Almost every mistake made choosing an extinguisher for an oil fire comes from one word doing two jobs. Cooking oil and fat in a kitchen are class F. Fuel oils, petrol, diesel, lubricating and hydraulic oil are class B. The agents are different, the techniques are different, and applying the class B answer to a fryer has injured people.
Why cooking oil is its own class
A fryer holds oil at a temperature above the point at which it will ignite on its own. When it catches, the whole volume is already hot and will re-ignite the moment a flame front is removed unless it is cooled or sealed for long enough to fall below that point. That behaviour is unlike any other liquid fire, which is why class F was created and why wet chemical exists.
Fuel oils behave as ordinary class B
Petrol, diesel, kerosene and lubricating oils burn from the vapour above the surface and respond to a foam blanket or to carbon dioxide. They are usually at or near ambient temperature when they ignite, so removing the flame is sufficient and the fuel does not carry enough stored heat to relight on its own.
Where a building has both
A commercial kitchen with a generator or a plant room has class F at the range and class B at the plant. These need separate provision at the two points rather than an averaged choice in the corridor between them. This is one of the clearest cases where a provision plan has to be drawn on a floor plan and not written as a total.
The one instruction that covers both
Turn off the heat or the supply first if it can be reached safely. A fryer that is still heating and a fuel line that is still feeding will both defeat a portable extinguisher, and both controls are usually a few steps further from the fire than the extinguisher is.
Questions people ask about oil fire extinguisher
What is a petrol fire extinguisher?
Foam or carbon dioxide, both rated class B. A petrol fire extinguisher is not a distinct product; it is a class B rated unit.
Is a diesel fire extinguisher different?
No. Diesel is class B like petrol, though it is harder to ignite and more likely to be a spill fire under a vehicle than a vapour fire above a tank.
Can wet chemical be used on fuel oil?
It is designed for the chemistry of cooking fats. Most wet chemical units carry no class B rating at all, so it is not the provision for a fuel risk.
What extinguisher for cooking oil is the right one, and what happens with the wrong one?
Wet chemical, which is the yellow panel, and nothing else does the job. Cooking oil above its auto ignition temperature relights the instant a covering is removed, so the agent has to seal the surface, which is what the potassium based solution does by reacting with the oil. Water or foam will eject burning oil from the pan violently, carbon dioxide blows across the surface and does not stop reignition, and powder makes a mess without sealing. A pan fire with the heat off and a fire blanket laid over it is the other correct answer, and the two are not alternatives to each other.