Kernersville Fire Rescue and responded to a residential in the 1300 block of Union Cross Road shortly after 0200 hours on January 14, 2009. Occupants had been evacuated by two civilians returning home from work at a nearby Dell computer plant. First arriving units initiated offensive operations and began primary search to ensure that all occupants were out of the residence.
Less than 15 minutes into initial operations, an explosion occurred resulting in partial collapse of the building. Kernersville Firefighter Jay Coleman and three firefighters from the Winston-Salem Fire Department were caught in the collapse, but were able to self-extricate. Firefighter Coleman suffered minor injuries.
Chief Walt Summerville of Kernersville Fire Rescue reported “as we entered the building and began to ventilate and to flow air by moving hose lines, the heated gases got the air it needed”. Chief Summerville believes the explosion was a backdraft, which was caused by a build-up of smoke in the crawl space of the home.
Explosion Captured on Video
A Kernersville police officer’s dashboard camera caught a burning home as it suddenly exploded. The police car was (appropriately) positioned a considerable distance from the house and provides a view of Side A from the Alpha/Delta corner. Watch the video several times to get a general sense of what happened Then download and print the B-SAHF Worksheet and identify any key indicators that might have be visible in the video.
Post fire video and an interview with Firefighter Coleman are available on the WGHP Fox Channel 8 web site.
At this point, information available about this incident is limited to news reports and video. However, we will be in touch with Kernersville Fire Rescue in an effort to obtain more detailed and fire behavior focused information about this incident. More to follow!
Important Lessons
An initial look at the limited information available about this incident points to several important considerations:
Conditions can vary widely in different compartments. In this incident (like many others) flaming combustion is visible in one location, while extremely under-ventilated backdraft conditions exist elsewhere.
Backdraft can occur in an entire building, one or more habitable compartments, or in a void space.
Backdraft indicators may be pronounced, they may be subtle, or may not be visible from firefighters working positions.
What is a smoke explosion? Is it the same thing as a backdraft or is it a completely different phenomenon? In one form or another I have encountered this question several times during the last week. In one case, I was asked to review a short article about an incident involving a smoke explosion that was submitted to FireRescue magazine. In another case, I was surfing the web and came across the following video titled large smoke explosion close call on firevideo.net. What happened in this incident? Was it a smoke explosion or a backdraft?
For many years, the term smoke explosion was a synonym for backdraft. In fact, if you look up the definition of smoke explosion in the National Fire Protection Association (NFPA) 921 (2007) Guide for Fire and Explosion Investigation, it says “see backdraft“. However, smoke explosion is actually a different, and in many respects more dangerous extreme fire behavior phenomenon.
Smoke explosion is described in a number of fire dynamics texts including Enclosure Fire Dynamics (Karlsson and Quintiere) and An Introduction to Fire Dynamics (Drysdale). However, Enclosure Fires by Swedish Fire Protection Engineer Lars-Göran Bengtsson provides the most detailed explanation of this phenomenon. Paraphrasing this explanation:
A smoke or fire gas explosion occurs when unburned pyrolysis products and flammable products of combustion accumulate and mix with air, forming a flammable mixture and introduction of a source of ignition results in a violent explosion of the pre-mixed fuel gases and air. This phenomenon generally occurs remote from the fire (as in an attached exposure) or after fire control.
In some cases, the fire serves as a source of ignition as it extends into the void or compartment containing the flammable mixture of smoke(fuel) and air. This was the case in Evanston, Wyoming, where two firefighters died as the result of a smoke explosion in a two-story wood frame townhouse (see National Institute for Occupational Safety and Health (NIOSH) Report F2005-13). In other cases, firefighters may unintentionally provide the source of ignition. On 26 March 2008, a Los Angeles City firefighter was killed when he attempted to force entry into an electrical room filled with smoke from a manhole fire in the adjacent street. (see LAFD News and Information). Battalion Chief John Miller, Commanding Officer of the LAFD Arson/Counter-Terrorism Section reported:
This combustible smoke accumulated in the confined area of the electrical room. When Firefighter Lovrien attempted entry into the room, a spark was generated when the composite blade of the rotary saw struck the locking mechanism of the door… Investigators have concluded that unburned combustible gases, from a fire in the electrical vault located in the street at the front of the building, accumulated in the electrical room. These products of combustion reached its explosive limit and was ignited by a spark from the forcible entry attempts
Conditions Required for a Smoke Explosion
The risk of a smoke explosion is greatest in compartments or void spaces adjacent to, but not yet involved in fire. Infiltration of smoke through void spaces or other conduits can result in a well mixed volume of smoke (fuel) and air. Smoke explosion creates a significant overpressure as the fuel and air are premixed and ignition results in a very large energy release. Several factors influence the violence of this type of explosion:
The degree of confinement (more confinement results in increased overpressure)
Mass of premixed fuel and air within the flammable range (more premixed fuel results in a larger energy release)
How close the mixture is to a stoichiometric concentration (the closer to an ideal mixture the faster the deflagration)
Potential Smoke Explosion Indicators
It is very difficult to predict a smoke explosion. However, the following indicators point to the potential for this phenomenon to occur:
Ventilation controlled fire (inefficient combustion producing substantial amounts of unburned pyrolysis products and flammable products of incomplete combustion)
Relatively cool (generally less than 600o C or 1112o F) smoke
Presence of void spaces, particularly if they are interconnected
Combustible structural elements
Infiltration of significant amounts of smoke into uninvolved compartments in the fire building or into exposures
Smoke Explosion and Backdraft
A smoke explosion requires a relatively cool mixture of fuel (smoke) and air within its flammable range to come into contact with a source of ignition. On the other hand a backdraft requires introduction of air to an hot, extremely ventilation controlled fire where the concentration of gas phase fuel (smoke) is high and oxygen concentration is low. Both result in an explosion, but the initiating event and indicators that may be observed by firefighters and fire officers are considerably different.
Have another look at the video and see what you think: Smoke explosion or backdraft? Remember that both of these phenomena can occur in a building, a compartment, or even a small void space. Look closely at the building, smoke, air track, heat, and flame (B-SAHF) indicators. Check CFBT-US Resources more information on extreme fire behavior and reading the fire.
In the Grading the Fireground on a Curve, published in the September issue of Firehouse magazine, Battalion Chief Mark Emery warned of the hazards of assuming that limited volume and velocity of visible smoke indicates a growth stage fire. He correctly identified that compartment fires may enter the decay phase as fuel is consumed or due to a lack of oxygen.
Emery cites National Institute for Occupational Safety and Health (NIOSH) Death in the Line of Duty reports 98-F07 and F2004-14, in which firefighters initiated offensive fire attack in commercial buildings and encountered rapidly deteriorating fire conditions due to changes in the ventilation profile. Concluding the introduction to his article, Emery observes “Unless you know which side of the fire growth curve you are entering, advancing into zero-visibility conditions is really a bad idea”.
I agree with BC Emery’s basic premise that appearances can be deceiving. However, this article points to two interrelated issues. The hazards presented by ventilation controlled fires and the dangerous conditions presented by enclosed buildings. In Smoke Burns,originally published on Firehouse.com I discussed the hazards of ventilation controlled fire and the relationship of burning regime to extreme fire behavior phenomena such as flashover and backdraft. The hazards presented by ventilation controlled fires are compounded when the fire occurs in an enclosed structure (a building with limited means of access and egress). Captain Willie Mora has written extensively on Enclosed Structure Disorientation on Firehouse.com.
BC Emery illustrated how appearances can be deceiving using data and still images from a full scale fire test in a warehouse in Phoenix, Arizona conducted by the National Institute for Standards and Technology (NIST). NIST conducted these tests as part of a research project on structural collapse. However, the video footage and temperature data from this test is extremely useful in studying the influence of ventilation on fire behavior and fire behavior indicators (Building, Smoke, Air Track, Heat, and Flame (B-SAHF)). The full report and video from this test is available on-line from the NIST Building Fire Research Laboratory (BFRL).
As an oxidation reaction, combustion requires oxygen to transform the chemical potential energy in fuel to thermal energy. If a developing compartment fire becomes ventilation controlled, with heat release rate limited by the oxygen available in the compartment, pyrolysis will continue as long as temperature in the compartment is above several hundred degrees Celsius. Pyrolysis products in smoke are gas phase fuel ready to burn. Increased ventilation at this point, may cause the fire to quickly transition to the fully developed stage (ventilation induced flashover). However, if the fire continues to burn in a ventilation controlled state and the concentration of gas phase fuel (pyrolysis products and flammable products of incomplete combustion) increases sufficiently, increased ventilation may result in a backdraft.
I take issue with BC Emery’s illustration of the growth side of the fire development curve as the value side of the cure and the decay side of the curve as the no value side of the curve. Depending on resources, a fire on the growth side of the curve may exceed the offensive fire control capability of the fire department. Conversely, a fire on the decay side of the curve which is limited to a single compartment or series of compartments may be effectively controlled using an appropriate tactics in an offensive strategic mode. However, Emery’s discussion of the more subtle indicators of burning regime that may warn firefighters of a ventilation controlled fire is right on track. For more information on fire behavior indicators and fire development, see Fire Behavior Indicators and Fire Development Parts I and II on Firehouse.com.
