The trailers, which were adjacent to the isomerization unit where the blast occurred, were heavily damaged or destroyed, as were the walls and roof of an adjacent metal warehouse used to store catalyst, according to CSB.
"The placement of the trailers in close proximity to an operating refinery unit is one factor we will be focusing on as the investigation proceeds," CSB member John Bresland said.
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.
CSB also will be looking into company reports of a small fire in the isomerization unit the day before the explosion. Bresland, however, said there was no evidence linking the two events.
Investigators Focusing on "Raffinate Splitter"
The isomerization unit, part of BP's 1,200-acre refinery complex in Texas City, Texas, converts the chemicals pentane and hexane into isopentane and isohexane in order to boost the octane rating of gasoline. The unit works by heating feedstock in the presence of a catalyst.
CSB investigators are focusing on equipment associated with a "raffinate splitter," which comprises a distillation system that prepares the feed stream for the isomerization reactor.
Preliminary results of the investigation point to a release of flammable liquid and vapor in that area of the plant.
"Process equipment within the explosion site shows substantial thermal damage and limited blast damage, consistent with a flash fire of flammable hydrocarbon in the open atmosphere," Selk said.
The distillation equipment was being restarted following maintenance work on the reactor a few days earlier, according to CSB. Pentane and hexane are highly flammable.
CSB Will Look at Refinery's Safety Record, Procedures
The CSB investigation likely will include:
- Examination of blast patterns to determine the origin of the explosion;
- Review of the design of the isomerization unit equipment;
- Examination of plant safety and operating procedures, past accidents, maintenance procedures and oversight and inspection; and
- Review of the adequacy of applicable regulations and industry standards for the placement of temporary structures such as trailers in refineries.
As part of their investigation, CSB officials have interviewed plant operators on duty at the time of the accident and have examined homes and businesses -- many of which had broken windows as a result of the blast -- about a half-mile north of the plant. The agency says it likely will hold public hearings in Texas City later in the investigation.
CSB is an independent federal agency charged with investigating industrial chemical accidents. The agency, which sent about a dozen inspectors to Texas City, says its investigation could take up to 2 years.