Pool Fire Extinquishment
LLL Fire Gallery with Water Mists

400
Wal Fires (10)
350 ‘ti ;
Critical droplet size
E sap | Reid Quench ~ (teom alt fires)
a fyes / tro
i)
2
3 200
g 0 1
3 100 7
s Bs of
so
7
° o 100 200 500 400 500 600 700 800
Oroplet diameter, microns
Figure 9: Rapid quenching results for wall fires.
Pool Fire Extinquishment
LLL Fire Gallery with Water Mists
400
3x3 fires (40) Critical droplet size
380, —————— *~ (from oll fires) L/L)
£ Ropid Quench
300 Critical totol flow /
oO Byes / fino Na
_ 780
2
S 200
@ 150
—
5
2 109
> ogy
50 sho “gt
°
o 100 200 + «300 ~400~=«s00.-=«00~=«700~=«OO
Droplet diameter, microns

Figure 10: Rapid quenching results for 3x3-ft pool fires


where, ry = droplet radius, cm
Py = density of water, 1 g/cm®
g = gravitational constant, 980 cm/sec”
1) = ambient air viscosity, 3x10* poise (in cgs units)

The time, t,, to fall the vertical distance, L, (plume height) is considered the characteristic
time for loss of water droplets from the air, i.e.,

9nL

t= L/y, —
2e,9r,

v d

The extinguishment requirement is now taken as t,27,, which in essence infers that water
entrained in the air will be transported into the plume before it can fall vertically onto the floor
This leads to

ty e
2 > raf L]] 2e]| BY 2 x (10)
vy, P49 en, ry ry

where the cgs value of the constant, K, for diesel oil is about 0.016 sec-cm.

It is seen that the ratio (t/t,) will increase as B increases and as ry decreases. Also, linear
burning rates of pool fires generally increase with increasing pool size (references 1-3). Thus,
larger droplets of water might be utilized for ‘large’ pool fires, but smaller droplets of water would
be required for ‘small’ pool fires. This suggests that water mist sprays might be more applicable to
the rapid quenching of ‘large’ pool fires than to ‘small’ pool fires.

The extinguishment requirement that t,21,, leads to a droplet size criteria that
ty < 0.016B. aly
Data from the literature suggests values of B would range from 0.001 to 0.01 cm/sec, which
indicates a need for water spray nozzles to produce particles of radii less than 0.0040 to 0.0125

cm (80 to 250 microns diameter). These calculated droplet sizes do fall within the range of sizes
available with Class I commercial water mist spray nozzles (reference 4).


Diffusion Flame
. an (Fire Plume)
pe Horizontal
Air Flow L
———— -
» Oil Pool
Oe

[ee

Figuret. Representation of an oil pool fire.

POOL FIRE EXTINGUISHMENT

900
800
700
600
500
400
300
200

100.
SEPO2

DEGC

TEMPERATURE,

0 1 2 3 4 5 6 7
TIME, MIN

— Tray 1 — Tray 2 — Tray 3

Figure 2: Example of a ‘yes' rapid quench extinquishment.


and are coated with a 1 to 1.5 in (2.5 to 3.8 em) thick fireproof material. The pool fire tests were
carried out in a section of the crosscut about 24 ft (7.3m) in length.

The water mist nozzles used in the experiments were commercially available spiral or
impingement types that were ceiling mounted and operated according to specifications. The spiral
nozzles produce a conical shaped water distribution pattern, with the larger droplets forming the
outer edge of the cone, while the impingement nozzles produce a more even droplet size
distribution pattern. Nozzle droplet sizes are referred to by the nozzle's Dvg, which is defined as
having 90 pct of the water spray volume as droplets less than the Dvys. Depending on the type of
nozzle and the operating pressure, the spray discharge from a single nozzle could be from 6 ft
(1.8m) to 18 ft (5.5m) with a water flow rate from 4.9 to 50.5 gallons per minute (gpm). The
number of nozzles used for an individual fire test generally depended on the number (2 to 12)
needed to cover the area of the 24 ft (7.3m) long fire test section.

The fuel oil was contained in two sizes of trays (3x3-ft and 5x7-ft) which normally contained
3 to 5 gallons of diesel fuel oil. The fire trays were located in one of four locations: (1) centered
under a spray nozzle; (2) off-center between neighboring nozzles; (3) against a single wall; (4)
against two walls (i.e., a corner). Numerous thermocouple positions and several gas sampling
points enabled the course of the fire to be followed as well as the spread of fumes and heat within
the test chamber. Several windows in the walls of the chamber allow for visual observation of the
fire test; however, due to smoke generation, it generally requires the use of an infra-red sensitive
videocom.

In carrying out a pool fire test, the oil in the pan is ignited and allowed to burn for one
minute prior to activating the water spray nozzles. Tray thermocouples (3 to 5 of them), which
were located directly over the surface of the fuel, generally registered temperatures of 700 to
800°C during this free burning stage. The temperature-time behavior of the tray thermocouples
defined the extinguishment behavior of the fire. In the case of rapid quenching, the tray
thermocouple temperatures quickly decrease to less than 100°C within one minute (Figure 2).
Figure 3 depicts a pool fire that took about 4 minutes to be extinguished. Although this fire was
eventually extinguished by the nozzle sprays, it is considered a 'no' extinguishment in terms of the
rapid quench process being considered in this report. Figure 4 is another example of a 'no’
extinguishment - this time the fuel apparently burned to extinction.


Water Flow Rate Criteria

In addition to a water droplet size requirement, it will be necessary to have sufficient
quantities of water entrained into the fire plume. Equation 3 indicates the required concentration
of water in the air being entrained by the plume. Assuming that the fire can occur somewhere in a
room having a total volume V, and cross-sectional area, A,, the total mass of water to be
suspended in the air will be

M,, = (5/8)p,A,L (12)
Here, for convenience, it is assumed that the room height and the fire plume height are the same.

Dividing M,, by the Stokes' droplet settling time, t, (equation 9), yields the required total
mass flow rate of water into the room, i.e.,

‘ 2 r?
M, = M/t, = (5/8) p,a,L| 2 (23)
“ wl Ty a onL
or when expressed in cgs units
M,, = 540 A,r? (g/sec) (4)

For a room of cross sectional area 18x24-ft (4x10* cm’), which is about the effective area of
the fire test chamber at the LLL fire suppression facility, the critical total mass flow rate becomes

M,, = 2.2x108 ry? (g/sec) (15)

It should be noted that the r, that appears in the above expressions for required total water
flow rate refers to the critical droplet size, i.e., mist spray nozzles must produce the critical flow
rate as droplets with radii <r,, the critical droplet size. As described in the next section, this
apparently was not always the case for the PRL pool fire tests.

EXPERIMENTAL

All experiments were conducted in the Fire Suppression Facility (FSF) located at the LLL
facilities near Fairchance, PA. The FSF was constructed to simulate an underground diesel fuel
storage area. The main entry is 78 ft (23.8m) long , with double steel self-closing doors at each
end. A crosscut extends 40 ft (12.2m) from the center of the main entry, and is closed-off with a
permanent concrete block stopping. All entries are 18 ft (5.5m) wide and 7 ft (2.2m) in height,


water droplets to enter the plume as it entrains the air needed to maintain the combustion
processes. As long as the critical water droplet size and water concentration in the air can be
maintained, rapid quenching should be promoted. It might be expected that, except for the
potential complications of a hot surface acting as a flame holder, rapid quenching of pool fires
should be somewhat independent of obstacles in the room containing the pool fire.

By reasons of the same argument given above, it might be easier to extinguish large pool fires
by a rapid quench process than small fires. The horizontal air flows generated by small pool fires
would likewise be small, and possibly require a critical droplet size too small to be produced by
commercial mist spray nozzles. For example, from equation 11, a small pool fire having a linear
burning rate of 0.0001 cm/sec, would require nozzles capable of producing an adequate supply of
25 micron diameter water droplets. In this case, it might be better to extinguish the fire by direct
injection of large water droplets into the fire plume, a process that would have greater chance of
success when aerodynamic forces generated by the fire plume are relatively weak.

CONCLUSION

The parametric approach to rapid extinguishment of pool fires that is presented here is reasonably
consistent with the extinguishment results for fuel oil pool fires interacting with water mists
produced by a variety of spray nozzles. The order of magnitude agreement between calculated
and observed critical total water flow rates and critical water droplet sizes is quite encouraging for
the simple approach taken; however, further testing of the predictive capabilities of the model
needs to be validated through more realistic pool fire/obstacle scenarios. An important aspect of
a simple parametric approach to fire extinguishment is that it could be a starting point for a more
realistic treatment of the coupling of pool fires with the water mists, but perhaps more significant
is that a simple parametric model can be a starting point for design of pool fire tests as well as the
approach to analysis of the extinguishment results.

REFERENCES

1. Kuchta, J. M., Investigation of Fire and Explosion Accidents in the Chemical, Mining, and
Fuel-Related Industries - A Manual, Bulletin 680, U.S. Bureau of Mines (1985) p.41.

2. Kung, H. C. and P. Stavrianidis, Buoyant Plumes of Large Scale Pool Fires, Proceedings of
the 19th Symposium (International) on Combustion, The Combustion Institute (1982) p.905

3. Seeger, P. G., On the Combustion and Heat Transfer in Fires of Liquid Fuels in Tanks, Ch.3 in
Heat Transfer in Fires, P. L. Blackshear, ed., Wiley & Sons, New York (1974) pp. 92-128.

4. National Fire Protection Association, Standard for the Installation of Water Mist Fire
Protection Systems, Publication 750 (1996) pp 30-35.


WATER MIST EXTINGUISHMENT OF POOL FIRES:
A PARAMETRIC APPROACH

by
Robert F. Chaiken and Alex C. Smith

U.S. Department of Health and Human Services
National Institute for Occupational Safety and Health
Pittsburgh Research Laboratory
P.O. Box 18070,

Pittsburgh, PA 15236


Pool Fire Extinquishment

LLL Fire Gallery with

Water Mists

400
AN Fires (52
350 , .
E 300 Rapid Quench a Critical droplet size
ci yes / fino
250
z
2 200 of
va)
& 80 u 6 a
5 Critical total flow
> 100 eo
$ edges oa) of
so 1,
a7
°
Cr re ee ee ee en ed
Droplet diameter, microns
Figure 5: Rapid quenching results for all fires.
Pool Fire Extinquishment
LLL Fire Gallery with Water Mists
400
Cenlered Fires (20) |
380 _- Critical droplet size |
& 300 | RoPid Quench (from all fires) ; i
° fyes / fo '
250 |
2
2 200
2 150 iti
€ We Critical total flow
S 100 sho [3
> es 1 of
50 170;
sh
°
° 100 200 300 400 500 600 700 800
Droplet diameter, microns

Figure 6: Rapid quenching results for centered fires.


POOL FIRE EXTINGUISHMENT

DEGC

TEMPERATURE,
w
3
ra

0 1 2 3 4 5 6 7
TIME, MIN

——~ Tray 1 —— Tray 2 — Troy 3

Figure 3: Example of a ‘no' rapid quench extinguishment.

POOL FIRE EXTINGUISHMENT
800

700
600
500

DEGC

400
300
200
100

TEMPERATURE,

IDECO2A

4 5 6 7 8 9 0 11 12:13
TIME, MIN

— Troy 1 —— Tray 2 — Tray 3

Figure 4: Example of non-extinguishment; also a 'no' rapid quenching result.


RESULTS AND DISCUSSIONS

Figure 5 depicts the results of all the pool fire tests on a single plot of total water flow rate,
gpm, versus droplet size, Dv), '. The data points are indexed with the number of ‘yes' and
number of ‘no' rapid quenching observed (represented by the #yes/#no values shown near each
point). The continuous curve shown in the plot represents the theoretical critical total flow rate
as determined by empirical fit (eyeball) of equation 14 to the data. The equation as shown with
the droplet diameter given in microns is

M,(gpm) = 0.0059 D? (16)
or in cgs units with the droplet radius given in cm,
M,(gm/sec) = 1.5x107 r42 (7)

The ‘observed’ theoretical constant coefficient is about an order of magnitude smaller than that
calculated equation 15).

The vertical dotted line in figure 5 represents the ‘observed’ theoretical critical droplet
diameter for extinguishment by rapid quenching. From the literature (reference 1) a linear oil
burning rate of about 0.01 cm/sec might be expected for the pool sizes used in these fire tests.
This would yield a calculated critical droplet diameter of 250 micron (equation 11) which is about
30% less than the ‘observed’ theoretical value. It is noted that if the data of figure 5 was based on
the root-mean square diameter of the actual droplet size distribution rather than the Dvg, value, it
would probably have resulted in a smaller "observed" critical drop diameter, and in better
agreement with the calculated value of 250 microns.

It should be noted that the above results are based on the 52 fire tests, independent of the fire
location and pool size parameters. Visually from figure 5, there doesn't seem to be any clear
trends to be noted other than those based on droplet size‘and/or the total flow rate, at least for the
case of the rapid quench process. This conclusion is also indicated from similar data plots where
the size and location parameters have been sorted (figures 6-11). Unfortunately, sorting the data
this way reduces the‘number of fire tests in a given category, and hence, the significance of any
observation. However, from the sorted plots it is seen that the 'observed' theoretical results from
all the fires are reasonably consistent with the sorted data.

An argument to support these finding might be that the rapid
quench mechanism as depicted in the model depends solely on there being an adequate supply of
suitably sized water droplets in the air surrounding the fire. The pool fire itself will cause the

‘From the functional form of equation 15, it would be appropriate to plot gpm versus the root-
mean square diameter of the droplet’size distribution as produced by the various nozzles.
Unfortunately, the size distribution data was not available at this time.


With all the air supplied by sideways entrainment, the horizontal air mass velocity
requirement is given by

m, = 16m, = 2nrLp,v, (4)

where va = linear velocity of the air,
L = height of plume.

The fuel burning rate can be written in terms of the linear rate of fuel vaporization, B, and the
fuel density, p, as

or

leh .

The time, t,, it takes for air to travel an average horizontal distance, r/2, into the plume can
then be expressed as

p,L
t=
b 16p,B

T, will vary with pool size, plume height and fuel evaporation rate which in turn will vary
with pool size.

The competing action to water entrainment is the fall-out of droplets from the air at the
Stokes terminal velocity, v4,

> 2
= LP WIE (8)

Vv,
a on


gpm

Volume flolw,

Pool Fire Extinguishment
LLL Fire Gollery with Water Mists

400
5x7 Fires (12) Crilical droplet size
sof ree (from of fires)
300 Ropid on ‘ilicol total flow
Wes / foo ~
a0
200
180 6
100 1%
‘Bp % ols
so TM 4
a
0
3 io aloe nwo —d~«SDS~C«O

Droplet diameter, microns

Figure 11: Rapid quenching results for 5x7-ft pool fires.


If sufficient water is entrained into the plume to lower the temperature of the plume below
some ignition temperature, T;,, the plume flame will be extinguished, leaving the liquid fuel at a

temperature T, < T,,, the boiling point of the liquid fuel. The mass flow rate of water required
will be given by

fh (c, (T.,-T,) +H] 2 Hom, (1)

where, tn, = mass flow rate of water, g/sec
th, = mass flow rate of fuel, g/sec
T, = initial water temperature, ~300 K
T,, = extinction flame temperature, ~1300 K
c, = heat capacity of liquid water/steam, ~0.5 cal/g-K
H,, = heat of vaporization of water, ~500 cal/g
H, = heat of combustion of fuel, ~ 10‘ cal/g

For the combustion reaction, this leads to
m= 10m, = (5/8) m, (2)

where m, = mass flow rate of air into the plume

For a given volume of air, V,, the concentration of water in the air required for
extinguishment becomes

m,/V, = (5/8)mJ/V, = (5/8)p, = 7.5 x 104 g/cm? (3)

where, p, = ambient air density, g/cm*

However, if there is a hot object in contact with the liquid that acts as a flame holder,
permanent extinguishment of the fire might not be achieved. The flame holder could rekindle the
pool fire until it too is cooled, perhaps by direct impingement of water onto the flame holder.
This direct process will not be considered in the current treatment of extinguishment. Only the

indirect entrainment of water is considered, which as will be shown later, leads to the rapid
quenching of the fire.

The indirect rapid quenching process just described will involve a coupling between the size
of the fire and the size of the water droplets that will be entrained in the horizontal air flow
(figure 1). This coupling will in turn involve the horizontal air flow rate which promotes water to

enter the plume, and the rate of settling of droplets from the air which promotes water not to
enter the plume.



INTRODUCTION

The National Institute for Occupational Safety and Health (NIOSH) at its Pittsburgh
Research Laboratory (PRL) is interested in the application of water mist sprays to extinguish
diesel fuel fires in underground mine diesel refueling areas. To this end experimental studies have
been carried out on the extinguishment of diesel fuel fires at PRL's large enclosed fire suppression
facility (FSP) at its Lake Lynn Laboratory (LLL). Two sizes of shallow fuel pans (3x3 ft and
5x7 ft) contained the fires, and a number of commercial mist spray nozzles (at roof locations) and
spray/fire locations were tested (52 fires in all). Visual observations of the pool fire
extinguishment process by the overhead sprays with an infra-red sensitive videocom suggests that
there are two distinct actions involved in extinguishment due to two different transport
mechanisms by which the water droplets can enter the fire plume.

First, is a direct injection of water droplets into the plume from overhead. This is believed to
involve larger water drops and leads to an extinguishment time of several minutes. Second, is an
indirect injection process involving small water drops which are entrained in the sideways air flow
that feeds oxygen to the fire. This second process apparently leads to relatively fast quenching of
the fire in less than | minute - a highly useful attribute for a fire protection system.

This paper describes a relatively simple parametric model for the indirect water injection
process, and defines the critical spray conditions for achieving rapid quenching of the pool fire.
While the model is consistent with the pool fire data obtained to date, the PRL studies are
continuing with more complex pool fire scenarios. It remains to be seen if this simple parametric
approach applies to these future fire tests.

THEORETICAL

Critical Drop Size

In the two-dimensional representation of figure 1, an oil pool fire is considered to be a cone
whose base is the vaporizing liquid oil surface having an area 7’, above which exists a fire plume
(diffusion flame) of height, L. The rising plume induces a horizontal air flow into the side of the
plume supplying the oxygen necessary for propagating gas phase combustion reactions. Water
droplets suspended in the air around the plume will tend to follow the horizontal air flows into the
plume. The rate of air entrainment by the plume will be determined by the overall combustion
reaction which is assumed to be

C,Hi, + 44.N, +11 O, ---> gas products + heat
100g fuel + 1584g air ---> CO,, H,O, N, + 10* cal/mole fuel

It takes 16g of air to burn 1g of fuel oil.


gpm

Volume flolw,

Pool Fire Extinquishment

LLL Fire Gallery with Woter Mists

seo
fyes / fino
250
200 oy)
iB
180 BQ
1 Critical total flow

4 *

Te Critical droplet size
Ropid Quench (trom at fires)

og!

° 100

wo 300 xo 800680
Droplet diameter, microns

700

Figure 7: Rapid quenching results for off-centered fires.

Volume flolw, gpm
2 283 8 38

Pool Fire Extinguishment
LLL Fire Gallery with Water Mists

A
Corner Fires (9 Critical dropiet size aI
~~ (rom alt fires) j
Ropid Quench
fyes / no
oft Critical tolal flow
1
ot
‘Big 4b 9
17
0 vo 200~=«S0DSOS:«C«ODSCCODSSC7OS«0D

Droplet diameter, microns

Figure 8: Rapid quenching results for comer fires.