Investigators, who previously had been blocked from entering the refinery's isomerization unit due to concerns over safety and evidence preservation, after examining the unit said they were unable to identify a diesel pickup truck thought to be a possible trigger for the explosion.
CSB officials found the demolished remains of 6 to 10 mobile trailers and about 30 cars and trucks in the debris, according to CSB investigation manager Bill Hoyle. According to BP, most of those who perished on March 23 were in one of the trailers planning turnaround work on a section of the isomerization unit, and CSB has said the proximity of the trailers to the epicenter of the blast is one factor they'll be analyzing during their investigation.
Hoyle said CSB isn't sure how many vehicles may have been running when, according to witness accounts, a geyser-like release of hydrocarbon liquid and vapor shot up from a 100-foot tall atmospheric vent stack in the northwest corner of the isomerization unit just seconds before the explosion.
Wearing air-purifying respirators -- to protect against residual benzene vapors -- and fire and biohazard protective gear, CSB investigators spent about 3 hours in the isomerization unit on April 1. Investigators, who were expected to continue examining the blast site on April 4, took about 200 pictures of the site on the first day.
"The isomerization unit equipment is structurally intact for the most part, but calcium silicate insulation and metal cladding litter the area and also dangle overhead," Hoyle said. "Some areas of the isomerization unit remain unsafe for entry."
According to CSB, investigators inspected the blowdown drum -- a vessel approximately 20 feet high and 10 feet in diameter that receives hydrocarbons vented from the raffinate splitter and other equipment -- connected to the atmospheric vent stack where the vapor release is believed to have occurred. Investigators determined and documented the position of critical valves on the blowdown drum and other unit equipment.
The inspection of the blast site likely will play a critical role in CSB's root-cause investigation, in addition to witness interviews and reviews of regulations, standards and company safety procedures and records.