Mine Air Quality

http://www.minesafe.org/underground/gases.html

Ventilation

VENTILATION RESPONSIBILITIES

Mine operators should appoint competent persons to measure and record:

• air quality and temperature;
• air distribution in work sites;
• atmospheric composition and compliance with air quantities;
• air maintenance plans; and,
• advice given to managers on all potential problems with air quality.

In underground mines competent persons should:

• regularly inspect, test and record atmospheric conditions;
• analyse atmospheric contaminants and air quantities and determine if they comply with appropriate Australian Standards;
• inspect, test and record wet and dry bulb temperatures at sites where temperatures are identified to have an adverse effect on the safety and health of staff;
• calibrate and maintain all metering and monitoring devices;
• select and position primary and auxiliary fans and record air levels at the parameters once every three months;
• record air volume and pressure in the mine at regular intervals;
• update ventilation plans as required to ensure current information is available in cases of emergency;
• identify and deal with equipment defects or deficiencies in air volume or contaminants; and
• provide technical advice.

With crushing or screening plants:

• regularly inspect and test workplaces to determine and maintain atmospheric contaminants at levels as low as are reasonably possible;
• ensure dust suppression and collection systems are effective;
• operate, calibrate and maintain all metering devices; and
• identify and deal with equipment defects or air contaminant levels exceeding appropriate standards.

Hazards

MINE AIR QUALITY

Air is a mixture of gases in the natural atmosphere. The main constituents of air are:

• nitrogen 78%
• oxygen 21%
• carbon dioxide 0.03%
• other gases 0.9%.

Of these, the gases of greatest interest are oxygen and carbon dioxide.

Oxygen (O2)

Oxygen is an odourless, tasteless and colourless gas essential for humans, plants and animals. An oxygen level of less than 17% is hazardous. Dilution from other gases, or by sulphide ores and carbonaceous shales which oxidise slowly, can also deplete oxygen volume. Timber decay and rust on iron deplete oxygen content and can also produce carbon monoxide or carbon dioxide which may contaminate mine air.

Carbon Dioxide (CO2)

CO2 is a colourless gas with a pungent smell in high concentrations, non-explosive in air, denser than normal air and can be found at floor level. At concentrations greater than 10%, CO2 causes loss of consciousness. The rate of breathing doubles at a concentration of 3%. Mine fires and slow combustion of timber, blasting, breathing, burning of flame lamps, breakdown of carbonate ores and burning diesel fuel increase carbon dioxide levels in the air. Surveys at Lightning Ridge Opal Fields in 1992 found significant concentrations of carbon dioxide on some fields, particularly, in blind shafts and newly drilled bore holes. For more information, see recommendations in the clause on abandoned workings in this section before entering newly drilled or blind Caldwell holes.

ATMOSPHERIC CONTANIMANTS

The main contanimants in air are:

Dust

Airborne dust affects health and safety of underground staff and is dangerous in excessive amounts if staff breathe it in over a sufficient length of time. Take precautions to minimise potential for dust to become airborne, particularly when using machinery and shot-firing underground. When dust is airborne, velocity of ventilating air currents should be strong enough to dilute and remove dust and any fumes. Dust includes:

• pulmonary dust (harmful to respiratory system): silica (quartz, chert) silicates (asbestos, talc, mica and sillimanite); metal fumes (nearly all);
• toxic dust (poisonous to body organs, tissue, etc): ores of arsenic, lead, mercury, tungsten, nickel, silver (principally the oxides and carbonates);
• explosive dust (combustible when airborne): metallic dust (magnesium, aluminium, zinc, tin and iron); and inert dust (harmful effect).

Gases

Carbon Monoxide (CO)
CO is a colourless, tasteless and odourless gas, lighter than air, easily absorbed into the blood stream and very toxic at low concentrations. It is explosive in air between concentrations of 12.5% and 74%. and can be detected with gas detector tubes. The main sources of CO are diesel emissions, blasting operations, and any incomplete combustion. Petrol engines create carbon monoxide and are prohibited for use underground or located adjacent to intake airways.

Sulphur Dioxide (SO2 )
Sulphur dioxide is a non-combustible, non-flammable toxic, colourless gas with a strong sulphurous suffocating odour. It is very poisonous and can irritate eyes and respiratory passages. SO2 in high concentrations is dangerous to breathe over a long time. Principal sources of SO2 are fires in sulphide ore bodies, diesel engines, blasting and burning rubber. The gas may be detected by smell at concentrations of 0.003% or by gas detector tubes and gas instruments.

Nitrous Fumes or Oxides of Nitrogen (NOx)
The term nitrous fumes includes all nitrogen oxides and in particular nitrogen dioxide, nitric oxide and nitrogen peroxide. All are toxic, having a pungent smell and an irritating effect on the air passage. Any air with sufficient nitrous fumes to cause appreciable irritation of the air passage should be regarded as dangerous. Main sources are diesel exhausts and partial detonation of explosives. Detection is by odour and should be taken as a warning not to proceed. Gas detector tubes are also available.

Other gases
Other gases which may be present in mines include methane, aldehydes and hydrogen sulphide.

DIESEL EXHAUST FUMES

Before diesel engines are used underground, carry out the following checks to ensure compliance with National Occupational Health Safety Commission (NOHSC 1003). Atmospheric contaminants to be considered include:

• carbon dioxide;
• carbon monoxide;
• nitrogen dioxide;
• nitric oxide;
• hydrogen sulphide;
• sulphur dioxide;
• aldehydes (as formaldehyde); and
• respirable combustible dust.

Undiluted exhaust gases from a diesel engine should be measured for CO and oxides of nitrogen or NO (new engines should be able to achieve about one third of the CO limit). Diesel engines should be fitted with an appropriate conditioner or scrubber. Airflows in which diesel engines operate underground should have been determined by the dilution required to achieve the atmospheric limits specified in by the NOHSC 1003 Standard. This requires knowledge of the swept exhaust volume of the engine and the maximum raw exhaust gas concentrations for the duty cycle). Sufficient quantity of air for ventilation should be available when engines in one specific area of a mine are operating. Contaminant levels are not being exceeded and appropriate monitoring methods, such as detector tubes, are in place.

EXPLOSIVE ATMOSPHERES

In addition to methane, carbon monoxide and hydrogen sulphide, other materials can create explosive atmospheres in mines. These include:

• acetylene, a colourless gas usually stored in bottles for oxy-acetylene cutting, which is
• explosive in air mixtures ranging from 3% to 82%;
• oxygen, stored in bottles, which can easily rupture and should be stored away from grease and electrical equipment;
• fuel vapours, which are easily ignited by flame if not properly flushed by a current of air; and
• vapours from fast-drying agents and paints which should be stored in well-ventilated areas in mines - if possible away from air that workers breathe and not in dead ends.

Mine Gases

Important aspects of mine gases are:
Density relative to air - is the gas lighter or heavier than air.
Effects of personnel - is the gas poisonous or suffocating
Flammability - Location (explosive limits)
Methods of detection

Gas Information - numbered to the following key

1. Name of Gas
2. Chemical symbol
3. Composition
4. Density relative to air
5. Characteristics
6. Effect on men and animals
7. Combustibility
8. Flammable (explosive) limits in air
9. Detection
10. Where gas is found in mines
11. Remarks

A flammable gas is one which, when mixed with air between certain limits, will propagate a flame away from the source of ignition. These limits are known as the upper and lower flammable limits The flammable limits are sometimes loosely referred to as the explosive limits because pressure is often associated with the flame propagation. There are two other terms you should be familiar with and they are:

TLV = Threshold Limit Value. This is a time- weighted average concentration for a normal 8hr workday to which nearly all workers may be repeatedly exposed day after day without ill effects.

MAC = Maximum Allowable Concentration. This is the concentration of a contaminant that by law must not be exceeded in the mine atmosphere. -

1 Oxygen

2 O ²

3 An element

5 Colourless Odourless Tasteless

6 Essential to life. Oxygen constitutes 20.93% by volume in air. Deficiency leads to asphyxia, the early symptoms of which are dizziness, palpitations, breathlessness and weakness of limbs. With more severe deficiency there is unconsciousness, eventual cessation of breathing and death.

7 High doses of oxygen, used in normal circumstances, do not cause harm. Breathing pure oxygen can be hazardous when the body is exposed to pressures greater than one atmosphere above normal pressure and when exposure is prolonged (greater than 48 hours).
Permissible Lower Limit for coalmines 19%
Not combustible in air but is that constituent of air which supports ordinary combustion.

8 Non-flammable Oxygen is the essential gas used in all mine rescue breathing apparatus. When liquid or compressed oxygen comes into contact with organic or other easily oxidisable materials, an explosion can take place.

9. A constituent of normal air. One-third of the gas evolved during battery charging is oxygen, the remainder being hydrogen.

1 Nitrogen

2 N ²

3 An element.

4 0.97

5 Colourless Odourless Tasteless

6 Non-poisonous but it does not support life. Air contains 78.08% nitrogen. Problems occur when its concentration is increased and it displaces oxygen, causing asphyxiation.

7 Incombustible in air.

8 Non-flammable.

9 Unnecessary in mines.

10 A constituent of air and some "damps".

11 In gas analysis of mine atmospheres "nitrogen, etc", includes other inert gases which are present in small amounts, e.g. argon. Used to make inert, mine atmospheres to extinguish fires by displacing oxygen (Inertisation).

 

1 Hydrogen

2 H ²

3 An element.

4 0.07

5 Colourless odourless tasteless when pure (see note in 10 below)

6 Non-poisonous. Will displace oxygen and cause asphyxiation.

7 Burns with bluish flame in air or oxygen, forming water vapour.

8 Forms flammable mixtures with air. Lower flammable limit is 4% and upper flammable limit is 74%.

9 No simple test for detection.

10 Found behind seals for a few days after a mine fire is sealed off. Two-thirds of the gas evolved during battery charging is hydrogen, which is usually accompanied by an irritant electrolyte mist.

11 One of the gases formed during distillation of coal, e.g., in mine fires. See also "water gas".

 

1 Carbon Dioxide

2 CO ²

3 A compound of carbon and oxygen.

4 1.53

5 Colourless; has slight pungent smell and "soda water taste.

6 Not only does it displace oxygen, but it also exerts poisonous affects on the body. It disturbs the body's pH balance and interferes with normal biochemical reactions. The body attempts to compensate by increasing the breathing rate in an attempt to "breath out' the excess carbon dioxide. At rest 2% concentration increases the respiration by one-half, 5% to three times normal and 8% concentration, violent panting and fatigue to the point of exhaustion from respiration occurs. Concentrations greater than 10% lead to intolerable panting, severe headaches and collapse.

7 Incombustible and will not support combustion.

8 Non-flammable.

9 It will extinguish safety lamps due to reduction of O² content of air. Can be identified and determined with a detector tube or an interferometer.

10 A constituent of "black damp", "after damp" or "' Bottom gas". Owing to density, tends to accumulate on floors and in lowest parts of workings. Gas from some outbursts is almost pure CO2. In mines where this is the dominant seam gas escaping from floor breaks, it can form gas layers at floor level. Care should be taken when entering dip headings.

11 Formed by oxidation or combustion of coal, timber, etc., and by breathing of men and animals. Can be given off in almost pure form by strata in some coal mines and also in some metal mines or other underground workings or caves. A constituent of gases given off by mine fires and explosions and by blasting, of internal combustion engine exhausts, and accompanies smoke. Also produced by action of acid waters on carbonate rocks.

1 Carbon Monoxide

2 CO

3 A compound of carbon and oxygen.

4 0.97

5 Colourless odourless tasteless

6 Carbon monoxide is a highly insidious poison, which combines with the compound in blood (haemoglobin) which normally carries oxygen. Its combination prevents haemoglobin from carrying oxygen to the tissues, thus depriving the brain and other vital organs of oxygen.

Carbon monoxide combines- much more readily with haemoglobin than does oxygen, so that relatively small concentrations of carbon monoxide can combine with and "saturate" large amounts of haemoglobin.

Death rapidly occurs when a person's blood is saturated 80% or more. Where massive atmospheric concentrations occur, death may be almost immediate, without any preliminary symptoms. Early symptoms include shortness of breath, palpitations on exertion, increasing headaches, disturbed judgment and loss of power in the legs. As exposure continues, mental confusion and collapse occur followed by unconsciousness and possible death. Atmospheric concentrations of 0.2% are fatal in less than one hour, even at rest.

Carbon monoxide poisoning can be cumulative; a series of short-term exposures to carbon monoxide can lead to progressive saturation of haemoglobin. The process is slowly reversible; if effected persons are removed from carbon monoxide exposure, the gas will slowly disassociate from the haemoglobin. Medical treatment involves administering pure oxygen, sometimes under increased pressure in a hyperbaric chamber.

7 Burns with bluish flame in air, forming carbon dioxide.

8 Forms flammable mixtures with air. The lower flammable limit is 12.5%. The upper flammable limit is 74%.

9 Can best be identified and determined with a detector tube. Rescue teams may also use small warm-blooded creatures particularly canaries for detection purposes.

10 After methane or coal dust explosions, after firing explosives, during mine fires. Small amount may be found in the exhaust gases of diesel engines but is rapidly diluted by the ventilating air. Diesel equipment must not be used where resulting CO concentration exceeds 50 parts per million (0.005%).

11 Formed by incomplete combustion of coal, timber or lumber, oil etc by methane explosions and some explosives. Presence in mine atmospheres usually indicates incipient heating or active fire A constituent of "water gas" "producer gas" "white damp" and "after damp", and of exhaust gases of internal combustion engines. Also accompanies smoke.

 

1 Sulphuretted Hydrogen or Hydrogen Sulphide

2 H ² S

3 A compound of hydrogen and sulphur

4 1.19

5 Colourless, sweetish taste, powerful unpleasant odour resembling that of rotten eggs. Nasal sensitivity to odour decreases with continued exposure.

6 Hydrogen Sulphide is highly poisonous and acts on the body in a similar way to hydrogen cyanide: it interferes with the utilisation of oxygen in the body's tissues.

Atmospheric concentrations above 500 parts per million (0.05%) cause rapid loss of consciousness and early death. Very high concentrations may cause immediate death.

Atmospheric concentrations between 20 parts per million (0.002%) and 50 parts per million (0.005%), may irritate the eyes, higher concentrations will irritate the upper respiratory tract.

The N.H.M.R.C. recommended T.L.V. is 10 ppm (0.001%).

7 Burns with blue flame in air.

8 Forms flammable mixtures with air. Lower flammable limit 4.5%, Upper flammable limit 45%.

9 Can be determined and identified with a detection tube. Odour is detectable at less than 1 part per million (0.0001%) in air, at 50 parts per million (0.005%) odour is powerful but rapidly lost due to nose becoming insensitive, so one must not rely on odour as a warning of dangerous concentrations of the gas.

10 May be given off by certain strata (e.g., brassy tops) or from stagnant water. Sometimes found in vicinity of heating coal. Tends to be associated with faults.

11 Its presence is usual due to natural chemical action on pyrites or other sulphides. Sometimes it is due to heating of coal or strata containing sulphides.

 

1 Sulphur Dioxide

2 S0 ²

3 A compound of sulphur and oxygen.

4 2.26

5 Colourless, acid taste, pungent "burning sulphur "smell.

6 Poisonous. Combines with water on the body's surfaces to form sulphurous acid and will cause severe irritation to the eyes and respiratory tract. 20 parts per million (0.002%) in air causes irritation of the eyes and coughing. The effect on the eyes and upper respiratory passages are immediate, but effects on the lungs may be delayed. Concentrations of 400 parts per million (0.04%) to 500 parts per million (0.05%) are dangerous to life, even for short exposures. N.H.M.R.C. Recommended T.L.V. is 2PPM (0.0002%)

7 Incombustible

8 Nonflammable

9 Can be identified and determined with detector tubes. Its typical odour can be detected by smell at 3 parts per million (0.0003%) to 5 parts per million (0.0005 %).

10 In vicinity of heating. Very small quantities may occur in sulphide metal mines due to slow oxidation of pyritic material.

11 Usually formed by heating or burning of coal containing sulphur compounds or of "brassy tops" in an adequate air supply.

 

1 Nitrogen Dioxide

2 NO ²

3 A compound of nitrogen and oxygen

4 1.6

5 If sufficiently dense is reddish brown in colour, acrid smell, and acid taste.

6 Poisonous. Very soluble and combines with water on the body's surfaces to form nitric acid and nitrous acids. Highly irritating to the respiratory system. The effect on the upper respiratory system is immediate; however, the effect on the lungs is often delayed. A severe lung reaction may occur several hours after exposure and can lead to death.

A concentration of 100 parts per million (0.01%) in air, which is the least amount to cause coughing, is dangerous if inhaled for only a few minutes. Any atmosphere in which nitrogen dioxide is noticeable by either smell, irritation or colour, should be regarded as dangerous. The N.M.R.C. Recommended T.L.V. is 3 ppm (0.0003%).

7 Incombustible but will support combustion.

8 Non-flammable.

9 Can be identified and determined with detector tubes.

10 In working places immediately after shotfiring. A small amount is found in the exhaust gases of diesel engines but is rapidly diluted and dispersed by the ventilating air. Diesel equipment should not be used where resulting concentration exceeds 5 parts per million (0.0005%).

11 The formation of this gas in dangerous quantities is usually due to the detonation or burning of explosives and due to diesel equipment if poorly ventilated or improperly maintained.

 

Nitrous Fumes

Definition: Term applied to mixtures of oxides of nitrogen as found in mines, mainly nitrogen dioxide (NO2) and nitrogen peroxide (N2O4) with nitric oxide (NO).

Remarks: See Nitrogen Dioxide. Found in working places immediately after shot firing (blasting). A small amount is found in the exhaust gases of diesel engines but this should be rapidly diluted and dispersed by the ventilating air.

 

Water Gas

Definition: Combustible mixture of gases.

Remarks: Variable composition - typical composition of undiluted water gas about 45% each of carbon monoxide and hydrogen with smaller amounts of carbon dioxide, methane, nitrogen and oxygen but, when due to formation in a mine is usually mixed with air or "after damp". Formed in a mine when water is hosed on incandescent masses of coal or coke as when extinguishing a fire. The water gas formed could produce a secondary gas explosion as the lower and upper limits of flammability in air is 6% to 9%, and 70% respectively. Water gas burns with a blue flame forming carbon dioxide and water vapour. Water gas is very poisonous by reason of its high carbon monoxide content.

 

Producer Gas

Definition: Combustible mixture of gases.

Remarks: Produced commercially as a gas fuel, being formed by action of air passing through a layer of incandescent fuel (coal, coke or charcoal). A typical composition of the gas from a producer gas generator is roughly 10% CO ² , 15% CO, 74% N ² , and up to 1 % of other gases CH4 , etc. An identical mixture of gases can be formed in mine fires. It is capable of explosion with air and is very poisonous due to its CO content.

 

Aldehydes

A particular group of organic compounds of carbon, hydrogen and oxygen. Small amounts of aldehydes are found in the exhaust gases of diesel engines and are the chief cause of irritation to eyes and nose.

 

Fumes

Definition:

a) The gas and smoke, more especially the noxious or poisonous gases, given off by the explosion or detonation or burning of explosives.

b) Fume is also a term applied to metals or metallic compounds that have been volatilised at high temperatures in the form of small particles. Also applied to fumes from acids, and the electrolyte mist formed when charging batteries.

The character of the fume is influenced largely by the completeness of detonation and the degree of confinement of the charge. See "nitrous fumes".

 

Other Miscellaneous Gases

Type (a) Occluded gases of coal seam consisting of ethane (C2H6), propane (C3H6 ), etc., in very small quantities associated with the methane.

Type (b) Gases from distillation of coal occurring during a mine fire which include various saturated and unsaturated hydrocarbons of the aromatic and aliphatic series - includes benzene (C6H6) which has a distinctive odour and can sometimes be detected in mine fire gases.

NOTE:Persons affected by any of the noxious gases listed above should be removed to fresh air as quickly as possible. They should meanwhile be treated with oxygen, and if necessary by assisted respiration. In conditions in which poisoning by gases and damage listed is likely to occur, a deficiency of oxygen may also exist in the atmosphere so that the symptoms experienced may be combined with, or may be due to, those caused by insufficient oxygen.

Persons who recover after treatment, as above, even it they do so very quickly, should be advised to see their doctor.

Methane (CH 4) Accumulation and Layering

A compound of carbon and hydrogen. It is colourless, odourless, and tasteless. It. forms a flammable mixture with air. Lower flammable limit 5%, upper flammable limit about 15%. The most easily ignited mixture in 7,5% and the most explosive mixture 9.8%. It has a relative density of 0.55 - almost half as light as air.

Effects on persons

Non poisonous but will displace oxygen and may cause asphyxiation (suffocation) because it does not support life. Lives have been lost, as a result of this gas, by asphyxiation. A specific risk is that of being overcome when examining a roof cavity containing an oxygen deficient atmosphere due to the presence of methane. The person overcome could then fall a considerable distance and suffer severe injury.

The methane hazard

Methane is the most common contaminant gas found in underground coalmines. It has cost more loss of life than any other gas found in mines. It is a lurking enemy, which can strike unexpectedly.

As a mine deputy, you would be expected to examine your district atmosphere in order to detect the presence of methane, and to take appropriate action to dilute and render harmless any accumulations or thin layers detected. This is a most important part of your study of mine gases. Your life and the lives of your co-workers depend on your skill in neutralising the hazard of Firedamp ignitions. You must know where to examine and what to do.

Where it is found in mines

During the formation of a coal seam, methane is produced along with carbon dioxide, higher hydrocarbons and other inert gases. The amount of gas in a coal seam and associated strata depends on how easily the gas was stored in the surrounding strata and in the coal seam itself during the coalification process. It can exist as free gas in cracks and fissures in the coal bed or be adsorbed on the surfaces of the interior passageways within the coal structure (pores). The gas is released by mining activity in the seam itself or in adjacent seams. The rate at which methane is liberated is highly variable among coal seams. Some seams are more "gassy" than others.

Methane Liberation

There are three broad general headings that the method of methane liberation may be classified under, they are:

1. Exuding or bleeding from the coal seam and associated strata.
2. Blowers
3. Outbursts

Of the three methods of methane liberation listed above, exudation or "bleeding off' produces by far the greatest volume of methane liberated by mining operations. As coal is extracted whether from first workings, pillar extraction panels or longwall units, fissures are formed by forces exerted by relaxation of the strata as the removal of the coal seam creates goaf areas. The gas so released is mainly diluted to almost undetectable limits by the ventilating air current coursing through the working place. In this way vast quantities of potentially dangerous gas is diluted and removed from the mine as it is silently liberated from the seam and associated strata during coal extraction. However, considerable quantities of methane are liberated in places and areas where the ventilating current of air is very weak or almost non-existent. Such places as goaf areas, sealed-off old workings, roof cavities etc. and methane occupying these unventilated locations can constitute a potential hazard during falling atmospheric pressure conditions.

The gas so released can be methane or carbon dioxide or a mixture of both. When this mixture is predominantly carbon dioxide it is so heavy that it flows along the floor and the term "Bottom Gas " is applied. As a potential deputy you will be called to monitor and control the emission of methane caused mainly by processes (1) and (2) listed above. Detection and appropriate control action are the most important aspects of a mine deputy's duty with respect to the safety and health of the mineworkers under his supervisory control.

Accumulation

Methane has a relative density of 0.55, when liberated it tends to migrate to the roof or high points in the mine workings. Just as water lies in pools in hollows in a mine roadway and will flow to the lowest points in the mine working, methane behaves in a converse way. It flows to cavities in the mine roofs and to the high points in the workings; Water flows downhill - Methane flows uphill.

Barometric effect

Gas will accumulate in any unventilated area of a mine. This includes goaf areas which are impossible to ventilate properly and the gas accumulations here are subject to the effects of atmospheric pressure. That is, as the barometer rises, air will be pushed into goaf areas and the gas therein will be diluted; when the barometer falls as it must do eventually, then this mixture of gas and air will be forced out into the active mine workings. This is the reason why the barometric effect must be studied in conjunction with mine ventilation and particular care taken when the barometer is high because it will fall in time. (Remember also the mine will show the effect of variation before the instrument itself can begin to record the change).

Poorly ventilated corners, or cavities, can present a hazard, however good the general ventilation, and a falling barometer may rapidly bring forward the fringe of flammable gas onto the coalface from the back of the goaf, and this may cause a layer to stream along the face.

Layering of firedamp

In inclined roadways, methane will travel on the roof in a thin layer, often only a centimetre or so in thickness) and under the effect of its buoyancy against the ventilation in some cases for considerable distances.

Layering is particularly dangerous, as it is difficult to detect. It leads to gas accumulations in unexpected places and, in the event of an ignition, the explosion can be transmitted over a considerable distance involving a number of gas accumulations acting as a fuse to any roof cavity accumulations within the explosive range.

Close to the perimeter of an airway the air velocity is lower than towards the middle. The combined effect of this low velocity of airflow close to the perimeter with the buoyancy of the firedamp emitted from the roof may result in a tendency for layers to form at roof level. This tendency to layer will be the less where the airway velocity is high and where the roof of the airway is rough causing turbulence.

Formation of layers

Methane emitted at roof level tends, by buoyancy, to remain near the roof except to the degree that it mixes by turbulence (eddying within the general forward flow) and by drag against the underlying airflow. The degree of turbulence of an air stream increases with its velocity, and the degree of entrainment between a layer and the main air stream depends on their relative velocity.

For a given rate of outflow from a feeder and given dimensions of airway, there is a critical ventilation velocity below which a layer will tend to form and above which layering is prevented.

For feeders of equal emission rate in the same roadway the tendency to layer will be the greatest at places where the velocity is the lower.

Velocity of ventilation

It is normally (and correctly) advised that ventilation in inclined workings should be ascensional on the grounds that the movement of the air and the buoyancy of the gas work together to carry away firedamp.

Nevertheless, although firedamp is carried uphill by its buoyancy, it may remain as a layer without mixing, due to low relative velocity. For similar air velocities and similar rates of emission, a layer may extend over a much greater length in an ascensionally ventilated road than in a descensionally ventilated road. On the other hand, in a descensionally ventilated road, if the gradient is steep enough, a layer could back against the ventilation.

The first line of the defence against layering is good ventilation, so that firedamp emitted at a high concentration is diluted as rapidly as possible through the 15% to 5% flammable range.

Adequacy of ventilation depends on air velocity (m/s), not only on air quantity (m3/s). The low air velocity near the periphery of an airway may be further reduced by such obstructions as projecting cable hangers or pipes. Therefore additional tests for gas should be made near to such obstructions.

Special efforts must be made to prevent the formation of a layer connecting an accumulation of gas with a possible source of ignition. Such sources include not only electrical equipment and drillholes, but also any intense frictional contact between two very hard materials.

Detection of layer

In examining a gate or roadway for layers, tests with a methanometer and probe should be made within 10 mm of the roof every 5 to 10 m along the roadway, starting at the face.

Each of these tests should comprise (a) roof samples (left hand, centre and right hand) and (b) a general body sample. A roof concentration appreciably higher than the general body concentration indicates a tendency to layering. In this latter event, additional measurements should be made to locate the layer (its length, its depth, and its source) with a view to dispersing it.

Removal of a layer / accumulation

In dealing with a layer the immediate first step usually is the erection of one or more brattice sheets of wings. The purpose of these is to increase the velocity of flow at roof level by obstructing the flow in the middle and upper parts of the airway. It is important that the sheets or wings are correctly positioned: for instance, the first one should be sited immediately upwind of the main feeder. Further sheets downwind of the feeder, and a few metres apart, may be needed to complete the dispersal. In any case, if there is more than one large feeder there will need to be more brattice sheets Approved venturi air movers offer less resistance to airflow and can be used to break up gas layers.

An accumulation at a high point in a roadway may be treated similarly. It is advisable that a brattice sheet should deflect more than the minimum necessary quantity of air, in case it is not efficiently replaced when disturbed by passing traffic.

A gas filled cavity should be stowed full without delay. If this is impractical it may be possible to clear the cavity of gas by means of a brattice sheet or by means of a venturi (earthed and exhausting through a tube from the top of the cavity) or possibly by a hose connected in to the side of a ventilation duct, or by piping leading from the cavity to the return airway.

More reliable than the preceding control methods would be sufficient increase in the district airflow if this can be achieved.

Alternatively, or additionally, it may be possible to reduce the make of firedamp by modification of the firedamp drainage arrangements if firedamp drainage is practised at your mine. Any such arrangements would only be authorised by a senior mine official.

General body detection

If methane is detected in the general body of the ventilating current, then Mining Legislation and the Mine Gas Management Plan prescribe immediate action. Such action would include isolating power from all electrical apparatus, the cessation of any shotfiring in the immediate vicinity or any place on the return side of the gas hazard, and the withdrawal of workmen to a fresh air base when the statutory withdrawal percentage is reached.

Removal of a layer / accumulation

In dealing with a layer the immediate first step usually is the erection of one or more brattice sheets of wings. The purpose of these is to increase the velocity of flow at roof level by obstructing the flow in the middle and upper parts of the airway. It is important that the sheets or wings are correctly positioned: for instance, the first one should be sited immediately upwind of the main feeder. Further sheets downwind of the feeder, and a few metres apart, may be needed to complete the dispersal. In any case, if there is more than one large feeder there will need to be more brattice sheets Approved venturi air movers offer less resistance to airflow and can be used to break up gas layers.

An accumulation at a high point in a roadway may be treated similarly. It is advisable that a brattice sheet should deflect more than the minimum necessary quantity of air, in case it is not efficiently replaced when disturbed by passing traffic.

A gas filled cavity should be stowed full without delay. If this is impractical it may be possible to clear the cavity of gas by means of a brattice sheet or by means of a venturi (earthed and exhausting through a tube from the top of the cavity) or possibly by a hose connected in to the side of a ventilation duct, or by piping leading from the cavity to the return airway.

More reliable than the preceding control methods would be sufficient increase in the district airflow if this can be achieved.

Alternatively, or additionally, it may be possible to reduce the make of firedamp by modification of the firedamp drainage arrangements if firedamp drainage is practised at your mine. Any such arrangements would only be authorised by a senior mine official.

Key Elements of Sub-section Methane

1. Methane is a light gas.

2. Methane is a flammable gas.

3. Methane in large volumes can be suffocating - due to displacement of oxygen in the mine air.

4. Methane is stored in the coal seam and associated strata.

5. Methane is given off during coal extraction by:

a) Exuding or bleeding off quietly and imperceptibly.

b) Gas blowers or jets.

c) Instantaneous outbursts.

6 Methane liberated will migrate to high points in the mine workings.

7. Methane can layer at the roof of mine workings and back up against the airflow.

8. Detection and appropriate action make for safe working conditions.

9. Methane can invade the mine workings during falling barometric conditions from goaves and sealed-off areas of the mine.

10. Once diluted by the ventilating air current methane cannot separate out from the air stream.