A brief power failure shortly before a deadly gas pipeline explosion in a San Francisco suburb briefly increased pressure in the pipe, according to a preliminary investigative report released Wednesday by federal officials.
The National Transportation Safety Board's first report on the incident does not identify the cause of the Sept. 9 blast that killed eight people and destroyed 37 homes.
Much of the material in the report was previously released, but Wednesday was the first time the board had laid out its timeline of events surrounding the explosion
PG&E was quick to praise the report, saying it is critical for the people of San Bruno to get to the bottom of the accident.
"We thank the National Transportation Safety Board for today’s release of its preliminary report on the tragedy in San Bruno. Although a final report and a conclusive set of findings are likely to be many months down the road, this initial release of information is an essential first step. We welcome it, and appreciate the painstaking efforts of the NTSB experts to conduct a thorough and comprehensive investigation to determine the root cause of this terrible accident."
Investigators said that after the power failure, an electronic valve opened completely and briefly increased pressure by 15 pounds-per-square-inch just 11 minutes before the blast in a San Bruno neighborhood.
At 6:11 p.m., when the blast occurred, Pacific Gas & Electric Co. monitors recorded a drop in pipeline pressure, likely occurring after it had ruptured and released the gas that eventually created a giant fireball.
Experts said that small increase in pressure would not be enough to rupture the 30-inch steel pipe but could have worsened a preexisting hole.
The NTSB found that PG&E sent its first crews out at 6:45 p.m. to shut off valves that controlled the flow of gas. One manual valve was turned off at 7:20 p.m. and the second at 7:40 p.m., one hour and 29 minutes after the explosion.
PG&E president Chris Johns said Tuesday the company wants to replace its manual valves with automatic or remote controlled valves to increase safety on the gas pipeline.
Rep. Jackie Speier, who has introduced a bill that would require automatic shutoff valves, said the report by the NTSB and the statement by Johns should not divert attention from what she saw as human failure in the delay to dispatch crews to turn off the valves.
NTSB officials said a more detailed report will be released in coming months after scientific tests are conducted on the pipe segments.