Woods Cross » A massive gasoline tank exploded at the Silver Eagle oil refinery here Monday night, sending four to the hospital with severe burns and forcing the evacuation of nearby homes.

The flames set off by the blast hovered over this city all evening, licking dangerously close to at least two more gasoline tanks as firefighters worked to douse the blaze.

About 50 employees were working at the refinery when the explosion rocked the site at 5:25 p.m., said South Davis Metro Fire Deputy Chief Jeff Bassett.

Silver Eagle workers found their injured colleagues and pulled them to the front of the refinery to await medical evacuation. All four were conscious and

breathing when they were found, Bassett said. He did not know how close the workers were to the tank when it exploded, a blast strong enough to blow out the walls of a nearby shed.

"As they were coming to us, some had their shirts . . . melted off," he said.

Bassett said the four men, ages 30-50, suffered third-degree burns and were transported to University Hospital. One was in critical condition; two in serious; and one in fair condition, said Chris Nelson, spokesman at University Hospital.

Meanwhile, fire crews ordered a mandatory evacuation for a half-mile radius around the refinery, affecting about 100 homes west of 800 West between 2100 South and 2600 South, Bassett said.

South Davis Metro fire crews joined with the fire teams from several refineries in an effort to snuff out the blazing tank, which has the capacity to hold 42,000 gallons of gasoline. Bassett did not know how much gasoline was in the tank at the time of the explosion or how the explosion occurred.

Fire crews were dousing the tank and surrounding tanks, which also contained gasoline, with constant streams of water.

They feared that those tanks might also explode.

Flames were towering over the refinery at 2355 S. 1100 West and clearly visible to residents of Bountiful to the east.

Around 7 p.m., fire crews introduced flame-retarding foam into the burning tank through a valve.

By 9:20 p.m., firefighters were making progress putting out the flames, and Woods Cross police said the evacuation had been reduced to voluntary, meaning people could return to their homes but were advised not to.

As of 11:15 p.m., firefighters said they would spray foam into the tanks until the fire was out, which might not be accomplished until sometime this morning.

People who had been evacuated were able to wait out the danger at Woods Cross High School at 600 W. 2200 South. The American Red Cross Greater Salt Lake Area chapter was hosting the evacuation center at the gym and was prepared to house people there overnight. By 9:30 p.m., the evacuees had left the high school.

Eva Webster was one of about 40 people who went to Woods Cross High School gymnasium on Monday while firefighters battled flames. Webster said she has lived in her home since 1970 and had never before evacuated.

"This is just a chance to visit," Webster said, as she sat next to people from her LDS ward. "We'll have Family Home Evening."

Brandon Patterson, who lives about 150 yards from the refinery, said he heard a boom and felt his house shake but thought it was his daughter slamming a door.

Then Patterson looked outside and saw the blaze.

"The flames were going way above the houses," Patterson said.

Patterson and his three young children also evacuated to the high school.

Patterson still wore the necktie he wore to work earlier in the day.

Woods Cross resident Jill Hardy didn't hear the explosion from her home about a mile from the refinery. She was notified of the evacuation order by an automated 911-based emergency notification system alert ordering her and her family to go to the high school, where she went with the three daughters, ages 7, 12 and 14.

This story was recasts to eliminate reference to "reverse 911," the trade name for an interactive communications product.

The family recalled an explosion at the refinery a year ago, and Hardy says she is getting tired of the danger.

"It's kind of scary living down there, but it goes with the territory," she said.

The refinery, which has been in its current location since the early 1950s, was hit by two fires in 2007. Neither caused any injuries.

On Aug. 15, a four-man crew immediately cut off the pipeline that feeds the distillation unit after it caught fire. The contents of the unit burned out within an hour, according to the refinery manager.

In a May 6 blaze, crude oil caught fire inside a furnace. Firefighters contained the blaze within 15 minutes and had it extinguished 30 minutes later as refinery workers shut off oil valves, a South Davis Metro Fire official said.

And a diesel line erupting inside a furnace caused a 2005 blaze.

No one was injured in that fire.

-- Tribune reporters Nate Carlisle and Pamela Manson contributed to this story.