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Husky Energy Superior Refinery Explosion and Fire

Overview

During shutdown of the fluid catalytic cracking (FCC) unit at Husky Energy Superior Refinery, air was inadvertently introduced into hydrocarbon-containing equipment. An explosion occurred in the primary and sponge absorbers. Vessel fragmentation propelled debris that punctured asphalt storage Tank 101. A major asphalt release and fire followed. The incident injured 36 workers, caused approximately $550 million in property damage, and led to evacuation and shelter-in-place actions.

Incident Snapshot

Field Value
Facility / Company Husky Energy Superior Refinery (now Cenovus Superior Refinery)
Location Superior, WI
Incident Date 04/26/2018
Investigation Status The CSB's investigation was approved December 23, 2022.
Accident Type Oil and Refining - Fire and Explosion
Final Report Release Date 12/29/2022

What Happened

  • Operators began shutting down the FCC unit on the morning of April 26, 2018 during a planned turnaround.
  • Air flowed from the regenerator through the reactor and main column into the gas concentration unit during shutdown.
  • Oxygen accumulated and mixed with hydrocarbon vapors inside the primary and sponge absorbers, and pyrophoric material provided an ignition source.
  • Just before 10:00 a.m., the primary and sponge absorber vessels exploded.
  • Explosion debris struck asphalt storage Tank 101 and punctured the tank wall.
  • Around 12:00 p.m., the asphalt ignited at the storage tank.
  • The main asphalt fire was extinguished shortly before 7:00 p.m.
  • Evacuation orders were lifted at 6:00 a.m. the next day.

Facility and Process Context

  • Fluid catalytic cracking (FCC) unit
  • primary absorber
  • sponge absorber
  • main column
  • gas concentration unit (GasCon)
  • reactor
  • regenerator
  • spent catalyst slide valve
  • regenerated catalyst slide valve
  • main air blower
  • auxiliary air blower
  • wet gas compressors
  • asphalt storage Tank 101
  • debutanizer
  • flare system
  • reactor steam barrier
  • main column gas purge

Consequences

  • Fatalities: 0
  • Injuries: 36 refinery and contract workers were injured and sought medical attention, including 11 OSHA recordable injuries.
  • Environmental release: Husky Superior Refinery reported that it released 39,000 pounds of a flammable hydrocarbon vapor mixture and approximately 17,000 barrels of asphalt during the event.
  • Facility damage: Approximately $550 million in property damage occurred; the explosion fractured the primary and sponge absorbers and propelled over 100 metal fragments into the refinery; one fragment punctured asphalt storage Tank 101.
  • Operational impact: The refinery was shut down for a turnaround; fires were extinguished before midnight; evacuation orders were lifted the next day.

Key Findings

Immediate Causes

  • An inadvertent introduction of air into hydrocarbon vapor resulted in an explosion.
  • Air flowed from the regenerator, through the reactor and main column, into the gas concentration unit, where it mixed with hydrocarbon and exploded inside the equipment.
  • The oxygen also reacted with existing pyrophoric material inside this equipment, creating the ignition source for the explosion.

Contributing Factors

  • A severely eroded spent catalyst slide valve allowed more air to pass from the regenerator into the reactor.
  • The refinery's shutdown procedure instructed operators to maintain the regenerator at a higher pressure than the reactor, which drove air into process equipment that contained flammable material.
  • The shutdown procedure did not include instructions for purging air out of the main column.
  • The refinery did not have a means to detect the presence of oxygen in its equipment during the FCC unit shutdown.
  • The refinery's approach to minimize flaring likely contributed to the wet gas compressor's continued operation during the shutdown.
  • The refinery's process hazard analyses did not effectively identify or control hazards inherent in FCC unit transient operation.
  • The refinery's operator training program did not effectively prepare the operators to shut down the FCC unit safely.
  • The process vessels that exploded were constructed from a grade of steel that was susceptible to brittle fracture, contributing to the severity of the incident.
  • The fire that resulted from the refinery's inability to contain and control the hot, ignitable asphalt spill contributed to the severity of the incident.
  • The refinery's slide valve standard did not properly describe the flow isolation capability of slide valves.
  • The refinery had normalized or otherwise accepted the spent catalyst slide valve's erosion rate over its five-year turnaround cycles.
  • The refinery did not identify or control the potential explosion hazard from accumulating air within the gas concentration unit during extended wet gas compressor operation.

Organizational and Systemic Factors

  • Husky Superior Refinery's FCC technology-specific process knowledge was not sufficient to safely shut down the FCC unit for a turnaround.
  • Husky Superior Refinery was not aware that its FCC unit shutdown procedure was not aligned with the technology licensor's guidance.
  • Husky Superior Refinery did not perform a technical review of its FCC unit operating procedures with its process engineers, the licensor, or outside consultants for at least 25 years prior to the incident.
  • Essential information describing how to shut down an FCC unit safely was not incorporated into the refinery's operating procedures, process hazard analysis, and operator training material.
  • Husky Superior Refinery's policies did not include requirements or guidelines for performing PHAs on its operating procedures.
  • Husky Superior Refinery's response to the 2013 and 2016 PSM and RMP compliance audit findings regarding the overall quality of the refinery's operating procedures was ineffective.
  • Husky Superior Refinery's operator training program did not effectively prepare the FCC operators to shut down the unit safely or respond to abnormal situations properly.
  • Husky Superior Refinery's use of external technical experts was limited to assessing and improving FCC unit performance during normal operation.
  • Husky Superior Refinery's policies did not require periodic technical review of its critical operating procedures by multidisciplinary teams.
  • Husky Superior Refinery's FCC technology-specific process knowledge did not adequately address unit safety in transient operations, including shutdown for a turnaround.

Failed Safeguards or Barrier Breakdowns

  • The energy dissipation capacity during brittle crack propagation was very limited compared to the available energy.
  • Husky Superior Refinery did not establish or maintain a reactor steam barrier between air and hydrocarbons during the shutdown.
  • Husky Superior Refinery did not properly purge the main column overhead receiver during the shutdown to remove oxygen from the system.
  • Husky Superior Refinery relied entirely on catalyst slide valves to keep the air and hydrocarbon systems separated during the shutdown.
  • The refinery's process hazard assessments and layer of protection analysis study did not identify that the slide valves alone were not adequate safeguards to prevent an incident of this magnitude.
  • Husky Superior Refinery did not have a means to detect the presence of oxygen in its equipment during the FCC unit shutdown.
  • The refinery's differential pressure instrument was not configured to measure or display the negative numbers needed to establish and maintain the steam barrier during a shutdown.
  • The original plant design did not allow for operators to establish a proper reactor steam barrier as UOP described in its General Operating Manual.
  • The spent catalyst slide valve lost differential pressure.
  • The steam barrier was compromised.

Recommendations

  1. 2018-02-I-WI-R1 | Recipient: Cenovus Superior Refinery | Status: Closed – Acceptable Action | Establish safeguards to prevent explosions in the FCC unit during transient operation (including startup, shutdown, standby, and emergency procedures). Incorporate these safeguards into written operating procedures, including a reactor steam barrier or similar inert gas flow, main column purging with non-condensable gas, monitoring of oxygen and critical operating parameters, documentation of consequences of deviation, and inclusion in operator training curricula.
  2. 2018-02-I-WI-R2 | Recipient: Cenovus Superior Refinery | Status: Closed – Acceptable Action | Based on licensor input and good industry practices, determine the appropriate point(s) in the FCC unit's shutdown procedures to shut down all wet gas compressor(s) and incorporate this information into shutdown procedures and operator training material.
  3. 2018-02-I-WI-R3 | Recipient: Cenovus Superior Refinery | Status: Open – Acceptable Response or Alternate Response | Develop and implement a slide valve mechanical integrity program that addresses erosion and ensures proper functioning of the slide valves during shutdown, including monitoring, inspection, acceptance criteria, process data analysis, preventive maintenance, and evaluation during turnarounds.
  4. 2018-02-I-WI-R4 | Recipient: Cenovus Superior Refinery | Status: Closed – Acceptable Action | Develop emergency procedures for responding to a loss of catalyst slide valve function, such as excessive leakage or failure to close on demand.
  5. 2018-02-I-WI-R5 | Recipient: Cenovus Superior Refinery | Status: Closed – Acceptable Action | Develop guidance for analyzing operating procedures to improve transient operation hazard evaluations during PHAs and incorporate the guidance into PHA procedural documents and policies.
  6. 2018-02-I-WI-R6 | Recipient: Cenovus Superior Refinery | Status: Closed – Acceptable Action | Develop and implement an FCC unit operator, supervisor, and manager training program based on licensor guidance and available industry guidance, including written training materials and simulator training for board operators.
  7. 2018-02-I-WI-R7 | Recipient: Cenovus Superior Refinery | Status: | Incorporate lessons learned from this incident into Emergency Response Team training materials, including the proper response to liquids potentially stored above their flash point and the ignition risk of pyrophoric material inside asphalt storage tanks.
  8. 2018-02-I-WI-R8 | Recipient: Cenovus Energy | Status: Closed – Acceptable Action | For all Cenovus operated refineries with FCC units, develop and implement an FCC unit-specific PHA guidance document as part of each FCC unit's ongoing PHA update/revalidation cycle, updated with new industry knowledge and including transient operation safeguards.
  9. 2018-02-I-WI-R9 | Recipient: Cenovus Energy | Status: Closed – Acceptable Action | Develop and implement a technology-specific knowledge-sharing network program across all Cenovus operated refineries, including an FCC technology peer network that meets at least annually to discuss process safety topics.
  10. 2018-02-I-WI-R10 | Recipient: Cenovus Energy | Status: Closed – Acceptable Alternative Action | Include and maintain the FCC technology licensors' operating manuals in the process safety information packages for all FCC units.
  11. 2018-02-I-WI-R11 | Recipient: Occupational Safety and Health Administration (OSHA) | Status: Open – Awaiting Response or Evaluation/Approval of Response | Develop guidance documents for performing process hazard analysis on operating procedures to address transient operation hazards in facilities with Process Safety Management (PSM) covered processes.
  12. 2018-02-I-WI-R12 | Recipient: Environmental Protection Agency (EPA) | Status: Open – Acceptable Response or Alternate Response | Develop a program that prioritizes and emphasizes inspections of FCC units in refineries that operate HF alkylation units and verify FCC unit safeguards that prevent explosions during transient operation.
  13. 2018-02-I-WI-R13 | Recipient: American Petroleum Institute (API) | Status: Closed – Acceptable Alternative Action | Develop a publicly available technical publication for the safe operation of FCC units, covering hazards, safeguards, monitoring, emergency operating procedures, PHA guidance, training topics, process safety assessments, and lessons learned from this incident and the ExxonMobil Torrance investigation.
  14. 2018-02-I-WI-R14 | Recipient: American Petroleum Institute (API) | Status: Open – Acceptable Response or Alternate Response | Modify the appropriate existing recommended practice to include information about the purpose, design, maintenance, and testing of additional FCC catalyst slide valve components, including the slide valve body.
  15. 2018-02-I-WI-R15 | Recipient: American Petroleum Institute (API) | Status: Open – Acceptable Response or Alternate Response | Incorporate lessons learned from the incident into appropriate API products, including the flammability of heated asphalt and the ignition risk of pyrophoric material inside asphalt storage tanks.
  16. 2018-02-I-WI-R16 | Recipient: Honeywell UOP | Status: Closed – Acceptable Alternative Action | Participate in the API committee that develops a technical publication for the safe operation of FCC units.

Key Engineering Lessons

  • FCC unit shutdowns require explicit safeguards to prevent air and hydrocarbon mixing during transient operation, not reliance on catalyst slide valves alone.
  • A reactor steam barrier or similar inert gas barrier, together with main column purging and oxygen monitoring, is needed to control transient shutdown hazards.
  • Slide valves can erode and lose their ability to maintain the catalyst barrier; their mechanical integrity must be monitored and managed.
  • Differential pressure instrumentation must be capable of showing the pressure relationships needed to verify the steam barrier during shutdown.
  • Operating procedures and PHA reviews must address transient operation scenarios, including shutdown, standby, and emergency conditions.
  • Operator training should be based on licensor guidance and include hands-on or simulator practice for FCC transient operations.
  • Heated asphalt stored above its flash point is ignitable, and pyrophoric material inside asphalt tanks can be an ignition source after tank damage.

Source Notes

  • Priority 1 final report facts were used as the primary authority for causes, consequences, and recommendations.
  • Priority 3 recommendation status summaries were used to update recommendation statuses and implementation details.
  • Priority 4 supporting documents were used only where they added incident sequence or context consistent with the higher-priority final report.
  • Where source documents differed on exact wording or detail, the higher-priority final report terminology was preferred.

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