Fire detection and suppression in mines
June 2017, IS & Ex
Sperosens’ solutions for the South African mining industry include deep mine distributed telemetry systems, coupled with carefully chosen fire detection and suppression options catering to unique customer requirements.
Distributed sensor networks monitor the quality and safety of air in underground mines and are primarily focused on four application areas:
• Monitoring for safe working environments for humans.
• Monitoring for early detection of underground fires.
• Monitoring for explosive atmospheres in fiery mines.
• Monitoring for optimising of ventilation system performance.
The challenge in underground mining telemetry is the long distances underground. Without the benefit of wireless devices due to RF propagation constraints, it has to be cable based systems.
From a central vertical shaft, mining workings are typically spread in a radius of 6 to 10 km, with as many as 15 separate vertical levels. The total distributed area to be covered becomes very large and in a typical mine it is not uncommon that 300 km of telemetry cable is installed.
Communication protocol deployed is serial digital communications, and must be rugged enough to withstand harsh physical and electromagnetic conditions, from shaft hub to the furthest working areas.
The collective data from all the levels are fed into the mine’s fibre optic backbone in the shaft and communicated to the control centre on surface. Modern computer systems in the control centre provide the functions of data logging, alarming, displaying, trending and reporting.
The underground telemetry infrastructure is utilised for all four monitoring applications listed above simultaneously, without fear of clash of interest. This is very good use of infrastructure.
Suppression system rational design principles
Fire risks on many industrial installations, including mining, are unique and cannot be controlled using a set of easily interpretable fixed rules from a handbook.
To design these applications a process known as rational design must be followed, which considers the unique parameters associated with the risk, and then applies a suitable international guideline or standard (such as SANS standards and the various NFPA standards) in search of a practical solution.
This rational design process is normally done by a professional engineer or engineering technologist registered with ECSA, and with fire credentials and experience.
Fire suppression principles
The easiest method to kill a fire is to remove the heat, in other words to cool the area down. The next best option is to displace the oxygen, and hence smother the fire.
Cooling down is the most reliable method, since after cooling down a fire cannot re-ignite spontaneously due to residual heat. This means that the energy source that generated the heat in the first place must also be removed. The most economical way of cooling is water, or a water foam mixture.
Displacing the oxygen is usually done in cases where cooling down is not an option, i.e. for fires in the presence of energised electrical equipment and oil fires. The methods of displacing the oxygen is to use an inert gas to displace oxygen, or to use a high density foam blanket or dry chemical powder to cover the burning object effectively preventing oxygen from reaching the burning fuel.
Fire detection linked to a suppression system
A fire suppression system is only useful when it is linked to a reliable activation system, better described as a fire detection system, or an early warning system that provides warning signals indicating that a fire may occur.
It is far better to prevent a fire than to suppress one. A good fire detection system should therefore have a pre-alarm that initiates a predetermined protocol to cool down or shut down equipment when a risk for ignition is detected. If pre-alarming fails, only then must a fire detection system actually release the suppressing agent and also shut down the energy source that generated the heat in the first place (i.e. electrical current or rotating or moving machinery).
The best technique to obtain early warning is to place temperature sensors or infrared scanners at the correct locations. A temperature sensor placed correctly will give immediate indication of a local temperature that poses an immediate risk. A temperature trend over time can be used to warn of an increasing risk to trigger preventative measures. An IR scanner is able to analyse a larger surface area for developing problems.
A temperature sensor placed directly on a bearing housing gives immediate reading of imminent bearing failure. An IR scanner observing a rotating conveyor belt drive pulley surface, will give indication of a belt that is slipping and heating up the pulley surface.
An actual flaming fire is best detected with a suitable flame sensor that gives a fast and accurate alarm. In open environments, indirect sensing of smoke and gas is an unreliable method to get immediate detection of an actual fire.
Distributed temperature sensing (DTS) using a fibre optic cable is becoming more affordable and feasible. DTS makes it possible to guard against temperature hot spots over very long distances, up to 10 km per instrument. Typical applications are the full length of a conveyor belt or the full length of cable, road or rail tunnels.
Application-based suppression techniques
The normal approach on conveyor belts is to protect the areas of greatest risk. These are normally the so-called head end and tail end. The head end is the one end of the belt containing the large pulleys that drives the belt. The tail end is at the opposite end where the belt loops back around. The reason for the higher risk in these two areas is the likelihood of belt slip over driven pulleys, as well as bearing seizure of free running pulleys. In such events the belt can catch fire due to friction or the bearings can overheat igniting lubricant materials. Conveyor belt heads and tails can be protected by a foam system (using NFPA 16 as guide-line), of by water spray (Deluge using NFPA 15 as a guideline). Generally a deluge system requires larger volumes of fire water than a foam system and is utilised as such.
Full conveyor belt suppression systems are much less common than local area protection systems. However, the mining industry is moving towards this goal, which requires distributed sensing, selective zone activation and much higher fire water supply requirements. Correct conveyor fire suppression system design also requires cooling of the steel supporting structure. If a steel supporting structure of a conveyor belt installation is lost in a fire it is difficult and time consuming to replace the steel structure lost and the consequent long period of operations interruption may be a serious business loss.
Power transformers are another high fire risk. Especially oil cooled transformers. Extinguishing a burning oil filled transformer is practically impossible. However, early warning of an overheating transformer can release a NFPA 16 or NFPA 15 suppression system cooling the unit down (after isolating the electrical supply side). An interesting fact is that the addition of a foam mix such as AFFF to the fire water increases the suppression/cooling effect of the same volume of water by a factor 4 due to better penetration of the mixture which overcomes surface tensions much better, hence cooling hot areas more effectively.
Electrical switchgear cubicles often catch fire due to overloading or ageing contactors that heat up. Here, introducing water is not an option, hence displacing oxygen is the chosen method. Two techniques are used, i.e. introducing an extinguishing gas or dry chemical powder (DCP) into the cubicle. Common gases that are used are Novec123 or FM200 or NAFS125.
A new technique that is fast becoming popular for fire suppression is high pressure water mist, which is when water is broken down into droplets smaller than 50 m using high pressure (> 100 Bar) and suitable nozzles.
The advantages of water mist are:
• Due to the small droplet size the total surface area of a specific volume of water introduced as mist into a fire zone, compared to the same volume of conventional water spray, is enlarged by a factor 100.
• Fast evaporation of the mist droplets when entering the fire zone forming steam, extracts 2200 kilojoule of energy from the volume around the fire, which does not happen so effectively in the case of water spray which normally has to come into contact with the hot surface before evaporation takes place.
• Thirdly, 1 litre of liquid water turns into 1650 litre of steam when it evaporates. This huge expansion of volume displaces the air (oxygen) from the fire zone.
Water mist therefore combines the effects of cooling and oxygen displacement in a very effective way. Other advantages of water mist suppression are that it requires greatly reduced volumes of fire water, while the pipework concerned is cheaper and easier to install due to the smaller diameter when compared to water spray piping requirements.
Applications for water mist include:
• A replacement for conventional sprinkler systems and gaseous fire extinguishing systems in many applications. Water mist causes very little damage to furniture and equipment and is not harmful to personnel that may be exposed to the mist).
• Computer rooms and data centres (the nature of the very clean water used means it can be used on operating computers without damaging the electronics or the need to shut down the systems).
• Power generation equipment and paper mills are important applications. Water sprinklers and water spray can severely damage super-heated machinery on contact. Water mist prevents this from happening.
For more information contact Marihette Hattingh, Sperosens, 086 177 3767, email@example.com, www.spero.co.za