Sidebilder
PDF
ePub

are fired electrically. For example, in a certain mine using permissible explosives the yield of lump coal was increased 20 per cent when the rib holes were fired by electricity at the same instant. Wet holes can be fired more easily by electricity than by any other method. When electric firing is used the shots can be exploded while the person who is firing the shots is at a safe distance; moreover, the explosion follows immediately after the lever or switch has been thrown, and there is no waiting, as there is in the other methods of firing, such as in the burning of a fuse or squib. With electricity, misfires and hangfires are rare. When fine coal dust is present in a bituminous mine, or explosive gas in any mine, no other method than electric firing should be employed, because squibs and fuse yield sparks that will ignite gas or dust and cause explosions and because they must be ignited in the open air by open lights.

The shots may be exploded singly with a magneto machine or with a safety-contact dry-cell battery while the men are in the mine, or they may be all exploded at the same time by an electric current from a generator or dynamo after all the men have left the mine. Delayed fires are very rare when electricity is used.

DEFECTIVE INSULATION.

When firing permissible explosives electrically, misfires are probably caused most often by defective insulation or improper connections between the detonator legs and the lead wires. If some of the insulation is rubbed off the detonator legs near the detonator, the current is likely to be short-circuited through the explosive and to set fire to the explosive before the detonator explodes. A common trouble is the insulation on the legs being rubbed off with the tamping bar or with sharp particles in the tamping. Another trouble is the loss of insulation on the lead wires.

When a misfire occurs the wires should be immediately disconnected from the battery or firing machine and carefully examined for defective insulation and places where the current may be shortcircuited. When the system of firing shots from without the mine is employed, inequalities in the roof may cut through the insulation of the wires that run along near the roof, especially in thin coal beds, and cause grounds. Careful search is often required to find these grounds.

DEFECTIVE CONNECTIONS.

In making connections the insulation should be carefully scraped off the legs and leading wires until the wires are bright; the ends should then be firmly twisted together. Merely looping them together is not sufficient.

ELECTRIC FUSES.

Electric fuses having iron in place of copper wires have been in- · troduced recently. Although they are cheaper, they are not as satisfactory for all purposes, for iron is not as good a conductor of electricity as copper, and therefore a stronger current is required to explode this type of electric detonator.

FROZEN EXPLOSIVES.

High explosives are very sensitive at a temperature a few degrees above their freezing point, especially if they have just been thawed. For this reason care should be exercised in using explosives that have been frozen and then thawed.

The attempt to fire frozen permissible explosives is a frequent cause of accidents. Because many of these explosives freeze easily it is difficult to have them always ready for use and the miner is therefore apt to use partly frozen explosives. When so used parts of the charge often fail to detonate, they are either set on fire or thrown down with the burden, to become a menace to the miner loading the coal or to the force in the breaker or to the consumer. Frozen high explosives may be set off by friction on the side of the hole or by coming in contact with a metal tamper; friction will sometimes explode them when a detonator will not. For this reason it is dangerous to break frozen explosives in the hands.

NOXIOUS GASES.

Unless the ventilating current is brought directly to the working face, noxious gases from the blast may gather there. More injurious and dangerous gases are set free when the explosives are not completely detonated, as when a weak detonator has been used or a partly frozen explosive has formed the whole or a part of the charge.

IGNITIONS.

Some permissible explosives can be ignited by the touch of a naked flame. When burning they may explode. Others may burn instead of exploding, if one tries to shoot them while they are frozen. If fired in a hole that is drilled almost through into other workings, they may cause a blown-out shot and set fire to gas or to dust in suspension, because practically they are then fired in the open or without tamping. Again, permissible explosives give off gases that burn, and these may be set on fire if a naked light is brought near them just after the shot has been fired, especially if the light touches the crevices opened in the coal by the shot or blast. The coal also is sometimes set on fire in this way. For these reasons the miner should

stay away from the face for at least 5 minutes after the shot has been fired, by which time the gases that burn will have become so mixed with the air that they can not be set on fire.

DRY-CELL BATTERIES WITHOUT SAFETY CONTACT BUTTONS.

Several accidents have occurred with permissible explosives when dry-cell batteries without safety contact buttons have been used. In some instances the wires were accidentally touched to the binding posts before the men had all left the face and these men were injured by the blast. Recently several men have been injured and a few killed through carelessness in carrying such batteries in sacks with electric detonators and explosives; the contact made between the battery and the detonator wires resulted in an explosion. While showing, a miner how to connect the wires to the battery one man was killed because he accidentally touched the second wire to the second post before he was ready.

The dry-cell battery is light and convenient as well as cheap, and hence is preferred to the magneto firing battery, which is usually heavy and cumbersome and tiresome to carry, but the dry-cell battery should always be provided with some means of preventing an accidental contact of the leading wires with the binding posts, and this is easiest done by the use of a safety contact button. Such batteries are on the market at present and can be obtained from dealers in explosives.

SHOOTING FROM THE TROLLEY WIRE.

In some mines the shots have been fired electrically by means of lead wires thrown over the bare trolley wires of the haulage system. This method is extremely dangerous and should not be permitted. The power on the wires is always much more than is required to fire the shots, there is sure to be an arcing (flashing) of the lead wire, and there is a probability that it will be fused on account of its small size and consequent high resistance. The flame from the arc will then ignite any gas or dust present.

TIME FOR SHOT FIRING.

In some mines permissible explosives are fired while the miners are in the mine, both by the miners and by special shot firers, using fuse, detonating squibs, and electric firing; in other mines the shots are fired with electricity after everyone has left the mine. Opinions differ as to the relative merits of these plans.

Some mine managers and superintendents in whose mines shots are fired while the men are in the mine object to the other method because they say that if, on account of gas and dust, it is unsafe to

fire shots in a certain mine while the men are in the mine, it is also unsafe to fire the shots from the surface while the men are out of the mine, as the shots will ignite the gas or dust in either case. They maintain that the mine should be put in such a condition (dust wet down and removed and gas carried off safely) that explosions will be impossible. They also argue that the simultaneous firing of a large number of shots will be more liable to cause explosions of gas and dust and roof falls than the firing of shots singly when the men are in the mine. To this argument the reply is made that there have been no serious explosions when permissible explosives have been fired from the surface, though at least one instance is recorded of an explosion resulting from firing blackpowder shots from the surface.

When there is no check on the manner in which the holes are drilled and charged, the firing from the surface will prove to be the safest; but when the shots are carefully inspected and charged and the dust and gas properly cared for, there should be no danger in firing shots when the men are in the mine if the proper permissible explosives are used and competent shot firers employed to fire the shots. However, if shots are fired entirely from the surface, after all the men have left the mine, there can be no possible danger to life and little to property if proper precautions are used to prevent the making of connections before the time for firing the shots.

SPECIAL SHOT FIRERS FOR PERMISSIBLE EXPLOSIVES.

It has been found by experience that permissible explosives yield the most satisfactory results, that is, the largest yield of merchantable coal and the maximum of safety, when they are fired by special shot firers. The permissible explosives are so unlike black blasting powder that their properties should be understood by the users. As they are relatively new explosives the average miner has not had an opportunity to learn their nature or how to use them. When allowed to shoot them the miners almost always use fuse and detonator; this does not yield the best results, because the explosives are not as properly detonated as when electric detonators and electric firing apparatus are employed. When these explosives are being tested at the Pittsburgh experiment station they are fired with electric detonators and batteries, and they are considered to have passed the tests only when they have been fired by these means; therefore, they should be fired by these means when used in the mines for, as pointed out in Miners' Circular 6 of the Bureau of Mines, permissible explosives are such only when used as prescribed.

Other reasons why permissible explosives should be fired by shot firers are as follows: If a shot firer is employed the shots will gener

ally be fired by means of the electric battery instead of by fuse and detonator. The shot firer will be apt to see that the explosive is fresh, in proper condition, and properly fitted to the electric detonator; that the detonator is of the right strength; that all connections are properly and safely made; and that the shot is fired only when all persons are away from the face. He will be sure to see that the hole is not run into the solid, for he will measure the charge and fire the shot. He will be sure to see that the hole is not overcharged, for he is not interested in the loading of the coal and hence does not desire to have it broken up. He must wish to have the shot do the work it is planned to do but no more. He will be sure to fire no shot unless the working place is well timbered and will take no risks in going into dangerous working places, as he will not risk himself in places where a miner often goes in a hurry to load a car. When the shot firer fires the shots they will be fired systematically and not intermittently as they are when the miner fires them. There will be no gas ignited as there often is when fuse is used and the miners fire their own shots. Since the shot firer will be a picked, experienced miner, he will not handle the detonators and explosives carelessly, but will use all possible care in taking the explosives into the working places and in preparing the charges, and he will not endanger the lives of others or his own, as miners often do, when making up their own charges or when carrying the detonators or explosives.

The yield of coal per pound of permissible explosive used will be greater, and the coal will be of better quality if permissible explosives are used. The work, if properly done, will tend to improve mine discipline, since every shot will be fired in turn and every miner will know when to expect the shot firer and will accordingly be prepared for him, so that there will be no waiting for holes to be drilled or for stemming to be prepared, but all supplies needed will be ready at the face when the shot firer arrives. What has been said above refers entirely to such mines as those in which the shots are fired by the shot firers while the men are in the mine.

In those mines using permissible explosives where the shots are fired by electric means, after everyone has left the mine, the practice is, of course, somewhat different. As a rule all the shots are carefully inspected by shot firers before they are tamped by the miner. The latter makes the connections to the electric wires at the face of the working place and the entrance to the same, but the shot firer attends to all other details, such as switches, line insulators, etc. The shot firer throws in all switches and fires the shots from the surface. After the shots have been fired he makes an inspection of the entire firing system, and examines all working places for shots that have failed, and removes such defects as have caused shots to misfire.

« ForrigeFortsett »