In a safety alert, the US Mine Safety and Health Administration said the accidental explosion at the unnamed plant caused two of the injured workers to be thrown into the air, with one landing almost 20 feet away.

The explosion took place after a spark from the welder landed into the open end of a nearby pipe assembly containing flammable PVC pipe joint vapour that was heavier than air.

The agency made three key findings in its investigation.

While it was previously thought that glue vapours dissipate soon after the pipe is glued, MSHA said in this case the pipe was glued five weeks before the accident occurred.

The agency also took on the assumption that a system must be enclosed for an explosion to occur.

“In this case, two ends of the pipe assembly were open. As long as heat and PVC vapours are present, the potential for fire and explosion is present,” MSHA said.

An assumption that the vapour would have evaporated if the ends of the pipe assembly were opened was also proven not to be correct.

“Two ends of the pipe assembly were cut and left open overnight; however, both open ends were turned upward creating a trapped pipe section.

“The PVC glue vapours, being heavier than air, settled in the low-lying pipe section and could not escape.”

For best practice guidelines, MSHA said installers should be trained on the manufacturer’s procedures for installation and use of the PVC pipe cement, and must thoroughly understand the fire and explosion hazards associated with the vapours produced by this cement.

“Cutting and welding activities should not be conducted in the vicinity of open pipes unless contents of pipe have been ventilated, made inert or the pipe ends capped to prevent hot sparks from entering the pipe.”