Used to weld Gas tanks at a old garage by my Folks house. Old man(Less than my Current Age) sat me down with a Briggs and Stratton tank needed a seam brazed, showed me to open every opening on the tank light the fumes and go to welding while flaming!! Bigger tanks as off a car or pickup he would torch weld full of gas or would fill with water, just depended on price of gas that day.
Older story was a small garage in Portage Des Sioux MO had a regular client, asphaltic oil spreader truck, used in chip and seal operations, they had worked on this nuisance many times. Summer day, old dog asleep under another truck, mechanic goes to replace a broken off mudflap bracket ahead of the rear axle, truck had sat inside the garage all night. Short Story longer and more boring guy got geared up, had spot prepped and angle iron clamped up on tank frame, struck a slight arc to know had ground. He woke up in a haze momentarily afterword's, head hurting ears ringing, unable to hear much at all, laying on floor could make out the dog high tailing it across a nearby farm field. Then realized was REALLY bright, like daylight, then really came out of it knowing the roof was gone.
Seems the first arc was enough to set off the gasses inside the Open and vented tank, which was now a flat sheet of steel dripping bitumen with insulation and lagging scattered everywhere, one side wall of the garage was GONE, mechanic was lucky to escape minor injuries remained deaf in one ear after, luck was the tank seam was UP and he was Under when it exploded limiting his injury. Truck was destroyed, garage closed never reopened, mechanic retired. Dog WAS found later and would not ever get near a garage shop ever again.