Thanks folks, for the education. I see now that water is more appropriate - presumably to get rid of the heat rather than starve oxygen? Gas is for confined spaces, like jet engine nascelles, where HFCs▸ replaced Halon some 20 years ago.
Excluding oxygen is a strategy that only works in the right circumstances. The general principles I remember are to separate fuel from burning fire and from ignition source (including heat which vaporises the fuel). Water can do those, and extract heat, but also protect fire-fighters with a water screen.
I get quite twitchy when I see someone take a gas torch to the underside of a car. A long time ago I saw an exhaust fitter doing that who, for speed, put the torch behind him over his shoulder so he could take off his goggles and see the bracket he was cutting. Of course he pointed it at the side of the petrol tank, with the predictable result that petrol flowed out, caught fire, and ran down his back.
His colleagues were quick to get him away and beat that fire out, but were left with a car on a hoist, petrol on the floor, and fire licking up at the tank. They made various attempts at it with CO
2 and water extinguishers, but each time someone put out the fire on the ground or under the tank they would shift to the other site and the fire relit. No-one though to use two at once - and then the manager came haring out of his office with a dry powder extinguisher and put it out in one second.
I was very impressed by that, and came away with several learning points (as the
RAIB▸ would put it). I didn't come away with any new tyres, as the fire brigade arrived and sent all us customers home so they could get all the staff in the office and treat them to some "learning points" of their own.
Dry powder (commonly baking soda) is of course excluding oxygen in a sense, but by creating its CO
2 locally. Good for small fires, and it does stick to surfaces. I've used a very small one on a chip-fryer in a neighbour's house, and it worked but was rather messy. Mind you he still thanked me, for obvious reasons, despite an off-duty fireman turning up and demonstrating how to do it with just wet cloths and no extra mess to clean up.