L I i 

Bureau of Mines Report of Investigations/l978 

Methane Accumulations 

in Coal Mine Roof Cavities 

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

Cecil D. Andrus, Secretary 

BUREAU OF MINES 
This publication has been cataloged as follows 

i 

Vinson, Robert P 

Methane accumulations in coal mine roof cavities / by 

Robert P. Vinson, Edward D. Thimons, and Fred N. Kissell. 

[Washington] : U.S. Dept. of the Interior, Bureau of Mines, 

1978. 

15 p. : ill., diagrams ; 27 cm. (Report of investigations - Bureau 

of Mines ; 8267) 

Bibliography: p, 15. 

1, Methane - Measurement, 2. Coal mines and mining - Safety 

measures. 3. Mine gases. 4. Mine explosions, 1. Thimons, 

Edward Du, joint author, 11. Kissell, Fred Nu, joint author, 111. 

United States. Bureau of hlines. IV. Title. V, Series: United 

States, Bureau of Mines. Report of investigations - Bureau of 

Mines ; 8267. 

TN23.U7 no. 8267 622.06173 

U.S. Dept, of the Int, Library 

w 
CONTENTS 

Page 

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

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

Test facilities .......................................................... 2 

Experimental procedure .................................................. 3 

Results and discussion ................................................... 5 

Brattice aid in ventilation of cavities .................................. 10 

Layering number .......................................................... 12 

Conclusions .............................................................. 14 

References ............................................................... 15 

ILLUSTKATIONS 

Roof cavity ......................................................... Airflow in cavity ................................................... 

Sampling probe locations and depths ................................. 

Methane concentrations in 114.7 -cu-ft cavity at 100-fpm air velocity 

Methane concentrations in 114.7-cu-ft cavity at 300-fpm air velocity 

Methane concentrations in 114.7-cu-ft cavity at 600-fpm air velocity 

Methane concentrations in 57.3-cu-ft cavity at 100-fpm air velocity . 

Methane concentrations in 57.3-cu-ft cavity at 300-fpm air velocity . Methane concentrations in 57.3 -cu-ft cavity at 600-fpm air velocity . 

Methane profiles in 114.7- and 57.3 -cu-ft cavities .................. Air velocity versus effective ventilation rate ...................... 

Airflow with brattice hung in cavity ................................ 

Effect of brattice in 114.7-cu-ft cavity ............................ 

Effect of brattice in 57.3-cu-ft cavity ............................. 

Layering number versus average methane concentration ................ 

Methane concentrations at roof of tunnel ............................ 

TABLE 

1 . Layering number and methane concentration in 57.3 -cu-ft cavity ...... 13 
METHANE ACCUMULATIONS IN COAL MINE ROOF CAVITIES 

by 

Robert P. Vinson, Edward D. ~hirnons,' and Fred N. Kissel13 

ABSTRACT 

The Bureau of Mines investigated the presence and extent of methane accumulations in mine roof cavities using a full-scale mine model. Methane was 

released at a constant rate into the roof of a cavity built onto a 700-ft wind 

tunnel. Two box-shaped roof cavities were used in the study: one had a volume of 114.7 cu ft; and the other had a volume of 57.3 cu ft. Methane concentration was measured at 28 sampling points in the cavities using 21 

different combinations of gas emission rates and air velocities. Brattice 

hung in the cavity helped reduce methane concentrations without auxiliary 

ventilation. It was also found that the effective ventilation rate increased 

by a factor of 40 in the large cavity and by a factor of 20 in the small 

cavity when air velocity was increased from 100 to 600 fpm. 

INTRODUCTION 

Any location in a coal mine where 5 pct or more of methane is present 

constitutes a potential site for an explosion and roof cavities are likely 

locations. Unfortunately, little is known about the buildup of methane in 

roof cavities. Some research has been done using scale models where no air 

flows past the cavity so that methane disperses only by diffusion (5),' a rare 

situation in mines. 

No known cavity-model investigations have been made that simulate air and 

methane flow conditions normally encountered underground. These Bureau of 

Mines studies were undertaken to determine the extent and manner of methane 

buildups in coal mine roof cavities with air velocities in the range normally 

found in coal mines and with various methane emission rates. 

lphysicist . "~u~ervisor~ physicist. 

"physical research scientist. 

All authors are with the Pittsburgh Mining and Safety Research Center, Bureau 

of Mines, Pittsburgh, Pa. 

4Underlined numbers in parentheses refer to items in the list of references at 

the end of this report. 
TEST FACILITIES 

Studies were conducted at the Mining Enforcement and Safety Administration (MESA) Technical Support wind tunnel at Bruceton, Pa. The tunnel is 

700 ft long, 8 ft wide, and 5 ft high. A roof cavity was simulated by a 

wooden, box-shaped structure built onto the top of the tunnel. Initially, 

this box was 8 ft wide, 4 ft long, and 4 ft high (fig. 1). The top 5 inches 

of the box was used as a gas chamber into which gas was fed through eight 

equally spaced gas-dispersing pipes. A 4- by 8-ft section of pegboard separated this gas chamber from the cavity. Fifteen equally spaced 1/16-indiameter holes were kept open in the pegboard; the remainder were sealed shut. 

This arrangement allowed gas to disperse evenly through the roof of the 

cavity. Gas issuing from these holes into the cavity was deflected 90' by 

small baffles placed directly in front of the holes to prevent gas jets from 

disturbing the established air current in the cavity (fig. 2). 

I Tunnel 

+-- Airflow 

FIGURE 1, - Roof cavity, 
Not to 

scale 

FIGURE 2, - Airflow in cavity. 

windows in case of an explosion. Gas flows into the cavity from 2 to 8 cfm 

were measured with a rotameter. Gas flows less than 2 cfm were measured with 

a wet -test meter. Airflow through the tunnel was adjusted by a regulator near 

the exhaust fan. Tunnel air velocity was determined from anemometer 

traverses. 

Gas concentrations in the cavity were measured with two five-point probes, 

each constructed of five lengths of 1/4-inch copper tubing. An air pump 

pulled gas samples from the gas chamber and cavity into the probes, through 

1/4-inch plastic tubing, and into an infrared analyzer. Two analyzers were 

used to measure methane: one in the zero- to 2-pct range; and the other in 

the zero- to 100-pct range. The point in the cavity to be sampled and the 

analyzer to be used were determined by opening and closing the appropriate 

valves. 

EXPERIMENTAL PROCEDURE 

The test procedure consisted of establishing the air velocity through the 

tunnel at 100, 300, or 600 fpm. Gas was then released into the gas chamber at 

a controlled constant rate. While gas was released, air samples were taken 

through the probe permanently placed in the center of the cavity. Gas 
measurements were taken at five levels: one in the gas chamber and four in 

the cavity at equidistant depths. Gas samples were taken from these five 

points periodically to determine when the methane-air mixture in the cavity 

reached steady-stateconditions. Once steady state was reached, the second 

probe was positioned at one of six other sampling locations (fig. 3). This 

probe also took a sample from the gas chamber and at four depths in the 

cavity. With seven sampling locations and four sampling depths in the cavity, 

28 gas measurements were made in the cavity for each gas emission and airflow 

combination. A wide range of methane concentrations in the cavities was 

obtained by varying the CH4 flow rates. Smoke was used to observe airflow 

patterns in the cavity (fig. 2). 

Top view of cavities 

FIGURE 3. - Sampling pcbe locations and 

Sampling- depths. 

probe 

depths 

114.7-cu-f t 

cavity It 

Samplingprobe 

depths 

57.3%~-ft 

cavity 

-7 

A' 

0 

SarnplingI --- 111 

!U Gas chamber 4 

probe 

locations 

A 

r 

Secti?n 

A-A 

0 

0 

0 0 
RESULTS AND DISCUSS ION 

The objective cf this study was 

cavities and to observe the effect of 

emission on these accumulations by va 

concentrations were sampled at four d 

to measure methane accumulations in roof 

cavity size, air velocity, and methane 

.rying these three parameters. Methane 

epths in both cavities and at seven 

100 I 

300fpm, 114.7- cu-f t cavity - - 

DISTANCE UP INTO CAVITY, in DISTANCE UP INTO CAVITY, in 

FIGURE 4, - Methane concentrations in FIGURE 5, - Methane concentrations in 

114.7-cu-ft cavity at 100- 11 4.7-cu-ft cavity at 300- 

fprn air velocity, fpm air velocity. 
I I 

600 fpm, 114.7-cu-ft cavity - - 

- - 

- - 

- - 

locations at each depth 

(fig. 3). An unweighted 

arithmetic mean of the 

concentrations at each depth 

was calculated. 

The mean methane concentrations in the 114.7- 

cu-ft cavity and in the 

57.3-cu-ft cavity are shown 

in figures 4 through 6 and 

figures 7 through 9. 

Steady-state methane concentrations were higher in 

the large cavity than in the 

small cavity at air veloci0 14 28 42 ties of 100 and 300 fpm. This is shown graphically in 

DISTANCE UP INTO CAVITY, in figure 10, which is a typiFIGURE 6. - Methane concentrations in cal methane profile as 

114.7-cu-ft cavity at 600- measured in both cavities. 

fpm air velocity. Figure 10 also shows the 

effect of the circular air 

current in the cavity. This is seen at the bottom of the downstream side of 

the cavity where the concentration profile is bent up, which indicates that 

fresh air is entering the cavity in this area. This agrees with the circular 

air current in figure 2. The effect of the cavity air current is much 

stronger in the shallow cavity because the concentration profile is bent up 

from the base to the roof of the cavity on the downstream side. In the large 

cavity, however, the effect of the air current is not as strong because only 

the bottom portion of the profile is bent up. The concentration profile near 

the roof on the downstream side of the large cavity is actually bent down. 

This may be the result of either a dead space of no airflow or a small, 

reverse, circular air current induced by the main airflow in the cavity. 

At an air velocity of 600 fpm, methane concentrations in the small 

cavity were slightly higher, but no special significance is attached to this 

less -than-1-pct increase. 
100 I i 

- 100 fpm, 57.3-cu-ft cavity - 

90 - I 1 

300 fpm, 57,3-cu-f t cavity - 

0 7 13.75 20.5 

DISTANCE UP INTO CAVITY, in 

FIGURE 7, - Methane concentrations in 

57.3-cu-ft cavity at 100- 

fprn air velocity. 

0 7 13.75 20.5 

DISTANCE UP INTO CAVITY, in 

FIGURE 8. - Methane concentrations in 

57.3-cu-ft cavity at 300- 

fprn air velocity. 
+ 600 fpm, 57.3-cu-ft cavity 0 

a 

d 20 

I 

0 

W 

a FIGURE 9, - Methane concentrations in 57.3- 

a 

10 cu-ft cavity at 600-fpm air 

w velocity, 

> 

a 

0 7 1 3.7 5 20.5 

DISTANCE UP INTO CAVITY, in 

The low methane concentrations in both cavities at 600 fpm were due to 

the strong circular air current in the cavity (fig. 2); a less pronounced 

circular air current also existed at 100 and 300 fpm. 

The effect of this circular air current in the cavity is shown in figure 11, which is a plot of the air velocity and the effective ventilation 

rate. The effective ventilation rate is the quotient of the methane flow 

divided by the average methane concentration in the cavity. In the 114.7- 

cu-ft cavity, the average effective ventilation rate increased by a factor of 

40 when the air velocity was raised from 100 to 600 fpm. In the 57.3-cu-ft 

cavity, the average effective ventilation rate increased by a factor of 20 

when the air velocity was raised from 100 to 600 fpm. 

Although the methane concentrations decreased substantially at 600 fpm, 

the concentrations in both cavities were above 5 pct when methane flows were 

4 and 8 cfm. 

Comparison of methane concentrations measured at the same air velocity 

and different methane inputs with methane concentrations measured at the same 

methane input and different air velocities shows that air velocity in the 

tunnel has more influence on methane concentration than does methane input. 

High air velocities substantially reduce the possibility of dangerous methane 

concentrations in roof cavities. 
+ Airflow 

Methane concentration profile 

Cross section 

114.7-cu-ft cavity 

4cfm CH4 

300-fpm air velocity 

Sect ion 

A-A' 

KEY 

CH4 ,per 

FIGURE 10; - Methane profiles in 114.7- and 57.3-cu-ft 

cavities. 
KEY 

114.7-cu-ft cavity - 

0 57.3-cu-f t cav i t y 

EFFECTIVE VENTILATION RATE, cfm 

FIGURE 11. - Air velocity versus effective ventilation rate. 

BRATTICE AID IN VENTILATION CAVITIES 

When cavities occur in main haulageways or main intake airways provisions 

are normally made to ventilate them. Common practice is to run a section of 

ventilation tubing from the cavity roof into a return airway that has a lower 

air pressure. Cavities in return airways can be kept free of methane by placing a baffle in the cavity, which forces the air to sweep out the cavity. 

An experiment was conducted to determine if the simulated cavities could 

be kept at nonflammable methane concentrations by a brattice sheet hung the 

full width of the cavity (fig. 12). Gas samples were taken from the center 

probe and compared with samples from the same location but without the 

FIGURE Airflow with brattice hung in 

cavity. 

- L~rattice cloth hung 

full width of cavity 
- 

I 

- 57.3-cu-ft cavity 

300- fpm air velocity - 4-cfm gas flow 

- 

- 

No baffle 

- 

- 

- 

- 

- 

DISTANCE UP INTO CAVITY, in DISTANCE UP INTO CAVIT

FIGURE 13. - Effect of brattice in 114.7-cu-ft FIGURE 14. - Effect of brattice in 57.3-cu-ft c

cavity. 
brattice inthe cavity. Both sets of gas sampl-es were taken with a tunnel air 

velocity of 300 fpm and a gas input of 4 cfm. The comparisons for both cavities (figs. 13-14) show n dramatic reduction in methanc concentration with the 

use of brattice. 

The volumes of gas at 5 pct or greater concentration were calculated by 

gas samples taken from the center probe. IJhen the brattice was hung in the 

smal-1 cavity, this volume was reduced from 31.5 to 7.2 c.u ft; when it was hung 

in the large cavity, volume was reduced from 88 to 18 cu ft. These concentrations might have been reduced further if the brattice had been hung nearer the 

roof of the cavity. 

LAYERING NUMBER 

Whcn methane flows into a mine entry from the roof, it tends to remain 

concentrated at the roof (1). This phenomenon, knob- as Izyering, has been 

the subject of many theoretical and practical studies (1-4). "As a phenomenon, 

layering depends on a dimensionl-uss combination of independent variables, such 

as velocity, gravitational acceleration, density, density difference, volume 

of fluid, and width of roof" (L) . 

To better understand layering, a dimensionless number using these variables has been derived by Bakke and Leach (L) . For methane in air, it is 

where L = layering number, dimensionless ; 

U = ventilation velocity, fpm; 

V = methane input, cfm; 

and D = width of layer (same as width of tunnel), feet. 

This number gives an estimate of the propensity to layer. The higher the 

layering number, the lower the probability that layering will occur. A horizontal entry with a layering number of 5 or more is generally considered safe; 

that is, layering will not be a problem. 

Although it is not related to roof cavities, the layering expression was 

applied to determine what value of L would correspond to safe methane concentrations (below 5 pct) in the cavities used in this study. Quite often, as in 

this investigation, the point of methane emission is the roof of the cavity. 

Figure 15 is a plot of average percent methane in the cavity versus layering 

number for all the tests run, both in the large and small cavities. 

The graph shows that as L increases, the methane concentration decreases. 

The layering number reached 20 before gas concentrations in the cavity 

approached safe levels; however, it should not be assumed that the layering 

number can be used to determine methane accumulations in cavities. In three 
The length of a methane layer extends from the point of emission to that 

point downstream in the entry where the methane is completely dispersed. For 

practical purposes, however, layer length is usually defined as the distance 

from the point of methane emission to the place at the mine roof where the 

mean concentration is 5 pct. Based on this definition of layer length, no 

layering occurred in the tunnel downstream from the cavity. Figure 16 shows 

that the highest methane concentration at the roof was 4 pct. This was at the 

downstream edge of the cavity, which was considered to be the point of methane 

emission into the tunnel. Fifty ft downstream from the cavity, methane concentrations were less than 1 pct at the roof. 

20.2 

16.2 

16.2 

8.1 

8.6 

.52 

8 

1 

8 

.25 

.41 

6.19 

.78 

31.7 

2.34 

300 

600 

300 

306 

100 
FIGURE 16. - Methane concentrations at roof 

of tunnel. 

DISTANCE FROM CAVITY, ft 

CONCLUSIONS 

This study demonstrated that methane accumulates in coal mine roof cavities into which methane flows from the roof regardless of the air velocity, 

and that the methane concentrations are a function of the air velocity in the 

airway below the cavity. Higher air velocities produce pronounced circular 

air currents in the cavities, which, in turn, reduce the methane concentrations. It was found that this velocity effect is more important in determining the methane concentrations that will exist in a cavity than the rate of 

flow of methane into the cavity. This conclusion is supported by the 

layering-number theory. Higher than normal layering numbers are needed to 

maintain safe methane concentrations in an airway if the airway has roof 

cavities. Higher layering numbers can most readily be obtained by increasing 

the air velocity in the airway. It was also found that brattice hung in the 

cavity reduced methane concentrations without auxiliary ventilation. 
REFERENCES 

1. Bakke, P., and S. J. Leach. Methane Roof Layers. Safety in Mines Res. 

Est., SMRE Res. Rept. 195, 1960, pp. 1-62. 

2. James, T. E., and L. J. Purdy. Experiments With Methane Layers in a Mine 

Roadway. Min. Eng., v. 121, No. 21, 1962, pp. 561-575. 

3. Jones, I. 0. A Report of Model and Full Scale Experiments on Firedamp 

Roof Layers. Min. Eng., v. 128, No. 102, March 1969, pp. 369-377. 

4. Raine, E . J. Layering of Firedamp in Longwall Workings. Trans. Inst. 

Min. Eng., v. 119, July 1960, pp. 579-597. 

5. Titman, H. , A. F. Roberts, and F. R. Brookes. The Accumulation of Methane 

in Roof Cavities. Safety in Mines Res. Est., SMRE Res. Rept. 235, 1965, 

pp. 1-22.