Umm, Spark will do nothing on a sealed tank.
You can try dropping lit matches in the tank all day if you like, you will never get anything significant in the way of a BOOM..
OK,... I have to pipe up on this one,...
I agree with you completely that a sealed system will not ignite but if there is liquid gas leaking around the top of the sender unit, that can certainly ignite. Even a spark inside a sealed gas tank can ignite if there is air (O2) in the tank and there always is. But I believe the whole gas danger thing is over rated. If you're working on those screws and you can't smell gas (mine isn't leaking yet so I'm gonna do mine before it does) then I really don't think there is a snowball's chance in hell of any kind of explosion and I'm gonna prove it by using my Dremel tool when I do mine. (just to be an a$$)
My friends wife's Grandfather delivered gas,... smoking cigarettes while standing right beside the big hose filling the underground tank. He was fine, for decades, then one day it all went bad,...RIP. I had friends that rinsed out their dirt bikes sponge air filters in gas smoking the whole time, flicking the ashes in the gas then putting out the cigarette in the gas.
I was working on a gas leak on my 85 626,... I lifted up the rear end almost two feet then detached the filler and filler vent tubes and tipped the tank down a foot to get access to the weld around the vent fitting where it was leaking. I was told everything is safe if there is no spark or open flame so a soldering gun is fine. (except for the sparking in the switch for my Weller soldering gun so I put an elastic band on the trigger to hold it on). It was kinda working except I couldn't get enough heat to melt the solder at the weld spot,... So I was figuring that the whole gas explosion thing was way over rated an decided to open the garage door (it was winter), cool the place down, air it out and then give go with my Bensomatic micro torch.
I got it lite and got the flame about 8" away from the fitting and POOF,... I jumped back two feet then stared at a 12" blue flame jetting straight out horizontally from the 2" filler fitting. It was making a serious jetting sound as it sucked air in the vent fitting to feed the jet blast. I just sat there in the snow three feet away like a deer caught in the headlights with my mouth and eyes wide open and just waited to see what was gonna happen next. Then the flame slowed down and shrunk until it was right at the fitting,.. then another small puff and the flame was now inside my tank,... the whole inside of my tank was on fire and was giving off a very pretty orange glow that shone out the filler neck. I figured it was about time to do some thing,.. so I reached behind me and scooped two hand fulls of snow and made a snow ball to plug the filler neck which I figured would be pretty hot,... I jammed it in there sealing the end with my palm and used my other thumb to plug the filler vent,... Then I turned my head away, pinched my eyes shut and waited,... I may have prayed but I don't recall,... After what felt like ten minutes but was probably about three seconds, I heard my final poof and the fire was dead.
I even managed to get the snowball out and put in some gas line antifreeze then blew through the vent to blow all the black smoke out of the tank. So I salvaged my tank and the 1/3 tank of gas and finished up my soldering with the Weller soldering gun.
The really stupid thing was that I had a fire extinguisher about 4 feet away,... I don't know if I forgot or if I was seriously trying not to waste all that gas and the tank,... I dunno,... I tend to learn the hard way,... I'm just lucky I've lived through each of my life lessons.
I'm pretty sure the addition of ethanol to gas changed the game quite a bit. I think that's why the flame was blue at first, just the alcohol vapors.