A common question about flammable liquid fires is which extinguisher you must not use to put one out. The answer comes first because it is the answer that prevents harm: never use water on a flammable liquid fire. Doing so does not extinguish the fire — it spreads it, often violently.

This page explains why water is so dangerous, sets out what to use instead (foam first, with CO2 and dry powder as alternatives for specific scenarios), and works through the practical specifics for the flammable liquids most UK premises actually deal with: petrol, diesel, paint and solvents.

Which extinguisher should you NOT use on flammable liquids

Do not use an ordinary water-jet extinguisher on a flammable-liquid fire. Use equipment specifically rated and suitable for the fuel. Wet chemical products primarily address Class F, although some also have a tested B rating; check the exact unit.

The hazards depend on the fuel and the discharge method, as the following sections explain.

Why water on flammable liquids is so dangerous

An ordinary water jet can spread a burning liquid, splash fuel and overflow a container. Many hydrocarbon fuels float on water, while some solvents mix with it; their behaviour is not identical.

With sufficiently hot liquids, rapid steam formation can also violently eject burning material. This is a particular danger with hot cooking oil, rather than a universal explanation for every petrol or alcohol fire.

A portable extinguisher must be selected for the specific liquid and situation. Do not infer that a water-based product is suitable simply because another formulation has passed a fire test.

Foam itself is water-based and can be suitable where its formulation and rating match the fuel. Electrical dielectric testing is a separate matter from Class B fire performance. See our cooking-oil guide for hot-fat hazards.

What to use — foam, first

The standard answer for a flammable-liquid fire in a UK workplace is a foam extinguisher. The cream-labelled foam unit produces a layer of expanded foam that floats on top of the burning liquid, sealing the surface from the air. With no oxygen reaching the fuel, the fire suffocates. The water content of the foam also cools the fuel, reducing vapour generation.

The technique matters. Sweep the foam gently across the surface to build a continuous blanket; do not aim the discharge directly into the fuel, which will splash burning liquid out of the container. The blanket needs to be unbroken — once a continuous foam layer covers the fuel surface, the fire is starved. If a gap opens up (someone walks through it, or the foam dissipates before a hot spot has cooled), re-ignition is a real risk.

A 6 L foam extinguisher with a 113B rating handles a tray-scale Class B fire — a tin of paint, a small fuel spill, a contained workshop incident. A 9 L unit rated 183B handles larger incidents. Above that, you are into the territory of fixed suppression systems or the fire and rescue service. The detail on foam itself, including the regulatory situation for older AFFF foam containing PFOA, is on our foam fire extinguisher page.

CO2 for small contained fires near electrics

CO2 extinguishers are also rated for Class B and are the right choice when the burning liquid is near energised electrical equipment — a contained solvent fire on a workbench beside a power supply, a small fuel-on-charger fire in a workshop, a benchtop spill near electronics.

The advantages over foam in this scenario: CO2 is non-conductive, so it is safe to use on or near live electrics, and it leaves no residue on the equipment. The disadvantages: CO2 cools very little, so re-ignition is more likely, and the gas dissipates fast in any normally-ventilated space.

CO2 is best at the small end of the scale. For anything beyond a tray-scale fire, foam handles it better. Use CO2 when the electrical hazard tips the choice. The product detail is on our CO2 fire extinguisher page.

Dry powder for suitable outdoor risks

ABC dry powder is rated for Class B and is the appropriate choice for outdoor flammable-liquid fires — vehicle fires, forecourt incidents, large workshop or yard scenarios. Powder smothers the fire and interrupts the chemical chain reaction at a molecular level; electrical suitability depends on the particular unit’s marked limits and the assessed hazard.

Indoors, dry powder runs into the visibility and respiratory problems that BS 5306-8:2023 (clause 5.4.3) specifically cautions against — the discharge cloud reduces visibility within seconds, and inhalation is genuinely unpleasant. For most UK workplaces, the practical pattern is to keep foam as the indoor primary for Class B and reserve dry powder for outdoor or vehicle use. The full picture is on our dry powder fire extinguisher page.

Wet chemical — check any additional Class B rating

Wet chemical is primarily selected for cooking oils and fats. It should not be assumed to handle petrol, diesel or solvents merely because it is described as chemical.

Some models also carry a tested Class B rating; Protec, for example, lists a 13A/133B/75F unit. Use only the fuel applications permitted by the actual product, within a competent assessment and training.

Petrol, diesel and kerosene specifics

The most common flammable liquids in UK premises split roughly into two groups by behaviour.

Petrol ignites readily — at room temperature, petrol vapour above an open container is normally above its lower flammable limit, which means a single spark is enough. A petrol fire is fast and aggressive; the vapour pressure keeps generating fuel for the flames as the liquid heats. Foam extinguishers handle petrol fires well within their rated capacity, but the effective window is short — the longer the fire burns, the more the surrounding air heats, and the harder it becomes to establish a stable foam blanket.

Diesel and kerosene ignite less readily. Both have higher flash points than petrol — diesel typically above 55°C, kerosene above 38°C — although a spray or mist can ignite below the bulk liquid’s flash point. Once burning, they behave similarly to petrol in terms of extinguisher selection, but the fires are usually slower to start and easier to control. Foam works well; CO2 handles small contained fires; dry powder is the outdoor choice.

For all three, the primary safety control is not the extinguisher but the storage and ignition control: keeping fuel in approved containers, away from ignition sources, with appropriate bunding and spill control. A fire extinguisher is the response of last resort, not the first line of defence.

Paint, solvents and spirits

The flammable liquids most non-fuel UK premises encounter:

Oil-based paints, varnishes and stains

Class B; the carrier is normally white spirit or another petroleum-derived solvent. Foam handles paint fires effectively; the foam blanket seals the liquid surface. Beware of metal paint tins becoming pressurised in a fire — they can rupture and spray burning paint.

Solvents — white spirit, methylated spirits, isopropanol, acetone

Class B with varying flash points and behaviours. Methylated spirits and isopropanol are water-miscible, which means foam designed for fuel fires (designed to float on hydrocarbon fuels) can sink into them rather than forming a blanket — alcohol-resistant (AR) foam is the right specification for premises that handle significant quantities of polar solvents. Standard foam works for hydrocarbon solvents.

Spirits and alcohols (whisky, brandy, etc.) in commercial settings

Same issue as methylated spirits — water-miscible, requiring AR foam for effective coverage. Distilleries, bars and restaurants handling substantial alcohol stock should consider this in their fire risk assessment.

Adhesives, contact cement, sealants

Variable. Many are flammable in liquid form and produce flammable vapours; once cured, the fire risk reduces but doesn't disappear. Foam covers most.

Aerosol products

Aerosol cans exposed to fire may rupture violently and become projectiles. Withdraw, warn others and tell the fire and rescue service; do not approach to tackle surrounding flames.

Storage and ignition control — the better safety control

Worth saying directly: extinguisher selection matters less than not having the fire in the first place. UK fire safety law (the Regulatory Reform (Fire Safety) Order 2005, plus the Dangerous Substances and Explosive Atmospheres Regulations 2002 — DSEAR — where dangerous substances create fire or explosion risks at work) requires you to identify and control the flammable-liquid risk through your fire risk assessment. The standard expectations include:

  • Storing flammable liquids in approved containers, in dedicated cabinets or stores, separated from ignition sources.
  • Bunding for larger quantities — secondary containment that catches a spill before it spreads.
  • Good ventilation to disperse vapours, particularly in workshops and garages.
  • Hot work permits for cutting, grinding or welding near stored flammables.
  • Spill kits and trained users to deal with leaks before ignition.
  • Smoking restrictions and signage in flammable storage areas.

A premises with proper storage controls and a sensible spill response rarely sees a flammable-liquid fire reach the point where extinguisher choice is the question. The extinguisher is the backup; the controls are the primary defence.

Frequently asked questions

Which extinguisher should you NOT use on flammable liquids?

Do not use an ordinary water jet. Select equipment specifically rated and compatible with the fuel; the name of the agent alone does not establish suitability.

What extinguisher do I use for petrol?

A compatible Class B-rated foam, powder or CO2 unit may be specified. Suitability depends on the setting, spill and electrical risks. Only trained people should consider a small fire where escape remains safe.

Can I use a wet chemical extinguisher on petrol?

Only if the particular product is explicitly rated and suitable for that fuel. Many wet chemical units are intended for A/F risks; some have an additional B rating. Never assume.

Why doesn't water work on flammable liquids?

An ordinary water jet can spread, splash or overflow burning fuel. Hot liquids can also cause violent steam generation. Water-miscible solvents behave differently from hydrocarbon fuels, so a single mechanism does not describe every case.

What's the difference between Class B and Class F?

Class B covers flammable liquids and liquefiable solids; Class F specifically covers cooking oils and fats. Choose the product’s tested rating and instructions for the actual fuel.

What about diesel?

Diesel usually has a higher flash point than petrol, but spraying or misting can create an ignition hazard below the bulk-liquid flash point. Control leaks, ignition sources and storage, and select compatible Class B equipment.

Where this connects

For the products themselves — chemistry, sizes, ratings — see our foam fire extinguisher page, our CO2 fire extinguisher page, and our dry powder fire extinguisher page. For where these sit in the wider type system, the fire extinguisher types hub is the place to start. For the wider fire classification context, see our UK fire classes guide. For the universal four-step technique, see how to use a fire extinguisher (PASS). For the maintenance and inspection picture, see fire extinguisher servicing and inspection.

If your team needs the basics of fire safety — what to do, what not to do, who is responsible — the online fire safety awareness training course covers it in 90 minutes, RoSPA-approved and CPD-accredited.

Sources & references

Official guidance and standards checked on 15 October 2026.

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Mark McShane
Mark McShane
Health & Safety Training Specialist, Online CPD Academy

Mark writes about workplace health & safety, fire safety awareness and accredited online training for Fire Safety Awareness Training, part of Online CPD Academy.