From MSHA accident file…
The two (10) ten-inch metal pipes being removed were part of a closed-loop chiller system that had been abandoned. One pipe supplied chilled water underground (supply and shown as blue in the drawing), and the other returned water to the surface (return and shown as red in the drawing).
The chiller pipe system ran from the “chiller buildings” on the surface to the 1600-foot underground level.
Both the supply and return pipes had isolation valves. However…