THE FINAL report into a massive fire and subsequent explosions that took place in June 2019 at the Philadelphia Energy Solutions (PES) Refinery in Philadelphia, Pennsylvania has been released by the US Chemical Safety and Hazard Investigation Board (CSB).
Causing damage to property worth an estimated US$750m, and the closure of a refinery capable of producing 335,000 bbl/d of crude oil, the cause of the accident has been confirmed as a ruptured corroded pipe elbow in the firm’s hydrofluoric acid (HF) alkylation unit which released approximately 2,268 kg of highly toxic hydrofluoric acid (HF) into the plant. Along with the HF, an estimated 306,600 kg of hydrocarbons were also released during the event, of which an estimated 275,800 kg was combusted.
The escaping HF and hydrocarbons vapourised in the air, causing a large vapour cloud to engulf part of the plant. Although control workers activated a number of systems to contain the acid, water pumps designed to reduce airborne HF through vapour suppression did not turn on. The cloud then ignited, causing a large fire, which resulted in three secondary explosions.
It took around 40 minutes into the release before a refinery worker was able to manually turn on the water pump that supplied the HF mitigation water cannons, and a day for the fire to be extinguished. Five workers and a firefighter experienced minor injuries during the incident and response, but remarkably there were no fatalities.
“This is one of the largest refinery disasters worldwide in decades in terms of cost. The local community in Philadelphia fortunately was not seriously harmed, but given the refinery’s location, it could have been much worse,” said CSB Interim Executive Authority Steve Owens. "This incident should be a wake-up call to industry to prevent a similar event from occurring in the future.”
According to the CSB, the report determines that the ruptured pipe elbow – which had already been blamed for the incident during a factual update in November 2019 – had corroded faster than other piping in the firm’s alkylation unit due to the higher concentration of nickel and copper it contained.
Carbon steel with high nickel and copper content is known to corrode faster from contact with HF than carbon steel with lower nickel and copper content, the report said, but at the time of installation in 1973, the limits on nickel or copper content had not been set by the American Society for Testing and Materials (ASTM).
Over the next few decades, however, the limits changed, and by 1995, a revised standard by the ASTM meant that the pipe elbow no longer met the new requirements due to its high levels of nickel and copper. But, said CSB Supervisory Investigator Lauren Grim, a comprehensive evaluation of unit piping was not carried out by the refinery owners despite regulations requiring companies to determine that their equipment is safe to operate after industry standards are updated. “To prevent catastrophic incidents companies and industry trade groups must ensure process safety when new knowledge on hazards is published,” she said.
In its report, the CSB also highlighted the refinery’s lack of remotely-operated emergency isolation valves installed in the HF alkylation unit to isolate nearby hydrocarbon sources that could then flow through the failed elbow. “Although these valves are not explicitly required by the current American Petroleum Institute (API) standard on Safe Operation of Hydrofluoric Acid Alkylation Units, if PES had installed such valves, the release from the pipe elbow could have been minimized and the subsequent explosions could have been prevented,” the CBS said in its report.
Despite 46 HF alkylation units currently in operation throughout the country’s 155 petroleum refineries, the CSB’s investigation found that there is no federal regulatory requirement for refineries to analyse inherently safer design strategies to reduce the risk of serious accidental releases, such as using sulfuric acid as a catalyst instead of HF.
Sulfuric acid is still a highly corrosive compound that can cause skin burns upon contact, but if released it remains a liquid and therefore does not present the same risk to surrounding communities as HF. One of the major hazards representing serious concern during the incident was 117,000 people residing within a mile of the refinery being exposed to clouds of vapourised HF, the CSB said.
Alternatively, some companies like Chevron are using an ionic liquid catalyst in its alkylation process at its Salt Lake City site, which combines light olefins such as butylene with isobutane to produce high-octane gasoline.
Along with providing a number of recommendations to mitigate further accidents of this nature, such as encouraging owners of refineries to instigate safer processes, the report does acknowledge that some practices are not as complete as they could be. “The current, 5th edition of API RP 751 Safe Operation of Hydrofluoric Acid Alkylation Units, has gaps that, without correction, could lead to consequences similar to those experienced at PES in the event of a fire or explosion event in an HF alkylation unit,” the report said.
The 40 C.F.R. Part 68 (EPA Risk Management Plan) and the ASTM A234 standard also need revising said the CSB.
The full report can be read here; https://www.csb.gov/assets/1/6/pes_final_report_published_october_2022.pdf
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