Around 1:25 pm, a technician noticed a strong gas smell he could identify as propylene along with the presence of a fog cloud in the middle of the steam cracker, yet without being able to precisely locate the leak’s source. He returned to the control room and sounded the alarm. Detection alarms on several explosimeters within the zone lit up on the safety control panel; the control panel operator called the fire safety team at 1:28 pm and activated the internal emergency plan at 1:33. Technicians working outside were evacuated and the site’s sprinkling systems were sequentially turned on between 1:28 and 2:20 pm in order to create a water screen around the gas cloud and cool potential flash points. Classified Facilities Inspection authorities were informed of the incident via the alarm relay around 2:30 pm.
Some 15 minutes later, a vertical gaseous spray was observed in a cluster of above-ground pipes situated approx. 8 m off the floor. About 3:35 pm, an opening could be identified on a liquefied butane pipe made of carbon steel, 4″ (101.6 mm) in diameter, 500 m long and operating at a pressure of 18 to 20 bar. At 3:40 pm, depressurisation of the line’s contents towards the flare system began; shortly before 3:50 pm, the circuit was isolated and the leak reduced in magnitude. The emergency plan was lifted at 4 pm, and the operator proceeded by issuing a press release. This leak incident was estimated to have lasted slightly less than 2½ hours; according to the operator, gas concentration only reached 20% of the lower flammable limit. At the time of this event, the line in question had been filled with liquefied gas yet isolated at its ends by valves placed in the closed position. The tube burst due to thermal expansion of the liquid at a pressure below the set pressure of the exhaust valve installed on the line (48 bar relative pressure). The steam cracker was kept running for a few days until its scheduled shutdown for 6 weeks. An examination of the suspected piping revealed an opening on the upper generator along a so-called “fish mouth” longitudinal path extending approx. 50 mm long by 20 mm wide (equivalent diameter of approx. 30 mm), with a considerable loss of thickness in the rupture zone. The piping, which had no heat insulation, also displayed a general external corrosion over the entire cross-section.
The cluster containing the defective pipes lay directly underneath another rack, where a refrigerated ethylene pipeline was running. Highly-localised external corrosion had been exacerbated by the drops originating from melting ice enveloping the outer surface of the refrigerated ethylene pipe above the defective pipe.
The investigations conducted by the Classified Facilities Inspectorate would confirm the good working order of gas detection sensors, which responded sequentially between 1:28 and 1:30 pm, starting from the sensor positioned closest to the leak and continuing to the furthest sensor. From a regulatory standpoint, the operator indicated that the pipeline was exempt from periodic recertification procedures, simply requiring a periodic inspection; moreover, this inspection had been scheduled for 2009. A visual assessment of the damaged pipe section 14 months prior had not detected even the slightest trace of corrosion. Several accidents caused by corrosion underneath heat insulation had already occurred at the site; the operator implemented an inspection plan adapted to the likelihood and consequences of external pipeline degradation, including:
– A control protocol for all pipes containing inflammable, toxic or corrosive products was adopted (supplemental visual inspection and non-destructive control);
– Detailed pipe inspection practices were extended, extending to the pipe structures (i.e. racks);
– Quick detection and treatment of specific aggressions affecting the facilities was emphasised;
– A quality assurance process dedicated to the pipe lining was designed (paint, heat insulation, fire retardant, coolant) in order to provide a minimum application standard.
