Hitting a major marker in its efforts to improve the safety of its natural gas system, PG&E on Tuesday disclosed it is nearly finished checking its vast network of gas pipelines, in some instances subjecting pipes to pressure tests to verify their integrity.

The utility said that when it concludes its safety check by the end of April, it will have completed seven of the 12 safety recommendations issued by the National Transportation Safety Board in the wake of the September 2010 lethal explosion of natural gas in San Bruno that killed eight people and destroyed 38 homes.

PG&E and its critics agree that much work remains to transform its pipelines into a truly safe network. And it remains unclear who will pay for the improvements, which by some estimates could reach $5.3 billion in the coming few years.

"What PG&E has done is good, it is necessary -- they have done some positive things," said state Sen. Jerry Hill, a San Mateo County Democrat whose district includes San Bruno and who has been a vocal critic of the utility. But he added that the utility still must "figure out what needs to be done long-term to keep the system safe, how much will it cost, when it must be done."

PG&E's Nick Stavropoulos, executive vice president of the utility's gas operations, said that while PG&E is "is very proud of what we've been able to accomplish," challenges remain.

"This really is the beginning of where we want to be to make the gas system safe," he said in an interview with this newspaper.

The San Bruno explosion was caused in great measure by PG&E's inadequate record keeping, failures to properly test gas pipes and flaws in its risk assessments, according to the safety board's investigation. The explosion occurred when a pipe running beneath a San Bruno neighborhood burst along a faulty weld and ignited.

The nearly completed pressure tests on some sections of pipeline and record reviews of other sections were undertaken on more than 6,000 miles of gas pipelines, said Brittany Chord, a spokeswoman for PG&E.

By the end of 2013, PG&E projects it will have hydro-tested about 550 miles of gas pipelines throughout its systems, going beyond the safety board's recommendations. That includes more than 160 miles tested in 2011, about 185 miles in 2012 and roughly 205 miles the utility intends to test this year.

Chord said some of the pressure tests revealed weaknesses and that parts of the pipeline were replaced as a result.

"There have been some instances of hydro-test failures," she said. "But that's the whole point of a hydro test. We want to find any cracks or problems that would compromise system safety."

Stavropoulos added that the tests "give us a lot of comfort level that the pressures at which we are operating the system are well within industry standards."

But not all experts are impressed with the tests.

Theo Theofanous, a chemical and mechanical engineering professor at UC Santa Barbara who served on a major federal panel assessing pipeline risks, said PG&E's update is encouraging but that "before we all start jumping up and down with joy, we need to know more detail."

In addition to the nearly completed pipeline safety tests, PG&E, as part of its response to the NTSB recommendations, also has developed new procedures for undertaking work on the gas transmission system and has upgraded its public awareness plan for natural gas safety.

Safety board recommendations still on its to-do list include conducting an overall risk analysis, a threat assessment, developing new software systems and adding automated controls of gas valves and sensors. The NTSB also has recommended that PG&E hydro-test any gas pipes that have no record of prior hydro tests.

Terry Williams, a spokesman for the NTSB, said the board is "pleased with the progress PG&E has made so far" and is "looking forward to the completion of the final five recommendations."

PG&E is also building a natural gas control center in San Ramon that will become the nerve center of the utility's gas operations. That complex is due to open by midsummer, Stavropoulos said.

"We are making real progress that can be seen and felt by our customers, employees and regulators," Stavropoulos said.

Contact George Avalos at gavalos@bayareanewsgroup.com, 408-373-3556 or 925-977-8477. Follow him at twitter.com/george_avalos.