Work on a pipeline control center triggered electrical problems and led to a rise in gas pressure before the blast, the NTSB says. It's not clear what role the event played in the pipeline failure.
Maintenance work at a pipeline control center triggered electrical problems and a rise in gas pressure just before a major transmission line exploded last month in San Bruno, killing eight and setting off an inferno that destroyed dozens of homes, according to a preliminary National Transportation Safety Board report released Wednesday.It was not clear what, if any, role the pressure change played in the catastrophe, one of the nation's deadliest natural gas pipeline failures in decades. But the link between maintenance trouble and a sudden increase in pressure just before the blast was among the fresh details disclosed by investigators.
Crews were working on a backup power supply system in Milpitas, about 40 miles from the blast site, the NTSB said. During the work, electrical voltage to a pipeline control system dropped, interrupting a signal to a pressure control valve, the report says.
"The loss of the electrical signal resulted in the regulating valve moving from partially open to the full open position as designed," the report says.
Pressure climbed to 386 pounds-per-square-inch gauge in the 30-inch San Bruno pipeline at the time of the blast, above the specified maximum operating pressure of 375 psig, the NTSB said. The pipeline operator, Pacific Gas & Electric, said the allowable pressure on the pipeline was 400 psig, the report says.
Experts said the timing of the pressure change could prove important, particularly if some other underlying weakness in the pipe is discovered. But they stressed the increased pressure was nowhere near what would typically be needed to cause a catastrophic failure. "It tells me we have some defects in that section of the pipe," said Bob Bea, UC Berkeley engineering professor.
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View entire article here: http://www.latimes.com/news/local/la-me-sanbruno-pipeline-20101014,0,6454827.story
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