3.3 Key Physical & Chemical Behavior

3.3 Key Physical & Chemical Behavior

To safely respond to hazardous materials incidents, Awareness-level responders must understand how chemicals behave in the environment. The physical state, density, and thermal properties of a substance dictate where it will travel, how easily it will ignite, and whether it poses an immediate explosive risk. This section explains key physical and chemical properties and details the dangerous mechanism of a Boiling Liquid Expanding Vapor Explosion (BLEVE).

Vapor Density

Vapor density is the relative weight of a given volume of pure gas or vapor compared to an equal volume of dry air at the same temperature and pressure. Air is assigned a baseline value of 1.0.

• Vapor Density Less Than 1.0 (Lighter than Air): These gases will rise and disperse upward in the atmosphere. Examples include hydrogen (0.07), helium (0.14), methane (0.55), and anhydrous ammonia (0.60). Outdoors, these gases tend to dissipate quickly unless trapped by a ceiling or roof structure.
• Vapor Density Greater Than 1.0 (Heavier than Air): These gases will sink and pool in low-lying areas, such as trenches, ditches, basements, and sewers. Examples include propane (1.56), chlorine (2.45), and gasoline vapors (3.0 to 4.0). Heavier-than-air gases pose a severe risk of creating invisible flammable or toxic pools that can travel long distances to an ignition source and flash back.

Knowing the vapor density allows responders to position themselves upwind and determine if low-lying areas must be evacuated.

Specific Gravity

Specific gravity is the ratio of the density of a liquid compared to the density of pure water at a specified temperature. Water is assigned a baseline value of 1.0.

• Specific Gravity Less Than 1.0 (Floats on Water): Liquids with a specific gravity less than 1.0 will float on the surface of water. Examples include gasoline (0.72), diesel fuel (0.85), and most hydrocarbon oils. Responders must recognize that applying water to these fires can cause the burning liquid to float and spread, expanding the fire zone.
• Specific Gravity Greater Than 1.0 (Sinks in Water): Liquids with a specific gravity greater than 1.0 will sink to the bottom of a water body. Examples include chlorinated solvents (like trichloroethylene) and concentrated sulfuric acid. These materials are difficult to detect visually in deep water and can cause severe ecological damage to aquatic beds.

Thermal and Volatility Properties

A chemical's response to heat dictates its volatility and ease of ignition. Key terms include:

• Flash Point: The minimum temperature at which a liquid gives off enough vapor to form an ignitable mixture with air near the surface of the liquid, but will not sustain combustion. At the flash point, a spark will cause a brief flash, but the fire will go out.
• Fire Point: The temperature at which a liquid produces enough vapor to support continuous combustion once ignited (typically a few degrees higher than the flash point).
• Boiling Point: The temperature at which a liquid transitions into a gas. At this temperature, the vapor pressure of the liquid equals the surrounding atmospheric pressure. Materials with low boiling points (such as liquefied gases) vaporize extremely rapidly at ambient temperatures, producing massive vapor clouds.
• Ignition Temperature (Autoignition Temperature): The minimum temperature at which a substance will self-ignite in the absence of an external spark or flame. For example, gasoline has a flash point of -43°C (-45°F) but an ignition temperature of 280°C (536°F). It can flash at room temperature if a spark is present, but it will not self-ignite unless heated to 280°C.

Boiling Liquid Expanding Vapor Explosion (BLEVE)

A Boiling Liquid Expanding Vapor Explosion (BLEVE) is a catastrophic failure of a pressurized container holding liquefied gas heated above its boiling point.

The BLEVE Mechanism

1. External Heating: A pressurized tank of liquefied gas is exposed to fire.
2. Boiling and Pressurization: The liquid boils, raising pressure. The relief valve vents gas, which often ignites as a torch fire.
3. Metal Weakening: As liquid drops, the tank wall above it (vapor space) is no longer cooled. Direct flame impingement heats this dry metal to failure.
4. Catastrophic Failure: The weakened shell stretches and tears open.
5. Instantaneous Vaporization: The pressure drop causes the pressurized liquid to flash into gas, expanding violently (e.g., propane expands 270 times).
6. Fireball: The expanding gas ignites, creating a fireball and projecting metal shrapnel.

Signs of an Impending BLEVE

• High-pitched ringing or whistling sound from the relief valve, showing it cannot vent pressure fast enough.
• Discoloration or bulging of the tank shell in the vapor space.
• Steam rising from wet tank surfaces.

Tactical Safety

Awareness responders must not attempt to fight the fire or cool the tank.

• Evacuation: Initial isolation must be at least 1,600 feet (500 meters), extending to 1 mile if a BLEVE is imminent.
• Positioning: Never stand at the ends of the tank, as they fail by launching end caps lengthwise. Approach from the sides.