October 13, 2022

CSB Report Highlights Corrosion Concerns in Refineries' HF Alkylation Units

The corrosion and rupture of a piping component caused a massive fire and explosions at a refinery in June 2019 in Philadelphia, Pennsylvania, according to a final report published on Tuesday by the U.S. Chemical Safety and Hazard Investigation Board. CSB’s investigation found that when the corroded pipe elbow ruptured, process fluid was released into the refinery’s alkylation unit. The release included propane and more than 5,000 pounds of hydrofluoric acid, or HF. The leak formed a large vapor cloud comprising propane, HF, and other hydrocarbons that ignited minutes later to cause the fire, which in turn caused three explosions. The largest explosion occurred when a vessel containing butylene, isobutane, and butane ruptured, launching a 38,000-pound fragment off-site and across a nearby river. Two other fragments weighing approximately 23,000 pounds and 15,500 pounds landed within the refinery.

CSB’s report notes that the steel pipe elbow that ruptured was susceptible to corrosion from HF in the process fluid due to its high concentrations of nickel and copper. The pipe elbow had corroded faster than adjacent piping components, which contained lower concentrations of nickel and copper.

“Carbon steel with high nickel and copper content is known within the industry to corrode faster from contact with HF than carbon steel with lower nickel and copper content,” the report states. However, “[at] the time of the incident, published industry standards and recommended practices did not require refineries to conduct 100% component inspection of carbon steel piping in HF service to identify any piping components corroding and thinning faster than others.”

Following the incident, the American Petroleum Institute’s recommended practice for the safe operation of HF alkylation unites was revised to include a new requirement for refiners to inspect all carbon steel piping components and welds in HF alkylation corrosion zones. CSB anticipates that this revision “should help prevent future failures of steel piping with high nickel and copper content in HF alkylation units.”

Other issues that contributed to the refinery incident in Philadelphia included a lack of remotely operated emergency isolation valves to isolate large hydrocarbon sources and the failure of safeguards in the HF alkylation unit. According to CSB, elements of the refinery’s water spray HF mitigation system were damaged during the incident, which meant the water pumps could not be remotely activated to suppress released HF. The agency found that it took 40 minutes for a worker to manually turn on a water pump from the time the release began.

Following its investigation, CSB is recommending that EPA develop a program to prioritize and emphasize inspections of refineries’ HF alkylation units. CSB also recommends that EPA initiate prioritization under the Toxic Substances Control Act to evaluate whether HF is a high priority substance for risk evaluation, and that ASTM International incorporate supplementary requirements for piping used in HF service into its standard for carbon and alloy steel pipe fittings.

CSB’s report on the 2019 incident can be downloaded as a PDF. Further details are available in an agency press release.