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Engineering Controls Database

Guidelines for the Control and Monitoring of Methane Gas on Continuous Mining Operations – Measuring Gas Levels Outby the Face – Cap Lamp-mounted Personal Monitor

The introduction of conventional mining methods, which increased the rate of mining, was an important step in the mechanization of mining. The intermittent nature of the conventional mining process halted the extraction process for ore-loading and usually allowed time for methane gas to be dispersed. However, the introduction of continuous mining machines in the 1940s produced a constant flow of ore from the working face of the mine and resulted in an increase in methane levels.

The number of face ignitions increased as more continuous mining machines were placed underground. Methane levels were found to be dangerously high. In some cases, methane concentrations measured 20 ft from the mining face exceeded the lower explosive limit (5% by volume) [USBM 1958]. The need for better face area ventilation was recognized to reduce the potential for explosions.
Excessive levels of methane gas can affect the safety of the underground work force. Available methane control systems have been challenged in recent years by mining developments which include the use of continuous mining machines.

Most mining accidents today generally involve only a few individuals. However, the infrequent occurrence of gas explosions puts the lives of the entire underground workforce at risk. In the past 10 years, explosions have led to 65 fatalities and 18 injuries with major explosions occurring at the Sago Mine in West Virginia in 2006 (12 fatalities and 1 injury), the Darby No. 1 Mine in Kentucky in 2006 (5 fatalities and 1 injury) and, most recently, at the Upper Big Branch Mine in West Virginia in 2010 (29 fatalities) [NIOSH 2011].
The U.S. Bureau of Mines (USBM) was formed in 1910 following a series of underground explosions that resulted in many fatalities and injuries [Kirk 1996]. The agency was responsible for conducting scientific research and disseminating information on the extraction, processing, use, and conservation of mineral resources. The USBM research program for mining health and safety was transferred to NIOSH in 1996. Since that time, NIOSH has established a ventilation test gallery where techniques for methane control and monitoring are evaluated under a variety of conditions that simulate airflow near the working face of a continuous mining section. Airflow patterns and methane concentrations are studied in a detailed manner that is not possible in a working underground mine.

Measuring Gas Levels Outby the Face

Regular monitoring for methane gas is required near faces where methane concentrations are usually highest and the potential for methane ignitions is the greatest. However, methane gas can also accumulate in areas outby the face where methane concentrations are not monitored on a regular basis. It is not practical to place sampling instruments in all areas where miners work and travel. Providing methane monitors that can be worn by the miner is one way to protect workers regardless of their work location.

Cap Lamp-mounted Personal Monitor

A personal monitor that uses a cap lamp-mounted methane sensor was also tested (Figure 1) [Chilton et al. 2003]. The monitor is powered by the cap lamp battery and continuously monitors methane as long as the cap lamp is operating. Audible and visual (buzzer and blinking cap lamp) signals are initiated when methane levels exceed preset limits.
Figure - 1 - Cap lamp-mounted methane monitor.

Figure - 1 - Cap lamp-mounted methane monitor.

Since this monitoring device has no visual readout, a test protocol had to be developed for measuring instrument accuracy. Evaluations were based on the time between application of the calibration gas through a calibration cup to the sensor head and the initiation of the alarm signals. Signal amplification was adjusted using a potentiometer so that the alarm time for 1% calibration gas was between 15 to 25 seconds. When the alarm times were within this range the instrument was considered calibrated.

After calibration with 1% calibration gas, the response of the instrument to other concentrations was determined. Calibration gases (0.6% to 2.5% by volume) were applied to the sensor heads using the calibration cup, and the time for obtaining an alarm signal were recorded (Figure 2).
Figure - 2 - Instrument alarm times (calibration gas 0.6% to 2.5%).

Figure - 2 - Instrument alarm times (calibration gas 0.6% to 2.5%).

• Alarm times increased with decreasing methane concentrations and decreased with increasing concentrations.
• Below a “threshold concentration” (e.g., less than 0.8% methane) alarm times were much longer (greater than 1 minute).

After calibration, two instruments were operated for 10 consecutive workdays. Twice each day, 1% calibration gas was applied to the sensor heads. The resulting response times are shown in Figure 3.
Figure - 3 - Instrument readings for 10 workdays.

Figure - 3 - Instrument readings for 10 workdays.

• One instrument displayed little drift in response time, but time for the second to alarm increased significantly after 3 days. This second instrument was recalibrated after 7 days.
• The cap lamp-mounted methane monitor provides a convenient way to continuously monitor methane levels in all areas where a miner works. However, without a visual readout to show concentrations, checking the instrument performance is more difficult.
• The alarms were difficult to recognize.
o The cap lamps blink rapidly when signaling an alarm. The rapid blink rate can be detected by a second individual. But the person wearing the lamp may not detect the signal, especially if the worker is moving and/or rapidly moving his or her head.
o The aural signal was 60 to 68 dBA at a distance of 1 ft from the instrument. It is unlikely the alarm could be heard when working near mining equipment having background noise levels of 85 dBA or higher.

NOTE: The above control information is taken directly from the following publication:
NIOSH [2010]. Information circular 9523. Guidelines for the control and monitoring of methane gas in continuous mining operations. Morgantown, WV: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2010-141.
Chilton JE, Taylor CD, Timko RJ [2003]. Evaluation of IYONI II methanometers. In: Proceedings of the 30th International Conference of Safety in Mines Research Institutes. Johannesburg, South Africa: South African Institute of Mining and Metallurgy, pp. 615–640.

Kirk WS [1996]. The history of the Bureau of Mines. In: U.S. Bureau of Mines Minerals Yearbook, 1994. Washington, DC: U.S. Bureau of Mines.

NIOSH [2011]. Ventilation and explosion prevention highlights.

USBM [1958]. Auxiliary ventilation of continuous miner places. By Stahl RW. Washington, DC: U.S. Bureau of Mines, Report of Investigations, No. 5414.
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