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Thread: NIST/UL Studies and PPA

  1. #1
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    Default NIST/UL Studies and PPA

    Hijacked from a discussion on another website:

    Just wondering if anyone knows of any departments that have changed their policies on positive pressure attack after familiarizing themselves with the results of the studies.
    The results indicated that strong control of all ventilation is the best way to prevent ventilation induced flashover. PPA seems to contradict this entirely. PPA proponents say we have to find the seat of the fire via outside survey, which IMO, is not always possible to do accurately. Nor can we always accurately determine the extent of fire this way. On top of that, with modern construction and contents, the seat of the fire is not really in a single place. If we arrive to find a ventilation controlled fire (very common due to speed of modern fire development), there is heat and fuel (fire gases in smoke) all around us. It will light up as soon as air is added. This heat and fuel is not limited to the original fire area. It is anywhere in the building that is not protected by a closed door. Modern fires also develop a great deal of heat energy and pressure. If the fan is not strong enough or the exhaust opening is not big enough, PPA fails. How do we know if it will work or not? We don't. We have to try it and see what happens. This comes straight from the experts in the field of PPA. Trial and error is not a sound method on the fire ground. The environment is too volatile and the risks too high.
    All right, I got off track a little bit. I clearly don't endorse PPA, never have for reasons stated. But has anyone else changed their opinion of it?

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    I've not been a fan of PPA for many reasons, many of which are related to being in the Northeast where we have a lot of balloon frame, people regulate their heating/cooling with windows and often close off sections of a home to minimize heating space. This on top of the standard reservations about increasing air flow into the building before fire control. But... In the past few years, much more research and better explanations have me at least considering some of the validity of my arguments. Without a doubt the UL/NIST studies at least on the surface would indicate that increasing air flow to the fire in an attempt to push the heat and smoke out would likely cause an even greater intensification than just opening the door?

    I'm interested to see the testing that looks at PPA with regard to some of the recent study conclusions. On one hand some of teh UL/NIST data shows the increase in internal building pressure due to heat from the fire, which explains why PPA may not have "pushed" fire into voids as many of us had anticipated. I suspect that the fan would prevent bi-directional flow out the entry point, but would require absolute control of the exhaust. Possibly the "cleaner" path to the seat would allow water on the fire quicker countering the increase in intensity from the air influx? I think I'd want to have a very small structure and nearly a guarantee we could be on the eat quick before trying this, though many places do this as a matter of routine already. Saw some video of LAFD (?) talking about closing the entry door, so that blows the PPA out there, though my understanding was that they were not a big proponents of PPA as many thought they were.

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    I think the cleaner path to the fire is what so many really like about PPA. But that doesn't necessarily translate to quicker knockdown. The advance may be quicker once started but it may start later, due to time spent gauging effectiveness of the fan. Plus, the guy setting up and monitoring fan could have been used to assist in stretching line. Seems like a wash, at best, to me as far as time factor. The time it takes to get a charged line from the entry door to the fire area in a typical house fire is just not that long. The line may need to be opened almost immediately upon entry in an open floor plan type house with fire in an unenclosed section. If it is taking a long time to get line in place, maybe the department's training time and energy would be better spent drilling on zero/low visibility line advance.

    As far as bi-directional flow back out through entry point, this is one of the key points targeted by proponents of PPA. They tell us we must monitor entry point for a period of time to ensure fan and exhaust point are working together properly to vent air. More wasted time, by the way. If not working properly, fan use must be discontinued or delayed until exhaust point is made larger. Way more wasted time. Some even suggest cutting wall area below window with chainsaw to accomplish this. Guess what I'm going to say? It has to do with time, which is not our friend at a working fire. While all of this is being done, the charged hoseline is still on the front lawn waiting for the go ahead. Then I'm supposed to believe that PPA allows faster advance and knockdown?

    Proponents of PPA can't seem to agree on the size of the exhaust point. I've seen anywhere from one half the size of the entry point to three times the size of the entry point. That's quite a range (factor of six). And why is entry point compared to exhaust point the important ratio? What about the size of the fire area, amount of smoke generated and the amount of heat energy created? Shouldn't those things factor into the formula? None of them can be quantified by an outside survey.

    Quite simply, IMO, the risk vs reward equation just doesn't hold up with PPA. Volatility of the fire environment being one of the main reasons. Another being the time factor. By the time a fan is set up and results evaluated, and possibly re-evaluated after enlarging the exhaust opening, the fire could have been knocked down with an aggressive interior attack.

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    We run into this a bit with a bordering FD who seem to get the fan in place pretty quick. All our guys get a little antsy when we hear the fan start. We used to carry fans as if they'd be primary tools (on the tailboard) but as you note, we never hand enough hands to put them in place early, as we focused on the stretch and search with limited crew. Thankfully too, as we had very limited training for the potential for harm vs. good that the fan can bring. Controlling outlets was always an issue with so many open windows or closed off second floors, leading me to believe that the only places that really took to PPA were where the houses relied oon central air HVAC systems minimizing the windows as a factor(Southeast and Southwest?).

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    To me the danger from improper placement and use of PPV during PPA far outweigh the benefits that supporters always claim.

    My most common use of PPV? During live fire training I will use it to clear smoke and help the next training fire get going faster!! Of course shutting it off before the attack crew enters.
    Crazy, but that's how it goes
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    Quote Originally Posted by RFDACM02 View Post
    We run into this a bit with a bordering FD who seem to get the fan in place pretty quick. All our guys get a little antsy when we hear the fan start. We used to carry fans as if they'd be primary tools (on the tailboard) but as you note, we never hand enough hands to put them in place early, as we focused on the stretch and search with limited crew. Thankfully too, as we had very limited training for the potential for harm vs. good that the fan can bring. Controlling outlets was always an issue with so many open windows or closed off second floors, leading me to believe that the only places that really took to PPA were where the houses relied oon central air HVAC systems minimizing the windows as a factor(Southeast and Southwest?).
    __________________________________________________ __________________________________________________ ____________

    Interior layout and status of interior doors are definite wild cards, IMO. I can't say that either factor would matter much because I just don't know. But I don't like not knowing.
    RFDACM02 likes this.

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    What a joke of a thread.

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    Quote Originally Posted by RedEye View Post
    What a joke of a thread.
    What awesome and amazing insight. Thank you for adding so much to the topic and with such clarity.
    Crazy, but that's how it goes
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    Quote Originally Posted by FyredUp View Post
    What awesome and amazing insight. Thank you for adding so much to the topic and with such clarity.


    You know how fire chiefs are, just have to let them crow

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    Quote Originally Posted by RedEye View Post
    What a joke of a thread.
    Some day maybe you'll be lucky enough to be a firefighter. You'll need to understand stuff like this should that day come.

    Once again you are displaying the attitude that has probably contributed to your inability to land a job with one of the 20 departments you've tried over the past 8 years.
    FyredUp and RFDACM02 like this.

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