The National Transportation Safety Board earlier this month released its preliminary report on the accident of the Sept. 9 gas pipeline explosion in San Bruno that killed eight people and damaged 55 homes.
The report came a day after Pacific Gas and Electric Co. announced its new natural gas pipeline safety measures, known as Pipeline 2020.
The explosion of a 30-in. diameter natural gas pipeline released 47.6 million standard cubic ft of natural gas, creating a crater 72 ft long by 26 ft wide and throwing a 28-ft segment of the pipe 100 ft away.
Investigators are looking at the sequence of events that led up to the explosion and PG&E’s response. Before the accident, PG&E crews were working on an uninterruptible power supply system about 40 mi away when the power supply system malfunctioned, resulting in the regulating valve moving from partially open to full open and increasing the pressure to 386 lbs per sq in. gauge (psig). The pipeline’s maximum allowable operating pressure was 400 psig and the specified maximum operating pressure was 375 psig.
Within minutes of the fluctuation, the pipeline exploded. Thirty-four minutes later, PG&E dispatched a crew to close the mainline valves.
NTSB shipped the ruptured segment to Washington and radiographed the girth welds and seams, microbiologically tested the pipe surface, performed ultrasonic wall thickness measurements, magnetic particle inspection of welds and seams and 3-D laser scans of the pipe pieces. The 50-year-old pipe was 0.375-in. thick, coated with hot applied asphalt and cathodically protected. Technicians still plan to perform hardness and microhardness testing, optical fractographic analysis and electron microscopy of the fracture surfaces. The final report could take a year.
In the meantime, PG&E plans to install hundreds of automatic over-pressure protection control valves and upgrade pipeline infrastructure, particularly in heavily populated areas.
The utility also plans to invest $10 million in launching an independent, nonprofit entity to perform research and development of new inspection technologies. Completing the enhancements could take ten years, according to PG&E spokesperson Katie Romans.
“This could help upgrade best practices for the entire state and the country,” Romans says.
