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FIRE – What is it and how it develops
Fire is an exothermic chemical reaction, i.e. it releases heat, between a fuel and an oxidizing substance.
For fire/combustion takes place is necessary in simultaneous the combination of three basic elements: fuel, oxidizing (oxygen) and activation energy (heat).
It’s enough that a heat source contacts with a fuel in the presence of air to such a reaction occurs.
The three basic elements of the fire are typically represented by a triangle, known as the Triangle of Fire.
The propagation of heat is given by:
Driving – Passing, through contact, from one material to another. This heat transfer is as most striking as how much better the conductor material is (ex.: metals). This phenomenon is quite understandable when holding an iron bar at one end and put the other near a heat source, after a few minutes begins to feel the temperature rise at the end where handles the bar.
Convection – The difference in density of hot and cold gases causes air currents. To understand this process just think that, if we have an oil heater running, we feel more heat on top of the heater, at a distance of 50 cm, than laterally, at a distance of 10 cm.
Radiation – Heat is transmitted by electromagnetic waves in the infrared range. This way to heat spread justifies why the sun warm us or the operation of the microwave. Note that in the first case, from the Sun to Earth, there is a huge void and, in the second case, doesn’t see anything on fire but the food is heated.
Stages of a fire:
The development of a fire is a phenomenon quite uncertain, depending primarily on the following factors: Type of fuel; Provision of fuel; Replacement of air.
However, four phases are distinguished, namely: Outbreak; Propagation; Sustained combustion; Decline in flames.
Outbreak – As the name indicates this is the initial phase of a fire. Its duration is dependent primarily on the quality and quantity of combustible material.
Propagation – At this stage, the active combustion rapidly conveys to the neighbour fuels, the propagation process is continuous, corresponding to a gradual rise in temperature in the compartment where the fire develops.
Typically, between 500 and 600° C, a phenomenon occurs which leads to all fuels self ignite. This phenomenon is called the “Flash Over”. The “Flash Over” determines the end of the propagation phase.
Sustained combustion – During this phase, the temperature in the compartment remains practically constant at its maximum.
At this stage there is a common to exist fuel in large quantities, the fire being controlled by the amount of oxygen (air) available – it’s, in this case, a fire controlled by ventilation.
Decline in flames – As the fuel is consumed, the flames decrease in intensity and hence the heat release rate decreases – the decline of the flames can be anticipated if the energy dissipation is greater than its production, causing lowering the temperature in the compartment until the normal temperature.
Classes of fires:
Combustible materials, when burning, do not always lead to fires with the same characteristics.
We all know that the cooking gas, butane, doesn’t burns like a match, not only by the colour of the flame as in the amount of heat released.
The study of various fuels has led to the standardization of the fuels and fires, creating four classes:
Class A – Fires arising from the combustion of solid materials. Ex.: Wood, Fabric, Rubber…
Class B – Fires arising from the combustion of liquid materials. Ex.: Petrol, Alcohol, Oil…
Class C – Fires that result from combustion of gases. Ex.: Butane, Propane, Acetylene…
Class D – Combustion of metals. Ex.: Sodium, Potassium, Magnesium…
Fumes are, in most cases, the main enemy during the development of the fire. Expand very rapidly, especially from the lower areas to upper floors, hindering the visibility and irritating the respiratory system. The fire, in addition to heat and smoke, produces different toxic gases and can cause death long before the flames get closer.
The main gases released during combustion are:
Carbon monoxide – Lighter than air, is toxic (prevents oxygen reaching the brain) and combustible;
Carbon dioxide – Heavier than air, is suffocating (causes acceleration in respiration facilitating the absorption of other toxic gases) and even is a good extinguishing agent;
Hydrogen sulphide – Affects the nervous system causing dizziness and pain in the respiratory tract;
Nitrogen dioxide – Is very toxic and causes paralysis of the throat.
The spread of fumes produced by fire is one of the most dangerous consequences, difficult or turns impossible a correct evacuation, because it hinders the visibility, creates panic and cause poisoning that can lead to death.
By detection it’s understood everything that can be done to discover a fire and locate it at a certain point , it’s obvious that the earlier the detection and the location is made better it is.
The detection can be done by people or by adequate equipments, the fire detectors.
But, by itself, the detection is of no use if not followed by a notice to users of where it’s starting the fire and eventually to the Fire Department.
This notice can be automatically triggered by specific equipment (fire alarm central) or can be given by a person.
The warning of a fire may take various types of signs: sirens, bells and optical/acoustic panels; these are some of the signalling modes the warning may be given.
The telephone connection to a Fire Department is another way of warning that should be given in conjunction with the ones above.
The fundamental characteristic of any detection is that it should be as early and quickly as possible, so that any active plan for evacuation and fire fighting takes place.
Any detection must be followed by an alarm and that by a plan for emergency/evacuation of people and fire fighting.
Automatic Fire Detection
The automatic fire detection system fitted with fire detectors has advantages over human detection by normally be faster, able to carry out simultaneously on many areas or spaces and still be able to exercise in places not accessible to people.
This detection is usually supervised by a central, in case of a location of an outbreak of fire or of a confirmed alarm (action of a manual push button), give alarms for which is scheduled and can still perform some functions:
- Transmit the alarm to a distance;
- Make a phone call the Fire Department;
- Close or open doors;
- Stop machine, open exhausters, control lifts;
- Start the operation of fixed fire extinguishing systems;
- Other tasks depending on the characteristics of the protected building.
The Automatic Fire Detection Systems are basically composed of:
- Command and reception Switchboard – Fire detection central;
- Automatic passive devices – These devices are designated as passives because it’s an external action that gives rise to the alarm – ex.: Fire in case of fire detectors and confirmed alarms in case of push buttons;
- Optical and acoustic warning devices – Alarm sirens and optical indicators(light signals);
- Electrical wiring:
- Class A – Loop or Ring;
- Class B – Radial – doesn’t return to the panel (switchboard).
There are basically three types of systems:
- Conventional – The activation of a device indicates the presence of an outbreak of a fire within an area of the protected space.
- Analogue/Addressable – The activation of a device indicates the exact location of an outbreak of a fire – these systems are ideal for protecting large areas.
- Hybrid (Analogue + Conventional) – These systems are basically used when remodelling and/or expansion of buildings.
- Electrical wiring:
The differences in performance of the different types of devices should be recognized so that the applications can be made correctly.
There are two types of thermal detectors, fixed temperature detectors and thermo-velocimetric detectors.
- Fixed temperature detectors Has a defined value of temperature at which the internal sensing element must be heated before acting;
- Thermo-velocimetric detectors This type of detector goes into alarm if the ambient temperature increases by 10° on every 5 minutes.
Are smoke detectors sensitive to combustion products that are absorbed or scattered by infrared radiation, visible and/or ultraviolet light.
Most optical detector detects smoke by the principle of scattering, also known as Tyndall effect:
- The detector consists of a smoke chamber configured as a labyrinth. This element prevents the entry of ambient light inside the chamber, where there is a transmitter and a receiver of infrared light positioned so that the light sent by the sender, under normal conditions, does not reach the receiver.
- When smoke enters the chamber the emitted light is scattered by smoke particles causing some light to reach the receptor triggering the alarm. The greater the amount of smoke, the greater the amount of light reaching the receiver.
UV flame detector:
The UV flame detector is designed to protect high-risk areas where a possible fire will spread quickly, with few or no early stage of fire or flame, where inflammation is almost instantaneous, like: flammable liquids, fuel gases, petrochemicals products, etc.
Infrared flame detector:
The infrared flame detectors are used to detect open fire indoors or outdoors.
These detectors are especially suitable for liquid and gas fires that do not produce smoke and fire on material that contains carbon and develop thick smoke.
They are usually applied to large industrial warehouses, oil refineries, machine rooms, power stations, printing plants, timber storage, underground tunnels, etc.
Manual Call Points:
Fire Manual Call Points are for the user, confronted with the presence of an outbreak of fire, should trigger the fire alarm.
The operating principle is based on breaking the glass embedded, which results in sending an alarm signal to the central; lighting also a LED for signalling activity.
- Prohibition – Prohibit a behaviour/action;
- Obligation – Imposes a behaviour;
- Warning – Warn of a danger or risk:
- Rescue and Relief – Give directions about emergency exits or means of relief and rescue.
It’s essential to achieve a proper signalling of all escape routes, exits and emergency exits.
Intended to ensure rapid evacuation without panic, it’s essential that, from any point where we are, is always visible an emergency sign.
Extinguishing media, essential in a first intervention in case of fire, should be carefully marked, ensuring that, in any way, the signs shows their exact location.
Will be used for this signalling the signs of equipment, at 2 or 2.2 meters of the soil (or higher when in large areas). When the equipment is not visible will be used signs with arrows indicating the direction to go to reach the fire fighting equipment.
By undertaking a risk analysis identifies the hazards, which should be marked using the warning signs of danger.
To minimize the danger, the duty will be to prohibit all behaviours and actions that may exacerbate the risk, using for such the danger signs.
To ensure the use of all necessary protective equipment, the duty will be signalling them with the signs of obligation.
This is guaranteed, in case of fire, the best evacuation conditions reducing the risks of panic and, consequently, the loss of lives.