Pacific Gas and Electric Co. is relying on what many experts call an inferior method to check for corrosion problems on nearly three-fourths of the utility's natural gas pipelines running through urban areas - including the part of the line that ruptured in San Bruno, with deadly results.

Federal and state regulators have reached no conclusions about what caused the Sept. 9 explosion in the Crestmoor neighborhood that killed at least four people and destroyed 37 homes. But one of the issues they are looking at is whether the 30-inch line was weakened by corrosion - a potential problem that PG&E was concerned about in 2009.

The pipe that ran under the San Bruno neighborhood was part of PG&E's network of 1,107 miles of lines in residential and urban areas - lines that must be checked every seven years under a 2002 law that Congress passed in reaction to a string of pipeline disasters.

According to PG&E's filings with the California Public Utilities Commission, the utility intends to inspect 72 percent of its urban lines by 2014 using what some analysts say is a flawed and outmoded method.

That method starts with running an electric current through a pipeline. Workers walk the pipeline route with devices that resemble ski poles and insert them in the ground.

If everything is fine with the pipe, the device will register an electronic signal. But corrosion in the metal can block that signal, so if the device detects a weaker response in one area, it points to something that could be wrong with the pipe.

Used in San Bruno

This electronic mapping method is how the San Bruno pipe was tested in November 2009, and PG&E says it passed. At the time, according to a PG&E filing with the state Public Utilities Commission, the utility knew the line was at greater risk of corrosion because compressor oil and other contaminants had turned up in sections of the pipe running from San Francisco to Milpitas.

However, the ski-pole method - known as "direct assessment" in industry parlance - is far from foolproof, many experts say.

"Direct assessment is certainly not the gold standard," said Carl Weimer, executive director of the Pipeline Safety Trust, a nonprofit organization created out of a 1999 gasoline pipeline rupture in Bellingham, Wash., that killed three people.

"We have always been pretty skeptical about direct assessment," Weimer said, "but it was one of the compromises that ended up in the regulations."

What it misses

Richard Kuprewicz, a pipeline safety consultant in Redmond, Wash., said the electronic mapping can only estimate the extent of corrosion, and only in areas that the poles can reach.

Also, corrosion is not the only problem that can weaken pipes. Cracking and fatigue due to age and elevated gas-line pressures are also dangers, Kuprewicz said. And the direct assessment method "is not a real good tool for stress corrosion cracking," he said.

Jim Hall, a former chairman of the National Transportation Safety Board who is now a private consultant, said the technique is so antiquated that it should not be allowed in urban areas. The safety board is the agency taking the lead in investigating the San Bruno disaster.

"That's essentially an old technology," Hall said of direct assessment. "I don't think is an effective method for a pipe that old."

Older pipes, such as the line in San Bruno that dates back to at least 1956, are more prone to cracks and fatigue that may not be detected through electronic mapping, Hall said.

The pipeline safety advocates agree that the better method of checking for corrosion, cracks and similar problems is to use an automated probe known as a pipeline inspection gauge, or "pig."

Better technology

A pig is typically a cylindrical instrument as long as 25 feet that measures ultrasound vibrations or magnetic field waves to check for corrosion or tiny cracking that would escape direct-assessment techniques, the analysts said.

PG&E, however, has used pigs to inspect only a fraction of its urban pipelines. In a filing with the PUC, the utility said that by the end of 2010, it will have used the devices to check just 69 miles of the 1,107 miles of lines under cities and towns in Northern and Central California.

It told the commission that from 2011 through 2014, it intends to increase its use of pigs to check an additional 235 miles - including a 32-mile stretch of the pipeline that ruptured Sept. 9.

That stretch, however, will start several miles south of San Bruno and extend to the South Bay. The rest of the San Bruno line will be among the more than 800 miles of pipeline where PG&E will continue to use the ski-pole method.

PG&E's reasons

PG&E spokeswoman Katie Romans said installing pig devices to check lines can take as long as four years because pipes have to be modified and infrastructure put in place.

And using pig systems is not feasible on some lines, she said, because the diameter of the pipe is too small or because of sharp turns in the line. In those cases, she said, direct assessment is an accepted alternative.

"It is a federally approved method" to check for pipe problems, Romans said. The utility has to consider "the shape and diameter of the pipeline" in deciding which technology to use, she said.

Mike Florio, a gas expert with the consumer group The Utilities Reform Network, said direct assessment has at best a mixed track record.

"I don't think there is any doubt that pigging is more comprehensive and accurate than direct assessment," Florio said. "That's why they (PG&E) have this program to convert to allow pigging. The obvious question is, is direct assessment good enough in the meantime, or do we have a bigger problem than that?"

Hall, the former safety board chairman, said utilities are "clearly driven for economic reasons to try to sell direct assessment as an effective means of providing safe operation."

Advanced pig devices, he said, are "certainly superior and particularly should be in all of the (urban) areas. Hopefully, they will re-evaluate that in light of this tragedy."