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Rational design principles
April 2017, Fire & Safety

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 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 a raging fire. A good fire detection system should therefore have a pre-alarm doctrine that initiate 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 slips which is heating up the pulley surface, eventually igniting the belt.

An actual flaming fire is best detected with a suitable flame sensor which gives fast and accurate alarm of a fire. In open circumstances 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 continuously 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.

Fire suppression principles

The easiest method to kill a fire is to remove the heat, in other words to cool down. The next best option is to displace the oxygen, hence to 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.

Application-based suppression techniques

The normal approach is to protect the areas of greatest risk. Full suppression systems are much less common than local area protection systems. However, the industry is moving towards this goal, which requires distributed sensing, selective zone activation and much higher fire water supply requirements. Correct fire suppression system design also requires cooling of the steel supporting structure. If a steel supporting structure of i.e. a transformer 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 a 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 of four 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. Introducing water is not an option, hence displacing oxygen is the only option. Two methods are used: 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 growing fast is high pressure water mist for fire suppression. Water mist is per definition when water is broken down into droplets smaller than 50 m using high water pressure (> 100 Bar) and suitable nozzles. The functioning 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 of 100. Hence the cooling effect due to droplet size alone is 100 times better.

• In addition the very fast evaporation of the mist droplets when entering the fire zone forming steam, extracts 2200 kilojoules of energy from the volume around the fire, which does not happen as 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 litres 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. One major advantage of water mist suppression is that it requires much reduced volumes of fire water, while the pipe work concerned is cheaper and easier to install due to smaller diameter piping compared to water spray piping requirements.

Applications for water mist are:

• A replacement for conventional sprinkler systems and gaseous fire extinguishing systems in many applications. Water mist causes 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 severely damages super-heated machinery when it gets in contact. Water mist will prevent this from happening.

Sperosens is a leading supplier of total fire solutions to the South African industry. Solutions include carefully chosen detection and suppression options, catering for each unique customer requirement. Additionally the Verifi Fire System Status Monitoring solution provides an encompassing, accurate, always-available status of various critical systems such as fire detection and suppression. Verifi will monitor everything from water pressure to servicing of fire extinguishers.

For more information contact Marihette Hattingh, Sperosens, 086 177 3767, marihette.hattingh@spero.co.za, www.spero.co.za


Credit(s)
Supplied By: Spero Sensors & Instruments
Tel: +27 12 665 0317
Fax: 086 562 6511
Email: info@spero.co.za
www: www.spero.co.za
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Further reading:

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