The U.S. Chemical Safety Board (CSB) (Washington, DC, USA) recently released a factual update that identified the cause of a fire and series of related explosions at the Philadelphia Energy Solutions (PES) Refinery (Philadelphia, Pennsylvania, USA). According to the CSB’s report, the fire that occurred at the refinery on June 21, 2019, was apparently caused by a corroded pipe elbow that ruptured in the refinery’s alkylation unit.
During the early hours of June 21, a pipe elbow—one that the CSB claims “had corroded to about half the thickness of a credit card”—ruptured at approximately 4 a.m. This rupture caused flammable process fluid containing hydrofluoric acid (HF) to release from the PES alkylation unit. The release of HF resulted in the formation of a ground-hugging vapor cloud that ignited around 4:02 a.m., causing a large fire in the alkylation unit.
While the fire continued to rage, a series of explosions occurred in the alkylation unit, with the first transpiring at 4:15 a.m. and the second at 4:19 a.m. At 4:22 a.m., a third explosion—the largest of the series—occurred with the violent rupture of the V-1 Treater Feed Surge Drum, with fragments landing in both the PES refinery unit and in the Schuylkill River. According to PES estimates, about 676,000 lbs of hydrocarbons were released during the event, of which an estimated 608,000 lbs were combusted.
“Since 2015, the CSB has investigated three major incidents at refineries that utilize HF for alkylation. Incidents in Superior, WI, and Torrance, CA, fortunately did not result in an HF release,” said CSB Interim Executive Dr. Kristen Kulinowski. “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.” At a news conference, Kulinowski said that the CSB will evaluate whether more robust reviews of corrosion mechanisms, as well as the use of HF in the refining process, are needed.
In its post-accident analysis, the CSB identified HF in the process fluid as a likely cause of the ruptured pipe elbow. The CSB states that “the elbow that ruptured corroded faster than the rest of the piping in this part of the process.” Moreover, the factual update revealed that while corrosion rates in the alkylation unit had periodically been monitored, the failed elbow had not. The failed piping was made from carbon steel with a high percentage of nickel and copper content, which has been found to corrode at a faster rate in HF environments than carbon steel with a lower percentage of those metals present.
“Corrosion is not a new issue for the CSB. In its prior investigation of a 2012 Chevron Refinery fire we determined that corrosion caused the rupture of a piping component,” said CSB Supervisory Investigator Lauren Grim. “Similarly, the 2009 Silver Eagle refinery fire was also caused by the failure of piping that had thinned due to corrosion.”
The CSB says that its investigation of the PES Refinery fire and explosions is ongoing, with a final investigation report detailing additional findings and recommendations forthcoming. On June 26, PES announced that it would be shutting down its refining complex, and on July 22 they filed for bankruptcy.
A factual update and an interim animation from the CSB that provide additional detail about the June 23 incident at PES Refinery can be found by clicking the links above.
Source: U.S. Chemical Safety Board, www.csb.gov