Plains said the pipeline had one valve to shut it down if oil flowed in the opposite direction and three valves controlled by operators in its Midland, Texas, control room.
Plains defended its approach to manually shutting down the system, saying it's the standard across the country for liquid pipelines.
"It is much safer for operators who understand the operations of the pipeline to shut it down following a planned sequence of steps than for a computer to automatically close a valve on oil that is traveling in confined space at high pressure," Patrick Hodgins, the company's senior director of safety, said Saturday. "This is all standard operating procedures within our industry."
While it's not known if an auto shut-off valve would have detected the leak and reduced the size of the spill, environmentalists have criticized the lack of such a device, saying it could have averted or minimized the disaster.
"Everyone is pretty mystified why the pipeline didn't automatically shut down when the leak occurred," said Linda Krop, chief counsel of the Environmental Defense Center.
Santa Barbara County regulations sometimes exceed state and federal standards, requiring additional environmental analysis or imposing conditions to further protect health and the environment, Drude said. One additional requirement is a valve that can detect changes consistent with a leak and automatically shut down.
The county successfully fought another operator that didn't want to install automatic shutdown valves on a pipeline from an offshore drilling platform, Drude said.
However, when there was a leak on that line in 1997, an operator overrode the automatic shutdown, and it continued spewing crude into the Pacific Ocean a couple miles from shore. The 10,000 gallon spill fouled 21 miles of shoreline and killed more than 150 birds.
Richard Kuprewicz, president of Accufacts Inc., which investigates pipeline incidents, said such valves aren't always effective, though newer, more sophisticated "smart" models provide more accurate signals that can trigger shutdowns.
A Plains employee discovered the leak early Tuesday afternoon, about three hours after mechanical issues with the pipeline, according to the company. The pipe was restarted for about 20 minutes before a pump failed and then it was shut down because of changes in pressure.
The company said it was looking into whether those earlier problems led to the leak.
A surge in pressure from starting up a system could cause a leak or exacerbate one, but it's too soon to tell, Kuprewicz said.
"In the past, surge pressures have caused pipes to rupture. But there were other failures, too," he said, speaking in general and not about the Plains incident. "If that were the case, that would become fairly evident ... pretty quickly."
Plains All American subsidiaries have reported at least 223 accidents along their lines and spilled a combined 864,300 gallons of hazardous liquids since 2006, according to federal records. The company has been subject to 25 enforcement actions by federal regulators and tallied damages topping $32 million.