Overruling PG&E’s plea, state regulators Wednesday said that they want the utility to replace or perform water pressure tests on 705 miles of its oldest natural gas pipes, declaring they have “become increasingly uncomfortable” with the utility’s claim that it can set safe pressure levels for those pipes using “assumptions.”

If the recommendation from the staff of the California Public Utilities Commission is approved, the hydro-tests could disrupt gas service for untold numbers of PG&E’s customers, cost the company upward of $350 million and take five years to complete. Replacing pipe also could inconvenience customers and typically costs far more per mile.

The commission had ordered PG&E to calculate safe pressure levels for its gas lines based on the pipes’ construction, inspection and other records. In response, PG&E argued last week that it “does not believe it will find specific records of every component” and asked to base the pressure levels on its assumptions about the lines, using what files it has.

But that request drew a rebuke Wednesday from Richard Clark, director of the commission’s safety branch. “Hydro-testing or replacement of potentially 705 miles of pipeline segments will be costly and disruptive to PG&E’s operations,” Clark acknowledged in a letter to the utility. However, he added, “we do not believe that reliance upon indirect evidence of the material condition of PG&E’s natural gas transmission system is sufficient.”

Asked about Clark’s letter, PG&E spokeswoman Brittany Chord said the company “will be working with the commission to better understand the impacts this may have on PG&E’s operations.” She added that “we have already taken many steps to further enhance the safety of our system,” which includes reducing the gas pressure in 10 transmission pipes at the commission’s direction.

Clark’s letter is the latest of several signs that the commission, which had been accused in the past of being too cozy with PG&E, is becoming more assertive with the company. The agency has launched public investigations of PG&E, threatened it with fines and made several public comments about not wanting PG&E’s customers to be saddled with huge bills related to the Sept. 9 San Bruno gas line explosion, which killed eight people and destroyed 38 homes.

Gov. Jerry Brown also recently appointed three new people to the five-member commission. And during a recent hearing, one of them — Catherine Sandoval — expressed concerns about PG&E setting its pipe pressure levels based on mere assumptions.

Paul Clanon, the commission’s executive director, said a final decision on the matter probably will be issued by an administrative law judge or Commissioner Mike Florio, another Brown appointee who is overseeing inquiries into PG&E’s gas operations.

Clanon noted that his agency has been aware for months that the issue would be a major one to wrestle with, adding, “now the question is teed up with the commission.”

In the wake of disaster

Concerns about PG&E’s pipe pressure levels were prompted by the San Bruno accident. While investigating the blast, federal investigators discovered that the pipe, which was riddled with welding defects, burst at a pressure that was less than what PG&E thought was safe.

In addition, the utility’s records were found to have mistakenly described the ruptured segment as not being welded along its length, an error investigators said could have fooled the company into thinking the pipe was stronger than it really was.

In January, the commission ordered PG&E to locate records proving that it has set safe pressure levels for all 1,805 miles of its urban gas lines. But PG&E failed to meet the March 15 deadline to find all of the files, especially for its older pipes, many of which were installed when record-keeping rules were more lax than they are today.

Because it didn’t meet that deadline, PG&E could be fined $3 million. It also could be fined millions more if the PUC determines its record-keeping practices violated the law.

In addition, the San Bruno blast is being investigated for possible civil or criminal violations by the offices of the U.S. attorney, California attorney general and San Mateo district attorney. On Wednesday, The Associated Press reported that federal prosecutors in San Francisco had asked the state utilities commission for a wide array of documents related to the explosion — including PG&E’s maps, reports and emergency plans.

PG&E has vowed to hydro-test or replace 152 miles of pipe with characteristics similar to that of the burst San Bruno segment this year. However, the directive that it hydro-test or replace an additional 553 miles, would mean more work and expense.

Costly tests

Experts have estimated that hydro-testing, which involves forcing water at high pressure into pipe segments to look for leaks, costs $150,000 to $500,000 per mile. The procedure also typically requires removing a line from service for a week or longer. While gas can be trucked in to an affected neighborhood or often rerouted through other pipes, it’s not clear whether every affected customer could temporarily receive gas that way.

Some experts also have said pressurized water can cause undetected flaws to worsen.

Despite those potential problems, Assemblyman Jerry Hill, D-San Mateo, said he believes hydro-testing is the best way for PG&E to know what pressure level its pipes can withstand.

“The assumptions that were used historically by PG&E brought us to the events in San Bruno last September,” he said. “Assumptions don’t work when it comes to public safety.”

Contact Steve Johnson at 408-920-5043.