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Choosing the best fire suppression technology is not an easy task. It even involves discussing risks and operations with insurance companies. The most relevant concern of a fire safety engineer is the protection of life which entails the safe evacuation of personnel.
The starting point of a suppression system is a risk analysis to reduce the potential occurrence of a fire. This is followed by the control of the damage and the recovery effort or emergency response associated with the means of fire suppression adopted.
The quality of installation, efficiency and maintenance of the suppression system adopted cannot be over-emphasised.
The phase out of halons, due to environmental concerns, has lead to forceful development of new fire prevention strategies and technologies that are efficient, as well as environmentally friendly technologies.
Fire protection halons were phased out of production in developing countries due to the quest to regulate the use of ozone depleting substances(ODS) as reflected in the Montreal Protocol,1987(London Amendment 1990, and Copenhagen amendment1992).
Fire suppression agents have two (2) toxicological aspects to them:
- The toxicity of the agent
- The toxicity of combustion products of the agent.
Several new fire suppression systems have been developed such as inert and halocarbon gaseous systems, water mist systems, gas and aerosol generators.
Fire has been extinguished with water since ancient times. Water in the normal form is not a suitable suppression medium of all classes of fire. The efficiency of water in suppression is enhanced by its use of water in form of mists.
Survey by Mawhinney and Richardson in 1996 showed that about 50 agencies worldwide are involved in the research and development of water fire mist and suppression systems.
"Water mist in fire suppression does not behave like true gaseous agents and is affected by fire size, the degree of obstruction, ceiling and the ventilation conditions of the compartment. To effectively suppress a fire, a water mist system must generate and deliver optimum sized droplets with an adequate".
1.2: Objectives and Structure of Dissertation
This project aims at studying the water mist as a replacement for halons systems in the extinguishment of fires. This replacement is a direct consequence of the phase out of halons due to environmental issues and the need to find a "drop-in" replacement or a suitable alternative in areas where high level of fire safety is required and the cost of fatalities is too high.
2.1: Overview of Fire Suppression
To suppress fires, the type of fire needs to be identified. The class of the fire to be extinguished also determines the type of extinguisher that can be used. There are six (6) types of fires:
- Class A FIRES: These involve flammable or combustible solids such as wood, rubber, fabric, paper and some plastics.
- Class B FIRES: These are fires involving flammable and combustible liquids or liquefiable solids such as oil, alcohol, petrol, paint and liquefiable waxes.
- Class C FIRES: These are fires involving flammable gases such as natural gas, hydrogen, propane, butane.
- Class D FIRES: These are fires involving combustible metals, such as sodium and potassium.
- Class E FIRES: "These are fires involving any of the materials found in Class A and B fires, but including electrical appliances, wiring, or other electrically energized objects in the vicinity of the fire, with a resultant electrical shock risk if a conductive agent is used to control the fire." http://www.sqa.org.uk/e-learning/FirstLineO2CD/page_06.htm
- Class F FIRES: These fires involve cooking fats and oils, especially in industrial kitchens. The temperature of these fats and oil on fire is much greater than that of other flammable liquids.
2.2: Means of Fire Suppression
The aim of fire suppression is to provide cooling, control the spread of the fire as well as extinguish the fire.
The behaviour of a fire is charcterised by the fire triangle which has fuel, oxygen and heat as its three sides.
Combustion process is represented by:
Fuel + O2 HEAT H2O + CO2 ……….eqn2.1
The combustion process is an exothermic reaction, involving a fuel and oxygen. The ratio of fuel to air must be within the flammability limits of the fuel for combustion to occur. The Lower Flammability Limit (LFL) is the minimum concentration of fuel vapour in air, below which a flame cannot be supported in the presence of an ignition source.
The Upper Flammability Level (UFL) is the maximum concentration of fuel vapour in air, above which a flame cannot be supported. Stoichiometric Mixture is the ratio of fuel in oxygen that requires minimal energy to support a flame.
A branch of the triangle must be removed for the fire to be extinguished. Fires can either be smoldering or flaming combustion. Smoldering occurs when solids such as wood or plastics burn at or on the surface. It usually involves the release of toxic gases and can be difficult to extinguish. Flaming combustion is a gas phase phenomenon that involves the release of visible and infrared radiation. This type of fire generates much more heat.
The extinguishing of a fire involves either chemical or physical mechanisms.
- Physical mechanism: Involves the removal of one side of the fire triangle. This can be done by either blanketing the fire (causing the fuel and air to be separated) or by removing the heat source using an agent with a high heat capacity/ latent heat of vaporization (this will cool the flame by absorbing the heat). Physical mechanism could be thermal or dilution. Thermal physical effect involves adding non-reactive gas to a fire plume leading to a reduction in the flame temperature. This is achieved by the distribution of the heat generated to a larger heat area. The heat capacity of the introduced agent determines the efficiency of the process. On the other hand, for dilution physical effect, the collision frequency of oxygen molecules with the fuel is lowered when the additional gas is introduced into the fuel-air mixture. This effect is quite minimal and negligible.
- Chemical mechanism: This is the use of an extinguishing agent or its degradation product to disrupt the chain reaction for sustaining combustion. This entails inhibition by halogen atoms.
Most good suppressants apply both the physical and the chemical mechanisms.
The type of hazard associated with an area determines the fire protection system that will be put in place. Halons have been used in a wide range of applications. Other alternatives include:
- Water Sprinkler Systems: This is a very common type of fixed protection that offers safe protection to limit structural damage. The cost of installing water sprinkler systems into existing structures is quite expensive. They are better at protecting structures than its contents . The reliability of water sprinkler system has encouraged its wide use. Accidental discharge is uncommon with water sprinkler systems. Water sprinklers have a much slower response than other systems. They also cause a considerable secondary damage. They cannot be used on live electrical equipment and flammable liquids, but they are used widely in computer and control rooms as well as storage rooms in the USA.
- Detectors: This involves the use of high sensitive smoke detection. This is not exactly an active fire protection approach but it serves as an initiator to other fire protection systems .
- Carbon dioxide: Carbon dioxide is widely used in gaseous based fire extinguishing systems. There are two types of carbon dioxide system depending on the manner by which they are stored. These are high pressure and low pressure carbon dioxide systems. It is a clean agent and has a good penetrating ability. This makes it safe for use on live electrical equipment. They are also used in unoccupied spaces such as computer and control rooms. Carbon dioxide causes very minimal direct or secondary damage and allows the installation being put back to immediate use after a fire. It is however toxic and cannot be used in total flooding situations. Carbon dioxide cannot also be used in situations where weight and space are important. High concentrations of carbon dioxide are required for extinguishment and as such they are bulky and heavy. They cannot be used in manned areas because they reduce the oxygen concentration to levels below life support and thus cannot be set in automatic mode. Carbon dioxide systems are generally fast acting and cost effective. Carbon dioxide has also found use in record storage, flammable liquid fires, chemical processing equipment, turbine generators, marine applications, computer rooms and shipboard machinery.
- Inert Gases: inert gases in use for fire suppression are majorly argon and nitrogen mixtures. These are electrically non-conductive fire suppressants. The mechanism behind their use is the lowering of the oxygen concentration of air to that below the lower flammability point (LFL). They are not liquefied gases and they are bulky because they are stored at high pressure. The concentration of inert gases released in the hazardous area is high because they have densities that are similar to that of air. Their response time is not very fast and so they are not efficient in situations where the rate of fire spread is high. Inert gases do not decompose thermally and thus they form no breakdown products . Inert gases can cause an extreme decrease in the composition of oxygen in the body accompanied by an increase in the concentration of carbon dioxide leading to loss of consciousness or death and as such health and safety issues have to be considered in its use. Inert gases have found wide acceptance because they pose no environmental problems. They are not ozone depleting substances neither do they contribute to global warming. They are employed in computer and control rooms, record storage, flammable liquid fires and shipboard machinery .
- Halocarbon Gases: These are hydrofluorocarbons and perfluorocarbons with zero ozone depleting potentials. They are however greenhouse gases and are governed by the Kyoto protocol and hence its release counts towards the national emissions inventory of global warming gases. Halocarbons are electrically non-conductive, are clean agents and are not bulky in terms of space and weight.
- Foam Systems: Foam systems could be low, medium or high expansion systems. Foam systems are efficient for extinguishing liquid pool fires and large cable fires. In this case, the foam acts as a barrier between the fire and the supply of oxygen. The use of chemical dispersants to clean up after its use has limited the wide use of foam systems. Furthermore the use of smoke detectors for its activation limits its speed of response. They cannot be used to protect any substance that reacts violently with water. Foams systems are often used with water sprinklers. This increases the efficiency of the systems. Foam systems have found use in the extinguishment of flammable liquid fires, engine compartments and shipboard machinery.
- Dry Powder: Powders have very high response time for extinguishing fires but have no cooling effect. They thus become ineffective as soon as it settles . They are limited in application to extinguishing flammable liquid fires as well as engine spaces.
- Fine Solid Particulates: This system is used in combination with halocarbon gases and inert gases . "They have the advantage of reduced wall and surface losses relative to water mist and particle size is easier to control". They however pose problems to sensitive equipment and cannot be used for explosion suppression applications because they are generated at high temperatures. Fine solid particulates can only be used in unmanned areas because of the problems associated with inhalation of particulate substances.
- Water Mist: This employs the use of fine water sprays, usually less than 100 microns in diameter. Water mists can be used on flammable liquid fires, as well as electrical equipment. They are not as effective on small or slow burning fires. Water mist installations pose problems in their design and fabrication.
- Hybrid Systems: Hybrid systems combine one or more of the above fire protection system. A common example of this is the combination of water mist systems and carbon dioxide.
There are two methods of applying fire extinguishing agents-Total Flooding and Local Application.
- Total Flooding: They are operated automatically and manually. It entails applying an extinguishing agent to an enclosed space to achieve a concentration of the extinguisher that is capable of putting out the fire. This method is the most common system of application
- Local Application: The agent is applied directly onto the fire plume or the affected enclosure. Portable fire extinguishers are the most common forms of this approach. This method is also known as "streaming application".
There is an increase in the need for the phasing out of halons and this has brought the search for the perfect or "drop-in" replacement. The department of trade and industry in 1995 listed checklists for the selection of alternatives to halons in critical uses situations as:
- Fire fighting effectiveness: This involves the speed of fire suppression, the post fire hold time, the ability of the alternative to permeate, the elimination of the risk of reignition, the suitability of the alternative to the fire hazard.
- Ease of Installation: Ease of maintenance, pipe work, and cost of installation, cost of refill, floor space and weight, system re-instate time, and availability of the extinguisher.
- Hazards to occupants: Toxicity, noise levels, pressurisation, inhalation, visibility, safety as regards electrical work, thermal decomposition products .
- Discharge effect on equipment: water damage, clean up, corrosion, thermal shock.
- Environmental acceptability: Ozone depletion potential, atmospheric lifetime, and global warming potential.
- Discharge damage: This entails clean up of the agent after use, water damage, thermal shock and corrosion.
Esso Australia, while looking for alternatives to halons on their installations considered the following issues :
- Effectiveness at extinguishing fires
- Environmental effects (a zero ozone depleting and global warming potential) of the agent before use and after thermal decomposition.
- Toxicity level and a safety margin between its No Observed Adverse Effects Level (NOAEL value) and the extinguishing concentration required
- Third party approval from regulatory bodies and safety partners such as International Maritime Organisation (IMO), NFPA, and EPA or Underwriters laboratory Organisations.
- Level of engineering required to modify an existing halon protected installations.
- Availability as regards to installation and maintenance at a reasonable cost.
2.2: Health and Safety Issues
Considering the health and safety in the UK, there is no specific regulation as regards choice of fire extinguishing systems. Otherwise fire risks and risk from the use of extinguishment can be categorised under risks at work. The Management of Health and Safety at Work Regulations 1992 stipulates all risks at work are to be assessed and prevented where ever it is reasonably practicable, controlled.
In cases where fire extinguishing systems contain toxic substances then the Control of Substances Hazardous to Health Regulations 1988 (COSHH regs) will also apply. The basis of the two regulations is the prevention rather than control of the risk.
2.3: Environmental regulations
The International Maritime Organisation (IMO) has prohibited the use of new halon systems from 1994, but accepts the use of existing ones. The EU has banned its use onboard vessels by the end of 2003.
The following are regulations that are put in place to phase out the use of halons.
- The Montreal protocol on Substances that Deplete the Ozone layer- the Montreal protocol, signed by 25 countries on the 16th of September, 1987 is an international treaty for the control of the production and use of ozone depleting substances. It involves the restriction and eventual prohibition of the production, distribution and use of Ozone Depleting Substances. A copy of this document is attached in Appendix 1.
- The EC regulations: This European legislation was put in place to further tighten the restriction on the ban of ozone depleting substances. EC Regulation 3093/94 came into force on the 23rd of December 1994. EC Regulation 3093/94 is directly binding in all EU Member States and does not require any national implementing legislation. The new Regulation EC 2037/2000 came into force on 1 October 2000, replacing the Regulation 3093/94. "The enforcement requires the use of bodies such as the HM Customs and Excise concerning import of controlled substances. The Department of the Environment proposes to implement these arrangements through enforcement regulations made under both the Environmental Protection Act 1990 s.140 and the European Communities Act 1972."(EC REGULATION) The new requirements are applicable to the production, distribution, use and recovery, and control of hazardous substances. The regulations also require the recovery of used controlled substances from certain equipment, such as fire protection systems, for disposal or recycling, during servicing and maintenance procedures of equipment. A copy of the regulation is attached to Appendix 2.
- The Victorian Environment Protection Legislation for the Control of Ozone Depleting substances (Victorian Government Gazette No.S57, 1990) this piece of legislation depicts the Australian government's compliance, reliance and advocacy to the implementation of the Montreal protocol on the phasing out of halon use .
- Environmental Protection agency: Under the Clean Air Amendment, the United States Environmental Protection agency, EPA analysed various substances that could substitute fire extinguishing agents that destroy the ozone layer. These substances also have low global warming potential and low Atmospheric lifetime. The SNAP program (Significant New Alternatives Policy) is used by the EPA to replace the use of halons with environmentally friendly systems in the United States. The Clean Air Act was signed into law in 1990. With this Act, the US banned the production and import of new halons 1211, 1301 and 2402 from the 1st of January 1994 in compliance with the Montreal Protocol. The US government also imposed excise tax on halons through specialized training and proper recycling and disposal.
Chapter Three: Halon Systems
Halon is the generic name for bromine contained halogenated hydrocarbons. Halons systems were first installed in the late 1960s and early 1970s.
In the gaseous form, halons are excellent fire extinguishers. Halons are mostly employed in situations where fire safety standards are high.
Halons are identified by a four digit number. The numbering system is assigned by the number of carbon, number of fluorine, chlorine and bromine atoms respectively.
Halon 1301, containing carbon, fluorine and bromine is used in total flooding applications while halon 1211, containing carbon, fluorine, chlorine and bromine is used as hand held portable extinguishers. The two common halon types described are effective in extinguishing classes A, B and C fires. These halons are preferred because they exhibited: high efficiency in suffocating combustion, availability in volume at reasonable cost, high storage stability, low electrical conductivity, as well as acceptable toxic properties.
3.1: Characteristics of Halons
Halons interfere with the chemical reactions which take place during a fire. The properties of halons allow for its use in most situations and thus most of its applications are linked to particular characteristics.
These principal applications include:
- Clean fire fighting agent: Halons leave no residue after use. This eliminates secondary damages and keeping loss caused by the fire to a minimum .
- Electrically non-conductive: This property makes it suitable for safe application on fires involving electrical equipment. It will prevent exposure of fire fighters to electric shock.
- Low toxicity: This property makes halons acceptable and in most cases halon flooding systems are set in automatic mode by default. They can also be used to extinguish fires while people are present in the protected room. Halon flooding systems do not displace so much oxygen which can lead to suffocation
- Rapid response: Halons are effective for rapid knockdown of flames. This property is mostly essential for class B fires involving liquid and liquefiable solids.
- Low concentration requirement: This means low quantity or amount of halons are required for extinguishment. It minimizes weight and space allowance .
- Gaseous state: This allows for good penetration and effective extinguishment in confined spaces.
- Boiling point: The boiling point of about -4 allows it to be discharged (in the case of hand-held extinguishers) as a liquid for a while before it vaporises. This is a key requirement in some manual fire fighting applications.
- Low heat of vaporisation: Halons will not condense to form water or ice in halon flooding systems.
- The most important advantage of halons is in its cost effectiveness. Halon fixed systems are the most cost effective of all extinguishing systems.
3.2: Extinguishing Mechanisms of Halons
Halons extinguish fires both chemically and physically. Chemically they interfere with the chemical reactions that take place during the fire. This characterises halons as inhibitors. Radicals released during combustion to keep the fire burning are suppressed chemically by halons. This reaction is anti-catalytic.
When halons are heated during combustion, they produce free radicals which compete with those produced by the original combustion process . Halon 1301 produces bromine radicals which react with hydrogen free radicals to produce hydrogen bromide. The hydrogen bromide then reacts with hydroxyl radical to form water and bromide. The bromide released reacts with the combustion fire again and the whole cycle is repeated.
"The hydrogen and hydroxyl free radicals produced by combustion are greatly reduced in concentration by combining with the halogen free radicals produced by halons" .
Where RH is the combustible fuel,
XBr is a halon agent
RH + O2 ENERGY OH + R ……………….eqn3.1
XBr ENERGY Br + X……………………………eqn3.2
RH + Br HBr + R………………eqn3.3
HBr + OH H2O + Br………………eqn3.4
RH ENERGY R + H…………………………………eqn3.5
H + Br HBr…………………………………eqn3.6
The combination of bromine and hydroxyl radical is also an ozone destructive reaction:
HOBr UV Br + OH……………………………..eqn3.7
OH + O3 HO2 + O2......................................eqn3.8
Br + O3 BrO + O2……………………………eqn3.9
BrO + HO2 HOBr + O2 …………………………..eqn3.10
3.3: HALONS AND THE OZONE
3.3.1: The ozone layer
The earth is enclosed by the atmosphere. This atmosphere is made up of a mixture of numerous gases in varying proportions. The atmosphere is further subdivided into three regions depending on temperature. These regions are: Mesosphere, Stratosphere and Troposphere. The word ozone is from a Greek word, ozein, for "to smell". "It is an allotropic form of oxygen having three atoms in each molecule. It is a pale blue, highly poisonous gas with a strong odour".  In its thickest part in the stratosphere, it is only a trace gas..
Ozone is highest in concentration, about 97%, in the stratosphere (15-60 kilometers above the Earth's surface) where it absorbs the ultraviolet radiation from the sun. Ozone is also highly concentrated at the Earth's surface in and around cities. The buildup of ozone on the earth's surface in and around cities is a result of industrial activities and is toxic to organisms living at the Earth's surface.
Table 3.1 shows the percentage volume composition of the constituents of atmospheric air
0 to 4%
*variable gases http://www.physicalgeography.net/fundamentals/7a.html
Ozone is very reactive and a stronger oxidising agent than oxygen. It is used in purifying water, sterilising air, and bleaching certain foods.
Ozone is formed when an electric spark is passed through oxygen. Ozone is prepared commercially by passing cold, dry oxygen through a silent electrical discharge .
Ozone formed in the atmosphere is from nitrogen oxides and organic gases emitted by automobiles and industrial sources . This is achieved by short wavelength ultraviolet. This is actually a health hazard, and it may cause crop damage in some regions. Ultraviolet wavelengths less than 200 nanometer reacts with oxygen molecules to make ozone.
O2 UV O + O………………eqn3.11
O + O2 O3 + Heat…….eqn3.12
The heat released here is absorbed by the atmosphere and results in a rise in temperature of the atmosphere.
"The structure of ozone has 3 oxygen atoms, but steric hindrance prevents it from forming a triangular structure, with each O atom forming the expected 2 bonds. Instead each atom of oxygen forms only 1 bond, with the remaining negative charge being spread throughout the molecule".
Ozone is very unstable. It is decomposed either by collision with monoatomic oxygen or by ultraviolet radiation on it. The decomposition causes ozone to form oxygen molecules. Heat is also released to the atmosphere by this reaction
O + O3 O2 + O2………….eqn3.13
O3 UV O2 + O + Heat……………….eqn3.14
Ozone is decomposed in the stratosphere to prevent highly energetic ultraviolet radiation from reaching the surface of the earth.
3.3.2: Halons and ozone depletion
The ozone layer is mainly depleted by compounds containing chlorine and bromine. Halogens are a chemical family containing fluorine, chlorine, bromine and iodine; any carbon compound containing them is known as a halocarbon. While all halogens have the ability to catalyze ozone breakdown, they have an unequal impact on the ozone layer.
The quantity of halons released into the atmosphere is small relative to the number of gases present in the atmosphere. Yet they are more active in destroying the ozone or disrupting the ozone balance for two reasons:
- Ozone is in a constant state of imbalance, as it is destroyed and produced by natural processes. This process is controlled by solar input that does not undergo significant fluctuations.
- The stability of halons makes it transportable from the troposphere to the stratosphere where halogens are made active and broken down very fast, destroying ozone in the stratosphere.
The impact is
described as depletion potential of the halocarbon. The OZONE DEPLETING
POTENTIAL (ODP) is a simple measure of its ability to
destroy stratospheric ozone. The ODP of compounds are calculated with
reference to the ODP of CFC-11, which is defined to be 1. Thus ODP is a
relative measure. A compound with
an ODP of 0.2 is, roughly speaking, one-fifth as "bad" as CFC-11.
The ODP of a compound "x" is expressed mathematically as the ratio of the total amount of ozone destroyed by a fixed amount of compound "x" to the amount of ozone destroyed by the same mass of CFC-11:
Global loss of Ozone due to x
ODP(x) == …………………..eqn3.15
Global loss of ozone due to CFC-11.
The above expression depicts that the ODP of CFC-11 is 1.0 by definition. The uncertainties experienced in evaluating the global loss of ozone due to a compound are eliminated here since the mathematical expression is a ratio. Evaluating the ODP of a compound is affected by the following:
- The quantity of chlorine or bromine atoms in a molecule.
- The nature of the halogen, as bromine is a more ozone- destructive catalyst than chlorine.
- Atmospheric lifetime of the substance: The atmospheric lifetime of the halon is the time it takes for the global amount of the gas to decay to 36.8% of its original concentration after initial emission. Compounds with low atmospheric lifetimes have lower ODP because it is destroyed in the troposphere.
- Molecular mass of the substance: This is because ODP is evaluated by comparing equal masses and not number of moles.
Table3.2 gives time-dependent and steady-state ODPs for some halocarbon in wide use.
Ozone Depletion Potential
Halons as catalysts alter the rate of the combustion reaction without permanently being altered by the process, and so can react over and over again. This way, a molecule of chlorine or bromine can degrade over 100000 molecules of ozone before the end of its life. The inactive compounds formed by halogens afterwards are known as "reservoirs". These reservoirs can release active halogens when attacked by sunlight.
The stability of the reservoir compounds determines the potency of halons in ozone depletion. Hydrogen fluoride, HF, is so very stable that fluorocarbons have relatively no known impact on ozone. Bromine reservoirs, such as HBr and BrONO2, are much more easily broken up by sunlight; making bromine up to 100 times more effective at destroying ozone than chlorine. Most bromocarbons released to the atmosphere are man-made (methyl bromide fumigants and halon fire extinguishers).
3.3.3: IMPACT OF OZONE DEPLETION
The basic impacts of ozone depletion are majorly environmental. "Exposure to higher amounts of UV radiation could have serious impacts on human beings, animals and plants" .
UV radiation is harmful because it causes premature ageing of the human skin. One of the major health dangers of ozone depletion is skin cancer since UV-B radiation is known to cause certain types of the disease and white-skinned people are at greatest risk . Similarly, the exposure of the eyes to UV radiation can lead to eye diseases such as cataracts. Research by the Environment, Canada in 1993 showed that 10% thinning of the ozone layer is expected to result in almost two million new cases of cataracts per year, globally .
Research has also shown that very high levels of UV radiation can have an adverse effect on the human immune system.
This invariably means that UV radiation will reduce the human body's resistance to diseases such as cancer and increase autoimmune problems and allergies.
"The World Health Organisation is concerned that if the body's immune system is suppressed by solar radiation, it won't be able to fight off common infections and diseases. This means that people could even die from illnesses that would not normally have proved fatal." http://www.ozonedepletion.co.uk/health-effects-ozone-depletion.html
Crops are affected by increased UV, resulting in reduced growth, flowering and photosynthesis. Planktons are threatened by increased UV radiation. Since planktons are basically the first in aquatic food chain and decrease in the planktons will disrupt the food chains and result in a shift in species.
Most construction materials including wood, plastic are degraded by UV radiation. Replacing or protecting such materials has a negative economic impact.
3.4: Halons and Global Warming
Halon 1301 (CF3Br) Bromotrifluoromethane
Halon 2402 (C2F4Br2) Dibromotetrafluoroethane
ODP-Ozone Depleting Potential
GWP- Global warming Potential
Though the total emissions of halons are relatively small globally, their Global Warming Potential (GWP) cannot be overlooked.
3.5: Phasing Out and recovery of Halon systems.
The use of halons is not illegal. To continue use of halons, the requirements of the EC regulation 3093/94 concerning the recovery and prevention of leakages must be observed. The EC regulation 3093/94 requires that ozone depleting substances must be recovered if practicable for recycling or destruction during the service and maintenance of equipment. Section 33 of the EPA 1990 states that it is illegal to "treat, keep or dispose-off controlled waste in a manner likely to cause pollution to the environment or harm to human health". Section 34 states the duty of care that imposes a responsibility on persons who has control of waste at any stage from production to disposal. This duty of care involves ensuring that their waste is safely and legally disposed off and well documented. Disposing it off entails transfer to properly authorized persons along legitimate routes towards proper recycling or disposal.
Halons can be sold to critical users who still need them through fire equipment suppliers or as advised by the Halon Users National Consortium (HUNC).
The HUNC Ltd was formed by a number of halon users with Government support to advice on disposal of halons as well as helps find halons to keep critical systems running. Its long term purpose is to put an end to the use of halons in a responsible way.
CHAPTER FOUR: WATER MIST
The National Fire Protection Association 750, Standard on Water Mist Fire Protection Systems, 2006 edition defines water mist as a water spray whose diameter, Dv0.99 as measured at the coarsest part of the spray in a plane about one metre from the nozzle at its minimum design pressure is less than 1000microns.
The recent economic and industrial interest in water mist technology is driven by two (2) circumstances:
- The rise in need for low-weight-impact replacement sprinkler systems on commercial ships driven by International Maritime Organization (IMO) regulations requiring a modification of most commercial marine vessels. This encouraged the search and development of low-water-demand, high-efficiency mist systems.
- The phase-out of halons and the search for alternative agents that possess most or all of the qualities of a clean total flooding agent.
4.1: GENERATION OF WATER MIST
Water mist generation is categorised based on the mechanism used in producing the droplets. These are:
- Impingement Nozzles: these are used in generating droplets required for extinguishing class A fires where large droplets are required. Mists produced by impingement nozzles are also effective for suppressing hydrocarbon pool and jet fires.
- Twin-fluid Nozzles: this employs the use of compressed air and water. This is used widely in industrial spray systems. It operates on a low pressure range and is disadvantaged cause of its cost and low discharge pressure.
- Pressure Jet Nozzles: It produces fine droplets with wide spray angles for wider projection. The applied pressure determines the size and distribution of the water droplets formed. This increased pressure results in finer sizes. It is widely used to generate mist for suppression of class A and B fires. It can also be used effectively on some class C fires involving electrical equipment. This method is expensive cause of the cost of operating at a high pressure.
- Flashing of Superheated Liquid: When superheated liquid is released suddenly under pressure, ultra fine water droplets of water are produced. Mists generated this way are effective at suppressing dust explosions. It is disadvantaged because it is difficult to control the direction of projection of water mist generated.
- Combination of Pressure and Improvement Nozzles and Pressure Jet Nozzle with nitrogen gas inserted in the water line.
4.2: Characteristics of water mist systems
Water mist is a fine spray of water droplets usually with a diameter of less than one millimetre. Thus, water mist systems work by using smaller quantities of water than normal sprinkler systems. The water droplets from water mists possess a high surface area to volume ratio which invariably increases their ability to absorb more heat. The smaller the droplet sizes, the more efficient the system is. This is so because a larger surface area is provided by small droplet sizes for evaporation and heat extraction.
The National Fire Protection Association 750, Standard on Water Mist Fire Protection Systems, 2006 edition defines the following classes of water mist according to the droplet sizes as:
- Class I mist- 90% of the volume of the spray =200microns
- Class II mist- 90% of the volume of the spray =400microns
- Class III mist- 90% of the volume of the spray =400microns
Mist sprays are formed from nozzles by three (3) different mechanisms :
- Colliding water jets
- Creating a swirl in the spray
- Direct droplet formation from a turbulent jet of water
The most common mechanism is the direct droplet formation from a turbulent water jet. The jet speed and the diameter of the nozzle determine how the droplets are formed.
There are four ways by which droplets are formed from a turbulent water jet. These are:
- "Rayleigh break up" Regime: The diameter of droplets formed is bigger than the nozzle diameter because the droplets are formed far away from the nozzle .
- Atomisation: the droplets are formed just outside the nozzle. This produces droplets that are much smaller in diameter than the nozzle .
- First wind-induced break up: The droplet diameter is almost the same as the diameter of the nozzle hole. Formation of droplet occurs at a considerable distance away from the nozzle outlet .
- Second wind-induced break up: The droplet is formed a short distance from the nozzle and this result in droplet sizes that are smaller in diameter than the nozzle hole .
The variables Reynolds number and Ohnersorge number are determining factors for the formation of droplet sizes. The Ohnersorge number, Oh is a relationship between viscous forces and surface tension while the Reynolds number, Re is a relationship between inertia forces and viscous forces . These numbers are defined by the following equations:
Re =…eqn 4.1
Oh = …eqn 4.2
Where, ? = density of the fluid
? = velocity of flow of the fluid
d = diameter of nozzle
µ = dynamic viscosity of the fluid
s = surface tension
The energy required to produce smaller diameter sizes is more than large diameter droplets as well as carry them to the fire, due to drag and hydrodynamic effect of the fire. Larger droplets penetrate better than small ones. Large droplets can splash the fuels in pool fires while small droplets with low momentum would not penetrate the fire . There is no one-size droplet distribution for all fire scenarios. Water mist system with mixed droplet size distribution is more effective than uniform droplet sized distribution.
"The size of the water mist droplet depends on the nozzle's orifice design and pressure. All three pressure systems—low, intermediate, and high—can be used for fixed (total flooding) and local (streaming) applications."
Low-pressure systems are used in large open-room areas and enclosures. They operate at pressures 175psi or below. They are usually used as where fire fighting nozzles need to be embedded in the floor. Examples include airplane hangars and fuel truck garages for putting out fire below planes and vehicles.
"Intermediate systems use pressure in the 175- to 500-psi range. Total flooding systems using intermediate pressure generate water droplets that provide good circulation and prolonged hang time throughout the protected space, demonstrating enhanced flame-cooling and high oxygen-deleting characteristics."
"High pressure systems can withstand pressures greater than 500 psi. In general, this type of system requires fewer nozzles and less water to achieve successful fire suppression results than low- and intermediate-pressure systems. The high pressure generates substantially smaller water droplets at the nozzle than the other two systems, enhancing the surface area and heat-absorbing capacity of the droplets."
Water mist flooding systems' efficiency is generally determined by - spray momentum, droplet size, and flux density. The enclosure effects, dynamic mixing, additives and location of the system also aid the efficiency of the system.
The spray momentum is the mass, velocity and direction of spray relative to the fire plume. It affects the rate at which the surrounding air is encroached into the fire as well as the depth of penetration of the droplets. This spray momentum is determined by the droplet size, the discharge pressure, cone angle, and the ventilation conditions of the compartment as well as the geometry of the compartment. The spray momentum of the mist has to be equal in strength and opposite in direction to the momentum of the plume for water mist not to be carried away by the fire plume.
The flux density is the quantity of water spray in a unit volume applied to a unit area. This flux density of the water mist must be high enough for the fuel to cool below its flammability limit. The enclosure effect is the ability of a compartment to capture heat and restrict the movement of combustion products. This effect aids the suppression of shielded areas fires in heavily obstructed compartment.
Dynamic mixing is achieved in the course of operation of a water mist extinguisher. This reduces the oxygen level in regions closer to the fuel surface. And hence increases the convective mixing of water vapour, fuel gases and the water mist, resulting in increased effectiveness of the mist system. This dynamic mixing is further influenced by ventilation conditions of the compartment, spacing and characteristics of the nozzles, as well as the spray characteristics.
The dynamic mixing created
The National Fire Protection Association 750, Standard on Water Mist Fire Protection Systems, 2006 edition defines the following as types of water mist extinguishers:
- Engineered water mist systems: systems that require individual design and calculations for the determination of flow rates, pipings, area covered by each nozzle, number and types of nozzles and nozzle placement.
- Wet pipe water mist systems: the automatic nozzles attached to the piping system containing water discharges immediately from nozzles operated by heat from a fire.
- Local application water mist system: this is the only non total flooding system. It is usually in form of hand-held extinguishers. It discharges directly on the subject in enclosed and or outdoor situations.
- Preaction water mist systems: the automatic nozzles attached to the piping system contain air, usually under pressure, with additional detection system installed in the same zones as the mist nozzles.
The detection system acts to open a valve allowing water to flow into the piping system and discharges through all opened nozzles in the system.
- Dry pipe water mist systems: the automatic nozzles are attached to the piping system containing air, nitrogen and inert gases under pressure. The release of the air, allows the water pressure to open a dry pipe valve. The water then flows into the piping system and out through any open nozzle.
The types of water mist systems listed above are all total flooding systems except the local application system.
There are two types of water mist total flooding systems :
- Single Fluid/ High pressure system-uses lone pipe to supply water mist nozzles
- Twin fluid System (Low pressure)-uses dual piping to supply water and compressed air or inert gas separately to water mist nozzles
Single fluid systems are more difficult to fabricate because a specific droplet size, spray momentum must be maintained; but this is compensated for since only a high pressure water source is required.
Twin fluid water mist systems are disadvantaged because there is need for a sufficient quantity of compressed air as well as higher cost since two supply lines are required for air and water.
Water mist hand-held extinguishers are perfect for class A fires, and situations where class C fire hazards exist .
Components of water mist systems
The components of a water mist system include:
- Water source
- Additive source(foam)
- Compressed gas source
- Detection device
Water supply for 30minutes is necessary for a water mist system. Water is usually stored in pressurized containers. Foam concentrates added to the water supply improves the efficiency of the water mist system by repressing buried ordinary combustibles and liquid fuel spill fires. "The resulting thin layer of foam solution blanketing the fuel spill reduces the amount of vaporization and inhibits the amount of radiant heat energy absorbed by the fuel."
Water mist nozzles can be automatic, nonautomatic or hybrid type nozzles (edition). The different types of nozzles used in water mist systems are: Air-atomisation nozzles, High-pressure single orifice nozzles and Low-pressure single fluid nozzles. The type of the nozzle determines the diameter of fine spray generated. They are thermally activated, using quick-response glass bulbs or opened by valves either manually or automatically by an electrical, hydraulic, or pneumatic signal. They consist of an assortment of nozzles of different sizes, depending on the fire hazard. Nozzles have total flooding and local applications.
The pipings required for water mist extinguishing systems are usually small-diameter, stainless-steel or copper/copper alloy pipings. 
Pumps or high pressure accumulators, containing air or nitrogen, supply the pressure necessary for the system. "Fire pumps for water mist extinguishing systems are designed to exceed flow rate and pressure demands by a minimum of 10 percent". Two types of pumps are used depending on the pressure of the water mist system. Low and intermediate pressure water systems use centrifugal pimps while positive displacement pumps are used in high pressure systems.
Automatic smoke or fire detection systems are usually used with water mist systems.
4.3: EXTINGUISHING MECHANISMS OF THE WATER MIST SYSTEM
Water mists do not act like a complete gaseous agent in the course of extinguishing fires. The distance between the flame and the nozzle as well as the water spray rate influence the effectiveness of the water mist system in extinguishing fires. Some water droplets do not reach the fire; some penetrate the fire plume while others cool the fuel or burning surfaces. Other droplets wet adjacent areas and prevent the spread of fires.
"It has been observed that fire extinguishment with water mist in an open environment has a direct impact, cooling the fuel surfaces rather than cooling the fire plume itself."
Braidech et al identified two mechanisms by which water mists extinguish fires:
- Displacement or dilution of oxygen and fuel vapour: this displacement depends on the fire size, the volume of the compartment and the ventilation conditions of the surrounding area. When the fire size is increased, more oxygen is consumed by the fire and also the formation of water vapour is increased thus effecting the extinguishment. The properties of the fuel determine the efficacy of oxygen dilution in fire extinguishment .
- Heat extraction: this involves gas-phase cooling of the fire plume and wetting of the fuel surface. This cooling is achieved by the conversion of water into steam.
As the water droplets vaporise to steam, the radiation from the fire and burning substance is absorbed. As the water droplets vaporise to steam, the radiation from the fire and burning substance is absorbed. The vaporization of the water droplets is determined by the surrounding temperature, the surface area of the droplets, the velocity of the droplets and the heat transfer coefficient of the mist system. The fire is extinguished when the adiabatic flame temperature is equal to or below the flammability limit of the fuel .
The brain behind this extinguishment is the "Fire point" theory which involves making an energy balance for the flames. "This thermal quenching theory is based on energy balance at stochiometry in the flame. This can be represented as" :
Xw [L + gw dT]
=XF?HC- SXPgp dT - XN2gN2 dT - SXdi ?Hdi ……eqn 4.3
where 1550 is the adiabatic flame temperature of hydrocarbon flame,
?Hdi = heat of dissociation
?HC = heat of combustion
L = latent heat of vaporisation of water
Cgp = molar heat capacity of combustion products
Clw = molar heat capacity of liquid water
CN2 = molar heat capacity of nitrogen
XN2 = mole fraction of nitrogen
Xp = mole fraction of combustion products
Xf = mole fraction of fuel
Xw = mole fraction of water
Wetting of the fuel surface reduces the rate of combustion of the fuel and prevents re ignition when the fuel is cooled. "Fuel wetting is the main mechanism for extinguishment of fuels that do not produce combustible mixtures of vapour above the fuel surface".
Secondary mechanisms of fire extinguishment by water mist systems include Kinetic effects, that is, dynamic mixing and radiation attenuation. Water acts as a thermal barrier when water mist reaches the surface of the burning fuel. It also acts to absorb radiant energy from the fire and re-radiates it at a minimized intensity. This action prevents the spread of fire to unignited zones.
Generally speaking, the process of the extinguishment can be summarized as- water droplets absorb the heat from the fire and evaporate. This evaporation causes the flame to cool. Oxygen is displaced by the expansion of the water vapour while heat is extracted by the reduction of heat transfer around the fire plume due to suspended water droplets in the air.
The mechanisms are basically independent of each other but only the combination of these mechanisms result in fire suppression.
Small diameter mist droplets behave like gases, and thus have airflow patterns of movement. This movement is exhibited around obstructions just like gaseous fire extinguishers. "The main fire extinguishing mechanisms of the smaller water particles are radiant heat attenuation and oxygen displacement. Droplets over 50 µm diameter are projected into the fire zone because of their greater momentum" .
Mists are directed to the flame zone by entrainment. This produces local dilution of the concentration of oxygen as well as slows down the reaction kinetics, cooling the liquid fuels below flashpoint temperatures.
4.4: Water mists, its uses and limitations.
Water mist systems and extinguishers have no ozone depleting potential nor do they contribute to global warming. Their atmospheric lifetime presents no environmental concerns. Water mist fire suppression systems have shown effective applications in combating Class A, B, C, and F fires with effective cooling, much less water requirement and damage than conventional water sprinklers, and less clean-up time than most other extinguishing agents.
Fine water mists do not conduct electricity as normal streams of water. This favours its use on electrical equipment and makes it a good alternative to halons in this sense. Very small amounts of water are required to achieve suppression and control. Water mist systems extinguish and cool rapidly preventing re-ignition as well as protecting life and property. Water Mist also removes toxins, corrosive gases and smoke from the atmosphere, this mechanism is known as Smoke-scrubbing.
They can be used on flammable liquids but not on reactive metals. The following are areas where water mist extinguishment can be put into use as replacement for halons:
- Telecommunications and control rooms
- Archive storage
- Flammable liquid hazards
- Shipboard machinery
- Protection for Electric Transformers and industrial hazards.
- Protection for Combustion and Gas Turbines
- Printing presses
- Protection for Baking Ovens, Fat Fryers and Industrial Cookers
Water mist cannot be applied in all situations to replace halons. They are usually used in combination with other inert gases extinguishing agents to improve the efficiency of the suppression of hydrocarbon pool fires. It can also be used with chemical additives but this increases the operating cost and equipment as well as corrosivity and toxicity of the mist. Examples of such situations are in switchgear and control rooms, as well as flare snuffing on oil platforms. Unlike most other gaseous agents, water mists do not require enclosure doors and windows to be closed. Water mist can extinguish fires where there were possible cases of a cross draught- this is considered as a distinct advantage. Water mists also extinguished fires within 45 seconds depending on the fire size and type. Water mist is effective at suppressing backdrafts in compartment fires. A backdraft occurs when a fire is starved of oxygen and when oxygen is allowed in by the action of opening the door or a window, the result is an explosive and violent combustion. This is because the fuel gas which was at a high temperature heats up and expands. Water mists system act by diluting the fuel gas in the compartment and reducing the total hydrocarbon mass fraction.
Water mist systems are more expensive than other gaseous alternatives to halons, especially in equipment costs. Water mists systems are also favoured cause of the ease in obtaining water as well as recharging the systems. The implementation of water mist systems is quite challenging because it a rapidly evolving technology, thus it involves continual research by industries to catch up with the technology.
Water mist is well suited for large space application. It becomes more expensive when applied to small spaces because of redundancy as regards piping and pumps. Water mists systems are generally more expensive to install cause of its high maintenance cost and expensive stainless steel pipework.
4.4.1: Water mist and Shipboard Machinery
Water mists have made tremendous entry into three (3) main applications:
- Protection of turbine and diesel powered machinery;
- Protection of passenger cabins aboard ships ; and
- Protection of machinery spaces aboard ships.
Ships are exposed to a wide range of risks, thus adequate fire safety measures must be put in place. This is very essential since external emergency responses are limited at sea. Water mist systems have been adopted for use on some ship to protect passenger, storage as well as machinery spaces. The system uses filtered sea water automatically fed from the ship and is usually combined with a foaming agent for use in machinery spaces.
In the event of a fire, an alarm signal is displayed in the engine control room through a fire detector and the value for the affected space is opened either automatically or manually. The pressure before the valve then drops to activate the sprinkler pumps and the affected space is engulfed with high pressure water mist.
4.4.2: Water Mist and the Civil Aviation.
Water mist systems could be used in aircraft whether in the passenger cabin, the cargo space or engine space. Water mist systems to be used on an aircraft have to be designed specifically for its use.
In cabin areas, the water mist will be used to prevent the spread of fire from an external pool fire, cool the fuel gases and provide extended time for the evacuation of passengers. The weight problem as regards water mist systems is compensated for by partitioning the cabin into different water spray zones. Water mist discharged within each zone was triggered by a sensor within the zone. Zoned water mist systems are directed to the hazard and it invariably improves the quality of fire suppression achieved. This zoning system also improves visibility within the cabin, reduced the production of toxic gases and maintained high oxygen concentration in the cabin.
In engine nacelle, class B fires, involving highly flammable liquids and highly explosive fires are the major concerns. "Weight restrictions and the efficiency of water mist systems under cold operating temperatures are two major concerns for the use of water mist systems in aircraft engine nacelles." (pro3) The use of additives, such as antifreeze, in water mist systems allows for their use at temperatures below ambient. The level of obstruction in the engine nacelle prevents its use as total flooding system in this area. "The performance of water mist in these spaces was dependent on the nozzle location, water mass flow, spray characteristics and temperature" (pro3)
A cargo fire is usually deep seated and so a high pressure water mist system will be required to reduce the spread of the fire till the airplane gets to safe landing. In this scenario, water mists systems tend not to be effective in extinguishment.
4.4.3: Water mist and Electronic equipment
Fires involving electronic equipment are usually slow growing and smoky. The electrical conductivity of water limits the use of water mists systems in the extinguishment of electrical rooms and situations. Water mists systems have exhibited some cogent uses in extinguishing electrical equipment. Water sprays are better at extinguishing fires involving hot cable fires due to its efficient cooling, while other methods, failed to conductors and temperatures exceeded the auto-ignition temperature of the plastics . In addition, evacuation of the compartment may not be necessary and the electronic equipment can continuously be operated during discharge of the water mist system, especially if a zoned water mist system is used. On the contrary, when halocarbon gaseous agents are used, the compartment has to be evacuated completely due to high concentrations of corrosive gases generated by the agent in fire suppression, thus disabling the operation suppress a flaming cable fire once the heat of combustion had penetrated to the copper of the room .
"In order to enhance the effectiveness of water mist as a halon alternative to protect facilities with substantial amounts of electronic equipment, the National Research
Council of Canada (NRC) has initiated the IntelMist™ project. The basic principle of the IntelMist™ system is the use of state-of-the-art fire detection technology to control a zoned water-mist fire suppression system, so that water can be applied to the smallest possible area directly associated with a fire.
The design parameters of a zoned water-mist fire suppression system examined in the IntelMist™ project included the water droplet size, spray angle, spray momentum, nozzle location as well as flow rate."(pro3)
4.4.4: Water mist and Standards
There is no UK standard or UK approved land based water mist system standard. The standards available are guidelines for installation of water mists systems. These standards include:
- National Fire Protection Association 750, Standard on Water Mist Fire Protection Systems (NFPA 750). This is an American standard that provides details of the design requirement. The document nevertheless states in its scope that,
"This standard does not provide definitive fire performance criteria nor does it offer specific guidance on how to design a system to control, suppress or extinguish a fire."
- FM global, FM5560 Approval Standard for Water. FM Global, a leading commercial insurance company in the UK that helps clients to support risk management including fire risks. This standard also shows overriding requirement of testing. In its scope, the standard also says,
"Approval standards are intended to verify that the product described will meet stated conditions of performance, safety and qualify useful to the ends of property conservation."
- BSI DD8489 and DD8458. British standard institute is due to publish these standard. This is a code of practice for design and installation of fixed fire protection systems-commercial and industrial water mist systems. This code specify test requirements against which a system is tested against successfully before its design and installation. The document also states in its scope that,
"This draft for development gives recommendations for the design, installation, commissioning and maintenance of water mist system and gives performance criteria for fixed water mist system for specific commercial and industrial hazards."
These three (3) documents show that water mist is seen by all standards as risk and fire test specific and thus users or designers are at their own risk. The potential limitations of the design of a water mist system are:
- Each risk requires its own specific design. Systems designed to protect a particular hazard are not necessarily applicable to other hazards.
- The systems are designed to meet fire tests and are not generic protection.
- Changes in surrounding area can negate the design and thus lower the efficiency of the protection. This will require a review and redesign of the installed system.
- Equipment is not interchangeable. The replacements of parts of the system have to be the same specification or a complete re-design may be necessary.
- It requires rigid maintenance procedures and cleaning of the nozzles.
It must be recognised that there are some special applications where there are no alternatives to halon based fixed systems. These are manned spaces where the fire protection system needs to be in an automatic mode of operation. These include military defence installations and special control rooms. This is also true for transport systems where low space and weight requirements are critical. These include the civil aviation industry and shipboard machinery.
Furthermore the European Commission has issued proposals to remove the critical use exemption for halon based aviation safety applications
Further development of a cabinwater mist system, however,was discontinued
after an industry-wide cost benefit study concluded that the cost of outfitting a fleet
of aircraft with passenger-compartment water mist systems would be too high,
compared to the benefits .
Good progress on improving water mist effectiveness in fire suppression has
been made over the last decade. New methods include the combination of both total
flooding and local applications, cycling water mist discharge mode, hybrid water
mist systems, as well as the intelligent water mist system that combines zoned
water mist application with intelligent detection. In order to further apply these
new technologies in fire suppression, however, more research efforts are needed,
including studies on optimum cycling frequency in the cycling discharge mode;
reliable and cost-effective intelligent water mist systems, as well as the performance
of hybrid water mist systems in the practical applications.(pro3)