In defending themselves as they were being grilled by federal investigators about a lack of automatic shut-off valves near the site of the San Bruno pipeline explosion, PG&E officials warned that abruptly stopping gas flow comes with its own set of potential dangers.
PG&E said a drop in pipeline pressure, which occurs when the gas flow is shut down, would extinguish pilot lights in homes, creating “the potential for gas to re-enter that house through this migration of low pressure. The gas could intrude back into the house and create an explosive situation,” said Keith Slibasager, PG&E’s gas systems manager.
Asked about automatic shut-off valves, which operate without human intervention, PG&E Senior Vice President Edward Salas said they could present a danger if they work based on “simplistic or incomplete” information. The utility has said it prefers remotely controlled shut-off valves.
“What we’re trying to do is maintain pressure within a tolerance of safety,” Salas testified. “An uncontrolled shutdown can be as hazardous to public safety in many cases as a rupture.”
PG&E officials said they considered that fact the night of the San Bruno explosion, when it opted not to cut the gas flow at the utility’s Milpitas terminal, which serves the entire Peninsula and San Francisco, Slibasager said. Utility workers shut off manual valves near the San Bruno site about 90 minutes after the explosion.
Cutting service at Milpitas would have caused pilot lights to extinguish for more than a million residents and “would have created a much more widespread safety issue,” Slibasager said.
PG&E has said it plans to install automatic and remote shut-off valves on more of its lines, but the technology has to be well-managed, Salas said.
“You need to have full situational awareness so you know you’re not creating any unintended consequences by shutting down a line,” Salas said.