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:
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.
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:
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.
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:
There are two methods of applying fire extinguishing agents-Total Flooding and Local 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:
Esso Australia, while looking for alternatives to halons on their installations considered the following issues [14]:
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.
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.
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.
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:
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 [2]. 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" [3].
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
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". [10] 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
Gas Name |
Chemical Formula |
Percent Volume |
Nitrogen |
N2 |
78.08% |
Oxygen |
O2 |
20.95% |
*Water |
H2O |
0 to 4% |
Argon |
Ar |
0.93% |
*Carbon Dioxide |
CO2 |
0.0360% |
Neon |
Ne |
0.0018% |
Helium |
He |
0.0005% |
*Methane |
CH4 |
0.00017% |
Hydrogen |
H2 |
0.00005% |
*Nitrous Oxide |
N2o |
0.00003% |
*Ozone |
O3 |
0.000004% |
*variable gases https://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 [7].
Ozone formed in the atmosphere is from nitrogen oxides and organic gases emitted by automobiles and industrial sources [7]. 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".[7]
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.
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:
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[8]:
Global loss of Ozone due to x
ODP(x) == …………………..eqn3.15[8]
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:
Table3.2 gives time-dependent and steady-state ODPs for some halocarbon in wide use.
Compound |
Formula |
Ozone Depletion Potential |
|||
|
|
10yr |
30yr |
100yr |
Steady State |
CFC-113 |
CF2ClFCl2 |
0.56 |
0.62 |
0.78 |
1.10 |
Carbon tetrachloride |
CCl4 |
1.25 |
1.22 |
1.14 |
1.08 |
Methyl Chloroform |
CH3CCl3 |
0.75 |
0.32 |
0.15 |
0.12 |
HCFC-22 |
CHF2Cl |
0.17 |
0.12 |
0.07 |
0.05 |
Halon-1301 |
CF3Br |
10.4 |
10.7 |
11.5 |
12.5 |
https://stason.org/TULARC/science-engineering/ozone-depletion-intro/18-What-is-an-Ozone-Depletion-Potential.html
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).
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" [4].
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 [5]. 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 [4].
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."[5] https://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.
Chemical Name |
Atmospheric Lifetime |
ODP |
GWP |
Halon1211(CF2ClBr)Bromochlorodifluoromethane |
16 |
3 |
1860 |
Halon 1301 (CF3Br) Bromotrifluoromethane |
65 |
10 |
7030 |
Halon 2402 (C2F4Br2) Dibromotetrafluoroethane |
20 |
6 |
1620 |
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.
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)[2].
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.
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:
Water mist generation is categorised based on the mechanism used in producing the droplets. These are:
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:
Mist sprays are formed from nozzles by three (3) different mechanisms [15]:
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:
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 [15]. 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
Aµ = 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 [17]. 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."[1]
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."[1]
"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."[1]
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:
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.[1]
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 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 [13].
Components of water mist systems
The components of a water mist system include:
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."[1]
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.[1]
The pipings required for water mist extinguishing systems are usually small-diameter, stainless-steel or copper/copper alloy pipings. [1]
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"[1]. 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.
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."[17]
Braidech et al identified two mechanisms by which water mists extinguish fires[17]:
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 [17].
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" [15]:
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"[17].
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 Aµm diameter are projected into the fire zone because of their greater momentum" [6].
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.
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:
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.
Water mists have made tremendous entry into three (3) main applications:
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.
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.
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 [80]. 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 [83].
"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 IntelMista project. The basic principle of the IntelMista 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 IntelMista project included the water droplet size, spray angle, spray momentum, nozzle location as well as flow rate."(pro3)
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:
"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."
"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."
"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:
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 [70].
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)
fire suppression technology. (2017, Jun 26).
Retrieved November 21, 2024 , from
https://studydriver.com/fire-suppression-technology/
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