Ch 6 Reading Smoke Flashcards
Smoke Defined
Smoke leaving a structure has four key attributes:
- Volume
- Velocity (pressure)
- Density
- Color
A comparative analysis of these attributed can help the ISO determine the size and location of the fire, the effectiveness of fire streams, and the potential for a hostile fire event, such as flashover
Smoke is better defined as …
… the products of incomplete combustion and pyrolytic decomposition that include an aggregate of particles, aerosols, and fire gases that are toxic, flammable, and volatile
Smoke at a building fire …
… is being developed by two sources:
- Incomplete combustion. Materials contributing smoke through incomplete combustion are actually on fire but the rapid oxidation hasn’t fully reduced the fuel
- Pyrolitic decomposition. Materials contributing smoke through pyrolytic decomposition aren’t necessarily burning but are being chemically degraded by heat.
- Pyrolysis often leads to combustion
- *Most of the smoke at a typical building fire is not coming from burning materials, but from materials breaking down w/out burning (pyrolysis)
- **the term Off gassing is often used for pyrolysis
More accurately, surfaces that are smoking - not burning- are off aggregating. That is, they’re releasing a mix of particulates and aerosols as well as gases
Particulates
- The solids (particulates) suspended in smoke are “high surface to mass” materials.
- Soot and ash are more prevalent solids in smoke
- Soot is carbon (officially “carbon black”) and carbon can support flaming.
- Ash can carry tremendous volumes of heat and cause other materials to ignite
- Carbon will add a flat (dry) black color to smoke, whereas ash will add a dirty white color. Other solids in smoke include dust, fibers, and pulp
- Most interior fires, the volumetric composition of smoke is mostly particulate matter
Aerosols
- An aerosol can be defined as a suspended or propelled liquid
- As a structure fire, the liquids in smoke include primarily moisture and hydrocarbons (oil and tar), although there are also acids, aldehydes, and ketones.
- Water vapor is a by product of combustion (flaming) and pyrolysis (materials steaming off saturated moisture)
- As materials break down, vapors become more of a true, non dissipating white (usually indicating ammonia and other chemicals)
- Hydrocarbons give smoke a satin (wet) black color.
- Hydrocarbons can self-ignite as low as 450 degrees F, but they often do not ignite because the particulates have made the smoke too rich to burn
Gases
- Significant quantities that affect fire behavior are carbon monoxide (CO), hydrogen cyanide (HCN), benzene (C6H6), acrolein (C4H4O), and hydrogen sulfide (H2S)
- Smoke is ignitable as low as 450 degrees F and has a collective flammable range of 1% to 74% in air
- The modern structure fire smoke environment is extremely flammable (explosive, even) and ultimately dictates fire behavior.
Predicting Fire Behavior
Complete combustion of common materials renders heat, light, carbon dioxide, and water vapor (none of which can burn)
Smoke Ignition Triggers
- Ventilation limited fires do not allow the open flaming to complete a reaction w/pure air, which in turn leads to increasing volumes of carbon monoxide as well as smoke products
- Two triggers may cause accumulated smoke to ignite: the right temp and the right mixture
- Smoke gases that are below their ignition temp (but above flashpoint) need only a proper air mix and a sudden spark or flame to complete their ignition (piloted ignition)
- Once smoke gases reach ignition temp they don’t need a spark or flame to ignite, only the right mix of air
Compartmentalized Fire Growth Phases
Compartmentalized fires are influenced by several factors that can lead to a six phased growth model (ventilation controlled model). Factors include:
- Size of individual compartments
- Type, quantity, and continuity of the fire load (contents and combustible finishes)
- The presence or lack of smoke and fire control systems
- Available flow path
- Size and status of exterior openings (closed, partially or fully opened)
6 Phases of Ventilation Controlled Fire
1) Ignition (incipient) phase: The ignition phase includes the event(s) that brings together heat, fuel, and oxygen to start the self sustaining process of combustion
2) Initial growth phase: The initial growth phase has also been labeled the fuel controlled phase; fire growth is controlled by the proximity of burning fuels to other usable fuels; air is abundant and the fire will grow in an upward and outward fashion as flames touch other materials.
Exothermic radiant heat waves are released and start heating up other fuels in the area of the fire. Pyrolysis begins and smoke production becomes abundant. The convection smoke column can also cause fire spread in the initial growth phase
3) Ventilation limited phase: Compartmentalized fire condition where open flaming decrease because smoke production displaces and limits available combustible air, although heating continues to produce smoke. The condition is considered dangerously explosive because heat and volatilized fuels are readily available, only air is missing. Introduction of air will cause the fire to rapidly transition into an explosive growth phase
4) Explosive growth phase: Rapid fire growth phenomenon that occurs when combustible air is reintroduced into a ventilation controlled fire, leading to smoke flame over and room flashovers.
In an explosive growth phase, the room of origin will likely flash over but the fire growth may include ignition of all the smoke in the building (a true flame over). Opening a door will cause this reaction.
5) Fully developed phase: The explosive growth event typically leads to total flame involvement of the interior flow paths
6) Decay phase: Available fuels become consumed and the fire begins to wane. Also known as “fuel limited phase”
Hostile Fire Events
- Flashover
- Backdraft
- Smoke explosion
- Flame over
Ghosting
Hostile event warning sign and is characterized as the intermittent ignition of small pockets of smoke, usually seen as fingers of flame that dance through the upper smoke layer.
Ghosting is not necessarily a hostile fire event but serves as a warning sign of impending flashover or flame over
Flame over
Typically originates at the seat of the fire and travels along the heat flow paths
Smoke explosion
Hostile fire event that occurs when a spark or flame is introduced into a pocket of smoke that is below ignition temp but above flash point. Result is a split second ignition and rapid expansion of that pocket with no sustained burning.
Flashover
Sudden hostile fire event that occurs when all the surfaces and contents of a space reach their ignition temp nearly simultaneously resulting in full room fire involvement.
Most cases this occurs because the room itself (walls, ceilings) can no longer absorb heat and begins to reflect radiant energy back into itself.
The super heated upper gas layer expands downward and rapidly heats floor level fuels to their ignition temp
