Fire Damp

Fire-damp is composed chiefly of methane with varying percentages of other gases. The fire-damp actually given off by coal or other strata has a composition lying between the following approximate limits

 

Methane from 70 to 98 % Inflammable gas Ethane, propane, etc From 0 to 2 % Ethylene, acetylene, hydrogen sulphide From 0 to a trace Carbon Dioxide From 0 to 4% Uninflammable gas Nitrogen From 0 to 15% Oxygen From 0 to a trace

Occurrence of Fire-Damp

Fire-damp is the natural gas given off by coal and carbonaceous strata in coal mines, or it may occur under great pressure in porous rocks adjacent to a coal seam, where it has been imprisoned by an overlying bed of non-porous rock. In such cases the gas may be stored under enormous pressure both in the coal and the adjacent strata. Seepage, blowers, feeders, and outbursts release this pressure. Seepage is the steady oozing out of gas from coal and other strata through pores or tiny fissures. Blowers or feeders are issues of gas through fissures or holes which often have a hissing sound and frequently cause great accumulations. Explosions or out bursts often occur when coal is being worked containing gas under enormous pressure. As soon as the face is cut or broken great blocks of coal are projected violently from the face, with fatal results to the men working on the face. The gas is also associated with petroleum in oil districts under great pressure.

Fire-damp being a very light gas tends to rise when the rate at which it is being given off is greater than the rate at which it can diffuse into the airway. Consequently, when ventilation is weak, it accumulates in rise workings, especially near the roof, where it fills cavities, breaks fissures, etc., and occurs at the face of ripping, cauches, or brushings.

Properties of Fire-Damp

Fire-damp is not poisonous, but it does not support life. It has no colour, but may have a taste or smell according to the traces of gas associated with the odourless and tasteless methane. The practical tests applied underground for the detection and estimation of "fire-damp" really give the percentage of inflammable gas present and neglect the uninflammable constituent. Now methane constitutes from 97 to 100 % of the inflammable portion of fire-damp, and from 70 to 98 % of the whole.

Methane is very inflammable, and when mixed with air in certain proportions forms an easily ignited explosive mixture. Now an inflammable mixture is one that, when once ignited, will continue to burn of itself independent of the source of ignition.

Any mixture of methane and air containing between 5 and 14.8% of methane is inflammable under the conditions which normally prevail in mines. These figures compose what is called the explosive range of methane and air.

The lower limit of inflammation of methane in still air is about 5.25 %.; below this proportion of methane the mixture does not explode, but burns round the flame or source of ignition, forming a second flame or cap over the other.

The upper limit of inflammation of methane in still air is about 14.8%., and above this proportion the mixture does not explode, but continues to burn around the source of heat.

The most violently explosive mixture of methane and air is that which contains 9.4 %, methane. With decreasing or increasing amounts of methane the explosion is less violent. The inflammability of Methane is affected by various factors, some of which can be made to depress the lower limit below the normal figure. Our present knowledge regarding the limits of inflammability of methane can be summarised from the point of view of its application to coal-mining problems as follows:-

Under the conditions that prevail in coalmines the lower limit of inflammability of methane may be taken as 5% and the upper limit as 14.8 %. The figure for inflammable gas will probably be lower according to the nature and amount of gases present other than methane. In practice, however, only small amounts occur of these other inflammable gases, and have therefore relatively little effect on the inflammability of fire-damp.

Ethane and Propane are very similar in general properties to methane since they are members of the same series (paraffins). They are heavier and are more inflammable than methane. They are also ignited at a lower temperature and have a lower limit of inflammability than methane. Though they only occur m small amounts they tend to make fire-damp more easily ignited and consequently more dangerous.

Ethylene or Olefiant Gas also occurs in small quantifies some e-damps. It has no colour, but has a smell resembling ether. It is slightly lighter than air, and when mixed with air is very inflammable. It is not poisonous, but does not support either life or combustion. Since it is more inflammable than methane, its presence in fire-damp makes the latter more explosive and easier to ignite.

Gas Caps

If a flame be introduced into a mixture of methane and air having less than about 5.3% of methane, a secondary flame or "cap - coloured pale blue, is formed above the other flame. If the flame is lowered until it has lost nearly all its luminosity, the cap will be more easily seen. The size and intensity of the cap are governed by the following conditions: —

The last two conditions do not appreciably affect the size of the cap under practical conditions, but advantage is taken of the effect of different fuels in certain special fire-damp detectors such as alcohol and hydrogen flame detectors.

With a standard-sized testing flame and a "known" oil the percentage of inflammable gas can be estimated to the nearest ½% between 4 and 5% by an ordinary miner, whilst a more experienced man can detect as low as 1% with even greater accuracy. Moreover, if a spirit safety-lamp be used, a recognisable cap can be obtained for as low as 1/2 to ¾% of methane.

Detection and Estimation of Inflammable Gas by the Ordinary Flame Safety-Lamp

Before going underground the observer must see that the safety-lamp is in order, glass and shield intactgauzes in position, and bottom securely locked to the top. He should blow around the top and bottom of the glass, and if the flame is not disturbed the lamp is ready for use. A superficial examination of the lamp should be made before each test, as the safety of those underground depends upon the intactness of the safety lamp in the presence of an explosive mixture. If an ordinary flame safety - lamp be introduced into an explosive mixture the flame is extinguished and does not pass through the gauze. If the observer becomes accustomed to a certain height of flame for lighting purposes he can detect the presence of a small percentage of gas without lowering the flame; with increasing amounts of gas the flame becomes longer and longer and is said to climb. This test is very useful in the hands of an experienced man, but the use of the standard testing flame gives more accurate results. The standard testing flame is one-tenth of an inch high by three-eighths of an inch wide, and should be almost non-luminous. It should be observed whether the particular oil in use gives a fuel cap on the testing flame. If so, it appears as a small indistinct cap along the upper edge of the testing flame; but with a little practice it can be distinguished from the ordinary gas cap.

In all gas tests the lamp must be introduced very carefully and cautiously and should be entirely withdrawn as soon as the cap reaches the top of the glass; but if the gas does fire in the lamp it should not be snatched away but deliberately smothered out with some textile material or extinguished in water. Now the size of the cap varies according to the different kinds of oil in use, but when a person becomes accustomed to a certain oil the height of the testing flame will indicate with fair accuracy the percentage of gas present.

Details of Caps Formed on a Testing Flame 0.375 inches wide by 0.1 inches High, Burning Colza Oil

At 5.5% and upwards the gas fires in the lamp and flame is extinguished. With light mineral oils such as benzene or colzaline the caps are slightly larger than those indicated, but the errors are on the right side.

THE MINES ACTS AND REGULATIONS RESPECTING CERTAIN PERCENTAGES OF INFLAMMABLE GAS

The Coal Mines Acts and General Regulations are very stringent as regards the use of electricity, methods of lighting, types of explosives used, and the introduction and storage of materials in mines where inflammable gas has occurred or is likely to occur in quantities indicative of danger and where an explosion of such gas has already occurred causing personal injury within a certain period preceding the date in question. In such cases electricity should not be used except in the case of approved safety lamps, telephones, and signalling apparatus. For lighting only approved safety lamps should be used, and blasting operations should be performed with "Permitted" explosives only.

It should be carefully noted that the Coal Mines Act uses the term "inflammable gas" and not "fire-damp ," since fire-damp may contain nitrogen and carbon dioxide, The former term is well chosen, since it is only inflammable gas that explodes and which can be detected by the "cap in the oil safety lamp. The fireman (examiner or deputy) must report whether he has found any inflammable gas during his inspection.