Page 327 - Advanced Mine Ventilation

304 Advanced Mine Ventilation

The valves are commonly held open by pneumatic pressure supplied from a
1 00
compressor or compressed air tank via a / 4 nylon line. As long as pressure is supplied
to the valve, the valve remains open, allowing gas to flow. In case of a massive roof
fall, the nylon line will be broken first and all boreholes will be shut. This line is
fastened firmly on top of the pipeline in such a manner that the pressure line could
be broken either by a roof fall or by pipe movement. Loss of pressure would then auto-
matically close the valves.

18.3.2 Sectionalization of the Pipeline
The most important safety feature to be incorporated into the pipeline design is auto-
matic shut-off valves. However, even if the gas wells are shut in at the instant of
rupture, a certain volume of gas will be released into the mine if the system is under
positive pressure. The volume released will be related to the operating pressures of
the pipeline, the diameter of the pipe, and the length of the pipeline. For each atmo-
sphere of positive pressure, a volume of gas equal to the volume of the pipeline will
be released. For an 8-inch diameter pipe under 15 psig, the volume released will be
350 cubic feet per 1000 feet of pipe. For all practical purposes, the gas will be released
instantly. The amount of gas that can be safely released will depend on the quantity of
air flowing in the entry into which the gas is released. It is generally agreed that longer
pipelines should be sectionalized by one-way valves in lengths of no more than
1000 ft.
It is unlikely that all gas wells drilled in a panel will operate at the same pressure and
flow rate. Also, design of the piping system will probably require that the gas from
more than one well will be delivered to the main gas transmission line. The condition
may, therefore, develop in which the gas pressure from an inby well may be greater
than from the well(s) outby. This may cause a pressure differential from the main trans-
mission line to the gas well instead of vice versa, and in turn may allow gas to flow into
the coal seam. To guard against this occurrence, one-way check valves are also
installed at the mouth of each degasification borehole.

18.3.3 Water Traps on the Pipeline

Most degasification boreholes produce water with gas. Commercially made water traps
are used at each degasification borehole and at the bottom of the vertical venthole.
They dump water automatically whenever water builds up in the pipeline. These traps
work well when the line pressure is above atmospheric pressure. Water traps are usu-
ally installed at a low point in the pipeline. Draining water is easy if the pipeline is
hung from the roof bolts.

18.3.4 Flame Arrestors
If the flow of gas is high, it must be discharged on surface. If the flow of gas is small, it
can be discharged in the mine airway. A flame arrestor must be used to prevent the
flame from entering the mine if the gas is ignited on surface.