TORRANCE ( — ExxonMobil has postponed Saturday’s plan to restart a Torrance refinery damaged by an explosion last year until Monday night.

The company cited unexpected operational delays as the reason.

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ExxonMobil had begun notifying about 11,000 residences and businesses near the refinery of the planned restart, which was expected to happen between 7 p.m. Saturday and 7 a.m. Sunday.

The company is now required to renotify them of the new restart time, according to officials with the South Coast Air Quality Management District.

As a safety precaution, ExxonMobil will shut down a pollution-control device for six hours during the 12-hour overnight period, from 7 p.m. Monday to 7 a.m. Tuesday.

That shutdown will result in up to 600 pounds of excess particulate emissions. But AQMD does not expect it to expose residents to unhealthy levels.

An AQMD hearing board has approved the restart plan. Regulators said they will be keeping a close watch on the operation.

“We are taking a number of steps to protect nearby residents when the refinery starts up and resumes operations,” said Wayne Nastri, acting executive officer of the AQMD. “One of those measures includes deploying an air monitoring network to measure fine particulate levels in the air around the refinery during the startup process.”

The refinery has been largely out of operation since Feb. 18, 2015, when an explosion caused extensive damage to the facility, injured four people and led to spiking Southern California gas prices.

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Under the restart agreement approved by the AQMD hearing board, ExxonMobil must pay about $5 million in penalties for air pollution violations that caused by the blast.

The refinery was sold to New Jersey-based oil refining company PBF Energy in September. The $527.5 million deal is expected to close in the second quarter of 2016.

The 750-acre refinery has a capacity of 155,000 barrels per day. With the purchase, PBF will increase its total capacity to about 900,000 barrels per day, according to the company.

Federal authorities blamed a breakdown in safety procedures for causing the 2015 explosion. According to the U.S. Chemical Safety Board, the trouble began six days before the blast when a problem developed with a piece of equipment known as an expander, forcing the plant’s “fluid catalytic cracking” unit to be shut down.

That shutdown led to steam being forced into a reactor, and some was leaking from an open flange that was preventing plant employees from carrying out repair work, the board found.

When a supervisor reduced the flow of steam, it caused hydrocarbons to flow into the plant’s electrostatic precipitator, where the hydrocarbons were ignited, causing the explosion.

State regulators issued 19 citations against ExxonMobil and proposed penalties totaling $566,600 in response to the explosion.

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Cal/OSHA officials said a 2007 safety review found problems with flammable vapor in the plant’s electrostatic precipitator, but no corrective actions were taken. Regulators noted that the plant’s fluid catalytic cracker had not been working properly for as long as nine years prior to the blast.