Fog attack
Lance C. Peeples
Firefighter
Webster Groves, Missouri
The results obtained by Commander John P. Farley in “Fog Attack for Ship Fires” (July 1996) seem to contradict the anecdotal reports of experienced structural firefighters (see “Return of the Solid Stream” by Andrew A. Fredericks, September 1995, for example). The problem I see with Farley`s methodology is that two independent variables are introduced into the experiment: the application of water into the overhead flame front and the form in which the water is applied–fog vs. straight stream.
Since the experiment applies only to water in the form of fog into the overhead fire and does not use a straight stream to repulse the overhead flame front, we do not know whether it was the mere application of water to the overhead flame front or the form that explains the results obtained. For the results to be valid, a straight stream must be used to repulse the overhead flame front, and the results must be compared with those obtained when water is applied to the overhead in the form of fog.
I urge Farley to repeat the experiment in this modified form and report the results. Until more definitive test results become available, I would encourage structural firefighters to adopt the following tactics:
Use solid (as opposed to straight) streams flowing an adequate volume of water to meet the fire threat at hand. (A 1516-inch nozzle with 50 psi nozzle pressure on a 134-inch attack line flows approximately 180 gpm and is a powerful tool.)
Apply the water directly to the burning material. (Note: A flaming overhead is the burning material, and a solid stream applied here will extinguish the overhead fire and provide a “raindrop” effect with water drops bouncing off the ceiling and extinguishing the base fire with a minimum of steam production and thermal inversion.)
Ventilate ahead of the line`s advance.
I also wish to comment on “Fighting Fire with Water–Thorton`s Rule and the Exterior Fog Attack: A Perspective,” by John D. Wiseman, Jr. (July 1996). From Wiseman`s discussion, it becomes evident that the theoretically correct (but low sounding) flow rates arrived at in the Iowa experiments were obtained using Lloyd Layman`s theory of indirect attack.
One of the disadvantages of the indirect method of attack, however, is that it cannot be used in an occupied building without scalding the victims. Since modern firefighting practice usually dictates an interior attack coupled with a primary search for possible victims, the low application rates derived from the Iowa formula (and its indirect application technique) become suspect.
If we question the Iowa formula and the indirect method of water application used to derive it, we must substitute another method of calculating required fire flow. One such method is the National Fire Academy formula, where
GPM = Length 3Width
3 3 number of involved floors
In our previous hypothetical examples, the required fire flow using the NFA formula becomes
1. Bedroom 10 feet 312 feet / 3 = 40 gpm
2. Living Room 20 feet 320 feet / 3 = 133 gpm
These fire flows seem to be more consistent with the application rates dictated by an aggressive interior attack (at least in my experience).
In his article, Wiseman seems to criticize 134-inch and two-inch attack lines (and their consequently high flow rates) as being unavoidably responsible for steam production and thermal inversion. However, steam production and thermal inversion can be minimized by
avoiding the use of fog when making an interior attack,
shutting the nozzle off as soon as the fire “darkens down,” and
venting opposite the nozzle`s advance.
In summary:
Exterior attacks are contraindicated in potentially occupied structures.
The Iowa formula was empirically derived using an exterior or indirect technique of water application and, consequently, research needs to be conducted to determine the validity of the formula when an interior attack is used.
The National Fire Academy fire flow formula`s application seems to be more consistent with currently accepted interior attack techniques.
Thermal inversion and steam production can be minimized by avoiding the use of fog, venting, and shutting the nozzle off when the fire is “darkened down.”
(Editor`s note: The author cited the following references in addition to Wiseman`s article:
Royer, K., “Iowa Rate of Flow Formula for Fire Control,” Fire Engineering, July 1996.
Fornell, D.P. Fire Stream Management Handbook. (Fire Engineering Books, Saddle Brook, New Jersey, 1991, pp. 84-90.)
