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A 1 1 1

5 ATASA^

NBSIR 81-2256

Analysis of Fire Data, Large-Scale

Tests, and Small-Scale Tests for

Conveyor Belts Used in

Underground Coal Mines

Emil Braun

Robert E. Meade

Lee R. Smith

Center for Fire Research

National Engineering Laboratory

U.S. Department of Commerce

National Bureau of Standards

Washington, DC 20234

December 1980

Final Report

Issued May 1 981

Sponsored in part by

U.S. Bureau of Mines

Pittsburgh, Pennsylvania 15213

-QC—

100

"

,U56

81-2256

1981

and

Mine Safety and Health Administration

Triadelphia, West Virginia 26059

NBSIR 81-2256

ANALYSIS OF FIRE DATA, LARGE-SCALE

TESTS, AND SMALL-SCALE TESTS FOR

CONVEYOR BELTS USED IN

UNDERGROUND COAL MINES

Emil Braun

Robert E. Meade

Lee R. Smith

Center for Fire Research

National Engineering Laboratory

U.S. Department of Commerce

National Bureau of Standards

Washington, DC 20234

December 1980

Final Report

Issued May 1981

Sponsored in part by

U.S. Bureau of Mines

Pittsburgh, Pennsylvania 15213

and

Mine Safety and Health Administration

Triadelphia, West Virginia 26059

HATIONAL BURKAV)

OW aTANUAKUa

UBKART

JUN 1 5 1981

n h (lcc ' c

noAW

c.->

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

TABLE OF CONTENTS

Page

LIST OF FIGURES iv

LIST OF TABLES iv

Abstract 1

1. INTRODUCTION 1

2. FIRE INCIDENCE 2 2.1 Health and Safety Analysis Center 3

2.2 Lee Engineering 3

2.3 Allen Corporation of America 5 2.4 British Fire Experience 11

3. SCENARIOS 13

3.1 Hot Roller 13

3.2 Stuck Belt 17

3.3 Electric Arc 18

3.4 Fluid Ignition Sources 19

4. LARGE-SCALE TESTS 19

5. SMALL-SCALE TESTS 21

6. ACCEPTANCE TEST - MSHA (CFR TITLE 30 §18.65) 22

7. DISCUSSION 22

8. CONCLUSIONS 26

9. RECOMMENDATIONS 2 6

10. ACKNOWLEDGMENTS 27

11. REFERENCES 27

APPENDIX A: Review of Qualification Tests for Conveyor Belts

in Coal Mines A-l

APPENDIX B: Computation of Temperature Profile for the Hot

Roller Scenario B-l

APPENDIX C: Heat Transfer Calculations for an Electric Arc

Near a Conveyor Belt C-l

APPENDIX D: Ignition, Flammability, and Smoke Generation

Characteristics of Conveyor Belts Used in Mines D-l

APPENDIX E: Evaluation of the MSHA Test for the Flammability

of Conveyor Belts E-l

APPENDIX F: Standard Test Procedure for the Flammability of

Conveyor Belts for Use in Underground Coal Mines F-l

iii
LIST OF FIGURES

Page

Figure 1. Summary of Consolidated Coal Company data on causes

of underground coal mine conveyor belt fires - 1952 to 1970 7 Figure 2. Allen Corporation data on the distribution of underground

coal mine conveyor belt fires by duration - 1950 to 1977 . . 8 Figure 3. Summary of Allen Corporation data on causes of underground

coal mine conveyor belt fires - 1950 to 1977 9

Figure 4. Calculated temperature profile across a stationary roller

in contact with a conveyor belt 16

LIST OF TABLES

Table 1. Summary of Health and Safety Analysis Center injury data

on coal mine fires 4 Table 2. Relative frequency values of various ignition sources

for conveyor belt fires in underground coal mines

(Allen Corporation) 10

Table 3. Number of incidents of conveyor belt fires in underground

coal mines from three U.S. sources and Great Britain .... 12

Table 4. Predicted hot roller temperature and the instantaneous

interface contact temperature for SBR conveyor belts .... 15

Table 5. The effect of airflow rate on afterflame time on

selected conveyor belts tested in the MSHA test (30:18.65)

using a natural gas burner 23

Table 6. The afterflame time of selected conveyor belts tested on

the MSHA (30:18.65) apparatus as a function of time to

steady-state air velocity 24

iv
ANALYSIS OF FIRE DATA, LARGE-SCALE TESTS, AND SMALL-SCALE

TESTS FOR CONVEYOR BELTS USED IN UNDERGROUND COAL MINES

Emil Braun, Robert E. Meade, and Lee R. Smith

Abstract

An investigation into the requirements and fire test

performance of conveyor belts intended for use in underground coal mines was conducted. The purpose of this study

was to develop recommendations for the Mine Safety and

Health Administration (MSHA) on a test method suitable for

measuring the fire hazard potential of a conveyor belt in

a coal mine environment. A review of fire incidence data,

large-scale tests, and small-scale tests was conducted to

provide the necessary information. The incidence data was

analyzed with the goal of developing scenarios that could

form the basis for the development of a suitable new test

or to evaluate the appropriateness of the existing test.

Large-scale tests were reviewed to determine

anticipated belt fire performance under "realistic" enduse conditions. The tests showed the effect of geometry,

input energy, and ventilation control on belt fire performance. Small-scale test results provided information

on specific fire properties such as ease of ignition,

flame spread, and smoke generation potential.

Key words: Accident data; coal mines; conveyor belts;

fire safety; fire tests; flame spread; ignition; test

methods.

1. INTRODUCTION

At the request of the Mine Safety and Health Administration (MSHA) and

the Bureau of Mines (BOM) , the Center for Fire Research (CFR) has conducted

an investigation into the requirements and fire test performance of conveyor

belts intended for use in underground coal mines. The aim of this study was

to develop recommendations for MSHA on a test method for measuring the fire

hazard potential of conveyor belts in a coal mine environment.

1
The 1969 Federal Coal Mine Health and Safety Act [1]^ requires that the

Secretary of the Department of the Interior establish and maintain an incidence

data base for coal mine operations and issue regulations to insure the health

and safety of coal mine workers. The 1977 Amendments [2] to this Act have

transferred the regulatory authority to the Secretary of Labor. The incidence

data base is intended to make available the information necessary for determining priorities and assessing the impact of various regulations. Fire incidents

involving conveyor belts exposed to incidental fires are included in this data

base

.

Scenarios were developed from this data base and other sources of fire

incidence data. Review of the data obtained from small-scale and large-scale

fire tests of conveyor belts and conveyor belt systems provides additional

information for determining the effectiveness and appropriateness of the current regulation on conveyor belt fire performance contained in Title 30, Code

of Federal Regulations.

The Code of Federal Regulations Title 30 [3] currently describes a test

method and acceptance criteria for approval of conveyor belts used in underground coal mines. The test method defines: number of specimens tested,

specimen orientation, ventilation conditions, ignition source, and exposure

time. Measurements are made of the after flame time and afterglow time of

each specimen. Acceptable performance is an average afterflame time of less

than 60 seconds and an average afterglow time of less than 180 seconds.

This report summarizes the available data and presents recommendations

as to the appropriate actions to be followed by MSHA in regulating the fire

performance of conveyor belts in underground coal mines.

2. FIRE INCIDENCE

Incidence data was obtained from three sources. The MSHA's Health and

Safety Analysis Center (HSAC) located in Denver, Colorado, furnished information on all reportable incidents from 1972 to 1977. A more comprehensive

collection of fire incidence data was obtained from the Allen Corporation [5]

;

and William Jamison [4] of Consolidated Coal Company provided a summary of

national conveyor belt fire incidents from 1952 until 1970. These data were

analyzed to define "typical" fire scenarios involving underground coal mine

conveyor belts.

Numbers in brackets refer to the references at the end of this report.

2
Several summary reports received from England were also reviewed. Gross

comparison were made between United States experience and British experience

as reported by the National Coal Board.

2.1 Health and Safety Analysis Center (HSAC)

Under provisions of the Federal Coal Mine Health and Safety Act of 1969,

the HSAC is required to maintain a reporting system. Data has been compiled

since 1972 on accident, injury, illness, employment, and production for anthracite and bituminous coal mines and preparation plants in the United States.

Fire incidence data is accepted into the system if it meets the requirements

of reportability.

A reportable fire incident is one in which (1) an individual is injured

or (2) the fire persist for at least 30 minutes from the time of discovery.

This latter requirement means that fires successfully extinguished, without

injury, in less than 30 minutes are not reported. This is independent of the

extent of damage to equipment or productivity.

Accident data obtained from HSAC covers the period 1972 to the third

quarter of 1977. There were a total of 148,781 reported mine injuries, with

approximately 1/4% (431) attributed to fires (see table 1) , and 50% of these

(216 injuries), occurring in underground coal mines. Of these 216 injuries,

eight were as a result of conveyor belt fires. This represents a total of

six conveyor belt related accidents reported to HSAC during the period 1972

to 1977. According to HSAC, underground coal mines experience approximately

one reportable conveyor belt fire every year. The primary causes of these

incidents were frictional heating of a jammed roller or failure in the

electrical system (i.e., power line arcing).

2.2 Lee Engineering

In a recent meeting of the National Fire Protection Association (NFPA)

Mining Safety Committee, W. B. Jamison [4] of Consolidated Coal Company, Lee

Engineering Division, presented a summary of conveyor belt fires recorded by

the U.S. Bureau of Mines covering the period July 1952 to July 1970. No

information was presented on the manner in which the data were obtained nor

what fraction of all fire incidents these represented. However, the data was

an attempt at defining failure modes of a conveyor belt system that result in

the initiation of a fire.

3
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Figure 1 is a summary of these data showing the frequency of various

failure modes. It can be seen that after 1960 the total number of fires began

to decline. What is more significant is that during this period the number of

fires caused by stuck belts remained relatively constant. To a large extent,

the decline in total incidents over this time period, 1960-1970, can be attributed to a reduction in fires caused by the "other" and "hot roller" classifications. The "other" category would include frictional heating that ignited

coal or wood supports as well as failures that ignited oil or other combustibles found around a conveyor system. Reduction of these type of fires are

primarily the result of improved maintenance and housekeeping.

2.3 Allen Corporation of America

The Allen Corporation recently completed an annotated bibliography of

coal mine fire reports [5]. The report covers the period 1950 to 1977.

Incidence data was obtained from MSHA field offices as well as HSAC and various periodicals. The data base, therefore, includes all reportable and some

nonreportable fires. The bibliography does not include explosions or ignitions and fires resulting from explosions. However, fires that may have later

caused an explosion are included.

The Allen Corporation compiled records of a total of 1,014 underground

coal mine fires. Of this total, 154, or 15%, were thought to have originated

with the conveyor belt system. An analysis of all underground coal mine fires

shows that 92% of the incidents are free of injuries and 97% produce no deaths.

Comparable figures for conveyor belt fires yield 94% injury free and 97% death

free incidents. This indicates that conveyor belt fires have approximately

the same probability of causing an injury or death as all fires in coal mines.

Based on Allen Corporation data, there have been 10 fire incidents involving

injuries and six fire incidents involving deaths in coal mines since 1970.

During the same period, fire incidents involving conveyor belts have resulted

in no injuries or deaths.

A summary of failure modes for the 154 conveyor belt incidents is shown

in figure 3. This distribution does not differ greatly from that derived from

the previous source. Both figures 1 and 3 show a maximum number of incidents

between 1960 and 1965 followed by a steady decline in succeeding years.

Following the implementation in 1952 of the Coal Mine Health and Safety Act,

the frequency of reported conveyor belt fires increased dramatically. The

passage of the 1969 safety legislation was accompanied by as dramatic a decrease in conveyor belt fires. McDonald [5], the principle investigator for

Allen Corporation, noted that the number of fires, overall, followed a pattern

5
that was independent of production, which has remained relatively constant.

The 1952 legislation coincided with an increase in mechanization in the coal

mine industry. This appears to have mated high powered equipment susceptible

to fires with manpower unskilled in the proper use of modern mining equipment

It may be possible that by 1969 a skilled workforce had developed with an increased awareness of Safety. It has been assumed that further decreases in

succeeding years may have had as much to do with incident reporting requirements in the 1969 legislation as with any real improvements in worker safety.

Prior to the 1969 Coal Act, reportable fire incidents were not defined. The

1969 Coal Act established a minimum time interval between fire detection and

extinguishment before a fire incident becomes reportable.

Figure 2 summarizes the incident data from 1950 to 1977 by fire duration

Three fire categories are shown: fires that were extinguished within a halfhour of discovery; fires that took at least a half-hour to extinguish; and

fires whose total duration from discovery to extinguishment were unknown or

not reported. The data indicates that the percentage of known fires whose

duration exceeded the half-hour minimum has remained relatively constant.

Since 1969, the percentage of fires of unknown duration have decreased, while

the number of fires extinguished within a half-hour have increased slightly.

This suggests that fire safety in underground coal mines has been steadily

improving. The decrease in the number of reported incidents does not appear

to be due to the elimination of fires of less than 30 minute duration.

Table 2 is a relative frequency tabulation of the data in figure 3. The

frequency values in the table are computed relative to the data for each

interval. It indicates that the bulk of the incidents, 65%, were caused by

one of three failures:

• Stuck belt.

• Hot roller.

• Electrical arcing.

In addition to the dramatic decrease in the total number of incidents,

figure 3 shows that no conveyor belts have ignited from stuck belts being

heated by operating drive motors since 1972.

With the decline in the total number of conveyor belt fires leveling off

at approximately two fires per year, it is of importance to note the severity

of these fires as a measure of fire control capabilities and to determine the

likelihood of occurrence of a serious mine fire originating in the conveyor

system. Wanchek [6] analyzed the extinguishment requirements for underground

6
Figure 1 - Summary of Consolidated Coal Company data on causes

of underground coal mine conveyor belt fires - 1952 to

years

1952-1953

1954-1956

1957-1959

1960-1962

1963-1965

1966-1968

1969-1970

Number of Incidents

5 10 15 20 25

mmrn

n 1 1 r wmmKmM HIM

—

H» m

Unknown | Hot Roller

H Other stuck Belt

H Electrical

30

-i

1970

7
Figure 2 - Allen Corporation data on the distribution of underground

coai mine conveyor belt fires by duration - 1950 to 1977

years

1950-1952

1953-1955

1956-1958

1959-1961

1962-1964

1965-1967

1968-1969

1970-1971

1972-1973

1974-1975

1976-1977

Percent

0 20 40 60 80 100

1 1— 1 1 1

63 less than 1/2 Hr.

greater than 1/2 Hr.

Unknown

8
Figure 3 - Summary of Allen Corporation data on causes of underground

coal mine conveyor belt fires - 1950 to 1977

years

1950-1951

1952-1953

1954-1956

1957-1959

1960-1962

1963-1965

1966-1968

1969-1971

1972-1974

1975-1977

Number of Incidents

0 5 10 15 20 25 30

i r 1 1 1 1

| | Unknown | Hot Roller

Other Stuck Beit

Electrical

9
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10

1975-77
coal mine conveyor belt fires from 1960 to 1977. He found that in 14% (10)

of the fires occurring between 1960 to 1969, secondary measures had to be

employed before the fire was successfully extinguished. In nearly all cases

the fire area had to be sealed and in one case the entire mine was sealed.

A similar review of fire incidents between 1970 and 1977 showed that 10% of

the fires required protracted firefighting before the fire was extinguished.

It should, however, be noted that between 1970 and 1977 only one serious fire

is represented by the 10% figure.

United States incidence data clearly shows a downward trend in the number

of underground coal mine conveyor belt fires. This may be caused by several

factors such as improvements in personnel training, equipment, maintenance, or protective devices. However, it may also be an artifact of the reporting

system. Prior to the 1969 Coal Act there were no specified conditions that

required the submittal of an incident report. Since the issuance of the Act,

an operator is required to report a mine fire only if it is not extinguished

within 30 minutes of discovery or if any injuries occurred.

2.4 British Fire Experience

Since very little documentation exists for nonreportable fires in the

U.S., several reports were obtained from Her Majesty's Chief Inspector of

Mines and Quarries, London, England [7,8]. These reports present summary

fire data on British experiences with underground conveyor belts. Their

reporting system is more comprehensive in that any incident is reportable.

British data indicate that, while there are fluctuations in the number of

mine fires for a given year, the total number of conveyor belt fires has

remained fairly constant from year to year. Table 3 is a summary of these

data compared to the three sources of U.S. data over corresponding years.

Of those values listed in the last column of table 3, approximately one-third

(61) of the British conveyor belt fires were caused by hot rollers and a little less than one-third (53) were caused by overheating of the conveyor

system brakes. While failures in the braking system of U.S. conveyors have

not been reported, table 2 indicates that U.S. experience with hot roller

failures has been very much the same.

While comparisons of this kind can be revealing, it should be pointed

out that the data have not been normalized against any measurable factors

such as man-hours, tons produced, or linear feet of conveyors in operation.

It is not clear if the difference between Britain and the U.S. is due to production variations, safety measures, or differences in reporting requirements.

11
Table 3

Number of Incidents of Conveyor Belt Fires in Underground

Coal Mines From Three U.S. Sources and Great Britain

Year

1968-69

1970-71

1972-73

1974-75

1976-77

TOTAL

HSAC

a Allen Corporation^ Con SolC

14 10

9 l

6 13 -

2 2 -

3

6 32

4-

11

Great Britain**

50

48

40

43

181

Includes only fires which caused injury or which were not extinguished

within 30 minutes.

May include fires that were reported but did not meet HSAC acceptance

criteria (a).

Consolidated Coal Company's summary of U.S. Bureau of Mines data.

Includes all fires which occurred.

Only 1 year.

12
3 . SCENARIOS

In appendix A several conveyor belt qualification tests are reviewed.

Initial design considerations that guided the development of these tests were

deduced from an analysis of the test conditions. Several scenarios were

deduced from the operational conditions of the test methods. A scenario is a sequence of critical events (component failure, ignition, growth, and spread

of a fire) that can result in a fire loss.

It is shown in appendix A that existing test methods appear to be

concerned with two regimes of fire exposure; small-scale tests which anticipate incipient fire exposures no greater than 500 W, and large-scale tests

that use ignition sources of approximately 100 kW. In addition, the smallscale tests provide little, if any, energy feedback mechanism that may

increase flame spread.

In order to determine the appropriateness of these various test methods,

the data presented in the preceeding section will be used to describe several

possible scenarios.

The incidence data can be classified into four categories based on the

method of ignition: hot roller, stuck belt, electric arc, and fluid ignition.

The first two involve the frictional generation of heat while the others are

concerned with mechanical failures in adjacent machinery such as electrical

faults in power lines or hydraulic failures in equipment.

3.1 Hot Roller

The first scenario considered is a hot roller failure. A hot roller on

a conveyor system occurs when the bearings in the roller arm seize, jamming

the normally freewheeling roller. Continued operation of the conveyor system

results in frictional heating of the roller drum. The maximum temperature

attained by the drum is a function of belt speed, tension, and load as well

as the surface characteristics of the drum and belt. During prolonged operation of the system, the roller drum will attain a steady state temperature

determined by a heat balance between convective heat losses to the surroundings and heat generation by the moving belt. As long as the conveyor system

continues to operate, the belt will not absorb enough energy during each pass

over the jammed roller to ignite. Considering the speeds at which these belt

systems operate, the contact time of any section of belt with the hot roller

drum is small compared with the time between encounters.

13
When the system is stopped, heat transfer from the roller raises the

conveyor belt to its ignition temperature thereby initiating a small fire.

It can be shown, however, that this is not a probable scenario. The rate

of heat transfer from the roller to the conveyor belt is not high enough to

offset the heat losses to the surrounding atmosphere.

The interaction of the conveyor belt and the drum can be treated in a semi-analytical manner (appendix B) . Over very short time periods (several

seconds) after the system is shut down, the initial contact temperature

between the drum and belt can be approximated by assuming that the system is

analogous to two semi-infinite solids. Table 4 is a tabulation of the interface contact temperature for a styrene butadiene rubber (SBR) conveyor belt

for various roller drum temperatures.

A drum temperature of at least 425 °C would be required to obtain a contact temperature of 350 °C at the belt/drum interface. (Pyrolysis of PVC

and SBR begin at approximately 200-250°C and 320-360°C, respectively [9]

.

The actual ignition temperature of,

a

specific PVC or SBR conveyor belt cannot

generally be predicted.) This temperature would persist for only a short

period of time. The temperature across the drum and conveyor belt rapidly

decreases, due to convective heat losses to the surrounding atmosphere.

Typical calculated temperature profiles from the center of a drum to the

outer surface of a conveyor belt are shown in figure 4 for 5 minutes and 30

minutes after shut-down.

It can be seen that, unless an exothermal reaction is initiated within

the conveyor belt, it will not ignite. The temperature profiles indicate

that ambient conditions are rapidly approached. v .

An alternative scenario that could lead to conveyor belt ignitions

assumes that the hot roller ignites any coal, coal dust, or oil present

instead of the belt. Self-heating studies [10,11] have shown that moderate

elevations in environmental temperatures can cause coal to ignite. The continued operation of a belt system would provide more than sufficient energy

to heat and hold a jammed roller at an elevated temperature until the surrounding coal dust ignites. Once again, as long as the system is in operation the intermittent encounters of the belt with the burning coal will not

result in ignition of the belt. When the system is shut down, the resultant

prolonged exposure to the flaming coal dust can result in the ignition of the

conveyor belt. Such a scenario has been postulated to explain the Schlazel

and Eisen Colliery disaster in Germany, October 1977 [12].

14
Table 4

Predicted Hot Roller Temperature and the Instantaneous

Interface Contact Temperature for SBR Conveyor Belts

Drum Temperature (°C)

150

200

260

315

370

425

480

540

Interface Temperature (°C)

127

170

216

261

305

349

393

440

15
gure

4

-

Calculated

temperature

profile

across

a

stationary

roller

in

contact

with

a

conveyor

belt

(Go) 9JrHDJ9dUU8J_

Distance

from

the

center

of

the

roller

(cm)
Fire growth on the conveyor belt is strongly dependent on the energy

transfer from the ignition source as well as the feedback energy from the

burning belt. There exists for each conveyor belt a minimum required exposure

time at any given exposure intensity to develop a growth rate that is self2 sustaining. Belts can ignite with exposure conditions as low as .35 W/cm in

approximately 2 hours (appendix D) . Any increase in exposure intensity

decreases the time to ignition. The rate of heat transfer and duration of

exposure from the burning coal to the conveyor belt would have to be of this

order of magnitude before it could be expected to ignite the belt.

3.2 Stuck Belt

Of the two frictional heating failure scenarios, the stuck belt appears

to have been eliminated as a failure mechanism. This may be due to the introduction of "zero slip switches" and sequence switches that shut down conveyor

operations when a discrepancy between drive speed and belt speed exists. Some

countries (e.g., Britain, Canada, and Australia) have addressed this same

problem by developing a drum friction test for belts.

Generally, a belt is stalled by a roof fault (cave-in) , a failure in the

sequence switches of a multibelt system, or loss of tension around the drive

section. Without the "zero slip switch" or with a defective one, the drive

drum continues to operate. Frictional heating of the belt begins immediately.

Long-term operation of the drive with the belt in a stalled condition will

result in either separation or ignition of the belt.

Ignition can occur at two locations. One possible point of ignition is

the drum-belt interface prior to belt separation. This type of ignition would

produce underside burning of the belt that could spread until a belt edge is

reached. Continued fire growth may then take place on the upper surface of

the belt.

Another possible location for ignition is the hot mass of material

frictionally removed from the belt and deposited on the surface of the lower

belt. The fire produced in this manner could also spread between the overhead and the return loops of the conveyor system. Since these two belt lines

could be thought of as an idealized tunnel, the close proximity of the "roof"

to the "fuel" source would tend to promote rapid and extensive flame spread.

Recently, Factory Mutual Research Corporation has demonstrated the importance

of roof-to-belt distance as a controlling mechanism for flame spread [13]

.

They found that, as the belt was moved towards the roof, flame spread was

more extensive and rapid.

17
The British incident data appear to indicate that they still have a problem in controlling these types of incidents in spite of the fact that

they employ a drum friction test for belt qualification.

3.3 Electric Arc

' ‘ i

" Sources for electrically induced ignitions are present throughout a coal mine. The majority of equipment in underground coal mines is electrically operated from various power sources, AC and DC, with voltages above

4KV not uncommon. In the belt area, voltages from 600 to 1,000 volts are

common, with currents as high as 800 amperes. Electrical cables can be

found suspended from the roof along haulage lines.

An electrical cable can be dislodged from its restraining hook by a minor roof fault or deterioration of the restraints. The cable could then

make contact with the belt system and subsequent operation of the belt system

would cause abrassive removal of the insulation until the conductors are

exposed. If two conductors are exposed or the insulation is degraded to the

extent that an arc can occur because of the potential difference between the

conductors, one would expect the circuit breaker protecting this line to trip

and remove the hazard. However, a low current arc could be produced that

would not draw sufficient current to trip the circuit breaker. An arc established in this manner could persist indefinitely. This could represent a significant source of ignition energy for the conveyor belt assuming the gap

did not increase. In an analogous manner, any arcing that occurs between the

conveyor frame and exposed conductor could be a potential source of ignition

energy.

How serious the problem is can be assessed by computing the magnitude of

heat transfer from the arc to the belt surface. A simple analysis, appendix

C, shows that ignition of the belt surface is possible provided the arc persists for several seconds and the conveyor belt is stationary. This would be

the case if the arc were drawing less current than the breaker set point value.

In heavy industrial applications this could be possible because breakers are

designed to ignore start-up spikes. Several cycles of over-current are necessary before a circuit breaker would trip. The transition from initial sparking to arc is very rapid and, therefore, may not trip the breaker.

Prolonged exposure to an arc would not only ignite a conveyor belt but

also provide additional external energy that would induce accelerated fire

growth.

18
3.4 Fluid Ignition Sources

The most severe source of ignition by fluid combustion is expected to be

a failure in an adjacent hydraulic system that forms a flaming spray. This

"torch" could impinge on a belt. If the conveyor system is stationary, the

flaming hydraulic fluid could ignite the belt. Measurements of energy release

perpendicular to the spray axis has shown that significant amounts of energy

2 are radiated (1.6 W/cm ) [14] by a flaming spray of hydraulic fluid. Direct

impingement of the flaming spray onto the conveyor belt would be even more

intense

.

Two exposure conditions arise with this type of failure. In the first

exposure condition, the conveyor belt, in motion, is exposed to the spray.

The normal speed of the belt is rapid enough that it passes through the flame

without igniting. However, the relatively cold belt forces some of the

unburned fluid to condense on its surface. This could cause additional problems with some other failure mode (i.e., hot roller). In addition, the flaming spray could ignite the coal seam or coal dust previously deposited by

the conveyor system. The subsequent development of this type of fire could

overshadow any fire performance properties of the belt.

The second exposure condition involves a stationary belt. It is shown

in appendix D that it is possible to ignite a conveyor belt meeting MSHA

2 (§18.65) requirements with a heat flux as low as .86 W/cm in 4 to 10 minutes.

2 Prolonged exposures at heat fluxes as low as .34 W/cm could also result m

ignition of a belt. Preliminary measurements of the heat flux [14] outside

of the spray cone perpendicular to the spray axis show that it is possible to

ignite some belts with a noncontacting spray if it persists for an hour.

Direct contact with the flames should greatly reduce the exposure time necessary for ignition. Following ignition, fire growth could be rapid, because

of the continued presence of the flaming spray. Under these conditions, the

conveyor belt represents the means for spreading the fire and not the source

of the initial ignition.

4 . LARGE-SCALE TESTS

Several researchers have conducted large-scale fire tests, in an effort

to clearly establish flammability differences between various grades of conveyor belts. These tests have provided some information on belt performance

under "real end-use" conditions.

19
Early large-scale tests were performed by Mitchell, et al., in 1967 [15].

Using a 1.2 meter diameter corrugated steel duct, he exposed 4.6 meter long

belts to a set of burners whose combined heat release rate was 74.2 kW. He

showed that under steady state conditions, variations in the ventilation rate

from neutral air to approximately 150 meters per minute increased the time to

ignition. However, aftter ignition the rate of flame spread increased with

increasing ventilation rate. Experiments with specimens preheated with an electrically heated steel plate indicated that while the flame spread rate

was unaffected, the likelihood of establishing a propagating flame increased

by the preheating.

Mitchell scaled up his work to a 3 meter wide by 2.1 meter high mine entry

using a 30 meter long conveyor system. Employing a similar burner arrangement,

he found that under comparable ventilation conditions, the rate of flame propagation was lower for the larger cross-sectional opening. He also noted that

variations in belt width did not affect flame spread rate as it had in the

smaller scale tests.

Similar tests conducted by Warner in 1974 [16] employed a 4.6 meter wide

by 1.8 meter high corrugated steel simulated entry way as the test gallery.

Although Warner used a 62 kW source, his results qualitatively agreed with

Mitchell. The differences are probably due to the different geometric configuration employed by Warner. Mitchell's tests were conducted using a single

belt run with ignition on the surface of the belt. Warner, on the other hand,

designed his tests around an actual conveyor system. He had two belt layers

and ignited the belt on the top surface of the lower layer. His effective

ceiling height was the top belt layer and not the mine ceiling. While

Mitchell's results were confined to new belts, Warner tested used belts as well.

He found that new belts burned more readily than used belts of the same make

and that the addition of coal and grease retarded ignition but enhanced flame

propagation.

The effect of the ceiling on flame propagation was further explored by

Buckley and Vincent [13] in a 1978 series of large-scale single layer belt

tests. A 2.4 x 2.4 m (8 x 8 ft) gallery with a 23 m (75 ft) horizontal run

was constructed of brick and contained a 7.6 m (25 ft) single layer length of

belt on a steel frame. Tests were conducted using an ignition source with an energy release rate of 53 kW to 212 kW. They found that at the maximum energy

release rate of 212 kW none of the belts would propagate a flame when the belt

to ceiling distance was 1.8 meters. This series also included a nonfireresistant rubber belt that required preheating and applications of coal dust

and grease before it would propagate a flame at an ignitor setting of 106 kW.

20
Flame propagation occurred on several belts when the conveyor belt

system was moved to a distance of .84 meters from the ceiling. Buckley and

Vincent were able to rate belts according to flame propagation rate and

destruction distance versus the energy release rate of the ignitor and exposure time. They also noted that carcass material did not appear to affect

the belt's burning characteristics although a more systematic experimental

design is necessary to verify this point.

Grumbrecht [17] also reported results of large-scale tests conducted in

Germany and the subsequent development of a reduced size test for conveyor

belts. His results do not differ greatly from the preceeding researchers.

Differences in test results of the various researchers can probably be attributed to the use of extremely large ignition sources of different intensities

and duration, as well as geometric factors associated with the tunnel and belt

mounting techniques. Grumbrecht ' s work clearly shows that if the igniting

fuel load is large enough, virtually any belt will ignite and propagate a flame.

5. SMALL-SCALE TESTS

Appendix D describes the result of small-scale tests performed at NBS on

selected belt samples. The tests measured the ignition, flammability, and

smoke generation characteristics of conveyor belts.

A piloted ignition test was assembled that exposed a small belt sample

to a reasonably uniform heat flux field. It was found that, as the heat flux

increased, the belt's time to ignition decreased. Under high heat flux exposures, little difference was noted in the ignition delay time between the

belts tested. Under low intensity conditions, there was a large spread in

ignition delay time. The large-scale belt tests demonstrated similar behavior.

Under low exposure conditions, very few belts ignited and propagated a flame.

But, as the feedback mechanism was enhanced (i.e., the belts were moved closer

to the ceiling and the intensity of the source was increased) , differences in

belt performance became less distinct until conditions were such that all of

the belts propagated a flame.

Buckley also noted that his optical smoke sensors indicated near 0%

transmission shortly after the initiation of the test indicating large production of smoke. This was verified in the modified NBS Smoke Density Chamber

under more controlled conditions.

21
While conveyor belts are troughed, their primary orientation is horizontal.

Therefore, it was felt that as a measure of the belt's fire spread potential,

the Flooring Radiant Panel would be a useful test. It was observed (see appendix D) that some belts that easily passed the MSHA test, exhibited marginal

flame spread performance in the flooring radiant panel test.

6. ACCEPTANCE TEST - MSHA (CFR Title 30 §18.65)

Appendix E describes the work performed using the MSHA test for the

flammability of conveyor belts. Various parameters of the test were varied to

determine their effect on test results. Variations in such parameters as the

gas used as the fuel, the exposure time, and flame height had very little

impact on mean values of after flame and afterglow time. However, variations

in the airflow rate across the specimen produced significantly different

results. Table 5 shows the changes in afterflame time as a function of the

airflow rate for several selected belts. Belts with very short afterflame

times at 91 MPM (300 FPM) had long afterflame times at reduced air velocity.

Table 6 shows the effect of airflow acceleration on afterflame times. The

more rapidly the speed approaches 91 MPM (300 FPM) , the shorter the afterflame

time. In effect, the very rapid application of airflow, as would be the case

if a damper were employed, blows out the flame. The use of a larger orifice

(1.9 mm) in the test burner also appeared to produce longer after flame times

than the smaller orifice 1.1 mm found in a natural gas burner.

As expected, afterglow was also affected by the airflow rate. The number

of specimens exhibiting glowing behavior decreased with decreasing airflow rate.

It can be anticipated that those factors affecting lab to lab

reproducibility will be:

• the manner of airflow application and

• the type of burner.

7. DISCUSSION

The Code of Federal Regulations requires that all underground conveyor

belt systems be equipped with slippage and sequence switches as well as with

automatic fire warning devices. Inspection of a belt must also be performed

within 2 hours after a planned or unplanned shutdown. The examination is

22
Table 5

The Effect of Airflow Rate on Afterf lame Time

on Selected Conveyor Belts Tested in the

MSHA Test (30:18.65) Using a Natural Gas Burner

Belt Belt

Number Type Airflow Rate - Mpm

0

(sec)

30

(sec)

60

(sec)

90

(sec)

5 PVC 6 3 1 2

8 SBR 3 1 1 1

13 PVC 63 3 3 3

14 PVC 634 33 10 3

18 SBR 3 3 3 1

26 SBR 199 159 6 2

32 PVC 3 1 1 0

34 SBR 351 14 3 3

39 SBR 10 3 2 2

42 SBR 21 3 2 2

45 SBR 153 3 2 2

23
Table 6

The Effect of Air Blower Time to Steady-State Flow

on the Afterflame Time of Selected Conveyor Belts Tested

on the MSHA <30:18.65) Apparatus as a Function of Time

to Steady-State Air Velocity

Belt

Number

Belt

Type

,

’ . *t

; \

'

’ Acceleration

<( ' i 2

Rate - Mpm

,270 365 545 2700

(sec) (sec) (sec) (sec)

13 PVC 12 7 8 3

14 PVC 162 163 51 3

) 26 SBR 82 116 72 3

34 SBR 111 145 46 3

i

24
primarily concerned with identifying sources of ignition such as hot rollers

or incidental coal fires.

Fire suppression systems are also mandated by statutory provisions within

the Code, however, the extent of coverage by a suppression system depends on

the fire-resistant classification of the belt used in the haulage system. A

belt classified as fire resistant, according to CFR Title 30:75.1101, need

only have a suppression system covering 50 feet of the belt beginning at the

belt drive unit. Nonfire-resistant belts require 150 feet of coverage.

It is apparent from a review of the accident data that the combined

introduction and use of safety switches, suppression and detection systems,

and fire standards for belting materials have reduced the frequency of

reported conveyor belt fires.

This multilevel approach to fire safety, however, has masked the real

impact of any single safety related element including the fire performance

requirements imposed on the belt. (It was noted that the U.S. fire experience

in this area of underground coal mine operations appears remarkable in contrast

to the British data. However, as was pointed out earlier, the British require

the reporting of all fire incidents while the U.S. reporting system ignores

any fire that was extinguished within 30 minutes and caused no injuries.)

While it may appear unlikely, a failure in either the detection or

suppression system could seriously reduce the margin of safety built into the

overall system. The increase in the risk due to such a failure was clearly

demonstrated in the large-scale tests conducted by Warner and Buckley. They

have shown that under the appropriate geometric and thermal conditions an MSHA

approved belt can propagate a fire.

The scenarios developed from the incidence data indicate that fires remote

from the drive-end of a conveyor system could occur. These could be detected

by the fire detection systems but they would not be within the range of the

suppression system. Given all of this information, it can be concluded that

the U.S. should be experiencing a high incidence of conveyor belt fires. The

accident statistics indicate a counter trend. There has been no serious fire

incident involving a conveyor belt in the past 10 years.

25
8. CONCLUSION

Coal mine fires involving conveyor belts were reviewed in order to

develop a set of scenarios that describe the sequence of events leading to a serious fire loss. Large-scale tests were also reviewed. This review indicated that meeting the fire standards administered by MSHA does not guarantee

acceptable fire performance if the fire detection or suppression system fails

and a significantly large source of energy is present.

Small-scale tests indicated that for incidental fires, belt performance,

as measured by MSHA (CFR 30 §18.65) appears to be adequate. However, largescale tests show that with a significant increase in the ignition source the

level of safety provided by the existing belt test is greatly reduced.

A quantitative evaluation of risk was not made because quantities such as the total number of nonreportable fires, the frequency of operation of the suppression system and the fire fighting capabilities of the miners could not be

assessed. However, the decrease in the number of reported incidents could be

taken as an indication of the effectiveness of the overall system.

9. RECOMMENDATIONS

Assuming the current effectiveness of the detection and suppression system

and that sequence switches and zero slip switches are properly maintained, the

current fire performance requirements for conveyor belts appear to be adequate.

However, in order to improve reproducibility of the test method, it is

recommended that the MSHA test procedure (§18.65) be modified to:

• require that the airflow not be applied until the afterflame has

extinguished and

• clearly define the use of an artificial gas burner with a 1.9 mm

orifice.

Appendix F is a modified test procedure that should improve repeatability and

reproducibi 1 i ty

.

In addition, in order to assess the effectiveness of fire safety

measures, HSAC should change its requirements for a reportable fire incident

26
to include any incident that exhibited fire or smoke regardless of

extinguishment time or occurrence of injury.

10. ACKNOWLEDGMENTS

This work was partially funded by the Mine Safety and Health

Administration, U.S. Department of Labor, and the U.S. Bureau of Mines, U.S.

Department of Interior.

11. REFERENCES

[1] Federal Coal Mine Health and Safety Act of 1969, Public Law 91-173,

December 30, 1969.

[2] Federal Mine Safety and Health Amendments .Act of 1977, Public Law 95-164 , November 9, 1977.

[3] U.S. Code of Federal Regulations Title 30, Chapter 1, Part 18, Paragraph

18.65, 1978.

[4] Jamison, W. B. , Lee Engineering, Private communication.

[5] McDonald, L. B. , An Annotated Bibliography of Coal Mine Fire Reports,

Allen Corporation of America, Final Report, U.S. Bur. Mines Contract No.

J0275008 (February 1979).

[6] Wanchek, G. A., Summary of Underground Coal Mine Conveyor Belt Fires,

1960 to 1970 and 1970 through 1977, draft report, Mine Safety and Health

Administration, U.S. Department of Labor.

[7] Department of Trade and Industries, Report of H. M. Chief Inspector of

Mines and Quarries for 1972, Her Majesty's Stationery Office, London,

England (1973) .

[8] Health and Safety Executive, Mines and Quarries, 1975, Her Majesty's

Stationery Office, London, England (1976)

.

[9] Madorsky, S. L. , Thermal Degradation of Organic Polymers, Interscience

Publishers, John Wiley & Sons, Incorporated, New York (1964).

27
[10] Mitchell, G. , NBS, Private communication.

[11] National Coal Board, Report of the Committee on Roadway Conveyors, Her

Majesty's Stationery Office, London, England (October 1955).

[12] Mines Inspectorate, Nordrhein-Westfalen, Report on the Mine Disaster at

the Schlagel and Eisen Colliery in Hecten, Recklinghausen on October 27,

1977, Dortmund, West Germany (March 1978) .

[13] Buckley, J. L. and Vincent, B. G. , Conveyor Belt Testing Program, U.S.

Bureau of Mines, Contract No. H0252085, Final Report (1978).

[14] Loftus, J. J. , NBS, Private communication.

[15] Mitchell, D. W. , Murphy, E. M. , Smith, A. F., and Polack, S. P. , Fire

Hazard of Conveyor Belts, U.S. Bureau of Mines, RI 7053 (1967).

[16] Warner, B. L. , Suppression of Fires on Underground Coal Mine Conveyor

Belts, Walter Kidde and Company, Incorporated, U.S. Bureau of Mines,

Contract No. H0122086, Final Report, NTIS PB-250 368 (September 1974)

.

[17] Grumbrecht, K, , Flammability of Conveyor Belts, Gluckauf-Forschungshefte,

Vol. 37, No. 4 (August 1976).

[18] Wilde, D. G., A Review of Test Methods for Fire-Resistant Conveyor Belts,

International Symposium, Fire Safety of Combustible Materials, University

of Edinburgh, England (October 1975)

.

28
APPENDIX A

• .• a‘ Z }

• • • . Review of Qualification Tests for Conveyor Belts in Coal Mines

This report represents a review of test procedures and methods used by

various international organizations to assess the fire hazards of conveyor

belts. The test methods and acceptance criteria from eight countries,

including the U.S.A. and International Standards Organization (ISO) , are

critically reviewed (see table 1)

.

1 . TEST METHODS

The various test methods currently in use to evaluate the flammability

characteristics of conveyor belts for underground mine application can be

divided into two groups. The first category of tests measures the belt's

resistance to frictional heating, while the second group measures the belt's

ability to limit flame spread when exposed to a standard heat source for a predetermined period of time.

1.1 Drum Friction Test

Drum friction testing of conveyor belts represents part of the

qualification testing programs of several countries. There are three basic

test procedures currently in use: South Africa [1], Poland [2], and the

National Coal Board [3] in England. Until the late 1950s, the United States

[4] also employed a drum friction test for belt qualification similar to the

Polish test procedure. In addition, the European Community Countries (FCC)

[5] are currently attempting to standardize the frictional heating test for

application to all ECC coal mines. Table 2 is a summary of the four test

procedures. A review of table 2 shows that, allowing for metric to English

conversions, the U.S. Bureau of Mines and South African tests are virtually

identical. They differ primarily with regard to the acceptance criteria

and the number of specimens tested.

Both standards require that the belts do not exhibit any flaming or

glowing during the 2-hour test period. If any flaming or glowing is noted,

the belts are rejected. The Bureau of Mines had a further stipulation that

the friction drum itself not achieve a temperature in excess of 250°C. If

this temperature was reached, the belt was also rejected. (MSHA no longer

applies this test for belt qualifications.)

A-l
Table 1. List of countries and the type of test procedures

applied to coal mine conveyor belts

Country Test Method

Small-Scale Large-Scale Drum Friction

Belgium X

France X

a

Germany X

b

X

Great Britain X

b

X X

Israel x

a

Netherlands X

South Africa x

a

United States X

Adopted ISO R340

.

Similar, but not identical, to ISO R340.

A2
Table 2. Specifications of various drum friction tests

Specifications

European

Community

Countries

South

Africa

National

Coal

Board

0 . S . Bureau

of Mines

Specimen Size (WxL) 150mm x 1M 225mm x 15M 152mm x 4.6M 229mm x 1.5M

Number of

Specimens Tested 6 2 2 1

Drum Size (DIA) Steel - 210mm Steel - 450mm Steel - 203mm Steel - 457mm

Initial Drum

Temperature

<30® C — — —

Drum RPM 190 110 190 110

Air Velocity 120M/min 90M/min 152M/min 91M/min

Minimum Initial

Air Temperature

5® C —

— —

Maximum

Ventilation Rate

of Test Room <30M/min — — —

Total Load

on Belt

Kg min Kg min Kg min Kg min

3570 105 140

175

60

3030

10 Fails

22.5

3445.5

59.5

75.5 91.5

107.5

123.5

15

1515

15

1515

1515

3268 105 141

177

120

10

10

10 Fails

22.5

3445.5

59.5

75.5

91.5

107.5

123.5

15

1515

15

15

15

1515

Acceptance

Criteria

No Flame

or Glow

No Flame

or Glow

1) No Flame

or Glow

2) Max. Drum

Temp. <300® C

1) No Flame

or Glow

2) Max. Drum

Temp. <250® C

Comments

Test 3 w/air

3 w/o air

Reverse

Specimen

Surface

Test 1 w/air

1 w/o air

A3
The Bureau of Mines tests rely on single sample testing for belt

qualification. The South African test procedure also uses single sample

testing, but the test procedure requires that a belt be tested with both

surfaces in contact with the friction drum. This results in two test

specimens per sample.

All of the other drum friction tests are based on the ECC test (see

table 2) . This test differs in almost every aspect with the previous two

tests. The specimen size is smaller, and the friction drum is smaller and

operates at a higher speed. The initial belt loading is higher by nearly

13 kg and a total of six specimens are tested per sample. However, the most

important difference is that belts are tested until they mechanically fail.

Throughout this test period, none of the specimens can exhibit any signs of

flaming or glowing. All of the tests specify an air jet to be applied to

the friction drum during the entire test period. Several of the test

procedures require testing with and without air.

1.2 Flame Test

There are five different types of flame test standards applicable to

conveyor belts that are used in coal mines (see table 3) . Three standards

are based on small-scale tests, while the remaining two tests can be classified as large-scale tests. This distinction is based, primarily, on specimen size and burner energy output.

The three small-scale tests subject a specimen, 25 mm wide by 200-300 mm

long to a small ignition source (less than 500 watts) ; either a spirit burner

or a bunsen burner. While specimen orientation (i.e., the direction of longitudinal axis) is different for the ISO test, the major difference between

these tests can be found in the post exposure period. Immediately upon the

removal of the burner, the test administered by MSHA [6] applies an airflow

of 1.5 mps parallel to the specimen surface in the direction of flame spread.

ISO and the German tests apply an airflow perpendicular to the flame spread

axis with a vertically oriented specimen. This is done 60 seconds after the

removal of the flame. The purpose of this is to determine if the additional

air movement would tend to drive the belt into continued glowing or flaming.

A minor difference also occurs in the actual exposure time. MSHA requires

that the specimen be exposed to the bunsen burner for 1 minute. The exposure

times found in the ISO test, 340 [7], and the British Standard, 3289 [8], are

45 and 30 seconds, respectively. While the actual acceptance criteria for

each test differs in absolute terms, they all require that flaming and glowing

to cease before a given time limit.

A4
Table 3. flame test for conveyor belt qualification for underground coal mines

Test Method ISO-R340 ECC Germany DIN 22 118 Specifications Germany DIN 22 103

orlClAn

BS 3289 30 CFR Part 18 Propane Grill

Test

Specimen Size (WxL) 25mm x 200mm 25mm x 305mm 13mm x 152mm "Normal" x 4M 90mm x 1.2M

Humber of Specimens Tested 12 4 2 1

Specimen

Orientation

(Longest Axis)

Vertical Horizontal Horizontal Horizontal Horizontal

Burner Type Spirit Burner Barthel-Spirit

Burner Bunsen Burner 450mm Sq. Frame

52 Jet Franke Bunner

(Similar to Mekker)

Fuel

952 Ethanol

5Z Methanol

90Z Propane

10Z Propene 95Z Ethanol Propane Natural Gas

Flame Length 150-180mm 152mm 76asn (.13 Kg/rnia)* 10ILIA

Exposure Time 45 sec. 30 sec. 60 sec. 10 min. 15 min.

Specimen Distance Above Burner 50mm 51mm 25.4mm 130ma 40m

Chamber Airflow Rate

1.5 M/s (60 sec. After Exposure)

1.5 M/e (After Test Exposure)1.5 M/s (After Test Exposure)

.5 M/s (During Test Exposure

Direction of Airflow

Normal to Flame

Spread Parallel to Flame Spread Parallel to FlameSpread Parallel to Flame Spread

Acceptance Criteria Afterflame Time:<45 sec. Avg. <15 sec. Each

Afterflame and

Glow:

<3 sec.

Afterflame:

<1 min. Afterglow:

<3 min.

Self-Extinguish

With a Portion of Sample Intact Across

Entire Width

Self-Extinguish

With a 40cm Section

to Remain Intact

* Specifies mass consumption rate rather than flame length.

A5
The two large-scale tests listed in table 3 are vastly different from

the previous three tests. This is true for specimen size, exposure conditions,

and acceptance criteria. The propane grill test, being considered by ECC [5]

and the British [9] , employs a rather large burner configuration releasing

approximately 99.7 kw of energy. The specimen in this test is also large in

comparison to the previous set of tests, the normal full width of the conveyor

belt by 4 m long. The Germans [10] also have a large-scale test that uses a large Mekker type burner. Specimen size is 90 mm wide by 1.2 m long. Test

exposure time is 10 minutes for the ECC test and 15 minutes for the German

test. In both cases, the acceptance criterion requires that a minimum portion

of the test specimen remain intact at the completion of the test (see table 3)

.

2. FIRE SCENARIOS AS RELATED TO A TEST METHOD

All safety standards are promulgated on the premise that their enforcement

would significantly improve safety in the end use environment. This is even

more nearly valid for fire safety standards, since their implementation is

designed to reduce the likelihood of a given set of circumstances from resulting in a fire loss. Fire safety standards require the use of materials or

designs that intervene into the flow of events that begin with the initial

failure of a product and end with an injury or financial loss due to fire. An

analysis of the current set of test methods for conveyor belts aimed at deducing the initial design considerations (i.e., scenario) that led to their

development would be helpful, if for no other reason, than to allow us to question their suitability to function as an indicator of fire safety in today's

world.

2.1 Small-Scale Fire Tests - Scenarios

Consider first the small-scale fire tests described above. What does it

mean if a conveyor belt passes one of these tests? In order to answer that

question, one has to realize that the probability for a conveyor belt failure

depends on the nature of the ignition source (i.e., the rate of energy release

and the total exposure time) as well as on the thermal characteristics of a given conveyor belt and the geometric relationships between the belt and the

environment.

In the various small-scale test procedures, the ignition source is defined

by stipulating the fuel, the fuel delivery rate (i.e., flame height, flame surface area, and burner bore size) , and the total exposure time. By means of the

acceptance criterion, materials are qualified for the intended end use. It is

A6
clear that the anticipated ignition source was thought of as being small with

an energy release rate on the order of 500 watts or less. Furthermore, from

the test procedure, it can be seen that the ignition source was intended to

have little or no influence on the development of a flame front along the conveyor belt specimen. Observations are made for a predetermined time, after

the removal of the burner, to ascertain the likelihood of self-sustained burning and/or glowing on the test specimen.

A basic assumption underlying all of these test procedures and, therefore,

the scenario, is that the conveyor system remains stationary during the

critical time that the thermal energy impinges on the belt. Only one of these

small-scale tests considers the impact of starting up the conveyor system

following the input of thermal energy—that is MSHA's test described in the

Code of Federal Regulations (30:18.65).

Following the low-level ignition of a conveyor belt, the observations

made during the test seem to relate to three possible outcomes, if selfsustained combustion occurs on the conveyor belt, either flaming or smoldering.

The possible hazards resulting from this are:

• that the persistent flames or glowing will ignite explosive gas

mixtures that may be found in coal mine operations,

• that the burning conveyor belt will spread the combustion process to

the surrounding coal, or

• that the burning conveyor belt alone represents a hazard to the

individuals working in a coal mine.

In the latter case, the hazard may be exposure either to the flames or,

more likely, to smoke.

A possible scenario that would correlate with the operational conditions

defined in the test procedures requires that a small ignition source appear.

This could be the result of a smoldering piece of coal on the conveyor belt,

a jammed roller igniting coal dust and grease, or an electric failure that

produces a short duration arc. Smoldering coal releases approximately 3.6

watts of thermal energy [11] for a lump of about 400 gm. A smoldering lump

on a belt would only transfer a fraction of this energy into the conveyor

belt, but smoldering could spread to other pieces of coal.

A7
A jammed roller that produces enough energy to cause frictional ignition

of trapped coal dust and grease represents a very real threat of ignition of

the conveyor belt. It is not readily possible to determine the energy release

rates of a coal dust and grease mixture.

A short duration arc represents another possible low-level ignition

source. However, for the arc to be of significant consideration, it must persist for several minutes, which, in the mine environment, is certainly possible.

Experiments have shown that a low current, high voltage arc transfers energy

to a horizontal surface at an average rate of approximately 2-3 watts [12]

.

Mine operations require high currents to run most of the equipment at voltages

below 1 kv. Under comparable conditions, high current arcs should produce

several times the energy release rate of the low current arc. However, the

size of the initial gap would have to be smaller before the arc could establish

itself

.

Once the exposure time has ended a finite observational period begins.

In the MSHA test, this is augmented by the introduction of an air current.

This appears to be intended to measure the belt's ability to glow or reignite

assuming that self-sustained burning has not developed during the exposure

time. Taking this test condition into account modifies the assumed scenario

for we are no longer limiting the hazardous condition to the vicinity of the

ignition source but are attempting to limit the impact of a burning belt to

other areas of the mine as the conveyor system is started up and moved through

the shaft.

2.2 Large-Scale Tests - Scenarios

It should now be clear that an adequate definition of the ignition source

is required. Its size determines the extent to which energy feedback from the

surrounding surfaces must be considered. One of the implied assumptions of the

small-scale tests is related to the incidental nature of the ignition source.

The opposite assumption can be drawn from the test procedures of the largescale tests. No large-scale test is currently used in the United States; however, Great Britain and Germany use a large-scale test whose ignition source

releases approximately 100 kw of thermal energy. This represents a 200 fold

increase over the small-scale tests. This rate of energy release continues

for 10 to 15 minutes instead of 1 minute or less for the small-scale tests.

This means that the interaction between the burning conveyor belt and chamber

walls and ceiling must be considered in developing a scenario. Although we

are not in a position to compare results from various test methods, the extent

of energy feedback will affect efforts towards correlating various test methods

(13, 14, 15, 16) . A8
The criteria of both large-scale tests are, in essence, identical. They

require that at the end of the test period the conveyor belt self-extinguish

and leave a section of the belt undamaged.

Consider, for example, a large drum of solvent on fire. Assuming that

the belt becomes ignited from this source, flame propagation along the conveyor belt is dependent upon the relationship between the conveyor belt,

source location, draft size, size of burning pool (i.e., energy release rate),

and airflow conditions. The extent of flame spread is controlled by the total

energy incident on the unburned portion of the conveyor belt. In this case,

the total incident energy is the sum of the energy released by the burning

pool incident on the belt and the energy feedback from the combustion of the

belt. As the pool diminishes in intensity or the flame front propagates

further from the burning pool, the pool fire represents less of a driving

force. Continued burning of the conveyor belt becomes more dependent on

energy feedback. If the energy feedback from the surroundings falls below a minimum level that is dependent on the type of belt, the geometry of the mine,

and the presence of flame-retardant additives, the belt will self-extinguish.

The large-scale tests described above define an ignition source in terms

of energy release and duration. The shaft cross sectional area is defined as well as the initial airflow conditions. The correlation of test performance

and end-use performance relies on the realistic assessment of the strength

and duration of a probable real fire. For similar geometric conditions, the

large-scale tests establish a baseline for hazardous ignition sources. They

would eliminate or greatly reduce extensive flame propagation from ignition

sources below the energy release and duration levels of the tests. To carry

this one step further, however, the importance of energy feedback cannot be

over emphasized once ignition has occurred, the actual energy input into a conveyor belt may be higher than one would assume from an analysis of the

ignition source alone.

2.3 Drum Friction Testing - Scenarios

Of the three types of flammability tests applied to conveyor belts, the

drum friction test is the simplest for which to develop a set of conditions

that describe the mode of failure. The drum friction test simulates frictional heating of the conveyor belt due to a stalled belt caused by a roof

fault or loss of tension. It is not clear why all of the test methods apply

an airstream aimed at the drum. Perhaps this is an effort to duplicate the

airflow characteristics found in some sections of a mine's haulage system.

However, at least for U.S. mines, portions of the haulage system are defined

A9
as neutral air (i.e., no forced airflow). Under these conditions, the

externally applied airstream unnaturally lowers the operating temperature of

the drum, but does increase the likelihood of observing afterglow.

The drum friction test method is no longer used in the United States for

conveyor belt qualification. This may be because of the introduction of a "zero-slip" switch that shuts the system down if the belt stalls while the

drive is still applied.

3. DISCUSSION OF TEST PARAMETERS

The basic question that must always be addressed relates to the

appropriateness of a given test procedure. A test method is appropriate if

it measures a parameter known to relate to the hazard. Parameters that have

been used in other fire tests and might be used for conveyor belts are:

• self-sustained burning;

® ease of ignition;

• rate of heat release;

• critical radiant flux;

9 smoke generation;

• toxicity of combustion products.

Clearly, the current set of flame tests for conveyor belts addresses only one

aspect of fire performance characteristics, self-sustained burning. (The drum

friction test measures an ignition characteristic under one given form of

energy input.) While self-sustained burning is recognized as an important

parameter, it must be realized that the probability of such performance for a given conveyor belt is a function of the characteristics of the ignition source

and the operational environment as well as the properties of the conveyor belt.

While measurements of all the parameters may not be valid for each

application, perhaps one or another may prove to be more flexible than the

current test method. Most of the items on the list are self-explanatory.

However, critical radiant flux is a relatively new test concept. The basic

idea is that one is able to determine the minimum level of feedback energy

necessary to sustain a flame across the surface of a specimen. This is accomplished by applying a varying radiant flux field to the specimen and observing

the extent of flame spread. This method would allow the acceptance criterion

to be established according to a well defined scenario. In addition, as the

A-10
scenarios are redefined, the acceptance criterion could be adjusted (within

the limits of currently available equipment)

.

4 . SUMMARY

In addition to the Mine Safety and Health Administration's test method

for conveyor belts used in underground coal mines, several international tests

were reviewed. The differences between large-scale and small-scale tests were

discussed in relation to the development of scenarios applicable to coal mine

fires that spread along a conveyor belt.

Only the drum friction test assumed that the conveyor belt was the first

item to ignite, while the flame tests presupposed the existence of an external

source

.

A short discussion of alternative measurement parameters was presented.

5 . REFERENCES

[1] South African Bureau of Standards, "Safety Requirements for Conveyor

Belting," SABS 971-1971.

[2] Polish National Standards Committee, "Fire Tests for Underground

Conveyor Belting," PN-76 G-04030.

[3] National Coal Board, "Fire-Resistant Conveyor Belting," Number 158/1971,

Part 7.

[4] Bureau of Mines, Department of the Interior, "Mechanical Equipment for

Mines," Center for Fire Research, Title 30, Schedule 28, Part 34 (1957).

[5] Mine Safety and Health Administration, "First Report on Tests and

Criteria of Flammability of Conveyor Belts," Appendix I (1974).

[6] Code of Federal Regulations, Title 30, Schedule 2G, Part 18 (1977) .

[7] International Organization for Standardization, "Flame Resistance of

Conveyor Belts—Specifications and Method of Test," ISO R340 (September

1973)

.

A- 11
[8] British Standards Institution, "Specification for Conveyor Belting for

[9] Underground Use in Coal Mines," B.S. 3289 (I960).

National Coal Board, "Method of Test for Fire Resistance of Conveyor

Belting—Propane Burner Test" (1975) .

[10] German Standards, "Conveyor Belts With Textile Plies for Coal Mining

Underground, Burning Characteristics and Test Methodology," DIN 22 118.

[11] Iki, S. and Higuchi, K., "Spontaneous Combustion of Powered Coal," J.

Ofc. Faculty of Engineering, University of Tokyo, Series 9, pp. 56-57

(1971)

.

[12] Peacock, R. D., NBS , personal communication.

[13] Denyes, W. and Quintiere, J. G. , Experimental and Analytical Studies

of Floor Covering Flammability With a Model Corridor, Nat. Bur. Stand.

(U.S.), NBSIR 73-199 (1973).

[14] Quintiere, J. G. , A Characterization and Analysis of NBS Corridor Fire

Experiments in Order to Evaluate the Behavior and Performance of Floor

Covering Materials, Nat. Bur. Stand. (U.S.), NBSIR 75-691 (1975).

[15] Quintiere, J. G. and Bromberg, K., Calculations of Radiant Heat Flux in

the Proposed Floor Covering Flame Spread Test Apparatus, Nat. Bur. Stand.

(U.S.), NBSIR 75-706 (1975).

[16] Buckley, J. L. and Vincent, B. G., Conveyor Belt Testing Program, U.S.

Bureau of Mines, Contract No. M0252085, Final Report (1978).

A- 12
APPENDIX B

Computation of Temperature Profile for the Hot Roller Scenario

The thermal interaction of a conveyor belt with a jammed roller arm is

analyzed by dividing the problem into three distinct parts. The initial part

is concerned with the contact temperature at the interface between the steel

roller and the belt shortly after the conveyor system is halted. The second

concerns the thermal behavior of the steel roller long after the system is

halted. The third part uses the steel roller surface temperature as a boundary

condition for the calculation of the temperature profile across the thickness

of the belt.

A jammed roller is frictionally heated by the moving belt. The roller's

temperature rises until a steady state condition is established at a given

temperature, t constant throughout the roller. This temperature is determined by the balance between the energy into the steel roller, which is a function of the tension, load, and surface characteristics of the belt and

roller, and the heat losses from the steel roller to the surrounding environment. The contact time of any portion of the belt with the jammed roller is

very short and, therefore, its temperature is also given by a steady state

temperature, t^ . This value is determined by all the frictional forces along

the entire system and concurrent heat losses.

For a very short time after the conveyor system stops operating, the hot

jammed roller and that portion of the conveyor belt in contact with the roller

will act as semi-infinite solids. If it is assumed that there is no contact

resistance between the roller and the belt, each will instantaneously come to

some intermediate contact temperature at the interface. Myers [1] has solved a similar analytical model and has shown that the contact temperature, t , is:

1 . CONTACT TEMPERATURE

t

c

B-l
where

t

^ = Initial temperature of the roller,

tj. = Initial temperature of the belt.

(Kpc) = Thermal inertia of the roller.

(Kpc)^ = Thermal inertia of the belt.

K, p, and c are the thermal conductivity, density, and specific heat,

respectively. Typical values for the thermal inertia of the roller and belt

are used--1463 J

2 /sec-cm4 -C2

[2] and 89 J

2 /sec-cm4 -C2 [3]

.

The results of

this computation are shown in table 1 for various initial roller temperatures,

t and an initial belt temperature, t^ , of 38°C.

2. TRANSIENT ROLLER TEMPERATURE

With the conveyor system idle, the belt no longer frictionally heats the

roller. The roller begins to lose thermal energy to its surroundings. Since

only a small arc of the roller surface area is in contact with a small section

of belt, the predominant mode of heat transfer from the roller is via convection

to the atmosphere. It will be assumed that the presence of the belt, therefore,

does not significantly modify the roller's thermal behavior. Furthermore, the

tables presented by Welty [2] for the transient case for a long finite cylinder

have been used. From these generalized tables, the surface temperature of the

roller was calculated (see figure 1) . Each curve represents the surface temperature of the roller assuming a different initial steady state temperature.

3. TRANSIENT TEMPERATURE PROFILE FOR A BELT

A one-dimensional model of a conveyor belt will be assumed. The surface

temperatures for the roller shown in figure 1 will be used as the boundary

transient contact temperature for one face of the belt. This will yield somewhat higher temperatures across the belt thickness. To insure that the model

does not underestimate the predicted temperatures, the opposite face of the

belt will be assumed to be insulated rather than incorporating a convective

heat transfer. This assumption may, in fact, not be to far removed from reality, if a layer of coal is assumed to be present on the belt opposite the contact point of the belt and roller. For this case Myers [1] provides a series

solution for the temperature distribution as a function of time. The equation

in nondimensional terms is:

B-2
Table 1. Hot roller temperature and the instantaneous

interface contact temperature for a conveyor

belt initially at 38°C

Drum Temperature (°C)

150

200

260

315

370

425

480

540

Interface Temperature (°C)

127

170

216

261

305

349

393

440

B-3
Figure

1

-

Calculated

transient

surface

temperature

for

a

stee

cylinder

at

four

initial

temperatures

(Oo) 9JrUDJ9dUU9| 90DJ-Jns

B-4

J

Time

(min)
where

t(X,6) = ire -e P(X)

L onrr

1 - *£

n n

P(X)

Qn^T

n

e-Qn6

t = Temperature , nondimensional, as a function of time and position.

Qn = (2n+l) 2

tt 2 / 4 .

P(X) = (2n+l) sin [ (2n+l) itX/2] .

otT The nondimensional time 0 is defined as where a is the thermal diffusivity,

T is time, and L is the thickness of the sample in cgs units. This equation

was evaluated using a computer for the numerical calculations. The results of

the calculation for two cases of initial drum temperature are shown in figure

2 in dimensional terms.

4. CONCLUSIONS

It has been shown that the difference between drum temperature and

interface contact temperature increases with increasing drum temperature. At

a drum temperature of 150 °C, the interface temperature lags by 23°C, while at

a drum temperature of 540°C, the interface temperature is 440°C, a difference

of 100°C.

The calculations show that surface drum temperatures decrease approximately

100 °C within 5 minutes of the removal of a heat source.

Drum and belt interface temperatures are a reasonable approximation of

the belt's cross-sectional temperature.

5 . REFERENCES

[1] Myers, G. E., Analytical Methods in Conduction Heat Transfer, McGraw-Hill

Book Company, New York (1971) .

[2] Welty, J. R. , Engineering Heat Transfer, John Wiley and Sons, New York

(1974) .

[3] Ho, C. Y., Desai, P. D., Wu, K. Y., Havill, T. H., and Lee, T. Y.,

Thermophysical Properties of Polystyrene and Poly (Vinyl Chloride), NBSGCR-77-83 (March 1977).

B-5
Figure 2-

Calculated

transient

temperature

profile

ofa

conveyor

belt

exposed

toa

time

dependent

surface

temperature

CO

COoco

<D JQ

COCOO

l_oa

<Doco

CO

(Oo) 9Jn+DJ9dUU81 o

CO OJ OJ t—i -eH

6^

CO

CO

<Dco

<D

COCOOi-oa

<DOco-J- co

(Oo) 9Jn+DJ9dUU91 oo

ID

COoE3L- TDa

'<z

B-6
APPENDIX C

Heat Transfer Calculations for an Electric

Arc Near a Conveyor Belt

An electric arc is a self-sustained discharge having a low voltage drop

and capable of supporting large currents. The arc is established either by

the separation of contacts or by a transition from a higher voltage discharge.

At atmospheric pressure, an arc is characterized by a small intensely brilliant

core surrounded by a cooler region of glowing gases. The electrodes are at

the boiling temperature of the electrode material, while the gas temperature

in the core region is approximately 7000 °C [1] . (The boiling temperatures of

copper and aluminum electrodes in air are approximately 1900 °C and 3100°C,

respectively.

)

An estimate of the heat transferred to a conveyor belt in close proximity

to an arc is approximated by assuming that heat is transferred only via radiation. Both the arc and the conveyor belt are assumed to have an emissivity

of 1 and that the arc is assumed to be an opaque sphere. In addition the

exposure is assumed to be in the center of the belt and edge effects are,

therefore, ignored. This simplified problem yields an upper value on the rate

of energy transferred to the conveyor belt.

Figure 1 is a simplified description of a spherical source, an arc, and

a plane substrate, a conveyor belt. The relevant parameters are shown. They

are

:

1. r^, the radius of the arc core.

2. r

2

, the radius of that portion of the conveyor directly below the arc.

3. h, the distance between the center of the arc and the conveyor belt.

The rate of heat transfer is computed by:

Q = F A

1 a

C-l
igure 1-

Schematic

drawing

of

thesi

mp Iifi

ed oL0£0

Li_ a>

C-2
where

2 Q = Rate of heat transfer, W/cm .

F = View factor.

= Surface area of the arc as seen by the conveyor belt.

-8 4 2 a = Stefan-Boltzman constant (5.67 x 10 J/K m sec).

= Initial temperature of the arc (°K).

T

2 = Initial temperature of the conveyor belt (°K).

Because the arc is opaque, A. represents the hemispherical surface area facing

. 12 the conveyor belt. This is A^ = 2 tt r^. The rest of the arc surface releases

energy to the environment but not the conveyor belt. The view factor for this

geometry is given by Siegel and Howell [2] as:

Figure 2 is a plot of the incident heat flux on the surface of a small

segment of the conveyor belt versus the arc core size. The core temperature,

T^ is assumed to be 7300°K, and the conveyor belt temperature, T

2

, is 300 °K.

The incident heat flux is computed for various distances between the arc

and the conveyor belt. The incident heat flux on the conveyor belt decreases

as the arc is moved further from the belt and as the arc size decreases.

Figure 3 shows the incident heat flux as a function of distance from the

center of the arc for three arc diameters. The center of the arc is centered

10 cm above the plane surface. The decrease in incident heat flux is rapid

with increasing distance from the arc.

F = 1/2

where
Figure

2

-

Energy

incident

on

a

conveyor

belt

as

a

function

of

arc

core

size

for 5

distances

between

the

arc

and

belt

o

GOo

COo

o

CVJ o

(3 uuo/m) xnii tD9M +U9picui

C-4

Arc

core

radius

(mm)
Figure

3

-

Incident

heat

flux

on

a

plane

surface

for

three

arc

core

diameters

10

cm

above

the

plane

(3UJ0/M) xnu +09M +U9PI0UI

C-5

Distance

on

the

plane

-from

arc

core

center

(cm)
Based on the data presented in appendix D, a belt exposed to the incident

energy from a small diameter arc (0.2 mm) 1 mm from its surface would ignite

within 10 seconds. As the distance is increased, the time to ignition increases.

References

[1] Cobine, J. D., Gaseous Conductors Theory and Engineering Applications,

Dover Publications, Incorporated, New York (1958) .

[2] Siegel, R. and Howell, J. R. , Thermal Radiation Heat Transfer, McGraw-Hill

Book Company, New York (1972)

.
APPENDIX D

Ignition, Flammability, and Smoke Generation Characteristics

of Conveyor Belts Used in Mines

1. INTRODUCTION

Three test methods were used to evaluate 28 conveyor belts and these

results were compared to test results obtained using the standard MSHA fire

resistance test for self-extinguishment [1] . First the modified NBS smoke

box [2] was used to characterize the smoke generating potential of these belts.

This could be an extremely important fire parameter considering that belts are

used in long confined areas with limited escape routes. Because many areas in

an underground coal mine are under forced airflow conditions, large quantities

of released smoke could spread to other sections of a mine.

Second, the Flooring Radiant Panel Test [3] measures a material's

resistance to propagate a flame when a moderate external source of heat is

present. In this case, the external source of heat could be wood lagging or

the coal itself.

Third, an ignition apparatus was assembled to measure the ignition

potential of these conveyor belts under a range of exposure conditions. These

results could, for example, be used to determine the suitability of a conveyor

belt based on an assumed energy release rate for a developing fire. Materials

with very low ignition delay times may prove unsatisfactory for a specific

application because belt ignition may occur before a fire detection system

could respond. Once mine operators are made aware of a fire the ignition of

the haulage system may seriously hamper escape and rescue procedures.

2 . TEST MATERIALS

A total of 28 conveyor belts were obtained from various companies. The

cover composition of 14 belts was styrene butadiene rubber (SBR) while the

remaining 14 were polyvinyl chloride (PVC) . The SBR belts were either singleply or triple-ply construction using a polyester fiber carcass. The PVC belts

were impregnated on a carcass of polyester fiber except number 14 which is a polyester cotton 2-ply construction. With the exception of eight belts, all

the tested belts were reported to have been formulated to comply with MSHA

requirements. This may indicate the presence of flame-retardant compounds.

D-l
Conveyor belt thickness varied from .371 cm to 1.151 cm (see table 1)

.

3.1 3 . TEST PROCEDURES

MSHA Self-Extinguishment Test

The MSHA test is a modification of ASTM D-635. In this test, a small

specimen, 15.2 cm x 1.3 cm, is subjected to a bunsen burner flame for 1 minute.

The burner is then withdrawn, and a forced airflow along the specimen's long

axis is produced by turning on an exhaust fan. The test is continued until

all glowing and flaming on the specimen has stopped. Afterglow and afterflame times are recorded. The acceptance criteria require that no flaming

shall persist on the specimen for longer than 1 minute after the removal of

the burner and afterglow shall not exceed an average of 3 minutes for four

specimens.

3.2

NBS Smoke Box - Horizontal

This test method measures the smoke generation potential of nominal 7.5

cm x 7.5 cm solid specimens exposed to a radiant flux level of approximately

2 2.5 W/cm . The smoke produced by the specimen burning in the closed chamber

is measured by a light source-photometer combination. The attenuation of the

light beam is expressed in terms of optical density for the exposed specimen

area, volume of chamber and length of light path. The smoke box used in this

test series has been modified from NFPA 258 to use a horizontal specimen and

2 to record weight loss at a reduced incident flux level of 2.0 W/cm .

The concurrent measurement of optical density and weight loss combined to

give a single number that is the optical density normalized for differences in

weight loss [2]

.

3.3

Flooring Radiant Panel

This test method exposes a specimen placed horizontally to a radiant

2 2 energy gradient that varies along its 1 m length from 1.1 W/cm to 0.1 W/cm

[3], The specimen is preheated by the radiant heat for 2 minutes, at which

time a small pilot flame is applied at the high flux end of the test specimen.

The maximum distance burned corresponds to the critical radiant flux (CRF)

necessary to support continued flame progagation. The higher the CRF value,

the better is the fire performance of the material. The conveyor belts were

tested according to the currently accepted test procedure (ASTM E 648)

.

D-2
Table 1. Physical characteristics of conveyor belts

: Number Cover Composition Thickness Density

(cm) (Kg/m3

)

1 pvc

a .666 614

2 PVC 1.151 629

3 PVC .371 570

4 PVC .777 619

5 PVC .932 582

6 PVC .815 619

7 PVC .688 623

8 PVC .996 622

9 SBR

b

(3-ply) 1.014 617

10 SBR (3-ply) .907 617

11 SBR (3-ply) 1.014 609

12 PVC* 1.006 599

13 PVC* .828 574

14 PVC* 1.029 581

15 PVC* .724 617

16 PVC* .747 567

17 PVC* .688 408

18 SBR (1-ply) .772 646

19 SBR (1-ply) .823 641

20 SBR (1-ply) .808 647

21 SBR (1-ply) .676 642

22 SBR (1-ply) .556 652

23 SBR (1-ply) .808 641

24 SBR (1-ply) .587 631

25 SBR (1-ply) 1.016 613

26 SBR* (1-ply) .907 544

27 SBR (1-ply) .765 635

28 SBR* (1-ply) .645 574

Polyvinyl Chloride

Styrene Butadiene Rubber

Not formulated to meet requirements of MSHA §18.65

D-3
3.4 Ignition Tests

An apparatus was assembled to measure the ignition delay time of conveyor

belt samples under various radiant flux exposures, airflow velocities, and

pilot ignitions. Figure 1 is a schematic representation of the apparatus. It

consists of an electrically heated quartz panel which provides a flux field

that is uniform over the 5.1 cm diameter of the specimen. The specimen and a heat flux meter are mounted in a slide shelf that allows for rapid interchange

of the specimen for the heat flux meter below the radiant panel. A pilot flame

ignition source is mounted .64 cm above the center of the specimen. This

distance was selected based on work previously performed on floor covering

materials [8]

.

The entire assembly is mounted in a mineral board tunnel that

2 has a square cross-sectional area of 2120 cm and an overall length of 120 cm. A pair of exhaust fans are mounted at one end of the tunnel to allow for the

introduction of various airflow rates across the surface of the specimen.

To test a specimen, the radiant panel is heated so as to provide the

desired incident heat flux as measured by the heat flux meter. This setting

is checked before and after each test. The total heat flux at the center of

the specimen is the sum of that supplied by the radiant panel and by the pilot

ignition source. This value is reported because invariably ignition occurred

directly below the pilot source.

4. DISCUSSION

4.1 MSHA Self-Extinguishment Test

For purposes of establishing a baseline of performance, all of the samples

were tested according to the procedure outlined in the Code of Federal

Regulations 30:18.65. Table 2 is a summary of these results. Of the 28 conveyor belts tested, 20 had been previously tested and approved by MSHA (numbers

1-11, 18-25, 27). The remaining eight belts (numbers 12-17, 26, 28) were not

intended for use in underground coal mines and, therefore, had not previously

been tested according to this procedure.

The tests indicate that all previously approved belts consistently met

the acceptance criteria. Three belts which had not been previously approved

and were, therefore, not formulated to meet the acceptance criteria were found

to perform as well as the previously accepted belts.

D-4
D-5
Table 2. Test results on conveyor belts using the Mine Safety

and Health Administration test method 30:18.65,

average of 4 specimens

Belt Average Afterflame Average Afterglow

Number Time Time

(sec.

)

(sec.

)

1 4.0 0

2 0 0

3 3.8 0

4 4.4 0

5 3.6 0.6

6 2.6 0

7 1.6 5.0

8 2.4 3.0

9 2.4 0

10 11.0 0

11 2.6 3 12 .4 0

13 4.4 2.0

14 206.0 0

15 3.6 1.2

16 241.8 180

17 >65.0

*

18 1.0 2.0

19 4.0 0

20 2.0 0

21 2.6 0

22 1.2 0

23 3.6 0

24 7.8 0

25 4.2 0

26 >65.0

*

27 .4 0

28 >65.0

*

Specimen manually extinguished

D-6
The difference between measured values and acceptance criteria for all

the belts meeting the criteria was so large that there could be no question

as to acceptance. For example, afterflame times on individual specimens never

exceeded 36 seconds and the longest afterglow time was 8 seconds. The average

value for each sample was, therefore, very low.

The same was true of the failing belts. The failure criterion was

exceeded for each specimen tested. These samples, which were not flame retardant or designed to meet MSHA requirements, exhibited persistent and sometimes

vigorous burning characteristics. Because of the size of the flames and the

quantity of smoke produced, three belts (numbers 17, 26, 28) were manually

extinguished. No afterglow times were recorded for these belts and the afterflame times represent the time of manual extinction of the specimen.

Belt performance for this set of samples was either clearly acceptable or

not acceptable. No samples tested exhibited borderline behavior. Such clear

cut distinctions were not noted in the other fire tests performed on the belt

samples

.

4 . 2 Smoke Data

In the modified NBS smoke box (horizontal specimen) , two sizes of square

samples were used. A square 5.1 cm on a side and a smaller square 2.5 cm on

2 a side were tested under an external flux of approximately 2.0 W/cm . The

standard size specimen was not used because preliminary tests indicated that

the amount of smoke produced from this size overwhelmed the .51 cu m smoke box.

It was found that many of the 5.1 cm samples exceeded the maximum possible

density reading. Dm, of 1080.

Table 3 shows the maximum optical density, Dm, obtained for 14 belt

materials using a 26 sq cm (5.1 by 5.1 cm) size sample. The Dm values for

most of these belts was 1080. Comparisons between belts could not, therefore,

be done using the Dm values.

In order to analyze this data, two methods for calculating the Mass

Optical Density were used. The method employed by Breden and Meisters [3]

required the use of the maximum optical density and the weight loss at that

time in the following equation:

D-7
Table 3 . Maximum optical density, D^j, for 5.1 cm by 5.1 cm conveyor belt samples tested in the modified NBS

smoke density chamber

Belt

Number Dm

1 1060

2 1080

3 1080

4 1060

5 1080

6 1025

7 1080

8 1045

10 1080

19 1080

21 1080

23 1080

28 1080

29 1080

D-8
MOD V i \ J- ' lam log io t

where

5 3 V = Chamber volume (5.1 x 10 cm )

.

L = Light beam path length through the smoke (91 cm)

.

AM = Sample weight loss (g)

.

T = % Light transmittance.

2 MOD = Mass Optical Density (cm /gm)

.

The MOD value calculated in this manner yields a maximum MOD (MOD^)

.

Chien and Seader [4] have suggested an alternative method of computing the

MOD value. The ratio of the rate of change of the optical density.

A log, „ (100)

AOD _ T

At At

to the rate of change of the mass loss, ~, multiplied by the constant of

V proportionality —. This would yield a rate dependent MOD (MOD ) . By this

J_i K

method, comparisons between different specimen sizes could be made even if

the total amount of smoke exceeded the maximum measureable value. This would

enable one to determine the viability of scaling up the results from the smoke

density chamber.

Table 4 lists the MOD^ and MOD

R values for the 28 conveyor belts using

a 2.5 by 2.5 cm specimen. In addition, the ratio of MODR to MOD^ has been

calculated. This ratio indicates that while there may be several exceptions

the manner in which the MOD value is calculated is not very important. The

average ratio for all 28 samples is 1.16 ± .34. This shows that computations

based on the rate of smoke evolution and the rate of mass consumption yield

approximately the same values as maximum final state value method. Since for

many belts the true maximum values are not known, the rate method of computation

will be used.

D-9
Table 4. Comparison of mass optical density (MOD) calculated

by the maximum values, MODjj,, and the rate values,

M0Dr , for a 2.5 cm by 2.5 cm specimen exposed to a 2.0 W/cm2 external radiation field in the horizontal

position

Belt #

MODm -3

(cm 2 /g) x 10

MOD

r (cm 2 /g) x 10

3 MOD /MOD

r m

1 6.26 8.96 1.43

2 5.99 4.87 .81

3 8.51 11.2 1.32

4 5.51 6.50 1.18

5 4.96 5.71 1.15

6 6.14 5.32 .87

7 5.97 5.88 .99

8 5.08 5.10 1.03

9 6.15 5.32 .87

10 6.77 6.38 .94

11 5.73 5.60 .98

12 3.77 6.33 1.68

13 3.07 5.60 1.83

14 4.56 6.44 1.41

15 4.14 6.33 1.53

16 10.4 18.7 1.80

17 7.64 8.34 1.09

18 3.86 5.60 1.45

19 5.88 5.21 .88

20 5.85 5.60 .96

21 6.11 7.73 1.26

22 5.39 7.62 1.41

23 5.60 6.78 1.21

24 4.03 5.88 1.46

25 5.16 4.09 .79

26 5.44 2.86 .53

27 4.31 5.15 1.20

28 7.41 3.30 .45

D-10
A comparison of data from different specimen sizes, table 5, indicates

that for about one-third of the samples tested specimen size did not have a significant effect on MODR values. However, for the remaining two-thirds of

the samples, variations between specimen sizes were noted. No clear explanation currently exists to explain the data, however, Chien and Seader [5] have

shown that variations in incident heat flux can yield different MODs. For

example, they showed that for cellulose, an increase in external heat flux

reduces the MOD value. They found the reverse effect to be true for a polyurethane sample. The same may be valid for the two specimen sizes tested here.

Although the external heaters were set to produce an incident flux of approxi2 mately 2 W/cm , allowance was not made for the pilot flame. Because all of

the samples were tested in the presence of a pilot flame, the net incident

flux per unit surface area was higher for the 2.5 by 2.5 cm specimen than for

the 5.1 by 5.1 cm specimen. The actual direction of the shift would depend

on the fillers and carcass material added to the base polymer. More detailed

work in this area would have to be done to verify this assumption.

Figure 2 summarizes the smoke data and demonstrates that, as in other

areas of fire safety, an improvement in one aspect of fire performance may do

little to improve other fire characteristics. It is not surprising to note

that, as measured by the MOD, material improvements necessary to meet MSHA

requirements have not resulted in parallel improvements in decreased smoke

generating potential.

4.3 Critical Radiant Flux

The Flooring Radiant Panel Test represents a more severe exposure

condition than defined by MSHA's §18.65. However, except for the parameter

value reported and the exposure condition, these tests measure similar fire

properties of a material. Extinguishment in the MSHA test and the Flooring

Radiant Panel Test is controlled by the level of energy feedback from the

flame front to the material substrate. Since the source is removed (after a 60-second application to the specimen) in the MSHA test, extinguishment is

controlled by feedback from only the flame front, whereas, in the Flooring

Radiant Panel Test the feedback mechanism is enhanced by the presence of the

gas panel. In either case, extinguishment occurs when the minimum flux into

the substrate falls below some critical value. The major conceptual difference between these tests is that one reports the time it takes for the feedback energy to decrease to this critical value and the other reports the value

of the critical radiant flux (CRF) . The rank order of materials should be

approximately the same. That is, those materials with long after flame times

D-ll
Table 5 . Comparison of mass optical density (MODr ) as calculated by the rate method for two

different specimen sizes exposed to a 2.0 W/cm 2 external radiation field in the

horizontal position

Specimen Size 2.5 by 2.5 cm 5.1 by 5.1 cm

Belt

Number

MOD

r (cm 2 /g) x 10

MOD

r (cm2 /g) x 10

MOD (2.5)

r MOD (5.1)

r

1 8.96 7.45 1.20

2 4.87 8.29 .59

3 11.2 7.00 1.60

4 6.50 7.73 1.19

5 5.71 9.13 .63

6 5.32 4.26 1.25

7 5.88 7.47 .79

8 5.10 4.88 1.05

9 5.32 4.41 1.21

10 6.38 30.6 .21

11 5.60 18.8 .33

12 6.33 3.42 1.85

13 5.60 3.95 1.42

14 6.44 8.96 .72

15 6.33 4.54 1.39

16 18.7 6.79 2.74

17 8.34 2.80 2.98

18 5.60 6.63 .84

19 5.21 6.53 .80

20 5.60 5.34 1.05

21 7.73 5.71 1.35

22 7.62 7.54 1.01

23 6.78 2.95 2.30

24 5.88 3.39 1.73

25 4.09 4.77 .86

26 2.86 5.60 .51

27 5.15 3.38 1.52

28 3.30 4.85 .68

D-12

i

J
Figure 2 - Range of Mass Optical Density values for conveyor belts that

’Pass’ and ’Fall' MSHA requirements showing the range and average value

(dashed lines)

Mass Optical Density (MOD)

0 lOOOO 20000 30000 40000

D-13
should have low CRF values. (Of course, the same would not be true for

afterglow times.)

Since belt performance in terms of acceptability on MSHA's test method

was either very good or very poor it was not possible to compare test results

directly. However, table 6 summarizes the data obtained on the Flooring

Radiant Panel for selected belts and compares them to afterflame time results

on the MSHA test. These belts are listed in order of decreasing CRF values.

The data shows that with the increased exposure intensity of the Flooring

Radiant Panel Test, a range of values were obtained for belts meeting MSHA

fire resistance requirements. The data extended over 90 percent of the

2 Flooring Radiant Panel's test range (.11-1.1 W/cm ). Based on afterflame time

MSHA test results were all contained within the lower 10 percent of the accept

able range (0-60 seconds) . It was found that belt number 13 which had a short

2 afterflame and afterglow times on MSHA's §18.65 had a CRF value of .21 W/cm . Although, in general, the two tests yield a comparable ranking of materials,

the correlation is not as good as originally anticipated.

In the initial development and selection of criteria for this test,

Benjamin and Adams [6] determined that two criteria were necessary. For

residential and commercial applications, a floor covering material with a min2 imum CRF of .25 W/cm would be acceptable, and in institutional buildings the

2 minimum CRF should be .50 W/cm . The primary distinction between these two

types of occupancies is the mobility of the occupants. A higher level of performance is required for those applications where the occupant's movements are

physically or medically restricted, such as prisons or hospitals.

Coal mines also represent a physically restricted environment. Escape

routes are few and rapid movement in the mines is sometimes hampered by the

low ceiling. In the event of a fire, smoke would be an additional element

that would tend to further impede escape. For these reasons, some of the

materials currently meeting MSHA requirements may pose a risk not assessable

by MSHA §18.65.

4.4 Ignition Characteristics

Ignition experiments were performed on the conveyor belt samples using a radiant panel and a nonimpinging diffusion flame pilot as described in section

3.4. Results from these experiments gave some indication of the ignition

potential of these belts.

D-14
Table 6. Critical radiant flux (CRF) obtained on the

Flooring Radiant Panel Test and MSHA test

results of selected conveyor belts

Belt

Number

CRF

(W/Cm2

) Afterf lame

(sec)

3 >1.1 3.8

9 >1.1 2.4

11 >1.1 2.6

12 .80 0.4

1 .62 4.0

8 .57 2.4

13 .21 4.4

14 .19 206.0

17 <.11 >65.0

D-15
Table 7 is a tabulation of the ignition delay times for SBR belts exposed

to various intensities of incident heat flux. The data shows that, at high

heat flux levels, the ignition delay of these belts fall into a very small

range of values, 12 to 17 seconds. As the incident heat flux is reduced, the

. 2 width of the range of values increases. At 2.16 W/cm , the range is 69 to 103

2 seconds, while at 1.26 W/cm , the range is 168 to 439 seconds. This represents

2 a 7 to 1 increase in the width of the range . Below 1.26 W/cm there is only

one exposure level and its range width is approximately the same--295 seconds

2 2 at . 92 W/cm as compared to 271 seconds at 1.26 W/cm . This suggests that

differences in belt fire performance are more readily apparent at low exposure

levels.

However, those belts not formulated to meet the requirements of MSHA

§18.65 did not have the highest or lowest ignition delay times. A plot of

ignition delay time against specimen thickness for a given level of incident

heat flux (see figure 3) indicates the importance of specimen thickness in

defining belt response to incident radiation. These data further reinforce

the idea that distinctions in belt performance are more readily apparent at

low exposure levels.

Table 8 is a tabulation of ignition test results for PVC belts under

various exposure conditions . Similar results were obtained with the PVC belts

as compared to those with the SBR belts. The width in the range of values

increased with decreases in the incident heat flux. A reduction in incident

2 2 heat flux from 2.16 W/cm to 1.26 W/cm represents a 5-fold increase in the

spread of ignition delay times. Further reductions in incident heat flux do

not produce as dramatic an increase in the width of ignition delay times.

PVC belts consistently ignited sooner than SBR belts at the same level

2 of exposure. At 2.16 W/cm , the shortest ignition delay time for an SBR belt

was 55 seconds, while the longest ignition delay time for a PVC belt was 40

seconds. At lower levels of exposure, some overlap in extreme values was

noted

.

Ignition delay times for PVC belts did not appear to depend on belt

thickness, although most of the PVC belts were impregnated and the SBR belts

were of a single or multi-ply construction.

Several conveyor belts were selected for determining belt ignition

characteristics during low-level, long-term exposures to incident radiant

energy. As in the previous ignition tests, piloted ignition was studied.

Table 9 lists the results of these tests for the selected belts. The minimum

D-16

l
Table 7. Ignition delay time (seconds) for SBR conveyor belts

exposed to various intensities of incident heat flux

Incident Heat Flux W/cm2 Belt

Number 00 -o- .92 1.26 2.16 3.1 3.3 5.3

9 410 315 55 21 16

10 365 295 70 20 17

11 470 300 100 24 16

18 519 271 78 33 15

19 546 227 93 43 12

20 552 238 103 44 13

21 413 186 87 33 12

22 361 168 69 33 13

23 404 232 84 37 12

24 398 198 79 36 14

25 656 270 97 35 13

26* 623 439 87 30 12

27 557 222 90 43 14

28* 392 257 78 36 12

* Not formulated to meet requirements of MSHA §18.65

D-17
Figure

3

-

Ignition

delay

time

-for

SBR

conveyor

belts

exposed

to

radiant

flux

field

with

a

pilot

flame

a

(09S) QUUJ4- AD|9P UO!4!U6l

o o o o o o o o

o o o o o o o 1^- tO ID CO OJ

D-18

Specimen

thickness

(cm)
Table 8. Ignition delay time (seconds) for PVC conveyor belts

exposed to various intensities of incident heat flux

Incident Heat Flux W/cm2 Belt

Number .84 1.26 2.16 3.3 5.:

1 240 108 33 14 7

2 132 120 40 17 7

3 195 123 24 14 6

4 120 240 30 15 5

5 205 157 5 8 5

6 346 130 23 16 7

7 295 127 19 17 6

8 189 63 12 15 7 12* 203 185 15 9

•k Not formulated to meet requirements of MSHA §18.65

D-19
Table 9. Minimum incident heat flux necessary for

ignition and the ignition delay time for

selected conveyor belts

Belt Incident Ignition

Number Heat Flux Delay Time

W/cm2 min

1 .34 109

2 .40 77

3 .58 80

4 .45 169

12 .47 180

18 .48 176

20 .69 83

26 .54 83

D-20
incident heat flux necessary for the establishment of a flame on the conveyor

belt was determined by conducting ignition tests at progressively lower exposure levels. The ignition delay time was recorded as the time for the establishment of a flame on the conveyor belt at the minimum exposure level. While

the results show that both PVC and SBR conveyor belts ignited from low-level

exposures, flame spread behavior was different. SBR conveyor belts ignited

below the pilot and flames spread over the entire specimen surface. While

PVC conveyor belts established a flame on the specimen surface, flames were

confined to the center of the specimen and did not spread over the entire

surface

.

5. CONCLUSIONS

The fire properties of 28 conveyor belts were evaluated using four

different test procedures. The current MSHA test measures a belt's performance

in terms of an unclear combination of ignitability and flame spread.

The NBS modified smoke box indicated that the smoke generated by belts

meeting MSHA requirements was equivalent to belts not meeting those requirements. Since no requirements exist to limit the maximum smoke generating

potential of conveyor belts, the belts demonstrate no systematic effort to

inhibit smoke production.

The Flooring Radiant Panel Test indicated that belts acceptable for use

in underground coal mines could propagate a flame under a moderate exposure

level. This exposure level is somewhat more severe than MSHA's current test

procedure.

Ignition tests demonstrated that for low-level exposures (less than

2 1.26 W/cm ) of SBR belts, thicker belts provide more ignition resistance than

thinner belts.

6 . REFERENCES

[1] Code of Federal Regulations, Title 30, Part 18.65 (1977).

[2] Breden, L. and Meisters, M. , The effect of sample orientation in the

smoke density chamber, Nat. Bur. Stand. (U.S.), NBSIR 76-1030 (May

1976) .

D-21
[3] Hartzell, L. G., Development of a radiant panel test for flooring

materials, Nat. Bur. Stand. (U.S.), NBSIR 74-495 (May 1974).

[4] Seader, J. D. and Chien, W. P., Mass optical density as a correlating

parameter for the NBS smoke density chamber. Journal of Fire and

Flammability, Vol. 5, 151 (April 1974).

[5] Chien, W. P. and Seader, J. D. , Smoke development at different energy

flux levels in an NBS smoke density chamber. Fire Technology (August

1974)

.

[6] Benjamin, I. A. and Adams, C. H. , Proposed criteria for use of the

critical radiant flux test method, Nat. Bur. Stand. (U.S.), NBSIR

75-950 (December 1975)

.

[7] DenBraven, K. , Radiative ignition of some typical floor covering

materials, Nat. Bur. Stand. (U.S.), NBSIR 75-967 (December 1975).

D-22
APPENDIX E

Evaluation of the MS HA Test for the Flammability of Conveyor Belts

1 . INTRODUCTION

This report describes the work performed on the MSHA Test Method for the

Flammability of Conveyor Belts, as described in Title 30 of the Code of

Federal Regulations (§18.65). The description of this test method has been

found to be vague in several critical areas. Terms such as natural gas, airflow, and cabinet configuration are undefined. In addition, no statement of

precision is given for the various test parameters.

This work investigated the interaction of the sample with various test

parameters in the context of the existing acceptance criteria, which is an average afterflame time of less than 1 minute and an afterglow time of less

than 3 minutes. The impact that variations in test parameters have on the

test results were determined by testing a set of conveyor belt samples

obtained from various manufacturers. In addition, the ignition source was

characterized in terms of energy absorption by a simulated specimen.

£

2. IGNITION SOURCE

The ignition source is described in the regulations as a PittsburghUniversal bunsen type burner with a burner tube having an inside diameter of • 11 mm. The fuel is specified as natural gas and is used to produce a blue

premixed flame 7.6 cm high. The composition or heat content of natural gas

and the portion of the flame that is measured to determine flame height are

not defined. In addition, it is implied that a natural gas orifice (1.1 mm)

is to be used in the burner. It was found, however, that both the apparatus

obtained by NBS and that in use by MS HA employ a Pittsburgh-Universal bunsen

burner with an artificial gas orifice (1.9 mm).

2.1 Heat Release Measurements for Different Methane Concentrations

Three concentrations of methane were used to measure the energy an

exposed specimen may receive. To evaluate the energy release rate in the

flame, the gas flow rate was measured using a calibrated in-line flow meter.

Both the artificial and natural gas burners were used. After a premixed blue

E-l
flame was established with an inner cone height of 7.6 cm, the gas flow was

recorded. Using the measured gas flow rate and the heat of combustion for

methane, the rate of energy release was calculated assuming complete combustion. It was found that for methane concentrations of 93, 97, and 99 percent

the minimum calculated rate of energy release was 13.7 J/sec, 14.3 J/sec, and

14.6 J/sec, respectively, for both burners.

Tests were also run to determine what fraction of energy release in the

flame may be transferred to a specimen. An approximate way to determine this

quantity is to measure the energy absorbed by a copper slug having the same

dimensions as the sample and mounted and exposed to the flame in a similar

manner. Measurements were made with different methane concentrations using

a copper bar, 2.5 x 1.27 x 0.63 cm, mounted on a rigid plastic support.

Figure 1 shows that the energy absorbed by a specimen exposed to a flame from

the artificial gas burner is slightly greater than when the specimen is

exposed to the natural gas burner flame. However, only approximately 12-14

percent of the total energy available can be expected to be transferred to

the test specimen.

2.2 Heat Release Measurements for Different Flame Heights

Measurements of the energy absorbed by the simulated specimen were also

made at several flame heights (inner cone height of 6.3 to 8.9 cm) using 93

and 99 percent methane. The data (see figure 2) indicates that changes in

flame height for the artificial gas burner produced slight differences in the

heat transfer rate at inner cone heights of 7.0 cm and 99 percent methane and

7.6 cm and 93 percent methane. Throughout the remainder of the range tested

no discernible difference in heat transfer rate was noted.

The natural gas burner showed a systematic difference in the heat transfer

rate as the flame height was adjusted and, as previously shown, the methane

gas concentration was varied. However, the data for both burners was within

±10 percent of their respective means.

3. COMPARISON OF ARTIFICIAL AND NATURAL GAS BURNER

Due to the apparent discrepancy between the apparatus in use by MSHA and

the implied statements in the regulations, tests were conducted comparing the

performance of a group of conveyor belts exposed to a premixed blue flame produced by an artificial gas burner and a natural gas burner. (The belts used

E-2
Figure 1-

Heat

transferred

toa

specimen

exposed

toa

flame

from

an

artificial

burner

and a

natural

gas

burner

E-3

Methane

Concentration

(%)
Figure

2

-

Effect

of

variations

in

flame

height

on

heat

absorption

rate

for

natural

gas

burner

and

artificial

gas

burner

(09S/P) UO||dJOSqD |D8H

(09s/p) uoudjosqo *D9H

E-4

Inner

cone

flame

height

(cm)

Inner

cone

flame

height

(cm)
in this report are described in appendix D, with the addition of samples

obtained from the Factory Mutual tests [1], table 1.)

The operating conditions outlined in the regulations were followed with

the additional stipulations that: the flame height was adjusted to produce a premixed blue inner cone of 7.6 cm, and the gas used had a methane content of

93 percent. This was chosen to follow as closely as possible the equipment

and procedures used by MSHA

.

3.1 Afterflame Time

Figure 3 is a graph of test results comparing the artificial gas burner

with the natural gas burner. The results indicate, that for the same flame

height, the artificial burner yields longer afterflame times. For three

samples, the results were significantly different in that these samples failed

the acceptance criteria when exposed to a flame from the artificial gas burner

but passed the acceptance criteria when exposed to a flame from the natural

gas burner.

3.2 Afterglow Time

Figure 4 shows the results of afterglow time measurements, in the range

of zero to 10 seconds, made on those samples that exhibited glowing after the

flame extinguished. With few exceptions, the afterglow persisted for a longer

time after an exposure to the natural gas burner than the artificial gas

burner. For the majority of belts, use of the artificial gas burner produced

no measurable afterglow, while with the natural gas burner afterglow times

were as high as 6 seconds. There were three samples that had an afterglow

time greater than 10 seconds. Only one of these samples, #16-PVC, had a longer

afterglow time with the artificial gas burner, 180 seconds as compared to zero

seconds when exposed to the natural gas burner. However, in all three cases,

independent of the burner employed, the samples would have been rejected

because they had not met the afterflame criterion.

4. IGNITION TIME

In order to determine the sensitivity of a material's performance to

variations in ignition time, tests were conducted at three exposure times and

the resulting after flame and afterglow times were recorded. The average afterflame time for the 29 belts listed in appendix D are tabulated in table 2.

E-5
Table 1. Description of Conveyor Belts Used in

the Factory Mutual Study

: Number Cover Material Thickness mm

30 PVC 10

31 PVC 8

32 PVC (1-ply) 10

33 SBR (3-ply) 11

34 SBR (3-ply) 11

35 Neoprene (3-ply) 12

36 Neoprene (3-ply) 12

E-6
Figure

3

-

Effect

of

burner

type

on

afterflame

time

-wV - '•X . , -’SOJ OJ 1-H -tH | (09S) aiuLii'x - joujng |DjniDN

(09s) uulux’T - Jeujng |Djn*DN

E-7

0

2

4

6

8

10

12

0

50

100

150

200

250

Artificial

Burner

-1.9mm

(sec)

Artificial

Burner

-1.9mm

(sec)
Figure

4

-

Effect

of

burner

type

on

afterglow

time

T O

00

eta cU-iib-U. © zz ! I q_I I oct ooa > m © > go ClCOZCLCO

0O-SI 10

0

$ 0

O OJ

0

m an o®m o

i

o 00 O ^

OJ o

(09s) uuuun- Jeujng idjiiidn

E~8

Artificial

Burner

-1.9mm

(sec)
Table 2. Effect of Ignition Exposure Time on Average

Afterflame Time Using an Artificial Burner

and 93 Percent Methane

Exposure Time

Belt # 45 sec 60 sec 75 sec

1 2.3 2.3 3.0

2 0.7 1.0 1.0

3 3.0 2.8 2.5

4 4.3 5.8 4.8

5 2.0 2.0 2.0

6 3.0 5.0 4.3

7 3.0 5.0 3.6

8 2.0 1.0 1.0

9 1.0 6.3 5.0

10 2.3 4.3 2.6

11 2.3 3.0 5.0

12 0.0 0.0 0.0

13 3.0 5.0 17.3

14 120.0 240.0 240.0

15 3.0 3.3 5.0

16 145.3 158.3 191.0

17 213.0 105.3 120.0

18 2.2 3.0 2.3

19 5.3 5.3 4.3

20 4.8 5.0 14.0

21 1.6 3.0 5.3

22 2.6 5.0 2.0

23 1.0 4.3 6.6

24 2.0 3.0 4.0

25 2.6 6.3 8.6

26 39.0 111.0 99.0

27 1.6 4.0 6.0

28 49.0 120.0 120.0

29 3.0 3.3 3.3
The afterglow times for those samples that also exhibited an afterglow are

listed in table 3.

Exposure times of 45, 60, and 75 seconds were used. The test procedure

followed by MSHA was used. This included the use of an artificial gas burner

with fuel containing 93 percent methane.

It was found that for most samples the exposure time had a marginal

effect on after flame time and, with the exception of one sample, had no effect

on afterglow time. Sample number 17 exhibited prolonged glowing after a 45-

second exposure to the ignition source. Increasing the exposure time eliminated afterglowing. This sample, however, also had afterflame times in excess

of the acceptance criterion for all three exposure conditions.

There were only two samples, numbers 26 and 28, that were acceptable at

a 4 5- second exposure time but not acceptable when the exposure time was

increased to 60 and 75 seconds. With a mean exposure time of 60 seconds, it

would seem that fluctuations about this mean would not appear to introduce

significant deviations in the final results. Furthermore, differences in the

type of burner or fuel used should also introduce minimal deviations. The

total energy absorbed by a specimen can roughly be approximated by using the

data in figure 1. Using a fuel containing 93 percent methane, the heat transfer rate for an artificial gas burner is 1.8 J/sec. Calculations show that a specimen may absorb approximately 81, 108, and 135 J for 45, 60, and 75-second

exposure times, respectively. The use of any fuel with a higher concentration

of methane could not be expected to transfer any more energy than the 75-second

exposure. For example, using a fuel with a methane concentration of 99 percent and an artificial gas burner, the energy transferred is approximately

125J for a 60-second exposure.

5 . SAMPLE ORIENTATION

An artificial gas burner using 93 percent methane as a fuel was used to

evaluate the changes in afterflame time as the specimen was varied from its

prescribed horizontal position. In addition to the horizontal position, four

angles were selected. Two angles, 5 and 10 degrees above the horizontal,

were chosen to represent downward burning samples, while two angles, 5 and 10

degrees below the horizontal, were chosen to represent upward burning samples.

Each specimen was positioned in the cabinet so that the end of the sample was

located 2.5 cm above the burner.

E-10
Table 3 Effect of Ignition Exposure Time on Average Afterglow Time

Exposure Time

Belt # 45 sec 60 sec 75 sec

1 4.0 4.5 6.3

3 1.8 4.3 2.5

6 5.8 5.5 7.0

7 4.7 5.3 4.0

12 0.3 2.0 1.3

17 220.0 0.0 0.0

29 102.3 138.0 144.0

E-ll
Eight belt samples were selected for testing. The results (see table 4)

indicate that a ±10-degree variation in mounting angle have no significant

effect on the final results. In no case did these variations cause a sample

to move from the acceptable category to unacceptable or vice versa.

6 . AIRFLOW

The current test procedure calls for the introduction of an airflow

along the specimen's longitudinal axis on removal of the burner. The airflow

is applied in the direction of flame spread at 90 Mpm (300 FPM) . The timing

of afterflame begins with the start of the blower and afterglow measurements

begin with the extinguishment of flaming combustion on the specimen.

Variations in the speed of the flowing air as well as the time it takes to

achieve steady-state flow can be expected to affect the final results.

6.1 Steady-State Flow Rate

Tests were conducted on a total of 45 conveyor belt samples; the 29

listed in appendix D, and seven conveyor belts from the Factory Mutual test

series (see table 1) plus nine additional belts supplied by MSHA (see table 5)

Four flow rate conditions were used with a natural gas burner and 93 percent

methane fuel. With the exception of the airflow, the standard test procedure

was followed.

Figures 5 and 6 summarize the afterflame and afterglow times obtained for

no forced flow and forced flows of 30, 60, and 90 Mpm. Figure 5 shows the fre

quency distribution of afterflame times over four ranges; less than 5 seconds,

between 5 and 25 seconds, between 25 and 60 seconds, and greater than 60

seconds. As the steady-state flow rate increased, the fraction of specimens

exhibiting less than 5 seconds afterflame increased. Under nonforced flow

conditions, 56 percent of the samples had afterflame times of less than 5 seconds, while at 90 Mpm the number increased to 93 percent. Also, the number of

samples with an afterflame time greater than 60 seconds decreased from 22 percent at nonforced flow to 7 percent at 90 Mpm. With increasing flow rates the

distribution of samples between the two extremes decreased until the samples

either exhibited very short afterflame times or exceeded the acceptance level

of 60 seconds. In terms of the acceptance criterion, no difference exists

between 60 and 90 Mpm induced flow.

E-12
Figure 5 - Frequency distribution of afterflame time for all belts

tested under various induced air flow rates

0 0.2 0.4 0.6 0.8

E-15
Figure 6 - Frequency distribution of afterglow time for ail belts

tested at several induced air flow rates

No Flow

30 Mpm Flow

60 Mpm Flow

90 Mpm Flow

A-fterglow Time (seo) none

less than 180

greater than 180

E-16
Figure 6 is a frequency distribution of afterglow times. The data was

divided into three categories; no afterglow, an afterglow time less than or

equal to 180 seconds, and an afterglow time greater than 180 seconds. The

number of samples exhibiting an afterglow increased with increasing flow rate

until 60 Mpm. At 90 Mpm the afterglow was seen on fewer samples, 67 percent

as compared to 69 percent. The number of samples having no afterglow also

increased when the flow was increased from 60 to 90 Mpm, to 24 and 31 percent,

respectively. The number of nonacceptable samples was also lowest at the

highest airflow rate in contrast to the other three conditions.

Maximum afterflame time occurred under the nonforced flow regime, while

afterglow was most persistent at 60 Mpm.

6.2 Time to Steady-State Flow

While the regulations define the steady-state airflow in the cabinet

during testing and MSHA has defined inlet port configuration, the lag time

from the initiation of the blower motor to the establishment of steady-state

flow is undefined. The importance of defining the time lag to steady-state

flow was investigated by selecting four conveyor belt samples (13, 14, 26,

and 34) and exposing them to several different airflow acceleration rates.

These samples were selected because under standard test conditions they

exhibited approximately the same afterflame time, 2 to 5 seconds. (A natural

gas burner was used with 93 percent methane.)

The maximum acceleration rate obtainable on the NBS apparatus was 2700

2 Mpm . The acceleration rate was varied by varying the rate of applied voltage

2 to the blower motor, thereby, achieving acceleration rates as low as 270 Mpm .

Figure 7 shows the effect of variations in the lag time to steady-state

flow on afterflame time. The data indicates that as the lag time increases

(i.e., acceleration rate decreases) the afterflame time also increases. Of

the four samples tested, three samples had after flame times in excess of the

2 acceptance criterion when the airflow acceleration rate was below 400 Mpm . This is consistent with the results described in figure 5 that shows that as the flow rate was reduced the afterflame time also increased.

6.3 Air Inlet and Exhaust Port Configuration

While the regulations define air inlet port design, they are silent when

it comes to the design of the exhaust port. Variations that exist in air inlet

and exhaust port designs between MSHA and various industry laboratories have

E-17
Figure

7

-

Effect

of

startup

time

of

air

blower

on

afterflame

time

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o LO o ID OJ

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E-18

200

600

1000

1400

1800

2200

2600

Air-flow

Acceleration

(Mpm

2)
been assumed to be one cause of variability in test results. The inlet port

on the NBS apparatus is made from a 20.0 cm long cone with a large end diameter of 19.0 cm and a small end diameter of 10.2 cm. The exhaust port is made

from a 10.2 cm circular cylinder 19.0 cm long with the blower motor mounted in

the end of the exhaust port. The MSHA apparatus, about which the regulations

were written, has a circular tapered inlet port having a maximum diameter of

40.6 cm and a minimum diameter of 2.6 cm (ASME 16-8%). The exhaust port

is rectangular (22.6 x 27.9 cm) tapering to a 15.2 cm diameter connector for

the blower mounting.

The effects of port configuration on afterflame time were investigated

by conducting tests on MSHA and NBS equipment using conveyor belt samples

supplied by MSHA. The same test procedures were followed and the operators

were rotated. The materials were two multi-ply SBR conveyor belts from different manufacturers. Ten specimens were tested in each direction, warp and weft,

for each belt.

The test results are tabulated in table 6. A comparison of operator

performance on the two apparatuses indicates that no real difference exists

except for material 2 in the weft direction. The data obtained on the MSHA

apparatus shows a marked dependence on the operator. However, the difference

was not so large that the material's acceptability was affected. The same

might not be true had the tests been conducted using a marginal sample.

A comparison between apparatuses, however, shows that the NBS equipment

produced shorter afterflame times than the MSHA equipment. The differences,

while not affecting acceptability, can be traced to the longer lag time in

the establishment of steady-state flow in the MSHA system. The MSHA system

required approximately twice the amount of time to achieve steady-state flow.

The previous data indicated that a reduction in the acceleration rate increased

the afterflame time. It is estimated that the MSHA system has an acceleration

2 2 rate of 1100 Mpm . This compares with 2700 Mpm for the NBS system. The

extent of the dependence of after flame on acceleration cannot be predicted,

but the data indicates that the shift in the data in table 6 is in the correct

direction.

Inlet and exhaust port design appear to have a minimal, if any, effect

on material performance, as long as the sample is in the center of the flow

pattern.

E-19
Table

6.

Comparative

Results

of

Two

Conveyor

Belts

Tested

on

MSHA

and

NBS

Equipment,

With

Two

Operators

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E-20
7 . CONCLUSIONS

Various parameters of the MSHA test were varied to determine their effect

on test results. Variations in such parameters as the gas used as the fuel,

the exposure time, and flame height had very little impact on mean values of

after flame and afterglow time. However, variations in the airflow rate and

acceleration affected afterflame times. Also, the use of a large orifice

burner (1.92 mm) appeared to produce longer afterflame times than the results

obtained with a natural gas burner.

Afterglow was also affected by the airflow rate and burner orifice size.

The number of specimens exhibiting glowing behavior decreased with decreasing

airflow rate. The larger orifice burner produced shorter afterglow times than

the natural gas burner.

\ 8 . RECOMMENDATIONS

In order to improve the reproducibility of the test method, it is

recommended that the MSHA test procedure be modified to:

\

' i • require that the airflow not be applied until flaming on the specimen

extinguishes and

• the use of an artificial gas burner be clearly defined.

E-21

APPENDIX F

Standard Test Procedure for the Flammability of Conveyor

Belts for Use in Underground Coal Mines

1.

SCOPE AND APPLICATION

This standard provides a test method to determine the flammability of

conveyor belting intended for use in coal mine operations.

2.

DEFINITIONS

a. "After flame" means the continuation of flaming after the removal of

the ignition source.

b. "Afterglow" means the continuation of glowing of parts of a specimen

after flaming has ceased.

c. "Sample" means four test specimens, two cut parallel to the warp and

two cut parallel to the weft.

d. "Specimen" means a 15.2 x 1.3 cm by full thickness (6 x .5 inches by

full thickness) section of conveyor belting or hose.

e. "Test criteria" means the maximum average afterflame time and

afterglow time which a sample may exhibit in order to pass the test.

f. "Item" means a conveyor belt submitted for testing under the

provisions of the Coal Mine Health and Safety Act.

3.

GENERAL REQUIREMENTS

a. Summary of test method . Conditioned specimens shall be suspended

one at a time horizontally in a prescribed cabinet and subjected to a standard

flame along their free edges for a specified time under controlled conditions.

After the exposure period and extinguishment of flames on the sample, a standard airflow shall be applied along the longitudinal axis of the specimens for

a specified time. The afterflame and afterglow times shall be recorded.

F-l
b. Test criteria . The test criteria when the testing is done in

accordance with the prescribed Sampling Procedures (4) and Test Procedures

(5) are:

1. The average afterflame time of four specimens shall not exceed 1

minute.

2. The average afterglow time of four specimens shall not exceed 3

minutes

.

4 . SAMPLING PROCEDURES

Four specimens shall be cut from an item submitted for testing.

a. Two specimens shall be cut in the warp direction such that no warp

and weft sections are common to each.

b. Two specimens shall be cut in the weft direction such that no warp

and weft sections are common to each other or the warp specimens.

5. TEST PROCEDURE

a. Apparatus . The following test apparatus has been found to be

acceptable for this test. An alternate test apparatus may be used with prior

approval of the Mine Safety and Health Administration.

1. Test chamber . The test chamber shall be a steel cubical cabinet

with inside dimensions of 53.3 cm (21 in) in each direction. The front of

the cabinet shall be a close-fitting door with a transparent insert to permit

observation of the entire test. A mirror shall be mounted on the back wall

of the test chamber to permit a rear view of the specimen through the viewing

door. An air inlet nozzle shall be mounted on one side of the chamber [an

ASME flow nozzle with a 40.6 cm-21.6 cm (16-8^ in) reduction or suitable

equivalent] , and an exhaust port with an electric fan shall be mounted on the

opposite wall. The electric fan, with an airflow control, shall be able to

produce an airflow across the horizontal surface of the specimen of 91 Mpm

(300 fpm) . A capped access port shall be provided for inserting an anemometer

to measure airflow velocity. A suitable cabinet 'for use in this test method

is detailed in figures 1 and 2.

F-2
Figure

F-3
.

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iaq

-

1

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5

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17

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holes120*

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2. Specimen holder . A suitable specimen holder for use in this test

method is detailed in figure 2. It shall be located in the central portion of

the cabinet. It shall permit suspension of the specimen with its long axis

horizontal and the short axis inclined at 45° to the horizontal. The lower

edge of the specimen shall be 2.5 cm (1.0 in) above the highest point of the

barrel of the gas burner when the burner is in the test position. A clamp

shall be provided to facilitate mounting of the specimen. A 20-mesh iron wire

gauze, with a maximum of 13 cm (5 in) on a side, shall be clamped in a horizontal position .64 cm (1/4 in) below the lower edge of the specimen and with

about 1.3 cm (1/2 in) of the specimen extending beyond the edge of the gauze.

3. Burner . A bunsen type burner with an inside tube diameter of

11 mm (0.4 in) shall be used as the ignition source. It shall have an orifice

plate in its base with a diameter of 1.90 ± .05 mm. The input line to the

burner shall be equipped with a needle valve for adjusting the height of the

flame. The burner shall be mounted in a slide guide which is affixed to the

floor of the cabinet to permit rapid application and withdrawal of the burner

from below the specimen. The slide guide shall have a stopping mechanism that

shall insure that the burner is directly beneath the specimen when the external

burner control arm is moved to apply a flame to the specimen. The burner shall

be connected to the gas source by rubber or other suitable flexible tubing.

4 . Fuel . The gas used as fuel for the bunsen burner shall contain

at least 96 percent methane by volume.

5. Stopwatch . A stopwatch or similar timing device shall be used to

measure time to 0.1 second.

6. Scale . A linear scale graduated in mm or 0.1-inch divisions or

similar device shall be used to measure the height of the bunsen burner flame.

A suitable gauge may be permanently affixed to the inside wall of the cabinet

or affixed to the burner tube for this purpose.

7. Anemometer . An airflow measuring instrument capable of recording

an airflow of 91 Mpm (300 fpm) ± 5 percent shall be used. A suitably calibrated hot wire-type anemometer should be acceptable.

8. Electric fan . A variable speed electric fan shall be mounted on

the exhaust port. The fan shall be fitted with a vent line for exhausting

smoke and/or toxic gases produced by testing. The fan shall have a variable

speed control that is preset to produce an airflow velocity of 91 Mpm (300 fpm)

along the surface of the specimen parallel to the longitudinal axis. This

F-5
shall be measured using the anemometer described in 5(a)(7). The electric

exhaust fan shall be equipped with an on/off switch that applies and removes

full power to the fan's speed control.

b. Conditioning . The specimen, prior to testing, shall be conditioned

in air at a temperature between 15 °C (60°F) and 32 °C (90°F) and a relative

humidity less than 55 percent for at least 24 hours.

c . Testing .

1. Calibration . The airflow velocity and flame height shall be

verified at the beginning of each testing period or after any unusual interruption in gas, electric, or service.

i. Airflow velocity . The airflow velocity is adjusted by

inserting the anemometer through the capped hole provided for this purpose.

The bunsen burner is moved to its test position. The anemometer is positioned

with its sensor 2.5 cm (1.0 inch) above the center of the bunsen burner, and

the bunsen burner is withdrawn. The electric fan's power switch is placed in

its "on" position and the variable speed control is adjusted to produce an

airflow velocity of 91 Mpm (300 fpm) . Power is then interrupted to the fan

with the speed control set at 91 Mpm (300 fpm) and the anemometer is withdrawn.

Note ; No further adjustments are necessary unless there is a fluctuation in the power source to the fan or the variable speed control has

been tampered with. In either case, the airflow velocity calibration procedure must be repeated.

ii. Burner adjustment . With the exhaust fan turned off and

using the gas specified in 5(a)(4), adjust the needle valve in the gas line

to the burner to produce a flame with a hard blue inner cone of 7.6 cm (3 in)

in height. It may be necessary to adjust both air and gas supplies to achieve

this condition. A linear scale graduated in 0.1 inch divisions or similar

device is used to measure the height of the burner flame.

Note ; Prior to preforming burner adjustments, the orifice plate

located in the base of the burner should be inspected to verify that it is

clean and unobstructed and is of the correct size.

F-6
2 . Specimen burning and evaluation .

i. Specimens shall be mounted in the test cabinet one at a time.

The cabinet door shall be closed and the burner flame impinged on the free end

of the specimen for 1 minute. Flame impingement is accomplished by moving the

burner under the specimen for this length of time, and then removing it.

ii. Beginning with the removal of the burner, after flame time

and afterglow time shall be measured.

iii. After the specimen ceases to flame, electrical power is

supplied to the exhaust fan. The sample shall remain in the air current for

at least 3 minutes to determine the presence of afterglow. If a glowing specimen exhibits reignition within the additional 3 minutes, the duration of

flame shall be added to the original afterflame time. The test is terminated

when all flaming and glowing on the specimen ceases.

iv. The products of combustion shall be exhausted before opening

the door and removing the specimen.

3. Report . The values of afterflame and afterglow times shall be

reported for each specimen, as well as the average times for each set of four

specimens.

F-7
NBS-M4A (rev. 2-80

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

4.

TITLE AND SUBTITLE

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publication Date

NBSIR 81-2256 December 1980

ANALYSIS OF FIRE DATA, LARGE-SCALE TESTS, AND SMALL-SCALE TESTS FOR CONVEYOR

BELTS USED IN UNDERGROUND COAL MINES

5.

AUTHOR(S)

Emil Braun, Robert E. Meade, and Lee R. Smith

6.

PERFORMING ORGANIZATION (If joint or other than NBS. see instructions) 7. Contract/Grant No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

8. Type of Report & Period Covered

Final Report

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State, ZIP)

10. SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bliography or literature survey, mention it here)

An investigation into the requirements and fire test performance of conveyor belts

intended for use in underground coal mines was conducted. The aim of this study was to develop recommendations for the Mine Safety and Health Administration (MSHA) on a test method suitable for measuring the fire hazard potential of a conveyor

belt in a coal mine environment. A review of incident data, large-scale tests,

and small-scale tests was conducted to provide the necessary information. The

incident data was analyzed with the goal of developing scenarios that could form

the basis for a suitable test or to evaluate the appropriateness of the existing

test

.

Large-scale tests were reviewed to determine anticipated belt fire performance

under "realistic" end-use conditions. The tests showed how geometry, input energy,

and ventilation controlled belt fire performance. Small-scale tests were used to

provide information on specific fire properties such as ease of ignition, flame

spread, and smoke generating potential.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) Accident data; coal mines; conveyor belts; fire safety; fire tests; flame spread;

ignition; test methods.

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

[X~l Unlimited

106 | | For Official Distribution. Do Not Release to NTIS

T I Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

15. Price 20402.

[X] Order From National Technical Information Service (NTIS), Springfield, VA. 22161 $11.00

USCOMM-DC 6043-P80

1II