9
CONCLUSIONS AND DISCUSSIONS
Barometric pressure at the mine changes on a daily bases as
a function of changes in surface air temperature. These two
variables are inversely related. Depending on the weather
conditions, changes in barometric pressure can be large
enough to change leakage flow direction of air and explo-
sive gases across the seals.
The isolation seals used at the mine are not fully air-
tight. Depending on the weather conditions, they will allow
some ingress of fresh air into the gob or egress of explosive
gases into the work environment. For example, when a seal
was evaluated on August 31, 2021, it was observed that an
increase of 1016 Pa) (0.30 in Hg) in barometric pressure,
increased the pressure differential across the seal (P3-P2)
from –50 Pa (–0.2 in wg) to 425 Pa (1.7 in wg), creating an
out-gassing condition.
As part of this research, two pressure chambers were
established at the mine: one in a mined-out area, and
another near an active longwall panel. In both cases,
Kennedy stoppings were installed to establish a chamber.
When the chambers were pressurized with nitrogen, leak-
age through the stopping became a problem, particularly in
the 1st chamber. To overcome the problem, the stopping
was reinforced and sealed. This reduced the leakage, but
was not sufficient to hold pressure differentials greater than
500 Pa (2 in wg).
Based on the lessons learned from the 1st chamber, the
stopping for the 2nd chamber was constructed in compe-
tent ground at about 3 m (10 ft) in front of a permanent
seal, and 8.5 m (28 ft) from the nearest intersection. Under
these conditions, the stopping held pressure differentials
up to 800 Pa (3.2 in wg). Beyond this pressure, leakage
through the access doors’ gasket was detected. To overcome
the problem, the door design was modified to include a
“bolt-on” rod assembly to swing the door against the frame
when the chamber is closed.
When the access doors of the 2nd chamber were
reversed, the stopping was able to withstand pressure dif-
ferentials in the range of 1250–1500 Pa (5.0 to 6.0 in wg).
Under these conditions, leakage around the doors was
reduced significantly. Nitrogen leaked into the entry more
through the chamber’s hairline fissures (rib &back) rather
than the Kennedy stopping.
Pressure chambers can be used to reduce the risk of self-
heating of coal by reducing the ingress of fresh air into the
gob. However, this requires a well- engineered design and
construction of pressure chambers. The design must include:
• Proper site selection for the chamber
• An MSHA approved stopping design
• Access doors designed to open inward into the cham-
ber when opening the chamber
• A reliable nitrogen injection and monitoring system,
and
• A safe operating procedure to open and close the
access doors, to purge the chamber after each test,
and to operate the chamber.
To uphold high pressure differentials across the cham-
ber walls and to minimize the ingress of air into the gob, a
continuous flow of nitrogen into the chamber is required.
Based on barometric pressure variations measured at the
mine, a continuous nitrogen flow of 0.02 m3/s (40 cfm)
at about 10 kPa (3 psi) gage pressure at the injection point
would be sufficient to maintain a positive pressure in the
chamber.
DISCLOSURE
This study was sponsored by the Alpha Foundation for the
Improvement of Mine Safety and Health, Inc. The views,
opinions, and recommendations expressed herein are solely
those of the authors and do not imply any endorsement by
the ALPHA FOUNDATION, its directors, and its staff.
REFERENCES
[1] Bessinger, S.L., Abrahamse, J.M., Bahe, K.A. &
Palm, A.T. 2005. Nitrogen inertization at San Juan
Coal Company’s longwall operation. SME Annual
Meeting, Salt Lake City, UT.
[2] Calizaya F., Nelson M. G., Bateman, C., and Jha.
[3] 2016. Pressure Balancing Techniques to Control
Spontaneous combustion. SME Annual Meeting,
Preprint 16-050. Phoenix, AZ.
[4] Chalmers, D.R. 2008. Sealing design. Proceedings
of the 12th North American/US Mine Ventilation
Symposium, Reno, NV: University of Nevada.
pp. 219–223.
[5] Grubb, J.W., 2008. Preventative Measures of
Spontaneous Combustion in Underground Coal
Mines. Ph.D. dissertation, Colorado School of
Mines, Golden, CO.
[6] Ray, S.K. &Singh, R.P. 2007. Recent Developments
and Practices to Control Fire in Underground Coal
Mines. Fire Technology (CMRI Dhanbad, India).
43:285–300.
[7] Smith, A.C. &Lazzara, C.P. 1987. Spontaneous
Combustion Studies of U.S. Coals, USBM RI 9079.
[8] Timko R.J. &Derick L. 1995. Detection and Control
of Spontaneous Heating in Coal Mine Pillars-A Case
Study. RI 9553.
CONCLUSIONS AND DISCUSSIONS
Barometric pressure at the mine changes on a daily bases as
a function of changes in surface air temperature. These two
variables are inversely related. Depending on the weather
conditions, changes in barometric pressure can be large
enough to change leakage flow direction of air and explo-
sive gases across the seals.
The isolation seals used at the mine are not fully air-
tight. Depending on the weather conditions, they will allow
some ingress of fresh air into the gob or egress of explosive
gases into the work environment. For example, when a seal
was evaluated on August 31, 2021, it was observed that an
increase of 1016 Pa) (0.30 in Hg) in barometric pressure,
increased the pressure differential across the seal (P3-P2)
from –50 Pa (–0.2 in wg) to 425 Pa (1.7 in wg), creating an
out-gassing condition.
As part of this research, two pressure chambers were
established at the mine: one in a mined-out area, and
another near an active longwall panel. In both cases,
Kennedy stoppings were installed to establish a chamber.
When the chambers were pressurized with nitrogen, leak-
age through the stopping became a problem, particularly in
the 1st chamber. To overcome the problem, the stopping
was reinforced and sealed. This reduced the leakage, but
was not sufficient to hold pressure differentials greater than
500 Pa (2 in wg).
Based on the lessons learned from the 1st chamber, the
stopping for the 2nd chamber was constructed in compe-
tent ground at about 3 m (10 ft) in front of a permanent
seal, and 8.5 m (28 ft) from the nearest intersection. Under
these conditions, the stopping held pressure differentials
up to 800 Pa (3.2 in wg). Beyond this pressure, leakage
through the access doors’ gasket was detected. To overcome
the problem, the door design was modified to include a
“bolt-on” rod assembly to swing the door against the frame
when the chamber is closed.
When the access doors of the 2nd chamber were
reversed, the stopping was able to withstand pressure dif-
ferentials in the range of 1250–1500 Pa (5.0 to 6.0 in wg).
Under these conditions, leakage around the doors was
reduced significantly. Nitrogen leaked into the entry more
through the chamber’s hairline fissures (rib &back) rather
than the Kennedy stopping.
Pressure chambers can be used to reduce the risk of self-
heating of coal by reducing the ingress of fresh air into the
gob. However, this requires a well- engineered design and
construction of pressure chambers. The design must include:
• Proper site selection for the chamber
• An MSHA approved stopping design
• Access doors designed to open inward into the cham-
ber when opening the chamber
• A reliable nitrogen injection and monitoring system,
and
• A safe operating procedure to open and close the
access doors, to purge the chamber after each test,
and to operate the chamber.
To uphold high pressure differentials across the cham-
ber walls and to minimize the ingress of air into the gob, a
continuous flow of nitrogen into the chamber is required.
Based on barometric pressure variations measured at the
mine, a continuous nitrogen flow of 0.02 m3/s (40 cfm)
at about 10 kPa (3 psi) gage pressure at the injection point
would be sufficient to maintain a positive pressure in the
chamber.
DISCLOSURE
This study was sponsored by the Alpha Foundation for the
Improvement of Mine Safety and Health, Inc. The views,
opinions, and recommendations expressed herein are solely
those of the authors and do not imply any endorsement by
the ALPHA FOUNDATION, its directors, and its staff.
REFERENCES
[1] Bessinger, S.L., Abrahamse, J.M., Bahe, K.A. &
Palm, A.T. 2005. Nitrogen inertization at San Juan
Coal Company’s longwall operation. SME Annual
Meeting, Salt Lake City, UT.
[2] Calizaya F., Nelson M. G., Bateman, C., and Jha.
[3] 2016. Pressure Balancing Techniques to Control
Spontaneous combustion. SME Annual Meeting,
Preprint 16-050. Phoenix, AZ.
[4] Chalmers, D.R. 2008. Sealing design. Proceedings
of the 12th North American/US Mine Ventilation
Symposium, Reno, NV: University of Nevada.
pp. 219–223.
[5] Grubb, J.W., 2008. Preventative Measures of
Spontaneous Combustion in Underground Coal
Mines. Ph.D. dissertation, Colorado School of
Mines, Golden, CO.
[6] Ray, S.K. &Singh, R.P. 2007. Recent Developments
and Practices to Control Fire in Underground Coal
Mines. Fire Technology (CMRI Dhanbad, India).
43:285–300.
[7] Smith, A.C. &Lazzara, C.P. 1987. Spontaneous
Combustion Studies of U.S. Coals, USBM RI 9079.
[8] Timko R.J. &Derick L. 1995. Detection and Control
of Spontaneous Heating in Coal Mine Pillars-A Case
Study. RI 9553.