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Old 10-27-2016, 07:34 AM
Machine Elf Machine Elf is online now
Join Date: Feb 2007
Location: Challenger Deep
Posts: 10,644
Originally Posted by pkbites View Post
Got a new exhaust put on my Land Rover yesterday. I'm watching the guys work under mine and other cars, using blow torches and welding equipment, back by where the fuel tanks are.

All I could think of was "just one small leak of fumes and Ka-Fucking-Bm!

I've never been sent to such a call. Does anything like that ever happen?
Inside the fuel tank, the air/vapor mixture is typically too rich (too much fuel vapor to actually ignite, sort of like leaving the choke on after your lawn mower has already warmed up). When fires happen while refueling a car (generally due to discharge of static electricity), the standard instruction is to just back away; the only thing that's burning is the combustible mixture of air and fuel vapor right at the filler neck (where vapor has been exiting the tank as you fill it with liquid), and the flame won't propagate down the filler neck into the tank. Under these circumstance, things don't generally go badly unless the panicked driver pulls the filler out and sprays liquid fuel all around the general vicinity. The upshot of all this is that performing hotworks on a car's underbody isn't likely to light off anything inside the fuel tank.

Moreover, oxyacetylene torches work pretty fast on the thin metal of an exhaust pipe; any given cut is over pretty fast, so there's not a whole lot of stray warm exhaust gas to dump heat into adjacent components. It would take a dopy, inattentive mechanic to inadvertently point the torch at the surface of the tank for a prolonged period (not saying this has never happened) - and even then I wouldn't expect much of a bang (see previous paragraph).

The only time I would expect a hazard is if a significant quantity of gasoline gets spilled on open ground, where it could conceivably form a combustible mixture and reach an ignition source. Disconnecting a hose from the tank could allow the hose (not the tank) to empty itself on the floor, but this wouldn't be much - only as much fuel as is held inside the hose between the tank and the engine, an ounce or two. If the car is six feet up on a lift, I'm not even sure the flames from such a fire would reach the underbody of the car.