I find your post incoherent. Can you be a lot more specific in what you are talking about. What KIND of tank and what does gasoline have to do with welding? Why is there an open stream of fuel? What are you talking about? Be specific. Be descriptive.
I am just curious about normal natural gas stove, as there is a countinous stream of natural gas from the tank to the stove, and there is fire, why doesn't the fire burn back through that fuel stream into the gas tank? Is it because there is no oxygen in the tank?
However it's worth looking at a Bunsen burner. The gas and air combine at the bottom just above the jet but the flame is normally at the top of the tube. So in this case there is a short length of tube with both gas and air in it. So why doesn't the flame shoot down the burner tube to the jet? I believe this is down to the flame speed. If you have a long thin length of flammable material a flame will normally propagate along it at a certain speed called the flame speed. The flame speed is quite low for a natural gas in a Bunsen Burner tube (it's much slower in a regular wax candle and much higher in something like Detonating cord!). If the material is also moving at the same speed as the flame the flame appears stationary. So in the case of the Bunsen burner if you turn the gas flow rate down too low (eg below the flame speed) the flame can indeed burn down the tube to the jet.