By OGJ editors
HOUSTON, Oct. 28 -- Investigators from the US Chemical Safety and Hazard Investigation Board (CSB) Oct. 27 identified six key safety issues underlying the Mar. 23 explosions and fire at BP PLC's 446,400 b/cd Texas City, Tex., refinery. The incident, which was contained within the plant's 24,300 b/cd isomerization unit, killed 15 workers and injured 170 others (OGJ Online, Mar. 23, 2005).
The six identified safety issues were:
-- Trailers were placed in an unsafe location, too close to an isom process unit. All of the fatalities occurred in and around trailers that were as close as 121 ft from the release of flammable material from a blowdown drum. One trailer 600 ft from the explosions was heavily damaged, and 39 other trailers were either damaged or destroyed.
-- The unit's raffinate splitter should not have been started up due to existing malfunctions of the level indicator, level alarm, and a control valve.
-- The raffinate splitter tower had a history of abnormal start-ups that included recurrent high liquid levels and pressures.
-- On Mar. 23, a blowdown drum vented highly flammable material directly to the atmosphere. The drum was never connected to a flare since its construction in the 1950s. The previous owner of the refinery, Amoco Corp., replaced the isom unit blowdown drum in 1997 with identical equipment; Amoco refinery safety standards recommended connecting the drum to a flare when such major modifications were undertaken, but this was not done.
-- Between 1995 and Mar. 23, 2005, there were four other serious releases of flammable material from the isom blowdown drum and stack that led to ground-level vapor clouds, none of which ignited.
-- In 1992, the US Department of Labor's Occupational Safety and Health Administration cited a similar blowdown drum and stack at the Texas City plant as unsafe because it vented flammable material directly to the atmosphere, but the citation was dropped and the drum was not connected to a flare system.
In May, BP issued a statement blaming personnel mistakes for the refinery explosion. A BP investigative team concluded that personnel failures before and during start-up of the isom unit in the plant led to the explosion and fire (OGJ Online, May 18, 2005).
This month, BP agreed to pay more than $21 million in penalties under a settlement agreement with OSHA for violations connected with the explosion (OGJ Online, Oct. 10, 2005).
What happened
CSBIB investigators released three computer animations of the start-up of the isom unit, the vapor cloud formation, and the subsequent explosions. "The simulations showed a vapor cloud that blanketed much of the nearly 5-acre isom unit just before the cloud was ignited, most likely by an idling diesel pickup truck," investigators said.
The process simulation depicts liquid hydrocarbon flows through a complex of piping connecting a heat exchanger, a furnace, the raffinate splitter tower, and the blowdown drum. "As the temperature and fluid levels increase inside the tower, the animation shows pressure-relief valves directing overflow to the blowdown drum and attached vent stack," investigators said. "The drum rapidly fills, finally causing a geyser-like release of flammable liquids from the top of the vent stack. The vaporizing liquid falls to the ground, where it forms a vapor cloud."
Don Holmstrom, CSB lead investigator, noted, "The first rule of oil refinery safety is to keep the flammable, hazardous materials inside piping and equipment. A properly designed and sized knockout drum and flare system would have safely contained the liquids and burned off the flammable vapors, preventing a release to the atmosphere."
Investigators found evidence that BP evaluated connecting the raffinate splitter to a flare system in 2002 but ultimately decided against it, Holmstrom said. Following the incident, BP said it would eliminate blowdown stacks that vent directly to the atmosphere at all US refineries.
Investigators presented new details on the 16 previous start-ups of the raffinate splitter from 2000 onward. They found 8 start-ups with tower pressures of at least double the normal value and 13 start-ups with excess liquid levels. BP did not investigate any of these abnormal start-ups.
"Investigations of these incidents could have resulted in improvements in tower design, instrumentation, procedures, and controls," Holmstrom stated.
He said there was no supervisor with appropriate experience overseeing the unit's start-up. "Operators did not follow the requirements of start-up procedures, including opening the level control valve for the splitter tower," he said. "This omission allowed the tower level to rise rapidly for 3 hr to 15 times its normal level. Operators were misled by the malfunctioning level indicator on the tower and a separate high-level alarm, which failed to activate."
Investigators also stated that a number of equipment problems made it unsafe to start up the raffinate splitter. "Proper working order of key process instrumentation was not checked as required by the start-up procedure," Holmstrom stated, adding, "Managers turned away technicians and signed off on the instrument tests as if they had been done."
Investigators also found that BP's traffic policy allowed vehicles unrestricted access near process units. On Mar. 23, there were running vehicles, including a diesel pickup truck, as close as 25 ft from the blowdown drum. "A total of 55 vehicles were in the vicinity of the drum, investigators determined, and one likely served as the ignition source for the explosions," Holmstrom said.
CSB earlier this week issued two recommendations to leading US petrochemical trade organizations, calling on the American Petroleum Institute to develop new safety guidance that would establish minimum distances for occupied trailers away from hazardous areas of process plants. The board also called on API and the National Petrochemical & Refiners Association to immediately contact their members urging "prompt action to ensure the safe placement of occupied trailers away from hazardous areas of process plants," before the new API safety guidance is completed.
A final public report of the investigation is expected in 2006.
