Limited curtailments this winter, up to 300 new shutoff valves, and new leaks identified and repaired on parts of the major natural gas transmission pipeline system in the greater San Francisco Bay Area were documented in reports released late Monday by Pacific Gas and Electric Co. (PG&E) and the California Public Utilities Commission (CPUC).
A survey of 16 miles of gas transmission pipes in the San Bruno area turned up no problems needing immediate repair, the combination utility told the CPUC in its report, which covers actions directed by state regulators in a resolution passed Sept. 23.
Looking at 15.93 miles of transmission piping in San Bruno where a sector of the utility's 30-inch diameter Line 132 failed Sept. 9, causing an explosion and fire that killed eight people and destroyed 37 homes, PG&E indicated that it used three instruments to survey the transmission mains in the suburb about 10 miles south of San Francisco: close interval survey (CIS), direct current (DC) voltage gradient, and pipeline current mapper (PCM) tools. The survey included Lines 101 and 109, in addition to Line 132.
The other areas covered in the 23 pages of text and maps were the automatic shutoff valves, an accelerated survey of the entire PG&E backbone gas transmission system, and the contingency plans for curtailment of some industrial customers if lower operating pressure levels are continued into the winter season. Curtailment estimates are preliminary and will not be finalized until there is a final determination on the allowable operating pipeline pressures, the utility said.
Estimates used three different daily average temperatures (42, 37 and 32 degrees F) and assumed a new plan for beefing up pipeline capacity in the three peninsula lines (101,109 and 132). At a 300 pounds-per-square-inch-gauge (psig), PG&E said it could not meet either a cold winter day (CWD) or abnormal peak day (APD).
"On an APD, 100% of all San Francisco and peninsula noncore customers will need to be curtailed, along with some large core customers," the utility told the CPUC. It further noted that the extent of the curtailments can be reduced if Line 101 and/or Line 109 are operated above 300 psig. "To avoid core customer curtailment, L101 and L109 must both operate at pressures above 300 psig, or L101 must operate at a pressure near 375 psig.
"PG&E will develop a final curtailment plan when operating pressures are finalized and system capacity for the winter is known."
Under its new Pipeline 2020 long-term revision of transmission safety and maintenance programs, PG&E assessed the need for automatic or remote shutoff valves to replace the numerous manual ones on its transmission pipelines. The preliminary analysis identifies at least 300 potential locations for the added equipment. These findings will be reviewed by the CPUC staff and a third-party natural gas expert that the utility will retain.
As part of the overall Pipeline 2020 program, PG&E also has hired another third-party firm to review and refine its preliminary analysis.
Kirk Johnson, vice president for gas engineering and operations, said PG&E has been "working intensely" to reexamine its transmission system and pipeline operational practices. "[This] filing shows how much progress we have made in a short period of time, but it also points to significant work still ahead of us."
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