The ‘07 Series Highway Tank Trailers:

Backbone of the Chemical Transport Industry

CarolinaFireJournal - By Glenn Clapp
By Glenn Clapp CHMM, CFPS
10/26/2015 -

Our discussion in this issue will continue our series begun in our previous installment regarding highway transportation tank trailers. While we took a look at the ’06 series non-pressure tank trailers — the most numerous types of tank trailers seen on our roadways — in our last discussion, in this iteration of our series we will concentrate on the ’07 series of low-pressure tank trailers.


Pictured represents an 07 series low-pressure tank trailer.

The ’07 series consists of the older MC (Motor Carrier) 307 and newer DOT (Department of Transportation) 407 trailers. While not as common on our highways as the ’06 series tank trailers, the ’07 series trailers are still very numerous and have several unique characteristics that warrant our study of such vessels before we encounter them in the response arena. The ’07 series of tank trailers are normally utilized to transport flammable or combustible liquids, such as mild corrosives, poisons and even non-hazardous commodities. Such tank trailers have capacities in the area of 6,000 gallons to 7,000 gallons, and although they are termed “low-pressure” their working pressure of up to 40 psi can present considerable dangers to responders.

Identifying the ‘07

The ’07 series of tank trailers can be easily identified in the field by their specific characteristics. Of course the specification or “spec” plate located on the trailer frame can always be consulted if it is safe to do so, as the trailer specification will be visible at that location. The cross-section of ’07 series trailers is smaller than that of ’06 series trailers, mainly due to the higher densities of many of the products transported as compared with the usual hydrocarbons carried in ’06 series units. Whereas the outside edges of ’06 series trailers extend to the outer edges of the trailer dual or “super single” wheels/tires, the outside edges of ’07 series tank trailers extend only to about halfway of the width of the trailer wheels/tires.

The ’07 series trailers are normally either circular or horseshoe-shaped in cross section and may be of insulated double shell construction with a smooth exterior finish or may be non-insulated with structural stiffening rings visible. Most ’07 series tank trailers are constructed as one single chamber; however multiple chamber versions may be encountered.

The most visible distinguishing characteristic of ’07 series trailers is the presence of the dome area — or areas in multiple chamber units — on the top of the tank area. In the case of single chamber versions, the dome area is usually located amidships and a side ladder is provided for access.

The dome area is provided with a “crash box” structure providing spill protection to catch small spills and overturn protection to prevent damage to the dome area in the event the trailer overturns. The dome area will normally have multiple devices and fittings present, which may be integrated with or separate from the dome lid itself. One such fitting is a pressure relief device, which when activated allows excess pressure to escape when a specific pressure is reached. A vacuum breaker is also normally provided, which functions in the opposite manner of a pressure relief device in that the breaker activates to prevent a destructive vacuum from forming within the tank itself and possibly imploding the vessel.

A third type of device that is usually present within the crash box is an air inlet, which is simply a connection and valve that allows an air line to be connected to the trailer to pressurize the tank to aid in the offloading of product. Multiple devices may be combined in what is termed a “christmas tree” configuration. Newer DOT 407 tank trailers will also have vapor recovery systems much like those seen on ’06 series tank trailers. When the internal tank valve is opened, a vapor recovery valve is also opened that ensures vapors from the product being transferred to or from the tank trailer will not be released to the atmosphere. A vapor recovery hose is attached from the loading or receiving vessel to the vapor recovery system on the trailer and any vapor will be captured in the appropriate vessel.

The Cleanout

Another fitting that will normally be present on the top of an ’07 series tank trailer is a three-inch cleanout. The cleanout consists of a three inch section of pipe with a lugged cap affixed and is provided for just the purpose that the name describes — cleaning out the trailer. It also has an ulterior purpose. If an ’07 series tank trailer is overturned and the trailer valves cannot be used for offloading product the cleanout can be used for that function. Just as in instances when we must offload product from an ’06 series tanker through fittings or drilling the tank prior to uprighting to avoid structural failures, we may do the same with ’07 series tank trailers.

To perform such an offloading, we employ the use of a product recovery valve — commonly termed a “Betts Valve” — that is a commercially available device that allows hazmat responders and/or cleanup company personnel to control the removal of the cleanout cap and subsequent removal of product. The product recovery valve consists of a hat-shaped gasket, gasket backplate (two halves), valve body, internal yoke assembly with external handwheel and end fitting; and attached product removal pipe and valve.

To install the product recovery valve, the gasket is pulled over the cleanout cap and onto the pipe itself. The two halves of the gasket backplate are then installed around the gasket and joined with the provided screws to compress the gasket. Set screws in the backplate are then tightened to “bite into” the gasket. The valve body and yoke/handwheel assembly can then be disassembled and the body installed onto the gasket/backplate assembly using a supplied band clamp. The yoke is then visually lined up with the cleanout cap lugs and inserted into the valve body with the end fitting secured to the valve body with a second band clamp. The handwheel is then turned counterclockwise until the cleanout cap is opened. Product can then be offloaded through the product removal pipe and valve.

Internal Valve

Just as with the ’06 series tank trailers, ’07 series trailers have an internal valve — or valves if the tank is multi-chambered — inside the tank itself and an external hand-operated valve or valves for the offloading of contents. The internal valve system utilizes hydraulic pressure to open the valve, which is normally held closed by spring pressure. Small pipes to the internal valve, or valves to provide the required hydraulic pressure, connect an externally mounted hydraulic jack. A handle is inserted into the proper receptacle on the jack, which is then pumped up and down to pressurize the system and open the valve or valves. A bleed valve on the jack can be opened to reduce the system pressure and close the valve or valves.

The system piping is also extended to the driver’s side front of the tank trailer where it terminates with a shear nut that acts as the emergency shutoff for the trailer. The shear nut has an integrated annular groove machined in that allows an emergency responder to snap the nut off using a tool, which then bleeds the hydraulic pressure off of the system and closes the internal valve or valves. The external valve or valves can be of several designs, acting as a redundancy in the system.

As we have discussed above, the MC 307/DOT 407 series of highway transportation tank trailers differ markedly from ’06 series units. Although we are slightly less likely to encounter ’07 series tank trailers than ’06 series tankers, numerous ’07 series trailers are plying our highways and roadways every day. By familiarizing ourselves with the characteristics of and products transported by such tank trailers, we can ensure that we are ready to competently respond to a highway transportation incident involving one of these “backbones of the chemical transport industry.”

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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