PERU – Two excavation company workers escaped serious injury Friday morning when a dump-truck driver accidentally ruptured a propane tank behind a new elementary school.

The subsequent blast sent flames 75 feet into the air, witnesses said. Fire enveloped the truck and caused minor burns to driver Robert Hack, 60, of Greene, said Chris Stanford, a state fire investigator.

Hack escaped out the passenger door and was taken by Med-Care Ambulance to Central Maine Medical Center in Lewiston. A hospital spokeswoman said Hack was treated and released Friday afternoon.

The blast cracked five windows in the school’s kitchen, Stanford said. K & K Excavation site superintendent Dean J. Adams, who was standing about 50 feet away, was knocked off his feet but suffered no apparent injury.

“Somebody was looking out for him today, because he was very lucky, by chance,” Stanford said. “He was closer to it than the windows and the windows cracked. These guys got really lucky.”

At about 7:20 a.m., Hack was backing toward a loam pile where Adams was standing when the truck struck the 1,000-gallon liquid propane tank’s filler pipe and hood, breaking it.

Propane vapor immediately spilled out, setting off a high-pitched sound, Peru fire Chief Bill Hussey said at the scene.

Initially, the too-rich-to-burn white vapor enveloped the dump truck and Adams started toward it to get Hack out, then realized the vapor was dissipating and tried to get away, Stanford said.

“Dean wanted to go to the truck and get his guy out, but he knew enough about the vapor cloud dissipating around the truck and backed off, and that’s when it ignited and blew up,” Stanford said.

Within 30 seconds to a minute of the rupture, the dissipating vapor mixed with air reaching its combustion point and was ignited by the truck’s engine, Stanford said.

“If Adams was any closer or the pipe snapped off in another direction, he would have been seriously burned,” he said.

Fire Chief Hussey said K and K employees used fire extinguishers to put out the burning truck. Firefighters shut off the propane valve into the school, but couldn’t shut off the ruptured pipe because its valve was destroyed in the blast.

That’s why firefighters couldn’t extinguish the 3- to 4-foot-high fire, which was still burning strong as of 1:30 p.m. and could burn for days.

“By allowing it to burn, it doesn’t allow it to escape,” Hussey said.

Peter Holmes, an inspector with the Maine Oil, Solid Fuel, Natural Gas and Propane Board advised firefighters not to extinguish the fire with water. He feared that it would flush propane, which is heavier than air, out of the tank and across the ground where it could spread toward the school, ignite and cause an even bigger problem, Stanford said.

Area responders have trained for propane explosion fires, and Hussey and Rumford fire Chief Richard Coulombe said they believed Friday’s major incident was a first for the River Valley area.