Under Pressure:

Our discussion this issue will once again expand upon our series examining highway transportation tank trailers. We will extend our previous coverage of the ’06 series of non-pressure tank trailers, the ’07 series of low-pressure tank trailers, and the ’12 series of corrosive liquid tank trailers to include coverage of the MC 331 series of high-pressure cargo tank trailers and also straight “bobtail” units of the same specification.


MC 331 high pressure tank trailer.

An additional idiosyncrasy of MC 331 units is the presence of a stencil stating “QT” or “NQT” on the tank itself.

As you have probably already noticed, the MC 331 series is unlike the previous tank trailer specifications in that the specification is solely a Motor Carrier (MC) specification without a corresponding Department of Transportation (DOT) specification. Although MC 331 tank trailers and bobtail units only comprise approximately 10 percent of the tank trailers/vessels on our roadways and are involved in roughly one percent of highway transportation hazmat incidents, the facts that the contents are under high pressure and also pose significant hazards to responders and citizens alike drive our need to discuss this particular mode of transport.

The MC 331 series of tank trailers and bobtail units are normally utilized to transport liquefied compressed gasses — gasses that are liquefied through the application of pressure — such as liquefied petroleum (LP) gas, anhydrous ammonia and sometimes chlorine. Other liquefied compressed gasses may be carried in MC 331 series units, however LP gas and anhydrous ammonia comprise the vast preponderance of commodities transported. Such vessels are also easily identified by their rounded or hemispherical ends — such a shape is the optimal configuration for containing high pressures — and round cross-sections; and can carry from approximately 2,500 gallons of product in bobtail configuration to 15,000 gallons of product in trailer form. MC 331 tanks are of single-shell, non-insulated construction and are rated from 100 psi to 500 psi working pressure. The top two-thirds of MC 331 units are painted white or a reflective color in order to minimize the heat absorption of the vessels.

Just as with the ’06,’07 and ’12 series of tank trailers we have discussed in our series on highway transportation, MC 331 units have distinctive safety features designed to ensure the safe transport and transfer of product. MC 331 units have internal valves — also normally equipped with an excess flow feature to prevent an unchecked mass release of product — that are usually cable operated — although responders may occasionally encounter pneumatically operated internal valves in the field. Such internal valves will be equipped with one emergency shutoff for units of 3,500 gallons capacity or less — usually located at the driver’s side front of the tank or a similar visible location — or two emergency shutoffs for units in excess of 3,500 gallons capacity — normally located at the driver’s side front and passenger’s side rear of the tank. The emergency shutoffs will be of a “T” handle, paddle, or other similar configuration that can be operated to close the internal valves for cable-operated internal units or an air valve that can be opened to release air pressure and hence close the internal valves for pneumatically operated units to cease the release of product. Fusible links — in cable-operated systems — or plugs — in pneumatically-operated systems — are also installed in both the emergency and normal operating control systems and are designed to fail at approximately 250 degrees Fahrenheit to ensure that any flow of product is stopped in the event of a fire. Additional safety features include pressure relief valves in the uppermost portion of the tank that are designed to relieve internal pressures of 110 percent or greater of the working pressure of the tank, loading gates — also called Dixie Gates for the company that developed them — that block the intakes/discharges and lock the trailer brakes to prevent movement of the unit when the gates are repositioned to allow access to the intakes/discharges, and break-away external plumbing with shear points to reduce the likelihood of a release in a collision.

Differentiating Between Bobtail
and Trailer MC 331U

Bobtail units are most often seen in the field as LP gas delivery trucks used to transport LP gas to the end user to be stored in residential or commercial tanks for heating or other similar purposes. Bobtail units may also be utilized for the transportation of anhydrous ammonia, however such a use is not seen as frequently as the aforementioned LP gas deliveries. Trailer MC 331 units are normally used to transport larger quantities of product over greater distances than bobtail units. Bobtail units are often equipped with a remote emergency shutoff in the form of a box with an emergency button worn on the belt of the person offloading the product. Depressing the button shuts the internal valves and ensures that the person at the end of the discharge hose offloading the product does not have to return to the truck to affect a shutoff.

Hazmat response personnel should also become familiar with the technical characteristics of MC 331 units in non-emergency modes of operation, such as the routine transfer of product. MC 331 units are usually equipped with either a liquid pump or an air compressor for the offloading of product. Liquid pumps are usually mounted underneath trailer-type MC 331 units, while bobtail units normally have the pump mounted at the rear. A hydraulic power unit driven off of the tractor or truck engine normally powers such pumps. Air compressors are sometimes used to offload product by creating a pressure differential between the offloading and receiving vessels. In addition to the internal valves, MC 331 units also have external manually operated secondary valves that are designed as “checks and balances” to the internal valves. To offload product, the operator first opens the internal valve with a cable-operated lever or pneumatic switch as appropriate, opens the external secondary valve, and transfers product into the receiving vessel. Just as with pressure rail tank cars, MC 331 units have both liquid and vapor lines and valves. Liquid lines protrude into the lower liquid space of the tank, while vapor lines protrude into the upper vapor space of the tank. A spray fill — sometimes called a vapor spray — line is also present on some MC 331 units to allow for a decrease in the amount of time required to fill a tank. As we all know from filling SCBA cylinders, the heat generated by compressing a product is our enemy in terms of achieving quick fill times due to the fact that we do not want to overheat and compromise the vessel containing the product. Spray fill lines allow operators to pump liquid product into a line that terminates at approximately three-fourths of the height of the vessel. The liquid then rains down the sides of the tank, cooling the vessel in the process. All valves are required to be labeled as to their nature — liquid, vapor, vapor spray, etc. on MC 331 units, which is of great value to responders operating in emergency situations.

Additional fittings and devices on MC 331 units may include pressure gages, temperature gages, and product level gages. Such gages are normally recessed into the tank itself to reduce the likelihood of shearing and associated product loss during a collision or rollover. In any MC 331 emergency, responders need to know the pressure inside the vessel to determine if the pressure is within the working pressure limit of the vessel — found on the trailer or unit specification plate — and if the pressure is increasing, decreasing or remaining static. Temperature gages allow responders to track the temperature of the product contained within the tank and in the absence of a functioning pressure gage allow responders to determine the internal pressure by noting the temperature and then utilizing the Handbook of Compressed Gases to derive the corresponding pressure. Product level gages usually take the form of magnatel gages in present times, which use a float and magnet to communicate the liquid product level in percent of full capacity through the pressure vessel while not compromising it. Responders may still occasionally encounter rotogages — especially on older bobtail LP gas delivery MC 331s — that consist of a pointer with bleed valve and associated scale. To determine the level of liquid product, the operator opens the bleed valve with the pointer in the highest position and then rotates the pointer downwards until liquid product flows out of the bleed valve. The level of liquid product then corresponds to the level indicated on the dial. In addition, MC 331 units may also have sample fittings in the same recesses as the gages to allow for the sampling of product.

An additional idiosyncrasy of MC 331 units is the presence of a stencil stating “QT” or “NQT” on the tank itself. QT indicates that the tank has been quench tempered by heating the metal up and then cooling it rapidly with a cooling media to increase the inherent tensile strength. NQT indicates that the tank material was not quench tempered. While quench tempering does increase the tensile strength of the material and therefore allows tanks to be constructed lighter by using thinner tank material, it also makes the material slightly more brittle when exposed to high temperatures. Quench tempering also results in a tank that is more susceptible to corrosion, which is why NQT tanks are allowed to transport all grades of anhydrous ammonia and LP gas, while QT tanks are only allowed to transport anhydrous ammonia with minimum moisture content of 0.2 percent by weight and noncorrosive LP gases.

As we have discussed above, the MC 331 series of high-pressure highway transportation tank trailers and bobtail units present unique response considerations due to the nature of the products contained within and the specific characteristics of the units themselves. Although MC 331 series units are very stoutly constructed and are unlikely to fail in an accident or collision, hazmat response personnel should be thoroughly acquainted with the characteristics and operating principles of such vessels and the properties of the products transported therein so that if we ever encounter an incident involving an MC 331 trailer or bobtail truck we will be ready to safely and competently address the situation.

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

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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