Page 328 - Advanced Mine Ventilation

Gas Transport in Underground Coal Mines 305

Sources for ignition are many. These could be frictional sparks from falling roof or
from an incendive spark producing metal; electrostatic discharges; lightning; a care-
lessly tossed lit cigarette or match; a bullet either from a hunter or someone using
the pipe as a target. Compressors have caught fire and have exploded.
The wire gauze in a flame safety lamp is a common example of a flame barrier. The
purpose of the gauze is to absorb heat and to restrict the propagation of flame beyond it.
There are many other types and designs of barriers; these include crimped metal arres-
tors, molecular seals, hydraulic traps, inert gas filling, and injection systems and de-
vices that, on sensing flame, discharge an extinguishant into the flame path.
Regardless of the type of barrier used, it is obvious that they are needed. Commer-
cially made flame arrestors do a good job.

18.3.4.1 Some Useful Guidelines

1. Every point of discharge of methane from a pipeline into the atmosphere or into a compressor
or pump should be fitted with a flame arrestor.
2. Each device should be securely anchored in an accurate seating so that it cannot be displaced
by an explosion and so that flame cannot propagate past the seating.
3. The pressure drop across the flame arrestor should not be high, and it should be monitored.
Weekly inspection is necessary, and regular cleaning is recommended.
4. Devices should be constructed of materials having ignition temperatures above 1500 F; in

other words, a noncombustible material. The devices should not be constructed of asbestos,
wire gauze, or plate perforated with circular holes. Deformation of asbestos by moisture can
result in failure of asbestos containing parts to function properly and to provide safety.
Compared with other arrestor materials, wire gauzes have a limited effectiveness for quench-
ing flame, are easily damaged, are highly resistant to gas flow, and are readily clogged by dirt
and ice. Similarly, perforated plate arrestors are highly resistant to gas flow and thin plates do
not protect against violent explosions. Crimped metal arrestors are preferred; they have high
resistance to mechanical and thermal shock and low resistance to gas flow.
5. Flame arrestors should be installed within 5 ft but not more than 10 ft from the point of
discharge. The most likely source of ignition will be a compressor, lightning, or flame at a
pipe outlet. Flame velocity increases with distance and with obstacles such as T’s and bends.
Up to 10 ft, flame speed is not affected greatly by obstacles.
6. Each device should contain a supporting system that prevents flow of methane through the
device after flame has been in the device for 60 or more seconds. Continued flow of methane
can result in reignition after extinguishant has been expelled or overheating and damage to an
arrestor.
7. Flame arrestors should be approved by the National Fire Protection Association, Factory
Mutual Research, or Underwriter’s Laboratories. Shop-made devices cannot be relied on
because they are not subjected to tests or quality control and at the critical period may not
perform as expected.
8. Gas may be discharged underground if the point of discharge is in the return and is sur-
rounded by a fence so located that methane content in the air at each part of the fence is
2% or less. Often, a diffusing-type water spray is incorporated to saturate the air to prevent
an ignition.