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Class B fires involve flammable liquids and gases—like gasoline, diesel, alcohol, and propane. These fuels ignite easily and spread rapidly, making them especially dangerous. The right extinguisher can stop a small fire before it turns destructive. That’s where the Class B fire extinguisher comes in. Designed for flammable or combustible liquids, this type of extinguisher is a critical tool for anyone responsible for fire safety in industrial, commercial, or even residential settings.
In this guide, we’ll break down what a Class B fire is, how to identify the right extinguisher, and how to use it safely.
Fire Classes and Fire Extinguisher Types
To choose the correct extinguisher, it’s important to understand how fires are classified. The five basic fire classes are:
- Class A: Fires involving ordinary combustible materials like wood, paper, and cloth.
- Class B: Fires fueled by flammable liquids and gases such as gasoline, alcohol, or oil-based paints.
- Class C: Fires that start from energized electrical equipment like outlets or appliances.
- Class D: Fires involving combustible metals such as magnesium or titanium.
- Class K: Fires related to cooking oils and fats, commonly found in commercial kitchens.
Each fire class requires a specific extinguisher. Class B fires cannot be put out with water, as it causes the burning liquid to splash and spread. Instead, they require extinguishing agents designed to cut off oxygen or stop the chemical reaction that fuels the fire.
What Is a Class B Fire?
A Class B fire starts when a flammable liquid or gas ignites. These materials have a low flash point, which means they can catch fire even at relatively low temperatures. Common fuel sources include gasoline, petroleum greases, solvents, oil-based paints, propane, and alcohol-based chemicals. These substances not only burn hot and fast—they often release toxic fumes and thick smoke that make fires harder to control.
Class B fires are frequently seen in environments like chemical storage rooms, fuel transfer stations, auto repair shops, and commercial kitchens. Because the fuel source is often a liquid or gas, using the wrong extinguisher—especially a water-based one—can actually spread the fire instead of putting it out.
When to Use a Class B Fire Extinguisher
Class B fire extinguishers are designed to handle fires involving flammable or combustible liquids and gases. These extinguishers work by cutting off the fire’s oxygen supply or interrupting the chemical reaction that fuels the flames.
Use a Class B extinguisher for fires involving:
- Gasoline, diesel, and other petroleum-based fuels
- Oil-based paints, solvents, and cleaning agents
- Alcohol-based liquids or chemicals
- Propane, butane, and other flammable gases
- Grease and fuel vapors in commercial kitchens or workshops
They are most effective in locations where these substances are stored or used regularly, including chemical storage areas, garages, industrial workspaces, and fuel transfer stations.
When Not to Use a Class B Fire Extinguisher
Using a Class B extinguisher on the wrong type of fire can cause serious harm or make the fire worse. These extinguishers are not designed for every fire class.
Do not use a Class B extinguisher on:
- Fires involving ordinary combustibles like wood, cloth, or paper (Class A fires)
- Fires caused by energized electrical equipment such as outlets, wiring, or appliances (Class C fires)
- Fires involving combustible metals like magnesium or titanium (Class D fires)
- Fires fueled by cooking oils or fats in deep fryers (Class K fires)
- Any fire where water or foam might spread the burning material, such as certain gas or solvent spills in confined areas
Always match the extinguisher to the fire class. If you’re unsure whether the fuel source is compatible, avoid using the extinguisher and evacuate the area.
How Class B Fire Extinguishers Work
Class B extinguishers interrupt the fire triangle—fuel, heat, and oxygen—by either smothering the flames or chemically stopping the combustion process. Different extinguishers use different methods depending on the agent inside.
Foam fire extinguishers, for example, work by forming a thick foam blanket over the burning liquid. This blanket blocks oxygen and cools the fuel, making it difficult for flames to re-ignite. Carbon dioxide extinguishers release a cloud of CO₂ gas that quickly displaces the oxygen surrounding the flames. Dry chemical extinguishers, on the other hand, use a powder (often monoammonium phosphate or sodium bicarbonate) to interrupt the fire’s chemical reaction and block oxygen from reaching the fuel.
Each extinguishing agent has specific strengths. Foam is ideal for liquid fires but less effective in enclosed electrical spaces. CO₂ works well in clean areas like labs and server rooms, while dry chemicals offer a broad-use solution across Class A, B, and C fires.
How to Identify a Class B Fire Extinguisher
To ensure you’re using the correct fire extinguisher, look for the classification label printed clearly on the body. A Class B fire extinguisher will feature the letter “B”, often accompanied by a symbol showing a gasoline can or flame over liquid. Many extinguishers are multipurpose and list multiple classes—A, B, and C—on the label.
Portable fire extinguishers designed for Class B fires come in various sizes and can be installed in homes, workshops, commercial kitchens, and industrial spaces. Placement matters. Make sure extinguishers are mounted in easy-to-reach, clearly marked locations near areas where flammable or combustible liquids are stored or used.
How to Use a Class B Fire Extinguisher
If you’ve been trained and it’s safe to proceed, follow the PASS method:
- Pull the pin to break the tamper seal
- Aim the nozzle at the base of the flames, not the tips
- Squeeze the handle to release the extinguishing agent.
- Sweep the nozzle from side to side, covering the entire base of the fire.
Keep a distance of 6 to 8 feet, and move closer only as the fire begins to subside. Aim low, stay calm, and be ready to leave if the fire re-ignites or spreads beyond your control.
Why Class B Fires Need More Than a Basic Fire Extinguisher
Not all extinguishers are created equal. Using the wrong one can worsen the fire or cause injury. Water fire extinguishers are especially dangerous around flammable liquids, while wet chemical extinguishers are designed specifically for cooking oil fires, not gas or solvent fires.
Good fire protection includes more than just having the right extinguisher. Flammable liquids should be stored in ventilated, cool areas away from ignition sources. Containers must be properly sealed, labeled, and stored upright in safety-approved cabinets. Routine inspections and preventative measures, like installing automatic extinguishers or fire monitoring systems, can prevent small hazards from becoming disasters.
How to Maintain Class B Fire Extinguishers
Regular maintenance keeps your extinguishers reliable and ready during an emergency. A few quick checks each month, and a yearly professional review, can make all the difference in a high-risk moment.
Here’s how to keep your extinguisher in working condition:
- Inspect monthly for visible damage. Look for signs of corrosion, dents, or a clogged nozzle that could affect performance.
- Check the pressure gauge. Make sure the needle is in the green zone and that nothing looks out of place.
- Confirm the pin and tamper seal are intact. If the seal is broken or missing, the extinguisher may have been discharged.
- Schedule annual professional servicing. A licensed technician can recharge, test, or replace units as needed to meet local fire codes.
- Replace or recharge after any use. Even a partial discharge means the extinguisher may not function properly in a second emergency.
- Track the expiration date. Most extinguishers last 10 to 15 years—beyond that, they may be unreliable or non-compliant.
- Make sure everyone knows where extinguishers are and how to use them. Clear signage, regular drills, and basic training help people respond quickly and safely.
Taking these steps doesn’t just meet code—it builds real preparedness. Keeping your Class B extinguishers in top shape protects people, property, and peace of mind.
Stay Fire-Ready with the Right Equipment
Class B fires can escalate quickly—but with the right extinguisher, knowledge, and precautions, they’re also highly preventable. Whether you’re storing flammable liquids in your workshop or managing fire safety in a commercial kitchen, a Class B fire extinguisher is a frontline defense against disaster.
Want help choosing the correct fire extinguisher for your property or facility? Contact B&W Fire Security for expert recommendations, professional inspections, and reliable fire protection solutions tailored to your needs.
