In a closed and rigid/strong container an explosion will occur.
In a soft container (like the Hindenburg)(whether gas or liquid) it will burn at the edges.. where there is air/oxygen.
In a long tube, like an 'empty' pipeline (gas or vapor + air, not full of liquid or 100% gas), there will be a fire until the speed of the blaze exceeds 'sonic' speed. At that point the gas/air will be compressed for several feet in front of the fire and that compressed pocket will explode, rupturing the pipeline. Then the pressure drops and the fire resumes, traveling down the pipe... The result is a timed series of explosions in the pipeline at specific distances. The distance between explosions can be predicted from the size of the pipe and the flammability of the liquid vapor (or gas). The cure is to put screens across the pipe at those predicted explosion distances (or less) so the fire is cooled and stopped before the (next) explosion can occur.
Ref: Flame arresters: https://en.wikipedia.org/wiki/Flame_arrester