Federal investigators have determined that the natural gas in the pipeline that exploded in San Bruno was running at a higher pressure than the maximum limit PG&E has told the public it maintained.
At the time of the explosion, the utility's computers showed that the gas was at least 386 pounds per square inch, according to a source familiar with the investigation. PG&E spokesman Jeff Smith said Thursday that the maximum the company operated the line at was 375 psi, a figure that PG&E Vice President Geisha Williams also cited in a news conference Monday.
Overpressurized lines have been a cause of explosions in other pipeline disasters.
On Friday, Smith said PG&E could not discuss the exact pressure in the 30-inch transmission line at the time of the explosion because it is under review by the National Transportation Safety Board.
"That's part of the ongoing NTSB investigation. We don't have a comment on that," Smith said.
He noted the pipe had a "maximum allowable operating pressure" of 400 psi.
The steel line was built in 1956. It blew a 167-foot crater in the ground, destroying 37 homes, killing four people and leaving three still unaccounted for. Asked if PG&E had ever changed the maximum pressure rating on the 54-year-old pipe as it aged, Smith said he didn't know and that the company is investigating that issue.
The Mercury News reported Friday that NTSB investigators took extensive data from the computer system at a PG&E gas pipeline control building in Milpitas. That facility sent gas along the doomed pipeline, known as Line 132, north toward San Francisco. Computer systems in such facilities maintain and monitor the pressure of natural gas in the line, which runs under thousands of homes and near schools, stores and roads.
Pipeline safety experts said the fact that the pressure in the pipe exceeded PG&E's maximum pressure level is not automatically cause for alarm. But they said if the pipe had flaws -- such as corrosion or cracked welds -- running the pressure close to the limit could be a significant factor in the explosion.
"Between 375 and 400 psi still sounds safe, but it's all premised on a defect-free line," said Bob Bea, a professor of engineering at UC Berkeley with extensive pipeline experience.
"Here's where the demands on the pipeline from internal pressure have to be matched with a set of capacity questions. Was the steel brittle? Did we have a combination of corrosion and fatigue?"
Bea said in reviewing photographs he noticed the pipe broke cleanly where it ruptured, suggesting it broke along a weld. The NTSB said immediately after the explosion that the pipe had an unusual series of welds, indicating it was made of several smaller pipe sections.
Bea noted two factors that could have contributed to the explosion. First is a sewer pipe replacement project that the city of San Bruno contracted for in 2008. In that work, contractors used a technique that shakes the surrounding ground, known as "pipe bursting," to replace the sewer pipe running directly under the gas transmission line at the point where it exploded Sept. 9. PG&E has said it inspected the work before and after and found no damage to the pipeline.
The second is corrosion from liquid in the line. In 2009, PG&E asked the California Public Utilities Commission to approve a $3.2 million project to reduce liquid in the line, PG&E's Smith said. The liquid was "mostly oil" but also some water. Smith said the project was completed in November 2009 and "since that time, we have inspected it and found no evidence of any liquid."
Water in the line could have come from the soil, through a hairline crack in the welds, Bea said, or it could have come from the natural gas itself, which contains water that is removed with various devices called dehydrators.
Bea said he would be surprised if PG&E was still operating the pipeline at the same maximum allowable operating pressure as when it was first installed, during the administration of President Dwight Eisenhower.
"As you get older, you don't get stronger," he said.
Another pipeline expert said that older pipes can retain their integrity for decades, but it is imperative that they are well-maintained.
"What's important is whether or not the pipeline has the integrity to handle the pressures," said Rick Kuprewicz, a pipeline safety consultant in Redmond, Wash.
PG&E has said the section of pipe that exploded was last inspected in March, through a leak survey in which technicians walked along it with leak detectors. In November, the utility performed corrosion tests and found no problems, PG&E Vice President Geisha Williams told San Bruno residents at a town-hall meeting last Saturday.
But PG&E was not able to use state-of-the-art technology to inspect the line. That equipment -- a device known as a "smart pig" that is put inside the line to measure corrosion, cracks and other defects -- could not be used in the line because of its bends and angles. Numerous PG&E lines in the Bay Area are too old or incompatible for smart pigs and cannot use them unless they are retrofitted at a cost of millions of dollars each.
Assemblyman Jerry Hill, D-San Bruno, said Friday he has been following the pressure question closely.
"I am concerned," said Hill, a licensed contractor. "With a 50-year-old pipe, you'd think you would exercise caution and not run it too close to the limit. It seems like every time we turn around there's another problem developing, or we're getting inaccurate information. Where are the regulators?"
U.S. Rep. Jackie Speier, D-San Mateo, has been meeting with federal and state regulators and pipeline experts all week. She said she also has been following the pressure issue.
"I remember hearing that it exceeded 375 psi," she said, declining to elaborate.
Meanwhile Friday, a San Francisco law firm filed a class-action lawsuit against PG&E in San Mateo County Superior Court seeking the establishment of a trust to immediately disburse the $100 million fund promised by the utility to help rebuild the devastated San Bruno neighborhood.
"I don't think it belongs in the hands of PG&E," said the only named plaintiff, Steven Dare, a 43-year-old personal trainer who rents a home near the explosion site.
His lawyer, William M. Audet, called the $100 million pledge "a PR move. We want it to be a real pledge and put into some sort of court escrow so residents can use it," Audet said.
Mercury News staff writer Pete Carey contributed to this report. Contact Paul Rogers at 408-920-5045.