The ‘06 Series Highway Tank Trailers: Workhorses of Highway Hauling

CarolinaFireJournal - Glenn Clapp
Glenn Clapp CHMM, CFPS
08/07/2015 -

In structural fire fighting, we continually drill to our personnel the idea that we have to “know our enemy,” with our enemy being the very structure that we are trying to save. In that setting, knowledge of the “enemy” in the form of an understanding of building construction and how structures react in fires helps to ensure our safety and effectiveness in the structural fire fighting realm.


The same concept applies to hazardous materials responses in the highway transportation context in that the highway transportation tank trailers we are most likely to encounter are what we refer to as the ’06 series of tank trailers — the older MC (Motor Carrier) 306 and more modern DOT (Department of Transportation) 406 series of trailers. We should be familiar with the characteristics of such ’06 series trailers so we have a solid working knowledge of them prior to encountering an emergency situation.

The ’06 series tank trailers are truly the workhorses of highway hauling, as they are mainly used to transport the flammable and combustible liquids that power our cars, trucks, airplanes and other modes of transportation — products such as gasoline, diesel fuel, ethanol, jet fuel, etc. This reliance on the ’06 series of tankers is shown by the statistical evidence that of the highway transportation tankers we encounter in hazmat incidents, 57 percent are ’06 series vessels.  Such trailers are technically termed “non-pressure” trailers, however that title is actually a misnomer as MC 306 trailers have a working pressure of three pounds per square inch and DOT 406 trailers four pounds per square inch. While four psi of pressure sounds very minimal, it actually can be hazardous to responders in that four psi acting on the surface area of a dome lid can be substantial if opened incorrectly. 

Let us now turn our focus to the characteristics of an ’06 series tank trailer that can be helpful in identifying the vessel and can add to our knowledge that can be used in the field. The ’06 series trailers are elliptical in cross section, with the extreme edges of the tank projecting to a point even with the outside edges of the trailer tires. A mounted ladder at the front or rear of the trailer leads to a walkway containing dome lids for accessing the interior area of the trailer. The raised area surrounding the walkway serves as overturn protection for the dome covers and also contains the vapor return lines that allow the transfer of vapor to equalize pressure when loading or unloading product and preclude the venting of vapor into the atmosphere. Such trailers normally contain multiple chambers for the hauling of product, however responders may occasionally encounter a single-chamber vessel. 

A solid bulkhead, or a double bulkhead in which two bulkheads are separated by a void space may separate chambers. The ’06 series trailers are usually constructed of aluminum in a single-shell configuration and have an approximate capacity of 9,000 gallons. The detailed information regarding the trailer specifications can be found on the specification plate, which is a metal plate affixed to the lower side of the trailer.

The ’06 series trailers also have safety features that serve to prevent overfilling of the compartments. An electronic system — often referred to as a “Scully System” — is connected to the trailer at the fill terminal. Optical liquid level sensors will then shut the flow of product off prior to reaching critical levels.  Additional measures include visual level indicators in each compartment consisting of a threaded rod with washers located at three levels, each of which corresponds to the gallonage markings stenciled on the side of the overturn protection adjacent to each dome lid. 

Each compartment on an ’06 series trailer is equipped with two valves to lend redundancy to the system. An internal main valve is held closed by spring pressure and is opened by cable tension (older trailers) or air pressure (newer trailers). The main valves are operated by levers or pneumatic valves located in a control compartment — usually located on the right side of the trailer. When the compartment door is opened, a plunger is released that bleeds the air pressure from the trailer brake system to prevent the driver from moving the trailer and opens up the vapor vent system. Closing the door normally forces the main operating lever closed on a cable operated system or closes the main pneumatic valve on an air-operated system, thereby closing any open main valves. The internal valves themselves also have a groove machined into the valve body at the point at which the valve projects from the underside of the chamber to allow the exposed portion of the valve and the attached piping to break away in the event a vehicle under-rides the trailer. On ’06 series trailers carrying flammable liquids with cable actuated main valves, fusible links are incorporated into the system to initiate a valve closure if a fire occurs. Airline fittings that will melt when exposed to flame serve the same purpose on pneumatic systems. 

The secondary portion of the valve system is located on the intake and discharge lines located underneath the trailer in the form of lever-operated valves. To discharge product, the operator first pulls a pin and raises a hinged bar that obstructs the intakes/discharges when in the down position. Doing so bleeds the air pressure from the trailer brakes to “lock the trailer down.”  Discharge hoses are connected to the outlets if gravity off-loading or to a tractor-mounted pump if off-loading product in that manner. The internal valves are then opened, followed by the external valves to allow the product to flow into the receiving tank. 

Now that we have discussed the characteristics and operation of ’06 series tank trailers, let us now turn to the areas we may encounter during hazmat responses involving such modes of conveyance. In instances in which we arrive on scene and an ’06 series tanker is leaking product, the easiest method for stopping the flow of product is by activating the emergency shutoff located on the driver’s-side front of the trailer. On cable activated main valve systems, the emergency shutoff will be a paddle-type or handle arrangement that will close the main valves when activated. On pneumatic systems, a pneumatic valve serves the same purpose. On trailers with cable-actuated systems, the cables will sometimes be pulled taught by vehicular impact or other damage. In these instances, the best course of action to close the main valves that are held open by the action of the cables is to cut the cables with bolt cutters or a similar cutting tool. Other options for stopping the flow of product include operating the cable handles or pneumatic valves in the control compartment or operating the secondary valve handles.

Another topical area that warrants discussion is overturns of ’06 series trailers. Such an overturn often results in product leaking from one or more of the dome lids. If the leak is due to a repositioning of or crimp in the gasket within the dome cover rim, a piece of cribbing can be used to tap on the cover rim to re-seat the gasket. The bolt securing the rim can also be tightened, however we should remember that the parts we are working with are low-strength in nature and can easily fail, leading to a much larger problem. The pressure of the liquid product inside the trailer acting on either of the inherent pressure relief devices in a trailer overturn may also lead to a loss of product. DOT 406 trailers employ two pressure relief devices, consisting of a three psi safety relief valve and a five psi safety relief valve — which is the spring loaded dome lid itself. If we encounter a leak from the three psi valve, we can sometimes strip the outside covering off of a blood pressure cuff and position the bladder between the relief valve and the dome lid latch. Inflating the bladder may then stop the leak. In the case of a leak from the dome lid itself, we can utilize commercially available dome clamps to engage the dome cover rim and tighten down the dome lid latch. Only enough pressure should be used to stop the leak, and opposing wooden wedges can be inserted between the dome lid and the latch to further secure the leaking area. 

Our final area of discussion concerns “drilling” an ’06 series trailer following a rollover incident. An aluminum ’06 series trailer cannot be up-righted in such a situation without first off-loading the product inside to lessen the weight of and reduce imparted stresses on the trailer. As hazmat technicians, we can perform the measures needed to off-load the product — hazmat cleanup companies usually can do the same. To begin the process, we would ground the trailer to equalize the electrical potential between the trailer and the earth to eliminate static arcs. If the product were a flammable or combustible liquid, we would then foam the tanker and surrounding area with compatible foam to suppress vapors and reduce the chance of ignition. A pneumatic hole saw is then used to drill holes in appropriate locations in the tank trailer, after which a “stinger” pickup tube is inserted and the product pumped off.

One may then ask what the appropriate locations for drilling an ’06 series trailer are. The answer is specific to the trailer involved. Within each compartment on an ’06 series trailer, there are baffles to reduce the movement of liquid product.  Each baffle is like a bulkhead, with the exception that it contains a circular hole in the center to allow for some flow of product and for maintenance access. A semi-circular vapor cutout exists in the top of the baffle, with a corresponding liquid cutout in the bottom of the baffle. If the trailer has two black dots on the exterior rear of the tank at the three o’clock and nine o’clock positions, it denotes that there are three-inch drain holes cut in the baffles in corresponding locations. If the trailer were rolled 90 degrees onto its side, we would then have to drill between each of the baffles in each chamber to off-load the maximum amount of product if the drain holes are not present due to the fact that only half of the product can be accessed in the adjacent baffled areas within each chamber. If the drain holes were present, we would only have to drill once in each chamber.

In conclusion, the highway transportation tank trailer we are most likely to encounter in hazmat responses is the MC 306/DOT 406 non-pressure trailer.  These workhorses of highway hauling have specific characteristics that enable the safe transport of flammable and combustible liquids and common product control techniques that can be used to stop or at least reduce the release of product in emergency situations. Hazardous materials responders should be cognizant of and regularly refresh themselves in this information to ensure their safe and effective response to incidents involving ’06 series trailers. As always, stay safe out there and be sure to visit the North Carolina Association of Hazardous Materials Responders website at       

Glenn Clapp is Past President of the North Carolina Association of Hazardous Materials Responders and is a Fire Training Commander (Special Operations) for the High Point Fire Department. He is a Technician-Level Hazmat Instructor, a Law Enforcement Hazmat Instructor, and is a Certified Hazardous Materials Manager and Certified Fire Protection Specialist.
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