Field study of longwall coal mine 

ventilation and bleeder performance

S. Schatzel, R. Krog and H. Dougherty

National Institute for Occupational Safety and Health, Pittsburgh, PA

Abstract

Longwall coal mine operators in the U.S. are required to ventilate multipanel longwall districts, but may 

have little or no knowledge about what happens to the ventilation air between the inlet evaluation points 

(IEPs), bleeder evaluation points (BEPs) and bleeder fans. The effectiveness of bleeder performance 

can directly influence the ability of a ventilation system to remove and dilute coal bed methane emissions. 

U.S. coal mining stakeholders have acknowledged their belief that the T-junction split at the longwall 

face tailgate corner is very important in controlling the distribution of ventilation air. To obtain direct 

measurements of bleeder performance, a tracer gas, sulfur hexafluoride (SF6), was released into the ventilation air stream on active and inactive longwall panels. Testing was performed on multiple longwall 

panels that included various phases of longwall development and variable path lengths of ventilation 

air transport to bleeder fan installations. Changes in the T-junction ventilation air distribution over the 

life of a longwall panel, and its variable effect on panel ventilation airflow are discussed. These findings 

will assess the effectiveness of commonly applied ventilation strategies for improving air distribution 

and ventilation controls to meet statutory requirements.

Introduction

Longwall coal mines in the U.S. use bleeder systems to 

dilute methane concentrations where panels and gobs connect 

to form return ventilation airways. Many modern U.S. longwall mines use bleeder fans to remove methane-laden air from 

these areas inby the active longwall face. The use of bleeder 

ventilation networks in U.S. mines was documented by Kingery 

and Dornenburg (1957) as applied to room-and-pillar mines. 

Since that time, large-dimension longwall panels have evolved 

in the U.S. coal mining industry. Improvements in auxiliary 

methane removal systems, such as directional drilling, with the 

addition of ventilation-assisting technologies such as bleeder 

fans, have allowed mine operators to mine large-dimension 

longwall panels in moderately gassy to highly gassy conditions (Thakur, 2006). As a consequence, methane loading and 

air movement in bleeder systems have become increasingly 

important and complex phenomena in underground coal mines. 

Consequently, the movement of ventilation air through longwall 

bleeders will interact with many aspects of methane drainage 

such as gob gas ventholes (GGVs), bleeder fans and sealed 

district ventilation.

Statement of problem

Although longwall ventilation technology has addressed 

deeper and gassier coal beds, increasingly large longwall 

panels and larger-volume coal bed methane reservoirs remain 

issues of concern. A series of U.S. National Institute for Occupational Safety and Health (NIOSH) stakeholder meetings 

were conducted to identify problem areas to be addressed by 

future research in coal mine ventilation and methane control. 

Input from industry, regulatory agencies, labor and academia 

prior to research proposal development identified ventilation 

air transport in bleeders as a high-priority area for future 

investigations. 

Airflows are typically measured weekly at inlet evaluation 

points (IEPs), bleeder evaluation points (BEPs) and bleeder 

fans. Unfortunately, these measurements are too infrequent 

to discern fluctuations in flow ranges that occur as a result of 

changing ventilation conditions. Even when measured flow data 

are available from these locations, vast expanses are present in 

the gob where flow rates and directions (e.g., former gateroad 

entries between gobs) are unmeasured and poorly defined. Other 

factors influencing the transport of methane-laden air in bleeder 

systems include rib spalling, roof-to-floor convergence and 

changing T-junction air distributions with increasing distance 

to the bleeder fan during panel retreat. The direct determination of these parameters and their influences on longwall panel 

airflow and transport in the bleeder ventilation network are 

often not possible. Also, high methane concentrations in the 

gob can result in increased methane emissions in the longwall 

returns and bleeders, which can make conforming to the 2% 

methane-in-air federal statutes very challenging (Mine Safety 

and Health Administration, 2008).

Research design and goals

To acquire data for this study, a method to quantify airflows in inaccessible areas of a longwall mine was needed. 

Tracer gas testing techniques have been used successfully to 

describe airflow movements in mine gobs, fractured overburden, around and though mine seals and air movement in face 

sections (Thimons and Kissell, 1974; Timko and Thimons, 

1982; Timko et al., 1986; Vinson and Kissell, 1986; Schatzel 
et al., 1999; Mucho et al., 2000). In addition to the tracer 

gas mine applications reported in the literature, the American 

Society of Testing Materials developed tracer gas testing standards applicable to mine voids (ASTM E 2029-99; ASTM E 

741-00). The complexity of mine void space, limited access 

and the variable flow rates of air and gas mixtures can create 

substantial challenges for tracer evaluation approaches (Grot 

and Lagus, 1991). 

This work used a series of tracer gas releases to describe 

airflow patterns and movements at the longwall bleeder/gob 

interface and to define ventilation air pathways, rates and 

volumes of movement at these interfaces. This work also 

identified the distribution of ventilation air at the longwall 

tailgate T-junction. A single tracer gas, sulfur hexafluoride 

(SF6), was chosen to describe ventilation air movement. Tracer 

gas sampling was conducted underground using tube bundles 

attached to permissible vacuum pumps and at the surface from 

operating GGVs and from a bleeder fan site. This study presents 

ventilation parameters at early, late and completed phases of 

panel extraction. The tracer gas measurements were designed 

to be redundant with the tracer gas slug passing by multiple 

sampling sites underground and at the surface. An important 

goal of this study was to describe intake air distributions and 

airflow rates on active and inactive panels, including bleeder 

sections and transport pathways to the surface.

Description of study site

The study site was an underground coal mine operating a 

single longwall in southwestern Pennsylvania. The mine operates in the Pittsburgh coal bed and utilizes a three-entry gateroad 

system with weekly production of about 151,000 mt (166,000 

t). The nearest intake air fan provides 106 m3/s (225,000 cfm) 

into the section. The shaft diameter is 2.4 m (8.0 ft). The 

study site is located on one panel of a four-panel district (Fig. 

1), where overburden depth ranged from 240 to 270 m (800 

to 900 ft). 

Figure 1 — Study site for tracer gas testing. Longwall configuration for test 1 is shown.

 A bleeder fan producing 129 m3/s (273,000 cfm) 

at 3.86 kPa (15.5 in WG) at the start of the study was located 

adjacent to longwall panel 1 (LW#1) on a bleeder shaft with 

diameter of 2.4 m (8.0 ft). Airflow around the back end of the 

panel is separated into an outer loop (Bleeder 1) and an inner 

loop (Bleeder 2) on its way to the bleeder fan. Two rows of 

76-cm (30-in.) pumpable cribs were installed throughout the 

length of the tailgate No. 3 entry, closest to the longwall block 

(numbered left-to-right when facing inby). This configuration 

was generally successful in keeping the entry open until the 

longwall face passed. Pumpable cribs were installed in the 

headgate No. 3 entry adjacent to longwall panel 4. The panel 

block width is 410 m (1,350 ft), entry width is 4.9 m (16 ft) 

and entry height is 2.6 m (8.5 ft). Each panel had at least three 

surface GGV sites removing gob gas via a coal bed methanepowered exhauster using a modified diesel internal combustion motor. Methane contents for the Pittsburgh Coal Bed in 

southwestern Pennsylvania are given in Diamond et al., 1985.

Methodology

Tracer gas was released from lecture bottles containing about 

34 L (1.2 cu ft) of high-purity SF6. An experimental protocol 

was developed for each tracer gas release utilizing the manual 

sampling of ventilation air with an underground tube bundle 

system. Tube bundles were 1.3 cm (0.5 in.) OD polyethylene 

tubing attached to SKC Inc. Aircheck vacuum pumps, which 

are U.S. Mine Safety and Health Administration (MSHA) 

permissible. The experimental protocol called for concurrent 

surface gas sampling to identify potential pathways of the 

tracer gas through the underground ventilation network and 

to the atmosphere. Tube bundles were installed in the panel 

3 headgate No. 2 entry and the tailgate No. 3 entry, adjacent 

to the No. 3 panel block. Four locations for sample retrieval 

were chosen in the headgate and tailgate entries, as shown in 

Fig. 1. Filters were installed on the tubing at the inby, open 

ends of each sampling line. The tubing outby, open ends were 

covered with electrical tape until a test was performed. 
For gas sampling, the SKC pumps were attached to the 

lines and gas samples were retrieved from the exhaust side of 

the pumps, with one pump attached to each tubing line. The 

length of each sampling tube is shown in Table 1. 

Table 1 — Sample tubing lengths, volumes, and transit 

times. 

Location Length,

m (ft)

Volume,

L (cu ft)

Transit 

time, min

Headgate

HG1 2,290 

(7,500) 194 (6.85) 63

HG2 1,830 

(6,000) 155 (5.47) 42

HG3 920 (3,000) 77.4 (2.73) 19

HG4 460 (1,500) 38.7 (1.37) 8

Tailgate

TG1 2,100 

(6,900) 178 (6.29) 55

TG2 1,650 

(5,400) 139 (4.91) 42

TG3 920 (3,000) 77.4 (2.73) 22

TG4 460 (1,500) 38.7 (1.37) 9

 Sampling 

pump stations were located in the No. 3 crosscut on both the 

headgate and tailgate sides, just inby the submains. All gas 

samples were collected in 15-ml glass vials that had been 

previously evacuated at NIOSH’s Office of Mine Safety and 

Health Research (OMSHR) in Pittsburgh, PA. 

Transit times for the movement of samples through the 

polyethylene tubing were computed by determining friction 

factors following testing at NIOSH and during pump tests 

performed in the field after installation. These tubing transport 

times were subtracted from the tracer gas arrival times at each 

sampling site to determine tracer gas movements. Additional 

underground tracer gas sampling was conducted manually by 

NIOSH staff by capturing samples of ventilation air in the bottle 

samples. Samples were taken in this manner at the tracer gas 

release points and in the bleeders.

In addition to the underground sampling sites, the experimental protocol called for concurrent gas sampling at surface 

sites. Sampling underground and at the surface was conducted 

to identify potential pathways of the tracer gas through the 

underground ventilation network and to the atmosphere. 

Sampling was conducted at bleeder fan exhausts that were 

within the same ventilation network as the study panel. Gas 

sampling was also performed at GGV sites on the study panel 

and selected adjacent sites where borehole exhausters were 

actively removing gob gas. Gas samples were pumped into 

the glass vials by NIOSH project staff using hand-operated, 

high-vacuum, plastic pumps.

Samples collected during the field study were analyzed for 

SF6 concentrations by gas chromatography. The analytical 

method for this study is based on NIOSH method 6502 (1994) 

using a chromatograph with an electron capture detector (ECD), 

typically with a Ni-63 source (Fig. 2). 

Figure 2 — In-house gas chromatographic analyses of gas 

samples for determination of SF6 concentrations.

 The analytical column 

was a stainless steel packed column with either a Chrompak 

Carboplot or a molecular sieve stationary phase. 

The tracer gas was injected into the ventilation airstream 

as short duration slugs. The movement of the tracer through 

the ventilation airways was anticipated to be relatively rapid, 

such that gas sampling could generally be completed in about 

one working shift. A gas chromatograph was also located at 

the mine site, so that gas samples could be analyzed quickly, 

allowing modifications to be made to gas sampling frequency 

and duration. Additional data collection included measurements 

of barometric pressure, temperature and humidity.

The exact volumes of tracer gas released during each test 

were computed, as were the gas volumes recovered from different sampling locations. The air distributions at key experiment 

locations for early, late and completed phases of longwall panel 

extraction were calculated. To compute retrieved tracer gas 

volumes at the sampling locations, plots of SF6 concentration 

against time were integrated to yield the recovered volume of 

gas. This method has been discussed by others and is summarized in the equation below (Hartman et al., 1997): 

 Q = Qg/ (CavTt) (1)

Where:

Q = airflow, cfm

Qg= tracer gas quantity, cu ft

Cav = average concentration of tracer gas, ppb

Tt = duration of measurable tracer gas concentration 

sampling, min

Discussion and results

Test 1. The first of three planned tracer gas releases was 

a relatively large-volume release using four lecture bottles of 

99.99% SF6. The four vessels were released sequentially and 

continuously over a three-minute period. The volume of release 

was calculated to be 6.52 moles or 156 L (5.51 cu ft) at the time 

of release at ambient conditions. The tracer gas monitoring 

design for test 1 included 11 underground locations and five 
surface locations. In addition to the headgate and tailgate pump 

stations, underground tracer sampling included the tracer gas 

release point and bleeder locations 1 and 2 (Fig. 1). On the 

surface, sampling locations included GGVs 2-1, 2-3, 3-1, 3-2 

and the bleeder fan. For GGV sampling, only boreholes with 

operational exhausters were included. 

The closest sampling location to the release point was 

HG4, where the highest quantity of tracer gas was expected 

to be recovered prior to leakage to the belt entry. At this 

location, 165 L (5.84 cu ft) of SF6 was measured, equaling 

106% of the released gas volume. It is assumed that the error 

is the result of some tracer gas leakage that occurred from the 

lecture bottle prior to release through the regulator and from 

infrequent sample analysis between the peak and tail of the 

concentration plot (Fig. 3).

Table 2 — Tracer gas statistics and airflow rates as determined or measured for test 1.

Location

Tracer gas recovered, v Tracer gas 

duration, min

Average tracer 

concentration, 

ppb

Airflow

L cu ft m3/s cfm

HG1 nd nd 89 366 nd nd

HG survey 

site1 70.8 150,000

HG2 133 4.68 96 325 71.0 150,000

HG3 161 5.69 74 450 80.5 171,000

HG4 166 5.87 57 569 85.2 181,000

HG survey 

site1 89.7 190,000

TG1 nd nd 133 247 nd nd

TG survey site1 12.3 26,000

TG2 21.9 0.775 140 223 11.7 24,800

TG3 20.0 0.705 141 239 9.87 20,900

TG4 16.6 0.588 153 208 8.73 18,500

TG survey site1 8.73 18,500

Bleeder 11 38.4 1.36 80 275 29.2 61,800

Bleeder 22 16.5 0.584 100 208 13.2 28,000

Bleeder fan 

survey site1 128 273,000

Bleeder fan 80.0 2.83 452 22.9 128 273,000

nd = not determined

1 Measured value

2 Measured value and survey site

To discern the arrival times of the tracer gas, a concentration of 10 ppb was selected as the minimum for inclusion into 

the tracer gas integration. The drops in tracer gas volume and 

maximum concentrations are distinguishable from the curves 

as the tracer migrates through the headgate. Between HG2 

and HG1, the air splits, with some flow directed towards the 

bleeders and bleeder fan and with the remainder going towards 

the face and the belt entry. At the tailgate T-junction, the air 

splits again, with ventilation air either flowing inby towards 

the bleeder fan or outby towards the submains (Fig. 1).

Airflow measurements were made at survey sites, but airflow measurements were not made at each of the tracer gas 

sampling points. The expression shown as Eq. (1) assumes a 

conservation of mass that is satisfactory for the first sampling 

location after the release, but is an erroneous assumption at 

other headgate and tailgate locations due to leakage out of the 

monitored entry (Fig. 1). Leakage rates can be estimated from 

confined spaces using tracer gas and monitoring concentrations 

over time (Timko and Thimons, 1982; ASTM, 2000). Plots of 

leakage rates from HG4 to HG2 and from TG2 to TG4 were 

made using known average SF6 concentrations. Although the 

graphical method did not successfully estimate magnitudes of 

leakage, since the monitoring sites were not confined spaces, 

the plots showed a linear loss of airflow in both the headgate 

and tailgate entries. Linear flow estimates at monitoring locations were made using the ventilation survey airflow data. Figure 3 — Concentration and time plot for samples 

retrieved from headgate sampling locations outby the 

face, test 1.

Flow rates, durations, average SF6 concentrations and the 

recovered volumes of tracer gas from each sampling location 
are shown in Table 2. Flow data and the location of ventilation 

survey points are plotted on Fig. 4. 

Figure 4 — Airflows measured at survey sites and determined at monitoring locations, 

test 1.

Only SI units are included 

on the figures to make the information more legible. A consistent rate of tracer gas loss exists in the headgate and tailgate 

entries outby the face location, while airflow leakages change 

significantly inby the face. Sampling at the bleeder locations 

was terminated earlier than at other underground locations, due 

to logistical issues. Unfortunately, this schedule terminated 

sampling before the tracer gas tail had dropped below the detection limit, about 1 ppb, and the tracer slug characterization 

is less precise than for the other monitoring locations. 

The only surface location that recovered SF6 in test 1 was 

the bleeder fan. No tracer gas was recovered from any of the 

operational GGV sites on panels 2 and 3. This is significant, 

because it shows a lack of interaction between the mine’s ventilation system and the longwall gob. Sampling at the GGVs 

was conducted for about seven hours during test 1 and samples 

were retrieved once on the morning following the release. 

Figure 5 — Longwall panel configuration for tests 2 and 3.

Tracer gas migration rates and permeabilities through the 

fractured gob overburden for a nearby Pittsburgh Coal bed 
mine have been previously reported (Mucho et al., 2000). 

Reports have documented tracer gas movement to the ventilation system and to the bleeder fan during a borehole release 

test when an adjacent GGV ceased operating. Migration rates 

in the fractured overburden were 0.02–0.09 m/s (5.0 to 19 

ftpm) (Schatzel et al., 1999). It is possible that infrequent 

sampling may have contributed to the lack of detection at the 

GGVs during test 1. Much greater flow volumes are present 

in the ventilation network than are produced from the GGVs, 

such that low volume, short duration and accumulations of 

tracer gas can be expected at the GGVs, if they occur. After 

the gob fractured and caved, overburden near the gateroads 

showed apparent high migration rates and high permeabilities 

for movements parallel to the gateroads (Schatzel et al., 2008). 

 Airflow quantities at the survey sites were measured by 

mine personnel in the headgate and tailgate entries where 

tracer gas sampling was conducted. Data gathered from the 

tracer gas monitoring activities are shown for all sampling 

locations where tracer gas was recovered and where airflow 

data are available. The survey sites show flow measurements 

made in headgate and tailgate entries directly in line with test 

sampling locations. More air in the panel 3 intake flowed to 

the face and to the belt entry compared to the amount of air 

continuing inby in the headgate towards the bleeders. Consequently, in this early phase of longwall panel extraction, 

the mine operator has chosen to utilize more intake air in the 

face and tailgate gateroads than is used to dilute methane in 

the gob or in the bleeders.

Airflow on the longwall face at shield 17 was measured 

to be 39.5 m3/s (83,700 cfm). About 14% or 21.9 L (0.775 

cu ft) of the SF6 was collected at TG2, suggesting that only 

a small portion of the airflow that was directed to the panel 3 

headgate actually reached TG2. The tailgate corner can be a 

critical location for mine safety, where longwall face methane 

concentrations are generally highest and frictional ignitions 

can occur in certain strata. 

In addition to the overall available airflow directed towards 

the tailgate, the distribution of gas at the T-junction is another 

important parameter. Although airflow was not measured at 

TG1 inby the longwall face, airflow measurements are available 

from the panel 3 longwall face at the time of test 1. Airflow 

on the face was 34.8 m3/s (73,800 cfm) just before the tailgate 

corner at shield 186, and a flow of 12.3 m3/s (26,000 cfm) was 

measured in the panel 3 tailgate just outby the face. The air 

quantity at the tailgate T-junction can be estimated by the difference as 22.6 m3/s (47,800 cfm) with a distribution of 1.8:1 

for inby:outby directions. Leakage of airflow to and from 

the gob is expected to be considerable in this region and the 

calculated T-junction distribution is intended to be an estimate 

for airflow at this location.

Another interesting feature of the test 1 data set is movement of airflow and SF6 to the bleeders and the bleeder fan. 

Once the tracer gas reached the bleeder monitoring sites, 

25% of the released gas was captured at Bleeder 1 and 11% 

at Bleeder 2, accounting for about 36% of the air that entered 

the panel 3 headgate. 

Table 3 — Tracer gas statistics and airflow rates as determined or measured for test 2.

Location

Tracer gas recovered, v Tracer gas 

duration, min

Average tracer 

concentration, 

ppb

Airflow

L cu ft m3/s cfm

HG1 nd nd nd nd nd nd

HG2 nd nd nd nd nd nd

HG3 nd nd 350 34.9 nd nd

HG survey site1 nd nd

HG4 65.2 2.30 80 159 85.4 181,000

HG survey site1 97.2 206,000

TG1 nd nd 338 42.5 nd nd

TG2 nd nd 330 12.0 nd nd

TG3 nd nd 338 38.8 nd nd

TG survey site1 9.34 19,800

TG4 4.22 0.15 78 62.3 14.4 30,600

TG survey site1 14.4 30,600

Bleeder 12 15.6 0.55 236 45.1 24.3 51,500

Bleeder 22 1.60 0.06 220 11.5 11.8 25,000

Bleeder fan survey site1 127 268,000

BF5 50.9 1.80 532 117 127 268,000

nd = not determined

1 Measured value

2 Measured value and survey site

Test 2. Tracer gas test 2 was conducted on the number 3 

panel about 3.5 months after test 1, when the longwall face had 

advanced to the position shown in Fig. 5. With the longwall 

face at this position, only tube bundle sampling locations HG4 

and TG4 were outby the face; all others were at inby locations. 

The volume of the tracer gas release was reduced for test 2 

compared to test 1. The total released SF6 volume was 2.90 

moles or 68.9 L (2.43 cu ft). Surface sampling for tracer gas 

included the bleeder fan and active GGVs 2-1, 2-3, 3-1 and 
3-2. The volume of tracer gas recovered at HG4 totaled 65.2 

L (2.30 cu ft) for a recovery of 95% of the released gas. No 

gas was recovered at any of the monitored, active GGV sites. 

No ventilation flow measurement was made just outby the 

face as in test 1 to determine leakages from intake to belt entry. Since the airflows at the mouth of the headgate were very 

similar in tests 1 and 2, the rates of leakage from intake to belt 

were assumed to be similar for this test. This yielded a flow 

of 85.4 m3/s (181,000 cfm) at HG4. In the tailgate, measured 

data were available to produce a new plot of changing flow 

rates including TG4. Prior to test 2, damage to some of the 

sampling lines was indicated. However, the shortest lines in 

the headgate and tailgate gateroads used for HG3, HG4, TG3 

and TG4 were shown to be intact during these pump tests, and 

data from these locations are included in this report (Table 3). 

Figure 6 — Changing SF6 concentrations over time from the bleeder fan monitoring site during 

tests 3 (a) and 1 (b). 

a

b

Despite the short distance from the mouth of the section to 

the face compared to test 1—about 533 m (1,750 ft) compared 

to 1,790 m (5,880 ft)—less intake ventilation air, 33.1 m3/s 
(70,200 cfm) was measured at shield 17 in test 2. 

The fraction of tracer gas recovered at the outby tailgate 

sampling site TG4 was 4.22/68.9 L (0.149/2.43 cu ft), or 6% of 

the released gas. This value is about half of the 14% measured in 

test 1, suggesting that more airflow was being directed towards 

the bleeder to dilute methane concentrations from the more 

extensive longwall gob present in test 2. Airflows measured at 

the Bleeder 1 and 2 sites were quite similar to those measured 

in test 1: 29.2 m3/s (61,800 cfm) versus 24.3 m3/s (51,500 

cfm) at Bleeder 1; 13.2 m3/s (28,000 cfm) versus 11.8 m3/s 

(25,000 cfm) at Bleeder 2 (Tables 2 and 3). The proportions 

of tracer gas recovered at the Bleeder 1 and 2 monitoring sites 

were 23% and 3%, respectively, showing a greater volume 

of the air being directed to the outer bleeders. However, the 

percentages of intake air reaching the Bleeder 1 and Bleeder 

2 locations were less for test 2, despite a higher proportion of 

the air at the headgate corner being directed inby towards the 

bleeder. The tracer gas slug measured at the bleeder fan did 

not display multiple peaks representative of the differing flow 

rates and migration paths observed in test 1.

Estimates were made of the face air distribution and flows at 

the tailgate T-junction. Airflow quantities of 28.7 m3/s (60,800 

cfm) and 9.34 m3/s (19,800 cfm) were measured on the face 

just prior to the tailgate T-junction at shield 186 and at the 

tailgate survey site, respectively, suggesting that the quantity 

of airflow inby the T-junction was about 19.3 m3/s (41,000 

cfm). This value produced an inby:outby flow distribution at 

the tailgate corner of about 2.1:1. This estimate is close to the 

previous T-junction flow distribution from test 1.

 Test 3. The third tracer gas test was conducted after panel 

3 had been completed and the longwall face on panel 4 had 

retreated to the position shown in Fig. 5. The test 3 release 

was 2.99 moles or 69.1 L (2.45 cu ft) of tracer gas over a 

three-minute period. The former headgate sample lines were 

now positioned on the tailgate side of the panel and were 

designated TG1 to TG4. None of the former tailgate gateroad 

sampling sites from tests 1 and 2 were included in test 3. During pretest preparations, it was noted that some of the tubing 

lines that had been in panel 3 tailgate number 3 entry had been 

compromised during mining on panels 3 and 4. Preliminary 

findings suggested the repair of the tubing was not successful. 

Consequently, the number of underground sampling locations 

included for analysis was reduced. The release location was 

modified from that used in prior tests and the tracer gas release 

point was relocated just inby shield 20 on the panel 4 longwall 

face. The bleeder monitoring locations were modified from 

tests 1 and 2 to include an additional bleeder sampling location to capture any tracer gas that migrated down the former 

gateroad between panels 2 and 3 to the inner bleeder pathway 

(Fig. 5). Surface monitoring locations included the bleeder 

fan and operational GGVs 2-1, 2-3, 3-1 and 3-2.

Due to the change in the test layout, the most desirable site 

to determine the amount of recovered tracer gas was at the 

bleeder fan. At this location, 56.6 L (2.00 cu ft) of SF6 was 

captured, representing 82% of the released gas. Initially, tracer 

gas recovery at the bleeder fan site was thought to account for 

all of the released gas, suggesting that none of the gas moved 

out of the multipanel longwall section towards the submains 

and the mine operator’s primary ventilation fans. A ventilation pathway out of the test study panel was later confirmed 

by mine management, which accounts for the relatively low 

quantity of recovered tracer gas. 

Two ventilation parameters were of primary interest for 

test 3. The first was a measurement of airflow rates between 

longwall gobs. Although damage to the tubing lines had made 

direct measurements of this flow rate impossible, estimates 

were made. Airflows on the panel 4 longwall face measured 

35.4 m3/s (75,000 cfm) at shield 19 and 31.6 m3/s (67,000 

cfm) at shield 173 during test 3. The airflow just outby the 

panel 4 tailgate corner measured 15.9 m3/s (33,600 cfm). The 

remainder of the airflow, about 15.7 m3/s (33,400 cfm) was 

directed inby in the number 2 and 3 entries. At the panel 4 

bleeder locations (corresponding to the bleeder locations from 

tests 1 and 2), flow rates were 5.62 and 4.25 m3/s (11,900 and 

9,000 cfm) in the outer and inner bleeders, respectively (Fig. 

5). Using the estimate for the tailgate gateroad flow rate from 

the face towards the panel 4 bleeders, it was determined that 

23.3 L (0.822 cu ft) was captured, a quantity that is somewhat 

higher than expected, since it accounts for about one third of 

the released volume of tracer gas. Air passing behind the shield 

line may have contributed to errors in airflow measurements at 

shield 173. Also, the TG3 site was not located at the T-junction 

corner and airflow rates certainly dropped moving inby the 

face such that airflow quantities at TG3 were not equivalent 

to airflows at the T-junction. 

The final ventilation parameter reviewed for test 3 was the 

distribution of airflow at the panel 4 T-junction. Although the 

flow estimate at TG3 was high, it was likely more valid for a 

location nearer the longwall face T-junction. If we accept the 

estimate of 15.8 m3/s (33,400 cfm) of air flowing inby from 

this location, then the inby:outby airflow rates were roughly 

15.8 m3/s /15.9 m3/s (33,400 cfm/33,600 cfm) or essentially 

1:1. This distribution of airflow results in far less dilution 

at the inby tailgate corner in test 3 than in the prior tests, a 

critical area for methane emission management. If the inby 

flow estimate is too high, the inby:outby flow would be more 

directed towards the submains and away from the gob. 

Figure 6a shows a plot of SF6 concentration versus time for 

the bleeder fan site during test 3. Essentially, a single peak was 

formed by the migration of tracer gas towards the bleeder fan 

monitoring site. Five different pathways through the gateroads 

and former gateroads to the bleeder fan have been identified, 

but they failed to produce any identifiable separate peaks. For 

comparison, Figure 6b shows the same methane concentration plot for test 1 when three migration pathways produced 

distinctive peaks on the plot. In test 1, the very different flow 

rates at Bleeders 1 and 2 (Table 2) produced distinct pulses of 

tracer gas migrating to the bleeder fan. The fastest pathway 

was the air split, which crossed the longwall face and then 

split through the Bleeder 1 and 2 locations as multiple small 

peaks. The next group of peaks represents the airflow, which 

moved inby the longwall face headgate corner, split between 

the set-up room and back bleeder, then split again at the Bleeder 

1 and 2 locations, creating the series of peaks shown in the 

figure. The late-arriving, dilute group of three peaks may be 

related to tracer gas that moved into the gob, migrated back 

into the gateroad and eventually reached the bleeder fan. In 

test 3, the gob was much more extensive than in test 1. This 

increased migration distance for intake air traveling inby the 

longwall face, combined with the greater gob surface area, 

may have negated initial differences in airflow rates for the 

differing pathways of air movement to form a single peak of 

tracer gas accumulation. This interpretation implies that more 

extensive longwall gobs tend to modify and dilute independent 

ventilation air masses. 

Summary and conclusions

A series of tracer gas tests were implemented in a cooperative research effort between NIOSH’s Office of Mine Safety 
and Health Research and a southwestern Pennsylvania coal 

mine operator. The tests were designed to identify airflow 

characteristics and patterns in longwall panel ventilation networks, especially where flows affected or contributed to air 

movement in the bleeders. The tests evaluated three phases 

of longwall extraction: early panel, late panel and completed 

panel. Tracer gas releases were made near the mouth of the 

headgate into intake air when the study panel was active and 

on the longwall face of the next panel in the set once the study 

panel was completed. Tracer gas sampling was conducted at 

multiple headgate and tailgate locations, at the surface GGVs 

and at the bleeder fan. No tracer gas was recovered from any 

of the GGVs, although slow gas migration rates through the 

fractured gob and intermittent gas sampling at the surface may 

have contributed to this result. 

Airflow computations were made at sampling locations 

around the panel and were reported using the results of the 

tracer gas tests. Tracer gas recoveries from the tests ranged from 

about 82% to 106% of the total released gas. The distribution 

of tracer gas in the bleeders accounted for 26% to 36% of the 

released gas distributed at these monitoring locations while the 

study panel was active. The lower proportion of ventilation air 

recovered in the bleeders was associated with the late phase 

of panel extraction. Air distributions were estimated at the 

active panel tailgate T-junctions. On the active study panel, 

the ratios of airflow quantities inby and outby the longwall 

faces were roughly 2:1 with about 9.34 to 12.3 m3/s (19,800 

to 26,000 cfm) of air flowing outby. Once the study panel was 

completed and the next and final panel in the set was being 

mined, the air distribution at the tailgate corner was roughly 

1:1 inby:outby with about 15.8 m3/s (33,400 cfm) flowing 

outby. Airflow patterns in the gateroads outby the longwall 

face and along the face did not change greatly throughout 

the monitored cycle of panel extraction. The loss of intake 

airflow inby the face and into the gob appears to greatly affect 

the quantity of intake ventilation air that reaches the bleeder 

locations. Interaction of this ventilation air with the gob is 

also suggested by the loss of independent air masses reaching 

the bleeder fan that is observed at the earlier phase of panel 

extraction. Controlling airflow at the T-junction is a critical 

safety parameter and this study suggests that the inby:outby 

flow distribution is not consistent throughout the monitored 

cycle of panel extraction. Data from this study implies that 

the number of longwall gobs present may be a more important 

influence on T-junction air distribution than the distance from 

the face to the bleeder fan.

The additional roof support installed in the gateroad of 

panel 1 is thought to have produced increased air resistance 

for ventilation air in test 3 compared to the prior two tests. 

This is the most likely cause of the change in inby:outby air 

distribution on the active face T-junction. The decreased proportion of ventilation air flowing inby in this scenario can be 

more problematic in the case of a gassy coal bed or a frictional 

ignition event. The 2:1 inby:outby air distribution at the tailgate 

T-junction that was in effect for tests 1 and 2 is the preferred 

ventilation design. This distribution should be effective at 

keeping the tailgate corner free of gob gas incursions, thereby 

diminishing the potential of frictional ignitions in this region.

Disclaimer

The findings and conclusions in this report are those of 

the authors and do not necessarily represent the views of the 

National Institute for Occupational Safety and Health.

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