IRII 9380 I 

Fire Detection for Conveyor Belt Entries 

By Charles D. Litton, Charles P. Lazzara, 

and Frank J. Perzak 

i:: 

AA76-COM M-lO6B-Exhi bit# 2 
Mission: As the Nation’s principal conservation 

agency, the Department of the Interior hasresponsibility for most of our nationally-owned public 

lands and natural and cultural resources. This 

includes fostering wise use of our land and water 

resources, protecting our fish and wildlife, preserving the environmental and cultural values of 

our national parks and historical places, and providing for the enjoyment of life through outdoor 

recreation. The Department assesses our energy 

and mineral resources and works to assure that 

their development is in the best interests of all 

our people. The Department also promotes the 

goals of the Take Pride in America campaign by 

encouraging stewardship and citizen responsibility for the public lands and promoting citizen participation in their care. The Department also has 

a major responsibility for American Indian reservation communities and for people who live in 

Island Territories under U.S. Administration. 
Report of Investigations 9380 

Fire Detection for Conveyor Belt Entries 

By Charles D. Litton, Charles P. Lauara, 

and Frank J. Perzak 

RI 
Libnuy of Congress Cataloging in Publication Data: 

Litton, C. D. (Charles D.) 

and Frank J. Pen& 

Fue detection for conveyor belt entries / by Charles D. Litton, Charles P. Lazzara, 

p. an. - (Report of investigations; 9380) 

Indudes bibliographical references @. 14). 

Supt. of Docs. no.: I 28.23:9380. 

1. Cop1 mines and mining--Frrrs and fm prevention. 2 Fm dctectorr--?'er;ting. 

3. Mine haulagcSPfely applicanecf. I. Lezzara, Qwrler P. 

(Frank J.) III. Title. IV. Serier: Report of investigations ~~t~ States. 

of Miner); 9380. 

43 622 622" 
CONTENTS 

Page 

Abstract ........................................................................... 1 

Introduction ........................................................................ 2 

Descriptionofexperiments .............................................................. 3 

Smolderingcoalfires ................................................................. 5 

Flamingcoaffves .................................................................... 6 

Growth rates ..................................................................... 6 

Carbon monoxide and smoke generation ................................................. 7 

Styrene-butadiene rubber belt fires ....................................................... 8 

Polyvinylchloridebeltfires ............................................................. 9 

Nomograph usage-an example ........................................................ 12 

Detector spacing-an example ......................................................... 13 

Fire detection ....................................................................... 10 

Sensor spacings and alarm thresholds ................................................... 10 

Conclusions ........................................................................ 13 

References ......................................................................... 14 

L5 

16 

Appendix C.-Smoke optical density ...................................................... 19 

Appendix D.-Effective entry cross section and air splits ........................................ 20 

AppendixE.-list ofsymbols ........................................................... 22 

Appendix A.-Heat-release rates ......................................... : ............... Appendix B.-Replacing point-type heat sensors .............................................. 

ILLUSTRATIONS 

1. Schematic of Lake Lynn aboveground fire gallery ........................................ 4 

of7.53m2 .................................................................... 6 

Production constant for CO for flaming coal fires ........................................ 7 

8 

of7.53 m2 .................................................................... 8 

9 

11 

12 

18 

18 

2. Times to produce CO and smoke alarm levels for smoldering coal for an entry cross section 

3. 

4. Production constant for smoke for flaming coal fires ...................................... 5. Tmes to produce CO and smoke alarm levels for flaming coal fires for an entry cross section 

6. 

7. 

8. Nomograph for smoke sensor alarm thresholds and spacings ................................ B-1. Thermal alarm times as function of air velocity for surface fire gallery tests ..................... B-2. Thermal alarm times as function of entry cross section at velocities of 0.254 and 0.508 m/s ......... 

Average times Gom belt ignition to onset of belt flame spread for SBR conveyor belts ............. Nomograph for CO sensor alarm thresholds and spacings .................................. 

TABLES 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

-1. 

Description of conveyor belt tested ................................................... Measured and predicted CO levels for smoldering coal dust just aming................. 

Estimated optical density levels at time of smoke alarm ................................... Large-scale gallery test data for ignition of rubber belts ................................... 

Times to belt ignition and peak fire intensities for PVC conveyor belt ......................... 

e~eriments ......................... 

Large-scale gallery test data for SBR belt fires .......................................... 

Location of sensors along example belt entry ........................................... , X, k,---,,,, and k, for combustibles use 

4 

5 

5 

7 

15 
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 

degree Celsius 

centimeter 

degree Fahrenheit 

foot 

square foot 

foot per minute 

gram 

gram Per €?'am 

gram per cubic meter 

gram per second 

kilogram 

kilogram per meter (linear) 

kilojoule 

kilojoule per gram 

kilojoule per meter 

kW 

kW/min 

m 

m-l 

m2 

m3 

min 

mm 

Pm 

3/min 

m/s 

PPm 

S 

V 

W 

kilowatt 

kilowatt per minute 

meter 

inverse meter 

square meter 

cubic meter 

minute 

millimeter 

micrometer 

cubic meter per minute 

meter per second 

part per million 

second 

volt 

watt 
FIRE DETECTION FOR CONVEYOR BELT ENTRIES 

By Charles D. Litton,' Charles P. Lazzara,' and Frank J. Pemk3 

ABSTRACT 

This U.S. Bureau of Mines report details the results of a series of large-scale experiments where 

small coal fwes were used to ignite the conveyor belt at air velocities ranging from 0.76 to 6.1 m/s. In 

the tests, electrical strip heaters imbedded within a pile of coal were used to heat the coal to a point 

of flaming ignition. The flaming coal subsequently ignited the conveyor belt located approximately 5 to 

10 cm above the coal pile. During the tests, temperature, CO, and smoke levels were continuously 

measured in order to determine both alarm time and level as the fire intensity progressed through the 

stages of smoldering coal, flaming coal, and flaming coal plus flaming belt. 

Analysis of the data leads to certain conditions of air velocity and sensor alarm levels that are 

required for early detection of conveyor belt entry fues. Two nomographs are presented, which define 

sensor alarm levels and sensor spacings as a function of belt entry cross-sectional area and belt entry 

air velocity. 

'Supervisory physical scientist. 

%upervisory research chemist. 

'Research chemist. 

Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA. 
2 

INTRODUCTION 

Fire represents one of the most severe hazards in 

underground coal mines. The heat and combustion 

products liberated are carried downstream from the fire by 

the ventilation airflow, eventually contaminating areas of 

a mine far removed from the fire. The ventilation airflow 

serves to dilute the combustion products, thus lowering 

their concentration. The higher the airflow, the greater 

the dilution. Combustion products also spread more 

rapidly at higher air velocities than at lower air velocities. 

These effects are somewhat obvious. The effects that the 

airflow has on the growth and burning characteristics of 

the fire are not so obvious. 

For many fires that develop within conveyor belt 

entries, it is found that coal heats to the point of flaming 

because of frictional overheating in the belt drive area or 

near idlers along the belt structure. When the conveyor 

belt is stopped, the coal fire then spreads to the conveyor 

belting, and if the conveyor belt has poor flame-resistant 

properties, the flame will begin to propagate along the 

exposed surfaces of the conveyor belt. As the surface area 

of the burning conveyor belt increases, so does the total 

fire intensity, along with increases in the levels of smoke 

and CO that are produced. Typical fires in belt entries 

develop in three distinct stages: 

1. Early smoldering stages of coal heated, due to 

overheated equipment or friction, to the point of flaming; 

2. kly flaming stages of a small coal fire, which 

ignites a stationary conveyor belt; 

3. Combined coal and conveyor belt fire, which increases in intensity to the point of sustained belt flame 

spread. 

The time it takes for the fire to develop through these 

various stages depends upon many factors. The duration 

of the smoldering stage depends upon the temperatures of 

the overheated equipment, the quantity of coal involved, 

and the proximity of the source of heating to the exposed 

surfaces of the coal pile. The size of the coal (i.e., dust 

or lumps, or a mixture of the two) also has an effect. This 

stage of development may take minutes or hours before 

the coal begins to flame. During this stage, CO and 

smoke are produced, with the quantities produced depending upon the size of the coal, the mass of the coal, the 

temperature of the coal mass involved, and other factors. 

Once ignited, the coal fire intensity begins to increase. 

The rate of increase depends upon the air velocity and the 

surface area of coal available for burning. Subsequent 

ignition of the conveyor belt depends upon the proximity 

ch~~~teristi~ of 

the belt material, and the air velocity. Once the belt 

ignites, usually near the lateral edges of the belt, the flame 

will begin to spread over the surface of the belt in the 

vicinity of the source coal fire. The rate of spread, locally, 

depends upon the air velocity and the flame-spread 

characteristics of the belt material. If the belt material has 

poor flame-resistant properties (i.e., it propagates flame 

easily), the combined local coal and belt fire will attain 

sufficient intensity so that the flame begins to spread away 

from the original ignition area along the exposed surfaces 

of the belt and in the direction of the airflow. If the belt 

has good flame-resistant properties (i.e., it is difficult to 

propagate flame), local burning will occur only in the 

vicinity of the coal fire, with no propagation of the flame 

along the belt surfaces. For a conveyor belt with poor 

flame-resistant properties, the time it takes for the fire to 

begin to propagate downstream, away from the ignition 

area, depends upon the air velocity and the flame-spread 

characteristics of the belt material. 

In general, if the fire reaches a size sufficient to begin 

flame spread down the belt, the effectiveness of control 

and extinguishment procedures dimiihes rapidly. In 

addition, the levels of smoke and CO produced begin to 

approach dangerous levels, and lethal levels may subsequently result during the propagation stage. Consequently, 

for fires in belt entries, all evacuation and control procedures should be implemented prior to the onset of belt 

flame spread. 

Clearly, any measure that can be taken to reduce or 

eliminate the possibility of the occurrence of belt fires 

should be done. For instance, diligent housekeeping procedures to eliminate coal spillage in a belt drive-belt 

takeup area reduces the potential for the source coal fire 

to develop. Maintaining slippage switches to reduce the 

occurrence of frictional heating also reduces the potential 

for development of the fire. Along the belt entry, continuous vigilance for overheated idlers, which can serve as the 

initiator for the fire, is beneficial. Use of belt materials 

that have superior fire-resistance characteristics will reduce 

the possibility of belt flame spread (1): Automated extinguishment systems that are activated in the early stages of 

fire development can reduce the potential for belt flame 

spread. 

The occurrence of any fire at any stage of development 

represents a potential hazard to underground personne 

If, and when, a fire develops, the detection of that fire at 

the earliest possible moment is paramount to secure the 

. 

41talic numbers in parentheses refer to items in the list of references 

preceding the appendixes at the end of this report. 
3 

safety of underground personnel and ~ucce~~fully control 

and extinguish the fire. It is obvious that detection of any 

developing fire prior to open flaming is always desirable. 

If the duration of this stage of development is long 

(several minutes to an hour, or longer) and a sufficient 

mass of coal (or other combustible) is involved, the 

probability of detecting the fire during this stage will be 

high. However, a small flaming coal fire may result from 

an intense smoldering stage that may last only a few 

minutes and may initially involve a small mass of coal. 

For this situation, the probability of detecting the fire in its 

smoldering stage is reduced. 

In general, a flaming coal fire follows the smoldering 

stage of development, During this flaming stage, the fire 

may grow in intensity until, eventually, the conveyor belting 

is ignited. The probability of detecting a fire in this stage 

of development depends upon how fast the fire grows 

and at what fire size belt ignition is achieved. The slower 

the growth rate of the flaming coal fire, the higher the 

probability that it can be detected prior to belt ignition. 

It is imperative that the relative times for transition of 

the fire from one stage to the next, along with the levels of 

CO and smoke produced during each stage, be quantified 

as accurately as possible. To obtain this information, the 

U.S. Bureau of Mines conducted a series of large-scale 

gallery tests at air velocities from 0.76 up to 6.1 m/s using 

a small coal fire to ignite rubber and polyvhyl chloride 

(PVC) belt materials. This work was done as part of the 

Bureau’s program to enhance mine safety. 

DESCRIPTION OF EXPERIMENTS 

The large-scale experiments for belt fire detection were 

conducted in the Bureau’s aboveground fire gallery located 

at Lake Lynn Laboratory. The fire gallery consists of a 

27.4-m-long tunnel constructed of masonry block walls, a 

steel arched roof, and a concrete floor. The tunnel is 

coupled to a 1.8-m-diameter, 3,500-m3/min axivane fan via 

a 6-m-long tapered transition section. The ventilation flow 

can be varied by adjusting the pitch of the fan blades 

and/or by throttling the fan intake. A schematic of the 

gallery is shown in figure 1. The cross-sectional area of 

the tunnel is 7.53 m2. The interior walls and roof of 

the tunnel are covered with ceramic blanket insulation. 

Tunnel distances are measured from the junction of 

the fire tunnel and transition section, designated as the 

O-m mark. A typical conveyor belt frame, 21 m long and 

1.5 m wide, is centered in the tunnel. The frame consists 

of a 0.4-m-diameter tail pulley and 0.W-m-diameter 

troughed idler assemblies spaced at 1.2-m intervals. 

A small coal pile fire, located just downstream of 

the tail pulley, was the ignition source for the tests. The 

0.5-m-deep coalbed, supported on a steel grate, consisted 

of about 320 kg of 5 cm or smaller pieces of Sewickley 

seam coal (35% volatile matter, 14% ash), with a minimum of fines. The top surface of the pile was 0.6 m long 

(along the length of the belt structure) by 0.9 m wide. To 

initiate a coal fire, six electrical strip heaters (three from 

each side) were imbedded about 5 cm below the top surface of the pile. Each strip heater was 1.9 em wide by 

49 cm long (heated length of 40 cm) and was rated at 

1,OOO W at 240 V. The voltage to the strip heaters was 

controlled by a variable transformer. For a test, the heater voltages were maintained as follows: 0 to 5 min, 80 V; 

5 to 15 min, 140 V, and 15 min to shut off, 190 V. The 

heaters were turned off after the coal fire ignited the belt 

sample and the belt fire was well developed in the ignition 

area. 

Typically, a 6.2-m-long sample of conveyor belting, with 

the top cover up if applicable, was placed on the rollers of 

the belt structure, stretched over the coal pile, and 

fastened to the tail pulley. The distance from the top 

surface of the coal pile to the bottom surface of the belt 

sample was 5 to 10 cm, and the distance of the belt sample 

to the tunnel roof was about 1.2 m. Thermocouples were 

imbedded just below the top surface of the belting, starting 

at a point above the coal pile and continuing 4.6 m downstream, to determine when the fire spread out of the 

ignition area. 

The gallery was instrumented with thermocouples to 

measure gas temperatures. An array of 12 thermocouples, 

connected in parallel and distributed over the crosssectional area of the tunnel, was located at 24.4 m to 

measure the average temperature of the stratified gas exit 

stream. 

A gas and smoke sample averaging probe was positioned at a tunnel distance of 25.9 m, about 21.4 m 

downstream of the coal pile fire. This probe, constructed 

from nominal icm-diameter steel pipe, had four inlet 

ports spaced along the vertical height of the tunnel in 

order to estimate the average smoke and gas concentrations in the exit gas stream. The sample was analyzed for 

smoke, CO, and CO,. In addition, a smoke and a CO 

detector were located near the roof of the tunnel near 

exit at the 26.7-m tunnel position. 

connected to a 60-channel microprocessor that trans 

the data to a computer for storage. The data were log 

The outputs of the thermocouples and analyzers were 
4 

END VIEW 

Figure 1cSchematic of Lake Lynn aboveground fire gallery. 

at 15s intervals and displayed on a computer terminal. 

After a test, time-temperature traces and gas concentration plots were generated from the stored data. The experiments were also recorded on videotape. 

The experiments were conducted at gallery airflows of 

0.76, 1.52, 4.06, and 6.1 m/s. The airflow was measured 

prior to the start. of a test by a handheld anemometer 

across the width of the belt sample (at a height of 25 cm 

above the belt) and at three locations along the sample 

length, and then these values were averaged. The average 

airflow near the exit of the tunnel was also measured. The 

airflow fluctuated, especially at the high flows, but was 

within +15% of the selected value. 

A description of the conveyor belting tested is given 

in table 1. All the belts were obtained new from cooperating belt manufacturers. Belt R1 is considered to be 

non-fire-resistant because it failed the current small-scale 

Federal approval test for fxe-resistant belting (2). Belts 

R4, R11, and P1 passed the test and were considered 

fue-resistant. Belts R11 and 1 were test& at all four 

airflows9 while belt R4 was te 

belt R1 at airflows of 0.76, 1.52, and 4. 

Table 1 .--Description of conveyor belt tested 

Width, Thickness, Weight, AreBelt Construction m mm kg/m resistant 

Quality .- R1 . . . &ply SBR, 1.07 15 17.8 NFR 

7-mm top cover, 

2-mm bottom 

cover. 

R4 . . . Chloroprene, 1.07 9 14.3 FR 

solid woven, 

3-mm top cover, 

2-mm bottom 

cover. 

R11 . . &ply SBR, 1.07 11 14.9 FR 

5-mm top cover, 

2-mm bottom 

cover. .. 

P1 . . . Solid woven PVC 1.07 11 14.2 FR 

FR Fire-resistant. Passed Federal approval test (2). 

NFR 

PVC Polyvinyl chloride. 

SBR Styrene butadiene rubber. 

Non-fire-resistant. Failed Federal approval test (2). 
5 

SMOLDERING COAL FIRES 

In the experiments, the period of smoldering of the 

coalbed prior to flaming is controlled primarily by the time 

required to raise the surface temperature of the strip 

heaters to temperatures sufficient to ignite the coal. For 

all tests conducted, the average time from ,the start of the 

test untii the time that flames were first oherved on the 

coal pile was 23.1k3.0 min. On the average, the time of 

smoldering (as measured from the time of first visible 

smoke until the time of flaming) was 10 min. It is during 

this period that low levels of smoke and CO are produced, 

and even though the duration of this period is controlled 

by the heater temperature, the levels of CO produced just 

prior to flaming can provide some insight as to the 

influence of air velocity on the generation rates of CO in 

these experiments. 

At any point in time, the levels of CO produced wichin 

an entry with a defined airflow rate can be expressed as 

Gco ppmC0 = -, 

V&O 

3 

where Ga = generation rate of CO, ppmm , S 

vo = air velocity, m/s, 

and A,, = entry cross-sectional area, m2. 

In a dynamic situation, Ga is not constant, but 

increases with time. Ga may also depend upon v,. To 

determine if Gm is air velocity dependent, the average 

levels of CO existing just prior to flaming were measured 

and Gco was determined from equation 1. Also, the 

change in Gay AGa, measured during the smoldering 

interval, At, was put in the form 

where a = average rate of production of CO during 

At. 

The value of a was tabulated for all tests and then 

the average value at each air velocity was determined. A 

le~t-squ~es regression of the data yielded the expression 

(3) 

then be written as 

(4) 

where the time, t? is measured from the onset of 

smoldering. 

Using equation 4, the measured and predicted levels of 

COY just prior to flaming, are compared in table 2. 

Table 2-fWeasured and predicted 

CO levels for smoldering coal 

dust just prior to flaming 

v, m/s g, rnin GO, PPm 

Measured . - __ - - - Predicted 

0.76 11.8 6.7 7.4 

1.52 11.2 4.6 4.7 

4.06 8.7 3:3 2.5 

6.1 6.2 1.3 1.6 

Equation 4 can also be used to predict the time, (tJm, 

a smoldering period would have to exist until certain levels 

of CO are formed that are equal to the CO sensor alarm 

thresholds, CO,, in parts per million. The equation that 

defines this time is given by 

For smoke sensors, previous data (3) from tests in an 

intermediate-scale fire tunnel indicate that the average rate 

of smoke production, G,, wh subscript "D" refers to 

optical density of the smo appendix C), from 

smoldering coal fires is 0.024 times the rate of CO 

production. By using one-half this value for an increased 

safety factor, the time, (tJ,, for the smoldering coal fire 

to produce some specified smoke alarm threshold level, 

DA, (units of inverse meters) is given by the expression 

In the large-scale gallery tests, the average times of 

actual smoke detector alarm, as measured from the time 

of first visible smoke, can be used in equation 6 to determine the approximate levels of optical density existing at 

the time of alarm. These levels are computed in table 3. 

Table 3.Xstimated optical density level8 

t time of smoke alarm 

Air velocity, Average time to Estimated optical 

m/s smoke sensor density, rn-l 

0.76 8.3 0.063 

1.52 7.0 .035 

4.06 9.2 .032 

alarm, min 
6 

For the two tests conducted at an air velocity of 

6.1 m/s, coal flame occurred at 8.4 min in the first test 

and at 4.0 min in the second test, For these two tests, the 

estimated smoke optical densities just prior to flaming are 

0.026 m-I and 0.013 m-l, respectively. Based upon the 

estimated alarm values from table 3, the levels of 0.026 

and 0.013 m1 were below the alarm threshold level for the 

smoke sensor. At this velocity, a smoldering period of 

about 14.0 min would have been sufficient to produce 

alarm prior to the onset of flamii as shown in figure 2. 

Figure 2 is a plot of the approximate smoldering times 

necessary to produce the indicated CO and smoke alarm 

levels as a function of the air velocity. Figure 2 also shows 

the average smoldering times, f, observed in the experiments at each air velocity. These data indicate that at 

higher air velocities, the duration of the smoldering period 

decreases. When the velocity exceeds about 2.54 m/s, the 

smoldering stage would not be detected by either 5 ppm 

CO sensors or 0.044 m-l smoke sensors. 

The generation rates of CO and smoke discussed above 

are specific to the arrangement of the experiment. For 

example, if more heaters were used, the coal surface area 

subjected to heating would increase, thus increasing the 

rates of production of CO and smoke. The surface area 

of a typical bottom roller along a conveyor belt is about 

0.5 m2 compared with the total surface area of the strip 

heaters used in the ' ents of about 0.10 m2. Thus, 

if the surface temp of the roller reached 500" C, 

typical of the heater surface temperature of the strip 

heaters, about five times more CO and smoke would be 

produced than experimentally observed. Fewer heaters 

(less surface) would have produced less CO and smoke. 

If the surface temperature of the heaters was lower than 

that used in the experiments, less 60 and less smoke 

would be produced. The experimental arrangement was 

v,, m/s 

0 2.54 5.08 762 

20 

Data from iargescale tests 

500 1,000 1,500 

v,, ft/min 

Figure P.-Tlmes to produce CO and smoke alarm levels for 

smolderlng coal for an entry cross sectJon of 7.53 m? 

intentionally sized to create a small coal fire sufficient to 

ignite the belt within a reasonable time. 

FLAMING COAL FIRES 

GROWTH RATES 

In the large-scale gallery tests, the times from the 

instant of flaming ignition of the coalbed until 

that flames were first observed on the conveyor belt were 

measured. The fire intensity at the moment of belt ignition was then calculated from the CO, and CO gas levels 

as outlined in appendix A. At the time of belt ignition, 

the coal fires were still quite small, so there exists some 

degree of uncertainty in the measured gas levels. 

The total heat-release rates from the coal fire, 

at the time of belt ignition; the time elapsed fr 

time, ama; and the ratio of fire inte~ity to ventilation 

velocity, Qdv,,, at the time of belt ignition for tests 

conducted with rubber conveyor belting at indicated v, 

are shown in table 4. 

sing the average values of amU at ea 

a least-squares regression of the data yields 
7 

Table 4.4.arge-scale gallery test data for ignition 

of rubber belts 

Belt 

R11 . . R1 ... R11 . . R4 ... R1 ... Rll . . R1 ... Rll . . 

Test vo* QCoa, 

m/s kW 

81A 0.76 20 

.82 .76 25 

78 1.52 60 

79 1.52 50 

84 1.52 40 

85 1.52 30 

77 4.06 140 

80 4.06 95 

tBP 

min 

8.5 

13.0 

26.0 

12.0 

11.0 

6.0 

24.5 

20.0 

PCOALV 

kW/min 

2.4 

1.9 

2.3 

4.2 

3.6 

5.0 

5.7 

4.8 

QCOALA 

kJ/m 

26.3 

33.1 

39.1 

33.1 

16.6 

19.6 

34.6 

23.3 

Ull R7 6.10 NA 24.6 NA NA 

NA Not available. 

From column 7 of table 4, it is also found that the average ratio of fire intensity to ventilation velocity is 28.2 at 

the time of belt ignition for the rubber conveyor belts. 

This means that the fire intensity sufficient to ignite the 

belt is a linear function of air velocity. 

CARBON MONOXIDE AND SMOKE GENERATION 

For open, flaming fires, the generation rates of CO and 

smoke are dependent upon the total heat-release rates via 

the expressions 

and 

Gco = Bco* Qcom (9) 

where B, BD = production parameters for CO and 

smoke, respectively. 

Because this stage of fire development is that of open, 

flaming combustion, the rates of production of CO and 

smoke depend upon the stoichiometry of the fuel-air 

mixture that is reacting. As the fuel-air mixture decreases 

from its stoichiometric level toward its lean limit of 

flammability, the levels of CO and smoke produced will 

also decrease. At the other extreme, as the fuel-air 

mixture increases above its stoichiometric level toward its 

rich flammability limit, the levels of CO and smoke 

produced will increase. 

In the early stages of fire growth, excess air is usually 

available for combustion of the fuel. As a result, such 

fires will generally burn on the lean side of the stoichiometric level. Further, the rates of production of CO and 

smoke are sensitive to the fuel-air mixture on the lean 

side, and it is these rates of production that determine the 

ability of GO and smoke fue sensors to detect fires in 

their early stages of flaming. 

To determine what, if any, effect air velocity has on the 

production of CO and smoke, the data were analyzed as 

a function of the air velocity. These data were supplemented by a series of tests of small coal fires in an 

intermediate-s~le fue tunnel at air velocities ranging 

from 0.53 to 7.7 m/s. During the intermediate-scale fire 

tests, the optical density of the smoke was also measured. 

Figure 3 is a plot of the combined data for CO from 

both the large-scale gallery and the intermediate-scale 

tunnel tests showing that the coal fires produced less CO 

at the higher air velocities. As a result of this behavior, 

the times to reach an alarm level of CO, during the growth 

stages of a coal fire, depend not only upon the growth rate 

of the fire, but also upon the rates of production of CO, 

both quantities being dependent upon the ventilation air 

velocity. 

Data for the smoke optical density acquired in the supplemental intermediate-scale tests are shown in figure 4. 

For smoke, the production rate shows a similar dependence on air velocity as observed for CO. Previous data for 

smoke production from coal fires were obtained at an air 

velocity of 0.38 m/s and yielded a value of 0.036 for BD, 

which agrees with the data of figure 4 (4). 

As was the case for the smoldering coal fires, equations 

7 through 10 may be used to estimate the times for small 

flaming coal fires to generate alarm levels of CO and 

smoke. For CO, the expression is 

while for the smoke, the expression is 

The resulting curves are shown in figure 5, where the time 

is measured from the instant of flaming ignition, assuming 

no smoldering exists prior to flaming. The results of this 

analysis are similar to those for the smoldering case. 

Figure 5 also shows the average times, denoted by 4, at 

each air velocity, between coal ignition and belt ignition. 

v,, m/s 

0 2.54 5.08 7.62 

4.8 

4.0 

3.2 0 

V 

m 

2.4 

I .6 

I I 

0 500 1,000 1,500 

v,, ft/min 

Figure 3.-Production constant for 60 for flaming coal fire 

The curve is defined by the expression B, = 4.80 e5J7ko mJ*. 
8 

vo, m/s vo, m/s 

0 2.54 5.08 762 

0.04 I 1 

.03 

P 

m 

.02 

0 Dota from intermediatescale tunnel 

- Least-squares regression 

I I .o I 

0 500 1,000 1,500 

vo, ftlmin 

Figure 4.--Production constant for smoke for flaming coal 

fires. The curwe is defined by the expression B, = 0.037 ea%'+. 

0 2.54 5.08 262 

.- c 

20 

30 1 

E 

c10 

0 

KEY 1 

-*- 0,. 0.044 m-1 

0 Data from largescale tests 

/- I' I I 

500 1,000 1,500 

vo, ft/min 

Figure 5.-Times to produce CO and smoke alarm levels for 

fiaming coal fires for an entry cross section of 7.53 m2. 

STYRENE-BUTADIENE RUBBER BELT FIRES 

Once the small coal fire ignites the belt, the total heatrelease rate increases dramatically because of the additional fuel supplied by the belt. The total fire intensity 

during this stage of fire development is due to both the 

coal fire and the styrene-butadiene rubber (SBR) belt fire. 

To determine the rate of growth of the belt fire and the 

ratio of total fire intensity to ventilation airflow at the 

beginning of belt flame spread, tern, the total fire intensity, 

a,,,,, was determined at the time the belt flame spread 

began. The coal fire intensity at this time was determined 

from equation 8 and subtracted from the total fire 

intensity. This fire intensity difference was then assumed 

to be due only to the burning belt, and when this quantity 

is divided by the difference in time between t, and tBI, 

the growth rate of the conveyor belt portion of the fire can 

be determined. The data used to make these determinations are shown in table 5. 

From the data in table 5, it is found that the average 

ratio of total fire intensity to vent~ation velocity was 323, 

independent of the air velocity, at tBm. It was also determined that the fire growth rate for the belt could best be 

put in the for 

where QS,, = heat-release rate of SBR conveyor 

belt fire, kW, 

and (TSBR = growth-rate parameter for SBR conveyor belt fire, kW/min. 

A least-squares regression analysis of the average values of oSBR obtained at each air velocity yielded the 

expression 

aSBR = (14.0 +- 1.5 v,) v,. (14) 

The average times at each air velocity between belt 

ignition and belt flame spread are shown in figure 6. As 

the air velocity increases, this time gradually decreases. 

For the frre-resistant rubber belting (Rll), the rates of 

generation of CO were found to be constant with a B, 

value of 5.68, independent of the air velocity. Using 

previous data (3) for smoke levels, the 

belts is 0.062. 
9 

VO , rn /s 

0 2.54 5.08 7.62 

0 Data for SBR 

conveyor belts 

0 

.- c 

E 

.. 

n 

m t 

I 

2 

m t 

v,, ft/min 

Figure 6.4verage timer from belt ignitlon to onset of belt flame spread for SBR conveyor 

belts. The curve is defined by the expression tBR - t,,, = 17-0.8~" m/s. 

Table 5.4rge-scale gallery test data for SBR belt flres 

Belt Test V, %&$ tBmv &JAL.* &BRI tBFS- tBI, %BR* bAdvo' 

m/s kW min kW kW min kW/min kJ/m 

R11 . . 81A 0.76 250 24.0 61 189 15.5 12.2 328 

R1 ... 82 .76 245 30.5 78 167 17.5 9.5 321 

R11 .. 78 1.52 480 44.5 142 338 18.5 18.3 315 

R1 . , . 84 1.52 515 23.0 73 442 11.0 38.1 338 

R11 . . 85 1.52 490 23.0 73 417 17.0 24.5 321 

R1 . . . TI 4.06 1,270 37.5 201 1,069 13.0 82.2 312 

Rll .. 80 4.06 1,320 35.5 190 1,130 15.5 72.9 325 

R11 . . 83 6.1 1,970 36.5 259 1,711 12.0 142.6 323 

POLYVINYL CHLORIDE BELT FIRES 

For tests conducted with VC conveyor belting, it was 

found that the P1 belt was more readily ignitable by the 

small coal fires than the SBR belt. However, it was 

also found that the P1 belt did not propagate flame. The 

data for this series of tests with the P1 belt are shown in 

table 6. At ignition of the P1 belt, the ratio of coal fire 

intensity to ventilation velocity was dependent upon the air 

velocity according to the expression 

-0.13~~ coAL/vo 2 27.5e 

It was also found that once the P1 conveyor belt 

ignited, its rate of fire growth was less than that for 

e SBR belt. For 

parameter was foun 

~pvc = (7.0 + 0.95 v,) V, Y2 , 

where apVC = ~o~-rate parameter for 

veyor belt fire, kW/m 

the resultant fue ~owth rate given by 

where Qwc = heat-release rate of 

belt fire, kW. 
10 

Table B.-TImer to belt Ignition and peak fire Intensities 

for PVC conveyor belt 

T8St V, tBt* dam da&,, tp,, opvc, uPVc* 

m/s min kW kJ/m min kW kW/min 

86 . . . 1.52 6.6 39.0 25.7 15.1 130 15.3 

101 . . 1.52 6.0 27.0 17.8 25.9 118 5.9 

102 . . 4.06 10.7 34.0 8.4 20.0 233 25.1 

105 . . . .76 6.6 20.0 26.3 17.5 80 7.3 

106 . . 6.10 18.0 80.0 13.1 24.5 217 33.4 

107 . . 4.06 20.8 92.0 22.7 28.3 127 16.9 

For the P1 belt, the CO data yielded an average value 

of B, = 11.2, a factor of 2 greater than that for the rubber belts. Again, using previous data (3) for the relative 

smoke level, the B, value for the P1 belt is 0.072. 

For both the PVC and SBR belts, the time from belt 

ignition necessary to produce CO and smoke alarm levels 

is very short because of the large growth rates of the belt 

fires. Burning belts typically produce sufficient CO and 

smoke to meet or exceed sensor alarm levels within 

several seconds from the time that they are ignited. 

FIRE DETECTION 

When a fire occurs, three events must take place in 

order for the fire to be detected. 

1. The fire must be large enough to produce alarm 

levels of the fire characteristic that is to be detected. For 

instance, if the fire characteristic to be detected is CO and 

the alarm level is 5 ppm of CO, then the fire must be 

large enough to produce 5 ppm of CO within the ventilation airflow. This implies, then, that a finite amount of 

time must elapse before this event can occur. The estimated times for specified alarm levels of CQ and smoke, 

based upon the data obtained in the experiments, are 

shown in figures 2 and 5. Lower alarm levels will require 

less time for this event to occur, while higher alarm levels 

will require more time. 

2. Once a characteristic alarm level has been reached 

at the fire source, then this level of CO or smoke must be 

transported from the fire to the sensor location by the 

ventilation airflow. For fires along conveyor belt entries, 

the maximum transport time is equal to the sensor spacing 

divided by the ventilation air velocity. For instance, if the 

sensor spacing is 304.8 m (1,OOO ft) and the air veIocity is 

1.016 m/s (200 ftlmin), then themaximum transport time 

is 300 s (5 min). In general, the location of fires along 

conveyor belt entries is most uncertain. As a consequence, 

the probability that a fire will occur very close to a sensor 

is the same as for a fire occurring one sensor spacing from 

the sensor. On the average, then, CQ or smoke will have 

to be transported a distance equal to one-half the sensor 

spacing, t,. The transport time, $, in minutes, then, can 

be defined by 

maximum distances of about 30.5 m from the belt drive 

area. For these cases, the transport time is relatively short 

owing to the small distance involved. 

3. Once the above level of CO or smoke reaches the 

sensor, then the sensor takes a finite amount of time to 

respond, t,. In general, CO or smoke sensors have 

response times in the range of 30 to 60 s. 

The total time that elapses until the fire is detected is 

the sum of these individual times. For CO, this detection 

time, (tD)CO? is 

while for smoke, this detection time, (tD)D, is 

for sensors located along a belt entry at some specified t,. 

For sensors near the belt drive area, the times to detect 

a fire are given by 

and 

For fire detection along conveyor belt entries downstream of the separately protected belt drive area, the 

detection times at low air velocities are limited by the 

transport time of the CQ or smoke to the sensor. At 

higher air velocities, detection times are limited by the 

time it takes to produce alarm levels of CO and smoke 

owing to the lower production rates of CO and smoke (see 

figures 3 and 4) and to greater dilution at the higher 

airflows. 

SENSOR SPACINGS AND ALARM THRESH 

Both the spacing and the alarm threshold used for 

a given sensor should e capable of satis~g some 
11 

500 

450 

400 

350 

300 

250 

225 

200 

175 

minimum constraints. By using the data in tables 4 and 6, 

the testwide average time from flaming ignition of the coal 

until ignition of the belt occurs is 14.25 min. By using this 

information, the following constraint may be placed upon 

the use of belt entry fire detection systems and its impact 

evaluated: The system nzust detect a small, flaming coal 

fire within a time, measured from the moment of ignition 

of the coal fire, of 14.25 min or less. 

By using this constraint and equations (ll), (12), (18), 

and (19), a determination can be made, for either CO or 

smoke sensors, as to the spacings and alarm thresholds of 

those sensors in a belt entry as a function of entry air 

velocities and entry cross-sectional areas. 

The results of this determination, assuming t, = 1.0 

min, are shown in figures 7 and 8 for CO sensors and 

smoke sensors, respectively. Each of these figures is a 

nomograph that uses the belt entry cross-sectional area 

and air velocity to determine the sensor alarm levels for 

either 305-111 (1,OOO-ft) or 610-m (2,OOO-ft) spacings. These 

are the maximum allowable spacings for CO and smoke 

sensors. In an actual situation, the spacing may be 

somewhat less, depending upon the total length of the 

- 

- 

- 

- 

- 

- 

- 

- 

- 

1,250 

1,000 

900 

800 

700 

600 

500 

400 

300 

250 

200 

I50 

I00 

as= 1,000 ft 

5 

6 

7 

8 

9 

10 

P 

50 

25 

00 

75 

entry to be protected. These maximum spacings would be 

used only if the length of entry is an exact multiple of 

either 1,OOO or 2,000. Further, if the belt entry contains 

more than one belt drive, the distance between any two 

belt drives would contain sensors at some specified 

spacing, while the distance from the second belt drive to 

the tailpiece would contain sensors, possibly at a different 

spacing, but not exceeding either 1,000 or 2,000 ft. (See 

the “Detector Spacing-an example” section.) 

Each nomograph is actually a composite of two 

nomographs. The left-hand side is for maximum sensor 

spacings of 1,000 ft while the right-hand side is for 

maximum sensor spacings of 2,OOO ft. In the nomograph 

for CO sensors shown in figure 7, the maximum alarm 

threshold for CO is limited to 10 ppm for sensors spaced 

at 1,OOO-ft intervals and 8 ppm for sensors spaced at 2,000- 

ft intervals. 

Figure 8 is a duplicate of figure 7, except that the vertical alarm scales are in units of inverse meters of optical 

density. In figure 8, the 0.044-m-’ value corresponds to a 

class 2 smoke detector and the 0.022-m-’ value to a class 1 

smoke sensor. The afarm scale is limited to a maximum 

1,500 

1, = 2,000 ft 

1,250 

co,, PPm 

I 

1,000 

900 

2 800 

3 

4 

5 

6 

7 

700 1 600 

- 

8 

8 

300 

25 

20 

01 

Figure 7.4omograph for CO sensor alarm thresholds and spacings. 
12 

value of 0.044 m-l. The minimum value has been extended 

to include an alarm threshold of 0.011 ml, or twice as 

sensitive as a class 1 sensor. Reliable smoke sensors at 

this high level of sensitivity (0.011 m-l) may or may not 

even be available, but this alarm level is included for 

completeness. 

The manner in which these nomographs are to be used 

is as follows: 

1. Determine the entry cross-sectional area, A,, in 

square feet. It is recommended that the value for A, 

be the geometric cross-sectional area, which is the product of entry height and width. (See Appendix D for 

modifications.) 

2. Determine the entry air velocity, v,, in feet per 

minute. The value used should be representative of the 

average velocity measured along the length of the entry. 

(See Appendix D for modifications.) 

3. For 1,0004 spacings, draw a straight line from the 

left-hand v, scale to the value of A,. This line intersects 

the CO alarm scale, or the smoke alarm scale, at the 

appropriate alarm level for this combination of v, and A,. 

4. For 2,OOO-ft spacings, draw a straight line from the 

right-hand vo scale to the value of A,. This line intersects 

1,500 r R,= 1,000 ft 450 500 [: 

t I, 250 

I, 000 0.01 I 

800 "i 500 

.022 

044 

400 t 

350 

300 

25 0 

200 

I50 

00 

O.( I 

300 

250 

225 

200 

I75 

50 

25 

00 

75 

the CO alarm scale, or the smoke alarm scale, at the 

appropriate alarm level for this combination of v, and &. 

5. When the indicated alarm level falls between two 

values, the lower value should be used. 

NOMOGRAPH USAGE-AN EXAMPLE 

Mine " X desires to reconfigure its ventilation system so 

that belt entry air may be used to ventilate a working 

section. The average cross-sectional area of the belt entry 

is 100 ft2. With the new configuration, the average air 

velocity in the belt entry is expected to be 400 ft/& but 

under certain conditions, the average air velocity may be 

as high as 700 ft/min. The mine operator proposes to use 

5 ppm CO sensors spaced at intervals of 1,000 ft. Will this 

sensor alarm level and spacing be adequate? 

From figure 7, the mine's entry cross section and 

average velocity of 700 ft/min yield an alarm level of 

4 pprn for both 1,000-ft spacings and 2,0004 spacings. At 

the average velocity of 400 ft/min, the nomograph yields 

an alarm level of 7 ppm for 1,000-ft spacings or 6 ppm for 

2,00043 spacings. 

/&= 2,000 ft 

I, 500 r 

c 1.250 

0.01 I 

.022 

.04 

0. 

1,000 

900 

800 

700 

600 

500 

400 

300 

250 

200 

0, 2 

07 

Figure 8.~0mograp for smoke sensor alarm resholds and spacings. 
13 

A similar analysis may be made for smoke sensors 

using figure 8. At either the average or maximum velocity, 

class 2 smoke sensors may be spaced at 2,000-ft intervals. 

This analysis indicates the following: 

1. At the air velocity of 400 ft/min, 7 pprn CO sensors 

at 1,000 ft or 6 ppm CO sensors at 2,000-ft spacings may 

be used, or class 2 smoke sensors at 2,00043 spacings; 

2. At the air velocity of 700 ft/min, 4 pprn CO sensors 

at 2,000-ft spacings, or class 2 smoke sensors at 2,000 ft 

spacings would be required. 

DETECTOR SPACING-AN EXAMPLE 

The following belt entry is to be prote ted by CO 

located 200 ft inby the headpiece. Belt drive 2 is located 

3,800 ft inby the headpiece of belt drive 1. The tailpiece 

from belt drive 2 is located at a distance of 7,000 ft from 

the headpiece of belt drive 1. It was determined that CO 

sensors should be used at maximum spacings of 1,OOO ft. 

The entry cross section is 100 ft2 and the air velocity is 

400 ft/min. From the previous example, the CO alarm 

level should be at 7 ppm. What are the actual spacings of 

the sensors along this entry? 

The distance from drive 1 to drive 2 is 3,600 ft. 

Because drives will be protected separately, one sensor 

will be installed downstream of each drive at approximately 100 ft. Then, the first sensor will be located 300 ft 

sensors. The entry is 7,100 ft in length. Be c t drive 1 is 

inby the headpiece. Another sensor will be located at 

3,900 ft inby the headpiece. To determine the number of 

sensors and their spacings, divide 3,900 minus 300 by 

1,000, which is 3.6. Any time this division falls between 

two int3ger values, the next highest integer value is the 

number of sensors required, with the last one at the end 

point (in this case, 100 ft past drive 2, or 3,900 ft). In 

between, sensors will be spaced at intervals defined by 

t, = 3,600/4, or 900 €t. 

The distance from the end of the entry (7,100 ft) to the 

sensor at 3,900 ft is 3,200 ft. This distance divided by 

1,000 is 3.2, which means that four sensors are required 

along this length, but their spacing is 3,200/4, or 800 ft. 

The number of sensors and their locations are summarized 

in table 7. 

Table 7.4ocation of sensors along example 

belt entry 

Sensor Location, t? 

1 ...... 2 ...... 3 ...... 4 ...... 5 ...... 6 ...... 7 ...... 8 ...... 9 ...... 

300 

1 ,m 

2,100 

3,000 

3,900 

4,700 

5,500 

6,300 

7,100 

CONCLUSIONS 

The data have provided significant insight into the 

phenomena of fires that develop within conveyor belt 

entries. In general, both coal fires and subsequent belt 

fires before the onset of belt flame spread were found to 

grow at rates that increase with increasing air velocities. 

The rates of CO and smoke production were found to 

decrease as the air velocity increases. 

For smoldering coal fires, the duration of the smoldering stage decreases as the air velocity increases, while the 

length of time from ignition of the coal until ignition of 

the belt increases as the air velocity increases. Once the 

SBR rubber belt ignites, the time to reach a stage of sustained flame spread decreases gradually as the air velocity 

increases. For the PVC belt, flame spread did not occur. 

A constraint was proposed that may be used to define 

the conditions for use of proposed GO and smoke fire detection systems. For CO or smoke fire sensors, this constraint defines the sensor spacings and alarm thresholds 

for a range of air velocities an entry cross-sectional areas. 

This constraint, derived from the data presented in this 

report and designed to approximate worst-case conditions 

for ignition of conveyor belting by a small precursor coal 

fire, defines the condition for sensor usage so that fire 

detection and alarm occurs just prior to ignition of conveyor belting. 

It is extremely important to realize that if these data 

and subsequent constraints can be accepted as approximate worst-case conditions, then fires that develop via 

some other scenario will generally be detected earlier in 

their stage of development, thus providing more time for 

subsequent evacuations and control. 

It is also extremely important to realize that this worstcase scenario can happen and that evacuation of personnel 

should be as rapid as possible owing to the short periods 

of time (10 to 20 min) that may be available until belt 

flame spread begins along with the untenable levels of 

combustion gases and smoke that result. 
14 

REFERENCES 

1. Lazzara, C. P., and F. J. Pemk. Conveyor Belt Flammability 

Tests: Comparison of Large-Scale Gallery and Laboratory-Scale Tunnel 

Results. Paper in Proceedings of the 23rd International Conference of 

Safetyin Mines Research Institutes (Washington, DC, Sept. 11-15,1989). 

BuMines OFR 2749,1989, pp. 138-150; NTIS PB 89-225262/AS. 

2. U.S. Code of Federal Regulations. Title Winera1 Resources; 

Chapter I-Mine Safety and Health Administration, Department of 

Labor, Subchapter &Testing, Evaluation, and Approval of Mining 

Products; Part 18-Electric Motor Driven Mine Equipment and Accessories; Subpart C-Inspections and Tests; Section 18.65-Flame Test of 

Conveyor Belting and Hose, July 1, 1990, pp. 116-117. 

3. Litton, C. D. Relationships Between Smoke and Carbon Monoxide and Their Implication Toward Improved Mine Fire Detection. 

Paper in Proceedinp of the 23rd International Conference of Safety 

in Mines Research Institutes (Washington, DC, Sept. 11-15, 1989). 

BuMines OFR 2749,1989, pp. 77-83 NITS PB 89-225262/AS. 

4. Egan, M. R Coal Combustion in a Ventilated Tunnel. BuMines 

IC 9169,1987,13 pp. 

5. Litton, C. D. Guidelines for Siting Product-of-Combustion Fire 

Sensors in Underground Mines. BuMines IC 8919,1983,13 pp. 
APPENDIX A.-HEAT-RELEASE RATES 

(A-1) where ACO, = CO, produced by fue, ppm, 'COP 

kco 

The fire heat-release rates may be calculated from 

measurements of the CO and CO, produced. When calculated on the basis of gas data, the resultant heat-release 

rate is assumed to be the total or actual heat-release rate. 

The total heat-release rate is calculated from the 

exuression 

Mco, = generation rate of CO, from fire, g/s, 

and Ma = generation rate of co from fire, g/s. 

Ma, and Mc0 are gven by 

Mco, = 1.97. v,,A0 ACO,, (A4 

and Mco = 1.z.10-~ VA ACO, (A-3) 

kco, = stoichiometric yield of CO,, g/g, 

Combustible ka kJ/g x, h,g/g kcnglg = 3.67.X,, where X, = mass fraction of 

Sewickley Seam coal 30.0 0.712 2.61 1.66 

R1 belt.. . . . . :. . . . 36.8 .785 2.88 1.83 

carbon in fuel, 

kco = stoichiometric yield of Co, g/g, RII beit . . . . . . . . . . 28.7 .638 2.34 1.49 

= 233 &, P1 belt . . . . . . . . . . . 23.4 .517 1.90 1.21 
16 

APPENDIX B.-REPLACING POINT-TYPE HEAT SENSORS 

There exists a particular situation regarding the use of 

CO or smoke sensors in place of point-type heat sensors 

along a belt entry. The US. Code of Federal Regulations, 

Title 30, Part 75, Subpart L, (in summary form) states that 

if the belt entry air velocity is 0508 m/s (100 ft/min) or 

greater, point-type heat sensors must be spaced at intervals 

not to exceed 15.24 m (50 ft). If the belt entry air velocity 

is less than 0508 m/s, point-type heat sensors may be 

spaced at intervals not to exceed 38.1 m (125 ft). The 

minimum alarm actuation temperature for a point-type 

heat sensor is 57.2' C (l.35" F). Other fire sensors (i.e., 

CO or smoke) may be used, provided they yield protection 

equivalent to that provided by point-type heat sensors. 

The intent of appendix B is to demonstrate that CO or 

smoke fire sensors, when used at appropriate spacings and 

with appropriate alarm levels, can provide protection 

equivalent to, or better than, point-type heat sensors. In 

order to do this, it is assumed that if a CO or smoke fire 

detector system can be shown to be able to detect a developing fire in a time less than the time for a point-type 

heat sensor to detect the same developing fire, then the 

CO or smoke detection system satisfies the intended definition of equivalence. 

Clearly a point-type heat sensor detects only open, 

flaming fires. Because of this, there exist two possible 

paths of tire development that can result in detection by 

point-type heat sensors. The first path is that of an open, 

flaming coal fire that increases in intensity until enough 

heat is produced to cause the point-type heat sensor to 

alarm without ignition of the conveyor belt. The second 

path is that of open, flaming coal fires that ignite the 

conveyor belt within the time frames observed in the tests 

reported here. The burning conveyor belt fire then increases in intensity until it is large enough to produce an 

alarm by the point-type heat sensor. Because this latter, 

second path of fire development was found to occur very 

rapidly in the tests conducted, only this fire scenario will 

be considered in the derivations that follow. 

The expression that relates the fire intensity, Qn 

thermal sensor spacing, and sensor alarm temperature, T,,, 

is given as follows (5):l 

9 

1.75 H/W 

&T 

where po = density of air, 1.201 

Co = heat capacity of air, l.088-lO-~ 

kJ 

g ' "C' 

To = ambient temperature, 183" C, 

TA = heat sensor alarm temperature, 572 C, 

H = entry height, m, 

W = entry width, m, 

and eT = spacing between point-type heat 

sensors, m. 

For v,, < 0.508 m/s, the allowable spacing of thermal 

sensors is 38.1 m. A fire can occur anywhere between two 

sensors so that, on the average, tT is one-half of 38.1 m or 

19.05 m. For v,, L 0.508 m/s, tT is set equal to one-half 

of 15.24 m or eT = 7.62 m. 

In principle, H/W can have any value. However, it is 

limited for a specified value of tT to some maximum 

vdue. This value is derived from the assumption that the 

fire size at thermal alarm can be no greater than the fire 

size predicted on the basis of complete mixing. It may be 

less, but it cannot be greater. The expression defining this 

constraint is 

TA - To 1.75 H/W 

eT 9 ~o~ovoA0 

I pocov&o - To), 

which reduces to 

or 

- 

'Italic numbers in parentheses refer to items in the list of references 

preceding this appendix. 
17 

For a given value of A,, H and W have a range of 

values as long as HW = A,. To address this range, it is 

assumed that the absolute minimum entry dimensions are 

H = 1.2i9 m and W = 3.048m (4 X 10 ft). The minimum value of H/W, (H/W)-, at any other larger entry 

cross section is 

(HrW)& = - 3.716*~.4~ = 1.486/A0. (B-4) A, 

The maximum values of HNir at any larger entry cross 

section are defined above at the specified values of eT. 

Because several possible values of H/W are allowed for 

any value of A,, an average value is obtained by integrating over the range of values. This average value is 

then used in equation B-1 to estimate the average fire 

intensity necessary to produce thermal alarm for a given 

value of A,. 

Then, hvo calculations are made: The first calculation 

is the time required for a combined coal and belt fire to 

reach the specified value of Q, (equation B-1) at a 

specified air velocity; the second calculation is the time 

required for the fire to reach the maximum value of QT, 

defined by the right-hand side of equation B-2. These 

times are called t,, and tMm, respectively, and represent 

the extremes that should be expected for point-type heat 

sensors to alarm. 

The results of these computations for the crosssectional area of the surface fire gallery (A, = 7.53 m3) 

as a function of the air velocity are shown in figure B-1. 

The measured thermal alarms at each air velocity are also 

shown. At air velocities of 0.76 and 1.52 ds, the 

measured thermal alarm times tend to fd closer to the 

estimated minimum values, while at the air velocity of 

4.06m/s, the measured thed alarm time corresponds 

to the maximum value. The reason for this is because of 

stratification of hot product gases that occurs at the lower 

velocities, while at the higher velocity, this stratified layer 

does not form. 

The approximate minimum thermal alarm times that 

could be expected at air velocities of 0.254and 0.508m/s, 

as a function of entry cross section, are shown in figure B-2. It is emphasized in this figure that the minimum 

thermal alarm times are always greater than the average 

time to belt ignition of 14.25min. It was this average time 

that was used as the constraint leading to figures 7 and 8; 

namely, that CO or smoke sensors shall detect a small, 

flaming coal fire in a time less than 14.25 min. Any 

combination of sensor alarm level and spacing obtained 

from the nomographs of figures 7 and 8 satisfies this 

contraint, and by definition, will detect the developing fire 

more rapidly than the estimated minimum time for pointtype heat sensors to detect the same fire, as shown in 

figure B-2. 

Consequently, any mine desiring to replace point-type 

heat sensors with either CO or smoke sensors should 

use the CO and smoke sensor nomographs in figures 7 

and 8. 
18 

4c 

3c 

.- c 

E 

t20 

10 

0 

vo, m/s 

2.54 5.08 

I 

KEY 1 

500 1,000 

vo, ft/min 

Figure B-l.-Thermal alarm times as function of air velocity for 

surface fire gallery tests. (k 7.53 m2.) 

743 9.29 13.94 18.5 

40 

.- c 

E 

c t 

80 100 I50 200 

A*, ft2 

Figure B-2.-Thermal alarm times as function of entry cross 

section at velocitles of 0.254 and 0.508 m/s. Average time to belt 

ignition for all tests was 14.25 min. 
19 

APPENDIX C.-SMOKE OPTICAL DENSITY 

The most widely measured smoke property is the light 

extinction coefficient, K. The physical basis for light 

extinction measurements is Bouguer-hbert's law, which 

relates the intensity, Iox, of an unattenuated incident 

beam of monochromatic light of wavelength X and the 

intensity of light, I,, transmitted through a path length of 

smoke, L, by the expression 

I,/I~' = eWa. (C-1) 

When this equation is expressed in terms of base 10, 

where D = optical density, m-', 

and K = 2.303.D. (C-3) 

Both K and D depend not only upon the wavelength of 

light, but also upon the size (diameter) of the smoke 

particles and their concentrations. When smoke is 

assumed to obscure visibility, the percent obscuration, 0, 

is related to the transmission of light by the expression 

Here, the parameter, T, has been used for transmission 

because it represents some average value over all the 

wavelengths visible to the human eye and also because it 

represents some average value over all the particle 

diameters that form the smoke. 

In this report, a smoke sensor with an alarm threshold 

of D, = 0.044 m-l represents an obscuration of 9.6% over 

a 1.0-m path. A smoke sensor with an alarm threshold of 

D,= 0.022 m" represents an obscuration of 4.9% over a 

1.0-m path. 
APPENDIX D.-EFFECTIVE ENTRY CROSS SECTION 

AND AIR SPLITS 

CROSSCUTS 

When €ire sensors are installed in entries that contain 

crosscuts, the additional volume of entry space due to the 

crosscuts may increase the contaminant travel time between sensors and also dilute the contaminant concentrations. Both of these effects may seriously degrade the 

early warning capability of CO and smoke sensors. To 

offset this problem and retain the necessary early warning 

capability, the following procedures are recommended: 

1. Determine the number of crosscuts and their 

approximate spacing along the entry to be protected. 

Divide the number of crosscuts, m, by their spacing, ex, 

and designate the resulting number as N,: 

m Nx = -. 

FQr instance, if there are crosscuts on either side of the 

entry and they occur at 100-ft intervals, then m = 2, 

4, = 100, and N, = 0.02. If there are crosscuts along 

only one side of the entry, then m = 1 and Nx = 0.01. 

2. Determine the average depth, d,, of the crosscuts. 

3. If 4, is the distance between fire sensors, then the 

total volume of space along the entry, V, with the crosscuts included is 

where it is assumed that the crosscuts have the same cross 

section as the entry, 4. Thus, 

Now the straight-line distance between sensors remains the 

same and the net effect of the crosscuts is to increase the 

effective entry cross section, A, to a larger value ~ven by

using the e~ression given in 

equation D-4, the effects of crosscuts are overestimated. 

Therefore, it is suggested that a more reasonable value for 

A, is given by 

This expression (equation D-!5) is sufficient to account for 

both the increased travel time and dilution effects due to 

crosscuts. When figures 7 and 8 are used to determine 

sensor alarm levels, the value Am, rather than &, should 

be used. 

For instance, if m = 2, d, = 25 ft, and 4, = 100 ft, 

then A,, = (lS)*/* A. = 1.22 A,. If & = 100 ftz, then to 

calculate the proper sensor alarm level at a given spacing, 

A, = 122 ft2 should be used. From figure 7, if the air 

velocity is 300 ft/min and the entry cross section is 100 ft2, 

then for 1,000-ft spacings the CO alarm level should be 

9 ppm. However, including crosscuts, for which & = A, 

= 122 ft2, a CO alarm level of 7 ppm for sensors space 

at 1,OOO-ft intervals is obtained from figure 7. 

PARALLEL ENTRIES 

In some mines, two individual entries may exist that are 

not separated by stoppings. In these cases, the conveyor 

belt haulage system is usually located in one of the entries, 

but because no stoppings exist, the contaminants from a 

fire may be diluted by ventilating air from the parallel 

entry that does not contain the conveyor belt. To address 

this situation, the entry cross section, 4, should be replaced by an effective entry cross section, which is the sum 

of the cross section of the individual entries. 

For instance, if two entries have the same cross section, 

then the effective cross section, A, for determining sensor alarm levels is A, = 20%. This is probably the most 

frequent situation. But if the entries have different cross 

sections A, and A,, then A,, = A, c A,. 

As an example, if two of these parallel entries exist and 

are of equal cross section, A, = 100 ft2, then A, = 200 ft2 

should be used for determining sensor alarm levels. From 

figure 7, if v, = 150 ft/min and A, = 100 ft2, 10-pp 

alarm levels could 

A, = A,, = 200 ft 

cates 6-ppm a~ar~ levek for 1,~-ft spacings. 

used for 1,OOO-ft spacings. 

ue to parallel entries, figure 
21 

COMBINED CROSSCUTS AND PARALLEL 

ENTRIES 

When parallel entries contain crosscuts, the crosscuts 

exist on only one rib of each entry since there are no 

stoppings between the two entries. In this situation, 

m = 1 for each entry. The effective cross sections of each 

entry due to the crosscuts are 

[ 1.0 + d, 11/2 A,. and (AEX)l = (D-7) 4x2 

For instance, if d,, = d, = 25 ft and ex, = 4% = 

100 ft, then (Ad, = 1.12 & and (Adl = 1.12 A,. The 

total net effective cross section, A,, is A, = (Ad, 

+ (Ad, = 1.12 (4 + A& 

In the above example, if A, = A, = 100 ft2 and 

contains crosscuts, then A, = 224 ft2. From figure 7, at 

v,, = 150 ft/min and A, = 224 ft2, a 6 ppm alarm leve 

could be used at spacings of 1,OOO ft. 

AIR SPUTS 

An air split is defined as any junction along a belt entry 

where ventilating air is either diverted to another entry 

(thus reducing the air velocity) or ventilating air from 

another entry is diverted into the belt entry. For purposes 

of determining sensor alarm levels, the length of airway 

between any two air splits should be treated as a distinct 

entry. 

For instance, if A, = 125 ft', no parallel entries exist, 

the entry contains no crosscuts, and the airflow between 

two air splits is 200 ft/min, then from figure 7, at a 

spacing of 1,OOO ft, the CO alarm level is 9 ppm. If, at the 

next air split, ventilating air is diverted to the belt entry 

from another entry, thus increasing the air velocity to 350 

ft/min along the next section of entry, figure 7 would 

indicate that the CO sensor alarm level should be 

decreased to 6 ppm. The net effect of air splits Ls a 

change in the sensor alarm levels along a belt entry, based 

upon the changes in ventilation air velocity that occur because of the air splits. 
22 

APPENDIX E.-LIST OF SYMBOLS 

total entry cross-sectional area including parallel 

entries, m2 

effective entry cross-sectional area when cross 

cuts are included, m2 

total entry cross-sectional area when both parallel entries and crosscuts are included, m2 

nominal cross-sectional area of conveyor belt 

entry, m2 

nominal cross-sectional area of parallel entry, m2 

3 CO production rate constant, PPm*m 

s min 

growth-rate parameter for coal fire, kW/min 

growth-rate parameter for PVC conveyor belt 

fire, kW/min 

growth-rate parameter for SBR conveyor belt 

fire, kW/min 

3 PPm*m 

M CO production constant, 

m-lem3 

M smoke production constant, 

M heat capacity of air, - 

g*"C 

GO sensor alarm threshold, ppm 

smoke optical density, m-l 

smoke sensor alarm threshold, m-l 

of crosscut, 

increase in GO due to fire, 

crease ue to fue, 

change in CO production rat~during smoldering 

ppm ., m' stage of coal fue, 

S 

time from onset of smoldering coal to onset of 

flaming coal, min 

3 CO production rate, PPm.m 

s 

m-lem3 

smoke production rate, 

entry height, m 

heat of combustion of coal or conveyor belt, 

S 

kJ/g 

heat of combustion of CO, kJ/g 

smoke attenuated light intensity, W 

unattenuated light intensity, W 

light extinction coefficient, m-l 

stoichiometric yield of CO, g/g 

stoichiometric yield of COB g/g 

path length of light, m 

CO or smoke sensor spachg, m 

point-type heat sensor spacing, m 

crosscut spacing, m 

wavelength of light, pm 

generation rate of CO, g/s 

generation rate of CO, g/s 

number of crosscut 

light obscuration, d~ensio 

densi~ of air, g/m3 
23 

APPENDIX E.4IST OF SYMBOLS-Contlnued 

QQwc 

T 

heat-release rate of coal fie, kW 

heat-release rate of PVC conveyor belt fie, kW 

heat-release rate of SBR conveyor belt fire, kW 

(tD), time to detect fire by CO sensor dowastream 

fie, min. 

(tD)D time to detect fire by smoke sensor downstream 

of fie, min 

heat-release rate of fie, kW 

heat-release rate of combined coal and conveyor 

4 average time that coal fue will burn before conveyor belt ignites, min. 

belt fires, kW t,, estimated maximum time for point-type heat 

sensor to alarm, rnin 

light transmission, dimensionless 

alarm temperature for point-type heat sensor, "C 

I tMIN estimated minimum time for point-type heat 

sensor to alarm, min 

ambient temperature, 'C t,, time to peak fire intensity, min 

time, rnin t It response time of fire sensor, min 

sensor alarm time (general), min 

time required for coal fie to produce a given 

time coal fire smolders before flaming ignitioq 

min 

alarm level of CO, rnin 4 transport time of contaminants, min 

time required for coal fie to produce a given V, entry volume, m3 

"0 air velocity, m/s 

W entry width, m 

alarm level of smoke, min 

time at which conveyor belt ignited minus time 

at which coal began to flame, min 

g of carbon 

g of fuel C carbon mass fractio~ time at which conveyor belt flame spread began 

minus time at which coal began to flame, min 

* U.S. GPO: 1891-511-01ff42.036 
U.S. Department of the Interior 

Bureau of Mines 

2401 E Street, N.W., MS #9800 

Washington, R.C. 20241-0001 

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