A corroded pipe caused the June 21 explosion at PES Holdings LLC's 335,000-barrel-per-day Philadelphia refinery that destroyed the plant's alkylation unit and ultimately led to the facility's shutdown, the U.S. Chemical Safety and Hazard Investigation Board found, and a release of hydrofluoric acid immediately preceding the disaster has the board examining the industry's use of the catalyst.
PES filed for bankruptcy for the second time in 18 months following the disaster, after executives were paid almost $4.5 million in retention bonuses.
Alkylation units make high-octane components for blending into gasoline, and the process relies on an acid catalyst derived from hydrofluoric acid, or HF. The federal Centers for Disease Control and Prevention notes that although the substance is a weak acid, it is "extremely harmful due to its ability to penetrate tissue."
An Oct. 16 factual update from the board, known as the CSB, said a pipe elbow "had corroded to about half the thickness of a credit card" and "appears to have ruptured in the refinery's alkylation unit, releasing a process fluid that included over 5,000 pounds of hydrofluoric acid." The CSB said two minutes after the rupture, a cloud of ground-hugging vapor that had formed from the leak ignited, causing the explosion. The board noted the piping of the 20,000-bbl/d alkylation unit was susceptible to corrosion from the HF, and that while the pipe thickness had been monitored, the thickness of the pipe elbow that failed had not been monitored.
"Since 2015, the CSB has investigated three major incidents at refineries that utilize HF for alkylation. Incidents in Superior, Wis., and Torrance, Calif., fortunately did not result in an HF release," CSB Interim Executive Authority Dr. Kristen Kulinowski said Oct. 16. "That was not the case here in Philadelphia. Though the main tank holding HF was not breached, HF was a component of the process fluid released from the alkylation unit. We are lucky there were no serious injuries or fatalities."
According to the U.S. Environmental Protection Agency, exposure to as little as 1 part per million of HF can lead to short-term, detrimental health effects. Exposure to as little as 12 ppm over the course of eight hours can cause "irreversible or other serious, long-lasting effects," the EPA said, while exposure to concentrations as low as 95 ppm over the course of 10 minutes can cause similar effects. Life-threatening exposure occurs at concentrations of 22 parts per million over eight hours or 170 ppm over 10 minutes.
Kulinowski said the CSB, an independent, nonregulatory federal agency, is considering stronger reviews of corrosion mechanisms and the use of HF in the refining process.
The CSB's investigation of the incident is ongoing.