Week 9 - Fire and Explosions

What does fire require?

Fuel, oxygen and heat. The fire will quickly stop if one of these is eliminated.

What are the two types of fire?

With flames - burning of gas from vaporisation of liquids and pyrolysis (chemical breakdown of solids caused by heat), flames will burn through materials

With smoulder - direct burning of solid fuel, production of charcoal, fires may be limited to smouldering if limited oxygen and convert to flaming if oxygen introduced

What is the process of spreading?

1. Conduction

2. Convection

3. Radiation

Conduction

Small scale molecular movement within a substance, movement of fire to immediate adjacent areas, e.g. flame moving down a matchstick

Convection

Currents within gas/liquids, movement of gas, flame and heat upwards, when the flames dance up into the air and come into contact with something, ignite it, and expand it

Radiation

Transference of heat, can raise temperature of surrounding objects to point of combustion if fire grows, i.e. we might not see much evidence of flame, but through the exchange of feat, something can burst into flames

What is the development/life of a fire?

1. Incipient ignition - low heat, starts slowly and builds up (unless there is an accelerant), some smoke

2. Growth - more heat, lots of smoke, start to see flames, flames spread, heat begins to build, fire starts to take hold

3. Fully developed/flashover - reached maturity/peak, massive heat and smoke output, extensive flame, after this fire will start to die down as it eats up all of the available materials to burn through

4. Decay - decreased heat and flame, considerable smoke with increased smouldering

Questions for fire investigation

• Where was the origin of the fire?

• What is the fire’s cause?

• What contributed to the spread and development?

• Who or what was responsible?

• Was it accidental or deliberate?

Who conducts a fire investigation?

Depends on suspicions, fatalities and damage involved, could be fire service, police, insurance industry, coroner, government

Processing the site

Cordoning and restricting access, health and safety protocols, external examination, systematic internal searching and visual inspection

Fire dogs

Effective at smelling out accelerant and can cover vast areas much faster than humans or machines. They usually have pads on their feat to protect them, and small/light dogs are used to cover large areas quickly. Dogs are sent in only after the fire and heat has stopped.

Eyewitnesses as sources

Especially fire-fighters. Can report nature and location of fire within building, where it started, how it spread, nature of fire fighting activities, when and what happened

The seat of the fire

Determination of cause will require identification of seat of fire (the point of origin). Evidence of ignition sources is usually the starting point

Fatal fire scenes

Any death is treated as a crime until proven otherwise. Examination of the body on scene and then in post-mortem to establish cause of death (people who were alive when the fire started will usually have internal scarring of the esophagus due to inhaling hot air, so if these scars aren’t present it could indicate they were not there when the fire started). Why didn’t they escape and what prevented them?

Examination at post-mortem to determine death

• Carbon monoxide poisoning

• Asphyxiation from smoke

• Burning

• Examination for non-accidental injuries

• We expect to find people in a curled up state, arms up to chest and hands in fist due to muscle contraction, so if they aren’t in this state, why?

What is an explosion

A sudden and violent release of physical or chemical energy, often accompanied by the emission of light, heat and sound. The chemical reaction causes a big bang and emission of light

What is an explosive?

Any substance that is capable of producing an explosion, the thing that starts the event

What has to be present for an explosion to occur?

1. Fuel to burn

2. Dispersion

3. Oxygen - to sustain the fire

4. Ignition - source of heat, spark

5. Confinement - within an enclosure or structure

Types of explosions

Chemical - explosive materials e.g. gunpowder, two chemicals may interact with each other

Physical/mechanical - cylinders of gas overheated in a fire, metal container would explode due to the containment of gas inside it

Thermal - liquid brought rapidly to boiling point through contact with something else significantly hotter

Electrical - heating effect of a potential electrical fault where a spark is produced and catches something flammable

Nuclear

Questions for explosion investigations

• Was it an explosion, what exploded/was involved

• Accident or intentional

• Nature of device

• Place used to store explosives

• Similarly of devices and incidents - people have patterns in making explosives, like following a recipe, so can identify if an explosion matches a similar batch of explosives used in a previous event

Phases of an explosion

1. Shock front - the initial explosion where the air is pushed out

2. Overpressure phase - blast wind

3. Vacuum phase - the air pushed out has to return, so the air is sucked back in which can cause damage like the blast wind can

4. Normalisation

Beruit, Lebanon 2020 example

• Huge amount of fertilizer incorrectly stored

• Fire started in neighbouring warehouse and caused fertilizer to ignite, causing a massive explosive

• Huge amount of destruction came as a result of this

What is a high order explosive?

The more industrial type that is relatively stable, tested and tried. Produces a true shockwave, difficult to home manufacture/replicate outside of factory settings, relatively stable

What is a low order explosive?

Usually homemade, requires rigid containment in small, confined areas (e.g. pressure cookers), easy to home manufacture

How do explosion crime scenes differ from other crime scenes?

• Risk of health and safety - engineer required to verify that rescue operation and investigation can take place (time-consuming, slow)

• Attendance of bomb disposal team and anti-terrorist police

• Need to declare scene safe from further explosives, dangerous structures and substances released (asbestos)

• Cordoning 1.5x distance of furthest observable debris so can be huge areas - essential for preserving life and preventing more injuries

• Importance of finding debris, especially the explosive device

Debris

Bomb parts, detonators, explosive wrappings providing trace, containers (bags, cases), manner in which bomb was triggered. Items lodge in soft receptors (tyres, furnishing, bodies), hard surfaces might have shrapnel marks and give insight into direction of blast/travel. Problems of finding initial cause in the midst of damage, damage patterns might be misleading or confusing

Glasgow school of art example

• Three flammable gases passed over the projector causing an electrical spark, flames spread onto the foam behind the projector and continued to spread through the ventilation system made up from voids throughout the building which allowed hot gases and smoke to travel vertically across the building

• Flames travelled up the walls and across the ceilings, and intensified by the influx of oxygen from the windows, building also wasn’t fire resistant as made from canadian pine which is highly flammable

• Reports of individuals trapped in elevator, but discovered it empty

Boston marathon 2013 example

• 2 bombs explode near the finish line within 8-12 seconds of each other around 180m apart

• Bombs contained pellets and nails, bombs were contained in pressure cookers hidden inside backpacks

• Detonated near the finish line of the marathon where there was a bottleneck of people

• Left the backpacks and walked away unharmed