Page 332 - Advanced Mine Ventilation

Gas Transport in Underground Coal Mines 309

Cathodic protection is a widely used means in the gas industry as a safeguard
against corrosion. Basically, it can be applied by imposing a voltage on the pipeline
by an independent electrical source or by connecting the pipeline to a buried metal
which is higher in the galvanic series than the metal of the pipeline. For steel lines,
zinc or magnesium rods are commonly used. The rods are buried alongside the pipe-
line and bonded to it. Zinc ribbons are often used by laying them alongside the pipeline
when it is buried. For coal mine applications, buried rods or zinc ribbons bonded to the
line will probably be the most practical application.

18.6 Compressors

If the drained gas is to be sold commercially, a compressor will be necessary to put the
gas into the commercial lines. However, transporting the gas to the surface will depend
on the size and length of the pipelines and the in situ gas pressures. The gas pressures
in the coal seams have been sufficient to allow methane drainage without the need for
compressors. However, when the gas is commercially marketed, compressors can be
used on surface but the underground pipeline should not be below atmospheric
pressure.
Main reasons against installing compressors underground are

1. Any oil-lubricated compressor can catch fire or cause an explosion. Auto ignition is possible
because the flash point of the lubricant is 450 F. This temperature can be exceeded if there is

(a) loss of coolant water or (b) the discharge pipe becomes blocked. This can be prevented
only if all safety devices and switches are properly installed and functioning.
2. Any oil-lubricated compressor can tolerate not more than 130 mg of H 2 S in 100 cubic feet of
gas.
3. Any oil-lubricated compressor can neither tolerate slugs of water nor can it tolerate pipe scale
or other abrasive material.
4. Any oil-lubricated compressor is adversely affected by dust; the dust and oil forms an abra-
sive emulsion that attacks the vanes and housing which have close tolerances.
5. Cavitation or overheating can destroy a compressor should it continue to operate without gas.
6. At pressure below atmospheric pressure, oxygen is drawn into the line, and explosive con-
dition could develop in the compressor.
7. A negative pressure in the pipeline also makes all water traps inoperative, and the pipeline
gets filled up with water stopping all gas production.

18.7 Surface Discharge of Gas

When the gas is not used commercially and discharged on surface, certain safety mea-
00
sures become necessary. Typically a 4 diameter stack is used. The safety measures are
listed below:
1. The vent stack must be about 20 ft high above the ground and the top 10 ft should be noncon-
ductive, such as, PVC.