Low-visibility fires
where heat and heavy smoke conditions obscure the seat of the
fire.
The conditions (heat, smoke, and fire gases) associated with
these fire scenarios typically do not prevent initial entry into
the fire compartment. However, the time it takes to maneuver
within the fire space to locate and directly attack the seat of
the fire does present a significant threat, due primarily to the
state of the fire. Uncontrolled, these fires could continue to
grow rapidly, potentially resulting in flashover conditions.
ONBOARD STUDY
The Naval Research Laboratory (NRL) initiated a study onboard
the Navy`s full-scale fire test ship ex USS
SHADWELL to
determine the benefits and drawbacks of using an offensive fog
attack vs. a traditional straight-stream attack to extinguish a
growing/ steady state class A fire threat within the confines of
a ship.
One of the first objectives for this study was to develop a
realistic fire threat that would provide a great challenge to
attack teams attempting a traditional direct attack using a
straight stream. The fire threat also had to be repeatable to
allow for evaluation and comparison of the test results
independently of fire-related variables.
Pre-fire preparations include ventilating the fire for 10 to
15 minutes to meet pre-flashover conditions. Ventilation was then
secured. The fire threat developed represented a growing/ steady
state class A fire that had multiple fire source locations
dispersed about the steel fire compartment, which created
near-flashover conditions in the fire space. Flames rolling
across the overhead and upper layer temperatures in the range of
500° to 600°C (932° to 1,112°F) typified the fire threat
used.
The fire compartment volume was approximately 2,600 cubic
feet, and the fuel load consisted of three wood cribs, six
particleboard panels, and 18 newspaper-filled cardboard boxes.
The wood cribs were initiated by n-heptane pool fires. To
provide further realism, obstructions were placed between the
fire sources and the entry point to the fire compartment. This
forced the attack teams to advance well into the fire space
before being able to apply water directly on the seat of the
fires (see Figures 1 and 2). For each test, the attack team
entered the fire compartment via the joiner door (JD) 2-16-0
with a single 3.8-cm (1.5-in.) handline equipped with a 360-lpm
(95-gpm) variable fog nozzle and conducted a direct
straight-stream attack or an offensive fog attack.
Offensive Attack
For the offensive fog-attack method, the attack team entered
the fire compartment approximately 1.2 to 1.8 m (4 to 6 ft),
took a crouched position, set the vari-nozzle to the medium fog
pattern (60 degrees), and discharged the stream upward at a
45-degree angle into the flaming overhead in front of them. A
series of two or three short bursts, two to three seconds in
duration, was generally sufficient to achieve fire knockdown.
After fire knockdown, the attack team adjusted the spray pattern
to a straight stream and advanced to the seat of the fire to
complete final extinguishment.
Straight-stream Attack
For the straight-stream attack, the attack team immediately
advanced to the fire sources and applied agent directly on the
seat of the fires. A short-burst nozzle technique was also used
to help minimize excessive water usage and steam production.
Test Results
To compare the test results, measures of performance to
evaluate heat, steam, and fire threat experienced by the attack
team, as well as the water usage were developed. The five
measurements determined to best demonstrate the effectiveness of
a given attack method were as follows:
1. wood crib temperatures (thermocouples),
2. average of overhead temperatures (thermocouples) in the
fire space,
3. upper vs. lower (calorimeter) total heat flux in the fire
space,
4. average of upper layer temperatures (three strong
thermocouples) vs. average of lower temperatures
(three strong thermocouples) in the fire space, and
5. cumulative total water usage.
The wood crib thermocouples showed when the fire was knocked
down, when it flared up, and when it was finally extinguished.
The average overhead temperatures showed the thermal threat
existing in the overhead and how well it was controlled. The
calorimeter and thermocouple string data demonstrated how much
the thermal balance within the fire compartment was disturbed.
Based on the results of these quantitative measures and the
qualitative observations made during the fire attacks, it was
determined that this offensive fog attack, using a medium-angle
fog directed 45 degrees upward at the flaming overhead and
discharged in short bursts, was the best method for approaching
this particular fire scenario.
Control of the overhead fire threat was best shown by the
average overhead temperature data. With the offensive fog-attack
method, the overhead temperatures were immediately reduced by
200° to 250°C (392° to 482°F) and then continued to cool for
the duration of the fire attack. Because the cooling of the
combustion gases was accomplished first, flashover potential was
mitigated, and a single attack team was able to safely complete
final extinguishment within five to 10 minutes after entry into
the fire space. In contrast, the data for the traditional
straight-stream tactic showed that the overhead temperatures
were reduced initially but quickly rebounded to their original
values. Since the attack teams were not able to get control of
the fire, the conditions for flashover evolved, requiring the
attack team to retreat out of the fire space. These conditions
usually presented themselves within two minutes.
The offensive fog attack also resulted in the least amount of
disturbance to the thermal layer. The disturbances of the
thermal balance within the fire compartment were best shown by
comparing the total heat flux measured by the calorimeters
mounted 0.9 m and 2.4 m (3 ft and 8 ft) above the deck in the
fire compartment (see Figure 3). The key indicator of
significant disturbances in the thermal balance was the upward
spike in the 0.9-m (3-ft) heat flux that approached or met the
heat flux for the 2.4-m (8-ft) calorimeter. This indicated total
compartment mixing with steam.
During the offensive fog test, the initial attack actually
cooled the upper layer enough to result in a 14.2-kW/sq m (1.25
Btu/sq ft-) drop in the 2.4-m (8-ft) heat flux level. While
steam was produced, it was described as more of a moist
"sweaty" type of steam rather than a hot penetrating
steam. In contrast, for the straight-stream test, there were
several instances in which the thermal balance was disturbed
sufficiently to cause the upward spikes on the 0.9-m (3-ft) heat
flux plot (see Figure 3). For all of the straight-stream
evolutions, the thermal balance was disturbed sufficiently to
impose a serious heat and steam threat to the attack team
members. To highlight the significance of this finding, it was
noted that none of the attack team members suffered burns during
the offensive fog attack tests, whereas many of the
straight-stream attacks resulted in burns to the hands, wrists,
face, neck, and back.
It is acknowledged that the demands put on manpower and
resources and the particular fire scenario will dictate the
optimum firefighting attack strategy. It is believed, though,
that the offensive fog attack strategy is the best method to
safely maintain a rapid, continuous, and aggressive response to
a fire when entry to the fire space can be made but direct
access to the fire seat cannot be gained. The situations in
which a fog attack should be considered include a horizontal
approach to a free-burning fire where (1) the overhead gases are
burning, (2) the seat of the fire is obstructed and water
streams cannot be applied directly to the fire seat, or (3)
multiple fire seats are growing within the fire space such that
one fire seat could grow out of control while water is being
applied to another fire seat.
Another possible situation that may warrant a fog attack
would be a fire scenario in which smoke or near-homogeneous
thermal conditions obscure a visual or infrared image of the
fire but the sounds emanating from the burning fuel bed provide
a reasonable indication of an immediate fire threat.
References
Farley, J.P.; J.L. Scheffey; C.W. Siegmann III; T.A.
Toomey; and F.W. Williams. "1994 Attack Team Workshop:
Phase II--Full Scale Offensive Fog Attack Test."NRL Ltr Rpt
6180/0798.2, Nov. 17, 1994.
Grimwood, P.T. April
1992. Fog Attack, Firefighting Strategy & Tactics--An
International View. FMJ International Publications Ltd., April
1992.
COMMANDER JOHN P. FARLEY is test team leader and
ex-USS Shadwell project officer at the U.S. Naval Research
Laboratory`s Technology Center for Safety and Survivability in
Mobile, Alabama.
http://www.chemistry.nrl.navy.mil/publications/6180publ.html
http://www.chemistry.nrl.navy.mil/6180/index.html
