INSPECTION AND PATROLLING The surface buildings of mine plants and parts of the underground workings susceptible to fire should be inspected regularly to discover any places that may have developed a serious fire risk. When this risk is realized, action should be taken to correct it by removing or fireproofing some or all of the flammable material or by guarding against whatever sources of ignition there may be. Inspections of this type are regular and decidedly worthwhile functions of fire-prevention officials in communities, industrial plants, and public service units. Although similar inspections are made at some mines by operating officials or by insurance-company inspectors, in many instances they are not thorough and recommended improvements are neglected. Certain fire risks may be accepted when lives are not endangered, but at least the inspections should be made so that the risks are known. Where defective conditions are known and where property is unprotected by the presence of employees on their regular duties, patrols should be made so that incipient fires can be extinguished before they get beyond control. Automatic alarms or sprinkler systems may often be more effective than patrols, and it is likely that such installations should be more widely used in mining operations than they are at present. Specific examples of inspection and patrolling by individual companies follow: 1. A continuous carbon monoxide sampler is connected to the hoist room and mine office to sound an alarm automatically in the event of an incipient mine fire. 2. Fire patrols go through each mine after the shift preceding any idle period, and, except where it is absolutely necessary, all electric power is cut off during the idle periods. 3. Welding and cutting areas are patrolled for at least 8 hours after the work is completed. GUARDING AGAINST IGNITION One common method of preventing fires is to remove or exclude sources of ignition from places where the surroundings are exceedingly flammable. Of the various causes of mine fires discussed here, electric arcs, open lights, matches, heating appliances, and welding equipment are the most common sources of ignition. The following standard recommendations are made by fire-prevention authorities as a result of experience and study. In locations where there are flammable vapors, gases, or dusts, vaporproof lamps should be installed, and wiring should be of a type approved by the National Electric Code. Switches should be at a safe distance from places where these dangers prevail. All light and power circuits should be provided with fuses or other protective devices of proper type and size; the circuits should not be overloaded. Circuit breakers should be installed on all main and branch powerlines not protected by fuses; they should be set as low as is practicable with the maximum demand of the equipment operated from that line. Electric wiring should be installed in accordance with a method recognized as suitable for the purpose by the National Electric Code. Defective electric wiring should be repaired promptly to conform to the standards of the code. Conductors whose insulation is unserviceable because of age, mechanical injury, or exposure to heat, moisture, or vapors should be replaced without delay. Wiring installed for temporary use should be replaced or removed. All joints of wiring should be properly soldered and taped. Conductors of "open wiring" systems should not be in contact with each other or with pipes, woodwork, or other combustible material. All extension lights should be provided with suitable wire guards. Extension cords should not be left in contact with pipes, nails, or combustible materials. Doors or covers of fuse and switch cabinets should be kept closed. To prevent heating, switches should make good contact and should not be overloaded. Where oil switches are used, a nonflammable type of oil is preferable. Starting resistance units of direct-current motors should be located away from combustible material, or the material should be suitably protected. Smoking should be prohibited in the vicinity of readily flammable material of any kind. Open lights or torches should not be taken into mines or into any location containing readily flammable material and should not be left unattended close to combustible material. Heating appliances should not be placed close enough to combustible material to Fixed welding and cutting operations should be confined within a well-ventilated fire- A suitable fire extinguisher should be carried with each portable welding or cutting unit. The area where the work is done should be wet down or wood surfaces protected when possible. The area should be inspected for evidence of any fire by a designated person 30 minutes after work has been completed. Metal containers with self-closing covers should be provided for oily rags and other waste material. GASES IN METAL AND NONMETALLIC MINES GASES ENCOUNTERED-HAZARDS Pure, dry air at sea level contains the following gases: 35 Oxygen (O2), 20.95 percent; nitrogen (N2), 78.09 percent; carbon dioxide (CO2), 0.03 percent; and argon (A), 0.93 percent by volume. Traces of helium (He), neon (Ne), krypton (Kr), and xenon (Xe) are also present, but these gases, as well as argon, are usually included with nitrogen because they are chemically and physiologically inert and for practical purposes may be considered as part of the atmospheric "nitrogen." Mine air also contains 1 percent or more of water vapor, depending on the temperature, barometric pressure, presence of liquid water to form the vapor, and the moisture content of the intake air entering the mine. The weight of a gas or a mixture of gases varies with the temperature and pressure and, to a slight extent, with the associated water vapor in the air. Dry air at sea level at 70° F. weighs 0.075 pound per cubic foot,36 a figure that is quite generally used as a standard for air density. The specific gravity of a gas or a mixture of gases is the ratio of the weight of a specific volume of the gas to the weight of an equal volume of air at the same temperature and pressure. Thus, the specific gravity of air is stated arbitrarily to be 1. The specific gravity of a gas lighter than air is less than I and of a gas heavier than air is greater than 1. The quantities of air breathed and of oxygen consumed by averagesize men at rest and exercising both moderately and vigorously 37 are given in table 5. Certain gases occur in metal and nonmetallic mines much more commonly than is ordinarily supposed. Some of these gases occur naturally in the rock or earth; others are formed by chemical action in the mine workings or are products of mine fires, explosions, or blasting. A brief description of the common gases follows. TABLE 5.-Approximate rate and volume of respiration and of oxygen consumed by man 35 Humphreys, W. J., Physics of Air: McGraw-Hill Book Co., Inc., New York, N. Y., 3d Forbes, J. J., and Grove, G. W. (revised March 1954 by McElroy, G. E., Watson, H. A., 36 McElroy, G. E., Engineering Factors in the Ventilation of Metal Mines: Bureau of 37 Henderson, Y., and Paul, J. W., Oxygen Mine Rescue Apparatus and Physiological Effects on Users: Bureau of Mines Tech. Paper 82, 1917, 93 pp. NITROGEN Nitrogen (N2) is a colorless, odorless gas slightly lighter than air, which when breathed suffocates a human being in much the same way that water does. Mixed with oxygen in the approximate percentage of 79 to 21, as in ordinary air, it dilutes the oxygen and is breathed normally. It is not combustible and will not support combustion. High-nitrogen gases sometimes issue from the strata in metal mines. CARBON DIOXIDE Carbon dioxide (CO2) is a product of oxidation and combustion of organic compounds and of the respiration of men and animals. It is a heavy, colorless, and odorless gas often found mixed with nitrogen in unventilated workings, particularly in dips and winzes. It will not burn or support combustion. It has been found emanating from the rock strata in underground workings of certain mines, notably in the Cripple Creek, Colo., and Tintic and Park City, Utah, districts, as well as in numerous other metal-mining regions.38 Carbon dioxide is a constituent of blackdamp, and traces of it (0.03 percent) are always present in normal air. Carbon dioxide is a respiratory stimulant. It is therefore physiologically active and cannot be classed with inert gases, although it is not highly toxic. Its property of stimulating breathing is utilized in certain resuscitation apparatus. One half of 1 percent (0.5 percent) of carbon dioxide in normal air causes a slight increase in lung ventilation; a man exposed to this percentage of carbon dioxide will breathe a little deeper and a little faster than when in pure air. If the air contains 2 percent of carbon dioxide, the lung ventilation will be increased about 50 percent; if the air contains 5 percent, it will be increased about 300 percent, and breathing is laborious; and 10 percent cannot be endured for more than a few minutes.40 Carbon dioxide in air has these effects when the oxygen content remains about normal and the subject is at rest. Moving around or working would naturally increase the symptoms, and they would be much more dangerous than when a man is resting. Low oxygen content of the air and temperatures above 80° F. increase the effect of carbon dioxide so that serious effects may be felt at lower concentrations than those given. Concentrations of more than 5 percent of carbon dioxide are usually accompanied by an appreciable lowering of the oxygen. Solid (dry ice) or liquid carbon dioxide sublimes at 109.3° F. below McElroy, G. E., Rock-Strata Gases in Mines of the East Tintic Mining District, Utah : 1928, 24 pp. Bureau of Mines, Some Pertinent Information About Mine Gases: Inf. Circ. 6983, 1938, 15 pp. Committee, International Mine Rescue Standardization Conference, Mine Rescue Standards, a Tentative Study: Bureau of Mines Tech. Paper 334, 1923, 44 pp. “International Critical Tables, McGraw-Hill Book Co., Inc., New York, N. Y., vols. 1 and 3, 1928. Tiffany, J. E., A New Permissible Blasting Device : Bureau of Mines Rept. of Investiga tions 2920, 1929, 8 pp. Air in which men work or travel in mines should be improved when it contains less than 19.5 percent of oxygen, more than 0.5 percent carbon dioxide, or is contaminated with combustible, noxious, or poisonous gases. CARBON MONOXIDE Carbon monoxide (CO) is the gas responsible for more than 90 percent of the fatalities from mine fires; it is not present normally in air but is formed in the burning of any combustible carbonaceous substance, such as timber, explosives, or petroleum products, especially where there is smoldering or incomplete combustion. It is found in the exhaust gases of internal-combustion engines and in gases generated by explosives. Carbon monoxide is a colorless, odorless gas slightly lighter than air. The most important characteristic of carbon monoxide is it poisonous or asphyxiant action upon man, even in very low concentrations. Carbon monoxide is combustible, and air that contains 12.5 to 75 percent of carbon monoxide will explode if ignited.43 Carbon monoxide can be detected with various types of carbon monoxide detectors and by use of canaries and mice, but it cannot be detected with the flame safety lamp until the concentration becomes so high that persons without respiratory protective equipment would be overcome almost immediately. The main effect of carbon monoxide on the body is that it combines with the hemoglobin of the blood and prevents the hemoglobin from carrying oxygen to the tissues. The affinity of carbon monoxide for hemoglobin is about 300 times that of oxygen. The more common effects of mild carbon monoxide poisoning are shortness of breath, headache, dizziness, muscular weakness, and nausea. Occasionally exposure to this gas causes loss of memory and visual, speech, and hearing disturbances and even psychoses, neuritis, and paralysis. However, these effects result only from the most severe exposure and usually after prolonged unconsciousness. The physiological effects of various concentrations of carbon monoxide and significance of time exposure, as recognized by the Bureau of Mines and the National Safety Council, are given in table 6. TABLE 6.-Physiological effects of carbon monoxide Concentration of carbon monoxide, percent by volume Physiological effects 0.01. .02. .04-0.05. .08-0.10 .15-0.20. .40 or more. Nonappreciable effect after several hours of exposure. Symptoms of mild poisoning (headache) after 111⁄2 hours of exposure. Unconsciousness after 1 to 111⁄2 hours of exposure; death after 2 hours. The generally recognized maximum allowable concentration " for an 8-hour exposure to air containing carbon monoxide and the normal 43 Coward, H. F., and Jones, G. W., Limits of Flammability of Gases and Vapors : Bureau of Mines Bull. 503, 1952, 155 pp. 44 American Medical Association Archives of Industrial Hygiene and Occupational Medicine, Threshold Limit Values for 1953: Vol. 8, 1953, pp. 296-297. |