MOUNT PLEASANT — Milton Langley was working at the athletic facilities near Patriots Point when he heard the muffled blast Friday morning.

Sirens from emergency vehicles ensued.

“I heard an explosion I initially thought was a tire blowout,” Langley said. “I figured it was a wreck down around the curve. That turned out not to be.”

Five workers were injured by flying glass after the explosion near an electrical conduit they were working on, a Mount Pleasant fire official said.

One of the men suffered serious injuries just after 11 a.m. at the end of a dirt street off Patriots Point Road. The scene was just west of Harry Hallman Boulevard, which leads to Waterfront Park. Trees and shrubs hide it from the view of motorists on the main road.

All of the workers were taken to area hospitals, but their exact conditions were not immediately known. Early reports indicated one worker was critically injured in the blast with facial injuries.

Battalion Chief Steven Drozd of the Mount Pleasant Fire Department said the workers were using compressed nitrogen to fill a pipe holding the electrical lines. The pipe runs under Charleston Harbor.

“Some guys were doing a procedure in a confined area,” Drozd said. “A piece of equipment failed, causing some injuries to some of their employees.”

The dirt road is gated, and writing on the pavement at its mouth indicates that it’s a facility for S.C. Electric & Gas. The site includes two manholes.

Robert Yanity, an SCE&G spokesman, said the crew was contracted to do work for the utility, but he could offer no further details.

Some of the men were working under ground, Drozd said, but the ones who got hurt were above ground when the glass device failed.

There was no fire.

An Airgas trailer hauling compressed nitrogen was parked at the scene. Vehicles for a contractor from Boston, UTEC Constructors Corp., also were there.

Sgt. Chip Googe of the Mount Pleasant Police Department said there would be no police investigation into the ordeal.

The accident didn’t greatly affect traffic in the area. Most of the emergency vehicles had left the scene an hour later.

Reach Andrew Knapp at 937-5414 or twitter.com/offlede.