Class B is flammable liquid: solvents, thinners, petrol, spirits, many paints and adhesives. What burns is the vapour above the surface rather than the liquid itself, and that single fact drives everything about how these fires are fought. An extinguisher that disturbs the surface makes more vapour; an extinguisher that seals the surface stops the fire.
The two agents that cover class B
Foam, which forms a blanket over the liquid and holds it, and carbon dioxide, which displaces the oxygen above it. Dry powder is also rated for class B and interrupts the flame chemistry, but it does not seal, so a hot liquid can re-ignite behind it. For a contained spill or tray fire foam is generally the better provision; for a fire on or near energised equipment carbon dioxide is.
Why the surface must not be disturbed
Driving a jet into a pool splashes burning liquid, which spreads the fire and can carry it onto a person. Foam is applied off a vertical surface or the near edge so that it flows across. This is the one technique that a building holding class B risk has to teach rather than assume.
Provision follows the containment
Guidance on dangerous substances asks first how much is kept, in what, and where. A workshop that keeps solvent in a closed metal cabinet in the quantities it needs for a day has a small class B risk and a modest provision. The same workshop with open drums has a large one, and no portable extinguisher makes that acceptable.
Reading the rating rather than the name
Class B ratings are expressed as a number and a letter on the label, and the number relates to the size of test fire the unit extinguished. Two foam units of the same nominal capacity can carry different class B ratings. Where a risk assessment has specified a rating, the replacement at service time has to meet it.
Questions people ask about flammable liquids fire extinguisher
Is a liquid fire extinguisher the same as one for flammable liquids?
No, and the phrase is a common trap. Water and foam are liquid agents rated mainly for class A. What covers flammable liquids is the class B rating printed on the label, not whether the contents pour.
Can powder be used instead of foam?
It is rated for class B and will knock the flame down. It does not cool or seal, so it is a poorer choice where the liquid is hot or the container will stay hot, and it is refused in occupied interiors on visibility grounds.
Does a small quantity of solvent need its own extinguisher?
It needs to be assessed. Very small quantities in closed containers may be covered by the general provision. The assessment has to say so rather than leave it unaddressed.