A Propylene Fractionator was equipped with 2 shell and tube-type reboilers (one in service, one on standby). The standby reboiler was being brought on stream to allow the operating reboiler to be taken off-line for cleaning. The Operations Supervisor opened the manual tubeside isolation valves to establish a flow of hot quench water to the standby reboiler in preparation for the reboiler switchover. Three minutes later the standby reboiler shell failed catastrophically. The escaping propane/propylene mixture caused a boiling liquid expanding vapour explosion (BLEVE) and fire, releasing approximately 13.6 tonnes (30,000 lb) of flammable hydrocarbons to atmosphere. The fire burned for 3.5 hrs and the plant remained shut down for 18 months.

Basic Cause:
Overpressure of the reboiler shell during warmup due to thermal expansion of trapped (blocked in) propane/propylene liquid.