2.2 Vapor Suppression, Dispersion & Dilution

2.2 Vapor Suppression, Dispersion & Dilution

NFPA 470 Tactical Requirement: Operations-level responders must control atmospheric hazards associated with volatile hazardous liquids and toxic vapor releases using defensive tactics including foam application (vapor suppression), water fog streams (vapor dispersion), and controlled dilution.

When volatile liquids or gases escape containment, they create ignitable or toxic atmospheric vapor clouds. Firefighters and hazardous materials responders face two distinct threats: the liquid pool on the ground and the expanding vapor cloud overhead. Defensive atmospheric control tactics isolate these hazards to prevent catastrophic flash fires, BLEVEs, or toxic off-site exposures.

---

Vapor Suppression (Foam Application)

Vapor suppression is the physical application of a foam blanket over a spilled liquid hazardous material to prevent or reduce the release of flammable or toxic vapors.

Mechanisms of Foam Suppression

1. Vapor Barrier: The continuous foam layer caps the liquid surface, suppressing volatile gas molecules from escaping into the atmosphere.
2. Oxygen Exclusion: The foam blanket separates atmospheric oxygen from the flammable liquid surface.
3. Cooling: The water content in the foam drains down to cool the liquid surface and surrounding hot pavement.

FOAM SUPPRESSION LAYER
[   Firefighting Foam Blanket (Vapor Barrier)   ]
=================================================
[ Volatile Liquid Pool (Hydrocarbon or Solvent) ]
=================================================
  Pavement / Ground

Firefighting Foam Concentrates & Chemical Compatibility

Selecting the correct foam concentrate is critical. Applying an incompatible foam concentrate will cause the foam blanket to break down instantly, releasing flammable vapors.

1. Aqueous Film-Forming Foam (AFFF)

• Chemistry: Synthetic fluorinated surfactant foam.
• Application: Designed specifically for hydrocarbon fuels (gasoline, diesel, jet fuel, kerosene, fuel oil). It forms a thin aqueous film that floats ahead of the foam blanket across the hydrocarbon surface.
• Limitation: Ineffective on polar solvents. Polar solvents dissolve the aqueous film and destroy the foam bubble structure.

2. Alcohol-Resistant AFFF (AR-AFFF)

• Chemistry: Synthetic foam containing a water-soluble polymer.
• Application: Required for polar solvents and water-miscible liquids (ethanol, methanol, acetone, isopropyl alcohol, methyl ethyl ketone, ethyl acetate).
• Mechanism: When AR-AFFF contacts a polar solvent, the polymer precipitates out of the foam matrix to form a tough polymeric membrane (physical barrier) between the liquid fuel and the foam blanket, protecting the bubbles from chemical destruction.
• Versatility: AR-AFFF can be used on both polar solvents and hydrocarbon fuels (typically proportioned at $3%$ for hydrocarbons and $3%$ or $6%$ for polar solvents).

3. Fluoroprotein (FP) & Film-Forming Fluoroprotein (FFFP)

• Chemistry: Protein base enriched with fluorinated surfactants.
• Application: Bulk fuel storage tanks and refined petroleum fires. Excellent fuel-shedding capability and superior burnback resistance.

---

Foam Expansion Ratios & Application Methods

Foam expansion ratio refers to the volume of finished foam produced from one volume of foam solution (water/concentrate mixture).

$\text{Expansion Ratio} = \frac{\text{Volume of Finished Expanded Foam}}{\text{Volume of Water/Foam Solution Supplied}}$

1. Low Expansion Foam ($< 20:1$)

• Expansion Range: Typically $3:1$ to $15:1$.
• Equipment: Standard fog nozzles, smooth bore nozzles, or low-expansion air-aspirating foam pipes.
• Tactical Use: Outdoor hydrocarbon spills, highway accidents. Excellent reach, high wind resistance, and rapid stream throw distance.

2. Medium Expansion Foam ($20:1$ to $200:1$)

• Expansion Range: Typically $30:1$ to $120:1$.
• Equipment: Medium-expansion foam nozzles containing fine mesh screens.
• Tactical Use: Suppressing deep liquid pools, toxic chemical spills, and vapor mitigation in ditches or open drainage channels. Provides a thick insulating layer.

3. High Expansion Foam ($200:1$ to $1000:1$)

• Expansion Range: $200:1$ up to $1000:1$.
• Equipment: Powered high-expansion foam generators utilizing high-capacity fans.
• Tactical Use: Enclosed or confined spaces (ship holds, basements, aircraft hangars, underground utility vaults, warehouse fires). Fills large three-dimensional volumes rapidly. Easily blown away by outdoor wind.

Foam Blanket Application Techniques

Responders must NEVER plunge a foam stream directly into a liquid spill pool, as plunging causes agitation, splashing, and increased vapor generation.

1. Roll-On Method: Direct the foam stream onto the ground immediately in front of the spill pool, allowing the expanded foam to roll gently across the liquid surface.
2. Bank-Down Method: Direct the foam stream onto an elevated structure or barrier (such as a tanker wall, retaining dike, or vehicle side), allowing the foam to slide down smoothly onto the spill pool.
3. Rain-Down Method: Angle the hose stream high into the air, allowing the foam to fall softly over the spill like natural rainfall. Recommended for large open pools.

---

Vapor Dispersion Tactics

Vapor dispersion is a defensive technique that uses water spray patterns (fog streams) to push, direct, dilute, or disperse airborne flammable or toxic vapor clouds in open air.

VAPOR DISPERSION (Water Fog Stream)
             Wind Direction --->
     [ Hose Team ]=====( Wide Fog Pattern )=====> [ Airborne Vapor Cloud ]
                                                 (Disperses & Dilutes)
     ===================================================================
                         [ Liquid Spill Pool ]  <-- DO NOT SPRAY POOL DIRECTLY!

Operational Rules for Dispersion

• Nozzle Pattern: Deploy wide-angle fog streams ($45^\circ$ to $60^\circ$ patterns) operating at standard pressures ($100\text{ psi}$ nozzle pressure) to maximize air movement (venturi effect) and water droplet surface area.
• Positioning: Hose crews must operate upwind or crosswind of the vapor cloud outside the flammable envelope (monitoring Lower Explosive Limit / LEL continuously).
• CRITICAL SAFETY WARNING: Never spray water streams directly into the liquid spill pool. Direct water contact with volatile liquid pools causes turbulence, rapid thermal reaction, violent boiling, splattering, and a massive sudden spike in vapor production.

---

Dilution Tactics & Environmental Risks

Dilution is the application of water to a water-soluble hazardous material to reduce its concentration below hazardous levels (e.g., diluting an acid to raise pH, or diluting a water-soluble solvent above its flash point).

Limitations and Hazards of Dilution

1. Massive Runoff Volume: Diluting a $500\text{-gallon}$ chemical spill down to a safe $1%$ concentration requires adding $49,500\text{ gallons}$ of water. This creates an unmanageable volume of toxic runoff that quickly overflows containment dikes.
2. Exothermic Reactions: Adding water to concentrated strong acids (such as Sulfuric Acid) causes extreme exothermic boiling, spattering, and violent acid misting.
3. Regulatory Authorization: Dilution is strictly restricted under environmental protection laws. Responders must obtain explicit approval from the Incident Commander and State/Federal Environmental Protection Agencies before attempting dilution.