Nick Snow
Washington Editor
WASHINGTON, DC, July 10 -- The Feb. 16, 2007, fire at Valero Energy Corp.' McKee Refinery in Sunray, Tex., likely was caused by water that leaked through a valve, froze, and cracked an out-of-service section of piping, reported the US Chemical Safety Board July 9.
The refinery did not have an adequate program to identify and protect from freezing out-of-service or infrequently used piping, CSB said.
CSB said 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 believed to have been out of service since the early 1990s when Ultramar Diamond Shamrock Corp. owned the refinery.
The piping crack released high-pressure liquid propane that ignited, causing a massive fire, which injured four workers and forced Valero to evacuate the refinery and shut it down for 2 months (OGJ Online, Feb. 21, 2007).
Ultramar Diamond Shamrock had not identified hazards that could arise from the dead-leg, had not removed the piping, isolated it from the process with blinds, or protected it against freezing temperatures. Nor had officials at the McKee plant applied Valero's policies on emergency isolation valves to control its valves, CSB said.
CSB has 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. It 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 the independent refiner-marketer already has implemented safety measures throughout its system, including the creation of a corporate process safety management and reliability department.
Dead leg hazard
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, CSB investigators said.
"The elbow was part of a 'dead-leg' formed when the piping was taken out of service. It was not intended to have any flow of liquid through it.
High-pressure liquid propane flowed through the leaking valve and was released through the fractured elbow, escaping from the pipe at an initial rate of 4,500 lb/min. It quickly created a huge flammable vapor cloud that drifted toward a boiler house where 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 report. The pipe bridge's supports, which were not fireproofed, collapsed, severing process pipes essential to the refinery's operation, it said.
"Valero and industry standards require fireproofing of structural steel supports up to a maximum of 50 ft from possible fuel sources. The collapse of a nonfireproofed bridge 77 ft 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 lb of toxic chlorine to be released from three cylinders stored 100 ft away, CSB report said. The chlorine, which was used to disinfect cooling water, could have posed a serious threat to emergency response teams had they not already been evacuated.
In addition, the fire threatened a large spherical tank containing up to 151,000 gal 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.
"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.
Actions already taken
Mayo said Valero has replaced the McKee plant's damaged propane deasphalting unit with a redesigned model that has remotely operable shutoff valves and other upgraded control systems designed to reduce the risks of such incidents.
The company is reevaluating the fireproofing of pipe racks and other structures at its refineries, Valero's safety director said, and by yearend will have completed a switch from chlorine to a safer bleach solution to treat cooling water.
Contact Nick Snow at [email protected].