CSB Safety Video: Wake Up Call - Refinery Disaster in Philadelphia

U.S. Chemical Safety and Hazard Investigation Board (CSB), October 27, 2022

The U.S. Chemical Safety and Hazard Investigation Board (CSB) has released a new safety video detailing the events leading to the fire, explosions, and toxic hydrofluoric acid (HF) release at the Philadelphia Energy Solutions (PES) refinery in Philadelphia, Pennsylvania, on June 21, 2019. The incident was caused when a pipe elbow catastrophically failed due to hydrofluoric acid corrosion, allowing flammable process fluid to escape and form a large ground-hugging vapor cloud about ten feet high that engulfed a portion of the refinery’s HF alkylation unit. The cloud soon ignited, causing the fire and series of explosions. In the end, over 5,000 pounds of highly toxic HF was released, a 38,000-pound vessel fragment was launched off-site, and approximately $750 million in property damage was caused. 

The CSB released its final investigation report on the PES incident earlier this month, identifying several key safety issues which contributed to the incident, including:

  • Mechanical Integrity: The CSB determined that the pipe elbow that failed had corroded faster than other piping in the HF alkylation unit due to it containing higher nickel and copper content than other piping in the unit.

  • Verifying Safety of Equipment after Changes to RAGAGEP: When the pipe elbow was initially installed in 1973, the standard set by American Society for Testing and Materials (ASTM) for carbon steel piping did not specify limits on nickel or copper content. Over the next decades that standard changed, and by 1995, the ASTM standard had been revised enough that the pipe elbow no longer met ASTM’s requirements due to the elbow’s high levels of nickel and copper.

    CSB Supervisory Investigator Lauren Grim said, “A comprehensive evaluation of unit piping never occurred despite regulations from both the Occupational Safety and Health Administration (OSHA) and the Environmental Protection Agency (EPA) 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.

  • Remotely Operated Emergency Isolation Valves: The CSB found that there were no 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. As a result, the CSB recommended that API update its standard on Safe Operation of Hydrofluoric Acid Alkylation Units to require installation of remotely operated emergency isolation valves on the inlets and outlets of all hydrofluoric acid containing vessels, and any hydrocarbon containing vessels meeting defined threshold quantities. 

  • Safeguard Reliability in HF Alkylation Units: On the day of the incident, pumps designed to spray large volumes of water to suppress an HF release failed to activate early in the incident as the elements to remotely operate the pumps were damaged by the fire and explosions. Forty minutes elapsed from the time the release began before a worker was able to manually turn on a water pump. In the meantime, highly toxic HF escaped from equipment and vaporized into the air. As a result, the CSB recommended that API update its standard on Safe Operation of Hydrofluoric Acid Alkylation Units to require that critical safeguards and associated control system components be protected from fire and explosion hazards, including radiant heat and flying projectiles.

  • Inherently Safer Design: Of the 155 U.S. petroleum refineries currently in operation in the United States, 46 operate HF alkylation units. Hydrofluoric acid is highly toxic and is one of the eight most hazardous chemicals regulated by EPA’s Risk Management Program (RMP). Alternative alkylation technologies have been developed, such as a solid acid catalyst and new ionic liquid acid catalyst alkylation technology. Replacing highly toxic chemicals with less hazardous chemicals is an “inherently safer design” approach. Additionally, some refinery alkylation units use sulfuric acid as a catalyst instead of HF. Although sulfuric acid is highly corrosive and can cause skin burns upon contact, it remains a liquid upon release and does not present the same risk to surrounding communities as HF, which vaporizes upon release and has the potential to travel offsite.

    The CSB’s investigation found that there is no federal regulatory requirement for refineries to analyze inherently safer design strategies to reduce the risk of serious accidental releases. Technologies are being developed that could be safer alternatives to HF alkylation, and refiners should periodically evaluate these available alkylation technologies.  Therefore, the CSB recommended that the EPA:
    • Require petroleum refineries to conduct a safer technology and alternatives analysis (STAA) as part of their Process Hazard Analysis under EPA’s RMP rule, and evaluate the practicability of any inherently safer technology; and
    • Initiate prioritization under the Toxic Substances Control Act (TSCA) to evaluate whether hydrofluoric acid is a high priority substance for risk evaluation, and if it is, conduct a TSCA risk evaluation of HF and implement any identified risk mitigation requirements.


About the CSB

The CSB is an independent, non-regulatory federal agency whose mission is to drive chemical safety change through independent investigations to protect people and the environment. The agency’s board members are appointed by the President and confirmed by the Senate. CSB investigations look into all aspects of chemical incidents, including physical causes such as equipment failure as well as inadequacies in regulations, industry standards, and safety management systems. The CSB does not issue citations or fines but makes safety recommendations to companies, industry organizations, labor groups, and regulatory agencies such as OSHA and EPA. For more information, please visit

Comments and Discussion

There are no comments yet.

Add a Comment

Please log in or register to participate in comments and discussions.

Inspectioneering Journal

Explore over 20 years of articles written by our team of subject matter experts.

Company Directory

Find relevant products, services, and technologies.

Training Solutions

Improve your skills in key mechanical integrity subjects.

Case Studies

Learn from the experience of others in the industry.


Inspectioneering's index of mechanical integrity topics – built by you.

Industry News

Stay up-to-date with the latest inspection and asset integrity management news.


Read short articles and insights authored by industry experts.

Expert Interviews

Inspectioneering's archive of interviews with industry subject matter experts.

Event Calendar

Find upcoming conferences, training sessions, online events, and more.


Downloadable eBooks, Asset Intelligence Reports, checklists, white papers, and more.

Videos & Webinars

Watch educational and informative videos directly related to your profession.


Commonly used asset integrity management and inspection acronyms.