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NTSB: Malfunction caused gas pressure spike

 | Published on 10/14/2010


A power supply malfunction at a Pacific Gas and Electric terminal in Milpitas caused pressure to spike, then drop dramatically in a 30-inch natural gas pipeline just as it exploded in San Bruno Sept. 9, according to the National Transportation Safety Board.

The NTSB released its preliminary report yesterday of its investigation into the integrity of the pipe and the cause of a fire that killed eight and destroyed 35 homes in the Glenview neighborhood.

Approximately 47.6 million standard cubic feet of natural gas was released as a result of the rupture, according to the report.

PG&E was working on its uninterruptable power supply, or UPS, system at the Milpitas Terminal in the hours before the explosion. The power supply from the UPS system to the supervisory control and data acquisition, or SCADA, system malfunctioned causing 24 volts of direct current to drop to seven volts of direct current to the SCADA system, according to the report.

“Because of this anomaly, the electronic signal to the regulating valve for Line 132 was lost,” according to the report. Line 132 is the transmission line that runs through the Glenview neighborhood that has been capped since a section of the pipe exploded.

State Sen. Leland Yee, D-San Francisco/San Mateo, does not see the malfunction as an isolated incident, however.

“This report suggests to me that this is not an anomaly,” Yee said. “There are power failures all the time. PG&E needs to win back the confidence of the public.”

The Milpitas Terminal is about 40 miles south of where Line 132 burst.

The specified maximum operating pressure for the ruptured pipeline, installed in 1956, was 375 pounds per square inch gauge, according to the report. PG&E, however, indicated to the NTSB that the maximum allowable operating pressure for the line was 400 psig.

“The loss of the electrical signal resulted in the regulating valve moving from partially open to the full open position as designed. The pressure then increased to 386 psig. The over-protection valve, which was pneumatically activated and did not require electronic input, maintained the pressure at 386 psig,” according to the report.

Assemblyman Jerry Hill, D-San Mateo, suggested the power supply malfunction PG&E suffered Sept. 9 should make the utility company consider creating greater redundancy in the system so that if one system fails, a backup system would kick in to keep the power supply constant.

“You can’t be too prepared,” Hill said.

At about 5:45 p.m., the SCADA system indicated the pressure at Martin Station in Milpitas exceeded 375 psig, according to the report. The SCADA system indicated the pressure at Martin Station continued to increase until it reached about 390 psig at about 6 p.m.

At 6:08 p.m., it dropped to 386 psig. At 6:11 p.m., the pressure at Martin Station decreased from 386 to 361.4 psig; within one minute the pressure dropped to 289.9 psig, according to the report.

The first calls into 911 regarding the explosion in the Glenview neighborhood came in at 6:11:59, according to an incident history report of the fire.

Lawmakers have been critical of PG&E’s failure to turn off the gas to Line 132 for an hour and 46 minutes after the pipe exploded.

“PG&E did not dispatch crews until 6:45 p.m., 33 minutes after 100-foot high flames in Glenview were clearly visible from (Highway) 101, more than 10 miles away. Why?” U.S. Rep. Jackie Speier, D-San Mateo, said in a press release yesterday.

On Tuesday, PG&E indicated automatic shutoff valves would be installed throughout its system.

“That’s a step in the right direction but it should not divert our attention from the human failure, the 33-minute delay it took for someone at PG&E to dispatch crews to turn off the valves. Remote shutoff valves require human intervention. PG&E must assure us that human error won’t be a part of its emergency response,” Speier said in the press release.

The ruptured pipeline fractured length-wise and at welds that held the sections of pipe together, according to the NTSB report. The thickness of the pipe walls were “fairly uniform” and corrosion was not listed as a factor for the pipe’s failure.

The investigation is ongoing, however, with a final report on the incident not expected to be completed for about a year.