Superior Refinery Explosion 2018: Causes, Failures, and Lessons Learned

Name: Transient Hazards: Explosion at the Husky Superior Refinery
Uploaded: 2026-02-11T06:36:41.626752+00:00
Channel: USCSB
Description: Summary and key takeaways on Superior Refinery Explosion 2018: Causes, Failures, and Lessons Learned, covering Incident Overview Date: April 26, 2018 Location

USCSB

Feb 11, 2026

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YouTube video ID: sFhkzK7jkKg

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Incident Overview

Date: April 26, 2018
Location: Superior Refinery, Superior, Wisconsin
Event: Large explosion during a maintenance shutdown punctured an asphalt tank, igniting a massive fire.
Impact: 36 workers injured, ~US$550 million in damage, 39,000 lb of flammable hydrocarbon vapor released, 2,500+ residents evacuated, shelter‑in‑place order in Duluth, MN.

FCC Unit Shutdown Procedure

The fluid catalytic cracker (FCC) cracks heavy hydrocarbons into lighter products.
Normal operation relies on a balanced pressure between the reactor (hydrocarbon side) and the regenerator (air side) using slide valves.
For shutdown, operators stopped hydrocarbon flow, closed both slide valves, and halted catalyst circulation.
One slide valve was eroded; catalysts fell through, leaving an open path for air to flow backward into the reactor.

Chain of Events Leading to the Explosion

Air Back‑flow: Without a reactor steam barrier, air from the regenerator entered the reactor and downstream equipment, creating a flammable air‑hydrocarbon mixture.
Ignition: Around 10:00 a.m., the mixture found an ignition source, causing a large explosion.
Debris Impact: Debris traveled ~200 ft, puncturing an above‑ground asphalt storage tank.
Asphalt Spill & Fire: Hot asphalt spilled, flowed through the plant, and ignited around noon, producing a massive plume of black smoke.
Fire Suppression: Emergency responders used dry chemical extinguishers and water; the fire was contained by 7:00 p.m., much faster than the typical multi‑day burn.

Six Safety Issues Identified by the CSB

Transient‑Operation Safeguards – No reactor steam barrier; pressure differential forced air into the reactor; lack of nitrogen/gas purge.
Process Knowledge – In‑house FCC expertise was isolated; no external technical review for 25+ years; shutdown procedures contradicted licensor guidance.
Process Safety Management (PSM) Systems – Incomplete safety information, inadequate hazard analyses for transient states, insufficient operator training.
Industry Knowledge & Guidance – Lessons from a similar 2017 Exxon‑Mobil explosion were not applied; no unified FCC safety publication existed.
Brittle Fracture of Vessels – Older steel grades shattered under explosion pressure, sending high‑velocity fragments; newer, tougher steel could have reduced projectile hazards.
Emergency Preparedness – Near‑miss with hydrofluoric acid (HF) storage; asphalt spill not treated as a flammable liquid quickly enough, allowing fire to spread.

Recommendations Made by the CSB

Cenovus Energy (new owners): Implement FCC‑specific safeguards for transient operations and embed them in written procedures.
Cenovus: Create a technology‑specific knowledge‑sharing network, including an FCC peer network.
OSHA: Develop guidance for Process Hazard Analyses that address transient‑operation hazards.
American Petroleum Institute (API): Publish a publicly available technical guide on safe FCC operation.
EPA: Prioritize inspections of FCC units paired with HF alkylation units to verify explosion‑prevention safeguards.

Aftermath

Refinery remained offline for over four years.
36 workers sought medical attention.
Community disruption for thousands of residents.
The incident is classified as a serious near‑miss involving HF, underscoring the potential for far worse consequences.

Key Lessons for the Industry

Never rely solely on equipment designed for normal operation during shutdowns. Dedicated safeguards (steam barriers, inert gas purges) are essential.
Maintain up‑to‑date process knowledge through regular external reviews and peer networking.
Ensure PSM systems cover transient states with robust hazard analyses and operator training.
Adopt modern materials to avoid brittle fracture in extreme events.
Treat all spills of flammable liquids, even high‑viscosity materials like asphalt, as immediate fire hazards to enable rapid response.

The Superior refinery disaster shows that inadequate safeguards, outdated procedures, and insufficient process knowledge during shutdowns can turn a routine maintenance event into a catastrophic explosion and fire, emphasizing the need for robust transient‑operation safety systems across the refining industry.

Frequently Asked Questions

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Helpful resources related to this video

If you want to practice or explore the concepts discussed in the video, these commonly used tools may help.

Process Safety Management Handbook Recommended

Provides comprehensive guidance on building and maintaining effective PSM systems, helping refineries prevent incidents like the Superior explosion

Amazon →

Chemical Engineering Safety Book

Covers fundamental safety principles and case studies for chemical plants, offering lessons directly applicable to refinery operations

Amazon →

Industrial Fire Fighting Equipment

Essential tools for quickly suppressing refinery fires such as the asphalt blaze, reducing damage and downtime

Amazon →

Hydrofluoric Acid Safety Kit

Contains protective gear and neutralizing agents for HF handling, critical after near‑miss events involving HF storage tanks

Amazon →

Refinery Operations Training Manual

Provides detailed procedures for safe shutdowns and startups, addressing the procedural gaps that led to the explosion

Amazon →

Links may be affiliate links. We only include resources that are genuinely relevant to the topic.

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Full Transcript YouTube

[Music]
Voiceover: April 26, 2018,
the Superior Refinery in Superior, Wisconsin.
A large explosion erupted during a maintenance shutdown,
puncturing a tank of asphalt
that spilled into the refinery.
The asphalt later ignited, causing a huge fire.
The incident injured 36 workers,
caused roughly 550 million dollars
in damage to the facility,
and released 39,000 pounds of
flammable hydrocarbon vapor into the air.
Over 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.
Owens: Refinery shutdowns as well as startups
can be particularly dangerous
because processes are not in normal operation mode.
Our investigation found that critical safeguards
were not in place during this shutdown
and that the procedures followed
at the refinery were not correct.
The result was a massive explosion and fire
that injured dozens of workers,
caused hundreds of millions of dollars in damage
and threatened the surrounding communities.
The explosion and fire thankfully didn't compromise
a nearby storage tank
that contained highly toxic hydrofluoric acid.
But the CSB considers this incident
to be a serious near miss involving HF.
If hydrofluoric acid had been released,
this could've been much worse.
Voiceover: Shortly before 6:00 a.m.
on the day of the incident,
operators took steps to shut down
the refinery's fluid catalytic cracker or FCC unit
for periodic maintenance and inspection.
The FCC unit uses heat and a solid catalyst
to break or crack heavy hydrocarbons from crude oil
into smaller hydrocarbons
which can then be blended into gasoline
and other products.
Slide valves control the flow of catalysts
between a reactor which contains flammable hydrocarbons
and a regenerator which contains air.
It is always critical to prevent
mixing of the hydrocarbons and air
because a dangerous flammable mixture can form.
Under normal operation, the catalyst
circulates continually between the reactor and regenerator
through a carefully controlled balance of pressure.
To initiate the shutdown of the FCC unit,
refiner workers stopped the flow of hydrocarbons
to the reactor and closed the two slide valves
connecting the reactor to the regenerator.
Circulation of the catalyst stopped.
But one of the two slide valves was eroded.
Catalysts from the reactor
fell through the valve to the regenerator
leaving an open path for air or hydrocarbons
to flow between the two pieces of equipment.
Without other critical safeguards in place,
air from the regenerator flowed backwards
through the slide valve into the reactor.
The air then moved into the main column
and other equipment downstream of the reactor
where it formed a flammable mixture with hydrocarbons.
At approximately 10:00 a.m. the hazardous mixture
of air and hydrocarbons found an ignition source,
and there was a large explosion.
Debris from two vessels flew into the air.
One piece traveled about 200 feet
and hit a large above-ground storage tank,
puncturing the side of the tank and releasing hot asphalt.
Asphalt poured out of the tank,
over the containment berm and into
the refinery's operating units
where it spread along the ground.
At around 12:00 p.m. the released asphalt
ignited at the storage tank, and a fire broke out.
The fire traveled along the stream of asphalt
to the refinery's crude and FCC units.
The fire produced a huge plume of black smoke,
leading to the evacuation
of a portion of the city of Superior.
Typically this type of asphalt fire can burn for days.
A plan to extinguish the massive asphalt fire
was developed by the refinery's emergency response team.
The refinery began to address the fire
together with the Superior Fire department.
They used a combination of
dry chemical fire extinguisher and water
to attack the fire from multiple directions
and surround it within a containment area.
With the fire contained,
emergency responders were able to put out
the asphalt fire at around 7:00 p.m.
The asphalt fire had been extinguished
in a matter of hours
as opposed to days as had been initially anticipated.
The evacuation order was lifted,
and Superior residents were allowed to return home.
Nevertheless 36 Husky employees and contract workers
sought medical attention for injuries.
The Chemical Safety Board launched an investigation
and identified six safety issues
that led to the explosion and fire.
They are:
transient operation safeguards;
process knowledge;
process safety management systems;
industry knowledge and guidance;
brittle fracture during extreme events;
and emergency preparedness.
The first safety issue is transient operation safeguards.
At the time of the incident,
the Superior refinery was shutting down the FCC unit.
Shutdowns like startups, stand-by's and emergencies
are known as transient operations,
meaning activities that take place
outside of normal process operations.
Transient operations can pose unique hazards
that are not present during normal operations.
An FCC unit presents atypical hazards
during transient operations
because air and flammable hydrocarbons
are present within interconnected equipment,
which increases the likelihood of an explosion.
Therefore during the shutdown of an FCC unit,
certain FCC specific safeguards must be in place
to ensure that air and hydrocarbons do not mix.
But the CSB found that at the Superior refinery these
safeguards were not implemented or were ineffective.
One such safeguard in to create a reactor steam barrier.
This is done by injecting a large amount of steam
into the reactor,
creating a higher pressure in the reactor
than in the connected equipment.
This prevents air from the regenerator
from flowing into the reactor
where it can then enter the main column
and other equipment
on the hydrocarbon side of the process.
However Superior refinery's shutdown procedure
did not call for a reactor steam barrier
but instead instructed the board operator
to maintain the regenerator at a higher pressure
than the reactor at all times.
Following these instructions
set the refinery on a path toward disaster
as this hazardous pressure differential
ensured that air from the regenerator
would flow into the reactor
and onward into the hydrocarbon side of the process
where a flammable mixture formed.
Another critical safeguard that should be in place
during an FCC unit shutdown
is to use natural gas or nitrogen
to purge air from the main column
to the flare system to prevent oxygen accumulation
inside the equipment.
But the CSB found that even though Superior refinery
had the ability to purge air out of its main column,
the refinery had not incorporated this safeguard
into its operating procedures for transient operations
or its operator training program.
So it was not done.
Instead the refinery overly relied on
the catalyst's slide valves
to keep the air and hydrocarbon systems
separated during the shutdown.
But slide valves are designed to control flow
of catalysts during normal operations,
not to prevent flow of air and hydrocarbons
during a shutdown.
And one of the slide valves was eroded after years of use.
Therefore without other safeguards in place,
air was readily available to pass
from the regenerator through the worn slide valve
into the reactor
and onward into the hydrocarbon side of the process.
Yersiz: During transient operation
for any process involving flammable materials,
operators need to understand the potential
for air to enter and accumulate inside equipment.
Written procedures should be in place that outline
safeguards meant to prevent air and hydrocarbon mixtures,
and operators should be trained to understand
why those safeguards are critical.
Voiceover: To that end the CSB made a recommendation
to the new owners of the refinery, Cenovus Energy,
to establish safeguards to prevent explosions
in the FCC unit during transient operation
and incorporate these safeguards
into written operating procedures.
The second safety issue found by the CSB is
process knowledge.
The CSB found that throughout
the history of the Superior refinery,
under several owners its FCC expertise was mostly in-house
and with minimal engagement with other refineries.
In addition the refinery's use of
external technical experts was limited to
assessing the FCC unit's performance
during normal not transient operations.
The Superior refinery required its operations department
to review and recertify its operating procedures annually.
But the refinery had not performed the technical review
of its FCC unit operation procedures
with its process engineers, the FCC technology licensor
or outside consultants for at least 25 years
prior to the incident and possibly
since the unit was commissioned around 1960.
As a result, the refinery's shutdown procedures
which instructed operators to maintain
the regenerator at a higher pressure
than the reactor at all times
directly contradicted the FCC technology licensor's
guidance and created the dangerous condition
that enabled the explosion to occur.
Yersiz: The Superior refinery had serious errors
in its FCC unit operating procedures
that directly led to the explosion.
But refinery employees did not have sufficient process
knowledge of FCC unit technology to recognize the problem.
Had the refinery engaged with other sites
or outside experts to review its operating procedures,
the technical errors could have been identified and
corrected, and the explosion could have been prevented.
Voiceover: Therefore the CSB
made a recommendation to Cenovus Energy
to develop and implement a technology-specific
knowledge-sharing network program
across all Cenovus operated refineries,
which at a minimum
includes an FCC technology peer network.
The third safety issue highlighted by the CSB
is process safety management systems.
OSHA's Process Safety Management Standard
and the EPA's Risk Management Plan Rule
require facilities like the Superior refinery
to implement process safety management systems.
Process safety management or PSM systems
are meant to identify, evaluate
and control process hazards.
But the CSB found that the
refinery's PSM systems were inadequate.
For instance, the refinery's process safety information
did not include the operating manual
published by the FCC technology licensor.
Process hazard analyses performed at the refinery
did not effectively identify hazards
present during transient operations of the FCC unit
nor outline ways to control those hazards.
The refinery's operating procedures lacked
clear instructions and were not technically evaluated.
And the operator training program at the refinery
did not properly prepare operators
to shut down the FCC unit safely.
Yersiz: The deficiencies we found at
Superior refinery's process safety management systems
directly contributed to the explosion.
To prevent catastrophic incidents, companies should
ensure that their PSM systems
are effective at identifying, evaluating
and controlling process hazards
not just during normal operations
but during transient operations as well.
Voiceover: As a result of its findings,
the CSB made a recommendation to OSHA
to develop guidance documents
for performing process hazard analysis
on operating procedures to address transient operation
hazards in facilities with PSM covered processes.
The fourth safety issue found by the CSB
is industry knowledge and guidance.
The explosion at Superior refinery occurred
less than one year after the CSB released its report
on a similar explosion at the Exxon-Mobile refinery
in Torrance, California.
Industry trade groups distributed lessons learned
from the California incident
as well as the CSB's safety video on the explosion.
But despite these educational efforts,
employees at the Superior refinery did not learn
or apply these key lessons in a way that
could have prevented the explosion at their refinery.
For example, the workforce appeared to
understand that flammable hydrocarbons had flowed
from the California refinery's main column
into the regenerator and subsequently exploded, but they
did not recognize that the reverse was also possible.
Therefore they did not realize a similar explosion
was preventable by implementing a reactor steam barrier.
In addition there is currently no industry publication
that establishes common basic process safety expectations
for all FCC units,
although the refining industry has accumulated many
years of FCC process safety knowledge from its members.
This information is scattered, complex and not
easily accessible to some refinery employees.
Yersiz: The CSB believes that
a publicly available technical publication
outlining recognized and generally accepted
good engineering practices for safe FCC operation
and incorporating lessons learned from this
and other similar CSB investigations
could help drive important safety improvements
in FCC units across the United States.
Voiceover: Therefore the CSB made a recommendation
to the American Petroleum Institute
to develop a publicly available technical publication
for the safe operation of FCC units
that is applicable to both new and existing units.
The fifth safety issue outlined by the CSB
is brittle fracture during extreme events.
The two process vessels that exploded
were constructed from a grade of steel
that is no longer recommended for new equipment.
The vessels failed by brittle fracture
which means that they shattered like breaking glass.
When the vessels failed,
the force of the explosion sent over 100 metal fragments
from the shattered vessels around the refinery.
One large piece struck and punctured
the nearby asphalt tank.
Had the process vessels been made of a new grade of steel
with better toughness properties,
they would have more likely torn open
rather than shattered,
resulting in fewer if any dangerous projectiles.
Yersiz: While some vessels may be deemed fit for service,
they're not usually assessed
for their performance during an explosion.
Selecting materials of construction
that are not susceptible to brittle fracture
can help lessen the impact to the surroundings
should an extreme event occur.
Voiceover: The sixth and final safety issue
noted by the CSB is emergency preparedness.
The Superior refinery uses highly toxic
hydrofluoric acid or HF in its alkylation process.
The HF storage tank was about 150 feet
away from the two vessels that exploded
which is about 50 feet closer than
the asphalt storage tank that was damaged.
The HF tank was not punctured or otherwise compromised,
but the CSB considers this incident to be
a serious near miss event with hydrofluoric acid.
Explosion debris did, however,
puncture the asphalt storage tank,
causing over 17,000 barrels of hot asphalt
to spill out into the refinery
where it accumulated for approximately two hours
before it ignited.
Yersiz: It was only a matter of time
until the spilled asphalt caught on fire.
Had emergency responders treated the asphalt
as a flammable liquid,
they could've taken more effective action sooner
to control the asphalt spill before it ignited.
Voiceover: The result is that the initial explosion
escalated into a large fire
that caused hundreds of millions of dollars
in property damage.
In addition smoke from the fire
and the potential for an HF release
prompted the city of Superior evacuation order,
causing disruption to over 2,500 members
of the surrounding community.
The refinery remained shut down
for more than four years following this incident.
In response the CSB made a recommendation to the EPA
to develop a program that prioritizes
and emphasizes inspections of FCC units
in refineries that operate HF alkylation units
to verify that safeguards that prevent explosions
during transient operations are in place.
Owens: The incident at the Superior refinery
should serve as a wakeup call to other refineries,
especially those that have
hydrofluoric acid alkylation units.
During transient operations like shutdowns and startups,
refineries must have necessary safeguards in place
and follow correct procedures
to prevent disasters from occurring.
Thank you for watching this CSB safety video.
Voiceover: For more information, please visit CSB.gov.
[Music]