For years I treated diesel like it was non flammable. Use a cutting torch to cut not empty tanks in half, then bail the liquid out. Diesel, like everything else, only burns as a gas. Sparks, flame, and drops of liquid steel don't ignite the liquid.
The moral of this story is that one night I was in the middle of cutting a 275 gallon tank with cutting torch. My son and his friend wanted into the garage. I had the tank on forks, and moved the tractor. A very small amount of diesel splashed on the gravel. The first subsequent sparks ignited it. On gravel it could vaporize enough to ignite. I had a driveway in flame! The extinguisher was close at hand thankfully.
On another occasion I was decommissioning a 10,000 gallon tank. It had maybe 15 gallons of nasty sludge in it. I cut a 4' square hole in the end of the tank, allowing me to stand where I could look inside the tank. In a tank this big that much oil isn't very deep. I decided to burn it rather than risk spilling it. A pollution equivalent to heating your house a couple days. I splashed a bit up the wall of the tank, and lit it with the acetylene torch. It lit instantly, and gradually grew as the increasing heat vaporized even more. Ultimately flame blasted out 10 feet horizontal, then thirty feet vertical. It was 200 yards from the nearest neighbor, but I was sure I was going to be in terrible trouble. A few people came to see what was happening, but no firemen, or police ever came.
Diesel is manageable but not fireproof. Be aware that you can purge the tank of air, but outside the tank, all bets are off.
Willie