The ’12 Series Highway Tank Trailers: A Possibly Corrosive Situation

This discussion is a continuance of the series pertaining to highway transportation tank trailers, in which we have previously covered the ’06 series non-pressure tank trailers and ’07 series low-pressure tank trailers. image

In this installment of our series, we will focus on the ’12 series of corrosive liquid tank trailers. The ’12 series of tank trailers consists of the older MC (Motor Carrier) 312 and newer DOT (Department of Transportation) 412 specifications of trailers.  While the ’12 series of trailers are not as prevalent on our roadways as the ’06 or ’07 series trailers, the very nature of the products carried in such vessels and the specific characteristics of the trailers underscore the need for our close study of such modes of conveyance.

As their name implies, the ’12 series of tank trailers are utilized to carry corrosive materials —defined as substances that can degrade metal or skin. Although acids such as sulfuric acid come to mind when we think of corrosives, bases — also known as caustics — that are on the high end (greater than seven) of the pH scale can be equally as harmful as acids if not more so and can also be transported in ’12 series tankers. The ’12 series of tank trailers generally carry 3,000 to 7,000 gallons of product and are rated at working pressures of up to 50 psi, although most ’12 series trailers are rated at working pressures in the 25 to 35 psi range.

The ’12 series of tank trailers can be easily identified in the field by their specific characteristics, as dictated by the properties of the materials transported therein. To begin with, the cross-section of ’12 series tank trailers is even smaller than that of their ’06 and ’07 series counterparts due to the density of products carried. Using the density of sulfuric acid as an example, the specific gravity of sulfuric acid is 1.8, yielding a density of approximately 15 pounds per gallon. As the density of gasoline — one of the most common products transported in ’06 series tankers — is approximately six pounds per gallon, if a ’12 series trailer was constructed to equal the diameter of an ’06 series trailer the unit would be grossly overweight for use on even modern roadways.  The ’12 series of tank trailers are circular in cross-section, and the density of the products transported dictates the use of stiffening rings to provide increased rigidity. The stiffening rings may not be visible on trailers that are insulated and have external cladding, leading to a smooth outward appearance. Such trailers are also usually designed with one internal chamber; however multiple chamber examples also exist. Each chamber will have a dome lid and other devices on top contained within a “crash box” that provides overturn and sometimes spill protection. On single-chamber trailers, the crash box is normally located at the rear of the unit. The exact devices and layout vary between the MC 312 and DOT 412 specifications. In addition, the interior of ’12 series tank trailers is lined to prevent the corrosive products carried from compromising the structure of the tank itself.

On MC 312 series tank trailers, a dome lid is provided within the crash box for access and top loading of product. The dome lid may be of double-lid construction with a smaller lid positioned on top of the main lid. A pressure relief device — and possibly a rupture disc — is installed for the release of excess internal pressure and a pressure inlet is provided to allow the tank trailer to be pressurized with air or nitrogen to force product through a siphon tube and associated discharge. The discharge tube normally extends over the top of the crash box and often terminates at a level even with the bottom of the tank, with quarter-turn valves used to control the flow of product. The discharge tubes of MC 312 series tank trailers may be constructed of PVC piping to reduce compatibility concerns.

The newer DOT 412 series of trailers exhibits some similarities with the older MC 312 series in the crash box area in that a pressure relief device, pressure inlet, and siphon tube with associated discharge piping are provided. The DOT 412 series of trailers differ from that point, however, as PVC piping is not utilized, a single dome lid is normally fitted, and an internal valve similar to that fitted on ’07 series trailers is employed. The internal valve is opened through the use of hydraulic pressure generated by a remotely operated jack and an emergency product shutoff is located at the driver’s side front of the trailer in the form of an extension of the hydraulic line that operates the internal valve terminating in a shear nut that can be snapped off with a tool to release the hydraulic pressure to affect a closure of the internal valve — or multiple internal valves if multiple chambers are provided. A quarter-turn valve is also present in the discharge tube for redundancy of product control.

One area of particular concern in the arena of ’12 series tanker product control is that of the sump present at the bottom of the trailer tank between the rear axles. The sump is usually affixed to the tank by bolts and a leak emanating from that area is oftentimes the result of a loose sump or gasket/liner failure. An additional area of concern also arises from the corrosive nature of the products carried and the vapors generated by them. This nature of the possible products encountered often necessitates the use of vapor-protective hazmat suits with self-contained breathing apparatus utilized for respiratory protection (e.g. Level A PPE) while performing emergency response operations in the hot zone.

The ’12 series of tank trailers may also be constructed as vacuum vessels — both tractor-drawn and straight truck varieties. Such vessels are equipped with pumps utilized to create a vacuum to allow products to be drawn into the tank through connected hoses. These vessels are often used by hazardous materials cleanup companies for the collection of spilled or released products and may be constructed to a dual DOT 407/DOT 412 specification.  Such “dual spec” tankers are provided with two specification plates and a hinged cover that is flipped and latched to cover the specification plate that is not indicative of the use.

As we have discussed above, the MC 312/DOT 412 series of highway transportation tank trailers present unique response considerations due not only to the design of the vessels themselves but also the very nature of the products transported therein. By familiarizing ourselves with the characteristics of and products transported by such tank trailers, we can ensure the adequacy of our response capabilities to these type incidents that are “corrosive” in nature. 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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