Page 326 - Advanced Mine Ventilation

Gas Transport in Underground Coal Mines 303

18.2.1 Instrumentation for Detecting Leaks and Ruptures

There are many ways to detect leaks. Most common tools are discussed below.

18.2.1.1 Methane Detectors
In addition to the commonly used hand-held methanometers, recording methanometers
with a variety of operating ranges are also available and in use in the mining industry.
It may be necessary to use two sensing heads, one downstream and one upstream of a
monitored point, to indicate small gas influxes into the airstream. If massive influxes of
methane occur, a single detector, or a series of detectors, downstream of the source
may be sufficient.
18.2.1.2 Sonic Leak Detectors
An intrinsically safe detector is available to detect pipeline leaks at flow pressures
greater than 1 psig by means of the change in sound level caused by gas escaping
from a pipeline. Two systems using this principle are available. The first is a hand-
held receiver which can be used during pipe inspections. The instrument is now being
used by the gas industry and by telephone and electric power companies to detect leaks
in pressurized overhead transmission lines. However, its potential for testing mine
degasification pipelines is questionable at flow pressures less than 1 psig.
The second type of system is basically the same, except that an audio tone is
imparted to the gas stream in the pipeline by an installed speaker. Leaks are then easier
to detect with the receiving element. This setup may be suited for use as an automatic
detection system whereby the speaker and detector elements would be permanently
installed, the detector relaying a signal when a change in the audio tone is observed.

18.2.1.3 Oxygen Detectors
If the pipeline is operated under pressure less than atmospheric, breaks in the line would
result in oxygen (mine air) being introduced into the line. Also, there is reason to expect
that oxygen may be introduced into the gas stream by migration through the coal rib into
the gas well. If oxygen is introduced into the line, an explosive mixture in the line could
occur. And if the line is connected to a compressor, serious fire or explosion hazards can
manifest. An oxygen analyzer in the line is one method of alerting mine personnel of
such leaks. Such analyzers are commercially available and could be used.

18.3 Other Preventive Measures for Safe Gas Transport

18.3.1 Automatic Shut-Off Valves
If a massive rupture of the pipeline occurs, it is imperative that the gas wells feeding
gas into the pipeline be shut off. If this is not done, and the gas continues to flow un-
interrupted from the ruptured line, the mine entries can be filled with an explosive gas
mixture. Automatic shut-off valves are commonly used. Basically, the system involves
the installation of an automatic (fail closed) valve as close as possible to the gas well.