A class B fire is a fire fuelled by flammable or combustible liquids including petrol, diesel, oil, alcohol, paint solvents, and grease. These fires burn at the liquid’s surface, spread rapidly through spilled fuel, and can travel along vapour trails far from the original ignition point. Water does not extinguish Class B fires. It makes them significantly worse.
Class B fires behave differently from ordinary solid-fuel fires. They spread faster, reignite easier, and create explosion hazards that do not exist in most Class A incidents. Understanding flash point, vapour density, BLEVE risk, and the correct suppression agent is what separates a contained incident from a refinery-scale disaster.
For the full picture of all 6 fire classes and the right extinguisher for each, see the complete classes of fire guide.
Table of Contents
- What is a Class A fire?
- What materials cause a Class A fire?
- What causes a Class A fire?
- Spontaneous combustion and hidden ignition
- Why Class A fires smoulder and reignite
- How to extinguish a Class A fire
- What 13A, 21A, 34A and 55A ratings mean
- Where Class A fires commonly occur
- Class A fire prevention checklists
- Class A vs Class B vs Class C fires
- Frequently asked questions
What is a class B fire? — the direct definition
A class B fire is any fire fuelled by a flammable or combustible liquid such as petrol, diesel, kerosene, alcohol, oil-based paint, or grease where the liquid’s surface vapour ignites and the fire spreads rapidly with the flow of the fuel.
Unlike a solid combustible fire, a Class B fire does not burn deep into the material itself. The liquid surface continuously releases vapour, and it is this vapour-air mixture that actually burns. Because liquids flow, the fire can spread across floors, drains, work surfaces, and containment areas within seconds. Vapour movement also allows ignition to occur far from the original spill source, particularly with petrol and solvents that produce dense, ground-hugging vapours.
Flammable liquid vs combustible liquid
Not all Class B liquids carry the same ignition risk. The difference comes down to one physical property: flash point.
Flash point is the lowest temperature at which a liquid releases enough vapour above its surface to form an ignitable mixture with air. The liquid itself does not burn directly. The vapour above the liquid burns. That is why petrol remains dangerous even on a cold winter morning. Petrol has a flash point of about -43°C, meaning it produces ignitable vapour at virtually any normal atmospheric temperature.
Diesel behaves differently. Its flash point sits around 52°C. At room temperature, diesel produces far less ignitable vapour, which is why diesel spills are generally less immediately volatile than petrol spills.
| Property | Flammable Liquid | Combustible Liquid |
|---|---|---|
| Flash point | Below 37.8°C (100°F) | At or above 37.8°C (100°F) |
| Fire risk at room temperature | Immediate, vapour always present | Lower, needs pre-heating |
| Common examples | Petrol (-43°C), acetone (-18°C), ethanol (13°C) | Diesel (~52°C), kerosene (~65°C), lubricating oil (~200°C+) |
| Storage classification | Strictest controls | Lower-tier controls |
| NFPA designation | Class I flammable liquid | Class II / Class III combustible liquid |
Both categories still fall under Class B fire classification systems. Understanding the distinction helps identify which environments carry the highest immediate ignition risk, particularly in fuel handling, transport, and industrial storage facilities.
What materials cause a class B fire?
Common Class B liquids in everyday and commercial settings
- Petrol has a flash point of -43°C and creates one of the fastest-spreading Class B fire hazards found in vehicles, petrol stations, generators, and garages.
- Diesel burns less readily because of its ~52°C flash point, but large diesel spills in workshops or storage depots still create major flammable liquid fire risks.
- Kerosene and paraffin are widely used in heaters and aviation systems, with flash points ranging from 38°C to 72°C depending on grade and refinement.
- Alcohols including ethanol, methanol, and isopropanol appear in laboratories, distilleries, pharmaceutical facilities, and even hand sanitiser production lines.
- Acetone, with a flash point of -18°C, is common in nail salons, plastics manufacturing, and chemical laboratories.
- Oil-based paints, lacquers, varnishes, turpentine, and white spirits create major Class B fire hazards in paint shops, printing facilities, and vehicle respray operations.
- Lubricating oils, hydraulic fluids, and heated grease also fall into Class B classifications under many fire standards once sufficient vapour generation occurs.
Class B liquids in industrial settings
Large-scale industrial Class B fuels include crude oil, petroleum products, aviation fuel, industrial solvents, adhesives, resins, and printing inks.
Refineries, tank farms, airports, chemical plants, pharmaceutical facilities, marine terminals, and military airfields all manage high-volume Class B fire risks daily.
Cooking oils and animal fats are not Class B fires. They fall under Class F fire classification because their combustion behaviour at elevated temperatures differs significantly from standard flammable liquid fires. See the full all 6 fire classes reference for the Class F distinction.
What causes a class B fire? — ignition sources and high-risk scenarios
Common ignition sources for Class B fires
Most Class B fires begin when vapour from a flammable liquid encounters heat, flame, or electrical energy.
Common ignition sources include cigarettes near petrol storage, pilot lights near solvent cabinets, electrical switch sparks, static discharge during fuel transfer, welding sparks, grinding operations, overheated engine components, and industrial heaters.
Oil-soaked rags stored in confined spaces can self-heat and ignite spontaneously through oxidation. Poor housekeeping in workshops remains one of the most common causes of small commercial Class B fires.
Why Class B fires travel (vapour density explained)
Petrol vapour is roughly 3 to 4 times heavier than air. Instead of rising and dispersing, it sinks and flows along the ground like invisible water. Vapour enters drains, pits, inspection channels, basements, and low corners where ventilation is poor.
A petrol spill in one part of a workshop can release vapour that travels 10 to 20 metres before reaching an ignition source. Once ignition occurs, the flame front races back along the vapour trail to the original fuel source. This is known as vapour trail ignition or flash fire propagation.
That behaviour makes Class B fires fundamentally more dangerous than most Class A fires. The ignition source and the fuel source are often physically separated.
High-risk locations for Class B fires
High-risk Class B environments include:
- Petrol stations and fuel dispensing zones
- Vehicle repair workshops and spray booths
- Paint shops and printing facilities
- Chemical warehouses
- Pharmaceutical plants
- Airports and aviation fuel systems
- Marine fuel handling areas
- Oil refineries and tank farms
- Domestic garages storing petrol cans or solvents
The specific dangers of class B fires
Rapid surface spread
Class B fires burn at the fuel surface. When a liquid spills, the burning surface area expands instantly across the entire spill footprint. A 5-litre fuel spill can turn a small ignition source into a floor-wide fire within seconds.
Boilover — the Class B hazard in large fuel tanks
Boilover occurs in large crude oil or heavy fuel storage tanks after prolonged surface burning. Heat slowly moves downward through the oil layer. If water exists at the tank bottom from condensation, rainwater, or suppression runoff, the advancing heat front eventually reaches it.
Water instantly converts to steam and expands violently beneath the burning oil. The result is a massive eruption that throws burning oil hundreds of metres outward from the tank. Boilover events have destroyed refinery infrastructure and caused multiple firefighter fatalities in documented petroleum storage incidents worldwide.
BLEVE — when Class B meets a pressurised container
A BLEVE is a Boiling Liquid Expanding Vapour Explosion. It occurs when a pressurised container holding flammable liquid or gas is heated externally by fire.
As temperature rises, internal pressure increases rapidly. If the vessel fails before pressure relief can stabilise conditions, the liquid flashes instantly into vapour and erupts as a fireball explosion.
This is why firefighters cool nearby LPG cylinders, fuel drums, and aerosol containers with water during Class B incidents. The water is not used to extinguish the burning fuel. It is used to stop the container from rupturing catastrophically.
Toxic smoke and fume production
Class B fires generate highly toxic combustion products. Burning petrol, diesel, solvents, and synthetic hydrocarbons release carbon monoxide, benzene, toluene, xylene, and dense particulate matter.
Exposure near an active Class B fire without breathing apparatus creates immediate inhalation hazards even before direct flame exposure occurs.
Why you must never use water on a class B fire
Water is denser than most flammable liquids. When sprayed onto burning fuel, the water sinks beneath the liquid instead of smothering it.
At the fuel-water interface, the trapped water absorbs extreme heat and flashes into steam almost instantly. Steam expands to roughly 1,700 times its original liquid volume within milliseconds. That violent expansion launches burning fuel droplets outward like flaming shrapnel, dramatically increasing fire spread.
A second problem happens before steam generation even begins. The force of the water stream physically pushes and disperses the burning liquid across floors, drains, and surrounding surfaces.
This is why a small burning petrol tray can become a workshop-wide fire within seconds after someone reaches for a hose.
The only legitimate role for water in a Class B fire scenario is cooling nearby pressurised containers to prevent BLEVE conditions. Water must never be used directly for suppression of the burning liquid itself.
How to extinguish a class B fire — correct agents and techniques
Foam extinguisher: the primary Class B agent
Foam extinguishers suppress Class B fires by creating a blanket over the fuel surface. That blanket blocks oxygen, cools the surface, and prevents vapour release that could reignite the fire.
Traditional AFFF foam systems dominated industrial Class B suppression for decades because they spread rapidly across hydrocarbon fuels. However, AFFF contains PFAS chemicals linked to persistent environmental contamination. Many industries are now transitioning toward fluorine-free F3 foam alternatives to meet evolving environmental and procurement requirements.
Foam compatibility matters. Standard AFFF works effectively on hydrocarbon fuels like petrol, diesel, and kerosene. It fails on polar solvents such as ethanol, methanol, acetone, and isopropanol because those liquids absorb the foam’s water content and destroy the foam blanket almost immediately.
Facilities handling alcohols or solvents require alcohol-resistant AR-AFFF or modern AR-F3 formulations specifically designed for polar solvent protection.
DCP / ABC dry powder extinguisher
Dry chemical powder extinguishers interrupt the combustion chain reaction at flame level. They provide rapid knockdown on fuel surface fires and remain widely used in workshops, fuel depots, and industrial environments.
Their limitation is re-ignition risk. Powder does not cool the liquid fuel. If vapour release continues after discharge, the fire can reignite quickly.
Speciality Geochem’s ABC powder fire extinguisher range is designed for Class B suppression in automotive, industrial, and commercial settings.
CO₂ extinguisher
CO₂ extinguishers suppress fire by displacing oxygen around the flame zone. They leave no residue, making them useful in electrical rooms, server areas, and enclosed machinery spaces.
In open environments, CO₂ disperses rapidly and re-ignition risk increases once oxygen returns to the fuel surface.
For enclosed fuel-related electrical hazards, a CO₂ fire extinguisher remains one of the cleanest suppression choices available.
What NOT to use on a Class B fire
- Water
- Standard water mist not rated for Class B
- Wet chemical extinguishers designed for Class F cooking oil fires
A properly rated foam fire extinguisher remains the preferred suppression method for most liquid fuel fires.
Understanding the Class B rating on fire extinguisher labels
The number before the letter “B” on a Class B extinguisher indicates the size of standardised heptane test fire the extinguisher can suppress under EN 3 testing conditions.
A 55B extinguisher must extinguish a 55-square-metre heptane fire tray. An 89B extinguisher must suppress an 89-square-metre test fire.
Higher B ratings indicate greater extinguishing performance and are essential in larger fuel-risk environments.
| Class B rating | Test fire area | Practical application |
|---|---|---|
| 34B | 34 sq metres heptane | Small vehicles, domestic garage |
| 55B | 55 sq metres heptane | Workshops, vans, small commercial |
| 89B | 89 sq metres heptane | Industrial units, fuel handling areas |
| 144B | 144 sq metres heptane | Large industrial, fuel storage, refineries |
| 233B | 233 sq metres heptane | Specialist high-risk applications |
Selecting a domestic 34B extinguisher for a fuel storage facility is like bringing an umbrella into a cyclone. The rating must match the fuel load and spill risk present in the environment.
Class B fire prevention — practical steps by environment
General Class B fire prevention principles
Class B prevention focuses on eliminating ignition sources and controlling fuel spills before vapour accumulation occurs.
- Store flammable liquids in approved sealed containers
- Use ventilated flammable storage cabinets
- Maintain minimum 6 air changes per hour in storage areas
- Prohibit smoking and open flames near fuel zones
- Bond and earth metal fuel containers during transfer
- Install vapour detection in enclosed fuel-handling spaces
- Inspect daily for leaks and vapour buildup
- Keep fuel containers sealed when not in use
- Dispose of solvent-soaked rags in sealed metal bins
Workplace Class B fire prevention checklist
- Conduct flammable material risk assessments
- Use hot work permit systems
- Train staff on flash point hazards
- Install extinguishers with suitable B ratings
- Use automatic foam suppression for large fuel stores
Class B prevention is less about dramatic firefighting and more about disciplined control of vapour, ignition, and containment. A workshop floor soaked with fuel vapour is a sleeping dragon waiting for one spark.
Class B fire vs class A and class C — key differences
| Class A | Class B | Class C | |
|---|---|---|---|
| Fuel | Solid combustibles | Flammable or combustible liquids | Flammable gases |
| Burns at | Surface + deep interior | Surface only but spreads with fuel flow | Point of release |
| Extinguish by | Cooling with water or foam | Smothering with foam or DCP | Shut supply + DCP |
| Water | Primary agent | Never use | No |
| Re-ignition risk | High from smouldering | High from exposed fuel surface | High until supply sealed |
| Unique hazard | Deep-seated reignition | Vapour trail ignition, boilover, BLEVE | Gas accumulation explosion |
For the full breakdown of Class D metal fires, electrical hazards, and cooking oil fires, see the complete fire classification guide.
For a deeper look at ordinary combustible fires, read the supporting class A fire article.
Frequently asked questions — class B fires
What is a class B fire?
A Class B fire is a fire involving flammable or combustible liquids such as petrol, diesel, alcohol, solvents, oils, and paints. The vapour above the liquid ignites and burns at the fuel surface. These fires spread rapidly because liquids flow across floors and containment areas. Vapour can also travel away from the original spill source before ignition occurs.
What materials are involved in a class B fire?
Class B fires involve petrol, diesel, kerosene, acetone, ethanol, isopropanol, oil-based paints, varnishes, solvents, lubricants, aviation fuel, hydraulic oils, and similar combustible liquids. Industrial Class B environments include refineries, fuel depots, chemical plants, paint facilities, and aircraft refuelling areas.
What is the difference between a flammable liquid and a combustible liquid?
The difference is flash point. Flammable liquids have flash points below 37.8°C and produce ignitable vapour at normal temperatures. Combustible liquids have higher flash points and generally require heating before significant vapour formation occurs. Petrol is flammable. Diesel is combustible.
What is flash point and why does it matter for Class B fires?
Flash point is the lowest temperature at which a liquid releases enough vapour to ignite in air. It determines how easily a fuel can start a fire. Petrol’s flash point is around -43°C, making it dangerous even in cold conditions. Diesel requires much higher temperatures before ignition risk rises sharply.
What fire extinguisher should be used on a class B fire?
Foam extinguishers are the primary choice for most Class B fires because they smother vapour release and cool the fuel surface. ABC powder extinguishers provide rapid knockdown. CO₂ extinguishers work well in enclosed spaces and around electrical equipment.
Can you use water on a class B fire?
No. Water must never be used directly on a Class B fire involving flammable liquids. Water sinks beneath the fuel and causes explosive steam generation that spreads burning liquid outward.
Why does water make a class B fire worse?
Water converts into steam when trapped beneath burning fuel. Steam expands roughly 1,700 times in volume, violently ejecting burning liquid droplets in all directions. Water streams also physically spread the fuel before steam expansion even begins.
What is foam fire extinguisher and how does it work on Class B fires?
A foam extinguisher creates a blanket across the liquid surface. That blanket blocks oxygen, suppresses vapour release, and cools the fuel. Foam prevents reignition more effectively than dry powder because it seals the liquid surface after flame knockdown.
Can a CO₂ extinguisher be used on a class B fire?
Yes. CO₂ extinguishers can suppress Class B fires by displacing oxygen around the flame zone. They work best in enclosed spaces because open-air wind conditions disperse the gas rapidly.
What is BLEVE and when does it occur in a Class B fire?
BLEVE stands for Boiling Liquid Expanding Vapour Explosion. It occurs when a pressurised container exposed to fire overheats and ruptures violently. LPG cylinders, fuel drums, and aerosol containers are common BLEVE risks.
What is boilover and which class B fires are at risk?
Boilover occurs in large crude oil or heavy fuel storage tanks. Water trapped beneath the fuel converts into steam after prolonged heating and violently ejects burning oil outward across a large area.
What is vapour trail ignition and how does it happen?
Vapour trail ignition occurs when heavy flammable vapour travels away from a liquid spill and reaches a distant ignition source. The flame then races back along the vapour cloud to the original spill location.
What is the difference between a class A and a class B fire?
Class A fires involve solid combustibles like wood, paper, and fabric. Class B fires involve flammable liquids like petrol and solvents. Water is effective on Class A fires but dangerous on Class B fires.
What does “55B” or “89B” mean on a fire extinguisher?
The number represents the size of standardised heptane fire the extinguisher can suppress during testing. A 55B extinguisher can extinguish a 55-square-metre heptane tray fire under EN 3 test conditions.
Why can’t standard foam extinguishers be used on alcohol fires?
Alcohols and polar solvents absorb the water content inside standard foam blankets, destroying the foam layer rapidly. Alcohol-resistant AR-AFFF or AR-F3 foam is required for those fuels.
What is AFFF foam and why is it being phased out?
AFFF stands for Aqueous Film Forming Foam. It has been widely used for hydrocarbon fuel fires but contains PFAS chemicals linked to environmental contamination and health concerns. Many facilities are transitioning toward fluorine-free F3 alternatives.
How do you prevent a class B fire in a workshop or garage?
Store fuel safely, ventilate enclosed spaces, remove ignition sources, inspect regularly for leaks, use approved containers, and keep suitable Class B extinguishers nearby. Grinding or welding should never occur near exposed fuel vapour.
What is a class bravo fire?
“Class Bravo fire” is the NATO phonetic alphabet term for a Class B fire. It refers to fires involving flammable or combustible liquids.
Are petrol fires class A or class B?
Petrol fires are Class B fires because petrol is a highly flammable liquid with a flash point around -43°C.
What class of fire is a diesel fire?
Diesel fires are Class B fires. Diesel is classified as a combustible liquid due to its higher flash point compared with petrol.
Conclusion
Class B fires move fast, spread unpredictably, and punish bad decisions immediately. A spilled fuel tray can become a workshop-wide fire before most people finish reaching for the wrong extinguisher. Vapour travel, BLEVE conditions, boilover hazards, and rapid reignition make these incidents fundamentally different from ordinary combustible fires.
The difference between containment and catastrophe usually comes down to three things: the correct suppression agent, the correct extinguisher rating, and disciplined fuel-handling practices before ignition ever occurs.
Speciality Geochem manufactures BIS-certified foam fire extinguisher and ABC powder systems rated for Class B fire protection across industrial, automotive, defence, and fuel-handling sectors with more than 28 years of supply experience.
For the full reference covering all 6 fire classifications and extinguisher selection guidance, see the complete fire classification guide.