[chris]

A release of hydrocarbons and hydrofluoric acid in the alkylation unit at the Philadelphia Energy Solutions refinery in South Philadelphia formed a ground-hugging vapor cloud that ignited at about 4 a.m. on June 21, 2019, setting off three explosions minutes apart.

The largest was a boiling liquid expanding vapor explosion, which threw a vessel fragment roughly 2,000 feet across the Schuylkill River.

Five workers were hurt, none seriously, and nobody was killed. The company estimated that 5,239 pounds of hydrofluoric acid escaped, about 3,271 pounds of it into the air.

Federal investigators traced the failure to a pipe elbow installed in 1973. It had corroded far faster than the rest of its piping circuit because its nickel and copper content did not meet composition limits adopted in later years.

The refinery announced it would shut down that month and filed for bankruptcy a month later.