Mounting pressure in a major natural-gas line immediately preceded the deadly San Bruno blast, though a healthy pipeline should have been able to withstand the surge, investigators and experts said Wednesday.
The National Transportation Safety Board is investigating the failure of a 30-inch PG&E natural-gas line that exploded in the suburban Glenview neighborhood Sept. 9. The blast and ensuing conflagration killed eight people, injured dozens, and destroyed 37 homes and damaged 18.
More than a month later, the exact cause of the line’s failure remains a mystery. But a report released Wednesday provides the clearest timeline of facts so far, and it has prompted experts to say the pipeline was somehow weakened prior to the explosion.
About an hour before the San Bruno explosion, PG&E work on the power supply system in Milpitas, 39 miles southeast of the blast, caused a malfunction. The failure of the power supply system caused the pressure of natural gas in the 54-year-old pipeline near the blast site to approach high levels.
At San Francisco’s Visitacion Valley station, the pressure reached 390 pounds per square inch about 11 minutes before the explosion and was at 386 pounds at the time of the explosion, according to the report released Wednesday. The maximum operating pressure for the ruptured pipeline was 400 pounds per square inch, according to PG&E.
UC Berkeley engineering professor Robert Bea said based on information in the report, it is clear that if the pipeline were in its original state then the surge of pressure should not have caused it to rupture.
"The pipeline must have developed defects that lowered the burst pressure to the point where this pressure spike could trigger the burst of the pipeline," Bea said, adding that defects could include flaws caused by welding, time, corrosion, cracking or faulty steel.
Assemblyman Jerry Hill, D-San Mateo, said the news should prompt PG&E and regulators to re-evaluate the maximum operational pressure of pipelines that may have changed through the years.
"That pipeline was rated 400, but it’s clear that there have been changes in the last 55 or 60 years, whether it’s the fatigue, the possible welds, the repairs that may have been allowed," Hill said.
The investigation is trying to determine what caused the weaknesses in the pipeline that made it unable to handle the sudden influx. The NTSB is examining the seams and welds that held the pipeline segments together.
Among the findings so far is one perfect circumferential fracture where pipe segments were welded together.
Another theory rests on the results of a microbiological examination. Scientists are studying whether bacteria resting in the broken pipeline ate away at the metal, causing the pipe to corrode. Those samples are "currently being analyzed," according to the NTSB.
A conclusive set of findings may take months. Since the explosion, PG&E has inspected the three major pipelines that serve San Francisco and the Peninsula, according to company President Chris Johns. Also, PG&E is conducting aerial inspections and ground-leak surveys of the entire gas transmission system.
"It is critical to the people of San Bruno, our customers and the industry that we get to the bottom of this accident and take the necessary steps to prevent such a tragedy from ever happening again," Johns said in a statement.
Hundreds of firefighters had been battling a geyser of flames soaring out of a broken natural-gas line in San Bruno for fully 33 minutes before Pacific Gas and Electric dispatched a crew to isolate the ruptured pipe section. About 55 minutes after that, the crew managed to close both the downstream and upstream valves, finally cutting off the source of the fuel, according to a report by the National Transportation Safety Board.
But the first fire from the transmission system had also caught fire to the neighborhood’s gas distribution system — the jungle of small lines that feed each individual home. That system was not directly attached to the transmission system, so its fuel source was not cut when the transmission line’s was.
The distribution lines continued to burn well into the night. It took more than five hours for PG&E to finally cut off the fuel to that system, the report states. The fires in the distribution system were extinguished at 11:30 p.m.
Millbrae Fire Division Chief Ron Lavezzo, who was at the site of the fire all night long, said he doubted the burning distribution system actually caused serious problems — the damage had already been done by the shower of fire from the transmission line. He noted that until the distribution system was finally cut at 11:30, flames from the distribution lines could still be seen burning at each home in the neighborhood, "like candles," he said.
"This is a step in the right direction, but it should not divert our attention from the human failure, the 33-minute delay it took for someone at PG&E to dispatch crews to turn off the valves," Rep. Jackie Speier, D-Hillsborough, said in a statement. "PG&E must assure us that human error won’t be a part of its emergency response."
— Katie Worth and Brent Begin
5:40 p.m. At about this time, while working on the uninterruptable power supply system at a gas transmission terminal 39 miles southeast of the rupture site, the power supply is interrupted. Power fades from 24 volts of direct current to 7 volts.
5:44 p.m. Up until about this time, pressure detected at the Martin Station in Visitacion Valley, downstream of the rupture site, is about 375 pounds per square inch, the normal load, PG&E says.
5:45 p.m. Pressure at the Martin Station begins to increase.
6 p.m. Pressure reaches 390 pounds per square inch.
6:08 p.m. Pressure drops to 386 pounds per square inch.
6:11 p.m. 30-inch natural-gas transmission pipeline ruptures, 47.6 million cubic feet of natural gas is released and catches fire.
6:12 p.m. Pressure at Martin Station drops to 289.9 pounds per square inch.
6:45 p.m. PG&E dispatches a crew to isolate the ruptured pipe section.
7:20 p.m. Crew closes upstream valve.
7:40 p.m. Crew closes downstream valve.
11:30 p.m. PG&E stops gas flow to distribution system.
11:31 p.m. Distribution line fires self-extinguish.