By Braden Reddall
RICHMOND, California (Reuters) - The wall of a pipe figuring in the investigation of Chevron Corp's Richmond refinery fire last month had thinned to 20 percent of its original design thickness, the Chemical Safety Board (CSB) said on Tuesday.
While the cause of the thinning was not yet certain, Chevron had recently replaced another pipe in that part of the refinery, CSB Managing Director Daniel Horowitz said at a city council meeting in the northern California city.
"We know Chevron was concerned about corrosion in this section," he said.
The pipe had not been inspected with ultrasound during the refinery's maintenance shutdown in November 2011, even though Chevron's own best practice guidelines said it should have been, Horowitz said.
The August 6 fire at the only crude distillation unit at the plant, which can process 245,000 barrels of oil per day, meant thousands of nearby residents had to seek hospital attention and also caused a spike in regional gasoline prices.
California's second-largest refinery first spewed out a white vapor cloud that CSB officials said 18 workers had to escape from, before fire broke out and the cloud was replaced by a pitch-black plume visible all around San Francisco Bay.
A 4 foot (1.2 meter) section of carbon steel pipe, which had been carrying a hydrocarbon known as gas oil at a temperature of about 600 degrees Fahrenheit (315 degrees C), had been sent to a lab for further study, Horowitz said.
The pipe's wall had thinned to as little as 1/16th of an inch, versus 5/16th originally, and the thinning had occurred from the inside, he added.
CSB officials had a new video of the fire outbreak that they said gave a better view of how the vapor cloud dispersed, and which was played at the city council meeting, attended by about 50 residents and the refinery's general manager, Nigel Hearne.
Hearne offered no further detail on the investigation or the potential cause of the fire, but reiterated Chevron's regrets.
"Clearly we fell short of the high standard that we set ourselves," said Hearne, who took charge of the 110-year-old refinery this month a year ago.
(Reporting by Braden Reddall in Richmond; Editing by Clarence Fernandez and David Holmes)