leakage into F Butt which resulted in an average airflow velocity of 5.8 cm/s in F-Butt. This
was eliminated in experiments 2-9, and there was no measurable airflow greater than 1.5 cm/s
prior to the ignition of the fire.

The POC spread rate in each experiment was determined from the alarm time of the CO
sensors along the mine entry roof. The CO alarm time is the time at which the CO
concentration achieves Sppm above ambient. The calculated POC spread rate is an average
over the 15 and 30 m spacing between sensors. Figure 2 shows the POC spread rates towards
stations $3 and S6 for each experiment. It is apparent from Figure 2 that the more intense
fires of experiments 1-6, average fire intensity of 330 kW, have a higher associated POC
spread rate than the less intense fires of experiments 7-9, average fire intensity of 30 kW.
Table 3 lists the average POC spread rate for these two average fire intensities at the three
locations. Based upon the limited data in Table 3, the POC spread rate is proportional to the
fire intensity to the 0.36 power.

The less intense fires resulted in slower mixing over the cross section of the mine entry. This
was confirmed from a comparison of the measured CO concentration near the roof and at
midheight. Figure 3 shows the measured CO concentration at entry roof and midheight at S1,
30 m from the fire, for experiment 9. Figure 4 shows the disappearance in the roof layer
stratification at $2, 60 m from the fire, for experiment 9.

The average of the CO alarm times at the sensor stations can be used to determine the alarm
times for CO sensors at fixed sensor spacings for the two average intensity fires considered.
Sensor spacing is equal to twice the distance of the measurement station from the fire, since
POC can spread in two directions. The results for experiments 1-6 for an average fire
intensity of 330 kW, and for experiments 7-9 for an average fire intensity of 30 kW are shown
in figure 5. Interpolation of the data in figure 5 for the 330 kW and 30 kW fires with the 14.25
min time rquired for a developing coal fire to ignite a conveyor belt (7) implies maximum
sensor spacings of 183m and 105m, respectively, would be necessary for fire detection at very
low airflow conditions. Although these results are not strictly invariant with respect to airway
dimensions, the results have wide applicability, since the SRCM is not atypical of coal mines.

Thermal Sensor

One type of fire sensor used as part of an atmospheric mine monitoring system is a thermal
sensor. The minimum alarm temperature for a fixed-temperature sensor is 5? C (10). The
effectiveness of this type of fire sensor for early fire detection was investigated as part of the
low flow experiments. For experiments 5 and 6 a thermocouple was placed at S1 and S4, and
for experiment 7 a thermocouple was placed at $2. The thermocouples, which simulate
thermal fire sensors, were positioned near the roof. For the high intensity fire experiments,
5 and 6, the maximum temperature rise above ambient was 21 and 20 C at the station 30 m
from the fire. The specific maximum temperatures were 36 and 30° C at the stations. These
values are considerably less than the minimum 57° C alarm temperature. If a thermal sensor
alarm is defined in an analogous manner to the definition for a smoke sensor as the ambient
value plus ten standard deviations of the signal noise, then a comparison of thermal and CO
sensor alarm times can be made. The thermal detection lags the CO alarm time by 14.7 min
and 14.2 min at stations $1 and S4 for the 296 kW fire of experiment 5, and by 5.2 and 10.3


TIME, min

50

40

w
3}

Root 4

CO, ppm

y
o

= Midheignt

10

ae a
oO 09 1.8 27 36
TIME, 103s

Fig. 4 Measured CO at airway roof and midheight
at station S2 for experiment 9

40 T T T
o
Key
1D 30KW, experiments 7-9 J
sor & 330 KW,experiments 1-6
20F . 4
a
10 o 4
o rn
a a
1 1 1
0 50 100 150 200

SENSOR SPACING, m

Fig. 5 CO alarm time dependence upon sensor spacing
for average 330kW and 30kW fires


can be evaluated from the known ventilation conditions. In the absence of imposed mine
ventilation, the buoyancy generated flow produced by the fire defines the sensor spacing
criteria. It was determined that for the 14.25 min required for a developing coal fire to ignite
a conveyor belt, a sensor spacing of 183 m and 105 m would be required for average 330 kW
and 30 kW fires respectively under zero airflow conditions.

REFERENCES

1. Edwards, J. C. and G. S. Morrow, "Evaluation of Smoke Sensors for Mining Use,"
U.S. Bureau of Mines RI 9586, 1995, 17pp.

2. Edwards, J. C. and G. F. Friel, "Comparative in-Mine Evaluation of Carbon Monoxide
and Smoke Sensors," U. S. Bureau of Mines RI 9622, 1996, 11pp.

3. Mulholland, G. W. and B. Y. H. Liu, "Response of Smoke Sensors to Monodisperse
Aerosols," J. of Res. of the National Bureau of Standards, 85, no.3, May-June, 1980,
pp.223-238.

4. Edwards, J. C. "Detection of Smoldering Combustion of Coal with an Odor Meter,”
Proc. 12th WVU International Mining Electrotechnology Conference, Dept. of Electrical
and Computer Engineering, West Virginia Univ., Morgantown, WV, July 27-29, 1994,
pp. 150-154.

5. Friel, G. F. and J. C. Edwards, "Ultrasonic Detection of Mine Fires,” Proc. 12th
WVU International Mining Electrotechnology Conference, Dept. of Electrical and
Computer Engineering, West Virginia Univ., Morgantown, WV, July 27-29, 1994,
pp.144-149.

6. Friel, G. F. and J. C. Edwards, "Mine Fire Detection by Ultrasonic Ranging Systems,”
U. S. Bureau of Mines RI 9624, 1996, 1Spp.

7. Litton, C. D., C. P. Lazzara, and F. J. Perzak, "Fire Detection for Conveyor Belt
Entries," U. S. Bureau of Mines RI 9380, 1991, 23pp.

8. Edwards, J. C., G. F. Friel, R. A. Franks, and J. J. Opferman, "Mine Fire Detection
Under Zero Airflow Conditions," Sixth International Mine Ventilation Congress.
Pittsburgh, PA, May 18-22,1997. Proceedings Ch 52, pp.331-336.

9. Underwriters Laboratories (Northbrook,IL). UL268 Standard for Smoke Sensors for
Fire Protective Signaling Systems. Feb. 1986, pp.26-31.

10. Custer, R. L. P. and R. G. Bright, "Fire Detection: The State-of-the-Art," U. S.
Dept. of Commerce NBS Technical Note 839, 1974, 110pp.

11. D. J. Rasbash, "Sensitivity Criteria for Sensors Used to Protect Life,” Fire
International, 49, 1975, pp.30-44.

12. Jin, T. "Studies of Emotional Instability in Smoke From Fires,” J. of Fire
Flammability, vol. 12, April, 1981, pp.130-142.


min for the 264 kW fire of experiement 6. For the low intensity 30 kW fire of experiment 7,
the temperature rise 15 m from the fire was 11°C to a maximum of 17°C, although the
measured CO exceeded 45 ppm. These results show that thermal sensors are limited in their
applicability to mine fire protection.

Smoke Sensors

A relative comparison was made of optical and ionization type smoke sensors which operate
in a diffusion mode or a pump mode with the first detection of smoke by a light obscuration
device at station S2 under zero airflow conditions. For smoke sensors A-C the alarm was
defined as a ten standard deviation change in the signal from its ambient value. The
manufacturer set alarm was used for sensors D and E. It was determined that smoke sensors
A, C, and E and a diffusion mode CO sensor consistently detected a diesel fuel fire within a
88 s time span of each other. Because of the lack of dilution from ventilation air, the
maximum optical density was between 0.82 and 1.30 m' for the 330 kW average fire intensity
experiments 1-6, and between 0.24 and 0.28m' for the 30 kW fire intensity experiments 7-9.
The equivalent visibility is between 0.6 and 1.0 m for experiments 1-6, and between 2.9 and
3.3 m for experiments 7-9. The visiblity is calculated from the optical density based upon
visibility studies (11) . These values are less than the 4 m visibility required for escape from
a building for someone familiar with the surroundings (12).

A comparison was made of the alarm time of each smoke sensor and the diffusion CO sensor
Sppm alarm times. Table 4 shows the average time interval by which the smoke sensor alarm
leads the diffusion mode CO alarm time at S2. A minus sign in Table 4 denotes a time lag. The
330 kW average fire intensity experiments 1-6 are in one group, and the 30 kW fire
experiments 7 and 8-9 are in separate groupings. For experiment 7 the fire is 15m from S2,
whereas the separation is 60 m for the other experiments.The results in Table 4 show that a
smoke sensor, in the case of sensors A, C, and E, is a viable candidate for a mine fire sensor
in place of a CO sensor. Diffusion mode smoke sensor C alarmed earlier than the diffusion
mode CO sensor at S2 in eight of the nine experiments, and can be more effective for mine fire
detection than a diffusion mode CO sensor if the alarm time for the smoke sensor is defined
as a ten standard deviation change of the analog signal from its ambient value.

CONCLUSION

Early mine fire detection can be best accomplished with CO or smoke sensors. The
equivalency of CO concentration of 5 ppm above ambient to a smoke optical density of
0.022m' was demonstrated for smoldering and flaming coal combustion. It was determined
that smoke sensors can be reliable indicators of smoldering and flaming coal combustion for
an optical density less than 0.011m". From an operational point of view in terms of sensor
alarm definition, smoke sensors with a continuous analog output have an advantage over those
with preset alarm levels. For diesel fuel mine fire experiments it was demonstrated that a
diffusion mode ionization type smoke sensor alarmed earlier than a diffusion mode CO sensor
under normal ventilation conditions. A similar result was found for 30 and 330 kW average
intensity diesel fuel fire experiments under zero airflow conditions. Thermal detection was
shown to be inadequate at a 30 m distance from a 300 kW fire under zero airflow conditions.
For imposed mine fire ventilation conditions, sensor spacing in a mine fire detection system


som —>A\ 30m —|
Key
18 Room
Brattice Station
AB Fire
Brattice
Fig. 1 Plan view of mine section
030
Se 025 oo key
id Experiment
g . . o1
€ 2 : 22
‘ ° 020: 33
a
E .
& ors . te
2 . 27
2 . <8
= on
° % 40.10
° -
eg] o oe + 8 “
x ° 008 . e
° ° °
a ee ot

foo 80 60 40 20 0 20 40 60 80 100

$3-— DISTANCE, m +36
Fig. 2 POC spread rate dependence upon distance
from fire
50
—
40F a
/
Ye
30+ Tail
€
&
&
g 20} rey 4
Root
== Midheight
10 F 7
\ j
° 1.8 27 36
TIME, 103s

Fig. 3 Measured CO at airway roof and midheight
at station Sl for experiment 9


OVERVIEW OF MINE FIRE DETECTION
John C. Edwards
CDC, National Institute for Occupational Safety and Health
Pittsburgh Research Center, P.O.Box 18070
Pittsburgh, PA 15236

Several experimental programs were conducted at the Pittsburgh Research Center to investigate
the level of mine fire detection and alarm capability possible using state-of-the-art technology.
These programs involved comparison of the response and alarm time of optical and ionization
type smoke sensors to smoldering and flaming coal combustion in a smoke chamber. Coal
combustion experiments conducted in the smoke chamber demonstrated that a CO
concentration of 5 ppm above ambient corresponded to an optical density of 0.022m'. For
the four smoke sensors for which a continuous analog signal was available, a smoke sensor
alarm was defined as the average background signal plus ten times the peak-to-peak noise.
This alarm criterion resulted in the association of the alarm for three of the four sensors with
a smoke optical density of 0.011m", and 0.033m" for the fourth smoke sensor. The smoke
sensors used in the smoke chamber studies were incorporated into large scale diesel fuel fire
experiments conducted under normal ventilation conditions in the Safety Research Coal Mine
(SRCM) at the Pittsburgh Research Center. These experiments showed that a diffusion mode
smoke sensor alarmed earlier than a diffusion mode CO sensor; and, based upon the
measurement of the optical transmission at the location of a pump mode ionization type smoke
sensor, that a pump mode ionization smoke sensor alarmed at an average smoke optical density
of 0.021m" for the twelve experiments conducted. A series of 30 and 330 kW diesel fuel fire
intensity experiments were conducted under zero airflow conditions in the SRCM. It was
determined that :(1) thermal sensors are inadequate at 30 m from a 300 kW fire;(2) a diffusion
mode ionization smoke sensor can be more effective for mine fire detection than a diffusion
mode CO sensor;(3) recommendations can be made for sensor spacing in a mine entry based
upon the measured CO buoyancy induced spread rates along the mine roof and the time for
a developing coal fire to ignite a conveyor belt.

INTRODUCTION

The safety of miners is dependent upon an atmospheric mine monitoring system which can
provide early warning in the event of a fire in a mine. Common sensors used in a mine
include CO, smoke, and thermal. NIOSH’s Pittsburgh Research Center has evaluated (1)
smoke sensors in a smoke chamber, as well as in a mine environment (2). Smoke sensors can
be classified into two types based upon their operational principle-optical or ionization. Their
sampling method will be either diffusion- or pump- controlled.

Ionization smoke sensors contain a radioactive source that ionizes the air. The oppositely
charged ions form a current between two charged electrodes. Diffusion of smoke particulates
into the path of the ion current reduces the ion current through attachment of the ions to the
smoke particles. This process reduces the mobility of the ions, and thereby increases the ion’s
probability of recombination. The ion current reduction is amplified as a measurable signal.

Optical smoke sensors operate on the principle of scattering or absorption of light over an
optical path through which the smoke particles migrate. For optical scattering, the sensor is
located to the side of the optical path to measure the amount of light scattered by a smoke


Table 1. Smoke Sensors

Smoke Detector Type Sampling Mode
A optical pump
B optical diffusion
Cc ionization diffusion
D ionization diffusion
E ionization pump

Table 2. Diesel Fuel Fires

Experiment Fuel, L ‘Area, sq.m | Burn Time,s | Heat Release
Rate, kW
1 4 0.58 250 495
2 6 0.37 589 315
3 12 0.37 1,222 303
4 18 0.37 1,910 291
5 24 0.37 2,506 296
6 24 0.21 2,816 264
7 2 0.047 2,024 30
8 2 0.047 2,024 30
9 3 0.047 3,036 30

Table 3. POC Spread Rates

Average Fire POC average spread rate, m/s at
Intensity, kW
30m 60m 90 m
330 0.22 +0.03 0.13 + 0.05 0.06 + 0.01
30 0.08 +0.02 0.04 +0.006 0.04 + 0.02

Table 4. Low Flow Experiments Smoke Sensor Alarm Average
Lead Time Relative to CO Sensor Alarm Times

Experiments A B c D E
16 21 23 13 4 18
7 NA 16 14 64 45
8.9 41 -130 16 566 67



sensor B alarmed an average 63s prior to the CO sensor. Smoke sensor D alarmed an average
17 s before the CO sensor for those experiments for which the minimum optical density was
greater than 0.15m’. For the reduced ventilation airflow experiments, the smoke sensors
alarmed at an average earlier alarm time with respect to the diffusion mode CO sensor than
they did under normal airflow ventilation conditions. Smoke sensor A alarmed an average 17
s after the CO sensor for the normal ventilation experiments, but an average of 61 s before the
CO sensor for the reduced ventilation experiments. At one location in the mine entry smoke
sensor E and a diffusion mode CO sensor were positioned adjacent to a light obscuration
device. The optical device consisted of a halogen lamp source and a photovoltaic cell separated
by a distance of 1 m, and mounted horizontally and transverse to the airflow. The diffusion
mode CO sensor alarmed at an average optical density of 0.085m’, and smoke sensor E
alarmed at an average optical density of 0.021m" for eight normal ventilation airflow
experiments and three reduced ventilation airflow fire experiments. In this series of in-mine
diesel fuel fire experiments, the smoke sensors were shown to be able to complement CO
sensors when they are to be used as part of an atmospheric mine monitoring system.

MINE FIRE DETECTION UNDER ZERO AIRFLOW CONDITIONS

The strategy for sensor deployment in a mine has been investigated (7) under the constraint
of known ventilation conditions and expected growth rates of fires. The presence of mine
entry crosscuts was shown (2) to increase the transport time of CO through entrainment
established by vortex creation in the crosscut. The entrainment results in a dilution of the POC
at their concentration profile front. This was quantified as an analytic relationship between
measured transport time and calculated transport times based upon the inclusion and exclusion
of crosscuts. In the absence of positive ventilation airflow, there is a need to understand the
POC spread rates induced by buoyant flow induced by the fire.

To measure the buoyancy induced POC spread rates, six experiments with an average 330 kW
and three experiments with a 30 kW diesel fuel fire were conducted in the SRCM (8). Fig.
1. shows a schematic of the mine entry in which the experiments were conducted. The mine
section in which the experiments were conducted was nearly free of elevation changes. F Butt
has an average height of 2.0 m and a width of 4.6 m. Room 10 has an average height of 2.2
m and width of 2.8 m. Room 18 has an average height of 1.7 m and width of 4.0 m. For each
of the nine experiments, except 7, the fire was located as shown in figure 1 midway between
stations $1 and $4. For experiment 7 the fire was located midway between stations S1 and S2.
To measure the POC advance from the fire, a pair of diffusion mode CO sensors were
suspended from the roof at each of the stations, S1-S6. One sensor inlet was 0.4 m from the
roof, and the other’s was at midheight in the entry. The measurement station separation
between adjacent stations other than S1 and $4, which are 60 m apart, was 30 m.

At station S2 the light obscuration device was used to indirectly measure the smoke optical
density. For comparison of alarm time response a diffusion mode CO sensor and smoke
sensors A-E were positioned at S2.

Table 2. lists the experimental conditions for each diesel fuel fire in terms of the fuel quantity
that underwent combustion; the surface area of the fuel fire; the burn time of the fuel; and
the average heat release rate for the fire. For experiment 1 there was a small air quantity


change from the background equal to ten times the peak-to-peak noise. With this alarm
criterion, sensors A, C, and E alarmed at a smoke optical density less than 0.011m' for
smoldering and flaming coal combustion. Sensor B alarmed at 0.022m' for smoldering coal
combustion, and at an optical density between 0.022m' and 0.033m' for flaming coal
combustion. Sensor D did not have a continuous analog output signal, and the manufacturer’s
set alarm was used. For smoldering coal combustion sensor D’s alarm occurred at an average
optical density of 0.12m'", and for flaming coal combustion the alarm occurred at an average
optical density of 0.077m". For each of the sensors for which a continuous output signal was
available, the sensor indicated a response to the smoke in the chamber coincident with the
onset of smoke as measured by optical attenuation. This comparison showed the advantage
of a continuous analog signal from a smoke sensor so that an alarm value can be readily
identified to correspond with the smoke optical density.

The optical type smoke sensor A showed that at an optical density of 0.022m' the sensor’s
output signal was greater for smoldering coal combustion than for flaming coal combustion.
The ionization type smoke sensor C showed the opposite effect. This is in agreement with
previous research (3 ).

As an ancillary measurement, the CO in the chamber was monitored. This measurement
established a correspondence between average CO concentration of Sppm above ambient with
an optical density of 0.022m". This is significant for establishing equivalent alarm levels for
CO and smoke sensors.

Evaluation of the performance of smoke sensors must also account for the environment in
which they are deployed. Mine environments can be humid, and temporarily have high dust
concentrations when preventive rock dusting operations are in use. These features, as well as
pressure and temperature effects, need to be incorporated into the sensor performance
evaluation.

IN-MINE COMPARISON OF CO AND SMOKE SENSORS

The development of a strategy for mine fire detection requires an evaluation of the relative
response of CO and smoke sensors to an in-mine fire. An experimental study was conducted
in the SRCM with a series of nine diesel fuel fires (2) under normal, linear airflow velocity
between 0.8 and 1.2 m/s,and three reduced ventilation conditions, linear airflow velocity
between 0.2 and 0.4 m/s, to compare the response of five types of commercially available
smoke sensors with the more commonly used diffusion mode CO sensors. The sensors were
located near the roof in a mine entry of average cross section 2.0 m high and 4.6 m wide. The
fire source is about 360m before the first sensor. This provides for some dilution of the
leading edge of the POC before they reach the sensor.

For the in-mine experiments, the manufacturer set alarm was used for sensors D and E. For
sensors A, B, and C, the alarm value was defined as a ten standard deviation change in output
signal from the ambient signal. Smoke sensors B, C, and E, consistently alarmed prior to the
diffusion mode CO sensor located adjacent to it. For the normal ventilation airflow
experiments, 3-9, with an average fire intensity of 330 kW, ionization type smoke sensors C
and E alarmed an average 61 and 46s prior to the CO sensor, and the optical type smoke


particle. In the case of optical absorption, the reduction of transmitted light due to absorption
and scattering is measured.

The smoke sensors evaluated in the laboratory and under in-mine fire conditions as
characterized by type and sampling mode are listed in Table 1. These are commercially
available smoke sensors intended for mine or industrial use.

The smoke particle size is dependent upon the mode of combustion. Smoldering combustion
produces relatively larger smoke particle sizes than flaming combustion of a given material.
This is significant from smoke monitoring in terms of the use of an ionization or an optical
type sensor. Ionization sensors are more sensitive to the smaller smoke particles associated
with flaming combustion, whereas optical sensors are more sensitive to larger smoke particles
associated with smoldering combustion (1,3).

Novel candidate fire sensors have been evaluated. One type of sensor is an odor monitor
which responds to H,S, a product gas of combustion of sulfur containing coal. With respect
to coal combustion it was determined that an odor monitor and a pump mode ionization type
smoke sensor alarmed at nearly equivalent times for smoldering coal combustion, whereas for
flaming coal combustion, the smoke sensor alarmed earlier than the odor monitor (1,4). A
second novel candidate for mine fire detection is an ultrasonic ranging system. It was
demonstrated (5,6) with diesel fuel fires and smoke candle experiments in the PRC’s Safety
Research Coal Mine (SRCM) that the acoustic sensor responded to both heat and smoke as the
result of the refraction and absorption of ultrasonic waves transmitted through the heated air
and smoke. Ina fire research tunnel experiment it was shown that the acoustic sensor alarmed
at an optical density of 0.025m' in response to smoke from a coal fire. This is slightly greater
than the 0.022m" alarm associated with mine fire smoke detection.

The implementation of mine fire sensors as part of a mine fire detection strategy will depend
upon the growth rate of the fire, and the mine ventilation conditions. Mine fire sensor
spacings based upon known ventilation conditions can be specified to assure alarm settings to
be consistent with fire growth rates. In the case of zero, or low, ventilation the buoyancy
induced spread of products-of-combustion (POC) from the fire will determine the sensor
spacings. Experimental programs at PRC have addressed the comparative response of CO and
smoke sensors under normal and zero airflow conditions (2,8). This paper will discuss both
the laboratory and in-mine evaluation of smoke sensors, and how recommendations can be
made for fire sensors in zero airflow conditions.

LABORATORY EVALUATION OF SMOKE SENSORS

A protocol was developed to evaluate the response of smoke sensors to smoldering and flaming
coal combustion. Coal was selected because it is the most common fuel source in a mine. A
smoke chamber was constructed according to UL268 specifications (9) and used for evaluation
of five smoke sensors (1). The evaluation procedure led to reproducible rates of change in the
optical density characteristic of smoldering and flaming coal combustion, as well as in the
output signal from a standard ionization chamber. The optical density of the smoke was
measured with a light obscuration device consisting of a light source and a photocell. For the
laboratory experiments, an alarm was defined for sensors A, B, C, and E as an output signal