CSB officials, who have yet to enter the isomerization unit that was the epicenter of the March 23 fire and explosion at BP's 1,200-acre refinery in Texas City, Texas, are analyzing eyewitness accounts that describe a release of hydrocarbon liquid and vapor shooting from an approximately 100-foot tall atmospheric vent stack in the northwest corner of the isomerization unit.
"They observed this liquid and vapor falling toward the ground seconds before a powerful explosion," CSB investigation manager Bill Hoyle said, adding that all information coming from the CSB at this point is preliminary and subject to change.
Officials from CSB and BP believe the highly flammable liquid and vapor described in eyewitness accounts ignited, although investigators have yet to determine how. A report in the April 1 Houston Chronicle claims several witnesses saw a driver try to shut off the engine of an idling truck as the plume of liquid and vapor shot out of the vent. Some observers are speculating that a spark from the truck may have set off the explosion.
CSB investigators hoped to enter the isomerization unit -- which produces components that raise the octane in gasoline -- March 31, according to agency spokesperson Daniel Horowitz.
In the meantime, CSB officials have been interviewing witnesses and cataloguing blast damage at the ultracracker unit and the aromatics recovery unit, which are two of four sections within the isomerization unit. The two units, located about 100 yards from the presumed epicenter of the blast, sustained "minor to moderate damage" to their control rooms and instrumentation sheds, according to CSB.
CSB investigators have taken pictures and measured the damage around the ultracracker and aromatics recovery units and will use the data -- through computer modeling -- to help determine the size and nature of the vapor cloud explosion.
Flammable Vapor May Have Ignited Near Vent Stack
As for what essentially is Ground Zero of the blast, CSB investigators have studied schematics to determine that the 100-foot-tall atmospheric vent stack was connected to pressure relief devices for the raffinate splitter distillation column, which comprises a distillation system that prepares the feed stream for the isomerization reactor.
Witness evidence, according to CSB, points to possible ignition sources on the ground near the vent stack.
At the time of the explosion, which occurred around 1:20 p.m., the raffinate splitter subunit was being restarted as part of a maintenance turnaround on the isomerization unit. Company officials say the isomerization unit had been offline for a turnaround since late February to change the isomerization catalyst, a procedure that takes place every 10 years. The unit, however, receives general maintenance every 1 to 2 years.
Three other units near the isomerization unit were undergoing turnarounds or recently had come back online following a turnaround, according to BP. In an industry in which refineries often run non-stop for several years, maintenance turnarounds are common.
CSB's Horowitz said investigators also are looking into the presence of mobile trailers near the epicenter of the blast. According to BP, most of those who perished on March 23 were in one of the trailers planning turnaround work on the ultracracker unit. The trailer and a nearby catalyst warehouse were severely damaged in the explosion and subsequent fire.
CSB investigators were not sure how many trailers had been present or what materials the trailers were made of, but CSB investigation manager Steve Selk noted that typical mobile trailers would be much less resistant to blast effects than permanent, protected structures that often are found at large refineries and chemical plants.
In addition to CSB, officials from OSHA and BP and the Paper, Allied-Industrial, Chemical and Energy Workers International Union are conducting investigations at the refinery.
Nine Workers Still in Hospital After Explosion
Workers at the Texas City refinery and at BP plants throughout the world observed a moment of silence and held a memorial service on March 30 at 1:20 p.m. to remember and honor the 15 workers who died in the explosion.
As of March 30, nine workers injured in the blast remained in the hospital, according to BP.