Maintenance work caused gas pressure to rise just before San Bruno explosion, report finds

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.

The NTSB said it would conduct more forensic tests on the retrieved parts of the pipe. A final report on the cause of the blast is expected next year. PG&E said it welcomes the NTSB investigation and has taken a number of steps to improve safety and inspections of its gas system.

Assemblyman Jerry Hill (D-San Mateo), who represents San Bruno, said the NTSB report raises questions about PG&E maintenance procedures and whether adequate power protection systems are in place for crucial control systems. He also noted the report found broken gas distribution lines to burning homes weren’t shut off until more than five hours after the blast. Given the gas-fed fire risks in such blasts, as well as earthquakes, automatic gas shutoff valves may be needed for every home in the state, he said.

The NTSB did not say whether corrosion was found outside or inside the pipe. But walls of the pipeline segment that ruptured were found to be a fairly consistent thickness of between .36 inches to .38 inches. The original thickness when the pipe was installed in 1956 was .375 inches.

The report noted that the segment of pipe that burst, at a dip in the terrain, was constructed of four small pieces welded together. The breaks occurred along some of the short segments. In one case, the weld around one of the short sections ruptured, the report said.

rich.connell@latimes.com

ralph.vartabedian@latimes.com