Often there's these stories on TV about someone accidentally damaging a large diameter natural gas pipeline while digging a ditch etc. and then a huge raging fire. What I'm wondering about is how frequently are shutoff valves installed on these lines. My intuition says ~30 miles apart at most. If so, how long does 30 miles worth of piped gas take to burn itself out. I'm just trying to get a rough estimate here. In a flowing interstate pipe I think gas velocity is somewhere about 20 m/sec? Though not sure how to estimate the burn time after the valve is shut off.