Natural gas from the mains of the City of Pittsburg is used to represent
that found in the mines by actual analysis. A typical analysis of this
gas is as follows:
Volumetric Analysis of Typical Natural Gas.
Hydrogen gases 0
Carbon dioxide 0.1
Oxygen 0
Heavy hydrocarbons 0
Carbon monoxide 0
Methane 81.8
Ethane 16.8
Nitrogen 1.3
The volume of gas used is measured by an accurate test meter reading to
one-twentieth of a cubic foot. The required amount is admitted near the
bottom, to one or more of the 20-ft. divisions of the gallery, from a
2-in. pipe, 14 ft. long. The pipe has perforations arranged so that an
equal flow of gas is maintained from each unit length.
Each 20-ft. division of the gallery is further equipped with an exterior
circulating system, as shown by Fig. 1, thus providing an efficient
method of mixing the gas with the air. For the first division this
circulating system is stationary, a portion of the piping being equipped
with heating coils for maintaining a constant temperature.
The other divisions have a common circulating system mounted on a truck
which may be used on any of these divisions. Valves are provided for
isolating the fan so that a possible explosion will not injure it.
In the center section of each division is an indicator cock which is
used to provide means of recording pressures above and below
atmospheric, or of sampling the air-and-gas mixture. The first division
of the gallery is equipped with shelves laterally placed, for the
support of coal dust.
The cannon in which the explosive is fired is placed in the concrete
head, the axial line of the bore-hole being coincident with that of the
gallery. This cannon (Fig. 2) is similar to that used in the ballistic
pendulum. The charge is fired electrically from the observation room. To
minimize the risk of loading the cannon, the charger carries in his
pocket the plug of a stage switch (the only plug of its kind on the
ground), so that it is impossible to complete the circuit until the
charger has left the gallery. That portion of the first division of the
gallery which is not embedded in concrete, has a 3-in. covering made up
of blocks of magnesia, asbestos fiber, asbestos, cement, a thin layer of
8-oz. duck, and strips of water-proof roofing paper, the whole being
covered with a thick coat of graphite paint. The object of this covering
is to assist in maintaining a constant temperature.
[Illustration: PLATE VII.
Fig. 1.--Bichel Pressure Gauges.