MFA #18: Like Herding Bees with a Whip - Why you shouldn't even try to push fire

Continuing resistance to the idea of applying water to a structure fire from the exterior comes from those who believe that a fire attack via an interior route is inherently better.  It's not.  We all used to think so, and it was the approach taught and practiced for decades, but now we know better.  The NIST and UL studies have shown that flowing water into a compartment containing fire only makes things better, making the most direct route to the fire the best choice.  

I was once a staunch proponent of the “attack from the unburned side” theory of hoseline placement, believing that the air entrained by a hose stream would assist with the movement of products of combustion out of the structure.  This approach was bolstered by the (mis)understanding that attacking the fire head-on would propel that same toxic mix deeper into the building, worsening the situation for any occupants.  In my last blog (MFA #17: The "Steaming Victims" Issue - How water flow makes everything better at http://community.fireengineering.com/profiles/blog/show?xg_source=a...), I did my best to explain why exterior streams do not cause interior conditions to deteriorate.  This post will look at the same topic from the opposite direction.  That is, I will attempt to show why initial water application from the interior offers no advantage, all things being equal.

Before we get too far, though, we need to come to an understanding about the concept of “pushing fire”.  I have to admit I once harbored fears that flames could be sent deeper into a structure by an ill-directed fire attack; that the fire would actually spread and involve other areas.  Upon further reflection, especially in light of the fire dynamics research that failed to demonstrate such an effect even when they tried, that idea now seems far-fetched.  Still, it was a fundamental belief that shaped my behavior, and that of many other firefighters/incident commanders.  Also, despite this profound revelation, for many of us (or at least me), “fire” in this context referred to not only flames, but all of the related badness: smoke, steam, and heat.  

While one of the most important findings of the fire dynamics experiments was the consistent and repeatable improvements to interior conditions when water was applied to a fire in a manner that did not obstruct the exhaust path (i.e., straight stream, directed upward, and held still), there were several instances of the opposite effect when that technique was not followed.  The Spartanburg, SC test burns showed a dramatic, if brief, fireball emanating from a window when a wide fog stream was directed into another, nearby window that had been releasing hot smoke.  In a side-by-side demonstration at the same burn series, a straight stream directed into a second floor window is shown to darken down a fire-filled room, while a fog stream played into another room on the same floor pushes flames down the stairwell to the first floor (https://www.youtube.com/watch?v=P1kjCf_8vFo&index=2&list=PL... ).  In the Governors Island experiments, the movement of even a straight stream across a venting second floor window caused a similar “back flow” of products of combustion out of an open door a story below.  

These examples beg the question: if positioning a hose stream so as to cover a venting window could redirect the outflow of heat and gases, would not the same principle work in the reverse?  In other words, could a hose stream operated from the interior be used to augment the exhaust of those same toxic substances, creating a cool-air flow path that actually protects the advancing firefighters?  To find evidence about the effects of hose streams on interior conditions, we need to go back over a decade to the findings of a group of firefighters who performed experiments without the benefit of computer models, real time thermocouple data capture, or government funding.  Jerry Knapp and his associates at the Rockland County Fire Training Center in Pomona, NY designed an apparatus to measure air flow from water streams.  Their laboratory consisted of a modified cargo container, into or out of which water could be directed through window- or door-sized openings, with resultant air flow measurement accomplished using a repurposed HVAC instrument.  An additional experiment used an array of hanging strings to indicate the directions of air movement as hose streams were directed past.  The results of these tests appeared in an article in the February 2004 issue of Fire Engineering (http://www.fireengineering.com/articles/print/volume-157/issue-2/fe...)

There were several enlightening phenomena demonstrated by these real-world researchers.  One was the significant air flow that could be accomplished with a hose stream directed out of a window, a practice commonly referred to as “hydraulic ventilation”.  Straight streams, whether from a solid bore or combination nozzle, were shown to exhaust over 500 cubic feet per minute (c.f.m.), which increased almost 50% if the hose stream were moved around to cover more of the opening.  A stream set on fog pattern, positioned so as to nearly fill the exhaust opening, moved about 10,000 c.f.m., roughly equivalent to that of a ventilation fan designed for such a purpose.  Considering that a “typical” room measuring 10 by 12 feet contains less than 1000 cubic feet, this technique can quickly refresh an interior atmosphere.  (It also shows that we can certainly pull fire and its components with a hose stream, though it would be best to first complete extinguishment as the nozzle operator is in the new flow path.)

Regarding our current topic, the applicable finding from these "street" experiments involved the air flow caused by aiming the hose stream into a room, rather than out of an opening.  Even with an exhaust ahead of the nozzle and an air intake behind, directing water into a room with a straight stream caused no significant air movement.  On the other hand, the use of a fog stream lead to violent turbulence, with air circulating back onto the hose team.  This effect contradicted our collective experiences with the operation of fog nozzles outdoors, where the passing air is effectively unidirectional and forward, providing enough protection and cooling of the hose crew to allow for the close approach to burning structures.  Indoors, though, despite an open door beyond the hose stream, this “churning” would bring products of combustion back onto attacking firefighters. 

The explanation for this finding offered by the NY firefighters was the “overpressurization” of the room by the fog stream, exceeding the capacity of even an unusually large (door-sized) exhaust port, and causing the very substances we are attempting to avoid to instead turn on the nozzle operator with a vengeance.  Their recommendation was to utilize straight streams as the only interior nozzle pattern due to the lack of such an ill effect.  Now, over ten years later, we await the results of the latest UL Fire Attack Study, in which the use of interior hoselines will be subjected to the sophisticated and accurate measurements to which we have recently become accustomed (and with which, I was glad to see, Jerry Knapp is again a participant.)  Could there be demonstrated some combination of stream pattern and/or water flow that allows for fire attacks from the interior to again become the preferred route?  Maybe, but until its data is collected, analyzed and reported, no current evidence exists that supports that practice, no matter how strongly we may believe in its value.

As always, no one is suggesting that every fire is best fought through a window or door from outside.  Sometimes the quickest route is through the structure.  Also, the initial water application, while effective at reducing temperatures and reversing fire growth, is usually insufficient for complete extinguishment, so interior hoseline placement will still need to be undertaken immediately in most instances.  Finally, there are plenty of other vital tasks that require us to enter as soon as we are able, not the least of which is search and rescue.  To that can now be added hydraulic ventilation, a tactic for which I hope you have a new appreciation.

MJC

The author can be reached at markjcotter@comcast.net