We deal with burning ordinary combustibles (Class A materials) quite regularly. As a result, we don’t give them much thought. In fact, the science of extinguishing Class A materials is quite simple. We must apply enough water, often measured in Gallons Per Minute (GPM) to absorb the heat, often measured in British Thermal Units (BTUs), faster than the fire is producing it. Unfortunately, our strategies, tactics, and tasks to achieve this end, are not always as simple as the science. In some cases, we are left to try to protect exposures while the fire consumes all of the available Class A fuels. Flammable/combustible liquids and gases (Class B materials) are not seen quite as much. As a result, we tend to move a little slower and more deliberately when dealing with burning Class B fuels.
Class B fires also involve a little more science than the simple GPMs versus BTUs. This additional science may take the shape of tactics such as foam application or isolation of fuel valves.
Class C materials are energized electrical equipment. With these we frequently move at a pace that can be marked with the changing of the seasons, if we are moving at all. Transformers or trees on power lines often receive little attention from the fire service. We may position an engine to isolate the scene; however, we never apply water to Class C materials.
Combustible metals (Class D materials) are often as mysterious as Big Foot, UFOs, and the Loch Ness monster combined. They include magnesium, sodium, lithium, titanium, as well as others. Often times we have trouble even identifying the presence of these materials. For instance, we may not realize an airbag system inside a vehicle that is burning contains magnesium until the hose stream hits the burning piece of magnesium. What follows is a brilliant explosion of white sparks and flaming pieces of the combustible metal which indiscriminately burn through whatever material on which they may land.
Since water application often intensifies Class D fires, we have to utilize quite a bit of science for proper extinguishment. One option may be to apply Class D dry powder to the material. Remember, dry powder is not dry chemical. Portable dry chemical extinguishers are designed to combat Class A, B or C fires. They are not designed for Class D fire application. Class D fires require a Class D fire extinguishing agent which frequently contains a material specific chemical designed for suppression of the combustible metal near its location.
For example, sodium chloride dry powder extinguishers work well on Magnesium, Aluminum, and Titanium; however, it has little effect on Lithium, which frequently requires another type of dry powder extinguisher such as copper or graphite based. These portable extinguishers are marked by a yellow star or otherwise indicate their intended use for Class D fires.
Although we may initially arrive to a Class D fire in quick attack mode, combustible metals (Class D fuels) may require us to pull out the hazardous materials reference books in order to proceed safely. However different the science may be for each individual Class D material, there is one area of science on which they will all agree to follow. Just like Class A fuels, Class D fuels’ surface to mass ratio is inversely proportionate to the energy required for ignition. A lower surface area of Class D fuels require a higher energy for ignition to occur. Conversely, the higher the surface area, the lower amount of energy required for ignition. Think of a popular Class A material, a wooden log and imagine that instead of wood, it is constructed of a combustible metal. The combustible metal log (just like a wooden log) would require someone to hold a flame to the material for an extremely long period of time for ignition to occur. Therefore, it requires a high amount of energy to ignite the material. If we take our same wooden log and grind it down to sawdust it has a much higher surface area than our log. The sawdust also requires a much lower energy for ignition as we would only need to hold a flame to a sawdust pile for a few seconds to obtain ignition.
Finely ground combustible metal behaves in the same manner. In other words, masses or large blocks of combustible metals will readily conduct heat away from the ignition point — meaning they do not represent high fire risks. However, when machine shavings and other finely ground dust or pieces of combustible metal are present, they can be ignited with sources of relatively low energy.
Don’t try this at home Adding water to burning oil from a chip pan can greatly exacerbate a fire. The following sequence of events results in an “explosion” (deflagration, not detonation): Oil gets so hot that it catches fire all by itself Water is poured into the burning chip pan Water is denser than oil, so it sinks to the bottom of the chip pan. As the water touches the bottom, it is heated above its boiling point and instantly vaporizes. The water vapor expands rapidly, ejecting a fireball of burning oil out of the chip pan and into the air where its surface area increases greatly and combustion proceeds much faster When all the oil burned, there is no more fuel to supply the fire. The experiment pictured was created using a cake of wax. http://wikipedia.org/wiki/Chip-pan
Our case study comes from the recently released NIOSH report dated July 6, 2010 and reviews a line of duty death investigation. As this report summarizes, a 33-year-old male firefighter died and eight firefighters, including a lieutenant and a junior firefighter were injured, as a result of a dumpster explosion at a foundry in Wisconsin. The incident occurred at a recycling dumpster on the grounds of a foundry which produced aluminum sand castings from various aluminum alloys. The 70,000 square foot foundry has been in business for over 73 years and currently employs over 100 people. The metal casting facility melts about 375,000 pounds of aluminum each month. On the evening of Dec. 29, 2009, dispatch reported a dumpster fire at a foundry. The first-in engine arrived eight minutes later to find a dumpster outside the building emitting two-foot high bluish green flames from the open top and a 10 inch reddish orange glow in the middle of the dumpster. A 1.75 inch hand line was stretched and the IC climbing an attic ladder examined the contents of the dumpster. He noticed aluminum shavings, foundry floor sweepings, and a 55 gallon drum in the dumpster. After observing the contents and color of the flames, the IC believed that metal cutting fluids and/or oils were burning. Two foundry employees were on scene and reassured the IC that no magnesium was in the dumpster. Approximately 700 gallons of water was flowed into the dumpster with no affect on the fire which led the IC to implement a foam operation. Approximately 100 gallons of foam solution was applied and just as the IC was calling to discontinue the operation, the contents of the dumpster started sparking then within seconds, exploded sending shrapnel and barrels into the air. The explosion killed one firefighter and injured eight others.
Of the NIOSH recommendations contained in this investigation, there are several worth reviewing. First, NIOSH identifies that high risk sites such as foundries, mills, processing plants, etc. should be pre-planned and updated annually. In this case, a walk through was conducted but no documentation existed outlining the presence of explosive hazards.
Second, all firefighters working in an atmosphere that could potentially contain combustible metals should wear full PPE, including SCBA. Remember that some heavy metals can be fatal if they enter the bloodstream or their smoke fumes are inhaled.
Third, manufacturing facilities that use combustible metals should implement measures such as a limited access disposal site and container labeling to control risks to emergency responders from waste fires. This should be enforced by the fire marshal, code enforcement division or fire chief; however, it is the responsibility of all firefighters to make the enforcement authority aware of the hazard.
Finally, fire departments should ensure that specialized training is acquired for high risk sites with unique hazards, such as combustible metals.
When this incident occurred, it is possible that many of us would have approached the scene similarly. Your author here sure remembers hearing about this call and thought that he surely would have been faced with the same exploding dumpster. How many of us have filled up dumpsters with water with the bottom plugged? We should take note of any signs that indicate the presence of combustible metals, such as strange color smoke or flames. Foam is often the answer when dealing with flammable liquids, not combustible metals. Keep in mind for every 100 gallons of foam (the stuff that comes out of the nozzle) at three percent concentration, there is only three gallons of foam concentrate (the stuff in the buckets). The other 97 gallons is still water. When dealing with combustible metals, if water is not the answer, neither is foam.
Most importantly, we should strive to properly document the presence of combustible metals on our pre-incident surveys. These can be invaluable to the IC during the initial planning at an incident and can serve as a warning for all of us who may be the first to arrive at an incident such as this. Moreover, obtaining this information pre-incident can be much more reliable than the information we may receive from plant workers during the emergency, who are afraid they are going to be in trouble because a fire ignited on their watch (i.e. “There are no combustible metals in there”).
We should all strive to not be a victim of complacency and remember to wear all of our personal protective equipment and self-contained breathing apparatus at every fire, even the small ones in the containers.
Be safe and do good.