By Erwin Seba
HOUSTON (Reuters) - The U.S. Chemical Safety Board has concluded a 2015 explosion at a Torrance, California, refinery then owned by ExxonMobil Corp was preventable, according to a report issued on Wednesday.
"Weaknesses in the ExxonMobil Torrance refinery's process safety management program" led to the explosion, according to the federal watchdog's probe of the incident.
The blast resulted in minor injuries to four workers and a lengthy shutdown of part of the refinery, which contributed to a spike in the state's gasoline prices.
The explosion occurred when volatile hydrocarbons flowed backward through an idled gasoline-producing fluidic catalytic cracking unit (FCCU) to a pollution control device called an electrostatic precipitator (ESP), the CSB found.
The generation of sparks by the ESP ignited the hydrocarbons, setting off the explosion.
The weaknesses found by the board included operating the FCCU without pre-established limits for shutdown, the board said.
Also, the CSB said Exxon relied on safeguards that could not be verified and said a safety-critical safeguard had been degraded.
"This explosion and near miss should not have happened," said CSB Chairperson Vanessa Allen Sutherland in a statement. "The CSB's report concludes the unit was operating without proper procedures."
Exxon said in a statement: "We are confident we understand the cause of the blast and have worked cooperatively with the Chemical Safety Board and staff to fully understand their findings and recommendations."
Exxon could not immediately be reached for additional comment.
PBF Energy Inc, which acquired the refinery last year, "has already implemented a number of measures that address the CSB's recommendations," spokesman Michael Karlovich said in a statement. "We plan to complete two studies later this year that will address the remaining recommendations."
The blast blew a large piece of debris 80 feet (24.4 m) to near alkylation unit settler tanks containing toxic hydrofluoric acid, which the board called a "near-miss event."
Hydrofluoric acid is a highly toxic chemical that can seriously injure or cause death at a concentration of 30 parts per million. As a gas it forms a ground-hugging cloud.
The board has asked a federal court to enforce subpoenas against Exxon for safety information about safeguards to prevent or mitigate a release of hydrofluoric acid, according to the report.
Residents near the refinery have pushed local and state officials in California to ban the use of hydrofluoric acid, which is used in refineries to make octane-boosting gasoline additives.
Exxon said there was no evidence indicating the blast "posed any risk" to the alkylation unit "or risk of harm to the community."
The Torrance refinery supplies 20 percent of the gasoline in Southern California and 10 percent statewide.
The CSB has no regulatory authority and does not assess fines. It determines root causes of chemical plant accidents and provides recommendations to companies, industry organizations and regulatory agencies.
(Additional reporting by Liz Hampton in Houston; Editing by Gary McWilliams and Matthew Lewis)