From the basic description of the conditions, a reasonable inference would be that the propane supply was not shut off upstream of the severed section of the downstream gas piping. At least that was my interpretation of the limited information provided.

The pressure in the propane supply train and the differential pressure created by flowing the gas to the relatively lower atmospheric pressure is the driving force for the propane gas flow in the pipe and out of the propane burner. If the driving pressure is not maintained, then the flow to the burner, severed piping or not, would not allow gas to flow to burner. The pressure wave in the gas travels at the speed of sound, so the change in pressure can be considered "instantaneous."

The activation of the fire suppression system is contingent on the temperature sensitive eutectic metal to fuse for the presence of a fire with a heat release rate greater than that of the propane burner or burners. A severed section of gas piping may occur without the activation of the fire suppression system.

Again, 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