Home > Industrial > PG and E electrical work caused valve failure prior to pipe blast

PG and E electrical work caused valve failure prior to pipe blast

Knoxville, TN – In a preliminary report on its investigation into the fatal explosion of
a Pacific Gas and Electric natural gas transmission pipeline, the US National
Transportation Safety Board confirmed Wednesday that work the utility was
conducting at a terminal upstream of the blast caused a regulating valve to
fail on the line.

     The NTSB noted that just before the September 9 blast and fire, PG&E was
working on its uninterruptible power supply system at the Milpitas Terminal,
located about 39 miles southeast of the accident site along its
30-inch-diameter Line 132.

     During that work, the power supply from the system to the supervisory
control and data acquisition system malfunctioned, which caused the valve to
move from partially open to fully open. The pressure on the line then
increased to 386 psig. An over-protection valve, which was pneumatically
activated, then maintained the pressure at 386 psig.

     NTSB said the maximum operating pressure on Line 132 was 375 psig, but
NTSB noted PG&E said the maximum allowable operating pressure was 400 psig.

     Downstream of the blast site, the SCADA system at the Martin Station
exceeded 375 psig and rose to about 390 psig by 6 pm local time, NTSB said. At
6:08 pm it dropped to 386 psig. At 6:11 pm, the time of the explosion, the
pressure at the Martin Station dropped to 361.4 psig, and then to 289.9 psig a
minute later. The amount of gas released during the incident was 47.6 million
standard cubic feet, NTSB said.

     PG&E had previously confirmed that it was performing the work at
Milpitas, but had not released further details as to effects on its system.
The blast and resulting fire resulted in eight fatalities, more than 50
injuries and 37 homes destroyed.

      The preliminary report does not reach any conclusions about the cause
of the rupture, but it does list the state of PG&E’s system before and after
the incident and details laboratory work performed so far on the section of
damaged pipeline the agency removed from the site.

Story via Platts.com

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