US Chemical Safety Board issues final report on 2023 fire at Marathon renewable diesel facility in Martinez, California

The U.S. Chemical Safety and Hazard Investigation Board released March 13 its final investigation report into the Nov. 19, 2023, fire at the Marathon Martinez Renewables facility in Martinez, California. 

The incident seriously injured one employee, who suffered third-degree burns over most of his face and body, and resulted in approximately $350 million in property damage to the facility. 

During the startup of a renewable diesel hydroprocessing unit at the facility, a metal tube in a fired heater ruptured and released renewable diesel and hydrogen, sparking the fire.

As a result of the incident, the process unit was shut down for nearly a year before restarting operations in November 2024.

The CSB’s final report outlines six critical safety issues and offers recommendations to prevent similar incidents in the future.

The CSB’s investigation found that the rupture occurred due to a combination of factors, including a misaligned valve left open, which caused the process flow to bypass the heater, and combustion issues resulting from blocked air supply to burners.

The combination of these conditions led to dangerously high temperatures in the fired heater, ultimately causing the tube to overheat and rupture.

“This tragic event underscores the importance of having proper safeguards in place for fired-heater operation, which can be particularly hazardous,” said CSB Chairperson Steve Owens. “A series of safety failures contributed to the severity of this incident, including the lack of appropriate guidance for when to shut down the heater remotely instead of putting a worker at risk of harm.”

The six key safety issues identified by the CSB are:

• Safe operating limits: The facility lacked effective alarms to warn personnel when tube-metal temperatures exceeded safe limits, leading to unsafe troubleshooting practices, such as not shutting down the fired heater remotely.

• Worker proximity to fired heater: The field operator was exposed to danger by being sent to the fired heater when safer options, such as remotely shutting down the heater, were available.

• Low flow through fired heater: The safety system failed to detect low-flow conditions due to diverted process material, which Marathon had not identified in its hazard analysis.

• Burner operation: Blocked air supply to burners led to improper air-fuel mixing, causing afterburning that contributed to the overheating and tube rupture.

• Valve misalignment: A misaligned valve caused process material to bypass the heater, contributing to the overheating and tube rupture.

• Corporate oversight: Marathon did not ensure that the facility met the company’s standards before restarting operations as a renewable diesel facility, resulting in safety deficiencies.

The CSB determined that the incident was caused by a combination of technical failures and unsafe operational practices at the facility.

Specifically, a valve misalignment diverted process flow and blocked air inlets led to afterburning, which caused the heater tube to overheat and rupture.

The severity was worsened by the presence of a field operator near the heater, who had been instructed to turn off some of the heater’s burners manually—an operation that placed him immediately next to the fired heater.

Despite dangerous conditions, personnel continued troubleshooting rather than shutting down the heater remotely.

The incident resulted in the operator suffering third-degree burns to over 80 percent of his body and remaining in critical condition for several months.

In response to the findings, the CSB is issuing several safety recommendations, including:

• Marathon Martinez Renewables: Implement engineering safeguards such as combustibles analyzers to detect and prevent afterburning and install alarms that trigger remote shutdowns and clear personnel from the area when unsafe limits are exceeded.

• Marathon Corp.: Update corporate standards to include guidance for alarming tube-metal temperatures and establishing remote-shutdown protocols to ensure personnel evacuation. Additionally, revise the “Heater Application Standard” to address flow-diversion conditions and include safeguards to detect and prevent afterburning and ensure the Martinez facility adheres to Marathon corporate safety standards.

• American Petroleum Institute: Revise API RP 556 “Instrumentation, Control, and Protective Systems for Gas Fired Heaters” to include requirements for responding to high tube-metal temperatures with alerts and predefined actions, including requirements to clear personnel from the area when unsafe limits are exceeded as well as design requirements to protect heaters from flow diversions and afterburning.

The CSB is an independent federal agency charged with investigating incidents and hazards that result, or may result, in the catastrophic release of extremely hazardous substances.

The agency’s core mission activities include conducting incident investigations, formulating preventive or mitigative recommendations based on investigation findings and advocating for their implementation, issuing reports containing the findings, conclusions and recommendations arising from incident investigations, and conducting studies on chemical hazards.

The agency’s board members are appointed by the president subject to Senate confirmation.

The board does not issue citations or fines but makes safety recommendations to companies, industry organizations, labor groups and regulatory agencies such as OSHA and EPA.