POCA, W.Va. - Two workers were injured Monday when highly flammable gas used in welding exploded at a West Virginia industrial site, officials said.

Fire crews were sent at about 3:20 p.m. to Airgas, a distributor of specialty gases in Poca, outside of Charleston. Putnam County emergency management director Frank Chapman said the explosion involved about 50 tanks of acetylene that were at Airgas waiting to be refilled. What caused the tanks to explode wasn't known.

Chad Jones, a firefighter with the Bancroft Volunteer Fire Department, said four cylinders continued to burn Monday evening and that crews were letting them "burn out." They were dousing other tanks with water to keep them from exploding, said Jones, whose station was one of several to respond to the scene.

The tanks were being stored in a bay behind the facility. Jones said after the first tank exploded, "it was like a chain reaction," with fireballs shooting 100 to 150 feet in the air. A nearby business was evacuated, and windows were shattered in the back of the Airgas plant, Jones said.

Doug Barker, chief financial officer at nearby Clark Truck Parts, told The Associated Press over the phone that "we felt our building shake like it's never come close to shaking before from a storm or anything. It was enough to make us run."

Barker said he and another company official bolted from their offices, and he ran to the road and saw dark smoke in the air. Soon afterward, they heard several smaller explosions and saw fire, he said. Barker also saw three or four ambulances speed by and heard a lot of sirens.

Clark Truck Parts is about half-mile from Airgas, Barker said. He said there are some homes between the two industrial sites.

Dave Castro, manager of the TransWood trucking company about a quarter-mile from Airgas, said he also felt his building shake.

"It felt like a truck ran into the building," he said.

He said he drove toward Airgas to check on his wife, who works at another company nearby, and could see the back of the plant on fire. He said the burning area was about the size of a house, and every 15 seconds or so a black ball of smoke would rise from a tank or drum "and explode like a firework."

Acetylene is used in welding canisters. Airgas calls itself the largest U.S. supplier of industrial, medical and special gases. The company also lists on its website propane, often used in backyard grills; hydrogen, helium; and nitrous oxide, or the "laughing gas" used during some dental procedures.

Company spokesman Doug Sherman said the explosions occurred in a concrete-enclosed storage area located outside of the main plant. He said the injuries were not life-threatening, and the fire had been extinguished.

Jones said the company was sending a hazmat crew from Kentucky.

"It's a hazardous job they do every day," Jones said. "Something went wrong today."

A U.S. Occupational Safety and Health Administration crew was at the scene.

Chapman said the explosion involved residue of the gas left in the tanks. He said the blast would have been much worse if the tanks were filled. He said both workers suffered second and third-degree burns.

Chapman said the cause of the explosion was being investigated and that the blasts and fire were the first problem he knows to be reported at Airgas.

The injured workers were taken to Cabell Huntington Hospital for treatment. A hospital official did not know their conditions.

Airgas Inc. is based in Radnor, Pa., and has more than 15,000 employees at 1,110 locations including retail stores, gas fill plants and distribution centres, according to Hoover's database on companies. It is the largest distributor of packaged gases in the US, with a 25 per cent market share and with sales of nearly $5 billion in fiscal 2012.

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