Water which leaked through a valve, froze and cracked an out-of-service section of piping was the likely cause of a Feb. 16, 2007 fire at a Texas oil refinery, the US Chemical Safety Board said.
The piping crack released high-pressure liquid propane which ignited, causing a massive fire which injured four workers and forced Valero Energy Corp. to shut down and evacuate its McKee Refinery in Sunray, Tex., CSB said as it issued its final investigation report of the incident. It also released a 13-minute safety video, including 3-D computer-generated animation depicting the accident scenario.
CSB also asked the American Petroleum Institute to develop a new recommended practice to protect refinery equipment from freezing and to improve existing practices related to fireproofing, emergency isolation valves and water deluge systems.
The federal agency, which investigates industrial chemical accidents but does not issue citations, also called on Valero to improve freeze protection, fireproofing, hazard analysis and emergency isolation procedures at its 17 North American refineries.
Mike Mayo, Valero's corporate safety director, said that the independent refiner-marketer already has implemented safety measures throughout its system after conducting its own investigation of the accident. "These steps included the creation of a corporate process safety management and reliability department to develop and oversee safety and related process reliability measures at all our refineries," he said. The CSB's findings were consistent with Valero's, he noted.
Protection was inadequate
In its final report, the CSB concluded that the accident occurred because the refinery did not have an adequate program to identify and protect out-of-service or infrequently used piping from freezing. Officials at the McKee plant also did not apply Valero's policies on emergency isolation valves to control valves, it said. Current industry and company standards do not recommend sufficient fireproofing of structural steel against jet fires, it added.
"This was a significant accident that seriously burned three people, shut down a major oil refinery for two months and contributed to gasoline shortages hundreds of miles away in Denver. The CSB investigation points to a number of areas where oil industry practices should be improved to reduce the likelihood and the severity of process-related fires," said John Bresland, the agency's chairman.
"Fireproofing, remotely operable shutoff valves and effective water deluge systems can spell the difference between a small, quickly contain fire and a massive blaze that cripples a large industrial facility," he observed.
CSB said that the fire occurred in the refinery's deasphalting unit, which uses high-pressure propane as a solvent to separate gas oil from asphalt. The propane leaked from an ice-damaged piping elbow which is believed to have been out of service since the early 1990s, the agency's investigators said.
They said that refinery employees were not aware that a metal object had wedged under a manual valve's gate above the piping elbow, allowing liquid to flow through the valve. Piping above that point contained liquid propane at high pressure and small amounts of water were entrained in the propane, they indicated.
Dead-leg was formed
"The elbow was part of a 'dead-leg' formed when the piping was taken out of service. This was a section that remained connected to the process but was not intended to have any flow of liquid through it. Dead-legs can pose special hazards in refineries that should be carefully managed," explained CSB Investigations Supervisor Don Holmstrom.
He said that Ultramar Diamond Shamrock Corp., which owned the refinery at the time, did not identify hazards which could arise from the dead-leg when it was created in the 1990s and did not implement safeguards such as removing the piping, isolating it from the process by using metal plates known as blinds, or protecting it against freezing temperatures.
Water seeped past the leaking valve and built up inside the piping elbow's low point, Holmstrom continued. A cold snap in early February 2007 probably caused the water to freeze, expand and crack the piping. On Feb. 16, the daytime temperature increased and the ice began to melt.
At 2:09 p.m., high-pressure liquid propane flowed through the leaking valve and was released through the fractured elbow. Investigators estimated that it escaped from the pipe at an initial rate of 4,500 pounds/minute, quickly creating a huge flammable vapor cloud which drifted toward a boiler house where CSB investigators believe it contacted an ignition source, the agency said.
Plant employees were not able to shut off the supply of fuel to the fire once it started because Valero's procedures requiring installation of remotely operable shutoff valves had not been implemented, Holmstrom said. "Such valves are especially critical in high-pressure service to prevent large inventories of flammable material inside process equipment from contributing to a fire," he noted.
Pipe bridge collapsed
The growing fire caused a pipe flange on a large propane-filled extractor tower to fail, igniting a powerful jet fire that was aimed directly at a major pipe bridge carrying liquid products throughout the refinery, according to the CSB's report. The pipe bridge's supports quickly collapsed because they were not fireproofed, severing process pipes that were essential to the refinery's operation, it said.
"Valero and industry standards require fireproofing of structural steel supports up to a maximum of 50 feet from possible fuel sources. The collapse of a non-fireproofed bridge 77 feet away from the source of the jet fire indicates that industry practices need to be revised," Holmstrom said.
The fire also caused an estimated 5,300 pounds of toxic chlorine to be released from three one-tone cylinders stored 100 feet away, the CSB report said. The chlorine, which was used to disinfect cooling water, could have posed a serious threat to emergency response teams if they had not already been evacuated.
In addition, the fire threatened a large spherical tank containing up to 151,000 gallons of highly flammable liquid butane, investigators said. Valves controlling a water deluge system designed to cool the sphere became inaccessible to operators and could not be opened as a result of the growing fire, they said.
"The consequences of this accident could have been even more serious under different circumstances. Refineries should minimize the presence of hazardous substances near units where they may be exposed to fire hazards and should ensure that emergency systems remain operable if a disaster strikes," Bresland said.
Already took action
Mayo said that Valero already had begun to remove the unused pipelines before the CSB issued the report with that recommendation. The company also replaced the McKee plant's damaged propane deasphalting unit with a new, redesigned model which has remotely operable shutoff valves and other upgrade control systems that are designed to reduce the risks of such incidents, he said.
As a result of lessons learned from the McKee fire, the company is re-evaluating the fireproofing of pipe racks and other structures at its refineries, Valero's safety director said. It also will have completed a switch from chlorine to a safer bleach solution by the end of the year to treat cooling water, he indicated.
Mayo said that the steps are part of Valero's history as a refining safety leader. The company actively participates in the US Occupational Safety and Health Administration's Voluntary Protection Program, with 10 of its 16 refineries designed VPP Star Sites, the program's highest designation, he said.
Valero also has an employee injury rate which is well below the industry average and has reduced reported injuries by 56% since becoming part of OSHA's VPP program, he continued. "Safety is our top priority and an ongoing effort for Valero. Our goal is to eliminate incidents like the fire at McKee altogether, which is why we continually take steps to guard against and mitigate the risks associated with our processes," Mayo said.
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