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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:
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:
-
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 Č
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 Č
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 Č
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 Č
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 OČ 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 Č 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 Č
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 Č
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 Č , 15% CO,
74% N Č , 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.
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