Firefighting - Principles and Considerations Flashcards
Define the term Backdraught
This is a ventilation driven event where a fire in an under ventilated compartment has lead to the flammable products of combustion accumulating above the upper explosive limit.
The admission of air when an opening is made this leads to the fire gas concentration lowering to within flammable limits which can lead to a sudden and rapid deflagration moving through the compartment and out of the opening.
List the signs and symptoms of a Backdraught
(i) Fires in tightly closed compartments.
(ii) Windows, doors and door handles hot to touch.
(iii) Rattling sounds or smoke pulsating around openings.
(iv) Smoke being drawn into openings and large air movements heading into openings.
(v) Ghosting tongues of flames seen in the compartment (from the exterior).
(vi) Turbulence in the smoke layer.
(vii) Whistling and roaring sounds (denoting high velocity air flow).
(viii) Dark oily deposits and condensation running down inside of windows (visible form exterior)
(ix) A change in fire conditions with fats moving smoke seen from exterior to exit at high velocity as if under pressure and steady darkening of the smoke colour.
What measures can be taken to deal with or control a backdraught?
(i) Ensure isolation or confinement of the fire compartment (anti-ventilation)
(ii) Ensure exterior jet is applied to openings before interior deployment where a Backdraught exists.
(iii) Ensure correct door entry procedures being especially vigilant to apply water towards the ceiling/gas cooling between each open and close cycle.
(iv) Consider ventilation tactics (could it be best to allow the Backdraught to occur, if no risk of further fire spread, public, FRS personnel etc).
(v) Consider additional control measures.
Define the term flashover
A temperature driven event where:
The total thermal radiation from hot fire gashes causes all exposed combustible surfaces to pyrolyse and ignite, when there is adequate ventilation.
This cause a sudden and sustained transition from a growing fire to a fully developed fire.
List the signs and symptoms of a flashover
(i) High heat conditions or flaming combustion overhead.
(ii) Existence of ghosting tongues of flame in the smoke layer.
(iii) Sudden lowering of the neutral plane.
(iv) lack of water droplets falling back to the floor following gas cooling / water directed at ceiling.
(v) Change in smoke issuing from a window, with increasing velocity (as if under pressure and darkening of smoke toward black).
(vi) Sudden appearance of pyrolysis at low/floor level.
(vii) Sound of breaking glass as windows or glazing fails from exposure to heat (possible increase in neutral plane).
What measure can be used to control a flashover
(i) Direct external firefighting techniques at openings of the compartment containing the fire before internal deployment.
(ii) Ensure proper door entry procedures are adhered to.
(iii) Consider ventilation tactics.
(iv) Ensure a covering jet is in place to deal with or prevent secondary fire spread.
(v) Employ a combination of direct firefighting techniques and gas cooling by interior teams to control conditions and locate and extinguish fire spread.
List and describe the four phases of a fire
INCIPIENT
An accidental fire normally begins with an incipient phase, may occur very slowly but in some instances can occur quickly.
GROWTH
Incorporates fault rapid fire development, normally includes the flashover unless restricted by lack of fuel, O2 or heat.
FULLY DEVELOPED
Period of burning where max heat is released, maximum amount of energy released.
DECAY
Much if the fire load continues to be released but to a lesser degree as burning rate decreases.
Describe the Coanda ventillation phenomena
The tendancy of a strem of hot fire gases to stay attached to the exterior surface of a build when exiting an opening, rather than follow a straight line in its original direction.
The effect of convection, fire comparment pressurisation and wind casue the hot gases to be expelled from an external opening and usually move vertically (although can move downward).
The Coanda effect encourages venting products to be drawn back towards the face building faciliting fire spread to other areas/comparments of the building.
Describe the Trench Effect ventillation phenomena
coand, flasover, incline
Phenomena that can produce a fire plume that accelerates up an incline surface.
This is a combination of the Coanda effect and a flashover.
(i) Can occur when a fire develops on an incline surface (~25 degrees).
(ii) Flames deflect towards the surface (coanda) and heat the combustible materials further up the incline.
(iii) as pyrolysis and heating of the fire gases commences, this leads to a subsequent ignition.
(iv) rapid fire development continues towards the top of the incline slope until fuel is depleted.
Describe the factors that need to be taken into consideration when developing a plan to tackle a fire at an industrial location.
16 points
(i) Identify the location of the fire and material(s) involved (potential for flashover/backdraught?)
(ii) Construction - type, stability, size, age, maintained?
(iii) Occupants - identify last known position, in need of rescue?
(iv) Question responsible person, witness to gather information on the incident.
(v) Identify utilities incl renewbale and isolation points.
(vi) Identify security features that may compromise safe access and egress from scene of ops.
(vii) Identify presence and effectiveness of fixed installations and take control where neccessary.
(viii) Identify fire engineer solutions and access effectiveness against intended purpose.
(ix) Use available plans to inform tactical decision making.
(x) Identify exposure risk
(xi) Use TIC to check for fire spread.
(xii) Identify/locate storage of hazmats if in use.
(xiii) Identify internal hazards eg machinery, refridgeration stroage, internal sub-zero structures (sandwich panels) and brief BA crews well on these hazards if present.
(ivx) Consider weather - wind direction for smoke, water run off, public safety.
(xv) Assess status of compartmentation, MoE, protected shafts.
(xvi) Check building systems indicator/alarm panel to assess fire location.
Define Fire Gas Ignition
The movement/leakage of gases from the fire comparment to a remote compartment OR pyrolysis occuring in a remote compartment which falls within a flammable range and may ignite due to an ignitionsource remote to that of the original fire.
What are the signs and symptoms of a fire gas ignition
(i) Indication of heat from floors/walls/ceilings in a room with no fire.
(ii) pyrolysis of any surface and those near/or are compartemt boundries
(iii) Accumulation of fire gases in a compartment remote to that of the original fire.
Describe the Piston Effect ventillation phenomena
Creates a potetnially large movement of air in a shaft or tunnel when an object moves into the enclosure.
e.g. train moving into a tunnel displaced air is confied by the tunnel walls creating:
- an area of higher pressure in front of the train and around sides.
- low pressure behind which is filled by the pressurised air escaping from around the sides.
IC must beaware of and manage these flow paths to minimise potential for sudden and rapid fire growth.
Describe the Stack Effect ventillation phenomena
- Movement of air into and out of structure and chimneys via buoyancy.
- This occurs due to contrast between internal and external air density resulting from temp and moisture differences.
- Generally air warmer inside buildings therefore cooler air is drawn in through openings at lower levels, the warm air (fire gases) rises and exit through opening higher up the building (or accumulate).
Describe the Dust Explosion phenomena
The rapid combustion of fine combustible particles suspended in the air in sufficient concentrations and often (but not always) occurs in an enclosed location.
Ventilation by Ff’s can creat or disturb combustilble dusts/powders and therefore muct be considered as part of a ventilation strategy.
