Page 329 - Advanced Mine Ventilation

306 Advanced Mine Ventilation

18.3.4.2 Discharge From the Pipeline

When the gas from the methane drainage boreholes cannot be marketed, it is dis-
charged into the atmosphere. Gas-into-air discharge practices in German coal mines
are summarized below.
At German mines, gas may be discharged underground or on the surface. Under-
ground discharge is in a “dilution area” where the methane content is reduced to
less than 1% and then mixed into the return air ventilation current. The “dilution
area” is a fenced-in zone where the pipelined gas is discharged into metal tubing on
the exhaust of an auxiliary blower fan. On the surface, the gas is discharged into a
chimney from whence it is released into the air.

18.4 Ventilation

Adequate ventilation in the airways with gas pipeline is the last line of defense against
a potential explosive mixture. Accordingly, the amount of ventilation that should be
provided is directly related to the amount of gas flowing in the pipeline. Typically,
the quantity of air moving over the pipeline is sufficient to dilute all of the gas that
could be released by rupture of the pipeline without causing accumulation of over
1% methane in the air current. For example, an air quantity of 70,000 CFM would
be needed to dilute a flow of 1 MMCFD in the pipeline. Automatic shut-off valves
reduce the gas flow very quickly. If the ventilation air is not adequate, methane mon-
itors and alarms are needed to monitor the situation.

18.5 Corrosion of Steel Pipelines for Methane Drainage

In some mines, steel pipes are still used for gas transport. Damaging corrosion can be
expected in metallic pipelines that remain in use for a long period. The definition of
“long” depends on the environment, the type of metal, and on the adequacy of the anti-
corrosion measures taken. It may range from days to years. The minimum require-
ments for the protection of metallic pipelines from external, internal, and
atmospheric corrosion are available from the natural gas industry.
The gas industry has learned that whenever the surface transmission lines approach
coal mining operations, the corrosion problem is accentuated and increased and pro-
tective measures have to be taken. The reason for this is that stray electrical currents
are common near and in coal mines utilizing trolley haulage systems, waterlines,
and communication systems. It is a fact that stray electrical currents will cause corro-
sion problems in underground degasification systems. In addition to the stray currents,
the coal mine water may also be more conducive to corrosion than water in other
locations.
Broadly defined, corrosion is an electrochemical reaction between a metal and its
environment. The result of that reaction is the deterioration of the metal. Most metals