Verdict: pipe walls thinned due to corrosion that went uninspected for years

Published:  11 April, 2014

CSB releases analysis showing cause of the rupture and hydrogen blast in 2009 Silver Eagle Refinery accident in Woods Cross, Utah.

A massive explosion and fire at the Silver Eagle Refinery on November 4, 2009, in Woods Cross, Utah, which damaged homes in a nearby neighbourhood, was caused by a rupture in a pipe that had become dangerously thin from corrosion, the US Chemical Safety Board (CSB) has reported.  The CSB has released a detailed expert metallurgical report that was commissioned in the course of its ongoing investigation of the incident.

The catastrophic rupture occurred in a ten-inch pipe at the bottom of a reactor in the Mobil Distillate Dewaxing unit. It led to a massive release of hydrogen, which caught fire immediately and exploded, sending a blast wave across the refinery into a subdivision. The blast wave damaged over 100 homes, many with shattered glass. Two of the homes were severely damaged, including one which was displaced off its foundation.

See video of the explosion here:

There were four workers near the process unit at the time of the explosion. They were blown to the ground but were not seriously injured. Another worker had been taking readings next to the pipe that failed just one to two minutes before the release.

The metallurgical failure study and analysis performed for the CSB by Exponent - a Texas-based engineering and scientific consulting company - details findings from laboratory examination of pipe segments recovered after the incident (read the analysis here). The report also examines the history of the pipe that ruptured, determining that the component that failed had no record of ever being inspected for corrosion as it thinned over the years.

CSB Chairman Rafael Moure-Eraso commented, “The findings in the Exponent report are all too familiar: Mechanical integrity programs at refineries repeatedly primarily emphasise inspection strategies rather than the use of inherently safer design to control the damage mechanisms that ultimately cause major process safety incidents.  This is the same syndrome we found in the Bay Area Chevron refinery fire of 2012 and the Tesoro refinery explosion and fire that killed seven in Anacortes, Washington, in 2010. Fortunately, there were no fatalities resulting from the explosion and it was only by chance no one was in the immediate area. But many lives were disrupted as residents in Woods Cross, just north of Salt Lake City, had to move out of homes pending repairs.”

CSB Investigation Lead Dan Tillema said, “The metallurgical analysis details the same kind of sulfidation corrosion at the Silver Eagle Refinery that we found in the Chevron accident; sulfur compounds in the process stream corroded a steel piping segment, causing the pipe walls to become severely thin.  This incident is also similar to Chevron in that, while sulfidation is a well-known damage mechanism at refineries that requires regular inspection and monitoring, the segment that failed has no record of ever being inspected.”

The CSB investigation team notes that the examination of the ruptured pipe segment and adjacent piping clearly indicated wall thinning had occurred in the piping component.  The elbow adjacent to the pipe segment that failed was noted to have an original thickness of 0.719-inch.  A 2007 thickness measurement of the elbow indicated a wall thickness of 0.483-inch, indicating years of thinning had taken place.  The adjacent straight-run segment that failed was found to have a wall thickness as low as 0.039-inch and there were no records of any previous inspection.  The CSB’s investigation previously noted records indicating other serious widespread mechanical integrity deficiencies and gaps across the refinery at the time of the incident and will address these issues in the final report.

Dr. Moure-Eraso said, “This is an investigation where we have had to delay its completion due to, ironically, a pressing series of accidents in the oil production and refining sector.  However, I want people to know that work has been continuing as this report shows, and that the CSB is working hard to assure refineries and indeed all chemical operations are operated more safely.”

The November explosion was the second accident at the refinery that year. On the evening of January 12, 2009, two refinery operators and two contractors suffered serious burns resulting from a flash fire. The accident occurred when a large flammable vapour cloud was released from an atmospheric storage tank which contained an estimated 440,000 gallons of light naphtha. The vapour cloud found an ignition source and the ensuing flash fire spread up to 230 feet west of the tank farm.

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