The root causes of an explosion at the ExxonMobil Refinery in Torrance, Calif., can be traced to multiple process safety management deficiencies. The U.S. Chemical Safety Board says the February 18, 2015, explosion occurred in the refinery’s electrostatic precipitator (ESP), which is used to control air pollution. The blast dispersed large quantities of catalyst dust up to a mile away from the facility.

The preliminary investigation report from the CSB found the sequence of events that led to the explosion at the refinery began on February 12, 2015. On that day, problems with an expander caused the refinery’s fluid catalytic cracking (FCC) unit to be put into an idled condition referred to as safe park. With the FCC unit shut down, steam was forced into the reactor to prevent hydrocarbons from flowing back from the main distillation column.

On the morning of the accident, this steam was escaping through an open flange on the expander, preventing operators from continuing their maintenance work. It had traveled through a leaking slide valve connected to the reactor.

An outside supervisor reduced the amount of steam being forced into the reactor so that work could continue. However, at the time, workers were unaware that hydrocarbons were leaking into the main distillation column from interconnected equipment. As the pressure of the steam dropped, the hydrocarbons flowed back into the reactor, out through the leaking slide valve and eventually into the ESP. There the hydrocarbons found an ignition source and exploded.

At ExxonMobil, two workers were injured when the explosion occurred. The CSB also found that large pieces of debris from the explosion were thrown into other units of the refinery directly surrounding the ESP. One of these pieces of debris hit scaffolding in the refinery’s alkylation unit, narrowly missing a tank containing tens of thousands of pounds of modified hydrofluoric acid (HF). The CSB determined that had the debris struck the tank, a rupture could have been possible, resulting in a potentially catastrophic release of extremely toxic modified HF into the neighboring community.

CSB Chairperson Vanessa Allen Sutherland said, “Hydrofluoric acid can pose a severe hazard to the population and environment if a release occurs. After HF acid vaporizes, it condenses into small droplets that form a dense low-lying cloud that will travel along the ground for several miles.... [HF]can cause severe damage to the respiratory system, skin and bones of those who are exposed, potentially resulting in death.”

While the investigation into explosion is ongoing, CSB investigators already have identified multiple process safety management (PSM) deficiencies that it says helped contribute to the accident. For instance, in order to perform work to bring the FCC unit back online, ExxonMobil determined it needed to deviate from several existing procedures. This required a document called a variance, which is a written temporary deviation from normal operating procedures. The variance used was created in 2012 to address problems with the expander. CSB investigators found that ExxonMobil did not conduct a management-of-change review before implementing this outdated variance even though conditions within the FCC unit had changed over the previous three years.

Also, ExxonMobil performed inadequate process hazard analyses, says CSB. PHA could have identified more effective safeguards against the flow of hydrocarbons such a blind or de-inventorying the main distillation column. Investigator-in-charge Mark Wingard said, “Although our investigation found two different process hazard analyses that considered a combustible mixture igniting in the electrostatic precipitator, no effective safeguards were implemented at the refinery to mitigate this threat.”

CSB presented its preliminary findings at a public meeting on January 13 in Torrance.