Wednesday, January 26, 2011

Federal investigators cite defective welds as likely cause of fatal San Bruno blast

SAN FRANCISCO, Calif. - A federal report issued on Jan. 21 says that the natural gas pipe that ruptured in San Bruno on Sept. 9, killing eight people and destroying 38 homes, had numerous flawed welds. According to some experts cited, the pipe did not meet even the welding standards in place when it was installed in 1956.

The metallurgical report shed new light on the blast, identifying for the first time a particular seam weld as the site of the rupture. It found that the weld was half as thick as it should have been.

The problem seam weld in San Bruno, the NTSB report notes, developed a crack "consistent with ductile overstress from the root of the weld." A surge eventually ripped apart the line at the seam at a time when pressure ran up to 386 pounds per square inch. The pressure surged because of a malfunction of the pipeline system's power source before the San Bruno explosion.

The report noted that there were several circumferential, or girth, welds, and all of them had similar flaws. One of them also failed in the accident, but the report did not specify the cause of the "overstress" on those welds.

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