FCC UNIT EXPLOSION — ASPHALT TANK FIRE — $550M DAMAGE — MASS COMMUNITY EVACUATION
Husky Energy Superior Refinery FCC Unit Explosion and Asphalt Fire
Husky Energy (now Cenovus Energy)
📍 Superior, WI
Incident: April 26, 2018  •  CSB Report: December 23, 2022
0
Fatalities
Hydrocarbon Vapors / Hot Asphalt (FCC Unit — Fluid Catalytic Cracking)
Chemical Involved
21
CSB Recommendations
📋 Incident Summary

On April 26, 2018, a malfunctioning slide valve in the fluid catalytic cracking (FCC) unit at the Husky Energy Superior Refinery in Superior, Wisconsin, allowed air from the unit's regenerator to mix with hydrocarbons and travel through the main column and into the gas concentration unit, where the mixture found an ignition source and exploded. Two vessels in the FCC unit catastrophically failed, propelling metal fragments up to 1,000 feet. One fragment punctured an adjacent hot asphalt storage tank, causing 17,000 barrels of asphalt to spill and ignite, producing multiple large fires.

Thirty-six workers were injured. More than 2,500 residents of the City of Superior were evacuated from their homes, and the City of Duluth, Minnesota, issued a shelter-in-place order. Estimated property damage exceeded $550 million — one of the costliest refinery incidents in U.S. history.

The CSB identified six key safety issues: inadequate maintenance of process safety information for the FCC unit, deficient operating procedures and operator training for transient operations, failure to implement transient operation safeguards, inadequate emergency preparedness, susceptibility to brittle fracture, and gaps in industry-wide guidance for FCC unit operations.

🔎 Key Findings
Finding 01
Slide Valve Malfunction Allowed Air-Hydrocarbon Mixing in FCC Unit
A malfunctioning slide valve failed to maintain separation between the regenerator (air) and reactor (hydrocarbon) sides of the FCC unit during a transient operation. Air was inadvertently directed into the hydrocarbon side, forming a flammable mixture that traveled through the main column to the gas concentration unit where it ignited.
Finding 02
Two FCC Vessels Catastrophically Exploded — Debris Struck Adjacent Asphalt Tank
Two vessels in the FCC unit catastrophically failed due to the internal deflagration, propelling metal fragments up to 1,000 feet. One fragment struck a nearby hot asphalt storage tank, breaching it and causing 17,000 barrels of asphalt at high temperature to spill and ignite, creating multiple large secondary fires.
Finding 03
Process Safety Information for the FCC Unit Was Inadequate
Husky Energy did not maintain adequate process safety information for its FCC unit, including documentation of the unit's operating envelope during transient operations and the consequences of process deviations such as air-hydrocarbon mixing events.
Finding 04
Operating Procedures and Training for Transient Operations Were Deficient
Husky's operating procedures and operator training for the FCC unit did not adequately address the hazards of transient operations — the periods during startup, shutdown, and non-routine operations when the risk of process deviations is highest.
Finding 05
$550M in Property Damage — 2,500+ Residents Evacuated
The explosion and asphalt fire caused an estimated $550 million in on-site property damage and required the evacuation of more than 2,500 residents of Superior, Wisconsin, and a shelter-in-place order for the City of Duluth, Minnesota — demonstrating the severe community consequence potential of refinery incidents.
🔍 Root Causes
1
Malfunctioning Slide Valve Allowed Air-Hydrocarbon Contact During Transient Operation
The initiating event was a slide valve malfunction that allowed air from the FCC regenerator to reach the hydrocarbon-containing side of the unit. The malfunction occurred during a transient operation — a period of elevated process risk — without adequate procedural or engineering safeguards to prevent or detect the air ingress.
2
Transient Operation Safeguards Were Not Implemented
Husky Energy had not implemented engineering or procedural safeguards specifically designed to protect against air-hydrocarbon mixing events during FCC unit transient operations. Industry knowledge of this specific hazard existed, but Husky had not translated that knowledge into actionable controls at the Superior refinery.
3
Brittle Fracture Susceptibility Contributed to Vessel Failure Severity
The CSB identified that susceptibility to brittle fracture in FCC unit vessels contributed to the severity of the catastrophic failure — a materials engineering and inspection issue that is relevant to FCC units industrywide.
☑ CSB Recommendations
→ Husky Energy / Cenovus Energy
Develop and implement comprehensive process safety information, operating procedures, and operator training specifically addressing the hazards of FCC unit transient operations, including air ingress scenarios and slide valve function during non-routine operations.
→ Husky Energy / Cenovus Energy
Implement engineering safeguards to prevent or detect air-hydrocarbon mixing in the FCC unit during transient operations; conduct a formal process hazard analysis of FCC unit transient operation scenarios.
→ American Petroleum Institute (API)
Develop or strengthen API standards and guidance for FCC unit transient operation hazards, including air ingress event prevention, detection, and mitigation; address brittle fracture susceptibility in FCC unit vessel management.
→ State and Federal Regulators
Review regulatory frameworks for major refinery process units to ensure requirements adequately address transient operation hazards, emergency preparedness for off-site community impacts, and brittle fracture risk management.
💡 Lessons Learned
FCC units are among the highest-consequence process units in petroleum refining — and their greatest hazard window is during transient operations: startups, shutdowns, and non-routine operating modes. The Husky explosion occurred during a transient condition that the facility had not adequately characterized in its PSI, procedures, or training. Transient operations require the same rigor as normal operations — and often more.
A single equipment failure cascaded from a slide valve malfunction to two vessel explosions to a secondary asphalt tank fire — illustrating how process safety incidents rarely stop at their initial cause. The 17,000-barrel asphalt fire resulted from FCC debris striking an adjacent tank, not from a process deficiency at that tank. Siting and debris-trajectory analysis must be part of facility risk assessments for high-energy processes.
The evacuation of 2,500 residents and the shelter-in-place order for a neighboring city confirm that refinery explosions are not contained events. Refineries are embedded in communities, and the consequence of a major process incident extends far beyond the facility fence. Emergency preparedness planning must account for large-scale community evacuation scenarios — and be coordinated in advance with local emergency management.
The $550 million property damage figure from a single FCC unit incident demonstrates the scale of financial consequence in petroleum refining. Process safety investment — in procedure quality, training depth, and transient operation safeguards — is not a cost center; it is protection against consequences that no insurance policy fully covers.
Brittle fracture susceptibility in aging process vessels is a Mechanical Integrity issue that can turn a manageable incident into a catastrophic one. Facilities operating aging FCC units and high-energy process equipment should include fracture toughness assessment in their MI programs — particularly for vessels that could fail in a way that produces high-velocity debris.
PSI: Process Safety InformationSOP: Operating ProceduresTRN: TrainingMI: Mechanical IntegrityPHA: Process Hazard AnalysisEP: Emergency Planning
🔨 Safety Meeting Toolbox Talk
►Does your facility have documented process safety information specifically covering the hazards of transient operations — startups, shutdowns, and non-routine modes — for your highest-energy process units?
►When did your facility last conduct a formal PHA of its FCC unit or equivalent high-energy process unit that specifically examined transient operation scenarios and air-hydrocarbon mixing hazards?
►Has your facility conducted a debris-trajectory and siting analysis for high-energy process units that could produce projectiles in a catastrophic failure? Are adjacent tanks, equipment, and community infrastructure included in that analysis?
►Does your emergency preparedness plan address large-scale community evacuation scenarios and include pre-coordination with local emergency management agencies? When was the last joint drill conducted?
Immediate Action Items
✓Identify all transient operations in your highest-hazard process units and confirm that current operating procedures specifically address the hazards unique to those non-routine periods; revise procedures where transient hazards are not explicitly addressed.
✓Review your most recent PHA for each major process unit and confirm that transient operation scenarios — startup, shutdown, equipment failure during transition — were systematically evaluated; schedule revalidation if the last PHA predates significant operational changes.
✓Conduct a siting and consequence analysis for catastrophic failures in high-energy process units, including debris trajectory modeling for vessels that could fail energetically; ensure adjacent equipment and community areas are considered.
✓Contact your local emergency management authority and confirm that your facility's emergency plan and community notification procedures are current and have been exercised within the last 12 months.
🔗 PSM Failures Behind This Incident

This incident traced to breakdowns across 6 PSM elements (PSI · SOP · TRN · MI · PHA · EP). Each represents a documented gap that process safety documentation and consulting can close before a similar event occurs at your facility.

Process Safety Information (PSI)
Accurate, complete Process Safety Information is the foundation every other PSM element depends on. When PSI is missing or wrong — chemistry data, equipment specs, P&IDs — the entire hazard analysis is built on a flawed base.
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Operating Procedures (SOPs)
Operators cannot reliably hold safe operating limits without clear, current, enforced procedures. Deviation from acceptable operating conditions — a root cause here — is a direct consequence of SOP failure.
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Training & Operator Competency
Workers must understand process hazards — not just the steps on the page. Training records, refresher frequency, and verified competency are all OSHA PSM requirements that gaps here violated.
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Mechanical Integrity (MI)
Equipment must be designed, inspected, and maintained to operate safely in its intended service. Mechanical integrity failures — degraded equipment, missed inspections, deferred repairs — contributed to loss of containment here.
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Process Hazard Analysis (PHA)
A structured PHA or HAZOP study exists to identify exactly these scenarios before they occur. When PHA is absent, superficial, or overdue for revalidation, hazards operate unseen until they kill someone.
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Employee Participation
OSHA PSM requires workers to be meaningfully involved in hazard analyses and procedure development — not just trained on the finished product. Active participation catches gaps that management alone misses.
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