we are with the vessels transporting hazardous materials on our roadways, the more effective and efficient we will be in resolving hazmat incidents in that arena, therefore ensuring the safety of the public and ourselves. We should also not discount the presence of the “other guys” — those highway hazmat transportation trailers that are not as frequently encountered as others but present their own unique sets of characteristics and hazards.

As always, stay safe out there and be sure to visit the North Carolina Association of Hazardous Materials Responders website at

Glenn Clapp is a past president of the North Carolina Association of Hazardous Materials Responders; and is the emergency manager for the City of High Point and a safety officer for the High Point Fire Department with the rank of Battalion Chief.  He has almost 20 years of fire service and emergency management experience and is a Technician-Level Hazmat Instructor; and is a Certified Hazardous Materials Manager and Certified Fire Protection Specialist.

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