Texas City refinery accident investigation: some $1.7 billion to be spent
Houston, TX — BP Products North America Inc. ’s Texas City, TX, refinery experienced a major explosion on Mar. 23, 2005, during the start-up of the isomerization plant. The accident claimed the lives of 15 workers, injuring many more. The explosion was believed to have been caused by undetected overflow of liquid from the raffinate splitter. The facility is BP’s largest and most complex refinery with a daily rated capacity of 460,000 barrels and can produce up to 11 million gallons of gasoline daily.
The Isomerization (ISOM) unit converts raffinate, a low octane blending feed, into higher octane components for unleaded regular gasoline. The unit has four sections including a splitter that takes raffinate and fractionates it into light and heavy components. The splitter consists of a surge drum, fired heater reboiler and a fractionating column 164 feet tall.
The explosion and fire occurred after personnel responsible for the startup greatly overfilled the raffinate splitter tower and overheated its contents, which resulted in over pressuring its relief valves. Liquid was pumped into the tower for almost three hours without any liquid being removed or any action taken to achieve the lower liquid level mandated by the startup procedure. Liquid level in the tower just prior to the loss of containment was at least 20 times higher than it should have been. Activation of the automatic liquid level control, as mandated in the startup procedure, would have prevented this occurring.
A decision late in the start up to begin removing liquid from the tower exacerbated the incident. Rapid heat exchange between the overheated liquid being removed from the bottom of the tower and the liquid feed continuing to flow into the tower (the two streams pass through a heat exchanger) caused significant vapor generation as the feed entered the tower. Vaporization of the liquid feed low in the tower pushed liquid up the tower and out of the unit, over pressuring the relief valves and ultimately overwhelming the adjacent blow down unit.
Detailed process modeling
New Jersy-based Process Systems Enterprise Ltd . (PSE) was called upon to provide expert information for the team investigating the accident. PSE was selected during the investigation after an initial scoping showed that the complexity of the modeling task and short timescales required were beyond the capabilities of standard dynamic flowsheeting tools. In addition to its gPROMS technology, PSE has a consulting group with a track record in detailed process modeling.
Working from first principles, with no a priori assumptions regarding the magnitude or direction of system liquid and vapor flows, PSE developed a high-fidelity gPROMS model of the raffinate splitter. The model included a detailed two-phase hydraulic representation of the tray active area and downcomers for all trays, as well as a full tray-by-tray vapor–liquid equilibrium calculation over the column.
Particular emphasis was placed on the effects of hydrostatic pressure. This led to understanding the latter’s role on initially suppressing boiling at the bottom of the column. It was also crucial in permitting the identification of the sequence of events that took place once hot liquid flow from the bottom of the column was allowed to exchange heat with inlet feed. This effectively transferred heat to parts of the column that were subjected to much less hydrostatic pressure, and were therefore closer to boiling.
Detailed dynamic simulations showed that, contrary to expectation, hydrocarbon liquid did in fact fully reach the overhead line at the top of the column. This was to some extent caused by vaporization in the bottom trays—from reboiler input—lifting liquid up the column, a process accelerated by vaporization at the feed tray.
The model pressure profile predictions closely matched the observed DCS/PI process data recorded during the period leading up to the incident. Further analysis suggested that it was likely that a 1
Company accepts responsibility
Ross Pillari, president of BP Products North America Inc. said, “The (final investigation) report clearly describes the underlying causes and management system failures which contributed to the worst tragedy in BP’s recent history. We accept the findings, and we are working to make Texas City a complex that attains the highest levels of safety, reliability and environmental performance.” The company has set aside $700 million to compensate victims of the explosion and has worked to resolve accident claims arising. Settlements have been reached with the families of most of the workers who died and with many workers who suffered serious injuries.
BP has an agreement with the U.S. Occupational Safety and Health Administration that resolves over 300 separate alleged violations of OSHA safety regulations. BP paid a fine of $21.3 million. The company agreed to a number of corrective actions, including the hiring of process safety and organizational experts at the refinery. Under the agreement, BP does not admit the alleged violations or agree with the way OSHA has characterized them. BP is cooperating with the U.S. Chemical Safety and Hazard Investigation Board (CSB), the U.S. Environmental Protection Agency, and the Texas Commission on Environmental Quality regarding the explosion and related concerns.
On CSB’s recommendation, BP has voluntarily appointed an independent panel to assess and make recommendations for improvement of safety management and safety culture at the company’s five U.S. refineries. Former U.S. Secretary of State James A. Baker is chairman of the panel.
The company has endorsed a CSB recommendation urging the industry to revisit existing standards for the use of temporary buildings inside refineries and other processing plants. BP has established a new standard for its refining operations and plans to share it with others in industry.
Based on process modeling, the investigation team estimates that about 1,100 barrels of liquid was discharged to the blow down unit which has a capacity of 390 barrels. Most of the liquid was released into the petroleum sewer system. An estimated 50 barrels overflowed the tower and led to the formation of a hydrocarbon vapor cloud at ground level.
Based on inspection of the sewer system and process and explosion modeling, the investigation team concluded that the sewers were not the primary route for the formation of the hydrocarbon vapor cloud that subsequently exploded. The damage observed to the sewer system was the result of secondary fires caused by the main explosion.
The source of ignition is not known.
BP’s recently released final report confirmed the critical factors—previously indicated in a May 17 interim report—which led to the explosion and greatly increased its consequences:
Failure to follow procedures, leading to greatly overfilling the raffinate splitter tower;
Venting of liquids caused by overfilling and overheating liquid in the tower; leading to a liquid release to atmosphere and the subsequent explosion;
Many personnel too close to the source of relief—a blow down unit, congregated in and around temporary trailers which were inappropriately sited; and
Continued use of a blow down unit for light-end hydrocarbon use when inherently safer options were available and could have been installed.
Some of the actions recommended by the investigation team have been completed, with many underway. Texas City site manager Colin Maclean has established a special project team to plan and drive execution of the improvement program. BP says that it will install modern process control systems on major units, transition to a more powerful maintenance management system, improve worker training, remove blow down stacks and implement the other recommendations contained in the final report. The project team will also develop plans for reconfiguring and simplifying the operation of the refinery.
The company expects to invest an estimated $1 billion dollars to improve and maintain the facility over the next five years, which together with victim compensation totals some $1.7 billion.
For prior Control Engineering coverage, click here .
— Richard Phelps , senior editor, Control Engineering
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