Thread: Backdraft??????

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    Default Backdraft??????

    Ok, let's hear from everyone......Had a fire last night in a double wide mobile home (OK, to be PC... manufactured home ). Fire started on an outside wall and worked up into the eave and then into the attic, The attic is devided into two halves by a pair of plywood walls in the attic. Fire was rolling all the way down the back half of the home, and the front half of the attic was only showing smoke from the eaves. Entry was made into the house two or three times for a quick search and to locate any fire in the house, as it turned out, all the fire was in the attic, except where the roof had caved in prior to our arrival. So we're on the front porch getting ready to go back in when suddenly, whoosh......boom! Enough force to blow out a section of the roof in the front half of the attic (about 8' X 12'), it also blew down a small section of sheetrock inside a bedroom. The concussion knocked one firefighter back a couple of feet and knocked the helmet off of another firefighter. No glass was blown out in the house, the front door was open and it slammed shut from the force. Here are few pics to show what happened:

    This pic shows point of origin.
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    Top corner of the attic where the plywood structure was breached by the fire, I'm guessing that this was the ignition point for the backdraft, or whatever it was. Note that both layers of plywood were burned through.
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    Last edited by arhaney; 05-06-2005 at 12:47 AM.
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    Not sure if this could have been a backdraft, maybe just a change in pressure or flashover. I would think a backdraft would have blown more out.

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    End result, a large hole in the roof! Nothing like that warm fuzzy feeling you get when you hear a boom and get pelted by plywood, shingles, nails and trim.
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    Oaky..maybe. Roof picture helped! Any thing inside that might have gone boom and went airborne?

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    Btw, the insulation on the side that blew out had a very light burn right on top from the flashfire, some fire was seen venting. So what do you guys think? I feel very lucky in that we were not inside at the time!
    Last edited by arhaney; 05-06-2005 at 01:01 AM.
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    Originally posted by Frosty42
    Oaky..maybe. Roof picture helped! Any thing inside that might have gone boom and went airborne?
    Nope, didn't even get warm enough to melt any plastic in the room below the large hole. Very little heat in the entire house when I first made entry, but the smoke was very low and thick. Not rolling or anything. The release definitely came from the attic.
    Last edited by arhaney; 05-06-2005 at 01:04 AM.
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    Another shot to give everyone a better idea of the size of the hole.
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    Based on that, I would say yes. Although most "mobile tenaments" are older and use smaller dimensional building materials, this one looks like it had more beef to it. It did take some force to blow the roof out like that.

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    Talk about a "Oh S#!t" moment, one of our guys thought that I was inside, he got pretty excited! But I was walking up on the porch right behind him when it happened, he busted down that door quick! I think we all needed a change of undies after that one!
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    What made the small explosion occur? Was an opening made in the ceiling just prior to the event? Did the smoke look oxygen deprived? (Yellowish, brown structural smoke). But being that it was night time I guess that could be hard to see. You said the fire was confined to a portion of the attic? I suppose if the attic had been buring for sometime before it was noticed and you guys arrived, there could have been a small backdraft, especially if the ceiling was breeched and allowed oxygen to enter the attic. Given the volumes of CO that is given off in confined spaces, ie. walls and attic, it could have been the culprit of the explosion. Just some thoughts, based on what I have seen of the incident.

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    erics99, Nothing was done on our part to start the explosion. The only thought I had was that when the plywood on the end burned through. (second picture) The attic was split in two halves, front and rear, the fire was in the rear half. There was a 2 or 3" air gap between the plywood that ran the length of the attic. The only place that this was breached was in the very end as shwon in that picture. Thanks everyone in advance for your thoughts, ideas and comments!
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    Originally posted by arhaney
    erics99, Nothing was done on our part to start the explosion. The only thought I had was that when the plywood on the end burned through. (second picture) The attic was split in two halves, front and rear, the fire was in the rear half. There was a 2 or 3" air gap between the plywood that ran the length of the attic. The only place that this was breached was in the very end as shwon in that picture. Thanks everyone in advance for your thoughts, ideas and comments!
    I see. Well then, possibly even a smoke explosion. The gases (CO) in the front half of the attic could have been within their flammable range and the flames from the rear lit them off when the plywood burned through. At any rate, probably not something that happens to often! Good thing nobody was hurt.

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    Plywood panels are renowned for 'smoke explosions' (FGIs - Fire Gas Ignitions). Where they are gradually heated they give off large amounts of light (often white) smoke that may accumulate in parts of the structure. This smoke is highly explosive!

    I am still trying to imagine your description of the split between to halves of the attic inline with where the fire breached the plywood panel. It looks like the fire came through from the attic to the exterior? Are you saying there a breach of the plywood in the attic to the uninvolved area? If so then yes this was probably a smoke explosion.

    There are three basic requirements that must be met before a smoke explosion can occur; they are:

    1. A contained smoke layer that consists of enough unburned pyrolyzates that places the mixture within its limits of flammability. For example, the flammability limits for carbon monoxide are 12.5% and 74%, for methane the range is between 5% and 15%, (SFPE, 1995, 3-16).

    2. To ignite the flammable mixture an ignition source is needed; there is a minimum amount of energy that will ignite the layer.

    3. The last requirement is enough oxygen to support combustion.

    NB; It is also possible for super-heated gases to ignite (auto-ignition), without a source of ignition being introduced, as they exit and mix with an air supply at various points, doorways, windows, stair-shafts etc. This form of Fire Gas Ignition may also be termed a smoke explosion if the ignition burns back into the compartment.

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    WHITE SMOKE CONDITIONS for some interesting images of dangerous smoke accumulations.

    To rule out a backdraft I would look closely at the smoke conditions at the point of entry (door) .... was the door open at the time of the event? Were there any indications at all of pulsating smoke movements or did the speed of the air-flow in at the lower halve of the door suddenly escalate just prior?

    This is a good warning of how smoke accumulations in adjacent compartments (sometimes are some way from the fire) may ignite with some ferocity. You had it on a small 'training' scale here but it will stick in your mind. The ventilation of the roof prior to entry may have prevented this event but that is an ideal approach made with hindsight so no criticism of your handling of this fire is meant. Neither would have been too comfortable on that rood when it blew!

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    Sounds like a smoke explosion to me to.

    I had my self a little smoke explosion experience bout a year ago.
    We were fighting a fire on the second floor of a 9-apartment building with 3 apartments on each floor. We had searched all the apartments and ended up in the apartment to the left on the second floor trying to put out the fire. The thing is all of the apartments exept the middle one on the second floor had been vented and in that one all the windows were closed and so was the door. After a while we got short on brething-air and decided to move out and just when we got out of the left apartment the one in the middle blew. The door came loose and sent me and my partner flying down the stairs. After a 20 foot flight I ended up between my partner and the door at the bottom of the stairs. Neither of us got hurt to bad but that was a real wake up call... Had we just opened up a window in that apartment the explosion had hopefully been avoided....
    Fredrik

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    Paul, Thanks for the link on the smoke.
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    Look on the bright side. At least you weren't standing on the roof when and where it decided to blow out.
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    Paul, I think it is unlikely that a backdraft could occur from autoignition of gasses at a building opening. How do you prevent a backdraft? By opening up the top. The backdraft is also unlikely to occur when the roof is vented.

    Most backdrafts that I have been involved in were predictable when the incident was reviewed. Example: Three-story taxpayer. Common basement. Well involved fire in the basement. Companies arrive and are conducting primary on the upper floors. Team is attempting to locate fire. W/o warning, loud explosion, side wall of the building comes down into the alley. Huge amount of yellow smoke. Fire goes to free burning stage and causes extensive damage to basement and first floor.

    Investigation determined, during review of the incident with the fire fighters, that the smoke explosion occurred less than a minute after a FF opened a rear exterior basement door. But at the time, there was strong consideration of a gas leak or an explosive device (by the FD, not by me).

    Don't forget that overpressures as little as .5 psi can cause significant structural damage.

    There is little doubt that this incident involved a backdraft. Methinks that the introduction of air may have occurred when a piece of the drywall on the ceiling or wall fell out due to thermal degradation. I am glad that no one was hurt.

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    George .... for the record - a 'smoke explosion' is not a 'backdraft'. They are two different phenomena associated with rapid fire progress and generally demonstrate different causal factors.

    Backdraft - is air to fire.
    Smoke explosion - is ignition source to accumulated fire gases.

    In simple terms ....

    Ths situation sounds like the second of two events.

    Auto-ignition at an opening can most certainly lead to another type of Fire Gas Ignition (FGI) (type of smoke explosion) and we can repeat this event in training using small 'dolls house' demonstration units. The actual causal factor here often involves an exterior wind gust that blows the flame off of the opening just prior to an ignition occurring inside the compartment. It is also possible for the auto-ignition to burn back into the compartment as gases therein head back towards a lean mix.

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    I'm not sure that we are not talking about a cultural thing here. I think we use the two terms synomously her in the US (NFPA 921, 2004 ed. pg. 151, Kirk's 5th ed. page 48). I understand and agree with what you said about them, though.

    As far as FGI, I know what it is and I understand the phenomenon. But can you have a FGI that causes the type of structural damage that is being discussed here? I think not, due to the fact that, to cause the damage, the event must be confined. I know it can burn back in, but I'm not sure it can initiate a deflagration.
    Last edited by GeorgeWendtCFI; 05-06-2005 at 08:16 PM.

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    Take your point George. It is a cultural definition but recent definitions here in Europe are being formalised by FIRENET to differentiate the THREE TYPES OF EVENT as a basis for ISO standards.

    Yes a 'smoke explosion' can be extremely destructive, sometimes moreso than a backdraft. However, I would agree with you that 'burn-back' through an opening is rarely so.

    The importance of differentiating events from a firefighting perspective is that smoke explosions often occur some way from, or adjacent to, the fire compartment, whereas backdrafts normally involve the fire compartment itself.
    Last edited by PaulGRIMWOOD; 05-06-2005 at 08:45 PM.

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    Wow, Great Thread Boy's!

    Who says there is no valuable discussion at FH anymore? I still learn something new every day. Good links Paul.

    And AR, good job on the fire and the pictures. The damage looks pretty minimal. A startling occurance to be sure, but it looks like a good job from here. I suspect an underwear change was required by all.
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    Alan, I know we talked about this and I am kind of thinkin about Paul's smoke explosion, (I know that it and backdraft are used synonymously here)but in my experience that a backdraft usually puffs smoke and that you were in the build that had to introduce air and move it around. Also there are noted high heat conditions and as you did say LOTS of smoke. The bottom line is that you all were able to walk away and go home unhurt, albeit with stained underdrawers.
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    Originally posted by GeorgeWendtCFI
    I'm not sure that we are not talking about a cultural thing here. I think we use the two terms synomously her in the US (NFPA 921, 2004 ed. pg. 151, Kirk's 5th ed. page 48). I understand and agree with what you said about them, though.

    As far as FGI, I know what it is and I understand the phenomenon. But can you have a FGI that causes the type of structural damage that is being discussed here? I think not, due to the fact that, to cause the damage, the event must be confined. I know it can burn back in, but I'm not sure it can initiate a deflagration.
    While some books here in the US use them synomously, I have also seen sources that define their differences. For example, a guy name Dave Dodson who teaches a great class on reading smoke clearly defines a smoke explosion and a backdraft. Perhaps he took the term from Europe, but obviously they are distinct events so I'm not sure why the US uses the terms synomously.

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    What you have to realize is that, as great as NIST is, there are Eurpoean countires that are years ahead of the US in terms of fire research.

    Paul, I agree 100%.

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