No matter what the product, odds are very good that it got to the shelves at the store via a commercial vehicle. In fact, national statistics show that nearly 70% of commercial freight activity involves transport by truck. With this much activity, so comes the increased potential of serious accidents and fires involving these vehicles. Nationwide, fatal fires with trucks make up more than 17% of all fatal highway fire incidents, a frequency that is six times greater than that of other motor vehicles (see Photo 1). Handling these fires can be difficult, but focusing on the basics of compartment fire behavior can help in handling these incidents.
These commercial vehicles can be broken down by class. There are eight classes, based upon gross vehicle weight rating (GVWR) of the vehicle. For our use, it is easier to keep them broken into three broad categories:
Light Duty: Commercial vehicles weighing between 6,000 – 14,000 pounds GVWR
Medium Duty: Commercial vehicles weighing between 14,001 – 26,000 pounds GVWR
Heavy Duty: Commercial vehicles weighing over 26,001 pounds GVWR (see Photo 2)
There are very little similarities between each truck, as many of them are custom-built for usage as ordered. But there are common inherent hazards that are important to deal with when facing a fire within one of these vehicles. We will look closely at each of these based upon the systems they interact with.
One of the primary hazards rescuers will encounter will be the electrical systems that power the vehicles. The most common system will be the 12-volt system, but it will look much different than the automobile 12-volt system. Commercial vehicles will utilize multiple batteries to provide enough power to turn the engine over and provide voltage for the rest of the vehicle. Batteries will be wired either in Series or Parallel. Series wiring involves sending a path of current through each battery in a single direction. In this case, disconnecting the negative post closest to the frame will break the loop and shut down the electrical system. Parallel wiring involves sending paths of current through each battery concurrently. In this case, the rescuer must disconnect all of the negative connections to power down the system (see Photo 3).
The primary fuel source for these types of vehicles will be diesel fuel, but many other types are still in service. Diesel fuel has four properties that make it popular for use: it is combustible, and provides a more efficient “burn” in the cylinder when it is atomized; it removes heat from the fuel system components; it acts as lubrication for fuel injection parts; and the viscosity of the fuel limits gelling, aiding in flow in colder weather, as long as water can be kept from contaminating the fuel system. Next to the fuel tank in newer vehicles is the Diesel Emission Fluid Tank (DEF Fluid). This material is a water-based Urea solution that is injected into the exhaust as it moves through the engine, where it vaporizes and decomposes to form Ammonia and Carbon Dioxide, reducing emissions from the vehicle (see Photo 4). While this fluid is reported to be non-flammable, it has proven to be corrosive to some metals. Fuel/fluid fires and releases should be treated as a volatile mix of flammables and corrosives and should be treated accordingly.
Liquid fuel loads for these vehicles do not only sit in saddle tanks on the sides of the vehicle; based upon application, there may be other powered components on the vehicle. For example, tractor trailers that haul perishable goods carry them in refrigerated trailers, or “Reefer Units,” so to speak. It is important to note the debate over flammability of the refrigerant used in these systems: mainly R134A (see Photo 5). This has replaced R-12 as the primary refrigerant in motor vehicles. The properties of concern to this material include Hydrogen Fluoride and Carbonyl Halides when it is heated. However, it is generally considered to be “non-flammable.” That being said, some studies have proven that the potential for combustibility will increase when dealing with elevated temperatures (fire, for example). So do not be surprised when a hostile event results from this material being exposed within the fire.