Comprehensive Fire Science, Physical Dynamics, and Incident Investigation Notes 2 week thursday.

Consensus Standards and Regulatory Enforceability

• Consensus Recommendations: Professional organizations aggregate research, internal data, and expert knowledge to generate standards by consensus. Consensus signifies that the majority of domain experts agree on a specific protocol or safety practice.

• OSHA and NFPA Relationship:

  • The Occupational Safety and Health Administration (OSHA) is a federal regulatory authority.

  • The National Fire Protection Association (NFPA) produces consensus safety standards.

  • When OSHA explicitly references specific NFPA standards in its regulations, those referenced consensus standards become legally binding regulatory requirements.

Measuring Flammability: Flash Point Testing Methods

• Flash Point Definition: The lowest temperature at which a liquid produces sufficient vapor near its surface to form an ignitable mixture with air upon exposure to an ignition source.

• Closed Cup Test:

  • Example: Pensky-Martens closed cup test.

  • Procedure: A liquid sample is enclosed within a closed container (cup) equipped with a heat source. Heat is applied to increase the sample's temperature until an introduced open flame ignites the accumulated vapor. The liquid temperature at ignition is recorded as the flash point.

  • Characteristics: Prevents vapors from escaping into the open atmosphere. Because vapors remain trapped and concentrated, closed cup tests yield lower, more conservative flash point temperatures.

• Open Cup Test:

  • Procedure: The liquid sample is heated in a container open to the ambient atmosphere.

  • Characteristics: Vapors diffuse, expand, and disperse into surrounding air. The open atmosphere allows greater oxygen interaction but disperses vapor density, resulting in higher measured ignition temperatures compared to closed cup tests.

Flammability Classes of Ignitable Liquids

• Ignitable liquids are categorized into three main classes based on hazard severity, flash point, and boiling point:

  • Class 1 (Flammable Liquids):

  • Highest hazard level. Ignites readily at lower temperatures.

  • Examples: Gasoline, hand sanitizer.

  • Subdivisions (e.g., Class 1A, Class 1B): Divided based on boiling point criteria while maintaining similar flash point ranges.

  • Class 2 (Combustible Liquids):

  • Moderate hazard level. Requires higher heat input to generate ignitable vapors.

  • Class 3 (Combustible Liquids):

  • Lower hazard relative to Class 1. Higher flash points requiring significant external heating.

  • Examples: Kerosene, motor oil, industrial grease.

Flammability Limits: LEL and UEL

• Explosive Limits (Flammability Limits): Expressed as a percentage of vapor volume in air ($\text{vol\%}$).

• Lower Explosive Limit (LEL): The minimum concentration of vapor in air below which flame propagation does not occur upon contact with an ignition source.

• Upper Explosive Limit (UEL): The maximum concentration of vapor in air above which flame propagation does not occur.

• Flammable Range Example (Gasoline):

  • LEL of Gasoline: $1.4\%$

  • UEL of Gasoline: $7.6\%$

  • Ignition Condition: Gasoline vapor will ignite or burn only when its concentration in air is between $1.4\%$ and $7.6\%$.

• Off-Ratio Vapor Mixtures:

  • Too Lean ($< LEL$): Vapor concentration is below $1.4\%$. Contains insufficient fuel or excess air/oxygen to sustain combustion.

  • Too Rich ($> UEL$): Vapor concentration is above $7.6\%$. Contains excess fuel vapor and insufficient oxygen/oxidizer to sustain combustion.

• Fire Tetrahedron Requirements: Continuous combustion requires fuel, heat, and an oxidizer (oxygen).

Physical Properties of Vapor and Liquids

• Vapor Pressure:

  • Quantified measure of a liquid's tendency to evaporate and release vapors at standard ambient conditions.

  • Evaluated at Standard Temperature and Pressure ($STP$: $14.7\,PSI$, $760\,mmHg$, $1\,atm$).

  • Vapor-Liquid Equilibrium: The point where the rate of liquid molecules evaporating into gas equals the rate of vapor molecules condensing back into liquid.

  • Comparative Examples:

  • Kerosene: Vapor pressure $\\approx 5\,mmHg$ at room temperature (low volatility, minimal vapor release).

  • Toluene: Vapor pressure $\\approx 23\,mmHg$ at room temperature (higher volatility, significant vapor release).

• Vapor Density:

  • The mass per unit volume of a gas or vapor relative to ambient air (where air density = $1.0$).

  • Behavior:

  • Vapor Density $< 1.0$ (e.g., $0.6$): The gas is lighter than air and floats/rises upward.

  • Vapor Density $> 1.0$ (e.g., $1.6$): The gas is heavier than air, causing it to sink, accumulate in low-lying areas, trenches, or confined spaces, and form hidden flammable atmospheres.

• Specific Gravity:

  • The ratio of the density of a liquid relative to pure water (where water specific gravity = $1.0$).

  • Behavior:

  • Specific Gravity $< 1.0$: The liquid is less dense than water and floats on its surface.

  • Specific Gravity $> 1.0$: The liquid is denser than water and sinks to the bottom.

Principles of Heat Transfer and Buoyancy

• Thermal Buoyancy:

  • The upward force exerted on air warmed by combustion due to a reduction in its density relative to cooler surrounding air ($\text{hot air rises}$).

  • In an enclosed room fire: Hot air rises toward the ceiling, displacing cooler air downward. Continuous thermal recirculation mixes smoke and hot gases throughout the volume, progressively raising ambient room temperatures.

• Modes of Heat Transfer:

  • Convection: Heat transfer through moving fluids, which include both liquids and gases (air).

  • Conduction: Heat transfer through direct physical contact within solid materials (e.g., structural metal beams, steel components).

  • Radiation: Heat transfer through electromagnetic waves across open space without requiring a physical medium.

Pyrolysis and Flashover Phenomena

• Pyrolysis (Pyrolyzation):

  • The chemical decomposition of organic materials induced by thermal energy in the absence or insufficiency of oxygen.

  • Heat breaks down solid components into volatile gases and smoke prior to active flaming combustion.

• Flashover:

  • A rapid, near-instantaneous transition in a compartment fire where every exposed combustible surface within the room reaches its autoignition temperature simultaneously.

  • Driven by intense radiant heat trapping along upper ceiling layers. Flashover converts a localized compartment fire into total surface involvement, creating a massive fireball.

Fire Investigation Techniques and Burn Patterns

• Structural Burn Patterns:

  • Fire naturally burns upward and outward from its origin.

  • V-Pattern: Formed when an object burns directly adjacent to a vertical wall. The narrow point indicates the origin, widening toward the ceiling.

  • U-Pattern: Formed when the burning object is located slightly away from the wall surface, preventing direct thermal tip contact.

  • Draft Line Distortion: Airflows toward ventilation openings (doors, broken windows) pull flames and alter char patterns, creating heavy localized damage distant from the true origin.

• Accelerant Analysis:

  • Liquid pour patterns on flooring (e.g., carpet burns showing localized liquid pooling) suggest intentional accelerant use (arson).

  • Specially trained accelerant detection canine units (sniffer dogs) sample suspect residue.

• Char Depth Measurement:

  • Exposure to thermal energy turns organic material into carbon char.

  • Char depth increases inward over time as burning continues. Comparative char measurements across structural wooden members establish a relative timeline of which areas burned first.

Case Study: The 1987 King's Cross Station Fire

• Event Overview:

  • Date: November 18, 1987 ($11/18/1987$).

  • Location: London Underground, King's Cross Station (Piccadilly Line).

  • Casualties: $31$ fatalities.

  • Initial Incident: A small localized fire was reported under a wooden escalator step. London Fire Brigade arrived within 10 minutes to attend to a routine small escalator fire.

• Historical Context & Escalator Conditions:

  • Over $400$ escalator fires were recorded across the London Underground network in the 30 years preceding 1987.

  • Escalator steps were constructed of structural wood.

  • Despite active smoking bans, commuters regularly ignited matches and cigarettes prior to stepping onto escalators.

• Fuel and Ignition Source:

  • Lit matches or glowing cigarette embers dropped through narrow gaps between moving escalator steps.

  • Ignition occurred within accumulated debris in the escalator pit: wood dust/particulates, clothing lint, footwear debris, and heavy uncleaned mechanical grease accumulation.

  • Grease acts similarly to a high-flashpoint Class 3 combustible liquid; while resistant to direct ignition alone, grease mixed with fibrous dust forms an easily ignitable wick matrix.

• The Trench Effect Mechanics:

  • Investigated over 7 months using computational fluid dynamics on the Cray-2 supercomputer (the single supercomputer operating in the UK at that time) and validated via physical scale models.

  • Escalator Geometry: Inclined at an angle of $30^\circ$ inside a narrow trench-like inclined shaft.

  • Coanda Effect / Fluid Dynamics: Instead of rising vertically by standard buoyancy, accelerating hot combustion gases traveled along the inclined trench shaft. Pressure dropped above the step surfaces, forcing flames and superheated gas to lie flat against and adhere to the incline of the wooden steps.

  • Preheating Void: Flames attached to the underside of the escalator steps superheated the wooden assembly to temperatures exceeding $500^\circ\text{C}$, driving intense pyrolysis and off-gassing volatile fuels without immediate flaming in the upper ticket hall.

  • Final Transition: Accumulated superheated pyrolyzed gases trapped in the ceiling void above the ticket hall reached autoignition temperature, culminating in a catastrophic flashover. A massive jet of flame erupted up the escalator shaft into the ticket hall.

Fire Safety Controls and Risk Assessment

• Hierarchy of Controls Applied Post-Incident:

  • Elimination / Administrative: Banning smoking across indoor public transit facilities, platforms, and access points.

  • Substitution / Engineering Controls: Total removal of wooden escalators across transport networks; replacement with non-combustible metal step assemblies.

  • Engineering Controls: Installation of automatic heat detectors and thermal sensors beneath escalator runs; installation of automatic water sprinkler arrays underneath internal escalator step shafts.

• Baseline vs. Controlled Risk Assessments:

  • Hazard risk evaluations are performed first without controls to quantify baseline risk severity, then re-evaluated with implemented controls to verify risk reduction efficacy.

Questions & Discussion

• Question: Why do open cup flash point tests yield different results than closed cup tests?

  • Answer: In open cup tests, vapors disperse into ambient air, requiring higher fuel vapor generation and temperature to achieve an ignitable mixture. Closed cup tests contain the vapor, yielding lower, more conservative flash point readings.

• Question: What causes vapor to sink into low spots or float in air?

  • Answer: Vapor density relative to air ($\text{Air} = 1.0$). Vapors with a density greater than $1.0$ sink into low points, trenches, and confined spaces, whereas vapors with a density under $1.0$ rise.

• Question: Why did the fire in the King's Cross station travel horizontally along the steps instead of rising vertically?

  • Answer: The $30^\circ$ inclined trench shaft caused accelerating hot gases to drop in pressure above the steps, pinning the flames flat to the wooden steps via the trench effect rather than allowing normal upward buoyant plume rise.

Incident Analysis Assignment Guidelines

• Assignment Deliverable Requirements:

  • Due Date: Submission due by next Thursday.

  • Document Format: Word Document (.docx).

  • Page Length Limit: Maximum $1$ page total per incident.

  • Typography Rules: Calibri font, size $11$, double-spaced.

  • Required Content Sections:

  • Brief Event Summary: Concise overview detailing Who, What, Where, Why, How, Date, and Time.

  • Fire Behavior Analysis: Characterization of fuel load, flame spread rate, thermal movement, predictability, and fluid behavior.

  • Post-Incident Changes: Specific regulatory, legislative, engineering, or organizational safety rule updates resulting directly from the investigated event.