Page 323 - Advanced Mine Ventilation

300 Advanced Mine Ventilation

18.1 Construction of Pipeline

A gas pipeline in underground coal mines is fraught with many dangers as listed
below:

1. Roof and rib falls damaging pipeline.
2. Floor heaving and movement of pipeline.
3. Electrochemical corrosion (steel pipes only).
4. Leakage from bad fittings or improper assembly.
5. Water removal from pipes.
6. Dangers from mobile mine equipment that can crush the pipeline.
A good design should protect the pipeline from all above and other unknown
dangers.

18.1.1 Pipeline Material
Mild steel pipes and casing (externally flush) were commonly used since 1940s. They
suffer from many drawbacks. Some of them are listed below:
1. Very heavy in large sizes (above 5 in diameter) and liable to hurt workers.
00
2. Must be on floor and prone to get damaged by both floor heaving and roof fall. The pipes,
often, need wooden support to keep them in alignment.
3. In mines, using DC current for haulage, the steel pipes are corroded by ground current. They
need cathodic protection in most cases.
4. They are also vulnerable to chemical corrosion inside the pipeline.
During 1970s, the author first replaced steel pipes by high-density polyethylene
(HDPE) pipes that had many advantages as listed below:

1. It is lighter than steel and can be supplied in 20e200 ft length depending on the diameter. A
crew of four can easily lay more than 1000 ft in a shift of 8 h compared with about 300 ft for
heavy steel pipes.
2. It is noncorrosive. It does not need any cathodic protection.
3. It cannot be punctured by roof falls even if it is closed shut by the falling rocks.
4. It is nonconductive and immune to ground current.
5. It can be laid on floor or preferably hung from the roof.
Table 18.1 shows the specifications for some HDPE pipes. Early designs of plastic
pipes, such as Aldyl-A by DuPont, were discarded in favor of new HDPE pipes.

18.1.2 Fitting/Couplings on the Pipeline

Couplings on a 20e30 ft long joint are fused onto the pipe by “thermal butt fusion”
machines. They melt the pipe and flange and fuse them together. Back up steel flanges
are used to bolt the two joints of plastic pipe together. Small diameter pipes, such as 3 00
pipes, come in a coiled length of 220 ft and can be laid much faster.