A distillate dewaxing unit at an oil refinery was undergoing hot hydrogen regeneration of the catalyst when an explosion occurred. Catalyst regeneration is a periodically performed procedure, in which the normal liquid hydrocarbon feed is stopped and a hydrogen-rich gas mixture is fed through the catalyst bed for which the normal operating temperature is raised from 700F to 800F. During the catalyst regeneration process the reactor pressure is increased from normal operating levels just below 600 psig to about 640 psig. A pipe failure occurred as a sudden and complete rupture of the 10-inch diameter line at the exit of one of the two reactors. Security video revealed that the release rapidly exapnded and the hot gas mixture ignited shortly after rupture. A shock wave from the resulting explosion expanded through the adjacent neighborhood, causing varied degrees of blast damage to residential homes. Four workers near the process unit at the time of the explosion were not seriously injured.
An earlier process report indicated that sulfiding was conducted to activate the catalyst several years prior to this incident. As well, sulfidation corrosion is a known issue in oil refinery process streams operating between 450F and 1000F where sulfur of H2S is present. Evidence suggests sulfidation corrosion as the primary cause of the wall thinning that resulted in the rupture and that turbulent flow at an elbow likely caused the most significant wall thinning at the rupture location just downstream of that elbow.
Quantitative analysis of piping and elbow specimens were consistent with specifications current in 1966 for ASTM A213 Grades T11 and T12 alloy steels.
- Process Equipment
- Process Vessels
An investigative communication notes that "mechanical integrity programs at refineries repeatedly emphasize inspection strategies rather then the use of inherently safer design to control the damage mechanisms that ultimately cause major process safety incidents." Regarding the similarity of this accident to others, it is also noted that "while sulfidation is a well-known damage mechanism at refineries that requires regular inspection and monitoring, the segment that failed has no record of ever being inspected."
NOTE: This record is based upon an investigative report and related communications and will be updated, as appropriate, when additional investigative reports are completed, released and reviewed. Additional details regarding mechanical integrity programs and procedures in place at the time of the accident are expected.