On April 26, 2018, a large explosion occurred at the Superior Refinery in Superior, Wisconsin during a maintenance shutdown. The explosion caused a tank of asphalt to spill into the refinery, which later ignited and resulted in a massive fire. This incident caused injuries to 36 workers, approximately $550 million in damage, and released 39,000 pounds of flammable hydrocarbon vapor into the air. Over 2,500 residents were evacuated from their homes in the city of Superior, and the city of Duluth, Minnesota issued a shelter in place order. The Chemical Safety Board (CSB) conducted an investigation and found that critical safeguards were not in place during the shutdown, resulting in the explosion and fire.
Shutdown Procedure
The incident occurred during the shutdown of the refinery's Fluid Catalytic Cracker (FCC) unit for maintenance and inspection. The FCC unit is responsible for breaking down heavy hydrocarbons into smaller ones that can be used to produce gasoline and other products. During the shutdown, operators stopped the flow of hydrocarbons to the reactor and closed the slide valves connecting the reactor to the regenerator. However, one of the slide valves was eroded, allowing air to flow back into the reactor. This created a flammable mixture of air and hydrocarbons, which ignited and caused the explosion.
Fire and Evacuation
Debris from the explosion punctured a storage tank, releasing hot asphalt that spread throughout the refinery's operating units. The released asphalt ignited, causing a fire that traveled to the crude and FCC units. The fire produced a large plume of black smoke, leading to the evacuation of a portion of the city of Superior. The refinery's emergency response team and the Superior Fire Department worked together to extinguish the fire using dry chemical fire extinguishers and water. The fire was contained within a few hours, and the evacuation order was lifted.
Safety Issues
It was identified that six safety issues that led to the explosion and fire:
- Transient operation safeguards
- Process knowledge
- Process safety management systems
- Industry knowledge and guidance
- Brittle fracture during extreme events
- Emergency preparedness
It was found that specific safeguards were not implemented or were ineffective during the shutdown of the FCC unit. The refinery did not have a reactor steam barrier in place to prevent air from flowing into the reactor. Additionally, the refinery did not use natural gas or nitrogen to purge air from the main column, which could have prevented the accumulation of oxygen. The CSB also discovered that the refinery lacked process knowledge and did not engage with external experts to review its operating procedures. These errors in process knowledge and operating procedures directly contributed to the explosion.
Recommendations
Several recommendations were made to prevent similar incidents in the future. They recommended that the new owners of the refinery, Cenovus Energy, establish safeguards for transient operations in the FCC unit and incorporate them into written operating procedures. They also suggested that Cenovus Energy develop a technology-specific knowledge sharing network program across all their refineries to ensure process knowledge is up-to-date. These recommendations aim to improve safety measures and prevent accidents like the one that occurred at the Superior Refinery.
Safety Issues and Recommendations
1. Process Safety Management Systems
- It was found that the process safety management systems at the Superior Refinery were inadequate.
- PSM systems are meant to identify, evaluate, and control process hazards.
- Deficiencies in the refinery's PSM systems directly contributed to the explosion.
- Companies should ensure their PSM systems are effective during both normal and transient operations.
2. Industry Knowledge and Guidance
- Despite educational efforts, employees at the Superior Refinery did not apply key lessons from previous incidents.
- No industry publication establishes common basic process safety expectations for all FCC units.
- It was recommended that the development of a publicly available technical publication for safe FCC operation.
3. Brittle Fracture During Extreme Events
- The exploded vessels were constructed from a grade of steel no longer recommended for new equipment.
- Brittle fracture caused the vessels to shatter, sending dangerous projectiles around the refinery.
- Selecting materials not susceptible to brittle fracture can help lessen the impact during extreme events.
4. Emergency Preparedness
- The use of highly toxic hydrofluoric acid (HF) in the alkylation process poses a serious risk.
- The incident was a serious near-miss event.
- Emergency responders should treat flammable liquids with urgency to prevent escalation.
- It was recommended that the development of a program to prioritize and inspect FCC units in refineries with HF alkylation units.
This article highlights the importance of effective process safety management systems, industry knowledge and guidance, prevention of brittle fracture, and emergency preparedness in preventing catastrophic incidents in refineries.