By R. J. limko. F. N. Kissell, and E. D. Thimons 

UNITED STATES DEPARTMENT OF THE INTERIOR 

UNITED STATES DEPARTMENT OF THE INTERIOR 

Donald Paul Hodel, Secretary 

BUREAU OF MINES 

Robert C. Horton, Director 
Library of Congress Cataloging in Publication Data : 

Timko, Robert J 

Evaluating ventilation parameters of three coal mine gobs. 

(Information circular ; 9109) 

Bibliography: p. 16. 

Supt. of Docs. no.: 128.27: 9109. 

1. Mine ventilation. 2. Mine gases. 3. Coal mines and mining-Safety measures. 4. Mine 

shafts. I. Kissell, Fred N. 11. Thimons. Edward D. 111. Title. IV. Series: Information circular 

(Irnited States. Bureau of Mines) ; 9109 

TN295.U4 [TN301] 622 s [622'.8] 86-600132 

t . 
CONTENTS 

Page 

Abstract ....................................................................... Introduction ................................................................... Mine A ......................................................................... Background ................................................................... Borehole examination ......................................................... Results ...................................................................... Conclusions .................................................................. Mine B ......................................................................... Background ................................................................... First gob evaluation ......................................................... Second gob evaluation ........................................................ Gas analyses of left and right inby positions ................................ Conclusions .................................................................. Mine C ......................................................................... Background ................................................................... Evaluation ................................................................... 4 South .................................................................... 

2 S~~the.............e*eee**ee*e~ee~**eee*a*~**e*~**ae*eee~~*e*eee*e***a**e 

Conclusions ................................................................... 4 South .................................................................... 3 South .................................................................... 2 S~~th.......e..........e~~eeee~e~~***eee~eeee~~~~*e~~ee~~~~~e~~~e~~e~a~

Summary ........................................................................ References ..................................................................... 

ILLUSTKATIONS 

1 . Ventilation schematic and sampling stations of Mine A gob .. ................ 2 . Sealed gob and sampling stations of mine B ................................. 3 . Mine B seals with urethane foam seal outby on roof and ribs ................ 4 . Standing water located outby Mine B seals .................................. 5 . Sealed gob and sampling stations of Mine C ................................. 

TABLES 

1 . Differential pressure change with depth .................................... 2 . SF6 detection time and average gas velocity through the gob from the horehole to sampling stations 2-9 ............................................. 3 . Distances from borehole to western sampling points ... ...................... 4 . Gas concentrations behind seal A and seal D. Mine B. ....................... 
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 

ft foot in wag. inch, water gauge 

ft3 cubic foot min minute 

f t 3/h cubic foot per hour PP~ part per billion 

ft/min foot per minute PPm part per million 

ft3/min cubic foot per minute Pet percent 

h hour psi pound per square inch 
EVALUATING VENTILATION PARAMETERS 

OF THREE COAL MINE GOBS 

By R. J. Timko,' F. N. Kis~ell,~ and E. D. Thimons3 

ABSTRACT 

The Bureau of Mines used sulfur hexafluoride (SF6) tracer gas to evaluate the effectiveness of gob ventilation and/or sealing practices at 

three coal mines, each having different problems associated with their 

mined-out areas. The purpose of these ventilation studies was to better understand whether current techniques employed for ventilation or 

sealing are successful at minimizing the potential for gob fires and explosions. The work performed at each mine is discussed: One is a longwall operation that uses ventilation to carry off hazardous gases, and 

two are rooeand-pillar operations, each with far different concerns, 

that seal off the gobs to isolate them from the main mine ventilation. 

In all cases employment of the Bureau's SF6 tracer gas technique resulted in answers to questions raised by the mine operators concerning 

the effectiveness of their gob ventilation or sealing practices in 

preventing mine fires. 

physical scientist. 

2~esearch supervisor. 

3~upervisory physical scientist. 

Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 
INTRODUCTION 

Coal mining involves the fracturing and 

removal of a certain portion of the coal 

seam; a production goal is to remove the 

maxirnum amount of coal without compromising safety of the miners. High extraction techniques, such as longwalls, 

have become common. In room-and-pillar 

operations, retreat or second mining is 

frequently performed. This involves systematic removal of coal pillars that 

sufficient quantities of methane (5 to 15 

pct in air) are present. Even without 

the presence of methane, problems can occur if blackdamp (C02) and/or whitedamp 

(CO) leak through permeable seals. 

This report gives an overview of investigations at three different coal 

mines having very different gobs. 

1. At Mine A, which employs mainly 

longwall extraction techniques, it was 

remain after the primary mining cycle, necessary to ventilate the remaining gob. 

which results in controlled caving of the Research determined the permeability as 

strata above the mined-out area, or gob. well as average air velocities through 

To prevent the mixing of explosive con- the gob. 

centrations of methane (5 to 15 pct CH4 2. At Mine B, which uses roomin air) and oxygen-rich mine air, mine and-pillar extraction, the gobs were 

operators may employ one of two techniques: (1) continue to ventilate the 

gob to keep the methane concentration 

low, or (2) seal off the section to isolate it from the main mine ventilation. 

The Mine Safety and Health Administration 

(MSHA) has regulations or specifications 

regarding either technique. For ventilation, MSHA requires that sufficient air 

be used to carry off hazardous gases 

(5) 

MSHA's specifications for gob sealing 

require that seals be made of substantial 

and incombustible materials that are 

sealed. The operators had been taking 

gas samples from two seals in the same 

small gob. They were concerned because 

sample analyses did not agree. Research 

was performed to determine if continuity 

existed across the gob and to evaluate 

the performance of the seals. 

3. At Mine C, which also uses the 

room-and-pillar method, the mined-out 

gobs were sealed. The particular gob 

studied was very large, with three separate but interconnected sections, having 

a total of 18 seals separating it from 

the active mine. The coal seam pitched 

strong enough to prevent explosion propa- downward so that the methane concentragation. Because of varying pressure dif- tion was different behind each set of 

ferentials, mineral friability, and seals. These tests examined the potenstrata convergence, seals are never corn- tial interconnection of the gob entries 

pletely airtight. Their purpose is to 

reduce the potential for combustion. On 

the one hand, they restrict the passage 

of mine air containing high quantities of 

oxygen into gobs that have a potential 

for spontaneous combustion; on the other 

hand, they reduce the passage of explosive gases liberated from the gob to the 

areas of the active mine. 

If ingassing of mine air to the gob 

occurs, fires can be started through 

spontaneous combustion, which usually 

begins slowly but can accelerate to hazardous proportions before detection. If 

outgassing of gob gas into active areas 

as well as the performance of the section 

seals. 

Sulfur hexafluoride (SF6) was the 

tracer gas used in all gob evaluation 

work. SF6 is a colorless, odorless, and 

tasteless gas having a threshold limit 

value-time weighted average of 1,000 ppm. 

This low toxicity means that workers can 

be repeatedly exposed to 1,000 ppm of SF6 

for a normal 8-h day and 40-h week without encountering adverse effects (1). 

SF6 is stored in small, low-pressure 

(300-psi), metal lecture bottles. The 

gas has a volume of approximately 1.5 ft3 

at atmospheric pressure. The Bureau has 

occurs, explosions can happen where developed techniques For releasing and 

sampling SF6, as well as for reducing the 

4~nderlined numbers in parentheses re- data obtained (3-4). - - Typically, the SF6 

fer to items in the list of references at is released, then sampled by punctllring 

the end of this report air-evacuating sampling tubes, which are 
similar to tubes used for blood collec- concentrations are determined through 

tion. These tubes are then returned electron-capture gas chromatographic 

to the laboratory where actual gas analysis. 

MINE A 

BACKGROUND 

Mine A, located in southwestern Pennsylvania, was required to ventilate, 

rather than seal, the gobs remaining 

after longwall mining. The gob area comprised four longwall panels whose perimeters contained entries to properly ventilate the gob. The longwall gob being 

evaluated was in the second developed 

panel, which was 490 ft wide and 3,900 ft 

long. Typical seam height in this mine 

was 60 in. 

At times, gob outgassing of methane in 

Mine A was a problem. Therefore, the 

mine had begun sinking boreholes into the 

longwall panels prior to mining. The 

topography above the mine made actual 

borehole placement difficult. The borehole in this gob was driven to the centerline of the gob, approximately 500 

ft from the eastern bleeder entries. 

The objective of this investigation was 

to measure gas velocities through the 

caved gob. This was accomplished by capping the borehole and injecting SF6 down 

a tube through the borehole into the gob. 

Several underground sampling locations 

around the perimeter of the gob were set 

up to detect the SFg. The total time required for the SF6 to traverse the gob 

and enter the ventilation airstream could 

be converted to an average velocity, expressed in feet per minute. 

BOREHOLE EXAMINATION 

Prior to beginning the evaluation, the 

the hole. The tubing served two purposes. First, it was used to release the 

SF6 just above the caved gob. Second, 

SF6 samples were taken from the tubing 

at specific intervals during the test. 

Static pressure at the top of the borehole was 3.0 in w.g. The pressure difference between the top of the borehole 

and the bottom of the tube was measured 

as the tubing was lowered down the hole. 

Two hoses were connected to a ~w~er' Magnehelic pressure gauge: one from the 

borehole top, and another from the fixed 

end of the tube descending the borehole. 

Table 1 shows that pressure difference increased somewhat linearly from 

0 in w.g. at the borehole top to 0.08 

in w.g. at 500 ft. 

At 570 ft, or approximately 200 ft 

above the coal seam, an obstruction was 

encountered, and the pressure differential began to climb rapidly. The tubing 

was then raised slightly to return the 

pressure differential to its original 

value (0.08 in w.g.). 

SF6 was then injected through the tubing. To ensure that the SF6 would be released into the gob and not remain in the 

tubing, the tube was then purged with 

nitrogen. However, problems were encountered with the nitrogen purge, and 

SF6 release was not completed until approximately 3.5 h after the release 

start. This was the official start time 

of the test. Sampling took place at 15- 

min intervals at nine underground bleeder 

entry stations at the gob perimeter, as 

well as at the borehole. 

flame arrester and check valve were removed from the borehole. The borehole S~eference to specific manufacturers 

was then capped and 0.25-in-ID, semirigid does not imply endorsement by the Bureau 

tubing was lowered through the cap into of Mines. 

TABLE 1. - Differential pressure change with depth 

- - -- - - -- 

'~ctual values were limited by the resolution of the instrument. 

Depth, ft: 

Surf ace. ......... 100. ............. 200. ............. 

Pressure differential, 

in w.g. 

'0.00 

.04 

.04 

Depth, ft: 

300.............. 

400.............. 

500.............. 

Pressure differential, 

in w.g. 

0.04 

008 

.08 
tected no SF6, probably because the 

borehole was between it and the fan. 

Stations 2-6 detected SF6 within 3 h 

after gas release. Stations 7-9 first 

detected SF6 between 40 and 57 h after 

the test began. Sampling was halted 

after 144 h. 

where V = average gas velocity, ftlmin, 

D = distance from borehole to 

station, it, 

and T = detection time, min. 

TABLE 2. - SF6 detection time and average gas velocity through the 

gob from the borehole to sampling stations 2-9, Mine A 

SF6 flow 

from borehole to-- 

East side: 

Station Z... 

Station 3... 

Station 4.. . Station 5... 

Station 6.. . West side: 

Station 7... 

Station 8. .. Station 9... 

l~ndirect path, via station 7. 

2-dimensional distance, ft 

800 

2,000 

2,100 

3,600 

'4,350 

3,900 

3,900 

3,600 

Last sampling before detection 

Time, min 

NAP 

3 0 

45 

30 

105 

2,395 

2,430 

2,7 25 

1st sampling showing SF6 

Gas velocity, 

ft/min 

NAP 

66.67 

46.67 

120.00 

41.43 

1.63 

1.58 

1.32 

Time, min 

45 

9 0 

105 

90 

165 

3,397 

2,741 

3,420 

Gas velocity, 

f t/min 

17.78 

22.22 

20.00 

40.00 

26.36 

1.15 

1.42 

1.05 
Looking at only stations 7-9 (on the 

west side of the gob), one might assume 

that the overall 5 was no more than 1.63 

ft/min. However, the wide range in 7 at 

stations 2-6 (18 to 120 ft/min) indicated 

that the SF6 was not traveling directly 

to those stations through the gob but 

must have entered the bleeder entries at 

some point and was carried along the airway to these sampling stations. If SF6 

was to take the shortest two-dimensional 

path to the bleeder entries (500 ft), 

it would enter the bleeder airstream at 

position A in figure 1. If 5 for stations 7-9 had been realistic, travel time 

from the borehole to position A should 

have been approximately 6 h; but in actual testing SF6 was detected at station 

2, just downstream from position A, after 

only 45 min. 

Bleeder entry air velocity was measured 

using an anemometer at 300 ft/min, and 

travel time from station 1 to station 6 

was as follows: 

From To Time, min 

Station I..... Station 2..... 2.7 

Station 2 ..... Station 3..... 4.7 

Station 3..... Station 4..... 4.0 

Station 4..... Station S..... 8.3 

Station 5 ..... Station 6..... 2.3 

Station I..... Station fi..... 22.0 

From this information, it was possible 

to approximate the gas velocity through 

the gob to position A. Since ventilation 

air traveled from position A to station 2 

in only 2 min, then the maximum time for 

the gas to move through the gob to the 

bleeder entry (at position A) is 43 min; 

this results in a 7 through the gob of 

12.8 f t/min, which is a minimum figure. 

This is approximately 10 times the velocity toward the west side of the gob 

(range 1.05 to 1.63 ft/min). The wide 

variation from the east to west sides of 

the gob may be due to several factors, 

including gob caving, proximity to the 

bleeder, borehole position, and atmospheric pressure differences in the surrounding bleeder entries. 

Once SF6 entered the bleeder entries, 

it acquired entry velocity and moved 

toward the fan. SF6 was detected at 

stations 2-6 within the same sampling 

interval, indicating that the gas took 

the shortest possible path from the borehole to point A. The SF6 concentrations 

peaked rapidly, then reapproached zero 

asymptotically. No "second spike" or 

subsequent increase in SF6 concentration 

was ever recorded at stations 2-6; such 

a change would have indicated a direct 

flow of SF6 from the borehole, through 

the gob, to that station and could have 

given researchers an average gas velocity 

throughout the entire gob. 

Assuming that the true gob velocity was 

12.8 ft/min, the ventilation paths to the 

western bleeder sampling stat ions ( 7-9) 

may have been so tortuous that the air 

required a greater time to permeate the 

gob. In an attempt to place a third dimension in the gob, the equation 

D = R x T was used where R was 12.8 

ft/min, T was the gas detection time 

(min), and D was the total distance (ft). 

From table 3, it is apparent that the 

distances derived were unrealistically 

large. This confirms the results of 

table 2. The velocities in the western 

side of the gob were significantly lower 

than those of the eastern side. 

CONCLUSIONS 

The objective was to determine the 

ability of ventilation air to adequately 

enter and ventilate a caved gob. The 

lone exhausting borehole was capped and a 

small-diameter tube placed down the hole 

from the surface to a position just above 

the actual caving. SFg, followed by an 

inert purge gas, was released into the 

gob. Nine underground bleeder evaluation 

stations, in addition to the borehole 

location, were sampled for SF6 over 

a total of 6 days. SF6 was detected 

in less than 3 h at the eastern Sampling points (nearer the borehole), 

whereas detection required 2 days at 

the western sampling points farthest from 

the borehole. 

----------- 

6~s shown in figure 1, the path of SF6 

to station 6 was not through the gob 

(hence, not via station 7), and collld not 

have been throuqh the bleeder entry from 

station 5, hut a likely through a 

parallel bleeder entry (shown). 
TABLE 3. - Distances from borehole to western 

sampling points 

The following conclusions were derived 

regarding the permeability of the caved 

gob. This gob was permeable, since SF6 

was detected after a short sampling time. 

The location of the borehole tended to 

enhance gas escape toward the eastern 

working areas even though mine atmospheric pressures were lower on the western side of the gob. If air quantities 

from the respective entries remained the 

same when the borehole was reopened, then 

one side of the gob was being ventilated 

more efficiently than the other. 

The location of the borehole appeared 

to be a critical factor in the performance of this gob. Multiple boreholes in 

each panel, or possibly a more strategic 

Station 7: 

Minimum..... 

Maximum..... 

Station 8: 

Minimum..... 

Maximum..... 

Station 9: 

Minimum.. . . . Maximum..... 

location of the single borehole, would 

improve gob ventilation. 

If multiple boreholes are not feasible, 

an alternative would be to ensure that a 

measurable pressure differential existed 

across the gob, and that a perceptible 

quantity of air was actually flowing into 

the gob. Since air movement is directly 

related to pressure dif fei-ential, if a 

differential exists across the gob, some 

air movement will take place. There was 

a pressure differential across the gob in 

Mine A. This, in conjunction with the 

borehole, was apparently sufficient to 

minimize problems associated with gob 

ventilation. 

MINE B 

BACKGKOUND 

Approximately 700 ft past the 90" right 

Research at Mine B, an underground coal turn, a vertical fault was encountered. 

mine in western Colorado, was performed Due to the extent of the vertical dison a sealed gob. The coal in this par- placement, the development was halted, 

ticular seam is prone to spontaneous com- and the section was retreat-pillar-mined 

bustion. The gob was in a section driven back to the four-entry area. Seals were 

90" westward from the mains for approxi- then installed in each of the four enmately 700 ft before executing a 90" tries. Because of the quantity of overright turn and continuing development burden and the resultant strata converparallel to the main entries. Initially, gence, each seal had to be constructed of 

five entries were driven; two intakes, wood blocks. These pine blocks were 

two returns, and one belt entry. After built to an approximate thickness of 3 

the 90" right turn, only four entries ft. Sprayed-on rigid urethane foam was 

were driven for just under 400 ft, after applied to the seal faces as well as to 

which the section was expanded to seven the ribs and roof for 10 to 19 ft outby 

entries (fig. 2). (fig. 3). Sampling ports in seals A and 

Gas detection 

time, min 

2,395 

3,397 

2,430 

2,741 

2,725 

3,420 

Distance from borehole, ft 

2-dimensional 

3,900 

3,900 

3,850 

3,850 

4,000 

4,000 

3-dimensional 

30,656 

43,481 

31,104 

35,084 

34,880 

43,776 
LEGEND Had SF6 been detected in the return, it 

FIGURE 2.-Sealed gob and sampling stations of Mine B. 

D made it possible to take gob samples 

remotely via tubing. 

The objective of these tests was to 

determine why the oxygen concentration at 

sampling point 5 was much higher than expected. This was a concern because of 

the potential for spontaneous combustion. 

SF6 was released directly into seal D 

through the sampling port. To determine 

seal performance and possible migration 

of SFg, gob air samples were taken behind 

the seals from the sampling ports at 

points 2 and 5. Additionally, SF6 samples were taken from positions just outby 

the left and right seals (points 3 and 

4, respectively), and from the left 

and right returns (points 1 and 6, 

respectively). 

Prior to this research, another SF6 

tracer gas survey at the same location 

had been conducted by an MSHA technical 

support group. MSHA found that detectable SF6 was neither permeating the large 

vertical fault that had previously halted 

mining in the section, nor entering a 

nearby return entry. Because of the 

proximity of the main return to the 

sealed section, a large pressure 

differential existed between the two. 

would have indicated that the area-was 

actively being ventilated rather than being sealed. 

Two unknowns were investigated: (1) the 

performance of the seals in prohibiting ingassing and outgassing, and (2) the 

potential for the voids inby the seals to 

be directly interconnected, thus enabling 

gas to migrate from one seal to another. 

Since oxygen inflow is directly related to seal performance, solving the 

first unknown would enable researchers to 

reach conclusions about the high oxygen 

concentrations. 

Underground sampling took place at six 

different locations (fig. 2). Any SF6 

that escaped from behind the seals would 

be detected at one or possibly two Sampling points (I or 6). A considerable 

amount of standing water in front of 

seals A-C (fig. 4) made it difficult to 

examine seal A for air leakage. A solution to this problem was to connect sampling tubing to the seal port and extend 

this tubing approximately 500 ft outby 

the seal (from point 2 to point 2'). 

This length of tubing also allowed the 

sampling station to be on the return side 

of the intake air split, eliminating the 

possibility of SF6 reentrainment. To 

sample for SF6 behind seal A, mine personnel aspirated the tubing, then began 

sampling gob air. 

Tubing was also used to sample from 

seal A outby (point 3 to point 3') and 

seal D inby (point 5 to point 5') positions. This was done to avoid overlapping of SF6 samples at different positions. Personal sampling pumps were used 

to continually pull air through the 

flexible tubing in all cases. 

FIRST GOB EVALUATION 

Once SF6 was released behind seal D, 

sampling began. A problem immediately 

surfaced at point 5. The rate at which 

the sampling pump was pulling air suggested the tubing had a restriction somewhere in-line. Shortly thereafter, water 

was visible in the flexible tubing. SF6 

samples taken at this position were considered unreliable because of the large 

quantity of water that was collecting 
FIGURE 3.-Mine B seals with urethane foam seal outby on roof and rlbs. 

inside the tubing. This implied that not 

all of the SF6 was injected behind the 

seal. 

Within 30 min after SF6 was released 

behind seal D, 60 pct of the tracer gas 

had already passed sampling point 6. Because of this rapid recovery of SF6, it 

appeared that much of the SF6 was released outby the seal (at point 4) as a 

resul t of the restriction. 

At sampling points 2, 4, and 6, results 

were obtained for approximately 30 min. 

Sampling point 4, closest to the SF6 release, showed an extremely high SF6 concentration immediately following the SF6 

injection into seal D. SF6 concentrations at points 6 and 3 (via 3 ') peaked 

shortly af ter point 4. No SF6 was detected at sample point 2 (via 2'). 

The quantity of SF6 recovered was determined by the equation: 

where QSF~ = quantity of SF6 recovered 

(ft3> 

CZSF~ = mean concentration of SF6 

in one part of air 

T = sampling time (min) 

and Qa 1 ,- = airflow (ft 3/min) 

At point 6, considered the most reliable 

sampling position, 0.67 ft3 or 60 pct of 

the SF6 released was detected within 35 

min. Concentrations at points 3 and 4 

averaged 23 ppb (sampled for 30 min) and 

9,800 ppb (sampled for 65 min), respectively. The air quantities flowing past 

each sampling point were unknown. No SF6 

was detected at point I. Since all SF6 

exiting the section had to pass sampling 

point 6, the sum of air quantities at 

points 3 and 4 should have equaled the 
g y 

0.67 ft3 found in the return. Modifying 

equation 1 slightly enables solving it 

for airflow, rather than for SF6 recovered. The calculated average airflow 

passing the right and left outby positions was 1,050 ft3/min. 

Because of injection problems, the results obtained in this portion of the 

test were limited. However, it was seen 

that no SF6 migrated to point 1 or from 

seal D inby (point 5) to seal A inby 

(point 2). 

SECOND GOB EVALUATION 

Because of the obvious sampling problems at point 5, researchers examined the 

copper tubing extending through seal D 

into the gob. The inby end of the tube 

was found to be almost totally collapsed 

and under water. The copper tube was repaired and reinserted into seal D, and a 

second bottle of SF6 was injected into 

the seal. Again, air sampling took place 

at all six sampling positions. This 

time, SF6 concentrations at point 4 averaged 320 ppb for 35 min. At point 6, 

mean SF6 concentrations were 90 ppb for 

105 sampling minutes. 

After the second release, point 6 

showed only a 29-pct recovery of the 

total SF6 injected. Assuming that the 

air quantity previously determined at 

point 4 (1,050 ft/min) was correct, now 

only 4.5 pct of the SF6 released was recovered at that point. Therefore, point 

4 was sampling less gas than the return. 

Since point 4 was within the urethanesealed roof and rib area, SF6 injected 

into the gob was probably leaking around 

the urethane seal and into the return. 

No SF6 was detected at point 2 throughout both these tests. Again, any air migration from point 5 to point 2 was either extremely slow or nil. 

After the release, the concentration 

at sampling point 5 remained almost constant (slope 0.02). Knowing this, an 
assumption was made about the relative 

size of the void behind seal D. Mean SF6 

concentration averaged 2.6 vol pct. The 

actual quantity of SF6 released was 1.01 

ft3. By proportion, the approximate size 

3f the void behind seal D was determined 

to be 39 ft3. 

Results of this second phase of testing 

provided more information. Assuming that 

the derived volume behind seal D was correct, the likelihood of gas migration 

from one entry to another was very remote. Therefore, there was little chance 

that SF6 released inby seal D would be 

detected in any other gob location. 

GAS ANALYSES OF LEFT AND RIGHT 

INBY POSITIONS 

In addition to the SF6 evaluations, gas 

concentrations behind both seals A and D 

were examined. Again, there appeared to 

be little correlation between data received from the two inby seal positions 

(points 2 and 5, respectively). Seal A 

exhibited characteristics commonly found 

in sealed gobs--depressed 02 and N2 

and elevated quantities of C02 and 

hydrocarbons (table 4). The air behind 

seal A appeared to be part of the gob 

atmosphere. 

Sampling behind seal D did not produce 

the same results. Initially, 02 samples 

resembled the ambient mine atmosphere. 

Samples obtained through the repaired 

sampling port during the second gob 

evaluation, which started at 10:30 a.m., 

showed a decrease in 02 and an increase 

in C02 and the hydrocarbons. Although 

the concentrations of various gases 

linked to gob atmospheres did increase, 

the elevated quantities were still low 

enough to support the belief that air was 

probably leaking through seal D. Since 

hydrocarbon concentrations remained relatively low, caving behind the seal must 

have been complete and very close to the 

seal. Only a remote connection, due to 

slightly elevated C02 and CH4, appeared 

to exist between the void and the overall 

gob volume. 

CONCLUSIONS 

In Mine B, after mining had been completed, standard practtce was to seal the 

section to eliminate the potential for 

spontaneous combustion. Seals were made 

of pine blocks, hitched into the floor 

and ribs, and coated with spray-applied, 

rigid urethane foam. In addition, to 

reduce the possibility of gob gas escape 

into the working areas of the mine, extensive sealing was performed on the roof 

and ribs outby the seals. 

In the particular section examined, 

four seals separated the gob from the 

active workings. The two outside seals 

had sampling ports through which gob gas 

was sampled. 

TABLE 4. - Gas concentrations behind seal A and seal D, Mine B 

Time 

SEAL A 

Concentration, pct 

02 1 Ar 1 N2 Ico~IcoICH~ 

10:15. . . .... 10~55. ... 0. 

11:30 ....... 13:OO.. a ...* 

14:30. e.... 

10:15 ....... 10:40....... 

10:55....... 

11:30....... 

13:OO ....... 14:30....... 

Concentrat ion, ppm 

C~H~IC~H~IC~H~OIC~H

SEAL D 

9.0 

8.4 

8.1 

5.0 

8.9 

ND No gas detected. 

20.8 

19.3 

19.0 

17.0 

16.6 

18.7 

0.71 

070 

.70 

070 

070 

0.93 

091 

089 

089 

.89 

59.9 

59.0 

58.8 

58.8 

59.7 

78.1 

76.4 

75.0 

74.4 

74.4 

091-7601 

8.4 

8.8 

8.9 

8.9 

8.5 

-- - 

0.1 

1.3 

1.8 

2.7 

3.0 

1.8 

ND 

ND 

ND 

ND 

ND 

ND 

ND 

ND 

ND 

ND 

ND 

21.9 

23.0 

23.4 

23.5 

22.1 

ND 

2.1 

3.3 

5.0 

5.1 

2.5 

200 

210 

200 

200 

190 

ND 

15 

20 

10 

10 

20 

95 

100 

100 

100 

90 

ND 

9 

12 

10 

8 

5 

ND 

15 

22 

25 

20 

12 

ND 

7 

9 

10 

10 

6 

150 

165 

165 

165 

155 

60 

65 

65 

65 

60 
The objective was to determine why gas 

samples taken from the two outside seals 

differed so radically, one exhibiting 

high methane and low oxygen, the other 

just the opposite. 

A sampling strategy was developed. In 

both tests, SF6 was injected inby the 

seal displaying ambient air properties 

(seal D). The results of these tests indicated the presence of an extensive 

cave-in just inby that seal. This caving 

was tight enough to separate the area immediately behind the seal from the rest 

MINE 

BACKGROUND 

In Mine C, unanticipated gob gas problems were occurring behind gob seals that 

separated three different, supposedly 

interconnected, mine sections from the 

working areas of a coal mine. Each section had several seals, one of which had 

a port for gob gas sampling. Section 2 

South exhibited high methane (approx. 50 

pct), section 3 South had methane concentrations just above the explosive range 

(approx. 17 pct) , and section 4 South approximated ambient air. Because of the 

potential danger associated with section 

3 South, the Bureau of Mines was asked to 

perform SF6 ventilation tests on all 

three sections. The objectives were to 

determine seal performance and attempt to 

verify the existence of an air interconnection between the three sections. 

Mine C is located in southeastern Colorado. Sections 2 South, 3 South, and 

4 South (fig. 5) are located in a coal 

seam 4 to 11 ft thick, which pitches down 

toward the southwest at a 7-pct slope. 

The main entry (not shown) was driven 

in a westward direction. Several sections were developed southward from the 

main. The three sections to be examined 

appeared to be interconnected by an 

east-west development, referred to as 

Panel A. Panel A develdpment took place 

7~he gob area associated with Panel A 

was where all examinations in this report 

took place. This area will be referred 

to as the gob. 

of the gob. Gas analyses showed only a 

remote connection between the space inby 

seal D and the rest of the gob. Samples 

taken inby seal D showed gas concentrations close to those in ambient air. 

Those samples taken inby seal A were more 

typical of gob gas samples in that they 

had higher concentrations of hydrocarbon 

gases and C02, and depressed quantities 

of 02. This implied that the area immediately inby seal D was almost completely 

isolated from the rest of the gob. 

at the southerly termination of all three 

sections. 

Development, and some retreat pillar 

mining had been completed in the 3 South 

and 4 South sections before they were 

sealed. Approximately 800 ft of the 

southernmost portion of 2 South was also 

sealed. The enclosed gob encompassed 

nearly 6.5 million ft3. An active section still existed in the 2 South development, just north of the sealed gob. 

This subsection was being developed westward and was referred to as Panel B. 

The gob was isolated from the active 

mine by 16-in-thick concrete block seals. 

A cementitious sealant was applied to the 

outby and inby faces. Rigid urethane 

foam was spray-applied to the perimeter 

of each seal for added tightness. A 2-in 

ball valve was located in the far left 

and right seals of each section. A 0.25- 

in-OD copper tube for gob gas sampling 

was also located in the left or No. 1 

seal of each section. It was estimated 

that the copper tubes extended into the 

gob for at least 50 ft. 

All three sealed sections were ventilated on return air. An intake split 

from the main entry ventilated Panel €3, 

then ventilated the seals across 2 South. 

Both 3 and 4 South seals were ventilated 

by the south leg of the main entry returns. Brattice-wing curtains, just outby each seal, forced return air to sweep 

the seal face, helping to ensure proper 

seal ventilation. 

To start the evaluation, on separate 

days SF6 was released into the No. 1 seal 
Stopping with door * Stopping 

- Intake air 

, 390 - Return air 

Scale, ft VA Caved area 

Gob sample tube 

CS Return and remote 

gob sampling locations 

FIGURE 5.-Sealed gob and sampling stations of Mine C. 

of each of the three sectio~ls through 

the 0.25-in gas sampling tube. On the 

first day of testing, SF6 was injected 

into 4 South; on the second day, into 

3 South; and on the third, into 2 South. 

Immediately following the SF6 release, a 

positive displacement pump, approved by 

MSHA for sampling in methane atmospheres, 

was connected to each tube and run until 

the line was completely purged. This ensured that the SF6 samples would be drawn 

Erom the gob atmosphere and not from the 

tube itself. 

For these tests, samples were collected 

in two dif Eerent ways--direct sampling by 

evaluating ambient air in the returns, 

and remote sampling, using a personal 

sampling pump to pull air from inby the 

seals through the gob sampling port. 

Semirigid tubing was connected to the gas 

sampling port that protruded from the 

seal. The sampling pump was connected to 

the outby end of the tubing. 

An in-line plastic tee was placed in 

the tubing at an arbitrary distance between the seal and pump. The semirigid 

tubing was connected to two legs of the 

tee. The third leg had an airtight septum, through which samples would be extracted at previously arranged specific 

time intervals. 
EVALUATION 

4 South 

In the 4 South section, 1.16 ft3 of SF6 

was released behind the No. 1 seal. The 

seal was ingassing as the SF6 was being 

released. The SF6 concentration at this 

location showed little change over the 

entire sampling period. The gas concentrations measured at this point indicated 

a very low rate of diffusion. 

The 4 South return sampling position 

began indicating the presence of SF6 117 

min after release and then continuously 

for the remainder of the test. The quantity of SF6 leaking through the seals 

during the test averaged 0.06 ft3/h. 

Thirty minutes after the 4 South return 

began detecting SFg, it was also found in 

the 3 South return. The average SF6 flow 

volume at the 3 South return for the test 

was 0.01 ft3/h. This yields a dilution 

ratio of 6: 1 (from 4 South to 3 South), 

which enabled researchers to determine 

the contribution of 4 South air leakage 

when the 3 South seals were evaluated the 

following day. No SF6 was detected in 

the 3 South gob; thus, it was highly unlikely that any SF6 found in the 3 South 

return was leaking through the 3 South 

seals. For the remainder of the test, 

SF6 was detected at both the 4 South and 

3 South return positions, with values at 

3 South roughly trailing 4 South peaks by 

20 min. 

3 South 

On the second day, 1.15 f t3 of SF6 was 

released behind the No. 1 seal of the 3 

South section, which outgassed throughout 

the SF6 release. SF6 concentrations behind the No. 1 seal in 3 South were initially fairly low and increased throughout the sampling period. This was 

contrary to what had been expected, since 

diffusion caused by the kinetic energy of 

the gases at the molecular level should 

tend to dilute the gases and thus reduce 

the concentrations. 

SF6 was detected in the 3 South return 

immediately after release. Average SF6 

flow rate through 3 South seals was 

0.04 ft3/h. Because of the low SF6 

concentration detected in the 4 South 

return during this evaluation, all SF6 

sampled in the 3 South return was considered to be leaking through 3 South 

seals. If this leak rate had continued 

unchanged, all the SF6 in the 3 South gob 

should have been depleted in just under 

26 h. However, as the 3 South section 

approached equilibrium, * leakage through 

the seals rapidly diminished. 

Average air leakage from the 3 South 

seals was 1,525 ft3/min. ~f SF^ was 

leaking evenly through all six seals, the 

average air leakage was slightly more 

than 250 ft3/min per seal. 

Remote gob samples taken behind the 4 

South seals showed that SF6 concentrations decreased by a factor of 1,500 

over a 24-h period. This large reduction 

took place even though very little concentration change was noted throughout 

the second day's sampling time. 

2 South 

On the third day, 1.13 ft3 of SF6 was 

released behind the 2 South seal. This 

seal also was outgassing during the SF6 

release. Again, the average SF6 concentration began at a low value and increased throughout the test. Assuming 

that the highest concentration attained 

signified equilibrium in the gob, the 2 

South gob appeared to equilibrate much 

faster than the 3 South gob. 

As in the 3 South test, SF6 was detected in the 2 South return immediately 

after release. Return concentrations 

remained high throughout the test. Average SF6 flow rate through the seals was 

0.18 ft3/h. At this rate, SFgwas projected to fully deplete in slightly over 

6 h. Again, as the gob air began to approach equilibrium, SF6 flow through the 

seals diminished rapidly. 

The air leakage through the seals was 

calculated by comparing the concentrations behind and in front of the 

*~~uilibrium in this context means the 

samplinq time in which SF6 concentrations 

fall, then maintain a constant vallle, 

with no measurable additional diffusion 

taking place. 
seals. If the SF6 was leaking through 

only the one seal with the sampling port, 

leakage was 3,569 f t 3/min. If all seven 

seals were leaking, the average airflow 

was 509 ft3/min. 

Remote gob samples taken inby the 3 

South seals were surprising, in that the 

average SF6 concentration increased, 

rather than decreased, compared with 

levels from sadples taken the previous 

day. In fact, the values were almost 

twice as high as the day before. At the 

same time, remote gob samples also showed 

that the concentration in the 4 South gob 

continued to decrease very slowly. In 

both cases, the concentration changed 

very little during sampling, showing 

little additional gas dilution throughout 

the gob. 

CONCLUSIONS 

4 South 

Because the SF6 concentration behind 

the 4 South seal did not decline measurably throughout the f irst-day test, 

the gas was apparently being released 

into a problem area, probably into some 

tightly caved or blind area where little 

mixing was able to take place. However, 

since SF6 leakage did occur through the 

seals, the air in the gob was moving. 

The sample probe, therefore, may have 

been poorly located and may not have indicated what was actually happening in 

the gob. 

Following the first day of sampling, 

only very small quantities of SF6 were 

detected in the 4 South returns. During 

initial SF6 release, the seal was ingassing, but it began outgassing during the 

next two sampling days. It was probable 

that, during hours when no SF6 sampling 

was taking place, more SF6 outgassed into 

the return. Mine conditions did not appear to be sufficiently severe to cause 

pressure changes that would have created 

an appreciable change in the mine 

atmosphere. 

Twenty-four hours after the SF6 release 

in 4 South, samples detected no SF6 in 

the 3 South gob. Even after more than 48 

h, the 2 South gob sampling position 

still had not detected any SF6 from 

4 South. Based on this, there appeared to have been no low-resistance air 

interconnection between sections. 

The seals themselves appeared adequately constructed to preclude largescale gob gas leakage. However, 

a periodic visual inspection was 

recommended to ensure that the seals, and 

especially their mortar sealants, remained solid. 

3 South 

It was not determined why, after SF6 

was released behind the 3 South seal, the 

concentration increased throughout the 

sampling period. According to gas diffusion laws, because of the random motion 

of molecules and their associated kinetic 

energy, the net flow of gas is from high 

concentrations to low. One explanation 

proposed was that if the sampling probe 

was at the roof and the gob atmosphere 

was static, the SF6, which is more than 4 

times as dense as air, would require a 

long time to dilute back to the probe. 

However, chis would be valid only if 

there was little air movement in the gob; 

the immediate detection of SF6 in the 3 

South return suggests air movement behind 

the seals. 

During the 2 South evaluation, SF6 concentrations in the 3 South gob were 

stable but approximately twice as high as 

the initial peak when SF6 was released 

into 3 South. SF6 continued to increase 

from the time gas was released into the 

section until equilibrium was established. Since no SF6 change occurred in 

3 South during sampling the day after 

SF6 release in 3 South, the gob was 

considered to be at equilibrium. 

Again, an open interconnection between 

4 South and 3 South did not appear to 

exist. No SF6 was detected in 3 South 

even as late as 24 h after its release in 

4 South. The distance from the release 

point in 4 South to the sample point in 3 

South was approximately 1,300 ft. Thus, 

a diffusion rate of less than 1 ft/min 

was required to detect SF6 in the 3 South 

gob. Still, no SF6 was found. 

Initially, it was thought that the increase in SF6 concentration 24 h after 

release into 3 South was due to the 
mixing of gas in 3 South and 4 South 

sections. No reasonable explanation 

could be given for the large difference 

in concentrations between the two sections. It seemed logical that, if the 

sections were readily interconnected, 

equilibrium would have provided a common 

SF6 value for both locations. 

The seals associated with the 3 South 

section also appeared to be well constructed. During testing, their leakage 

values approximated what was expected 

from most concrete block seals. Although 

appreciable quantities of SF6 existed 

behind the seals, only minute quantities 

were sampled in the return. 

2 South 

As in the 3 South gob, SF6 released 

into 2 South gob also exhibited characteristics that cannot be readily 

explained by gas diffusion laws. The SF6 

concentrations behind the seal appeared 

to stabilize near the end of the test. 

This equilibration occurred much faster 

in 2 South than in other sections. The 

inference here was that the unobstructed 

volume behind 2 South appeared to be less 

than the volume behind 3 South seals. 

Looking at the mine map, this theory 

might be valid if all stoppings shown in 

2 South were still in place. 

No SF6 was detected in the 2 South gob 

prior to gas release in that section. 

Since the first release of SF6 was 48 h 

earlier, it was evident that no readily 

accessible passageway existed between the 

4 South and 2 South sections. The average air velocity required for transmission of SF6 from 4 South to 2 South within 48 h would have been slightly more 

than 1 ft/min. 

Section 2 South was apparently not as 

well sealed as the other two sections, as 

proven by the higher average air leakage 

through the seals. At several places 

across the seal face, small areas of high 

(greater than 10 pct) methane concentrations were detected. These data bore 

out suppositions based on prior visual 

inspection. Recoating these seals with a 

sealant would probably have eliminated 

the additional leakage. 

SUMMARY 

Two options, ventilating or sealing, 

are available to mines having abandoned 

areas. If the gob is to be ventilated 

after abandonment, there should be several bleeder entries around the gob, as 

well as a pressure differential across 

the gob, to ensure adequate air movement. 

To further enhance gob ventilation, at 

least one borehole should be driven into 

the section before or after mining. 

Gobs that liberate excessive quantities 

of methane, or are prone to spontaneous 

combustion, are usually sealed (2). 

Seals must be constructed of substantial, 

incombustible material and must be 

able to arrest an explosion. A section that is to be sealed after mining should have a minimum number of 

entries, consistent with good health 

and safety practices. Seals are not 

100 pct efficient; therefore, minimizing their number reduces the potential for gob leakage. Boreholes also 

enhance the performance of sealed gobs. 

These are developed vertically from 

the surface, or horizontally through a 

seal to the return. 

Each mine examined was faced with a 

different situation that required specific information to determine gob performance. All of the research was performed using sulfur hexafluoride (SF6) 

gas as the primary air tracer. Mine A 

ventilated its gobs; research determined 

both the average gob air velocity and the 

locations around the gob that were more 

thoroughly ventilated. Mine B sealed its 

gobs, but was receiving conflicting gas 

sampling results from two seals isolating 

the same gob; research found a restriction behind one seal that altered sampled 

gas concentrations. Mine C has a large 

gob, containing three sections with a 

total of 18 seals. This gob was experiencing different methane concentrations 

in each section; results showed that the 
individual sections were not inter- the seals in each section were also 

connected. Average leak rates through determined. 

REFERENCES 

1. American Conference on Governmental 

Industrial Hygienists. Threshold Limit 

Values for Chemical Substances and Physical Agents in the Work Environment With 

Intended Changes for 1983-84. Cincinnati, OH, 1983, 93 pp. 

2. Foster-Miller, Inc. Improved Ventilation of Sealed Gobs - Phase I Report. Ongoing BuMines contract 50308029; 

for inf., contact R. J. Timko, TPO, 

Div. Health and Saf . Technol. , BuMines, 

Pittsburgh, PA. 

3. Thimons, E. D., R. J. Bielicki, and 

F. N. Kissell. Using Sulfur Hexafluoride 

as a Gaseous Tracer To Study Ventilation 

Systems in Mines. BuMines RI 7916, 1974, 

22 PP* 

4. Timko, R. J., and E. D. Thimons. 

Sulfur Hexafluoride as a Mine Ventilation 

Research Tool--Recent Field Applications. 

BuMines RI 8735, 1982, 15 pp. 

5. U.S. Code of Federal Regulations. 

Title 30--Mineral Resources; Chapter I-- 

Mine Safety and Health Administration, 

Dep. Labor; Subchapter 0--Coal Mine Safety and Health; Part 75--Mandatory Safety 

Standards-Underground Coal Mines; July 1, 

1981, pp. 451-566. 

-; U.S. GOVERNMENT PRINTING OFFICE: 1986-605-017140.098