"Blowing down" fuel tanks
I have emptied a few fuel tanks in my time for various reasons whether to scrap or to repair/modify the tank. The last two vehicles were gasoline powered and I used some rubber fuel tubing to hook up the output line to a jerry can. I then put gentle air pressure to another hose connected to the tank. Only a few psi from a blow gun to gently push the gasoline into the jerry can. Worked fine. No spillage, no mess and most importantly no fire. I have another job to do - blowing down the main tank on my 21.5' glass Wellcraft to get old gas out of it. The hoses unfortunately will be long seeing as how its a boat.
Today I read a warning on a container holding 20 L of methyl ethyl ketone (MEK) I use for gun wash - not too different from gasoline except the RVP is lower) to not blow down the container. No explanation why not. I'm aware of the static electricity problem with moving volatile fuels like gasoline (refueling jerry cans in plastic truck box liners, etc.). In plastic tanks on boats there isn't much you can do about it. I'd be interested in hearing anyone's thoughts/experiences on this issue as I have no wish to destroy my boat or anything else. Wish it was a diesel. Thanks in advance.
Other than the static issue which could be real when air goes through plastic, I was thinking that normally it's very difficult to get a gas tank to explode because whatever gas is in the tank will vaporize and fill any free space - no oxygen, no explosion. When that airliner went down of the East coast years ago because of a short in the tanks the FAA directed that they had to keep a minimum of 10% fuel in the tanks until all the planes got checked and fixed. When you blow it down you introduce nice clean air with plenty of oxygen so if there was an ignition source it might go off. I used to keep a tennis ball in my truck with holes in it. When the truck driver ran out of fuel 10 lbs of air through the tennis ball held in the fuel filler would make bleeding much easier.
Thanks Komatsu150 that makes sense. I guess the mixture normally in a gasoline tank is much too rich to burn even with a little oxygen there. Maybe the safest thing to do is blow down a gasoline tank with nitrogen or other inert gas to make sure there is no oxygen there like the pipeliners do. I like your idea of a tennis ball to charge the tank for bleeding but I guess you were talking diesel fuel. Reminds me of a story on the Apple pipeline north of Calgary in Alberta where a catskinner digging a dugout for a rancher ripped into the line and it was in a gasoline cycle. Cat was doused in gasoline but no fire ensued. Engineers thought that the he got hit with so much gasoline that the surrounding air/fuel mix was too rich to burn. A very lucky man.