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Part 4: A Matter of Time

Presented by:
Dr. John DeHaan

The time factors of any fire are often the most critical factors an investigator had to establish and yet they are the most difficult. Establishing the fuel load is often pretty straight-forward by direct examination of the remains, interviews with owners, tenants, or fire-fighters, or review of inventories, bills of lading or even surveillance tapes, the investigator can usually figure out what fuels were there. Establishing possible ignition/heat sources may be more of a problem. Barring the discovery of a broken Molotov cocktail, evidence of a lightning strike or massive electrical or mechanical failure, sources of heat may be small, subtle, or missing completely. To rule out various ignition mechanisms, it will be critical to identify the first fuel ignited (even as a series of hypothetical scenarios) and to establish time factors for each.1 We normally think of "time" in a fire as: "At what time did the fire start?" But in the absence of witnesses or surveillance videos we normally cannot answer that question and we have to establish other factors related to time. These factors may include: time to ignition, time to involve the first fuel ignited, time to spread to other fuel packages, time to flashover, and time to suppression. Each one of these "times" is important to the overall reconstruction of the event and requires different information to be available to the investigator, some from the scene and some from background knowledge. Let's look at each of them.

The time to ignition is the time delay between contact of the ignition (heat) source with the first fuel to be ignited and the moment combustion is established. (Even here, we have to establish what we consider to be combustion is it first open-flame or is self-sustaining glowing (smoldering) combustion enough?) In its simplest form, time to ignition is dependent on (1) the nature of the firstfuel ignited and (2), the rate at which heat is supplied to the fuel. Some fuels ignite faster than others given the same heat source and the faster the rate of heat transfer, the faster the fuel will heat up. The nature of the fuel is a really broad concept­from its chemical and physical properties to its physical state. Its chemical and physical properties include its thermal conductivity (k), density (p) an its thermal capacity (c) (often taken together as thermal inertia­[kp c]), plus its melting point, boiling point, and color. Its physical state, if it is a liquid fuel, is whether it is in the vapor state at ambient starting conditions, and if it is a solid fuel, whether it is finely divided (thermally thin like crumpled sheets of paper, or thermally thick like a stack of phone books). It would be nice to generalize and say that the larger the ignition source the shorter the ignition time (and in fact, if all other conditions are the same, tig is inversely proportional to q"2) but that is not true in the general sense. A match (with a heat release rate ­HRR­ of 50 W) will ignite paper faster than a cigarette with its 5 W HRR. A very small amount of energy released in an electric arc (even a low voltage one) will ignite a fuel/air mixture if it is correctly mixed and in contact with the arc, but it will not ignite most solid fuels. The important features of the ignition source are: heat release rate, energy content (and therefore temperature), and whether radiant, conductive, or convective transfer processes predominate. The radiant heat output of a source goes up in proportion to its temperature raised to the fourth power (T4) so the hotter it is (the larger T is), the much higher the heat output is. The glowing coal of a puffing cigarette has a temperature of around 1800°F but its HRR is only 5 W and its radiant and convective transfer are so low that direct contact between the coal and the fuel to be ignited is going to be necessary for ignition. The radiant heat output from a fire is more significant than its convective heat output if the fuel to be ignited is in front of the fire instead of directly over it.

For a solid fuel to be ignited by an external source, enough heat has to be transferred to it so that its surface temperature reaches its ignition temperature. This is where [kp c] comes in. If a fuel has a low thermal conductivity (like urethane foam), heat losses to its interior are going to be minimal, so the surface temperature goes up quickly, faster than if the same amount of heat were applied to a fuel with a higher conductivity. This means the fuel will ignite in a shorter time and flames will spread across it more quickly. The same is true for a fuel with a low density or thermal capacity, where a given amount of heat will cause a larger increase in temperature than the same amount of heat would produce in a fuel with a high density or thermal capacity. So to establish time to ignition we need to know: the nature and condition of the first fuel ignited, the nature, HRR and temperature of potential ignition sources, and their position and distance from the fuel. Most fuels will reach smoldering combustion before (i.e., at a lower temperature) than flaming, so time to ignition has to consider whether a glow or a flame is needed to advance the fire. Another factor that must be considered is the duration (life) of the ignition source. Ignition requires not only enough heat in the source but enough contact and prolonged enough contact between fuel and source, so that adequate energy is transferred. An electric arc typically lasts only a fraction of a second, a burning match only a few seconds, a burning piece of toast just 3 minutes, a cigarette about 15 minutes. The question to be asked is: "Is this enough time to ignite the fuel involved?"

Once the first self-sustaining combustion is established, a new time frame becomes important­how long does it take before the first fuel package is fully involved, or at least involved enough to cause spread to other fuels. This will depend first on whether smolder or flame is occurring. Obviously combustion rates for smoldering are on the order of 0.1mm/second (0.25 in/min) where flame spread rates can be 0.01m/sec (0.4 in/sec) for solid fuels and as high as 10m/second (40 ft/sec) for premixed vapor/air mixtures. Variables include the chemical and physical characteristics of the fuel involved (such as [kp c]), the physical state of the fuel, and the geometry, shape and orientation of the components. There are too many variables to discuss here but the issue is to realize that there will be a time delay, ranging from a second to many hours between initiation of first combustion and the time at which the fire is big enough to spread. An appreciation of time factors has to be gathered from first-hand observations of test fires and also from data on furniture tests such as those in James Quintiere's book and on the BFRL.NIST web site. Such furniture tests show that it takes about three minutes for a modern sofa to become completely involved from one point of ignition by open flame, and about ten minutes to burn to completion. These times are dependent on the size of the compartment, where heat reflected from nearby walls or ceilings causes a heat feedback effect that increase fire growth rates.

What of spread from the first fuel package to others in the vicinity? Here the same concepts hold as described earlier, but on a larger scale. The question is: "How big is the fire from the first fuel package (its HRR)?" (Note that all fires at this stage have basically the same flame temperature whether it is a pool of gasoline, a wooden desk, or a polyurethane foam sofa burning.) The HRR determines how tall the flame plume is (from the Heskestad relationship) and how intense the radiant heat flux is at various distances (from the relationship: q" = Xr Q/ 4p d2 where Xr is the radiant heat fraction, Q is the heat release of the fire and d is the distance between the fire and the receiving surface). We also need to know the position of other fuel packages in relation to the first fire. Much of the initial heat energy of a fire is in the convective plume so fuels immediately above the fire will be ignited soonest (heated by direct flame contact or convective transfer). Fuels alongside the first fire will be heated predominantly by radiant heat so they will not be involved until the fire is quite large. In considering fire spread, two factors that get overlooked are the possibility of "drop-down" ignition and the role of wall coverings. When adjacent walls are painted plasterboard, there will not be much contribution, but when the wall is easily ignited wood paneling, there can be extremely rapid horizontal and vertical flame spread. Ignition of furnishings can then be triggered by pieces of burning paneling falling or simply by increased radiant heat. Separation between packages is very important. One rule of thumb is that a 1 MW fire in one chair will not spread to another chair more than 1 m (3.3 feet) away by direct radiant heating. A good example of this would be a scene where one chair has burned completely away leaving adjacent chairs scorched, but otherwise undamaged, since the first fire was simply too low in HRR and too far away to bring about ignition. Fires can also spread by ignition of carpets or other materials at floor level.2 Once again, times of spread to adjacent furnishings are dependent on the size of the compartment, where heat reflected from nearby walls or ceilings causes a heat feedback effect .

Time to flashover is finally being recognized as the critical factor it is in many investigations and reconstructions. Flashover is the transition to the point at which all of the fuels in the room are fully involved, heat fluxes and temperatures are at a maximum throughout the room, the most intense fire is no longer linked to location of fuel packages but instead is taking place where the ventilation-driven mixing of air and pyrolysis gases are best. Such fires are not survivable. Time to flashover is often the same as time to detection since fires are detected in many cases only as they go to flashover, as windows shatter, flames vent from doors and windows, and the shudder and rattle of the turbulent combustion makes their presence known to people inside and outside the building. The time required for a fire to progress to flashover is controlled by the HRR of the fire, the size of the compartment, the nature of walls and ceilings (as far as thermal losses) and the area and height of any ventilation openings. The "average" (bedroom) size room requires a fire of, say, 600800 kW to go to flashover. That can be created by a single over stuffed foam chair fully involved after ten minutes, a PU foam mattress partly involved after three minutes, or within seconds of half a gallon of gas being poured on the floor and ignited. The sooner additional packages can be involved, the sooner this trigger (critical) HRR can be reached, and the shorter the time to flashover. This is often the role of an accelerant3 Applied in moderation, the HRR of a "pour" of a flammable liquid may not be sufficient to trigger flashover but it serves as a big match. Lasting only a minute or two, it manages to ignite enough of the furnishings in the room that the critical HRR is achieved and flashover occurs sooner and more surely than would occur if the fire were to grow from one item to the next by ordinary routes of propagation.

It was once thought that if a fire in a room went to flashover in less than 15 minutes, it could not have been accidental in cause and had to have had help (i.e., an accelerant) Unfortunately this is not true. For the reasons outlined in a previous column, a fire of sufficient HRR to trigger flashover can occur within a very few minutes if a single item of modern furniture is ignited by open flame. Numerous experiments by this author and others have demonstrated that an 8 x 8 x 8 compartment, with a moderate fuel load, can go to flashover in less than 3 minutes from open flame ignition of modern furniture where larger rooms (12 x 16 x 8) can be fully involved in less than five minutes if the first fuel package is suitable. It should be noted that the time difference between an accelerated fire in a room and a non-accelerated fire involving the furnishings may be a matter of a very few minutes, and it is rare that witnesses can help determine the actual time of ignition to that sort of precision. The investigator has to be careful before deciding that a "fire was so fast, it had to have been accelerated."

Assessment of all furnishings in a room (including carpets and wall finishes and their potential heat release rates) must be carried out. A reconstruction of the arrangement of the room (on paper if not in actual fact) is critical to this assessment. This may be done by witness statements, physical remains, burn indicators, or pre-fire photos or even videos.

Other times may be important to fire reconstructions: time to smoke detector or sprinkler activation, time to evacuate, etc. But a major one is time to suppression, defined as the time between ignition and the time at which the fire is suppressed. This unfortunately is very hard to establish because it is not reported in run reports or even radio communications transcripts, so it is often an estimate from fire crews. (Only interviews with fire crews can reveal what was burning on arrival and on entry (not often the same time), how the fire was attacked, and how long it took to bring under control and extinguish (at least in critical areas). There are those who think that "The fire did so much damage, it had to have been arson." This ignores the time factor. Any fire that burns uncontrolled long enough can result in tremendous damage, even complete consumption, no matter how it was started. Despite the difficulty of extracting this information, it really is critical to accurate reconstruction and should be on the investigator's checklist for time really is the essence (of thorough investigation).

1 Testing of hypotheses as the key to the scientific method as it is applied to fire investigation will be discussed in a future column.

2 See previous column about the ignitability of new generation synthetic carpets

3 A future column will address the role of accelerants.

Dr. John DeHaan has been a criminalist for some 32 years. He has worked at county, State, and Federal forensic labs. He is a native of Chicago and his Bachelor of Science degree in physics was from the University of Illinois at Chicago. He has been involved with fire and explosion investigations for over 30 years, and has authored dozens of papers on fires, explosions, and their investigation and analysis. He is probably best known as the author of the textbook Kirk's Fire Investigation (now in its Fourth Edition). His doctorate (in 1995), from the University of Strathclyde in Glasgow, Scotland, was on the Reconstruction of Fires Involving Flammable Liquids.

He is a member of NFPA, and served on its 921 Technical Committee from 1991-1999. He is a member of the IAAI and serves on its Forensic Science Committee. He holds a diploma in Fire Investigation from the Forensic Science Society (United Kingdom) and one from the Institution of Fire Engineers (U.K.). He is a Fellow of the American Board of Criminalistics in Fire Debris Analysis and a member of the Institution of Fire Engineers. He retired from the California Department of Justice in December 1998 and is now the president of his own consulting firm, Fire-Ex Forensics, Inc., Based in Vallejo, California, where he now serves as a consultant in fire and explosion cases all over the U.S., Canada and overseas.

 

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