I am not sure this is an absolute "No." It depends on the size of the break in the pipe and the local pressure field and momentum at the break.

Think of your propane gas stove appliance. The use of propane in gas stove cooking appliances is delivered as a "pre-mixed" flame (i.e. characteristic blue flame). The pre-mixing is done by two openings in the gas line that allows air entrainment into the gas flow with the gas pipe at the location of the two holes. The size of the holes is controlled by an adjustable aperture (i.e. a cover on the openings). The area of the openings and the velocity of the gas across the holes determine the amount of pre-mixing, the structure of the flame, flame color, and the products of combustion. Also, think Bunsen Burner that can produce both a diffusion flame, a partially pre-mixed flame, and a fully pre-mixed flame.

If the "severed" section is on the order of the size of a hack-saw with the same size distance between the two severed pipe sections, most likely, you would just get the infiltration of air with the driving force of a pressure difference between the ambient outside pressure and the lower pressure in the flowing gas. If the "severed" section gas a much larger gap on the order of an inch, then the release propane gas would have some time to expand and dissipate the pressure and flow field.

As is frequently the case, the answer is "it depends" and the specific details matter.

Douglas J. Carpenter, MScFPE, CFEI, PE, FSFPE
Vice President & Principal Engineer
Combustion Science & Engineering, Inc.
8940 Old Annapolis Road, Suite L
Columbia, MD 21045
(410) 884-3266
(410) 884-3267 (fax)
www.csefire.com