Last Christmas Eve, a sprinkler pipe at a United States Steel Corp. plant south of Pittsburgh fell from the ceiling in a building at the 120-year-old complex. Within minutes, an area the size of a football field was engulfed in flame.

The falling pipe severed another delivering oil to a compressor, a machine sucking highly combustible gas through the plant. The leaking oil caught fire, the compressor began to vibrate violently and a flood of gas from coking-coal ovens was ignited, according to a 59-page report on the accident…

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