Fire detection and prevention in underground mines
Fire detection and prevention in underground mines
ABSTRACT
Underground coal mines are more prone to underground fires, this is due
to fact that coal itself is a combustible matter. Underground coal fires can be
severe that mining operation may have to cease. The severe consequences of
underground fires may be coal dust explosion and burning of coal seams. The need
for designing, implementing safety measures and systems to prevent and mitigate
the impact of underground fires is of utmost importance. Coal mines all over the
world are implementing new technologies to prevent, detect, suppress and
monitor underground fires. South African coal industry is yet to improve on
these technologies. This report elucidates on the causes of underground fires
in coal mines, the detection methods, the prevention methods, fire suppression methods,
alerting and evacuation during fires, latest technological trends and lastly the
need for training on fire awareness. The conclusion will give a brief
discussion on the new technology that I’m currently designing which I believe will
enhance and effectively monitor environmental conditions undergrounds.
INTRODUCTION
Underground fires are one of the major hazards in coal mines. The
impact of fire outbreaks can be severe. It may potentially lead to fatalities, injuries,
ill-health problems, accumulation of dangerous gases and fumes, loss of
revenue. The risks and impacts of fires in underground coal mines is
significant that regulations had to be drawn to manage fire hazard properly.
According to mine health and safety act (MHSA ACT;1996), “the management of
underground fires can be achieved by an assessment of the fire risks involved,
monitoring of fire risk controls, good mine ventilation system design, the
preparation, implementation and enforcement of mine standards”.
UNDERSTANDING THE CONCEPT OF UNDERGROUND COAL FIRES
The basic fire theory describes the three constituents required to be
simultaneously present for a fire to occur. The figure 1 above shows the fire
triangle that illustrates the requirements of fire, where oxygen which will be
air, source of fuel which will be coal or other combustible materials, and a
source of heat. The presence of these constituents in a coal mine is monitored.
To prevent fires, one of these constituents must be removed. However in a
coal mine it is not possible to remove the presence of fuel which is coal since
it is the commodity being mined and also oxygen which is air cannot be removed
since ventilation is mandatory to sustain health and safety working
environment. The only thing that can be easily removed or managed is the presence
of the source of heat, there are many ways to monitor, control and eliminate the
source of heat.
Sources of heat are not limited to the following listed herein :-
- Naked flames
- Electrical sparks
- Spontaneous combustion
- Hot work
- Explosives
- Combustion engines
- Contraband
STAGES OF FIRE
INCIPIENT
This is the initial stage where the constituents of fire are combined in suitable proportions to initiate a chemical reaction that result in fire. In this stage there’s a high chance of successful fire suppression.
GROWTH
This is the second stage, where there is a continuous supply of factors
that affects the growth of fire, these factors are fuel and oxygen. In this
stage heat is increasing and its one of the dangerous phases if not managed
properly and on time.
FULLY DEVELOPED
This is the third stage, where the fire has reached its maximum
potential, where all consumable constituents of fire have been ignited or
consumed. The is the most dangerous stage of them all as there are high
temperatures involved and fire suppression is not easily accomplished, especially
in coal mines. This will be explained later in fire suppression section.
DECAY
This is the last stage where there is a reduction in fuel and oxygen,
resulting in the fire to cool down, temperature is reduced but does decrease
gradually slowly. This stage is the longest stage of them all, however it’s not
too dangerous.
The following objectives were generated:-
- Identify sources of fire
- Identify fire detection systems used in coal mines
- Discuss fire prevention systems
- Outline new technological trends
- Analyse fire management systems
The literature review of this project is limited to only underground coal
mines. It will focus on fire detection, fire prevent, fire suppression, fire management
systems and latest trends.
Research methodology is important as it creates a logic that is used in
this project to join data and information gathered to the conclusion that will
be drawn. It outlines procedures and methods used to collect relevant information
and assist in analysing and meeting the objectives of this report.
This project took 7 weeks. 3 weeks of literature review, 3 weeks of results
and one week of final editing.
This research is based on the principles of qualitative research, where
literature was reviewed to gain insight of the chosen topic and was utilised to
forge and gain detailed understanding of mine fires.
Data was collected from the following sources
- Academic articles
- Mining gazzate
- Industry news from company websites
- Academic textbooks
- Articles from industry organisation ( SAIMM, Etc)
FIRE DETECTION
Underground fires are a serious hazard, according to mine health and
safety act (MHSA 1996) “it is the duty of care of mine employees to prevent and
report any accidents associated with fires and also a duty of care for the employer
to provide and maintain a safe working environment”.
Fire is a hazard, therefore all underground coal mines are required to treat
it as such and conduct a proper, effective risk assessment that has adequate
measure to prevent, detect and suppress underground fires. Before fire
detection is implemented, there are crucial steps to be done to ensure that
fire detection application is correct, suitable and specific to its operations
[5]. This is one of the reasons why the risk assessment is a crucial tool. Risk
assessment identifies areas where there’s a likelihood of fires occurring, it provides
appropriate controls to manage the risks and prevents the outbreaks of underground
fires.
Figure 2 about shows a risk management cycle of underground fires. Risk
management is a reasonable, organised method of recognising, evaluating,
assessing, treating, monitoring, and communicating risks linked with any
activity, or occupation in a way that will allow organisations to minimise
losses and maximize opportunities. Since fire is a hazard, risk management is
crucial for coal mines to ensure safety of underground workers.
Risk identification will enable us to identify areas underground with the
risk of fires, a survey should be conducted to identify sources of ignition and
fuel in all underground areas. This will help locate areas with the significant
likelihood of fire occurrences.
Risk assessment provides a clear view to the following questions:
- Which areas are likely for underground fires to occur?
- What are the drivers of underground fires?
- What equipment and material can cause fire?
- What’s the fire magnitude that can be expected based on fuel sources and ignition sources?
- What is the likelihood of fire occurrences in those areas?
- Which measures are appropriate for certain types of fires?
General sources of underground fires are not limited to the following:
Blasting activities (misfires and blown-out shots) Explosives (handling, usage and storage) Conveyor belts Electrical installations Faulty electrical cables Underground machinery (non-flameproof) Spontaneous combustion Coal winning (coal cutter, CM, etc.) Hot works.
The methods can be classified into non-instrumental and instrumental detection.
These methods relies on a human senses to detect fires. These senses can be smell, touch, and sight. However it should be noted that human sense are not reliable tools as they cannot be quantified. Smoke “by smell” This is a very common and obvious method of detecting fire in the real world besides the mining environment. Smoke gives clear signs of active fires, advanced stages of heating and smoke from recently burned objects. Haze “by sight” This is not a common method. Not everyone can effectively use this sense in this context. It depends on the level of knowledge, training, and skills to detect and understand. During spontaneous combustion of coal, surface moisture of the coal is evaporated into the circulation air, together with water given off as product of combustion causes air to become warm and saturated with water vapour. When the evaporated air comes into contact with other ventilation currents, condensation takes place where a visible form of cloud of haze may form and becomes visible to see. Radiated heat from hot surfaces “by touch/ felt”
This is also an unlikely method, as it also requires a certain level of
understanding to be able to detect fire. According to Fauconnier (1975), “this is
where the heating that originates from sealed off areas are leaked or where
coal heaps generate heat during oxidation reaction can be felt as heat
radiated”.
Characteristic smell
This is where the smell given off by spontaneous combustion of coal is detected.
This also requires some sort of experience or training to be able to
differentiate the smells. This method can be very misleading as smell can
be impaired by some gases, for instance hydrogen sulphide impairs the sense of
smell within minutes.
This is the mostly used method of detection in most mines as instrumental
detectors provides accurate and quantifiable results. In this report the
researcher will focus on three instrumental detectors.
Chemical detectors
They can be classified into two categories namely detector tubes and length-of-stain
tubes.
Physical detectors
These devices are commonly used, they observe rates of temperature
increases as a result of incipient fires.
Physicochemical detectors
These detectors are mostly used in the detection of oxidation reaction
results [5]. They are not currently used in South African mines.
This report will focus on instrumental detectors as they are the most
reliable methods of detection. They have proven to provide substantial results
and information that can used to make decisions, for planning and improving systems
[10]. Detectors can be both prevention and mitigation method depending on their
application and functions. Those detectors that acts as prevention detect
conditions that might result to fires and those that act as mitigation method
detect conditions that are a result of incipient fires, for instance smoke.
Detectors are located in areas where there a likelihood of fire occurring
and where non-instrumental detection is not practical. This is process of
locating detectors should be based on a fire survey that must be conducted and all
the forms of risk assessments (baseline, issue based and continuous).
Fire signatures
Product resulting from fire causes changes in the surrounding atmospheric
conditions, according to Graffin (2010), these changes care called fire
signatures. Fire signature are useful properties that enables the detection of fires. They must be quantifiable and measurable for practical detection of fires.
Types of signatures normally used for fire detection are: gas, energy, and aerosols
signatures.
Heat detectors
These detectors are physical detectors. They observe, detect and react to
temperature increases. Figure 3 below shows an example of a typical heat
detector used in coal mines. Heat detectors are classified into two categories which
are namely:
Open fires from combustible material burning
Fires from spontaneous heating’s
Depending on the type of environment, different principles are used by
these heat sensors. They employ rate of rise a fixed temperature principles.
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Figure 3 typical heat detection system
Smoke detector
These type of detectors are part of chemical detectors and
physicochemical detectors . They react to smokes produced by combustion. These
product are not limited to the few listed herein. Figure 4 below will show all these
smoke detectors listed herein, how they are installed underground.
Ionizing type detector/ Ionization-type detectors
This detector uses common ionizations emitters to create ionization currents
that ionizes the air inside an electrode chamber to detect the type of smoke.
CO sensors or electrochemical detectors
These detectors are used to detect or sense carbon monoxide [2]. They are
not highly favoured instruments as they can easily be interfered with by the presence
of other gases and also tend to be relatively expansive
Solid state detector
This detector reacts to certain gaseous components of the products of
fire. Variation of the chemical formulation allows the cell to be made
sensitive to a particular group and insensitive to a particular group of gases.
Optical smoke detectors
This is a very complex detection system and generally confined to
laboratory use.
Aerosol detectors
These detectors reacts to fire aerosols and experts recommend it for
immediate detection. This is because aerosols is one of the earliest detected
fire signature. There are two types of aerosols namely visible and invisible
aerosols.
Flame detectors
These detectors reacts to different light variations as a result of fire.
They respond to radiant energy from electromagnetic spectrum.
Photoelectric-type detectors
They respond to particulate matter like ionization detectors. They use
the principle of photoelectric effect. Light and its detector are requirements
to measure radiant power . This is a good invention, however it is very expensive
to use. It also requires extensive level of knowledge, education and experience
to effectively use it. Its disadvantage is that it can be affected by the
environmental conditions. If there is too much dust and high air velocity, false
alarm may be given off.
Infrared sensing detectors
Quantum energy detector: this ones reacts to excited electrons that are in
a bound state and photon lux that is falling on sensing element
Thermal detector: these detectors react to energy that is absorbed by temperature
sensing materials
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Figure 4 elucidate the type of smoke detectors and how each
smoke detector can be position underground
Fire prevention is actions deemed necessary and suitable to inhibit the
initiation of a fire or stop the evolution of developing fire [5]. From the
risk assessment conducted, fire prevention plan must be designed, implemented and
maintained to ensure safety. All combustible materials and tools, installations
and mine infrastructure that poses a threat from fire risk must be identified
and officially recorded and registered as a fire hazard. Fire prevention measures
must be reasonable addressed.
There are many areas in underground coal mine that are considered to pose
a fire hazard and its significant risks [6]. Fire prevention is a broad topic,
hence the report will focus on general requirements for fire prevention to
minimise the risk fires associated with the above mentioned categories. These
areas are not limited to the following categories.
Mine infrastructure: (fuel storages , Refuelling bay, Electrical
installations, Workshops, Explosive magazine)
The following are general fire prevention control for mine
infrastructures .
Infrastructure to be installed according to mine’s risk assessment.
Infrastructure to be adequately ventilated.
Infrastructure must contain minimum storage of combustible liquids.
Infrastructure floor to be smooth, and impervious floors if necessary, prevent
fuel spillage
Dangerous goods, flammable liquids to be stored safely and segregated
Use fire retardant materials to build
Use insulated electrical cables.
Areas must be clearly demarcated of fire awareness and emergency
preparedness.
Mining areas: (hazardous area, Return airway, Sealed off areas). The following
are general fire prevention control for mining areas [13]
Use flame resistant material
Supply adequate ventilation “Ventilation in mines and underground
constructions is a significant part of fire prevention. Noxious fumes quickly
spread and they are often the most important risk regarding fire underground” (
)
Coal seam to be adequately stone dusted
Coal seam to be adequately wetted down
Clean floor to prevent spontaneous combustions
Use flameproof equipment and apparatus
Effectively clear out flammable gases and continuously check
Cables must be suspended, must not be I contact with water
Equipment and fixed installations
The following are general fire prevention control for mine equipment and fixed
installation [13].
Use flame retardant materials
Use flameproof enclosures
Prevent leakage of fuel and oils
Use circuit breakers where possible
Equipment maintenance
Monitor engine oil pressures, monitor devices temperatures
Pre-check equipment before usage
Provide earth leakage and overload protection for fixed installations
Flammable liquids and combustible materials
The following are general fire prevention controls for mine flammable liquids
and combustible materials [8].
Use fire retardant materials
Clearly demarcate flammable liquid and combustible material
Clearly demarcate storage areas
Supply adequate ventilation and sufficient illumination
Located adjacent to return airways if possible
Prevent leakages
No smoking signs, no ignition sources and no electrical installations nearby
No hot work to be permitted
Good housekeeping mandatory
Access restricted to authorised personal
Flammable substances must be stored in quantities not exceeding mines risk
assessment.
Training and alarm systems
These also form part of the fire prevention controls, but in this report
they will be further discussed in later sections.
THE SUPPRESSION OF UNDERGROUND FIRES
Fire suppression is important as it ensures that a safe evacuation and management
of underground fires. During this stage the fire is rapidly growing or it is
fully developed (as stages of fires were discussed in section 1). At this stage
the fire has progresses to be dangerously enough that these incidents can
occur: potential trapping, injury and killing of miners, poor visibility, fumes
can cause suffocation.
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However it should be noted that in most cases it is difficult if not
impossible to suppress coal seams once they have ignited and have reached fully
developed stage [14]. Many coal mines have been shut down because of these fire
accidents. One can with confidence say once a coal mine has reached a fully developed
fire stage all the equipment mentioned in this document will be sufficient to
supress it. Figure 6 and 7 below show the common types of equipment used to
suppress fires, whereas figure 5 show how to suppress fire by removing one of the
three constituents of fire.
Suppression systems are determined from the risk assessment report [4].
This is planned after the type, magnitude and likelihood of fires have been
estimated. Knowing this information enables engineers to select appropriate, suitable
fire suppression equipment for every scenario predicted.
FIRE EXTINGUISHERS
These are very commonly known and used. They are normally found in mobile
machinery, fixed installations, conveyor belt headgears, storages and waiting places.
They need a regularly maintenance to ensure that they are always in good
working conditions [15]. There are three classes of potable fire extinguishers.
Class A: used for suppression of fires from solid materials such as wood Class
B: used for suppression of fires from plastics and liquids
Class C: used for suppression of fires from gases (not widely used in the
mining industry)
FIRE HYDRANT
This is one of the most effective suppression system. It uses a hose
that’s connected into a reel and it’s permanently fixed into a water supply
system. Fire hydrants are normally found along the conveyor belt [15].
SPRINKLERS
Although in coal mines their function may not be to primarily suppress
fires but to suppress dust generated during coal winning operation and during transfer
point in a conveying system, their secondary function is to suppress sparks and
heat generated [7]. There are normally found on transfer points, CM cutter head,
and where there is a possibility of oxidation of coal.
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Figure 7shows a fire hydrant in a reel
There are two general types of alerting mechanisms namely: internal alerting
and external mechanisms.
This mechanism is about alerting the mine’s rescue services and employees
about emerging situations. Internal in this context being the inside the mine
works. There are two reasons for this alert mechanism, namely fire alert and evacuation
[16].
Fire alert systems
There are different fire alarm systems but they all have one common purpose
which is to protect personnel employed underground and within mine property. This
is done by giving an alert at an early stage so that any emerging fires can be
quickly suppressed at incipient stage. It must be emphasised that these alert
devices are installed to guarantee safety, therefore the demand for reliability
and quality are of utmost importance.
Evacuation alert system
This system is for alerting labour to evacuate the underground workings, this
only happens when the need for evacuation becomes necessary. These systems may
be manual or automatic. They may either alert by sound or light or both,
preferably both.
This mechanism is the last alerting resort. Procedures on alerting
external parties such as local rescue services to intervene with emerging
problems must be properly communicated. At this stage the fire would have been beyond
the mine’s internal rescue service capability to suppress.
Reliable early fire detection in underground mine can be hindered by
external factors that causes false alerts [16]. These false alert jeopardizes the
safety of mine employees in the event of fire, this is because
workers tend to get used to false alerts and then decide to ignore these
any alert. This can be dangerous because in the event of real fire these systems
will be ignored and could lead to catastrophic disasters. Means to reduce the risk
associated with false alerts should be addressed and implemented continuously.
As discussed in the above section 5.1(II), there is a need for evacuation
systems in case of an emergency. These system enables workers to leave the
underground working environment safely and according to the prescribed means
[9]. The successful implementation of these system will minimise the impact and
save lives.
These plans must be developed and implemented before operations are
undertaken. They are based on a baseline and continuous risk assessments. The plan
is not limited to the aspects listed below:
Use of personal rescuers
Procedures for reporting fires
Refuge bay procedures
Evacuation and escape route procedures
Medical and first aid procedures
It is important that workers be trained and given sufficient basic
knowledge of underground fires. This knowledge will enhance chances of survival
during occurrence of fires and prevent fires [13]. It must be noted that common
sense is not always common. This basically means that our level of
understanding and interpretation of information is not the same. So the
employer has to ensure that every single employee has the relevant information
and knowledge about fire prevention awareness and emergency preparedness.
Systems, procedures, and technology are not sufficient measures without
relevant training and information [5]. It is safe to conclude that training is
the most crucial aspect in dealing with underground fires.
Types of training
Baseline training: this is the initial training of basic underground fires
awareness
Refresher and periodical training: this training is done at least once a
year, to refresh, update, and provide updated information regarding fire awareness.
What aspects must be included in training awareness programmes? [1]
Basic understanding of fire theories
Basic understanding of prevention methods
Sources of fire and risk areas
Emergency preparedness
Fire suppression systems
Fire detection systems
Procedures and channels of reporting mine fires
Detailed and specific knowledge and information on each of the major fire
potentials, such as (Explosives, mobile equipment, fixed installations,
electrical installations and apparatus, flammable liquids, etc.)
Importance of emergency drills
Alert systems
Fire prevention measures
Legal aspects
LATEST TECHNOLOGY FOR FIRE DETECTION
Mining technology needs to progress with times. Depending on a type of
mining, the detection and prevention systems can be either cyclic or
continuously [17]. Lately, more research is based on automation mining and
mechanization, this means that the detection methods need to improve also. The
best detection method that can suitable synchronise with automation mining is
the remote detection systems [14].
These remote detection systems need to be able to effectively operate
remotely with minimum human interference, and also need to transmit data electronically
to control rooms. These instruments needs to be more reliable and provide
accurate feedback even in harsh underground conditions. This report will name on
two latest technologies, figure 8 and 9 will show pictures of these instruments.
Fibre Optic Linear Heat Detector
This detector uses an optic fibre cord that’s able to sense and withstand
harsh condition. It is less susceptible to false alerts and has proven to be
reliable. It detects fire and overheating along the length of the conveyor
belt. The figure 8 below, show how the fibre-optic cable is aligned in relation
to the conveyor belt.
Figure 8 shows illustrates the application of Fibre optic linear
detector
Benefits of optic fibre cable [11]
Increased safety and reduced false alarm
Can with stand harsh environment
Easy installation
Cheap maintenance
Reliable and accurate detection.
Thermal imaging camera detector
Figure 9 below shows the picture of the thermal image showing hotspots
among material on a moving conveyor bel. This detector is commonly applied in
Australia, Asia and Europe. It reacts to temperature increases, this enables
early detection. It uses a camera that senses the thermal conditions of the environment.
The camera continuously analyse thermographic images by use of infrared camera
[17]. This instrument is better suitable to be used in detection spontaneous combustion.
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Figure 9 image from the thermal imaging camera
8.0 DISCUSSION AND CONCLUSSION
Underground fires can be easily managed and prevented. Mines must train all
employees on fire awareness. The use of modern technology enables the mine to detect
fires early and supress it immediately at initial stages. Sources of ignition must
be closely monitored to prevent ignition of flammable gases and coal dust.
Mine fires may be a function of inadequate ventilation. Too much air supplied
in the working can minimise the occurrence of spontaneous combustion by
providing too much supply of air that will supress heat generated by oxidation
reaction of coal. The stockpiling of coal in the working faces must be
prohibited to prevent oxidation reaction.
Miners tend to get complacent about taking ventilation measurements
underground. This may be due to repetitive tasks during the examination of
workings that enables the miners to be familiar and easily estimate ventilation
readings based on visual inspections instead of taking actual measurement. This
behaviour tend to provide incorrect and unreliable feedback and information to the
VOHE department.
The need for accurate readings is of utmost importance to maintain health
and safety working environment underground. Mine excavations are not plane,
smooth, straight and exact. These factors may cause variation in the ventilation
calculations such as the quantity of air.
I propose that for future research, a modified technology that will
combine existing technologies such as velometer, thermometer, transmitter,
distometer, LED screens for readings, calculator, flammable gas measuring device
and flammable gas warning device to be designed and tested.
What will this device measure?
Air velocity
Air quantity
Average ambient temperature
Presence of flammable gas Purpose of this device
To measure all the above mentioned factors at once, at a specific point.
Calculate automatically the air quantity at a specific point (a distometer
will measure the dimensions of the heading to give area, air velocity will be measured
by the velometer).
Calculate the amount of air quantity and velocity required to dilute the gas
on specific point
Record data for number of days and immediately send the data (readings to
the control room)
To ensure that miners conduct actuals examinations at the prescribed times
on prescribed location. (Data sent to control will record the time of examination).
Benefits of this device
Will ensure that miners take actual readings at designated areas.
Will provide accurate readings (taking into consideration the actual dimension
of heading at a specific point).
Will provide numerous measurements at once (temperature, air velocity, air
quantity, flammable gas content)
Will immediately send or transmit data to control room.
Enhance productivity (less time taken spent on taking different
measurements, notifying control room, performing ventilation calculations).
Enhance safety (will be able to state and give required amount of air quantity
necessary to dilute flammable gases.
Save battery charging time and energy (only one battery will be used, instead
of different batteries for all the combined devices)
Although this technology requires an intense study and research, I
certainly believe it will enhance productivity, safety, ensure compliance to
legislation requirements and ensure reliability of accuracy of readings. If this
device can be implemented it can be utilised in two ways: installed in semi-permanent
or permanent areas (such as areas within the sealed off workings) and issued to
miners responsible for ventilation district.
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