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  1. #1
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    Default Scientific Study of PPV

    If you are a regular user of positive pressure ventilation, you will be very interested in reading the scientific study on positive pressure ventilation conducted and published by the National Institute of Standards and Technology. The abstract and link are posted below:

    Abstract
    Fire departments may use ventilation blowers or fans to pressurize a structure prior to suppressing a fire. This pressurization or positive pressure ventilation (PPV) tactic can assist in the venting of smoke and high temperature combustion products and make attacking the fire easier than without PPV. However, this tactic also provides additional oxygen to the fire and can increase the rate of heat and energy being released. PPV has not been characterized carefully enough to establish specific guidelines for optimum use. This study examined gas temperatures, gas velocities and total heat release rate in a series of fires in a furnished room. The use of the PPV fan created slightly lower gas temperatures in the fire room and significantly lower gas temperatures in the adjacent corridor. The gas velocities at the window plane were much higher in the PPV case than in the naturally ventilated scenario. This higher velocity improved visibility significantly. PPV caused an increase in heat release rate for 200 seconds following initiation of ventilation but the heat release rate then declined at a faster rate than that of
    the naturally ventilated experiment.

    http://www.fire.nist.gov/bfrlpubs/NIST_IR_7213.pdf


  2. #2
    Forum Member PattyV's Avatar
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    Very interesting read thanks for that.
    I always frowned on it from the point of view of it giving extra oxygen to the fire, but turns out it helps in the long run.
    "There are only two things that i know are infinite, the universe and human stupidity. And im not so sure about the former."

    For all the life of me, i cant see a firefighter going to hell. At least not for very long. We would end up putting out all the fires and annoying the devil too much.

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    MembersZone Subscriber arhaney's Avatar
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    Thanks for the Info, George.
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    Ahh the science behind the reason! Perfect, thanks George!
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    Very interesting, thanks George.

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    EuroFirefighter.com PaulGRIMWOOD's Avatar
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    Yeah thanks George.

    It is worth noting that the NIST research concluded that -

    1. The heat release from the fire was generally increased by about 2MW for up to 2-3 minutes following PPV - this is quite substantial.

    2. Whilst this HRR is potentially a problem it was reported that after 60-90s this increase in heat release would be directed out of the window and away from the advancing firefighters.

    3. 'Good effects' were general throughout the approach route from a firefighters point of view.

    4. The NIST research referred to other studies ie Stott (UK) and Ezokoye, Lan & Nicks (Austin, Texas) but failed to point out that these studies did note some relevant points -

    a) That PPV can cause ignitions of the fire gas layers (interior rapid fire progress) where the ventilation outlet is inadequate or restricted (Stott).

    b) That PPV may increase the burn rate and place trapped occupants in harms way if they are sited between the ventilation outlet and the fire itself (Ezokoye, Lan & Nicks).

    The NIST research does not appear to directly account for these possibilities.

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    Originally posted by PaulGRIMWOOD
    Yeah thanks George.

    It is worth noting that the NIST research concluded that -

    1. The heat release from the fire was generally increased by about 2MW for up to 2-3 minutes following PPV - this is quite substantial.

    2. Whilst this HRR is potentially a problem it was reported that after 60-90s this increase in heat release would be directed out of the window and away from the advancing firefighters.

    3. 'Good effects' were general throughout the approach route from a firefighters point of view.

    4. The NIST research referred to other studies ie Stott (UK) and Ezokoye, Lan & Nicks (Austin, Texas) but failed to point out that these studies did note some relevant points -

    a) That PPV can cause ignitions of the fire gas layers (interior rapid fire progress) where the ventilation outlet is inadequate or restricted (Stott).

    b) That PPV may increase the burn rate and place trapped occupants in harms way if they are sited between the ventilation outlet and the fire itself (Ezokoye, Lan & Nicks).

    The NIST research does not appear to directly account for these possibilities.
    Paul I think you are the only one to pick up the point of this post. If I were a Chief Officer, if I read this study and understood what I was reading, I would never allow PPV to be used again.

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    MembersZone Subscriber SamsonFCDES's Avatar
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    I will read it as soon as I can get to it.

    I did notice from the thread thought that the heat ouput increase of the fire was alarming after the PPV was kicked in!

    I will read the study, I am curoius to see if they icluded the mising type PPV fans.

    The ones that put a thick mist of water into the PPV airstream to decrease temps.

    Hmmmmm.....
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    MembersZone Subscriber ullrichk's Avatar
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    Originally posted by GeorgeWendtCFI

    if I read this study and understood what I was reading, I would never allow PPV to be used again.
    For fire attack using PPV, I was taught to open up first, then use PPV. If I use some judgement in choosing my ventilation opening, I shouldn't be sending combustion products toward any savable victims. I don't see anything terribly wrong with fire gasses igniting IF it's at the time and place of my choosing and the space where this happens is already untenable or unoccupiable (a residential attic, for example).

    The advantages to advancing firefighters from both safety and operational viewpoints shouldn't be understated, either.

    I'm not sure how well the physical configuration of the test burn describes, say, the corridor/bedrooms in a typical ranch house. Will 60 or so seconds of unvented intensified burning really put savable victims in greater danger if a properly chosen ventilation exit is used?
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    Originally posted by GeorgeWendtCFI


    Paul I think you are the only one to pick up the point of this post. If I were a Chief Officer, if I read this study and understood what I was reading, I would never allow PPV to be used again.
    Question for you George: Wouldnt you want a cool hallway to work in rather than have the gases igniting over your head potentially creating flame spread?

    This scenario looks like a basic apartment style room with no chance of vertical ventilation. Push the smoke and heat back into the room, locate and extinguish the fire. We would do the same with a fog nozzle. With our training in certain scenarious you would use a fog pushing the smoke and heat back into the room while drawing cooler air behind us.

    Have they done similar tests with vertical ventilation?
    Dave
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    Forum Member Dave1983's Avatar
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    No time to read the study now, but let me say this. We have been useing PPV for about 15 years and its a great tool if used correctly. The key is to create you exhaust point first, then once you have a hose line ready for entry then and only then should you start PPV.

    As stated above, its much the same as advancing with a fog nozzle and pushing the heat/smoke as you go. With PPV the effect just happens much sooner.

    Yes, you will intensify the fire some what (for a short period) BUT, with less heat and better visibility afforded by PPV, you can advance and control the fire much quicker the normal.

    As for victims, I guess you could push fire onto them with PPV (though Ive never heard of it). But since the majority of deaths are from smoke and not fire, why would clearing the smoke much sooner then normal not be a good thing?

    The main problem Ive seen with PPV are departments (ours included)that either didnt have a proper exhaust opening, didnt have a line ready or didnt check the attic right away and end up burning the building to the ground or at least burn the roof off. If your going to use PPV you sure as hell better be AGRESSIVE with your interior operations.
    Last edited by Dave1983; 05-12-2005 at 08:17 PM.
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  12. #12
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    Question for you George: Wouldnt you want a cool hallway to work in rather than have the gases igniting over your head potentially creating flame spread?
    I'd rather have a cool iced tea, also. However, I find the results of this study to be very compelling. Have you read it?

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    MembersZone Subscriber arhaney's Avatar
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    After reading the report I would have to say that I don't see any reason to STOP using PPV. If used properly, it's a wonderful tool to have in your toolbox. Will it work at every fire? No, no and no.......But it will work in a lot of them. We use it in about 50-60% of our SFD fires, with great success. It's so nice to have that hallway of room ahead of you just clear out as you make your way towards the seat of the fire. Allows for a much safer and faster attack on the fire in my book.
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    Very interesting reading George, thanks.

    We don't use PPV as a tactic here. Nor do we use a fog pattern during an interior attack. We use a fog stream to vent after knockdown. No fans until the fire is out. It is not a decision to resist change, but rather a team effort that works. There is a few points that would make me reluctant to use this method. Good reading.

    Both rooms flashed over within 10 seconds of each other under controlled conditions. Both occured under 5 minutes. I was told at one time to go find a study that put any time frame on flashover.





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    Originally posted by GeorgeWendtCFI


    I'd rather have a cool iced tea, also. However, I find the results of this study to be very compelling. Have you read it?
    YEEEEESSSS! Both scenarios varied to a degree but as Arhaney said is it compelling enough to stop using PPV? As far as I am concerned this is a closed test of a single room with only a window for ventilation. We just had a large house fire ($700,000 house value) where we used PPV to keep the flame (gasses) from spreading. The fire was ventilated from the basement (semi walkout 5x7 window), then attacked to knock down the bulk of fire from outside - interior crew then took down the rest of the fire. The room had already flashed over, blowing out the window thus venting itself. Hot gasses had made it up to the upper floors and with ppv we were able to push these gases back down to the ventilation point. House damage pegged at $300,000 and $170,000 for contents. (would have been less if doors to rooms were closed at time of fire!!)

    As for the rusults of the test - yes there are heat spikes, but if your dealing with those temps you will have an indirect attack until it becomes manageable. Victims wont need rescuing because they will be crispy critters - as stated in the test rooms flashed over in 5 minutes. Im not interested in going into a room that has flashed over until it is manageable - how do you get it to that point, indirect attack and PPV whether it be by fan or fog!

    Thoughts??
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    Oh Yeah - the main jist is here - its also explained in the graphs:


    The use of the PPV fan created slightly lower gas temperatures in the fire room and significantly lower gas temperatures in the adjacent corridor. The gas velocities at the window plane were much higher in the PPV case than in the naturally ventilated scenario. This higher velocity improved visibility significantly. PPV caused an increase in heat release rate for 200 seconds following initiation of ventilation but the heat release rate then declined at a faster rate than that of the naturally ventilated experiment.

    As I explained above - cooler corridor, of course you expect higher temps at the exit point, thats simple logic.

    So again thoughts on why PPV would be bad? I do realize that there are scenarios out there where ppv is bad, but this is a simple test to explain results.

    Give me some thoughts George!
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  17. #17
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    Give me some thoughts George!
    Fire development in a compartment is mainly defined by the rate of heat release of the fire. More so than the amount of fuel, more so than the size of the compartment, more so than any other factor.

    When you feed fresh air to a compartment fire, especially when you force feed fresh air to a compartment fire, you increase the rate of heat release. This could result in a faster spreading fire, and an increased risk of flashover. In a post flashover fire, you run the risk of spreading the fire rapidly to the next compartment. In a prefire flashover fire, you run the risk of hastening the flashover process and, as the study points out, increasing the risk to the unrescued occupants in upper areas of the structure.

    It is my opinion that there are very few department who utilize PPV in the correct manner enough times to be proficient in its use. The use of PPV by most departments increases the hazard instead of reducing the hazard.

    Anyone old enough to remember high-pressure fog? I am. It was going to be the panacea for the fire service. 30 gpm delivered at 600 psi would put out the fire w/o water damage in a much shorter period of time. Well, the same concept applies. The main hazard of hpf was that it force fed a ton of fresh air to the fire and increased the hazard instead of reducing it. Where is hpf today? Same place PPV will be in 5 years.

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    Forum Member FyredUp's Avatar
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    PPV is not a new idea to the fire service. It has been used on the west coast for many years, I would guess around 30. When used properly it can be an excellent tool. When used improperly it is as dangerous as any other tactic or tool used improperly in the fire service.

    To me right now this is nothing more than a smooth bore / fog, traditional / modern helmet, yellow / red fire truck debate. If this particular piece of equipment does not fit into your method of operation don't use it. But don't try and stop me from using a tool that has been successfully and properly used many times over the last 10 years.

    Just like with any other ventilation method there are things that need to be considered:

    1) Tactical evaluation of the proper ventilation method should take place at EVERY structure fire so the best method can be chosen.
    2) Venting should not be done until hoselines are in place.
    3) The inlets and outlets must be properly placed in consideration of where the fire is, where it is going, and where you want it to be vented to.
    4) The outlet for PPV must be created first before the fan is started.
    5) The outlet must not be bigger than the inlet for proper pressurization when using PPV.
    6) Irregardless of the type of ventilation, natural horizontal, natural vertical, PPV or negative pressure, the fire will usually incerease in size due to an influx of fresh air.

    I am not saying PPV is the only answer. I am saying it is a valuable tool in our ventilation tool box. We use PPV, negative pressure, roof venting and natural horizontal venting. The decision on which type to use is made based on the incident.

    Have I seen PPV fail or make the situation worse? Yes, but every time it was the result of improper application. I have also seen roof vents cut in the wrong place and fire pulled through a structure. Should I ban roof venting because of that? Of course not. Training, training, training, is again the answer for any style of ventilation.

    FyredUp

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    Here Here Fyredup - thats the point I was trying to get across. As I have said previously this is a closed test with very controled factors. It would be nice to see results in an actual structure. Maybe NIST can cook a couple of houses for us!

    George I think I understand where your coming from. Correct me if Im wrong. What your saying is if you have a victim in the vicinity of the fire and you use ventilation and PPV before attempting to retrieve the victim your basically going to be responsible for a well done corpse. Rescue should be initiated prior to ventilation, or at the same time but without ppv. Is that what your getting at??
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    Originally posted by Dave404
    Here Here Fyredup - thats the point I was trying to get across. As I have said previously this is a closed test with very controled factors. It would be nice to see results in an actual structure. Maybe NIST can cook a couple of houses for us!

    George I think I understand where your coming from. Correct me if Im wrong. What your saying is if you have a victim in the vicinity of the fire and you use ventilation and PPV before attempting to retrieve the victim your basically going to be responsible for a well done corpse. Rescue should be initiated prior to ventilation, or at the same time but without ppv. Is that what your getting at??
    Dave
    No.

    What I am saying is that I do not think that PPV is a safe tactic for structural fire fighting for the reasons I cited.

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