II. DESCRIPTION OF THE SEPTEMBER 9, 2010, SAN BRUNO GAS TRANSMISSION PIPE RUPTURE

The PG&E owned and operated transmission pipeline that ruptured under the San Bruno, California neighborhood on September 9, 2010, was installed in about 1956. The pipeline was constructed of longitudinally seam-welded pipe. When the pipe was installed, various segments of such pipe were joined together with circumferential welds, referred to as "girth" welds. The pipeline was buried underground. The San Bruno segment of this pipeline was located in a neighborhood the U.S. Department of Transportation classifies as "high consequence," meaning "any area where a potential impact radius of 660 feet would contain more than 20 buildings intended for human occupancy." (49 C.F.R. Section 192.903, and NTSB January 3, 2010 Safety Recommendation, p. 2, Appendix B).

The pipeline buried in the neighborhood in San Bruno was a 30 inch diameter transmission pipeline. It was used to transport large quantities of gas at high pressure from Milpitas north through the Bay Area Peninsula and to the City of San Francisco. PG&E identifies the pipeline as Line 132 (NTSB Preliminary Report, p.1, Appendix A). Line 132 is part of a series of three transmission pipelines that provide 100% of the natural gas consumed by residential, commercial and industrial customers, as well as power plants, in San Francisco and along the Peninsula. Line 132 originates at the terminus of a larger pipeline at Milpitas, California, at a PG&E owned and operated facility known as the Milpitas Terminal.

During the afternoon of September 9, 2010, just before the rupture, PG&E personnel were working on an uninterruptable power supply system at the Milpitas Terminal, about 39.33 miles southeast of the San Bruno rupture site. While the repair work was underway, the power supply malfunctioned. This caused the wrong voltage to be supplied to PG&E's supervisory control and data acquisition system ("SCADA"). In turn, this caused the loss of the electronic signal to the gas pressure regulating valve at the inlet of Line 132. The signal loss caused a regulating valve to move to full open, as it was designed to do under those conditions. Pressure in Line 132 increased to 386 psig. This pressure was limited to about 386 psig by an over-protection valve that required no electronic input. At about 5:45 p.m., PG&E's SCADA indicated that the gas pressure at Martin Station, downstream (north) of the San Bruno rupture location, exceeded 375 psig. At about 6:00 p.m., the pressure at Martin Station increased to about 390 psig. Pressure at Martin's station then decreased at 6:08 p.m. to 386 psig, at 6:11 p.m. to 361.4 psig, and within another minute to 289.9 psig (NTSB Preliminary Report, pp. 1 and 2, Appendix A.).

At approximately 6:11 p.m., Pacific Daylight Time, the 30 inch segment of Line 132 located in a residential neighborhood of San Bruno ruptured. The rupture created a crater about 72 feet long and 26 feet wide. Investigators later found an approximately 28 feet long pipe segment about 100 feet from the crater. The rupture released about 47.6 million standard cubic feet of natural gas. The escaping gas ignited moments after the rupture. The resulting fire destroyed 37 homes and damaged 18 other homes. The San Bruno rupture and fire killed eight people, injured numerous others, and necessitated the evacuation of many others from the area.  

The local fire department set up an Incident Command. The NTSB noted that "[t]he immediate response by local emergency responders, as well as three strategic drops of fire retardant and water by air, assisted in stopping the spread of the fire." (NTSB Preliminary Report, p.1, Appendix A.)

PG&E dispatched a crew at 6:45 p.m. in order to isolate the ruptured pipe and halt the gas flow. They closed the nearest upstream valve at about 7:20 p.m., and the nearest downstream valve at about 7:40 p.m. This stopped gas flow to the rupture location, and the fire from the ruptured line self-extinguished. Later that evening, PG&E isolated the area residences' gas distribution system, and by about 11:30 p.m. gas fires at damaged homes were extinguished. (NTSB Preliminary Report, p.2, Appendix A.)