Pacific Gas and Electric Co. asked for permission Thursday to raise natural gas rates by an average of $5.23 per month for residential customers to fund continued maintenance and modernization of its pipeline system.

The San Francisco-based gas and electric utility's filing requests $1.29 billion in 2015, or 13 percent more than it expects to spend on such activities next year.

It would cover things like replacing older pipelines in its 6,750-mile gas transmission system serving 4.4 million customers. It would also pay for testing and inspecting pipelines, strengthening levees, and installing additional automated and remotely operated safety valves.

PG&E's application will be reviewed by the California Public Utilities Commission, which has discretion to reduce the size of the proposed increase if it determines that some or all of the proposed spending is unjustified.

The Bay Area consumer advocacy organization TURN noted that the utility's request is separate from a proposed PG&E rate increase already under review by the CPUC.

"Overall, in the next three years, consumers could be looking at, on average, approximately $18 more per month," TURN spokeswoman Mindy Spatt said. "That's just gas."

She added that PG&E's inadequate gas pipeline maintenance and record-keeping contributed to the San Bruno pipeline explosion that killed eight people on Sept. 9, 2010. Ratepayers should not have to pay PG&E to clean up its own mess, she said.

Also Thursday, the CPUC levied a $14.35 million fine against PG&E for the company's failure to give the agency prompt notice that its records on a pipeline through the city of San Carlos were incorrect.

The CPUC's Office of Ratepayer Advocates plans to review and make a recommendation on PG&E's new request for a rate increase. Results won't be available for at least two months.

A news release by the company Thursday said the additional spending on infrastructure -- the utility's main source of income since it generally does not make money selling energy -- will help it meet some of the nation's toughest safety standards.

"Our plan to invest in 21st century infrastructure will serve customers by helping PG&E become one of the safest and most reliable gas utilities in the country," Nick Stavropoulos, executive vice president of PG&E for gas operations, wrote in the release.

The release noted that the spending would also support many well-paying jobs.