DULUTH — Emergency crews extinguished heavy fuel vapors in the second leak in two days at Cenovus Energy's Superior, Wis., refinery.

The Superior Fire Department said crews responded to a report of a heavy fuel release inside a process unit at 1 p.m. Thursday. Like the refinery did with the Aug. 23 propane leak, its own emergency crews used water fog streams to dissolve petroleum vapors while they reduced pressure in the system, the Fire Department said.

No fuel oil vapors have been found through air monitoring and the public isn't in danger, the Fire Department said.

Cenovus said the two incidents aren't related and both will be investigated.

The incident caused an equipment shutdown and "venting to the atmosphere," but no fires, said spokeswoman Kim Guttormson. She said the company's on-site emergency response team is "safely managing" the leak.

Nonessential Cenovus employees were evacuated, but no one was injured.

The refinery is still building up to full operations after a 2018 explosion rocked the city.

At the time of the explosion, the refinery, owned then by Husky Energy, was shutting down its fluid catalytic cracking unit for planned maintenance. The unit is a common piece of equipment at oil refineries used to refine crude oil into higher octane fuels.

A worn valve inside the unit allowed air to mix with hydrocarbons, leading to the explosion of two outdated vessels, spraying metal fragments up to 1,200 feet and puncturing a nearby asphalt storage tank. About 17,000 barrels of hot asphalt spilled and ignited, causing several fires.

In a final report, the U.S. Chemical Safety and Hazard Investigation Board laid out several safety recommendations for the new plant.

The $1.2 billion rebuild came with new safety measures, including pipes built to change color in the event of a leak, along with other leak detectors, and remote control water cannons built to address hydrogen fluoride vapors.