Ignition of gases & Blown out shot & Misfires

Ignition of gases

The main danger from explosives in U/G coal mine is the ignition of fire damp. It may take place in the following ways.

1. By the flame and hot gases:— Though this is a common cause, because of "lag on ignition", a flame, coming in contact with gas, does not ignite it instantaneously. If the flame is sufficiently cooled during the interval of lag on ignition, which is a fraction of a second, the gas will not ignite. If the heat energy generated during blasting of the explosive is effectively converted into   mechanical work, the explosion products are not hot enough to ignite the gas.

2. By incompletely detonated explosive: — Such explosive may continue, to burn like an ordinary combustible material.

3. By incandescent particles coming out of the shot hole after blasting, if such particles come in contact with coal dust or gas.

4. By the compression wave of the blast which may compress the gases in the cracks connected with the shot hole and raise the temperature of the compressed gas to such an extent as to ignite it. Such 20-fold compression is known to be sufficient to ignite all inflammable mixtures of fire-damp and air. The gas in the breaks and fissures connected with shot hole and leading to a fire damp pocket cannot be easily removed and the presence of breaks and fissures is one of the commonest causes in explosions of fire damp due to shot firing. If any break is found, the shot must not be fired.

Blown out shot

A blown out shot is one which has not done any useful work of blasting coal, but has ejected itself out of the shot hole. An undercharged hole will result in blown out shot. Similarly in an overcharged hole where only part of the charge is utilised in doing the useful work of blasting coal the excess charge may still be burning when the coal is broken, and it has the same effect of igniting gas or coal dust as an undercharged blown out shot.

Common causes of accidents

1. Not taking proper cover. This is the most common cause 01 personal injury due to explosives. It is essential that the shot-firer shall himself take adequate cover and see that all workmen in the vicinity of a shot are removed to a safe place. No place in direct line with a shot can be regarded as safe and every person should be protected by at least one right angled corner. All approaches to the danger zone should be guarded by sentries or otherwise so as to prevent anyone entering inadvertently.

2. Failing to warn persons in an adjoining place into which the blasted rock may be thrown, as is possible when two galleries are about to join and partition is thin.

3. Carelessness in handling detonators causing them to explode or to be lost in a mine.

4. Carelessness whilst charging a hole, e.g.  tamping too forcibly in the neighbourhood of the detonator, or ramming the primer cartridge into a hole of insufficient diameter.

5. Firing a shot when persons are at the shot hole due to instructions being misunderstood, or lack of proper sentries.

6. Returning to the face too early after firing a round of shots, if which is a "hang-fire" (i.e. a delayed ignition), or before authorised to do so by the shot-firer.

7. Dealing with misfired shots otherwise than in the prescribed manner.

Ignition of gas or coal dust is a major accident which may arise from the use of explosives if proper precautions, as laid down in the Regulations, are not taken.

Misfired shot

When a detonator fails to explode, or after exploding fails to blast the charge of the main explosive cartridge, it is known as "misfire".

The reasons for misfire are as follows:—

1. Defective firing exploder.

2. Defective detonator or bad quality explosive either due to bad manufacture or due to deterioration during storage.

3. Broken detonator leads, defective or broken shotfiring cable, bad connections between exploder and cable, between cable and detonator leads or at other places.

4. Short circuit of the cable or detonator leads due to poor or broken insulation.

5. Where fuse blasting is done, it may be due to (i) wet fuse, (ii) improper timing of fuse, so that blasting by one fuse may cut the fuse of another hole, and (iii) the fuse being drawn out during stemming.

To guard against misfire, the cable should be checked before blasting, the exploder should be examined once every three months by competent persons, and only good quality of explosives and detonators, not spoilt during storage, should be used. It is better to have two single-core cables for shot-firing separated by a good distance, instead of a twin core cable, which may be liable to short circuit if the insulation is bad. During use all the connections should be carefully made. For important blasting work, the circuit should be tested for continuity by a galvanometer, described, earlier.

Dealing with a misfired shot underground

If shots are fired with safety fuse, no person should enter the site of blasting for 30 minutes after firing. If elec. detonators are used, this time may be reduced to 5 minutes after cable is disconnected from the exploder.

All the entrances to the place should be fenced.

Another attempt should be made to blast it by making proper connections of the cable, if the misfired shot does not explode, the shot should be dislodged by drilling another relieving hole at least 0.3 m away from the misfired hole and by blasting it. The new hole should be drilled in the presence of the shot-firer who knows the direction of the misfired shot hole, so that during drilling the drill bit does not touch the misfired charge.

After the relieving shot has blasted¹'the rock, a careful search for misfired cartridges and detonators should be made in the presence of the shot-firer in the material brought down by the shot. If the misfired explosive is not traced, the material should be loaded in a separate tub, distinctly marked, for further search on the surface.

The misfired explosive and detonator, when traced should be destroyed on the surface.

Except in the case of coal mines where the Regulations currently in force prohibit the placing of a second charge in a misfired hole, the stemming may be sludged out with compressed air or water under pressure, and the hole re-primed and fired. The stemming should not, however, be scraped out.

In underground coal mine the yield of coal on an undercut face is nearly 5-8, te per kg of explosive and 1-2.5 te per detonator but with solid blasting the figures are 1.8 to 2.7 te per kg of explosive and 0.8 to 1.35 te per detonator.

Some of the slurry explosives of I.D.L. have produced the following results during experiments at a few coal mines on precut face (Pentadyne figures are for solid blasting).

	Coal yield (te)
	Per kg. explosive		per detonator
	Min.	Max.	Min.	Max.
Godye (PI)	4.4	10	1.5	2.6
Permadyne (P3)	3.1	6	0.9	1.80
Pentadyne (P5)	1.87	2.72	0.8	1.37

Maximum shots fired by a shotfirer

The mining Regulations fix the following limits on the number of shots to be fired by a shotfirer.

Underground coal mines

Deg. 1 gassy mines - 50 shots with single-shot exploder and 100 shots with multi shot exploder

Deg. 2 & 3 gassy mines — 40 snots with single-shot and 80 shots with multi shot exploders.

If a mining sirdar or over man is working as a shotfirer also and has more than 30 persons working under his charge, he can fire not more than half the number stated above.

Opencast mines

60 shots with single-shot exploder or with ordinary detonators and 120 shots with multi-shot exploder.

The D G M S can put restrictions on the above or give relaxat