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NTSB Cites Power Glitch in San Bruno Gas Pipe Explosion

A power glitch at the Pacific Gas & Electric Co. (PG&E) natural gas terminal in Milpitas, CA, opened up a pipeline control valve, which in turn caused a surge in pressure before the San Bruno, CA, gas pipeline ruptured last month, the National Transportation Safety Board (NTSB) reported Wednesday.

According to the NTSB's preliminary report, the blast, which killed eight people, released 47.6 MMcf of gas (see Daily GPI, Sept. 13). The ruptured pipeline was fractured length-wise and at welds that held the pipe sections together, but the report did not mention corrosion. The thickness of the pipe's walls was described in the report as "fairly uniform."

Line 132, a 30-inch diameter line that runs from the Milpitas terminal through the East Bay, was running at a maximum operating pressure of 375 pounds per square inch (psi) on Sept. 9, the day of the blast, the report stated. The maximum allowable operating pressure was 400 psi.

According to the report, a PG&E crew was dispatched to shut down the pipeline 34 minutes after the blast, and it took an hour and a half from the moment the line ruptured to completely shut it down, the board said. At 11:30 p.m. the utility isolated the gas distribution system and within a minute "fires from escaping natural gas at damaged houses went out."

In the lead-up to the explosion, NTSB investigators found that when a PG&E crew was working on an uninterruptible power supply (UPS), the gas pipeline's computer control system malfunctioned. Electrical power to the system fell, which caused the control valve to open and pressure to build in the line. The report said "nstead of supplying a predetermined output of 24 volts of direct current, the UPS system supplied approximately 7 volts or less to the SCADA [supervisory control and data acquisition] system. Because of this anomaly, the electronic signal to the regulating valve for Line 132 was lost. The loss of the electrical signal resulted in the regulating valve moving from partially open to the full open position as designed."

Pressure in the pipeline following the malfunction increased to 386 psi, and a pneumatically activated valve maintained the pressure at that level, the NTSB report stated. At about 5:45 p.m., the system was recording a pressure of more than 375 psi at another station in Daly City, CA. The pressure continued rising until it reached about 390 psi at around 6 p.m., said investigators.

Eight minutes later pressure dropped to 386 psi, and at 6:11 p.m., when the San Bruno explosion occurred, pipeline pressure had fallen to 361.4 psi. A minute later pressure stood at 289.9 psi.

An investigation is continuing concerning fractures in the pipeline and the pipe's metal strength, the NTSB said. An electron microscope analysis of the ruptured pipe segment also is being conducted.

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