mining application, the active
suppression system can only be
mounted on the auger outside the
production hole, thus far away from
the cutter head. As the cutter head is
most likely to be the only ignition
source and the auger hole the only
roadway for the ignition to propagate
in, it can be expected that the explosion
would exit the auger hole in due
course. This is, however, dependent on
the amount of fuel and oxygen
available and on the amount of
confinement achieved by the number
of auger flights trailing the cutter head.
The Kloppersbos explosion tunnel is
10 m long, with a 2 m dia. It is raised
700 mm above a cement floor with one
side sealed off by a steel plate. During
a suppressed explosion, the steel plate
acts as an emergency pressure-release
mechanism in the event of very high
pressures building up in the tunnel.
In one test a fuse cap was used to
ignite the methane/air mixture. In all
the other tests, a shielded detonator
was used to ignite the flammable gas
mixture. The fuse cap was initially
chosen for use because it produced a
flame that would not be seen or
recognised by the triggering
mechanism of the suppression system.
The detonator was shielded from
the triggering system; the reason for
using it was to create a more violent
methane explosion. The chamber
containing 23 m
3
of methane/air
mixture was obtained by placing a
plastic membrane 7 m from the closed
end of the tunnel. This amount of
methane/air mixture will produce
enough wind pressure to lift the coal
dust into the air, supply sufficient heat
to the coal dust particles for flame
propagation to take place and be
sufficient to ensure flame growth up to
5 m beyond the tunnel mouth.
For most of the active suppression
system tests conducted in the 10 m test
tunnel, an explosion mixture of 9%
methane/air per volume was used.
This was done to test the triggered
barrier system under simulated
worst-case scenarios. A small amount
of coal dust was placed on racks at the
open end of the 10 m tunnel.
The measure of success was defined
to indicate whether the flame
propagation was stopped inside the
tunnel opening (referred to as
“stopped inside”). The results of the
methane explosion tests are shown in
Table 1.
In every test, the flame was
stopped at the tunnel opening, with no
flame visible from the front and
perpendicular to the tunnel. Each
explosion was further captured on
camera and a record of the photos
and the video material was used to
evaluate the success of the flame
suppression.
The ExploSpot system registered a
methane ignition and opened the
extinguishing cylinders within 20 msec.
The suppressing agent sealed off the
tunnel opening completely within
30 msec, preventing the flame from
penetrating the suppressant material.
During these tests the suppressing agent
was initially dispersed at high pressure
(stored at 60 MPa in the cylinder) and
velocity into the propagating flame
front. From the tests conducted, it was
concluded that the auger
machine‑mounted ExploSpot system
was successful in stopping a methane
flame in the tunnel opening.
20 m test tunnel
A 20 m test tunnel was erected at the
Kloppersbos Research Facility to suit
the double-pass mining method
associated with the use of a continuous
miner. The tunnel was modified to
simulate the dimensions of mine
workings of medium seam height. A
full-scale model of the Joy 12HM9
continuous miner was used for test
purposes. The test tunnel is 20 m long
and 7 m wide, with a variable height of
2 – 6 m. It has a cement floor and
springs along both sides, on the outside,
supporting and guiding it. For the case
of an unsuppressed explosion, the
tunnel is able to lift up to 140 mm off its
base to provide an alternative escape
route for the expanding gases. For the
full-face conditions, the cross-sectional
area was approximately 21 m
2
.
The test sequence was carried out in
order of ascending difficulty. Three
main placements of the machine inside
the tunnel were tested, as well as
sub-conditions for the placements of the
boom (and thus ignition) and, of course,
the various methane concentrations.
The testing began with the 9%
methane/air explosion for full-face
Table 1: Results of the performance of the ExploSpot system
Test
Methane concentration (%) Initiator used
Visible flame
Comments
1
7.5
Detonator
None
One cylinder failed, flame stopped successfully
2
9
Detonator
None
Flame stopped successfully
3
9
Detonator
None
Flame stopped successfully
4
9
Fuse cap
None
Flame stopped successfully
5
12
Detonator
None
Flame stopped successfully
6
9
Detonator
None
Flame stopped successfully. Small flame could
be seen at the back of the tunnel.
Table 2: Full-face active suppression test results
Test
Methane concentration (%) Flame length (m)
Temperature increase (˚C)
77
9
6
80
78
9
6
no temperature rise
79
9
6
93
80
9
6
70
81
9
6
no temperature rise
82
12
5
58
44
|
World Coal
|
September 2014
