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Exploded pipe launched 100 feet, NTSB says

 | Published on 9/13/2010


The natural gas explosion that preceded a massive fire around Crestmoor Canyon in San Bruno launched a 28-foot section of pipe a distance of about 100 feet from underground out onto the street, the vice chairman of the National Transportation Safety Board said this weekend.

The explosion also created a crater 167 feet long and 26 feet wide, NTSB Vice Chairman Christopher Hart said.

“This really emphasizes the magnitude of what occurred here,” Hart said.

The depth of the crater is unknown because the soil is unstable on the floor of the hole, making it unsafe for a person to go in to measure, Hart said.

At about 6:15 p.m. Thursday, a 30-inch PG&E steel gas transmission pipeline ruptured, causing a massive explosion and fire that officially killed four people and hospitalized more than 50 others.

Saturday was the first full day of investigation for the NTSB. Ravindra Chhatre, the lead NTSB investigator for the San Bruno explosion and fire, has more than 30 years experience with pipelines, Hart said. I nvestigators found multiple seams on the section of pipe, Hart said. The NTSB initially reported that the section of pipe was seamless, he added.

“Longitudinal seams” found on the pipe means it started as a flat piece of metal that was curved and welded, Hart said.

The pipe also has “circumferential welds,” meaning that section was made of smaller segments of pipe, Hart said.

“So that opens another avenue of investigation, namely to determine why that segment had sub-segments,” Hart said.

Welding does not automatically mean that segment of pipe had undergone repairs, Hart said. Segmented pipe is more expensive than pipe not welded together from smaller pieces, he added.

“There are lots of reasons to have segmented pipe like this,” Hart said.

It’s too early to tell if the pipe was corroded, Hart said. It’s also unknown how deep underground the pipe was at the time of the explosion, he added.

There was no automatic shutoff valve on the pipe, Hart said. The locations of two manual shutoff valves on either end of the pipe are unknown, he said.

PG&E has provided the NTSB with information on the locations of other pipelines in the area, Hart said. The date of the last PG&E inspection of the pipe is unknown, he added.

The Pipeline and Hazardous Materials Safety Management Administration classifies the pipe as “Class 3”, which refers to “density of population in the vicinity of the pipe,” Hart said. Class 3 pipes are in areas with more than 47 homes per linear mile of pipe within 220 yards on either side of the pipeline, Hart said.

Large pipelines in residential areas usually means the pipeline was built before the area was heavily populated, Hart added.

The gas in the pipe was odorized to make it detectable to the human nose, Hart said. Witnesses have reported allegedly smelling gas in the area before the explosion.

The NTSB is asking anyone with information about the explosion to email sanbruno@ntsb.gov.

The California Public Utilities Commission has established a toll-free number and e-mail address for anyone who noticed the smell. People can call (800) 789-0550 or send an e-mail to SBFire@cpuc.ca.gov if they smelled the gas.

Hart has said he estimates it would be 14 to 18 months before NTSB issues a final report and recommendations. Lt. Gov. Abel Maldonado said in a statement Saturday that timeline was “unacceptable.”