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Pipe Corrosion Blamed in UT Blast

Friday, April 18, 2014

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Pipeline walls thinned by corrosion and uninspected for over 20 years caused a major explosion at a Utah refinery—a scenario that has become “all too familiar,” federal officials announced.

When a 10-inch pipe failed in November 2009 at the Silver Eagle Refinery in Woods Cross, UT, it caused a powerful blast that damaged 100 nearby homes and knocked four workers to the ground.

The workers were not seriously injured, but two homes were severely damaged, including one that was knocked off its foundation.

Silver Eagle Refinery
Security video footage obtained by the CSB

It took less than a second for the pipe the fail and the explosion to occur, releasing hydrogen and other gases and damaging 100 nearby homes. The circle indicates the area of the pipe rupture.

In a metallurgical report released April 10, the U.S. Chemical Safety Board determined that the pipe had become "dangerously thin" from corrosion, leading to a massive release of hydrogen, which caught fire immediately and exploded. The pipe was located at the bottom of a reactor in the mobile distillate dewaxing unit.

According to the report, the pipe segment that failed had no record of ever being inspected for corrosion in the 21 years since it had been installed.

It was the second incident at the plant that year. On Jan. 12, 2009, four people were injured in a flash fire after a large flammable vapor cloud was released from an atmospheric storage tank.

The failure study and analysis was performed by Exponent Failure Analysis Associates, a Texas-based engineering and scientific consulting company.

'Same Syndrome'

"The findings in the Exponent report are all too familiar," said CSB Chairman Rafael Moure-Eraso.

"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," Moure-Eraso said.

Examination of the ruptured Utah pipe segment and adjacent piping "clearly indicates" wall thinning, the investigation team said.

The failed segment had a wall thickness of 0.039 inch.

"Mechanical integrity programs at refineries repeatedly primarily emphasize inspection strategies, rather than the use of inherently safer design, to control the damage mechanisms that ultimately cause major process safety incidents," Moure-Eraso said.

Delayed Investigation

A rash of such incidents in recent years, in fact, slowed the Utah investigation, 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," he said.

Although the completed report was released April 10, it was submitted to the CSB in June 2013.

"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," Moure-Eraso said.

pipe corrosion
Exponent Failure Analysis Associates via CSB

Investigators discovered one end of the ruptured pipe wrapped around a reactor support beam.

Silver Eagle Refinery's website says the company is committed to safety and has "embarked" since the accident "on a bolt-by-bolt, pipe-by-pipe, thought-by-thought, action-by-action reevaluation of all its operations."

"Silver Eagle is now working constantly and effectively with a total commitment to safe thinking, safe actions, safe practices and safe processes. We understand that nothing is more important than the safety and wellbeing of our employees, our neighbors, our communities and our environment."

Multiple Gases Released

The elbow adjacent to the failed pipe had an original thickness of 0.719-inch. A 2007 measurement found a wall thickness of 0.483 inch, "indicating years of thinning had taken place," the report said.

After the rupture, high-pressure gas escaped from both ends of the failed pipe segment. One end ended up wrapped around a reactor support beam; the other was attached to the bottom of the reactor.

Only 0.66 seconds elapsed between the rupture and the explosion, according to the report, citing a security video. This time was used to calculate the amount of each type of gas released at the time of the explosion, which primarily included hydrogen, followed by nitrogen, methane, and hydrocarbons.

1966 Structure

The piping was installed and put into service in 1993, and the reactor pressure vessel was originally constructed in 1966.

Silver Eagle Refinery

Parts of the system were installed in 1993, but "no documentation has been produced indicating that the wall thickness of the straight segment was ever measured from build date to the time of the incident," the report said.

However, the report says it could not be determined if the elbow and straight segments were new when they were installed in 1993, or if they had been used with the reactor pressure vessel when it previously functioned as a lube stock hydrotreater.

"Regardless, to date no documentation has been produced indicating that the wall thickness of the straight segment was ever measured from build date to the time of the incident," the report noted.

Years of Thinning

Ultrasonic testing readings from 1993 on the elbow found thicknesses ranging from 0.671 to 0.805 inches. An October 2007 inspection sheet noted that the section had an "original thickness" of 0.791 inch.

It is unknown how the inspector determined the original thickness, since no documents or design drawings were ever produced, the report said.

By the time of the October 2007 inspection, the elbow was showing wall thinning from its 14 years in service, measuring 0.483 inch in some locations.

According to the report, the primary cause of the thinning was sulfidation corrosion; turbulent flow at the elbow likely caused the most significant thinning at the rupture location just downstream of the elbow, it said.


Tagged categories: Chemical attack; Corrosion; Explosions; Inspection; Oil and Gas; Pipeline; U.S. Chemical Safety Board

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