In a special equipment warning Wednesday, the agency said the failure occurred in an inflatable tent design unit manufactured by AL Lee located at an undisclosed operation. The shelter’s model number was 4042-35, manufactured March 21, 2008, and it had 12 high-pressure, 4500psi oxygen cylinders.

MSHA does not yet know the manufacturer of the fitting.

“On the day of the incident, the refuge alternative had been examined during the required pre-shift examination and no defects were identified,” the agency reported.

“Approximately two-and-one-half hours later, a scoop operator discovered debris scattered on the mine floor near the refuge alternative.”

Following an examination of the refuge alternative by the manufacturer, state inspectors and federal officials, the groups made a preliminary determination that there had been a catastrophic failure of a brass high-pressure fitting connected to one of the oxygen cylinder valves.

The failure, in turn, resulted in a rapid oxygen release, which pressurized the steel unit’s interior.

“The initial determination is that the pressure build-up inside the container forced open both the tent deployment door and the air-lock access door, ejecting a supply container and three 5-gallon water containers from the access door area onto a nearby rib,” MSHA said.

The agency will update the alert as more becomes known, and in the meantime has asked all mines to do a physical examination of all refuge alternatives to ensure correct fittings are installed on compressed gas systems. Shelter manufacturers should be called in to assist with the evaluations to ensure proper performance.