Under normal conditions, many gases are present in the mine. The mineís ventilation system is designed to bring in fresh air to disperse and remove harmful gases and to supply oxygen.
But during a disaster, the situation may be quite different. Fires and explosions may release dangerous gases into the atmosphere. And a disrupted ventilation system could result in an oxygen-deficient atmosphere and/or a buildup of toxic or explosive gases.II. Basics of Gas Detection
a. Gas Detection- Gas detection is an important part of any rescue or recovery operation. Your team will make frequent tests for gases as it advances inby the fresh air base. For your own safety, youíll want to know what harmful gases are present, how much oxygen is in the atmosphere, and whether or not gas levels are within the explosive range.
Knowing what gases are present and in what concentrations provides you with important clues as to what has happened in the mine. Test results can also give you an idea about existing conditions. For example, if you get carbon monoxide (CO) readings, that indicates thereís probably a fire. The amount of carbon monoxide indicates something about the extent of that fire.b. Gas Detector Requirements- Each mine rescue station is required by law to have gas detectors appropriate for detecting each type of gas that may be encountered in the mine, and two oxygen detectors.c. Portable Gas Detectors- The type of gas detection equipment most often used by mine rescue teams is the portable gas detector. Portable gas detectors include such devices as methane monitors, carbon monoxide (CO) detectors, and the multi-gas detectors used in conjunction with various tubes. The team uses these devices to test the mine air repeatedly as it advances inby the fresh air base.d. Air Sampling and Chemical Analysis- Another way to test for gases is to collect air samples, special syringes, evacuated bottles, (bottles from which air has been removed), or gas or liquid displacement containers. These samples are then sent to a laboratory for chemical analysis. Chemical analysis is also sometimes performed at the mine site with portable equipment.
Chemical analysis is generally a more time-consuming process than testing with a portable device, but its advantage is accuracy. A chemical analysis tells exactly what gases the sample contains, and in precisely what amounts.
A complete chemical analysis can also reveal the presence of gases that portable detectors are not designed to detect.
Air samples arenít taken as often as portable detector readings, but they are still an important part of rescue and recovery operations. For example, you may be required take air samples from ventilation shafts and return airways
This method is often used to get information about existing conditions prior to sending teams underground.
Air samples taken from behind sealed areas of the mine are analyzed to determine when itís safe to begin recovery work.III. Basic Gas Principles
In order to test for gases and to understand what the test readings mean, you should first know a little about the characteristics and properties of gases. After weíve discussed these general principles, weíll talk about specific gases you might encounter during rescue and recovery work.a. Description- To help you understand what a gas is, letís compare it with a liquid and a solid. A solid has a definite shape and volume. A liquid has a definite volume, but changes shape according to the shape of its container.
However, a gas is a substance with neither a definite shape nor volume. It expands or contracts to fill the area in which itís contained.b. Diffusion of Gases- The volume of a gas changes in response to any change in atmospheric pressure or temperature. For example:
The ventilating air currents in the mine also affect the gasís rate of diffusion. The rate of diffusion is greatly increased by higher velocities of air currents or by turbulence in the air.
Knowing the effects of air current, temperature, and pressure on a gas will help you determine its rate of diffusion. The rate of diffusion is how quickly the gas will be quickly mix or blend with one or more other gases and ho it can be dispersed.c. Atmospheric Pressure and Its Effects on Rate of Diffusion- Pressure exerted on a gas is usually atmospheric pressure. Atmospheric pressure is measured on a barometer. A rise in the barometric reading indicates an increase in pressure. A drop in barometric reading indicates a decrease in pressure. The atmospheric pressure varies within a mine, just as it does on the surface.
Atmospheric pressure affects the diffusion rate of a gas. For example, if the barometer rises, indicating increased pressure, gas responds by contracting.
A gas that 9 s squeezed into a smaller area like this is more concentrated, so it diffuses more slowly. As you might guess, itís much easier for concentrations of explosive gases to build up when the barometric pressure is high. And it is harder to disperse the gases by means of the mineís ventilation system.
On the other hand, when barometric pressure falls, the pressure on the gas is reduced. The gas responds by expanding. Once the gas expands, it is less concentrated, so it diffuses more quickly.d. Temperature and Its Effects on Rate of Diffusion- Under normal conditions, the temperature in a mine does not vary greatly, but fires and explosions will produce temperature variations. This is why itís important for you to know what happens to gases when temperatures change.
High temperatures (or heat) cause gases to expand, so they diffuse more quickly. Consequently, heat from a fire in the mine will cause oases to expand and be dispersed more easily.
Lower temperatures work the opposite way: Gases respond to cold by contracting and by diffusing more slowly.e. Specific Gravity or Relative Weight- Specific gravity is the weight of a gas compared to an equal volume of normal air under the same temperature and pressure. (This is also referred to as "relative weight.")
The specific gravity of normal air is 1.0. The weight of air acts as a reference point from which we measure the relative weight of other gases. For example, a gas that is heavier than air has a specific gravity higher than 1.0. A gas that is lighter than air will have a specific gravity less than 1.0.
If you know the specific gravity of a gas, you will know where it will be located in the mine and where you should test for it.
Gases issuing into still air without mixing tend to stratify according to the gasís specific gravity. Light gases or mixtures tend to stratify against the roof and heavy gases or mixtures tend to stratify along the floor.
Methane, for example, has a specific gravity of 0.5545. This is lighter than normal air. Knowing this, you can predict that methane will rise and collect in greater concentrations near the top or roof of a mine. This is why you test for methane near the top.
If the weight of a gas youíre testing for is heavier than normal air, youíll know to test for it near the bottom of the mine or in low places. Thatís because heavier gases tend to fall, so you can expect to find them in greater concentrations in low areas.
Besides helping you determine where to test for a gas, specific gravity also indicates how quickly the gas will diffuse and how easily it can be dispersed by ventilation.
In stir air, the ordinary process of diffusion is a very slow process. However, under usual mine conditions, ventilating air currents and convection currents produced by temperature differences cause a rapid mechanical mixing of gases with air. And once the gases are mixed they will not separate or stratify again.
Light gases, such as methane or hydrogen, diffuse rapidly and are fairly easy to disperse. Heavier gases such as carbon dioxide donít diffuse rapidly, so theyíre more difficult to disperse. Itís much easier to remove a concentration of a light gas like methane by ventilation than it is to remove the same concentration of a heavier gas like carbon dioxide. Specific gravity is not the only factor that determines how quickly a gas will diffuse or disperse. Temperature and pressure also affect it.
Remember that an increase in temperature makes a gas diffuse more rapidly. A decrease in temperature slows down the rate of diffusion.
Atmospheric pressure works just the opposite: An increase in pressure slows down the rate of diffusion. A decrease in pressure speeds it up. f. Explosive Range and Flammability- A gas that will bum is said to be "flammable." Any flammable gas can explode under certain conditions. In order for a flammable gas to explode, there must be enough of the gas in the air, enough oxygen, and a source of ignition. The range of concentrations within which a gas will explode are known as its "explosive range." Figures representing the higher and lower limits of the explosive range are expressed in percentages.
The amount of oxygen that must be present for an explosion to occur is also expressed as a percentage. When the necessary oxygen concentration approaches that found in normal air, the level is expressed simply as "normal air."
The explosive range of methane, for example, is 5 to 15 percent in the presence of at least 12.1 percent oxygen.g. Solubility- Solubility is the ability of a gas to be dissolved in water. Some gases found in mines are soluble and can be released from water. Sulfur dioxide and hydrogen sulfide, for example, are water-soluble gases. Both may be released from water.
Solubility is an important factor to consider during recovery operations. When a mine is sealed off for any length of time, water can collect in it. This water may have occurred naturally, or it may have been introduced during firefighting.
Whatever the case, pools of water can release water-soluble gases into the air when they are stirred up. Pumping water from such pools, or wading through them, can release large amounts of soluble gases- which would not otherwise be found in the mine atmosphere.h. Color /Odor/Taste- Color, odor, and taste are physical properties that can help you identify a gas, especially during barefaced exploration. Hydrogen sulfide, for example, has a distinctive "rotten egg" odor. Some gases may taste bitter or acid; others sweet. The odor of blasting powder fumes, together with a reddish-brown color, indicates there are oxides of nitrogen present.
Of course, you canít rely on only your senses to positively identify a gas. Only detectors and chemical analysis can do that. And many hazardous gases, such as methane, have no odor, color, or taste. But keep these properties in mind as we discuss each gas you may encounter in the mine. One or more of these properties may be your first clue that a particular gas is present. IV. Health Hazards
a. Toxic Gases- Some gases found in mines are toxic (poisonous). This can refer either to what happens when you breathe the gas, or what happens when the gas comes into contact with exposed areas of your body.
The degree to which a toxic gas will affect you depends on three factors:
Some toxic gases are harmful to inhale. A self-contained breathing apparatus (SCBA) will protect you from such gases, as long as your face-to-facepiece seal is tight. Other toxic gases harm the skin, or can be absorbed by the skin. As you might guess, an SCBA wonít protect you from such gases. If you wear your SCBA in petroleum-based fumes for prolonged or successive periods, the fumes can eventually permeate its rubber parts so that the apparatus no longer provides you with adequate protection. Your team may be forced to leave an area where such gases are detected. b. Asphyxiating Gases- "Asphyxiate" means to suffocate or choke. Asphyxiating gases cause suffocation. They do this by displacing oxygen in the air, thus producing an oxygen-deficient atmosphere. Since your self-contained breathing apparatus supplies you with oxygen, it will protect you against asphyxiating gases. V. Measurement of Gases
a. Threshold Limit Value (TLV)- The amount of a gas exposure for an 8 hour day for 5 days a week without any harmful effects.b. Ceiling Limit- The amount of gas at no time a person can be exposed to.c. Immediately dangerous to life or health (IDLH)- The maximum concentration of a gas, in case of SCBA failure, one could escape without any irreversible health effects.d. Parts Per Million (PPM)- The most accurate measurement of a contaminant in the atmosphere.
VI. Mine Gases & Their Components
a. AirChemical Formula: NoneSpecific Gravity: 1.000Source: AtmosphereCharacteristics: No color, odor, or taste
Pure dry air at sea level contains the following:
Chemical Formula: O2 Specific Gravity: 1.105Oxygen will not burn or explode
Source: AtmosphereCharacteristics: No color, odor or taste
Note: When another gas is introduced into the atmosphere of artificial environment, such as a mine, tunnel or man holes, oxygen is usually displaced causing asphyxiation.
Chemical Formula: CH4
Specific gravity: 0.555 Needs 12.5% O2 to ignite Explosive Range: 5-15% Ignition Temperature: 1100o-1300o F Source: Carbon products decaying in anoxic environment Characteristics: No color, odor or taste
d. Heavy Hydrocarbons (LEL)
Chemical Formula: C2H2
Specific Gravity: 0.9107Explosive Range: 2.4-83%Ignition Temperature: 581oF Source: Methane heated in a low oxygen atmosphere, Calcium carbide mixed with water
Chemical Formula: H2
Specific Gravity: 0.0695Needs 5% oxygen to ignite Explosive Range: 4.1-74%Ignition temperature: 1030o - 1130oFSource: Water on super hot fires, battery charging
Chemical formula: N2
Specific Gravity: 0.967Ceiling Limit: 810,000 ppmSource: Atmosphere, released from coal seamCharacteristics: No color, odor, or taste
h. Carbon Dioxide
Chemical Formula: CO2
Specific Gravity: 1.529Ceiling Limit: 1.5%IDLH: 50,000 ppmSource: Product of complete combustion, slow oxidation of carbon productsCharacteristics: No color or odor, acidic taste above 10%
I. Carbon Monoxide
Chemical Formula: COSpecific Gravity: 0.967Needs 6% O2 to igniteIgnition Temperature: 1100oF Explosive Range: 12.5-74%TLV: 50 ppmCeiling Limit: 200 ppmIDLH: 1500 ppmSource: Incomplete combustion, diesels, gasoline enginesCharacteristics: No color, odor, or tasteEffect on the body: 300 times more attracted to hemoglobin than oxygen, forms carboxyhemoglobin which prevents oxidation of cells
j. Hydrogen Sulfide
Chemical Formula: H2SSpecific Gravity: 1.191Ignition Temperature: 700oFTLV: 10 ppmCeiling limit: 15 ppm IDLH: 300 ppmSource: Sulfur dissolving in water in a poorly ventilated areaCharacteristics: Colorless, sweet taste, rotten egg smellEffect on the body: Paralysis of respiratory system
k. Nitrogen Dioxide Chemical Formula: NO2 Specific Gravity: 1.589TLV: 1 ppmCeiling Limit: 3 ppmIDLH: 50 ppmSource: Explosives after-product, diesel exhaustCharacteristics: Burnt powder odor, reddish brown in high concentrationsEffect on the body: Forms nitric acid in lungs causing pulmonary edema l. Sulfur Dioxide Chemical Formula: SO2 Specific Gravity: 2.264Source: Burning of sulfide ores, diesel exhaust, and gob firesTLV: 5 ppmCeiling Limit: 10 ppmIDLH: 100 ppmCharacteristics: Heavy sulfur odorEffect on the body: Same as nitrogen dioxide m. Smoke Tiny particles of solid and liquid matter suspended in air as a result of combustion Diesel Particulate Matter By-products of burning belts Carbon materials Usually noxious and toxic gases are present Can be carcinogen (cancer causing) n. Mine Damps The word damp is a derivative of the German word "damf" which means vapor. It was used by the immigrant German miners in the anthracite fields to describe a certain atmosphere condition.Black damp: Carbon dioxide, nitrogen, and low oxygen.White damp: Carbon monoxideFire damp: MethaneStink damp: Hydrogen SulfideAfter damp: By-products of a fire or explosion.
k. Nitrogen Dioxide
Chemical Formula: NO2
Specific Gravity: 1.589TLV: 1 ppmCeiling Limit: 3 ppmIDLH: 50 ppmSource: Explosives after-product, diesel exhaustCharacteristics: Burnt powder odor, reddish brown in high concentrationsEffect on the body: Forms nitric acid in lungs causing pulmonary edema
l. Sulfur Dioxide
Chemical Formula: SO2
Specific Gravity: 2.264Source: Burning of sulfide ores, diesel exhaust, and gob firesTLV: 5 ppmCeiling Limit: 10 ppmIDLH: 100 ppmCharacteristics: Heavy sulfur odorEffect on the body: Same as nitrogen dioxide
Tiny particles of solid and liquid matter suspended in air as a result of combustion
Diesel Particulate Matter
By-products of burning belts
Usually noxious and toxic gases are present
Can be carcinogen (cancer causing)
n. Mine Damps
The word damp is a derivative of the German word "damf" which means vapor. It was used by the immigrant German miners in the anthracite fields to describe a certain atmosphere condition.Black damp: Carbon dioxide, nitrogen, and low oxygen.White damp: Carbon monoxideFire damp: MethaneStink damp: Hydrogen SulfideAfter damp: By-products of a fire or explosion.