A Motiva Enterprises investigation into the dramatic Aug. 11 fire at its St. James Parish oil refinery found the blaze resulted from a small valve failure that could not have been foreseen and prevented.

The Houston company reached that conclusion in a recent report to the State Police after a monthslong probe into the four-hour blaze that sent black clouds billowing into the sky, forced the evacuation of 1,400 workers and contractors during an intermittent lightning storm and heavily damaged an important H-Oil unit.

The report, which summarized Motiva investigators' "root cause analysis," also disclosed a sizable oil spill and considerable emissions of pollution-forming volatile organic compounds occurred during the fire.

The Motiva probe rejected the idea that lightning from the thunderstorm sparked the blaze at the complex along the Mississippi River near Convent. Company investigators blamed packing inside a 1.5-inch valve, which they said was in a "nonstandard" arrangement based on what the valve's manufacturer recommends.

This packing, which is designed to seal the valve and control the flow of flammable, high temperature hydrocarbons in a pipe, broke down and was ejected, eventually sparking the fire.

"It is not possible to determine if this packing arrangement was the result of an improper installation or a possible manufacturing defect," the Feb. 23 Motiva report says.

Despite Motiva's uncertainty about the origins of the valve's packing, company investigators concluded the fire was not preventable because the valve failure could not have been anticipated. They noted there were no prior warnings of an impending failure and the valve design has a solid track record.

"Motiva has managed these valves in a consistent manner for the life of the H-Oil unit (over 30 years) with no similar failures," the report says.

The H-Oil unit handles the heaviest of the heavy materials left after oil goes through the 225,000 barrel-per-day refinery's first two main processing units, treating the leftover residue so it can ultimately be turned into a diesel-type material and fuel oil elsewhere in the plant.

Company investigators, however, did note a contributing factor to the blaze was a second control valve that also failed, allowing high-temperature hydrocarbons to sit next to the first valve's packing longer than normal.

In any case, after most of the first valve's packing was gone, hydrocarbons seeped out, sparked a flame that weakened the piping connected to a reactor vessel in the H-Oil unit until the pipe ruptured about 10:42 a.m. Aug. 11. The subsequent release of hydrocarbons "auto-ignited upon reaching a flammable mixture in the atmosphere."

The huge fire resulted, however, in no injuries. Several workers credited Motiva's safety procedures for lightning storms, which require workers to stop working and get off plant equipment when strikes are close-by.

The Motiva report was obtained recently through a public records request with State Police.

Greg Langley, spokesman for the state Department of Environmental Quality, said Tuesday the agency still has the fire under review. 

Angela Goodwin, spokeswoman for Motiva, added in an email Monday that the "root cause analysis identified corrective activities, which have been implemented, to prevent re-occurrence." She did not say what the corrections were.

The fire came in the middle of the planned breakup of Motiva's assets by the partners in the joint marketing venture, Royal Dutch Shell PLC and a Saudi Arabian Oil Co. subsidiary.

Motiva announced last month that companies reached an agreement to have Shell assume ownership of the partnership's Convent and Norco refineries and other assets. A Shell spokesperson did not return emails and calls for comment. 

The H-Oil unit is just one part of the crude oil refinery, which produces a variety of gasoline blends, jet and diesel fuel, heating oil, propane, butane, fuel oil and sulfur, a Motiva fact sheet says.

Motiva's fire report estimated that the fire released 21,421 pounds of sulfur dioxide, 470 pounds of hydrogen sulfide and 16,308 pounds of volatile organic compounds into the air. VOCs are known to react with other gases in the air and form pollution while some VOCs, like benzene, also have been shown to be carcinogenic.

The report indicates that some portion of the sulfur dioxide and hydrogen sulfide released during the fire were burned up and broken down in the plant's flare system.

Air tests by DEQ, the U.S. Environmental Protection Agency and Motiva at the time did not find hazardous air releases beyond federal exposure limits.

In addition to the air release, 1,696 barrels of oil, or about 71,232 gallons, also spilled onto the plant's grounds. Motiva reported that some of the oil, which the company said did not leave the facility's boundaries, was cleaned up with vacuum equipment and absorbent pads.

The soil contamination was worsened by heavy rain, which flooded the plant's wastewater system, the Motiva report says. 

Reuters reported that another fire broke out in the H-Oil unit March 18 while the refinery was trying to restart one of the unit's production trains for the first time since the August fire. The H-Oil unit had been operating at half-capacity since November.  

Reuters cited two anonymous sources "familiar with plant operations," but Goodwin, Motiva spokeswoman, would not confirm the fire was in the H-Oil unit or comment on its current status for "reasons of commercial confidentiality."

St. James Parish Sheriff Willy Martin and Francis Hymel, parish homeland security director, confirmed that a fire was reported at the plant March 18 and was extinguished by plant firefighters.

Follow David J. Mitchell on Twitter, @NewsieDave.