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Silver Eagle Refinery Flash Fire and Explosion and Catastrophic Pipe Explosion

Overview

The incident involved two related refinery accidents at Silver Eagle Refinery in Woods Cross, Utah. On January 12, 2009, a large flammable vapor cloud was released from atmospheric storage tank 105 containing light naphtha. The vapor cloud found an ignition source and caused a flash fire. Two refinery operators and two contractors were seriously burned.

On November 4, 2009, a 10-inch pipe off the bottom of a reactor in the Mobil Distillate Dewaxing Unit failed during hot hydrogen catalyst regeneration. Hydrogen gas was released, ignited, and caused a large explosion and fireball. The final report identified sulfidation corrosion as the primary cause of wall thinning in the ruptured piping.

Incident Snapshot

Field Value
Facility / Company Silver Eagle Refinery
Location Woods Cross, UT
Incident Date 01/12/2009
Investigation Status The CSB issued a technical report on April 14, 2014, and formally closed the investigation at a public meeting on January 28, 2015. The incident included a January 12, 2009 flash fire and a November 4, 2009 catastrophic pipe explosion at the Silver Eagle Refinery in Woods Cross, Utah.
Accident Type Oil and Refining - Fire and Explosion Investigation
Final Report Release Date 04/14/2014

What Happened

  • On the evening of January 12, 2009, a large vapor cloud was released from atmospheric storage tank 105, which contained an estimated 440,000 gallons of light naphtha.
  • The vapor cloud found an ignition source and the ensuing flash fire spread up to 230 feet west of the tank farm.
  • Two refinery operators and two contractors suffered serious burns and were hospitalized.
  • On November 4, 2009, an explosion occurred in the Mobil Distillate Dewaxing Unit (MDDW) of the Silver Eagle refinery.
  • The unit was undergoing hot hydrogen regeneration of the catalyst at the time of the explosion.
  • The catalyst regeneration procedure began at 9:00 p.m. and the reactor inlet temperature rose from 680°F to 806°F by 3:00 a.m.
  • At 9:11 a.m., approximately six hours into the procedure, the inlet temperature suddenly dropped and the exit temperature increased to over 1000°F.
  • This resulted from a sudden and complete rupture of the 10-inch diameter line at the exit of the South Reactor between 9:11 a.m. and 9:12 a.m.
  • High-pressure gas was released from both ends of the ruptured pipe and ignited shortly after rupture.
  • A shock wave from the resulting explosion expanded through the adjacent neighborhood and caused blast damage to residential homes.

Facility and Process Context

  • Silver Eagle refinery, located in Woods Cross, Utah.
  • Atmospheric storage tank 105 contained an estimated 440,000 gallons of light naphtha.
  • The Mobil Distillate Dewaxing Unit (MDDW) contained two catalyst bed reactors: the South Reactor (RTR30101) and the North Reactor (RTR30103).
  • The pipe failure occurred at the bottom of South Reactor R30101 and the discharge line ran from the bottom of R30101 up to Heat Exchanger EXC-30115.
  • The unit typically operated in the temperature range of 500°F to 800°F and was rated to 1000 psi at 800°F at the time of the incident.
  • The nominal system pressure was reported to be 625 psi at 735°F.
  • The incident piping was installed and put into service in the MDDW with the South Reactor in 1993.
  • The reactor pressure vessel was originally constructed in 1966.
  • On January 12, 2009, tank 105 was receiving up to three different streams of hydrocarbon liquids from the refinery, including light or low-boiling substances.
  • On November 4, 2009, the MDDW was undergoing hot hydrogen regeneration of the catalyst.

Consequences

  • Fatalities: None reported.
  • Injuries: On January 12, 2009, two refinery operators and two contractors suffered serious burns and were hospitalized. On November 4, 2009, four workers near the process unit were blown to the ground but were not seriously injured.
  • Environmental release: A large vapor cloud was released from tank 105 on January 12, 2009. On November 4, 2009, high-pressure hydrogen gas was released from the ruptured 10-inch pipe.
  • Facility damage: The January 12 flash fire damaged two structures, a shed and a lab facility, and spread up to 230 feet west of the tank farm. The November 4 explosion caused explosion damage to light structural elements on the adjacent hydro pad, and a section of the pipe wrapped itself around a steel support. The blast wave damaged over 100 homes, including two severely damaged homes.
  • Operational impact: The refinery was asked to shut down temporarily after the November 4 accident; four of the refinery’s five processing units were shut down and the fifth was to be brought down in the coming days.

Key Findings

Immediate Causes

  • The evidence is currently consistent with sulfidation corrosion as the primary cause of the wall thinning in the 10-inch piping that resulted in the rupture.
  • The rupture was in the straight segment base metal.
  • The vapor cloud found an ignition source.
  • The explosion occurred when a 10-inch pipe failed catastrophically.

Contributing Factors

  • Turbulent flow at the elbow likely caused the most significant wall thinning at the rupture location just downstream of the elbow.
  • The low silicon, chromium, and molybdenum contents of the straight segment, and the service temperatures and process stream chemistry, are all consistent with sulfidation corrosion.
  • It appears that corrosion monitoring of the straight segment was not conducted between 1993 and the time of the rupture.
  • The pipe showed evidence of significant thinning.
  • The significant thinning had not been detected by the refinery’s mechanical integrity program.
  • A lack of required documentation.
  • A lack of needed calculations of the fitness for service of various pieces of equipment.
  • The unit was undergoing a special operation to regenerate the catalyst.
  • Circulating high-pressure hydrogen gas inside the piping at 800 degrees Fahrenheit and 630 psi.
  • The pipe was open and pointing directly east toward the housing subdivision next to the refinery.
  • Vapor was seen escaping from atmospheric vents on the west side of tank 105.
  • Tank 105 was receiving up to three different streams of hydrocarbon liquids from the refinery, including light or low-boiling substances.
  • The primary feed into tank 105 had been sent from the #1 crude unit pre-flash accumulator for approximately three weeks prior to the incident.
  • Feeding tank 105 directly from this unit was a recent process change.
  • Workers were purging equipment with nitrogen to remove flammable liquid, with the intent to pressure the liquid into tank 105.
  • Reports from plant personnel indicated a history of vapor leaks from tank 105 both prior to and following tank repairs.
  • The floating roof on tank 105 may not have been equipped with the appropriate seal for use in the storage of light hydrocarbons of the type sent to the tank.
  • Possible gaps between the seal and the inner tank wall.
  • Possible integrity issues with the seal.
  • Possible design and structural integrity issues of the tank.
  • An open-flame furnace near the pipe was a likely ignition source.

Organizational and Systemic Factors

  • The refinery’s mechanical integrity program had serious deficiencies.
  • Prior to May 2009, mechanical integrity inspections, including thickness monitoring of pipes and vessels at the refinery, were completed by a contract company hired by Silver Eagle.
  • The refinery later replaced this company with a second outside inspection company, which remained active at the site.
  • The refinery was using ultimate tensile strength values rather than the industry-recommended stress tables.
  • A significant percentage of the pipes and vessels have no documented thickness readings at all.
  • Minimum thickness values for piping and equipment throughout the refinery have been miscalculated.
  • The refinery could be operating 17 years after the federal government enacted process safety regulations without an established and effective mechanical integrity program.
  • There was a lack of required documentation and a lack of needed calculations of the fitness for service of various pieces of equipment.
  • The CSB identified facility siting issues relating to the January 12 release and fire.
  • The CSB will be examining whether existing guidelines for the siting of hazardous facilities provide enough protection for residents.

Failed Safeguards or Barrier Breakdowns

  • corrosion monitoring of the straight segment
  • refinery’s mechanical integrity program
  • thickness monitoring of pipes and vessels
  • documented thickness readings
  • industry-recommended stress tables
  • process safety regulations
  • appropriate seal for use in the storage of light hydrocarbons
  • floating roof on tank 105
  • seal and the inner tank wall
  • design and structural integrity of the tank
  • existing guidelines for the siting of hazardous facilities

Recommendations

  1. Recommendation ID: Not provided
    Recipient: American Petroleum Institute
    Status: urgent recommendation
    Summary: update their guidelines for portable work buildings such as trailers

Key Engineering Lessons

  • Wall-thickness monitoring and fitness-for-service calculations must be adequate to detect severe thinning in high-temperature hydrogen service piping before rupture.
  • Corrosion mechanisms such as sulfidation corrosion must be evaluated using service temperature, process chemistry, and material composition for the specific piping segment.
  • Mechanical integrity programs must maintain documented thickness readings and effective inspection coverage for piping and vessels.
  • Tank venting, seal design, and floating-roof integrity must be appropriate for the specific hydrocarbon service being stored.
  • Recent process changes that alter feed composition or operating conditions should be reviewed for their impact on tank and process safety.

Source Notes

  • Priority 1 final report findings were used to resolve the November 4, 2009 pipe rupture cause and metallurgical conclusions.
  • Priority 4 supporting statements were used for the January 12, 2009 flash fire sequence, injuries, and preliminary mechanical integrity concerns where not contradicted by the final report.
  • The incident dataset combines two related Silver Eagle Refinery accidents described in the provided source extracts.
  • https://www.csb.gov/silver-eagle-refinery-flash-fire-and-explosion-and-catastrophic-pipe-explosion/

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