Foam Fire Extinguishers: How it Works

Fire safety is a critical aspect of any residential, commercial, or industrial setup. Choosing the right type of fire extinguisher can be the difference between a quickly controlled incident and a devastating disaster. Among the various types available, foam fire extinguishers have proven particularly effective for certain fire classes. This guide offers a deep dive into how foam fire extinguishers work, their composition, suitable use cases, limitations, and best practices — all written from the standpoint of a certified fire safety consultant.

What is a Foam Fire Extinguisher?

A foam fire extinguisher, also known as an AFFF (Aqueous Film Forming Foam) extinguisher, is a versatile fire suppressant designed to extinguish Class A (solid combustible materials like paper and wood) and Class B (flammable liquids like petrol or oil) fires.

These extinguishers release a foamy substance that forms a blanket over the burning material or liquid, smothering the flames and preventing reignition. They are commonly marked with a cream-colored label and are widely used in offices, garages, fuel stations, workshops, and warehouses.

Components of a Foam Fire Extinguisher

To understand how foam fire extinguishers work, it’s important to know what they contain:

• Water: Acts as the cooling agent.

• Foam concentrate (synthetic or protein-based): Responsible for forming the foam blanket.

• Compressed air or nitrogen: Provides pressure to discharge the contents.

• AFFF or AR-AFFF: These variations stand for Aqueous Film Forming Foam and Alcohol-Resistant Aqueous Film Forming Foam, respectively. AR-AFFF is especially designed for polar solvents like ethanol.

The combination of water and foam concentrate forms an aqueous film that cuts off the oxygen supply to the fire.

How Foam Fire Extinguishers Work – Step-by-Step Breakdown

When activated, the foam fire extinguisher undergoes the following process:

1. Discharge Initiation: Upon pulling the safety pin and pressing the handle, the pressure inside the canister forces the foam solution out through the nozzle.

2. Foam Formation: The foam solution exits the nozzle where it mixes with air to form a thick, frothy foam. The air entrainment creates small bubbles that help in forming a stable foam layer.

3. Application on Fire: For Class A fires, the foam penetrates porous materials, cools the surface, and prevents reignition. For Class B fires, the foam spreads across the flammable liquid, forming a barrier between the fuel and atmospheric oxygen.

4. Smothering and Cooling: The foam acts by:

• Smothering the fire (cutting off the oxygen),

• Cooling the burning surface (absorbing the heat).

The aqueous film spreads quickly, even beyond the visible foam, which is why foam extinguishers are highly effective against liquid fires.

Advantages of Foam Fire Extinguishers

From a professional fire consultant’s viewpoint, foam extinguishers provide a strategic advantage in mixed-risk environments.

Here’s why:

1. Effective on Multiple Fire Classes: Foam extinguishers can be used on Class A and Class B fires, offering flexibility where both solid combustibles and flammable liquids are present.

2. Prevents Reignition: The foam forms a seal over liquid fires, preventing flammable vapors from escaping and reigniting.

3. Rapid Knockdown Capability:  Foam works faster than water alone when dealing with flammable liquids.

4. Minimal Collateral Damage: Unlike powder extinguishers, foam causes less mess and is easier to clean up.

Limitations of Foam Fire Extinguishers

Despite their effectiveness, foam extinguishers have limitations that must be carefully considered during fire risk assessments:

1. Not Suitable for Electrical Fires (Class C): Although some foam extinguishers are tested to be dielectrically safe, they are not the preferred choice for electrical fires.

2. Unsuitable for Metal Fires (Class D) and Cooking Oil Fires (Class F): Foam reacts violently with burning metals and is ineffective against high-temperature oil fires in kitchens. Use a Class D extinguisher or a wet chemical extinguisher, respectively.

3. May Be Corrosive Over Time:  If left on surfaces, especially metal, foam residue can lead to corrosion. Prompt cleanup is essential.

Recommended Usage Techniques

Using a foam fire extinguisher effectively requires understanding the PASS method with foam-specific nuances:

1. P – Pull the pin.

2. A – Aim the nozzle at the base of the fire.

3. S – Squeeze the handle to release foam.

4. S – Sweep side to side across the fire surface.

For Class A Fires:

• Aim directly at the base of the fire.

• Maintain a safe distance of 1.5 to 2 meters.

• Sweep slowly to cover all burning material.

For Class B Fires:

• Aim the foam just in front of the fire, not directly into the flames, to avoid splashing.

• Allow foam to form a layer on the surface of the liquid.

• Avoid disturbing the foam once the fire is out.

Real-Life Scenario: Foam Fire Extinguisher in Action

Case Study: Petrol Station Fire Suppression

In 2021, a vehicle at a petrol station in Kent, UK, leaked fuel and caught fire. The quick-thinking staff deployed a nearby foam fire extinguisher. Thanks to its aqueous film-forming property, the fire was controlled within seconds. No casualties were reported, and the forecourt sustained minimal damage.

This incident demonstrates how foam extinguishers are crucial for locations with high flammable liquid risks.

Maintenance and Inspection Guidelines

Regular maintenance of foam fire extinguishers is vital for reliability. As per BS 5306-3 (British Standard), the following schedule is recommended:

Monthly Checks:

• Ensure the extinguisher is in its designated place.

• Confirm it’s not blocked or tampered with.

• The pressure gauge should be in the green zone.

Annual Servicing:

• Conducted by a certified technician.

• Involves internal inspection, re-pressurization, and seal replacement.

Extended Service (Every 5 Years):

• Discharge and refill the extinguisher.

• Pressure test the canister.

Always use approved refill agents to retain extinguisher effectiveness and compliance.

Locations Where Foam Fire Extinguishers Are Ideal

As a consultant, I recommend placing foam fire extinguishers in the following areas:

• Fuel stations

• Car workshops

• Garages

• Warehouses storing paint or solvents

• Boat engine rooms

• Offices (where both paper and cleaning liquids may be stored)

They are especially suitable where liquid spill fires pose a major threat.

Alternatives to Foam Fire Extinguishers

Depending on your fire risk assessment, alternatives may be more appropriate in certain scenarios:

A multi-class approach to extinguisher placement often provides the most comprehensive fire protection.

Key Differences: Foam vs CO₂ vs Dry Powder

Foam extinguishers strike a balance between effectiveness and cleanup ease, especially for locations where Class A and B risks coexist.

Compliance and Safety Standards

Foam extinguishers must meet international safety standards such as:

• EN 3 (European Standard) for portable fire extinguishers.

• BS 5306 (UK Standard) for fire extinguishing installations.

• NFPA 10 (USA) guidelines for fire extinguisher inspection and use.

• ISO 7165 – Global performance standard for fire extinguishers.

Choosing a certified model with compliance markings ensures safety and legal adherence.

Environmental Considerations of Foam Extinguishers

Some foam agents have come under scrutiny due to environmental concerns, especially those containing PFAS (Per- and polyfluoroalkyl substances).

Recommendations:

• Opt for fluorine-free foams (F3) where possible.

• Properly dispose of used foam and follow EPA or local environmental regulations.

• Use spill containment kits during use to prevent environmental runoff.

Manufacturers are increasingly producing eco-friendly foam variants to align with modern sustainability goals.

Conclusion:

As a Fire Safety Consultant, I consistently recommend foam fire extinguishers as a first line of defense in mixed-risk environments. Their ability to address both Class A and Class B fires makes them incredibly versatile.

However, their effectiveness depends on correct application, proper placement, and regular maintenance. Employers, facility managers, and safety personnel must ensure that all fire extinguishers, including foam types, are installed according to risk assessment outcomes and that staff are adequately trained in their use.

Frequently Asked Questions (FAQs)

1. Can foam extinguishers be used on electrical fires?

Not recommended unless dielectrically tested and approved. CO₂ is a safer choice.

2. How long does a foam extinguisher last?

Typically 5 years, but check the manufacturer’s guide. Regular servicing ensures functionality.

3. What does AFFF stand for?

Aqueous Film Forming Foam – forms a water-based film to smother flammable liquid fires.

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