UNITED STATES DEPARTMENT OF THE INTERIOR 
UNITED STATES DEPARTMENT OF THE INTERIOR 

Thomas S. Kleppe, Secretary 

BUREAU OF MINES 

Thomas V. Falkie, Director 
This publication has been cataloged as follows: 

L 

Vinson, Robert P 

Three coal mine ventilation studies using sulfur hexafluoride tracer gas, by Robert P.Vins on and Fred N. Sissell. 

[Washington] U.S. Bureau of Mines [I9761 

19 p. illus., table. (U,S. Bureau of hlines, Report of investigations 8142) 

Includes bibliography. 

1. Mine ventilation. 2. Sulphur hexafluoride, I. Kissell, Fred 

N., jt. auth. 11. U.S. Bureau of Mines. 111. Title. (Series) 

TN23.U7 no. 8142 622.06173 

U.S. Dept. of the Int, Library 

i 
CONTENTS 

Page 

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

Introduction ............................................................. 1 

First study: Air movement in a sealed area ............................... 2 

Air movement-- zone A ............................................. 2 

Leakage from upper seam into zones A and B .......................... 8 

Second study: Airflow test in a pillared-and-caved area ................. 13 

Third study: Stopping leakage ........................................... 16 

Conclusion ............................................................... 17 

References ............................................................... 19 

ILLUSTRATIONS 

Sealed zone A ....................................................... 2 

Sealed zone B . ...................................................... 3 

Double seals ........................................................ 5 

Sulfur hexafluoride concentration as a function of time at each 

sampling location ................................................. 6 

Sulfur hexafluoride concentration in a simple enclosed area . - A, Simple enclosed area; E3, SF, concentration at S, and S, due 

to diffusion; C, SF, concentration at S1 and S, due to air 

movement from S1 to S, ............................................ 7 

Sulfur hexafluoride concentration as a function of time at 

sampling locations in zone A ...................................... 8 

Sulfur hexafluoride concentration as a function of time at 

sampling locations in zone B ...................................... 8 

Air pressure across seals as a function of time in zone A ........... 9 

Air pressure across seals as a function of time in zone B ........... 9 

Carbon monoxide concentration as a function of time in zone A ....... 10 

Carbon monoxide concentration as a function of time in zone B ....... 10 

Methane concentration as a function of CO concentration ............. 11 

Methane concentration as a function of seal pressure ................ 11 

Sulfur hexafluoride concentration as a function of seal pressure .... 12 

Carbon monoxide concentration as a function of seal pressure ........ 13 

Ventilated gob ...................................................... 14 

Four-entry heading .................................................. 16 

Sulfur hexafluoride concentration at S1 and S, as a function of time 17 

TABLE 

1 . Air movement test in zone A ....................................... 4 
THREE COAL MINE VENTILATION STUDIES USING SULFUR 

HEXAFLUORIDE TRACER GAS 

by 

Robert Pa Vinson and Fred N. Ki ssel l 

AB STRAC T 

This report describes three coal mine ventilation studies by the Bureau 

of Mines in which sulfur hexaf luoride (SFG) was used as a tracer gas. One of 

these studies was conducted to determine air movement and leakage in a sealed 

area. Another was run to determine the ventilation efficiency of a bleeder 

system. Finally, a study was made of air leakage across permanent stoppings 

of parallel intake airways. These studies proved sulfur hexafluoride to be a 

useful addition to the equipment commonly used in coal mine ventilation 

analysis. 

INTRODUCTION 

Sulfur hexafluoride has been used to analyze ventilation in large buildings, for meteorological studies, in gas reservoir analysis, etc. (2, 4, 6). 3 

It is an ideal gas for this type of work because it is odorless, colorless, 

chemically and thermally stable, and safe to breathe at high concentrations 

(I). In addition, sulfur hexafluoride can be detected at low concentrations 

by gas -solid chromatography using elec tron-capture detection (1, 9-10). 

Recently, the Bureau of Mines has developed procedures for using SF, as a 

tracer gas for mine ventilation studies in cases where conventional equipment 

cannot be used (11-12). Except for some preliminary work, these studies were 

conducted in noncoal mines. Because coal mines have many different kinds of 

ventilation problems, particularly associated with ventilation of and/or leakage through abandoned areas, we felt it worthwhile to do a separate series of 

coal mine studies with SF,. 

This report discusses three tracer gas ventilation studies conducted in 

underground coal mines. The first study was conducted to investigate air 

movement and leakage in a sealed area. A second study was made to determine 

the effectiveness of a bleeder system. The third study was used to evaluate 

lphysicist. 

2~hy~ical research scientist. 3 Underlined numbers in parentheses refer to items in the list of references at 

the end of this report. 
i 

i old workings of C seam. 

These old workings are connected to the surface by a 

10-in-diam borehole. An air 

shaft and two tunnels, which 

FIGURE 1. - Sealed zone A. forme~.ly gave access to the 

upper seam from the lower, 

have been sealed. These seals and the seals enclosing zones A and B were constructed of cemented concrete block that was then sprayed with urethane foam. 

Adjacent ribs and roof were also sprayed. A pipe with valve was inserted into 

each seal for gas sampling. 

Air Movement --Zone A 

Zone A contained a high concentration of carbon monoxide. Because of the 

self-heating nature of the coalbed, efforts were made to contain and reduce 

the CO to prevent leakage into the mine atmosphere. The carbon monoxide concentration was greatest on the east side behind the seals at location 1A 

(fig. 1) An "intake" air pressure differential was maintained across 1A 

seals. This means that the air pressure on the active mine side of the seal 

is greater than the air press.ure on the sealed-zone side. This would tend to 

keep CO-contaminated air from migrating into the mine atmosphere in the innnediate vicinity. Intake pressure was sustained by channeling high-pressure 

intake air directly across 1A seals. 
FIGURE 2. - Sealed zone B. 

Normally, the other sealed locations were ventilated by lower pressure 

return air. This produced an "exhaust" pressure differential across these 

seals. This tended to pull air out of the sealed zone. The combination of 

exhausting and intaking pressure differentials would possibly pull COcontaminated air from the east side to the west side of zone A. Normally, the 

oxygen in the sealed zone was depleted, but air movement would bring in fresh 

oxygen, which would react with the coal and produce fresh CO. An attempt was 

made to reduce the possible air mwement at locations 2A and 3A by placing 
these seals on intake air pressure. This was done by building a second set of 

seals outby the first and connecting the air space between to intake air pressure by running tubing from an intake airway to the air space, as shown in 

figure 3 (3). These changes reducedthe seal pressure differentials. Table 1 

shows that exhaust pressures at 2A and 3A were lower than exhaust pressures at 

4A, 5A, and 6A, which were not connected to intake air. 

TABLE 1. - Air movement test in zone A, seal pressure 

in inches water gage 

- - -- - 

Static--No differential pressure. 

I --Intaking pressure differential. 

E --Exhaus ting pressure differential. 

X--No pressure measurements taken. 

- - - -- - -- 

Location 

Another factor relating to air movement within zone A was the position of 

the water level. If this level was maintained high enough, the entries behind 

1A seals would be blocked off from the other sealed-off entries. This would 

prevent air movement in the sealed area, the input of fresh oxygen would be 

reduced, and high CO concentrations might be confined to the entries imrnediately behind seals at 1A. However, there was no definite way of knowing where 

the water level was, or whether in fact, it was blocking the entries behind 1A 

seals. 

Center 1A 

X 

X 

I 0.02 

I 0.03 

I 0.07 

I 0.05 

X 

Tracer gas was used to determine if the flooding was blocking air in 1A 

and if not, whether the air was moving from east to west as shown in figure 1. 

For this purpose, 3.9 cu ft of SF, was injected through a pipe in the center 

seal of location 1A. Gas samples were then collected at various locations 

surrounding the enclosed area on day zero (the day SF6 was injected) and 1, 

3, 4, 8, 36, and 70 days after SF, injection. The sampling schedule was 

rather erratic because there were no established guidelines since this was the 

first test of this type. When SF, was detected, the location, date of sample, 

and measured concentration were plotted (fig. 4). Lines connecting the same 

location from day to day give the general history of the tracer gas at each 

measured location. 

On day zero, SF, was detected only at the left and right seals of lA, 

with the right seal showing its highest SF, concentration. In the following 

days, the SF, concentration declined as the gas moved away from the right 

seal. 

Left 1A 

I 0.06 

E 0.02 

I 0.02 

I 0.02 

I 0.07 

I 0.10 

I 0.04 

2A 

Static 

E 0.01 

E 0.01 

Static 

Static 

Static 

E 0.01 

3A 

E 0,30 

E 0.12 

E 0.05 

E 0.14 

E 0.10 

I 0.02 

Static 

4A 

E 0=39 

E 0.34 

E 0.15 

E 0.31 

E 0.29 

E 0.12 

E 0.12 

5A 

E 0.20 

E 0.24 

E 0.19 

E 0.36 

E 0.22 

E 0.10 

E 0.10 

6A 

E 0.18 

E 0.34 

E 0.21 

E 0.34 

E 0.20 

E 0.15 

E 0.08 
tracer reached peak concentration, it showed a constant decline. 

Samples were first obtained from behind the center seal of 1A on day 3. 

Because this was where SF, was injected, the concentration was very high, 

almost 10,000 ppb. This seal showed the same rapid decline in SF, as the 

other seals at 1A. 

The sequence in which SF, was first detected at locations 1A left to 4A 

is evidence that there was air movement behind the seals from east to west. 

The further west the seals were, the longer it took before SF, appeared 

(fig. 4). Only location 2A does not follow this sequence. This is probably 

because 2A is connected to the main body of sealed air by one entry (fig. 1). 

Also, the low exhaust pressures at 2A would tend to pull air at a much slower 

rate than other seals. The comparatively low SF, concentration is an indication that there was a weak connection to the main body of sealed air. 

Arrival times of the peak concentration of SF, at locations 1A left 

through 4A follow the same sequence as the first appearance of tracer gas at 

each location. This would also indicate that the main body of SF, was slowly 

moving to the west side of zone A. 

The last location at which SF, was measured was at the 5A seals. The 

area behind these seals is separated from the main zone of SF, injection by a 

50-ft coal barrier. Tracer gas migrated through the coal and to the seal 

sometime between the 8th and 36th day. 
TIME, days 

FIGURE 4. - Sulfur hexafluoride concentration as a function of time at each sampling 

location, 
The decline of SF, in 

the area of injection and 

the increase on the western 

eal side of zone A is positive 

evidence that air migrated 

from east to west behind the 

seals. If there were no air 

movement in zone A, the SF, then sample A would eventually diffuse 

equally throughout the 

sealed zone. For example, z 

0 let us assume a simple 

I- Complete dilution \ 

a enclosed area (fig. 5). If a 

tx 

t- volume of SF, is injected at 

z 

W S1, it will diffuse until 

0 

Z the SF, concentration at S1 

E and S, are equal. Figure 5g 

d! illustrates how, with time, 

V) 0 the S1 and S2 curves would 

TIME meet at a single asymptote. 

If air moved from S, to S,, r2 U the SF, concentra tibns would 

vary as shown in figure 5s. 

The S1 curve would gradually z #- 0 - -#+ decrease and cross the S2 

ta curve, which would gradually lx 

t- increase. This is essenz 

w tially the case at the 

0 

z Air movement from Sl to S2 zone A sealed area. 

E 

2 The zone A study area 

V) 

0 contained approximately 

TIME C 3.56 x lo6 cu ft of air 

space into which 3.92 cu ft 

FIGURE 5. - Sulfur hexafluoride concentration in a simple of SF6 was injected. If 

enclosed area. A, Simple enclosed area; B, there was air movement 

SF6 concentration at S, and S2 due to diffu- and the SF, dispersed evenly, 

sion; C, SF6 concentration at Sl and s2 d3e the concentration would 

eventually be 1.1 ppm to air movement from s1 to S2. throughout the enclosed zone. 

This, however, was not the case. In figure 4, concentration curves 1A cross 

2A, 3A, and 4A, in the same way as in figure 5C, indicating that air was moving from east to west, despite the reduced pressure differentials at 2A and 3A. 

It indicates also that the water level was not blocking off the region behind 

1A from the rest of the sealed area. Fresh oxygen was still entering at 1A 

and generating CO. 

Subsequent to these tracer gas tests, fresh water was piped into the zone 

to raise the level. This reduced the CO concentrations to a safe level, indicating that the raised water level had blocked air movement and reduced the 

oxygen influx. 
Leakage From Upper Seam Into Zones A and B 

The objective of this part of the study was to determine if air from the 

old workings of the upper seam was leaking into sealed zones A and B of the 

lower seam. There was reason to believe leakage was occurring. The upper 

seam was known to have methane gas. Methane was measured at seals 1B and 2B 

in the lower seam (zone B, fig. 2). To determine if there was air leakage 

between seams and if this was carrying the methane, SF, was injected into the 

upper seam through the surface borehole. Gas samples were then taken at seals 

enclosing zones A and B in the lower seam. These samples were then analyzed 

for SF,, methane, and carbon monoxide. Air pressure differentials across the 

seals were also measured at each sampling location. A recording microbarograph located outside the mine was used to keep track of atmospheric pressure 

changes during the test period. 

01 2345678910 

TIME, days 

FIGURE 6, - Sulfur hexafluoride concentration as a function of time at 

01 2345678910 

TIME, days 

FIGURE 7, - Sulfur hexafluor ide concentration as a function of time at 

sampling locations in zone A, sampling locations in zone B. 
Within 3 hours after injection, SF, was detected at seven sampling locations showing that air from the upper seam was, in fact, leaking into both 

sealed zones A and B in the lower seam. Later, SF, appeared at three more 

locations. 

The manner in which SF, varied at the seals is shown in figures 6 and 7. 

Most seals have a significant dip in concentration on day 2. On day 10, there 

was a rise in SF, concentration at five of the seals. This may be related to 

barometric pressure variations. As atmospheric pressure changes, there is a 

corresponding change in the mine air pressure. However, in the enclosed area, 

air pressure changes lag behind atmospheric changes. This in turn produces 

fluctuations in the pressure differentials across the seals. As the mine air 

---- pLcaaure -.. rises, the seals with exhaust pressure decrease and the seals with 

intake pressure increase. The opposite takes place with a drop in mine air 

pressure. Barometric pressure was dropping on days zero and 10, but was rising on day 2. Figures 8 and 9 show how the seal pressures varied during the 

test period. Seals 1B and 2B had unusually high exhaust pressures because 

they were in close proximity to one of the exhaust ventilating fans. With 

the exception of 4B, all seals having exhausting pressure showed a drop in 

pressure on day 2. 

Exhaust 

.2 I- Intake 

TIME, days 

FIGURE 8. - Air pressure across seals as a 

function of time in zone A. 

- 

7 

Exhaust 

.3 

012345676910 

TIME, days 

FIGURE 9. - Air pressure across seals as a 

function of time in zone B. 
Pressure variation at seal 3A was most dramatic, changing from exhausting 

pressure to intaking pressure from day zero to day 2 and then back to an 

exhausting pressure on day 10. Seals with intaking pressures (1A and 3B) 

showed higher intaking pressures on day 2. These changes fit in with the 

barometric pressure rise on day 2. The only exception is 2A, which remained 

static because of its weak connection to the main body of sealed air. 

Following injection into the borehole, tracer gas was detected at all 

seals with exhaust pressure differentials on day zero or day 1 (figs. 6 and 7). 

On day 2, the tracer declined or disappeared completely at these same seals 

following the barometric pressure rise. The relationship between seal pressure and SF, concentration was not so apparent on the 10th day. However, 

there was evidence to show that the relationship was still holding because 

there was an increase in SF, at four out of six seals that had higher exhaust 

pressures. This is another indication that air leakage across seals is influenced by atmospheric pressure changes. 

Gas samples were also analyzed for CO and CH4. The CO concentrations are 

shown in figures 10 and 11. Note that location 1A has by far the highest CO 

despite the fact that it was always intaking. This indicates that the CO 

source is close to the seals at this location because of the fresh air leaking 

in here. 

Figure 12 shows CO versus CH4 concentrations for all measurements on all 

seals. There seems to be little correlation aside from the fact that the very 

TIME, days TIME, days 

FIGURE 10, - Carbon monoxide concentra- FIGURE 11, - Carbon monoxide concentration as a function of time in tion as a function of time in 

zone A, zone B, 
Read lngs here mean 

zero CO 

Ill IIII I I1 

0005 0001 0.05 001 05 01 5 

CO, pct 

FIGURE 12. - Methane concentration as a function of CO 

concentration. 

Exhaust 

SEAL PRESSURE, ~nches water gage 

FIGURE 13; - Methane concentration as a 

pressure. 
highest methane concentrations corresponded to zero CO concentrations. This 

confirms that CO and CH4 originate in different locations. 

Figures 13, 14, and 15 show CH, , SF, , and CO concentrations, respectively , versus seal pressure for all measurements on all seals. Figures 13 and 14 

indicate that when the seals were exhausting, the CH, and SF, levels were generally higher than when the seals were intaking. However, in figure 15, CO 

levels are lower when the seals are exhausting, which lends strength to the 

notion that the SF, and CH, are not generated in the lower s2am (this is certainly true for the SF,), and that the CO comes from reactions with oxygen 

that leaks in through the lower seam seals when the seals are intaking. 

SEAL PRESSURE, inches water gage 

FIGURE 14. - Sulfur hexafluoride concentration as a function of seal pressure. 

I,OOC 

500 

I00 

P 

a 

a 

G 50- IL 

v, 

10 

- i I I I I I _ I - Il' 'I' 4 

- 

- a 

- 

- 

- 1 a - 

+ - 

- 

5- 

- - 

- - 

- i rReod~ngs here mean - 

1- - I I - A I - 

- Intake cSfati~ a Exhaust - - a - 

- - 

- i - 

- - 

I l 

a a 

- 

- - 

- 

- - - a - 

- I; - 

- 

- 

- 

- lea.. : - 

- i - 

\L 

0.3 0 0.3 0.6 0.9 1.2 1.5 

Sampling 4050cfrn~ \ stat ion 

LEGEND - Intake air =tR= Stopping with regulator 

W Return air * Curtain 0 200 400 

?t= 

=Kk 

Permanent 

Stopping with 

stopping 

door 

Overcast - 

Scale, ft 

FIGURE 16. - Ventilated 
three-quarters of the gob air (3,000 cu ft/min). Suppose further that this 

path is a square passage 1,200 ft long and width D, and that it has a friction 

factor of 250 X 10 lo. The Atkinson equation is as follows (I): 

where HF = friction loss, inches of water, 

V = velocity, ftlmin, 

A = area, ft2, 

P = passage perimeter, ft, 

K = friction factor for air of standard density (0.075 lb/ft3), 

and L = length, ft. 

If the width is D, the perimeter is 4D and the area is D~. If the 

Atkinson equation is solved simultaneously with the relation Q = AV, where Q 

is the air quantity in cubic feet per minute, then D ? 3 ft and V ? 300 ft/min. 

At this velocity, the SF, tracer would have traversed the gob in 4 min, but 

the actual time was 60 min. The same disparity between the calculated and 

actual velocities exists even with a tenfold increase in the friction factor 

or even assuming multiple short-circuit paths, each carrying smaller air 

quantities. From this, it may be inferred that an SF6 arrival time of 60 min 

and a pressure drop of 0.69 in w.g. are not consistent with the concept of a 

few major passages around the gob edge. Thus, a different sort of approach is 

indicated. 

A better solution might be to visualize the gob as having a permeable and 

an impermeable region and to estimate the relative size of the two regions as 

follows: If the SF6 traveled 1,200 ft in 60 min, the air velocity was 20 

ft/min. Since the air volume was 4,050 cu ft/min, the open area for flow was 

= 49050 ? 200 ft2. If this open area is in the permeable region, the size 

v 20 

of that region can be calculated, based on an assumed void volume. 

Miletich (8) has given void volumes (porosities) for longwall gobs in the 

Soviet Union. At a distance of 2,000 ft from the working face, typical values 

range is from 34% to 48%. If 40% is assumed, then the area of the permeable 

region is 

Although these numbers are very approximate, they do indicate that no 

short-circuit path existed in this gob and that the permeable region occupied 

a fair percentage of the total cross-sectional area. 
Return oir 

=il= Permanent stopping 

=#= Stopping with door 

FIGURE 17. - Four-entry heading. 

mometer measurements, 44% of the intake air, and 

should have been diverted. However, a differenc 

air measurements is not significant. 

S, and S, accounted for 3.6 

and 0.24 liters, respectively (fig. 18). This was 

61% of the amount released. 

Unmeasured SF,, 39% of 

the amount released, was 

diverted into two o rher 

entries at location A and 

entry 4. According to anetherefore 44% of the SF,, 

e of 5% between tracer gas and 

Measured air volume results also show that entry 2 lost 7,700 cu ft/min 

of air between the release and sampling stations, so the net leakage was from 

entry 2 into entry 3. Despite this, some air did leak the other way from 

entry 3 into entry 2 because the coal train traveling in and out of entry 3 

0.24 

created a temporary backflow into entry 2. This backflow forced - = 3.8% 6.3 

of the SF, from entry 3 into entry 2, even though the net flow was the other 

way. 

The first 12 stoppings were constructed of cemented concrete block, and 

the last 8 were constructed of thin sheet metal on wooden frames. The sheet 
TIME, min 

FIGURE 18, - Sulfur hexafluoride concentration at ST and S, as a function of time. 

metal stoppings had leaks, and they flexed with the air pressure change caused 

by passage of the train. Thus, train movement caused a small amount of air 

leakage around the metal stoppings into the adjacent entry. 

CONCLUSION 

In the first study, SF, showed that air movement was taking place within 

a sealed area. The direction of this air movement was found by analyzing the 

SF, concentration curves at each sampled location. In the second part of this 

study, the tracer gas was used to confirm that air leakage was taking place 

from old workings of another seam. It was also found that atmospheric pressure changes strongly affect both seal leakage and the measured concentration 

of gas samples taken from short sample pipes cemented into seals. 

The purpose of the second study was to determine if a pillared-and-caved 

area was being properly ventilated. By using tracer gas, it was possible to 

find the air velocity across the inaccessible area. With this information and 

an altimeter survey of the area, it was shown that the gob was properly 
ventilated and that there was no short -circuit path through the pillared -and - 

caved area. 

A third study using SF, showed that pressure pulses caused by a moving 

train were causing a small reverse leakage from the intake airway with the 

train to the adjacent airway. 

The successful use of SF, in these three studies indicates that SF, is 

potentially capable of helping to solve ventilation problems that do not 

respond to conventional methods of ventilation analysis. Its application 

should be considered when such problems are encountered. 
Arch. Ind. Hyg. and Occupational Med., v. 2, 1950, pp. 348-349. 

Miletich, A. F. Air Leakage Calculations in Mine Ventilation. Nedra 

Publishing House, Moscow, 1968, 146 pp. 

Niemeyer, L. E., and R. A. McCormick. Some Results of Multiple-Tracer 

Diffusion Experiments at Cincinnati. J. Air Pollution Control Assoc., 

v. 10, 1968, pp. 403-405. 

Saltzman, B. E., A. E. Coleman, and C. A. Clemens. Halogenated Compounds 

as Gaseous Meteorological Tracers. Anal. Chem., v. 38, 1966, 

pp. 753-758. 

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. 

Thimons, E. D., and F. N. Kissell. Tracer Gas as an Aid in Mine Ventilation Analysis. BuMines RI 7917, 1974, 17 pp. 

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